CHAPTER 3.THE VAPOR COMPRESSION CYCLE The refrigerating effect g2 is the quantity of heat that each unit mass of refrigerant absorbs from the refrigerated space.The unit for the refrigerating effect is the same as for the enthalpy,i.e.J/kg or kJ/kg If the mass flow rate of the refrigerant is denoted m (in kg/s)then the refrigerating capacity (in Swedish kyleffekt)(in W or kW)is Q=m(hak-hs) 3.07b The vapor is conveyed to the suction inlet of the compressor through the suction line,or the low-pressure line. 3.08 The compressor might be one of several types,e.g.reciprocating,rotary,scroll,screw, or centrifugal compressor.The purpose of the compressor is to remove the vapor from the evaporator and force it into the condenser.The compressor maintains such a low pressure in the evaporator so as to enable the refrigerant to vaporize,and at the same time maintain such a high pressure in the condenser as to allow the refrigerant to condense at the temperature prevailing there.The work input to the refrigerant in the compressor makes it possible to lift the heat absorbed in the evaporator to a higher temperature level,where the heat can be rejected to ambient air or to cooling water. The compressor creates the necessary pressure difference and together with the expansion device it maintains that difference between the low pressure side and the high pressure side of the cycle. The process in the compressor might often be considered as adiabatic,with no heat transfer.The work is commonly supplied by an electric motor,although internal combustion engines (especially in systems for automobile air-conditioning),steam engines,and steam turbines are sometimes used. The refrigerant enters the compressor as a slightly superheated vapor,point b.In the theoretical cycle the compression process is assumed to be isentropic,presupposing negligible heat exchange with the surroundings.In an isentropic process the entropy s is constant and the line b-cis coincides with a line of constant entropy,starting at point b and ending at the condensing pressure pi,point cis.Since work is done on the vapor during the compression,the enthalpy of the refrigerant is increased from hak to hIk is, theoretically.From the energy equation,Equation 3.05b,the required work of isentropic compression on a per unit mass basis is Ed=h1k.is -h2k 3.08a where the index d denotes that the values are ideal,from the diagram. With the mass flow rate m,the power needed to operate an isentropic compressor is 店=ia 3.08b 3:7