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Artcle by Dr. L.C.Singal, Prof. (ME Deptt), CEC
Landran, (Mohali), Punjab (India), Formerly Prof
in Chemical Engg. Deptt., Panjab University,
Chandigarh
Refrigeration is a process of producing low
temperatures as compared to the surrounding
temperatures.
It will be possible only if heat is transferred
from the low temperature region to a high
temperature region. Obviously it is not possible
in the natural manner because heat flows from
high temperature to low temperature like fluid
flows from high pressure to low pressure/
current flows from high voltage to low voltage/
gas flows from high concentration to the region
of low concentration. It means in refrigeration
one is trying to go against the natural process
as well as against the second law of
thermodynamics which states that heat cannot
flow from low temperature region to a high
temperature region without the use of an
external agent. The external agent in
refrigeration is the compressor which introduces
the most common method of refrigeration.
The most commonly used closed vapor
compression refrigeration system consists of six
main parts namely compressor, condenser,
expansion device, evaporator, piping and
circulating working substance called the
refrigerant. The other refrigeration systems
are
·
vapor absorption refrigeration,
·
gas refrigeration,
·
Steam jet water vapor refrigeration,
·
Vortex tube refrigeration,
·
Thermoelectric refrigeration,
·
Magnetic cooling.
·
Liquefaction of natural gases.
Fig.1 is the schematic diagram of a vapor
compression refrigeration cycle, Fig.2 is the
schematic diagram of a vapor absorption water
chilling unit Fig.3 is the schematic diagram of
a gas refrigeration cycle.
Fig.1 Simple Vapor Compression Refrigeration
System
Fig.2 Vapor Absorption Water Chilling Unit
Fig.3 Gas Refrigeration Cycle
VAPOUR COMPRESSION REFRIGERATION CYCLE
There are six main components in a refrigeration
system:
. The Compressor
•
The Condenser
•
The Metering Device or expansion valve
•
The Evaporator
•
Piping material
•
Refrigerant
•
COMPRESSOR:
It is heart of the refrigeration system as it
circulates the refrigerant in the system like
the heart of a human being circulating the blood
in the body.
•
Two different pressures exist in the
refrigeration cycle. The evaporator or low
pressure, and the condenser, or high pressure.
These pressure areas are divided by the other
two components. On one end, is the metering
device which controls the refrigerant flow, and
on the other end, is the compressor?
The compressor is the heart of the system. The
compressor does just what its name is. It
compresses the low pressure refrigerant vapor
from the evaporator and compresses it into a
high pressure vapor.
•
The inlet to the compressor is called the
“Suction Line”. It brings the low pressure
vapor into the compressor.
•
After the compressor compresses the refrigerant
into a high pressure Vapor, and the outlet of
the compressor is called the “Discharge Line”.
There are three types of compressors namely
reciprocating, rotary and centrifugal. The type
of compressor depends on the pressure difference
between the high pressure side (condenser) and
low pressure side (evaporator) of the
refrigeration system. This further depends on
the refrigerant selected for the application
under consideration.
THE CONDENSER
•
The “Discharge Line” leaves the compressor and
runs to the inlet of the condenser.
•
Because the refrigerant was compressed, it is a
hot high pressure vapor.
•
The hot vapor enters the condenser and starts to
flow through the tubes.
•
Cool air is blown across the out side of the
finned tubes of the condenser (usually air by a
fan or water with a pump).
•
Since the air is cooler than the refrigerant,
heat jumps from the tubing to the cooler air
(energy goes from hot to cold – “latent heat”).
•
As the heat is removed from the refrigerant, it
reaches its “saturated temperature” and starts
to change state, into a high pressure liquid.
•
The high pressure liquid leaves the condenser
through the “liquid line” and travels to the
“metering device” through a filter dryer to
remove any dirt or foreign particles.
The condenser can be free air cooled (domestic
refrigerator), forced air cooled (window air
conditioner), water cooled (Central air
conditioning plant in a library, cinema house
and evaporative cooled (ice plant unit or a cold
storage unit).
EXPANSION DEVICE
•
Metering devices regulate how much liquid
refrigerant enters the evaporator as per heat
load on evaporator.
•
Common used metering devices are, small thin
copper tubes referred to as “capillary tubes”,
thermally controller diaphragm valves”
(thermostatic expansion valves, called “TXV’s.
This valve has the capability of controlling the
refrigerant flow. If the load on the evaporator
changes, the valve can respond to the change and
increase or decrease the flow accordingly. The
TXV has a sensing bulb attached to the outlet of
the evaporator. This bulb senses the suction
line temperature and sends a signal to the TXV
allowing it to adjust the flow rate. This is
important because, if not all, the refrigerant
in the evaporator changes state into a gas,
there could be liquid refrigerant content
returning to the compressor. This can be fatal
to the compressor. Liquid can not be compressed
and when a compressor tries to compress a
liquid, mechanical failing can happen. The
compressor can suffer mechanical damage in the
valves and bearings. This is called” liquid
slugging”. Normally TXV's are set to maintain
10 degrees of superheat. That means that the gas
returning to the compressor is at least 10
degrees away from the risk of having any liquid.
The metering device tries to maintain a preset
degree of superheat at the outlet openings of
the evaporator. As the metering devices
regulates the amount of refrigerant going into
the evaporator, the device lets small amounts of
refrigerant out into the line and looses the
high pressure to low pressure.
•
Now we have a low pressure, cooler liquid
refrigerant entering the evaporative coil.
These are of
five type namely capillary tube (domestic
fridge), Automatic expansion valve (ice plant
unit), Thermostatic expansion valve (Library
refrigeration plant, theatre air conditioning
unit and many more), Low side float valve
(industrial cooling units) and high pressure
float valve (industrial cooling units). These
causes the required pressure drop between the
high and low pressure sides and also control the
flow of refrigerant as per cooling requirements.
THE EVAPORATOR:
The evaporator is where the heat is removed from
your house, business or products to be
cooled.
•
Low pressure liquid leaves the metering device
and enters the evaporator.
•
Usually, a fan will move warm air from the
conditioned space across the evaporator finned
coils.
•
The cooler refrigerant in the evaporator tubes,
absorb the warm room air. The change of
temperature causes the refrigerant to “flash” or
“boil”, and changes from a low pressure liquid
to a low pressure cold vapor.
•
The low pressure vapor is pulled into the
compressor and the cycle starts over.
•
Evaporators are two types i.e. flooded
evaporators necessitating the use of
accumulators to permit only vapors to the
compressor and dry expansion type evaporators.
Flooded types are used in industrial units
whereas dry expansion types are used in domestic
and commercial refrigeration units.
PIPING MATERIALS:
Pipe material should have high thermal
conductivity, low cost, easy working and
inertness with the refrigerant. Till date most
commonly used pipe material is soft copper with
all refrigerants except ammonia. The pipe
material used with ammonia is mild steel as
ammonia is highly corrosive to copper.
REFRIGERANT:
It is working substance in a refrigeration unit
like blood in the human body. Its selection
depends on many considerations like temperature
to be produced, latent heat, ozone depletion
potential, global warming potential, toxicity,
inflammability, inertness, corrosion, erosion,
action with water and lubricating oil, cost,
availability, leak detection and power
requirements for a certain amount of cooling
needed. Various commonly used refrigerants are
halogenated saturated hydrocarbons like R-134,
R-22 and inorganic compounds like ammonia and
air. Most common previously used refrigerants
like R-12 and R-11 has been banned because of
their high ozone depletion and global warming
potentials. Mixed refrigerants and zoetrope’s
are also in use. Refrigerants can be primary,
secondary and tertiary type depending where and
how these being used are. The same substance,
for example, air can be primary in aircraft
refrigeration; can be secondary as in a window
air conditioner and tertiary in a central air
conditioning plant.
VAPOUR ABSORPTION REFRIGERATION
Fig.2 Vapor Absorption Water Chilling Unit
VAPOUR ABSORPTION SYSTEM
The vapor absorption refrigeration system consists of:
•
Evaporator
•
Absorber
•
High pressure generator
•
Condenser
.
Evaporator
•
The refrigerant (water) evaporates at around 4oC
under a high vacuum condition of 754 mm Hg in
the evaporator.
•
Chilled water goes through heat exchanger tubes
in the evaporator and transfers heat to the
evaporated refrigerant.
•
The evaporated refrigerant (vapor) turns into
liquid again, while the latent heat from this
vaporization process cools the chilled water (in
the diagram from 12 oC to 7 oC).
The chilled water is then used for cooling
purposes.
Absorber
•
In order to keep evaporating, the refrigerant
vapor must be discharged from the evaporator and
refrigerant (water) must be supplied. The
refrigerant vapor is absorbed into lithium
bromide solution, which is convenient to absorb
the refrigerant vapor in the absorber. The heat
generated in the absorption process is
continuously removed from the system by cooling
water. The absorption also maintains the vacuum
inside the evaporator.
High Pressure Generator
•
As lithium bromide solution is diluted, the
ability to absorb the refrigerant vapor reduces.
In order to keep the absorption process going,
the diluted lithium bromide solution must be
concentrated again. An absorption chiller is
provided with a solution concentrating system,
called a generator. Heating media such as steam,
hot water, gas or oil perform the function of
concentrating solutions. The concentrated
solution is returned to the absorber to absorb
refrigerant vapor again.
Condenser
•
To complete the refrigeration cycle, and thereby
ensuring the refrigeration takes place
continuously, the following two functions are
required
•
To concentrate and liquefy the evaporated
refrigerant vapor, which is generated in the
high-pressure generator.
•
To supply the condensed water to the evaporator
as refrigerant (water)
•
For these two functions a condenser is
installed.
Six distinctions between the vapor compression and vapor absorption
refrigeration system are.
1.
For lower tonnage capacity vapor absorption
system is more expansive.
2.
For 50 TR cost becomes the same for the two
systems. For TR > 50 tons the vapor absorption
refrigeration system becomes more favorable.
3.
Vapor absorption systems are environmentally
friendly as there is no GWP and no ODP.
4.
A single unit of 7000 tons capacity is available
in VAS where as much less capacity say, 1000 TR
single units in VCS.
5.
In VAS, liquid coming from the evaporator has no
bad effect whereas in VCS the liquid going to
compressor will result in physical breakdown of
the compressor.
6.
In the event of lower temperature requirement in
VCS the cooling capacity decreases very
significantly whereas in VAS the cooling
capacity can still be same only by controlling
the heating rate in the generator.
Fig.3 Gas Refrigeration Cycle
Air or gas refrigeration system
The components of the air refrigeration system
are shown in Fig.. In this system, air is taken
into the compressor at point 1 from atmosphere
and compressed to condition 2. The hot
compressed air is cooled in heat exchanger up to
the atmospheric temperature condition3 (in ideal
conditions). The cooled air is then expanded in
an expander to point 4. The temperature of the
air coming out from the expander is below the
atmospheric temperature due to isentropic
expansion. The low temperature air coming out
from the expander enters into the cabin and
absorbs the heat. The cycle is repeated again.
The working of air refrigeration cycle is
represented on T-s diagrams in Fig.
COMPARISON OF VAPOUR COMPRESSION AND GAS REFRIGERATION CYCLE:
The comparison of vapor compression and gas compression
refrigerating cycles based on following
parameters:
1) Type of refrigerant used:
In vapor compression cycle liquids like Freon
and ammonia are used as the refrigerant. In the
gas cycle the gas like air is used as the
refrigerant.
2) Heat exchangers:
In the vapor compression refrigeration cycle
condenser and evaporator are the two heat
exchangers where the refrigerant gives up and
absorbs latent heat respectively. The
refrigerant undergoes change in phase in both
the heat exchangers. In the gas cycle the
refrigerant exchanges heat in the heat
exchangers, but there is no phase change of the
gas there is only sensible heat transfer
3) Efficiency of the cycle:
The efficiency of the vapor compression cycle is
more than that of the gas cycle. For producing
the same amount of refrigerating effect in the
gas cycle, large volume of gas is required;
hence the systems tend to become very large,
bulky and expensive, which are not affordable
for the domestic applications.
4) Cycle used:
The vapor compression cycle works on the reverse
Bray ton cycle while the gas compression cycle
works on reverse Rankine cycle.
5).
NET WORK: Gas refrigeration is used in
all types of aircrafts. There is already a
compressor and a turbine in each aircraft.
Therefore weight of equipment needed for air
conditioning is quite small i.e. around 1.3
times weight is required in vapor compression
systems. Work of expansion is recovered in gas
refrigeration. Therefore net work is employed in
finding COP.
6.
COP of VCR cycle is much more than that of
air (gas) refrigeration cycle.
7.
SAFE: Gas refrigeration is safe as
compared to that of vapor compression cycle.
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