The first requirement of installation is that the blower, at a predetermined speed, as fixed by the specifications under which the apparatus is installed, shall be sufficiently large to deliver the given quantity of air through the registers. Second, the heater outside or beyond the blower should have an excess of clearance area over the area of the fan inlet. Third, under ordinary conditions the heater should not be less than 20 coils in depth to maintain a temperature of 70° inside the building with zero weather outside. Fourth, the area of discharge from the bases of the coils should be approximately that of the steam-supply connections when working at very low or atmospheric pressure. Fifth, main and branch ducts should be so proportioned as to produce a uniform flow of air and reduce frictional resistance to a minimum. Sixth, metal ducts should be covered with some form of non-conducting material, especially where the delivery ducts are long.

In designing a blower system of heating and ventilation, it is considered good practice to provide for a maximum supply of air under the most unfavorable conditions likely to arise. In hot-blast heating, more failures result from lack of space around and between the pipes of the heater than from any other cause; lack of sufficient area there greatly reduces the amount of air that the fan is capable of delivering.

CLASSIFICATION OF BLOWER SYSTEMS.

The Plenum System.-The plenum system takes its name from the fact that the pressure of the air within the building is greater than that of the outer atmosphere. The blower forces air into the rooms in greater volume than it is removed by the ventilating fiues, the balance leaking out around windows and doors. This tends to check the in-leakage of cold air.

The heating apparatus employed in conjunction with a fan may be of any desirable type, that is, either steam, hot water, or hot air.

Buildings are commonly ventilated on the plenum system in the manner indicated in the accompanying illustration. A fan a, placed in what is usually called the fan room, in the basement of the building, takes its supply of fresh air from the outer atmosphere through a wire-screen window b in the fresh-air inlet room c. The fan wheel is revolved by a small vertical engine e, connected to the shaft of the fan wheel. The fan wheel, when in motion, forces fresh air into the main ducts, either through and between the surface of a steam

heated coil g and into d, or under the coil and into f. The air in d, then, is warm, and that in f is cold. Hot and cold branches h and j are taken from the main ducts and join the base of a vertical flue i, built in the wall; a mixing damper is placed at the point k to allow mixing the hot and cold air, to obtain the proper temperature without affecting the volume of air discharged. Each room to be ventilated is furnished with an outlet or vent flue v, which passes out through the roof to the atmosphere.

The Exhaust or Vacuum System.—The vacuum system takes its name from the fact that the air pressure within the building is slightly less than that of the outer atmosphere, because the fan removes air from the rooms and blows it to the outer atmosphere. Air may be moved and distributed by the exhaust or vacuum system with the same facility as by the plenum system; but the inward leakage of cold air that takes place through every hole and crevice in the walls, and around every loose-fitting window, produces cold drafts in the rooms.

Combined Plenum and Exhaust Systems.-Under all ordinary circumstances, the combination of an exhaust system with a pressure or plenum system is not to be recommended. The combination does not afford any decided advantage, and there are several substantial objections to it. The cost for flues, fans, power, motors, installation, and attendance is greater than in a single pressure system of equal aggregate capacity. For very large plants, however, a combination of the pressure and exhaust system is sometimes considered advisable.

GENERAL ARRANGEMENT OF APPARATUS.

Methods of Installation.-With reference to the element of heating alone there are three methods of installing blower systems: first, so that a constant volume of air is warmed by heating surfaces massed in a heater close to the fan, the air being distributed by a single- or double-duct system; second, so that the air is warmed or reheated, as the case may be, by separate radiating surfaces, arranged, as for indirect heating, at the base of the flues, a single duct serving to convey the fresh air from the fan to the heating surface; third, so that tempered air at from 70° to 75° is forced through a single-duct system into the rooms, where sufficient direct radiation is placed to give the required temperature and to provide against entire absence of heat in case of accident to the fan or main heater. Considered with reference to the method of distributing the warmed air, blower systems may be divided into two classes, viz., single-duct systems and

double-duct systems. With the former, the mixture of hot and cold currents of air takes place close to the fan, whereas with the latter, the hot and cold air are carried in separate ducts to the base of the flues in which the mixing is done.

Single-Duct System.-One of the methods of arranging the apparatus for use in connection with a single-duct system of distribution is illustrated in perspective in Fig. 1. The three-quarter housing steel-plate fan a, driven by the hori

zontal steam engine b, delivers air through its left-hand, top, horizontal discharge outlet into the brick receiving chamber in which, on a raised platform, is mounted a duplex heater cc', over and between the pipes of which the air is forced by the fan to the ducts beyond. Beneath the heater is arranged a by-pass controlled by the damper d, operated by the hand crank e outside the plenum chamber. The partition ƒ serves to separate the heated air from the cold air by-passed under the heater, while the partition g serves to form one side of the chamber in which the motors h, h that operate the mixing