Transcript Week 10 Refrigeration Cycles

Refrigeration Cycles
Mohamed Iqbal Pallipurath
Refrigerators and Heat Pumps
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Refrigerators: move
heat from colder
space
Heat pump: move
heat to warmer space
Both require work
input
Refrigerators and Heat Pumps
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Performance is
expressed in term of
coefficients of
performance
Carnot Cycle Model
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Reverse Carnot Cycle
Reverse Carnot Cycle
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Coefficients of performance for Carnot
cycles can be expressed in terms of
temperature
Ideal Vapor-Compression
Refrigeration Cycle
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1-2 Isentropic
compression in
compressor
2-3 Constant-pressure
heat rejection in a
condenser
3-4 Throttling in an
expansion valve
(irreversible process)
4-1 Constant-pressure
heat absorption in an
evaporator
Ideal Vapor-Compression
Refrigeration Cycle Analysis

Steady-flow

Where h1 = hg@P1 and h3 = hf@P3
Actual Vapor-Compression Cycle
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Differences

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Evaporator discharge
Higher specific volume
in compressor
Compressor losses
Condenser pressure
losses
What Refrigerant?
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Types:
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Chlorofluorocarbons (CFCs)
Ammonia
Hydrocarbons (propane, ethane, ethylene,
etc.)
Carbon dioxide
What Refrigerant?

Driving criteria:

Temperatures of where
heat is removed from
and where it is moved
to.
Heat Pumps
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
Reverse of
refrigerators
Bring heat from lower
temperature space to
higher temperature
space.
Energy source


Air
Water/soil
Gas Refrigeration Cycle
Gas Refrigeration Cycle
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Fluid always a gas, no phase change
Lower COP than vapor-compression systems
Used in aircraft and gas liquefaction