Air changes can be calculated by counting the people in a building or by calculating the building volume. It is always necessary to choose the process that produces the greater number of changes per hour (air replenishment).
To calculate fresh air requirements and air changes per hour, the following factors must be considered:
Number of occupants
Use of the space
Dry bulb temperatures
Relative humidity
Amount of fresh air admitted by infiltration
The efficiency of the unit
Formulas for calculation
Required air exchange rate = (number of occupants x 15 cfm)
Time for complete air exchange =
Air exchange per hour =
Problem:
Six people occupy a 10,000 cubic foot space. Calculate the necessary air exchange rate, the time required for one complete air exchange, and the number of air exchanges per hour.
A. Air exchange rate
B. Time for one complete exchange
C. Number of air exchanges/ hour
A. To calculate the necessary air exchange rate, multiply the number of occupants (6) by 15 cfm:
Air exchange rate = 6 occupants x 15 cfm = 90 cfm
B. To calculate the time required for one complete air exchange, divide the volume of the space (10,000 cubic feet) by the air exchange rate (90 cfm):
Time for one complete exchange = 10,000 cubic feet / 90 cfm = 111.11 seconds
C. To calculate the number of air exchanges per hour, divide 60 minutes (1 hour) by the time for one complete exchange (111.11 seconds):
Number of air exchanges per hour = 60 minutes / (111.11 seconds / 60) = 32.43 exchanges/hour (rounded to the nearest hundredth)