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Refrigeration

Refrigeration. Thermodynamics Professor Lee Carkner Lecture 21. Refrigeration. Generally involve 4 processes There is an input of heat and work and an output of heat W net,in + Q L = Q H We can write the coefficient of performance as: COP R = Q L / W net,in = Q L /(Q H -Q L ).

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Refrigeration

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  1. Refrigeration Thermodynamics Professor Lee Carkner Lecture 21

  2. Refrigeration • Generally involve 4 processes • There is an input of heat and work and an output of heat Wnet,in + QL = QH • We can write the coefficient of performance as: COPR = QL / Wnet,in = QL/(QH-QL)

  3. Carnot Refrigeration Cycle • Two isothermal and two isentropic processes • But serves as an upper limit on the efficiency of any refrigerator operating between two temperatures COPR,Carnot = 1 / (TH/TL -1)

  4. Ideal Refrigeration Cycle • The ideal refrigeration cycle consists of four processes: • Isobaric heat rejection in a condenser • Isobaric heat absorption in a evaporator • Similar to a reversed Rankine cycle • Less efficient but much cheaper and simpler

  5. Ideal Refrigerator Diagram

  6. Household Refrigerator

  7. COP of Refrigeration Cycle • The heat or work for each process is just Dh • The heats are: • qH = (h2-h3) • The COP can be written: • COPR = qL/win = (h1-h4)/(h2-h1)

  8. Solving Refrigerator Problems • For the four key points in the cycle, there are only two pressures • At point 1 (before compressor) the fluid is a saturated vapor and h1 = hg and s1 = sg at P1 • At point 3 (before throttle) the fluid is a saturated liquid and h3 = hf at P3 • Find all h’s to get qL, qH, w and COP

  9. Actual Refrigerator Cycles • The fluid may leave the evaporator superheated instead of saturated • The fluid may be subcooled (a compressed rather than a saturated liquid) when it exits the condenser • Can also define the isentropic efficiency of the compressor hC = (h2s-h1)/(h2a-h1)

  10. Actual Refrigerator Diagram

  11. Refrigerants • Want a fluid with a low boiling point • Many of these fluids are toxic • First safe refrigerants were developed in the 1920’s • Unfortunately, CFCs destroy ozone which blocks out solar UV • Newer, CFC-free refrigerants (like 134a) have become very common since the 1980’s

  12. Refrigerant Properties • Need 5-10 deg temperature difference between refrigerant and hot or cold chamber for good heat transfer • To avoid air leaks • For the condenser you need to keep a temperature above room temperature

  13. Next Time • Read: 11.6-11.9 • Homework: Ch 11, P: 18, 31, 42, 55

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