The application of forced blast for the warming of factories and shops is shown in Fig.. 20 and 21. The proportional heating surface in this case is generally expressed in the number of cubic feet in the building for each linear foot of 1-inch steam pipe in the heater. On this basis, in factory practice with all of the air taken from out of doors, there are generally allowed from 100 to 150 cubic feet of space per foot of pipe according as exhaust or live steam is used; live steam in this case indicating steam of about 80 pounds pressure. If practically all of the air is returned from the buildings to the heater, these figures may be raised to about 140, as a minimum and possibly 200 as a maximum, per foot of pipe. The heaters in table II may be changed to linear feet of 1 inch pipe by multiplying the numbers in column three (square feet of surface) by three.

Example. - A machine shop 100 feet long by 50 feet wide and 3 stories, each 10 feet high, is to be warmed by forced blast using exhaust steam in the heater. The air is to be returned to the heater from the building and the whole amount contained in the building is to pass through the heater every 15 minutes. What size of blower will be required and what will be the H.P. of the engine required to run it? How many linear feet of 1 inch pipe should the heater contain?

Fig. 18.

The total volume of air is

100 X 50 X 3 X 10 = 150,000 cubic feet.

This will require

150,000 = 1071 feet of pipe.

There are 10,000 cubic feet of air required per minute; hence from Table III we must use a 4 H.P. engine and a 90-inch blower.

If live steam were used, and the air changed every 20 minutes, we should need 150,000/ 200 = 750 feet of pipe, and 7500 cubic feet of air per minute, requiring a 2 1/2 H.P. engine and an 80-inch blower.

In using this method of computation judgment must be used which can only come from experience. The figures given are for average conditions of construction and exposure.