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Heat transfer & heat exchangers - Lecture 9: Refrigeration cycles


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
1 𝑡𝑜𝑛 2000 𝑙𝑏𝑚 of liquid water at 0℃ 32℉ into ice
at 0℃ in 24 hours, this amount is said to be 1 𝑡𝑜𝑛
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  1. Lecture 9 Refrigeration cycles
  2. The concepts WARM environment 푄 푒푠𝑖 푒 표 푡 푡 표표푙𝑖푛 푒 푒 푡 푄 푃 = = = 퐿 푅푒푞 𝑖 푒 𝑖푛 푡 푊표 𝑖푛 푡 푊푖푛 R 푊푖푛(required input) 푄퐿(desired output) COLD refrigerated space
  3. Reversed Carnot cycle 1 WARM medium at 푃 = 푄 − 1 퐿 4 3 Condenser isothermal isentropic Turbine Compressor isentropic isothermal 1 Evaporator 2 퐿 푄퐿 COLD medium at 퐿
  4. ideal vapor–comp. refrigeration cycle Saturated liquid 2 푃 푄 푄 3 isentropic isobaric 3 2 푊푖푛 푊푖푛 turbine isobaric 4 1 1 푄 4′ 4 퐿 푄퐿 Saturated vapor 푠 ℎ
  5. Cascade refrigeration WARM space Decrease in compressor work 푄 7 6 6 Condenser 푄 7 2 8 5 8 5 3 3 2 4 1 Evaporator 푄퐿 4 1 푄퐿 Increase in 푠 COLD space refrigeration capacity
  6. Multipurpose refrigeration WARM space 푄 3 Condenser 2 푄 2 4 3 Refrigerator 5 4 5 푄퐿,푅 푄퐿,푅 1 1 6 6 푄퐿,퐹 Freezer 푄퐿,퐹 푠 COLD space
  7. Absorption refrigeration WARM space Generator 푄 Rectifier Pure 3 Condenser 3 + 2 푄 푒푛 2 Absorber + 3 2 푄 Pure 3 Freezer Regenerator 푄퐿 푄 표표푙 푊 COLD space Cooling water
  8. Refrigerant selection • Designation of refrigerants 퐹 , 퐹 : 푅 Number of Carbon = + 1 Number of Hydrogen = − 1 Number of Fluorine = Number of Chlorine = 2 − − + 5 For example: 푅22 퐹2 푙 푅12 퐹2 푙2 푅134 푅134 2 2퐹4
  9. Refrigerant selection • Requirement for refrigeration selection: temperatures of condenser and evaporator, then toxic, corrosive, chemical, latent heat and the cost Saturated liquid 2 • ∆ ≈ 10℃ 푄 1 • Lower , higher 푃 푃 = 3 • ∆ ≈ 10℃ − 1 푊푖푛 • Higher 퐿, higher 푃 퐿 • 푃 > 1 푡 1 4′ 4 푄퐿 Saturated vapor 푠
  10. Reversed Brayton (gas) cycle 푞 푞 푃 = 퐿 = 퐿 푤푖푛 푤 표 ,푖푛 − 푤푡 ,표 푡 2 푊 푄 표 ,푖푛 1 3 푄퐿 푊푡 ,표 푡 4 푠 ℎ − ℎ 푃 = 1 4 ℎ2 − ℎ1 − ℎ3 − ℎ4
  11. Summary Power Refrigeration cycles cycles 푠 푠
  12. Energy analysis For condenser: 푄 = ℎ2 − ℎ3 2 푃 푄 3 푄 3 2 푊푖푛 푊 4 1 푖푛 1 푄 4 퐿 푄퐿 푠 ℎ
  13. Energy analysis For evaporator: 푄퐿 = ℎ1 − ℎ4 2 푃 푄 3 푄 3 2 푊푖푛 푊 4 1 푖푛 1 푄 4 퐿 푄퐿 푠 ℎ
  14. Superheating & Subcooling 2 푃 3 푄 3′ 3 2 2′ 3′ Increase in compression work 푊푖푛 Increase in specific 푊푖푛 refrigeration effect 1′ 4 1 1′ 4 1 푄퐿 푠 ℎ