Conditions for Highest COP:

Effect of Evaporator Pressure

Assume a simple saturation cycle 1-2-3-4 along Freon 12 as the refrigerant as indicated in given fig. for operating conditions of t_k = 40°C & t = - 5°C.

Now assume a change in the evaporator pressure equivalent to a decrease in the evaporator temperature to - 10°C. The changed cycle is indicated as 1'-2'-3'-4' in fig.

Therefore It is seen that a drop in evaporator pressure equivalent to a drop of 5°C in saturated suction temperature enhance the volume of suction vapour and therefore reduced the capacity of a reciprocating compressor and enhance the power consumption per unit refrigeration.

                                   Effect of Evaporator Pressure

It is observed that a reduction in evaporator temperature results in :

 (a) Reduction in refrigerating effect from (h₁ - h₄) to (h₁' - h₄')

 (b) Enhancement in the particular volume of suction vapour from v₁ to v₁'

 (c) Reduction in volumetric efficiency, because of enhancement in the pressure ratio, from ?_v to ?_v '

 (d) Enhancement in compressor work from (h₂ - h₁) to (h₂' - h₁') because of enhancement in the pressure ratio with change from steeper isentropic 1-2 to flatter isentropic 1'-2.

Since

  . . . (2.1)

and

W *  = m* w                     . . . (2.2)

Expressions for the dependence of unit power consumption and capacity can now be written as: