Fox Sonic Chokes

Fox Sonic Choke Diagram

Flow Regulation with No Moving Parts

Fox critical flow venturies or sonic chokes are used to maintain constant, stable gas flow rates when discharge pressure or conditions can vary. The sonic choke establishes constant flow rate unaffected by ΔP. With no moving parts, chokes are the simplest way to regulate gas flows. This is particularly true when high pressure, high temperature, cryogenic, explosive, or high purity gasses need to be regulated accurately.

Fox Flange Insert Sonic Choke
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How does Pressure Drop or ∆P Affect Flow? It Doesn’t.

As long as discharge pressure is below about 88% of upstream pressure (in psia), backpressure has 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 psia, and will deliver stable, uniform, accurate, unchanging gas flow rates.

Accuracy

Fox sonic chokes can be calibrated to ±0.25% traceable to NBS. Theoretical calibrations are accurate to ±2%.

Simplifying Gas Flow Control by Eliminating Flow Meters, and Control Valves

Systems designers often think that the only way to maintain uniform gas flow rates into a process is to provide for continuous flow management via a flowmeter, a process controller, and a control valve. If inlet pressure to the sonic chokes can be regulated accurately, all of the above equipment can be replaced with one simple sonic choke.

Fox Sonic Choke

Mechanical Properties of Fox Sonic Chokes

  • Line Sizes: 1/4 to 42 inch diameter (6mm to ~1m)
  • Temps: -350° to 2000° F (to 1090° C)
  • Materials:
    • Metals: Stainless, Monel, Hastelloy, Titanium, Alloy 20, Aluminum, or Any Machinable Metal
    • Plastics: PVC, CPVC, Teflon-Lined, Kynar-Lined
  • MAWP available to 10,000 psig (700 barg)
  • End Connections:
    • Threads: NPT, BSP, SAE 45° Flare, AN 37° Flare
    • Flanges: 150# to 3000# ANSI, DIN Flanges, or any custom-built flange to mate to any equipment
    • Tubing: Stub ends of any specified tubing, inch or mm
    • Food-Grade: Tri-clamp or other sanitary/hygienic ferrules
    • Weld: Butt weld prep ends
  • Available Processes: Electropolish, oxygen cleaning, passivation, anodizing, etc.

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
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