Polytropic efficiency of turbomachinery
Polytropic efficiency of turbomachinery
(OP)
Hi, members
Polytorpic efficiency or small-stage isentropic efficiency is easily found in any textbooks of turbomachinery introduction. Its definition and difference from the adiabatic or isentorpic efficiency are well understood. But I am eager to know why the polytropic efficiency is to be introduced there, and at what cases people use it rather than the isentropic efficiency.
Thanks.
Polytorpic efficiency or small-stage isentropic efficiency is easily found in any textbooks of turbomachinery introduction. Its definition and difference from the adiabatic or isentorpic efficiency are well understood. But I am eager to know why the polytropic efficiency is to be introduced there, and at what cases people use it rather than the isentropic efficiency.
Thanks.





RE: Polytropic efficiency of turbomachinery
RE: Polytropic efficiency of turbomachinery
Could you explain more why the positive displacement compressors prefer the adiabatic (or isentropic) efficiency, and the centrifugal and axial compressors prefer the polytropic efficiency.
In a textbook by Dixon, it is said that the polytropic efficiency is useful when two compressors with different pressure ratios are to be compared. But I cannot fully figure it out.
RE: Polytropic efficiency of turbomachinery
Empirical field experience has demonstrated that compression in reciprocating compressors closely follows a reversible adiabatic process (constant entropy). I have seen this in such gases as CO2, air, Nitrogen, Methane, Nitrous Oxide, and the common refrigerants. The GPSA Engineering Databook also is in agreement with this experience and recommends this method above all others when dealing with reciprocating machines.
What does your second paragraph refer to? You don't mention whether the two compressors are the same type (nor which type) nor do you define what it is that you are comparing. Please be clear and specific in your questions in order to understand what you are asking. I'll await your reply. Are we supposed to know who "Dixon" is? Cite the book title, publisher, year and/or country of origin.
RE: Polytropic efficiency of turbomachinery
Sorry about my insufficient information about the second paragraph. Here is the whole title of the textbook.
Fluid Mechanics Thermodynamics of Turbomachinery
S.L.Dixon
Pergamon Press, 3rd edition 1978
On page 38, he said that "The adiabatic efficiency, although fundamentally valid, can be misleading if used for comparing the efficiencies of turbomachines of differing pressure ratios."
I cannot understand why in such a case the polytropic efficiency is better. I look forward to your another reply.
RE: Polytropic efficiency of turbomachinery
One exception is of course air comprssors which have historically often been stated on an isentropic basis, but of course always use the same gas (air).
Furher complicating, If a Mollier Chart (or Real gas equations) are used to calculate performance, the polytropic impeller ratings must be converted to adiabatic head and efficiency in order to compute delta H along constant entropy lines. Again, since historically gas compressors state performance as polytropic, the results are converted back to a polytropic basis once the overall calculation is done.
The conversion is pretty easy since Work = Polytropic Head/Polytropic eff and also = Adiabatic Head / Adiabatic eff.
As long as you aren't going to change the gas, performance can be stated on either an adiabatic or polytropic basis.
All of the above comments apply to how manufacturers use base ratings when calculation machine performance.
RE: Polytropic efficiency of turbomachinery
RE: Polytropic efficiency of turbomachinery
Ie. A compression or expansion process caused by movment of a piston in a cylinder.
RE: Polytropic efficiency of turbomachinery
RE: Polytropic efficiency of turbomachinery
Polytropic process
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