HEATING AND VENTILATION. 694 HEATING AND VENTILATION. be temporary, a boiler of low etBciency maj' be have been largely- replaced by vertical radiators, permissible, to save a relatively large capital of either cast or wrouglit iron. For direct heat- outlay. The character of labor readily available , iiig, cast-iron radiators are most commonly used, as boiler attendants may have a very important For steam, the connections enter and le.ave the bearing upon the clioice of a boiler. Thus for vertical radiators at the base, but hot-water Flu. 6. BTEAM-HEATING — DIRECT-CIHCUiT SYSTEM. heating most residences and many school build- ings simplicity, coupled with a maximum of safety, even under neglect, may be of more im- portance than economy of fuel. Sectional boilers are advantageous hero, since a small portion, in- stead of the wliole boiler, may give way in case of accident. Under other conditions fuel economy may be put first, with the understanding that the continuous service of a skilled fireman will be insisted on. In large heating plants, par- ticularly central heating stations (see following page), automatic stokers may be used to feed coal to the furnaces and also mechanical means for removing the ashes. See Boiler. Piping and Radiators. Pipes for distributing both steam and hot water and for returning the condensed steam and the partially cooled water to the boiler and heater, respectively, are Fig. 7. sectional radiator for hot-wateh heating. generally of v.'rought iron, with screw joints formed by means of couplings. The pipe-lines and radiators are controlled by various steam, water, and air valves, some of which are auto- matic. In most isolated plants the circulation is maintained wholly by gravity. Originally the radiating surfaces were mostly of cast-iron pipe, in horizontal lines, but this was superseded by coils of wrought-iron pipe, and these, in turn, Fig. 8. radiator for indirect SYSTEM. radiators have connections at the top, as well as at the bottom. This allows the air, which collects at the highest point in all water-pipes, to be drawn off. The indirect radiators are of either cast or wrought iron, placed in a chamber, so the fresh-air supply to be warmed may be passed over or through them. Some of the essentials of radiator designs are ample radiating surface, free passages to minimize friation, and good drain- age in the case of steam radiators. To prevent the escape of heat where not needed for warming, steam and liot-water pipes are covered with as- bestos, felt, or other insulating materials. Yhere high-pressure steam is used for heating it is generally reduced before entering the distributing system, since steam at much over 212" F. would be unpleasant if not dangerous to those coming near or in contact with the radiators. Recent improvements in heating include the securing of a partial vacuum in the pipes and radiators before admitting the steam, thus lessening the trouble with air. It may also be mentioned that fans are employed to aid the circulation of air in the indirect systems of steam and hot-water heating. Electric Heating. Heat is developed by the ■ passage of an electric current through any con- ductor; the greater the resistance of the con- ductor the higher will be the temperature to which it is raised by the passage of the current. This is the principle upon which all electric heaters are constructed. These heaters have come into considerable use since 180.5 for heating street oars. Their general construction is as fol- lows: Wire of high resistance is imbedded in asbestos, fire-clay, or other refractory material and built up into plates of large radiating sur- face, these being generally inclosed between iron plates. The current is passed through the resist- ance wire, which at once becomes very hot and in turn heats up the refractory material and the inclosing iron. When the heater has been brought up to the required temperature the cur- rent may be turned all or partly off, and, owing to the nature of materials used, the heater will retain its heat for a long time. To heat water
