CHAPTER10 - cen84959_ch10.qxd 2:17 PM Page 551 Chapter 10...

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Chapter 10 VAPOR AND COMBINED POWER CYCLES | 551 I n Chap. 9 we discussed gas power cycles for which the working fluid remains a gas throughout the entire cycle. In this chapter, we consider vapor power cycles in which the working fluid is alternatively vaporized and condensed. We also consider power generation coupled with process heating called cogeneration. The continued quest for higher thermal efficiencies has resulted in some innovative modifications to the basic vapor power cycle. Among these, we discuss the reheat and regen- erative cycles, as well as combined gas–vapor power cycles. Steam is the most common working fluid used in vapor power cycles because of its many desirable characteristics, such as low cost, availability, and high enthalpy of vaporiza- tion. Therefore, this chapter is mostly devoted to the discus- sion of steam power plants. Steam power plants are commonly referred to as coal plants, nuclear plants, or natural gas plants, depending on the type of fuel used to supply heat to the steam. However, the steam goes through the same basic cycle in all of them. Therefore, all can be analyzed in the same manner. Objectives The objectives of Chapter 10 are to: Analyze vapor power cycles in which the working fluid is alternately vaporized and condensed. Analyze power generation coupled with process heating called cogeneration . Investigate ways to modify the basic Rankine vapor power cycle to increase the cycle thermal efficiency. Analyze the reheat and regenerative vapor power cycles. Analyze power cycles that consist of two separate cycles known as combined cycles and binary cycles.
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10–1 THE CARNOT VAPOR CYCLE We have mentioned repeatedly that the Carnot cycle is the most efficient cycle operating between two specified temperature limits. Thus it is natural to look at the Carnot cycle first as a prospective ideal cycle for vapor power plants. If we could, we would certainly adopt it as the ideal cycle. As explained below, however, the Carnot cycle is not a suitable model for power cycles. Throughout the discussions, we assume steam to be the work- ing fluid since it is the working fluid predominantly used in vapor power cycles. Consider a steady-flow Carnot cycle executed within the saturation dome of a pure substance, as shown in Fig. 10-1 a . The fluid is heated reversibly and isothermally in a boiler (process 1-2), expanded isentropically in a tur- bine (process 2-3), condensed reversibly and isothermally in a condenser (process 3-4), and compressed isentropically by a compressor to the initial state (process 4-1). Several impracticalities are associated with this cycle: 1. Isothermal heat transfer to or from a two-phase system is not diffi- cult to achieve in practice since maintaining a constant pressure in the device automatically fixes the temperature at the saturation value. Therefore, processes 1-2 and 3-4 can be approached closely in actual boilers and con- densers. Limiting the heat transfer processes to two-phase systems, how-
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