WERABE UNIVERSTY

Institute of Technology
Mechanical Engineering Department

25 March 2024 Nur. Jems

25 March 2024 Nure J.
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25 March 2024 Nur. Jems

 Chapter six
REFRIGERATION CYCLES
Introduction
• A major application area of thermodynamics is refrigeration, which is the
transfer of heat from a lower temperature region to a higher temperature
one. Devices that produce refrigeration are called refrigerators, and the
cycles on which they operate are called refrigeration cycles.
• The most frequently used refrigeration cycle is the vapor-compression
refrigeration cycle in which the refrigerant is vaporized and condensed
alternately and is compressed in the vapor phase.
• Another well-known refrigeration cycle is the gas refrigeration cycle in
which the refrigerant remains in the gaseous phase throughout.
REFRIGERATORS
• Cascade refrigeration: where more than one refrigeration cycle is used;

• Absorption refrigeration: where the refrigerant is dissolved in a liquid before it is compressed;

• Thermoelectric refrigeration: where refrigeration is produced by the passage of electric current

through two dissimilar materials.

• We all know from experience that heat flows in the direction of decreasing temperature, that is,
from high-temperature regions to low-temperature ones. This heat-transfer process occurs in nature
without requiring any devices.

• The reverse process, however, cannot occur by itself. The transfer of heat from a low-temperature
region to a high-temperature one requires special devices called refrigerators. Refrigerators are
cyclic devices, and the working fluids used in the refrigeration cycles are called refrigerants.
HEAT PUMPS
• Another device that transfers heat from a low-temperature medium to a high-
temperature one is the heat pump. Refrigerators and heat pumps are essentially the
same devices; they differ in their objectives only. The objective of a refrigerator is
to maintain the refrigerated space at a low temperature by removing heat from it.

• Discharging this heat to a higher-temperature medium is merely a necessary part

of the operation, not the purpose. The objective of a heat pump, however, is to
maintain a heated space at a high temperature. This is accomplished by absorbing
heat from a low-temperature source, such as well water or cold outside air in
winter, and supplying this heat to a warmer medium such as a house.
•h
 Cont.…
• The performance of refrigerators and heat pumps is expressed in terms of the
coefficient of performance(COP), defined as

• Notice that both COPR and COP HP can be greater than 1

• The cooling capacity of a refrigeration system—that is, the rate of heat removal
from the refrigerated space—is often expressed in terms of tons of refrigeration.

• The capacity of a refrigeration system that can freeze 1 ton (2000 lbm) of liquid
water at 0°C (32°F) into ice at 0°C in 24 h is said to be 1 ton. One ton of
refrigeration is equivalent to 211 kJ/min or 200 Btu/min.

• The cooling load of a typical 200-m 2 residence is in the 3-ton (10-kW) range.
THE REVERSED CARNOT CYCLE
• Carnot cycle is a totally reversible cycle that consists of two reversible isothermal and two
isentropic processes. It has the maximum thermal efficiency for given temperature limits,
and it serves as a standard against which actual power cycles can be compared.

• Since it is a reversible cycle, all four processes that comprise the Carnot cycle can be
reversed. Reversing the cycle does also reverse the directions of any heat and work
interactions. The result is a cycle that operates in the counterclockwise direction on a T-s
diagram, which is called the reversed Carnot cycle.

• A refrigerator or heat pump that operates on the reversed Carnot cycle is called a Carnot
refrigerator or a Carnot heat pump.
Working principles of reversed Carnot cycle
• The refrigerant absorbs heat isothermally from a low-temperature source at TL in

the amount of QL (process 1-2), is compressed isentropically to state 3

(temperature rises to TH ), rejects heat isothermally to a high-temperature sink at

TH in the amount of QH (process 3-4), and expands isentropically to state 1

(temperature drops to TL ).

• The refrigerant changes from a saturated vapor state to a saturated liquid state in

the condenser during process 3-4.

 Cont…
• Schematic diagram and Ts
The Ideal Vapor-compression Refrigeration Cycle
• Many of the impracticalities associated with the reversed Carnot cycle can be eliminated
by vaporizing the refrigerant completely before it is compressed and by replacing the
turbine with a throttling device, such as an expansion valve or capillary tube. The cycle
that results is called the ideal vapor-compression refrigeration cycle. The vapor-
compression refrigeration cycle is the most widely used cycle for refrigerators, air-
conditioning systems, and heat pumps. It consists of four processes:
• 1-2 Isentropic compression in a compressor

• 2-3 Constant-pressure heat rejection in a condenser

• 3-4 Throttling in an expansion device (isenthalpic process)

• 4-1 Constant-pressure heat absorption in an evaporator

Working principles of ideal VCRC
• In an ideal vapor-compression refrigeration cycle, the refrigerant enters the
compressor at state 1 as saturated vapor and is compressed isentropically to the
condenser pressure. The temperature of the refrigerant increases during this
isentropic compression process to well above the temperature of the surrounding
medium.

• The refrigerant then enters the condenser as superheated vapor at state 2 and
leaves as saturated liquid at state 3 as a result of heat rejection to the
surroundings. The temperature of the refrigerant at this state is still above the
temperature of the surroundings.
• The saturated liquid refrigerant at state 3 is throttled to the evaporator pressure by
passing it through an expansion valve or capillary tube. The temperature of the
refrigerant drops below the temperature of the refrigerated space during this
process.

• The refrigerant enters the evaporator at state 4 as a low-quality saturated mixture,

and it completely evaporates by absorbing heat from the refrigerated space. The
refrigerant leaves the evaporator as saturated vapor and reenters the compressor,
completing the cycle.
•g
• All four components associated with the vapor-compression refrigeration cycle are
steady-flow devices, and thus all four processes that make up the cycle can be
analyzed as steady-flow processes.

• The condenser and the evaporator do not involve any work, and the compressor
can be approximated as adiabatic. Then the COPs of refrigerators and heat pumps
operating on the vapor-compression refrigeration cycle can be expressed as
Example
1. A refrigerator uses refrigerant-134a as the working fluid and operates on an ideal
vapor-compression refrigeration cycle between 0.14 and 0.8 MPa. If the mass flow
rate of the refrigerant is 0.05 kg/s, determine (a) the rate of heat removal from the
refrigerated space and the power input to the compressor, (b) the rate of heat
rejection to the environment, and (c) the COP of the refrigerator.
Actual Vapor-compression Refrigeration Cycle

• An actual vapor-compression refrigeration cycle differs from the ideal one in

several ways, owing mostly to the irreversibilities that occur in various
components. Two common sources of irreversibilities are
➢ fluid friction (causes pressure drops)

➢ heat transfer to or from the surroundings.

• In the ideal cycle, the refrigerant leaves the evaporator and enters the
compressor as saturated vapor. In practice, however, it may not be
possible to control the state of the refrigerant so precisely.
• The compression process in the ideal cycle is internally
reversible and adiabatic, and thus isentropic. The actual
compression process, however, involves frictional effects,
which increase the entropy, and heat transfer, which may
increase or decrease the entropy, depending on the
direction.

• Therefore, the entropy of the refrigerant may increase

(process 1-2) or decrease (process 1-2”) during an actual
compression process, depending on which effects
dominate. The compression process 1-2 may be even more
desirable than the isentropic compression process since the
specific volume of the refrigerant and thus the work input
requirement are smaller in this case.
Cont….
• Therefore, the refrigerant should be cooled during the compression process whenever it is
practical and economical to do so.

• In the ideal case, the refrigerant is assumed to leave the condenser as saturated liquid at
the compressor exit pressure. In reality, however, it is unavoidable to have some pressure
drop in the condenser as well as in the lines connecting the condenser to the compressor
and to the throttling valve.

• Also, it is not easy to execute the condensation process with such precision that the
refrigerant is a saturated liquid at the end, and it is undesirable to route the refrigerant to
the throttling valve before the refrigerant is completely condensed. Therefore, the
refrigerant is subcooled somewhat before it enters the throttling valve.
• Schematic and T-s diagram
Tea Brake!
 Selecting the Right Refrigerant
• When designing a refrigeration system, there are several refrigerants from which to choose, such as
chlorofluorocarbons (CFCs), ammonia, hydrocarbons (propane, ethane, ethylene, etc.), carbon dioxide, air (in the
air-conditioning of aircraft), and even water (in applications above the freezing point).

• Hence refrigerants such as R-11, R-12, R-22, R-134a, and R-502 account for over 90 percent of the market in the
United States.

• Ethyl ether was the first commercially used refrigerant in vapor-compression systems in 1850, followed by 𝑁𝐻3 ,
𝐶𝑂2 , 𝑆𝑂2, 𝐶𝐻3 𝐶𝑙 , 𝐶𝐻4, 𝐶4 𝐻10, 𝐶3 𝐻8, isobutane, gasoline, and chlorofluorocarbons, among others.

• The industrial and heavy-commercial sectors were very satisfied with ammonia, and still are, although ammonia is
toxic. The advantages of ammonia over other refrigerants are its low cost, higher COPs (and thus lower energy cost),
more favorable thermodynamic and transport properties and thus higher heat transfer coefficients (requires smaller
and lower-cost heat exchangers), greater detectability in the event of a leak, and no effect on the ozone layer.
Criteria for selection of right refiregrant
• Temperatures of the two media (the refrigerated space and the environment) with
which the refrigerant exchanges heat.

• Refrigerant should be being nontoxic, noncorrosive, nonflammable, and

chemically stable; having a high enthalpy of vaporization (minimizes the mass
flow rate); and, of course, being available at low cost.
Innovative Vapor-compression Refrigeration Systems

• The simple vapor-compression refrigeration cycle discussed above is the most

widely used refrigeration cycle, and it is adequate for most refrigeration
applications. The ordinary vapor-compression refrigeration systems are simple,
inexpensive, reliable, and practically maintenance-free
Cascade Refrigeration Systems
• Some industrial applications require moderately low temperatures, and the temperature
range they involve may be too large for a single vapor compression refrigeration cycle to
be practical. A large temperature range also means a large pressure range in the cycle and
a poor performance for a reciprocating compressor. One way of dealing with such
situations is to perform the refrigeration process in stages, that is, to have two or more
refrigeration cycles that operate in series. Such refrigeration cycles are called cascade
refrigeration cycles.

• For two-stage cascade refrigeration cycle the two cycles are connected through the heat
exchanger in the middle, which serves as the evaporator for the topping cycle (cycle A)
and the condenser for the bottoming cycle (cycle B).
• Schematic diagram and T-s for cascade
Cont….
• Assuming the heat exchanger is well insulated and the kinetic and
potential energies are negligible, the heat transfer from the fluid in the
bottoming cycle should be equal to the heat transfer to the fluid in the
topping cycle. Thus, the ratio of mass flow rates through each cycle
should be
Multistage Compression Refrigeration Systems
• When the fluid used throughout the cascade refrigeration system is the
same, the heat exchanger between the stages can be replaced by a
mixing chamber (called a flash chamber) since it has better heat
transfer characteristics. Such systems are called multistage
compression refrigeration systems. A two stage compression
refrigeration system is
• A two-stage compression refrigeration system with a flash chamber.
Cont…
• In this system, the liquid refrigerant expands in the first expansion valve to the flash
chamber pressure, which is the same as the compressor interstage pressure. Part of the
liquid vaporizes during this process. This saturated vapor (state 3) is mixed with the
superheated vapor from the low-pressure compressor (state 2), and the mixture enters the
high-pressure compressor at state 9. This is, in essence, a regeneration process. The
saturated liquid (state 7) expands through the second expansion valve into the evaporator,
where it picks up heat from the refrigerated space.

• The compression process in this system resembles a two-stage compression with

intercooling, and the compressor work decreases. Care should be exercised in the
interpretations of the areas on the T-s diagram in this case since the mass flow rates are
different in different parts of the cycle.
GAS REFRIGERATION CYCLES
• Carnot cycle (the standard of comparison for power cycles) and the reversed Carnot cycle (the
standard of comparison for refrigeration cycles) are identical, except that the reversed Carnot cycle
operates in the reverse direction. This suggests that the power cycles can be used as refrigeration
cycles by simply reversing them.

• Gas refrigeration cycle is known as reversed Brayton cycle.

• Consider a gas power cycle show below then the surroundings are at T0 , and the refrigerated space
is to be maintained at TL. The gas is compressed during process 1-2. The high-pressure, high-
temperature gas at state 2 is then cooled at constant pressure to T0 by rejecting heat to the
surroundings. This is followed by an expansion process in a turbine, during which the gas
temperature drops to T4 . (Can we achieve the cooling effect by using a throttling valve instead of
a turbine?) Finally, the cool gas absorbs heat from the refrigerated space until its temperature rises
to T1.
Cont….
• Gas refrigeration cycle
Cont….
• All the processes described are internally reversible, and the cycle executed is the ideal gas
refrigeration cycle. In actual gas refrigeration cycles, the compression and expansion processes
deviate from the isentropic ones, and T3 is higher than T0 unless the heat exchanger is infinitely
large.

• On a T-s diagram, the area under process curve 4-1 represents the heat removed from the
refrigerated space, and the enclosed area 1-2-3-4-1 represents the net work input. The ratio of these
areas is the COP for the cycle, which may be expressed as
25 March 2024 Nure J.
 • h

25 March 2024 Nure J.