speedaire compressor cfm unknown

by BJ
(canada)

Is there a way I can roughly calculate the output. It is a reciprocating type V piston arrangement twin cylinder. I can't find the specs for it and am determining whether it is better as a boat anchor or what.
As for repairs, what? I am familiar with motorcycle engines and have pulled the heads on this unit and the flap valves "seem tight" and rust free....no scores in the cylinders and no visible wall wear. Just seems to take a lot longer to fill up the 20g tank(than it use to. Air filter is clean and free flowing and lines are (soap tested) tight and head seal externally is tight (soap tested).




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speedaire compressor cfm unknown

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Sep 08, 2010
 Rough Formula for Determining Compressor CFM
by: Terry

This formula can help find air compressor output in CFM when it is not known. If it has already been posted on the site, my apologies.

First, some things have to be known about the compressor pump in question:

1. Bore ( Piston diameter )

2. Stroke ( Distance the piston travels up and down )

3. Number of cylinders

4. RPM of pump ( divide pump pulley diameter by motor pulley diameter, and divide electric motor RPM by this figure for approximate pump RPM )

If a model number is found on the pump or is known, you may find this information the easy way on the internet, by searching that model #. If not, then you have to do it the old fashioned way. To do this requires some pump dissassembly. The head needs to be removed if possible, and:

1. the piston diameter measured. If you don't have sophisticated measuring tools, don't worry about it, you can use a tape measure here. This is close enough for your purposes.

2. Measure the stroke by rolling the compressor piston all the way to the bottom, then measure the distance to the top of the cylinder.

Now that you have your bore and stroke, you can apply the following formula you should have learned in school, that you thought you would never use.

To find the area of the piston bore, the formula is pi x radius squared. Radius is one half of piston diameter. Example if your piston diameter is 3 inches and your stroke is 2 inches:

pi (3.1414) x Radius squared (2.25) = 7.068 square inches.

Now that is known, multiply area ( 7.068) by the stroke (2) to get 14.136 cubic inches. for one piston displacement.

Multiply this by number of pistons. Such as for a 2 cylinder, would be 28.272 cubic inches.

Not there yet, though. This is for one pump revolution. We need to multiply this times pump RPM your compressor has. For example, the pump RPM is 1000 RPM. So, 1000 x 28.272 = 28272 cu. in.

Lastly, we need to determine CFM. To do this, divied the cubic inches by 1728 ( one cubic foot 12 x 12 x 12 )

CFM for this compressor pump would be 16.361 CFM

This is the amount of free air flow at ZERO PRESSURE.

Many variables make up the efficiency of the compressor from this point, but roughly, you can use 50% for many compressors to get cfm. So for 90 psi, you should have an approximation of 8.18 CFM.

Some compressors are more efficient and the percentage figure could be closer to 65% to 75%.

This formula works on 2 stage compressors, too. BUT, for a 2 stage compressor pump, just use the LP or big piston only to determine cu. in., NOT the smaller or HP piston.

Sorry for the length of the post. Hopefully this can help those that really want to know their pump's CFM

______________________________

Thanks Terry.

Bill
Dec 18, 2007
 Speedaire Compressor
by: Bill - Publisher

Tough question, BJ.

You don't indicate the horsepower of the motor, and that might help.

Industry standards suggest that for motors over 10 HP, each HP will generate about 4 CFM of compressed air at 90 PSI.

I know that this changes substantially when the motor HP is less than 10.

I've also read that using a figure of about 2.2 CFM of air discharge for each horsepower for a compressor under 10 horsepower is common. That source didn't say at what pressure that flow rate was achieved.

I've never been able to "nail down" a specific formula for determining the discharge rate of a specific compressor, mostly, I think, as there are so many variables. A host of different manufacturers, different piston sizes for different compressors with the same electric motor horsepower, different belt to sheave ratios and so on.

At this point, as noted, over 10 HP I use 4 CFM @ 90 PSI, and under 10 HP I go with 2 CFM per horsepower at around 90 PSI.

If someone has a formula to offer us, that would be great.

Cheers,

Bill

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