Displaced CFM vs. Delivered CFM

[57JoeFoMoPar]

Obviously, when purchasing a compressor you want to buy one that will supply enough air to suit your needs. However, it seems that some pumps either don't list delivered CFM on their specs. Is there some kind of formula to use to determine what the delivered CFM will be from the displaced CFM number? Are some pumps more efficient than others? What makes for an efficient pump? Am I on drugs asking this question

 

[Fixnair]

No you're not on drugs. Most people don't even think about about delivered CFM. And most box stores won't know what you're talking about.
The best compressor you could buy would only be about 75% efficient. The Chinese stuff I would venture to guess is 65%. That is 75% of actual piston displacement. (Bore & stroke X rpm)
The CFM ratings on the box store compressors are probably as inflated as their horsepower ratings.
Thanks for asking this question.

 

[zkling]

Fixnair hit the nail on the head with the efficiency remark for CFM differences. As for what makes an efficient pump and are some pumps more efficient than others? yes, but not drastically so. Efficiency in compressor pumps is going to be from thermodynamic and volumetric efficiency. Ideally you want the air to have the smallest change in temperature possible going through the pump. Colder air = denser air = higher mass flow rate for a given volumetric flow. The compression of the air itself causes heat to generated, depending on the frequency of compression, mass of the pump and dissipation from the pump all factor into the efficiency of the compressor.

No you're not on drugs. Most people don't even think about about delivered CFM. And most box stores won't know what you're talking about.
The best compressor you could buy would only be about 75% efficient. The Chinese stuff I would venture to guess is 65%. That is 75% of actual piston displacement. (Bore & stroke X rpm)
The CFM ratings on the box store compressors are probably as inflated as their horsepower ratings.
Thanks for asking this question.

I've actually found most of the belt driven units to be pretty accurate in terms of CFM rating, which I think is more like ACFM or ICFM.

 

[57JoeFoMoPar]

Thanks for the great answers guys. The 65%-75% number is what I was looking for. I'm looking to pick up a new compressor and want to make sure the one I buy is going to do everything I ask of it, including running a blast cabinet, straight line sander, DA sander and spray guns (on the hobbyist level, this will not a continuous use scenario).

The one I've been going back to is the Husky 80 gallon single stage. http://www.homedepot.com/p/Husky-80...H=RV-_-RV_homepage_rr-3-_-NA-_-203187346-_-N#
So with an advertised rating of 14 CFM at 90 PSI, could I actually expect only around 10 delivered CFM at 90 PSI? Would that be enough to do what I need it to do?

 

[DynoDale]

This isn't really related to the pump displacement vs. flow rating, but the compressor ratings are usually SCFM, which is close to the ICFM at the inlet of the compressor, when it is delivering the rated pressure in PSIG. The ACFM at the tank outlet or tool inlet is different: 14 SCFM is only about 2 ACFM at 90 psig. 14 SCFM at 90 psi is just OK for a 5 HP compressor; other 5HP units deliver 16-18 "CFM".

Air tool specifications are equally vague. The values are given as CFM but are probably SCFM, but sometimes assume a 25% duty cycle to "help" you size a compressor.

 

[bsaint]

How critical of a number are you looking for?

 

[57JoeFoMoPar]

This isn't really related to the pump displacement vs. flow rating, but the compressor ratings are usually SCFM, which is close to the ICFM at the inlet of the compressor, when it is delivering the rated pressure in PSIG. The ACFM at the tank outlet or tool inlet is different: 14 SCFM is only about 2 ACFM at 90 psig. 14 SCFM at 90 psi is just OK for a 5 HP compressor; other 5HP units deliver 16-18 "CFM".

Air tool specifications are equally vague. The values are given as CFM but are probably SCFM, but sometimes assume a 25% duty cycle to "help" you size a compressor.

Yeah, it seems pretty middle of the road as far as air output, but the price is right, the quality seems excellent (both from personal inspection and from friends I have that have Husky compressors) and the reviews are excellent. Plus I don't have to mess with it to make it work, so that's a big plus. I've just been having problems sizing the compressor because the numbers for these things are all over the place and often seem open to interpretation.

 

[57JoeFoMoPar]

How critical of a number are you looking for?

I'm not necessarily looking for some magic number as far as CFM, but I would like for this compressor to do everything I ask of it continuously without having to make many, if any breaks. That includes using a DA and straight line sander and a small blast cabinet. I'm a home hobbyist into auto restoration, so this won't be pro shop usage by any stretch (which is also why I can't justify the expense of a Quincy or Champion). But I also don't want to be standing around waiting for the compressor to catch up to me.

If I had to pick a "critical number", it seems that 12 CFM at 90 PSI would be about the max I would need.

 

[bsaint]

I'm not necessarily looking for some magic number as far as CFM, but I would like for this compressor to do everything I ask of it continuously without having to make many, if any breaks. That includes using a DA and straight line sander and a small blast cabinet. I'm a home hobbyist into auto restoration, so this won't be pro shop usage by any stretch (which is also why I can't justify the expense of a Quincy or Champion). But I also don't want to be standing around waiting for the compressor to catch up to me.

If I had to pick a "critical number", it seems that 12 CFM at 90 PSI would be about the max I would need.

"Delivered" CFM is going to be base on arbitrary control numbers. I would just do what someone here mentioned and de-rate displaced CFM by 25%.

If you can run your compressor in a cool area of the shop and buy one with an aluminum intercooler between stages and aluminum aftercooler, you can maintain high efficiency.

 

[justanengineer]

JMHO, but I think many are viewing this wrong. Rather than worry about what the volumetric flow (cfm) really is, Id worry more about durability (aka, protecting my investment) as I suspect theres more variation in that than in flow between expensive and cheap compressors. Yes, youre only a hobbyist, but we tend to get spurts of time where we focus and spend a few hours working our tools like the pros do, just in the evenings, weekends, and holidays instead of 8/5/50/yr, so they very well may see some rather hard work once in awhile. The duty cycle of the compressor you linked to is only 50% and yes, I know the manual suggests thats 30 mins/hr. IMHO, I wouldnt trust that to run more than 5 mins in 10, Ive seen too many of those burned up in <5 years to believe folks are really using them more than 30 mins/hr with any regularity, craigs is literally littered with them for sale.

Again, JMHO, but for the $850 price tag you should be able to find a very nice used Quincy, Champion, IR, Gardner Denver, Wayne, or other top quality compressor. The real difference between those cheapies and an industrial compressor - after a few years you throw the cheapie away, after 40 years you spend a few hundred bucks and rebuild the industrial compressor. My personal vote is for Quincy, they have excellent customer service and still sell parts for models like mine that are 50+ years old.

 

[mmouse]

I have a Champion R15. Based on the pressure and RPM I run, the manual says displacement is 12.8 cfm. However, using this calculator, I am getting 9.0 delivered (about 70%).
http://aircompeq.com/formulas.htm