Diagrams of Ideal and Real Processes

Figure 23 Control Volume for Second Law Analysis

Power Plant Components - h1012v1_104

Thermodynamics Heat Transfer and Fluid Flow Volume 1 of 3
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Thermodynamics SECOND LAW OF THERMODYNAMICS Solution: where = entropy added to the system ms_in p ms_out p p m (s_out s_in) s_in = 1.7088 Btu/lbm -°R (from steam tables) s_out = 1.8158 Btu/lbm°R (from steam tables) = Btu/lbm-^oR p/ m s_out s_in 1.8158 1.7088 = 0.107 Btu/lbm -°R p/ m = 10,000 (0.107) p = 1070 Btu/lbm -°R. = entropy added to the system p It should always be kept in mind that the Second Law of Thermodynamics gives an upper limit (which is never reached in physical systems) to how efficiently a thermodynamic system can perform. A determination of that efficiency is as simple as knowing the inlet and exit temperatures of the overall system (one that works in a cycle) and applying Carnot’s efficiency equation using these temperatures in absolute degrees. Diagrams of Ideal and Real Processes Any ideal thermodynamic process can be drawn as a path on a property diagram, such as a T-s or an h-s diagram. A real process that approximates the ideal process can also be represented on the same diagrams (usually with the use of dashed lines). In an ideal process involving either a reversible expansion or a reversible compression, the entropy will be constant. These isentropic processes will be represented by vertical lines on either T-s or h-s diagrams, since entropy is on the horizontal axis and its value does not change. A real expansion or compression process operating between the same pressures as the ideal process will look much the same, but the dashed lines representing the real process will slant slightly towards the right since the entropy will increase from the start to the end of the process. Figures 24 and 25 show ideal and real expansion and compression processes on T-s and h-s diagrams. Rev. 0 Page 77 HT-01

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