Compressor Duty Point

[RKJustice]

Hi,

A P&ID states the duty of a screw compressor as “x CFM at y psig”, compressing atmospheric air. To work out the compressor mass flow rate for start-up conditions where the downstream pressure is also at atmospheric then I would use x and y? Is this correct or should the mass flow rate be based on x and atmospheric pressure?

 

[zdas04]

Mass flow rate is constant through a compressor. CFM is not in any way constant. Calculate the density at atmospheric pressure and multiply it times your CFM number to get a mass flow rate. Then, just to prove to yourself that CFM is not constant, calculate the density at the discharge and divide it into your mass flow rate and it won't be the same CFM as the suction.

Now if the air compression industry had standardized on STP instead of local atmospheric pressure and temperature you could calculate the mass flow rate at suction or discharge and SCFM would be the same, because it has to be.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. —Galileo Galilei, Italian Physicist

 

[RKJustice]

Ok, thanks. It is a bit of a confusing way of stating the information, SCFM would not leave it open to interpretation.

 

[zdas04]

It is confusing. Many industry conventions are confusing. What doesn't help is that process compressors are rated in SCF, it is just air compressors that are rated in ACF at suction conditions. It is just one of those things.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. —Galileo Galilei, Italian Physicist