# SCFM result from joining two CP compressors

by Bob Palmer
(Avon, CT)

I have a 2HP 8 gallon CP unit and a 21 gallon 2.5HP CP unit. My question is, if I join the two tanks together will the SCFM output @ 90PSI (4.7) increase, decrease, or stay the same? The unit will be driven by the 2.5 motor.

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Bob, if you have just one motor running, then by combining the two compressors, what you have done is increase the tank size of a single air compressor.

Therefore, if the 2.5 motor outputs about 8-10 CFM at 90 PSI, you do not increase the motor / pump output by adding a tank.

What you do get is a larger reservoir of pre-compressed air, when both tanks are full, to use in an air tool, before the pressure in the tanks fall to the normal motor cut in pressure level.

By the same token, it will take the motor / pump longer to fill two tanks, so the end result is you can run an air tool a little longer before pressure drops too low to use it, but then you'll have to wait longer for the pressure to regenerate.

If you had both pumps running from two motors, then you will increase the supply of air.

Cheers,

Bill

## Compressed air for pipeline packing and driving pig against back pressure

by mithun
(United arab emirates )

Pipeline details 43km 52" pipeline with 14.7mm wall thickness to be packed to 5 bar(g) with compressor 900cfm @ 7 bar(g) rating.
How to calculate the time required for packing and number of compressors.

How to calculate the the time required for pigging and number of compressors with a drive pressure of 6 bar9g) against a back pressure of 5 bar(g) as above.
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All good questions, Mithun. And, sadly, way above my pay grade. Happy to post this for you in case a willing air engineer spots it and can offer help.

Cheers,

Bill

## Reciprocating air compressor

by veerapandiyan k
(India)

Given the dia and stroke of the compressor how to calculate the flow for double acting reciprocating compressor
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Interesting question. I hope someone with an engineering background can provide assistance here.

Bill

## Why does CFM decrease as PSI increases?

by Craig Buehlmaier
(Concord, NC)

I know CFM decreases as PSI increases, but normal logic would suggest that as pressure increases, it would generate more flow. Doesn't pushing something harder cause it to move faster?
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The higher the pressure of air in the tank, the faster it will flow to a lower pressure area, that's right.

Your compressor is an electro-mechanical device. As it pumps air into the tank, thereby increasing pressure in the tank, the compressor has to work harder against the rising pressure in the tank, hence the decreasing CFM as an output from the compressor.

Cheers,

Bill