REFRIGERATING ENGINEERING p C d T T P 6 T P2 a a ha=h。=h 92 9 h 片 h h2 hax k Fig.3.06.The simple cycle in T-s and p-h diagrams. The pressure in the evaporator is called the evaporating pressure.For a pure refrigerant, a certain evaporating pressure corresponds to a specific evaporating temperature according to the vapor pressure curve,the relation between saturated vapor pressure and temperature.Figure 3.03 illustrates this relation for some refrigerants. For example with ammonia,NH3,as the refrigerant,and presuming that the compressor maintains the evaporating pressure equal to 2,9 bar,then the evaporating temperature will be-10C.The evaporating temperature will decrease if the evaporating pressure is depressed. Under steady-state conditions,the evaporator is supplied with a continuous flow of refrigerant which is vaporized successively by the heat transferred from the refrigerated space.The pressure drop in the flow direction of the refrigerant is normally small and, consequently,the heat transfer to the refrigerant takes place under essentially constant pressure and constant temperature. In the evaporator it is a constant-pressure,almost constant-temperature process, indicated by the line a-b in the diagrams.At the inlet to the evaporator,point a,the refrigerant consists of a mixture of saturated liquid and saturated vapor having the average enthalpy hs.Heat transfer from the refrigerated space causes the liquid to evaporate and change the mixture of liquid and vapor into a saturated vapor.In practice, however,the cold vapor usually will continue to absorb heat from the refrigerated space and become slightly superheated before it leaves the evaporator.At the outlet,point b, the enthalpy is hak. According to the energy equation,Equation 3.05b,with=0 for the evaporator,the refrigerating effect (in Swedish koldalstring)is 92=h2k-hs 3.07a 3:6