REF & AC I
What is refrigeration
Refrigeration is a process of removing heat from a
low-temperature reservoir and transferring it to a
high-temperature reservoir. The work of heat transfer
is traditionally driven by mechanical means, but can
also be driven by heat, magnetism, electricity, laser, or
other means.
Principle of refrigeration
•Liquids absorb heat when changed from liquid to gas
•Gases give off heat when changed from gas to liquid.
What is the principle behind the working of a
refrigerator?
A refrigerator does not cool items by lowering their
original temperatures; instead, an evaporating gas
called a refrigerant draws heat away, leaving the
surrounding area much colder. Refrigerators and air
conditioners both work on the principle of cooling
through evaporation.
Purpose of refrigeration
• To keep food cold/ Food storage
• Refrigerators maintain medication temperatures of sensitive drug
• Most commonly used to achieve a more comfortable
interior environment, typically for humans and animals
• Used to cool/dehumidify rooms filled with heat-producing
electronic devices, such as computers and electronic
devices, and even to display and store some delicate
products, such as artwork
Refrigeration
HEAT TRANSFER. ... Heat transfer is the method by which heat flows. This is another basic principle of
refrigeration. The evaporator transfers heat into the refrigerant; the refrigerant transfers this heat to the
condenser; the condenser transfers the heat to a cooling medium (such as water or outside air)
Refrigeration cycle: The vapor compression refrigeration cycle is a common method for transferring
heat from a low temperature to a high temperature. The purpose of a refrigerator is the removal of
heat, called the cooling load, from a low-temperature medium.
What is the unit of refrigeration?
A ton of refrigeration (TR), also called a refrigeration ton (RT), is a unit of power used in some
countries (especially in North America) to describe the heat-extraction capacity of refrigeration
and air conditioning equipment.
Refrigerants, are chemical compounds that are alternately compressed and condensed into a liquid and
then permitted to expand into a vapor or gas as they are pumped through the mechanical refrigeration
system to cycle.
Essential requirements for refrigeration
• Size
• Space
• Type
• Reliability
• Power consumptions
Difference between refrigerator and air
conditioning
Refrigerator or cooling technology is a technology branch that
deals with phenomena and processes of body cooling. In this
sense, cooling means reducing the internal energy of a body by
removal of energy, which is manifested by lowering of its
temperature while Air conditioning is the process of changing
the air condition by removing the heat and moisture in order to
achieve a more comfortable inner environment.
The goal of this process is usually to distribute air-conditioned
air in different indoor spaces in order to achieve certain
comfort and air quality.
Air conditioners often use a fan to distribute the conditioned air to an occupied space such as a building or a
car to improve thermal comfort and indoor air quality.
Air conditioning
Air conditioning is a combined process that performs many functions simultaneously. It conditions
the air, transports it, and introduces it to the conditioned space. It provides heating and cooling from
its central plant or rooftop units. It also controls and maintains the temperature, humidity, air
movement, air cleanliness, sound level, and pressure differential in a space within predetermined
limits for the comfort and health of the occupants of the conditioned space or for the purpose of
product processing.
The term HVAC&R is an abbreviation of heating, ventilating, air conditioning, and refrigerating.
The combination of processes in this commonly adopted term is equivalent to the current definition
of air conditioning. Because all these individual component processes were developed prior to the
more complete concept of air conditioning, the term HVAC&R is often used by the industry
Air conditioning
Most air conditioning systems perform the following functions:
1. Provide the cooling and heating energy required
2. Condition the supply air, that is, heat or cool, humidify or dehumidify, clean and purify, and
attenuate any objectionable noise produced by the HVAC&R equipment
3. Distribute the conditioned air, containing sufficient outdoor air, to the conditioned space
4. Control and maintain the indoor environmental parameters – such as temperature, humidity,
cleanliness, air movement, sound level, and pressure differential between the conditioned space and
surroundings — within predetermined limits
Air conditioning systems can be classified according to their applications as (1) comfort air
conditioning systems and (2) process air conditioning systems
Application of refrigeration
Domestic refrigeration
Domestic refrigerator (also household refrigerator),an
appliance that is used for the short-term preservation of food
products in the home by means of refrigeration. Depending on
the type of refrigerating machine used, domestic refrigerators
are classified as compression-type, absorption-type, and
thermoelectric.
The first domestic refrigerators, which used vapor-compression
machines, appeared in 1910 in the USA. Absorption-type
domestic refrigerators were first produced in 1925 in Sweden.
The first thermoelectric domestic refrigerators were
manufactured in the second half of the 1950’s. In the USSR, the
first compression-type domestic refrigerators (the KhTZ-120)
were produced in 1939, the first absorption-type domestic
refrigerators (the Gazoapparat) were manufactured in 1945,
and the first prototypes of a thermoelectric refrigerator, which
uses thermoelectric cooling, were produced in 1951. The mass
production of Soviet compression-type domestic refrigerators
(the ZIL) began in 1951.
Domestic refrigerators
A domestic refrigerator is a metal cabinet with a built-in hermetically sealed refrigerating unit. Inside the
cabinet is a cold chamber with shelves for storing products. Heat insulation is placed between the walls of
the cold chamber and the case of the refrigerator. The air in the cold chamber is cooled by means of heat
transfer between the air and the cold surface of the evaporator. The necessary temperature conditions in
the refrigerator are provided by the brief periodic—that is, cyclic—operation of the refrigerating unit, which
is switched on by means of a thermal relay. Domestic refrigerators have a storage space of 20 to 800 liters.
Domestic refrigerators may be stationary or portable. Stationary refrigerators are classified as freestanding, wall-type, and built-in (that is, built into a kitchen or reception-room furniture unit). Portable
models are mainly absorption-type and thermoelectric refrigerators. In addition, absorption-type
refrigerators are classified, depending on the heat source, as electric, gas-fired, kerosene-burning, and
combination-type. Electric refrigerators are the most widely used. The limited use of gas-fired
refrigerators is due mainly to safety considerations and also to difficulties associated with the connection
of such refrigerators to a gas distribution system. Kerosene-burning refrigerators are used mainly aboard
ships and as portable appliances.
Domestic refrigerator
Most of the domestic refrigerators manufactured today are of the compression
type. Absorption-type refrigerators account for 5–10 percent of the total
production. In comparison with compression-type refrigerators, absorptiontype refrigerators are bulkier and heavier, consume more electrical energy
(more by a factor of 1.5–2), and have a smaller freezer compartment.
Thermoelectric refrigerators are not widely used, since they are expensive and
have higher power requirements than compression-type refrigerators
Commercial refrigerator
Commercial refrigerator means a unit of commercial refrigeration equipment in which all
refrigerated compartments in the unit are capable of operating at or above 32 °F (±2 °F).
Commercial refrigerator, freezer, and refrigerator-freezer means refrigeration equipment that (1) Is not a consumer product
(2) Is not designed and marketed exclusively for medical, scientific, or research purposes;
(3) Operates at a chilled, frozen, combination chilled and frozen, or variable temperature;
(4) Displays or stores merchandise and other perishable materials horizontally, semi-vertically, or
vertically;
(5) Has transparent or solid doors, sliding or hinged doors, a combination of hinged, sliding, transparent,
or solid doors, or no doors;
(6) Is designed for pull-down temperature applications or holding temperature applications; and
(7) Is connected to a self-contained condensing unit or to a remote condensing unit.
Industrial refrigeration
Industrial refrigeration covers a wide range of applications, from high temperature process chilling, through
to very low temperature applications such as those used in medical freezers or LNG liquefaction.
Transport refrigeration/marine
Transport refrigeration is essential in today's society, to preserve and protect food, drugs and
medical supplies for people worldwide
Trailers, large and small trucks but also
containers and rail carriages are refrigerated
in order to preserve perishable goods. Smaller
units are connected to the motor directly or
via an alternator. Bigger units have their own
diesel units.
Comfort Air conditioning
Comfort air conditioning systems provide occupants with a comfortable and healthy indoor environment in which to carry
out their activities. The various sectors of the economy using comfort air conditioning systems are as follows:
1. The commercial sector includes office buildings, supermarkets, department stores, shopping centers, restaurants, and
others. Many high-rise office buildings, including such structures as the World Trade Center in New York City and the
Sears Tower in Chicago, use complicated air conditioning systems to satisfy multiple-tenant requirements. In light
commercial buildings, the air conditioning system serves the conditioned space of only a single-zone or comparatively
smaller area. For shopping malls and restaurants, air conditioning is necessary to attract customers.
2. The institutional sector includes such applications as schools, colleges, universities, libraries, museums, indoor
stadiums, cinemas, theaters, concert halls, and recreation centers. For example, one of the large indoor stadiums, the
Superdome in New Orleans, Louisiana, can seat 78,000 people.
3. The residential and lodging sector consists of hotels, motels, apartment houses, and private homes. Many systems
serving the lodging industry and apartment houses are operated continuously, on a 24-hour, 7-day-a-week schedule, since
they can be occupied at any time.
4. The health care sector encompasses hospitals, nursing homes, and convalescent care facilities. Special air filters are
generally used in hospitals to remove bacteria and particulates of submicrometer size from areas such as operating rooms,
nurseries, and intensive care units
5. The transportation sector includes aircraft, automobiles, railroad cars, buses, and cruising ships. Passengers
increasingly demand ease and environmental comfort, especially for long-distance travel.
Process Air Conditioning Systems
Process air conditioning systems provide needed indoor environmental control for manufacturing,
product storage, or other research and development processes. The following areas are examples of
process air conditioning systems:
1. In textile mills, natural fibers and manufactured fibers are hygroscopic. Proper control of humidity
increases the strength of the yarn and fabric during processing. For many textile manufacturing
processes, too high a value for the space relative humidity can cause problems in the spinning
process. On the other hand, a lower relative humidity may induce static electricity that is harmful
for the production processes.
2. Many electronic products require clean rooms for manufacturing such things as integrated circuits,
since their quality is adversely affected by airborne particles. Relative-humidity control is
also needed to prevent corrosion and condensation and to eliminate static electricity. Temperature
control maintains materials and instruments at stable condition and is also required for workers who
wear dust-free garments.
3. Pharmaceutical products require temperature, humidity, and air cleanliness control. For instance, liver extracts require a temperature of 75°F (23.9°C) and a relative humidity of 35 percent.
If the temperature exceeds 80°F (26.7°C), the extracts tend to deteriorate.
Refrigeration and Heat pumps cycles
The Carnot Engine and Cycle
The Carnot cycle was introduced as the most efficient heat engine that can
operate between two fixed temperatures TH and TL. The Carnot cycle is
described by the following four processes.
Process Description
1-2 Reversible isothermal heat addition at high temperature,
TH > TL, to the working fluid in a piston cylinder device that
does some boundary work.
2-3 Reversible adiabatic expansion during which the
system does work as the working fluid temperature
decreases from TH to TL.
3-4 The system is brought in contact with a heat reservoir
at TL < TH and a reversible isothermal heat exchange
takes place while work of compression is done on the
system.
4-1 A reversible adiabatic compression process increases
the working fluid temperature from TL to TH
The Carnot Engine and Cycle
We often use the Carnot efficiency as a means to
think about ways to improve the cycle efficiency of
other cycles. One of the observations about the
efficiency of both ideal and actual cycles comes from
the Carnot efficiency: Thermal efficiency increases
with an increase in the average temperature at which
heat is supplied to the system or with a decrease in
the average temperature at which heat is rejected
from the system.
Refrigerator and a heat pump
A refrigerator may be defined as a device that operates in a thermodynamic cycle and transfers a
certain amount of heat from a body at a lower temperature to a body at a higher temperature by
consuming certain amount of external work. Domestic refrigerators and room air conditioners
are the examples. In a refrigerator, the required output is the heat extracted from the low
temperature body.
A heat pump is similar to a refrigerator, however, here the required output is the heat rejected to
the high temperature body.
A refrigerator or a heat pump that operates on the reversed Carnot cycle is called a Carnot refrigerator,
or a Carnot heat pump.
The Reversed Carnot Cycle
The 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.
The Carnot heat-engine cycle just described is a totally reversible cycle. Therefore, all the
processes that comprise it can be reversed, in which case it becomes the Carnot
refrigeration cycle. This time, the cycle remains exactly the same, except that the
directions of any heat and work interactions are reversed: Heat in the amount of QL is
absorbed from the low-temperature reservoir, heat in the amount of QH is rejected to a
high-temperature reservoir, and a work input of Wnet,in is required to accomplish all this. The
P-V diagram of the reversed Carnot cycle is the same as the one given for the Carnot
cycle, except that the directions of the processes are reversed, as shown in Fig.
The reversed Carnot cycle is the most efficient refrigeration cycle operating between two
specified temperature levels. Therefore, it is natural to look at it first as a prospective ideal
cycle for refrigerators and heat pumps. The reversed Carnot cycle is not a suitable model for
refrigeration cycles.
TS Diagram for a Reversed Carnot Cycle
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.
Schematic of a Carnot refrigerator and T-s diagram of the reversed Carnot cycle.
TS Diagram for a Reversed Carnot Cycle
Consider a reversed Carnot cycle executed within the saturation
dome of a refrigerant, as shown in the Fig. The refrigerant
absorbs heat isothermally from a low-temperature source at T in
the amount of Q (process 1-2), is compressed isentropically to
state 3 (temperature rises to T ), rejects heat isothermally to a
high-temperature sink at T in the amount of Q (process 3-4), and
expands isentropically to state 1 (temperature drops to T ). The
refrigerant changes from a saturated vapor state to a saturated
liquid state in the condenser during process 3-4.
L
L
H
H
H
L
The Reversed Carnot Cycle
Here QLis the magnitude of the heat removed
from the refrigerated space at temperature TL ,QH
is the magnitude of the heat rejected to the warm
environment at temperature TH, and Wnet, in is the
net work input to the refrigerator. As discussed,
QL and QH represent magnitudes and thus are
positive quantities.
Coefficient of Performance of Refrigerator
The efficiency of a refrigerator is expressed in terms of the coefficient of
performance (COP), denoted by COPR. The objective of a refrigerator is to
remove heat (QL) from the refrigerated space. To accomplish this objective,
it requires a work input of Wnet,in. Then the COP of a refrigerator can be
expressed as
COPR =
𝐷𝑒𝑠𝑖𝑟𝑒𝑑 𝑜𝑢𝑡𝑝𝑢𝑡
𝑅𝑒𝑞𝑢𝑖𝑟𝑒𝑑 𝑖𝑛𝑝𝑢𝑡
=
𝑄𝐿
𝑊𝑛𝑒𝑡,𝑖𝑛
The conservation of energy principle for a cyclic device requires that
𝑊𝑛𝑒𝑡,𝑖𝑛 = 𝑄𝐻 − 𝑄(KJ)
𝐿
Coefficient of Performance of Refrigerator
Then the COP relation becomes
COPR =
𝑄𝐿
𝑄𝐻 −𝑄𝐿
=
1
𝑄𝐻 𝑄𝐿 −1
Notice that the value of COPR can be greater than unity. That is, the amount of heat
removed from the refrigerated space can be greater than the amount of work input.
This is in contrast to the thermal efficiency, which can never be greater than 1. In
fact, one reason for expressing the efficiency of a refrigerator by another term—the
coefficient of performance—is the desire to avoid the oddity of having efficiencies
greater than unity.
Coefficient of Performance of a Heat Pump
The measure of performance of a heat pump is also expressed in terms of the coefficient of
performance COPHP, defined as
𝐷𝑒𝑠𝑖𝑟𝑒𝑑 𝑜𝑢𝑡𝑝𝑢𝑡
COPHP= 𝑅𝑒𝑞𝑢𝑖𝑟𝑒𝑑 𝑖𝑛𝑝𝑢𝑡 = 𝑊
𝑄𝐻
𝑛𝑒𝑡,𝑖𝑛
which can also be expressed as
COPHP=
𝑄𝐻
𝑄𝐻 −𝑄𝐿
=
1
1−𝑄𝐿 𝑄𝐻
COPHP= COPR + 1 for fixed values of QL and QH
This relation implies that the coefficient of performance of a heat pump is always greater than unity since
COPR is a positive quantity.
COP of Refrigerator and Heat Pump
The coefficients of performance of Carnot refrigerators and heat pumps are expressed in terms of
temperatures as
COPR, Carnot=
1
𝑇𝐻 𝑇𝐿 −1
COPHP, Carnot=
1
1−𝑇𝐿 𝑇𝐻
Notice that both COPs increase as the difference between the two temperatures decreases, that is, as TL
rises or TH falls.
Example
The food compartment of a
refrigerator, shown in the Fig, is
maintained at 4°C by removing
heat from it at a rate of 360
kJ/min. If the required power
input to the refrigerator is 2 kW,
determine (a) the coefficient of
performance of the refrigerator
and (b) the rate of heat rejection
to the room that houses the
refrigerator.
Solution The power consumption of a refrigerator is given. The
COP and the rate of heat rejection are to be determined.
Assumptions Steady operating conditions exist.
Analysis (a) The coefficient of performance of the refrigerator is
COPR =
𝑄𝐿
𝑊𝑛𝑒𝑡,𝑖𝑛
=
360𝐾𝐽/𝑚𝑖𝑛
1𝐾𝑊
2𝐾𝑊
60𝐾𝐽/𝑚𝑖𝑛
=3
That is, 3 kJ of heat is removed from the refrigerated space for
each kJ of work supplied.
(b) The rate at which heat is rejected to the room that houses
the refrigerator is determined from the conservation of energy
relation for cyclic devices,
360𝐾𝐽
60𝐾𝐽/𝑚𝑖𝑛
𝑄𝐻 = 𝑄𝐿 + 𝑊𝑛𝑒𝑡,𝑖𝑛 =
+ 2 𝐾𝑊
= 480𝐾𝐽/𝑚𝑖𝑛
𝑚𝑖𝑛
1𝐾𝑊
The vapor compression refrigeration cycle is a common method for transferring heat
from a low temperature to a high temperature.
The above figure shows the objectives of refrigerators and heat pumps. The purpose
of a refrigerator is the removal of heat, called the cooling load, from a lowtemperature medium. The purpose of a heat pump is the transfer of heat to a hightemperature medium, called the heating load. When we are interested in the heat
energy removed from a low-temperature space, the device is called a refrigerator.
When we are interested in the heat energy supplied to the high-temperature space,
the device is called a heat pump. In general, the term heat pump is used to describe
the cycle as heat energy is removed from the low-temperature space and rejected to
the high-temperature space.
34
The performance of refrigerators and heat pumps is expressed in terms of coefficient
of performance (COP), defined as
Desired output Cooling effect
QL
COPR 


Required input
Work input
Wnet ,in
COPHP 
Desired output Heating effect
Q

 H
Required input
Work input
Wnet ,in
Both COPR and COPHP can be larger than 1. Under the same operating conditions,
the COPs are related by
COPHP  COPR  1
Can you show this to be true?
Refrigerators, air conditioners, and heat pumps are rated with a SEER number or
seasonal adjusted energy efficiency ratio. The SEER is defined as the Btu/hr of heat
transferred per watt of work energy input. The Btu is the British thermal unit and is
equivalent to 778 ft-lbf of work (1 W = 3.4122 Btu/hr). An EER of 10 yields a COP of
2.9.
Refrigeration systems are also rated in terms of tons of refrigeration. One ton of
refrigeration is equivalent to 12,000 Btu/hr or 211 kJ/min.
35
The Vapor-Compression Refrigeration Cycle
The vapor-compression refrigeration cycle has four components: evaporator,
compressor, condenser, and expansion (or throttle) valve. The most widely used
refrigeration cycle is the vapor-compression refrigeration cycle. In an ideal vaporcompression refrigeration cycle, the refrigerant enters the compressor as a saturated
vapor and is cooled to the saturated liquid state in the condenser. It is then throttled
to the evaporator pressure and vaporizes as it absorbs heat from the refrigerated
space.
The ideal vapor-compression cycle consists of four processes.
Ideal Vapor-Compression Refrigeration Cycle
Process
Description
1-2
Isentropic compression
2-3
Constant pressure heat rejection in the condenser
3-4
Throttling in an expansion valve
4-1
Constant pressure heat addition in the evaporator
36
The P-h diagram is another convenient diagram often used to illustrate the
refrigeration cycle.
37
The ordinary household refrigerator is a good example of the application of this cycle.
COPR 
COPHP
Q L
h h
 1 4
Wnet ,in h2  h1
Q H
h h

 2 3
Wnet ,in h2  h1
38
Example
Consider an ideal refrigeration cycle which uses R-12 as working fluid. The temperature of the refrigerant in
the evaporator is -20oC and in the condenser it is 40oC. The refrigerant is circulated at the rate of 0.03kg/s.
determine the coefficient of performance and the capacity of the plant in rate of refrigeration.
For control volume analysed, the thermodynamic model is the R-12 table
Compressor T1 known, saturated vapour state fixed
P2 known (saturation pressure at T3)
From the first law
Wnet, in = h1 – h2
From the second law s2=s1
Solution
At T3 = 40oC, Pg = P2 = 0.9607MPa
From the R-12 tables, h1 = 178.61, s1 = 0.7082
Therefore, s2=s1= 0.7082
So, T2 = 50.8oC and h2 = 211.38
Wnet, in = h1 – h2 = 211.38 – 178.61 = 32.77kJ/kg
Expansion valve
Inlet state: T3 is known, saturated liquid, state fixed
Exit state T4
From first law
h4= h3= 74.587
Evaporator
Inlet state: State 4 known (as given)
Exit state : State 1 known
First law
qL = h1 – h4 = 178.61 – 74.587 =104.02kJ/kg
Therefore, the COPR = qL/Wnet,in =104.02/32.77 =3.17
The capacity of the plant in rate of refrigeration = 104.02 x 0.03 = 3.12kW
Example 11-1
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

kJ
h

238.41

1
Compressor inlet  
kg


T1  20o C
  s  0.9456 kJ
  1
kg  K
x1  1.0


kJ
Compressor exit
 h2 s  278.23 kg
P2 s  P2  900 kPa

o
kJ  T2 s  43.79 C

s2 s  s1  0.9456
kg  K 

kJ
h

101.61
3

Condenser exit  
kg

P3  900 kPa  
kJ
s3  0.3738
 
kg  K
x3  0.0

 x  0.358
Throttle exit   4
kJ

T4  T1  20o C   s4  0.4053
kg  K
 
h4  h3
State1
State 3
State 2
State 4
41
COPR 
QL
m(h1  h4 ) h1  h4


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
QL  m(h1  h4 )
kg
kJ 1Ton
(238.41  101.61)
min
kg 211 kJ
min
 1.94 Ton
3
COPR , Carnot
TL

TH  TL
(20  273) K

(43.79  ( 20)) K
 3.97
42
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
QL

4.715
COPR
4.715 hp
3.44 Ton
hp
 1.37
Ton

43
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) and heat
transfer to or from the surroundings. The T-s diagram of an actual vapor-compression
refrigeration cycle is shown in the Fig. 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. Instead, it is easier
to design the system so that the refrigerant is slightly superheated at the compressor
inlet. This slight overdesign ensures that the refrigerant is completely vaporized when
it enters the compressor. Also, the line connecting the evaporator to the compressor
is usually very long; thus the pressure drop caused by fluid friction and heat transfer
from the surroundings to the refrigerant can be very significant. The result of
superheating, heat gain in the connecting line, and pressure drops in the evaporator
and the connecting line is an increase in the specific volume, thus an increase in the
power input requirements to the compressor since steady-flow work is proportional to
the specific volume.
Actual Vapor-Compression Refrigeration Cycle
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. Therefore, the refrigerant should be cooled during the
compression process whenever it is practical and economical to do so.
Actual Vapor-Compression Refrigeration Cycle
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 case enters the
evaporator with a lower enthalpy and thus can absorb more heat from the
refrigerated 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
Actual Vapor-Compression Refrigeration Cycle
47
Vapour compression method of refrigeration
The most frequently used refrigeration cycle is the vapor-compression refrigeration cycle,
which involves four main components: a compressor, a condenser, an expansion valve, and an
evaporator, as shown in Fig.
Schematic of a basic vapour compression refrigeration system
Vapour compression method of refrigeration
Figure shows the basic components of a vapour compression refrigeration system. As shown in
the figure the basic system consists of an evaporator, compressor, condenser and an
expansion valve. The refrigeration effect is obtained in the cold region as heat is extracted by
the vaporization of refrigerant in the evaporator. The refrigerant vapour from the evaporator is
compressed in the compressor to a high pressure at which its saturation temperature is greater
than the ambient or any other heat sink. Hence when the high pressure, high temperature
refrigerant flows through the condenser, condensation of the vapour into liquid takes place by
heat rejection to the heat sink. To complete the cycle, the high pressure liquid is made to flow
through an expansion valve. In the expansion valve the pressure and temperature of the
refrigerant decrease. This low pressure and low temperature refrigerant vapour evaporates in the
evaporator taking heat from the cold region. It should be observed that the system operates on a
closed cycle. The system requires input in the form of
mechanical work. It extracts heat from a cold space and rejects heat to a high temperature heat
sink. Fig.
Vapour compression method of refrigeration
A vapour compression refrigeration system can
also be used as a heat pump, in which the
useful output is the high temperature heat
rejected at the condenser. Alternatively, a
refrigeration system can be used for providing
cooling in summer and heating in winter. Such
systems have been built and are available now.
HEAT PUMP SYSTEMS
Heat pumps are generally more expensive to purchase and install than other heating systems, but they save
money in the long run in some areas because they lower the heating bills. Despite their relatively higher initial
costs, the popularity of heat pumps is increasing. About one-third of all single-family homes built in the United
States in the last decade are heated by heat pumps. The most common energy source for heat pumps is
atmospheric air (air-to-air systems), although water and soil are also used. The major problem with air-source
systems is frosting, which occurs in humid climates when the temperature falls below 2 to 5°C. The frost
accumulation on the evaporator coils is highly undesirable since it seriously disrupts heat transfer. The
coils can be defrosted, however, by reversing the heat pump cycle (running it as an air conditioner). This results
in a reduction in the efficiency of the system. Water-source systems usually use well water from depths of up to
80m 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 underground water.
Ground source systems are also rather involved since they require long tubing placed deep in the ground where
the soil temperature is relatively constant. The COP of heat pumps usually ranges between 1.5 and 4,
depending on the particular system used and the temperature of the source. A new class of recently developed
heat pumps that use variable-speed electric motor drives are at least twice as energy efficient as their
predecessors.
HEAT PUMP SYSTEMS
Both the capacity and the efficiency of a heat pump fall significantly at low temperatures. Therefore,
most air-source heat pumps require a supplementary heating system such as electric resistance
heaters or an oil or gas furnace. Since water and soil temperatures do not fluctuate much,
supplementary heating may not be required for water-source or ground-source systems. However, the
heat pump system must be large enough to meet the maximum heating load. Heat pumps and air
conditioners have the same mechanical components. Therefore, it is not economical to have two
separate systems to meet the heating and cooling requirements of a building. One system can be used
as a heat pump in winter and an air conditioner in summer. This is accomplished by adding a reversing
valve to the cycle, as shown in Fig. in the next slide. A sa result of this modification, the condenser of
the heat pump (located indoors) functions as the evaporator of the air 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. Such dual-purpose units are commonly
used in motels.
Heat pumps are most competitive in areas that have a large cooling load during the cooling season
and a relatively small heating load during the heating season, such as in the southern parts of the
United States. In these areas, the heat pump can meet the entire cooling and heating needs of
residential or commercial buildings. The heat pump is least competitive in areas where the heating
load is very large and the cooling load is small, such as in the northern parts of the United States.
Heat Pump Systems
53
Vapour Absorption Refrigeration Systems
Another form of refrigeration that becomes economically attractive when there is a
source of inexpensive thermal energy at a temperature of 100 to 200°C is
absorption refrigeration. Some examples of inexpensive thermal energy sources
include geothermal energy, solar energy, and waste heat from cogeneration or
process steam plants, and even natural gas when it is available at a relatively low
price. As the name implies, absorption refrigeration systems involve the absorption of a
refrigerant by a transport medium. The most widely used absorption refrigeration
system is the ammonia–water system, where ammonia (NH3) serves as the refrigerant
and water (H2O) as the transport medium.
Other absorption refrigeration systems include water–lithium bromide and water–
lithium chloride systems, where water serves as the refrigerant. The latter two systems
are limited to applications such as air-conditioning where the minimum temperature is
above the freezing point of water.
Vapour Absorption Refrigeration Systems
To understand the basic principles involved in absorption refrigeration, we examine the NH3–H2O
system shown in the Fig. The ammonia–water refrigeration machine was patented by the Frenchman
Ferdinand Carre in 1859. Within a few years, the machines based on this principle were being built in
the United States primarily to make ice and store food. You will immediately notice from the figure that
this system looks very much like the vapor-compression system, except that the compressor has
been replaced by a complex absorption mechanism consisting of an absorber, 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 only thing they are set up to do), it is cooled and condensed in the
condenser by rejecting heat to the surroundings, is throttled to the evaporator pressure, and
absorbs heat from the refrigerated space as it flows through 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 NH3H2O . 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. 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 a source to vaporize some of 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.
Absorption Refrigeration Systems
Another form of refrigeration that becomes economically attractive when there is a
source of inexpensive heat energy at a temperature of 100 to 200oC is absorption
refrigeration, where the refrigerant is absorbed by a transport medium and
compressed in liquid form. The most widely used absorption refrigeration system is
the ammonia-water system, where ammonia serves as the refrigerant and water as
the transport medium. The work input to the pump is usually very small, and the COP
of absorption refrigeration systems is defined as
COPR 
Ammonia absorption refrigeration cycle
Desired output Cooling effect
QL
Q


 L
Required input
Work input
Qgen  Wpump ,in Qgen
56
Vapour Absorption Refrigeration Systems
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 with vapor-compression systems, absorption refrigeration systems have
one major 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. Therefore, absorption refrigeration systems
are often classified as heat-driven systems. The absorption refrigeration systems are much
more expensive than the vapor-compression refrigeration systems. They are more
complex and occupy more space, they 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, absorption refrigeration systems should be considered only
when the unit cost of thermal energy is low and is projected to remain low relative to
electricity. Absorption refrigeration systems are primarily used in large commercial and
industrial installations.
Multistage refrigeration systems
Multistage refrigeration systems are used where ultralow
temperatures are required but cannot be obtained economical
through the use of a single-stage system. The reason for this
is the compression ratios are too high to attain the temperatures
required to evaporate and condense the vapour. The following
two general types of systems are presently in use:
• Cascade
• Compound
Other Refrigeration Cycles
Cascade refrigeration systems
Very low temperatures can be achieved by operating two or more vapor-compression
systems in series, called cascading. The COP of a refrigeration system also
increases as a result of cascading.
The cascade system has two separate refrigerant systems
interconnected in such away that the evaporator from
the first unit cools the condenser of the second unit.
This allows one of the units to be operated at a lower
temperature and pressure than would otherwise be
possible with the same type and size of single-stage
system. It also allows two different refrigerants to be
used, and it can produce temperatures as low as
−50°F
59
The compound system
The compound system uses two or more compressors connected in series
in the same refrigeration system. In this type of system the first stage
compressor is the largest and for each succeeding stage the compressor gets
smaller. This is because as the refrigerant passes through each compressor, it
becomes a denser vapour. A two-stage compound system can attain a
temperature of approximately –80°F. A three-stage system (Figure) can attain a
temperature of –135°F efficiently. Compressor 1 pumps vapour into the
intercooler and then into the intake of compressor 2. This operation is repeated
between the second and third stages. In the third stage, the refrigerant vapour is
further cooled and travels to the evaporator for specific cooling use.
Compound refrigeration system (three-stage)
Multipurpose refrigeration systems
A refrigerator with a single compressor can provide refrigeration at several
temperatures by throttling the refrigerant in stages.
62
Liquefaction of gases
Another way of improving the performance of a vapor-compression refrigeration
system is by using multistage compression with regenerative cooling. The vaporcompression refrigeration cycle can also be used to liquefy gases after some
modifications.
63
Gas Refrigeration Systems
The power cycles can be used as refrigeration cycles by simply reversing them. Of
these, the reversed Brayton cycle, which is also known as the gas refrigeration cycle,
is used to cool aircraft and to obtain very low (cryogenic) temperatures after it is
modified with regeneration. The work output of the turbine can be used to reduce the
work input requirements to the compressor. Thus, the COP of a gas refrigeration
cycle is
COPR 
qL
qL

wnet , in wcomp , in  wturb , out
64
Methods of reproducing refrigeration
Evaporative cooling
As the name indicates, evaporative cooling is the process of reducing the temperature of a
system by evaporation of liquid (i.e. water). Human beings perspire and dissipate their
metabolic heat by evaporative cooling if the ambient temperature is more than skin
temperature. Animals such as the hippopotamus and buffalo coat themselves with mud for
evaporative cooling. Evaporative cooling has been used for centuries to obtain cold water in
summer by storing the water in earthen pots. The water permeates through the pores of
earthen vessel to its outer surface where it evaporates to the surrounding, absorbing its latent
heat in part from the vessel, which cools the water.
Evaporative cooling is based on the thermodynamics of evaporation of water i.e. the change
of liquid phase of Water into water vapor. This change phase requires energy which is called
the latent heat of evaporation. This is the energy required to change a substance from liquid
phase to gaseous one without temperature.
When evaporation occurs naturally it called passive evaporation and evaporation has to
controlled by means of some mechanical device, the system is called hybrid evaporative
system.
Thermoelectric Refrigeration
Thermoelectric refrigeration is a novel method of producing low temperatures and is based on
the reverse Seebeck effect. The Figure shows the illustration of Seebeck and Peltier effects.
As shown, in Seebeck effect an EMF, E is produced when the junctions of two dissimilar
conductors are maintained at two different temperatures T1 and T2. This principle is used for
measuring temperatures using thermocouples. Experimental studies show that Seebeck effect
is reversible. The electromotive force produced is given by:
E =α(T1 −T2 )
where α is the thermoelectroic power or Seebeck coefficient. For a constant cold junction
temperature (T2),
α= dE/dT
Illustration of Seebeck and Peltier effects
Thermoelectric Refrigeration
If a closed circuit is formed by the conductors, then an electrical current, I flows due to the emf and
this would result in irreversible generation of heat (qir= I2R) due to the finite resistance R of the
conductors. This effect is known as Joulean Effect. Due to different temperatures T1 and T2
(T1>T2), there will be heat transfer by conduction also. This is also irreversible and is called as
conduction effect. The amount of heat transfer depends on the overall thermal conductance of the
circuit. When a battery is added in between the two conductors A and B whose junctions are initially
at same temperature, and a current is made to flow through the circuit, the junction temperatures will
change, one junction becoming hot (T1) and the other becoming cold (T2). This effect is known as
Peltier effect. Refrigeration effect is obtained at the cold junction and heat is rejected to the
surroundings at the hot junction. This is the basis for thermoelectric refrigeration systems. The
position of hot and cold junctions can be reversed by reversing the, direction of current flow. The heat
transfer rate at each junction is given by:
Q= φI
where φ is the Peltier coefficient in volts and I is the current in amperes.
Thermoelectric Refrigeration
When current is passed through a conductor in which there is an initial uniform temperature
gradient, then it is observed that the temperature distribution gets distorted as heat transfer
takes place. This effect is known as Thomson effect.
POSITIVE THOMSON EFFECT
In metals such as zinc and copper, which have a hotter end at a higher potential and a cooler end at a lower potential,
when current moves from the hotter end to the colder end, it is moving from a high to a low potential, so there is an
evolution of heat. This is called the positive Thomson effect.
NEGATIVE THOMSON EFFECT
In metals such as cobalt, nickel, and iron, which have a cooler end at a higher potential and a hotter end at a lower
potential, when current moves from the hotter end to the colder end, it is moving from a low to a high potential, there is
an absorption of heat. This is called the negative Thomson effect.
Thermoelectric Refrigeration
The Joule-Thomson effect : The reduction of temperature experienced by a gas when throttled from a high
pressure to a lower one. This is known as the Joule-Thompson effect.
This expansion can be carried out:
-
Without external work being applied to a valve or passage through an orifice. The process is accompanied by a
small temperature drop.
-
With external work being applied to an expansion machine. In this case expansion corresponds to a
considerable drop in the temperature of the gas being expansion.
The Joule-Thomson effect (throttling effect is observed as refrigerant flows across the thermostatic expansion
valve. A similar effect is noticed as high pressure LPG gas flows out a cylinder.
The Joule–Thomson effect is widely used in cryogenic engineering and in the liquefaction of
gases.
RANQUE TUBE METHOD (RANQUE EFFECT)
The vortex tube, also known as the Ranque-Hilsch vortex tube, is a device that separates a compressed gas into
hot and cold streams. It has no moving parts.
Pressurized gas is injected tangentially into a swirl chamber and accelerates to a high rate of rotation. Due to the
conical nozzle at the end of the tube, only the outer shell of the compressed gas is allowed to escape at that end as
hot air. The remainder of the gas is forced to return in an inner vortex of reduced diameter within the outer vortex as
cold air. There are different explanations for the effect and there is debate on which explanation is best or correct.
One simple explanation is that the outer air is under higher pressure than the inner air (because of centrifugal force).
Therefore the temperature of the outer air is higher than that of the inner air.
Ranque-Hilsch Effect Tube ("vortex tube")
Expandable refrigeration
An expendable refrigeration system is for used in
trucks, railroad cars, and shipping containers that
transport perishable items. The three types of
refrigerants presently being used in an expendable
system are liquid nitrogen, carbon dioxide, and liquid
helium. The evaporator system and the spray
system are two types of expendable systems
commonly used in the Navy.
Expandable evaporator refrigeration system
In the expendable evaporator system, liquid refrigerant is stored in large metal insulated cylinders.
These cylinders are normally located in the front of the cargo vehicle. Each cylinder is equipped
with a temperature control to provide a temperature range of –20°F to 60°F. The temperature
control is connected to a temperature sensor. As the temperature rises, the switch operating the
control valve opens and liquid refrigerant flows into the evaporator. The evaporator can be blower
coils, plates, or eutectic plates. As it passes through the evaporator, the refrigerant vaporizes. The
vapor is pushed through the evaporator by the pressure difference between the cylinder and the
vent. When the selected temperature is attained, the refrigerant valve closes. The vapor that
has been used is then discharged from the evaporator at about the same temperature as the air in
the cargo vehicle. With this system, the refrigerant does not mix with air in the vehicle space.
An example of the expendable evaporator system is shown in the Figure. This example shows two
nitrogen cylinders located inside a truck body connected by a manifold to regulators and to
temperature control solenoid valves. The vaporizing liquid nitrogen flows into the vaporizers or cold
plates to refrigerate the true box.
Expandable evaporator refrigeration system
Spray Systems
In the expendable refrigerant spray system, liquid nitrogen or carbon dioxide is sprayed
directly into the vehicle space that is to be cooled. This system uses liquid containers, a
control box, a fill box, spray headers, emergency switches, and safety vents. The fill box is
normally located on the front of the vehicle. It contains the valves, gauges, and connections
that allow the liquid containers to be filled. The liquid containers are insulated cylinders
similar to thermos bottles. The control box contains the valves, gauges, and thermostats that
are necessary for safe release of the liquid to the spray headers. Once the liquid is received
at the spray headers, the nozzles spray it into the vehicle. The remaining two components
are primarily safety devices. These emergency interlock switches are attached to each door.
That means, whenever a door is opened, the system shuts down. The safety vent is a small
trapdoor that vents air directly to the atmosphere whenever the air inside the truck box
exceeds atmospheric pressure. A benefit of this system is that liquid nitrogen or carbon
dioxide replaces the oxygen inside the space being refrigerated. Therefore when
fruits, vegetables, meats, and fish are being refrigerated, they are also preserved by
the inert atmosphere.
The Fig. illustrates the spray system. Liquid nitrogen (dark red) supplied from a cylinder
inside the refrigerated space, kept under pressure (200 psi). Dark blue indicates low
pressure liquid refrigerant.
Primary and Secondary refrigerant
Refrigerant
It is a substance in the vapour state which used as a working fluid in
refrigerators. It is also a cooling agent refrigeration system.
Important practical issues such as the system design, size, initial and
operating costs, safety, reliability, and serviceability etc. depend very
much on the type of refrigerant selected for a given application.
Due to several environmental issues such as ozone layer depletion and
global warming and their relation to the various refrigerants used, the
selection of suitable refrigerant has become one of the most important
issues in recent times.
Classification of refrigerants
• Primary refrigerant
• Secondary refrigerant
Primary refrigerants cools substance or space directly by absorbing
heat. It is also known as direct expansion system.
They are used directly as working fluids, for example in vapour
compression and vapour absorption refrigeration systems. When used in
compression or absorption systems, these fluids provide refrigeration by
undergoing a phase change process in the evaporator.
Example are Ammonia, Freon, SO2, CO2 etc
Ammonia (NH3)
• Used for commercial purposes mainly in cold stores and ice
plants
• The boiling temperature of NH3 is -330oC
• It is colourless gas with a pungent smell
• Has good thermodynamics properties
• It is neutral to all metal, highly soluble in oil
• Its solubility increase with increasing pressure and decreasing
temperature
• Used free small and medium refrigerating capacities
• Volatile and toxic under ambient temperature
Sulphur Dioxide SO2
• Previously used in household refrigerator
• Toxic, non-explosive and non-flammable
and non-corrosive
• Irritant to the human body
• Non-mixable with oil
• Has pungent odour and low latent heat
value
Carbon Dioxide
•
•
•
•
•
Colourless gas at ordinary temperatures
It has a slight odour and acid taste
It has a boiling point of 5oF (15 oC)
It is harmless to breathe except in large concentrations
Carbon dioxide is used aboard ship and industrial
installation
• It is not used in household application
• The main advantage using carbon dioxide as a
refrigerant is that small compressors can be used.
• It is inefficient, compared other refrigerant
Freon group
• They are fluorocarbons of methane and ethane series
• They contain one or more of these halogens (chlorine,
bromine, fluorine)
• Non-toxic, non-flammable, non-corrosive, non-explosive, nonirritant to the human body and eyes
• Odourless and colourless
• Will not react with food product stored in the refrigerated space.
• Will not react with lubrication oil
• Has excellent thermodynamics properties
• Only disadvantage is the ozone layer is damaged
Freon 11(Trichloro monofluro methane (CCl3F)
• Has boiling point of 74.9oF (23.8oC)
• Used in centrifugal compressors for industrial and
commercial air conditioning plants in a larger level
factories and theatres
• It is also used for industrial process water and brine
cooling
• Its low viscosity and freezing point have also led to its
use as low temperature brine.
Freon 12 (Dichloro-difluro methane- CCl2F2)
• Most used in domestic and commercial refrigeration system ( in ice
cream, display cabinet, deep freezers, food locker plants, water coolers,
window air-conditioning unit etc)
• It has a boiling point of -21.6oF (-29.8oC)
• Most widely used and known of the Freon refrigerant
• It is widely used colourless gas with mild odour
• Heavier than air
• Does not dissolve in water
• Refrigeration effect per unit volume of ammonia is 1.5times that of Freon
– 12
• It does not react with ferrous metals, aluminium
• It attack copper, copper alloys, zinc, and bronze
Freon 13 (Mono chloro-trifluoromethane(CClF3)
• It has a boiling point of -41.4oF (-40.8oC)
• They are used in small refrigeration plants
• Used to obtain moderately low temperature (500oC)
• Generally used in cascade with Freon 12, Freon 22
or 522
Look for the characteristics of Freon 22, Freon 113, Freon 114,
Freon 500, Freon 503
Secondary refrigerant
Secondary refrigerants are those liquids, which are used for transporting
thermal energy from one location to other. Secondary refrigerants are also
known under the name brines or antifreezes
Antifreezes or brines are used when refrigeration is required at sub-zero
temperatures. Unlike primary refrigerants, the secondary refrigerants do not
undergo phase change as they transport energy from one location to other.
An important property of a secondary refrigerant is its freezing point.
Generally, the freezing point of a brine will be lower than the freezing point
of its constituents. The temperature at which freezing of a brine takes place
its depends on its concentration. The concentration at which a lowest
temperature can be reached without solidification is called as eutectic point.
The commonly used secondary refrigerants are the solutions of water and
ethylene glycol, propylene glycol or calcium chloride. These solutions
are known under the general name of brines.
Calcium Chloride (CaCl2)
• It is used only in commercial refrigeration plants.
• Simple used as a carrying medium for refrigeration
• Used in large installations were there is danger of
leakage
• Also used where the temperature fluctuates in the
space to be refrigerated
• They cooled down by direct expansion of the
refrigerant. It is then pumped through the material or
space to be cooled.
Ethyl Chloride (C5H5Cl)
• It not is commonly used in domestic
refrigeration units
• It is colourless liquid or gas with pungent
ethereal odour and sweetish taste
• It is neutral towards all metals
• It soften all rubber compounds and gasket
material
Refrigerant selection criteria
Selection of refrigerant for a particular application is
based on the following requirements:
i. Thermodynamic and thermo-physical properties
ii. Environmental and safety properties, and
iii. Economics
Thermodynamic and thermo-physical properties
a) Suction pressure: At a given evaporator temperature, the saturation
pressure should be above atmospheric for prevention of air or moisture
ingress into the system and ease of leak detection. Higher suction
pressure is better as it leads to smaller compressor displacement
b) Discharge pressure: At a given condenser temperature, the discharge
pressure should be as small as possible to allow light-weight construction
of compressor, condenser etc.
c) Pressure ratio: Should be as small as possible for high volumetric
efficiency and low power consumption
d) Latent heat of vaporization: Should be as large as possible so that the
required mass flow rate per unit cooling capacity will be small
In addition to the above properties; the following properties are also
important
e) Isentropic index of compression: Should be as small as possible so that the
temperature rise during compression will be small
f) Liquid specific heat: Should be small so that degree of subcooling will be
large leading to smaller amount of flash gas at evaporator inlet
g) Vapour specific heat: Should be large so that the degree of superheating will
be small
h) Thermal conductivity: Thermal conductivity in both liquid as well as vapour
phase should be high for higher heat transfer coefficients
i) Viscosity: Viscosity should be small in both liquid and vapour phases for
smaller frictional pressure drops
Environmental and safety properties
a) Ozone Depletion Potential (ODP): According to the Montreal protocol, the
ODP of refrigerants should be zero, i.e., they should be non-ozone depleting
substances. Refrigerants having non-zero ODP have either already been phasedout (e.g. R11, R12) or will be phased-out in near-future(e.g. R22). Since ODP
depends mainly on the presence of chlorine or bromine in the molecules,
refrigerants having either chlorine (i.e., CFCs and HCFCs) or bromine cannot be
used under the new regulations
b) Global Warming Potential (GWP): Refrigerants should have as low a GWP
value as possible to minimize the problem of global warming. Refrigerants with
zero ODP but a high value of GWP (e.g. R134a) are likely to be regulated in future.
c) Total Equivalent Warming Index (TEWI): The factor TEWI considers both
direct (due to release into atmosphere) and indirect (through energy consumption)
contributions of refrigerants to global warming. Naturally, refrigerants with as a low
a value of TEWI are preferable from global warming point of view.
d) Toxicity: Ideally, refrigerants used in a refrigeration system should be
non-toxic. However, all fluids other than air can be called as toxic as they will
cause suffocation when their concentration is large enough. Thus toxicity is a
relative term, which becomes meaningful only when the degree of
concentration and time of exposure required to produce harmful effects are
specified. Some fluids are toxic even in small concentrations. Some fluids are
mildly toxic, i.e., they are dangerous only when the concentration is large and
duration of exposure is long. Some refrigerants such as CFCs and HCFCs are
non-toxic when mixed with air in normal condition. However, when they come
in contact with an open flame or an electrical heating element, they
decompose forming highly toxic elements (e.g. phosgene- COCl2). In general
the degree of hazard depends on:
Other important properties
e) Flammability: The refrigerants should preferably be non-flammable and non-explosive. For
flammable refrigerants special precautions should be taken to avoid accidents.
f) Chemical stability: The refrigerants should be chemically stable as long as they are
inside the refrigeration system.
g) Compatibility with common materials of construction (both metals and nonmetals)
h) Miscibility with lubricating oils: Oil separators have to be used if the refrigerant is not
miscible with lubricating oil (e.g. ammonia). Refrigerants that are completely miscible with oils
are easier to handle (e.g. R12). However, for refrigerants with limited solubility (e.g. R 22)
special precautions should be taken while designing the system to ensure oil return to the
compressor
j) Ease of leak detection: In the event of leakage of refrigerant from the system, it should
be easy to detect the leaks.
Economic properties
The refrigerant used should preferably be inexpensive
and easily available.
Other classification
Azeotropic mixtures
Although it contains two or more refrigerants, at a certain
pressure an azeotropic mixture evaporates and condenses
at a constant temperature. Because of this, azeotropic
mixtures behave like pure refrigerants in all practical
aspects.
Azeotropic mixtures are designated by 500 series,
whereas zeotropic refrigerants (e.g. non-azeotropic
mixtures) are designated by 400 series.
Azeotropic mixtures:
R 500: Mixture of R 12 (73.8 %) and R 152a (26.2%)
R 502: Mixture of R 22 (48.8 %) and R 115 (51.2%)
R503: Mixture of R 23 (40.1 %) and R 13 (59.9%)
R507A: Mixture of R 125 (50%) and R 143a (50%)
Non-azeotropic/Zeotropic mixtures
Zeotropic mixtures have a gliding evaporation and condensing
temperature. When evaporating, the most volatile component will boil off
first and the least volatile component will boil off last. The opposite
happens when gas condenses into liquid.
Zeotropic mixtures:
R404A : Mixture of R 125 (44%), R 143a (52%) and R 134a (4%)
R407A : Mixture of R 32 (20%), R 125 (40%) and R 134a (40%)
R407B : Mixture of R 32 (10%), R 125 (70%) and R 134a (20%)
R410A : Mixture of R 32 (50%) and R 125 (50%)
The Pressure - Enthalpy Chart
Latent Heat of Fusion - The quantity of heat (Btu/lb) required to change 1 lb. of material from the solid
phase into the liquid phase.
Latent Heat of Vaporization - The quantity of heat (Btu/lb) required to change 1 lb. of material from the
liquid phase into the vapor phase.
Sensible Heat - Heat that is absorbed/rejected by a material, resulting in a change of temperature.
Latent Heat - Heat that is absorbed/rejected by a material resulting in a change of physical state
(occurring at constant temperature).
Saturation Temperature - That temperature at which a liquid starts to boil (or vapor starts to condense).
The saturation temperature (boiling temperature) is constant at a given pressure,*
and increases as the pressure increases. A liquid cannot be raised above its saturation temperature.
Whenever the refrigerant is present in two states (liquid and vapor) the refrigerant mixture will be at
the saturation temperature.
Superheat - At a given pressure, the difference between a vapor’s temperature and its saturation
temperature.
Subcooling - At a given pressure, the difference between a liquid’s temperature and its saturation
temperature.
The Pressure - Enthalpy Chart
The Pressure - Enthalpy Chart
Pressure - The vertical axis of the chart, in psia (see pink line). To obtain gauge pressure, subtract
atmospheric pressure.
Enthalpy - The horizontal axis of the chart shows the heat content of the refrigerant in Btu/lb.
Temperature- Constant temperature lines generally run in a vertical direction in the superheated vapor & subcooled liquid portion of the chart. In the saturated bubble, the constant temperature line is along the horizontal,
illustrating that the saturation temperature is constant at a given pressure (see black line).
Specific Volume - Constant volume lines extend from the red line saturated vapor line out into the superheated
vapor-portion of the chart at a slight angle from the horizontal axis.
Specific volume is expressed in cu.ft/lb. (see orange line).
Entropy - Entropy is the mathematical relationship between heat and temperature, and relates to the
availability of energy. These lines extend at an angle from the saturated vapor line. Their presence on the
chart is relevant in that vapor compression (in the ideal cycle) occurs at constant entropy (see dark blue line).
Quality - Lines of constant quality appear vertically, and only within the saturation bubble. The refrigerant
within the bubble is a mixture of liquid and vapor at saturation, and the quality is the percentage of the mixture
which is in the vapor state (see green line).
The Ideal cycle
If the operating temperatures and pressures are known, the refrigeration system can be
plotted on the P-H diagram. Let’s assume our system is operating at a -20ºF evaporator and
100ºF condensing.
Data Points and System Design Calculations
Refrigeration Effect (RE): This is the total heat transfer, in Btu/lb, from the
refrigerated space to the refrigerant. H1 minus H4 (H1 is the enthalpy of
the refrigerant at point #1 in Fig. 3, and so forth).
Heat of Compression (HOC):This is the amount of heat added to the
refrigerant from the compression process (represented by H2 minus H1
on the chart).
Heat of Rejection (HOR): This is the amount of heat that has to be rejected
at the condenser. The heat transferred to the refrigerant from the
refrigerated space (RE), and the heat transferred to the refrigerant during
compression (HOC). It is this value, plus some safety factor, from which the
condenser selection is made (represented by H2 minus H3 on the chart
Data Points and System Design Calculations
Note: For the purpose of this discussion, H1 will be considered the
point where the evaporation line intersects with the saturated
vapor line. In the real world, the location of H1 would be to the right
of the saturated vapor line, reflecting the superheated vapor at the
evaporator outlet. With an expansion valve maintaining a typical
amount of superheat (in the 4° - 6° range for low temperature
applications), the heat transferred to the vapor is minimal (less
than 1 Btu/lb).
Let’s look at a few typical system scenarios, plot them on the P-H diagram, and
then compare the performance measurements.
Typical Cycle #1 (Fig. 4): It is neither realistic nor safe to have a saturated vapor
at the compressor inlet. Because liquid cannot be present with superheated
vapor, some amount of superheat at the compressor inlet becomes the margin
necessary to insure the safety of the compressor
This is the result of an expansion valve set to maintain some amount of
superheat, plus the temperature increase the refrigerant vapor experiences in
the suction line.
The suction superheat will insure that the compressor is protected from liquid
flooding. The cost of this protection comes in the form of a larger compressor
volume requirement.
Typical Cycle #1A (Fig. 5): Using an open drive compressor, with a
20ºF vapor temperature at the compressor inlet, we see the benefit
of subcooling the liquid to 50ºF.
Note the change in refrigerant quality at the TEV outlet. Instead of
65% liquid, we now have 80% liquid. Because the difference
between liquid temperature and evaporator temperature has been
reduced, there is less refrigerant flashing during the expansion
process.
The benefit realized resulting from subcooling will be offset by the
higher suction vapor temperatures, and the volume requirement
penalty they impose.
Typical Cycle #2 (Fig. 6): The advantage of using a hermetic compressor is the elimination of
either belts or drive motor couplings which require precise alignment, and crankshaft seals. The
disadvantage is that there is now an electric motor in the refrigerant circuit (at least on suction
vapor cooled hermetic compressors).
In addition to the guaranteed system contamination problem when a hermetic motor burns, you
have the heat from the motor being transferred to the refrigerant vapor. An approximate 80ºF
temperature increase can be expected between the vapor entering the compressor service
valve, and the vapor entering the compressor cylinders. This brings the suction vapor
temperature up to 100ºF, with a corresponding increase in discharge temperature…now
approaching 300ºF.
This is the upper limit at which most compressor manufacturers agree
shouldn’t be exceeded. The higher suction vapor temperature also results
in a higher HOC, which raises the horsepower requirement.
Dirty condenser
Filthy condenser and TEV not adjusted
Typical Cycle #4 (Fig. 8): Not only is the condenser
filthy, but the TEV’s were never adjusted. They are
operating at abnormally high superheats, which in
effect have reduced the size of the evaporators. As
a result, the discharge air temperature in the glass
door frozen food display cases is too high, causing
the frozen juice to melt.
Deviation of actual cycle from ideal cycle
Irreversibilities that occur in various components. Two common
sources of irreversibilities are fluid friction (causes pressure
drops) in the tubes, valves and other accessories and heat
transfer to or from the surroundings.
The compression process occurs at a constant entropy ONLY in the
ideal cycle. In the real world, entropy will increase during the
compression process, resulting in even higher discharge
temperatures and HOC values
Compressors
Function of the compressor
It is a pump, like the heart in the circulation system of the human
body.
It is considered as the heart of the refrigeration system.
In the refrigeration system, it is responsible for pumping vapour i.e.
it is also referred to as vapour pump.
The compressor lifts/increase the pressure of the ref system from
suction pressure level (low pressure) to discharge level.
All compressors are powered or driven by an electric motor.
Compression ratio (CR)
The compression ratio is an engineering expression of the high side
absolute pressure divided by the low side absolute pressure. It is
expressed in absolute pressures.
Compression ratio (CR) =
𝑎𝑏𝑠𝑜𝑙𝑢𝑡𝑒 𝑑𝑖𝑠𝑐ℎ𝑎𝑟𝑔𝑒 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒
𝑎𝑏𝑠𝑜𝑙𝑢𝑡𝑒 𝑠𝑢𝑐𝑡𝑖𝑜𝑛 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒
NB: The difference between the discharge pressure and the
suction pressure is referred to as the lifts pressure
Performance of a compressor
The performance of a compressor is
influenced by numerous parameters including
the following:
• compressor speed,
• suction pressure and temperature,
• discharge pressure and temperature, and
• type of refrigerant and its flow rate.
Classification of compressors
Compressors used in refrigeration systems can be
classified in several ways: Based on working
principles or arrangement of compressor motor or
external drive.
The method of compression may be reciprocating, rotary and
centrifugal
Based on working principles
• Positive displacement type
• Roto-dynamic type
Positive displacement compressors
These compressors use the shaft work to
increase the refrigerant pressure by reducing
the compression volume in the chamber. The
compressors of this group are reciprocating,
vane (rotary), and screw (helical rotary)
compressors.
Positive displacement compressors
In positive displacement type of compressors, compression is
achieved by trapping a refrigerant vapour into an enclosed space and
then reducing its volume. Since a fixed amount of refrigerant is
trapped each time, its pressure rises as its volume is reduced. When
the pressure rises to a level that is slightly higher than the condensing
pressure, then it is expelled from the enclosed space and a fresh
charge of low-pressure refrigerant is drawn in and the cycle continues.
Since the flow of refrigerant to the compressor is not steady, the
positive displacement type compressor is a pulsating flow device.
However, since the operating speeds are normally very high the flow
appears to be almost steady on macroscopic time scale. Since the
flow is pulsating on a microscopic time scale, positive displacement
type compressors are prone to high wear, vibration and noise level.
Positive displacement compressors
Depending upon the construction, positive displacement
type compressors used in refrigeration and air
conditioning can be classified into:
• Reciprocating,
• Vane (rotary), and
• Screw (helical rotary) compressors
Reciprocating compressors
Reciprocating compressor is the workhorse of the
refrigeration and air conditioning industry. It is the
most widely used compressor with cooling
capacities ranging from a few Watts to hundreds of
kilowatts. Modern day reciprocating compressors
are high speed (≈ 3000 to 3600 rpm), single acting,
single or multi-cylinder (up to 16 cylinders) type.
Reciprocating compressors
Schematic diagram of a reciprocating compressor
Reciprocating compressors
• It consists of a piston moving back and forth in a cylinder, with suction and discharge
valves to achieve suction and compression of refrigerant vapour.
• The construction and working principles are similar two stroke engine, where suction
and compression are completed in revolution of a crank.
• The suction side of the compressor is connected to the exit of the evaporator, while
the discharge side of the compressor is connected to the condenser inlet.
• The suction (inlet) and the discharge (outlet) valves open and close due to pressure
differences between the cylinder and inlet or outlet manifolds respectively. The pressure
in the inlet manifold is equal to or slightly less than the evaporator pressure. Similarly
the pressure in the outlet manifold is equal to or slightly greater than the condenser
pressure. The purpose of the manifolds is to provide stable inlet and outlet pressures for
the smooth operation of the valves and also provide a space for mounting the valves.
Rotary compressors
The rotary compressors have replaced many smaller reciprocating compressors.
A rotary unit is much smaller and lighter weight than a comparable reciprocating
unit. A typical two blade rotary compressor operates as follows:
As the first blade passes the suction port, low pressure vapour is drawn into the
compressor
When the second blade passes the suction port, it seals the refrigerant between
the two blades, then starts a new cycle behind as it moves along. The gas is
trapped and compressed as the blades revolves.
As the first blade reaches the exhaust port, it pushes the compressed gas into
the exhaust port. This entire cycle repeats itself as the blade rotate between
intake and exhaust ports of the compressor.
Rotary compressor
Rotary compressors can have up to eight blades. This type of
compressor is often used as the first or booster compressor in a
cascade refrigeration system. They also provides large opening into
the suction line, so that substantial amounts of vapour can be drawn
in on each stroke maximizing the efficiency of the compressor. Small
clearance used in the construction of rotary compressors ensures
that low pressure vapour drawn into the unit is compressed and
pushed out on the discharge stroke.
A check valve is normally installed in the suction line to prevent high
pressure vapour and compressor oil from flowing back into the
evaporator.
Rotary compressors
Rotary vane compressor (courtesy: Hicks, Hargreaves & Co. Ltd)
Screw compressors
In the screw compressor, the screw comprises a set of precision –matched
helical rotor to trap and compress refrigerant vapour as they turn inside a
precisely machined compressed cylinder. The drive or the male rotor is
driven by the compression motor. The lobes on the rotor fit into the interlobe
spaces on the female rotor. As the rotor turns, low pressure vapour is drawn
into the intake side of the compressor, filling the interlobe spaces. As the
screw turn, the vapour is trapped, compressed and then discharged from the
outlet or exhaust port. Operation is continuous and extremely smooth
and vibration free. Oil injection is used on many screw compressors to help
seal clearance between the rotors and the compression cylinder. Screw
compressors are highly efficient.
Screw compressors
Describe the design and
operational principles of
single and twin screw
compressors?
Twin screw compressor (courtesy: Stal Refrigeration AB)
Roto-dynamic compressors
In roto-dynamic compressors, the pressure rise of refrigerant is
achieved by imparting kinetic energy to a steadily flowing stream
of refrigerant by a rotating mechanical element and then
converting into pressure as the refrigerant flows through a
diverging passage. Unlike positive displacement type, the rotodynamic type compressors are steady flow devices, hence
are subjected to less wear and vibration. Depending upon the
construction, roto-dynamic type compressors can be classified
into:
i. Radial flow type, or
ii. Axial flow type
Roto-dynamic compressor
Centrifugal compressors (also known as turbocompressors) are radial flow type, roto-dynamic
compressors. These compressors are widely used in
large capacity refrigeration and air conditioning
systems. Axial flow compressors are normally used
in gas liquefaction applications.
Centrifugal compressors
A centrifugal compressor resembles a long horizontal cylinder. A
drive shaft is mounted inside the cylinder and a series of
impellers are mounted to the shaft. The impeller spin very fast.
The refrigerant vapour is drawn in through vanes near the centre
of the impeller. As the impellers turns, vapour is thrown outward
against the outer wall of the impeller housing. The force exerted
on the vapour increases it pressure by a small, but significant.
The pressurized vapour the impeller body through slots in the
perimeter of the body exits. It is then picked up by the second
impeller and the process is repeated, further increasing vapour
pressure. Centrifugal compressors are used for some large
commercial applications.
Based on arrangement of compressor
motor or external drive
• Open type
• Hermetic (or sealed) type
• Semi-hermetic (or semi-sealed) type
Open type compressors
In open type compressors the rotating shaft of the compressor extends
through a seal in the crankcase for an external drive. The external drive
may be an electrical motor or an engine (e.g. diesel engine). The
compressor may be belt driven or gear driven. Open type
compressors are normally used in medium to large capacity refrigeration
system for all refrigerants and for ammonia (due to its incompatibility
with hermetic motor materials). Open type compressors are
characterized by high efficiency, flexibility, better compressor cooling
and serviceability. However, since the shaft has to extend through the
seal, refrigerant leakage from the system cannot be eliminated
completely. Hence refrigeration systems using open type compressors
require a refrigerant reservoir to take care of the refrigerant leakage for
some time, and then regular maintenance for charging the system with
refrigerant, changing of seals, gaskets etc.
Hermetic (Sealed) compressors
In hermetic compressors, the motor and the compressor are enclosed in the same housing to
prevent refrigerant leakage. The housing has welded connections for refrigerant inlet and
outlet and for power input socket. As a result of this, there is virtually no possibility of
refrigerant leakage from the compressor. All motors reject a part of the power supplied to it due
to eddy currents and friction, that is, inefficiencies. Similarly the compressor also gets heatedup due to friction and also due to temperature rise of the vapor during compression. In Open
type, both the compressor and the motor normally reject heat to the surrounding air for
efficient operation. In hermetic compressors heat cannot be rejected to the surrounding air
since both are enclosed in a shell. Hence, the cold suction gas is made to flow over the motor
and the compressor before entering the compressor. This keeps the motor cool. The motor
winding is in direct contact with the refrigerant hence only those refrigerants, which have high
dielectric strength, can be used in hermetic compressors. The cooling rate depends upon the
flow rate of the refrigerant, its temperature and the thermal properties of the refrigerant. If flow
rate is not sufficient and/or if the temperature is not low enough the insulation on the winding of
the motor can burn out and short-circuiting may occur. Hence, hermetically sealed
compressors give satisfactory and safe performance over a very narrow range of design
temperature and should not be used for off-design conditions.
Hermetic (Sealed) compressors
The COP of the hermetic compressor based systems is lower than that of the
open compressor based systems since a part of the refrigeration effect is lost in
cooling the motor and the compressor. However, hermetic compressors are
almost universally used in small systems such as domestic refrigerators, water
coolers, air conditioners etc, where efficiency is not as important as customer
convenience (due to absence of continuous maintenance). In addition to this,
the use of hermetic compressors is ideal in systems, which use capillary tubes
as expansion devices and are critically charged systems. Hermetic
compressors are normally not serviceable. They are not very flexible as it is
difficult to vary their speed to control the cooling capacity. In some (usually
larger) hermetic units, the cylinder head is usually removable so that the valves
and the piston can be serviced. This type of unit is called a semi-hermetic (or
semi-sealed) compressor.
Hermetic compressors
Semi-hermetic compressor
Hermetic compressor
Courtesy: Durham Bush Ltd and L’UniteHermetique S.A