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The Rate of air flow required for a vent hood is
calculated using the following definitions (Fig. 6):
Q = Rate of Air Flow in Cubic Feet Per Minute or
[ CFM ] Required for the Hood.
A = Area of Hood Opening = (L x W) in Feet
2
D = Clearance Height = Distance in Feet from
lower lip of hood to top of dishwasher
chamber.
P = Perimeter of Hood that is Open. This
depends on the hood design, as follows:
If there are four open sides (Island Design), the calculation of the Rate is as follows:
Q = 75 x A
If there are three or fewer open sides, the calculation of the Rate is as follows:
Q = 50 x A
As an alternate method, the Rate can be calculated as follows:
Q = 50 x P x D
Example:
L = 3 W = 3 D = 2
Perimeter Calculation Formula
Hood Design Corner Wall Island
Number of Open Sides 2 Sides Open 3 Sides Open 4 Sides Open
P = L + W L + W + W L + L + W + W
Dimensions Feet Feet Feet
NOTE: Any listed and labeled factory-built
commercial exhaust hood tested in accordance
with UL Standard 710 by a nationally recognized
testing laboratory, should be installed according
to the terms of its listing and the manufacturer's
installation instructions.
RATE OF EXHAUST FLOW CALCULATIONS
Based on the 1996 International Mechanical Code.
Rate Calculations
Hood Design Corner Wall Island
Number of Open Sides 2 Sides Open 3 Sides Open 4 Sides Open
Q = 75 x A
675 CFM
Q = 50 x A 450 CFM 450 CFM
Q = 50 x P x D 600 CFM 900 CFM 1200 CFM
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LENGTH
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➤
CLEARANCE
HEIGHT
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➤
WIDTH
Fig. 6