Fox Sonic Choke, Laval Nozzle or Critical Flow Venturi
Since 1961, Fox critical flow nozzles, also called sonic chokes, have been used to maintain stable gas flow rates in systems where maintaining stable, accurate gas flow rates is essential to overall system performance. By establishing a shock wave in the venturi, the sonic choke establishes fixed flow rates unaffected by ΔP or any fluctuations, surges, or changes in downstream pressure.
With no moving parts, sonic chokes provide the simplest and most reliable way to regulate gas flows. This is particularly true when high pressure, high temperature, cryogenic, explosive, or high purity gasses need to be regulated.
Industry uses many different names to describe this equipment. Here is a partial list of descriptions that can be used interchangeably:
- Sonic choke
- Laval Nozzle
- CVM (Critical Venturi Meter)
- Sonic Nozzle
- Critical Flow Venturi
Sonic chokes are converging/diverging nozzles whose operating principles are discussed in every fluid mechanics textbook. They offer the designer a remarkably simple way to regulate stable gas flow rates.
Simplifying Gas Flow Control by Eliminating Flow Meters, Feedback, and Control Valves
Systems designers often think that the only way to maintain uniform gas flow rates into a process where backpressure may fluctuate is to provide for continuous flow management via a) a flowmeter, which sends a signal to b) a process controller, which in turn adjusts c) a control valve. The aerospace industry, which cannot afford the weight and complications of such a system, embraced critical flow Venturis thirty years ago as the simplest and most reliable way to regulate gas flow rates. As long as inlet pressure to the sonic chokes can be regulated accurately, all of the above equipment can be replaced with one simple sonic choke. A simple, standard piece of equipment—a pressure regulator—has, when coupled with a sonic choke, become a gas flow regulator.
How does Pressure Drop Affect Flow?
It doesn’t. As long as discharge pressure is below about 88% of upstream pressure (in psia), backpressure has absolutely no effect on flow rate. Downstream pressure can fluctuate wildly with no impact on gas flow rates. This limitation is referred to as the ‘recovery’ of a choke. For Fox sonic chokes, recovery can be assumed to be 88- 90%. For example, with a fixed inlet pressure of 100 psia, a sonic choke can discharge into a backpressure that cycles abruptly from 20 to 80 to 20 psig, and will deliver stable, uniform, accurate, unchanging gas flow rates.
Because they are machined from solid bar, Fox chokes are often used in gas lines at 3000 psig or higher. With no moving parts, they are the ideal way to regulate gas flows that are very hot, or very cold. Fox chokes have been used with gasses up to 1500° F, and with Helium at – 400° F.
Fox sonic chokes can be calibrated to ±0.25% traceable to NBS. Theoretical calibrations are accurate to ±2%. There is no reason to ever install a flowmeter downstream of a sonic choke. The flow has been regulated with such accuracy that it does not need to be measured. Simple corrections enable calibration data to be converted to other temperatures and pressures.