Flow Coefficient, Cv factor
A Cv factor (C-sub-V) is a numerical expression associated with the number of gallons of water per minute that will pass through a valve with a 1psi differential between the valve's inlet and outlet.
It can be calculated by the formula: Cv = Q/22.8 * Ö{(T1*G)/(DP(P2+Pa))}
Where:
Cv = capacity coefficient
Q = required flow in standard cubic feet per minute (scfm) at 14.7psig, 36% relative humidity
G = specific gravity of the flowing medium (G = 1 for air)
T1 = absolute temperature (460 + degrees F)
P1 = inlet pressure (psig)
P2 = outlet pressure (psig)
DP = pressure drop (psi) static to static pressure
Pa = atmospheric pressure (psia)
A simpler alternative is the formula: Cv = flow rate (scfm) * A (constant from table below)
5psi is the maximum desired pressure drop across a solenoid valve that is used for compressible fluids such as air.
For EXAIR's standard testing conditions of 80psi and a maximum 5psi pressure drop across a valve, the A constant is 0.048
In sizing a valve to use for a particular application, the valve must have a Cv value equal to or higher than the calculated Cv value based on the required air flow volume, inlet pressure, and pressure drop across the valve.
|
Inlet Pressure |
"A" Constants for Various Pressure drop |
|
2 psi |
5psi |
10psi |
|
10 |
.155 |
.102 |
- |
|
20 |
.129 |
.083 |
.066 |
|
30 |
.113 |
.072 |
.055 |
|
40 |
.097 |
.064 |
.048 |
|
50 |
.091 |
.059 |
.043 |
|
60 |
.084 |
.065 |
.040 |
|
70 |
.079 |
.050 |
.037 |
|
80 |
.075 |
.048 |
.035 |
|
90 |
.071 |
.045 |
.033 |
|
100 |
.068 |
.043 |
.031 |
|
110 |
.065 |
.041 |
.030 |
|
120 |
.062 |
.039 |
.029 |
|
130 |
.060 |
.038 |
.028 |
|
140 |
.058 |
.037 |
.027 |
|
150 |
.056 |
.036 |
.026 |
|
160 |
.055 |
.035 |
.026 |
|
170 |
.054 |
.034 |
.025 |
|
180 |
.052 |
.033 |
.024 |
|
190 |
.051 |
.032 |
.024 |
|
200 |
.050 |
.031 |
.023 |