Environmental Engineering Reference
In-Depth Information
I = (ln 100/25)/2
(11.5)
I = 0.69 ACH
In perfluorocarbon determinations of air exchange, perfluorocarbons are
injected at a constant rate. The air exchange I is calculated from the ventila-
tion rate (Q), which is the ratio of the rate of injection (F) to the measured
concentration C.
Q = F/C
(11.6)
Since Q is expressed in m
/sec or cubic feet per minute (CFM), it must be
multiplied by appropriate time units and divided by the volume of the space
to obtain ACH values.
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III. Mechanical ventilation
Most large commercial, office, and institutional buildings constructed in
developed countries over the past three decades are mechanically ventilated.
Use of mechanical ventilation is often required in building codes and repre-
sents what can be described as good practice for building system designers
and architects. Increasingly, buildings are being designed to provide year-
round climate control. To ensure optimum operation of heating, ventilating,
and air-conditioning (HVAC) systems, windows are sealed so they cannot
be opened by occupants to provide ventilation. The availability of outdoor
air for space ventilation depends on the design and operation of HVAC
systems as well as air that enters by infiltration and exfiltration processes.
Mechanical ventilation is used in buildings to achieve and maintain a
comfortable and healthy indoor environment. Two ventilation principles are
used to accomplish this goal; general dilution and exhaust ventilation. Both
principles are used in most buildings. General dilution ventilation is the
dominant ventilation principle used to ventilate buildings. Local exhaust
ventilation is used for special applications: removing lavatory and kitchen
odors, combustion by-products, combustion appliance flue gases, etc.
A. General dilution ventilation
Ventilating buildings to provide a relatively comfortable, healthy, and odor-
free environment is based on the premise that a continual supply of outdoor
air can be introduced into building spaces. As ventilation air mixes with
contaminated air, contaminant levels are reduced by dilution.
In general dilution theory, a doubling of the air volume available for
dilution is expected, under episodic or constant conditions of contaminant
generation, to reduce contaminant concentration by 50%. If the volume of
ventilation air were to be doubled again, the original concentration would
be reduced to 25% of its original value. By decreasing ventilation air required
