Friday, October 11, 2013
Refrigeration and Air Conditioning
REFRIGERATION AND AIR CONDITIONING
Contents:
Refrigeration:
Definition
of refrigerator, VCRS, Domestic refrigerator.
Air
Conditioning: Window air conditioner, Split air conditioner.
1.
Refrigeration:
1.1.
Introduction:
Refrigeration may be defined as
the process of achieving and maintaining a temperature below that of surroundings.
The aim of the refrigeration is to cool some product
or space to the required temperature. One of the most applications of
refrigeration is preservation of perishable food products by storing them at
low temperature. Refrigeration systems are also used extensively for providing
thermal comfort to human being by means of air conditioning. Air conditioning
refers to the treatment of air so as to simultaneously control its temperature,
moisture, cleanliness, odour and circulation as required by occupants, a
process or products in the space.
1.2.
Units of Refrigeration- Tons of Refrigeration
The rating of the refrigerating machine
is given by a standard unit of refrigeration called ton of refrigeration (TR)
One
tone of refrigeration means capacity to freeze 1 ton of water from and at 00C
in 24 hours
1
TR= 211 kJ/min
1.3.
Vapour Compression Refrigeration system:
Figure
1 shows basic components of a vapour compression refrigeration system.
As shown in the figure the basic
system consists of evaporator, compressor, condenser and expansion valve. The
refrigeration effect is obtained in the cold region as heat is extracted by the
vaporisation 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 flows through 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 decreases.
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 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 sink.
1.4. Domestic
Refrigerators:
Refrigerators,
these days, are becoming the common item for house hold use, vendor’s shop,
hotels, motels, offices, laboratories, hospitals, chemists and druggist’s
shops, studios etc. They are manufactured in different size to meet the
needs of various groups of people. They are usually rated with internal
gross volume and the freezer volume. The freezer space is meant to
preserve perishable products at a temperature much below 0°C such as fish,
meat, chicken etc. and to produce ice and ice-cream as well. The refrigerators
in India are available in different sizes of various makes, i.e., 90,
100, 140, 160, 200, 250, 380 litres of gross volume. The freezers are usually
provided at top portion of the refrigerator space occupying around one-tenth to
one-third of the refrigerator volume. In some refrigerators, freezers are
provided at the bottom.
A domestic refrigerator consists of the following two main
parts:
1. The refrigeration system.
2. The insulated cabinet.
Figure shows a flow diagram of a typical refrigeration system
used in a domestic refrigerator.
Domestic
Refrigerator
A simple domestic refrigerator consists of a hermetic
compressor placed in the cabinet base.
The condenser is installed at the back and the evaporator is
placed inside the cabinet at the top.
The working of the refrigerator is as follows:
· The low pressure and low temperature refrigerant vapour (usually
R12) is drawn through the suction line to the compressor. The accumulator
provided between the suction line and the evaporator collects liquid
refrigerant coming out of the evaporator due to incomplete evaporation, if any,
prevents it from entering the compressor. The compressor then compresses
the refrigerant vapour to a high pressure and high temperature. The compressed
vapour flows through the discharge line into condenser (vertical natural
draft, wire-tube type).
· In the condenser the vapour refrigerant at high pressure
and at high temperature is condensed to the liquid refrigerant at high pressure
and low temperature.
· The high pressure liquid refrigerant then flows through the filter
and then enters the capillary tube (expansion device). The capillary
tube is attached to the suction line as shown in Figure. The warm refrigerant
passing through the capillary tube gives some of its heat to cold suction line
vapour. This increases the heat absorbing quality of the liquid refrigerant
slightly and increases the superheat of vapour entering the compressor. The
capillary tube expands the liquid refrigerant at high pressure to the liquid
refrigerant at low pressure so that a measured quantity of liquid refrigerant
is passed into the evaporator.
· In the evaporator the liquid refrigerant gets evaporated
by absorbing heat from the container/articles placed in the evaporative chamber
and is sucked back into the compressor and the cycle is repeated.
2. Air
Conditioning
Air
conditioning
is the process of improving the properties of air such as temperature and
humidity human comfort. More generally, air conditioning can refer to any form
of technological cooling, heating, ventilation, or disinfection that modifies
the condition of air.
2.1. Types
of Air Conditioning
Two types of air conditioning
systems are discussed.
a)
Window air conditioner
b)
Split air conditioner
2.2. Window
Air Conditioner
It is the most commonly used
air conditioner for single rooms. In this air conditioner all the components,
namely the compressor, condenser, expansion valve or coil, evaporator and
cooling coil are enclosed in a single box. This unit is fitted in a slot made
in the wall of the room, or often a window sill.
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. The whole assembly of the window
air conditioner can be divided into two compartments: the room side, which is
also the cooling side and the outdoor side from where the heat absorbed by the
room air is liberated to the atmosphere. The room side and outdoor side are
separated from each other by an insulated partition enclosed inside the window
air conditioner assembly (refer fig below).
In the front of the window air conditioner on the
room side there is beautifully decorated front panel on which the supply and
return air grills are fitted (the whole front panel itself is commonly called
as front grill). The louvers fitted in the supply air grills are adjustable so
as to supply the air in desired direction. There is also one opening in the
grill that allows access to the control panel or operating panel in front of
the window air conditioner.
Parts of Window air conditioning system
1) Blower: This is
the small blower that is fitted behind the evaporator or cooling coil inside
the assembly of the window air conditioner system. The blower sucks the air
from the room which first passes over the air filter and gets filtered. The air
then passes over the cooling coil and gets chilled. The blower then blows this
filtered and chilled air, which passes through the supply air compartment
inside the window air conditioner assembly. This air is then delivered into the
room from the supply air grill of the front panel.
2) Propeller fan or the condenser fan: The
condenser fan is the forced draft type of propeller fan that sucks the atmospheric
air and blows it over the condenser. The hot refrigerant inside the condenser
gives up the heat to the atmospheric air and its temperature reduces.
3) Fan motor: The
motor inside the window air conditioner assembly is located between the
condenser and the evaporator coil. It has double shaft on one side of which the
blower is fitted and on the other side the condenser fan is fitted. This makes
the whole assembly of the blower, the condenser fan and the motor highly
compact.
Limitations of Window Air Conditioner
1.
It has no control over humidity though it carries out dehumidification.
2.
Most of the window air conditioners do not provide heating for winter.
3.
No provision for humidification is possible in window air conditioner.
2.3. Split Air Conditioning
System
The
split air conditioner comprises of two parts: the outdoor unit and the indoor
unit. The outdoor unit, fitted outside the room, houses components like the
compressor, condenser and expansion valve. The indoor unit comprises the evaporator
or cooling coil and the cooling fan. For this unit you don’t have to make any
slot in the wall of the room. Further, the present day split units have
aesthetic looks and add to the beauty of the room. The split air conditioner
can be used to cool one or two rooms.
1) Evaporator Coil or the
Cooling Coil:
The cooling coil is a copper coil made of number turns
of the copper tubing with one or more rows depending on the capacity of the air
conditioning system. The cooling coil is covered with the aluminium fins so
that the maximum amount of heat can be transferred from the coil to the air
inside the room. The refrigerant from the tubing at very low temperature and
very low pressure enters the cooling coil. The blower absorbs the hot room air
or the atmospheric air and in doing so the air passes over the cooling coil
which leads to the cooling of the air. This air is then blown to the room where
the cooling effect has to be produced. The air, after producing the cooling
effect is again sucked by the blower and the process of cooling the room
continues.
After
absorbing the heat from the room air, the temperature of the refrigerant inside
the cooling coil becomes high and it flows back through the return copper
tubing to the compressor inside the outdoor unit. The refrigerant tubing
supplying the refrigerant from the outdoor unit to the indoor unit and that
supplying the refrigerant from indoor unit to the outdoor unit are both covered
with the insulation tape.
2) Air Filter:
The
air filter is very important part of the indoor unit. It removes all the dirt
particles from the room air and helps supplying clean air to the room. The air
filter in the wall mounted type of the indoor unit is placed just before the
cooling coil. When the blower sucks the hot room air, it is first passed
through the air filter and then though the cooling coil. Thus the clean air at
low temperature is supplied into the room by the blower.
3) Cooling Fan or Blower:
Inside
the indoor unit there is also a long blower that sucks the room air or the
atmospheric air. It is an induced type of blower and while is sucks the room
air it is passed over the cooling coil and the filter due to which the
temperature of the air reduces and all the dirt from it is removed. The blower
sucks the hot and unclean air from the room and supplies cool and clean air
back. The shaft of the blower rotates inside the bushes and it is connected to
a small multiple speed motor, thus the speed of the blower can be changed. When
the fan speed is changed with the remote it is the speed of the blower that
changes.
4)
Drain Pipe:
Due
to the low temperature refrigerant inside the cooling coil, its temperature is
very low, usually much below the dew point temperature of the room air. When
the room air is passed over the cooling due the suction force of the blower,
the temperature of the air becomes very low and reaches levels below its dew
point temperature. Due to this the water vapor present in the air gets
condensed and dew or water drops are formed on the surface of the cooling coil.
These water drops fall off the cooling coil and are collected in a small space
inside the indoor unit. To remove the water from this space the drain pipe is
connected from this space extending to the some external place outside the room
where water can be disposed off. Thus the drain pipe helps removing dew water
collected inside the indoor unit. To remove the water efficiently the indoor
unit has to be a tilted by a very small angle of about 2 to 3 degrees so that
the water can be collected in the space easily and drained out. If this angle is
in opposite direction, all the water will get drained inside the room. Also,
the if the tilt angle is too high, the indoor unit will shabby inside the room.
5)
Louvers or Fins:
The
cool air supplied by the blower is passed into the room through louvers. The
louvers help changing the angle or direction in which the air needs to be
supplied into the room as per the requirements. With louvers one easily change
the direction in which the maximum amount of the cooled air has to be passed.
There
are two types of louvers: horizontal and vertical. The horizontal louvers are
connected to a small motor and there position can set by the remote control.
Once can set a fixed position for the horizontal louvers so that chilled air is
passed in a particular direction only or one can keep it in rotation mode so
that the fresh air is supplied throughout the room. The vertical louvers are
operated manually and one can easily change their position as per the
requirements. The horizontal louvers control flow of air in upper and downward
directions of the room, while vertical louvers control movement of air in left
and right directions.
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