Determining CFM by the Heat Removal Method
The heat removal method is useful for determining cfm in installations where the ventilation of sensible heat is required.
In order to determine cfm by this method, you need to know the total Btu per minute, the average outdoor temperature, and the desired inside temperature. This information is then used in the fol- lowing formula:
Note that the cfm determined by the heat removal method deals primarily with sensible heat, not with radiant heat. The cfm obtained from the previous formula indicates the amount of air that needs to be passed through a structure or space in order to maintain the desired inside temperature.
Determining Air Intake
Adequate air intake area should be provided where fans are used to exhaust the air. The same holds true for fans used to supply air to a room (that is, adequate air exhaust area should be provided). The size of the air intake (or air exhaust) area depends on the velocity (fpm) of the entering or existing air and the total cfm required by the structure or space. This may be expressed by the following formula:
Doors and windows are suitable air intake areas if they are located close enough to the floor and provide a full sweep through the area to be ventilated. When you have determined the total free air intake area by the aforementioned formula, deduct the area for doors and windows that function as passageways for air. Fixed or adjustable louvers can be installed over the other intake areas.