If the air in the space is ashrae guideline 14 2014 pdf 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. 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. This table needs additional citations for verification.
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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.