Air changes per hour

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Air changes per hour

Air changes per hour (ACH) is a measure of air infiltration or air cleaning. It is a value representing the number of times each hour that an enclosure’s total volume of air is exchanged with fresh or filtered air. It is the total volume of air in a space that is turned over in one hour.

The term ACH is used in dilution ventilation

Dilution (equation)

The dilution equation, also known as the Basic Room Purge Equation, is used in industrial hygiene. It determines the time required to reduce a known vapor concentration existing in a closed space to a lower vapor concentration....
, air conditioning

Air conditioning

An air conditioner is an appliance, system, or Mechanism designed to extract heat from an area via a refrigeration cycle. In construction, a complete system of heating, Ventilation , and air conditioning is referred to as "HVAC." Its purpose, in a building or an automobile, is to provide comfort during either hot or cold...
, indoor air quality

Indoor air quality

Indoor air quality is a term referring to the air quality within and around buildings and structures, especially as it relates to the health and comfort of building occupants....
, industrial hygiene, air purification, containment and exhaust ventilation related applications. -------------------- ACH equation - SI Units

where:
N = number of air changes per hour
Q = Volumetric flow rate

Volumetric flow rate

The volumetric flow rate in fluid dynamics and hydrometry, is the volume of fluid which passes through a given surface per unit time Volumetric flow rate should not be confused with volumetric flux, as defined by Darcy's law and represented by the symbol q, with units of m3/, that is, m s-1....
of air in cubic meters per second

Second

The second , sometimes abbreviated sec., is the name of a units of measurement of time, and is the International System of Units SI base unit of time....
(m³/s)
Vol = Space volume

Volume

The volume of any solid, liquid, plasma, vacuum or theoretical object is how much three-dimensional space it occupies, often quantified numerically....
L x W x H , in cubic meters (m³)

-----------------------------------------

ACH equation - Imperial units

where:
N = number of air changes per hour
Q = Volumetric flow rate of air in cubic feet per minute (cfm)
Vol = Space volume

Volume

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Encyclopedia

Dilution (equation)

Air conditioning

Indoor air quality

Volumetric flow rate

Second

The second , sometimes abbreviated sec., is the name of a units of measurement of time, and is the International System of Units SI base unit of time....
(m³/s)
Vol = Space volume

Volume

The volume of any solid, liquid, plasma, vacuum or theoretical object is how much three-dimensional space it occupies, often quantified numerically....
L x W x H , in cubic feet
------------------------------ An air change does not represent a complete change of all air in the enclosure or structure unless it can be considered "plug flow". 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. In order to have an equilibrium

Equilibrium

For the opposite, see disequilibrium.Equilibrium is the condition of a system in which competing influences are balanced and it may refer to:...
pressure, the amount of air leaving the space and entering the space must be the same.

House air-tightness

The air-tightness of a home is measured in ACH; the number of times the home's air is replaced by outside air in an hour. Air outside a building is constantly infiltrating through cracks in a building shell and exchanging with inside air. ACH is the measure of the rate at which this occurs.

Typically air change rates, for existing homes, average between one and two per hour. Newer construction tends to be tighter, with exchanges below one. Tightly constructed homes may have an ACH of 0.35 to 0.25.

Standards

Various regulations, codes and standards prescribe the minimum required ACH for specific room uses in residential sleeping areas, offices, bath rooms, cooking areas, smoking lounges, laboratories

Laboratory

A laboratory is a facility that provides controlled conditions in which science research, experiments, and measurement may be performed. The title of laboratory is also used for certain other facilities where the processes or equipment used are similar to those in scientific laboratories....
, industrial spaces, clean rooms, animal facilities, nurseries, intensive care unit

Intensive Care Unit

An intensive care unit , critical care unit , intensive therapy unit or intensive treatment unit is a specialized department used in many countries' hospitals that provides intensive care medicine....
s, operating rooms etc.

Room - Building - Accomodation	ACH
Assembly and entrance halls	3 - 6
Commercial kitchens	20 - 60
Public bathrooms	6
Class rooms	3 - 4
Laboratories	6 - 12
Smoking rooms	10 - 15
Warehousing	1 - 2