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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