Basic Refrigeration Cycle
Reversed Carnot Cycle
Reversed Carnot Cycle
Refrigeration Cycle
Dr. M. Zahurul Haq
Professor
Department of Mechanical Engineering
Bangladesh University of Engineering & Technology (BUET)
Dhaka-1000, Bangladesh
zahurul@me.buet.ac.bd
http://teacher.buet.ac.bd/zahurul/
ME 415: Refrigeration & Building Mechanical Systems
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Coefficient of Performance, COP =
c Dr. M. Zahurul Haq (BUET)
Refrigeration Cycle
Basic Refrigeration Cycle
ME 415 (2011)
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Reversed Carnot Cycle
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Reversed Carnot Cycle
Carnot cycle demands that the expansion take place isentropically and that
the resulting work be used to help drive the compressor. Practical
difficulties, however, militate against the expansion engine:
the possible work that can be derived from the engine is small fraction
that must be supplied to the compressor.
practical problems such as lubrication intrude when a fluid of two
phases drives the engine.
In wet compression, the droplets of the liquids may wash the
lubricating oil from the walls of the cylinder, accelerating wear. Dry
compression takes place with no droplets and is preferable.
Liquid refrigerants may be trapped in the head of reciprocating
compressor by the rising piston, possibly damaging the valves or the
cylinder head.
ME 415 (2011)
ME 415 (2011)
Expansion Process in Carnot Cycle
During compression, droplets present in liquid are vaporised by the
internal heat transfer process which requires finite time. High-speed
compressors are susceptible to damage by liquid because of the short
time available.
Refrigeration Cycle
Refrigeration Cycle
Basic Refrigeration Cycle
Wet Compression in Carnot Cycle vs. Dry Compression
c Dr. M. Zahurul Haq (BUET)
c Dr. M. Zahurul Haq (BUET)
TL
QL
=
Wnet
TH − TL
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the economics of the power recovery have in past not justified the cost
of the expansion engine.
A throttling device, such as a valve or other restriction, is almost
universally used for this purpose.
c Dr. M. Zahurul Haq (BUET)
Refrigeration Cycle
ME 415 (2011)
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Basic Refrigeration Cycle
Ideal Vapour Compression Refrigeration Cycle
Basic Refrigeration Cycle
Ideal Vapour Compression Refrigeration Cycle
Ideal Vapour Compression Refrigeration Cycle
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c Dr. M. Zahurul Haq (BUET)
Refrigeration Cycle
Basic Refrigeration Cycle
ME 415 (2011)
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c Dr. M. Zahurul Haq (BUET)
Ideal Vapour Compression Refrigeration Cycle
Refrigeration Cycle
Basic Refrigeration Cycle
ME 415 (2011)
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Ideal Vapour Compression Refrigeration Cycle
Processes of VC System
QL
QH
Win
COP
=
=
=
=
Q41 = m(h1 − h4 )
Q23 = m(h2 − h3 )
W12 = m(h2 − h1 )
QL /Win
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1 → 2: Isentropic compression, Pevap → Pcond
2 → 3: Isobaric heat rejection, QH
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3 → 4: Isenthalpic expansion, Pcond → Pevap
Simple vapour compression cycle with pressure & enthalpy values for R134a
4 → 1: Isobaric heat extraction, QL
c Dr. M. Zahurul Haq (BUET)
Refrigeration Cycle
ME 415 (2011)
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c Dr. M. Zahurul Haq (BUET)
Refrigeration Cycle
ME 415 (2011)
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Basic Refrigeration Cycle
Ideal Vapour Compression Refrigeration Cycle
Basic Refrigeration Cycle
Effect of Evaporator Temperature
A theoretical single stage cycle using R134a as refrigerant operates with a
condensing temperature of 30o C and an evaporator temperature of -20o C.
The system produces 50 kW of refrigeration effect. Estimate:
o
R134a: RE = 50 kW, Tcond = 30 C
10
0.40
9
0.39
1
Coefficient of performance, COP
8
0.38
2
Refrigerant mass flow rate, m
7
0.37
Ref. flow rate
QL
COP
6
= Q41 = m(h1 − h4 ) = 50 kW
m = 0.345 Kg/s
= W12 = m(h2 − h1 ) = 12.5 kW
= QL /Win = 50.0/12.5 = 4.0
Win
COP
0.36
COP
5
0.35
4
0.34
3
0.33
2
0.32
1
0.31
0.30
0
-50
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Refrigerant flow rate (kg/s)
Example
Ideal Vapour Compression Refrigeration Cycle
-45
-40
-35
-30
-25
-20
-15
-10
-5
0
o
Tevap ( C)
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c Dr. M. Zahurul Haq (BUET)
Refrigeration Cycle
Basic Refrigeration Cycle
ME 415 (2011)
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c Dr. M. Zahurul Haq (BUET)
Ideal Vapour Compression Refrigeration Cycle
Basic Refrigeration Cycle
Effect of Condenser Temperature
10
10
0.40
9
9
0.38
0.34
7
Ref. flow rate
5
5
0.30
4
0.28
3
0.26
1
2
0.24
50
0
20
25
30
35
40
45
Tcond = 30 C
6
0.32
15
o
7
6
10
o
Tcond = 20 C
8
COP
0.36
COP
5
4
o
Tcond = 40 C
3
2
-50
-45
o
Tcond ( C)
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Refrigeration Cycle
-40
-35
-30
-25
-20
-15
-10
-5
0
o
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c Dr. M. Zahurul Haq (BUET)
Ideal Vapour Compression Refrigeration Cycle
11
0.42
Refrigerant flow rate (kg/s)
COP
0.44
11
0
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12
o
8
ME 415 (2011)
Effect of Evaporator & Condenser Temperatures
R134a: RE = 50 kW, Tevap = -20 C
12
Refrigeration Cycle
ME 415 (2011)
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Tevap ( C)
Refrigeration Cycle
ME 415 (2011)
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Deviation from Simple Cycle
Deviation from Simple Cycle
Deviations from Ideal Cycle
Super-heating & Sub-cooling
1
Refrigerant pressure drop in piping, evaporator, condenser, receiver
tank, and through the valves and passages.
2
Sub-cooling of liquid leaving the condenser.
3
Super-heating of vapour leaving the evaporator.
4
Compression process is not isentropic.
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Sub-cooling of liquid serves a desirable function of ensuring that 100%
liquid will enter the expansion device.
Super-heating of vapour ensures no droplets of liquid being carried
over into the compressor.
Even through refrigeration effect is increased, compression work is
greater & probably has negligible thermodynamic advantages.
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c Dr. M. Zahurul Haq (BUET)
Refrigeration Cycle
ME 415 (2011)
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Refrigeration Cycle
ME 415 (2011)
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