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.