Air Flow Formula Help

I need to know how many CFMs I will get with 150psi through a fitting with a 1/8th inch hole. The hose is about 2 feet long, if it matters.

Thanks

Reply to
bgreer5050
Loading thread data ...
32.4 CFM

google "air flow orifice"

DJ

Reply to
Mechanical Magic

Why 150 psi?

Bringing unregulated air to the orifice from a high pressure main may be creating an unnecessary saftey hazard for those near the orifice.

This would be a "choked flow" situation. Also known as "sonic flow".

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Choked flow is a limiting condition which occurs when the mass flux will not increase with a further decrease in the downstream pressure environment.

Or, if the downstream pressure is fixed at atmospheric pressure: the flow rate will not increase with a further increase in upstream pressure,

From:

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The critical pressure ratio is 0.5283

Flow through the orifice will increase until the upstream pressure becomes about:

14.7 psia / 0.5283 = 27.8 psia = 13.1 psig

So bringing an air pressure higher than about 15 psig to the upstream side of the orifice may be needlessly dangerous.

Actually, the post said a "hole" in a "fitting" -- not an orifice. It suggests a rough-edged opening -- with stress-concentration points. Bringing 150 psi to a jagged hole which may develop tears and allow the fitting to fly apart could be a really bad idea.

Adam

Reply to
Adam

This is just rubbish.

Reply to
me

Non-choked flow:

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Choked-flow:
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The problem in Adam's logic is that "locking outlet pressure to atmospheric and raising inlet pressure" is not the same as "dropping outlet pressure further (if possible)". This is a compressible gas, which inlet pressure directly effects, so the gas presented to the orifice has different properties when presented to the orifice.

David A. Smith

Reply to
N:dlzc D:aol T:com (dlzc)

Aha.

150 psi input = ~165 psia input

Pressure where the flow is sonic = 165 psia * 0.5283 = ~87 psia

Ambient pressure: ~15 psia

No diverging nozzle section implied.

Shock wave(s) between 87 psia and 15 psia.

Thanks, Adam

Reply to
Adam

Such a succinct reply.

Not informative beyond condemning, but succinct.

Still a valid question: Why 150 psi?

What is trying to be accomplished that _needs_ 150 psi air to the hole? Is it prudent to use unregulated air in this application? It may be expedient to avoid the cost of a regulator, but at what risk to the safety of those who will operate whatever is being designed?

Adam

Reply to
Adam

No, the minimum supply pressure where the flow is sonic is 15/0.5283 =

28.4psia.

For any supply pressure above this the flow rate is then just a linear function of the supply pressure.

Reply to
me

Trying to generous this holiday season.

Because that's what the OP said the supply pressure was.

Why do you assume 150 psia isn't regulated. On a daily basis I work with regulated air supplies of 100, 600, 1000 and 5000 psia. All of these systems are "regulated."

A hole in a fitting IS an orifice. It doesn not at all imply there are ragged edges. One can drill or ream a hole and even finish it such that there are no sharp edges. In fact we do just this at work to create inexpensive choked orifices in gas systems to provide an easily controlled flow rate of gas, be it air, hydrogen, oxygen, ethylene, etc. Choked orifices and venturis are used specifically for just this purpose all over the place. Once choked, the flow rate through the orifice or venturi is simply a linear function of the supply pressure, neglecting changes in supply temperature.

You are so worried about 150 psi and a hole in a fitting, why didn't you even ask about the pressure rating of the hose?

Reply to
me

You got a reasonable answer within three hours - without vituperation, and with a pointer thrown in from DJ. It may not be exact - but it must be in the ball park. I got a number a little higher than DJ's, on the back of an envelope so it's not that wild....

Brian W

Reply to
Brian Whatcott

Brian, Thank you for the comment. My answer was not meant to be accurate, to six decimal places. The problem was obviously posed by someone with limited knowledge of flow dynamics. My answer was intended to be appropriate for the poster. Since the OP has not been back, I must assume this was a needed WAG number. (I think I did 15 seconds google.)

I find it rather interesting that the group bantered, rather than asking for actual specifications. Every group is different. Most are interesting. DJ

Reply to
Mechanical Magic

"No" suggests disagreement.

The OP specified 150 psi.

The flow is sonic through the hole in their fitting.

The air pressure where the flow is sonic through the hole is ~87 psia.

With what are you disagreeing?

Adam

Reply to
Adam

They got a numerical answer.

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abusive or venomous language used to express blame or censure or bitter deep-seated ill will

A question was posted without context by someone of unknown background.

How is it vituperation to encourage prudence in a reply?

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Engineering: The application of scientific, physical, mechanical and mathematical principles to design processes, products and structures that improve the quality of life.

I submit that if the fitting blows up in the OP's face because he applied more pressure than necessary to a weakened fitting, then a numerical answer to the terse question didn't serve to "improve the quality of life" for the OP.

After the rancor generated from an attempt to encourage prudence, I checked the OP's posting history:

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Results 1 - 4 of 4 for group:sci.engr.mech author: snipped-for-privacy@yahoo.com

The OP's post in this thread didn't say _air_.

This post suggests that he might have meant argon:

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What I have read thus far indicates it is critical to weld in an Argon atmosphere so I have made a little manifold to flood the entire piece in Argon.

If the OP meant 150 psi of argon, maybe the numerical answer by DJ needs some tweaking.

As I read it, we are dealing with someone whose primary experience isn't in building high-pressure gas systems. That was also suggested by the need to ask the question that opened this thread.

Efforts to keeping him from hurting himself or others might be useful.

Merry Christmas. And a new year with all your fingers.

Adam

Reply to
Adam

Doesn't the question "Why 150 psi?" come under the heading of "asking for actual specifications"?

Other info that might have been helpful:

  1. 150 psi of what gas
  2. Temperature

It seems that those were assumed in the only answer that he got.

Indeed.

Adam

Reply to
Adam

The part I quoted you back, which you conveniently edited out. The flow does NOT become sonic when the supply pressure is 87 psia as you again state incorrectly. The flow through the orifice becomes choked when the pressure ratio across it becomes 0.5283 or less. Since the ehaust/downstream side is atmospheric, the flow becomes choked with the pressure upstream of the orifice reaches 14.7/0.5283 = 28.4 psia. It remains choked for all pressures above this. It would behove you to actually read what folks write.

In fact you later write "The OP's post in this thread didn't say _air_." READ the subject. What gas does it state? You needn't be so obtuse.

Reply to
me

**
** [Adam]
*** Adam seems to have attitude to spare - but he's a little short on aptitude possibly?

Brian W

Reply to
Brian Whatcott

Let's say it's a bit difficult to take someone's off topic comments seriously, when their on topic comments don't hold any water.

Reply to
me

I didn't say anything about the supply pressure being 87 psia. I said "where" not "when".

My post didn't address the issue of when the flow _becomes_ sonic as the upstream pressure increases. I gave the pressure which exists at the throat of a nozzle when the flow _is_ sonic at the throat and the upstream pressure is 165 psia -- for _any_ downstream pressure less than about 87 psia.

What pressure profile would you plot in the direction of air flow as it goes from 165 psia to atmospheric? Mine would have a short but smooth transition from 165 to 87 psia -- with 87 psia existing at the location of the hole -- and then an abrupt drop to atmospheric pressure.

After the fashion of Figure 1 on this page:

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or the image near the bottom of this page:

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Except there is no suggestion that the OP's apparatus has a diverging section; the pressure must make an uncontrolled drop from 87 psia to atmospheric.

How would your pressure profile differ?

OK.

Where do you expect an air pressure of 28.4 psia to exist in the OP's apparatus when it is operating at 150 psi?

I don't expect the pressure of 28.4 psia to exist within his apparatus at operating conditions. So I didn't see the point of mentioning it

-- especially after it was linked with "rubbish" when I mentioned it as the point where choked flow begins! You said better things than I did in my first post about what happens when the critical pressure is exceeded, but this limit is well below where the OP plans to operate his apparatus.

On the other hand, I do expect an air pressure of about 87 psia to exist in the hole when 150 psi is applied to the OP's fitting.

What is the pressure at the throat of a nozzle in choked flow when the upstream reservoir contains air at 165 psia?

Is it not ~87 psia?

Is that not the pressure in the OP's hole?

The question remains: With what were you disagreeing when you replied to my post with "No"?

The part that I snipped in my last reply went on to say:

Those observations seemed irrelevant when I snipped them. They still do. Nor are they contrary to what I said.

I did the "mea culpa" thing after misapplying the critical pressure ratio in my first post. It occurs to me that it wouldn't be inappropriate for carpers to come forth with their own mea culpas at this point.

This also applies to your statement above: "... when the supply pressure is 87 psia as you again state". In fact, I stated no such thing.

Indeed, the subject line does say _air_ formula.

It is bad form to have the subject line be the only source of relevant info for a post. It is much better for all relevant info to be included in the body of the message.

Others have said similar:

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Some users put their complete question in the subject line. Also a BAD, BAD idea. A subject line is the "appetizer" for other people to read your message. But the actual CONTENT should (read: has to) be in the BODY.

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it's just plain laziness and rudeness to have a sentence that begins in the subject line and ends in the body of the posting.

So we have a poorly-constructed, terse post with no context asking about a field-modification made (with an unknown level of care) to a fitting (of unknown material, weight or condition) that is to be used (for who knows how long) at a non-trivial pressure.

I didn't see evidence that the question was well-considered or that there would be meticulous attention to safety once an answer was in hand.

So 150 psi is not on par with the 5000 psi systems that you deal with in an industrial setting where there is adequate safety training. But

150 psi is still high enough to do serious damage if something goes wrong.

If the OP is actually using argon, do you suppose that the subject line would have said "Argon Flow Formula Help"?

I don't. Not based on the level of effort put into the construction of the post nor the OP's posting history -- gas flow doesn't seem to be his main area of interest. That the flow rate for argon is different from that for air may or may not occur to him.

The volume flow rate through the OP's hole would be lower for argon than for air at the same conditions. Is it lower by a significant amount? We don't have enough info to know what would be "significant" in this application.

But the subject line did say "_air_ formula", so we go with that.

You needn't be so condemning.

My main issue here is this: A machine might answer a question that is put to it. An engineer considers whether he is being asked the right question.

In my book, engineering involves more than having a knee-jerk reaction to spew a textbook answer to whatever looks like a textbook problem.

The OP's question was _not_ a textbook problem. It was about a real situation where real people can suffer real harm if something thrown together without adequate thought blows up.

I haven't found chapter and verse, but using 150 psi air where it might come into contact with an operator's skin is dangerous:

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U.S. Federal O.S.H.A. does not allow sources of compressed air where the dead-end pressure (compressed air pressure that comes in contact against your skin) to exceed 30 p.s.i.

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I still recommend to my clients that the back pressure relief type nozzles be used as it is a good safeguard from inadvertent dead end contact with nozzle.

We don't know the setting in which the OP's apparatus is to be used.

I was harangued for asking whether 150 psi might be excessive pressure for the job at hand.

I maintain that it is a reasonable question.

And it has gone unanswered.

The answer could well be that 150 psi is exactly the right pressure for the job at hand.

At the moment, we don't know whether 150 psi is overkill for the OP's application.

But we do know that:

  1. The OP received a numerical answer to his question.
  2. The OP hasn't posted again since then.

Hopefully, that isn't because something went awry when 150 psi air was applied to whatever he was building.

Adam

Reply to
Adam

Or maybe I have plenty of both.

Here is the Reader's Digest version of my other post:

A machine might answer a question that is put to it. An engineer considers whether he is being asked the right question.

Adam

Reply to
Adam

I am going to give you one try; perhaps you are blanking on the original explanation, repeated two or three times for you.

At a pressure over 28 psi abs, the variation of flow rate with source pressure is linear. Although the profile is not knowable from the data presented, and we can be reasonably sure it is not isentropic at least, still you can say that the pressure drops from 165 psia shortly before the aperture, to 15 psia shortly after the aperture. There is a shock wave "choking' the flow til the pressure in the aperture's channel drops below 28psia which is very roughly near the tail end of the aperture channel.

OK?

Brian W

Reply to
Brian Whatcott

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