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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.
Actual cubic feet per minute (ACFM) is a unit of volumetric flow. It is commonly used by manufacturers of blowers and compressors. [1] This is the actual gas delivery with reference to inlet conditions, whereas cubic foot per minute (CFM) is an unqualified term and should only be used in general and never accepted as a specific definition without explanation.
The AHAM-certified seal lists a rating for tobacco smoke, pollen, and dust removal, and a rating for room size. The clean air delivery rate (CADR) is a figure of merit that is the cubic feet per minute (CFM) of air that has had all the particles of a given size distribution removed.
Cubic feet per minute (CFM) is an often confusing term because it has no single definition that applies to all instances. Gases are compressible, which means that a figure in cubic feet per minute cannot necessarily be compared with another figure when it comes the mass of the gas.
Typically, mass air flow measurements are expressed in the units of kilograms per second (kg/s) or feet per minute (fpm), which can be converted to volume measurements of cubic metres per second (cumecs) [1] or cubic feet per minute (cfm). [2]
Displacement ventilation systems supply air directly to the occupied zone.The air is supplied at low velocities to cause minimal induction and mixing. This system is used for ventilation and cooling of large high spaces, such as auditorium and atria, where energy may be saved if only the occupied zone is treated rather than trying to control the conditions in the entire space.
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The infiltration rate is the volumetric flow rate of outside air into a building, typically in cubic feet per minute (CFM) or liters per second (LPS). The air exchange rate, (I), is the number of interior volume air changes that occur per hour, and has units of 1/h.