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    "title": "HEATING AND VENTILATION",
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    "verified_text": "heating and ventilation (see 13.160 and 27.1008).— the close association between heating and ventilation, especially in modern practice, makes it desirable to treat the two subjects together. i. heating when the temperature inside a building is maintained above that out-of-doors, a sufficient quantity of heat must be continu- ously supplied to replace that which passes by conduction through the walls and roof of the building and to warm the con- siderable quantity of in-leaking air. the heating system has to produce the proper amount of heat and to distribute it. the earlier forms of heating apparatus, such as the stove and the fireplace, deliver their heat to the room principally by radiation. in general, heating by convection is to be preferred from a standpoint of comfort in severe climates, and for this reason and because of the dirt and labour necessitated by their care, the stove and the fireplace are being largely superseded, especially in north america, by the warm-air furnace or by steam and hot-water systems, in great britain the fireplace is still an acceptable method of heating because of the relatively mild climate. warm air furnace—the warm-air furnace system satisfies the requirement of a source of heating removed from living quarters, gives a fairly successful distribution of heat throughout the house and ts very low in cost compared with steam and hot- water systems. it consists of a firepot in which the fuel is burned and a more or less extended flue for the gases of combustion, the whole being sur- rounded by a sheet-metal casing through which a current of air passes, absorbing heat from the hot surfaces of the firebox and flue and passing thence through metal ducts to the rooms above. air is supplied to the base of the furnace casing either from out-of-doors or, more economically, by a return duct from some central room of the house. the fundamental difficulty with the warm-air furnace is that the force producing the circulation of air through the system, being due only to the difference in the densitics of the heated air in the pipes and the unheated air without, is necessarily very small and consequently it is difficult to heat rooms remote from the furnace, particularly under adverse wind conditions. steam-leating.—this system consists of a boiler in which the steam is generated, a number of so-called radiators located in the rooms to be heated, and a system of piping which conveys the steam to the radiators and returns the water of condensation to the boiler. the vapour system.—the so-called vapour system is a steam system operating at a pressure of only a few ounces above atmosphere as compared with a pressure of one to five pounds for the ordinary steam system. in the vapour system the steam enters the radiator through a valve near the top and the displaced air and the water of condensation are conveyed away by return piping, the air being finally discharged from the system through a vent near the boiler. ventilation 335 to prevent the flow of steam from the radiator into the return pip- ing, a trap is usually installed at the discharge end of each radiator. for large buildings the vacuum steam system ts uscd very exten- sively. the arrangement of the radiators and piping is nearly the same asin the vapour system, and a vacuum pump is connected with the return piping. this increases the differential pressure through the system and promotes the rapid flow of steam to the radiators and the complete removal of air. hot-water system.—in the hot-water system, which is widely used in great britain, water is heated in a central heater, flows through pipes to the radiators, where it delivers its heat, and thence back to the heater. circulation is produced, in the sim- ple gravity type system, by the difference in weight of the column of heated water in the supply pipe and the column of cooler water in the return pipe. most large systems employ forced circulation, produced by a pump in the circuit, thus in- suring positive flow and permitting the use of smaller pipes. the hot-water system, because of the thermal capacity of the water contained in it, gives a less fluctuating output of heat to the rooms, but is for the same reason less readily capable of responding when the outside temperature changes. the hot- water system, using forced circulation, is frequently employed in heating groups of buildings because the heat output can be readily controlled from a central point by adjusting the tcm- perature of the water. steam is produced for a heating system by a boiler which usually has the furnace or firepot incorporated in it. where electricity is used for heating—a procedure sometimes employed where coal is expensive and electric current cheap-— the boiler is fitted with electric heating elements of the submerged resistance type. in recent years, due to the increasing cost and scarcity of anthracite coal, boilers have been developed which will burn bituminous coal successfully and without excessive smoke. (see bowers.) this is accomplished by the introduction of air at certain points in the furnace or by other means for burning the volatile matter before it is chilled by contact with the cold boiler surfaces. temperature control—temperature control is frequently applied to schools, offices and public buildings in order to secure constant room temperatures. there are several systems in use which regulate the tlow of steam to radiators and adjust the posi- tion of dampers, under the control of a thermo-stat. com- pressed air or electricity are used to furnish the power required. 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 a furnace instead of fireplaces. as understood in north america, however, the term refers to 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. either steam or hot water may be used as the medium for convey- ing the heat from the central plant. the pipes are carried either in tunnels, permitting ready access to them, or are buried a few feet below the surface enclosed in some form of conduit to insulate the pipe against excessive loss of heat and to protect it, the conduit sometimes consists of a thick-walled wooden box or tube in which case the wood serves as the insulating material; or it is made of con- crete or clay tile and a layer of magnesia or asbestos 1s applied to the pipe as an insulator. there are extensive systems of this type distributing steam commercially for heating, cooking and to some extent for power in new york city, detroit, pittsburg, rochester, st. louis and elsewhere. the largest areas covered are about one sq. m. in extent. business districts and good residential districts of high class are the desirable territorv. frequently the exhaust steam from electrical gencrating plants is utilised and is distributed at a pressure of from 2 to 10 |b. above atmosphere; but in some cities steam direct from the boilers at pres- sures up to 100 |b. or more is used. the consumption of heat by the individual buildings is metred either by a steam meter in the supply pipe or by a meter which measures the water of condensation as it is drained from the heating system of the building. losses of all kinds between the boiler outlet and the consumer's meter are, in the more efficient systems, from 15 to 20°, of the steam sent out from the boilers. hot water is used as the distributing medium in some cases, but is not regarded as commercially satisfactory, largely be- cause of the lack of a suitable means of measuring the amount of heat used by each consumer. 336 il. ventilation the modern conception of ventilation (see puntic irartit) distinguishes between the internal and the external effects of the atmosphere upon the body. those factors having an internal effect are chiefly the humidity of the air affecting the mucous membranes in the respiratory passages, the amount of dust and bacteria in the air and the nature and strength of odours. ex- ternally, the temperature, humidity and motion of the air play perhaps an even more important part by controlling the rate of heat loss from the skin. it is only recently that this cooling power of the air in ventilation has been quantitatively studied. in 1913 sheppard and fe. y. ifill established by tests a comfort zone, which showed, for still air, the relation between tempera- ture and humidity necessary for comfort. more recently ilough- ten, 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 de- termined the complete relations between temperature, humidity and air motion as they affect bodily comfort. an adequate system of ventilation must supply a sufficient quantity of air, properly heated and humidified, so that its cool- ing effect will be correct. dust and bacteria must be reasonably absent, and odours not excessive. in the more elaborate sys- tems of ventilation, such as are used in schools, theatres, auci- toriums and other densely occupied buildings, the air used for ventilation is drawn by a centrifugal fan over steam coils, passed through an air washer for cleaning and humidification, and forced by the fan through a system of ducts to the various inlet points of the rooms to be ventilated. air is withdrawn from each room, through properly located outlets, into an exhaust duct and discharged to the outside by the exhaust fan. arrangement of wentilating system—the air washer consists of a spray chamber, in which the air is humidified, followed by a series of baffles wet by sprays which remove most of the dust, and finally there are a number of dry bafiles which serve to re- move any entrained water. the ventilating system may be uscd for heating as well as for ventilation. artificial cooling is sometimes used in theatres by supplying refrigerated water to the air washer (see refrigeration). bibliograpity.—american society of heating and ventilating engineers, transactions (1923 et seg.); j. r. allen and j. h, walker, heating and ventilation (1922); a. h. barker, ifeating and ventila- fion (1913); l. a. harding and a. c. willard, afechanical equip- ment of buildings (1916); national district heating association, proceedings (1914 ef seg.) and handbook (1922); the heating and ventilating magazine. 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