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OT Air compressor CFM tp PSI conversion ???

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TIMW(PA)

11-22-2004 19:06:41

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Some day soon I would like to get my dream Air compressor for running a Sandblaster Air tools ect. I would like something over 20 CFM. Well, in shopping around I see the usual deceitfulness of business takes hold and nobody will list equal values of CFM at a standard pressure, Some will list CFM @175 others @ 135, 90 or 45 and so on. Then to muddy the waters we have CFM that includes the air in the tank for a minute or so and also Pump CFM that it will maintain. so my question is....
- Is there a formula that I can use to convert CFMs from one pressure to the other?
-And, also, A formula to calculate CFM from bore diameters and RPM?
TIA.....Tim
p.s. and any other input as far as good features to opt for would be gratly appreciated Tahnks

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JoeNever

02-14-2006 18:24:21

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Re: OT Air compressor CFM tp PSI conversion ??? in reply to TIMW(PA), 11-22-2004 19:06:41

hello, was just wondering if you ever got your answer, I was just begining my quest for that same answer, and crossed paths with this post, I have read the responses you received and I already feel like pulling hair out of my head. Dosn't anyone understand what the question is?
I have a Compressor small but anyway 4.1CFM @ 90PSI, Now I found a Airgun that is rated 8CFM @ 40PSI is there a way to find out what Airgun rating as follows ?__CFM @ 90PSI or 4.1CFM @ ?__PSI
so it can be compared to the compressor.

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Barry8N

11-23-2004 14:17:03

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Re: OT Air compressor CFM tp PSI conversion ??? in reply to TIMW(PA), 11-22-2004 19:06:41

Even by changing the psi to cfm etc, this probably won't help you. Most companies give the maximum cfm at a specific psi. You get into tank size, dual or single stage compressor head, etc. when determining compressor efficiency. So therefore, the compressor may not pump the psi needed at a certain cfm. The best thing to do is define your maximum requirements ( tool ratings and how many used at once) and go from there. Each tool has a minimum required cfm/psi rating. Just because a compressor will pump a certain cfm at a certain psi doesn't mean it can sustain this peak without overworking the unit. A nice large compressor and tank will have a shorter recovery time and will not constantly run.
If you are going to use many airtools, go with the highest rated cfm at the psi operating range of your tools. Also go with a 2 stage compressor when possible.

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Barry8N

11-23-2004 14:41:24

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Re: OT Air compressor CFM tp PSI conversion ??? in reply to Barry8N, 11-23-2004 14:17:03

I wasn't thinking before but one is a volumetric flow rate and the other is a pressure. They are related but not interchangeable as you want to do. As I said many factors come into play on compressor requirements and it gets complicated trying to evaluate the different brands and even requirements. Just go for max cfm at the needed psi rating you can afford or will need.
These internet sites may help with you compressor choices.
Link

Link

http://www.highbeam.com/library/doc0.asp?docid=1G1:10691010&refid=ink_puballmags&skeyword=&teaser=

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tim in pa

11-23-2004 10:03:54

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Re: OT Air compressor CFM tp PSI conversion ??? in reply to TIMW(PA), 11-22-2004 19:06:41

tim, although "cfm" is a volumetric flow rate, we can use the volume part for conversion purposes.

volume #1 (cubic feet) x pressure #1 (psi) = volume #2 (cubic feet) x pressure #2 (psi)

the problem however is the non-standardization of how the specifications were attained. some mfr's rate their pump only (best way for comparison), and some rate their's as a system (pump and reservoir) were the tank changes the values. rule of thumb - don't buy cheap air tools. poor construction (bad seals) requires more cfm.

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Fessman

11-23-2004 07:08:14

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Re: OT Air compressor CFM tp PSI conversion ??? in reply to TIMW(PA), 11-22-2004 19:06:41

Tim,

Not sure if this will help you. Attached is a page out of fluid power data book on hp to compress air. This may help in showing the relationship of PSIG to hp.

Fessman

hp to compress air

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Fessman

11-23-2004 07:32:25

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Re: OT Air compressor CFM tp PSI conversion ??? in reply to Fessman, 11-23-2004 07:08:14

Here is the picture, sorry about the size, I am working on my picture posting abilities.

Fessman

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Midwest redneck

11-23-2004 02:36:40

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Re: OT Air compressor CFM tp PSI conversion ??? in reply to TIMW(PA), 11-22-2004 19:06:41

2 years ago I sized up a 25HP rotary screw compressor for my building (employer). One rule of thumb to remember: for every 1HP of electricity you can get about 4CFM of compressed air out. (keep in mind you need to size your comp. according to free air) So for instance a 5HP compressor will give you 20CFM of free air, or in other words the compressor will continue to give you 20CFM at say 90PSI running continuously in the on/off state. If you need a 5HP compressor then get a 7.5HP to be safe.

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TIMW(PA)

11-22-2004 21:46:29

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I'm confused :) in reply to TIMW(PA), 11-22-2004 19:06:41

ok I know ya'll are trying to help and I appreciate it but I'm still not getting to where I want to go...Heres a real life example. I'm looking at a catalog. Pump manufacturer "A" says my 5 HP pump does 19.1 cfm @ 125 psi, ok, and pump manufacturer "B" says my pump does 18 cfm @ 90 psi. -Which one pumps more air?
-Is there some way that I can mathmatically figure out what pump "B"s' cfm rating would be at 125 instead of 90 so I can compare accurately?

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timw(pa)

11-23-2004 05:34:22

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Re: I'm confused :) in reply to TIMW(PA), 11-22-2004 21:46:29

thanks..Now that raises another question. Some manufacturers mess with the HP rating. It seems they are all "5hp" even though some of the motors are pulling 25 amps and some only 10. I know amps aint everything but it is part of the equation. Thats why I kind of wanted a way to figure the correct CFMs produced with RPM x Bore dia,stroke. Is there an equation for that? I know this seems a little anal but it is a big investment for me and I hate getting taken by marketing techniqes

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Stan(PA)

11-22-2004 21:56:32

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Re: I'm confused :) in reply to TIMW(PA), 11-22-2004 21:46:29

Quick and dirty: Pump "B" would put out about 13cfm at 125psi. (18x 90/125) I think the other equation will get you close using Inlet volumes.

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Stan(PA)

11-22-2004 21:21:29

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Re: OT Air compressor CFM tp PSI conversion ??? in reply to TIMW(PA), 11-22-2004 19:06:41

Tim, compressors are normally rated at INLET cfm, which is how much air it will move at ATMOSPHERIC pressure. In a nutshell, a 100cfm compressor would give an output at 100psi of about 14cfm. I believe an equation would go something like this: Rated Cfm ----- ----- ----- Desired Cfm/14.7
I might have myself all turned around, but I think this works..... ..Stan

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duey

11-22-2004 21:07:00

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Re: OT Air compressor CFM tp PSI conversion ??? in reply to TIMW(PA), 11-22-2004 19:06:41

I think you have some good answers.... the only thing I would add is that a chart might be of some comparative value. . . *IF* the efficiencies were all equal... and they never are.

As mentioned, you"re probably best off to just mind the CFM at a reasonable PSI... for sure, 18 CFM @ 175 is going to be higher CFM @ 90 PSI. Most mfg"s rate their air tools at 90 to 100 psi, that should help you figure what CFM you need.

Decide and then be happy with it!! duey

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Eric ATL

11-22-2004 20:40:18

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Re: OT Air compressor CFM tp PSI conversion ??? in reply to TIMW(PA), 11-22-2004 19:06:41

Forget about PSI. The ONLY thing to concern yourself with is how many CFM will the pump move. Look into DEVAIR compressors. I have a 220v, 5hp that will produce 25 CFM. The pump is everything. Not priced out of reason. HTH, Eric

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timw(pa)

11-22-2004 20:46:58

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Re: OT Air compressor CFM tp PSI conversion ??? in reply to Eric ATL, 11-22-2004 20:40:18

Yeah thats what i'm trying to figure out what pump is delivering what. one pump mftc. says my pump will do 25 CFM at 2psi then the next will say mine will do 1 CFM at 3,000 psi (exagerating) I need some way to compare apples to apples. I need some way to convert the CFM ratings so they are acurate at a standarnd PSI...Thanks Tim

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Tom Wy

11-22-2004 20:00:26

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Re: OT Air compressor CFM tp PSI conversion ??? in reply to TIMW(PA), 11-22-2004 19:06:41

I wont be too much help.
I dont think you can convert CFM to PSI??
CFM is cubic feet per minute which is a flow rate and psi of course is not.
I would pick out the tools you want to purchase/run and see what the air requirements are for each. DA (dual action) sanders require alot of air for continuous use. I have a 220v. 6hp compressor with a 60 gallon tank that is BARELY adequate (for home use). hth.

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timw(PA)

11-22-2004 20:42:09

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Determining CFMs at different pressures??? in reply to Tom Wy, 11-22-2004 20:00:26

No...I'm sorry I didn't write that well. I want a conversion for CFMs at different psi...e.g. if I have 20 CFM@135 psi, what would the CFMs be at 90 psi? is there a formula , rule of thumb etc

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