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Department of Mechanical Engineering
ME 322 – Mechanical Engineering
Thermodynamics
Lecture 29
The Vapor Compression Refrigeration
(VCR) Cycle
Refrigerator used for Cooling
CCOPinE
cycle
Q
W
energy sought
energy that costs
The concept of an efficiency being greater than 100%
makes people uneasy. Therefore, the conversion
efficiency for a refrigerator is called the Cooling
Coefficient of Performance (COPC). A refrigeration
system that is used for cooling is called a refrigerator.
Observation: may be >1 100%E E
HT
LT
2
Refrigerator used for Heating
HCOPoutE
cycle
Q
W
energy sought
energy that costs
The concept of an efficiency being greater than 100%
makes people uneasy. Therefore, the conversion
efficiency for a refrigerator is called the Heating
Coefficient of Performance (COPH). A refrigeration
system that is used for cooling is called a heat pump.
Observation: may be >1 100%E E
HT
LT
3
Analysis of the Carnot Refrigerator
R
1COP
/ 1
in inth
cycle out in out in
Q Q
W Q Q Q Q
For the Refrigeration cycle …
H
H, H,
1COP
1 /
1 1COP COP
1 / 1 /
out outth
cycle out in in out
HCarnot Carnot
in out L H H Lrev
Q Q
W Q Q Q Q
T
Q Q T T T T
For the Heat Pump cycle …
HT
LT
R,
1 1COP
/ 1 / 1Carnot
out in H LrevQ Q T T
R,COP LCarnot
H L
T
T T
4
Terminology
• Refrigeration cycle
– The cycle is operating in a refrigeration cycle
• The goal is to keep the cold space cold
– Transfer heat from a low-temperature source to the cycle
• Heat pump cycle
– The cycle is operating in a refrigeration cycle
• The goal is to keep the hot space hot
– Transfer heat to a high-temperature sink from the cycle
The words refrigeration or heat pump define the goal of the cycle.
5
The Vapor Compression Refrigeration Cycle
6
HT
LT
The Vapor Compression Refrigeration Cycle
• Two phase changes
– Boiling (evaporator)
– Condensing (condenser)
• Low temperature boiling fluids
Working fluid = Refrigerant
7
Refrigerants
Most refrigerants are halogenated hydrocarbons. The naming
convention adopted by ASHRAE is,
R(a-1)(b+1)d = CaHbClcFd c = 2(a – 1) – b – d
1 0 1
1 2 1
2
2 1
2 1 1 1 2 1
a a
b b
d
c a b d
c
Example: R22 (R022)
C
H
F
Cl
F chlorodifluoromethane
8
The Pressure-Enthalpy Diagram
9
Vapor Compression Refrigeration Analysis
2 1m h h
2 3m h h
1 4m h h
3 4h h
Performance
CCOP in
c
Q
W
10
P
h
The Ideal VCR Cycle on the P-h Diagram
2 1m h h
2 3m h h
1 4m h h
3 4h h1
23
4
1-2: Isentropic compression
2-3: Isobaric heat rejection
3-4: Isenthalpic expansion
4-1: Isobaric heat addition
/in inq Q m /c cw W m
/out outq Q m
11
Refrigeration Effect and Capacity
1 4inq h h Refrigeration Effect:
Refrigeration Capacity: 1 4inQ m h h
Refrigeration capacity is often expressed in tons of
refrigeration. Definition …
1 ton of refrigeration is the steady state heat transfer rate
required to melt 1 ton of ice at 32°F in 24 hours.
1 ton = 12,000 Btu/hr = 3.516 kW
12
VCR Cycle Irreversibilities
P
h
1
22s
3
4
Isentropic efficiency
of the compressor
Pressure drop through
the condenser
Pressure drop through
the evaporator
13
Practical VCR Cycle
P
h
1
22s
3
4
3T SCT
1T
SET
SCT = Saturated Condensing Temperature
DSC = Degrees of Subcooling = SCT – T3
SET = Saturated Evaporating Temperature
DSH = Degrees of Superheat = T1 – SET
Subcooling increases the
refrigeration capacity
Superheating provides a dry
vapor at the compressor inlet
14
Example
15
Given: A vapor compression refrigeration cycle is operating
with a saturated evaporating temperature of -20°F and a
saturated condensing temperature of 80°F. The refrigerant,
R22, leaves the condenser as a saturated liquid and enters
the compressor with 5 degrees of superheat. The pressure
drops through the evaporator and condenser can be
considered negligible. The compressor has an isentropic
efficiency of 85%. The cycle has a capacity of 15 tons.
Find: (a) the mass flow rate of the R22 (lbm/hr)
(b) the power requirement of the compressor (hp)
(c) the coefficient of performance of the cycle
Example
16
P
h
1
23
420 FSET
80 FSCT
15 tons1 5 FT SET
1 5 FT SET
4 20 FT
3 80 FT
2s
Example
17
15 tons
1 5 FT SET
4 20 FT
3 80 FT
P
h
1
23
420 FSET
80 FSCT
1 5 FT SET
2s
Example
18
15 tons
1 5 FT SET
4 20 FT
3 80 FT
Strategy: Build the property table
then do the thermodynamics!
The high and low pressures in the cycle
can be found since the saturation
temperatures are given.
The pressures at all four states are known!
P
h
1
23
420 FSET
80 FSCT
1 5 FT SET
2s
Example
19
15 tons
1 5 FT SET
4 20 FT
3 80 FT
P
h
1
23
420 FSET
80 FSCT
1 5 FT SET
2s
Example
20
15 tons
1 5 FT SET
4 20 FT
3 80 FT
P
h
1
23
420 FSET
80 FSCT
1 5 FT SET
2s
The property table is complete!
Example
21
15 tons
1 5 FT SET
4 20 FT
3 80 FT
P
h
1
23
420 FSET
80 FSCT
1 5 FT SET
2s
EES Solution (Key Variables)
12,000 Btu Btu
15 tons 180,000ton-hr hr
inQ
2545 Btu Btu
24.45 hp 62,225hp-hr hr
cW
Comparison (same units) …