a) Process 1-2: Isentropic compression in compressor
Vapour refrigerant is compressed to increase pressure from P1 to P2. This results in increase in temperature from t1 to t2.
P1=$P_E$ i.e.evaporator pressure
P2= $P_C$ i.e.condenser pressure
b) Process 2-3: Constant pressure heat rejection in condenser
Heat is rejected by superheated vapour refrigerant in condenser to the ambient air. This results in condensation of vapour into saturated liquid at pressure Pc.
c) Process 3-4: Throttling process in expansion valve
It is an isentropic process which results in pressure drop from Pc to P_E. This process involves increase in entropy which results in conversion of part of liquid refrigerant into vapour.
d) Process 4-1: Constant pressure heat addition in evaporator
Wet vapour refrigerant absorbs heat from air in refrigerating space. This converts wet vapour refrigerant into dry saturated vapour. This causes refrigeration.