29. The following rule may be used to compute the quantity of heat which will be given off by a current of hot air of a given volume per minute, in cooling through any given number of degrees:

Rule 3

Multiply together the given volume of the air in cubic feet, the number of degrees through which it is cooled, and the number given in columns 3 and 6 of Table 7, for the original temperature. Divide this product by the original temperature, and the quotient will be the amount of heat given off in heat units.

Example

A current of hot air having a temperature of 150°, and a volume of 400 cubic feet per minute, is cooled to 65° in passing through a certain room; what amount of heat is given off per minute?

Solution

From Table 7, the heat contained in 1 cubic foot at 150° is 2.3196 B. T. U. Applying rule 3,

Example

How much heat will be required to warm 400 cubic feet of air to a temperature of 150°, the temperature out of doors being 10°?

Solution

The weight of 1 cubic foot at 10° is .08451 pound. Hence, applying rule 2, the number of B. T. U. required to heat 400 cubic feet from 10° to 150° is

U = .23751 X -08451 (150 -10) X 400 = 1,124 B. T. U. Ans.

30. The following table gives, for various temperatures, the weight of air per cubic foot, and the heat units that will be given up per cubic foot, upon cooling, from the original temperature to 0° F. The table is computed on the assumption that the air remains at a constant pressure of 14.7 pounds per square inch.

Table 7. Properties Of Air