Fig. 10 shows that the controlled evaporating temperature does not change much while the expansion valve opening is varied. Hence, the evaporator pressure does not vary much until refrigerant is superheated. Heat transfer characteristics in superheated vapor region become drastically worse and after occurrence of superheated vapor in evaporator, the capacity controller makes evaporator pressure to be lowered. The phenomenon can be found in Fig. 10. The occurrence of superheated region is represented by degree of superheat (DSH) at evaporator outlet and it is shown that the evaporator pressure decreases rapidly with increase of DSH at evaporator outlet. The gas-cooler pressure, however, does not maintain the present pressure as expansion valve is closed. Small mass flow rate causes rapider increase of refrigerant quality in evaporator and refrigerant amount which is contained in evaporator becomes reduced. The excessive refrigerant moves to gas-cooler and gas-cooler pressure arises as time goes by. Near the optimal point, DSH at evaporator outlet is generally 0 and in this reason, evaporator pressure change is not so much significant compared with the pressure change of gas-cooler. Fig. 11 shows the absolute value of changed pressure when the expansion valve opening is changed by one step from the previous state. The below equation (Eq. (10)) shows how the changed pressure is calculated.