Critical Flow Orifices

copy-file

This note reviews my findings on orifice restriction of gas flow.

Theory: the downstream speed of a gas through an opening

is governed by Bernouillis theorem under the conditions of

adiabatic flow. The discharge rate reaches a maximum

however, when the pressure ration p2/p1 reaches a value rc

such that on substitution in the velocity formula yields the

velocity of sound. In c.g.s units the maximum flow is

where w is grams/sec, S2 is the aperture area, C is approx. 0.6,

a discharge coefficient, p1 the upstream pressure in dynes/cm²

[gamma] is Cp/C[gamma] = 1/4 for air, M is the molecular weight of air (29),

R is the gas contant per mole, 8.3 x 107 erg-deg-1.

Substitution of the above numbers yields

For example, if w1 is to be 100 cm³/min = 1.67 cm³/sec, then

the aperture area should be

the diameter is

Practice: The material on orifices in Chem. Eng. Handbook includes

comments about the importance of shape of orifices in determining

the flow. It would appear that the orifices should be constructed

with diameters of the opening of the order of those calculated

and tested under pressure conditions comparable to those expected,

ie atmospheric on one side, low pressure on the other.