What is Refrigeration? Types, Unit and Important terms in Refrigeration

 

• Refrigeration is defined as the process in which the removal of heat takes place in a confined space and maintaining the temperature at a level below the surrounding temperature.
• The equipment used to maintain low temperature is called a refrigerating system.
• Refrigeration is mainly used in household refrigerators to preserve food items and also in industrial freezers, cryogenics, and air conditioning.

• From the above figure R represents a refrigerating machine which will cool and maintain the body at the temperature below the surrounding.
• The refrigeration machine extracts heat from the cold body Q1 at the temperature of To with the aid of external agency Win (work input) and rejects the heat Q2 to the surroundings at high-temperature Ta.
• To make it easy to understand, let's take our own refrigerator at our homes. Nowadays no house is fulfilled without a refrigerator. The electricity we are providing as input to the refrigerator is work input and the output we are getting by refrigerator is the cooling effect.

Unit of Refrigeration

• The unit of refrigeration is expressed in terms of  "Ton" or "Tonnage" of refrigeration (TR).
  
  
  
• A tonne of refrigeration is defined as the amount of refrigeration effect produced by the uniform melting of one tonne (1000kg) of ice from and at 0°C in 24 hours.
• Since the latent of ice is 335kj/kg, therefore one tonne of refrigeration,

          1TR = 1000 x 335 KJ in 24 hours
 In actual practice, one tonne of refrigeration is taken as 210 KJ/min or 3.5 kW.

Types of refrigeration 

1. Ice refrigeration.
2. Dry ice refrigeration.
3. Evaporative refrigeration.
4. Refrigeration by Expansion of air.
5. Refrigeration by throttling of the gas.
6. Steam-Jet refrigeration.
7. Refrigeration by using Liquid gases.
8. Thermo-Electric refrigeration.
9. Vapour Refrigeration.

Important Terms In Refrigeration

1.Refrigerating Effect (N)

•  It is defined as the amount of heat absorbed by the refrigerant per unit time while it passes through the evaporator.

from the p-h diagram (process 4-1)

 

Refrigerating effect = Amount of heat absorbed/kg of refrigerant

        Qe or N = (h1-h4) KJ/Kg of refrigerant. 

2. Compression Work (w)

•   The amount of work done on the system by the compressor is called compression work.
•   The amount of compression work in terms of enthalpy from the p-h diagram (process 2-1)

    Compression work (w) = (h2 - h1) KJ/Kg of refrigerant

3.Co-efficient of performance (COP)

• It is the ratio of net refrigerating effect of the compressor work.

            COP = Net refrigerating effect/Compression Work  

     from the p-h diagram,

           COP = (h1-h4)/(h2-h1)

 4.Actual COP, Theoretical COP, and Relative COP

• If the values of the refrigerating effect and compression, work are measured from the actual test and the corresponding COP is known as actual COP.
• If the values of refrigerating effect and compression work are computed by applying the laws of thermodynamics and the corresponding COP is known as theoretical COP.
• The ratio of actual COP and the theoretical COP is known as relative COP.    

Here is some Fun fact about refrigeration:

In earlier days, before inventing the refrigerator people used various methods to preserve food. Here are some methods

Salting - In this method salt acts as a preservative. It draws out water from the food and slows down the bacteria growth. This process of removal of water is called osmosis.     

Root cellar - It is an underground structure where fruits and vegetables are placed inside it. It can be stored for up to weeks to months.

I hope you don't throw up after reading this fact. In the early Russian folk belief that placing a frog in milk can keep it from spoiling.