By Gary Wu
Mechanical Engineer
Millipore Corp.
Allen, Tex.
EDITED BY VICTORIA REITZ
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Flow rate versus upstream pressure for a 0.005-in.-diameter orifice. When upstream pressure is lower than 27.84 psia, the curve is no longer perfectly linear. |
Most mass-flow controllers are thermal based. That is, they use the thermal properties of a gas to directly measure the mass-flow rate. But for many control situations — such as when downstream pressure is too low, allowable pressure drop is too small, or gas (vapor) properties change — pressurebased controllers which use choked-flow phenomenon are the better choice.
Understanding the basic principles of choked flow at the valve opening lets designers take advantage of some built-in properties.
CHOKED FLOW BASICS
When gas travels through an orifice the flow velocity at the opening depends on upstream and downstream pressures Pu and Pd. Increasing the upstream pressure or decreasing the downstream pressure will increase velocity until it reaches sonic speed. When Pd /Pu < 0.528, pressure equals absolute pressure and increasing the upstream pressure or decreasing the downstream pressure further will not increase the velocity at the orifice. This point is called choked flow. For choked flow, the flow rate is a linear function of the upstream pressure, which changes the gas density. Downstream pressure changes do not affect the flow velocity and flow rate. Unless a specially shaped nozzle is used, the downstream gas velocity cannot surpass sonic speed. Downstream disturbances cannot travel backwards fast enough to influence the upstream flow. By controlling upstream pressure, the valve can deliver gas with a stable flow rate. A ball in the valve opening, for instance, can be adjusted to control upstream pressure.
CALCULATING RESPONSE
Designers have two options when dealing with choked-flow valves. They can calculate response time for the pressure change from initial pressure Pi to final pressure Pf based on the valve design, or design the valve to obtain a certain response time.
The response time is related to the dead volume V0 inside the valve, the orifice flow rate, and the initial and final pressures Pi and Pf.
First, transfer the gas in the dead volume V0 to the standard condition (14.7 psia and 70°F) volume Qi.
Some gas will then flow out of or into the valve with a flow rate •m, which is a function of P. In addition, P is a function of time t.
Assuming that completely closing the valve decreases the pressure and completely opening the valve raises the pressure, the standard gas volume is
The pressure change in the chamber depends on Q. It can be expressed as
Substitute the pressure change into the gas volume
Differentiating the equation gives
or
Integrate the equation to produce
Use the initial conditions, t = 0 and Q = Qi, to get C = lnQi. The equation can be written in terms of time
Substituting Qi from the first equation into the response time results in
To find the time needed for the pressure to drop from Pi to Pf, the equation can be expressed as
If the response time is known, dead volume of the valve is given as
In the case of a pressure increase, the equations become