Saturday, May 12, 2012

Production of low temperatures


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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