Fox Sonic Choke 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 P 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:

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 venturies 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.

Pressure/Temperature Ratings
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.

Accuracy
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.

Typical Applications of Sonic Chokes

Controlling Conveying Air Flow Rate in Dense Phase Pneumatic Transport Systems
Fox sonic chokes have been incorporated into hundreds of dense phase pneumatic conveying systems. In these systems, it is essential that air flow rates are both controlled accurately and remain fixed and stable, even if downstream pressures due to changing product or flow rates (sugar, sand, coal, etc.) vary. The sonic choke must, therefore, maintain uniform air feed rates, and hence uniform, controlled transport velocities, that are unaffected by any changes in product rates or characteristics.

Ultra-High Purity Gas Systems
Traditional flow meters used by the industrial gas industry would contaminate the ultra high purity gasses needed by the semiconductor and other industries. When filling customer tanks without a sonic choke, gas feed rate vary constantly as P varies. With a sonic choke, which are easily provided in ultra-high purity configurations, flow rates are easily controlled and the lack of moving parts eliminates the chance of contamination.

Rocket Engines, Chemical Lasers, and Superconductors
Fox sonic chokes have been used in these three applications for decades. Aerospace - Chokes are typically used to control hydrogen and oxygen, the fuel and oxidizer, in test-stand rocket engine firings as well as flight hardware. Pressure ratings are frequently 4000 psig or higher. Chemical Lasers similarly require precise flow control of very high pressure gasses. Airborne lasers require light weight flow controls, for which sonic chokes are ideal. Superconductivity research often requires controlling helium or hydrogen gas flow rates at below -300° F, which is quite simple for chokes.

Calibration Standard
Because sonic chokes are so simple, they have been preferred for years as a reference standard. One calibrated choke can be used as a reference to which other, less reliable flowmeters can be checked.