REFRIGERATING ENGINEERING The purpose of any expansion device is twofold: to accomplish the throttling process and maintain the pressure difference between the condenser and the evaporator to adjust the refrigerant flow rate to the evaporator;keeping the evaporator filled with refrigerant liquid,yet preventing the liquid to be carried over to the compressor where it might cause damages through liquid hammer. By this uncomplicated expansion device,the pressure as well as the temperature can be reduced from the level existing in the condenser (including subcooler)to the level wanted in the evaporator. As mentioned earlier,the cycle has a high pressure side and a low pressure side,and the boundary line is at the expansion device and the compressor. The entire heat extraction,i.e.the refrigerating effect,occurs on the low pressure side of the cycle,and the heat rejection takes place on the high pressure side. The process in the expansion valve completes the cycle.Here a throttling process takes place and we obtain the well known condition that the enthalpy of the refrigerant has the same value before and after the reduction of the pressure.This relation is readily obtained from the energy equation,Equation 3.05b,with q=0 and &=0,leaving ha=ha 3.10 Such a process is usually referred to as a throttling process.The thermodynamic state at the outlet of the expansion valve,i.e.the inlet to the evaporator,is located in the diagram by following the line of constant enthalpy from point d to the intersection with the line representing the evaporating pressure,point a. The pressure of the refrigerant is reduced in the throttling process and the liquid begins to vaporize,and a portion of the liquid turns into vapor.This portion must be supplied with the latent heat of vaporization.This energy is taken from the fluid itself,resulting in a reduction of the temperature,from Ts to T2.The low temperature of the cycle is created in the expansion device! Normally,the expansion valve is installed close to the evaporator,and when the refrigerant passes through the valve,we have completed one loop of the cycle,and the course of events will repeat again. (Equation 3.10 does not hold for one of the most frequent applications,namely household refrigerators.These domestic units are usually equipped with a capillary tube where the refrigerant in the expansion process is brought into heat exchange with the vapor leaving the evaporator.To analyze this we have to apply Equation 3.05b and the situation resembles that which is treated in sections 3.41-42.) 3.11 State point a represents a mixture of saturated liquid,at state a',and saturated vapor,at state a".Calculations on thermodynamic properties of such mixtures are strongly simplified through the introduction of the concept quality of the mixture,defined as the 3:10