## POLYTROPIC COMPRESSION

## POLYTROPIC COMPRESSION

(OP)

We know: PV^^n= constant and

We know in centrifugal compressor

My question is what is the difference between for example n=1.2 and n=1.3?

For a centrifugal compressor n=1.2 is better or n=1.3? which parameters define or specify

**n**(polytropic coefficient) is also**constant**.We know in centrifugal compressor

**n**could be higher than 1 (isotherm ) and lower than gamma Cp/Cv (isentropic).My question is what is the difference between for example n=1.2 and n=1.3?

For a centrifugal compressor n=1.2 is better or n=1.3? which parameters define or specify

**n**?
## RE: POLYTROPIC COMPRESSION

- go ahead and conduct the head calcs using 1.2 & 1.3 to determine the difference using same inlet/outlet conditions. you may be surprised at the results.

"For a centrifugal compressor n=1.2 is better or n=1.3? which parameters define or specify n? "

- the compressor does not care which is better. as far as what parameters, inlet/outlet conditions and gas composition.

## RE: POLYTROPIC COMPRESSION

Good advices from pmover. Consider to study the document attached .

Good luck

Pierre

## RE: POLYTROPIC COMPRESSION

Another document to support your work.

Pierre

## RE: POLYTROPIC COMPRESSION

n) and one equation. my question is how can we calculate polytropic head when we have notn? Or how can we calculatenwhen we have not head?## RE: POLYTROPIC COMPRESSION

read through the first document provided by pierreick and pay attention to page 6.

pierreick,

i long lost my copy of that document, so thank you. the second, i still have - obtained +20 years ago. fyi, both documents are valuable for personnel to use/learn from.

## RE: POLYTROPIC COMPRESSION

I read these two document but I haven't found my question.

Imagine I want to buy a centrifugal compressor. Is it important to fix polytropic coefficient

nfor manufacturer? For example imagine I want to buy a compressor with n equal 1.1 . Do manufacturer build any centrifugal compressor with my fixed poly tropic coefficient? Do I have any restriction to specify or identifyn?## RE: POLYTROPIC COMPRESSION

not really. it is more important to state the inlet/outlet conditions, gas composition, volumetric or mass flow, and other essential operating parameters. best to utilize the API-617 datasheet which can be purchased online. the GPSa databook is another resource.

"Do manufacturer build any centrifugal compressor with my fixed poly tropic coefficient? Do I have any restriction to specify or identify n?"

n is not necessary a requirement, but an option. you can specify n, but the mfg will take the gas composition, determine n, and size the compressor based on the operating parameters/criteria you provide.

contact a centrifugal mfg and discuss the matter with them. they will be glad to work with & discuss the matter with you.

## RE: POLYTROPIC COMPRESSION

## RE: POLYTROPIC COMPRESSION

One more observation about Perry's Theory of Compression, the graphic presentation has an additional curve for k > Cp/Cv with no further elaboration. You may want to do more research on that aspect of the graphic presentation.

## RE: POLYTROPIC COMPRESSION

You may use this calculator and data sheet to support your work .

My advice with 40 years + experience as an engineer , don't over specified , let manufacturer do their job .

http://yrsp.elliott-turbo.com/public/comp_perf/ind...

another good reference is GPSA (Gas Processors Suppliers Association) (chapter 13)

Good luck

Pierre