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Application
Engineering Basics
Orifice Flow Calculation
To determine
air flow through an orifice:
Where:
V = Velocity in feet per minute (fpm)
C = Orifice Coefficient
K = Constant = 14,786 when P is expressed in In. Hg
21,094 when P is expressed in PSIG
4,005 when P is expressed in In.
of Water
(Above constants are based on an
air density of 0.075 lbs/ft )
P = Pressure differential across the orifice
Q = Flow rate in cubic feet per minute (CFM)
A = Total orifice area expressed in square feet
VP = Velocity pressure (units are those of pressure)
Coefficient C for Orifices Under Vacuum or Pressure Flow
Area of Orifices
Orifice Diameter in Inches |
| Diameter
in Inches |
Square Inches |
Square Feet |
| 1/8 |
.01227 |
.000085 |
| 3/16 |
.02761 |
.00019 |
| 1/4 |
.04908 |
.00034 |
| 3/8 |
.11044 |
.00076 |
| 1/2 |
.19634 |
.00136 |
| 5/8 |
.30679 |
.00213 |
| 7/8 |
.60132 |
.00417 |
| 1.0 |
.78539 |
.00545 |
Orifice area
(in sq. inches) = .25 X  X (orifice diameter in inches)
Orifice area (in sq. feet) = Area in sq. inches ÷ 144
Orifice Pressure
Drop as a Function of Flow and Orifice Area (C=.65)
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