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polytropic efficienvy

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hassann

Civil/Environmental
May 7, 2018
51
polytropic efficiency formula used in compressors is equal to"

((?-1)/?)/((n-1)/n)) that ?=gamma

foe example when n=1.3 and ?=1.4
polytropic efficiency is:
(.4/1.4)/(.3/1.3)= .29 / .23= 1,24

polytropic efficiency for this compressors is equal 1.24.(greater than 1.00)

We know that the maximum polytropic efficiency for ideal process is equal to 1.00. Could someone please correct me on my interpretation of compression polytropic efficiency, greater than 1.00.


 
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That is polytropic exponent ( not eff.) used only for the purposes of computing compressor power and polytropic head.
 
as mentioned above the following
((?-1)/?)/((n-1)/n)) is polytropic eff
n is polytropic exponent.
? is isentropic exponent (gamma)
 
Ok
Its the other way around.
n/(n-1) = k/(k-1).Ep, where Ep = poly eff, as a ratio <1
See GPSA in the section on polytropic path compressor calculation
 

dear
georgeverghese (Chemical)
your formula (gpsa) is exately same as my formula.

(k-1)/k / (n-1)/n = n/(n-1) / k/(k-1)

therefor based on each formula polytropic eff is 1.24 > 1 !?
 
Pls note that n, the polytropic exponent, is always > k, the isentropic exponent.
For example, when k=1.18, Ep=0.751, and n = 1.255
See also fig 10-65 in Perry 7th edn.
 
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