Chapter 9 Refrigeration and Liquefaction Introduction n Refrigeration

Chapter 9. Refrigeration and Liquefaction (냉동과 액화) 고려대학교 화공생명공학과

Introduction n Refrigeration n n n Air conditioning of building Preservation of foods and beverages Manufacture of ice Dehydration of gases Purifications , separations Low temperature reactions Liquefaction n n Propane gases in cylinders Liquid oxygen for rockets LNG (Liquid Natural Gas) Separation of air

9. 1 The Carnot Refrigeration n Reversed heat-engine cycle n n n Heat is absorbed at low T Heat is rejected at high T Requires external source of energy : W Coefficient of Performance (COP)

9. 2 Vapor-Compression Cycle 4 3 Condenser Const. H throttling process 3 3’ Throttling Valve 1 Compressor Evaporator 4 1 2 Coefficient of Performance 2 Rate of circulation of refrigerant

A Typical Compressor

Vapor-compression refrigeration cycle on P-H diagram ln P Const. S 4 3’ 1 3 2 H P-H diagrams are more commonly used in refrigeration cycle than TS diagrams.

Example 9. 1 n A refrigerated space is maintained at 10 (o. F) and cooling water is available at 70 (o. F). Refrigeration capacity is 12, 000 Btu/h. The evaporator condenser are of sufficient size that a 10 (o. F) minimum temperature difference for heat transfer can be realized in each. The refrigerant is tetrafluoroethane (HCF-134 a), for which data are given in Table 9. 1 and Fig. G. 2 (App. G). (a) Whate is the value of COP for a Carnot Refrigerator ? (b) Cacluate COP and m for the vapor-compression cycle of Fig. 9. 1 if the compressor efficiency is 0. 80

Solution (a) 10 o F Minimum temperature difference : Tc = 0 o. F Th = 80 o. F

Solution (b) H 1, H 2, H 3, H 4 값을 알면 모든 답을 구할 수 있다. State 2 ln P H 1 = H 4 Const. S 4 3’ 1 2 3 0 o. F, saturation condition (V) P = 21. 162 psia H 2 = 103. 015 Btu/lbm S 2 = 0. 22525 Btu/lbm. F State 4 80 o. F, saturation condition (L) P = 101. 37 psia H 2 = 37. 987 Btu/lbm State 3’ Read H from Fig. G. 2. P = 101. 37 psia, S=0. 22525 H 3’ = 117 Btu /lbm H

Solution (b) n Compressor efficiency is 0. 8

Solution (b) n COP n Refrigerant Circulation Rate

9. 3 The Choice of Refrigerant In principle, COP of Carnot refrigerator is independent of the refrigerant. n Irreversibility in the refrigerator cause the COP to depend on the choice of refrigerant. n

The Choice of Refrigerant n Main characteristics n n n Vapor pressure of the refrigerant at evaporation T should be greater than 1 atm Vapor pressure condenser T should not be high. Additional characteristics n n n Toxicity Flammability Cost Corrosion properties Environmental consideration

Choice of Refrigerant Ammonia, Methyl chloride, Carbon Dioxide, Propane and other hydrocarbons n Halogenated Hydrogcarons (CFC, HCFC) n n n CCl 3 F (CFC-11), CCl 2 F 2 (CFC-12) Replacement n CHCl 2 CF 3 (HCF-123), CF 3 CH 2 F(HCF-134 a), CHF 2 CF 3 (HCF-125)

Cascade refrigeration systems n To overcome the limit of operation… n n Tc fixed : environments Two or more refrigeration cycle employing different refrigerant

9. 4 Absorption Refrigeration Condenser Throttling Valve Compressor Evaporator This part can be replaced by heat engine – a work producing device W

Schematic Diagram of an Absorption-Refrigeration Unit QH at TH Heat discarded at TS Condenser Regenerator Throttling Valve Heat Exchanger Evaporator Absorber Pump QC at TC Heat discarded at TS Li. Br Solution + Water + Ammonia

Ammonia Absorption Refrigeration Unit

Analysis Work required for the refrigeration cycle Heat required for the production of the work.

9. 5 The Heat Pump n Dual-purpose reversed heat engine n n Winter : Heating Summer : Cooling If COP = 4, five times work has to be done to the compressor n Economic advantage depends on the cost of electricity vs. oil and natural gases. n

9. 6 Liquefaction Processes n Liquefaction processes n By heat exchange at const. P n By an expansion process from which work is obtained (Adiabatic expansion) T n By a throttling process S

Liquefaction Processes For small-scale commercial liquefaction plant, throttling process is commonly employed. n Sufficiently low T and high P desired. n

Linde Liquefaction Process 4 3 7 Throttle Cooler Exchanger 8 10 Win 15 2 1 9 Liquid Gas Feed

Linde Liquefaction Process

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