8 replies to this topic

[panagiotis]

Hello everyone,

Could someone explain me the "real" difference between the actual head and polytropic head?

Actual head is calculated as the ration of polytropic head/polytropic efficiency.

Polytropic head depends on MW (for example), while actual head does not depend on MW.z
This is something not clear to me.

Edited by panagiotis, 26 October 2024 - 02:16 AM.

[breizh]

Hi,

Study these documents.

Breizh

Attached Files

•  Centrifugal compressor polytropic perf..pdf   10.3MB   14 downloads
•  Compressor efficiency.pdf   2.83MB   25 downloads
•  simulation compresseur.pdf   1.78MB   14 downloads

[panagiotis]

Hello Breizh,

Thanks, I had a fast look, but I could not see the difference between polytropic and actual head.

[panagiotis]

Maybe actual head is derived from the eulers equation. That's why does not depend on the gas characteristics?

[demank]

Dear Pangiotis., for compressor actual head, you may choose between these two. Isentropic (adiabatic) head which is related with isentropic efficiency; and Polytropic head which is related with polytropic efficiency.

 

Hello Breizh,

Thanks, I had a fast look, but I could not see the difference between polytropic and actual head.

[panagiotis]

Yes, I know about these 2. I am referring to the actual head.

[breizh]

Hi,

You are confusing people, what do you mean by actual head?

Breizh

[panagiotis]

Oh sorry, I mean the ratio of polytropic head divided the polytropic efficiency.

[PingPong]

Hello everyone,

Could someone explain me the "real" difference between the actual head and polytropic head?

Actual head is calculated as the ration of polytropic head/polytropic efficiency.

Polytropic head depends on MW (for example), while actual head does not depend on MW.z
This is something not clear to me.

You seem to be mixing up the required polytropic head, and the polytropic head that a given centrifugal compressor delivers.

 

If you want to compress a certain gas with a certain T1, a certain Z, a certain MW, and a certain Cp/Cv, from P1 to P2 then you can calculate the required polytropic head, which then depends on all the factors mentioned.

 

However once the compressor is manufactured the polytropic head it delivers depends mainly on the number of impellers and their tip speed, not on the above mentioned factors.

Some textbooks, articles or websites will inform you about "head coefficient" and "flow coefficient". The "head coefficient" is a function of the "flow coefficient". As a result the delivered polytropic head is not constant but varies with the volumetric flowrate through the compressor. This is visible in the shape of the performance curve of every centrifugal compressor between surge and stonewall.

 

There is a lot of friction between the fast rotating impeller and the gas causing additional heating on top of the heating due to compression.

That additional heating due to friction causes additional power consumption on top of the power required if the efficiency were 100 %.

 

The polytropic head is the power consumption if there were no friction (100% efficiency).

What you call the "actual head" is the actual power consumption, including power consumption due to additional heating by friction.

 

Head can be expressed in kJ/kg or m or BTU/lb or ft or ......., but it is always: energy per kg.

Edited by PingPong, 29 October 2024 - 02:21 PM.