Lines of constant pressure originate on the saturated water line. Since the temperature and pressure at which water boils are in a fixed relationship to each other, Figure 2.15.1 could equally be drawn to show enthalpy against temperature, and then turned so that temperature became the vertical ordinates against a base of enthalpy, as in Figure 2.15.2. To see how this works, perhaps it is best to start off with a diagram showing how the enthalpy content of a kilogram of water increases as it is heated to different pressures and evaporated into steam. A more practical approach is to define entropy as energy added to or removed from a system, divided by the mean absolute temperature over which the change takes place. That may be of some philosophical interest, but does not help very much in the calculation of actual values. The increase of entropy in the overall system always takes place in the same direction as time flows.
If the entropy of a system is calculated at two different conditions, then the condition at which the entropy is greater occurs at a later time. This compulsion to spread out has led some observers to label entropy as ‘time’s arrow’. It is, in some ways, a measure of the lack of quality or availability of energy, and of how energy tends always to spread out from a high temperature source to a wider area at a lower temperature level. Entropy values can then be listed and used in calculations in particular, calculations to do with steam flow, and the production of power using turbines or reciprocating engines. Rather, it must be calculated from things that can be measured. A sensor cannot detect it, and it does not show on a gauge. In some ways, it is easier to say what it is not! It is not a physical property of steam like pressure or temperature or mass.
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