The withdrawal of impure air from rooms is effected by fans connected to a system of vent flues extending upward to an attic space, or downward to a cellar or basement, if the latter is more convenient. Another method of accel erating the outflow of air through flues rising to the roof of a building is by the use of as pirating coils. These are simply coils of pipe, or radiators, placed in the vent-flues as low down as possible, the coil heating the air and thus causing it to rise. Theoretically the aspi rating coil is a more expensive method of mov ing air than the mechanical method, as far as fuel-cost is concerned. It is simpler, however, than the fan-system.
Fans are of two general types — the disc or propeller fan, and the centrifugal blower. The former is constructed somewhat like a ship's propeller, and the current of air that it produces is mainly in a direction parallel with the shaft of the fan. The centrifugal blower, as usually de signed, consists of a wheel with blades, some thing like a ship's paddle-wheel, enclosed in a casing. The air enters at the axis of the fan, and when the fan-wheel is revolved the air is discharged radially to the casing by the action of centrifugal force. Relatively speaking, the propeller-fan will move a large volume of air with small expenditure of power, but the pres sure at which it will deliver air is limited. The centrifugal fan will deliver air under a greater pressure and the power required is therefore greater. In some buildings, where the system is of ducts and flues, is long, and the cross-sec tions are comparatively small, to save space, quite a pressure is required to force the neces sary amount of air through them. For such situations the centrifugal blower is best adapted. When the ducts are short and of ample area, it is best to use the propeller type of fan.
Fans are driven usually by small steam engines or by electric motors. Sometimes gas engines have been used with success. Where an engine is used, it is necessary for the boilers to operate under a sufficient pressure to drive the engine, or at least under a higher pressure than is commonly used with the gravity-system of connecting radiators. If the steam exhausted by the engine is condensed in the heating-sys tem, as it usually is, a pump is necessary to re turn the condensation to the boilers. In large office buildings, public buildings, theatres, etc., where a skilled engineman is employed to care for the plant, the use of a pump, an engine, etc.,
does not present an objection. On the other hand, in the case of schoolhouses, large resi dences, churches, etc., which are apt to be looked after by less skilled attendants, an en gine, pump and other apparatus that must go with them are open to objection. In such cases electric motors can be used if current can be obtained from an electric-supply company. The entire heating system can then be operated on the simpler gravity system. Of course the cur rent must be paid for, but in many locations its cost will be more than offset by the greater simplicity of the motor-driven system.
One of the most important developments in recent years in connection with ventilation of buildings is the control of humidity in air sup plied for ventilating purposes. One method is to employ an evaporating pan in which a coil of pipe, containing steam at high temperature, is submerged in water. The high steam tempera ture causes the surrounding water to evaporate and add moisture to the air that is blown over the evaporating pan. An automatic device in the air duct controls the supply of steam to the coil in order that the humidity of the air may be maintained at the desired point.
Another method of adding moisture to air is by means of an air washer which is a large chamber of sheet metal through which the air is passed, with a receptacle for holding water at the bottom. A pump draws water from the bottom of the washer and forces it under pres sure through pipes to nozzles which deliver the water in a finely divided spray over the in terior of the washer. The air is drawn through the washer and comes in direct contact with the spray which not only cleanses the air but adds moisture to it as well. The amount of moisture absorbed can be varied by varying the tem perature of the water by means of a suitable heater, the apparatus also being under auto matic control.
In view of the fact that some of the water is vaporized, an amount of heat is abstracted from the air equal in amount to the latent heat of evaporation of the water. The result is that the air is cooled, due to partial evaporation, in passing through the washer. Air washers of this type are, therefore, used to cool the air and they have been used to a considerable ex tent in certain classes of buildings is manu facturing plants, where air cooling s necessary in hot weather.