The volume of air which a given fan will deliver depends upon the speed at which it is run and the friction or resistance through the heater and air ways. The pressure referred to in connection with a fan is that in the discharge outlet and represents the force which drives the air through the ducts and flues. The greater the pressure with a given resistance in the pipes the greater will be the volume of air delivered, and the greater the resistance, the greater the pressure required to deliver a given quantity.

Fan wheels of the same manufacture are usually made with a constant ratio between the diameter and width, although special &8226;22 forms are made where this does not hold true. All practical data on the action of fans is based on the results of tests, and from these the following relations have been found to be approximately correct:

(1) The volume of air delivered varies directly as the speed of the fan, that is, doubling the number of revolutions doubles the volume of air delivered.

(2) The pressure varies as the square of the speed, for example, if the speed is doubled the pressure is increased 2 X 2 = 4 times, etc.

(3) The power required to run the fan varies as the cube of the speed; again, if the speed is doubled the power required is increased 2 X 2 X 2 = 8 times.

The value of a knowledge of these relations may be illustrated by the following example.

Suppose for any reason it was desired to double the volume of air delivered by a certain fan. At first thought we might decide to use the same fan and run it twice as fast; but when we come to consider the power required we should find that this would have to be increased 8 times, and it would probably be much cheaper in the long run to put in a larger fan and run it at lower speed. In speaking of a fan as a 4 or 5-foot fan, the diameter of the propeller wheel is meant, but if we say an 80 or 100-inch fan we mean the height of casing in inches.

It has been found in practice that fans of the blower type having curved floats operate quietly and give good results when run at a speed corresponding to 1/2 ounce pressure at the discharge outlet; this gives a speed of about 3600 feet per minute at the circumference of the wheel. Higher speeds are accompanied with a greater expenditure of power and are likely to produce a roaring noise or cause vibration. A much lower speed does not provide sufficient pressure to give proper control of the air distribution during strong winds. The following table gives average capacities for various sizes of fans and the corresponding horse-power of engine required. If an electric motor is used multiply the horsepower given in the table by 1.3.

This is done because we can never tell exactly what the power required will be and it is well to have an excess to meet any emergency or unlooked-for conditions which may arise. In the case of a steam engine the steam pressure may be raised to meet any special requirements but a motor can only give out the original power for which it was designed.

 Nominal Size of Fan. Height of Housing inInches. Diameter ofFan Wheel inInches. Width ofHousing inInches. Ordinary Speed Giving 1/2 Ounce Pressure. Cubic Feet of AirDelivered perMinute. Horse-Power of Engine to Drive the Fan. 30 18 9 870 1000 1/2 40 24 12 580 1600 1 50 30 15 465 2600 1 60 36 18 390 4500 2 70 42 21 333 6000 2 1/2 80 48 24 293 8000 2 1/2 90 54 28 260 11000 4 100 60 32 233 12500 4 120 72 43 195 21500 7 140 84 . 48 167 28600 9 160 96 48 147 31800 10 108 54 130 40400 13 120 60 117 51000 16