CFM AND AIR CHANGES CALCULATIONS
CUBIC FEET PER MINUTE (CFM): It is amount of air in cubic meter supplied by filter or blower per unit time (minute).
CFM of blower or AHU filter is calculated for the purpose of Air-System Validation and AIR Changes Calculations.
A Calibrated Anemometer (make-Lutron, Met one etc.) can be used for the measurement of air velocity. From the calculation of CFM of blower or filter air velocity is measured firs in Feet per minute (or KM/Hrs, M / S).
Start the AHU, Put the Anemometer ON and measurement the Velocity of air after blower or after filter in ft / min. Take the reading from 4 corners and centre of filter surface. Calculate the Average Velocity of air. Convert the Air velocity in to CFM as follow:
CFM of Blower / Filter = Velocity of Air in ft / min x face area of filter.
In this way measure the CFM of all filters for that area and finally calculates the Air changes per hours.
AIR CHANGES: Air changes are that how many times whole volume of air from the room is replaced by fresh air per unit time.
Air changers are calculated by using following formula:
Example: Suppose CFM of one filter is 250 and there are 5 no. of filters in that area
then, Total CFM of filter = 5 x 250 = 1250 cfm.
Room Size = 10 ft x 15 ft x 8 ft = 1200 cubic ft.
Air changes per hours = 1250/1200 x 60
= 62.5 Air changer per hours.