Methods of Refrigeration

Different methods of refrigeration are

  1. Dry ice refrigeration
  2. Evaporative refrigeration
  3. Thermo-electric refrigeration
  4. Steam jet refrigeration
  5. Liquid gas refrigeration and
  6. Vortex tube system of refrigeration

1. Dry Ice Refrigeration:

  • Solid Carbon dioxide (CO2) is called dry ice and it has a peculiar characteristics that it changes from solid state to vapour state without getting converted into intermediate liquid state (sublimation). Due to the change of state, it absorbs heat equivalent to enthalpy of vaporization. The sublimation temperature of dry ice at atmospheric pressure is -78°C.
  • Dry ice is used to preserve foodstuff during transportation. Now a days it is universally used to preserve food in air-transportation. Dry ice slabs are usually packed in frozen food cartons on either side or on the top of the food packages, dry ice absorbs heat from the foodstuff and preserve them in the frozen state.

2. Evaporative Refrigeration:

  • Evaporative refrigeration makes use of the principle that when a liquid evaporates, it absorbs heat equivalent to its latent heat of vaporization from the surroundings, thereby cooling it.
  • Cooling of water in the earthen pitcher – the water coming out of the pores of the pitcher evaporates when it comes in contact with dry air, thereby cooling the water in the pitcher.
  • When a drop of spirit is put on the palm of hand, it evaporates producing cooling effect.
  • Evaporation cooling may be defined as the adiabatic transfer of heat from air to water.
  • Evaporation cooling may be defined as the adiabatic transfer of heat from air to water.
  • It is utilized in cooling towers where condenser water is cooled by spraying it from top and forcing a current of air from below. Another application is evaporative type of condensers. Yet another application is in desert coolers or room coolers. Dry air is passed through wet pads. Due to evaporation, air gets cooled. The principle is also utilized in making artificial snow.

3. Thermo-electric Refrigeration:

  • Thermo-electric refrigeration type employs Peltier’s effect. when two dissimilar metals are joined on either ends and a direct current is circulated through it, one joint gets cooled while the other gets heated.
  • Antimony (Sb) and Bismuth (Bi) are commonly used metals as they are electro-chemically opposite in their polarity. If the cold end is placed in a closed space, it gets cooled. If the magnitude of current is increased and a series of such strips are placed together a good cooling effect can be produced.

4. Steam Jet Refrigeration:

Steam Jet refrigeration

Schematic diagram of steam jet refrigeration

  • The principle of steam jet refrigeration is that the boiling point of water can be reduced by reducing the pressure. At standard atmospheric pressure, the boiling point of water is 100°C. If the atmospheric pressure is lowered, the boiling point is considerably reduced. For example, at pressure of 6.5 cm of water, the boiling point of water is considerably reduced to 5 cm of water, its boiling point reduces to 6°C.
  • The schematic diagram of a steam jet refrigeration system is shown in the figure. The pressure reduction in the system is achieved by a steam nozzle and ejector assembly. Due to the extremely high velocity of steam in ejector assembly all the gases and vapors present in the flash chamber are removed thereby creating partial vacuum. This results in the reduction of the boiling point of water.
  • Due to the evaporation of water at low temperature, the remaining water in the flash chamber gets cooled which is circulated to the refrigerated space and the warm water received from it is sprayed in the flash chamber.
  •  A make-up water connection is provided in the flash chamber to make good any water lost in evaporation. The steam is supplied to the nozzle from a boiler. The condenser condenses the steam and water vapour received from flash chamber. The condensate from the condenser is taken back to the boiler.

5. Liquid Gas Refrigeration:

  • Liquid gases which can be utilized for producing refrigeration should be non-toxic. Due to this limitation only nitrogen and liquid carbon dioxide can be utilized for the purpose.
  • Due to the evaporation of cooling gas, cooling is accomplished.
  • Liquid gas is filled in a well insulated cylinder and its quantity regulated by means of a valve.
  • Liquid gas is sprayed in the refrigerated space. Due to its evaporation, the temperature in the refrigerated space is lowered. This method is used for cooling the vehicles transporting food stuff. It may also be utilized for cold storage’s.

6. Vertex Tube System of Refrigeration:

  • Vertex tube is a simple straight piece of tube into which compressed air flows tangentially and is so throttled that the central core of the air steam can be separated from peripheral flow.
  • The central core of the air is separated either by uni-flow or counter flow method. The central core of the air steam in cold as compared to the hot gases at the periphery.

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