Does 2 hp compressor equal 8 cfm?

by Clive Haylock
(Sacramento, Ca, USA)

Hello I have been reading through your site trying to find information that will help me purchase the right air compressor.. from reading your info I think that I might understand it but i just want some clerification..

I am buying an air rachet that requires 5.5 scfm@ 90psi... you stated that the rule of thumb is 4 cfm@ 90 psi for every hp.. so it does that mean if i buy a 2hp compressor should i assume that it will provide me 8cfm@90psi per the rule of thumb? for example i have seen some compressors with the specs on the unit that states 3.5 scmf@ 90psi and 5.0 scfm@ 40psi and this has a motor with 1.5hp...inclosing, is there a calculation difference between scfm and cfm, i didn't see it in your postings..

Thank you...
Clive





Bill says...

Hello Clive. Glad to see that you've been reading the info on the site. That's gratifying.

Yes, the rule of thumb is that you will get about 4 CFM of compressed air for every HP of electric motor.

I also say in many places that this rule applies to compressors over 10 HP. Why? Because they will invariably powered by at least 220 Volts or more.

The compressor converts electrical energy into stored energy. That conversion process uses the electricity that's coming from the plug. In the typical DIY shop, that plug will have 110 Volts.

There isn't enough available energy in a 110 Volt energy supply to produce the compressed air capacity of 4 CFM per 1HP. If you were running 220, you would get that, approximately.

Have a read of these pages.

Cheers, and thanks for writing in.

Bill




Comments for
Does 2 hp compressor equal 8 cfm?

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Oct 20, 2011
 Explanation from the perspective of Elementary Thermo-Fluid
by: Frontier

Thermodynamic formulae:
Power = Pressure x (Flowrate)

Power - Watt
Pressure - Pascal
Flowrate(Q) - cubic metre/second

e.g.

1 HP = 745.7 Watt
Assume operating pressure - 4 bar = 4x100,000 Pa

745.7 = 400,000 x Q
Q = 0.00186425 cubic metre/second
1 cubic metre/second = 1x60x3.28^3 cfm = 2,117cfm
Therefore,
0.00186425 cu. m/s = 0.00186425 x 2,117cfm
Q = 0.00186425 cu. m/s = 3.9466 cfm

1 HP produce 3.9466cfm.
This is ideal (compressor 100%).
In the reality, compression efficiency 85%-95% depends on size and age of the equipment.

So, for average; taken compression efficiency 90%;
1 HP => 3.9466cfm x 0.90
1 HP produce 3.552cfm @ 4 bar.

Conclusion,
horse power cannot direct related to cfm.
different pressure for same cfm involve different mass transfer rate. For rules of thumb, better use Power = P x (dQ) and make sure all engineering units involved is standardised. S.I. or Imperial.


____________________

Yowza, Frontier!!!! Amazing stuff.

I still stand by "I do indicate in a number of places that the resulting 4 CFM per motor HP is an approximation. Not many folks, me included, have the desire or the capacity to undertake the math unless the actual output flow in CFM is critical. In which case, one would be better off getting an engineer involved."

Or... in the case of a need for specific and accurate output, maybe contact Frontier! :-)

Thanks, man.

B.
Jan 07, 2010
 compressor hp vs motor hp
by: Sorlozer

Actually Bill, I agree with your rule of thumb method, because there are times that we don't want to have all the computations just to know what to buy.

Also, that is not what I am trying to point out, what I am trying to point out is that the hp that you are referring to in your rule of thumb should not be mistaken.

If the reader applied the hp to motor hp, but without looking at the compressor hp, then there will be problems (assuming the compressor is reciprocating type).

If compressor hp is larger than the motor hp, then the motor will not have sufficient torque to move the piston, and that might cause overheat.

If the motor hp is greater than the compressor hp, then the performance of the motor hp is compensated, following the demand of the smaller compressor hp.

So, using the rule of thumb method, the hp should more likely refer to compressor hp, provided that the motor hp is the same, or larger than the compressor hp. If the hp of motor is less than the hp of the compressor, then there will be problems, and the rule of thumb method might not apply due to design problems.

Thanks Bill for the rule of thumb, for that is helpful.
Jan 06, 2010
 cfm vs cfm, and other warnings
by: Sorlozer

First of all, if you're interested in comparison between scfm and cfm, cfm / acfm is the actual flow rate in cubic feet per minute, while scfm is volumetric flow rate in standardized conditions, and stands for standard cubic feet per minute.

The formula for conversion is:

scfm = acfm*(Pactual/Pstandard)*(Tstandard/Tactual); where P is prssure, and T is temperature.

This formula is proven and can be derived using the Combined Gas law.

For hp using rule of thumb method as proposed by Bill, I have to warn that there is a difference between compressor hp and motor hp. Bill is definitely refering to both, not only motor hp, but also compressor hp, depending on the minimum, if both motor and compressor hp are different (I am assuming that the compressor hp is lower than the motor hp, in case the hp of both are not equal due to design considerations).

If you apply the rule of thumb to only one of the 2 components, then there will be discrepancies regarding the cfm, and that there will be disarray of data, and might mislead.

Just a warning though.
_____________

Good counsel.

I do indicate in a number of places that the resulting 4 CFM per motor HP is an approximation. Not many folks, me included, have the desire or the capacity to undertake the math unless the actual output flow in CFM is critical. In which case, one would be better off getting an engineer involved.

Thanks for your knowledgeable input. It elevates the information on the website for all to learn.

Cheers,

Bill

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