Effect of Suction Pressure:

The suction pressure (or evaporator pressure) reduced because of the frictional resistance of flow of the refrigerant. Let us assume a theoretical vapour compression cycle 1-2-3-4 while the suction pressure reduced from p_s to p_s' as indicated on p-h diagram in given fig.

It can be noted down that the decrease in suction pressure:

 (a) Decreases the refrigerating effect from (h₁ - h₄) to ( h₁' - h₄ '), and

 (b) Increases the work needed for compression from (h₂ - h₁) to ( h₂' - h₁').

                                         Effect of Suction Pressure

As the C.O.P, of the system is the ratio of refrigerating effect to the work done, so with the decrease in suction pressure, the total effect is to decrease in the C.O.P. of the refrigerating system for the similar refrigerant flow. That's why with the decrease in suction pressure the capacity of refrigerating of the system decreases & the refrigeration expense increases.