Principles
of refrigeration
In order to produce a low temperature a body should
be cooled, which means that the body should be so treated so as to deprive it
of its total heat content. The following methods may be adopted to achieve this
end.
(i)
By adding a salt
to ice or water.
(ii)
By boiling a
liquid under reduced pressure.
(iii)
By the adiabatic
expansion of gas doing external work.
(iv)
By utilizing the
cooling due to Peltier effect.
(v)
By absorption
and desorption of gases.
(vi)
By utilizing the
cooling due to Joule-Thomson effect.
(vii)
By the process
of adiabatic demagnetization of paramagnetic salts.
Cold caused
by adding a salt to ice
When a salt is added to ice there is a considerable
lowering of temperature. The salt added dissolves in water adhering to pieces
of ice and so more ice melts. The necessary heat for this process, the heat of
solution and the latent heat required to melt the ice, is extracted from the
mixture itself whose temperature consequently falls down. This process does not
however continue indefinitely for a stage is reached when salt no longer goes
into solution.
Air
conditioning
The seasonal variations of temperature, humidity,
composition of atmosphere and wind velocity have marked effect on growth,
longevity, efficiency and comforts of man. Temperature and relative humidity
are the two guiding factors in the comfort feeling. The process of adjusting
the condition of air by artificial means most comfortable for human beings is
called air conditioning. Complete air
conditioning involves the control of the following factors: temperature
75-77°F, relative humidity 60-65%, wind velocity 25-75 ft. per minute, fresh
air circulation- at least 25%, purification of air, deodorizing and activating
the air by isolation.
The air conditioning machine is essentially a
refrigerating machine with a difference in the design of the evaporator. When
the outside temperature is too high the air is cooled keeping the refrigerating
coil somewhere in the room. The air is sucked with the help of a suction pump
through the coil chamber and on being cooled is again distributed in the room.
Before being cooled the air is passed through a suitable filter which removes
dust particles, smoke, carbon dioxide and other poisonous gases. Proper
adjustment to desirable hygrometric condition of air (dehumidification) is
achieved simultaneously with cooling by causing the moisture to condense in the
coil chamber. The speed of the suction pump and rate of the cooling of the
refrigerating coil are adjusted within desired limits by automatic methods.
When heating is intended, a heating surface
consisting of an electric heater or a steam piping is installed in the machine
just above the cooling surface. Humidification is accomplished by sprinkling
water on the heating surface.
Cooling by
adiabatic expansion
If a highly compressed gas be allowed to expand
adiabatically doing some external work, it cools considerably. The energy
necessary for doing the work is supplied from its own kinetic energy or the
heat content of the gas resulting in this drop in temperature. If a cylinder
containing highly compressed carbon dioxide be suddenly opened and a piece of
cloth held before it, the issuing gas is found to be deposited in the form of
fine ice (dry ice or dry snow as it is called). This cooling and consequent
solidification occurs due to adiabatic expansion. This process is essentially
discontinuous and so it is not suitable for the commercial purpose of producing
low temperature. This principle of cooling was however, utilized by Claude and
Heylandt for the liquefaction of air under special arrangement.
Cooling due
to Peltier effect
 |
| Peltier |
It is well known that the junction between two
dissimilar metals is the seat of an e.m.f. A cooling is produced if a current
is passed across the junction direction of this e.m.f. This is known as Peltier effect and may be utilized in
producing cooling. The extent of cooling depends on the pair of metal sat the
junction. With junctions between ordinary metals the resulting cooling is small
and is not of direct practical importance. In recent years, development of some
semi conducting materials has made it possible to construct practically useful
cooling devices employing this effect.
Cooling by
absorption and desorption of gases
When a gas absorbed by a solid body a certain
quantity of heat called the heat of
absorption is given up. Conversely, it requires some amount of heat or
energy for desorption of the gas from the body to occur. A gas may be allowed
to be absorbed by a body. The heat developed during the process may be removed
by a suitable means. If the solid is then thermally insulated and the absorbed
gas is removed under adiabatic condition a cooling will result. Simon’s process
of liquefying helium is based on this process.
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