Refrigeration Cycles: Department of Mechanical Engineering H H Itui It Hashemite University
Refrigeration Cycles: Department of Mechanical Engineering H H Itui It Hashemite University
Refrigeration Cycles: Department of Mechanical Engineering H H Itui It Hashemite University
REFRIGERATION CYCLES
Dr Ali Jawarneh
Department of Mechanical Engineering
H h it U
Hashemite University
i it
Objectives
• Introduce the concepts of refrigerators and heat pumps
and the measure of their performance.
• Analyze
A l th
the id
ideall vapor-compression
i refrigeration
fi ti cycle.
l
• Analyze the actual vapor-compression refrigeration cycle.
• Review the factors involved in selecting the right
refrigerant for an application.
• Discuss the operation of refrigeration and heat pump
systems.
• Evaluate the performance of innovative vapor-
compression refrigeration systems.
systems
• Analyze gas refrigeration systems.
• Introduce the concepts of absorption-refrigeration
systems.
2
11-1 REFRIGERATORS
AND HEAT PUMPS
The transfer of heat from a low-temperature
region to a high-temperature one requires
special devices called refrigerators.
refrigerators
Refrigerators and heat pumps are essentially
the same devices; they differ in their
objectives only.
a steady-flow Carnot
cycle executed within 1-2 isothermal heat
the saturation dome addition in a boiler
of a pure substance
2-3 isentropic expansion
in a turbine
3-4 isothermal heat
rejection in a condenser
4-1 isentropic
compression in a
compressor
7
SOLUTION:
8
11-3 THE IDEAL VAPOR-COMPRESSION REFRIGERATION CYCLE
The vapor-compression refrigeration cycle is the ideal model for refrigeration
systems.
t U lik the
Unlike th reversed
dCCarnott cycle,
l ththe refrigerant
fi t iis vaporized
i d completely
l t l
before it is compressed and the turbine is replaced with a throttling device.
Schematic and T-s diagram for This is the most widely used
the ideal vapor-compression
vapor compression cycle
l ffor refrigerators,
fi t A-C
AC
refrigeration cycle. systems, and heat pumps.
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
If the throttling device were replaced by an isentropic turbine, the refrigerant would leaves the evaporator as saturated
enter the evaporator at state 4’ instead of state 4. As a result, the refrigeration vapor and reenters the compressor,
capacity would increase (by the area under process curve 4’-4 in Fig. 11–3) and the completing the cycle.
net work input would decrease (by the amount of work output of the turbine).
Replacing the expansion valve by a turbine is not practical, however, since the added 9
benefits cannot justify the added cost and complexity.
A schematic
diagram showing a
typical vapor
compression
refrigeration cycle.
The compressor raises the pressure of the refrigerant, which also increases the temperature. The compressed
high-temperature refrigerant vapor then transfers heat to the ambient environment in the condenser, where it
condenses to a high-pressure liquid at a temperature close to the environmental temperature. The liquid
refrigerant is then passed through the expansion valve where the pressure is suddenly reduced, resulting in a
vapor–liquid mixture at a much lower temperature. The low temperature refrigerant is then used to cool air or
water in the evaporator where the liquid refrigerant evaporates by absorbing heat from the medium being
cooled. The cycle is completed by the vapor returning to the compressor. If water is cooled in the evaporator,
the device is usually called a chiller. The chilled water could then be used to cool air in a building.
10
The ideal vapor-compression refrigeration cycle involves an irreversible (throttling)
process to make it a more realistic model for the actual systems.
Replacing the expansion valve by a turbine is not practical since the added
benefits cannot justify the added cost and complexity.
Steady-flow
energy balance
An ordinary
household
refrigerator.
fi t
The P-h diagram of an ideal vapor-
compression refrigeration cycle.
11
EXAMPLE:
A refrigerator uses refrigerant-134a as the working
fluid and operates on an ideal vapor-compression
refrigeration cycle between 0.12 and 0.7 MPa. The
mass flow rate of the refrigerant is 0.05 kg/s. Show the
cycle on a T-s
T s diagram with respect to saturation lines
lines.
Determine
(a) the rate of heat removal from the refrigerated space
and the popower
er inp
inputt to the compressor
compressor,
(b) the rate of heat rejection to the environment, and
(c) the coefficient of performance.
12
SOLUTION:
13
11-14 ACTUAL VAPOR-COMPRESSION
REFRIGERATION CYCLE
An actual
A t l vapor-compression
i refrigeration
fi ti cycle l diff
differs ffrom th
the id
ideall one iin
several ways, owing mostly to the irreversibilities that occur in various
components, mainly due to fluid friction (causes pressure drops) and heat transfer
to or from the surroundings. The COP decreases as a result of irreversibilities.
DIFFERENCES
Non isentropic
Non-isentropic
compression
Superheated vapor
at evaporator exit
Subcooled liquid at
condenser exit
Pressure drops in
condenser and
evaporator
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
transfer, which may increase or decrease the entropy
entropy, depending on the direction
direction. Therefore
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. 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. We do not mind this at all, however, since the refrigerant in this
t
case enters the
th evaporatort with
ith a lower
l enthalpy
th l and d th
thus can absorb
b b more h heatt ffrom th
the refrigerated
fi t d
space. The throttling valve and the evaporator are usually located very close to each other, so the
pressure drop in the connecting line is small.
15
EXAMPLE:
Refrigerant-134a enters the compressor of a refrigerator
as superheated vapor at 0.14 MPa and -10°C at a rate of
0 12 kg/s,
0.12 kg/s and it leaves at 0.7 0 7 MPa and 50°C.
50°C The
refrigerant is cooled in the condenser to 24°C and 0.65
MPa, and it is throttled to 0.15 MPa. Disregarding any
heat transfer and pressure drops in the connecting lines
between the components, show the cycle on a T-s
diagram with respect to saturation lines, and determine
(a) the rate of heat removal from the refrigerated space
and the power input to the compressor,
(b) the isentropic efficiency of the compressor,
compressor and
(c) the COP of the refrigerator.
16
SOLUTION
17
11-5 SELECTING THE RIGHT REFRIGERANT
• Several refrigerants may be used in refrigeration systems 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).
• R-11,
R 11, RR-12,
12, R
R-22,
22, R
R-134a,
134a, and R-502
R 502 account for over 90 percent of the market.
• The industrial and heavy-commercial sectors use ammonia (it is toxic).
• R-11 is used in large-capacity water chillers serving A-C systems in buildings.
• R-134a (replaced R-12, which damages ozone layer) is used in domestic
refrigerators and freezers, as well as automotive air conditioners.
• R-22 is used in window air conditioners, heat pumps, air conditioners of commercial
buildings, and large industrial refrigeration systems, and offers strong competition
to ammonia.
• R-502 (a blend of R-115 and R-22) is the dominant refrigerant used in commercial
refrigeration systems such as those in supermarkets.
• CFCs allow more ultraviolet radiation into the earth’s atmosphere by destroying the
protective ozone layer and thus contributing to the greenhouse effect that causes
global warming. Fully halogenated CFCs (such as R-11, R-12, and R-115) do the
most damage to the ozone layer. Refrigerants that are friendly to the ozone layer
have been developed.
• T
Two important
i t t parameters
t that
th t needd to
t be
b considered
id d iin ththe selection
l ti off a
refrigerant are the temperatures of the two media (the refrigerated space and the
environment) with which the refrigerant exchanges heat. 18
11-6 HEAT PUMP SYSTEMS The most common energy source for heat pumps
is atmospheric air (air-to- air systems).
Water-source systems usually use well water and
ground source (geothermal) heat pumps use earth
ground-source
as the energy source. They typically have higher
COPs but are more complex and more expensive
to install.
Both the capacity and the efficiency of a heat
E
Evaporator
t pump fall significantly at low temperatures
temperatures.
Therefore, most air-source heat pumps require a
Condenser supplementary heating system such as electric
resistance heaters or a gas furnace.
Heat pumps are most competitive in areas that
have a large cooling load during the cooling
season and a relatively small heating load during
A heat pump can be the heating season. In these areas, the heat pump
used to heat a house can meet the entire cooling and heating needs of
residential or commercial buildings.
in winter and to cool
The major
Th j problem
bl with
ith air-source
i systems
t is
i frosting,
f ti
it in summer. which occurs in humid climates when the temperature falls
below 2 to 5°C. Water-source systems usually use well
water from depths of up to 80 m in the temperature range
of 5 to 18°C, and they do not have a frosting problem.
They typically have higher COPs but are more complex
and require easy access to a large body of water such as
Evaporator underground water.
The condenser of the heat pump (located
indoors) functions as the evaporator of the air
Condenser
conditioner in summer. Also, the evaporator of the heat
pump (located outdoors) serves as the condenser of
the air conditioner. This feature increases the
competitiveness of the heat
pump. 19
EXAMPLE: A heat pump with refrigerant-134a as the working fluid is
used to keep a space at 25°C by absorbing heat from geothermal water
that enters the evaporator at 5050°C
C at a rate of 0.065
0 065 kg/s and leaves at
40°C. The refrigerant enters the evaporator at 20°C with a quality of 23
percent and leaves at the inlet pressure as saturated vapor. The
refrigerant loses 300 W of heat to the surroundings as it flows through
the compressor and the refrigerant leaves the compressor at 1.4 Mpa at
the same entropy as the inlet. Determine (a) the degrees of subcooling
of the refrigerant
g in the condenser,, ((b)) the mass flow rate of the
refrigerant, (c) the heating load and the COP of the heat pump
20
SOLUTION
Whatt do
Wh d we mean
by:The refrigerant
enters the
compressor at 200
subcooled by 3°C. kPa superheated
More information by 4
4°C
C as an
you need such example
as compressed
region
21
+0.3
22
11-7 INNOVATIVE VAPOR-COMPRESSION
REFRIGERATION SYSTEMS
• The simple vapor-compression refrigeration cycle is the most widely
used refrigeration cycle, and it is adequate for most refrigeration
applications.
pp
• The ordinary vapor-compression refrigeration systems are simple,
inexpensive, reliable, and practically maintenance-free.
• However, for large industrial applications efficiency,
However efficiency not simplicity,
simplicity is
the major concern.
• Also, for some applications the simple vapor-compression
refrigeration cycle is inadequate and needs to be modified.
• For moderately and very low temperature applications some
innovative refrigeration systems are used. The following cycles will be
discussed:
• Cascade refrigeration systems
• Multistage compression refrigeration systems
• M lti
Multipurpose refrigeration
fi ti systems
t with
ith a single
i l compressor
• Liquefaction of gases
23
Cascade Refrigeration Systems
Some industrial applications require moderately low temperatures, and the temp range they involve may be too
large for a single vapor compression refrigeration cycle to be practical
practical. A large temp 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.
25
SOLUTION
26
Multistage When the fluid used throughout the cascade
Compression
p refrigeration
g system
y is the same,, the heat
Refrigeration Systems exchanger between the stages can be replaced
by a mixing chamber (called a flash chamber)
since it has better heat transfer characteristics.
28
SOLUTION
29
30
Multipurpose Refrigeration Systems with a Single
Compressor
Some applications require refrigeration at more than one temperature. A
practical and economical approach is to route all the exit streams from the
evaporators to a single compressor and let it handle the compression process
for the entire system. Each evaporator operating at different temperatures.
Schematic and T-s diagram for a refrigerator–freezer unit with one compressor.
31
Liquefaction of Gases
Many important scientific and engineering processes at cryogenic temperatures (below
about -100°C) depend on liquefied gases including the separation of oxygen and nitrogen
from air, preparation of liquid propellants for rockets, the study of material properties at low
temperatures, and the study of superconductivity.
The storage (i.e.,
(i e hydrogen) and
transportation of some gases (i.e., natural
gas) are done after they are liquefied at very
low temperatures. Several innovative cycles
are used
d ffor the
th liquefaction
li f ti off gases.
Linde-Hampson system
for liquefying gases
gases.
32
33
11-8 GAS REFRIGERATION CYCLES
The reversed Brayton cycle (the gas
refrigeration cycle) can be used for
refrigeration.
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. The gas refrigeration cycle deviates from the reversed Carnot cycle because the heat
transfer processes are not isothermal. 35
The regenerative gas cycle is shown in Fig. 11–19. Regenerative cooling is achieved by
inserting a counter-flow heat exchanger into the cycle. Without regeneration, the lowest
t bi iinlet
turbine l t ttemperature
t is
i T0, the
th ttemperature
t off the
th surroundings
di or any other
th cooling
li
medium. With regeneration, the high-pressure gas is further cooled to T4 before
expanding in the turbine. Lowering the turbine inlet temperature automatically lowers the
turbine exit temperature, which is the minimum temperature in the cycle. Extremely low
t
temperatures
t can be
b achieved
hi dbby repeating
ti thithis process.
Gas refrigeration
g cycle
y with regeneration.
g
In gas refrigeration cycles, can we replace the turbine No; because h = h(T) for ideal gases,
by an expansion valve as we did in vapor-compression and the temperature of air will not drop
refrigeration cycles? Why? during a throttling (h1 = h2) process.
36
EXAMPLE:
EXAMPLE
Air enters the compressor (isentropic efficiency of 80%) of
gas refrigeration cycle at 12 12°C
C and 50 kPa and the turbine
(isentropic efficiency of 85%) at 47°C and 250 kPa. The
mass flow rate of air through the cycle is 0.08 kg/s.
A
Assumingi variable
i bl specific
ifi heats
h t for
f air,
i determine
d t i
(a) the rate of refrigeration,
(b) The net power input, and
(c) the coefficient of performance.
37
SOLUTION
38
39
EXAMPLE:
A gas refrigeration system using air as the working fluid has
a pressure ratio of 4. Air enters the compressor (isentropic
efficiency of 75%) at -7°C. The high-pressure air is cooled
to 27°C by rejecting heat to the surroundings. It is further
cooled to -15°C
15 C by regenerative cooling before it enters the
turbine (isentropic efficiency of 80%). Using constant
specific heats at room temperature, determine
(a) the lowest
lo est temperature
temperat re that can be obtained by b this
cycle,
(b) the coefficient of performance of the cycle, and
(c) The mass flow rate of air for a refrigeration rate of 12
kW.
40
SOLUTION
41
wnet=q
qh-q
ql
42
11-9 ABSORPTION REFRIGERATION SYSTEMS
The vapor When there is a source of
NH3+H2O,
which is rich in
inexpensive thermal energy at
NH3 a temperature of 100 to 200°C
is absorption refrigeration.
The high- Some examples include
pressure pure geothermal energy, solar energy, and
NH3 vapor waste heat from cogeneration or
process steam plants, and even natural
hot NH3 + H2O
gas when it is at a relatively low price.
solution, which is
weak in NH3 Liquid
NH3+H2O ammonia (NH3) serves as the
solution, refrigerant and water (H2O)
which is rich as the transport medium.
in NH3
System looks very much like the
vapor
vapor-compression system, except
that the compressor has been
replaced by a complex absorption
mechanism consisting of an absorber,
Ammonia absorption refrigeration cycle. a pump, a generator, a regenerator, a
valve, and a rectifier.
Once the pressure of NH3 is raised by the components in the
box ((this is the onlyy thing
g they
y are set up
p to do),
), it is cooled The fluid in the absorber is cooled to
and condensed in the condenser by rejecting heat to the maximize
i i th the refrigerant
fi t content
t t off the
th
surroundings, is throttled to the evaporator pressure, and liquid; the fluid in the generator is
absorbs heat from the refrigerated space as it flows through heated to maximize the refrigerant
content of the vapor. 43
the evaporator. So, there is nothing new there.
Here is what happens in the box:
Ammonia vapor leaves the evaporator and enters the absorber, where it dissolves
and reacts with water to form NH3 +H2O solution. This is an exothermic reaction;
thus heat is released during this process. The amount of NH3 that can be
dissolved in H2O is inversely proportional to the temperature.
temperature Therefore,
Therefore it is
necessary to cool the absorber to maintain its temperature as low as possible,
hence to maximize the amount of NH3 dissolved in water. The liquid NH3+H2O
solution, which is rich in NH3, is then pumped to the generator. Heat is transferred
to the solution from
f a source to vaporize some off the solution. The vapor, which is
rich in NH3, passes through a rectifier, which separates the water and returns it to
the generator. The high-pressure pure NH3 vapor then continues its journey
through the rest of the cycle. The hot NH3 + H2O solution, which is weak in NH3,
then passes through a regenerator, where it transfers some heat to the rich
solution leaving the pump, and is throttled to the absorber pressure.
Compared
C d with
ith vapor-compression
i systems,
t absorption
b ti refrigeration
fi ti systems
t h
have one major
j
advantage: A liquid is compressed instead of a vapor. The steady-flow work is proportional to
the specific volume, and thus the work input for absorption refrigeration systems is very small (on
the order of one percent of the heat supplied to the generator) and often neglected in the cycle
analysis The operation of these systems is based on heat transfer from an external source
analysis. source.
Therefore, absorption refrigeration systems are often classified as heat-driven systems.
44
• Absorption refrigeration systems (ARS) involve the absorption of a
refrigerant by a transport medium.
• The most widely
idel used
sed ssystem
stem is the ammonia–water
ammonia ater system,
s stem where here
ammonia (NH3) serves as the refrigerant and water (H2O) as the transport
medium.
• Other systems include water–lithium
water lithium bromide and water–lithium
water lithium chloride
systems, where water serves as the refrigerant. These systems are limited
to applications such as A-C where the minimum temperature is above the
freezing point of water.
• Compared with vapor-compression systems, ARS have one major
advantage: A liquid is compressed instead of a vapor and as a result the
work input is very small (on the order of one percent of the heat supplied to
the generator) and often neglected in the cycle analysis
analysis.
• ARS are often classified as heat-driven systems.
• ARS are much more expensive than the vapor-compression refrigeration
systems.
y They y are more complexp and occupypy more space,
p , theyy are much
less efficient thus requiring much larger cooling towers to reject the waste
heat, and they are more difficult to service since they are less common.
• Therefore, ARS should be considered only when the unit cost of thermal
energy isi llow andd iis projected
j d to remaini llow relative
l i to electricity.
l i i
• ARS are primarily used in large commercial and industrial installations.
45
From Ch 6
Schematic of
g
a heat engine.
46
The COP of actual absorption
refrigeration systems is usually less
than 1.
1
Air-conditioning systems based on
absorption refrigeration, called
absorption
p chillers, p
perform best
when the heat source can supply
heat at a high temperature with little
temperature drop.
where TL, T0, and Ts are
the thermodynamic
temperatures of the
Determining the refrigerated space, the
maximum COP of environment,, and the
an absorption heat source, respectively
refrigeration system.
47
EXAMPLE:
Heatt is
H i supplied
li d to
t an absorption
b ti refrigeration
fi ti system
t f
from a
geothermal well at 130°C at a rate of 5 x 105 kJ/h. The
environment is at 25°C, and the refrigerated space is
maintained
i t i d att -30°C.
30°C Determine
D t i th maximum
the i rate
t att which
hi h
this system can remove heat from the refrigerated space.
SOLUTION:
48
Summary
• Refrigerators and Heat Pumps
• The Reversed Carnot Cycle
y
• The Ideal Vapor-Compression
• Refrigeration Cycle
• Actual Vapor-Compression
• Refrigeration Cycle
• Selecting the Right Refrigerant
• Heat Pump Systems
• Innovative Vapor-Compression
• Refrigeration Systems
49