Chemical Engineering thermodynamics – II                                                                   Unit V - Refrigeration 
                CHEMCAL  ENGINEERING THERMODYNAMICS – II 
                 
                UNIT V – REFRIEGERATION 
                 
                Principles of refrigeration 
                Methods of producing Refrigeration 
                Liquefaction Process 
                Coefficient of performance 
                Evaluation  of the performance of vapor compression and gas refrigeration cycles 
                 
                                                        REFRIGERATION 
                 
                        Refrigeration  is  process  of  producing  and  maintaining  a  temperature  below  that  of  the 
                surrounding atmosphere.  This requires continuous absorption of heat at a low temperature level, 
                usually accomplished by evaporation of a liquid in a steady state flow process.  The vapor formed may 
                be returned to its original liquid state for re-evaporation, either by compressing and condensing or by 
                absorbing it with a liquid of low volatility from which it is subsequently separated at high pressure.  Thus 
                refrigeration is essentially an operation involving the pumping of heat from one temperature to a higher 
                temperature.  The complete series of processes that the working fluid - the refrigerant – undergoes 
                constitute  a  refrigeration  cycle.    A  typical  refrigeration  cycle  includes,  evaporation  of  the  liquid 
                refrigerant,  compression of  the  refrigerant  vapor,  condensation  of  the  vapor  into  liquid  and  finally 
                expansion of the liquid. 
                 
                CARNOT REFRIGERATOR 
                 
                        A refrigeration cycle is a reversed heat engine cycle.  Heat is transferred from a low temperature 
                level to  a higher temperature level.  According second law of thermodynamics this requires an external 
                source of energy.  The ideal refrigerator like the ideal heat engine operates on a Carnot cycle, consisting 
                of two isothermal steps in which heat lQcl is is absorbed at a lower temperature level Tc and heat lQ l is 
                                                                                                                 H
                rejected at higher temperature T  and two adiabatic cycles steps.  The cycle requires the addition of net 
                                                H
                work w to the system.  Since ∆U of the working fluid is zero from the first law of thermodynamics.,  
                 
                                               W = lQ l –lQcl  (1) 
                                                     H
                 
                COEFFICIENT OF PERFORMANCE 
                 
                        It  is  defined  as  the  measure  of  the  performance  of  a  refrigerator  and  is  the  ratio  of  heat 
                absorbed at the lower temperature to the net work. 
                                       Heat abosorbed at the lower temperature 
                                ω = --------------------------------------------------- 
                                                              Net work 
                                                    lQcl 
                               = ----------  (2) 
                                                     W 
                Dividing equation (1) by (Qc).,    w          QH 
                                                                                              ----   =   -----   -   1 
                                                                         Qc         Qc 
                St. Joseph’s College of Engineering   Department of Chemical Engineering                     Page 1 
                 
                Chemical Engineering thermodynamics – II                                                                   Unit V - Refrigeration 
                                                                      lQ l        T  
                                               H      H
                But for Carnot refrigerator   -------  =  ------ 
                                                                     lQcl          Tc 
                 
                                             w         T                 T   - Tc             1 
                                       H          H
                Therefore        -----   =  -----  - 1   = ------------   =   ----      
                                          lQcl        Tc                     Tc                ω 
                 
                                          Tc 
                        ω  =    ------------ 
                                      TH – Tc 
                 
                        Applicable to a refrigerator, operating on a Carnot cycle. 
                 
                According to this refrigeration effect per unit of work decreases as the temperature of the refrigerator 
                Tc decreases, and as the temperature of heat rejection T  increases. 
                                                                      H
                For refrigeration at a temperature level of 5⁰C and a surroundings of 30⁰C the value of ω for a Carnot 
                refrigerator is 
                                                   5 + 273.15 
                                ω = -----------------------------------   =  11.13 
                                       (30 +273.15) – (5 + 273.15) 
                 
                                                                   
                                             THE VAPOR COMPRESSION CYCLE 
                 
                St. Joseph’s College of Engineering   Department of Chemical Engineering                     Page 2   
                 
                Chemical Engineering thermodynamics – II                                                                   Unit V - Refrigeration 
                 
                                                                                                                     
                 
                 
                        A liquid  evaporating at constant pressure provides a means of heat absorption at constant 
                temperature.  Similarly condensation of the vapor after compression to a higher pressure provides for 
                the rejection of heat at constant temperature.  The liquid from the condenser is returned to  its original 
                state by an expansion process using a turbine or an expander.  When the compression and expansion 
                are isentropic, the sequence of processes constitutes the cycle as shown above.  This is just similar to 
                Carnot cycle except that the superheated vapor from the compressor must be cooled to its saturation 
                temperature before condensation begins. 
                 
                                                                            
                        On the basis of unit mass of fluid, the heat absorbed in the evaporator is 
                 
                               lQcl = ∆H = H  – H  
                                            2   1
                 
                Heat rejected  lQ l = H  – H
                                H    3    4 
                 
                St. Joseph’s College of Engineering   Department of Chemical Engineering                     Page 3 
                 
                Chemical Engineering thermodynamics – II                                                                   Unit V - Refrigeration 
                But W = lQ l – lQcl  =  (H  – H ) – (H  – H ) 
                          H            3    4    2    1
                 
                                      Qc                   H  – H  
                                          2   1
                COP = ω = -------  =  ---------------------- 
                                     W            (H  – H ) – (H  – H ) 
                                      3   4     2    1
                 
                In small units expansion is accomplished by throttling the liquid from the condenser though a partly 
                opened valve. 
                 
                                                                                                       
                                                                   
                The pressure drop in this irreversible process results from fluid friction in the valve.  But still because of 
                its simplicity and lower cost, it outweighs the energy savings possible with a turbine.  The throttling 
                process occurs at constant enthalpy.  The vapor compression cycle incorporating an expansion valve is 
                shown above.  Line 4- 1 represents the constant enthalpy throttling process.  Line 2 - 3 represents an 
                sloping in the direction of increasing entropy. 
                 
                        The dashed line 2 – 3’ is the path of isentropic compression. 
                For this cycle  
                 
                                             H  – H  
                                 2   1
                COP = ω =  ---------------------- 
                                    (H  – H ) – (H  – H ) 
                             3   4     2    1
                 
                But H  = H     throttling being a isentropic process 
                     4    1
                                                          H  – H  
                                        2   1
                              COP = ω =  ------------------ 
                                                        (H  – H ) 
                                       3    2
                St. Joseph’s College of Engineering   Department of Chemical Engineering                     Page 4