Page 234 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

The Carnot efficiency of a mechanical refrigeration system can be expressed by the reversible coefficient
of performance, COP :
REV

Because all the processes for a Carnot engine must be reversible, the COP REV gives the best theoretical
performance of a refrigeration system. Thus the net required power (compressor-expansion turbine) will

always be greater than that predicted by the equation above using COP REV . Nevertheless, it is clear that
as the temperature difference between the evaporator and condenser increases then the work required per
unit of energy removed in the evaporator (refrigerator) increases. Therefore, the operating costs for
refrigeration will increase as the temperature at which the refrigeration is required decreases. The
condensation of the working fluid will most often be achieved using cooling water, so a reasonable
condensing temperature would be 45°C (giving a 5°C approach in the condensing exchanger). Figure 8.4
illustrates the effect of the evaporator temperature on the reversible work required for a given cooling
load. This figure gives an approximate guide to the relative cost of refrigeration. The relative costs of

refrigeration at different temperatures are explored in Example 8.4.

Figure 8.4 Ideal Work for Refrigeration Cycles as a Function of Refrigeration Temperature