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Air changes per hour, or air change rate, abbreviated ACH or ac/h, 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. 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 or 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. In order to achieve equilibrium pressure, the amount of air leaving the space and entering the space must be the same. ACH equation in 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 ''L'' × ''W'' × ''H'', in cubic feet 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 (occupants per square foot, occupants per square meter) *''h'' = Ceiling height (ft, meters) ==Air change rate== Air change rates are often used as rules of thumb in ventilation design. However, they are seldom used as the actual basis of design or calculation. For example, laboratory ventilation standards indicate recommended ranges for air change rates,〔(【引用サイトリンク】title=Lab Ventilation ACH Rates Standards and Guidelines )〕 as a guideline for the actual design. Residential ventilation rates are calculated based on area of the residence and number of occupants.〔 Non-residential ventilation rates are based on floor area and number of occupants, or a calculated dilution of known contaminants. Hospital design standards use air changes per hour, although this has been criticized.〔(【引用サイトリンク】title=Engineers' Perspectives on Hospital Ventilation )〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Air changes per hour」の詳細全文を読む スポンサード リンク
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