Refrigeration_PPT KM.ppt

1. REFRIGERATION

2. • REFRIGERATION CYCLES
• Refrigeration is a process of maintaining a space or a body at
a temperature lower than that of its surroundings.
• To produce and maintain the low temperature, it is necessary
to transfer heat from the space to be refrigerated or the cold
body to the place where it is not highly objectionable.
• A refrigerator is a device that is employed to accomplish
refrigeration by the expenditure of external energy in the
form of work or heat or both.
• For the refrigerator to operate continuously, it must reject
heat to an external sink, usually the atmosphere.
• The working substance used in the refrigerator, which
absorbs the heat from the refrigerated space and rejects to
the sink, is called a refrigerant.
• The refrigerator is a device which consists of all the necessary
component to obtain the required cooling effect

3. • The heat removed from the refrigerated space
or the cold body is called “refrigeration effect”
or “capacity of the refrigerator”.
• The refrigeration effect is normally expressed
in tons of refrigeration.
• One ton of refrigeration is the amount of heat
that has to be removed from 1 ton of water at
00C to convert it into ice at 00C in 24 hours.
• In SI system of units this will be equal to
211kJ/min or 3.51kW.

4. Refrigeration cycles are classified into two types; namely
(i) Gas refrigeration cycles
(ii) Vapor refrigeration cycles.
Vapor refrigeration cycles
(a) Vapor compression cycle and
(b) (b) vapor absorption cycle.

5. Ideal Air Refrigeration
(Reversed Brayton Cycle or Bell – Coleman Cycle)
Assumptions made in the analysis of the ideal cycle
(i) The working fluid is air
(ii) Air behaves as a perfect gas.
(iii) All processes that the working substance undergoes are
reversible.
(iv) There are no pressure losses in the piping connecting the
various components and also in the heat exchanger.
(v) Flow is steady and one dimensional.
(vi) Changes in kinetic and potential energies of the working
substance are negligible.

6. Process 1-2:- Isentropic compression of air from state 1 to state 2.
During this process work is done on air by the surroundings.
Process 2-3:- Constant pressure cooling of air in the intercooler.
Process 3-4:- Isentropic expansion of air from state 3 to state 4.
During this process work is done by air on the surroundings.
Process 4-1:- Constant pressure heat addition to air in the cold
chamber ( Real refrigeration )

7. Let QR = Refrigeration effect per unit time
= Heat removed from the cold chamber per unit time
Applying SFEE to the cold chamber ( No changes in KE & PE) we have
m h4 = QR + m h1, where m is the mass flow rate of refrigerant through the cold
chamber.
or QR = m (h1 – h4) = m Cp(T1 – T4)
Similarly compressor work input= Wc= m (h2 – h1)= mCp(T2– T1)
And expansion work = We = m (h3 – h4) = mcp(T3 – T4)
Therefore net work input to the cycle = Wnet = Wc − We
Or Wnet = m Cp(T2 – T1) – m Cp(T3 – T4)
Coefficient of performance = COP = QR / W

9. Mechanical Vapor Compression Refrigeration
Cycle
• Limitations of Carnot Refrigeration Cycle With Vapor as a
Refrigerant:-
 Though the isothermal process of heat rejection and heat
absorption can be achieved in practice, it is extremely
difficult to achieve isentropic compression (process 1-2) and
isentropic expansion (process 3-4) in practice when the
vapor is wet.
In a mechanical vapor compression refrigeration cycle, the
isentropic expansion is replaced by a throttling expansion or by an
expansion through a capillary and the compression of wet vapor
is avoided.

10. Assumptions:-
(i) Isentropic compression
(ii) Dry saturated vapor entering the compressor
(iii) No pressure losses in the piping connections (viz., the condenser
and the evaporator.
(iv) The flow of the refrigerant is steady and one dimensional.
(v) Changes in kinetic and potential energies of the refrigerant, as it
flows through the various components, are negligible.

12. • Effect of different parameters on the
performance of vapor compression
refrigeration system
Sub cooling
Decreasing the evaporator pressure
Increasing the condenser pressure
Superheating the refrigerant

13. Sub cooling

14.  Sub cooling
 It can be seen form the T-s diagram that by
sub-cooling the refrigerant say up to 3’, the
refrigeration effect per unit mass has
increased from (h1 – h4) to (h1 – h4’)
 There is no change in work input or
compressor work.
 Thus sub cooling results in increase in COP
of the refrigeration system.
 Therefore, it is desirable to sub-cool the
refrigerant before it enters the throttle valve

15. Decreasing the evaporator pressure

16. Increase in condenser pressure
• Decrease in refrigeration effect and increase in
compressor work for the given condition of the
refrigerant entering the compressor and saturated
liquid entering the throttle valve.
• The refrigeration effect decreases from (h4-h1) to (h4
1-
h1) and compressor work increases from (h2-h1) to (h2
1-
h1). Thus the COP of refrigeration decreases.
• The volumetric efficiency of the compressor will
decreases because of increases in delivery pressure.
• This type of situation happens when we take the
refrigerator from a cold region to a hotter region where
the ambient temperature is higher.
• Thus, the discharge pressure should be kept as low as
possible so as to have higher COP which mainly depends
on the cooling medium available for heat rejection.

18. •.
Super heating
• Superheating the vapor at the
inlet to the compressor is
preferred because it ensures the
complete vaporization of the
liquid in the evaporator.
• For refrigerant like Freon -12,
maximum COP is obtained with
superheating of suction vapor.
• Increase in specific volume of
the suction vapor from v1 to v1
1
• The increase in refrigeration
effect from (h1-h4) to (h1
1-h4)
• Increase in specific work from
(h2-h1) to (h2
1-h1
1) and RE from
(h1-h4) to(h1
1-h4).

19. • REFRIGERANTS
• A refrigerant is the primary working fluid used for
absorbing and transporting the heat in a refrigeration
system.
• It is the heat transporting agent from the evaporator
to the condenser.
• Refrigerant absorb heat at a low temperature and
pressure from the space to be cooled and releases
the heat at a higher temperature and pressure in the
condenser.
• Most refrigerant undergo phase changes during heat
absorption process and heat releasing

20. • Desirable properties of refrigerants; -
• The saturation pressure at the evaporator temperature should
be slightly above atmospheric pressure to ensure that no air
leaks into the system.
• The pressure difference between the evaporator and the
condenser should be moderate. If the difference is more work
of compression increases and probability of leakage will
increases.
• The triple point and the critical point for the refrigerants should
be far from range of operation of the refrigeration system.
• Specific volume of the refrigerants should be small to reduce
the size of the compressor and to avoid the high flow velocities.
• The specific volume of the suction vapor required per ton of
refrigeration is an indication of the size of the compressor.
Reciprocating compressor are used for refrigerants with high
pressure and small suction volumes.
• High enthalpy of evaporation is required which reduces the
refrigerant flow rate.

21. • The freezing temperature should be sufficiently below
the operating temperature to avoid any blocking of the
refrigerants.
• The refrigerants should be acceptably safe for humans
and manufacturing processes with little or no toxicity
and it should not be flammable and explosive.
• The refrigerants should have reasonable solubility with
water. The presence of the moisture is very critical in
refrigeration system working below 00C. If more water
is present than can be dissolved by the refrigerants
then there is a danger of ice formation and subsequent
choking of valves or capillary tubes.
• The refrigerants should have high thermal conductivity,
low viscosity, low specific heats and easily available.
• Refrigerants should have low ozone depletion
potential.

22. Ammonia: -
• One of the oldest refrigerants, still widely used in
industrial application,
• An excellent from thermodynamic point of view- High
latent heat of vaporization (small compressor size),
excellent heat transfer characteristics & relatively
cheap.
• In the presence of water strongly attacks copper and
cuprous alloy hence ferrous alloys are used.
• It is non-miscible with mineral lubricating oils, irritating
odor and mildly toxic.
Carbon Dioxide: - It is used as dry ice or solid carbon
dioxide only in transport vehicles.
Water:- High freezing temperature of water limits its use
in vapor compression systems. Water is used as the
refrigerant in some absorption system and in systems
with steam jet compression.

23. • Halocarbon Refrigerants: -Although there are many
refrigerants in this category only few are important.
 Tricloromonofluoromethane (R-ll): - It is non-corrosive,
non-toxic and noninflammable. Its normal boiling point
is 23.7°C. Since it is a low-pressure refrigerant (high
specific volume), a centrifugal compressor is generally
required. It is generally used in large central air
conditioning systems.
 Dichlorodifluoromethane (R-12):- It is a clear, odorless
liquid with normal boiling point of -29.80C, thus
allowing positive evaporating pressure for wide range
of applications. This refrigerant has low latent heat of
vaporization, highly inert and stable. It has high ozone
depletion potential and green house potential
therefore; environmental concern will severely restrict
its use in future.

24.  Mono clorodifluoro methane(R-22): - Used in
unitary air-conditioners and heat pumps, chillers,
commercial and industrial applications. Its normal
boiling point (NBP) is -40.8° higher than R-12,
greater enthalpy of evaporation and vapor is
denser It has low ozone depletion potential than
R-12 and hence lower global warming potential,
hence slightly environmentally friendly.
Refrigerant 134a (R-134a): - Very similar to R-12
with NBP is -26°C, no chlorine atoms and hence
ozone depletion potential is zero and very less
(5%) global warming effect than R-12.

25. • The absorption system is a heat operated unit
which uses a refrigerant that alternatively
absorbed and released from the absorbent.
• In the basic absorption system, the
compressor in the vapor compression cycle is
replaced by an absorber - generator assembly
involving less mechanical work.
• The most popular aqua ammonia vapor
absorption refrigeration system is as shown.
VPAOR ABSORPTION REFRIGERATION SYSTEM

27. • Actual vapor absorption system: - In driving the
ammonia vapor out of the solution in the generator, it
is impossible to avoid some of the water vapor. This
water vapor going to the condenser along with the
ammonia vapor after condensation may get frozen to
ice and block the expansion valve. So an analyzer
rectifier combination is used to eliminate water vapor
from the ammonia vapor going into the condenser.
• Analyzer: - It is a direct contact heat exchanger
consisting of a series of trays mounted above the
generator. The strong solution from the absorber flows
downward over the trays to cool the outgoing vapors.
Since the saturation temperature of water is higher
than that of ammonia it is the water vapor which
condenses first. As the vapor passes upward through
the analyzer it is cooled and enriched by ammonia and
the liquid is heated

28. • Rectifier: - The function of the rectifier is to further
cool the vapor leaving the analyzer so that the almost
all water vapor is condensed and only dehydrated
ammonia gas is allowed to pass to the condenser. This
is a closed type of vapor cooler and is generally water-
cooled and may be of double pipe shell and coil or shell
and tube type. The condensed aqua is drained back to
the analyzer and generator.
• Aqua heat exchangers:- There are two heat
exchangers, one between the generator and absorber
and second one between receiver and evaporator. The
heat exchanger located between the absorber and
generator serves to cool the weak aqua and heat the
strong aqua solutions. The heat exchanger located
between the receiver and evaporator has two
purposes.