Methods of Refrigeration

Different methods of refrigeration are

Each of the above refrigeration methods is described briefly below.

1. Dry Ice Refrigeration:

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

Evaporative refrigeration utilizes the principle that when a liquid evaporates, it absorbs heat equivalent to its latent heat of vaporization from its surroundings, thereby cooling itself.
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 the hand, it evaporates, producing a 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 the top and forcing a current of air from below. Another application is the evaporative type of condensers. Yet another application is in desert coolers or room coolers. Dry air is passed through wet pads. Due to evaporation, the air gets cooled. The principle is also utilized in making artificial snow.

Thermo-electric refrigeration type employs Peltier’s effect. When two dissimilar metals are joined together at one end and a direct current is passed through them, one joint becomes cooled while the other becomes heated.
Antimony (Sb) and Bismuth (Bi) are commonly used metals as they are electrochemically opposite in their polarity. If the cold end is placed in a closed space, it gets cooled. If the magnitude of the current is increased and a series of such strips is combined, a significant cooling effect can be achieved.

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 a pressure of 6.5 cm of water, the boiling point of water is dramatically reduced to 5 cm of water, and its boiling point is reduced 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 the ejector assembly, all the gases and vapors present in the flash chamber are removed, thereby creating a partial vacuum. This results in the reduction of the boiling point of water.
Due to the evaporation of water at low temperatures, the remaining water in the flash chamber is cooled and then circulated to the refrigerated space. The warm water received from this space is sprayed back into the flash chamber.
A makeup water connection is provided in the flash chamber to replenish any water lost through evaporation. The steam is supplied to the nozzle from a boiler. The condenser condenses the steam and water vapour received from the flash chamber. The condensate from the condenser is taken back to the boiler.

Liquid gases used for producing refrigeration should be non-toxic. Due to this limitation, only nitrogen and liquid carbon dioxide can be utilized for this purpose.
Due to the evaporation of cooling gas, cooling is accomplished.
Liquid gas is stored in a well-insulated cylinder, and its quantity is regulated by 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 to cool vehicles transporting foodstuffs. It may also be utilized for cold storage.

A 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 stream can be separated from the peripheral flow.
The central core of the air is separated using either the uniflow or counterflow method. The central core of the air stream is colder than the hot gases at the periphery.