When air is to be moved against a very slight resistance, as in the case of exhaust ventilation, the disc or propeller type of wheel may be used. This is shown in different forms in Fig.. 24, 25 and 26. This type of fan is light in construction, requires but little power at low speeds, and is easily erected. It may be conveniently placed in the attic or upper story of a building, where it may be driven either by a direct or belt-connected electric motor. Fig. 24 shows a fan equipped with a direct-connected motor, and Fig. 27 the general arrangement when a belted motor is used. These fans are largely used for the ventilation of toilet and smoking rooms, restaurants, etc. and are usually mounted in a wall opening, as shown in Fig. 27. A damper should always be provided for shutting off the opening when the fan is not in use. The fans shown in Fig.. 25 and 26 are provided with pulleys for belt connection.

Fans of this kind are often connected with the main vent flues of large buildings, such as schools, halls, churches, theatres, etc., and are especially adapted for use in connection with gravity heating systems. They are usually run by electric motors, and as a rule are placed in positions where an engine could not be connected, and also in buildings where steam pressure is not available.

Fig. 22.

Fig. 23.

Fig. 24.

Table IV gives the air delivery per minute against slight resistance, and the proper size of motor for fans of the disc type.