CHAPTER 3.THE VAPOR COMPRESSION CYCLE In the equation q net amount of heat transferred to the working fluid =net amount of shaft work delivered from the working fluid h specific enthalpy for the working fluid w=velocity of the working fluid z elevation above an arbitrarily datum level g =acceleration due to gravity. In vapor compression systems,changes in the potential energy (gz)are mostly negligible,and so are normally changes in the kinetic energy(w/2),leaving the energy equation in the simple form q=a+h2-h 3.05b Important notice:Heat transfer and work per unit mass exposed to a process are designated by the lower-case letters g and &respectively.The unit for g and eis J/kg or kJ/kg.The total amount of heat transfer and work in J or kJ are designated by the capital letters O and E,and are obtained from g and &by multiplication with the mass m in kg involved in the process,e.g.O=m.q.The corresponding rates of energy transfer in W or kW are symbolized by a dot,i.e.Oand E,and the multiplication is now with the mass flow rate m in kg/s,e.g.Q=mq.(Later on,in sections 3.30 and 3.32,two volumetric quantities gv and in J/m'or kJ/m'are introduced.) B.Simple vapor compression cycle B1.The simple cycle in T-s and p-h diagrams 3.06 Figure 3.06 shows the cycle for a simple vapor compression system according to Figure 3.04 in T-s and p-h diagrams.As explained previously,the main part of the cycle is executed in the wet region(also called the liquid-vapor saturation region,the two-phase region or the vapor dome),where the refrigerant undergoes vaporization and condensation.The compressor and the expansion device maintain a high pressure side, with the pressure pi,and a low pressure side,with the pressure p2.The corresponding condensing temperature and evaporating temperature are denoted Ti and T2, respectively. 3.07 The evaporator is a container or a pipe system wherein the refrigerant vaporizes at a low temperature.The latent heat of vaporization is taken from the surroundings of the evaporator,in this case the air in a refrigerated space.Heat is removed from that space and the temperature in the space is kept at a low,desired,level. The compressor sucks away the formed vapor,thereby keeping the pressure at a low level,allowing the vaporization to be maintained.To make the heat transfer possible, from the air in the refrigerated space to the refrigerant,the boiling temperature of the refrigerant,i.e.the evaporating temperature,has to be below the temperature of the air in the refrigerated space. 3:5