The Ideal Vapor-Compression Refrigeration Cycle
Equipments:
condenser
3
2
Expansion
valve
1
4
evaporator
compressor
condenser
3
2
Expansion
valve
compressor
1
4
T
2
TH
TL
evaporator
3
1
4
s
evaporator
compressor
condenser
Capillary tube
h
2
1
3
4
s
qL
h1  h4
COP 

w
h2  h1
p
3
4
2
1
h
10-2 Refrigerant
Good refrigerant
High enthalpy of vaporization
Not too low evaporator pressure
Not too high condenser pressure
Nontoxic
Noncorrosive
Chemically stable
Low cost
•Ammonia
•CFC(chlorofluorocarbons)
•water
T
2
TH
T0
h2  h3
COPhp 
h2  h1
3
1
4
s
Heat pump for heating
Heat pump for refrigeration
Cascade Refrigeration Systems
condenser
evaporator
T-s Diagram
T
T0
s
10-4-3 Multistage Compression Refrigeration Systems
5
4
condenser
4
3
6
9
6
7
9
2
1
8
2
5
heat exchanger
Flash chamber
7
T
8
3
1
evaporator
s
10-4-4 Multipurpose Refrigeration Systems with a Single
Compressor
T
3
condenser
4
2
2
refrigerator
5
Alternative
path
3
1
4
5
1
6
Freezer
s
The vapor compression refrigeration cycle is a common method for transferring heat
from a low temperature to a high temperature.
The above figure shows the objectives of refrigerators and heat pumps. The purpose of a refrigerator is
the removal of heat, called the cooling load, from a low-temperature medium. The purpose of a heat pump
is the transfer of heat to a high-temperature medium, called the heating load. When we are interested in
the heat energy removed from a low-temperature space, the device is called a refrigerator. When we are
interested in the heat energy supplied to the high-temperature space, the device is called a heat pump. In
general, the term heat pump is used to describe the cycle as heat energy is removed from the lowtemperature space and rejected to the high-temperature space.
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The performance of refrigerators and heat pumps is expressed in terms of coefficient
of performance (COP), defined as
Desired output Cooling effect
QL
COPR 


Required input
Work input
Wnet ,in
Desired output Heating effect
QH
COPHP 


Required input
Work input
Wnet ,in
Both COPR and COPHP can be larger than 1. Under the same operating conditions,
the COPs are related by
COPHP  COPR  1
Can you show this to be true?
Refrigerators, air conditioners, and heat pumps are rated with a SEER number or seasonal adjusted
energy efficiency ratio. The SEER is defined as the Btu/hr of heat transferred per watt of work energy
input. The Btu is the British thermal unit and is equivalent to 778 ft-lbf of work (1 W = 3.4122 Btu/hr).
An EER of 10 yields a COP of 2.9.
Refrigeration systems are also rated in terms of tons of refrigeration. One ton of refrigeration is
equivalent to 12,000 Btu/hr or 211 kJ/min. How did the term “ton of cooling” originate?
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The Vapor-Compression Refrigeration Cycle
The vapor-compression refrigeration cycle has four components: evaporator,
compressor, condenser, and expansion (or throttle) valve. The most widely used
refrigeration cycle is the vapor-compression refrigeration cycle. In an ideal vaporcompression refrigeration cycle, the refrigerant enters the compressor as a saturated
vapor and is cooled to the saturated liquid state in the condenser. It is then throttled
to the evaporator pressure and vaporizes as it absorbs heat from the refrigerated
space.
The ideal vapor-compression cycle consists of four processes.
Ideal Vapor-Compression Refrigeration Cycle
Process
Description
1-2
Isentropic compression
2-3
Constant pressure heat rejection in the condenser
3-4
Throttling in an expansion valve
4-1
Constant pressure heat addition in the evaporator
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Q L
h1  h4
COPR 

Wnet ,in h2  h1
COPHP
Q H
h2  h3


Wnet ,in h2  h1
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Example 11-1
Refrigerant-134a is the working fluid in an ideal compression refrigeration cycle. The
refrigerant leaves the evaporator at -20oC and has a condenser pressure of 0.9 MPa.
The mass flow rate is 3 kg/min. Find COPR and COPR, Carnot for the same Tmax and
Tmin , and the tons of refrigeration.
Using the Refrigerant-134a Tables, we have

kJ
h

238.41
Compressor inlet   1
kg

T1  20o C
  s  0.9456 kJ
  1
kg  K
x1  1.0


kJ
Compressor exit
  h2 s  278.23
kg
P2 s  P2  900 kPa

o
kJ  T2 s  43.79 C

s2 s  s1  0.9456
kg  K 
State 3

kJ
h

101.61
Condenser exit   3
kg

P3  900 kPa  
kJ
s3  0.3738
 
kg  K
x3  0.0

 x  0.358
Throttle exit   4
kJ

o
s

0.4053
T4  T1  20 C   4
kg  K


h4  h3

State1
State 2
State 4
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COPR 
QL
m(h1  h4 ) h1  h4


Wnet , in m(h2  h1 ) h2  h1
kJ
kg

kJ
(278.23  238.41)
kg
 3.44
(238.41  101.61)
The tons of refrigeration, often called the cooling load or refrigeration effect, are
QL  m(h1  h4 )
kg
kJ 1Ton
(238.41  101.61)
min
kg 211 kJ
min
 1.94 Ton
3
COPR , Carnot
TL

TH  TL
(20  273) K

(43.79  ( 20)) K
 3.97
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Another measure of the effectiveness of the refrigeration cycle is how much input
power to the compressor, in horsepower, is required for each ton of cooling.
The unit conversion is 4.715 hp per ton of cooling.
Wnet , in
QL
4.715

COPR
4.715 hp

3.44 Ton
hp
 1.37
Ton
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Actual Vapor-Compression Refrigeration Cycle
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Heat Pump Systems
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Other Refrigeration Cycles
Cascade refrigeration systems
Very low temperatures can be achieved by operating two or more vapor-compression
systems in series, called cascading. The COP of a refrigeration system also
increases as a result of cascading.
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27
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Multistage compression refrigeration systems
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30
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Multipurpose refrigeration systems
A refrigerator with a single compressor can provide refrigeration at several
temperatures by throttling the refrigerant in stages.
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Thermoelectric Refrigeration Systems
A refrigeration effect can also be achieved without using any moving parts by simply
passing a small current through a closed circuit made up of two dissimilar materials.
This effect is called the Peltier effect, and a refrigerator that works on this principle is
called a thermoelectric refrigerator.
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Features of
Actual Vapor-Compression Cycle
The COP decreases – primarily due to increasing
compressor work input – as the
• temperature of the
Trefrigerant ↑
refrigerant passing
through the evaporator is
reduced relative to the
temperature of the cold
region, TC.
• temperature of the
Trefrigerant ↓
refrigerant passing
through the condenser is increased relative to the
temperature of the warm region, TH.
Features of
Actual Vapor-Compression Cycle
Irreversibilities during the compression process are
suggested by dashed line from state 1 to state 2.
• An increase in specific
entropy accompanies an
adiabatic irreversible
compression process. The
work input for compression
process 1-2 is greater than
for the counterpart isentropic
compression process 1-2s.
• Since process 4-1, and thus the refrigeration capacity,
is the same for cycles 1-2-3-4-1 and 1-2s-3-4-1, cycle
1-2-3-4-1 has the lower COP.
Isentropic Compressor Efficiency
The isentropic compressor efficiency is the ratio of
the minimum theoretical work input to the actual work
input, each per unit of mass flowing:
work required in an isentropic
compression from compressor inlet
state to the exit pressure
(Eq. 6.48)
work required in an actual
compression from compressor
inlet state to exit pressure
Actual Vapor-Compression Cycle
Example: The table provides steady-state operating
data for a vapor-compression refrigeration cycle
using R-134a as the working fluid. For a refrigerant
mass flow rate of 0.08 kg/s, determine the
(a) compressor power, in kW,
(b) refrigeration capacity, in tons,
(c) coefficient of performance,
(d) isentropic compressor efficiency.
State
1
2s
2
3
4
h (kJ/kg) 241.35 272.39 280.15 91.49 91.49
Actual Vapor-Compression Cycle
State
1
2s
2
3
4
h (kJ/kg) 241.35 272.39 280.15 91.49 91.49
(a) The compressor power is
 (h2  h1 )
Wc  m
kg 
kJ 1 kW

W c   0.08 (280 .15  241 .35)
 3.1 kW
s 
kg 1 kJ/s

(b) The refrigeration capacity is
 (h1  h4 )
Q in  m
kg 
kJ
1 ton
60 s


Qin   0.08 (241 .35  91 .49 )
 3.41 tons
s 
kg 211 kJ/min min

Actual Vapor-Compression Cycle
State
1
2s
2
3
4
h (kJ/kg) 241.35 272.39 280.15 91.49 91.49
(c) The coefficient of performance is
(h1  h4 )

(h2  h1 )

(241 .35  91 .49 )kJ/kg
 3.86
(280 .15  241 .35)kJ/kg
Actual Vapor-Compression Cycle
State
1
2s
2
3
4
h (kJ/kg) 241.35 272.39 280.15 91.49 91.49
(d) The isentropic compressor
efficiency is
c

Wc / m s (h2s  h1 )


c 
Wc / m
(h2  h1 )
(272 .39  241 .35)kJ/kg
 0.8 = 80%
(280 .15  241 .35)kJ/kg
p-h Diagram
►The pressure-enthalpy (p-h) diagram is a
thermodynamic property diagram commonly used
in the refrigeration field.
Selecting Refrigerants
Refrigerant selection is based on several factors:
• Performance: provides adequate cooling
capacity cost-effectively.
• Safety: avoids hazards (i.e., toxicity).
• Environmental impact: minimizes harm to
stratospheric ozone layer and reduces
negative impact to global climate change.
Refrigerant Types and Characteristics
Global Warming Potential (GWP) is a simplified index that estimates the potential
future influence on global warming associated with different gases when released
to the atmosphere.
Refrigerant Types and Characteristics
• Chlorofluorocarbons (CFCs) and Hydrochlorofluorocarbons
(HCFCs) are early synthetic refrigerants each containing
chlorine. Because of the adverse effect of chlorine on Earth’s
stratospheric ozone layer, use of these refrigerants is regulated
by international agreement.
• Hydrofluorocarbons (HFCs) and HFC blends are chlorinefree refrigerants. Blends combine two or more HFCs. While
these chlorine-free refrigerants do not contribute to ozone
depletion, with the exception of R-1234yf, they have high GWP
levels.
• Natural refrigerants are nonsynthetic, naturally occurring
substances which serve as refrigerants. These include carbon
dioxide, ammonia, and hydrocarbons. These refrigerants feature
low GWP values; still, concerns have been raised over the
toxicity of NH3 and the safety of the hydrocarbons.
Vapor-Compression Heat Pump Systems
• The objective of the heat pump is to maintain the
temperature of a space or industrial process above
the temperature of the surroundings.
• Principal control volumes involve these components:
•
•
•
•
Evaporator
Compressor
Condenser
Expansion valve
The Vapor-Compression Heat Pump Cycle
Performance parameters
Coefficient of Performance
(Eq. 10.10)
Carnot Coefficient of Performance
(Eq. 10.9)
This equation represents the maximum theoretical
coefficient of performance of any heat pump cycle
operating between cold and hot regions at TC and TH,
respectively.
Vapor-Compression Heat Pump System
The method of analysis for vapor-compression heat
pumps closely parallels that for vapor-compression
refrigeration systems.
Example: A vapor-compression heat pump cycle with R134a as the working fluid maintains a building at 20oC
when the outside temperature is 5oC. The refrigerant
mass flow rate is 0.086 kg/s. Additional steady state
operating data are provided in the table. Determine the
(a) compressor power, in kW,
(b) heat transfer rate provided
to the building, in kW,
(c) coefficient of performance.
State
h (kJ/kg)
1
2
3
244.1
272.0
93.4
TH = 293 K (20oC)
TC = 278 K (5oC)
Vapor-Compression Heat Pump System
State
h (kJ/kg)
1
2
3
244.1
272.0
93.4
TH = 293 K (20oC)
(a) The compressor power is
 (h2  h1 )
Wc  m
kg 
kJ 1 kW

W c   0.086 (272 .0  244 .1)
 2.4 kW
s 
kg 1 kJ/s

(b) The heat transfer rate provided to the building is
 (h2  h3 )
Q out  m
kg 
kJ 1 kW


Qout   0.086 (272 .0  93 .4)
 15.4 kW
s 
kg 1 kJ/s

TC = 278 K (5oC)
Vapor-Compression Heat Pump System
State
h (kJ/kg)
1
2
3
244.1
272.0
93.4
TH = 293 K (20oC)
TC = 278 K (5oC)
(c) The coefficient of
performance is
Q out
 
Wc
15.4 kW

 6.4
2.4 kW
Comment: Applying Eq. 10.9, the maximum theoretical
coefficient of performance of any heat pump cycle
operating between cold and hot regions at TC and TH,
respectively is
293 K
TH
 max 
 19.5
 max 
293 K  278 K
TH  TC
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