**TecKnow of the day**

Rule of Thumb for Air Flow.
Air quantities (cfm):400 cfm per ton of cooling is needed for normal comfort applications, 500 cfm per ton of cooling for heat pump and high sensible heat applications, and 350 cfm per ton of cooling for high latent heat applications.

These are the approximate amounts of air that should be moving through evaporators for three common applications. Say, for example, you have a comfort cooling application at a savings and loan office that has a 10-ton system. Therefore, you multiply 10 tons by 400 cfm per ton to reveal the total system airflow requirement of 4,000 cfm under normal operating conditions.

One cfm is needed per square foot (1 cfm/sqft) of floor area.
This is the average air quantity required for a room or an entire building. This number is based upon an averaged heat load calculation for comfort cooling. There is an assumption of an 8-ft ceiling, no unusual window areas, and average insulation. This rule of thumb provides about 7.5 air changes per hour.

This rule is a quick way to approximate the cooling load for a room or building and may be helpful in estimating room air quantities. For example, to estimate the number of tons required to cool a 1,600-sq-ft home, multiply 1,600 by 1 cfm/sqft to get 1,600 cfm of air. Using the 400-cfm/ton rule, divide 1,600 cfm by 400 cfm/ton to get 4 tons of required cooling.

Six to 10 air changes are needed per hour.

This is the average number of times in each hour that the air in the building or room (assuming 7- to 8-ft ceilings) is removed and replaced by circulating the air.