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Refrigeration Thermodynamics Professor Lee Carkner Lecture 21
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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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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 COP R,Carnot = 1 / (T H /T L -1)
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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
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Ideal Refrigerator Diagram
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Household Refrigerator
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COP of Refrigeration Cycle The heat or work for each process is just h The heats are: q H = (h 2 -h 3 ) The COP can be written: COP R = q L /w in = (h 1 -h 4 )/(h 2 -h 1 )
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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 h 1 = h g and s 1 = s g at P 1 At point 3 (before throttle) the fluid is a saturated liquid and h 3 = h f at P 3 Find all h’s to get q L, q H, w and COP
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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 C = (h 2s -h 1 )/(h 2a -h 1 )
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Actual Refrigerator Diagram
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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
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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
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Next Time Read: 11.6-11.9 Homework: Ch 11, P: 18, 31, 42, 55
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