Choked Flow vs. Critical Flow 1

[Melon00]

Can anyone explain the difference between choked flow and critical flow, if there is one?

 

[JLSeagull]

Several threads have discussed choked flow. Think in terms of sonic velicity.

We discuss critical pressure and critical temperature related to fluid phase limits. We also discuss critical speeds for large machinery. Please advise the context for critical flow.

 

[Melon00]

From what I see on the threads you speak of, the difference is that choked flow is when no more flow will go through your valve when decreasing the downstream pressure, with fixed inlet conditions, and critical flow is attributed with a pressure drop ratio (dP/P1) of 0.5 or higher.

Correct?

 

[JLSeagull]

I don't know the term critical flow. The Fisher Control Valve Handbook uses "critical flow" once in a paragraph as follows:
“…Although in actual service, pressure drop ratios can, and often will, exceed the indicated critical values, this is the point where critical flow conditions develop. Thus, for a constant P1, decreasing P2 (i.e., increasing P) will not result in an increase in the flow rate through the valve. Values of x, therefore, greater than the product of either FkxT or FkxTP must never be substituted in the expression for Y. This means that Y can never be less than 0.667. This same limit on values of x also applies to the flow equations that are introduced in the next section. …”

 

[israelkk]

When the pressure drop ratio is ~0.5 or higher the flow will no longer increse through valve/orifice (as long as the upstreem pressure is constant) even if the downstream pressure decreass. At this condition the flow is choked because the gas velocity is equal to sonic velocity.

 

[Melon00]

Okay, good. It sounds like critical flow is the same as choked flow. I was guessing that was the case, from the of the terms "choked" and "critical" used interchangeably in various texts.

 

[rmw]

The pressure drop ratio that will result in choked flow is referred to as the critical pressure drop. It sounds like some word shortning has gone on where "flow at critical pressure drop" has become "critical flow".

rmw

 

[ronfrend]

It seems as if we have been making a simple issue over-complicated. I have included a slide from one of our training courses explaining this issue.

Put simply, choked flow is that flow at which you cannot increase the flow by increasing dP. Flow below choked is called SUB-CRITICAL, flow above choked is called CRITICAL.

Hope this helps,

Ron

Ron Frend

http://www.predicon.net

 

[sterl]

Choke refers to the device: An orifice can be considered Choked if a reduction in downstream pressure will not increase flow.

Critical refers to the fluid pressures (usually ratio) at the point at which the device becomes "choked"...For gases, the consequence of critical pressure difference is that a reduction in downstream pressure does not increase the flow. The term critical velocity is also used, and its deviation from the stagnant speed of sound gives some indication of the localized departure from predicted velocity due to the geometry of the device that is choked.

 

[zdas04]

While this thread is just about beaten to death, there is one point in the answers above that is not quite correct. The "critical pressure ratio" is not equal to 0.5 except in a very rare coincidence. the ratio is:

P(crit)=P(upstream)*(2/(k+1))^((k/(k-1))

[both pressures in absolute terms]

This equation usually gives you a P(crit) greater than 1/2 P(upstream), so the 0.5 number is conservative and close enough for most applications (and it is easier to remember than 0.54 for a typical natural gas or 0.53 for air, but other gases can be much higher).

David