Air Compressor Sizing & Derating

[Krausen]

Gentlemen,
I am attempting to size a portable air compressor that will be used to push hydrotest water through a pipeline in order to move the water to another hydrotest segment (mostly uphill). Not having much experience in air compressor sizing, I went back to the basics as I understood. I estimated the inlet flow (ICFM) based on the polytropic thermo process P1*V1^n = P2*V2^n for a closed system. The delivered air (V2) would need flow at 50 cfm at a pressure of 175 psia (P2) to adequately move the water. P2 was estimated based on the worst-case total backpressure the compressor should see during operation (actual backpressure will vary based on location of the water-air interface). I assumed n = 1.25 as a typical polytropic constant for efficient recip compressors. For P1 I calculated 11.2 psia for the atmospheric pressure at 7000 ft, but understand this may need to be lowered due to the air filter on the intake side. The calc resulted in a compresssor ICFM (V1) = 450.

Does this calc make sense?

FYI, I was assuming these portable compressors would be reciprocating but I've learned many of the larger available ones are rotary screw types (not recips). Not sure how this will effect the process or polytropic constant . Due to the extreme site elevation, I'm also worried about how to derate the compressor & driver (likely a diesel engine).

Thank you in advance for any help on this.

 

[LittleInch]

Err no it doesn't add up to me.

At the pressures and temp you're talking about air is pretty close to ideal, hence V in this instance is 781. I didn't get the reason why you end up with a much smaller volume

You also ignore temperature which isn't valid unless you've got a big air cooler d/s your compressor.

There are many threads on here that talk about the wide and varied way in which vendors state volumes that their air compressor produces. Whilst many work on "standard" or inlet air volumes, there are many different standards and inlets which normally assume sea level conditions or they are quoted at a pressure, but then this normally assumes inlet at some "standard" condition.


best bet is to talk to some local suppliers. Tell them what you need (450cfm at 175 psia) and let them give you a machine which works, ditto for the driver. They've all been there and done it, but if you're bringing it in from somewhere else they might not get it.

A few of the more interesting posts are below, but if you search a bit more you'll find many others.

Zdas04 is particularly knowledgeable about air compressors at altitude so if you come across some of his posts then take heed.

http://www.eng-tips.com/viewthread.cfm?qid=44986

http://www.eng-tips.com/viewthread.cfm?qid=366707

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Krausen]

Little Inch - The smaller V1 volume (450 cfm) came from applying the polytropic exponent constant (n = 1.25) in both terms. I was under the impression that P1*V1^n = P2*V2^n was the governing equation, but I have not seen this referenced anywhere else so far. It appears most folks simply apply the P1*V1 = P2*V2 ideal gas relationship (ignoring temperature) and work directly with the vendor to size the compressor correctly.

I was trying to nail down the specifics of the compression process and equipment inefficiencies, but it appears this is unnecessary in the grand scheme. So long as the air compressor can do what it's rated to do. I am surprised to read about all the confusion surrounding compressor ratings, whether it's rated for scfm/icfm flow on the suction side or acfm/cfm FAD flow on the discharge side, etc. It all seems a little overkill to me, but I'm pretty inexperienced with compressors. In the end, all I need is the rated max discharge pressure and corresponding discharge cfm based on its location.

 

[LittleInch]

In fact temperature is assumed to be constant and standard conditions. Given that you use degees K, the temperature component is usually fairly small.

Yes the acfm, cfm, icfm, cfm at X pressure, debacle has gone on for a long time and shows no sign of getting any better.

The number of posts you get when you enter search terms like scfm air is evidence of this.

When talking to vendros / checking data sheets, make sure you really understand and the vendor understands which type of cubic foot you're talking about. See below for some useful posts.

http://www.eng-tips.com/viewthread.cfm?qid=339851

http://www.eng-tips.com/viewthread.cfm?qid=39164

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Krausen]

Thanks for your help and links to past threads LittleInch. Much appreciated.