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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
Wnet,in + QL = QH
We can write the coefficient of performance as:
COPR = QL / Wnet,in = QL/(QH-QL)
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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
COPR,Carnot = 1 / (TH/TL -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: qH = (h2-h3)
The COP can be written: COPR = qL/win = (h1-h4)/(h2-h1)
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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 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
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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 = (h2s-h1)/(h2a-h1)
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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