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Ebook Thermodynamics for engineers: Part 2

(BQ) Part 2 book "Thermodynamics for engineers" has contents: Power and refrigeration vapor cycles, power and refrigeration vapor cycles, thermodynamic relations, mixtures and solutions, combustion. | Chapter 8 Power and Refrigeration Vapor Cycles 8.1 INTRODUCTION The ideal Carnot cycle is used as a model to compare all real and all other ideal cycles against. The efficiency of a Carnot power cycle is the maximum possible for any power cycle; it is given by Note that the efficiency is increased by raising the temperature TH at which heat is added or by lowering the temperature Tt at which heat is rejected. We will observe that this carries over to real cycles: the cycle efficiency can be maximized by using the highest maximum temperature and the lowest minimum temperature. We will first discuss vapor cycles that are used to generate power, then vapor cycles that are used to refrigerate or heat a space. Chapter 9 will examine gas cycles with both power and refrigeration applications. 8.2 THE RANKINE CYCLE The first class of power cycles that we consider are those utilized by the electric power generating industry, namely, power cycles that operate in such a way that the working fluid changes phase from a liquid to a vapor. The simplest vapor power cycle is called the Rankine cycle, shown schematically in Fig. 8-la. A major feature of such a cycle is that the pump requires very little work to deliver high-pressure water to the boiler. A possible disadvantage is that the expansion process in the turbine usually enters the quality region, resulting in the formation of liquid droplets that may damage the turbine blades. The Rankine cycle is an idealized cycle in which losses in each of the four components are neglected. The losses usually are quite small and will be neglected completely in our initial analysis. The Rankine cycle is composed of the four ideal processes shown on the T-s diagram in Fig. 8 - l b : 1 2 3 4 --j 2: + 3: -+ 4: -+ 1: Isentropic compression in a pump Constant-pressure heat addition in a boiler Isentropic expansion in a turbine Constant-pressure heat extraction in a condenser If we neglect kinetic energy and potential energy .