Basic Documentation

Air Changes Per Hour
Recirculated air is also known as return air. During much of the year the required fresh
(outside) air must be substantially heated or cooled. However, the temperature and humidity
of the return air is much closer to the required room supply air conditions. Thus the use of
return air reduces the amount of energy required to provide the temperature and humidity
level of the supply air necessary to maintain the required room temperature and humidity.
Air Changes Per Hour
The total amount of air that is supplied to a room normally exceeds the required fresh air
component and is the room ventilation rate. The HVAC industry also uses expresses room
ventilation rates in terms of number of air changes per hour (ACH) for the room. The amount
of air necessary to achieve one air change for a room is equal to moving an amount of air
into, and simultaneously out of, the room that equals the room’s volume. Therefore, a room
ventilation rate of one air change per hour is equivalent to moving a quantity of air through
the room equal to the total volume of the room over the course of one hour.
As was stated, of the total amount of air normally supplied to a room, a certain minimum
amount must consist of fresh outside air. However, an occupant of a laboratory room is more
likely to suffer the effects of harmful airborne substances than merely the discomfort of
inadequate fresh outside air. Therefore, the room ventilation rates required for laboratory
rooms are not based on a minimum cfm of fresh air per occupant since that would be quite
low. Rather, laboratory rooms must maintain a relatively high room air change rate to ensure
a safe room environment. Since air that is exhausted from a laboratory room is normally
discharged outside and cannot be reused or recirculated
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, it follows that a laboratory facility
needs to use nearly all outside air to maintain the required room ventilation rate of the
individual laboratory rooms.
Dilution
Although the HVAC industry often uses the term air change per hour (ACH) with regard to
quantifying the ventilation rate, the term can be misleading. The air being supplied to a room
enters the room through a room air diffuser and then mostly mixes with the existing air in the
room. This does not produce a true change out of the room air but rather a continual dilution
and removal of any contaminants that are present in the room air. For example, consider a
1000 gallon water tank into which enough coloring dye is mixed to leave the water with a
distinct dark coloration.
Although clear water may be allowed to flow into the tank while an equal amount of water is
being drained from the tank, the water in the tank will not be totally clear even after 1000
gallons of clear water is allowed to flow through the tank. Although this is the equivalent of
one “tank change”, we know that the tank water will still possess some of the original
coloration. If we continue to add clear water into the tank while draining an equivalent
amount, we know that we’ll be continually diluting and reducing the coloration but the last
molecules of dye could be present for an indeterminate amount of time.
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Although the reuse of air exhausted from a laboratory room is not always specifically prohibited, using such air for recirculation
purposes requires ensuring removal of all hazardous substances which is technologically difficult and in the least very costly.
Thus this option where available is not generally cost effective.
Siemens Building Technologies, Inc. 5