HEATING AND VENTILATION. 696 HEATING AND VENTILATION. equable temperature, night and day. As an off- set 10 tile eeouomy of central heating there is the cost of installing and maintaining the distribu- tion system, and of making good the losses of lieat in transmission. Then it must be remem- bered that the care of isolated heating plants involves, oftentimes, no extra expense for labor, since they are tended by some member of the family, or some employee whose time is not fully occupied. Few central heating plants, except for colleges and other institutions having groups of buildings, were constructed until there came into vogue the utilization, for this pur- pose, of the e.haust steam from electric light- ing and power plants. The occasional early in- stallations were most frequent where a number of large customers could be secured near the station, particularly if steam could also be sold for power purposes. The latter was quite pos- sible, since live steam was supplied to the mains, and reduced in pressure at the premises to be heated. From 1895 to 1900 many heating in- stallations were added to central electric light- ing plants. Although still largely an experiment, at least in many details, this departure seems sure to extend rapidly. The combination adds stability to both the electric and the heating in- dustry: (1) By enabling the former to utilize a large percentage of the heat value of the fuel, which would otherwise go to waste; (2) by saving the entire fuel bill of the latter, so long as the exhaust steam is sufficient for the heating de- mand; (3) by making it easier to secure cus- tomers for both current (light and power) and heat, since many possible consumers either went without electric light and power, or maintained isolated plants, because they required a steam plant for heating purposes. On the other hand, where the exhaust steam is used for heating, non-condensing engines, with lower efficiency than condensing, are generally employed. At pres- ent the majority of electric stations use non-con- densing engines, so they are at liberty to choose between the more efficient condensing engine as a means of utilizing a portion of their exliaust steam and central heating installation to utilize the whole of it. The basis for choice rests upon a variety of local circumstances, including the cost of fuel, and the number and character of the population near the electric station. Station Eqiitpiiext for centra! plants is much the same as for isolated ones, only on a larger scale. When combined with electric plants, live steam must be made available when the de- mand for heat is greater than can be sup- plied by the exhaust steam. In such cases the pressure of the live steam is reduced before ad- mitting it to the mains. Back pressure on the en- gine must be eliminated as far as possible. This may be effected, at times of low demand for ex- haust steam, by wasting a part of the exhaust. If hot water is the heating medium, the exhaust steam is passed through a heater, then condensed and the warmed water distributed by pumping. Distribution Systems consist of supply pi[)es for the steam or hot water and return pipes for the latter, and corresponding branches, or ser- vices, for the several consumers, all of which must be carefully installed to prevent loss of heat in transmission. The pipe is generally of Avrought iron. Special provision must be made for expansion and contraction. The expansion joints are known as variators. They are made without packing, botli to save the trouble and expense of its renewal and to diminish the num- ber of manholes. Insulation is secured in a variety of ways. One of the most effective and cheapest means of insulation is bored pine logs. The large pipe is inclosed in regular compart- raentSj often formed of brick side walls and plank tops. The space between the walls and the pipe is packed with some non-conductor of heat, in addition to which the pipes may be wrapped with asbestos felt, ilore expansion joints, better in- sulation, and more thorough protection against ground water are required for steam than for hot-water pipes, on account of the higher tem- perature and consequent greater heat losses of steam, and also to prevent condensation. Inside the building line there is a shut-off" valve, and beyond that there may be a reducing or a regu- lating valve, to maintain the pressure at the de- sired point, regardless of its amount or variation in the street pipes. Xext in order conies the meter, here the steam is sold by direct measure- ment, or the condensed steam may be passed through a hot-water meter on the main waste pipe leading to the sewer. A hot-water supply may be metered, provided its temperature is kept fairly constant at the station. In some plants, however, the tempera- ture, rather than the volume, of the water is increased when more heat is required. Steam may be supplieil to either steam or hot-water radiators, but the lower temperature of hot water renders a previous installation of steam radiators inadequate for hot-water service, be- sides which a change in the piping system may be required. The temperature in liot-water sys- tems is sometimes regulated at the central sta- tion and sometimes it is left to the customer. In either case the regulation may be effected by a thermostat. In fact, thermostats may be em- ployed to a greater or less extent on nearly all systems of heating, isolated as well as central. The charges for central heating are most fre- quently based on the service for a whole season, modified by either the radiating surface supplied or the cubical space which that surface is de- signed to heat. Seasons, of course, vary both in length and in range of temperature, and the amounts of heat desired by different consumers also vary. On these and other accounts it is, a convenient basis, and it is more attractive to new consumers than any system of metering, with its indefinite promise as to the cost of service. The introduction of central heating plants for general municipal service was largclv due to Birdsill Holly, of Lockport, N. Y. Tlie 'first trial of his idea was made at Lockport. about 1880, when he organized a company for that purpose. Soon after, a plant with five miles of mains was installed in Xew York City. These, and a number of other Holly plants, supply steam for power, as well as for heat. At present a large number of both steam and hot-water plants are in use. They are operating under .a great variety of con- ditions and their location ranges from Atlanta, Ga., to Seattle, ^Yash. Some of the plants are serving customers in to'mis with only a few thousand inhabitants. ventilation. History of Ventilation. Although special openings were left in the roofs of Roman build- ings for the escape of air, their object was to
