Air compressor flow rate 1

[ctran]

Can anybody help me to figure out how to check on the capacity of an air compressor? The spec required is 223 ACFM at 125 psig. We used the orifice to measure the flow rate but it fails. The manufacture suggested to fill the system up to 125 psig then record the vent time. This method pass the spec, but somehow I still think it isn't correct. Do they try to cheat on me?
Really appreciate for your help!

 

[StoneCold]

ctran
Maybe the problem is similar to my own dispute with the air compressor vendor. The air compressor is rated at 223ACFM at 125psig. The ACFM they are talking about is on the suction side of the compressor, not the discharge. So the unit will really produce about 26 ACFM at 125psig on the discharge. Which to me is completely dis-honest way to measure the flowrate but that is how it is done.

Hope this helps you out.

Regards
StoneCold

 

[zdas04]

If you start off thinking that the manufacturer is cheating you, then you'll have a really hard time ever understanding what is going on (you can expect the manufacturer to go into "lawyer" mode pretty early).

The capacity of the compressor will be a function of discharge pressure, so you can't just fill a receiver (since the capacity will be different at the start than at the end).

One way to do it is to put a 125 psig backpressure regulator between the compressor and the receiver. Then isolate the outlet of the receiver and blow it down. Put a high quality electronic recording gauge on the receiver and fill it (make sure you also record temperature). The ideal gas law will give you the change in mass between each pressure point and the next (I would convert all the volumes to STP to let apples be compared to apples, but converting to mass also works).

That should give you very accurate and repeatable flow rate data.

David

 

[zdas04]

Stonecold,
There is nothing dishonest about it. The Air Compressor industry chose to rate their equipment based on volume flow rate at suction conditions many decades ago. For a compressor inside a building near sea level, suction ACFM is pretty close to the same as SCFM. In that case a compressor with a suction flow of 223 ACFM would (and does) have a discharge flow of really close to 223 SCFM.

The "problem" is that you were not conversant with the industry terminology and seem to have convinced yourself that you are being ripped off. You (as the specifying engineer) have an obligation to understand the terminology of the equipment you are specifying. If you needed 223 ACFM at 125 psig you should have specified a machine that could produce 1900 SCFM (or ACFM at suction conditions).

The manufacturer had the right to expect that an engineer with responsibility for purchasing an air compressor to understand his actual needs and call for a machine that could satisfy those needs.

David

 

[TD2K]

I'm with David, a "1500 cfm compressor" will typically mean 1500 acfm at inlet conditions.

 

[ctran]

On the contract they put "223ACFM at 125 psig". This makes people think of discharge more than suction. If I say 223 ACFM at suction, it couldn't be at 125 psig. I still think this is a manufacture error. Any idea?

 

[zdas04]

It is simply operator (specifier) error. The compressor capacity is a function of discharge pressure. If it can make 223 ACFM producing into 125 psig then it probably can make 350 ACFM discharging into 50 psig. Their terminology may be confusing, but it complies with industry norms.

By the way, I am not a compressor manufacturer, salesman, or in any way connected with the supply side of the compresison business. I am a compressor user and find your "let's find someone to blame for my screw up" attitude to be the source of your problems.

David

 

[StoneCold]

zdas04
I think that it would be clearer if they used SCFM of gas through the compressor. The ACFM @###psig makes you believe that the flowrate is at the stated pressure. Luckily my run in with this worked out fine other than re calibrating a meter twice, but it is a confusing terminology. I am not blaming anyone just wanted to point this out to ctran before he went off on a bender that was really just a terminology issue.


Regards
StoneCold

 

[zdas04]

It would be better if they used mass flow rate, but they don't. Every industry has terminology that sucks (I work in Oil & Gas and sometime in the dark ages they started using "M" for "thousand" and "MM" for million, now when the rest of the world uses "k" for thousand and "M" for million you never know what you have), but you just have to adapt to it. I'm sure that when you saw "223 ACFM at 125 psig" you thought you had a clear understanding of the parameters, it sucks that the convention was different, but that is the way it is. Sometimes it is hard to know what the heck the question should be or even that there is a question.

If you specify your compressor in SCFM or lbm/hr then if the vendor pretends not to know what you're talking about you have a basis for discussion. You could have also said you need "1900 ACFM at inlet conditions, with an operating discharge pressure of 125 psig", but you didn't know that you'd need a translator to order a danged little air compressor. The salesmen tend to be very knowledgeable on this subject, but very unhelpful (and I'm not sure why).

This particular confusion is why all purchase/sale agreements for industrial gases are either in lbm or SCF (which is really a surrogate for lbm), moles would be better but that is rare.

David

 

[JimCasey]

The air is inhaled through the intake valve and then compressed and expelled through the outlet valve.
Any air that is not expelled at the TDC pressure of the piston stroke re-expands on the inlet stroke and partially displaces incoming air. Thus the capacity of inhaled air drops off as the pressure increases. At some point the flow through the compressor will cease, as the discharge pressure system is pressurized to the peak compression pressure. All the piston would do at that point is to squeeze the same slug of air and then be driven back down as the air expands. No work done. It would be somewhat like a vacuum-cleaner speeding up when you plug the hose.

So it is necessary to state what the flowrate through the compressor is, but only meaningful at a specific developed pressure corresponding to that flow-rate. Actual cubic feet are inhaled, regardless of altitude or ambient conditions. So converting everything to SCFM or mass units makes the calculations easier, but may be misleading.

 

[Niwot]

Obviously, ACFM and SCFM are only the same at atmospheric condtions (example: 14.7 PSIA and 60F).
For what its worth, 223 ACFM @ 125 PSIG is equivalent to roughly 2000 SCFM @ 125 PSIG.

Having been the referee in disputes relating to compressor ratings/efficiency, pump curves, fan curves, etc., I will stay out of this one. There are many articles from independent sources that explain compressor ratings and what to expect in real world conditions.

 

[zdas04]

Niwot,
I see this is your first post at eng-tips.com. Welcome.

It is generally considered polite at this and most other sites to read the thread before commenting. Everything in your post has been said above, repeating it as though it has extra weight by coming from you is less effective than you might think.

David

 

[RotaryG]

Apoogies for butting in late...

If Ctran "intended" to buy a "223ACFM at 125 psig" compressor, then Ctran should have noticed (as a responsible purchasing engineer) that the motor rating offered bt the compressor vendor is too low. He then should have corrected the misunderstanding.

If Ctran "intended" to buy a "223ACFM at suction pressure", but the manufacturer mentioned "223ACFM at 125 psig" and if rest of the things are fine (such as the motor rating) Ctran does not have a reason to comply - except that he is dealing with shoddy manufacturers.

In either case, Ctran, should have noticed the "error" before purchase and taken appropriate action. I would say, manufcaturer has not pulled a fast one on Ctran...

I do not represent manufacturers..