DEPARTMENT OF MECHANICAL ENGINEERING

INSTITUTE OF TECHNOLOGY
HAWASSA UNIVERSITY

Refrigeration and Air Conditioning - EMEng

2092

Part 1 – Refrigeration
1.1. Basic concepts in refrigeration
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Basic Principles of Refrigeration & Air Conditioning

• Refrigeration may be defined as lowering the temperature of

an enclosed space by removing heat from that space and
transferring it elsewhere. A device that performs this function
may also be called an air conditioner, refrigerator, air
source heat pump, water source heat pump, geo thermal
heat pump or chiller (heat pump).
• Air conditioning it means the mechanical control of the
internal environment to maintain specified condition or to
provide a thermally comfortable temperature, humidity, air
cleanliness and freshness for the uses of the building.

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 1.1.1 Thermodynamic system

• Thermodynamics: Science of Heat (form of energy) Transfer

and its effect on properties of system.
• A thermodynamic system is defined as the quantity of matter
or a region in space upon which attention is concentrated in the
analysis of a problem.
• Everything external to the system is called the surroundings.
• Boundary separate the surroundings from the system. The
boundary may be fixed or flexible.
• System and surroundings together constitute the universe.
Size of the universe depends on the size of the system and
surroundings.

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• A system is classified as closed, open or isolated based on
the interaction of mass and energy through the system
boundary.
 A closed system: (also known as a control mass) consists
of a fixed amount of mass, and no mass can cross its
boundary. That is, no mass can enter or leave a closed
system. But energy, in the form of heat or work, can cross
the boundary. The volume of a closed system does not have
to be fixed.
 Isolated system: is a special case of closed system, even
energy is not allowed to cross the boundary. E.g. thermos

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  An open system: (also known as a control volume) is a
properly selected region in space. It usually encloses a
device that involves mass flow such as a compressor,
turbine, or nozzle. It can also involve heat and work
interactions
• Properties those observable behavior/characteristics
are which can be used for and
of a system physical conditions of defining
describing
the properties sometimes observable directly system.
and sometimes
These
are indirectly.

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• Properties are considered to be either intensive or extensive
properties.
 Intensive properties are those that are independent of the
mass or extent of a system, such as temperature, pressure,
and density.
 Extensive properties are those whose values depend on
the size or extent of the system. Total mass, total volume,
and total momentum are some examples of extensive
properties.
• Extensive properties per unit mass are called specific
properties. Some examples of specific properties are specific
volume (v = V/m) and specific total energy (e = E/m).

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• To know the characteristics of the system quantitatively refers
to knowing the state of system. Thus, when the properties of
system are quantitatively defined then it refers to the ‘state’.
• Any change that a system undergoes from one equilibrium
state to another is called a process
• Path :- the series of states through which a system passes
during a process is called the path of the process
• Cycles :- A system is said to have undergone a cycle if it
returns to its initial state at the end of the process. That is, for a
cycle the initial and final states are identical.

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• An isothermal process, for example, is a process during
which the temperature T remains constant; an isobaric
Process is a process during which the pressure P remains
constant; and an isochoric (or isometric) process is a process
during which the specific volume v remains constant.
• The prefix iso- is often used to designate a process for which a
particular property remains constant.

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 1.1.2. Heat and work

• Heat is energy transferred a system and its

surroundings by virtue
betweenof a temperature difference
The different only. transfer are: conduction,
modes
convection and radiation.of heat
 Conduction heat transfer takes place whenever a
temperature gradient exists in a stationary medium.
 Convection heat transfer takes place between a surface and
a moving fluid, when they are at different temperatures.
 Radiation is another fundamental mode of heat transfer.
Unlike conduction and convection, radiation heat transfer
does not require a medium for transmission as energy transfer
occurs due to the propagation of electromagnetic waves.
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Energy Transfer by Heat
• There are two forms of heat energy: sensible heat and latent
heat.
• Sensible heat is the form of heat energy which is most
commonly understood because it is sensed by touch or
measured directly with a thermometer. When weather
reporters say it will be 90 degrees, they are referring to
sensible heat.
• Latent heat cannot be sensed by touch or measured with a
thermometer. Latent heat causes an object to change its
properties. For example, when enough latent heat is removed
from water vapor (steam or humidity), it condenses into water
(liquid).
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• If enough latent heat is removed from water (liquid), it will
eventually freeze. This process is reversed when latent heat
is added.

Figure of sensible heat and latent heat on T-H diagram

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• Change of State:- An object that changes from a solid to a
liquid or liquid to vapor is referred to as a change of state.
When an object changes state, it transfers heat rapidly. It will
happen on constant temperature and Heat is required for
changing phase is called Latent heat.
Energy Transfer by Work
• Work, like heat, is an energy interaction between a system and
its surroundings.
• As mentioned earlier, energy can cross the boundary of a
closed system in the form of heat or work. Therefore, if the
energy crossing the boundary of a closed system is not heat, it
must be work.
• Work is the energy transfer associated with a force acting
through a distance.
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 1.1.3. Fundamental laws of Thermodynamics

• Thermodynamic laws are based on human observation of

natural phenomena; they are not mathematically derived
equations. Since no exceptions to these have been observed;
these are accepted as laws.
• The Zeroth law of thermodynamics states that when two
systems are in thermal equilibrium with a third system, then
they in turn are in thermal equilibrium with each other.
Equality of temperature is a necessary and sufficient condition
for thermal equilibrium.
• The First law of thermodynamics states that energy can be
neither created nor destroyed during a process; it can only
change forms.

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• The first law of thermodynamics, also known as the
conservation of energy principle.
• The net change (increase or decrease) in the total energy of the
system during a process is equal to the difference between the
total energy entering and the total energy leaving the system
during that process.
• That is,
(Total energy entering the system) – (Total energy leaving the
system) = (Change in the total energy of the system)
E in - E out= ∆E system
dQ=dU+đW

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• Second law of Thermodynamics may be defined in many
ways,
• Clausius’ statement of second law: It is impossible to
transfer heat in a cyclic process from low temperature to high
temperature without work from external source.
• Kelvin-Planck statement of second law: It is impossible to
construct a device (engine) operating in a cycle that will
produce no effect other than extraction of heat from a single
reservoir and convert all of it into work.

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Refrigerators Vs Heat Pumps
• The performance of a refrigeration system is expressed by a
term known as the ‘‘co-efficient of performance’’, which is
defined as the ratio of heat absorbed by the refrigerant while
passing through the evaporator to the work input required to
compress the refrigerant in the compressor; In short it is the
ratio between heat extracted and work done (in heat units)
Refrigerators
• The transfer of heat from a low-temperature medium to a high-
temperature one requires special devices called refrigerators.
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.

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• Refrigerators, like heat engines, are
working fluid cyclic devices. The used in
refrigerant. the refrigeration cycle is called
a

Figure of Refrigerators

• 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.

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Heat Pumps
• Another device that transfers heat from a low-temperature
medium to a high temperature one is the heat pump. The
objective of a heat pump, however, is to maintain a heated
space at a high temperature. This is accomplished by
absorbing heat from a low-temperature source, such as well
water or cold outside air in winter, and supplying this heat to
the high-temperature medium.
• The measure of performance of a heat pump is also expressed
in terms of the coefficient of performance COPHP, defined as

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Figure of Heat Pumps

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 1.1.4. Unit and rating of Refrigeration

• The rating of a refrigeration machine is obtained by

refrigerating effect or the amount of heat extracted in a given
time from a body or space.
• Ton of refrigeration is the term used to indicate the capacity
of the refrigeration and air conditioning system. One Ton of
Refrigeration is defined as the amount of cooling produced by
melting one ton of ice from and at 0 °C in 24 hours.

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Co-efficient of Performance (C.O.P.)
• The performance of a refrigeration system is expressed by a
term known as the ‘‘co-efficient of performance’’, which is
defined as the ratio of heat absorbed by the refrigerant while
passing through the evaporator to the work input required to
compress the refrigerant in the compressor; In short it is the
ratio between heat extracted and work done (in heat units)
Energy efficiency ratio (EER)
• It is the ratio of the cooling capacity of the unit, measured in
Btu/hr, to the power required to operate it, in watts. It is
evident that the EER is proportional to the COP, neglecting fan
power. Applying the conversion factor between BTUs and
watt-hours one sees that
EE𝑅 = 3.413 ∗ 𝐶OP.
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• Ideal Refrigeration Cycle: (The Carnot Principle) – it
is Reversed Carnot Cycle

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 1.1.5. Working of Refrigeration & Air-conditioning
systems
• The system by which air conditioners provide cooling is
called the Refrigerant Cycle. This system has four major
components common to all air-conditioning systems (see
figure below). These components and their basic functions are
listed below.

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1. Compressor
• Refrigerant is drawn from the evaporator and pumped to the condenser by
the compressor. The compressor also pressurizes the refrigerant vapor so
that it will change state (condense) readily.
2. Condenser
• The high-pressure refrigerant vapor releases heat through the condenser
coils as it condenses into liquid refrigerant. Making it easier to vaporize.
3. Metering Device (Expansion Device) (capillary tube)
• The metering device restricts the flow of liquid refrigerant from the
condenser to the evaporator. As refrigerant passes through the metering
device, its pressure decreases.
4. Evaporator
• The low-pressure liquid refrigerant absorbs heat as it vaporizes in the
evaporator coils.
• The process described above is the Refrigerant System or Refrigerant
Cycle. It is the system on which virtually all modern Air-Conditioning and
refrigeration is based.
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