Compressor rating question

I've been looking at a few second air compressors. I've always
understood the volume flow rate quoted in CFM to refer to the flow rate
at atmospheric pressure, not the flow rate at the compressor's maximum
working pressure. Can someone confirm this for me?
Many thanks,
Chris
Reply to
Christopher Tidy
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The volume is measured in SCFM (Standard Cubic Feet per Minute). The "standard" means referenced to standard temperature and pressure. But, the reading is only useful when the tank pressure is known. Single stage pumps lose a LOT of efficiency at higher pressures, that's why el-cheapo manufacturers give the SCFM rating at 40 PSI tank pressure. You want the rating at 90 PSI tank pressure. A crummy punp will lose half its volumetric efficiency at 90 PSI. A good single-stage punp will still lose 25% or so, going from 40 to 90 PSI.
If they didn't do this, ratings would be almost impossible to compare. One cubic foot of sea-level air would be compressed to 1/7th of a cubic foot at 90 PSI, which is 7 atmospheres of absolute pressure. That, of course, is neglecting any temperature effects.
Jon
Reply to
Jon Elson
If you only want to draw 40 psi you can get xx cfm. If you want to maintain 125 psi you can only get x cfm.I have not seen one rated at atmospheric pressure.most quote a lower pressure of 80 or 40psi then at max working pressure.
Christ> I've been looking at a few second air compressors. I've always
Reply to
MK1
Oh, god not again. We beat this to death and then some awhile ago, much much flames with mucho heat but very little light. I read thousands of lines of postings and didn't understand what the gurus were saying. Typical specs are given at 40psi and again at 90psi, but the way I read it none of this matters somehow. It doesn't really matter about the theory, a decent 5hp 2-stage compressor can keep up with a die grinder, that's all I really care about.
GWE
Reply to
Grant Erwin
Well actually I was looking at this second hand compressor:
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I know it's old, but I love projects. The whole thing is 28" long and has a motor rated at 0.75 hp at 1440 rpm. There are two small air receivers underneath you can't see. Basically I was wondering what kind of tools I could run off it. A few things I'm interested in are paint spraying, a die grinder, Cengar saw (a.k.a. shark saw, as used by the fire service) and inflating tyres. At a rough guess I reckon this machine probably has a flow rate of 6-8 CFM at atmospheric pressure. Any thoughts?
Best wishes,
Chris
Reply to
Christopher Tidy
Well actually my thoughts were more pragmatic too. I was looking at this second hand compressor:
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I know it's old, but I love projects. The whole thing is 28" long and has a motor rated at 0.75 hp at 1440 rpm. There are two small air receivers underneath you can't see. Basically I was wondering what kind of tools I could run off it. A few things I'm interested in are paint spraying, a die grinder, Cengar saw (a.k.a. shark saw, as used by the fire service) and inflating tyres. At a rough guess I reckon this machine has a flow rate of 6-8 CFM at atmospheric pressure. Any thoughts?
Best wishes,
Chris
Reply to
Christopher Tidy
Christopher Tidy wrote in news: snipped-for-privacy@cantabgold.net:

You can forget the spray painting and die grinder with 6-8 CFM, and probably the saw too. Just about any die grinder will eat more like 10 CFM (for a good one) an elcheapo may pull as much as 14-16 CFM.
Reply to
Anthony
My old 1/2 HP gives me 2CFM @ 80-100 PSI and with 10 gal. storage I can use a die grinder but not continuously. With minimal storage, the one you are looking at would be cycling very rapidly and put out about 3 CFM of usable air. By adding storage capacity it might be of some use. Gerry :-)} London, Canada
Reply to
Gerald Miller
snip-----
My honest opinion? It isn't worth your effort. 3/4 of a HP won't produce enough air on a continual basis to do much for you. What Grant said. I have a two stage 5 horse Quincy, rated 19+ CFM @ 175 PSI and it is just adequate for running a small blast cabinet. It will run most air tools with ease, that's not an issue. Small compressors aren't up to much. Been there, done that. That's why I own the 5 horse Quincy. Unless you have no prospects of a larger compressor, I'd pass on it and invest in something that will serve you better in the long term.
Harold
Reply to
Harold and Susan Vordos
Thanks for the opinions. It looks like this compressor would be a bit limiting, but I'm still tempted by it. I can't afford a 5 hp compressor, and although I have the space for it now, I'm likely to be moving and won't in the future.
I probably should have mentioned that when I talked about spraying I didn't mean cars or trucks, I meant small stuff which I've built. Probably I wouldn't want to spray things larger than 18" across. I can manage without the die grinder but the Cengar saw would be nice. I checked the Cengar website and the saw needs 5 3/4 CFM at 75-90 psi. Perhaps I might just manage it with this compressor, or am I pushing my luck? I figure that inflating tyres and blowing away debris would be fine with a little compressor like this.
I was also looking at a Broomwade compressor, which is of a similar size but I believe it is two-stage:
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I know these machines are projects in themselves, but I don't mind that. I like their "old school" appearance and construction.
Best wishes,
Chris
Reply to
Christopher Tidy
"CFM" refers to the cubic feet of free air per minute, regardless of the delivery pressure. It does not mean the compressed volume delivered. See my page, "Evaluating True Horsepower and CFM Ratings of Air Compressors":
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Reply to
Richard J Kinch
I question the CFM required to run your saw. Many manufacturers do not rate the CFM at 100% usage so you get a poor indication of air volume needed to run the tool. Either way anything less than a 3 HP compressor will allow very intermittent use of the tools you mention. I have a Porter Cable 7 HP, ( yeah right, really about 3 HP), compressor that pretty much does all I need it to do. It is rated at 9+ CFM at 90 PSI.. I had a real 2 HP compressor for years that really was inadequate for much air tool use. If you need a limited amount of air, a larger tank will get you by with a small compressor. Keep in mind that once the tank has run down, recovery time is very poor so you will not have air when you probably will want it! Greg.
Reply to
Greg O
snip------
I can't argue with that, Chris. As long as you don't mind using your tools on a limited basis, there's no reason why you can't get by. You could even sand blast with it, but not for a very long time. It's all in what you expect. If you can get it for the right price and dedicate some time to making it work well, using it will quickly tell you if you made the right decision, or not. On thing is absolute------you can't have too much air at your disposal, but you certainly can find yourself needing more than you have. Only you will know if you're happy, or not. Good luck, regardless of your decision. Let us hear from you and how things turned out.
Harold
Reply to
Harold and Susan Vordos
I'll let you know how it turns out. Thanks for all the advice and suggestions. This compressor is cheap and only 30 miles from my home, so there's not too much to lose if I buy it and later decide it isn't for me. Physically it's about the largest I can accommodate (at 28" long), but I guess I might be able to get a larger CFM rating from a more modern machine of about the same size. I'm hoping I might be able to get a small spray gun which will operate happily from this compressor for little jobs. Also the saws I'm referring to were originally powered from breathing apparatus cylinders when used by the fire service, so maybe connecting a larger reservoir to this machine would give acceptable performance with the saw. Also I could add a larger reservoir at a later date. I recently spent a while restoring a vacuum pump I got free and am very pleased with the result (perhaps I'll post some pictures here sometime) so this is an appealing project. I'll think it over and let you know how it works out.
Best wishes,
Chris
Reply to
Christopher Tidy
I'll say that some compressed air is a lot better than none. When I was a kid I had a little corn-popper compressor with about a 4 gallon tank. I could knock 5 wheel nuts off if I loosened them all first, but if I took three all the way off there wan't pressure enough to break the last ones loose.. but it would air up a tire just as good as a big one..
John
Reply to
JohnM
Thanks for another opinion, John. I'm still pondering the 3/4 hp machine but I suspect I not going to buy it. The guy wants more than I thought, although I'm still tempted as it looks like a nicely built machine.
By the way, can anyone summarise briefly explain the advantage of a two-stage compressor? Do you get better performance at higher pressures?
Best wishes,
Chris
Reply to
Christopher Tidy
snip------
Unless you have need for the higher pressures they generate, I don't think there are any other benefits. In my mind, it's another way to increase storage capacity. Regardless of the size of the tank, you have more air stored for use, so you can get more from a smaller tank. That means you can use air longer before the compressor kicks in, but that then becomes the limiting factor. At that point, you're limited to the capacity of the compressor, the chief reason to not buy one that is undersized for your needs.
Let us know how it goes, and if your decision was good for you.
Harold
Reply to
Harold and Susan Vordos
I decided to pass on the small compressor, even though it was rather nice. If it won't run a die grinder or Cengar saw, I won't find it too useful. I'm not in any hurry so I'll look out for something which can supply around 10 CFM, is nicely built and is on a trolley/can be fitted to a trolley. We'll see what turns up.
I did also see a big twin cylinder BroomWade pump on eBay which I thought I might be able to build into a decent compressor:
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I need to keep it fairly compact, which probably means a smaller air receiver than average. I'll keep a look out and think it over.
Thanks for the advice!
Chris
Reply to
Christopher Tidy
I can't help but think you made the right decision, Chris. I started out with a small compressor and grew to hate it the moment I put it to serious use. In a sense, I was lucky. One fine day the reed valve started leaking, providing the push I felt necessary to get me to purchase a serious compressor. I've never regretted my decision. Even if you must use a small receiver, the compressor may very well have the capacity to run your choices directly. That's the most important feature, at least to me.
The one on ebay looks like it has some possibilities----although I'm not familiar in the least with that particular machine. Strangely, Chris, it bears a strange resemblance to the Quincy (at least in color). I noticed it has unloading valves, a very good sign of a quality compressor.
Good luck, should you bid. Let us know if you get it.
Harold
Reply to
Harold and Susan Vordos

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