Air change
Air change
The quantity of infiltration of ventilation air in cubic meters per second divided by the volume of the room gives the number of so-called air changes during a given interval of time. Tables of the recommended number of such air changes for various rooms are used for estimating purposes.
Air changes per hour, abbreviated ACPH or ACH, or air change rate is a measure of the air volume added to or removed from a space (normally a room or house) divided by the volume of the space.[1] If the air in the space is either uniform or perfectly mixed, air changes per hour is a measure of how many times the air within a defined space is replaced.
In many air distribution arrangements, air is neither uniform nor perfectly mixed. The actual percentage of an enclosure's air which is exchanged in a period depends on the airflow efficiency of the enclosure and the methods used to ventilate it. The actual amount of air changed in a well mixed ventilation scenario will be 63.2% after 1 hour and 1 ACH.[2] In order to achieve equilibrium pressure, the amount of air leaving the space and entering the space must be the same.
Where:
- ACPH = number of air changes per hour; higher values correspond to better ventilation
- Q = Volumetric flow rate of air in cubic feet per minute (cfm), if using Imperial units, or cubic meters per minute if using SI
- Vol = Space volume L × W × H, in cubic feet if using Imperial units, or cubic meters if using SI
Ventilation rates are often expressed as a volume rate per person (CFM per person, L/s per person). The conversion between air changes per hour and ventilation rate per person is as follows:
Where:
- Rp = ventilation rate per person (CFM per person, L/s per person)
- ACPH = Air changes per hour
- D = Occupant density (square feet per occupant, square meters per occupant)
- h = Ceiling height (ft, meters)
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