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Mechanical Engineering Dept. CEME NUST 1
Ch-2:Vapor Compression Cycle
Book:
Refrigeration & Air-Conditioning by Wilbert F.
Stoecker / Jerold W. Jones
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Mechanical Engineering Dept. CEME NUST 2
Vapor Compression cycle
The Carnot Cycle
Ideal thermodynamically Reversible Cycle, first investigated by Sadi Carnotin 1824
A measure of the maximum possible conversion of heat energy into
mechanical energy
Heat rejected to low
temperature sink
Cool Liquid
Compressor
2 3
41
Turbine
Heat from high
temperature source
WorkWork
2 3
41Temperature
T1=T4
T2=T3
SA SBEntropy
Process 4-1: isothermal rejection of heat
Process 1-2: Adiabatic Compression
Process 2-3: isothermal addition of heat
Process 3-4: adiabatic expansion
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Mechanical Engineering Dept. CEME NUST 3
2 3
41Temperature
T1=T4
T2=T3
SA SBEntropy
Process 1-2: adiabatic compression
Process 2-3: isothermal addition of heat
Process 3-4: adiabatic expansion
Process 4-1: isothermal rejection of heat
Heat supplied during isothermal expansion (2-3)= T
2(S
B- S
A)
Vapor Compression cycle
The Carnot Cycle
work done = Heat supplied Heat rejected
= T2(SBSA) T1(SBSA)
= (SBSA)(T2T1)
Heat rejected during isothermal compression
(4-1) = T1(S
BS
A)
=
=
=
=
Efficiency increases as T2is increased and T1is decreased
Heat should be taken in, at as high temperature as possible and rejected
at as low a temperature as possible.
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Mechanical Engineering Dept. CEME NUST 4
A measure of the maximum performance to be obtained from a refrigeratingmachine
Vapor Compression cycle
Reversed Carnot Cycle (i.e. Carnot Cycle for Refrigeration Cycle)
Heat from low
temperature sink
Cool Liquid
Turbine
3 2
14
Compressor
Heat to high
temperature source
Work
Entropy
Net Work
23
4 1Temper
ature
1-2: Adiabatic compression
2-3: Isothermal heat rejection
3-4: Adiabatic expansion
4-1: Isothermal addition of heat or isothermal
expansion
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Mechanical Engineering Dept. CEME NUST 5
Carnot Cycle:
o A standard of comparison,
o A convenient guide to the temperatures
that should be maintained to achieve
maximum effectiveness
Heat absorbed from the low temperature source
in process 4-1 is the Refrigeration Step
Vapor Compression cycle
Reversed Carnot Cycle (i.e. Carnot Cycle for Refrigeration Cycle)
Entropy
Net Work
23
4 1Temperature
1-2: Adiabatic compression
2-3: Isothermal heat rejection
3-4: Adiabatic expansion
4-1: Isothermal addition of heat or
isothermal expansion
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Mechanical Engineering Dept. CEME NUST 6
Ratio of out put to input would be misleading for
a refrigeration system as the o/p in process 2-3 is
usually wasted
Vapor Compression cycle
Coefficient of Performance (COP)
=
Entropy
Net Work
23
4 1Temperature
1-2: Adiabatic compression
2-3: Isothermal heat rejection
3-4: Adiabatic expansion
4-1: Isothermal addition of heat
or isothermal expansion
=
=
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Mechanical Engineering Dept. CEME NUST 7
Area underline 2-3 represents the Heat
Rejectedfrom the cycle
Useful Refrigeration is the heat transferred inprocess 4-1, or the area beneath the line 4-1
Vapor Compression cycle
Conditions for Highest Coefficient of Performance
23
4 1Temperature
Entropy (S)
KJ / Kg.K
Refrigeration
Net Work
Area enclosed in rectangle 1-2-3-4 represents
the Net Work
Work done = Heat Rejected Heat Supplied
= T2(S2S3) T1(S1S4)
= (T2T1) (S1S4)= Area of rectangle
=
=
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Mechanical Engineering Dept. CEME NUST 8
Vapor Compression cycle
Coefficient of Performance (COP)
1-2: Adiabatic compression
2-3: Isothermal heat rejection
3-4: Adiabatic expansion
4-1: Isothermal addition of heat
or isothermal expansion
COP of the Reversed Carnot Cycle is entirely a
function of the temperature limits and can varyfrom zero to infinity
To obtain maximum possible COP
oCold body temperature T1should be as high as possibleoHot body temperature T2should be as low as possible
COP indicates that a given amount of
refrigeration requires only a small amount of
work
23
4 1
T
emperature
Entropy (S)
KJ / Kg.K
Refrigeration
Net Work
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Mechanical Engineering Dept. CEME NUST 9
Vapor Compression cycle
All refrigeration works against certain temperature limitations
Temperature Limitations
t
t
S
T
-20 oC = 253 K
30o
C = 303 K
2
1
3
4
Cold Room
Atmosphere
oCold room to be maintained at -20 oC or 253 K
oReject heat to the atmosphere at 30oC or 303 K
During Heat Rejection Process,refrigerant temperature must be
higher than 303 K
During the Refrigeration Process, refrigeranttemperature must be lower than 253 K
Q. Do we have the control over the temperature?
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Mechanical Engineering Dept. CEME NUST 10
To decrease tto zero, eitherUorAwould have to be infinite
Reduction of t can be accomplished by
increasing A or U in the heat exchange
equation:
we can keep the tas small as possible
Infinite values ofU andAwould also require an infinite cost
Vapor Compression cycle
Temperature Limitations
t
t
S
T
2
1
3
4
Cold Room
Atmosphere
Q = U A t
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Mechanical Engineering Dept. CEME NUST 11
Vapor Compression cycle
Carnot Heat Pump
Heat Pump rejects heat at a high
temperature
Refrigeration cycle absorbs heat at a
low temperature
Heat PumpRefrigeration system operates
for the purpose of delivering heat at a highlevel of temperature
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Mechanical Engineering Dept. CEME NUST 12
COP of Refrigeration Cycle with the same
temperatures would be: T1/(T2- T1).
Performance Factor
Vapor Compression cycle
Carnot Heat Pump
S
T
2
1
3
4
Net Work
HeatRejected
=
=
=
Performance
Factor
=
=
+
=
+ = +
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Mechanical Engineering Dept. CEME NUST 14
Cycle differs from the Carnot cycle by the
addition of areas xand y
If vapor/gas such as air is used as the refrigerant, cycle would differ from thefamiliar rectangle of the Carnot cycle.
Effect of area xis to increase the work required,
which decreases the COP.
Effect of area y is to increase the work required
and in addition reduce the amount of refrigeration
Vapor Compression cycle
Both these effects of areas xand yreduce the COP
Carnot Refrigeration Cycle for Vapor as Refrigerant
23
4
1
T
S
x
y
Atmosphere
Cold Room
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Mechanical Engineering Dept. CEME NUST 15
o liquid refrigerant may be trapped in the head of the
cylinder by the rising piston and may damage the
compression valves and the cylinder itself
With a reciprocating compressor, the wetcompressoris not suitable
Wet Compression versus Dry Compression
The Compression process 1-2 is called wet
compression
Vapor Compression cycle
Revision of the Carnot Cycle
T
S
23
4 1
Atmosphere
Cold Room
Saturated
Liquid
Saturated
Vapor
o Another possible danger of wet compression is that the droplets of liquid may wash
the lubricating oil from the valve of the cylinder thus increasing wear
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Mechanical Engineering Dept. CEME NUST 16
If the refrigerant entering the compressor is saturated vapor as point 1, the
compression from point 1-2 is called Dry Compression
Compression of a dry vapor results in atemperature at point 2 which is higher than
the condensing temperature.
Area of that part of the cycle which is above
the condensing temperature is called the
Super Heated Horn
Super Heated Horn represents additional work required by dry compression
Wet Compression versus Dry Compression
Vapor Compression cycle
Revision of the Carnot Cycle
2
3
4 1
Super
Heated Horn
T
S
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Mechanical Engineering Dept. CEME NUST 17
Expansion Engine is not found suitable
oWork derived from the expansion engine is a small
fraction of that to be supplied to the compressor
Carnot cycle demands that the expansion 3-4takes place Isentropically and
that the resulting work be used to help drive the compressor
o Difficulties such as lubrication intrude when a fluid
of two phases drives the engine
o Economics of the Power Recovery has not justified the cost of the expansion process
Expansion Process
Vapor Compression cycle
Revision of the Carnot Cycle
2
3
41
T
S
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Mechanical Engineering Dept. CEME NUST 18
Expansion Process
Vapor Compression cycle
Revision of the Carnot Cycle
2
3
41
T
S
A Throttling Device such as a valve or other
restrictions is almost universally used for this
purpose
No change in potential and kinetic energy andwith no transfer of heat, constant enthalpy
process i.e. h3 = h4i.e. process is Isenthalpic
Constant enthalpy throttling process is Irreversibleand during the process
entropy increases
o Friction is one of the biggest reasons for any process to be irreversible
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Mechanical Engineering Dept. CEME NUST 19
Vapor Compression cycle
Simple/Standard Vapor Compression System
6
1
2
3
4
5
7
8
1- Evaporator
to produce a heat transfer surface
through which heat can pass from
the refrigerant space into the
vaporizing refrigerant
2- Suction Line
carries the low pressure vapor from
the evaporator to the suction inletof the compressor
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Mechanical Engineering Dept. CEME NUST 20
Vapor Compression cycle
Simple/Standard Vapor Compression System
6
1
2
3
4
5
7
8
3- Compressor
To draw refrigerant vapor from the
evaporator and then it rises its
temperature and pressure to such a
point so that it may be easily
condensed with normally available
condensing media
4- Discharge Line or Hot Gas Line
delivers the high temperature, high
pressure vapor from the discharge
of the compressor to the condenser
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Mechanical Engineering Dept. CEME NUST 21
Vapor Compression cycle
Simple/Standard Vapor Compression System
6
1
2
3
4
5
7
8
5- Condenser
to provide a heat transfer surface
through which heat passes from
the hot refrigerant vapor to the
condensing medium, which is
either air or water
Energy rejected by the Condenser
comprises the heat energy removed by
each kilogram of refrigerant passing
through the Evaporator and the heat
energy added to each kilogram of
refrigerant passing through the
Compressor
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Mechanical Engineering Dept. CEME NUST 22
Vapor Compression cycle
Simple/Standard Vapor Compression System
6
1
2
3
4
5
7
8
6- Receiver Tank Reservoir which stores the liquid
refrigerant coming from the
condenser and supplies it to the
evaporator according to the
requirement
7- Liquid Line
carries the liquid refrigerant from
the receiver tank to the
refrigerant flow control valve
8- Refrigerant Flow Control or
Expansion Valve
to supply a proper amount of refrigerant to
the evaporator after reducing its pressure
considerably so that the refrigerant may
take sufficient amount of heat from the
refrigerant space during evaporation
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Mechanical Engineering Dept. CEME NUST 23
Vapor Compression cycle
Introduction to PH-Charts
The properties of the refrigerants can be listed in tables or they can be shown
on a graph
Most useful and commonly used in refrigeration work is called the Pressure
Enthalpy (P-h) or Mollier diagram
Condition of the refrigerant in any
thermodynamic state can berepresented as a point in the P-h chart
that represents the condition of the
refrigerant in any one particular
thermodynamic state
Once the state point has been located on
the chart, other properties of the
refrigerant for that state can be
determined directly from the chart
P
h
Saturated
Vapor Curve
Saturated
Liquid Curve
2 31
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Mechanical Engineering Dept. CEME NUST 24
Vapor Compression cycle
Introduction to PH-Charts
Property Lines on the Pressure
Enthalpy Diagram
P
h
Iso-ThermalLine
Iso-Pressure
Line
Iso-Quality Line
Iso-Entropy Line
Iso-Specific
Volume Line
Iso-Enthalpy
Line
Saturated Vapor Curve
Saturated Liquid
Curve
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Mechanical Engineering Dept. CEME NUST 25
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Mechanical Engineering Dept. CEME NUST 26
Vapor Compression cycle
Performance of standard vapor compression cycle
P
(kPa)
h, kJ/kg
23
1
Evaporation4
Condensation
Expansion
Condenser
Compressor
Evaporator1
Expansion
Valve
23
4
oWork of compression
oHeat rejection rate
oRefrigeration effect
oCOP
o Volume flow rate per
KW of refrigeration
With the help of ph-diagram, significant
quantities of the vapor compression
cycle will be determined:
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Mechanical Engineering Dept. CEME NUST 27
Vapor Compression cycle
Performance of standard vapor compression cycle
P
(kPa)
h, kJ/kg
23
1
Evaporation4
Condensation
Expansion
Work of compression
Change in enthalpy in process 1-2
W = ( h1 - h2)
Knowledge of the work of compression isimportant this term may be one of the largest
operating costs of the system
Heat rejection
KJ/kg
Change in enthalpy in process 2-3, ( h3 - h2) KJ/kg
This heat rejection value is used in sizing the condenser and calculating the required flow
quantities of the condenser cooling fluid
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Mechanical Engineering Dept. CEME NUST 28
Vapor Compression cycle
Performance of standard vapor compression cycle
P
(kPa)
h, kJ/kg
23
1
Evaporation4
Condensation
Expansion
Refrigerating Effect
Change in enthalpy in process 4-1
( h1 h4) KJ/kg
Knowledge of the magnitude of this term is
necessary because performing this process is
the ultimate purpose of the entire systemCOP
=
Volume flow rate per kW is usually expressed in cubic meter per second per kW
(m3/sec.kW).
Volume Flow Rate is rough indication of the physical size of the compressor Greater
the value of the term, greater must be the displacement of the compressor in m3/sec
Efficient refrigeration system has a low value of power per kW, but a high COP
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Mechanical Engineering Dept. CEME NUST 29
A standard vapor-compression cycle developing 50 kW of refrigeration using
refrigerant 22 operates with a condensing temperature of 35 oC and an
evaporating temperature of -10 oC. Calculate :
(a) the refrigerating effect in Kj/kg,
(b)the circulation rate of refrigerant in kg/s,
(c) the power required by the compressor in kW,
(d)the COP,
(e) the volume flow rate measured at the compressor suction,
(f) the power per kW of refrigeration
(g) the compressor discharge temperature.
Vapor Compression cycle
Example
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l
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Mechanical Engineering Dept. CEME NUST 31
Vapor Compression Cycle
PH-Diagram of superheated R-22 Vapor
C i l
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Mechanical Engineering Dept. CEME NUST 32
Vapor Compression cycle
Heat Exchangers
Condenser
Compressor
Evaporator6
Heat
Exchanger
23
45
1
P
h
23
1
Evaporation
5/
Condensation
Expan
sion
6
4
5
Sub Cooling
Super Heating
Heat exchanger sub cools the liquid from the
condenser with suction vapor coming from
the evaporator
h3 - h4= h1 h6
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V C i l
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Mechanical Engineering Dept. CEME NUST 34
Vapor Compression cycle
Actual Vapor Compression Cycle
P
h
23
14
Pressure Drop
Pressure Drop
Sub Cooling
Super Heating
Actual Cycle
Standard Cycle
Difference b/w actual and Standard cycle can be shown by superimposing the
actual cycle on the Ph-diagram of the standard cycle
Essential Differences between the actual and the standard cycle appear:
o In the pressure drops in the
condenser and evaporator
o In the sub cooling of the liquid
leaving the condenser
o in the superheating of the
vapor leaving the evaporator
V C i l
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Mechanical Engineering Dept CEME NUST 35
Vapor Compression cycle
Example
In the vapor compression cycle a throttling device is used almost universally to
reduce the pressure of the liquid refrigerant.
(a) Determine the percent saving in net work of the cycle per kg of refrigerant if
an expansion engine could be used to expand saturated liquid refrigerant 22
isentropically from 35 oC to the evaporator temperature of 0 oC . Assume
that compression is isentropic from saturated vapor at 0 oC to a condenser
pressure corresponding to 35 oC.
(b) Calculate the increase in refrigerating effect in kJ/kg resulting from use of
the expansion engine.