Starting with its defining relation the entropy

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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 refrigerant-134a, 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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