The size of the evaporator selected is dictated by the design temperature difference, namely the temperature difference between the entering fluid and the refrigerant. Typical values for this temperature difference are discussed in Sec. 6.16 on the selection of air-cooling coils. In many situations the choice of this temperature difference, and thus the size of the evaporator, is purely economic, with trends as shown in Fig. 6.13. An increase in evaporator area results in a lower temperature difference (high evaporating temperature) with the consequent saving in compressor power over the life of the facility.
The compressor energy costs which occur throughout the life of the system are reflected back to a present value using an applicable interest rate.