REFRIGERATION

Define refrigeration and refrigerant. What are the applications of refrigeration

in different fields?
➢ Refrigeration is a process of producing and maintaining lower temperature
compared to surrounding temperature.
➢ In order to maintain temperature continuously, the refrigeration system must
run on a cycle.
➢ Refrigerant is a substance used for producing lower temperatures.
➢ Examples are NH3, water, air, R-11, R-12, R-134 etc.
➢ Refrigerants absorb heat at a low temperature and reject heat at a higher
temperature .

Applications of Refrigeration
1. Domestic refrigeration
2. Chemical refrigeration
3. Industrial refrigeration
4. Transport refrigeration
5. Air-conditioning

Give the name of any four environment friendly refrigerants.

These include ethane, propane, isobutane, difluoromethane (HFC-32), and a blend
of four alkanes called R-441A (ethane, propane, butane, and isobutane).
These refrigerants are already being used in some places around the world

Explain the term 1 tonne of refrigeration. What are the methods of

refrigeration?
➢ It is the amount of heat that is to be removed from one tonne of water at zero
(0oC) in order to convert it into ice at 0 oC in one day (24 hours).
➢ Tonnes of refrigeration represents heat transfer rate.

1 T.R. = 3.5 kJ/s = 3.5 kW = 210 kJ/min

Methods for Refrigeration

1. Natural Refrigeration Methods
2. Natural ice for refrigeration
3. Evaporative Cooling
 4. Artificial Refrigeration Methods
5. Gas refrigeration system
6. Vapour Compression refrigeration system
7. Vapour absorption system

Define refrigerator and heat pump.

Clausius Statement: “It is impossible to construct a device which operates on a

cycle and transfers heat from low temperature body to high temperature body
without any external work.

TH : High Temperature Reservoir

TL : Low Temperature Reservoir

Refrigerator
➢ Refrigerator works on the Clausius statement.
➢ It absorbs the heat from the low temperature medium and rejects heat into the
high temperature medium by consuming external work.
➢ Refrigerator used to maintain a low temperature as compared to the
surroundings.
Heat Pump
➢ Heat Pump works on the Clausius statement.
➢ It absorbs the heat from the low temperature medium and rejects heat into the
high temperature medium by consuming external work.
➢ Heat pump used to maintain High temperature as compared to surrounding.

TH : High Temperature Reservoir

TL : Low Temperature Reservoir

Coefficient of Performance
➢ The efficiency of a refrigerator and heat pump is expressed in terms of the
coefficient of performance (COP).
➢ The value of COP can be greater than unity.
➢ Thermal efficiency can never be greater than 1.
➢ The COP represents the running cost of the refrigerator and heat pump.
➢ Higher the value of COP lower the running cost.
Relationships between COP of Refrigerator and COP of Heat Pump

Good practices to minimize the amount of energy consumed by refrigerator

1. Open the refrigerator door the fewest times possible for the shortest duration
possible.
2. Cool the hot foods to room temperature first before putting them into the
refrigerator.
3. Check the door gasket for leaks
4. Avoid unnecessarily low temperature settings.
5. Avoid excessive ice build-up on the interior surfaces of the evaporator.
Methods of Refrigeration:
1. Ice Refrigeration
2. Dry Ice Refrigeration
3. Evaporative refrigeration
4. Dissolution of salts in water

Methods of Refrigeration
1) Ice Refrigeration: In this method the ordinary ice is used for keeping the space
at temperature below the surrounding temperature. The temperature of ice is
considered to be 0 degree Celsius hence it can be used to maintain the
temperatures of about 5 to 10 degree Celsius. To use the ice for refrigerating effect
a closed and insulated chamber is required. On one side of the chamber ice is kept
while on the other side there is a space which is to be cooled where some material
to be cooled can be placed. If the temperature below 0 degree Celsius is required,
then the mixture of ice and salt is used. This method of cooling is still being used for
cooling the cold drinks, keeping the water chilled in thermos, etc.

2) Dry ice refrigeration: Dry ice is the solid carbon dioxide having the temperature
of -78 degree Celsius. Dry ice converts directly from solid state to gaseous; this
process is called sublimation. Dry ice can be pressed into various sizes and shapes
as blocks or slabs. Dry ice is usually packed in the frozen food cartons along with
the food that has to be kept frozen for long intervals of time. When the dry ice gets
converted into vapor state it keeps the food frozen. The process of dry ice
refrigeration is now-a-days being used for freezing the food in aircraft transportation.

3) Evaporative refrigeration
When a liquid evaporates, it absorbs heat from the surroundings equivalent to its
latent heat of vaporisation, and that results in lowering the temperature of
surroundings. For example, we feel cooling effect when there is evaporation of a
drop of spirit placed on the palm of hand. Likewise, the evaporation of moisture from
the skin of a human body helps to keep it cool. There is a common practice to cool
the water for drinking purposes by keeping it in the porous earthen pots. The water
evaporates through the pores and that produces cooling effect. The army people
keep small water containers made of metal and covered with water soaked namada;
the walls of the metallic container get cooled and that cools the water kept inside it.
In the refrigeration literature, there is mention of an experiment where a pump was
used to create partial vacuum over a container of ethyl ether. The liquid then boiled
by absorbing heat from the surrounding air. The cooling effect so created even
produced a small amount of ice.
The principle of evaporative refrigeration is employed in desert (room) coolers. The
dry atmospheric air is made to pass through water soaked packings. When this
water evaporates, it takes heat from the air causing it to cool.

With reference of Fig. 4.3, a volatile liquid (liquid nitrogen, liquid carbon dixoide)
contained in a flask evaporates and gets converted into gas. For evaporation, it
absorbs heat from the chamber and cooling effect is produced. The chamber is
insulated to restrict the infiltration of heat from outside. The liquid N₂ and CO2 are
non-toxic and as such the liquid gas refrigeration finds application for keeping the
perishable food articles cool when being transported.

4) Dissolution of salts in water

When certain salts are dissolved in water, they absorb heat and lower the
temperature of water and create a sort of refrigeration bath for cooling substance.
Sodium chloride lowers the water temperature upto - 20°C while calcium chloride
upto - 50°C. The salt can be recovered by evaporation of water from the solution.
The method of producing cooling effect by dissolution of salt in water could not
become feasible for commercial purposes because,
(i) the refrigeration effect produced is quite small
(ii) the process of regaining salt is cumbersome.
 DOMESTIC REFRIGERATOR
Explain construction and working of domestic refrigerators.
Refrigerator is a cyclic device which is used to maintain lower temperature as
compared to surrounding temperature.

1. COMPRESSOR:
➢ It is a mechanical device which transfers mechanical energy to working fluids
i.e. refrigerant which is coming from an evaporator.
➢ Compressor raises the pressure and temperature of the refrigerant.

2. CONDENSER
➢ It is a type of heat exchanger.
➢ The refrigerant enters into the condenser from the compressor.
➢ Condenser rejects the heat from working fluid (refrigerant) by means of
cooling coils made up of copper into the atmosphere.
➢ Due to heat rejection from refrigerant, it converts from gaseous state to liquid
state.
➢ After condensing refrigerant goes into the expansion devices.

3. THROTTLING/EXPANSION DEVICES
➢ In an expansion valve the pressure and temperature decreases which comes
from the condenser.
➢ It also regulates the flow of refrigerant into the evaporator and maintains the
flow rate equal to the rate of evaporation in the evaporator.
➢ We can regulate and control the temperature of the refrigerator using
expansion devices by varying the opening as per our requirements.

4. EVAPORATOR
➢ Refrigerant comes from a throttling device entering into the evaporator at very
low temp and pressure.
➢ In the evaporator refrigerant goes through cooling coils and heat is absorbed
by the refrigerant.
➢ Due to this temperature of the refrigerant increases and liquid refrigerant
expands and converts into vapors after that refrigerant goes to the
compressor.
➢ Evaporator works as a heat exchanger between storage space and cooling
coils.
➢ This cycle repeats continuously.
 Heating Effect obtained
on the surrounding
P2, T2 High Pressure,
P2, T2 High Pressure, High Temperature
High Temperature Vapour Refigerant
Liquid Refigerant

P1, T1 Low Pressure,

Low Temperature
P4, T4 Low Pressure, Vapour Refigerant
Low Temperature
Liquid Refigerant

Cooling Effect obtained

on the surrounding
1-2 Isentropic Process ( Compression )
2-3 Constant Pressure Heat Rejection
3-4 Throttling Isenthelpic

4-1 Constant Pressure Heat Absorption

 AIR-CONDITIONING

Define the term ‘air-conditioning’

What are the different applications of air- conditioning.?
Air-Conditioning is a process of controlling air temperature, humidity, ventilation,
filtration and air circulation in a space (Building or Vehicle).

Applications of air- conditioning:

1. Residential and office buildings
2. Hospitals and cinema halls
3. Libraries, museums, computer canters
4. Transport vehicles : Car, bus, aircraft etc
5. Food and process industries
6. Production shop laboratories.

(i) Dry air : It is a mixture of nitrogen, oxygen, and small amounts of some other
gasses.

(ii) Atmospheric air : Air in the atmosphere normally contains some water vapor (or
moisture), number of pollutants and is referred as atmospheric air.

(iii) Saturated air : Saturated air is air that holds water vapor at its highest level i.e.
just about to condense.

What is Psychrometry?
➢ Moist air is the mixture of water vapour and dry air.
➢ The properties of moist air are called Psychrometric properties.
➢ The science in which we deal with the Psychrometric properties is known as
psychrometry.

Psychrometric Properties/ Properties of Moist Air

In psychrometry, a psychrometer comprises a dry bulb and a wet bulb thermometer.

1. Dry Bulb Temperature (Tdb or T)

The dry bulb thermometer has a bare bulb which is directly exposed to air and
measures the actual temperature.
2. Wet Bulb Temperature (Twb)
➢ The bulb of the wet bulb thermometer is covered by a wick thoroughly wetted
by water.
➢ The temperature which is measured by the wet wick covered bulb is known as
wet bulb temperature.
➢ The air in the atmosphere contains moisture (water vapour).
➢ If we reduce the temperature of the air, moisture condenses.

3. Dew point Temperature (Tdp)

The temperature at which the first drop of dew is formed or condensation begins
when the air is cooled at constant pressure is known as dew point temperature and
denoted by Tdp.

4. Specific humidity (𝜔):

It is also known as absolute humidity or humidity ratio and denoted by 𝜔.
Specific humidity can be defined as the mass of water vapor present in a unit mass
of dry air.

Unit: Kg of water vapour/ Kg of dry air

5. Relative Humidity(ϕ): Relative humidity (RH) is a measure of how much water

vapor is in a water-air mixture compared to the maximum amount possible.

𝒂𝒄𝒕𝒖𝒂𝒍 𝒎𝒂𝒔𝒔 𝒐𝒇 𝒘𝒂𝒕𝒆𝒓 𝒗𝒂𝒑𝒐𝒖𝒓 𝒊𝒏 𝒂 𝒈𝒊𝒗𝒆𝒏 𝒗𝒐𝒍𝒖𝒎𝒆

ϕ= 𝒎𝒂𝒙𝒊𝒎𝒖𝒎 𝒎𝒂𝒔𝒔 𝒐𝒇 𝒘𝒂𝒕𝒆𝒓 𝒗𝒂𝒑𝒐𝒖𝒓 𝒊𝒏 𝒂 𝒈𝒊𝒗𝒆𝒏 𝒗𝒐𝒍𝒖𝒎𝒆
RH is a ratio of the humidity ratio of a particular water-air mixture compared to the
saturation humidity ratio at a given temperature.
6. Wet Bulb Depression (WBD)

WBD=Dry bulb temperature - Wet bulb temperature

7. Degree of Saturation (µ)

It is define as a ratio of actual specific humidity to the saturated specific humidity
𝑎𝑐𝑡𝑢𝑎𝑙 𝑠𝑝𝑒𝑐𝑖𝑓𝑖𝑐 ℎ𝑢𝑚𝑖𝑑𝑖𝑡𝑦
µ = 𝑠𝑎𝑡𝑢𝑟𝑎𝑡𝑒𝑑 𝑠𝑝𝑒𝑐𝑖𝑓𝑖𝑐 ℎ𝑢𝑚𝑖𝑑𝑖𝑡𝑦

8. Enthalpy of Moist Air

Explain the human comfort. What are the conditions for comfort air
conditioning?
Human Comfort refers to the control of temperature and humidity of air and its
circulation.
So that the resulting environment becomes human friendly.
General human comfort conditions are to maintained in the range of
➢ Temperatures 22 °C to 27 °C
➢ Relative humidity 40% to 60%
➢ Air velocity 5 m/min to 8 m/min

With the help of a neat sketch describing the working of window type
air-conditioners.
➢ Air-Conditioning is a process of controlling air temperature, humidity, quality
and ventilation in a space (Building or Vehicle).
➢ Air conditioning can be used in both domestic and commercial environments.
➢ This process is most commonly used to achieve a more comfortable interior
environment, typically for humans and other animals.
➢ Window air conditioner is sometimes referred to as a room air conditioner.
➢ It is the simplest form of an air conditioning system and is mounted on
windows or walls.
➢ It is a single unit that is assembled in a casing where all the components are
located.
 Window Air Conditioner

Compressor
➢ The refrigerant enters the compressor at low temperature and pressure in a
gaseous state.
➢ In the compressor temperature and pressure of refrigerant increases.
➢ The refrigerant leaves the compressor and enters the condenser.
➢ Since this process requires work, an electric motor may be used.

Condenser
➢ It is a kind of heat exchanger in which refrigerant of high pressure and
temperature enters which comes from the compressor. .
➢ The function of the condenser in a refrigeration system is to transfer heat from
the refrigerant to another medium, such as air.
➢ By rejecting heat, the gaseous refrigerant condenses to liquid inside the
condenser.

Throttling/Expansion valve
➢ High pressure refrigerant from the condenser enters the throttling device, the
pressure and temperature of the refrigerant drops down suddenly.
➢ Throttling valve also controls the amount of the refrigerant flowing through it.

Evaporator
➢ It is a kind of heat exchanger in which refrigerant of low pressure and
temperature enters which is coming from the throttling valve.
➢ The function of the evaporator is to absorb heat by the refrigerant from the
space to be cooled.
➢ By absorbing heat, the refrigerant converts from liquid state to gaseous state.
Filter Drier is used to remove the moisture from the refrigerant.
Drain Pan is used to contain the water that condensates from the cooling coil and is
discharged outdoors.
Propeller Fan is used in air-cooled condensers to help move the air molecules over
the surface of the condensing coil.
Fan Motor is located here. It has a double shaft where the indoor blower and
outdoor propeller fan are connected together.

Specification
A window air- conditioner is normally specified by the following parameters:
Capacity : 1, 1.5 and 2 ton etc
Overall dimensions : length x width x height
Power supply : AC, 220-240 volts
Control : site or remote

Application of Different Refrigerants

Refrigerant Application
NH3 Cold storage, ice plants, refrigerators
H2O Water, Li Br absorption system
CO2 Used as dry ice in transport
R-11 Central air conditioning
R-12 Domestic refrigerator
R-22 Window type air conditioner
R-113 Air conditioning

Desirable Properties of Refrigerant

Thermodynamics Properties
1. Specific heat of refrigerant in liquid phase must be low and in vapour phase
must be high.
2. Enthalpy of vaporization should be high.
3. Heat conductivity should be high.
4. Freezing point should be low.
5. Critical temperature should be high.
6. Specific volume of refrigerant at the inlet of the compressor should be low.
7. Temperature of the refrigerant at the outlet of the compressor should be low.
 Chemical Properties
1. It should not be toxic and non-flammable.
2. Refrigerants should not be miscible with oil or completely im-misicible with oil.
3. It should not attack material of construction of equipment

Physical Properties
1. Its viscosity should be low.
2. There is no leak tendency.

Basic Air - Conditioning Process

Process in diagram - Types of Air conditioning
OA Sensible Heating
Wet bulb
OB Sensible Cooling
temperature
OC Humidification
Water Vapour Pressure P =f(ω)

OD Dehumidification
Specific Humidity,ω

Dew point OE Heating and Humidification

temperature
W

OF Cooling and Dehumidification

et
bu
lb

Dew point Moisture

OG Cooling and Humidification
Content OH Heating and Dehumidification

Dry-bulb temperature, t

Points to remember:
➢ Relative humidity of saturated air will be 100%.
➢ Wet bulb temperature (WBT) is always less than dry bulb temperature (DBT)
except when the air is saturated.
➢ Wet bulb temperature is an indirect measure of the dryness of the moist air.
➢ For saturated air, DPT = WBT = DBT
Refrigeration
It is the process of producing lower temperature compared to surrounding

1. Refrigeration Machine

A refrigerating machine is a device which will either cool or maintain a body at a

temperature below that of the surroundings. Heat must be made to flow from a body
at low temperature to the surroundings at high temperature.

2. Refrigeration Effect (R.E)

It is the amount of heat, which is to be extracted from storage space in order to

maintain lower temperature.

Q1: Heat rejected to surrounding.

Q2: Storage space
W = Work done

Points to remember:
➢ There are two statements of the second law of thermodynamics, the Kelvin
Planck statement, and the Clausius statement.
➢ The Kelvin-Planck statement relates to heat engines and the Clausius
statement pertains to refrigerators and heat pumps.
➢ Above two statements are for a cycle not for a process.

3. Refrigeration Capacity (R.C)

R.C= Refrigeration effect x mass flow rate of refrigerant