The Refrigeration Cycle IDEAL CYCLE
An ideal reversible cycle based on the two temperatures of the system in Example 1.1 can be drawn on a temperature-entropy basis (see Figure 2.1).
(b) Entropy, s
Figure 2.1 The ideal reversed Carnot cycle: (a) circuit and (b) temperature-entropy diagram
In this cycle a unit mass of fluid is subjected to four processes after which it returns to its original state. The compression and expansion processes, shown as vertical lines, take place at constant entropy. A constant entropy (isentropic) process is a reversible or an ideal process. Ideal expansion and compression engines are defined in Section 1.2. The criterion of perfection is that no entropy is generated during the process, i. e. the quantity V remains constant. The addition and rejection of heat takes place at constant temperature and these processes are shown as horizontal lines. Work is transferred into the system during compression and out of the system during expansion. Heat is transferred across
the boundaries of the system at constant temperatures during evaporation and condensation. In this cycle the net quantities of work and heat are in proportions which provide the maximum amount of cooling for the minimum amount of work. The ratio is the Carnot coefficient of performance (COP).
This cycle is sometimes referred to as a reversed Carnot cycle because the original concept was a heat engine and for power generation the cycle operates in a clockwise direction, generating net work.
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