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Heating Systems

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HEATING SYSTEMS The Fireplace or Grate, of which various modifications and improvements have been made from time to time, consists essen tially of a basket which holds the fire (or andirons if logs are the fuel), an opening backed by brick which becomes heated by the fire and radiates heat to the room, and a chimney. The grate is an inefficient form of heating because most of the heat imparted to the air which is supplied to the fire passes up the chimney and is lost. In the best designs of open grates only about 2o% of the heat in the fuel is actually delivered to the room. Because of this, and because of the impossibility of comfortably heating a building in a severe climate with grates, and also because of the dirt and labour involved, the grate is not used as a primary means of heating in North America. In Great Britain and else where, where the climate is relatively mild, the fireplace is still in general use; the psychological effect of the open fire has also caused it to be retained even where more complete means of heat ing are provided.

The Stove.

The stove is an improvement over the grate from the standpoint of economy. The modern baseburner stove makes use of from 70 to 8o% of the heat in the fuel, but it delivers its heat almost entirely by radiation and is, therefore, not a particularly comfortable method of heating. The stove is being rapidly discarded in America, where formerly it was widely used, because of the attention and space it needs, its unsightly appearance and the fact that a separate stove is re quired in every room for satisfactory results.

The Warm Air Furnace.

The warm air furnace is the natural outgrowth of the stove. The furnace consists of a firepot and an extended flue, the whole being surrounded by a sheet metal casing (fig. I). Air passes through the casing, absorbing heat from the hot surfaces of the firepot and flue, and flows through pipes to the various rooms. In the simplest type, the so-called pipeless furnace, the heated air is delivered only to the room directly over the furnace and passes into the other rooms through open doorways by natural circulation. For any but the smallest houses, a furnace having separate pipes to the individual rooms is necessary for good results. The air supply to the furnace may be taken from outside, but in order to save fuel is usually partially or wholly recirculated from the rooms through a system of return ducts.

Heating Systems

The warm air furnace, properly installed, is a fairly satisfactory method of heating small homes and is the standard method for such purposes in many parts of America. It is economical in fuel consumption and humidification can be obtained by means of a pan of water placed above the dome of the furnace in the path of the warm air.

While many buildings are heated successfully by furnace sys tems without fans, very much more positive results are obtained when thermal circulation of the air is ignored, dependence being placed on electric driven centrifugal fans which discharge the recirculated air (or a mixture with fresh air) around the furnace and thence through trunk-line ducts to the various rooms. In such systems the fans usually operate continuously; the air tem perature being varied by regulating the rate of combustion. The trunk-line air ducts need not pitch upward away from the furnace and the fan pressure around the furnace ensures that gases can not enter the air stream through cracks. Adequate filtering out of air-borne dust is practicable with such systems. The furnaces practicably can be fired with gas, or with mechanically intro duced oil or coal ; the latter process almost universally being via a forced draught stoker.

The Steam Heating System.

This type consists of a boiler in which steam is generated, a number of so-called radiators located in the various rooms, and a system of pipes to convey the steam to the radiators and to carry the water of condensation back to the boiler. The heat which has been imparted to the water in the boiler to form the steam is given up as the steam condenses in the radiators and is transmitted to the air of the room. There are several forms of steam heating systems, differ ing principally in the arrangement of the piping. The simplest is the single-pipe system shown in fig. 2. There is a single hori zontal main pipe which leaves the boiler and circles the basement, branches leading to the individual radiators. In large buildings this main is often placed overhead, in the attic. Only one con nection is made to the supply-valve at the bottom of the radiator and the water of condensation must return against the steam to the horizontal main and thence back to the boiler. The air which is originally in the system is forced out by the steam through air valves which close automatically when the steam reaches them; they also prevent the escape of water. The steam pressure must exceed atmospheric pressure. Improved air-valves are available which prevent re-entry of air, so that after the air is expelled the system may operate at sub-atmospheric pressure.

The single-pipe system is a simple, fairly satisfactory system for such buildings as factories and warehouses and is also used in many houses. Its chief disadvantages are the noise, odour and drip which often accompany the action of the air-valves. It is necessary, to prevent water hammer, that the pipes shall be per fectly pitched for drainage. Radiator valves must be wide open or tightly closed.

The Two-pipe System

(fig. 3) , which is somewhat more costly to install, overcomes these drawbacks. There is a separate system of piping for the air and condensation, and the air is discharged at a single point in the basement. The supply-valve is placed at the top of one end of the radiator and the return connection is made at the bottom of the opposite end through a thermostatic trap which permits the air and condensation, but not the steam, to pass into the return pipe. This system when used in a small building will circulate with only a few ounces of pressure at the boiler and is, for this reason, often called a vapour system. It operates practically without noise and is more economical in fuel than the single-pipe system. It is possible to regulate the heat by partial closing of the radiator valve.

The Vacuum System

is used in large buildings. It is a two pipe system with a vacuum pump attached to the return piping. A partial vacuum is maintained, equivalent usually to about i oin. of mercury, and the circulation is greatly improved thereby. Radiators and piping located slightly below the boiler can be operated successfully, which would not be otherwise possible. The vacuum pump returns the water of condensation directly to the boiler or to a tank from which it is pumped to the boiler. The vacuum steam system is employed extensively for heating blocks of offices, large retail stores, hotels and similar buildings in America.

The piping for a steam-heating system must be carefully de signed to secure satisfactory op eration. The pipe sizes must be so chosen that the required quan tities of steam will flow at a low velocity to avoid noise and per mit proper drainage of the con densation. Accumulations of water at low points will cause the annoying tapping noise called "water hammer." Provision must also be made for the free expan sion of the pipes due to changes in temperature. In the vertical risers of tall buildings it is neces sary to provide loops which absorb the expansion by their flex ibility, or slip joints consisting of one sleeve which slides within another.

The Hot Water Heating System.

In this system heat is conveyed to the various rooms by hot water which vows through the radiators and recirculates to the heater. In the simple gravity type the circulation is produced by the difference in density of the hot water in the pipes supplying the radiators and that of the returning cooler water.

There are two different piping arrangements in common use.

In the two-pipe system the supply and return pipes are separate as shown in fig. 4. In the single-pipe system the supply for each radiator is taken from the top of the single main pipe and the return connection is made in the side or bottom at a point far ther along the pipe. If there are many radiators, the size of those farthest along the main pipe must be increased because they receive cooler water than those near the heater. An expansion tank, usually placed at the top of the sys tem, is required with hot water systems to accommodate the changes in the volume of the water with changing temperatures. Most hot water systems operate at a maxi mum temperature on leaving the heater of about 18o° F. A higher temperature is practicable if the water is subjected to pressure to prevent it from boiling. The fundamental characteristic of a gravity hot water system is that the force producing circulation, due only to the slight differ ence in densities of the water in the differ ent parts of the system, is small. The suc cessful design of a gravity hot water sys tem therefore requires skilful selection of the pipe sizes and the proper arrangement of the pipes so that the required quantity of water will flow to each radiator. The selection of the proper pipe sizes is thus of considerable importance and the design re quires more care than does that for a steam-heating system.

A hot water system gives a less fluctuating output of heat to the rooms than a steam system because of the thermal capacity of the circulating water. Mechanically circulating hot water heat ing systems respond as quickly as steam systems to changes in heat demand. Mechanically induced circulation of the water, very widely employed throughout the world, permits use of piping even smaller than that for a vacuum steam system. The energy required for the water-circulating pump is less than that for a vacuum pump to accomplish the same heat transfer, and the design of such a plant is much easier than that for gravity circulation.

The radiator temperature, un like that with steam, can be varied through a wide range and as a result room conditions are improved. Overheating with at tendant waste of fuel is much reduced, and there is much less corrosion of the pipes when kept full of water than when carry ing a mixture of air and water or vapour. When tall buildings are heated with hot water the excessive pressure due to high head can be avoided by trans ferring the heat from the water of a lower zone to that in an up per one. The cost of a forced circulation hot water system, par ticularly if the water is prevented from boiling, does not ex ceed that for a steam system. In many buildings the same pipes and convectors which carry hot water in winter, carry chilled water for cooling and dehumidifying in summer.

Heat Transmitters

from the circulating medium to the air are called radiators or convectors, though all of these devices tually transmit heat by both processes. They are made of steel, copper, brass, aluminium or of cast iron and may be combinations of all of these. The trend is away from exposed pipe coils or sectional castings toward comparatively small tubes upon which sheet metal fins are pressed very tightly. In some cases the tubes and fins, after having been sembled, are tinned so as to simulate a homogeneous ture. This is not only to resist corrosion but also because the rate of heat transfer between the tube and fin is dependent on the intimacy of the contact. There are many satisfactory heating installations in which enclosed convectors induce preciable air currents. A heat transmitter in an air supply duct leading to a room would be a vector and little if any radiant heat could be expected. Any heat transmitter within the room self having merely a steel ing between it and the room air heats by radiation as well as by convection. In Europe for many years rooms have been heated by pipes imbedded in the floors, walls or ceilings. While convection (heat transfer to moving air) plays a minor part in such a method of heat transmission, the principal heat transfer is by radiation. It has been proved that a person receiving radiant heat will be comfortable at an air perature five degrees or more cooler than that in effect when the heat is received via air currents. It has been demonstrated also that comfort will be improved and economy of fuel will be achieved to a greater extent from a large heat transmitting area at moderate temperature than from a comparatively small, wry hot heat transmitting surface such as that of a conventional steam radiator. In many American installations of panel heating hot water pipes imbedded in the plaster of the ceiling have proved successful in sub zero weather with water temperature not exceed ing 13o degrees. The water pipes usually have welded joints and are tested to high pressure before being covered with the plaster. Living rooms which can be heated comfortably without any regis ters or radiators appeal strongly to most housewives.

Fig. 5 shows the common form of cast iron radiator, built in sections of various sizes and assembled as required ; the sections are connected at both top and bottom. Radiators with connec tions between the sections at the bottom only are obsolete.

Fig. 6 shows a common form of convector-radiator designed for use within a room, usually below a window. Control of the output may be by a valve on the steam or water supply connection or by a damper. The vertical distance from the comparatively thin heating element to the warm air outlet affects profoundly the heating capacity. It is wise in any building to have all heat transmitters of the same general type. For instance, given radiators like fig. 5 in one section and those like fig. 6 in another, the heat lag due to greater storage in fig. 5 will cause complaints of improper temperature control.

Unit heaters, shown in fig. 7, usually having copper finned con vectors with electric fans, are especially effective for heating large manufacturing or storage rooms. The mechanically distributed warm air can be deflected toward the floor or spread in any direc tion desired. The heaters are available with either centrifugal or propeller fans, the operation of which can be cheaply controlled by electric thermostats.

Furnaces

for warm air heating may be of steel, provided that the combustion chamber is lined with refractory and protective material, but usually are built of cast iron sections, these often having flanges projecting into the air stream. The most economical warm air furnaces are those which have secondary surfaces above the combustion chambers to intercept the radiant heat and transfer it by convection to the air. Oil, gas, wood and coal can be burned with approximately equal efficiency in the ordinary cast iron furnace, but a special design in which the combustion chamber size is reduced, with a relative increase in the areas which cool the products of combustion, is desirable for gas alone. The use of electric driven forced draught coal stokers with warm air furnaces, even of small size, is practicable and re markable economies in fuel and in labour are accomplished. It is entirely practicable to have the stoker convey the coal from the bottom of a sloping fuel room all the way into the furnace, and to operate automatically in response to a thermostat in the space which is being heated. The ashes also are removed mechanically and deposited in a dust tight receptacle. Thermostatic devices with any of the fuels will prevent overheating, will maintain ig nition of the fuel, and will stop the feeding of fuel and sound an alarm in case of trouble.

Boilers

for steam or hot water heating are essentially alike, and may be of sectional cast iron or of welded or riveted steel. They are efficient largely in proportion to the thoroughness with which the water circulates within them around the heated sur faces, though for all fuels but gas the combustion chamber size is an important factor. Boilers which burn gas at the highest efficiency, like gas burning warm air furnaces, have reduced com bustion chamber volume in proportion to the secondary heat ab sorbing areas and therefore are not usually satisfactory for other fuels. Cast iron boilers comprising horizontal sections, pancake fashion, are satisfactory in the smaller sizes but when the sections are assembled vertically the upper connections should be rela tively large and so located that when the boiler is used for steam not only steam but also water can circulate freely from section to section. Otherwise each section will have a different water level when heated intensely with water circulation limited to each section, and with resultant low efficiency.

In any steam boiler ease of cleaning both sides of the heat transmitting surfaces is important, and for this reason the trend generally is toward welded steel construction, with straight fire tubes accessible for easy cleaning. Steel is far less likely to crack than is cast iron when either metal is subjected to rapid heating and cooling, and steel used in a boiler usually is con siderably thinner than the cast iron of a commensurate boiler would be. In most cases any steam boiler can be used for heating water: the essential difference being that with a water boiler no try cocks or water level glass are required and the pressure relief or safety valve may be much smaller.

Fuels.

The same fuels and mechanical combustion arrange ments can be made for boilers as for warm air furnaces though an additional safety control is employed, which will cause stoppage of the combustion in case of too high steam pressure, too high wa ter temperature, or too low water level in the boiler.

The efficiency of boilers and furnaces burning solid fuel under manual firing is probably not better than 5o%. An increase of 1 o% to i 5 % can be expected when an electric stoker is em ployed. Specially designed gas and oil burning boilers often achieve 75% average efficiency.

Chimneys.

Chimneys should extend to a level high enough above roofs or other obstructions to prevent back draughts fol lowing reaction-eddies in high winds. Chimneys in building in teriors rather than in outside walls always are preferred, since they will lose less heat and therefore will have adequate draught with lower temperature than if chilled by cold winds throughout their height. Brick chimneys frequently are built with poorly filled joints and are leaky and inefficient when in outside walls and dangerous fire risks when in the inside. Chimneys which serve gas burning boilers require minimum draught but may be drenched with water which condenses out of the cooling products of com bustion. When the wet brick and moisture freeze they disintegrate rapidly. All chimneys, therefore, which serve gas burning appli ances should have linings which are moisture-resistant as well as corrosion-resistant. Stainless steel, or heavily coated cast iron, or salt glazed vitrified tile, or moulded asbestos-portland cement such as Transite, have been used for lining chimneys from gas burning devices. With either gas or oil there is no bed of incan descent fuel, as with coal, and when the demand for heat is satisfied the combustion rate drops to that for a tiny pilot flame. If the chimney draught persists during the time there is no fire, the combustion chamber will be unnecessarily chilled by the large volume of air drawn through to satisfy the chimney draught. Therefore the more efficient automatic oil or gas and even coal burning appliances have devices for stopping the flow of air while no fuel is being burned, with ingenious timed anticipations and delays to suit each characteristic. Very interesting economies lave been accomplished not only by mechanically delivered air supply for combustion, but also by curtailing and controlling excessive chimney draught.

Humidification.

The atmosphere in a building is being more or less frequently renewed through leakage, and the outside air, though it may contain nearly the maximum possible amount of moisture at its original temperature, has its capacity for holding moisture greatly increased as it reaches the indoor temperature and therefore has a drying effect upon the skin and the mem branes of the respiratory passages as well as upon furniture, woodwork, etc. Health authorities advocate artificial humidifica tion to alleviate this condition and various devices have been contrived for this purpose. For hot water and steam systems they usually consist of a wick or pan attached to the radiator. In the warm air furnace system humidification can be quite easily ac complished by means of an evaporating pan located in the air passage of the furnace. That humidifying devices are not always effective is usually due to the lack of appreciation of the quantity of water that must be evaporated. For example, with an outside temperature of 2o°, about i gal. must be evaporated every 24 hrs. for each i,000 cu.ft. of room volume to bring the relative hu midity to 40%, which is about the proper point. In England and elsewhere, where the outside temperatures are not low and the climate is moist, humidification is rarely needed. Although the temperature required for comfort is somewhat lower when the air is humidified there is no appreciable saving in the amount of fuel required because of the heat which must be used to evaporate the water for humidifying.

Electric Heating.

Electricity is in many respects an ex cellent heating medium but its use is limited because of its cost.

When electricity is generated by means of the combustion of coal the process involves a loss of 75% or more of the heat in the fuel; and further losses in transmission and the cost of the transmission system make the cost of heating excessive as com pared with other methods. It is therefore used only for small rooms and as an auxiliary to other means of heating. Where the electricity is generated by water power, however, the cost is of ten very much less and in certain cases may compare favourably with that of coal, if the latter is high priced in that locality.

Automatic Temperature Control.

This is essentially a means for preventing overheating. A heat-sensitive element called a thermostat may be placed in some representative location in a small building, to control the opening or closing of the dampers of a manually fired furnace, or to open or close the power cir cuit to an electric motor from an oil burner or stoker or to actu ate the control valve of a gas burner. In larger buildings it is desirable to have a separate thermostat for every room, and this may operate a damper in an air duct or a valve on the steam or hot water heat transmitter. Transmission from the thermostat to the heat supply control may be by electricity or by compressed air or by combinations of both, and the dampers and valves may have small electric motors or air-actuated bellows. In most cases the arrangements are such that should the control system fail, heat will be supplied.

Central Heating.

In Great Britain, and in Europe generally, the term central heating usually refers to the heating of a build ing by means of one heating unit instead of fireplaces or stoves in every room. As understood in North America, however, it means the supplying of heat to a number of separate buildings from a central plant. When portions of a city are thus heated the term district heating is often synonymously used. The first central-heating system was installed at Lockport, N.Y., in 1877 by Birdsill Holly. Other systems were built from time to time in various cities and there are at present extensive systems in New York city, Detroit, Pittsburgh,, Rochester and St. Louis and in a number of smaller cities and towns in America. The areas cov ered are sometimes 1 or 2 sq.mi. or more in extent and include business districts and high class residential districts. Also many groups of factory and institutional buildings are heated from central plants. Either steam or hot water may be used as the medium for conveying the heat from the central plant. Control of the heat supply from the central plant by adjusting the hot water temperature is quite satisfactory in institutional work. A double set of underground pipes composing the supply and return sys tems is installed and the water is circulated by means of pumps. For commercial district heating, however, hot water has several disadvantages. It is impracticable to meter the amount of heat used by each consumer because of the lack of a suitable meter; and it is not feasible to supply tall buildings because of the high pressures that would be required to serve the topmost radiators. Consequently district heating in all of the larger cities is effected by the steam system.

Most of the first steam systems were designed with the idea of utilizing the exhaust steam from engines driving electrical generators and for steam pressures between 2 and iolb. per sq.in.; later it was sometimes found more desirable to use live steam direct from the boilers and distribute it at a higher pressure. The passing of the steam through engines or turbines, which drive electric generators, before distributing it for heat ing purposes is, from a standpoint of fuel consumption, an eco nomical procedure. The electricity thus generated is produced at a very low fuel cost as compared with the usual method of generation in which a large proportion of the heat of the fuel is carried away by water circulating through condensers (see ELEC TRICAL POWER GENERATION). On the other hand, a low pressure distribution system requires much larger pipes because of the greater volume of the low pressure steam and the cost of in stallation is therefore much higher. In fact, in large cities it would be impossible to install pipes large enough to carry, at pressures below iolb., the quantities of steam required. This fact, together with the added cost of the electrical generating machinery, has sometimes made it appear more expedient, commercially, not to generate electricity in the heating plant, notwithstanding the apparent economy of this method. There are, however, a number of heating plants in which some electricity is generated and the present development of boilers and turbines for higher steam pressures and higher exhaust pressures will undoubtedly have a considerable effect upon future practice in this respect.

The steam-distributing pipes are laid beneath the streets or alleys, either inside a conduit buried in a trench, or in a tunnel large enough for men to walk in. The latter method, while allow ing inspection of the pipe and facilitating repairs, is naturally much more costly and is justified only when several pipes are to be laid along the same route or where sub-surface conditions do not permit the use of the conduit construction. Various forms of conduits are shown in fig. 8. They are designed so as to prevent excessive loss of heat and to protect the pipe from water and from earth pressure. They are built of wood, vitrified clay, brick or concrete, and so constructed as to leave a space around the pipe to permit its free linear expansion due to changes in temperature. Heat insulation consisting of magnesia or asbestos I or Zin. in thickness, surrounds the pipe except in the case of the wooden conduit, in which the wood itself is the insulator. Underdrainage of the conduit is very necessary in order to carry off ground water. It is provided by a layer of crushed stone or coarse gravel below the conduit with a drain tile, laid with open joints, which leads to some sewer or other outlet.

The heat loss from well constructed underground pipes in dry soil is less than is generally supposed, owing to the insulating effect of the soil. It varies from 4o to 8o B.T.U. per hour per sq.ft. of pipe surface. Further reduction of the heat loss could be obtained by thicker insulation but would not warrant the in creased cost. In well maintained systems the efficiency of dis tribution, which is the ratio of the steam delivered to the con sumer to the steam sent out from the central plant, varies from about 8o% to as high as 95%, the latter being possible only for a small system serving a dense load.

The linear expansion and contraction of the pipes due to changes in temperature must be provided for. With a steam pressure of 2olb. per sq.in., for example, the pipe will increase in length about 1.6in. in each 1 oof t. from its length when cold. To provide for this movement there are two general types of expansion fittings. One of them, the slip joint, consists of a sleeve sliding within another, with suitable packing to prevent steam leakage; others make use of flexible diaphragms or corru gated copper sleeves. Expansion fittings are placed at regular intervals with anchor points midway between to control the direction of the movement. The life of the underground pipes depends upon the design of the conduit and the nature of the soil. The chief cause of damage is ground water which corrodes the pipe and is difficult to exclude entirely. Pipes installed in well constructed concrete or tile conduit in fairly well drained soil will last for 3o years or more.

The

distance over which steam can be transmitted is much greater than is commonly supposed, being limited only by the size of the pipe and by the pressure available. Pipes a mile or two in length are not uncommon and a distance of several miles is quite feasible, though probably not economical. The pressure loss can be closely estimated. The usual scheme of distribution consists of a central trunk main of large diameter near the plant and de creasing in size, with lateral branches at the intersecting streets. Some systems make use of feeder pipes which radiate to strategic points in the distribution system. Long distance gauges, electri cally operated, are used to record, at the plant, the pressure at remote points in the system. The condensation from the con sumers' radiators is returned to the plant in most small systems serving only a few buildings, but in large systems it is frequently discharged to the sewer, unless the cost of water is very high, because of the expense of return piping and the difficulty of re turning the water without pumping.

In addition to its use for heating buildings the steam is used in small quantities for other purposes. Cooking and laundry appa ratus use steam and are satisfactorily served from the central plant if sufficient pressure is available. Cooking equipment is usually designed for steam pressures of 251b. or higher. Some steam is used for the heating of water for domestic purposes and, in a few cases, for producing power in steam driven pumps, etc.

Central Heating Costs.

The charge for heat supply is based upon the amount of steam used, which is measured either by a steam flow meter located at the point of supply in each building, or by a meter which measures the amount of condensate drained from the radiators. The latter method is more common and is quite satisfactory. The heat is usually removed from the condensate before it is discharged to the sewer by passing it through an economizer which heats the water supplying lavatories, etc. The water is passed through the economizer, which consists of a cylindrical tank in which a coil or a bundle of tubes, through which the condensation flows, is submerged.

The overall efficiency of the central-heating system compares favourably with that of the individual boiler plant. The better boiler efficiency in the more elaborate and refined central plant balances, to a great extent, the losses in distribution. The efficiency of the modern central plant is from 75 to 85%; the efficiency of distribution is from 8o to 9o%; and about 9o% of the heat deliv ered to the consumers' buildings is actually utilized (there being some loss in the condensate). The combination gives an overall efficiency of perhaps 6o% in the average case. This is to be compared with an average efficiency of 5o% or less for the small coal-fired boiler and perhaps 65% for the large plant; and with 6o to 75% for gas or oil, varying with the size of the plant. The charges for heating service range from about $.6o to $1.25 per i,000lb. of steam, varying with the size of the building and local conditions. The cost of service is naturally higher in larger cities and where the cost of fuel is high ; it compares favourably with that of the individual plant, particularly where the space occupied by the latter has a rental value. The service is popular in cities where the luxury value is appreciated. To the large-build ing owner as well as to the householder the freedom from the dirt and nuisance of operating a boiler plant is attractive. There is also an advantage to the community in the reduction of smoke and of dirt from coal and ash hauling.

There are at present approximately 200 commercial district heating systems in operation in America with an aggregate annual revenue of about $2o,000,000.

Heating and Ventilation.

In 1913 Sheppard and E. V. Hill, in America, established by tests a comfort zone which showed, for still air, the relation between temperature and humidity necessary for comfort. More recently Houghten, Yaglou and others, in an investigation sponsored by the American Society of Heating and Ventilating Engineers, the U.S. bureau of mines and the U.S. public health service made elaborate observations of the relations between temperature, humidity and air motion as they affect comfort.' The experiments consisted in exposing a large number of persons to carefully controlled atmospheric conditions and record ing their sensations as to the relative warmth or coolness of the various conditions. Tests were made in still air and in moving air and with the subjects stripped to the waist and normally clothed. The results obtained give the true relationships between temperature, humidity and air motion as they affect human com fort, over a wide range of conditions. For example, with an air movement of roof t. per min. and with subjects normally clothed and slightly active, the sensible effect of the atmosphere is the same for the following conditions :— Dr. E. V. Hill, in America, has suggested a method of ascer taining the degree of perfection of the ventilation in any particular room on the basis of observations made of these several factors, namely, the amount and distribution of the air supply; the cooling effect as indicated by the temperature, humidity and air motion; and the factors of odours, dust, bacteria and other injurious sub stances. By arbitrarily assigning a scale of penalties for each one of these factors falling short of the ideal condition and combining the results, the degree of perfection of the ventilation as a whole can be computed. This method has been adopted by the American Society of Heating and Ventilating Engineers.

Systems of Ventilation.

To satisfy the requirements it is necessary to provide some means for renewing the atmosphere and it is evident that since the room temperature is a factor in ventilation, the heating and ventilating systems must be con sidered together. When no special ventilating equipment is pro vided, acceptable atmospheric conditions are sometimes secured merely through the natural circulation of air through the building, aided by the action of chimneys and open windows, . but when many people occupy a room some more positive means must be provided and the air supply must be properly warmed before introduction. There are many kinds of ventilating systems vary ing from the open window to elaborate fan arrangements, the choice depending upon the type of building and the standard of ventilation it is desired to maintain. The tendency is toward better air conditions in offices, factories and public buildings, steady progress being made in developing improved apparatus and methods. The health and efficiency of office and factory workers is demonstrably improved by a proper atmosphere; and in many manufacturing processes air conditioning is absolutely essential (see AIR CONDITIONING).

A complete system of ventilation for buildings in which the best results are desired consists of apparatus for warming, cleaning and humidifying the air, a fan and a system of supply ducts for Transactions of the American Society of Heating and Ventilat ing Engineers for 1923 et seq.

distributing it to the various rooms, and a system of exhaust ducts with an exhaust fan for removing the foul air. An additional feature, sometimes included, is the cooling of the air, in summer, by artificial refrigeration.

Types of Fan Systems.

There are several types of central fan systems in common use. The arrangement depends somewhat upon the extent to which the heating requirements are taken care of by the fan system. The various combinations are as follows: (I) fan system takes care of both the heating and ventilation; (2) it provides ventilation and part of . heating, remainder sup plied by radiators; (3) it provides ventilation, heat supplied by radiators.

In the first type the air must be delivered at a sufficiently high temperature to heat the rooms and, since the heating requirements vary, provision must be made to deliver air at different tempera tures to the different rooms. One common arrangement of this system is shown in fig. 9. The air drawn from outside passes through a tempering heater, is heated to about 4o°, then goes through the air washer, secondary heater and fan. Leaving the fan, part of the air passes through additional heaters which are adjusted to bring it to a temperature of from 8o ° to 16o°, depending upon weather conditions ; the remainder bypasses these heaters and is at a temperature of about 68°. Individual ducts lead to the various rooms and take their supply partly from a hot air chamber and partly from a tempered air chamber, the proportions being regulated by a mixing damper usually controlled by a thermostat located in the room itself. Thus the proper mixture of air is supplied to maintain the room at the desired temperature. In an alternative arrangement the fan system delivers air to a trunk duct with branch ducts to the various rooms and in each branch duct is located a heater, often thermostatically controlled from the room, which adds just the proper amount of heat to the air.

In the second type, when part of the heat is supplied by radi ators, the fan system is similarly arranged but the air is delivered at a lower temperature. In severe climates it is desirable to install radiators along the outside walls, particularly under large windows, to avoid local cold spots. This is the justification for this type.

In type 3, since no heating is done by the fan system, the air is supplied to all of the rooms at the same temperature, usually about 7o°, and the fan system is therefore simple in arrangement. A trunk duct with branches to the various rooms is used as shown in fig. 1 o.

It is evident that the first type of fan system must be operated whenever heat is required in the building, while the third type need be used only when ventilation is desired.

Exhaust System.

In modern systems a central exhaust fan is used to withdraw the foul air but in some systems the circula tion is produced only by the natural tendency of warm air to rise, and by the slight pressure produced by the supply fan. Provision is often made for partial recirculation of the air from the exhaust duct to the supply fan so as to avoid the waste of warm ing unnecessary quantities of fresh air during the heating up period. Also, it is held by some that partial recirculation even in times of full occupancy of the building is permissible, provided the air is passed through an air washer or filter.

The air supply is introduced, in ordinary rooms, usually at points along the inside walls well above head height. Sometimes the inlet grilles are placed in the ceiling, but this requires a much lower air velocity to avoid uncomfortable draughts. An air movement in the occupied zone of the room greater than about eft. per sec. is likely to cause discomfort. In theatres the air is introduced from above and withdrawn through small openings in the floor beneath the seats.

Air-conditioning apparatus (see AIR CONDITIONING) used in ventilation comprises the air washer and the air filter, either or both being used, depending upon the standard of ventilation desired. The function of the washer is to remove dirt and bac teria, to increase or reduce the moisture content depending on whether the water temperature is warmer or cooler than the dew point temperature of the air; and to cool the air in cases where artificial cooling is used. Convectors of cast iron or of finned tubes frequently are employed as heat transfer devices instead of air washers. When these cool and dehumidify the air, drains are necessary to remove the water condensed on the convectors and removed from the air.

Filters are more effective dust removers than are air washers and often are added to supplement or entirely to replace the ter. Air filters may be sieve-like, with air passages smaller than the dust particles they seek to arrest, in which case they clog quickly, or may have air passages larger than the dust particles, when they trap the dust by impact and eddy pockets which are viscous coated and have a comparatively long period of usefulness before becoming clogged. They may be in cell form, easily re placed and cleaned, and frequently are moved and cleaned matically. The most effective dust filters employ the electric precipitation principle in combination with viscous coated baffles.

The baffles may be of finely divided metal, treated animal hair, spun glass, etc.

BIBLIOGRAPHY.-A. H. Barker,

Heating and Ventilation (um) ; Bibliography.-A. H. Barker, Heating and Ventilation (um) ; L. A. Harding and A. C. Willard, Mechanical Equipment of Buildings; J. D. Hoffman, Handbook for Heating and Ventilation Engineers; National District Heating Association, Proceedings and Handbook (1922) ; The Heating and Ventilating Magazine.

(J. H. W.; S. R. L.)

air, water, heat, steam, system, temperature and pipes