S the size of the colored area the saved work input

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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 THERMAL-FLUID SCIENCES P P2 T Work saved P2 Px 2 P1 Polytropic 2 Px T1 Intercooling 1 Intercooling Isothermal P1 1 FIGURE 7–46 P-υ and T-s diagrams for a two-stage steady-flow 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 two-stage 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.

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