HEATING SYSTEMS
BY THE SAME AUTHOR
DOMESTIC SANITARY ENGINEERING AND PLUMBING
Dealing with Domestic Water Supplies, Pump and Hydraulic Ram Work, Hydraulics, Sanitary Work, Heating by Low Pressure, Hot Water, and External Plumbing Work.
With 277 Illustrations. 8vo, IDS. 6d. net.
LONGMANS, GREEN, AND CO.
LONDON, NEW YORK, BOMBAY AND CALCUTTA
HEATING SYSTEMS
DESIGN OF
HOT WATER AND STEAM HEATING APPARATUS
BY
F. W. RAYNES
CONSULTING HEATING AND VENTILATING ENGINEER
LECTURER ON HEATING AND VENTILATING, THE ROYAL TECHNICAL
COLLEGE, GLASGOW
WITH ILLUSTRATIONS
LONGMANS, GREEN, AND CO.
39 PATERNOSTER ROW, LONDON NEW YORK, BOMBAY, AND CALCUTTA
All rights reserved
PREFACE
THIS work presents the most modern practice in an im- portant branch of Engineering. It is freely illustrated with a large number of drawings, and some catalogue prints are also included where special drawings would possess little further advantage. It is not, however, the writer's intention to convey the impression that he favours the products of one maker in preference to those of another.
A special feature of the book is the large number of Charts that have been prepared, and the method adopted in sizing the pipes of different systems. Attention has been directed rather to the practical than to the theoretical aspects of the work, whilst in the sizing of pipes, the process in a large measure is merely a mechanical one. Consideration is also given to the economical aspect of heating problems, and especially in connection with the heating of works or of industrial buildings. The book is therefore intended for the busy professional or business man, as well as for the use of students.
The writer desires to thank all who have assisted either directly or indirectly in the production of the work.
F. W. E.
THE KOYAL TECHNICAL COLLEGE, GLASGOW.
CONTENTS
CHAPTER I GENERAL
PAGE
Ventilation — Impurity of air — Organic poisons — Ozone — Mechanical
ventilation — Heating systems 1
CHAPTER II HOT- WATER CIRCULATION
Gravity circulation— Circulating head — Velocity of gravity circulation — Dipped or trapped circuits — Irregular circulation — Forced circula- tion—Accelerated circulation 16
CHAPTER III
SYSTEMS OF PIPING FOR HOT-WATER GRAVITY APPARATUS
One-pipe up-feed systems — Two-pipe systems — Down-feed systems —
Accessory apparatus — Joints for copper pipes 33
CHAPTER IV
SMALL-BORE GRAVITY APPARATUS Systems of piping — Expansion tubes — Medium pressure systems ... 48
CHAPTER V
ACCELERATED HOT- WATER CIRCULATING SYSTEMS Heat generators— Various systems 56
CHAPTER VI
FORCED HOT-WATER CIRCULATING APPARATUS
One-pipe system with loop circuits — Two-pipe systems — Connections of heaters— Plants for high buildings — Duplication of Pumps- Methods of temperature regulation 69
viii CONTENTS
CHAPTER VII
LOW-PBESSUBE "LIVE" STEAM HEATING SYSTEMS Heat of steam — Drop of pressure in pipes — Water hammer — Gravity
PAOI
systems — False water lines ............. 82
CHAPTEE VIII FITTINGS FOB LOW-PBESSUBE STEAM SYSTEMS
Pressure reducing valves— Air valves— Steam traps— Beturn traps —
Pump receivers ....... ....... 95
CHAPTER IX
EXPANSION OF PIPES
Springing pipes — Expansion joints — Expansion bends — Application of
tensile strain in jointing ......... 109
CHAPTER X
ATMOSPHEBIC SYSTEMS OF STEAM HEATING Different piping systems— Begulation— Fittings used .... 119
CHAPTER XI
EXHAUST STEAM HEATING
Heat of exhaust steam— Belative cost of exhaust heating . -. 129
CHAPTER XII
EXHAUST STEAM HEATING-contom^ De-oiling processes-Back-pressure valves-Feed-water heaters-Boiler
sd pumps- Systems of exhaust heating . ..... 143
CHAPTER XIII
VACUUM AND VACUO-VAPOUB SYSTEMS OF STEAM
HEATING Merits and limitations-Various systems, and their special features . . 159
CHAPTER XIV ACCESSOBIES FOB VACUUM SYSTEMS
......... 17d
CONTENTS IX
CHAPTER XV HEATING SURFACES
Radiant heat — Convected heat — Resistances to the transference of heat
—Radiators— Humidifying Radiators— Radiator shields . . . 183
CHAPTER XVI
VENTILATING AND INDIRECT RADIATORS Size of ducts to radiators— Volume of air for gravity indirect heating . 192
CHAPTER XVII HEAT LOSSES FROM BUILDINGS
Heat transmission coefficients— Heat absorbed by air— Heat lost by
cooling surfaces— Effect of wind ........... 202
CHAPTER XVIII
QUANTITY OF HEAT EMITTED BY RADIATORS, PIPES, AND INDIRECT HEATERS
Heat emitted by direct surfaces— Heat emitted by indirect surfaces-
Temperature charts ......... ..... 209
CHAPTER XIX AREA OF HEATING SURFACE TO WARM BUILDINGS
Formulae for estimating surface — Size of ducts to gravity heaters — Size
of indirect heaters— Application of charts ........ 216
CHAPTER XX
SIZING PIPES FOR GRAVITY SYSTEMS OF HOT-WATER HEATING
General formulae — Resistance of pipe fittings — Tables— Equivalent resistance of pipes — Charts and their application ......
CHAPTER XXI SIZING PIPES FOR FORCED HOT-CIRCULATNG SYSTEMS
in pumping — Charts and their application— Special cases ... 255
X CONTENTS
CHAPTEK XXII THE SIZING OF PIPES OF STEAM-HEATING SYSTEMS
PAGE
General Formulae —Sizes of steam and return pipes — Sizes of drip
pipes— Steam charts and their application 269
GHAPTEE XXIII
BOILERS
General aspects of boilers— Efficiency of boilers — Bating of boilers —
Chimneys » 287
CHAPTEE XXIV THE TEMPERATURE CONTROL OF BUILDINGS
Automatic regulation — Mechanical devices — Compressed air — Water
power and other sources of energy — Thermostats — Automatic valves 302
APPENDIX . . . .... . . . . . ..... . . . 308
INDEX 321
TABLES
PAGE
I. LENGTH OP FURNACE COILS FOB SMALL-BORE APPARATUS . 53 II. APPBOXIMATE EXPANSION OF WATEB BETWEEN 40 AND 60
DEGBEES *..... 53
III. PBOPEBTIES OF METALS 110
IV. DIMENSIONS OF EXPANSION BENDS ........ 116
V. VALUES OF K FOB ESTIMATING CONSUMPTION OF STEAM . . 137
VI. COEFFICIENTS FOB BBICK WALLS 203
VII. HEAT TBANSMISSION COEFFICIENTS FOB DIFFEBENT KINDS OF
WALLS . . . . . . . . . 203
VIII. HEAT TBANSMISSION COEFFICIENTS OF GLASS AND OTHEB
SUBFACES . . . ' . . . .204
IX. APPBOXIMATE VALUE OF / WHICH INCLUDES MINOB LOSSES, ALLOWANCE FOB EXPOSUBE, ASPECT, HEIGHT OF ROOMS,
ETC . 207
X. VOLUME OF AIB DELIVEBED BY DUCTS PER HOUR .... 207
XI. HEAT EMITTED BY WROUGHT-IRON PIPES 209
XII. HEAT EMITTED BY RADIATORS 210
XIII. VELOCITIES OF AIB THROUGH DUCTS 220
XIV. VALUES OF c1} OR COEFFICIENTS OF FORMULA FOB SIZING
HOT-WATEB PIPES . . . 229
XV. APPBOXIMATE CAPACITY OF MAIN CIRCUITS OF LOW-PBESSUBE
GRAVITY HOT- WATER SYSTEMS 240
XVI. APPROXIMATE CAPACITY OF COMPOUND RISERS OF LOW-PRES- SURE GRAVITY HOT-WATER SYSTEMS 241
XVII. VALUES OF c2, OR COEFFICIENTS FOR STEAM PIPE FORMULAE 270
XVIII. SIZES OF RETURN PIPES FOR STEAM HEATING SYSTEMS . 273
Xll TABLES
PAGE
XIX. CAPACITY OF GRAVITY STEAM HEATING SYSTEMS .... 281
XX. CALORIFIC VALUE OF FUELS 290
XXI. PROPERTIES OF STEAM 308
XXII. WEIGHT OF DRY AIR 309
XXIII. LENGTH OF PIPE OFFERING THE EQUIVALENT EESISTANCE
OF PIPE FITTINGS 310
XXIV. PROPORTIONAL RESISTANCE OF PIPES OF VARIOUS SIZES . 311 XXV. HYDRAULICS MEMORANDA 311
XXVI. WEIGHT OF WATER AT DIFFERENT TEMPERATURES . . . 312
XXVII. WEIGHT OF METALS PER SQUARE FOOT 313
XXVIII. WEIGHT OF METALS PER SQUARE FOOT 313
XXIX. WEIGHT OF CAST-IRON PIPES 314
XXX. WIRE AND PLATE GAUGES 315
XXXI. LOGARITHMS . . . . . _. 316
XXXII. ANTILOGARITHMS . 318
CHARTS
PAOE
1. Temperature to which air is warmed by indirect heaters . . . 211
2. Ditto 212
3. Ditto -212
4. Ditto 213
5. Ditto 213
6. Ditto 214
7. Ditto 214
8. Ditto 215
9. Capacity of circuits of low-pressure hot- water systems .... 232
10. Ditto 233
11. Ditto 234
12. Ditto 235
13. Ditto 236
14. Ditto "". 237
15. Ditto 238
16. Ditto ...• 239
17. Capacity of circuits of forced hot- water circulating systems . . . 258
18. Ditto 259
19. Ditto 260
20. Ditto 261
21. Capacity of circuits of steam-heating systems 275
22. Ditto 276
23. Ditto 277
24. Ditto 278
25. Ditto 279
26. Ditto 280
27. Assumed efficiency of boiler • . . . 293
28. Ditto 293
29. Size of chimney 301
FORMULA
PAGE
1. Circulating head 20
2. Velocity of circulation 21
3. Length of expansion tubes . 53
4. Expansion of pipes 109
5. To find point of anchorage of pipes 110
6. Ditto 110
7. Temperature to which pipes may be raised without being over-
strained 117
8-12. Heat value of exhaust steam 132
13. Ditto 134
14. Mean effective steam pressure on piston of engine 136
15. Value of K or cut-off value 136
16. Approximate weight of steam consumed per indicated horse-power
per hour when exhausting into heating system 136
17-20. Percentage cost of exhaust steam heating .139
21, Area of ducts for ventilating radiators . . 193
22, Volume of air passing through flues to ventilating radiators . . 193
23, 26. Volume of air for gravity indirect heating 197, 199
24, 27. Temperature to which air requires to be raised in indirect
heating 197,199
25, 28. Total heat absorbed by air for indirect heating .... 198, 199
29. Volume of air in cub. ft. at any temperature desired 199
30. Heat lost by walls and other cooling surfaces 205
31. Average air temperature of rooms 206
32. 34, 35. Heat lost from buildings 206-208
33. Flow of air through ducts 207
36-40. Area of heating surface 216, 217
41. Area of ducts for indirect heaters 221
42. Face area of indirect heaters 221
43, 45. Capacity of gravity hot water circuits 228-230
44, 46. Diameter of circuits of gravity hot-water systems .... 228-230
47. Head absorbed by sharp elbow fittings 230
48. Length of pipe having equivalent resistance to sharp elbow fittings 231
49. Weight of water circulated in forced hot- water systems .... 255
50. Ditto 256
51. Horse-power absorbed by pipe friction 256
xvi FORMULAE
PAGE
52, 53. Horse-power absorbed by pump 256
54, Proportional length of a loop in terms of a main circuit .... 266
55, Weight of water circulating through branch loop in forced circulating
systems 266
56, 59. Capacity of steam pipes 269, 270
57, 60. Size of steam pipes 269, 270
58, 61. Drop of pressure in steam pipes 269, 270
62. Height of wave motion of condensation in return pipes .... 271
63. Permissible steam velocities in pipes 271
64. Sizes of drip pipes or bleeders 274
65. 66. Capacity of boilers 294
67, 68. Size of boilers 295
69. Velocity of flow in chimneys 299
70. Size of chimneys 300
CHAPTEE I
GENERAL
ON reviewing the development of heating and ventilating apparatus, one cannot fail to be struck with the progress made during the last few years. On the economical side of heating work, much, however, remains to be done, in order that a large proportion of the heat now lost can be utilized for the service of man.
Ventilation. — The advantages derivable from a plentiful supply of pure fresh air at a suitable temperature and humidity are widely recognized, although there is a sharp difference of opinion as to the precise form a plant should take to give the best results.
In Great Britain there has been no legislation up to the present to enforce the ventilation of public buildings. This is most unfortunate; for the heating and ventilating of these places are often notoriously bad. In a comparatively new school the writer inspected a short time ago, less than 5500 cubic feet of air were flowing through the fresh-air inlets of one of the rooms per hour. This was often occupied by fifty -four children. Thus less than 100 cubic feet of air per hour per person were accounted for by direct ventilation. Although this would scarcely represent the true state of affairs owing to air leakages, the case is sufficiently marked to show that little wonder need be expressed at the difficulty experienced by the children in performing their tasks, on account of the drowsiness produced by the stuffy and very humid atmosphere that envelops them. This case, of course, is aggravated in the summer time.
The ventilation of many other buildings is as bad as the case cited, and it is high time a certain minimum standard was enforced by law in the interest of the community at large.
2 * DFi(iN b HOT -vrrsjR & STEAM HEATING APPARATUS
In most of the American States 1800 cubic feet of air per person per hour is the minimum volume allowed for public buildings, and this is generally regarded as the smallest volume that should be provided to keep the atmosphere of a room reasonably pure.
Impurity of Air. — On some phases of ventilation opinions are undergoing a change. For example, the generally accepted standard of air, as regards its fitness to be inhaled, has been based upon its chemical impurity in terms of the contained carbonic acid gas. Koughly speaking, it has been considered important that in ten thousand parts of air there should be not more than twelve parts of carbonic acid gas, whilst for good ventilation the same volume of air should not contain more than six parts of carbonic acid gas.
Many physiologists now maintain that the percentage of carbonic acid gas is of no great moment, and the thing that really matters is the air temperature and percentage of moisture present. Quite recently, experiments on the physiological aspect of ventilation have been conducted, both here and abroad, the general conclusions being the same. The method adopted in these experiments has been to confine one or more subjects in a small chamber, the ventilation of which is under absolute control. During the period of confinement the car- bonic acid gas has been allowed to run up to over one hundred parts in ten thousand parts of air, and in some cases to over twenty times the amount that is likely to accumulate in ordinary rooms. It is stated that under these conditions the subjects experienced no discomfort.
Of the experiments made by Dr. Leonard Hill, of the London Hospital, the following observations were made by him some time ago to the Eoyal Commission appointed to inquire into the condition of weaving sheds : — " We have a small chamber which holds about 3 cubic metres. Into this chamber I put eight of my students, and seal them up. ... At the end of half an hour the wet-bulb temperature has gone up to 85 degrees" . . . when "their faces are congested with blood. The C02 has gone up to four, or even five, per cent., and the oxygen down to a corresponding extent. Well, in these con- ditions I put on three electric fans, and do nothing else than
GENERAL 3
whirl the hot air just as it is ... the students at once feel as comfortable as possible, but immediately the fans are stopped they feel as bad as ever, and beg for the fans to be started again. . . . All this tends to show that when you have a stationary moist air, warmed up around the body, you get discomfort ; when the fans are put on the air is stirred up, and the cool air is brought into contact with the body, and the discomfort ceases."
In quoting a few passages apart from their context, the writer does not wish to convey the idea that the doctor is not an advocate of a plentiful supply of air, but that he, along with other physiologists, contends that the poisonous effects of car- bonic acid gas have been overstated, whilst insufficient attention has been paid to the conditions of temperature and humidity.
For many years those interested in the problem of ventila- tion have not considered a specific quantity of carbonic acid gas in itself as injurious, but rather as an index of the contained organic impurity which may be harmful.
Organic Poisons. — It has generally been assumed that expired air contains matter of a poisonous nature, although on this point physiologists differ. Some experimenters, to prove there are toxic constituents in expired air, have collected liquid from the condensed vapours of expired air, and injected it into animals. The subjects of the experiments in some cases succumbed, but whether it was due to poisons or otfcer effects must . be left to the physiologists to decide. Other experimenters have failed to detect organic poisons in expired air, and in an address to the British Institute of Heating and Ventilating Engineers, Dr. Leonard Hill expressed the opinion that " the experiments which started this organic poison theory are really absurd. I cannot find that this organic poison exists."
Assuming the question of organic poisons in expired air is a disputable one, there is no doubt as to the presence of infective bacteria when people have colds, and other infectious complaints. It is, of course, impracticable to have an atmo- sphere free from infectious germs ; but the more effective the ventilation, or the purer the air, the less risk there is of infection, and where other things are equal, the better able the constitution becomes to resist the attack of poisonous germs.
4 DESIGN OF HOT-WATER & STEAM HEATING APPARATUS
Important as ventilation is, immunity from disease does not by any means entirely depend upon it. Suitable diet, sunshine, rest, and environment all play their part, as well as psychological and physiological factors.
From the experiments of Dr. Hill and others, it has been shown that a person may live in stale air for a prolonged time, without experiencing discomfort or apparently any ill effect, so long as it is kept circulating, and the temperature and moisture do 'not rise too high. Very few, however, would be satisfied with such an atmosphere because injurious effects had not been proved. If a person is run down in health a visit to some health resort, or a sea voyage, is often suggested; the con- sumptive has outdoor treatment. In each case, the intention is for the subject to be more or less continuously swept by a pure cool atmosphere. This is the idea to be incorporated in a system of ventilation, as advanced by the modern physiologist.
The problem of ventilation is not always a simple one, for it is not merely a matter of passing so many cubic feet of air into a building.
When people congregate in large numbers in a room the temperature of the latter is raised to an appreciable extent through the heat evolved by them. As soon as the air tem- perature approaches that of the body, the radiation of heat from the occupants is interrupted, whilst the circulation of air about them is retarded. This has the effect of causing the occupants to be enveloped in a very humid and stagnant atmosphere, and this it is that causes the drowsy feeling and discomfort so fre- quently experienced in a large audience.
Ideal System. — The ideal ventilating system is sometimes said to be the one that will reproduce natural conditions. This is not a practicable thing, even assuming the difference in the chemical purity of internal and external air to be ignored. External conditions are very changeable, and to these humanity adapts itself. When, however, it comes to internal tempera- tures, our civilization requires these to be adjusted to suit the individual, although it is the antithesis to the natural order.
The maintenance of uniformity of temperature in buildings has received considerable attention, and many special appliances have been devised to attain this end. From a physiological
GENKKAI. 3
standpoint this side of the problem may have been earned too far, but the saving effected in fuel by the prevention of over- heating has been greatly in favour of the installation of these fittings.
As regards the purity and suitability of the air that can be obtained in large structures this is very much a problem of cost for the handling and treatment of the air, the means adopted for its distribution, and the temperature at which it is admitted.
Ozone. — Another phase of ventilation is the use of ozone which is generated by a special machine. As an adjunct to a ventilating plant this has advantages for deodorizing purposes, and for imparting freshness to air when for economical reasons sufficient air changes cannot be effected. For ventilation, ozone is only used in a very diluted form, a high concentration being injurious in that it acts as an irritant on the respiratory tract.
As ozone is a very unstable quantity, its efficacy in ventila- tion will largely depend upon how it is introduced and upon the temperature of the air. If the air is ozonized and after- wards brought in contact with heating surfaces, it is possible that very little ozone will pass with the air into an apartment. More or less dust always gathers upon and floats about the heating surfaces, and with this the ozone combines, and all the more readily when it contains matter of an organic nature and is in a heated state. The use of ozone, in the writer's opinion, is better where the air does not require to be warmed.
On a large scale ozonized air is being used by the Central London Eailway Company, and when the scheme is complete, it will be capable of delivering 80,000,000 cubic feet of ozonized air per day through the tubes and into the stations. The proportion of ozone used is one part in ten million parts of air.
Heating and ventilating may be carried out either as separate units, or they may be combined. Each method has its special features according to the class of structure to be dealt with.
Natural Ventilation. — Natural or gravity ventilation, although suitable for small and one-storey buildings that are not very wide, is inadequate for large high buildings. As the term implies, it is entirely dependent upon the elements or forces of nature, and
6 DESIGN OF HOT-WATER & STEAM HEATING APPARATUS
as these are erratic in their action, the systems depending upon them act in the same way.
A combined arrangement of mechanical and gravity ventila- tion, if properly designed, will satisfy most conditions that may be demanded. The air before delivery into buildings may be purified, tempered, or cooled, its relative humidity adjusted to suit the changing weather conditions, a definite volume of air may be delivered to any point desired, and a system as a whole may be easily controlled.
Mechanical Systems. — In mechanical ventilation the air moved is frequently utilized as the conveyer of heat for warm- ing purposes, and such systems are often designated as "hot blast" ones.
In America, hot-blast heating for public buildings, as generally carried out, is looked upon by many with disfavour. The high temperatures to which the air is usually raised bring about chemical changes by decomposing some of the contained dust. Thus the air loses its freshness, and becomes to a certain extent vitiated before it enters a room. It is also thought that the overheating process devitalizes air by robbing it of properties that cool fresh air contains.
So far as Great Britain is concerned, the drawbacks of hot- air heating have never been so acute as in America. This, how- ever, is not due to the superior way in which the engineering side of the problem is handled, but rather to our mild climate, and to the lower temperatures to which we are accustomed.
Good results can be obtained by " hot-blast " or indirect heating, but to get these the temperature of the heating surfaces and that of the air must be kept down, whilst the heatiug surfaces should also be kept as free as possible from dust.
In large buildings independent heating and ventilating is now very much practised, direct heating surfaces being used to replace the heat lost by windows, walls, and other cooling surfaces. The air, under these circumstances, only requires to be raised at the fans to a temperature a little higher than that maintained in the rooms. Such an arrangement permits of the fans being stopped when a building is unoccupied, power is saved, whilst the temperature may be fully maintained by the direct heating surfaces ; further, it is very flexible in operation,
GENERAL 7
and the volume of air to any apartment may either be diminished or increased without affecting its temperature.
Hot-blast systems of heating and ventilating possess advan- tages for industrial buildings, in that the whole of the heating surfaces and power units are centralized. The initial cost is also lower than where the heating and ventilation are treated separately. There is no one system that will satisfy the requirements of all classes of buildings ; every case requires to be considered on its own bearings.
Mechanical ventilating plants are usually classed either as " vacuum," " plenum," or combined ones. Each system has its advantages and limitations. A " vacuum " system is understood to be one where the air is withdrawn from an enclosed space by locating fans in the outlet ducts, and where the air pressure of the space is slightly less than that outside.
Vacuum Systems. — Generally speaking, when a vacuum system is installed in an ordinary building, its success depends upon the location, number, and area of the fresh-air inlets. Badly designed vacuum systems are liable to produce un- pleasant draughts, owing to the inward leakage of air around windows, doors, and through other crevices; the air currents may move directly from inlets to outlets without being diffused over the greater area of a room.
For localized ventilation, a vacuum system is specially valuable, such as for the immediate and direct removal of dust and fumes that are produced in connection with dangerous trades. Smoke rooms and other apartments into which vapours are emitted can be better ventilated by a vacuum than by a plenum system.
Plenum systems of ventilation are those in which the air is propelled into rooms by locating fans or other air movers in the inlet ducts. It is assumed in these systems, that the air in the apartments exceeds in pressure the external atmosphere, and that any air leakage through irregular channels will rather be outwards than inwards. Whether this will be the case or not largely depends upon the features embodied in the design. That considerable outward leakage takes place in many plenum systems is well known, owing to the fact of the entering air sometimes exceeeding by 25 per cent, that recorded at the
8 DESIGN OF HOT-WATER & STEAM HEATING APPARATUS
outlet. On the other hand, a number of systems admits of more or less considerable inward leakage.
The principal merits attributed to plenum systems are, less liability to draughtiness, and the better diffusion of the inflow- ing air, whilst the air may also be " conditioned." The defects of these systems very largely arise through the outlet ventila- tion being controlled more or less directly by natural agencies. The extent to which this weakness becomes marked is governed by the arrangement of the outlet ducts. Where, for example, the outlet duct from a room terminates above a roof, the wind affects its rate of discharge and often upsets the balance of a system. Or again, where a number of outlet ducts terminate in the roof space before discharging into the external air, the difference in the power of their draughts is sometimes so marked as to cause the flow of air to be reversed in one in order to supply a duct with a stronger draught.
A failing common to both vacuum and plenum systems, is their easy disorganization by the opening of doors and windows. Some, to minimize this drawback, advocate the use of locked windows, but the remedy suggested is worse than the disease.
Combined Systems. — An installation that is the least liable to derangement is one in which the plenum and vacuum systems are combined. This combination permits of the control of both the inflowing and outflowing air, and when the propelling and extracting forces are properly balanced, the opening of windows (which is so often desirable) or doors will have no adverse effect upon a system at any other point. The initial and operating costs, however, are necessarily greater in the com- bined than in the separate systems, on account of the higher initial and operating costs. /*"
Heating Apparatus. — Buildings may be warmed by either of the following : —
(a) Open fires.
(b) Stoves.
(c) Hot-air furnaces.
(d) Hot- water apparatus.
(e) Steam-heating systems. (/) Gas fires.
(g) Electric heaters.
GENERAL 9
Open Fires. — If viewed only from an economical stand- point, open fires have nothing to recommend them. The heat usefully employed is but a small percentage of that which the fuel yields. All the heat, however, that passes into the chimney is not lost, as a portion is essential to produce the draught. Open fires have a cheerful effect, and it is this property that so strongly appeals to the average Britisher. The chimney also makes a good outlet ventilator. The chief drawbacks of open fires, in addition to wastefulness, are the amount of work they entail, their large share in the pollution of the atmosphere (especially in congested areas), and their over-heating and under-heating effects according to position in room. They are often combined with other forms of heating where economy is of secondary importance.
Hot -Air Furnaces. — In this country, heating by these fur- naces finds little favour, although with a well-proportioned and installed system fairly good results may be obtained. Furnace systems are of two lands, the first where the circulation of the heated air depends upon the force of gravity, the second where the air is propelled' or drawn over the heating surfaces by fans, and after ward's , forced through the distributing ducts to the apartments to be warmed. The drawbacks associated with furnace heating are often due to the furnaces being too small, to the over-heating of the surfaces, to structural defects, and to the ducts not being properly sized. When fans are employed, the air may be purified and otherwise treated. Furnace systems of heating are less costly than hot- water and steam installations, but they are less durable. Generally speaking, the heated air from furnaces is rather dry.
Hot- Water Gravity Apparatus. — Systems using hot water as the circulating medium* may be roughly divided into two classes, (a) open systems, and (b) sealed systems. Those which are open to the atmosphere are usually termed low-pressure installations irrespective of what the hydrostatic pressure at any point may be.
Sealed systems take two principal forms. In the first, the internal pressure may be raised to any extent desired, this being limited only by the strength of the apparatus. In the second the maximum internal pressure is limited, loaded valves
10 DESIGN OF HOT- WATER & STEAM HEATING APPARATUS
or mercury seals being used to afford relief. The principal purpose for sealing a system is to raise the boiling-point of the water to a temperature higher than that corresponding with atmospheric pressure.
Low-Pressure Systems. — For dwellings, schools, workrooms, offices, etc., low-pressure hot-water heating with direct surfaces is specially advantageous. The apparatus is easily managed, the temperature of the circulating water can be increased or dimin- ished to suit the weather, the heat emitted may be of a mild nature, all unpleasantness through the over-heating of the dust that accumulates on the radiators may be obviated, the tempera- ture of rooms can be readily controlled, and with ordinary care the apparatus is very durable.
The chief drawbacks associated with low-pressure apparatus are the large heating surfaces and pipes required. Neither do systems which hold large volumes of water, readily lend them- selves to automatic regulation.
Sealed Systems. — Small-bore hermetically sealed apparatus can be applied to a variety of uses. At one time this system was largely used for warming dwellings, churches, and other public buildings, but it is better suited for drying-rooms, where high temperatures are necessary, for boiling- pans, for bakers' ovens, and for other industrial uses.
Systems sealed with loading devices, such as valves and mercury seals, are suitable for buildings where for economical reasons moderately high water temperatures are adopted. On account of the higher temperature that can be obtained when compared with open systems, a smaller quantity of: heating surface is necessary, smaller pipes may be used, and in conse- quence the initial cost is less.
Hot- Water Apparatus with Forced and Accelerated Circula- tion.— As the energy producing the circulation of water in gravity systems is a very limited quantity, the use of these is in consequence restricted. In forced circulating systems, the water is moved by positive means, whilst in accelerated circu- lating systems the head producing movement is in excess of that in ordinary installations when erected under similar con- ditions. The principal features of these special systems are : Small pipes can be used in virtue of the quickened movement
GENERAL 11
of the water ; heat can be transmitted long distances without undue cooling of the water ; there is greater freedom as regards the way in which the pipes may be arranged ; and when instal- lations are of a large size they are less costly to erect.
Steam Heating Apparatus. — These may be divided into the older and the newer forms. The former includes " high " and " low " pressure systems in which the pressure in the return pipes is greater than that of the atmosphere. In the latter, the heating surfaces and the return mains are open to atmo- spheric pressure. The newer forms of apparatus have been developed at a remarkable pace the last few years, and most of the drawbacks incidental to the earlier forms have been removed.
High-Pressure Steam Heating is usually confined to places where high temperatures are required, such as drying-rooms, stoves, Turkish baths, etc. It is unsuitable for general heating work on account of the highly heated surfaces affecting the quality of the air.
Low-Pressure Steam Apparatus. — Although there is no precise definition of the term "low pressure," it is generally understood to be associated with a system in which the steam pressure is less than 10 Ibs. per square inch (gauge pressure). Usually the gauge pressure does not exceed 5 Ibs. per square inch.
Low-pressure steam heating is suitable for works, large public and private institutions, and for buildings that are irregularly heated, and where there would be danger of a water system being damaged by frost. Another feature of steam heating is that the boilers readily lend themselves to the automatic control of the rate of combustion.
The principal drawbacks to the early types of apparatus are : The temperature of the heating surfaces cannot be regulated by the valves — they must either be fully on or off ; the steam at all times must exceed in temperature 212°, irrespective of external conditions; they are less economical than corresponding systems in which water is used ; clicking or hammering sounds are often produced, and a certain percentage of the heating surfaces is ineffective.
Atmospheric Steam-Heating Systems. — These differ princi-
12 DESIGN OF HOT- WATER & STEAM HEATING APPARATUS
pally from the ordinary low-pressure apparatus in that the returns are open to the atmosphere, and no resistance is offered to the water of condensation other than that due to pipe friction ; the steam supply is also restricted to the amount a radiator or other surfaces can condense. The boiler pressure or other source of steam supply is limited to a few ounces per square inch, or to a greater pressure, depending upon the conditions the plant is required to satisfy.
The advantages of atmospheric systems over the earlier forms are : the lower temperature of the heating surfaces, the degree of regulation afforded by the radiator valves, and the units into which a plant can be divided with a simple arrange- ment of piping. Their operating costs are also less than those of ordinary steam systems. Defective appliances often cause waste of steam, but this can be avoided by the adoption of suitable fittings, and the proper regulation of the steam supply.
Vacuum and Vacuo Vapour Systems of Steam Heating. —These take manj forms, differing principally in their mode of operation and in the appliances that are used. Speaking broadly, they differ chiefly from atmospheric systems in the manner the differential pressure is produced to cause the circulation of the steam, and to remove the air from the heating surfaces. In vacuum systems some form of exhauster is employed, whilst in atmospheric systems the differential pressure is due to the direct fall of pressure upon the steam entering the heat- ing surfaces. The principal vacuum systems may be divided into two classes : the first in which an independent air line is employed for the removal of the air, and the second, that in which the air and water of condensation are conveyed by the same pipe.
A good vacuum system has a number of points in its favour, such as where the condensation is unable to gravitate to the point desired, and where exhaust steam is the heating medium. They may be designed to be very flexible in operation, and the heating surfaces may be maintained at a high or comparatively low temperature, according to the intensity of the heat desired. For example, assuming that it is found economical to operate a system under 15 inches of vacuum, it should be possible during the mild weather, by restricting the supply, to circulate the
GENERAL 13
steam at a temperature of about 180° Fahr., whilst with lower external temperatures, the steam may be increased until eventually its temperature in the heating surfaces coincides with the initial pressure.
All so-called vacuum systems, however, are not a success. For example, in a certain class of apparatus, where the vacuum generated depends largely on the 'curtailment of the steam supply, trouble may arise through imperfect drainage. In other cases, the cost of creating the vacuum is out of all proportion to the benefits that are obtained.
The efficiency of a vacuum system is chiefly dependent upon the form it takes, and upon the kind and quality of the fittings that are used.
Heating by Gas. — Of late the advantages of gas fires for heating purposes have been greatly discussed, but as a general means for warming buildings gas apparatus will take a different and more economical form. Gas fires, however, are convenient for heating rooms that are temporarily used, especially in mild climates, for they are clean and easily put into and out of use. The chief drawbacks associated with gas fires occur when they are of defective design. To be satisfactory, practically the whole of the heat they transmit should be from the radiant used. The design should prevent the exterior surfaces being raised to high temperatures, and ample provision should be made for carrying off the products of combustion.
Gas heaters that warm chiefly by convected heat, and those that permit the products of combustion to escape into the surrounding air, have nothing from a health standpoint to recommend them.
The smoke nuisance or aerial pollution that exists in industrial and densely populated centres cannot much longer continue, and so far as the domestic side of the problem is concerned, the trouble can be greatly minimized by the use of gaseous fuel for small hot-water and steam heating apparatus. Before this method of heating can be extensively adopted, gas, however, will require to be sold at a much cheaper rate. The latter aspect of the problem should not be a difficult one, especially in view of the lower qualities that can be used.
Another use for gas that is becoming more common is the
14 DESIGN OF HOT- WATER <fc STEAM HEATING APPARATUS
heating of water for domestic and other purposes, and its further adoption will doubtless increase as its value in this direction is realized.
Heating by Electricity. — This form of energy is a very con- venient one for heating purposes, but it is only in very special cases that it can be economically employed. Where electricity for its production is dependent upon fuel, it is placed at a disadvantage so far as heating is concerned, as only a very small percentage of the total heat energy of the fuel can be transformed into electrical energy.
For popularizing electricity for heating rooms, some municipalities sell it at a specially low rate, and undoubtedly more could be done in this direction. This is especially the case where the period of minimum load at a generating station coincides with the time the heat is principally wanted.
District and Large Central Heating Plants. — Eeference has been made already to the smoke-polluted atmosphere of towns and the large centres of industry. In the case of factories and large public or private institutions, the acute trouble aris- ing from the production of smoke can be remedied by the introduction of appliances that better regulate the combustion of fuel, by improved types of furnaces, and by the substitution of electrical power, oil and gaseous fuels for the coal consumed in small power plants.
On the domestic side, a good method of reducing the produc- tion of smoke is by district and large central heating systems in which either water or steam is circulated through the pipes. Such plants have also advantages over small independent systems in that the labour for attention is centralized, whilst a householder can procure heat at any time without any inconvenience.
To what extent district heating is practicable where the heating medium is conveyed through long underground pipes, depends principally upon the demand for this source of heat, the climate, and the length of the heating season. In America, district heating has been extensively adopted, but the same favourable results could not be obtained in Great Britain where underground pipes are of considerable length owing to its much milder climate. There are, however, numerous localized areas
GENERAL 15
in all our • towns, more or less congested, that lend them- selves admirably to isolated heating plants, whilst in other cases single groups of buildings could be economically heated from one central source.
District Heating can be carried out to greater advantage where the same station supplies both heat and electrical energy. The units of combined plants may be so arranged that the exhaust steam forms a large portion of the heating medium, and probably the best results would be obtained when the volume of the "exhaust" was just sufficient to supply the demand for heat during the average winter temperature. For lower external temperature the deficiency can be made good by the addition of " live " steam, whilst for milder conditions the surplus steam may be passed to exhaust, or sent to the con- denser.
District systems take two principal forms. In the first, either " live " or " exhaust " steam or a mixture of both is delivered from the central station to the heating surfaces. The water of condensation is either passed to waste or returned to the source of heat, the former being the more common practice. In the other system, water is circulated between the station and the heating surfaces, this in turn receiving its heat from either " live " or " exhaust " steam or from both.
As regards the relative merits of hot water and steam systems for district heating, each has its own special points, and the choice depends upon the chief conditions to be met.
Generally speaking, steam heating is advantageous for industrial centres in that the steam can be utilized for a variety of purposes in addition to general heating work.
Hot-water systems are largely used for residential districts. The water temperature can be varied at the station to suit the changing atmospheric conditions, and when condensing engines are used, this may be done by regulating the degree of vacuum at the condenser. Other features of water systems are, that the transmission losses are considerably less than those of steam plants, and less back-pressure is put upon the engines.
CHAPTER II
HOT- WATER CIRCULATION
THERE are three phases of circulation applicable to hot- water apparatus : (a) natural or gravity ; (&) accelerated ; and (c)
forced.
Gravity Circulation depends primarily upon the application of heat at a low point of an apparatus, when the pressures exerted by the flow and return columns are rendered unequal.
FIG. 1.— Tredgold's method of explaining circulation.
Although this explains the cause of gravity circulation in a general way, many do not appear to grasp the fact.
Probably Mr. Tredgold was the first who attempted to account for the circulation of water in a heating system, whilst Hood, in a later work, takes exception to his conclusions, and
HOT-WATER CIRCULATION
17
endeavours to show his reasoning is not correct. The explana- tions of circulation by these gentlemen are interesting and instructive in showing the points on which they differ.
Tredgold's account of circulation is as follows : " If the vessels A and B (Fig. 1) and the pipes connecting them be filled with water, and heat be applied to A, the effect of heat will expand the water in the vessel A, and the surface will in consequence rise to a higher level d, the common level being gf. The density of the fluid in the vessel A will also decrease in consequence of its expansion, but as soon as the column
FIG. 2. — Hood's method of explaining circulation.
ccl (above the centre of the upper pipe) is of greater weight than the column fe, motion will commence along the upper pipe from A to B, and the change this motion produces in the equilibrium of the fluid will cause a corresponding motion in the lower horizontal pipe B to A."
Hood makes the following statement : " Suppose the ap- paratus (Fig. 2) to be filled with cold water, and the two stop-cocks are closed. On applying heat to the vessel V, the wacer it contains will expand in bulk, and a part of it will flow through the waste pipe x, which is so placed as to prevent the water rising higher in the vessel V than in that of vessel B. The water which remains in the vessel V after it has been heated will evidently be lighter than it was before owing
c
1 8 DESIGN OF HOT-WATER & STEAM HEATING APPARATUS
to a portion having passed through the waste pipe x, although its height will remain unaltered. Suppose now the two cocks, g and k> to be simultaneously opened : the hot water in the vessel V will immediately flow towards E through the upper pipe, and the cold water in E will flow to V through the lower horizontal pipe, although by Tredgold's hypothesis, unless
I c
ff
-9.-6
FIG. 3. — Illustrating cause of circulation.
the water in vessel V rose to a higher level than that in the vessel E no circulation could take place." Mr. Hood con- tinues : "Assume heat to be applied to the vessel V (Fig. 2) : the heated particles rise through the colder ones, which sink to the bottom by their greater specific gravity, and they in turn become heated and expand like the others. As soon as the water in vessel V begins to acquire heat and to become
HOT- WATER CIRCULATION 19
lighter than that in vessel E, the water in the lower horizontal pipe is pressed by a greater weight at Z than at y, and it there- fore moves towards V with a velocity and force equal to the difference in pressure at the two points y and Z. The water in the upper vessel K would now assume a lower level were it not that the upper horizontal pipe furnishes a fresh supply from V to replenish the deficiency."
Upon the first perusal Tredgold' s explanation may not be very clear, but briefly expressed it is : that the circulation of water is due to the equilibrium of the " flow " and " return " columns being destroyed by the overflow of the expanded water as represented by gd in Fig. 1. Hood's views stated briefly are : that circulation is due to the greater density of the return water, and that the increment of expansion has nothing to do with it.
The reasons given by Tredgold and partly those by Hood are correct so far as they are carried, but Hood errs when he tries to prove that Tredgold is wrong. The overflow x in Hood's apparatus (Fig. 2) does not dispose of the increment of expansion when the stop-cock k is opened, and his view has been simply narrowed through the use of the additional cock g.
The following may aid in the elucidation of the point under consideration. Assume an apparatus with a circuit of indefinite height, as in Fig. 3, to be filled with water to the level gb, with the stop-cock s closed. If heat be applied to the boiler the water in the flow pipe may be expanded so as to reach the point I. So long as the stop-cock is closed, the pressures in columns A and E are in equilibrium, but no circulation occurs in the return pipe although the two columns differ in density and are in direct communication by means of the lower horizontal pipe. If, now, the stop-cock s be opened, the equilibrium is at once destroyed, and circulation begins owing to the increment gl being able to flow towards the return. Now, as the total pressure in column A has been diminished by an amount, say, cl, and the pressure in column E increased by the amount Id, the total head causing circulation is cl + bd, the sum of which is equal to the increment of expansion gl.
If an apparatus takes a form similar to that of Fig. 4, the effect is the same although the piping differs somewhat from
20 DESIGN OF HOT-WATER & STEAM HEATING APPARATUS
Fig. 3. For example, if nm represents the increment of expan- sion when the return is temporarily disconnected from the flow, it is apparent that it will produce two unequal forces when the passage through the upper horizontal pipe is clear.
Circulating Head, or the power that causes the movement
772,
FIG. 4.— Illustrating the increment of expansion.
of the water, is the same thing as the increment of expansion, and is represented by gl in Fig. 3 and mn in Fig. 4. It may be calculated by the formula —
where
h = circulating head in inches, H = height of circuit in feet,
w = average density of water per cubic foot in return, Wi = average density of water per cubic foot in flow.
HOT-WATER CIRCULATION 21
Circuit Height. — The height of a circuit in contradistinction to the circulating head is the vertical distance between the highest and lowest points where the water circulates. In the case of main circuits which do not dip beneath a boiler, the fire- bars are taken as the lowest point, although some assume it as the centre of the fire pot.
Example 1.— The height of a circuit is 20 feet, and the average temperatures of the flow and return water 160° and 130° F. Determine the value of the circulating head.
By formula 1 h = 12I{(W - 1 ).
\Wl
In the Appendix, p. 312, the weight of water for 160° and 130° F. is given as 60-99 Ib. and 61-56 Ib. per cubic foot respectively.
Substituting these values —
when h = 2-241, or, say, 2^ inches.
Velocity of Gravity Circulation. — If friction were absent, the velocity of circulation would be nearly equal to the velocity attained by a body when freely falling through a height equal to the circulating head. Pipe friction, however, is a more or less considerable quantity, and therefore requires to be taken into account.
The following general formula may be used for determining the velocity of circulation—
(2)
where V = velocity in feet per minute.
„ c = a coefficient that varies with the diameter.
„ H = effective circuit height.
„ d = diameter of circuit in inches.
„ I = length of circuit in feet.
„ w and Wi = the densities of the return and flow waters per cubic foot.
22 DESIGN OF HOT-WATER & STEAM HEATING APPARATUS
APPKOXIMATE VALUES OF c FOR SIMPLE CIRCUITS. Circuits of 1 to IJ-inch diameter c = 600 2 to 3 „ „ c = 700
3J- to 5 „ „ c = 750
Example 2. — Determine the velocity of circulation of a 3 -inch diameter circuit that takes the form of Fig. 4. Let its length be assumed as 360 feet, its effective height 20 feet, and the average temperatures of the flow and return waters 155° and 145° F. respectively. By formula 2 —
Hrf(^ - 1 )
. , V in*
V = c
For 145° F. wi = 61-29 lb.; for 155° F. w = 611 lb., and c is given for a 3-inch diameter pipe as 700.
Substituting values, V = 700 V 3377—
ouU
V = 700 x 0-0227 when V = 15' 9 feet per minute.
This calculation shows what a feeble current is obtained under the conditions given, although the circuit is of moderate height.
Dipped or Trapped Circuits. — It is sometimes necessary to dip or trap a circuit, and although, as a general rule, this should be avoided as far as it is practicable, yet dips can be introduced in many cases without greatly impeding the circulation. At all dips, vent pipes should be provided, for it is imperative that the air should escape freely when a system is charged with water, or when it is in operation.
The effect of dips is to reduce the circulating head, and the velocity of flow is directly proportional to the square root of that factor when the other conditions remain unaltered. The circulating head of trapped circuits may be estimated by formula 1 after first determining the average density of the ascending and descending columns of water.
HOT-WATER CIRCULATION
23
Figs. 5 and 6 give two trapped circuits where the rate of cooling is assumed to be as shown. For convenience, the densities of the water for the temperatures under consideration
|
1/42' |
H7' 150° |
767°| 7601 |
|
^h |
Tb to +— 60" 1 14-8 1491 ~ F |
C |
|
FIG. 5. — Trapped circuit. |
||
|
L |
I- |
|
|
mi780 pT . |
? |
76/1 |
|
*• — '"s |
160°f |
" C
FIG. 6. — Trapped circuit.
are here given, whilst the method of ascertaining the average densities per cubic foot of the ascending and descending columns of water is as follows : —
|
Temp. Deg.F. |
Weight lb. per cubic ft. |
Temp. Deg. F. |
Weight lb. per cubic ft. |
Temp. Dcg. F. |
Weight lb. per cubic ft. |
|
142 143 147 |
61-34 61-32 61-24 |
148 149 150 |
61-22 61-20 61-18 |
160 161 178 |
60-98 60-96 6059 |
PARTICULARS OP FIG. 5.
|
Ascending columns. |
Descending columns. |
||||
|
Column. |
Average temp. |
Density per cubic ft. in lb. |
Column. |
Average temp. |
Density per cubic ft. in lb. |
|
HA FG |
178° 148° |
60-59 61-22 |
BC DE |
160° 150° |
60-98 61-18 |
2)121-81 Average density = 60-905
2)122-16 Average density = 61-08
24 DESIGN OF HOT-WATER & STEAM HEATING APPARATUS
This simple method of ascertaining the average density is only applicable when the total height of the ascending columns is equally divided between AH and FG, and that of the descending columns between BC and DE, as in Fig. 5. Where the parts forming the columns are unequal, as in the ascending ones of Fig. 6, the procedure is rather different.
PABTICULAES OF FIG. 6. Ascending columns.
|
Column. |
Average temp. deg. F. |
Density per v cubic ft |
II eight of _ Foot- column ft. ~~ Ib. |
|
Sti PR |
178 147 |
60-59 X 61-24 X |
6 = 363-54 2 = 122-48 |
8)486-02 Average density = 60-752
The average temperature for the descending column MO may be taken at 160°, and the density corresponding with this is 60*98 Ib. per cubic foot.
Velocity of Circulation through Trapped Circuits. — The velocity in trapped circuits may be ascertained approximately by letting w and wi in formula 2 represent the average densities of the descending and ascending water columns.
Example 3. — Determine the probable rate of flow through a circuit that takes the form of Fig. 5, the height being 12 feet, length 220 feet, and of 2-inch bore. Assume the water to cool at the rate given.
By formula 2 —
IT \/ ^1
v = «v 2
On p. 23 the average density of the ascending columns was found to be 60*905 Ib., and that of the descending columns 61-08 Ib. per cubic foot. The value of c = 700.
Substituting values —
V = 700
V = 700 x 0-017, when V = 11*9 feet per minute.
HOT- WATER CIRCULATION 25
Example 4. — Assuming the length and diameter of circuit in Fig. 6 are the same as in the previous example, what will be the probable velocity of circulation if the water cools as shown ?
By formula 2 —
V =
The average density of the ascending columns was found on p. 24 to be 60752 Ib. per cubic foot, whilst that of the descending column is 60'9S Ib.
Substituting values —
/7 / 60-98
/ 8 x 2 x (7^7*9 - 1)
V = 700V 60 752 ;
220
V = 700 x 0-016, when V = 11*2 feet per minute.
To show the application of formula 2 to the problems indicated by Figs. 5 and 6, the rate of cooling has been con- sidered to have been equal over the whole of the circuits. In practice, however, this effect would not be realized, for -more heating surface is usually concentrated at one point than at another ; moreover, the cooling effect is greatest where the water is hottest. These factors therefore require to be taken into account, for a marked cooling of the water at a certain point will either impede or accelerate circulation. Were radiators connected to the circuit between the points DC, Fig. 5, the water may be cooled to a greater extent than shown, but the rate of circulation would be little affected. On the other hand, if the water in the pipe OP, Fig. 6, were cooled more than shown, this would adversely affect the circulation, owing to the greater resistance offered by the column PK.
Irregular or Spasmodic Circulation. — The principal causes for irregular motion in a heating system are due to the accumulation of air and to trapped circuits. The remedy for the first is simple, for all that is required is properly pitched pipes, and a well-vented system. As a rule, the pitch of
26 DESIGN OF HOT-WATER & STEAM HEATING APPARATUS
pipes should not be less than 1 inch in 10 feet, but should be greater for those of small-bore.
As already shown the adverse effect of dips is when they tend to equalize the pressure of the ascending and descending water columns. If this condition is realized, circulation ceases, and equilibrium is only destroyed either by the overheating at the boiler on the one hand, or by the abnormal cooling of some portion of the return water on the other. In badly designed systems, irregular circulation often occurs in branched circuits, and especially in those of small bore.
Forced Circulation. — A single circuit with forced circulation is given in Fig. 7. This form of circulation is especially suitable where a series of detached buildings is to be warmed from a
Relief k Vacuum Valve.
Water Feeder Water Supply.
Steam Supply _
^-y Steam Neater
Centrifugal Pum '
Steam Trap.
FIG. 7. — Forced circulating system.
central source; also for large buildings and structures where gravity circulation is unreliable or otherwise unsuitable. With forced circulation, the pipes may be run and fixed in any position desired, irrespective of dips. The greatest drawbacks associated with it are the cost of operation and the greater attention required, when compared with gravity circulation.
The speed of circulation should be governed by the extent of the resistance offered by the piping and appliances used,
HOT- WATER CIRCULATION 27
but the "critical" velocity may lie anywhere between 2 feet and 8 feet per second. The critical velocity is the one that gives the most economical results with reference to the cost of operation and to the initial outlay as a whole. In com- paratively small plants, the critical velocity is not so im- portant, and in these, the circulating speed is often fixed with reference to a given size of motor, and to the efficiency of the pump that it is proposed to use.
The power for circulating water by means of a pump varies roughly with the cube of the speed, whilst the amount of heating surface a given size of pipe will serve, varies directly with the velocity of circulation, provided the tempera- ture drop remains unaltered. Assume that water is moved through a circuit at the rate of 100 feet per minute in order to serve 500 square feet of heating surfaces, and one unit of power is absorbed. If now the velocity is raised to 400 feet per minute, this will supply four times the amount of heating
surface, or - -y00 - = 2000 square feet. The energy, how-
1 x 4003 ever, to produce this rate of flow would require ^3 = 64
units, or sixty-four times the original quantity. Thus it becomes clear that the saving effected in the initial cost of an apparatus by adopting very high circulating speeds may be offset by the greater operating charges.
Accelerated Circulation. — There are various ways in which the circulation of water may be accelerated, such as by pro- viding apparatus whereby the temperature of the water is raised above the normal amount, by the injection of steam or air into the flow pipe of a system, by producing a partial vacuum in a circulating tank through the condensation of steam, or by mechanically increasing the " circulating head." The general advantages derived from accelerated circulation are similar to those of forced circulation, but the force pro- ducing motion is not of so " positive " a nature, and hence the speed that can be attained is limited. Each particular method of acceleration has its own special merits and limitations, so that what may give the most satisfactory results in one case may be disadvantageous in another.
28 DESIGN OF HOT- WATER & STEAM HEATING APPARATUS
Although the same precautions with respect to the gradients of pipes need not be observed as in gravity systems, yet it is desirable to do so, for this permits accelerating apparatus to be operated by gravity circulation during the milder portions of a heating season.
A method of accelerating circulation by means of steam is shown in Fig. 8. It represents the principle introduced by
L
P V (
L = automatic vent.
T = expansion tank.
S = overflow and exhaust pipe.
R = return from tank T .
M = mixing pipe.
C = circulation.
D = condenser.
O = condensation return.
P = steam supply.
V = steam valve.
F = pipe to circulator.
Hi — reheater.
B = steam boiler.
FIG. 8.—" Reck System." *
Captain Reck, and is called the "Keck System." Steam is generated in the boiler B, from which it flows to the re-heater H, which takes the form of a straight-tubed calorifier with water- way ends. The steam that is not condensed in the re-heater H flows through the pipe P to the top of the apparatus, the pipe
* The reheater H is frequently omitted.'
HOT-WATER CIRCULATION 29
being returned to join at C, where the steam mixes with the water circulating through the flow-pipe F. The mixing of steam and water at C has the effect of substantially diminish- ing the pressure of the ascending columns when compared with that of the descending ones. The pipe M is directly joined to the expansion tank T, the latter providing space for the expanding water, and acting as a barrier to prevent the passage of steam into the water circuit. From the tank T, two pipes are taken ; that indicated by K completes the circuit for the water by joining with the re-heater H, whilst the pipe S serves as the overflow to return surplus water to the boiler, and also to convey any steam to the condenser D.
For any given installation, the rate of acceleration largely depends upon the height of that portion of the flow pipe M which represents the vertical distance between the circulator C and the expansion tank ; the greater this height, other conditions being equal, the more rapid the circulation. As a rule, how- ever, the vertical distance between the circulator and expan- sion tank (even in large plants) is limited to a few feet, for this also determines the steam pressure at which the boiler will require to be operated. Assuming, for example, that steam is not generated, or that it is cut off from the circulator by the valve V, it is obvious that the water will rise in the dipped portion of the pipe P to the level of that in the expansion tank. The pressure, therefore, must be adequate to dislodge this water before the steam can enter the circulator. It is essential to carry the steam pipe higher than the expansion tank in order to avoid the flooding of the boiler, whilst to prevent the water being siphoned from the expansion tank through a partial vacuum forming in the steam pipe, an automatic air valve is provided at L.
To permit of the escape of air from the apparatus shown by Fig. 8, an automatic relief valve is usually attached to the con- densers, or an open pipe may be provided immediately beneath so long as some form of mechanism is employed to control the steam automatically to the circulator. The upper por- tion of pipe K may also be vented to the atmosphere if desired. When steam is introduced into a body of water some means should be introduced to obviate hammering or cracking
30 DESIGN OF HOT- WATER & STEAM HEATING APPARATUS
sounds. In the " Eeck " apparatus this is done by the use of fine-mesh gauze, which has the effect of breaking up the steam into a very fine spray. Special forms of injectors may also be used for the same purpose.
The rate of circulation in a system resembling Fig. 8, may be ascertained by formula 2, provided that the condenser properly performs its function.
Example 5. — Determine the velocity of circulation when the height of the column F in Fig. 8 is 35 feet, and contains water with an average temperature of 170° F., column M 5 feet high, with a temperature of 212° F., whilst free steam occupies one-fifth the pipe area. Take the average temperature of the water in E as 180° F., the length of the circuit 300 feet, and its diameter 3 inches.
The density of the water at 212° F. is 5976 Ib. per cubic foot, whilst that of the steam may be taken as 0*038 Ib. per cubic foot. From these values the density of the mixture in
• AT -n u S9'76 * 4 °'038 ^01 n u- t
pipe M will be - — = -f- — = — = 47*81 Ib. per cubic foot.
5 o
The average density of the ascending columns may be obtained by the same procedure as in trapped circuits.
|
Column. |
Temp. deg. F. |
Density per cubic ft. |
Height of Foot- A column in ft. ~ Ib. |
|
F. M. |
170 212 |
60-78 47-81 |
X 35 = 2127-30 X 5= 239-05 |
40)2366-35 Average density per cubic foot of ascending column = 59-16 Ib.
The average density of the water of the descendin R for 180° F. will be 60*55 Ib. per cubic foot. By formula 2 —
g column
v =
For a 3 -inch diameter circuit c = 700, and its height is 40 feet.
HOT-WATER CIRCULATION Substituting values—
V=700
31
300
V = 700 x 0:097, when V = 68 feet per minute.
Suppose now that the circulator C is thrown out of use by closing the valve V, and the average temperatures of the water in F and E are 170° and 150° F. respectively. Where all the
irLscape.
Expansion <fc Separating Tank.
'ector
Hot Water Boiler
FIG. 9.— Accelerated circulating system.
other conditions remain unaltered, the velocity will be directly proportional to
- — 1.
rate of circulation would be
For the latter case, therefore, the
32 DESIGN OF HOT- WATER & STEAM HEATING APPARATUS
/ 61*2 v fQ / 60'7o
A / 60*55 /v
59-16 when V = 36*72 feet per minute.
Thus, under gravity for the conditions given the speed- of circulation is diminished by 46 per cent.
If a supply of compressed air is available the circulation could be accelerated by an injector as in Fig. 9. Air, however, is not so satisfactory or economical as steam for the accelerating agent.
CHAPTEE III
SYSTEMS OF PIPING FOR HOT-WATER GRAVITY APPARATUS
THE most suitable method of piping for a particular case will depend very much upon the size of the installation, upon the location of the heating surfaces with respect to one another, upon the facilities for pipe runs, and upon the resistance offered to the circulation of the water.
There are two distinct methods of piping, these being designated as " one-pipe " and " two-pipe " systems, and each may be arranged on either the up-feed or down-feed principle. It is not always desirable to adhere rigidly to any one piping system, but the circumstances of a case may be better met at times by a modification or combination of these systems.
One -Pipe Up-feed System. — This is the simplest form of piping, and single circuits are of equal bore for their whole length. The " one-pipe " system is advantageous, in that short circuiting cannot take place, and this makes it especially suit- able where the dipping or trapping of circuits is unavoidable. As a general rule, this system is better for supplying compara- tively small amounts of heating surfaces than for large ones owing to the cooling effect through the " flow " and " return " water being accommodated in the same pipe. A very pro- nounced temperature . drop can always be avoided by having the pipes of relatively large bore, but this may be undesirable unless another form of piping is unsuitable.
Fig. 10 gives a single main circuit. It is well to keep the horizontal portions of the risers as short as practicable, although there is no serious objection to their being rather prolonged as at A and B, provided the pipes are of suitable size and are given a good pitch. When joining the risers or branches to a
D
34 DESIGN OF HOT-WATER & STEAM HEATING APPARATUS
circuit it is desirable that the " flows " be taken from the top, whilst the returns re-enter at the side ; this arrangement aids the circulation through the " branch " circuits, besides tending to separate to some extent, the cooled from the hotter water. In Fig. 10 the highest part of the main circuit is shown at H,
FIG. 10. — One-pipe system : Single main circuit.
where an air pipe is provided ; from that point the circuit should have a downward pitch to where the boiler is located, and then be dropped to join with the lower connection.
Although in Fig. 10, H is made the highest part of the main circuit, this need not necessarily be so, but it is the better practice in a " one-pipe " system to make the highest point as near to the boiler as possible. In other words, it is advan- tageous to limit the length of the "flow" in order that the cooling water can gravitate directly to the source of heat.
In Fig. 11 a divided circuit is given, and this may with advantage be adopted in many cases. With divided circuits, however, or where a loop is introduced as a minor circuit, care should be exercised in making the return connections. Before joining with one common " return" it is a good plan to
PIPING FOR HOT- WATER GRAVITY APPARATUS 35
drop all the branches, as this causes the water to circulate iu the right direction and also aids its movements. With the exception of the short pipe D, the remaining portion of the
FIG. 11. — One-pipe system : Main circuit divided.
circuits acts as " returns," the air from the heated water being principally relieved by the expansion pipe. Pipes D and C should, of course, be larger than the others.
Fig. 12 gives a " one-pipe " system, where it is essential to dip the circuit beneath a number of doorways. A case of this kind sometimes occurs where corridors are heated by radiators, or where a heating apparatus is erected after a building is completed. The object of carrying the flow pipe above the top floor is to increase the circulating head, but in order for this to be attained, a temperature difference must occur in the upper ascending and descending columns. For a case like Fig. 12, an advantage would be gained if the distance between the pipes at x were considerably greater. The air pipe provided at y is principally of use when charging the apparatus. From the horizontal pipes, the air would tend to gather in the radiators, from which it could be discharged by opening the air valves.
36 DESIGN OF HOT- WATER & STEAM HEATING APPARATUS
Another "one-pipe" system is illustrated by Fig. 13, where the heating surfaces on two floors are served by a single circuit.
FIG. 12. — One-pipe system where circuit dips beneath doors.
£
FIG. 13. — One-pipe system.
PIPING FOR HOT- WATER GRAVITY APPARATUS 37
This is not an economical arrangement as regards the piping, and, unless of a large bore, the temperature difference between the first and last radiator would be pronounced. Instead of a large single circuit being employed for the case shown, it is better to provide an independent circuit for each floor, this being more economical, and also affording a greater degree of flexi- bility in operation. With separate circuits, stop valves could
FIG. 14. — Two pipe " up-feed " system.
be provided on the return pipes, and either the one or the other could be partially cut out of use when desired. Should stop valves, however, be fixed upon both flow and return pipes, a safety valve, or relief pipe, should be provided.
Two-Pipe System. — This is the oldest method of piping, and differs from the " one-pipe " or circuit system in that the various flow and return connections join with separate pipes. The main circuit is graded, whilst that of a simple " one-pipe '' system is of uniform bore.
38 DESIGN OF HOT- WATER & STEAM HEATING APPARATUS
A "two-pipe" system is specially suited for buildings where long circuits are required, and owing to the returns from the heated surfaces joining with the main returns, there is a nearer approach to uniformity of temperature in the first and last radiator on a circuit.
To be a success, a " two-pipe " system must be well designed, or some portion of it may not be heated properly through the short circuiting of the water. For this reason, the dipping or trapping of pipes should be avoided, as the water will necessarily circulate the more freely along the path that offers the least resistance.
A general arrangement of the piping is shown in Fig. 14. Here the highest point in the main circuit is at the centre H. As a rule, the radiators on the lower flats are the slowest to get hot, but by joining the risers, as at the upper left radiator, and properly sizing them, the heating up of the radiators on the different floors is more nearly equalized. The arrangement on the right tends to favour the heating surfaces on the higher floors, for a direct and easy passage is provided to those points. Only one main circuit is shown, although for many buildings it would be desirable or necessary to introduce two or more. The number of units into which a system should be divided will largely depend upon its size, the location of the boiler with respect to the heating surfaces, and the degree of regula- tion desired. Speaking from an economical standpoint, two or more main circuits will have an advantage over one in those cases where a boiler is centrally placed. On the other hand, if a boiler is located near to one end of a building, the cost of the piping begins to rise as the number of separate units is increased.
Pitch of Pipes.— For the main circuits of " one " or " two- pipe" systems, the horizontal piping should have a pitch of from 1 inch in 10 feet to 1 inch in 20 feet. The quicker the pitch the better. As regards the horizontal portions of risers, these should be given, where possible, a pitch of about 1 inch per foot.
Drop, Overhead, or Down-feed Systems are represented by Fig. 15. This method of piping is very suitable for high build- ings, where the heating surfaces on the different floors can be located immediately above one another. From the boiler the
PIPING FOR HOT- WATER GRAVITY APPARATUS
39
flow pipe is carried as directly' as possible to the highest part of a structure, whilst the radiators are supplied by the vertical pipes. The whole of the piping from the point P is arranged to take the form of returns. For some structures, this method of piping is not suitable, but where it can be adopted it is the most economical one. The principal advantages of " drop " systems arise through the greater freedom of circulation, and the smaller pipes that can be used, and to the fact that air valves on the heating surfaces are not required.
In Fig. 15, two arrangements of drop pipes are shown.
FIG. 15.— Overhead or down-feed system.
The use of single drop pipes necessitates their being of equal bore from end to end, whilst if they take the double form they may be varied in size. The upper horizontal distribut- ing, and the lower intercepting pipes are graded as well, and they may be arranged in various ways, according to the circumstances of the case in hand. To the right of Fig. 15 two radiators are shown on the lowest floor, these being supplied with separate branches instead of continuing the principal drop
40 DESIGN OF HOT-WATER & STEAM HEATING APPARATUS
pipes to the lowest points. There is much freedom with regard to the choice of details in overhead systems, but every care should be observed to prevent air being entrapped in the hori- zontal pipes.
Water Supply Connections. — The general arrangement with respect to expansion tanks, and of the water supply connections of heating apparatus, differs somewhat in Great Britain and in America. In the latter country, where a water service is under a suitable pressure, it is the custom to introduce the feed water at a point near the boiler, whilst its height is recorded by an altitude gauge. The expansion tank usually takes a cylindrical shape, and is provided with a gauge glass for observing the water-level at that point. In this country, the cold supply is usually delivered into the expansion tank, an automatic float cock being used for controlling the water supply.
Figs. 10, 11, and 13 show different methods of connecting the overhead feed tanks. The chief advantage of that given in Fig. 10, is that air is readily dislodged from the apparatus when it is charged with water. On the other hand, if overheating occurs, so as to generate steam, the water may be dislodged from the boiler into the expansion tank, when the surfaces of the metal are liable to damage through being burned.
The feed pipe in Fig. 11* is joined to the highest part of the flow, and the advantages derived from this connection are as follows : —
(1) The feed pipe acts as the principal air relief.
(2) If overheating takes place any steam or excess of
pressure is immediately relieved without displacing the water from the boiler.
(3) If stop valves are used on the circuits, the feed pipe
may be arranged to afford the necessary relief should
they be left closed when a fire is lighted.
The principal drawbacks likely to arise are : Some of the
pipes may get partially air locked when charging a system, and
undue loss of heat from the expansion tank may occur through
its contained water being raised to a high temperature. The
latter defect may be greatly minimized by protecting the tank
with a good insulating material, or the circulating of the water
between it and the flow pipes may be avoided by the method
PIPING FOR HOT-WATER GRAVITY APPARATUS 41
Air Pipe.
Overflow.
adopted in Fig. 13. In the latter case the feed pipe is trapped, but the merits associated with Fig. 11 are retained.
Fig. 15 shows the feed pipe joined with a drop pipe, instead of with the upper horizontal pipe. In general, it may be associated with the same defect mentioned in connection with Fig. 10, whilst to some extent the same merit is retained. There are other points to which the supply pipe may be attached, but the principal features likely to arise have been already considered. For example, the joining of the feed directly with the top of the boiler is similar in its effect as if it were connected with the flow pipe. In like manner, a direct con- nection with the lower part of the boiler is similar to joining the supply with a main return.
Expansion Tanks. — When automatic cocks are used for regulating the water supply they should be arranged to close when the water in the tanks is only a few inches deep, the remaining space being utilized for accommo- dating the expanding water. The levers of the automatic cock should be strong and rigid, that they may easily bear the strain when the floats are submerged. Fig. 16 shows an expansion tank where the water supply is delivered directly into a system from a service pipe, or by the aid of a pump. The tank is joined at the highest point either with a main or with a riser.
Fittings and Other Accessory Apparatus. — To aid in the even distribution of fluids throughout a system, special forms of fittings may be used. In some cases, these may be found useful, but generally speaking, if a system is properly designed they offer no decided advantages over the ordinary fittings.
FIG. 16.— Expansion tank.
42 DESIGN OF HOT- WATER & STEAM HEATING APPARATUS
Fig. 17 gives a special form of tee for a main or branch circuit, but for gravity circulating apparatus, a fitting of this class should be used very sparingly, owing to the resistance the projecting lip will introduce. If, on the other hand, the circu- lation is forced, there is very little objection to its freer use.
A special tee is also designed for risers so as to favour the lower heating surfaces where the circulating head is the least. The chief point in its favour is the neat or compact form it takes. As regards the common fittings, such as elbows and bends, long curved ones are the best to use, whilst the ends of the pipes should be properly reamed after being cut and screwed.
A failing that occurs through the use of unsuitable dimin-
Air
FIG. 17.
FIG. 18.
ishers is indicated at A, Fig. 18, where air is confined so as to reduce the effective area of a pipe. This defect, however, is easily avoided by pitching the pipes, and by the use of eccentric fittings as indicated at B, Fig. 18.
Air Pipes. — Where convenient, it is a good practice to ventilate the flow risers of a gravity system by air pipes, for by so doing, less air cocks may be used, and a system is the more readily freed from air. As a rule, single air pipes should not be smaller than f inch bore, and where an overhead horizontal air line is used to intercept the vertical pipes, care should be observed that it is not rendered useless through getting trapped at some point.
Air Valves. — For hot water apparatus air valves take different forms, but they may be classified under two heads as
PIPING FOR HOT- WATER GRAVITY APPARATUS 43
hand controlled, and automatic ones. Automatic valves are not free from mechanical defects, but they are the best appli-
FIG. 19. — Automatic air valve.
FIG. 20. — " Ideal " automatic valve. FIG. 21. — " Norwall " automatic valve.
ances as yet devised where air must be discharged from circuits as it tends to accumulate. Fig. 19 gives a common form of
44 DESIGN OF HOT-WATER & STEAM HEATING APPARATUS
automatic valve, and -Figs. 20 and 21 show other styles. For its action, Fig. 19 depends upon the air dislodging the water from the chamber shown, when the ball falls by its own weight and allows the air to escape. The screwed plug at the base of the appliance acts as a stop valve, and can be used as such when it is neces- sary to effect any repairs. Fig. 20 ope- rates in a similar way, but is an im- proved type.
The valve shown in Fig. 21 is intended either for hot water or steam apparatus. When used on the former, the pressure of the accumulating air exerts its force on the top of the float and depresses it, and in turn the orifice is opened for the air to escape. Upon being relieved the air has its place taken by water, when the float is again buoyed up to close the orifice. This valve is more suit- able for steam sys- tems.
Stop and Radiator Valves. — When stop valves are used on
circuits, fullway or the gate type should be adopted. Globe pattern valves are unsuitable for this purpose owing to the resistance they incur. Eadiator valves take different shapes, and if reference is made to the catalogue of a good firm of heat- ing apparatus manufacturers, most patterns will be found to
FIG. 22.— Angle valve.
PIPING FOR HOT-WATER GRAVITY APPARATUS 45
meet the conditions that arise in general practice. The angle
valve, Fig. 22, is a very convenient one, as it permits of a simple
and direct connection
being made between the
radiators and the supply
pipes.
A quick-opening valve is shown in Fig. 23, and this being provided with an index plate indicates the extent to which it is opened. It is made in the angle as well as in the straight form.
When open, both the radiator valves already shown depend upon stuffing boxes for their water-tightness, and as more or less trouble
through leakage occurs
FIG. 23.— Quick- National
ning regulating valve by "'ator Company.
at these points, valves have been designed with a view to remove this source of weakness. These have been produced in both the slow and quick opening types, the former being indicated by Fig. 24 and the latter by Fig. 25.
The principal feature of Fig. 24 is the prevention of the fluid coming in contact with the valve steam by means of the metal bellows, one' end of which is secured to the bonnet A, and the other to the disc holder F. From the construction of the valve it will be seen that the twisting of the stem imparts a vertical motion to the bellows, either to expand or to contract it, according to whether the valve is being closed or opened.
Joints for Copper Pipes. — No great difficulty is involved in the jointing of iron and steel pipes, but the joints of light copper pipes have given trouble from time to time. Owing to the rapid corrosion of iron and steel by certain classes of water, copper pipes are superseding those made of these metals, espe- cially when of small bore and in first-class work. So long as copper pipe of heavy gauge was used no difficulty occurred in
46 DESIGN OF HOT-WATER & STEAM HEATING APPARATUS
jointing, but thin pipes with screwed joints form a source of weak- ness. On thin copper pipes only fine threads can be cut, whilst to
FIG. 24.—" Sylphon " packless valve. FIG. 25.—" Triton " packless valve.
• By United States Radiator Cor- poration.
strengthen the joints it is the usual practice to sweat the pipe ends and fittings together with fine solder. The solder, how- ever, is often responsible for the defect, a galvanic action being
FIG. 26. — Leigh compression joint for light copper pipes.
set up between it and the copper, when the solder is corroded and the joints begin to leak.
PIPING FOR HOT-WATER GRAVITY APPARATUS 47
At the present time, the best way to fit up thin copper pipes is by the use of compression joints, in which soundness depends upon metallic contact, instead of the use of packing materials. The chief drawback associated with compression joints arises through the special fittings required, and to their higher initial cost. Fig. 26 gives Leigh's joint, in which one pipe end is expanded, whilst the other is slightly tapered, with a bead also formed upon it. For preparing the pipe ends
FIG. 27. — Compression joint for light copper pipes.
special tools or machines are necessary, and when this is done they are firmly drawn together by the screwed cap and sleeve piece, the washer W preventing the bead from being damaged. Another form of compression joint is given in Fig. 27. 'In this case, both ends of the pipes are expanded and drawn over the tapered ferrule by the flanges and bolts. In comparing these joints, it will be observed that Fig. 27 has an extra point at which leakage may arise, but it is the simpler of the two, and can be used for a "greater range of thicknesses than Fig. 26. It is only with pipes of a light gauge that the bead in the Leigh joint can be formed without in some measure reducing the substance of the material.
CHAPTER IV
SMALL-BORE GRAVITY APPARATUS
SMALL-BORE apparatus may take either a so-called " high pres- sure " or " medium pressure " form. The terms used are relative ones, but it does not necessarily follow that the working pressure in the one will be greater than that in the other. The chief distinguishing feature is this: the "high pressure" form is hermetically sealed, provision being made for the expansion of the water by special tubes, whilst in the " medium pressure " system loaded relief valves are employed.
Systems of Piping, — There are three different ways of arrang- ing the piping of small-bore apparatus : (a) on the single circuit
FIG. 28. — Left and right screwed joint for small-bore heating apparatus.
principle, (b) on the branched circuit principle, and (c) on the crossed circuit principle. The first method is not adopted for large buildings as a rule, for wThere two or more independent circuits are formed, an expansion tube and charging point would be essential for each. Branched circuits necessitate the use of stopcocks for regulating circulation, and special cocks are often required at certain points for charging the apparatus with water. Crossed circuits are better suited for larger buildings, and although this arrangement nominally divides a plant into a
SMALL-BORE GRAVITY APPARATUS
49
number of independent units, yet the crossing lias the effect of producing one long continuous circuit.
The small-bore apparatus was invented by Mr. Perkins of London in 1831, and is known as the Perkins system of high- pressure heating. The tubes entering into the construction of the apparatus are very strong, lap welded, and approximately of }g-in. bore. Before leaving the works these tubes are subjected to a hydraulic test of 4000 Ibs. per square inch, and are put together with left and right screwed joints. The threads
FIG. 29. — Single circuit, high-pressure heating.
are cut with a smaller pitch than that adopted for ordinary wrought iron and steel pipes, and when threading the pipe ends one is prepared with a square flat surface, whilst the other is shaped with a sharp edge. No jointing material is used, the two pipe ends are simply drawn together by a union socket with powerful pipe wrenches until the one cuts into the end of the other, as in Fig. 28.
50 DESIGN OF HOT- WATER & STEAM HEATING APPARATUS
Although the small-bore system was largely adopted formerly for warming buildings, it has for this purpose been superseded by low-pressure apparatus. It has, however, a large sphere of usefulness in industrial concerns, such as for drying rooms, heating bakers' ovens, heating water, and other purposes.
In Fig. 29 a single circuit is shown. The sealed expansion tube T is located at the highest point, its size being governed
FIG. 30. — Branched circuits, high -pressure heating.
by the capacity of the piping and the temperature to which the water is raised. There is considerable freedom as regards the arrangement of the piping, but the flow portion should be run as directly as possible to the highest point and kept free from dips. If dips are necessary, these should be formed in the return piping.
Under ordinary circumstances the water temperature in a small-bore system is raised to about 300° or 350° F., but when a circuit is of considerable length there is a very pronounced difference in its temperature when leaving and when re-entering the furnace. It is essential to add a little water from time to
SMALL-BORE GRAVITY APPARATUS
51
time, owing to loss through the porosity of the material, and as too little water causes the circulation to be broken, this state of affairs is soon indicated by the noise created. Water of course can only be added when a system has cooled down, the plugs O and P (Fig. 29) being removed and the water poured in at the latter point.
A branched circuit system is indicated in Fig. 30, and to distribute the water more or less evenly through the piping stopcocks, S, are used. These stopcocks are of special design, and when they are used to control two circuits from one point,
FIG. 31. — Crossed circuits, high-pressure; heating.
only one circuit at a time can be put out of use, or the flow of water may be divided between them. It is imperative that a path be always provided through which the water can flow.
Fig. 31 illustrates a system in which the circuits are " crossed." Although only two circuits are shown, the principle
52 DESIGN OF HOT- WATER & STEAM HEATING APPARATUS
introduced is the same irrespective of the number of units in which a plant may be divided. The furnace coils are bent into any suitable shape, and each acts as the heater for its particular circuit. If the pipes are traced it will be seen that the flow pipe of the upper coil joins with the upper circuit, whilst the return of the upper circuit joins with the lower coil. In a similar manner the flow from the lower coil of furnace joins with the flow of the lower circuit, whilst the return is connected with the upper boiler coil.
Not only are crossed circuits advantageous in distributing the heat more effectually over a building, but the charging of a system is simplified, and only one point for the location of expansion tubes is essential.
When charging Small-bore Apparatus it is necessary to intro- duce the water by a pump. A special fitting is located, as at C, Fig. 31, and is so arranged that water upon entering at L passes through the whole of the piping before overflowing at M. The fact that the water is made to flow from the pump in one direction is advantageous in that the whole of the air is dis- lodged from the piping.
Furnaces, — Although it is customary to make the furnace coils of the same tubes as those used for the circuits, still, tubes of a larger bore are sometimes used, but it is essential to avoid all weak points when these are introduced. The furnaces may be of iron or of brick construction, the former as a rule being used for small apparatus, and the latter for larger installations.
The size of a boiler is dependent upon a number of points which will be considered in a later chapter, but so far as the furnaces for small-bore apparatus are concerned, the length of coil to produce a given heating effect is often expressed as a fraction of the total length of piping. It is only a rough-and- ready method, however, but it is a convenient rule when only approximate values are required.
SMALL-BORE GRAVITY APPARATUS
53
TABLE I. LENGTH OF FURNACE COILS FOB SMALL-BORE APPARATUS.
|
Temp, of room Fahr. deg. |
Proportion of tube in furnace to total length in circuit. |
Temp, of room Fabr. deg. |
Proportion of tube in furnace to total length in circuit. |
|
50 to 75 75 to 95 |
A i |
95 to 140 140 to 200 |
1 1 |
Size of Expansion Tubes. — Ample provision should be made for the expansion of the water so as to obviate damage through overstrain. Water does not expand at a uniform rate, but is greater in high than in low temperatures. The expansion, how- ever, for very high temperatures does not appear to have been accurately determined, but the following values may be used in the design of heating apparatus.
TABLE II. APPROXIMATE EXPANSION OF WATER BETWEEN 40° AND 600° FAHR.
|
Temp. deg. Fahr. |
Volume. |
Temp. deg. Fahr. |
Volume. |
|
40 |
1-0000 |
400 |
1-1484 |
|
212 |
1-0433 |
450 |
1-1843 |
|
300 |
1-0869 |
500 |
1-2233 ' |
|
350 |
1-1156 |
600 |
1-3099 |
Expansion tubes often have a bore of 3 inches, but other sizes may be used, and where the length of one would be un- wieldy, two or three shorter tubes may be joined to give the requisite capacity.
For determining the length of expansion tube the following formula may be used —
L-J - - - - - • • (3)
where L = length of expansion tube in feet,
I = total length of tube (^-inch bore) in circuits, hearing
coils, and furnace in feet,
d = internal diameter of expansion tube in inches, e = a coefficient which varies with the maximum water
temperature.
54 DESIGN OF HOT-WATER & STEAM HEATING APPARATUS
For a maximum temperature of 300° F. e = 0'08
350° F. e = 010 400° F. e = 014 450° F. e = 018 500° F. e = 0-22 600° F. e = 0-30
Example 6. — Assume an installation consisting of 540 feet of small-bore tube for a maximum water temperature of 350°, what length of 3-inch diameter expansion tube would be re- quired ?
By formula 3 —
Substituting values when
L =
L==
d
d2
01 x 540
32 L = 6 feet.
Medium Pressure Small-bore Apparatus. — There is no differ- ence in the design of this and that of the "high pressure"
FIG. 32. — Expansion tank for small-bore apparatus.
arrangement. The only departure is one of detail, an open tank and loaded valve taking the place of the expansion tube. Fig. 32 shows the tank and valve for joining at the head of a circuit, whilst a section of the valve is given in Fig. 33. It takes a combination form, the upper part being loaded ac- cording to the pressure to be carried, whilst a vacuum valve is formed at the lower part. This device permits of the expanding
SMALL-BORE GRAVITY APPARATUS
55
water escaping into the tank, and returning to the apparatus as a cooling action sets in.
The small-bore heating system is quick in action owing to its small water capacity, and it is comparatively cheap to install.
FIG. 33.— Relief and vacuum valve.
When an apparatus is composed of |§-inch bore tubes its approximate capacity in imperial gallons may be obtained by dividing the total length of piping by 44. If its capacity in American gallons is desired, the total length of piping should be divided by 36'6, or say 37.
CHAPTEE V
ACCELERATED HOT-WATER CIRCULATING SYSTEMS
WHERE accelerated circulation is simply due to a high water temperature, the latter is usually obtained by some contrivance that partially seals a system. Under such circumstances an installation resembles the medium pressure small-bore appa- ratus, excepting that the ordinary form of piping is adopted instead of tubes of small diameter.
Sealing devices can generally be applied to any ordinary system ; they may be useful for increasing the heating capacity of an existing installation, and they are comparatively cheap. To be a success, however, ample boiler power is essential.
The most popular means of partially " sealing " a heating system at the present time is by mercury, the appliance used taking a simple form, but an apparatus cannot be subjected to a greater pressure than that for which the device is designed. Another feature of a mercury seal is that the apparatus on which it is employed comes within the category of a low pressure one. On the other hand, small-bore apparatus and those in which loaded valves are used, in order to conform with the provisions of the London Building Act, require the piping fixed three inches clear of woodwork and other inflammable material, whilst no such restrictions apply to low-pressure systems as generally defined.
When an apparatus is open to the external air, the boiling point of the water at the highest, level is dependent upon atmospheric pressure, the latter varying with altitude and the prevailing meteorological conditions, whilst the normal boiling temperature at sea level is 212° F. In a heating system the boiling temperature varies at different elevations, increasing as the hydrostatic pressure increases; if, therefore, in a boiler,
ACCELERATED HOT-WATER CIRCULATING SYSTEMS 57
water is raised to over 212° F., the excess heat beyond that coinciding with atmospheric pressures will, unless absorbed by the lower heating surfaces, cause the production of steam when the water reaches the highest level. By partially sealing a system, however, the boiling point is raised, and as the resistance due to the sealing device is usually equivalent to a pressure of 10 Ib. per sq. inch, the water, even at the highest part, may have its temperature increased to within a short distance of 240° F. The latter value is the boiling point for the pressure given. Thus, the water between leaving and re-entering a boiler may be subjected to a considerable temperature drop, which affects the rate of circula- tion. Where it is desired that the fall of temperature shall be further in- creased, this can be done by increasing the resistance of the sealing appliance. Figs. 34 and 35 give two mercurial sealing devices for accelerating circulation, and although they differ somewhat in form they operate precisely in the same way. Into the lower chamber of each appliance mercury is poured until it reaches the level of the small plug on the right, thus sealing the lower end of the double tube which communicates between the lower and upper compartments. The supply or expansion is joined with the top con- nection, and the bottom one is attached to some other part of the system, the precise point of junction depending upon the piping adopted. Assuming the water in a system is cold and the surface of the mercury at its normal level, upon the appli- cation of heat the expanding water presses on the mercury, dis- lodging it through the double tube to the upper compartment of the fitting ; at this stage the increased volume of water passes to the expansion tank, whilst the mercury endeavours to return to the lower level ; in fact, a partial circulation of the mercury is set up within the double tube through permitting
PIG. 34.— " Honeywell " heat generator.
58 DESIGN OF HOT- WATER & STEAM HEATING APPARATUS
the excess water to flow to the expansion tank. When con- traction occurs, very little opposition is offered to the return of the water, and what resistance there is, is represented by the depth of the mercury at the bottom of the appliance. To over- come this, the expansion tank only re- quires locating a few feet higher than the sealing device, the minimum distance being about 3 feet.
When comparing Figs. 34 and 35 it will be observed that the principal differ- ence between them is simply one of detail, special provision being made in Fig. 35 for dealing with the air. In Fig. 34 it would be necessary for any air to be forced through the mercury seal, un- less a special air chamber were used. Air pipes of course cannot be adopted on circuits when sealing appliances are used, although special automatic valves may be fixed for effecting the escape of air.
For an " overhead " or " drop " system the usual point of connection with a mercury seal is shown in Fig. 36, whilst that for an up-feed system is indicated in Fig. 37. The latter position is the better of the two, for the appliance works more smoothly, and there is less likeli- hood of the mercury being precipitated against the curved deflection plate.
In some cases, combination relief and FIG. 35. - " Klymax " vacuum valves take the place of mercury
heat generator. By Kel- , , , J
logg, Mackay, & Co. seals, but where these are adopted, a type should be selected that is sensitive and
reliable in action. For ordinary heating systems, the combi- nation type shown in Fig. 33 is not suitable, a superior arrange- ment being one where the vacuum valve opens by its own weight when the water tends to leave it. The chief advantages of valves are due to the facilities they offer in the way of adjustment with respect to a larger range of working pressures.
ACCELERATED HOT- WATER CIRCULATING SYSTEMS 59
Some of the literature on these appliances greatly overrate their value, but the extent to which the circulation is accelerated may be computed by formula 2.
A recent application of the mercury seal is shown in Fig. 38, and the important features of this system are, the location
FIG. 36. — Showing position of generator for " drop " system.
of the expansion tank, the relief of any excess pressure, and the means adopted for regulating the rate of combustion. The mercury trap is joined with the return on the left, the seal being just sufficient to hold back the pressure due to the head of water plus an additional 10 Ib. per sq. inch. To the top of the flow-pipe bend, the expansion tank is joined with a pipe of small bore, and from one end of the tank, another pipe com- municates with the diaphragm that operates the check and draught dampers of the boiler.
60 DESIGN OF HOT-WATER & STEAM HEATING APPARATUS
When a system like Fig. 38 is charged with water, the air in the tank is compressed, and this is subjected to still greater pressure as the temperature of the water is raised. Whatever pressure is created in the tank is at once transmitted to the diaphragm, whilst, by means of the perforated plate at the end of the lever, the dampers £may be set to operate at any
FIG. 37.— Showing position of generator for " up-feed" systems.
temperature desired. Should the diaphragm fail to act at any time, any excess of pressure is immediately relieved by water escaping through the mercury seal. The water may be added by joining a service pipe directly with a return, and where this form of connection is not permitted, a hand pump could be used.
ACCELERATED HOT-WATER CIRCULATING SYSTEMS 61
Another system in which the expansion tank is located in the boiler house is given in Fig. 39, and this presents a novel feature as regards the boiler draught control. Here it is preferable to place the expansion tank alongside the boiler, as
FIG. 38. — Accelerated system by Dongherly & Tablet.
the former is arranged to fall bodily and to rise through a short distance to impart motion to the lever. To the expansion tank is attached a balance weight, and for normal working conditions the contained air and water are proportioned to keep the tank in the higher position. If, however, the temperature continues to rise, further water enters the expansion tank, when, by virtue of the additional weight, it falls to the lower position by the aid of the gland joint shown. Upon a cooling action being set up, the water is displaced from the tank by the compressed air, when the tank is again raised by the balance weight, and the position
62 DESIGN OF HOT-WATER & STEAM HEATING APPARATUS
of the dampers is reversed. The gland joint only permits of a half-inch drop, but this distance is multiplied for operating the dampers by means of the lever arm. At the top of the expansion tank, a relief valve is provided.
The mode of operation and method of charging Fig. 39 is as
Draw-ofY'.
FIG. 39. — Accelerated system of hot-water heating.
follows : Before filling the apparatus with water, the expansion tank is disconnected by closing the stop valve shown, after which the whole of the pipes and radiators are charged in the usual way. Air is then pumped into the tank until sufficient pressure is produced to hold up the water in the highest heating surface, after which a few gallons of water are withdrawn to allow for the expansion that accompanies the application of heat. This being done, the stop valve is opened, when the
ACCELERATED HOT- WATER CIRCULATING SYSTEMS 63
water should rise to a predetermined point. It is more con- venient if the gauge used only records the pressure that accrues
ji
ji
Ji
FIG. 40.— "Reck" system
L = automatic vent. T = expansion tank. S = overflow and exhaust pipe. R = return from tank. M = mixing pipe. C = circulator. D — condenser.
O = condensation return.
P = steam pipe.
V — regulating valves.
F = flow pipe.
H = reheater.
G = return from heating surfaces.
B = low-pressure steam boiler.
from the heating up of the system, it being graduated for the index finger to point to zero when the water is cold, irrespective of what the static pressure may be. By the adjustment of the
64 DESIGN OF HOT- WATER & STEAM HEATING APPARATUS
balance weight, the dampers may be set to operate at any pressure desired.
In Fig. 8, page 28, the general principle of acceleration by the aid of steam is shown, whilst Fig 40 gives an apparatus on the " two pipe " principle in greater detail. An ordinary low-pressure steam boiler is used, which should be fitted with an automatic draught regulator, but the reheater H as a rule is discarded. For the apparatus to be a success the steam that escapes into the expansion tank must be readily condensed, or there will be a tendency to the equalization of the pressures in the pipes EG and MF, when the circulation will be greatly impeded.
Although in Fig. 40 a surface condenser is used to ensure the necessary differential pressure, the surplus steam may also be condensed by being brought into direct contact with the cooled return water, especially in those cases where no re- heater is used. To effect this, a condensing tank is placed at a certain point between the expansion tank and the circulator, the top of the former being joined with the overflow from the expansion tank, whilst the pipe from the bottom of the con- densing tank is taken to the circulator. The return pipe from the heating system is also connected with the con- densing tank so as to bring the steam and water in direct contact. The expansion tank should be properly proportioned and charged with water to a certain point, any deficiency through leakage or other cause being indicated by the gauge at the boiler. If desired, the risers may be vented to the atmosphere.
Where acceleration is due to the introduction or disengage- ment of steam, it is sometimes considered a drawback, in that the water is delivered in a highly heated state to the radiators. This point, however, is easily overrated, for the water, upon entering the heating surfaces, at once mixes with the cooler water and so lowers the temperature.
Fig. 41 gives a form of rapid circulating plant where acceleration is due to the liberation of steam bubbles in the ascending water columns. In an installation of this type it is necessary to limit the production of steam, otherwise it would be noisy and somewhat erratic in operation. The principal
ACCELERATED HOT-WATER CIRCULATING SYSTEMS 65
features in Fig. 41 are the provision of the "flow bottle," the method of operating the draught regulator, and the large " dip " or siphon that forms part of the expansion or relief pipe. By opening the valve V, the system works in the ordinary way, the increment due to expansion having direct relief whilst any air that is liberated passes directly to the expansion tank.
FIG. 41.—" Beck " system.
When the valve V is closed, the direct connection is cut off, and the only course left for relief is through the siphon or dip. As steam rises from the boiler it tends to gather in the " flow bottle " instead of being conducted into the horizontal pipes, and in this way its contained water is dislodged through the siphon, thus subjecting the two sides of the regulator diaphragm
F
66 DESIGN OF HOT- WATER & STEAM HEATING APPARATUS
to differential pressure. The dampers are automatically adjusted to regulate the generation of steam, but the order of the dampers is again reversed as soon as the steam in the " flow bottle " is condensed. It will be observed that the underside of the regulator is subjected to a constant pressure in virtue of its being joined with the pipe that leads directly to the expansion tank, whilst the pressure on the top of the diaphragm is the variable one.
It is usual with the system indicated in Fig. 41 to locate the " flow bottle " and the horizontal main pipes as high as practicable, and although the steam from the boiler is largely intercepted by the " flow bottle," yet the water in the hori- zontal flow mains may be maintained at a sufficiently high temperature, that upon its passage into the vertical risers steam will be disengaged. The production of the steam in this manner is due to the diminishing hydrostatic pressure to which the heated particles are subjected in ascending to the higher level, and as a mixture of steam and water gives a diminished density for the ascending columns, the circulation is accelerated throughout the installation.
Still another type of apparatus is indicated in Fig. 42, where acceleration is due to displacement by steam. At the head of the system, tanks T and M are shown, and from each the water is alternately dislodged, and delivered through the circuit, being finally received in the adjacent tank. In the tank T a float chamber is placed, this being arranged as it rises and falls to open one of the tanks to the steam supply, and the other to the exhaust pipe E. The non-return valves A, B, D, and H control the course the water must take, whilst at the same time they admit of the apparatus being operated by natural circulation when this is desired. A system may take different forms, and steam may either be supplied from a separate source, or the hot water boiler may be replaced by a calorifier, and a low-pressure steam boiler installed. The action of the tanks resembles a series of pulsations, but the circuits will require to be properly sized and adjusted for the water to be evenly distributed over the whole plant.
For dealing with the exhaust steam in Fig. 42, a small surface condenser C is shown, and into this the surplus water
ACCELERATED HOT-WATER CIRCULATING SYSTEMS 67
is conveyed that accrues from the condensation of the steam in the circulating tanks.
In Fig. 43 the circulating tanks of the last apparatus are more clearly shown, it being assumed that the tank T is open
FIG. 42. — Baker's system of circulation.
E = exhaust pipe. C = condenser. A, B, D, H = check valves.
S = steam supply. O = condensation.
T, M = circulating tanks.
to the exhaust pipe, whilst steam under a suitable pressure is being admitted to M. From the latter tank the water would be displaced, and, provided that the valves D and A are in order, would flow through H, complete the circuit and finally enter tank T. When, however, the water in T has filled the float
68 DESIGN OF HOT- WATER & STEAM HEATING APPARATUS
bucket and weighed it down, the position of the valve is reversed, the tank T being opened to the steam supply and M to the exhaust. Under the altered conditions, the water is now dislodged from the float, completes the circuit as before,
FIG. 43.
and enters M, when the float is buoyed up, and the position of valve again reversed. So far as the speed of the circulation is concerned in this case, it will be chiefly governed by the pressure of the steam and the resistance offered by the piping.
CHAPTER VI
FORCED HOT- WATER CIRCULATING APPARATUS
ALTHOUGH the application of external power to produce a positive movement of water is common to all forced systems, yet they differ in form, such as in the construction and arrange-
FIG. 44. — Forced circulating system.
ment of the heaters, the manner in which the connections are made, and in the arrangement of the piping. Fig. 44 gives a simple system in which the water is forced through the main
70 DESIGN OF HOT-WATER & STEAM HEATING APPARATUS
circuit, whilst the movement through the risers depends upon natural or gravity circulation.
Where a simple circuit system is adopted, the movement through the risers may be accelerated by the use of the fittings shown in Fig. 45. Here the nozzle for the flow riser is turned to face the stream, whilst that for the return riser is fixed the opposite way. This arrangement has the effect of converting the velocity into static pressure at the flow, and of producing a partial vacuum at the end of the return riser.
The fittings shown, however, have a restricted use in these
SECTION AL V/EW OF EJECTOR fIXED IN MAIN
SKT10HAL VIEW OF INJECTOR FIXED IN MAIN
FIG. 45. — " Acme " fittings by National Radiator Company.
systems, and it is more usual in a one-pipe arrangement to depend upon the use of long sweep fittings. In some cases, instead of joining the risers directly to the main circuits, subsidiary loops are formed, these being sized and arranged so that a positive movement of water through them is ensured. To the subsidiary loops the risers are joined. For intercepting the air in Fig. 44 an air vessel is shown, and from this it may be released by a hand-controlled valve, or an automatic air trap may be substituted for the vessel shown. The feed and expansion tank is joined to the return, close to the inlet
FORCED HOT-WATER CIRCULATING APPARATUS 71
side of the pump, but where there is circulating pressure the open tank requires to be replaced by a closed one with auto- matic feed as in Fig. 7. As a rule, an open tank can be used for all plants where the feed pipes can be joined as in Fig. 44.
Fig. 46 indicates how radiators are "shunted off" the main circuit. If a given circulating pressure is generated by the pump, it is clear that this will be absorbed by pipe friction as
|
t |
r t |
i G |
FIG. 46. — Forced circulation. " One-pipe " system with loop circuits.
the water completes the circuit. Let it be assumed that the circulating pressure at A is equal to a head of 30 feet, whilst that at D is 29'5 feet. In this case the fall of pressure is half a foot, and represents the head absorbed by the length of main between A and D and by the loop ABCD. Whatever weight of water is circulated, the velocity through OA will be greater than that through AD owing to the additional path provided. The proportion of the water circulating through the loop ABCD will depend upon the resistance introduced, but the velocity through
72 DESIGN OF HOT- WATER & STEAM HEATING APPARATUS
it will necessarily be less than where the water can take the direct course as from A to D. In like manner, the pressure head at E may be assumed as 22 feet, and that at H as 21 feet, thus giving a pressure drop of 1 foot of head. In order to diminish the frictional resistance from F to G, a double pipe is shown, whilst the risers are joined to this portion of the loop. A portion of a " two-pipe " system with forced circulation is shown in Fig. 47. With this arrangement, a much greater force is available to circulate the water through the branches than in the case of a " one-pipe " or circuit system. The extent of the differential pressure to produce motion in a branch circuit varies with its distance from the pump, and with the initial circulating
|
t r |
i |
1 1 t M |
^5 |
||
|
R N |
t |
FIG. 47. — Forced circulation. " Two-pipe " system.
pressure. For example, if the water upon leaving the pump has a circulating head of 28 feet, 24 feet at M, 20 feet at E, 16 feet at T, 12 feet at U, 8 feet at S, 4 feet at N, and zero at the inlet of the pump, the pressure difference between M and N would be 20 feet, between E and S 12 feet, and between T and U 4 feet. Thus it will be seen that the velocity of flow will vary considerably in the different branches, and that a given size near to the pump will serve a larger area of heating surface than one further removed, provided the remaining factors are equal.
To regulate the flow of water through the branches of a two- pipe system, some form of throttling device will be essential, for with standard sizes of tubes alone, the resistance of the
FORCED HOT- WATER CIRCULATING APPARATUS 73
various sections cannot be exactly proportioned. For this purpose, valves are sometimes used, or where cheaper means are required, special tees with throttling plugs can be utilized, or orifices of different diameters can be introduced.
For circulating water in a heating system, a centrifugal or turbine pump should be used. Piston types are not so suitable for regulating the speed of circulation, whilst some forms are rather noisy in action.
The more interesting aspect of circulating plants is that in which either " exhaust " or " live " steam is available for heating the water, and for works and other large buildings these systems often open up possibilities for economy and flexibility of operation that are unrivalled in any other form of heating apparatus.
Whether a " live " or " exhaust " steam heater can be the more advantageously employed, depends upon the case as a whole, such as the amount of steam required for warming purposes, the extent to which this varies throughout the heating season, and the type and size of the engines in use. Conditions frequently arise, however, where it is desirable to provide both forms of heaters, the live steam heater being the one used during the milder weather, and when the engines are stopped.
Connections of Heaters. — There are various ways of arranging the heaters and their connections. In Fig. 48, an arrangement is given that is sometimes suitable where exhaust steam is available, either from a condensing or non-condensing engine. Assuming in the first place that a non-condensing engine is in use, the back pressure valve B diverts the "exhaust" to the heater, whilst the separator G will aid in keeping the heater tubes in a cleanly state. To regulate the temperature of the water as it leaves the heater, the back pressure may be varied, or the stop valve adjoining the separator may be used, or the valve on the condensation pipe may be partially opened or closed. The latter procedure results in a portion of the heater tubes being thrown out of use, according to the depth to which the condensation rises in the casing. In order that the water temperature can be maintained when the engines are stopped, or when the " exhaust " is insufficient, provision is made for the admission of " live " steam. To prevent the latter from
74 DESIGN OF HOT- WATER & STEAM HEATING APPARATUS
escapiDg into the main "exhaust" when the stop valve is open, a non-return valve V may be added, or a combined stop and non-return may be used.
The introduction of " live " steam as in Fig. 48 is not the most economical method, owing to the whole of the water of condensation requiring to be pumped back to the boilers, and to the loss of heat that accrues through reducing the steam from a high to atmospheric pressure. This connection, how-
To Hot Well
FIG. 48.— Forced circulating system.
P = pressure reducing valve. F = flow pipe.'
B = back-pressure valve. R = return pipe.
C = pipe to atmosphere. H = steam heater.
E = exhaust from engine. A, D = steam gauges.
V = check valve. T = steam trap.
G = grease extractor. M = valve.
ever, is suitable where the heater is some distance removed from the source of supply, or where a "live" steam heater cannot be sufficiently elevated to permit of the condensation returning by gravity to the boiler.
Should the "exhaust" steam for a case like Fig. 48 be received from a condensing engine, the economy of the system would chiefly depend upon the relationship between the power and heating loads. This aspect of the problem is considered in the chapter on " Exhaust Steam Heating."
FORCED HOT- WATER CIRCULATING APPARATUS 75 With a condensing engine, the heater and connections as
fc
.C4
1
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III
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if gj3f
oc ^3 d jj .— < cj
•B §11111
S^o ft§ ® g
g o'o'S'3^5 * •S II II II II I! II
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shown in Fig. 48 may be considered in the light of a supple-
76 DESIGN OF HOT- WATER & STEAM HEATING APPARATUS
rnentary condenser to be used in the winter time, whilst the ordinary condenser is utilized in warm weather and when only small heating loads are required. There would be this difference, however, that whereas the general condenser would be operated under a vacuum, the heater or supplementary condenser would be under a small pressure, which would con- siderably increase the steam consumption of the engines. When, however, heating and power plants are considered as a whole instead of as separate units, there would be no loss in the combined efficiency, so long as the circulating water of the heating system condenses a certain percentage of the steam.
Fig. 49 gives a combination for utilizing the exhaust from one or more turbines where the average steam required for heating purposes is approximately equal to, or in excess of, that required by the power units. The condenser C and exhaust heater E are arranged so that either one or the other can be put in or out of use, or they may be operated together when the heating load falls appreciably below that of the turbines. In other words, the condenser C would deal with the " exhaust " that the heater E could not condense.
The arrangement of Fig. 49 is advantageous, as the turbines may be operated under a high vacuum for a large variation in the weather conditions. For example, where the " exhaust " is in- sufficient to supply the heating demand, the water may be circu- lated through the "live" steam heater H after first passing through the exhaust heater E. It is only a matter of manipu- lating the stop valves shown, for the temperature of the water leaving the heater H may be controlled by allowing the condensa- tion to accumulate, and to cover part of the heater tubes. On the other hand, where the volume of steam required for heating pur- poses is rather more than the turbines will supply when operating under the usual vacuum, the steam consumption may be raised to the right amount, either by altering the speed of the air pump, or by admitting air to lower the vacuum. The live steam heater H should have a capacity sufficient to do the whole of the heating work when the turbines are stopped, and provided it is located well above the boiler, the condensation can be returned without the intervention of a pump. It may, however, be essential to provide an injector at I to facilitate the return of
FORCED HOT-WATER CIRCULATING APPARATUS 77
the condensation, when the condenser is working nearly at its full capacity, or where the head space above the boilers is limited. The air pump A is assumed to be of the rotary form, coupled directly with an electric motor, whilst the circulating pumps P and M are of the centrifugal or turbine class, and driven in the same way.
Plants for High Buildings. — When forced circulating systems are used for very high buildings, it is desirable to divide the water circuits into two or more isolated units, in order that excessive strain be avoided at the lower levels. The steam, however, for heating the water may be supplied from a common source, and if exhaust steam is available it may be utilized as far as it will go. This idea is indicated in Fig. 50, where two principal water circuits are provided, each supplying a number of different floors ; but additional main circuits may be added in the same way. The engine M is assumed to be operating as a non-condensing one, and the water of condensation from C returned through a feed heater or economiser to the boiler. The arrangement of the valves permits of the separate use of either the "live" or "exhaust" heater, or the water may be circulated first through the one and then the other. It will be observed that the valves on the condensation pipe from the " live " steam heater are located in the boiler house, so that the temperature of the circulating water can be largely regulated from that point.
In forced circulating systems .the total pressure at any point is represented by the static plus the circulating pressure, so that if it is desired that the pumps shall not be subjected to the maximum strain they should be located at a fairly high level instead of at the lowest point.
In Fig. 50 the expansion tanks T are located near the pumps, and this position is convenient in that it helps to con- centrate the important parts. For forcing the water to the top of a system compressed air may be introduced into the expan- sion tank from any convenient source, or the pressure may be generated by the aid of a force pump. Each expansion tank should be provided with a gauge glass to indicate the volume of water it contains, whilst a pressure gauge and relief valve should also be added. As regards the piping for supplying the
78 DESIGN OF HOT- WATER & STEAM HEATING APPARATUS radiators, this may be arranged on either of the recognized
FIG. 50. — Forced circulating plant for high buildings. B = automatic vent. A = condensation pipe.
H = live steam heater. G = grease extractor.
E = exhaust steam heater. V = back-pressure valve.
P = circulating pump. M = engine.
T = expansion tank. C»= Condensation receiver.
FORCED HOT-WATER CIRCULATING APPARATUS 79
systems, the " overhead " being advantageous where a large number of floors is served by one principal circuit.
Duplication of Pumps. — When pumps of the centrifugal type are used for circulating water, and their duplication is desired, the piping for joining them should be arranged in series owing to the characteristics of these appliances. A series connection, with both pumps operating at the same time, permits of the head being divided between them, and are more simply con- trolled than when operating in parallel. Fig. 51 shows how the piping for duplicate pumps may be arranged, with the neces- sary stop valves in position. For running in series the stop valves 5 and 6 are closed, whilst by the further adjustment of the valves, either the one or the other pump can be used alone.
6
FIG. 51. — Showing valves for duplication of pumps.
Another form taken by a forced circulating system is shown in Fig. 52. This differs principally from the usual " two-pipe " arrangement in that a by-pass is provided at B between the flow and return risers, and in that the movement of water through the greater portion of the risers depends upon natural circulation. The principal object attained by this design is the utilization of water at a high temperature in the main circuits, whilst at the same time it is delivered to the heating surfaces at a much lower temperature. By circulating water at a higher temperature than is usually done, a smaller weight is required to produce a given heating effect, and the cost of operation is reduced. Into the base of each flow riser the highly-heated water is forced, this having its temperature reduced through mixing with the cooled return water in virtue of the by-pass B. The supply of heated water is controlled by a throttling device or regulated orifice at A, and so long as
80 DESIGN OF HOT- WATER & STEAM HEATING APPARATUS
it is correctly adjusted to suit the height of the risers and the requirements of the heating surfaces, the heated water will not make a short circuit with the main return. The risers when in pairs together with the by-pass make a complete circuit in themselves, whilst the positive and limited supply of the high temperature water serves the function of a separate heater. It will be conceived that in unit time every pair of risers will circulate a given weight of water, depending upon their height and the temperature drop allowed, and that if the volume
PIG. 52. — Forced circulating system with by-pass for risers as conceived by Captain Reck.
S = steam pipe. R = condensation outlet.
H = steam heater. A = regulating device.
P = circulating pump. B = by-pass.
>
passing from A falls below that amount, the deficiency will be made good by the admission of the return water at B.
An example will aid in making the matter clear. Assume the risers in Fig. 52 are sized to circulate 20 Ib. of water per minute for a temperature drop in the heating surfaces of 40° F., that is for a capacity of 40 x 20 = 800 B.Th.U. per minute. These same heat units would be supplied by 8 Ib. of water falling through a temperature of 100° F. Now, if 8 Ib. of water at the temperature of 240° F.'were delivered per minute into the base of the flow riser, the circulation would proceed apace until the capacity of 20 Ib, per minute were attained. The
FORCED HOT- WATER CIRCULATING APPARATUS 81
temperature of the return water would be 240 - 100 = 140° F., and as 20 — 8 = 12 Ib. of this would flow through the by- pass B, the temperature of the water in the flow riser would be reduced from 240 to an average of 180° F.
Methods of Regulating Temperature of Circulating Water. — The different methods of adjusting the water temperature to suit the changing weather conditions have been indicated in the figures considered, but where steam is the heating agent they may be briefly stated as follows : —
(1) The water temperature may be varied at the heater by increasing or decreasing the steam supply, by controlling the discharge of the water of condensation, and by regulating the pressure and. degree of vacuum.
(2) The water at the heater may be maintained at a constant temperature, and the velocity of circulation varied.
(3) Both the temperature and circulating speeds may be altered conjointly.
The best practice to adopt, will depend upon the general design, Figs. 49 and 50 being best served by the first method given, whilst Figs. 48 and 52 are more suitable for the second and third methods of regulation.
When a heater, for example, as in Fig. 48 or 52, is supplied with live steam, the first method of regulation is not so good, owing to the power absorbed by the pump being the same under all conditions of the external air. In other words, no economy of operation would be shown when only the minimum heating effect is required. On the other hand, by curtailing the discharging capacity of the pump, the power to drive it may be substantially reduced. The extent to which economy can be carried by an electrically-driven centrifugal pump depends upon the size and type of motor, and the regulation adopted. As a general statement, when a pump is run at a fairly constant speed, the power to drive it may be diminished by about 25 per cent., if the discharging capacity is reduced by one half through the partial throttling of the outlet valve.
CHAPTER VII
LOW-PRESSURE "LIVE" STEAM HEATING SYSTEMS
Heat of Steam. — As a heating agent, steam is specially con- venient, as it may be utilized for a large variety of purposes, both of an industrial and domestic nature.
The total heat of steam gradually increases with increase of pressure, but the latent heat value diminishes as the pressure increases. For example, at atmospheric pressure, the latent heat of steam is 970, and at 30 Ib. gauge pressure 928 B.Th.U. per Ib. In other words, each pound of steam during condensa- tion at, and from atmospheric pressure will yield 970 — 928 = 42 additional heat units to the condensation of the same weight at, the higher pressure. On the other hand, the total heat of steam per Ib. at atmospheric pressure is 1150 B.Th.U., whilst that at 30 Ib. per square inch is 1171 B.Th.U. Thus, more heat by 21 units is stored in the steam at the higher pressure; but it may not be available for heating purposes.
In this class of work it is usual to take the useful heat as equivalent to the latent heat value ; but this holds good only when the condensation occurs at the same pressure as the steam. If, on the other hand, the condensation becomes subjected to a lower pressure, re- evaporation will occur ; but heat will be lost, unless precautions are taken to prevent it.
Drop of Pressure in Steam Pipes. — To circulate steam through a system of piping a certain head or pressure is required, but the permissible velocity will be chiefly governed by the follow- ing considerations, viz. the flow of the steam and water whether in the same or in opposite directions, the height of the lowest heat- ing surfaces above the boiler, also by the direct return, or other- wise, of the condensation to the boiler. As a rule, when a gravity
LOW-PRESSURE " LIVE ' STEAM HEATING SYSTEMS 83
system is installed, only a small pressure drop should be allowed, and if this is observed there is less likelihood of the lower heating surfaces being flooded during the coldest weather.
Those unfamiliar with steam-heating work sometimes fail to realize what is meant by drop or fall of pressure. As already indicated, some force or energy is essential to cause steam to flow through pipes, and it is the absorption of this energy through friction that is responsible for the drop of pressure. For example, assume a circuit of a given length, where the boiler pressure is 5 Ib. per square inch ; if, by the time the steam has reached the far end of the circuit its pressure has fallen to 3 Ib. per square inch, the drop of pressure would be 2 Ib. per square inch. At the same time this differential pressure of 2 Ib. per square inch would be counterbalanced at the boiler return, by the water rising in it to a height of. 2-4 x 2 = 4*8 feet. It is the restoring of the equilibrium at the boiler return that is responsible for low-lying mains being sometimes flooded.
Water-Hammer in Steam Pipes. — It is important when arranging the pipes of steam-heating systems that no pockets are formed in which water will be retained, thereby interfering with the flow of steam. This may be avoided by adopting suitable fittings, by giving the pipes an adequate pitch, and by the use of drip pipes where the condensation tends to gather. Accumulations of water in steam pipes are detrimental and objectionable, in that they cause .water-hammer or snapping sounds. Steam coming in contact with cold water, is rapidly condensed, when the vacuum that ensues is responsible for the projection of the pocketed water in the direction of the vacuum. The violence with which the water is precipitated against bends, tees, valves, or other fittings, is often the cause of their being damaged.
Clicking or snapping sounds usually arise through the steam coming in contact with smaller volumes of water, and, although these are not usually accompanied with any marked strain, they should be avoided as far as possible. A certain amount of snapping occurs when steam is readmitted into a system, for the cold extended surfaces bring about a rapid condensation, and- produce a more or less differential pressure.
84 DESIGN OF HOT-WATER & STEAM HEATING APPARATUS
With a well-designed system, however, this is of short duration, the snapping ceasing when the pipes are heated.
Gravity Systems. — As the name implies, these are installa- tions in which the water of condensation simply gravitates from the heating surfaces to the boilers, in contradistinction to those in which the condensation is returned by some external agency, such as pumps, injectors, or other lifting appliances. Generally speaking, gravity systems may be divided into two principal divisions, viz. low-pressure ones, or those using steani
FIG. 53.— One-pipe system of steam heating.
above atmospheric pressure, and atmospheric systems, or those in which the steam falls within the heating surfaces to the pressure of the atmosphere.
The piping is arranged in different ways, and, as in low- pressure hot-water installations, may be on the up-feed or down- feed principle. Frequently, the terms " wet " and " dry " are used in connection with the returns, to indicate whether they are constantly charged with the water of condensation or not.
LO \V-PRESSURE "LIVE" STEAM HEATING SYSTEMS
85
One-Pipe Systems. — In Fig. 5.°> a one-pipe circuit system is shown, and from the point A the main should have a fall of about 1 inch in 10 feet. This arrangement permits the steam and condensation to flow in the same direction in the main, thus avoiding conflict with these fluids. Excepting for the short vertical return that joins with the boiler, the main circuit should be of uniform bore, whilst the vertical part of the return may be reduced by one or two sizes. At the point B, an automatic
FIG. 54. — Divided circuit.
air valve is placed to free the circuit of air, for, unlike water systems, the air tends to seek the lower, instead of the higher level. Neither is the air so effectively dislodged as in water installations, for the density of air is only a little greater than that of the steam. For example, at atmospheric pressure, the density of steam is 0'037 Ib. per cubic foot, whilst air at 212° F. weighs 0*059 Ib. per cubic foot. With one-pipe systems, only one riser is required for supplying the heating surfaces, the steam and condensation passing through the same pipe.
86 DESIGN OF HOT-WATER & STEAM HEATING APPARATUS
Owing to the varied forms that buildings take, divided circuits are rendered necessary, these being indicated in Fig. 54. From the point J, two sections are formed, but the steam should follow a denned course, otherwise the drainage may be interrupted and the efficiency of a system impaired. There is no difficulty, however, in causing the steam to flow in one particular way, so long as the returns are joined below the boiler water-line. From point P, the mains are supposed to have a downward pitch, whilst an automatic relief for air is provided for each return.
The number of units into which a system should be divided will depend upon the circumstances of each particular case, and
r?-*H
No.3
Not
r-^i
Water Line.
FIG. 55.—" Two-pipe " up-feed system.
the degree of control desired. For example, one floor may be provided with two or more circuits, each of which could be sub-divided into two or more parts, or one divided circuit may be used for two or more floors.
Two-Pipe Systems. — A " two-pipe " system of steam heating is given in Fig. 55. This differs principally from a "one-pipe " system, in that the steam mains are reduced in size as the heating surfaces are supplied ; separate pipes are employed to handle the condensation, and the principal returns are charged with water. In the risers Nos. 2, and 3, it will be observed that a separate return is used for each radiator. This arrangement is adopted to make a system more silent in operation, for when two or more returns from radiators are joined, as in
LOW-PRESSURE "LIVE STEAM HEATING SYSTEMS 87
risers No. 1, steam can enter the heating surfaces either through the inlet or outlet connections. The method indicated by riser Xo. 1, however, can be made to answer satisfactorily.
When diminishing the size of steam mains, eccentric fittings should be used, in order that true alignment may be preserved on the lower sides of the pipes. The common forms of diminish- ing fittings are unsuitable for steam pipes, for not only may they be the cause of snapping sounds, but the capacity of the mains may be much reduced.
With a two-pipe system, valves are attached to both the flow and return connections, and when stopcocks are introduced at the base of the risers, a simple arrangement for this is given on the right of Fig. 55.
Overhead or Down-feed Systems. — Fig. 56 gives an overhead system of piping, the steam from the boiler being directly con- veyed to the overhead mains, whilst from these, the various drop- pipes are taken to supply the heating surfaces. The lower horizontal mains are arranged to remain charged with water, for it is essential that the steam should only gain admission to the drop-pipes from their higher ends. Although only one steam main from the boiler is shown, two or more may be used, the extra provision of course depending upon the size of the plant and the number of units into which it is proposed to divide it.
Radiator Connections. — The radiator connections of steam apparatus may be arranged in any convenient way, so long as they do not trap or retain the water of condensation. It is not essential to make any difference in the form of connection for the higher and lower heating surfaces, as is frequently done with hot-water apparatus, for the equal distribution of steam is more readily effected.
Obstructions to Pipe Lines. — It frequently happens when piping a building that some obstruction presents itself that necessitates a break or alteration in the alignment of the pipes. This may arise through the intervention of girders, beams, and the like, or through structural work requiring the raising of pipes from a lower to a higher level. Such irregularities in the piping often demand the use of " drips," and as these transmit the steam-pressure directly where they discharge into the returns, allowance must be made for this in sizing the pipes,
88 DESIGN OF HOT- WATER & STEAM HEATING APPARATUS
or the drainage from the more distant mains may be appreciably retarded.
In Fig. 57 two methods are shown for relieving the conden- sation, where, on the one hand, the steam main is passed beneath a girder, and on the other, where it is bent over it. Where, however, circumstances are favourable, the method at B
_^JJU
F*~|
LL
Drip.
Water
Drip
Lint:
Air Valve
FIG. 56. — " Overhead " or down-feed system.
is the better one to adopt, for when " drip " pipes are used, suit- able points must be found for their discharge.
False Water Lines. — Occasionally boilers are placed in positions that demand the use of false water-lines, in order to seal the ends of the returns. Fig. 58 shows what is meant, but in this case a difficulty is often experienced in maintaining the
LOW-PRESSURE "LIVE/' STEAM HEATING SYSTEMS 89
"false" water-line, through a tendency to siphonage being brought about. An attempt is made to arrest this by the
FIG. 57. — Showing methods of draining pipes.
provision of the equalizing pipe between the steam main and the top of the loop, but it does not fulfil effectively the purpose that is sought.
FIG. 58. — Common l»ut defective method of making a false water-line.
The weakness arising in connection with the equalizing pipe of Fig. 58 is produced by the steam being brought into .intimate
90 DESIGN OF HOT- WATER & STEAM HEATING APPARATUS
contact with the condensation at the loop through the oscillation that occurs. Condensation of the steam is brought about, which results in the partial vacuum removing the contents of the trapped return.
Another method of arranging the equalizing pipe for a " false " water-line is shown in Fig. 59, where it is joined with a main return some 30 feet or so distant from the loop. The purpose of the long equalizing pipe is to enable a small volume of air to be trapped so that it may form a cushion between the steam and the condensation, whilst at the same time the neces- sary pressure is transmitted to the loop. This method, however.
FIG. 59. — False water-line provided with long equalizing pipe.
may not be proof against siphonage, but it will prolong the intervals between the actions, and is better than the practice shown in Fig. 58. To charge the equalizing pipe with air, water must first accumulate in the loop when the stopcocks A and B are closed. In a short time, the steam in the equalizing pipe is condensed, when, upon the opening of the valve V, air is readily admitted to supply the partial vacuum that is formed. The air valve V is now closed, and the valves A and B partially opened, so as to admit the steam pressure being gradually transmitted to the return.
LOW-PRESSURE
"LIVE" STEAM HEATING SYSTEMS
91
A better method of forming a " false " water-line is shown in Fig. 60, in which a constant discharge trap is employed. This method is specially suitable for large apparatus, and where the return water is subject to a pronounced cooling action. The water of condensation, it will be seen, enters at the bottom of the trap, in order that its water surface may be kept as steady as possible. As in Fig. 59, the equalizing pipe is joined with a return riser, which is well removed from the trap, and the connection is made so as not to drain the water of condensation from the return. To put the trap into action, the water is
FIG. 60.— Better method of making false water-lines.
allowed to rise to its normal height, when the valves 1, 2, and 5 are closed, and No. 3 on the by-pass partly opened. The steam in the equalizing pipe and trap is given time to condense, when the valve on the trap is opened to admit air. This being done, the air valve is closed as well as No. 3, whilst 1 and 2 are opened wide ; finally, valve 5 on the equalizing pipe is partially opened when the steam pressure is admitted to the trap.
Dry Returns. " Two-pipe " Systems. — In the preceding pages on steam heating, the importance of sealing the returns has been pointed out where a silent working and efficient system is desired. It is not convenient, however, nor yet practicable in
92 DESIGN OF HOT- WATER & STEAM HEATING APPARATUS
every case to introduce low-lying wet returns, and when these cannot be utilized it is advisable to arrange and size the piping, that the drop of pressure may facilitate the drainage of the condensation from the different branches.
Figs. 61 and 62 show two methods of treating the returns from four coils, and will aid in the elucidation of the point under consideration. Let it be assumed that each coil in Fig. 61
FIG. 61. — Method of treating return piping.
is of the same size, and that the pressure of the steam falls at any given but equal rate. This being granted, it will be clear that the steam pressure at the inlet of coil No. I will exceed by a certain amount that at the inlet of coil No. 4 ; similarly, the pressure in the return of Fig. 61 will be greater at the outlet of coil No. 1 than that of No. 4. The effect of this arrangement
FIG. 62.— Defective arrangement of return piping.
is to cause the surplus pressure at the first coil to drive the condensation in the direction desired.
On the other hand, when the returns are treated as in Fig. 62, the pressures at the outlet of the coils are the same as in Fig. 61, but the surplus pressure at the first coil holds back the condensation from the others until the hydraulic pressure is sufficient to overcome the resistance imposed. More or less
LOW-PRESSURE " LIVE STEAM HEATING SYSTEMS 93
uoise may be caused by the water surging backwards and forwards, whilst the heating capacity of the coils beyond the first would be somewhat impaired.
Cases where Special Appliances are required for Returning the Water of Condensation. — When possible, an apparatus should be erected so that the condensation can gravitate to the boilers, but in some cases this cannot be done, either because it is not practicable to place the boilers low enough, or because steam is used which requires to have its pressure reduced. Under the circumstances first named, it may only be essential to artificially return the condensation from the lowest surfaces,
sLdi
|
p ri |
=? H |
ne. |
||||
|
-< * ' |
< ' |
|||||
|
J n N |
ate_rLi |
|||||
|
j |
I'1' |
jU |
& _' |
FIG. 63. — A method of handling the condensation from low-level radiators.
whilst in the other case, the whole of the condensation would require to be handled. Occasionally, the condensation may be passed to waste, but where this is done an automatic feed is necessary to replace the water that is lost. In all systems, a certain volume of make-up water is essential to replace the leakage at valves and other fittings, but in a well-arranged gravity system, this loss should only be small.
A simple and comparatively cheap appliance for returning the condensation is an injector, and although its application is limited in low-pressure work, it will answer veiy well where the heating surfaces are not extensive, and where the water-line
94 DESIGN OF HOT-WATER & STEAM HEATING APPARATUS
of the boiler is frequently observed. In Fig. 63 a case is given where an injector is employed for returning the condensation to the boiler from a few radiators at a low level, whilst from the higher heating surfaces the condensation is returned by gravitation. From the steam trap T, the water is displaced by the terminal pressure of the steam, and with any suitable trap the water should be elevated about two feet for each Ib. per square inch of pressure. Thus, if the piping is sized for a drop of pressure of J Ib. per square inch, and the condensation requires to be raised through a height of 6 feet, the boiler pressure should not be less than f 4- 4 = 3| Ib. per square inch. The fittings that accompany a low-pressure injector are indicated in Fig. 63, V being a relief cock that requires to be opened when starting the appliance.
Other appliances for handling the condensation are shown in the following chapter.
General Remarks. — With respect to the merits and draw- backs of " one- " and of " two "-pipe systems, it is sometimes contended that the latter is the less noisy of the two, but this depends very much upon how the systems are designed. With a one-pipe system, larger pipes are necessary, owing to the steam and condensation travelling through the same channels ; but this is not always a serious drawback, as the capacity of the pipes increases rapidly with increase of size. The chief advantage of two-pipe systems is that smaller pipes can be used, although this is more than off-set by the extra length of piping required and by the use of additional valves.
A failing more or less common to all low-pressure systems of steam heating is that the valves must be either turned fully on or shut off. If a radiator valve were partly open in the case of a one-pipe system, the passage through it would be in- sufficient to admit steam and drain away the condensation. The result of this would be to hold up the water in the radiator until its pressure was sufficient to overcome that of the steam, whilst with the ordinary automatic air relief, the leakage of the condensation may be brought about.
The extent to which the failing occurs in two-pipe systems depends very much upon the height of the heating sur- faces, and upon the treatment of the return connections. If,
LOW-PRESSURE "LIVE" STEAM HEATING SYSTEMS 95
for example, the radiator connections are made as at E (Fig. 55), the closing of the inlet valve when the return one is left open would have the same effect as the single connection already described. On the other hand, if the inlet valve of a radiator on, say, No. 2 riser of Fig. 55, were closed, and the return valve left open, differential pressure would arise through the condensation of the steam, when water would rise in the return until a height was reached to restore equilibrium. Under ordinary circumstances the differential pressure produced in this way may be considered as nearly equal to the initial pressure of the steam. As an approximate guide it may be taken that each pound of differential pressure per square inch will raise the. condensation through a height of 2 feet 5 inches. For the most economical operation of a low-pressure plant it is necessary that the water of condensation be returned to the boiler at as high a temperature as possible, and at little or no cost. Eeturn traps provide a simple automatic means of handling the condensation from low situations where the ter- minal pressure is suitable. The principal cost of this method is in the initial outlay for the appliances, the operating and maintenance charges being comparatively small.
CHAPTER VIII
FITTINGS FOR LOW-PRESSURE STEAM SYSTEMS
Pressure Reducing Valves. — These appliances take various forms, the type to use depending upon the pressure to be carried in the heating system, the extent to which the pressure must be lowered, and the degree of sensitivity desired. In heating plants where very low pressures are necessary, some
form of diaphragm reducing valve is usually adopted, as this form enables a large differential pres- sure to be maintained between the inlet and outlet sides.
Fig. 64 shows a form of re- ducing valve for realizing on its outlet side a pressure as low as 1 Ib. per square inch. It is simple in construction, the valve of the equilibrium type being closed when the outlet pres- sure acting on the upper side of the diaphragm exceeds the upward effort due to the weight and lever. On the other hand,
.tiG. 64. — Pressure reducing valve ,
by Kieley and Mueller. when the pressure at the outlet
tends to fall, the lever exerts the
superior force, and the valve is opened to the flow of steam. This form, however, is only suitable for installations when the delivery is not subject to rapid fluctuations.
Another pattern of reducing valve is given in Fig. 65, which is intended for situations where the pressure on the outlet side exceeds 5 Ibs. per square inch, in other words, where a very
FITTINGS FOR LOW-PRESSURE STEAM SYSTEMS 97
sensitive valve is not imperative. At a point a few feet distant from the valve and on the low-pressure side a small pipe is taken, the other end being joined with the diaphragm chamber at the top of the valve. A stopcock should be pro-
:i-EY, CAL
FIG. 65. — Pressure reducing valve. By Kieley and Mueller.
CONNECT WITH LOWPBESSUDE SIDE
FIG. 66. — Reducing valves for low pres- sures. By Kieley and Mueller.
vided on this pipe, the throttling of which reduces pulsation, when a rapid change of pressure is brought about.
A construction giving a more sensitive form of reducing valve is shown in Fig. 66. Like the previous one, the dia- phragm is independent of the valves, this being advantageous in that it can be joined with the low-pressure side some dis- tance away, so as to respond to the average pressure of a system. The main valve is of the equilibrium form, the diaphragm being large enough to give any low pressure desired, whilst
98 DESIGN OF HOT-WATER & STEAM HEATING APPARATUS
any tendency to pulsation can be avoided by throttling the small pipe that communicates with the diaphragm.
Many reducing valves are very troublesome, so that every care should be taken to select a type suitable for the work in hand.
Air Valves. — The small difference between the densities of air and steam makes it a difficult matter to know the best point
FIG. 67. — Automatic air valve for steam.
for locating air relief valves on ordinary low-pressure systems ; for, instead of the air accumulating at one particular place, it tends to diffuse over -a large area. So far as heating surfaces are concerned, the air valves are usually placed from
one-third to two-thirds their height on the side opposite to the steam supply, whilst on the mains they are located at low points.
For the relief of air, automatic valves are universally adopted, and, although they differ in form, they mostly depend for their action upon the expansion and contraction prin- ciple. They are not, however, an ideal means for affording relief, but as yet there is nothing better to take their place.
A well-known automatic air valve is illustrated in Fig. 67, which ope- rates by the expansion and contrac- tion of a composition plug, C. Like all similar contrivances, it depends upon a decrease of temperature accompanying the accumulation of air, under which circumstances the plug
FIG. 68. — Automatic air valve. By National Radiator Co.
FITTINGS FOR LOW-PRESSURE STEAM SYSTEMS 99
contracts and the air escapes. For adjusting the valve, the screw B is used, this being slackened when the steam is first turned on, and being gradually screwed up until none escapes.
Another form of air valve is given in Fig. 21, p. 43, in which a float is used. Briefly explained, its action depends upon the expansion and contraction of the air confined in the annular space, which communicates with the float chamber by means of a small aperture near the bottom of the fitting. Upon the expansion of the air, the water from the annular space is dis- placed into the float compartment, the float is rendered buoyant and so the air outlet is closed. On the other hand, the con- traction of the air causes the water to leave the float chamber, and, in consequence, the float falls.
Still another type is indicated by Fig. 68. In this, a metallic vessel is used which contains a volatile fluid that is readily vaporized with heat. When in the gaseous state, pressure is exerted within the vessel, and as end deflection brings about its elongation, the outlet orifice is closed. The gathering of air, however, in the vicinity of the valve causes a cooling action to set in, when the vapour reassumes its fluid form, and the internal pressure is removed.
Radiator Valves. — These are usually the same as for low- pressure water heating, and are considered in an earlier chapter.
Steam Traps. — In low-pressure heating plants, steam traps are often necessary, in the first place, to receive and regulate the discharge of the condensation, and secondly to prevent waste of steam. These appliances take a variety of forms, but it is bad economy to procure a type in which the initial cost is the chief recommendation, for the loss through steam leakage may soon more than offset the difference in cost between the inferior and superior, appliance.
Fig. 69 gives a steam trap of the box type, where the valve, by means of a quick screw motion, is opened and closed by the falling and rising of the float. The interior of the trap is not subjected to the pressure of the steam, and, unless dis- charging against a head of water, may be open to the atmo- sphere. In construction, the float valve differs from the usual form in that the condensation is discharged through the floa
100 DESIGN OF HOT- WATER & STEAM HEATING APPARATUS
itself. When the trap is not in use, or when the condensation is flowing into it, the float falls, owing to the water entering it. If, however, the steam reaches the float, the water from the latter is dislodged, and in being rendered buoyant the valve is closed. After a short period, the steam in the float condenses, the water re-enters and causes it to fall, when a further
FIG. 69. — Steam trap. By Lancaster and Tongue.
discharge of condensation is effected, if any has accumulated at the inlet of the trap. On the other hand, if in the interval no condensation has occurred, the steam immediately reappears and brings about the buoyancy of the float. For the satis- factory working of the trap it is necessary that air should escape from the float, and for this purpose an adjustable air
FITTINGS FOR LOW-PRESSURE 'STtfAM &?&&&&
valve is provided. The air valve also serves the purpose of preventing irregular action through the re- evaporation of the condensation when it enters the float at a high temperature.
Another box trap is shown in Fig. 70. In this case, a bucket float that is pivoted to the casing on the right is used, the motion opening and closing a double-seated valve. The inlet is not shown, but it is usually located on one side of the outlet, the trap in this case being subjected to the full pressure
-
FIG. 70.— Steam trap : low-pressure type. By Kieley and Mueller.
of the steam. Upon the condensation being delivered into the trap, it overflows into the bucket, which, when nearly full, sinks by virtue of its weight, and so opens the valve. As the interior of the trap is subjected to the steam pressure, the water is displaced until the float is buoyed up by the surround- ing water, and the valve is again closed. The form of valve in Fig. 70 is only suitable for low pressure, a single valve with restricted orifice being generally adopted for higher pressures. For the removal of air, a hand-controlled valve is provided.
Where smaller traps are necessary, expansion forms are largely used ; but many of these are faulty through lack of sensitivity of the expanding parts.
Fig. 71 gives a trap of the expanding type, the valve being
OF SQTl- WATER & STEAM HEATING APPARATUS
closed when steam comes in contact with the inner tube. Here it is a case of differential expansion, the inner tube expanding in a greater degree than the body of the trap. In the absence of steam, the trap is open to the air, and the condensation drains freely away. In order to permit of the necessary move- ment of the valve, the trap should be of moderate length, the overall length for a f-in. size, being 2 ft. 10 in., whilst the external diameter is 1J inch.
Another type of expansion trap is shown in Fig. 72, and although its external form is similar to the previous one, its construction is entirely different. To impart a pronounced motion to the valve, a very expansive material is employed in the inner tube H, the action of the trap depending upon the differential expansion between this substance and the inner tube, and not that between the inner and outer tubes, as in Fig 71. When steam enters the trap, the composition within the tube is readily heated, and the expansion that results, forces along the piston D, which in turn, causes the tube to move bodily in the direction of the inlet, and so the valve is closed. This particular action may not be apparent at the outset, but when it is taken into account that the spring at C, is consider- ably stronger than that at J, and that the movement of the tube must be in the direction of the smaller resistance, the action will be the more readily conceived. Upon the condensation gathering in the trap, the tube begins to cool, and as the medium inside contracts, the piston D is forced back (owing to the spring at J, and the pressure of the steam), the valve is again opened, and the condensation discharged. The purpose of the spring at C is for setting the trap to suit the pressure of the steam, and also for preventing the trap being subjected to excessive strain.
When selecting a steam trap in which a sensitive expand- ing medium is employed, it is important to observe that this does not prove a source of weakness. All steam traps possess some drawback, but by carefully observing their construction, and knowing the requirements to be fulfilled, there should be no difficulty in selecting a trap of a suitable type.
Return Traps. — For automatically returning the water of condensation from low positions, return traps are often employed.
FITTINGS FOR LOW-PRESSURE STEAM SYSTEMS 103
104 DESIGN OF HOT- WATER & STEAM HEATING APPARATUS
These appliances vary in structural details, but the underlying principle is usually the same.
One form of return trap is given in Fig. 73. The various return pipes discharge into a receiving tank, which is located at the lowest point, and from this point the condensation is raised to the trap, which should be located not less than 3 feet above the boiler water- line. As the water accumu- lates in the return trap, a float is buoyed up, which opens a valve by means of which steam is admitted directly from the
FIG. 73.— Return trap. By Kieley and Mueller.
boiler. The effect now produced is the equalization of the boiler pressure on the inlet and outlet sides of the trap, when, on account of its position, the contained water is able to gravi- tate to the boiler. As the condensation is being discharged the float begins to fall, and when a given level is reached (governed by the slack motion of the lever) the steam supply is cut off. It at the end of the discharge, the return trap is not opened to
FITTINGS FOR LOW-PKESSURE STEAM SYSTEMS 105
the air, a partial vacuum will be created through the condensa- tion of its contained steam, which will assist in the elevation of the water, along with the terminal pressure of the steam.
't/37/OB VJLdlNnd
On the other hand, if an automatic valve is provided that will admit air when the steam supply to the trap is shut off, the raising of the condensation will depend entirely upon the pressure at the receiver.
106 DESIGN OF HOT- WATER & STEAM HEATING APPARATUS
With regard to the general aspects of return traps, in which a partial vacuum is, or is not used, each method has its own merits and limitations. The chief advantage arising from the formation of a partial vacuum in a return trap, is, that for any given case the condensation can be raised through a greater height than where the steam pressure of the container is solely relied upon. Any slight leakage, however, will soon destroy a
STEAM FROM BOILER TO PUMP
vViVt ^
LINE
FIG. 75. — Receiver showing float.
vacuum, and if the elevation of the condensation depends mainly upon it, the trap may be somewhat slow and uncertain in its action. There is no difficulty, however, in bringing about a rapid condensation in the return trap, and increasing the degree of vacuum, through the introduction of a water spray, but it has the drawback of further complicating the appliance.
Pump Receivers. — In works and large places, where power boilers are used, the latter are often utilized for supplying the steam to the heating system. In low-pressure heating, this necessitates the steam pressure being reduced, with the result
FITTINGS FOR LOW PRESSURE STEAM SYSTEMS 107
that the water of condensation requires to be handled by some special appliance. As already shown, this can be done by the use of return traps, but for returning large volumes of condensa- tion, pump receivers are more suitable.
In Fig. 74 a general view of a pump receiver is shown where
FIG. 76. — Pump and receiver. By Worthington Pump Co.
the condensation is assumed to gravitate to it. For its action, the appliance depends upon a float in the receiver which operates the valve by which steam is admitted to the pump. Thus the speed of the pump is automatically adjusted to deal with the varying rates of condensation, and the latter may be either directly returned to the boiler, or delivered to any other point.
108 DESIGN OF HOT-WATER & STEAM HEATING APPARATUS
In Fig. 75 the returns are indicated as discharging into the upper part of the receiver, but where " wet " returns are used it is often more convenient to join them at its base. To prevent the receiver under the latter conditions being subject to differential pressure and so interfering with the water-line, an equalizing pipe is joined to the upper part of the receiver.
Another pump-receiver is given in Fig. 76, and although it is similar in action to the previous one it differs in details of construction. In the receiver of Fig. 76 the float employed is of the bucket type, this being attached to one end of the lever, whilst a counter- weight is secured to the other end. The float is filled with water, but upon the condensation rising in the receiver to a predetermined height, the float is rendered buoyant by virtue of the counter-weight, the steam valve opening and putting the pump into action. The removal of the water- beyond a given point in the receiver is accompanied by the opposite movement of the float, and the steam supply to the pump is curtailed.
As a rule the automatic valves used in connection with receiver pumps do not entirely cut off the steam supply, for so long as a portion of a heating system is in use, the minimum condensation at any period will be sufficient for the pump to " creep." This is also advantageous in that the pump is more suitably maintained for speeding up at any moment desired.
CHAPTER IX
EXPANSION OF PIPES
DEFECTS frequently arise in a system of piping through inade- quate provision being made for its expansion, for where free movement is prevented, sufficient stress may be caused to distort a pipe permanently, and its fracture may be only a matter of a short time. In some cases, where more or less pro- vision is made, defects appear, for unless a pipe is arranged to expand in a particular direction and from specific points, con- siderable strain may be concentrated in the wrong place.
Provision for expansion may be made by arranging the pipes that they can be sprung, by means of expansion joints and special bends, and in some cases by subjecting the pipes to a tensile strain when they are jointed.
The Springing of pipes may be resorted to at branches and at changes of direction, where the pipes are free to move, but the extent of the movement requires to be regulated by the safe permissible strain.
Example 7. — A wrought-iron branch is taken from a main pipe as in Fig. 77, this being of 3 -inch bore and firmly anchored at B, whilst the main pipe is secured at A. Let it be assumed that the pipes expand in the direction of the darts, that the distance between Ax is 60 feet at 40° F., and that the steam pressure carried is 10 Ib. per square inch (equivalent tempera- ture 240° F.). Determine (a) how much the pipe L expands when heated from 40° to 240° F. ; (I) the distance the anchor B should be placed from x in order that no damage is done through the strain set up.
The increase of length due to expansion may be found by the formula —
r = 12fctf . y; .-• , . . . (4)
110 DESIGN OF HOT- WATER & STEAM HEATING APPARATUS
where r = expansion in inches, le = length of pipe in feet, c = coefficient of linear expansion, t = rise of temperature in degrees F.
To ascertain the minimum length of the strained portion / of Fig. 77, the following formula may be used : —
1 =
OY I =
UdLct
(5) (6)
|
9 |
} |
|
|
IP |
t |
\\ |
|
i |
rian. |
|
|
i |
~\A |
|
|
I |
\ • |
* x ff |
FIG. 77.
where / = minimum length of strained or deflected pipe, L = length of piping causing deflection in pipe I, E = modulus of elasticity in Ib. per sq. inch, d = external diameter in i aches of deflected pipe, / = safe working stress of rhetal in Ib. per square inch,
k
c and t are the same as for formula 4.
TABLE III. PROPERTIES OF METALS.
|
Metal. |
Linear coefficient of expansion, c. |
Modulus of elas- ticity Ib. per sq. in. & |
Safe working BtresH in Ib. per sq. in. |
Value of K. |
|
Cast iron Wrought iron |
0-0000062 0-0000068 |
18,000,000 26,000,000 |
4,500 12,000 |
0-057 0-043 |
EXPANSION OF PIPES
111
Using formula 4 for ascertaining the expansion for the case given, we have —
r = IZleCt
Substituting values r = 12 x 60 x 0 0000068 x (240 - 40) when r = 0*98, or nearly 1 inch.
For the second part of the problem, using formula 6
I =
Assume the external diameter as 3J inches, whilst k is taken from Table III.
Substituting values, I = 0-043V/3-5 x 60 x 200
I = 0-043 x 204-9 when / = 8'8, or say 8 ft. 10 in.
Thus, for Example 7, the point of anchorage B should be about 9 feet removed from the bend at x, whilst this length
|
1 |
|
|
D 1 |
|
|
f |
} |
|
'c 1 |
Flan G T\ |
|
1 |
|
|
1 |
1 |
FIG. 78.
would be deflected about one inch through the expansion of the pipe L. Although in Fig. 77 no other fixings are indicated than at A and B, it will be understood that roller or other suitable supports would be necessary at regular distances apart.
Example 8. — Determine the distances for the points of anchorage from the branch T for a case as represented by Fig. 78, where the pipes are firmly secured at C and D. The
112 DESIGN OF HOT-WATER & STEAM HEATING APPARATUS
pipes are of cast iron, the external diameter of L being 6 inches, that in the direction of D 5 inches diameter, and that towards F 4 inches diameter. Assume the pipes convey steam at 5 Ib. pressure per square inch (equivalent temperature 228° F.), whilst the length L at 48° F. is 30 feet.
For this example the minimum length I between D and T is determined by the aid of formula 6, where
I = Ic^/dLt. The value of k in Table III. is given as 0'057.
Substituting values, / = 0-057\/5 X 30 x (228 - 48)
I = 0-057 X 164 when I = 9'35, or say 9 ft. 4 in.
If now a fixing is used at F which prevents lateral, but permits of longitudinal, movement, the distance from T to F may be obtained as above. Or, as the length will vary directly as the square root of the diameter when the remaining conditions remain unaltered, it may be found by proportion, when
9-35 X y/4
/! = 8-36, or say 8 ft. 4 in.
Thus, for the arrangement in Fig. 78 and the conditions given, the anchorage D should be approximately 9 feet 4 inches from T, whilst the support F in the line should be about 8 feet 4 inches from the branch. The fixing at G, if permitting of lateral motion, may be located close to the branch, but if it only allows movement lengthwise its correct position may be ascer- tained by formula 6.
Expansion Joints. — In straight lengths of pipes where ample provision for expansion cannot be made by bends and the straining of pipes, expansion joints are generally used. For these appliances to work satisfactorily, they should be securely fixed, whilst the pipes themselves must be arranged to move in the direction of the expansion joints. This is readily done by anchoring the pipes at given points, and by providing roller or other movable fixings.
EXPANSION OF PIPES
113
There are two principal .types of expansion joints. The first is provided with a sleeve-piece, which slides through a "stuffing" box, whilst the second depends upon a flexible diaphragm or disc, the maximum strain upon which should be kept well within the safe elastic limit of the metal.
In Fig. 79 is shown a common form of sliding joint for
FIG. 79. — Expansion joint.
pipes of small bore, whilst a joint for larger pipes is given in Fig. 80. To the latter, lugs are attached in order that it may be readily secured in position.
The chief weakness of sliding joints is their tendency to
FIG. 80. — Webster expansion joint.
leak, either through the failure of the packing material or through the straining of the movable ends when the alignment is not perfect. To preserve a true alignment between the expansion joints and piping, different practices are adopted, such as the fixing of permanent guides to the joints, or by using ball and socket joints.
Their principal merit is in the length of movement they allow, but when fixing, care should be observed that the sleeves are well withdrawn before the pipes are attached.
I
114 DESIGN OF HOT- WATER & STEAM HEATING APPARATUS
Fig. 81 gives a disc or diaphragm expansion joint of the double type, expansion being provided for on both sides of the fitting. In construction, the body part consists of an inner and two outer cast-iron rings, whilst between these rings the outer edges of two copper diaphragms are clamped. The inner edges
OUTER RING
FIG. 81. — Diaphragm expansion joint.
of the diaphragms are passed through and spun over cast-iron " backing " rings, which limit the movement of the diaphragms and form a suitable attachment for the " slip " ends of the joints. In the figure, it will be observed that the outer and inner cast-iron rings are arranged to form rebates, in which the movement of the " backing " rings is confined.
Another form of diaphragm joint is shown in Fig. 82. In this, the movement is effected on each side of the appliance, One edge of a corrugated copper disc being connected with the end where the pipe is joined, whilst the other edge is secured between the large cast-iron flanges.
The diaphragm class of expansion joint has no stuffing box, and is therefore very suitable for positions difficult of access. For this reason, they have been largely used on underground pipes for conveying both water and steam.
Generally speaking, the double diaphragm joint provides expansion for 100 feet of pipe, whilst large sliding joints will
EXPANSION OF PIPES
115
take up the expansion from at least 300 feet of pipe. Taking the unit of length as 100 feet and the unit temperature range as 100° F., the expansion of wrought- and cast-iron pipes will be 0'82 inch and 0'75 inch respectively. Thus, if the maximum
PIG. 82. — Diaphragm expansion joint or ".variator."
temperature range in a system of wrought steam piping is 200° F., the expansion to be provided for each 100 feet should be not less than 0'82 X 2 = T64 in.
Expansion Bends. — Where space permits, expansion bends
PIG. 83.— Expansion bend.
FIG. 84. — Expansion bend.
are frequently used, two common forms being shown in Figs. 83 and 84. For these, copper instead of iron tubing should be adopted, as the latter is more liable to failure through the rigidity of the material, especially when the ends are screwed.
116 DESIGN OF HOT- WATER & STEAM HEATING APPARATUS
For either copper or iron expansion bends, flange joints are preferable, as, when ordinary screwed joints are used, any weakness introduced by the threads permits the whole strain to be concentrated at such points. When the ends of wrought- iron pipe are screwed into flanges such joints may be strengthened by welding. A few years ago the welding of such joints was not such a practicable thing as at the present time, but with the advent of the oxy-acetylene welding apparatus it can be simply and economically carried out. Moreover, when flanges are welded to pipes no previous threading need be done, the ends being slipped simply through the flanges and the blowpipe applied.
Should expansion bends be required on vertical pipes they may take the form of a helix.
The following table gives the principal dimensions of the expansion bends shown in Tigs. 83 and 84.
TABLE IV. DIMENSIONS OF EXPANSION BENDS.
|
Dimensions. |
||||
|
Internal diameter |
||||
|
in ins. |
A. |
B. |
||
|
ft. |
ins. |
ft. |
ina. |
|
|
2 |
1 |
6 |
2 |
6 |
|
2* |
2 |
0 |
3 |
0 |
|
3 |
2 |
4 |
3 |
9 |
|
4 |
2 |
9 |
4 |
9 |
|
5 |
3 |
0 |
5 |
3 |
|
6 |
3 |
6 |
5 |
6 |
|
7 |
3 |
9 |
6 |
0 |
|
8 |
4 |
0 |
6 |
6 |
|
9 |
4 |
6 |
7 |
0 |
Application of Tensile Strain during Jointing-. — This method consists of fixing pipes rather short, so that when two ends are drawn together they are subjected to a tensile strain. Upon the application of heat, however, expansion begins, and the tensile strain diminishes, falling to " zero " when a certain temperature is reached. If, however, the temperature continues to rise, the pipes in turn are subjected to a compressive strain, provided they are unable to move in a
EXPANSION OF PIPES 117
longitudinal direction. Fig. 85 will help to illustrate what is meant.
Example 9. — If a wrought-iron pipe 40 feet long is rigidly held at C and D (Fig. 85), determine the temperature range through which ic may be heated without overstraining it. Also
FIG. 85.
give the distance the flanges should be apart prior to the pipe being jointed.
If the temperature range is calculated from the neutral point or that of no strain, it may be obtained by the following formula : —
where tn = temperature from neutral point,
/ = maximum stress allowed per square inch, c = coefficient of linear expansion, E = modulus of elasticity in Ib. per sq. in.
For the values of/, c, and E see Table "III., which, for the case under consideration, will be 12,000, 0'0000068, and 26,000,000 respectively.
By formula 7 —
12,000 and substituting values, tn =
0-0000068 x 26,000,000 when tn = 68° F.
The distance between the flanges should be equal to the ex- pansion from the neutral point, and is obtained by formula 4, where
r = 12 fat.
118 DESIGN OF HOT- WATER & STEAM HEATING APPARATUS
The value of t here will be 68 as obtained above. Substi- tuting values
r = 12 x 40 x 0 0000068 x 68, when r = 0'22, or, say, -~ inch.
For the example given, the maximum temperature through which the pipe could be heated without exceeding the safe stress used would be 68 x 2 = 136° F.
If the flanges were closer or further apart prior to jointing, and the pipes were heated through the same range of tempera- ture, the strain would exceed 12,000 Ib. per square inch.
Tensile strain, it will be seen, is limited in application, but there are cases where it can be advantageously adopted, and especially when the pipes can be sprung a little from a bend at the same time.
CHAPTER X
ATMOSPHERIC SYSTEMS OF STEAM HEATING
THE term "atmospheric" is used in connection with installa- tions where the pressure falls to that of the atmosphere upon steam entering the heating surfaces. In other words, atmo- spheric systems are those in which the radiators or other heating surfaces and the return mains are open to the air.
When compared with ordinary low-pressure apparatus, atmospheric systems of steam heating have much in their favour, for they eliminate most of the drawbacks of the former whilst their installation is comparatively cheap. In the earlier atmospheric systems the boiler pressure carried was usually about 2 Ib. per square inch, whilst with more modern appa- ratus it is limited to a few ounces per square inch. More- over, much progress has been made in the design of the valves for regulating the steam supply, whilst the failings of the earlier forms have been overcome to a great extent.
A simple " atmospheric " system is given in Fig. 86, the boiler fittings being omitted. The water of condensation is drained to the receiver E, from the top of which an air- pipe is taken and joined with the boiler chimney, the draught of which is utilized to free the pipes from air. The receiver E is graduated in ounces per square inch, to indicate the boiler pressure, the water column being observed by a gauge glass.
When pressure is generated in the boiler the water from the latter is dislodged to the receiver, until the rising water column exerts the same pressure as the steam. To prevent the boiler plates being burned by too great a displacement of water, a steam relief is provided which comes into operation when a given pressure is exceeded.
It is the usual practice in these systems to join the steam
120 DESIGN OF HOT- WATER & STEAM HEATING APPARATUS
Water
jWater Supply.
FIG. 86. — Atmospheric system.
tn
ILu
ll
I
FIG. 87. — Atmospheric system.
ATMOSPHERIC SYSTEMS OF STEAM HEATING 121
supply to the highest parts of the heating surfaces, for by so doing, the air is the more effectively removed. No air valves are used, these being unnecessary with open returns.
The piping of " atmospheric " installations is much simpler than that of ordinary systems, and they are not subject to water hammer and similar sounds if carefully installed. For dividing into any given number of units, these systems are specially
Relay.
FIG. 88.— Atmospheric system.
advantageous, as the condensation from any radiator may be discharged into the most convenient return.
A piping system for large buildings is shown in Fig. 87. In this case, separate mains are used for each of the different floors, whilst any unit may be used independently of the re- maining ones. The ends of the steam mains are trapped before being joined with the returns, the depth of these being adequate to resist the greatest steam pressure to be carried.
Fig. 88 gives another method of piping for large buildings. Here the steam mains are run from the " header " along a wall
122 DESIGN OF HOT- WATER & STEAM HEATING APPARATUS
or on a basement ceiling, separate risers and mains being used to serve the radiators on the different floors. Where mains are prolonged, " relays " are often necessary, either through some obstruction intervening in the line of piping or through the mains having fallen to too low a level. These, however, pre- sent no difficulty so long as trapped connections are made between the lowest points of the steam mains and the returns. In Fig. 88 the steam risers and returns are connected with
FIG. 89. — Atmospheric system where steam is supplied from an external source.
one side of the radiators, and this arrangement answers satis- factorily for radiators of a moderate size. For large heating surfaces where the connections are desired as shown, a distri- buting pipe may be used inside them.
Fig. 89 gives a general arrangement of the piping and fittings used, where the steam supply is obtained from a district main. The pressure of the steam in the service pipe S may be anything between 1 Ib. and 5 Ib. per square inch, this depend- ing upon the distance from the district heating station. After entering a building the steam pressure is reduced to the desired
ATMOSPHERIC SYSTEMS OF STEAM HEATING 123
degree by the pressure-reducing valve shown. A cooling and condensing radiator is located at C, which serves to condense any steam that may reach that point, and to deprive the conden- sation of a large percentage of its heat before it is passed to waste. In general, no attempt is made in district heating to return the condensation, for unless the conditions are specially favourable, it is found much cheaper to waste it. From the cooling radiator the water of condensation passes through a meter, M, which records the weight of steam condensed, whilst the drip pipe from the steam main may join the cooling radiator in the manner shown.
Regulation of Atmospheric Systems. — From an economical standpoint, the success of these systems chiefly depends on the use of suitable appliances to prevent the waste of steam.
For radiator and other heating surfaces there are two modes of regulation. The first depends upon the use of " fractional " radiator valves, which are designed to admit varying volumes of steam according to the condition of the weather, whilst plates and pointers are frequently attached to indicate the extent to which they are opened. As one size of valve is used to serve varying amounts of heating surface, the valve or the piping should be arranged that only a limited volume of steam can enter a radiator, even when the valve is widely opened. In most cases, the steam supply is restricted, so that for a given external temperature, the heating surfaces cannot be filled to within 10 to 20 per cent, of their full capacity, the valves being partially closed as the weather gets milder, or as the con- densing capacity of the surfaces diminishes.
The prevention of waste by this form of regulation depends upon the radiator valve being properly manipulated by the occu- pants of a room, or by the use of thermostatic contrivances, any failure in either respect allowing steam to flow directly into the returns. On the other hand, if due attention is paid to the temperature of a room, or the thermostatic appliances remain in order, not only is waste avoided, but the temperature of the con- densation as it leaves the radiators is very appreciably reduced.
In the other mode of regulation, both the inlets and outlets of heating surfaces are governed, fractional valves, as before, being used for the steam supply, whilst some thermostatic or
] 24 DESIGN OF HOT-WATER & STEAM HEATING APPARATUS
fixed device is connected with the outlets. The principal advantage gained in this case is the reduced waste of steam.
A fractional radiator valve is shown in Fig. 90. The design of fractional valves should be such that the wire-drawing effect of the entering steam will not cut the valves and seatings, whilst the wear on the restricting orifices should be as nearly uniform as possible.
Fig. 91 gives a fitting for the outlet regulation of radiators.
FIG. 90.— Jenkins " fractional valve.
FIG. 91. -Webster Sylphon ther- motor.
As already indicated steam should not appear at the outlets of atmospheric systems when the regulation is correct, although this is likely to occur, to a more or less extent, in all systems dependent upon hand control. The opening and closing of the valve are effected by the contraction and expansion of the corrugated vessel, which is rendered the more sensitive by the volatile fluid it contains. So long as steam is absent the valve orifice is open, and providing for the escape of both condensation
ATMOSPHERIC SYSTEMS OF STEAM HEATING 125
and air. If, however, steam comes in contact with the metal vessel the valve is closed.
The boiler draught of these systems also requires to be under accurate control in order that the pressures may be adjusted to meet the varying demands for steam. In Fig. 92 one method of doing this is shown, although special forms of
FIG. 92. — Boiler of atmospheric system with automatic draught control.
diaphragms or other means are used. To a branch from the steam main S, the regulator and relief tube-are fixed, the depth of the water seal being proportional to the blow-off pressure desired. In the upper tube B, a float is placed which is con- nected with the dampers of the boiler, whilst the level of the water in the float tube is influenced by the pressure of the steam on account of the cross connection P. As the actual steam pressure is represented by the vertical distance between the water-line of boiler, and that of the receiver R, it is im- perative when fixing the latter that the zero point corresponds with the boiler water-line. The relief of steam when the pressure is excessive is effected through the smaller pipe that is joined at T as soon as the water-level is depressed to that level.
When steam is taken from an external source, as in Fig. 89,
J26 DESIGN OF HOT- WATER & STEAM HEATING APPARATUS
the water of condensation is sometimes discharged to a pipe receiver instead of to a condensing radiator as shown. Some central heating companies, however, require condensing radiators to be installed, for where meters are used there is less likelihood of steam passing away unrecorded when it has once entered a system. Fig. 93 gives a pipe receiver discharging into a con- densation meter.
The "Broomell" System of vapour heating is an atmospheric
Ret*
AirPipe.
i.
Drip.
Cupper Flo
FIG. 93.— Pipe receiver and meter.
FIG. 94. — Broomell's receiver.
one, the apparatus being installed on the lines laid down, although some of the fittings differ in construction from those already shown. For this apparatus the receiver is shown in Figs. 94 and 95, but it performs two other functions as well, viz. it regulates the boiler draught by the float arrangement
ATMOSPHERIC SYSTEMS OF STEAM HEATING 127
shown, and it affords relief when the pressure of the steam rises too high.
It will be observed in Fig. 95 that the return pipes enter a water seal, the air pipe from this going to a coudensiug coil which is located above the receiver. From the coil the air pipe is joined with the nearest flue. In the same figure the
-TO DRAFT REGULATION
TO STEAM SPACE OF BOILER
RETURN OUTLET
FIG. 95. — Broomell's receiver.
steam release valve is shown, which is opened by the raising of the adjustable rod when the float is buoyed up to it.
Fig. 96 gives the radiator valve that is used in the "BroomeU" apparatus, this being made in six different sizes for attaching to J-inch supply pipes. From the illustration, it will be seen that the seating of the valve contains four ports, all of which ar,e uncovered when the valve is opened wide. As less steam is required, one or more of the ports will be closed, the
128 DESIGN OF HOT- WATER & STEAM HEATING APPARATUS
degree of regulation being indicated by the relative position of the lever handle.
The special outlet fitting that is used is shown in Fig. 97. This appliance is attached to the outlet of each radiator or other
FIG. 96. — Broomell's radiator valve.
FIG. 97.— Broomell's radiator outlet connection.
surface, the small trap serving to break the full bore passage so far as the steam is concerned between the heating surfaces and the return pipes. At the same time the condensation can readily escape, whilst air or a little steam passes through the aperture in the lip.
CHAPTER XI
EXHAUST STEAM HEATING
ALTHOUGH "exhaust" heating is frequently combined with vacuum systems, this and the following chapter are confined to the utilization and value of exhaust steam, in ordinary " low pressure " and in " atmospheric " systems.
In works and other buildings where steam power is required, considerable economy may be often effected by combining the heating installation with the power plant employed. Before " exhaust " steam, however, can be delivered from the engines into a system of piping, some back pressure must be necessarily put upon them, and this has the effect of reducing their efficiency, owing to the mean effective pressure on the pistons being lowered. With a well-designed and suitable size of heating plant, the loss of efficiency of non-condensing engines will be very small, and may not realize 5 per cent, where 100 per cent, is taken as the basis when operating under normal conditions.
Generally speaking, it is economical to place back pressure on an engine for heating buildings when the heat required exceeds that available from the extra steam consumed by the engine in virtue of the added back pressure. To exhaust into a heating system, less than 2 Ib. per square inch of pressure can be used (depending upon the general design), but whilst a case may be simple to solve where non-condensing engines are in use, the problem becomes more involved if engines of the condensing type are installed. An example at this point will aid in the explanation of the above so far as non-condensing engines are concerned.
Example 10. — At a given speed a non-condensing engine develops 80 horse-power, and uses 30 Ib. of steam per horse-
K
f
130 DESIGN OF HOT- WATER & STEAM HEATING APPARATUS
power-hour when exhausting to the external air. If it is found that the steam consumption is increased by 10 per cent, when exhausting into a heating system, determine the smallest effective area of heating surface that will justify the increased consumption, also the advantage gained when the whole of the steam can be condensed in the heating plant. Let it be assumed that each square foot of heating surface transmits 250 B.Th.U. per hour, that each pound of exhaust steam yields 850 B.Th.U., the same weight of high-pressure steam 990 B.Th.U., and that 8 Ib. of water are evaporated per pound of fuel consumed.
In the first place the extra steam consumed by the engine
..,,,,. . 80 x 30 X 10
when exhausting into the heating system is -
= 240 Ib. per hour. This weight of high-pressure steam will
240 X 990 _
serve — .. — = 950 square feet of heating surface, lor a
heating area less than the above, it would be more economical to blow the exhaust to waste, and to take the steam for heating directly from the high-pressure boiler. The smallest effective heating surface should therefore exceed 950 square feet.
In the second place, the coal to be charged against the heating account when exhaust steam is used, is that required to generate the extra steam owing to the back pressure that is carried. In this case the additional fuel consumed for a period
v 9d.n of 1000 hours will be * = 30,000 Ib. or 13'35 tons.
o
Before the weight of the fuel for live steam heating can be found, the area of the heating surface that will condense the whole of the "exhaust" must be ascertained. The latter is done by multiplying the total steam consumpt by 850, and dividing the result by 250. The total steam consumpt will be (80 x 30) + 10 per x;ent. = 2640 Ib. per hour, whilst
as exhaust steam it will serve - OFTT = 8976 square feet
of heating surface.
Supposing now that the latter amount of surface were supplied by steam from a heating boiler at about 2 Ib. per square inch gauge pressure (latent heat value 966), the fuel
EXHAUST STEAM HEATING 131
, , , 8976 x 250 x 1000 cousumpt in 1000 hours would be -7^7; —
966 x 8
= 290,372 Ib. or 129'63 tons; that is, when taking the rates of evaporation of the two different boilers as equal. A saving is thus shown in favour of exhaust heating for the conditions given of 129*63 — 13'35 = 115'28 tons of fuel, which represents a substantial cash value.
In general, it is not often that the " exhaust " from large steam engines can be wholly utilized for warming purposes, and although different conditions will give different results, the example taken shows where the efficiency begins, and to what extent it may be carried.
Heat in Exhaust Steam. — During recent times, controversy has been rife upon the relative merits of "exhaust" and of "live" steam for heating work. Cases have been cited in which it is claimed that greater economy in the consumption of fuel has been obtained by running a power plant for the sake of the exhaust steam, rather than taking high-pressure steam direct from the boilers. Now, so far as