# ch10-lec - Learning Objectives 1. Analyze vapor power...

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Learning Objectives 1. Analyze vapor power cycles in which the working fluid is alternately vaporized and condensed. 2. Investigate ways to modify the basic Rankine vapor power cycle to increase the cycle thermal efficiency.

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Chapter 10: Vapor and Combined Power Cycles Overview In this chapter, we consider vapor power cycles in which the working fluid is alternatively vaporized and condensed 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 vaporization 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 Regardless of the fuel, the steam goes through the same basic cycle
10-1 The Carnot Vapor Cycle Carnot Vapor Cycle Because the Carnot cycle is the most efficient cycle operating between two specified temperature limits, it is natural to look at the Carnot cycle first as a prospective ideal cycle for vapor power plants It will turn out that the Carnot cycle is not a suitable model for vapor power cycles Consider a steady-flow Carnot cycle executed within the saturation dome of a pure substance, as shown below The cycle consists of the following four processes 1→2 Isothermal reversible heating in a boiler 2→3 Isentropic expansion in a turbine 3→4 Isothermal reversible condensation in a boiler 4→1 Isentropic compression by a compressor

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10-1 The Carnot Vapor Cycle Impracticalities of the Carnot Vapor Cycle 1. Limited maximum temperature Limiting the heat transfer process to two-phase systems, which makes isothermal heat transfer possible, severely limits the maximum temperature that can be used in the cycle (E.g. for water the critical temperature is 374 o C) Limiting the maximum temperature in the cycle limits the thermal efficiency An attempt to increase the maximum temperature will involve heat transfer to the working fluid in a single phase, which is not easy to accomplish isothermally
10-1 The Carnot Vapor Cycle Impracticalities of the Carnot Vapor Cycle 1. High moisture Content in Turbine The quality of the steam decreases during the isentropic expansion in the turbine (process 2→3 ) The impingement of liquid droplets on the turbine blades causes erosion and is a major source of wear In practice, steam with a quality less than 90% cannot be tolerated

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## This note was uploaded on 04/20/2010 for the course M E 320 taught by Professor Deinert during the Spring '08 term at University of Texas at Austin.

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ch10-lec - Learning Objectives 1. Analyze vapor power...

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