Assumption (1) is related to the behavior of point C in Fig. 1. If the present refrigerant state at gas-cooler outlet is C1,l, when the heat rejection pressure rises by small amount, it moves along the line T1. Assumption (2) determines the variation of shape of line AB in Fig. 1. If the assumption is applied, the slope of line AB will not change when the expansion valve is closed by small amount.