Absorption refrigeration is an established technology that uses low-quality
heat (e.g., hot water or low-pressure exhaust gas) rather than electric power
to drive the cooling cycle.
The vapor absorption refrigeration system comprises of all
the processes in the vapor compression refrigeration system like compression,
condensation, expansion and evaporation. In the vapor absorption system the
refrigerant used is ammonia, water or lithium bromide. The refrigerant gets condensed in the
condenser and it gets evaporated in the evaporator. The refrigerant produces
cooling effect in the evaporator and releases the heat to the atmosphere via
the condenser.
The
major difference between the two systems is the method of the suction and
compression of the refrigerant in the refrigeration cycle. In the vapor
compression system, the compressor sucks the refrigerant from evaporator and
compresses it to the high pressure. The compressor also enables the flow of the
refrigerant through the whole refrigeration cycle. In the vapor absorption
cycle, the process of suction and compression are carried out by two different
devices called as the absorber and the generator.
Refrigerant
A refrigerant is a substance or mixture, usually a fluid, used in a
heat pump and refrigeration cycle. In most cycles it undergoes phase
transitions from a liquid to a gas and back again.
Absorbent
The absorbent enables
the flow of the refrigerant from
the absorber to the
generator by absorbing it.
Absorber
The absorber is
a sort of vessel consisting of water that acts as the absorbent, and the previous absorbed refrigerant. Thus
the absorber consists
of the weak solution of the refrigerant (ammonia in this case) and absorbent (water in this case).
Generator:
Heat is supplied to
the refrigerant water and absorbent lithium bromide solution in the generator
from the steam or hot water. The water becomes vaporized and moves to the
condenser, where it gets cooled.
Condenser
A condenser unit used in central air
conditioning systems typically has a heat exchanger section to cool down and
condense incoming refrigerant vapor into liquid.
Refrigerant control
Refrigerant flow control is used to
fine-tune the temperature in refrigerant devices by
maintaining an optimal flow of refrigerant into its
evaporator.
Concentrated Solution:
A Solution with a large amount/concentration of absorbant and
only small amount of refrigerant is known as concentrated solution and also
known as weak solution.
Dilute Solution:
An absorbent solution dilute by large amount of dissolved
refrigerant is known as dilute solution.it is also known as strong solution.
Heat of condensation:
Heat evolved when a vapor changes to a liquid specifically.
Heat of dilution
Definition is - the
heat evolved per mole of solute when a solution is greatly diluted.
Evaporator:
Water as the
refrigerant enters the evaporator at a very low pressure and temperature. Since
very low pressure is maintained inside the evaporator the water exists in a
partial liquid state and partial vapor state. This water refrigerant absorbs
the heat from the substance to be chilled and gets fully evaporated. It then
enters the absorber.
Pump
When the absorbent
absorbs the refrigerant strong solution of refrigerant-absorbent (water-Lithium
bromide) is formed. This solution is pumped by the pump at high pressure to the
generator.
Heat
Exchangers:
Two heat exchangers
are provided to internally exchange heat from the higher temperature fluid to
the lower temperature fluid so that one is cooled and the other is heated.
Special Features of Water-Lithium Bromide Solution
Here are some special features of the
water and lithium bromide in an absorption refrigeration system:
1) Lithium bromide has great affinity
for water vapor, however, when the water-lithium bromide solution is formed,
they are not completely soluble with each other under all the operating
conditions of the absorption refrigeration system. Because of this, the
designer must take care that such conditions would not be created where
crystallization and precipitation of the lithium bromide would occur.
2) The water used as the refrigerant
in the absorption refrigeration system means the operating pressures in the
condenser and the evaporator must be very low. Even the difference of pressure
between the condenser and the evaporator must be very low. This can be achieved even without installing the
expansion valve in the system, since the drop in pressure occurs due to
friction in the refrigeration piping and in the spray nozzles.
3) The capacity of any absorption refrigeration system
depends on the ability of the absorbent to absorb the refrigerant, which in
turn depends on the concentration of the absorbent. To increase the capacity of
the system, the concentration of absorbent should be increased, which would
enable absorption of more refrigerant. Some of the most common methods used to
change the concentration of the absorbent are: controlling the flow of the
steam or hot water to the generator, controlling the flow of water used for
condensing in the condenser, and re-concentrating the absorbent leaving the
generator and entering the absorber.
In a water-lithium bromide vapor
absorption refrigeration system, water is used as the refrigerant while lithium
bromide (Li Br) is used as the absorbent. In the absorber, the lithium bromide
absorbs the water refrigerant, creating a solution of water and lithium
bromide. This solution is pumped by the pump to the generator where the
solution is heated. The water refrigerant gets vaporized and moves to the
condenser where it is cooled while the lithium bromide flows back to the
absorber where it further absorbs water coming from the evaporator.
The water-lithium bromide vapor
absorption system is used in a number of air conditioning applications. This
system is useful for applications where the temperature required is more than
32 degree F.
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