SEM314 Topic 7 - Steam Plant

SEM314 Topic 7 - Steam Plant - STUDY TOPIC: STEAM PLANT...

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STUDY TOPIC: STEAM PLANT Reference: Chapter 8 in the unit Text Book. Previously, you learned the basics of thermodynamics including basic definition, first and second laws of thermodynamics heat engine cycles. From these study, we know that the most efficient cycle (for heat engine) is the Cornot cycle for given temperatures of source and sink. If the wet vapour is used as working fluid, the Carnot cycle is shown in a T-S diagram below. Figure 1.1 Its essential features is: Process 4 −> 1: heat is supplied at constant temperature and pressure. Process 1 −> 2: the vapour expands isentropically. In doing so it does work, which is the purpose of the cycle. Process 2 −> 3: the wet vapour at state 2 has to be cooled to point isothermally. Process 3 −> 4: the rich wet vapour at state 3 returns to its original state at 4 by isentropic compression. Key points of this chapter The simplest form of saturated Rankine cycle has a high work ratio but a disappointingly low thermal efficiency. The thermal efficiency can be improved, in theory, by making the cycle more like a Carnot cycle, and by increasing the maximum temperature of the cycle.
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The main way the cycle can be made more like a Carnot cycle is to add more of the heat near the maximum temperature. The maximum temperature can be increased by increasing the pressure, and by superheating the system. More of the heat can be added near the maximum temperature by re-heating the steam and having two turbines, and by bleeding off part of the steam to heat the feed water. Steam turbines often have wet steam at the exit. The water drops can sometimes damage the turbine. The turbine or the whole cycle can be modified to prevent damage occurring. Re-heating the steam also has the advantage of making the steam drier at the turbine exit. The Rankine cycle Deficiencies of Carnot cycle as the ideal cycle for a vapour are: 1. Work ratio (Net work/Gross work) is low. 2. Practically it is difficult to stop condensation at point 3 and then compress is just to state 4. It is more convenient to allow condensation process to proceed to completion and then pump the liquid to boiler pressure at state 4 (see Figure 1.2). The advantage of pumping liquid are that the pump is smaller, the compression process is carried out more efficiently and the equipment required is simpler and less expensive than pumping the vapour. In this modified cycle, water entering the boiler is not at the saturation temperature corresponding to the boiler pressure. So, heat must be supplied at constant pressure to change water at state 4 to saturated water at state 5. This is no longer a constant temperature heating process, so efficiency of the cycle shown in Figure 1.2 is less than that of the Carnot cycle. This cycle is called Rankine cycle.
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SEM314 Topic 7 - Steam Plant - STUDY TOPIC: STEAM PLANT...

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