UNIT – 3

REFRIGERATION AND AIR CONDITIONING

THERMODYNAMICS

Thermodynamics in physics is a branch that deals with heat, work and temperature, and their
relation to energy, radiation and physical properties of matter.

Thermodynamic Terms

Thermodynamic 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.

Surroundings

Everything external to the system is called the surroundings. The system is separated from
the surroundings by the system boundary. The boundary may be fixed or flexible.

System and surroundings together constitute the universe.

 Isolated System – An isolated system cannot exchange energy and mass with its
surroundings.

 Closed System – Across the boundary of the closed system, the transfer of energy
takes place but the transfer of mass doesn’t take place.

 Open System – In an open system, the mass and energy both may be transferred
between the system and surroundings.

LAWS OF THERMODYNAMICS
Zeroth Law of Thermodynamics

The Zeroth law of thermodynamics states that if two bodies are individually in equilibrium
with a separate third body, then the first two bodies are also in thermal equilibrium with each
other.
First Law of Thermodynamics

First law of thermodynamics, also known as the law of conservation of energy, states that
energy can neither be created nor destroyed, but it can be changed from one form to another.

ΔU = q + W

Second Law of Thermodynamics

Any spontaneously occurring process will always lead to an escalation in the entropy (S) of
the universe.

ΔSuniv > 0

Third Law of Thermodynamics

Third law of thermodynamics states that the entropy of a system approaches a constant value
as the temperature approaches absolute zero.

REFRIGERATOR, HEAT PUMP AND HEAT ENGINE

Refrigerators, heat pumps, and heat engines are all devices that involve the transfer of heat,
but they serve distinct purposes and operate on different principles.

Heat Engine

It is defined as thermodynamic device used for continuous production of work from heat
when operating in a cyclic process is called heat engine.

It receives heat from a high-temperature source at temperature T1 (furnace, nuclear reactor,

solar energy etc.) It converts the part of this heat to work (mostly in the form of a rotating
shaft). It rejects the remaining waste heat to a low-temperature sink (the atmosphere, rivers
etc.).

Thermal Efficiency
It is defined as the ratio of the desired net work output to the required heat input is
called thermal efficiency.
Refrigerator

It is defined as the mechanical device that used for the transfer of heat from a low-
temperature medium to a high-temperature medium is called refrigerator.

The objective of a refrigerator is to maintain the refrigerated space at a low temperature by

absorbing heat from it and reject to higher-temperature medium.

Coefficient of Performance of Refrigerator

The COP of a refrigerator can be expressed as the ratio of refrigerating effect to the work
input.

Heat Pump

It is defined as the mechanical device that transfers heat from a low-temperature medium to a
high- temperature is called heat pump.

The objective of heat pump is to maintain a heated space at a high temperature. This is
accomplished by absorbing heat from a low-temperature source and reject to higher
temperature source.

Performance of Heat Pump

The COP of a heat pump can be expressed as the ratio of heating effect to the work input.
Relation between cop of heat pump and refrigerator

Difference between Coefficient of Refrigerator, Heat Pump and Heat Engine

Property Refrigerator Heat Pump Heat Engine

Primary Function Removes heat from a Transfers heat Converts heat

cold space from a cold source into mechanical
to a warmer space work

Direction of Heat From low to high From low to high From high to low
Transfer temperature temperature temperature

Energy Source Electrical energy Electrical energy Thermal energy

Heat Absorption Inside the Inside the building In the

Location refrigerator or space environment

Work Output No mechanical work No mechanical Produces

work mechanical work

Efficiency Coefficient of Coefficient of Efficiency is

Performance (C.O.P) Performance measured by the
measures efficiency (C.O.P) measures work done/output
efficiency

Example Household Air conditioner Car engine, steam

refrigerator engine, or gas
turbine

Environmental Typically consumes Can be energy- Generally emits

Impact energy to remove efficient if used waste heat and
heat for heating has lower
efficiency
REFRIGERATION
Refrigeration is defined as the process of achieving and maintaining a temperature below
ambient, with the aim of cooling a product or space to the required temperature.S

Unit of Refrigeration

The standard unit of refrigeration is ton refrigeration or simply ton denoted by TR. It is
equivalent to the rate of heat transfer needed to produce (or that results in the freezing) of
1 short ton (2000 lbs; 907kg) of pure ice at 32°F/0°C from water at 32°F/0°C in one day, i.e.,
24 hours.

Latent heat of ice (i.e., heat of fusion) = 333.55 kJ/kg

One short ton = 2000 lb = 907 kg

Heat extracted in 24 hours = Q = m x L = 907 x 333.55 = 302,529.85 KJ

Heat extracted / Second = 302,529.85 / (24 x 60 x 60)

1 ton = 3.517 kJ/s = 3.517 kW = 4.713 HP

Methods of Refrigeration

1. Dry Ice Refrigeration:

 Solid Carbon dioxide (CO2) is called dry ice and it has a peculiar characteristics that it
changes from solid state to vapour state without getting converted into intermediate
liquid state (sublimation). Due to the change of state, it absorbs heat equivalent to
enthalpy of vaporization. The sublimation temperature of dry ice at atmospheric
pressure is -78°C.

 Dry ice is used to preserve foodstuff during transportation. Now a days it is

universally used to preserve food in air-transportation. Dry ice slabs are usually
packed in frozen food cartons on either side or on the top of the food packages, dry
ice absorbs heat from the foodstuff and preserve them in the frozen state.

2. Evaporative Refrigeration:

 Evaporative refrigeration makes use of the principle that when a liquid evaporates, it
absorbs heat equivalent to its latent heat of vaporization from the surroundings,
thereby cooling it.

 Cooling of water in the earthen pitcher – the water coming out of the pores of the
pitcher evaporates when it comes in contact with dry air, thereby cooling the water in
the pitcher.

 When a drop of spirit is put on the palm of hand, it evaporates producing cooling
effect.

 Evaporation cooling may be defined as the adiabatic transfer of heat from air to water.

 Evaporation cooling may be defined as the adiabatic transfer of heat from air to water.
  It is utilized in cooling towers where condenser water is cooled by spraying it from
top and forcing a current of air from below. Another application is evaporative type of
condensers. Yet another application is in desert coolers or room coolers. Dry air is
passed through wet pads. Due to evaporation, air gets cooled. The principle is also
utilized in making artificial snow.

3. Thermo-electric Refrigeration:

 Thermo-electric refrigeration type employs Peltier’s effect. when two dissimilar

metals are joined on either ends and a direct current is circulated through it, one joint
gets cooled while the other gets heated.

 Antimony (Sb) and Bismuth (Bi) are commonly used metals as they are electro-
chemically opposite in their polarity. If the cold end is placed in a closed space, it gets
cooled. If the magnitude of current is increased and a series of such strips are placed
together a good cooling effect can be produced.

VAPOR COMPRESSION REFRIGERATION CYCLE

Vapour Compression Refrigeration Cycle is the most widely used refrigeration system. In
this system, the working fluid is a vapor.

Components:-

(A) Compressor: It is a mechanical device, similar to a heat pump, used to increase the
gaseous refrigerants pressure by decreasing its volume.

(B) Condenser: These are generally heat exchangers in the system that has radiator fins to
remove heat. A condenser is used to change the phase of refrigerant from vapour to liquid or
to reduce heat.

(C) Expansion Valve: The condensed refrigerant is pushed to an expansion valve to control
the rate of flow. Due to constant enthalpy, the pressure of the refrigerant reduces. This low
pressure is essential for an evaporator in the next step.

(D) Evaporator: This is where the refrigerant absorbs the heat from the system and removes
it. Usually, a fan or a blower circulates the warm air absorbed from the system to the tubes
carrying the cold refrigerant. This process decreases the system temperature.

Working
The vapor compression refrigeration cycle consists of four processes as discussed below:

Isentropic Compression in the Compressor (1 – 2)

In an ideal vapor compression cycle, the refrigerant enters the compressor at the state 1, as dry &
saturated vapor. This dry & saturated vapor at state 1 is compressed isentropically to superheated
vapor (state 2) as shown in Fig. by line 1 – 2. During the process, its temperature and pressure
are increased to state 2.

The work done during the isentropic compression per kg of refrigerant is given by

Constant Pressure Heat Rejection in Condenser (2 – 3)

The superheated vapor refrigerant is passed through the condenser where it rejects
its heat to the surroundings and gets condensed completely at constant pressure &
temperature, which is shown by line 2 – 3 in Fig.
Heat rejected per kg of refrigerant in the condenser is given by,

Isenthalpic Expansion in the Expansion / Throttle Valve (3 – 4)

The saturated liquid refrigerant is expanded by the throttling process through the
expansion valve orcapillary tube to low pressure and low temperature, which is shown by
curve 3 – 4 in Fig .

Constant Pressure Heat Absorption in Evaporator (4 – 1)

During evaporation, the liquid-vapor refrigerant absorbs latent heat of vaporization from
the cooling medium and changed into the saturated vapor refrigerant, which is shown by line 4
– 1 in Fig.
Heat absorbed per kg of refrigerant in evaporator or refrigerating effect is given by,

COP of the Simple VCR Cycle

COP of the simple vapor compression refrigeration cycle is given as

DOMESTIC REFRIGERATOR
The household refrigerator works on vapour compression refrigeration cycle. The common
type of domestic refrigerator has a cabinet shaped with compressor, the condenser and
receiver fitted in their basement. The expansion valve and evaporator coils are exposed in the
storage cabinet with the piping’s carrying liquid refrigerant passing through the body.
Generally, methylene chloride, Freon-12, and Freon-11 are used as the refrigerants. The
refrigerant is circulated through the system and undergoes a number of changes in its state
while passing through various parts of the system.
COUNSTRUCTION OF DOMESTIC REFRIGERATOR:

A domestic refrigerator consists of 5 essential parts.

1. COMPRESSOR: The low pressure and temp. Vapor refrigerant from evaporator is drawn
into the compressor through the inlet or suction valve, where it is compressed to a high
pressure and temp..This high pressure and temp. vapour refrigerant is discharged into the
condenser through the delivery valve.

2. CONDENSOR: The condenser or cooler consists of coils or pipes in which the high
pressure and temp. vapor refrigerant is cooled and condensed. The refrigerant while passing
through the condenser, gives up its latent heat to the surrounding condensing medium which
is normally air or water.

3. RECEIVER: The condensed liquid refrigerant from the condenser is stored in a vessel is
known as receiver from where it is supplied to the evaporator through the expansion valve.
4. EXPANSION VALVE: It is also called throttle valve or refrigerant control valve. The
function of the expansion valve is to allow the liquid refrigerant under high pressure and low
temp. to pass at a controlled rate after reducing its pressure and temp.

5. EVAPORTAOR: An evaporator consists of coils of pipe in which the liquid vapour

refrigerant at low pressure and temp. is evaporated and changed into vapor refrigerant at low
pressure and temp. During evaporating the liquid vapor refrigerant absorbs its latent heat of
vaporization from the medium which is used to be cooled.

WORKING PRINCIPLE:

 The low pressure vapor in dry state drawn from the evaporator during the suction
stroke of the compressor. During compression, the pressure and temp.is increased.
 When the high pressure refrigerant vapor enters the condenser, heat flows from
condenser to cooling medium, thus allowing the vaporized refrigerant to return to the
liquid state.
 After condensation, the liquid refrigerant is stored in the liquid receiver.
 Then it is passed through the expansion valve, where the pressure is reduced
sufficiently to allow the vaporization of the liquid at a low temp.
 The low pressure refrigerant vapor after expansion enters the evaporator where heat is
absorbed by it and the cycle is completed.

PSYCHROMETRY
The psychrometric is that branch of engineering science which deals with the study of moist
air i.e., dry air mixed with water vapour or humidity. It also includes the study of behavior of
dry air and water vapour mixture under various sets of conditions.

PSYCHOMETRIC TERMS

1. Dry air. The pure dry air is a mixture of a number of gases such as nitrogen, oxygen,
carbon dioxide, hydrogen, argon, neon, helium etc. But the nitrogen (78.03%) and
oxygen (20.99%) have the major portion of the combination.
2. Moist air. It is a mixture of dry air and water vapour. The amount of water vapour
present. in the air depends upon the absolute pressure and temperature of the mixture.
3. Saturated air. It is mixture of dry air and water vapour, when the air has diffused the
maximum amount of water vapour into it. The water vapours, usually, occur in the
form of superheated steam as an invisible gas. However, when the saturated air is
cooled, the water vapour in the air starts condensing, and the same may be visible in
the form of moist, fog or condensation on cold surfaces.
4. Degree of saturation. he degree of saturation in Psychrometry is defined as the ratio
of specific humidity of moist air and specific humidity of saturated air at the same
temperature. The degree of saturation is represented as μ. The degree of saturation is a
measure of moisture absorbing capacity of air. It gives the relationship between
normal specific humidity(ω) and maximum possible specific humidity (ω s).
μ = ω/ωs
 5. Humidity. It is the mass of water vapour present in 1 kg of dry air, and is generally
expressed in terms of gram per kg of dry air (g / kg of dry air). It is also called
specific humidity or humidity ratio.

ω = Mass of water vapour/Mass of dry air

6. Absolute humidity. It is the mass of water vapour present in 1 m3 of dry air, and is
generally expressed in terms of gram per cubic metre of dry air (g /m3 of dry air). It is
also expressed in terms of grains per cubic metre of dry air.
7. Relative humidity. It is the ratio of actual mass of water vapour in a given_volume of
moist air to the mass of water vapour in the same volume of saturated air at the same
temperature and pressure. It is briefly written as RH.

ϕ = Mass of water vapour in moist air/Mass of water vapour in saturated air

8. Dry bulb temperature. It is the temperature of air recorded by a thermometer, when

it is not affected by the moisture present in the air. The dry bulb temperature (briefly
written as DBT) is generally denoted by td or tdb.
9. Wet bulb temperature. It is the temperature of air recorded by a thermometer, when
its bulb is surrounded by a wet cloth exposed to the air. Such a thermometer is called
wet bulb thermometer. The wet bulb temperature (briefly written as WBT) is
generally denoted by tw or twb.
10. Wet bulb depression. It is the difference between dry bulb temperature and wet bulb
temperature at any point. The wet bulb depression indicates relative humidity of the
air.
11. Dew point temperature. It is the temperature of air recorded by a thermometer, when
the moisture (water vapour) present in it begins to condense.
12. Dew point depression. It is the difference between the dry bulb temperature and dew
point temperature of air.

DALTON'S LAW OF PARTIAL PRESSURES

It states, The total pressure exerted by the mixture of air and water vapour is equal to the sum
of the pressures, which each constituent Fould exert, if it occupied the same space by itself.
In other words, the total pressure exerted by air and water vapour mixture is equal to the
barometric pressure. Mathematically, barometric pressure of the mixture,

Pb = Pa+ Pv

where

Pa = Partial pressure of dry air, and

Pv= Partial pressure of water vapour.

PSYCHROMETRIC CHART
It is a graphical representation of the various thermodynamic properties of moist air. The
psychrometric chart is very useful for finding out the properties of air (which are required in
the field of air conditioning) and eliminate lot of calculations. There is a slight variation in
the charts prepared by different air-conditioning manufactures but basically they are all alike.
The psychrometric chart is normally drawn for standard atmospheric pressure of 760 mm of
Hg (or 1.01325 bar).

Though the psychrometric chart has a number of details, yet the following lines are important
from the subject point of view:

1. Dry bulb temperature lines. The dry bulb temperature lines are vertical i.e. parallel to the
ordinate and uniformly spaced.

2. Specific humidity or moisture content lines. The specific humidity (moisture content)
lines are horizontal i.e. parallel to the abscissa and are also uniformly spaced
3. Dew point temperature lines. The dew point temperature lines are horizontal i.e. parallel
to the abscissa and non-uniformly spaced.

4. Wet bulb temperature lines. The wet bulb temperature lines are inclined straight lines
and non-uniformly spaced.

5. Enthalpy (total heat) lines. The enthalpy (or total heat) lines are inclined straight lines
and uniformly spaced.

6. Specific volume lines. The specific volume lines are obliquely inclined straight lines and
uniformly spaced.

7. Vapour pressure lines. The vapour pressure lines are horizontal and uniformly spaced.
Generally, the vapour pressure lines are not drawn in the main chart. But a scale showing
vapour pressure in mm of Hg is given on the extreme left side of the chart.

8. Relative humidity lines. The relative humidity lines are curved lines and follow the
saturation curve. Generally, these lines are drawn with values 10%, 20%, 30% etc. and up to
100%. The saturation curve represents 100% relative humidity. The values of relative
humidity lines are generally given along the lines themselves.
PSYCHROMETRIC PROCESSES
The various psychrometric processes involved in air conditioning to vary the psychrometric
properties of air according to the requirement are as follows:

1 Sensible Heating The heating of air, without any-change in its specific humidity, is known
as sensible heating.

2 Sensible Cooling The cooling of air without any change in its specific humidity, is known
as sensible cooling.

3. Humidification and Dehumidification The addition of moisture to the air, without

change in its dry bulb temperature, is known as humidification. Similarly, removal of
moisture from the air, without change in its dry bulb temperature, is known as
dehumidification.
4. Cooling and Dehumidification This process is generally used in summer air conditioning
to cool and dehumidify the air. The air is passed over a cooling coil or through a cold water
spray. In this process, the dry bulb temperature as well as the specific humidity of air
decreases. The final relative humidity of the air is generally higher than that of the entering
air.

5. Heating and Humidification This process is generally used in winter air conditioning to
warm and humidify the air. It is the reverse process of cooling and -- dehumidification. When
air is passed through a humidifier having spray water temperature higher than the dry bulb
temperature of the entering air, the unsaturated air will reach the condition of saturation and
thus the air becomes hot.

AIR CONDITIONING
The air conditioning is that branch of engineering science which deals with the study of
conditioning of air i.e., supplying and maintaining desirable internal atmosphere conditions
for human comfort, irrespective of external conditions. This subject, in its broad sense, also
deals with the conditioning of air for industrial purposes, food processing storage of food and
other materials.

Factors affecting comfort Air Conditioning - Human Comfort

The four important factors for comfort air conditioning are discussed as below:
1. Temperature of air: In air conditioning, the control of temperature means the
maintenance of any desired temperature within an enclosed space even though the
temperature of the outside air is above or below the desired room temperature. This is
accomplished either by the addition or removal of heat from the enclosed space as and when
demanded. It may be noted that a human being feels comfortable when the air is at 21°C with
56% relative humidity.

2. Humidity of air: The control of humidity of air means the decreasing or increasing of
moisture contents of air during summer or winter respectively in order to produce
comfortable and healthy conditions. The control of humidity is not only necessary for human
comfort but it also increases the efficiency of the workers. In general, for summer air
conditioning, the relative humidity should not be less than 60% whereas for winter air
conditioning it should not be more than 40%.

3. Purity of air: It is an important factor for the comfort of a human body. It has been
noticed that people do not feel comfortable when breathing contaminated air, even if it is
within acceptable temperature and humidity ranges. It is thus obvious that proper filtration,
cleaning and purification of air is essential to keep it free from dust and other impurities.

4. Motion of air: The motion or circulation of air is another important factor which should be
controlled, in order to keep constant temperature throughout the conditioned space. It is,
therefore, necessary that there should be equi-distribution of air throughout the space to be air
conditioned.

Classification of air conditioning system

Air conditioning systems are classified as

1) Classification as to major function-

i) Comfort air-conditioning - air conditioning in hotels, homes, offices etc.

ii) Commercial air-conditioning- air conditioning for malls, super market etc

iiI) Industrial air-conditioning – air conditioning for processing, laboratories etc

2) Classification as to season of the year

i) Summer air-conditioning - These system control all the four atmospheric conditions for
summer comfort.

ii) Winter air-conditioning – This system is designed for comfort in winter.

iii) Year round air-conditioning – These system consists of heating and cooling equipments
with automatic control to produce comfortable condition throughout the year

3) Classification as to Equipment Arrangement

i) Unitary system

ii) Central system

 Air Conditioner
Windows air conditioners are one of the most widely used types of air conditioners because
they are the simplest form of the air conditioning systems. Window air conditioner comprises
of the rigid base on which all the parts of the window air conditioner are assembled. The base
is assembled inside the casing which is fitted into the wall or the window of the room in
which the air conditioner is fitted.

Components:

The refrigeration systems consists of :

a) Compressor - The compressor used in window air conditioner is of hermetic type. The
refrigerant vapour is compressed to high pressure and temperature in the condenser.

b) Air-cooled condenser - It is used to cool the high pressure high temperature refrigerant
vapour. It is continuous coil made of copper tubing. A propeller type fan is used to draw the
necessary air from atmosphere to cool the refrigerant.

c) Capillary tube - It is an expansion device. It is used to reduce the pressure and

temperature of the refrigerant.

d) Evaporator - It is cooling coil made of copper. A centrifugal blower is installed behind

the coil to deliver cool air from the evaporator to the room. The blower also sucks warm air
from the room and sends it to the evaporator through a Filter.

Working –

 The air moving inside the room and in the front part of the air conditioner where the
cooling coil is located is considered to be the room air. When the window AC is
started the blower starts immediately and after a few seconds the compressor also
starts. The evaporator coil or the cooling gets cooled as soon as the compressor is
started.
 The blower behind the cooling coil starts sucking the room air, which is at high
temperature and also carries the dirt and dust particles. On its path towards the
blower, the room air first passes through the filter where the dirt and dust particles
from it get removed.
 The air then passes over the cooling coil where two processes occur. Firstly, since the
temperature of the cooling coil is much lesser than the room air, the refrigerant inside
the cooling coil absorbs the heat from the air. Due to this the temperature of the room
air becomes very low, that is the air becomes chilled.
 Secondly, due to reduction in the temperature of the air, some dew is formed on the
surface of the cooling coil. This is because the temperature of the cooling coil is lower
than the dew point temperature of the air. Thus the moisture from the air is removed
so the relative humidity of the air reduces. Thus, when the room air passes over the
cooling coil its temperature and relative humidity reduces.
 This air at low temperature and low humidity is sucked by the blower and it blows it
at high pressure. The chilled air then passes through small duct inside the air
conditioner and it is then thrown outside the air conditioner through the opening in the
 front panel or the grill. This chilled air then enters the room and chills the room
maintaining low temperature and low humidity inside the room.

Refrigerants

The refrigerant is a heat-carrying medium which during their cycle (i.e comparison,
condensation, evaporation) in the refrigeration system absorbs heat from allow temperature
system and discards the eat so absorbed to higher temperature system.

Desirable properties of an Ideal Refrigerant :

A refrigerant is said to be ideal if it has all of the following properties:

1. Low boiling and freezing point,

2. High critical pressure and temperature,
3. High latent heat of vaporization,
4. High thermal conductivity,
5. Non-corrosive to metal,
6. Non-flammable and non-explosive,
7. Non-toxic
8. Low cost
9. Easily and regularly available,
10. Easy to liquefy at moderate pressure and temperature,
11. High coefficient of performance, end
12. Ozone friendly.

Classification of refrigerant:

Fluids suitable for refrigeration purposes can be classified into primary and secondary
refrigerants. Primary refrigerants are those fluids, which are used directly as working fluids,
for example in vapor compression and vapor absorption refrigeration systems.

1. Primary Refrigerant:-

The refrigerant which takes part in the refrigeration cycle is known as the primary refrigerant.
The refrigerants which directly take part in the refrigeration system are called primary
refrigerant. Primary refrigerants are used in domestic refrigerator and Air conditioning
system etc. Primary refrigerants are R-11, R-12, R-21, R-143a, etc.

2. Secondary Refrigerant

The refrigerants which are first cooled by primary refrigerant and then used for cooling
purpose are called as secondary refrigerant.. Secondary refrigerants allow the amounts of
environmentally harmful primary refrigerants to be minimized and contained in a restricted
area.
Examples of secondary refrigerants include water, air, hydrocarbons, ammonia, and carbon
dioxide, which are more environmentally benign than traditional refrigerants such as HCFCs.
They are safer and generally suitable for refrigeration systems.