Critical Orifices for Flow Control

Control of aerosol flow is necessary to achieve a desired exposure dose and to measure that dose through sampling. A relatively simple way to produce a constant, known flow rate is to use a critical orifice, through which the gas flows at the speed of sound. This is often helpful in stabilizing the flow rate of a sampling flow, such as the flow to a filter or to an optical particle counter or to an impactor for determining the particle concentration in an inhalation exposure chamber.

Air flowing through an orifice will reach the speed of sound in the orifice if the downstream pressure is less than half the upstream pressure. [These pressures are measured from zero, the absolute scale, so that atmospheric pressure is P=1atm or P=14.7lb/in², etc.] The use of a critical orifice is described in many books, including Aerosol Technology, by W. C. Hinds (1999). If the downstream pressure drops below half the upstream pressure, the flow rate will be unaffected. If the upstream conditions change, the flow will change, because the speed of sound is affected by pressure, temperature, and density. If a filter is upstream from the orifice and if the pressure drop across the filter is much smaller than half the upstream pressure, then the change in the flow rate will be fairly small. More details are given in the Chapter 22, "Instrument Calibration," by B. T. Chen, in the book Aerosol Measurement, edited by K. Willeke and P.A. Baron (1993).

Using equations available in either of the books mentioned above, one can find that the volume flow rate, Q (liters / minute, lpm) is related to the area of the circular orifice, Ao, by the equation:

Q (lpm) = 7.2 Ao(mm²).

This equation indicates the following hole diameters (d) would be needed for the indicated flow rates:

Drilling such small holes in thin material is a challenge, one which a supplier for CH Technologies has solved, using laser techniques.

With holes this small, it is advisable to protect the orifice from becoming clogged by using a filter upstream from it, where possible.

References

W. C. Hinds, Aerosol Technology, Wiley, New York, 1999.

K. Willeke and P. A. Baron, Aerosol Measurement, Van Nostrand Reinhold, New York, 1993.

by Douglas W. Cooper, Ph.D.