TYPES OF REFRIGERATION

1. Cyclic Refrigeration

In  the  cyclic  process  of  refrigeration  the  heat  is  removed  from  the  low temperature reservoir and is thrown to high temperature. As per the second law of thermodynamics  the  natural  flow  of  heat  is  from  the  high temperature  to  low temperature reservoir.  In the cyclic  refrigeration  process since the flow of heat is reversed,  the external  work has  to be done  on the system. The cyclic  process  of refrigeration is  also reverse of the thermodynamic  power cycle or Carnot cycle in which the  heat flows from high temperature reservoir to low temperature  reservoir; hence the cycle of refrigeration is also called as Reversed Carnot Cycle.

There are two types of cyclic process of refrigeration:

1 Vapour cycle 

2   Gas cycle.

  

The vapour cycle is classified into

1.1  Vapour compression cycle        1.2 Vapour absorption cycle.

1.1 Vapour Compression Cycle

            In a vapour compression system, an evaporator and a gas-liquid separator are received in a common casing, so that the gas-liquid separator and the evaporator are placed close to each other. Thus, it is possible to limit heart absorption of the liquid phase refrigerant from the atmosphere to reduce the heat loss upon discharge of the refrigerant from the gas-liquid separator. Also, it is possible to reduce pressure loss in refrigerant passage between the gas-liquid separator and the evaporator

 1.2 Vapour Absorption Cycle

Before the development  of the vapour  compression  system of refrigeration, vapour  absorption  system  was very widely  used.  The vapour  compression  system replaced  vapour  absorption  system  because  it  has  high  coefficient  performance (COP). The vapour  absorption system requires very less amount   of electricity but large amount of heat; hence it can be used very effectively in industries where very large stocks of excessive stem are available. In such cases there is not only effective utilisation of steam, but also lots of savings in electricity costs.

2. Gas Cycle

Just as the vapour are used for cooling in the vapour compression cycle and vapour absorption cycle, the gas is used cooling in gas refrigeration cycle. When the gas is throttled  from  very  high  pressure  to lower  pressure  in throttling  valve,  its temperature reduces suddenly while its enthalpy remains constant. This principle is in gas refrigeration system.

In the system instead of using Freon or ammonia as the refrigerant, the gas is used as the refrigerant. Throughout the cycle there are no phase changes of the gas, which are observed in the liquid refrigerant. Air is the most commonly used gas, also called as refrigerant in this case, in the gas refrigeration cycles.

2 Non Cyclic Refrigeration

In these methods, refrigeration can be accomplished by melting ice or by dry ice. These methods are used for small-scale refrigeration such as in laboratories and workshops, or in portable coolers.

2.1 Thermoelectric Refrigeration

A refrigeration effect can also be achieved without using any moving parts by simply  passing a small current through a closed circuit made up of two dissimilar materials. This  effect is   called Peltier effect, and a refrigerator that works on this principle is called a thermoelectric refrigerator.

Fig : 1.1 Thermoelectric refrigeration

Under steady-state operating conditions, heat will be transferred from the refrigerated space  to  the cold junction.  The other junction  will be at a temperature  above the ambient, and heat will be transferred from the junction to the surrpundings.     

  2.2 Magnetic refrigeration

       Magnetic refrigeration  is a cooling technology  based  on the magneto  caloric effect. This technique can be used to attain extremely low temperatures (well below 1K), as well as the ranges used in common refrigerators, depending on the design of the system.

 2.3 Other Methods

Other methods of refrigeration include the air cycle machine used in aircraft; the vortex  tube  used  for  spot  cooling,  when  compressed  air  is available;  and thermo acoustic  refrigeration using sound waves in a pressurised gas to drive heat transfer and heat exchange.