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Unformatted text preview: ate. Starting with its
defining relation, the entropy change of a substance can be expressed in terms
of other properties (see Section 7–7). But in general, these relations are too
complicated and are not practical to use for hand calculations. Therefore, using a suitable reference state, the entropies of substances are evaluated from
measurable property data following rather involved computations, and the results are tabulated in the same manner as the other properties such as υ, u, and
h (Fig. 7–10).
The entropy values in the property tables are given relative to an arbitrary
reference state. In steam tables the entropy of saturated liquid sf at 0.01°C is
assigned the value of zero. For refrigerant134a, the zero value is assigned to
saturated liquid at 40°C. The entropy values become negative at temperatures below the reference value.
The value of entropy at a specified state is determined just like any other
property. In the compressed liquid and superheated vapor regions, it can be T } P1
s ≅s
T1 1 ƒ@T1 } Compressed
liquid
1 2 T3
s
P3 3
Superheated
vapor Saturated
liquid–vapor mixture 3 } T2
s = sƒ + x2sƒg
x2 2 s FIGURE 7–10
The entropy of a pure substance is
determined from the tables
(like...
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 Spring '09

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