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Airflow through an Orifice

REFERENCE

The following table shows estimated flowrates for air through various size orifices at several supply pressures. These estimated flowrates refer to air discharging to

normal atmospheric pressure at 70°F through an "average" shape orifice with a smooth finished surface. It is estimated that this data is accurate to ±15%.

ORIFICE		AIRFLOW (standard cubic feet per hour - scfh)
Diameter (in)	Area (in²)	@ 20 psig	@ 25 psig	@ 30 psig	@ 45 psig	@ 75 psig	@ 115 psig
0.02	0.00031	4.8	6	7.2	10.8	18.1	27.7
0.03	0.0007	10.8	13.6	16.3	24.4	40.7	62.3
0.04	0.00126	19.3	24.1	28.9	43.4	72.3	110.8
0.05	0.00196	30.1	37.7	45.2	67.8	113	173.2
0.06	0.00283	43.3	54.2	65.1	97.6	162.6	249.4

CORRECTION FACTOR FOR GASES OTHER THAN AIR To correct for gases other than air, the following factors need to be calculated and the obtained value then multiplied by the flow rate for air. Correction factor for gas "x" = Square Root (28.96/molecular weight of gas*) Flow rate for gas "x" = (flow rate for air) x (correction factor for gas "x") * If the gas is a mixture, a proportional weighted average of molecular weights should be used.

SAMPLE CALCULATION USING HYDROGEN CHLORIDE A. Find the flow of air through a 0.02 orifice   Airflow in scfh = 767 x (orifice area in square inches) x (supply pressure of HCl in psig)     = 767 x 0.00031 x 619.7     = 149.32 scfh B. Calculate the correction factor for HCl versus air   Correction factor for HCl = Square Root (28.96 /molecular weight of HCl)     = Square Root (28.96 /36.46)     = 0.89 C. Correct flow for HCl   Flowrate for HCl = (flowrate for air) x (correction factor for HCl)     = (149.32) x (0.89)     = 133 scfh