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# Compressor Sizing

compressor

2 replies to this topic
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### #1 rpdlpascual

rpdlpascual

Brand New Member

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Posted 17 September 2018 - 09:18 PM

I am confused about compressor curves reading and I would just like to ask about the following:

1. Is polytropic head mathematically related to flowrate? I have been using the equation below and I cant seem to find the mathematical relationship of polytropic head with flowrate.

Head (poly) = (n/n-1) (ZRT/MW) ((P2/P1)^(n-1/n)  - 1)

Specifically, I would like to ask why the polytropic head (energy required to compress a fixed mass of gas) decreases as the flow increases.

2. I would also like to ask about compressor's system resistance curve which is constructed by plotting pressure drop versus flow rate.

dP = (0.5(density)(V^2)) (4fL/D)

Will the curve shift to the left when the flow is increased? or is it to the right?

Thank you very much!!

Edited by rpdlpascual, 17 September 2018 - 09:20 PM.

### #2 Bobby Strain

Bobby Strain

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Posted 17 September 2018 - 10:48 PM

The answer to your first question is yes. But one must be a rocket scientist to understand the relationship. The cure you see is simply plotting that relationship. The polytropic head equation is used to translate head to pressure ratio for the compressor. Item 2 has little to do with system dynamic losses. One determines the discharge pressure required to meet delivery pressure which may include pipe components resistance. Keep studying your fundamentals and you will eventually understand how we use them together in engineering applications. Be patient.

Bobby

### #3 breizh

breizh

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Posted 18 September 2018 - 02:17 AM

Hi ,

Let you consider the search engine in this forum and/or get a copy of textbooks like Perry's chemical engineers' handbook or others.

Good luck,

Breizh