In many studies the pre-cooler and the pre-heater (Fig. 5b) are installed to the conventional system, in order to improve the system efficiency.The temperature of the refrigerant from the condenser is slightly increased and decreased before entering the boiler and evaporator, respectively. Therefore, the required heat input and the cooling load of the system are reduced. Apart from using the mechanical driven pump, there were two alternate ways to return the liquid refrigerant to the boiler. The first alternative was proposed by Riffat and Holt. The principal of heat pipe is applied to an ejector. The condensed liquid refrigerant from the condenser returned through the wick by capillary action to the boiler. Another practical alternative was achieved by using the gravitational head difference between a condenser and a boiler. The requirement of the recirculation pump is eliminated. Thus, the system becomes the purely heat-operated cycle.