Air changes per hour

Air changes per hour, abbreviated ACPH or ACH, or air change rate is the number of times that the total air volume in a room or space is completely removed and replaced in an hour. 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 each hour. Perfectly mixed air refers to a theoretical condition where supply air is instantly and uniformly mixed with the air already present in a space, so that conditions such as age of air and concentration of pollutants are spatially uniform. 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. These systems range from a conceptual system of perfect displacement, which removes and replaces all air in a space, to a short circuit flow in which very little of the existing air is replaced. The actual amount of air changed in a well mixed ventilation scenario will be 63.2% after 1 hour and 1 ACH. In order to achieve equilibrium pressure, the amount of return air (air leaving the space) and the amount of supply air (air entering the space) must be the same. In Imperial units: 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) Vol = Space volume L × W × H, in cubic feet In metric units where: ACPH = number of air changes per hour; higher values correspond to better ventilation Q = Volumetric flow rate of air in liters per second (L/s) Vol = Space volume L × W × H, in cubic meter 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 (cubic feet per minute (CFM) per person or cubic meters per minute per person) ACPH = Air changes per hour D = Occupant density (square feet per occupant or square meters per occupant) h = Ceiling height (feet or meters) One cubic meter per minute = 16.