SEM314 Topic 3 - Working Fluids

SEM314 Topic 3 - Working Fluids - Topic 3 Working fluids...

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Deakin University 2003 Topic 3 Working fluids Throughout this topic you will need to refer to chapter 2 in your textbook. Topic 1 covered definitions/concepts such as ‘system’, ‘working fluid’, and ‘system boundary’. Topic 2 introduced the First Law of Thermodynamics, the closed system energy equation, and the open system (including steady flow) energy equation. Nowhere in this work was it necessary to know exactly what the working fluid was. This topic covers chapter 2 in your textbook where the working fluid is considered to be either steam or a perfect gas. As you work through this chapter, I suggest you tabulate the important formulas with the conditions under which they can be used, like ‘vapour only’, ‘perfect gas only’ and ‘either vapour or perfect gas’. A study of the derivation of each will show which category it belongs to. Steam The working fluid to which the phase change (liquid to vapour, or vapour to liquid) may occur, in the cases we are talking about, is considered in sections 2.1 and 2.2 in your textbook. Here, for the moment, we are just concerned with steam as the working fluid. A look around industrial complexes will soon convince you of its importance. Normally, the properties of a vapour do not conform to nice mathematical descriptions. The position of the saturated liquid line and the saturated vapour line on a P-V chart has been determined by experiments. Note: Every point, on a P-V chart (or other chart), represents a particular state of the working fluid, which is defined by the correspondent properties P and V. If the point is located on the left-hand side of the saturated curve, the working fluid is in sub-cooled liquid condition. If on the right-hand side of the saturated curve, it is superheated (vapour, for water we call it steam).
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3-2 Principles of Thermodynamics Deakin University 2003 Figure 3.1 Saturation curve of stream on P-V diagram The steam tables consisting of the saturated table and the superheated table, which are established by experimental testings, provide us with the values of the properties of steam or water in various states. Once the state of the steam or water is fixed, you are able to read out all the thermodynamic properties that relate to the steam or water at that state. Therefore, fixing the state is essential before you try to use steam tables or charts. To fix the state: For saturated steam or liquid , you have to know the value of at least one of its properties, for example, T or P; for superheated vapour , you need to know at least two properties. If the vapour is wet (i.e. it is a mixture of saturated liquid and saturated steam), you need to know two properties and then using the saturated steam table and formula, you can calculate all other properties you want to know.
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This note was uploaded on 07/26/2009 for the course ENG sem314 taught by Professor Dunno during the Spring '09 term at A.T. Still University.

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SEM314 Topic 3 - Working Fluids - Topic 3 Working fluids...

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