Calculation of air volume
The volume of air required to operate a ventilating and cooling system
is a very fundamental requirement. Before air volume is estimated
or calculated, careful consideration should be given to the following
factors.
The objectives
of the ventilation system
Be
sure that the decision you make will accomplish the most important
objectives or will overcome the most important problems. This should
be the overriding consideration in the selection of a system of
ventilation.
The budget
Availability
of money to finance the system is a factor that must be considered
early in the planning stage. The funds available will affect the
objectives you set and will influence the system of ventilation
selected to meet the objectives.
Alternate
methods of
calculating the required air volume
The
method selected for calculating air volume will relate closely to
the objectives to be accomplished by the system. Based on the objectives
and funds available, you can now consider the alternatives and select
the best method to use in calculating air volume requirements. A
description of these methods follows.
Rate of
air change method
This
is a time-honored approach to a determination of air volume requirements.
It is based on the theory that a complete change of air in a room
or building should be made at a certain time frequency. The rate
selected is frequently an arbitrary decision. It may be based on
experience with similar installations or may be established by a
health or safety code. Many fan manufacturers have published
charts that show recommended rates of air change for typical installations.
The formula for calculating the air volume in CFM
by the rate of air change method is as follows:
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Typical
layout of fans for rate of air change method
Although
the rate of air change method of calculating air volume has been
used for many years, American Coolair engineers have found it unsatisfactory
except in relatively small buildings or rooms. For jobs that involve
personnel comfort. this method is not recommended if the building
is over 50,000 cubic feet in content or more than l00' in length.
CFM per
square foot of floor area method
This method of calculation is a modern adaptation of the rate of
air change formula. Total air volume (cubic feet per minute) is
determined by multiplying the total square feet of floor area by
an arbitrary CFM per square foot figure. The figure selected may
be as low as 2 CFM or as high as 12 CFM per square foot. Four CFM
per square foot has been recommended as a minimum for summer ventilation
of large assembly type operations.
This method of calculation, like the rate
of air change method, is likely to produce unsatisfactory results
in many cases. Failure to control air distribution and air velocity
can be a major weakness in the whole concept. Selection of a CFM
per square foot figure should be based on experience and a proven
method of air distribution.
Rate of
air velocity method
This
method of calculating air volume needed for a system is highly recommended
by American Coolair. A breeze conditioning system can be highly
effective in providing personnel comfort in hot weather if the recommendations
outlined below are observed. It has been determined from field experience
that an average air velocity of 150 feet per minute (FPM) to 200
FPM is usually sufficient for personnel cooling under summertime
conditions. The CFM required to do the job is calculated by multiplying
the cross section of an area through which the air is to move by
the desired velocity. This is expressed in the following formula.
Air velocity
method of determining CFM
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