Refrigeration and Air Conditioning
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Vapor-Compression Refrigeration
Cycle
Most common refrigeration cycle in use today
There are four principal control
volumes involving these
components:
Evaporator
Compressor
Condenser
Expansion valve Two-phase
liquid-vapor mixture
All energy transfers by work and heat are taken as positive in the
directions of the arrows on the schematic and energy balances are
written accordingly.
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VCRS
The processes of this cycle are
Process 4-1: Two-phase liquid-vapor
mixture of refrigerant is evaporated
through heat transfer from the
refrigerated space.
Process 1-2: Vapor refrigerant is
compressed to a relatively high
temperature and pressure requiring work
input. Two-phase
liquid-vapor mixture
Process 2-3: Vapor refrigerant condenses
to liquid through heat transfer to the
cooler surroundings.
Process 3-4: Liquid refrigerant expands to
the evaporator pressure.
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Standard VCRS: P-h & T-s diagrams
Standard VCRS cycle on a P-h chart Standard VCRS cycle on a T-s chart
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VCRS
Engineering model:
Each component is analyzed as a control volume at steady
state.
Dry compression is presumed: the refrigerant is a vapor.
The compressor operates adiabatically.
The refrigerant expanding through the valve undergoes a
throttling process.
Kinetic and potential energy changes are ignored.
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VCRS
Applying mass and energy rate balances
Evaporator •
Qin (Eq. 1)
•
= h1 - h 4
m
The term Qin is referred to as the refrigeration capacity,
expressed in kW in the SI unit system or Btu/h in the English
unit system.
A common alternate unit is the ton of refrigeration which
equals 200 Btu/min or about 211 kJ/min.
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VCRS
Applying mass and energy rate balances
Compressor •
Assuming adiabatic Wc (Eq. 2)
•
= h 2 - h1
compression m
•
Condenser Qout
= h 2 - h3 (Eq. 3)
•
m
Expansion valve
Assuming a throttling h 4 = h3 (Eq. 4)
process
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VCRS
Performance parameters
Coefficient of Performance (COP)
• •
h1 - h 4
Qin m
COP = • •
=
Wc m h 2 - h1
Carnot Coefficient of Performance
TC
COP =
TH -TC
This equation represents the maximum theoretical coefficient of
performance of any refrigeration cycle operating between cold
and hot regions at TC and TH, respectively.
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PROBLEM
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
State 2
State1 kJ
Compressor inlet 1
h 238.41 Compressor exit kJ
kg h2 s 278.23
P2 s P2 900 kPa kg
T1 20o C s 0.9456 kJ kJ T2 s 43.79 C
o
x1 1.0 1 kg K s2 s s1 0.9456
kg K
State 3 kJ State 4
x4 0.358
Condenser exit 3
h 101.61
kg Throttle exit
o kJ
P3 900 kPa kJ T4 T1 20 C 4
s 0.4053
s3 0.3738 kg K
x3 0.0 kg K h4 h3
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QL m(h1 h4 ) h1 h4
COPR
Wnet , in m(h2 h1 ) h2 h1
kJ
(238.41 101.61)
kg
kJ
(278.23 238.41)
kg
3.44
The tons of refrigeration, often called the cooling load or refrigeration effect, are
TL
QL m(h1 h4 ) COPR , Carnot
TH TL
kg kJ 1Ton
3 (238.41 101.61) (20 273) K
min kg 211 kJ
min (43.79 (20)) K
1.94 Ton 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 4.715
QL COPR
4.715 hp
3.44 Ton
hp
1.37
Ton
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Features of Actual VC Cycle
Heat transfers between refrigerant and cold and warm regions
are not reversible.
Refrigerant temperature in
evaporator is less than TC.
Refrigerant temperature in
condenser is greater than TH.
Irreversible heat transfers
have negative effect on
performance.
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Features of Actual VC Cycle
The COP decreases – primarily due to increasing compressor
work input – as the
temperature of the refrigerant
Trefrigerant ↑
passing through the
evaporator is reduced relative
to the temperature of the cold
region, TC.
temperature of the refrigerant
passing through the
condenser is increased Trefrigerant ↓
relative to the temperature of
the warm region, TH.
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Features of Actual VC 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.
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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
W CV m
s h2 s h1 (Eq. 5)
c = =
h2 h1
W CV m
work required in an actual
compression from compressor inlet
state to exit pressure
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PROBLEM
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
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State 1 2s 2 3 4
h (kJ/kg) 241.35 272.39 280.15 91.49 91.49
(a) The compressor power is
W c m (h2 h1 )
kg kJ 1 kW
Wc 0.08 (280.15 241.35) 3.1 kW
s kg 1 kJ/s
(b) The refrigeration capacity is
Q in m (h1 h4 )
kg kJ 1 ton 60 s
Qin 0.08 (241.35 91.49) 3.41 tons
s kg 211 kJ/min min
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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 )
C.O.P
(h2 h1 )
(241.35 91.49)kJ/kg
C.O.P 3.86
(280.15 241.35)kJ/kg
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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 (h2 s h1 )
W c / m (h2 h1 )
(272.39 241.35)kJ/kg
c 0.8 = 80%
(280.15 241.35)kJ/kg
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Air conditioning
Definition
Maintaining a living space or an industrial facility at the
desired temperature and humidity requires some processes
called air-conditioning processes.
These processes include simple heating (raising the
temperature), simple cooling (lowering the temperature),
humidifying (adding moisture), and dehumidifying (removing
moisture).
Sometimes two or more of these processes are needed to
bring the air to a desired temperature and humidity level
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REFRIGERANT
A refrigerant is a substance or mixture, usually a fluid, used in a heat
pump and refrigeration cycle. In most cycles it undergoes phase
transitions from a liquid to a gas and vice-versa.
Ex.- Freon(R-22), Puron (R-410A).
A refrigerant should be non-flammable, non-toxic ,should have
dense vapour ,lower boiling point.
MAJOR PARTS OF THE AC
Evaporator
Compressor
Condenser
Expansion Valve
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HOW AIR CONDITIONER WORKS
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Types of AC
Window AC
Split AC
Central AC plant
Packaged AC
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THANK YOU
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