POLYTROPIC COMPRESSION 1

[142846]

We know: PV^^n= constant and 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?

 

[pmover]

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

 

[pierreick]

Hi ,
Good advices from pmover. Consider to study the document attached .
Good luck
Pierre

 

[pierreick]

Hi,
Another document to support your work.
Pierre

 

[142846]

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

dear pmover (Mechanical) probably I couldn't explain clearly. For adiabatic process because the gamma Cp/Cv is specific number, with knowing only the compression ratio, we can calculate or determine the isentropic head. But for polytropic process we have two unknown parameter (head and n) and one equation. my question is how can we calculate polytropic head when we have not n? Or how can we calculate n when we have not head?

 

[pmover]

142846,
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.

 

[142846]

For a centrifugal compressor n=1.2 is better or n=1.3

dear pierreick (Chemical)and pmover (Mechanical) thanks for explanation and sharing the documents.
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 n for 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 identify n?

 

[pmover]

"Is it important to fix polytropic coefficient n for manufacturer? For example imagine I want to buy a compressor with n equal 1.1 ."

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.

 

[chicopee]

For a centrifugal compressor n=1.2 is better or n=1.3? which parameters define or specify n? An interesting question since I am not in the business of manufacturing and testing air and gas compressors. I did a little research in my Kent Mechanical Engineer's Handbook "Power", 12th edition and on page 1-46 the paragraph titled Actual Performance is fig 21 presenting a set of curves. The ordinate is labeled Shp per million cu ft...... and the abscissa is exponent K varying from 1.0 to a max value of 1.4+.. On a test stand if you can calculate the shaft horsepower(Shp) from the electrical power or other method and knowing the air or gas supply, you can then get your exponent "n" or "k" as shown in my Kent ME Hdbk. Remember the "k" values shown in my reference are not equal Cp/Cv except for the max value in the abscissa of my reference.

 

[chicopee]

For further research into your questions, I reviewed my Perry's Chem.Engineer's Hdbk, fourth Edition, pg 6-15, Theory of Compression. Paraphrasing " The path of least amount of work input is during isothermal compression whereby n=1. For adiabatic compression in which no heat is removed or added, n=k=Cp/Cv. As it is impractical to design sufficient heat transfer equipment into most compressors, most machine will operate on a polytropic path whereby k will range between one(1) and Cp/Cv" during performance tests which should answer your question.
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.

 

[pierreick]

Hi ,
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/index.jsp

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