introduction to refrigeration and air conditioning

1. MENSCHEN FÜR MENSCHEN FOUNDATION
AGRO–TECHNICAL AND TECHNOLOGY COLLEGE
MANUFACTURING TECHNOLOGY DEPARTMENT
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Refrigeration and Air Conditioning Engineering
MEng 4220

2. CHAPTER ONE
INTRODUCTION AND BASIC CONCEPTS IN
REFRIGERATION
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PART I – REFRIGERATION

3. • Refrigeration is the removal of unwanted heat from a selected object,
substance, or space and its transfer to another object, substance, or space.
• Air conditioning deals with artificial tampering of the conditions of air that
may involve cooling as well as heating coupled with ventilation, filtration, and
air circulation.
Refrigeration And Air-conditioning
• Refrigeration and air conditioning are very closely interrelate
• Removal of heat lowers the temperature and may be accomplished by use
of ice, snow, chilled water or mechanical refrigeration.
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Refrigerators Vs Heat Pumps
• The transfer of heat from a low-temperature medium to a high-temperature one
requires special devices called refrigerators/Heat Pumps.
• Refrigerators/Heat Pumps are cyclic devices. The working fluid used in the
refrigeration cycle is called a refrigerant.
✓ The objective of a refrigerator is to
maintain the refrigerated space at a low
temperature by removing heat from it.
The objective of a heat pump is to maintain a
heated space at a high temperature.

5. Review of Basic Principles - Thermodynamics
Thermodynamicsystem
• 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.
• A system is classified as closed, open or isolated based on the interaction of
mass and energy through the system boundary.
• 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.
✓ A closed system(control mass)
✓ An open system(control volume)
✓ Isolated system 5

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• Properties are those observable behavior/characteristics of a system which
can be used for defining and describing the physical conditions of system.
• These properties are sometimes measured directly or observed indirectly.
• 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 such as 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).
Properties,stateand process
• State is Condition of the system at an instant of time. when the properties of system
are quantitatively defined then it refers to the ‘state’..

7. • Any change that a system undergoes from one equilibrium state to another is
called a process
• 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.
• Equilibrium state means the condition of balance.
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✓ Pure Substance Is a substance that has a fixed chemical composition throughout such as water,
air, and nitrogen. (Homogeneous in composition, Homogeneous in chemical aggregation,
Invariable in chemical aggregation)
• A pure substance does not have to be of a single element or compound.
• A mixture of two or more phases of a pure substance is still a pure substance as long as the
chemical composition of all phases is the same.
• A pure substance may exist in different phases. (solid, liquid, and gas.)
✓ A phase: is defined as having a distinct molecular arrangement that is homogenous
throughout and separated from others (if any) by easily identifiable boundary surfaces called phase
boundaries
Phases and pure substance
• A substance may have several phases within a principal phase,
each with a different molecular structure. carbon may exist as
graphite or diamond in the solid phase,

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Gibbs phase rule IV = C - PH + 2
where IV = the number of independent variables,
C= the number of components, and
PH = the number of phases present in equilibrium.
Single component (C= 1),
Two-phase (PH = 2) system,
One independent intensive property needs to be specified (IV = 1),
• At the triple point, however, PH = 3 and thus IV = 0.That is, none of the
properties of a pure substance at the triple point can be varied.
Properties of a pure substance
• The state of a simple compressible system is completely specified by two
independent, intensive properties.
✓ A simple compressible system is a system in the absence of electrical, magnetic,
gravitational, motion, and surface tension effects.
✓ Two properties are independent if one property can be varied while the other one is
held constant.
the state postulate

10. phase change process
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• An object that changes from a solid to a liquid or liquid to vapor is referred to as a change of phase
(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.
• The prerequisite to conversion of a solid to a liquid, and liquid to a gas is to overcome the
molecular forces.

11. ✓ Compressed liquid, or a sub cooled liquid (C-D) is a
liquid which is not about to vaporize (exists below the
saturation temperature.
✓ Saturation temperature: the temperature at which
vaporization takes place at a given pressure.
✓ Saturation pressure: The pressure at saturation
temperature is called the saturation pressure.
✓ Saturated liquid (D): A liquid about to vaporize (exists
at the saturation temperature).
✓ Quality : If a substance exists as part liquid and part
vapour at the saturation temperature, then its quality
is defined as the ratio of the mass of vapour to the
total mass.
✓ Saturated vapour (E): vapor that is about to condense
(exists at saturation temperature).
✓ superheated vapor (E-F]: A vapor that is not about to
condense (i.e. not a saturated vapor) (exists above
the saturation temperature. .
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• Some terms in L-V phase change process of pure substances. These are as follows:

12. Property Diagram
• The variations of properties during phase-change processes are best studied and
understood with the help of property diagrams.
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13. P-h diagram
• To understand the principle of operation of refrigerator and heat pump, a P-h diagram is most
commonly used.
• A P-h diagram shows all state variables of the refrigerant. On the horizontal axis enthalpy (h) is shown.
• The vertical axis shows pressure and horizontal axis shows enthalpy. The other lines show:
Blue: Temperature
Cyan: Entropy
Green: quality
Pink: Specific volume
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P= 5 MPa
v=0.01 m3/kg

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Zeroth law of thermodynamics
✓ When two bodies have equality of temperature with a third body then the two
bodies have equality of temperature with each other (they are in thermal
equilibrium).
zeroth law of thermodynamics,
✓ The basis of temperature measurement is the zeroth law of thermodynamics

16. • The first law of thermodynamics (conservation of energy) and energy
For closed stationary systems
For cycles
For open systems
Enthalpy: the combinations of thermodynamic property
First law of thermodynamics
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17. EnergyTransferbyHeatandwork
• When a body moves through a distance by the action of a force, Work is said to be done.
• Heat is thermal energy transferred between a system and its surroundings by
virtue of a temperature difference only. The different modes of heat transfer are
Conduction, Convection, Radiation heat transfer
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• Moving boundary work associated with
closed systems is expressed as :
• The work associated with steady flow devices in terms
of fluid properties.
• 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.
✓ Latent heat cannot be sensed by touch or measured with a thermometer. Latent heat
causes an object to change its phase.

18. Second law of Thermodynamics
• 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.
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• 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.

19. Heat Transfer
• From thermodynamics we can calculate the amount of energy required to change a system from
one equilibrium state to another equilibrium state since thermodynamics deals with systems in
equilibrium.
• But if we want to know the rate of heat transfer or how fast a system will change from one
equilibrium state to another then we have to know the subject of heat transfer.
Conduction
• When heat transfer occurs through a substance without any motion
of the substance, then the mode of heat transfer is called
conduction.
K - Thermal conductivity
dT/dx - Temperature gradient
Fourier’s law of heat conduction
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20. • The mode of energy transfer between a solid surface and
the adjacent liquid or gas that is in motion,
Convection
Newton’s law of cooling
h convection heat transfer coefficient, W/m2 · °C
As the surface area through which convection heat transfer takes place
Ts the surface temperature
T the temperature of the fluid sufficiently far from the surface.
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21. • Thermal radiation, is the form of radiation emitted by bodies because of their temperature. can
also be transferred without any medium, i.e. in the vacuum.
• To demonstrate the heat transfer rate by radiation, it is useful to introduce a substance that
emits radiation ideally, i.e. a blackbody.
✓ The radiation heat transfer rate for a blackbody is proportional to the fourth power of its absolute
temperature, i.e.
Radiation
• But most real material surfaces do not emit electromagnetic radiation ideally. And these
materials are called gray substances. Hence other important parameters like emissivity, should
be considered in quantifying the heat transfer.
• There is another important factor called the shape factor. The shape factor tells us how much
energy will leave from one surface and how much energy will arrive directly on the other surface
• The net radiant exchange between two surfaces at
temperatures T 1 and 7' 2 respectively can be expressed as
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22. Thermal circuit
• Thermal circuit or network is a useful concept for the analysis of heat transfer
problems. In the thermal circuit concept, equations of thermal resistance are
introduced for different modes of heat transfer.
For conduction
the thermal resistance is
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23. Thermal Resistance Concept
❖ Rearranging the equation for heat conduction through a plane wall as
where
• R is the thermal resistance (conduction resistance ) of the wall against heat
conduction.
• Note that the thermal resistance of a medium depends on the geometry and
the thermal properties of the medium.
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24. ❖ Rearranging the equation for heat conduction through a long cylindrical and
spherical layer
where
conduction resistance of the cylinder layer
conduction resistance of the spherical layer
where
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25. ❖ Convection resistance of the surface: Thermal resistance of the surface against
heat convection.
• When the convection heat transfer coefficient is very large (h → ∞), the
convection resistance becomes zero and Ts ≈ T∞
• That is, the surface offers no resistance to convection, and thus it does not slow
down the heat transfer process.
• This situation is approached in practice at surfaces where boiling and
condensation occur.
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26. ❖ Radiation resistance of the surface: Thermal resistance of the surface against
radiation.
Radiation heat transfer coefficient
Ts and Tsurr must be in K
• The definition of the radiation heat transfer coefficient enables us to express
radiation conveniently in an analogous manner to convection in terms of a
temperature difference.
• hrad depends strongly on temperature while hconv usually does not
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27. • For a surface exposed to convection and radiation simultaneously,
• The convection and radiation resistances are parallel to each other and may
cause some complication in the thermal resistance network.
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28. Thermal Resistance Network Plane wall
• consider steady one-dimensional heat flow through a plane wall of thickness L, area A,
and thermal conductivity k that is exposed to convection on both sides to fluids at
temperatures T∞1 and T∞2 with heat transfer coefficients h1 and h2, respectively.
• Note that the temperature varies linearly in the wall, and asymptotically approaches
T∞1 and T∞2 in the fluids as we move away from the wall.
rearranged as
• For one-dimensional, steady-state conduction in a plane wall with no heat generation, the heat
flux is a constant, independent of x.
Assuming T∞1 > T∞2.
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29. Adding the numerators and denominators yields
• The rate of steady heat transfer between two surfaces is equal to the
temperature difference divided by the total thermal resistance between those
two surfaces.
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30. • Once Q is known, an unknown surface temperature Tj at any surface or interface j can
be determined from
• The thermal resistance network for heat transfer through a two-layer plane wall
subjected to convection on both sides.
Multilayer Plane Walls
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31. • It is sometimes convenient to express heat transfer through a medium in an
analogous manner to Newton’s law of cooling as
for a unit area, the overall heat transfer
coefficient is equal to the inverse of the total
thermal resistance
• Overall heat transfer coefficient is usually used in heat transfer calculations
associated with heat exchangers and through windows 31

32. • The thermal resistance concept can also be used to solve steady heat transfer
problems that involve parallel layers or combined series-parallel arrangements
(Composite walls).
• Although such problems are often two- or even three-dimensional, approximate
solutions can be obtained by assuming one-dimensional heat transfer and using
the thermal resistance network.
Generalized thermal resistance networks
• Two assumptions commonly used in solving complex multidimensional heat
transfer problems by treating them as one-dimensional (say, in the x-direction)
(1) Any plane wall normal to the x-axis is isothermal (i.e., to assume the
temperature to vary in the x-direction only) and
(2) Any plane parallel to the x-axis is adiabatic (i.e., to assume heat transfer to
occur in the x-direction only).
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33. Even though either approach give different
results, they can be used in practice to obtain
satisfactory results.
Case (a) it is presumed that surfaces normal to
the x-direction are isothermal,
Case (b) it is assumed that surfaces parallel to
the x-direction are adiabatic.
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34. The total heat transfer is the sum of the heat transfers through each layer, we have
Utilizing electrical analogy, we get
where
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35. The total rate of heat transfer through this composite system
where
and
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36. • Now consider steady one-dimensional heat flow through a cylindrical or spherical
layer that is exposed to convection on both sides to fluids at temperatures T1 and
T2 with heat transfer coefficients h1 and h2, respectively,
✓ The thermal resistance network in this case consists of one
conduction and two convection resistances in series,
✓ just like the one for the plane wall, and the rate of heat
transfer under steady conditions can be expressed as
where
for a cylindrical layer
for a spherical layer
Thermal Resistance Network Cylinders and Spheres
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37. Multilayered Cylinders and Spheres
• The total thermal resistance is simply the arithmetic sum of the individual thermal
resistances in the path of heat flow
• Steady heat transfer through multilayered
cylindrical or spherical shells can be handled
just like multilayered plane walls
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38. The Overall Heat Transfer Coefficient
The rate of heat transfer between the two fluids express as
In the analysis of heat exchangers, it is convenient to combine all the thermal resistances in the
path of heat flow from the hot fluid to the cold one into a single resistance R
• A heat exchanger typically involves two flowing fluids separated by a solid wall.
• Heat is first transferred from the hot fluid to the wall by convection, through the wall by
conduction, and from the wall to the cold fluid again by convection.
The thermal resistance network
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39. Fluid Mechanics
• In air conditioning and refrigeration the main application of fluid mechanics is in the modelling
of piping and duct systems.
• The Bernoulli equation which is widely used for relating pressure, velocity and elevation of a
fluid is significant in designing piping and ducts of thermal systems.
• For steady frictionless and incompressible flow, the Bernoulli equation is stated as follows:
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40. Bernoulli’s Equation For Real Fluid
• Bernoulli’s equation earlier derived was based on the assumption that fluid is non-viscous
and therefore frictionless.
• Practically, all fluids are real (and not ideal) and therefore are viscous as such there are
always some losses in fluid flows.
• These losses have, therefore, to be taken into consideration in the application of Bernoulli’s
equation which gets modified (between sections 1 and 2) for real fluids as follows:
hL = Loss of energy between sections 1and 2
hpump, is the useful head delivered to the fluid by the pump.
hturbine, e is the extracted head removed from the fluid by the turbine.
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41. • The total head loss in a piping system is determined from
Where
i represents each pipe section with
constant diameter
j represents each component that causes
a minor loss.
• If the entire piping system being analyzed has a constant diameter,
where V is the average flow velocity through the entire system (note that V = constant since D = constant).
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42. The Moody Chart
• The friction factor in fully developed turbulent pipe flow depends on the
✓ Reynolds number and
✓ the relative roughness 𝜀/D, which is the ratio of the mean height of
roughness of the pipe to the pipe diameter.
• The relation combined the available data for transition and turbulent flow in
smooth as well as rough pipes into the following implicit relation known as the
Colebrook equation:
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43. • Lewis F. Moody produced Moody chart which presents graphical plot
Colebrook’s equation, i.e. the Darcy friction factor for pipe flow as a function of
Re and 𝜀/D over a wide range.
• Although it is developed for circular pipes, it can also be used for noncircular
pipes by replacing the diameter with the hydraulic diameter.
The friction factor is minimum for
a smooth pipe and increases with
roughness.
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Unit and rating of Refrigeration system
• 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, TR is the term used to indicate the capacity of the refrigeration
and air conditioning system.
• One TR is the amount of heat that is required to be extracted from one ton of water
at 0°C in order to convert it into equivalent ice at 0°C in a day.

46. Desired Result
COP=
Required Input
,
L
R
net in
Q
COP
W
=
L
R
H L
Q
COP
Q Q
=
−
,
H
HP
net in
Q
COP
W
=
H
HP
H L
Q
COP
Q Q
=
−
• The index of performance of a refrigerator or heat pump is expressed in
terms of the coefficient of performance, COP, the ratio of desired result to
input.
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✓ A higher COP value indicates a more energy-efficient refrigerator because it can
extract more heat or provide more cooling effect with less work input.

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• The performance of air conditioners and heat pumps is often expressed in
terms of the energy efficiency ratio(EER) or seasonal energy efficiency
ratio(SEER).
✓ EER (Btu/Wh) is defined as the ratio of the rate of heat removal from the cooled space by the
cooling equipment to the rate of electricity consumption in steady operation.
It is a measure of the instantaneous energy efficiency
• A device that removes 1 kWh of heat from the cooled space for each kWh of electricity it
consumes (COP = 1) will have an EER of 3.412.
✓ SEER (Btu/Wh) is the ratio of the total amount of heat removed by an air conditioner or heat
pump during a normal cooling season (in Btu) to the total amount of electricity consumed (in
watt-hours, Wh),
it is a measure of seasonal performance of cooling equipment.

48. Energy label
• The EU energy labels for household fridges and freezers use, as of 1 March 2021, a scale
from A (most efficient) to G (least efficient). The labels provide information on the
product’s
✓ energy efficiency class
✓ energy consumption
✓ storage volume(s)
✓ whether or not it has a freezer compartment
✓ noise emissions
• Other factors may apply to the label.
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