There are two classes of forced ventilation: (1) The plenum system, In which the air pressure in the building is slightly greater than that of the outer atmosphere; in other words, that system by which air is blown through the building by a fan or other blower placed at the inlet. (2) The vacuum system, or that method of removing foul air, and causing a consequent inrush of fresh air. by an exhaust fan placed at the outlet to the vent flue or stack. In the latter system, the air pressure in the building is slightly lower than that of the outer atmosphere. The plenum system is the more wholesome and preferable method.

The capacity of a fan, that is, the amount of air which it will deliver, depends considerably upon the construction of the fan, and upon the resistance to the flow of the air.

The following table shows a safe capacity for some leading forms of blowers and exhaust fans, operating against a pressure of 1 oz. per sq. in., or 1 3/4 in. water, nearly:

Diameter of

Wheel.

Ft.

Revolutions per

Minute.

Horsepower Required.

Capacity per

Minute.

Cu. Ft.

4

350

6.0

10,600

5

325

9.4

17,000

6

275

13.5

29,600

7

230

18.4

42.700

8

200

24.0

46,000

9

175

29.0

56,800

10

160

35.5

70,300

If the resistance is greater than above given, the required capacity may be obtained by increasing the number of revolutions, increasing the horsepower correspondingly. Or, if the resistance is less, the required capacity may be obtained by reducing the number of revolutions, with a corresponding decrease of power.

In selecting a fan, it is advisable to choose a large one, so that it may run easily, quietly, and economically. A small fan, which must be run at high speed, wastes too much energy, and often makes a disagreeable noise.

Fresh-air inlets, if near the floor, should be so arranged that the velocity of inlet will not exceed 2 ft. per second; a higher velocity is considered a perceptible draft. Higher velocities, however, are permissible if the inlets are located more than 8 ft. above the floor. Vent-flue inlets can safely be made to operate with a velocity of 6 or 8 ft. per second, without the air-current being disagreeably perceptible.