Lecture Refrigeration cycles The concepts Apparatus described Jacob Perkins in the patent of 1834 The concepts WARM environment 𝑄𝐻 R 𝐷𝑒𝑠𝑖𝑟𝑒𝑑 𝑜𝑢𝑡𝑝𝑢𝑡 𝐶𝑜𝑜𝑙𝑖𝑛𝑔 𝑒𝑓𝑓𝑒𝑐𝑡 𝑄𝐿 𝐶𝑂𝑃 = = = 𝑅𝑒𝑞𝑢𝑖𝑟𝑒𝑑 𝑖𝑛𝑝𝑢𝑡 𝑊𝑜𝑟𝑘 𝑖𝑛𝑝𝑢𝑡 𝑊𝑖𝑛 𝑊𝑖𝑛 (required input) 𝑄𝐿 (desired output) COLD refrigerated space The concepts • Cooling capacity of a refrigeration system that is the rate of heat removal from the refrigerated space, in terms of tons of refrigeration • Tons of refrigeration that is the rate of heat can freeze 𝑡𝑜𝑛 2000 𝑙𝑏𝑚 of liquid water at 0℃ 32℉ into ice at 0℃ in 24 hours, this amount is said to be 𝑡𝑜𝑛 𝑡𝑜𝑛 𝑜𝑓 𝑟𝑒𝑓𝑟𝑖𝑔𝑒𝑟𝑎𝑡𝑖𝑜𝑛 = 211 𝑘𝐽Τ𝑚𝑖𝑛 = 200 𝐵𝑡𝑢Τ𝑚𝑖𝑛 = 3.5𝑘𝑊 Reversed Carnot cycle WARM medium at 𝑇𝐻 𝐶𝑂𝑃 = 𝑄𝐻 𝑇𝐻 Condenser 𝑇𝐻 −1 𝑇𝐿 isothermal isentropic Turbine Compressor isothermal Evaporator 𝑇𝐿 𝑄𝐿 COLD medium at 𝑇𝐿 isentropic ideal vapor–comp refrigeration cycle Evaporator coils Capillary tube WARM environment isobaric isenthalpic Freezer compartment 𝑄𝐻 Condenser 𝑄𝐻 Compressor Expansion valve 𝑊𝑖𝑛 𝑄𝐿 −18℃ Condenser coils isentropic Evaporator isobaric 𝑄𝐿 COLD refrigerated space 3℃ 𝑊𝑖𝑛 Compressor ideal vapor–comp refrigeration cycle 𝑇 𝑄𝐻 isobaric 𝑄𝐻 turbine 𝑊𝑖𝑛 isobaric 4′ 𝑃 isentropic Saturated liquid 4 𝑊𝑖𝑛 𝑄𝐿 𝑄𝐿 Saturated vapor 𝑠 ℎ real vapor–comp refrigeration cycle WARM environment 𝑄𝐻 Condenser 2′ Expansion valve 6 Compressor 𝑇 𝑠 Evaporator 𝑄𝐿 COLD refrigerated space 𝑊𝑖𝑛 Cascade refrigeration WARM space Decrease in compressor work 𝑄𝐻 𝑇 𝑄𝐻 Condenser 𝐴 𝐴 8 𝐵 𝐵 𝑄𝐿 Increase in 𝑠 refrigeration capacity Evaporator 𝑄𝐿 COLD space Multistage compression refrigeration WARM space 𝑄𝐻 Condenser 𝑇 𝐴 𝐴 𝐵 9 𝐵 𝑠 Evaporator 𝑄𝐿 COLD space Refrigerant selection • Refrigerant is a heat carrying medium which during their cycle in a refrigeration cycle absorbs heat from a low temperature system and delivers it to a higher temperature system • Refrigerants: chlorofluorocarbons (CFCs), ammonia, hydrocarbons, carbon dioxide, air water… Refrigerant selection • Designation of refrigerants 𝐶𝐹𝐶, 𝐻𝐶𝐹𝐶 : 𝑅𝑥𝑦𝑧 Number of Carbon = 𝑥 + Number of Hydrogen = 𝑦 − Number of Fluorine = 𝑧 Number of Chlorine = 2𝑥 − 𝑦 − 𝑧 + For example: 𝑅22 𝐶𝐻𝐹2 𝐶𝑙 𝑅12 𝐶𝐹2 𝐶𝑙2 𝑅134𝑎 𝑅134 𝐶2 𝐻2 𝐹4 Refrigerant selection • Designation of refrigerants (other): 𝑅7𝑤 𝑤 ≅ molar weight For example: 𝑁𝐻3 𝑅717 𝐶𝑂2 𝑅744 𝐻2 𝑂 𝑅718 Refrigerant selection • Requirement for refrigeration selection: temperatures of condenser and evaporator, then toxic, corrosive, chemical, latent heat and the cost 𝑇 𝐶𝑂𝑃 = • ∆𝑇 ≈ 10℃ • Lower 𝑇𝐻 , higher 𝐶𝑂𝑃 𝑇𝐻 −1 𝑇𝐿 • ∆𝑇 ≈ 10℃ • Higher 𝑇𝐿 , higher 𝐶𝑂𝑃 • 𝑃 > 1𝑎𝑡𝑚 Saturated liquid 𝑄𝐻 𝑊𝑖𝑛 4′ 𝑄𝐿 Saturated vapor 𝑠 Reversed Brayton (gas) cycle WARM environment 𝑄𝐻 isentropic Heat exchanger isobaric Turbine isentropic Compressor isobaric Heat exchanger 𝑄𝐿 COLD refrigerated space 𝑊𝑛𝑒𝑡 Reversed Brayton (gas) cycle 𝑞𝐿 𝑞𝐿 𝐶𝑂𝑃 = = 𝑤𝑖𝑛 𝑤𝑐𝑜𝑚𝑝,𝑖𝑛 − 𝑤𝑡𝑢𝑟𝑏,𝑜𝑢𝑡 𝑇 𝑊𝑐𝑜𝑚𝑝,𝑖𝑛 𝑄𝐻 𝑊𝑡𝑢𝑟𝑏,𝑜𝑢𝑡 𝑄𝐿 𝑠 ℎ1 − ℎ4 𝐶𝑂𝑃 = ℎ2 − ℎ1 − ℎ3 − ℎ4 Reversed Brayton (gas) cycle 𝑇 Gas refrigeration cycle 𝐴 Reversed Carnot cycle 𝐵 𝑠 A reserved Carnot cycle produces more refrigeration (area under B1) with less work input (area 1A3B) Summary 𝑇 𝑇 Refrigeration cycles Power cycles 𝑠 𝑠 Energy analysis For compressor: 𝑊𝑖𝑛 = 𝑚 ℎ2 − ℎ1 𝜂𝑐𝑜𝑚𝑝 𝑊𝑖𝑠𝑒𝑛 ℎ2𝑠 − ℎ1 = = 𝑊𝑖𝑛 ℎ2 − ℎ1 𝑇 2𝑠 𝑃 𝑄𝐻 𝑄𝐻 𝑊𝑖𝑛 4 𝑊𝑖𝑛 𝑄𝐿 𝑄𝐿 𝑠 ℎ Energy analysis For condenser: 𝑄𝐻 = 𝑚 ℎ2 − ℎ3 𝑇 𝑃 𝑄𝐻 𝑄𝐻 𝑊𝑖𝑛 4 𝑊𝑖𝑛 𝑄𝐿 𝑄𝐿 𝑠 ℎ Energy analysis For throttling valve: ℎ4 = ℎ3 𝑇 𝑃 𝑄𝐻 𝑄𝐻 𝑊𝑖𝑛 4 𝑊𝑖𝑛 𝑄𝐿 𝑄𝐿 𝑠 ℎ Energy analysis For evaporator: 𝑄𝐿 = 𝑚 ℎ1 − ℎ4 𝑇 𝑃 𝑄𝐻 𝑄𝐻 𝑊𝑖𝑛 4 𝑊𝑖𝑛 𝑄𝐿 𝑄𝐿 𝑠 ℎ Energy analysis For the cycle: 𝑄𝐿 ℎ1 − ℎ4 𝐶𝑂𝑃 = = 𝑊𝑖𝑛 ℎ2 − ℎ1 𝑄𝐻 = 𝑄𝐿 + 𝑊𝑖𝑛 𝑇 𝑃 𝑄𝐻 𝑄𝐻 𝑊𝑖𝑛 4 𝑊𝑖𝑛 𝑄𝐿 𝑄𝐿 𝑠 ℎ Superheating & Subcooling 𝑇 𝑃 3′ 3′ Increase in compression work 2′ 𝑊𝑖𝑛 Increase in specific refrigeration effect 𝑄𝐻 1′ 𝑄𝐿 𝑠 1′ 𝑊𝑖𝑛 ℎ Superheating & Subcooling Condenser 1′ For heat exchanger: 𝑄 Heat exchanger 3′ Evaporator ℎ3 − ℎ3′ = ℎ1′ − ℎ1 ... Gas refrigeration cycle