Chapter 7

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Unformatted text preview: negligible. 3 The kinetic and potential energy changes are negligible, ∆ke ∆pe 0. Analysis Noting that h2 h1, the entropy of the steam at the inlet and the exit states is determined from the steam tables to be State 1: P1 T1 7 MPa 450°C h1 s1 3287.1 kJ/kg 6.6327 kJ/kg · K State 2: P2 h2 3 MPa h1 s2 7.0018 kJ/kg · K Then the entropy generation per unit mass of the steam is determined from the entropy balance applied to the throttling valve, { · Sgen 0 (steady) · ∆Ssystem 14243 Rate of entropy generation Rate of change of entropy · · Sin Sout 14243 Rate of net entropy transfer by heat and mass · m s1 · m s2 · Sgen · Sgen 0 · m (s2 s1) Dividing by mass flow rate and substituting gives sgen s2 s1 7.0018 6.6327 0.3691 kJ/kg · K This is the amount of entropy generated per unit mass of steam as it is throttled from the inlet state to the final pressure, and it is caused by unrestrained expansion. The increase of entropy principle is obviously satisfied during this process since the entropy generation is...
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This document was uploaded on 11/28/2012.

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