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,

 400 X (150-65) X 2.3196 = 525.78 B. T. U. Ans. 150

## 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

 Temperature.Degrees . F. Weight perCubic Foot.Pounds. B. T. U. Given up by 1 Cubic Foot of Air in Cooling to0° From 0 .08635 .0000 2 .08597 .0408 4 .08560 .0813 6 .08523 .1214 8 .08487 .1613 10 .08451 .2007 12 .08415 .2398 14 .08380 .2786 16 .08344 .3171 18 .08309 .3552 20 .08275 .3931 24 .08206 .4678 28 .08139 .5413 32 .08073 .6136 36 .08008 .6847 40 .07944 .7547 45 .07865 .8406 50 .07788 .9249 55 .07712 1.0074 60 .07638 1.0885 65 .07566 1.1681 70 .07494 1.2459
 Temperature. Degrees F. Weight perCubic Foot.Pounds. B. T. U. Given up by 1 Cubic Foot of Air in Cooling to0° From 75 .07424 1.3225 80 .07355 1.3975 85 .07288 1.4713 90 .07222 1.5438 95 .07157 1.6149 100 .07093 1.6847 110 .06968 1.8205 120 .06848 1.9518 130 .06732 2.0786 140 .06620 2.2013 150 .06511 2.3196 160 .06406 2.4344 170 .06305 2.5458 180 .06206 2.6532 190 .06111 2.7577 200 .06018 2.8587 210 .05929 2.9572 220 .05841 3.0521 230 .05756 3.1444 240 .05674 3.2343 250 .05594 3.3216 260 .05517 3.4069