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Unformatted text preview: ween the same pressure limits. cen54261_ch07.qxd 11/18/03 9:57 AM Page 310 310
FUNDAMENTALS OF THERMALFLUID SCIENCES
P P2 T
Work saved P2 Px 2 P1 Polytropic
2
Px T1 Intercooling 1
Intercooling Isothermal
P1 1 FIGURE 7–46
Pυ and Ts diagrams for a twostage
steadyflow compression process. υ s The size of the colored area (the saved work input) varies with the value of
the intermediate pressure Px, and it is of practical interest to determine the
conditions under which this area is maximized. The total work input for a twostage compressor is the sum of the work inputs for each stage of compression,
as determined from Eq. 7–57b:
wcomp, in wcomp I, in wcomp II, in
(n 1)/n
nRT1 Px
1
n 1 P1 (7–58) nRT1
n1 P2
Px (n 1)/n 1 The only variable in this equation is Px. The Px value that will minimize the
total work is determined by differentiating this expression with respect to Px
and setting the resulting expression equal to zero. It yields
Px (P1P2)1/2 or Px
P1 P2
Px (7–59) That is, to minimize compression work during twostage compression, the
pressure ratio across each stage of the compressor must be the same. When
this condition...
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This document was uploaded on 11/28/2012.
 Spring '09

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