Objective:

Tostudy the vapour compression and vapour absorption refrigeration systems.

Aim:
(a) To study the working principle and components of vapour comprcssion and vapour absorption
refrigeration systems.
(b) Todetermine the COP of VCRS.
Theory:
Ingeneral, refrigeration is defined as any process of hcat removal. More specifically, refrigeration is defined as
the branch of science that deals with the process of reducing and maintaining the temperature of a space or
material below the temperature of the surroundings. The system maintained at the lower temperature is known
as a reffigerated system while the equipnent used to maintain this lower temperature is known as a refrigerating
system.
flow from a low
In accordance with Clausius's statement of the second lavw of thermodynamics, heat does not
heat against
temperature region toa high-temperature region without the aid of external energy. This transfer of
the machine. A machine
areverse temperature gradient can be accomplished if mechanical energy is supplied to
process is
that maintains a space at a lower temperature than the surrounding is known as a refrigerator, and the
Such
known as refrigeration. Refrigeration, therefore implied the cooling or removal of heat from a system.
cooling may be obtained by any of the following principles.
chenical means, in which a chemical reaction (endothermic reaction) is carried out
which
i. By
absorbs heat for its completion. The heat required for the purpose is taken from the substance of
space to be cooled.
cooling medium
ii. By bringing the substance to be cooled directly or indirectly in contact with some
such as chilled water or ice.
be abstracted from
iii. By using mechanical or heat energy to operate a heat pump by which heat may
a low temperature region and rejected to a high temperature region.
and medicines
Refrigeration is widely used for the cooling of storage chambers in which perishable foods, drinks
are stored. Refrigeration also has wide application in submarine ships, aircraft, et.
Vapour Compression Refrigeration
refrigeration
The major difference in theory and treatment of vapor refrigeration system as compared to the air
to vapour
system is that, the vapor alternately undergoes a change of phase from vapour to liquid and liquid
heat from the
during the completion of a cycle. The latent heat of vaporization is utilized for carrying
sensible heat of the
refrigerator,which is quite high compared with the air-cycle, which depends only upon the
air.
alternately after condensing
The substances used do not leave the plant but are circulated through the system
the air, which is used for circulating
and re-evaporating. During evaporating, it absorbs its latent heat from
ouna thecold chamber. In condensino. it oives out its latcnt heat to the circulating water of thc cooler, the
machine is, therefore, known as a latcnt heat pump.
It absorbs its latent heat from the air and oives out in the condenser. The diagrammatic view of the system is
shown in the figure below. All the major parts are shown and discussed n deta.
Condeneer
High High temperature heat rejection ()High pressure
pressure (3
liquid vapor

Compres8or
Expanslon Work input
Device
Woormp
Low pressure
vapor
Low pressure
bwo-phase mixture Cevap Evaporator
Lowtermperature heat acquisilion
Figure 1Schematic diagram of vapor compression refrigeration
The vapour compression cyele consists of the following four processes:
(1) Reversible adiabatic compression from the saturated vapour to a
superheated condition.
(2) Reversible heat rejection at constant pressure (de-superheating and condensation of the
refrigerant)
(3) Irreversible is enthalpy expansion from saturated liquid to a low-pressure
vapour.
(4)Reversible heat addition at constant pressure.
System components and theirfunction
(1) Compressor: The Compressor is the most important part of the system. The compressor raises the
measure
of incoming vapour from the evaporator to a high pressure. Different types of compressors are:
a) Reciprocating compressor
b) Rotary compressor
c) Screw compressor
d) Centrifugal compressor
The selection of the above-mentioned compressor depends upon the usage. Usually, domestic retrigerators are
installed with reciprocating compressors. Ahermetic-type compressor is one in which the compressor and motor
are integral on one shaft, and they are both contained in a pressure shield housing. It is compact in size, quite,
low in cost and no problem of refrigerant gas leakage. The motor in a hermetic-type compressor is cooled by
refrigerant suction gas.
(2) Condenser: The function of a condenser is to remove heat from the superheated high-pressure refrigerant
vapour and to condense the vapour into a subcooled high-pressure liquid. This is accomplished by a cooling
medium, either air or water. The air-cooled condenser may be of a static cooled type where natural convective
motion of air surrounding heat is enough to cool the condenser or they may be of fan cooled type. The static
cooled condenser is used in domestic refrigerators. In commercial appliances and windows, air-conditioners'
 uscd in large system firms 5 tons and
usually fan-coolcd condenserS are uscd. The watcr condenscer is normally
above.
the condenser passes through
(3) Expansion Devices: As the high pressure sub-cooled refrigerant liquid from
refrigerant. mostly in liquid stage.
the cxpansion device, its pressure and temperature is reduced and outlct
For example, for constant
Expansion devices are of many types and are selected as per the requirement.
cooling load is high.
requirements, automatic expansion valves are used. If the variation in
cvaporator pressure
thermostatic valves are best suited.
few of them might also be used
Hermetic compressors are almost invariably uscd with capillary tube, though a
with thermostatic expansion valves.
mass flow rate of refrigerant by
(a) Thernmostatic expansion vahe: A thermostatic cxpansion valve controls the
cooling load. If the load is more,
sensing the evaporator outlet temperature. Thus, the valve is sensitive to the
To maintain the degree of super
the degree of the super hit of refrigerant coming out of the evaporator increases.
low, the valve closes, and less
heat to the preset level, more liquid is fed to the evaporator. When the load is
liquid is fed.
Selection of thermostatic expansion is done on the basis of the refrigerant used in capacity.
include any moving
(b) Capillary tube: It is the simplest and cheapest form of expansion device. It does not
operation device in
parts; hence, no maintenance is required. The capillary tube is supposed to be a single-point
the sense that the best control is achieved only at a given set of operating parameters.
at lower loads than the
Under varying loads, the capillary tube does not function satisfactorily. For example,
compressor.
designed value, the capillary tube may overfeed the evaporator, causing liquid to return to the
Under higher loads than designed, the capillary tube starves the evaporator, causing excessive return gas
superheat. Hence, for variation in cooling loads, it is not suitable.
or the area to be cooled and
(4) Evaporator: The function of the evaporator is to remove heat from the product
maintain it at any desired temperature. The liquid refrigerant inside the evaporator evaporates by absorbing heat
constructions
and converts into vapour refrigerant, and then it returns back into the compressor. Various types of
of the evaporator used in refrigerant systems e.g.
a) Finned tube evaporator coil
b) Bare tube soldered/clamped to the tank
c) Bare tube dipped in the liquid to be cooled
d) Shell &coil typeevaporator
The choice of particularconstruction depends on the type of appliance. However, in every case, the heat capacity
depends on these factors, viz., the temperature difference between the load and the refrigerant, heat transfer
coefficient, and arcas of the heat transfer.
(5) Drier: Ifby chance the refrigerant contains any water particle, then in low temperature region (i.c. at and
after the expansion valve) it forms ice and chokes the valve or bends thereby preventing the smooth passage of
refrigerant through it. To prevent this, a drier is used to remove any water particles carried into the refhigerant.
It is used between the evaporator and compressor.
 rctr1gcrant from
(0) Accumulator: I is fitted in betwecn the cvaporator and comprcssor. I prevents thc liquid
cntering the compressot.
V'apour Absorption Refiigeration
Absorption refigeration systems operate using thermal energy. They use the principle of liquids evaporating at
with
tatures when pressure is reduced. This basic principle is demonstrated in the experimental unit
de cxample of an ammonia-tvatcr solution, with the ammonia acting as a refrigerant. In the evaporator, the
iquldammoniaevaporates and withdraws heat from the enviropment. To keep the evaporation pressure low,
e ammonia vapor in the absorber is absorbcd by the water. In the next step, ammonia is permanently removed
om ne nigh-concentration ammonia solution to prevent the absorption process from being halted. For this
Pupose, the high concentration ammonia solution is heated in a generator until the ammonia evaporates agaim.
In the final step, the ammonia vapor is cooled in the condenser to the base level, condenses, and is returned to
the evaporator. The low concentration ammonia solution flows back to the
absorber. To maintain the pressure
differences in the system, hydrogen is used as an auxiliary gas. In process technology systems, the
resulting
waste heat can be uscd for cooling. In small mobile systems, such as a camping
refrigerator or minibar in a
hotel, the required heat is generated electrically or by gas burner. Another benefit of absorption
refrigeration
systems is their silent operation. The present experimental setup demonstrates the functional
principle of an
absorption refrigeration system with its main components: evaporator, absorber, boiler as generator with bubble
pump, condenser. The boiler can alternatively be operated with propane gas or
electrically. Another electric
heater at the evaporator generates the cooling load. The operating principle of the three
circuits; water circuit,
ammonia circuit and hydrogen circuit.

Cold Hot side

sicde

H2 gas

Figure 2 Schematic of the vapor absorption system (1)Boiler with bubble pump, 2)Condenser, 3)Evaporator.
4)Gas heat exchanger, 5) Absorber, 6) Liquid heat exchanger 7) Storage Tank)
Comparison between Vapour Absorption and Vapour Compression system
S.No. Vapour Absorption System Vapour Compression System
1 Chcapcr form of cnergy is uscd to opcrate Relatively costlier form of encrgy like mcchanical
thecycle. or electrical energy is required.
The only moving part involved in the Compressor and motor are required and that too of
system is small pump uscd to dcliver the higher capacity. Beccause of vibrations of
strong mixturc. compressor strong foundation is also required.
3 The vapour tcnpcrature lcaving the In vapour compression system the degree of
gencrator is slightly superhcated and is superheat is governed by the suction state of vapour.
controlled by the heat cxchangers provided
after the generator
4 Smaller condenser is required. Bigger condenser is required.
The capacity of the absorption system is Capacity of the compression system decreases
controlled bygenerator temperature even if apidly with the reduced evaporator pressure.
the evaporator pressure falls.
6 The COP of absorptionsystem isgenerally The COP of compresion system is generally much
much lower. higher.
The capacity of absorption system may run For higher capacity the parameters like inertial
into hundreds of tons and there is no bar on force of reciprocating masses has to be considered
the capacity. and hence for higher capacity plants the system
becomes costly.
8 Requires very less maintenance as the only Requires greater maintenance compared to vapour
moving part is smallcapacity pump. absorption system.
9 There is less wastage of refrigerant. Comparatively more amount of refrigerant is
wasted due toleakage through shaft seal.
10 For small units the absorption system is For small units the compression system is much
much bulkier as compared to compression compact as compared to absorption system. But for
system. But for high capacity plants the high capacity plants the system becomes bulkier
system becomes compact than than absorption system.
compression system.

Specification of VCRS test rig:

S.No. Component Name Specification
Capacity 0.3 T
Refrigerant R134a
3 Compressor Model No.: KCE 444 HAG
Make: Emerson
 Single Phase 230V. 50Hz, AC
Water Heater Total: IkW
Condenser (Air cooled) 3/8"OD ||" × 10" with 3 rows
6 Rotameter Fluid: RI34a at roonm temperature
Range: 4.2 to 42 LPH
7
Pressure Gauges 4No. Glycerin filled, SS body, panel mounted,
back connection
Range: 0-16 kg'cm', 2Nos.
0-28 kg/cm', 2Nos.
Temperature Scnsors PU00-Pencil Type, INo.
Pt100-Bare Typc, 4 Nos.
Tempcraturc Indicator Supply Voltage: 240 VAC,
Range: 0-100°C
11
Charging Valve 1/4" Flair connection hand shut-OFF valve
12 Energymeter
Power Supply
230 V, Make L&T
230 V, 50 Hz, Single Phase
Max Current: 15 A

Experimental Procedure:
1. Ensure that the drain valve of the test
2.
chamber is fully closed.
Fill clean water in the test
chamber up to the level marked. If the water is
should be close tO room temperature. If not, already filled, its temperature
switch on the load (Heater) and stirrer and
desired temperature is achieved. wait till the
3. Ensure that the
pressure shown on all the gauges is nearly
4. Ensure that all the equal.
switches on the panel are in OFF position before
5. Give power supply to the connecting the supply to the system.
trainer by connecting the three-pin plug to the
6. Switch on the mains socket in your laboratory.
ON-OFF switch on the control panel to start the
7. Switch on the compressor and stirrer using the supply to the trainer.
switch provided on the panel marked as
and "Stirrer". "Compressor"
8. Check the
temperature of the water (T5).
9. Byusing the selector switch on
the temperature indicator, note the
Note: Note readings after T2become above 55°C temperature TI, T2, T:. T4, and Ts.
for better results.
10. Note the pressures of R-134a gas in
the circuit by noting pressure Pi, P2, P3, and P4.
11. Note the number of blinks per
minute on the energy meter.
12. Repeat the above procedure to get
different sets of readings for different loads every 10 minutes till you
get fairly constant temperatures in the consecutive
readings.
13. Calculate the COP of the system.
14. Switch off all the switches after you complete the
experiment.
15. Disconnect the power supply.
16. Remove water from the water tank.

Observation and Calculation:

 Observations Table
Ts P, P: P: P. Clock
S.No. T T: T; T,

1
2

Calculations:

1. COP (Rev Carnot) =

The pressure is in cm-/kg.
So,
Absolute pressure P = P +1
chart.
By taking P get the corresponding value of T from the
(Temperature in Kelvin)
T = Saturation temperature at the suction pressure
Similarly find the P, and T;
(Temperature in Kelvin)
T, =Saturation temperature at the condenser pressure
2. Theoretical COP = -A
hy-hi
Where,
h =enthalpy (for gas) at temperature T;
h, = enthalpy (for gas) at temperature T;
h, = enthalpy (for liquid) at temperature T3
Precautions & Maintenance Instructions:

run the apparatus if the power supply is less than 180 volts andabove 230 volts.
1. Never
mains power supply before ensuring that all the ON/OFF switches on the panel
2. Never switch ON the
are in OFF position.
LP-HP cutoff switch.
3. Do not change the setting of the
level.
4. Water should be filled up to the marked
gently.
5. Operate the selector switch of the temperature indicator
6. Keep the stirrer in the on position for better results.
refrigerant charging valve. If this valve is opened slightly, the entire refrigerant will
7. Do not touch the
leak and lead to non-performance of the instrument. any
over, remove the water from the water tank so as to prevent corrosi0n of
8. Once the experiment is
part inside the test chamber.