What is Refrigeration??

Refrigeration may be defined as the process of achieving and maintaining a temperature below that of the surroundings, the aim being to cool some product or space to the required temperature.

Air Conditioning refers to the treatment of air so as to simultaneously control its temperature, moisture content, cleanliness, odour and circulation, as required by occupants, a process, or products in the space.

Art of Ice making by Nocturnal Cooling:

The art of making ice by nocturnal cooling was perfected in India. In this method ice was made by keeping a thin layer of water in a shallow earthen tray, and then exposing the tray to the night sky. Compacted hay of about 0.3 m thickness was used as insulation. The water looses heat by radiation to the stratosphere, which is at around -55°C and by early morning hours the water in the trays freezes to ice. This method of ice production was very popular in India.

Evaporative Cooling:

As the name indicates, evaporative cooling is the process of reducing the temperature of a system by evaporation of water. Human beings perspire and dissipate their metabolic heat by evaporative cooling if the ambient temperature is more than skin temperature. Animals such as the hippopotamus and buffalo coat themselves with mud for evaporative cooling. Evaporative cooling has been used in India for centuries to obtain cold water in summer by storing the water in earthen pots. The water permeates through the pores of earthen vessel to its outer surface where it evaporates to the surrounding, absorbing its latent heat in part from the vessel, which cools the water.

Cooling by Salt Solutions:

Certain substances such as common salt, when added to water dissolve in water and absorb its heat of solution from water (endothermic process). This reduces the temperature of the solution (water+salt). Sodium Chloride salt (NaCl) can yield temperatures up to -20°C and Calcium Chloride (CaCl2) up to - 50°C in properly insulated containers. However, as it is this process has limited application, as the dissolved salt has to be recovered from its solution by heating.

Artificial Refrigeration:

Refrigeration as it is known these days is produced by artificial means. Though it is very difficult to make a clear demarcation between natural and artificial refrigeration, it is generally agreed that the history of artificial refrigeration began in the year 1755, when the Scottish professor William Cullen made the first refrigerating machine, which could produce a small quantity of ice in the laboratory. Based on the working principle, refrigeration systems can be classified as vapour compression systems, vapour absorption systems, gas cycle systems etc.

Domestic refrigeration systems:

The domestic refrigerator using natural ice (domestic ice box) was invented in 1803 and was used for almost 150 years without much alteration. The domestic ice box used to be made of wood with suitable insulation. Ice used to be kept at the top of the box, and low temperatures are produced in the box due to heat transfer from ice by natural convection. A drip pan is used to collect the water formed due to the melting of ice. The box has to be replenished with fresh ice once all the ice melts.

Though the concept is quite simple, the domestic ice box suffered from several disadvantages. The user has to replenish the ice assoon as it is consumed, and the lowest temperatures that could be produced inside the compartment are limited.

QUIZZZZ

Q: Which of the following can be called as a refrigeration process?

a) Cooling of hot ingot from 1000oC to room temperature

b) Cooling of a pot of water by mixing it with a large block of ice

c) Cooling of human beings using a ceiling fan

d) Cooling of a hot cup of coffee by leaving it on a table

e) Cooling of hot water by mixing it with tap water

f) Cooling of water by creating vacuum over it

Q. The disadvantages of natural refrigeration methods are:

a) They are expensive

b) They are uncertain

c) They are not environment friendly

d) They are dependent on local conditions

Q. Evaporative cooling systems are ideal for:

a) Hot and dry conditions

b) Hot and humid conditions

c) Cold and humid conditions

d) Moderately hot but humid conditions

Q. Compared to natural refrigeration methods, artificial refrigeration methods are:

a) Continuous

b) Reliable

c) Environment friendly

d) Can work under almost all conditions