Mechanical refrigeration is the process of lowering the temperature of a substance less than that of its surroundings.
The simplest form of refrigeration observed in daily life is the use of ice. Ice absorbs heat from the surroundings and melts. During this process, the surrounding area becomes cool due to the loss of heat. The capacity of refrigeration is expressed in a tonne.
A tonne of refrigeration is defined as the rate of heat removal from the surroundings equivalent to the heat required for melting 1 tonne of ice in one day.
If the latent heat of fusion of ice is taken as 336 kilojoules per kg. one tonne is equivalent to heat removal at a rate of 14,000 kilojoules per hour.
Applications of refrigeration
Refrigeration is used extensively in air conditioning plants in the areas for comfort and processing areas. It is used in several other areas as given below.
- Removal of heat from chemical reactions.
- Preservation of thermolabile substances. For example, insulin, ACTH, pituitary hormones and vaccines are stored in the refrigerator. It is extremely important to store all blood products such as whole human blood at as low a temperature as possible above freezing point.
- Liquefy processing gas.
- Separation of vapour by distillation.
- Purification of products.
- Preferential freeze-out of one component from a liquid mixture.
Refrigerants
Refrigerant is a liquid which readily absorbs heat when evaporated at a low temperature and pressure and gives out heat on condensing at a higher temperature and pressure.
Refrigerants may be primary or secondary.
Primary refrigerants are those liquids that change from liquid to gas after absorbing heat.
A number of refrigerants are available, which permits the selection for a specific application. Examples of primary refrigerants and their chemical constitution are given in Table 1.
The choice of a suitable refrigerant depends upon many factors. although a few are mentioned below…
- The latent heat of vaporisation should be large so that the desired cooling effect is produced.
- At normal temperature and pressure, the refrigerant must be in the vapour phase and on compression and cooling it should be liquefied easily.
- In a condenser section, the pressure required to liquefy the vapour of the refrigerant must be so small as to maintain. refrigerant usually at room temperature.
- In the evaporator section, the vapour pressure of refrigerant liquid must be maintained at about -15°C greater than the atmospheric pressure.
- The specific volume of the refrigerant vapour should not be large.
In addition, several safety, and economic factors are considered.
Secondary refrigerants are those liquids, which act only as heat carriers.
Examples are brine, air and water. These are also known as antifreeze.
Brine Systems: Brine is a concentrated solution of salt. Brine can bring the temperature below 0 °C and will give a greater temperature gradient and faster cooling. Brine systems can transport heat and this produces These systems do not change state in this process. These are just particles which are carried with them. Sodium chloride solution is usually used. Calcium chloride solution can also be used.
Advantages: Brine acts as a cold reservoir and is used to average-out load fluctuations. It is easy to design pumps that can withstand brine pressure. It does not require complicated compressors.
Disadvantages: Brine systems have a relatively high freezing point and are corrosive. It requires additional equipment such as brine tanks, pumps, pipework etc. Corrosiveness of the brine to the metal container is an important consideration.
Glycerin, diethylene glycol, ethanol and aqueous solutions can be used instead of a brine system. These are not satisfactory for producing – 40°F, due to high viscosity and deposition of crystals. These are limited to small installations in which the required temperature is not too low.
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