Top 4 Systems of Refrigeration | Electrical Engineering

There are four refrigeration systems: 1. Vapour Compression Refrigeration System 2. Vapour Absorption Refrigeration System 3. Thermoelectric Refrigeration System 4. Compound or Multi-Stage Vapour Compression Refrigeration System.

1. Vapour Compression Refrigeration System:

This is the most important system from the standpoint of commercial and domestic utility and most practical form of refrigeration. The working fluid, known as a refrigerant, used in this refrigeration system readily evaporates and condenses or changes alternately between the vapour and liquid phases without leaving the refrigerating plant. During evaporation it absorbs heat from the cold body and in condensing or cooling or liquefying it rejects heat to the external hot body.

The heat absorbed from cold body during evaporation is used as its latent heat for converting it from liquid to vapour. Thus a cooling effect is created in the working fluid. This system of refrigeration, thus act as a latent heat pump since it pumps its latent heat from the cold body or brine and rejects it or deliver it to the external hot body or the cooling medium. According to the laws of thermodynamics, this can be done only on the expenditure of energy which is supplied to the system in the form of electrical energy driving the compressor.

The main advantages of this system of refrigeration over the other systems are the smaller size for a given refrigerating capacity (except for some recent development in air refrigeration system in the aircraft cooling), higher coefficient of performance or lower power requirements for a given capacity and less complexity in both design and operation. It can be used over a large range of temperatures.

The main disadvantages like toxity, inflammability and leakage of vapours have largely been eliminated by improve­ments in design resulting in greater safety and prevention of leakages and by development of non-toxic, nonflammable vapours to be used as refrigerants.

The vapour compression cycle is used in most of the modern refrigeration systems in large industrial plants requir­ing the rapid removal of large quantities of heat and in most of the domestic refrigerators. The vapour in this cycle is circulated through the various components of the system, where it undergoes a number of changes in its state or con­dition.

Each cycle of operation consists of the four fundamen­tal changes of state or processes:

(i) Expansion

(ii) Vaporisa­tion

(iii) Compression and

(iv) Condensation.

A simple vapour compression refrigeration system consists of the follow­ing five essential parts:

i. Compressor:

The low pressure and temperature vapour refrigerant from evaporator is drawn into the compressor through the inlet or suction valve A, where it is compressed to a high pressure and temperature. The high pressure and temperature vapour refrigerant is discharged into the condenser through the delivery or discharge valve B.

ii. Condenser:

The condenser or cooler consists of coils of pipe in which the high pressure and temperature vapour refrigerant is cooled and condensed. The refrigerant, while passing through the condenser, rejects its latent heat to the surrounding condensing medium which is normally air or water. Thus hot refrigerant vapour received from compressor is converted into liquid form in condenser.

iii. Receiver:

The condensed liquid refrigerant from the condenser is stored in a vessel, known as receiver, from where it is supplied to the expansion valve or refrigerant control valve.

iv. Expansion Valve or Throttle Valve:

The function of this valve is to allow the liquid refrigerant under high pressure and temperature to pass at a controlled rate after reducing its pressure and temperature. Some of the liquid refrigerant evaporates as it passes through the expansion valve, but the greater portion is vaporised in the evaporator at the low pressure and temperature.

v. Evaporator:

An evaporator consists of coils of pipe in which the liquid vapour refrigerant at low pressure and temperature is evaporated and changed into vapour refrigerant at low pressure and temperature.

During evaporation process, the liquid vapour refrigerant absorbs its latent heat of vaporization from the medium (air, water or brine) which is to be cooled.

2. Vapour Absorption Refrigeration System:

The vapour absorption refrigeration system is one of the oldest method of producing refrigerating effect. The refrigerant, commonly used in this system, is ammonia. This system uses heat energy, instead of mechanical energy as in vapour compression systems, in order to change the conditions of the refrigerant required for the operation of the refrigeration cycle.

In the vapour absorption refrigeration system, the compressor is replaced by an absorber, a pump, a generator and a pressure reducing valve. These components in vapour absorption refrigeration system, perform the same function as that of a compressor in vapour compression refrigeration system.

In this system, the vapour refrigerant from the evaporator is drawn into an absorber where it is absorbed by the weak solution of the refrigerant forming a strong solution. This strong solution is pumped to the generator where it is heated by some external source. During the heating process, the vapour refrigerant is driven off by the solution and enters into the condenser where it is liquified. The liquified refrigerant then flows into the evaporator and thus the cycle is completed.

A simple vapour absorption refrigeration system essentially consists of an absorber, a pump, a generator and a pressure reducing valve, in addition to condenser, receiver, expansion valve and evaporator, as in the vapour compression refrigeration system.

In this system, the low pressure ammonia vapour leaving the evaporator, enters the absorber where it is absorbed by the cold water. The water has the ability to absorb very large quantity of ammonia vapour and the solution so formed, is known as aqua-ammonia. The absorption of ammonia vapour in water lowers the pressure in the absorber which in turn draws more ammonia vapour from the evaporator and thus the temperature of the solution is raised.

Some form of cooling arrangement (usually water cooling) is used in the absorber for removal of heat from the solution evolved there. This is necessary so that the absorption capacity of the water is increased. The strong solution thus formed in the absorber is pumped to the generator by the liquid pump. The pump increases the pressure of the solution up to 10 kg/cm².

The strong solution of ammonia in the generator is heated by some external source such as gas or steam. During the heating process, the ammonia vapour is driven off the solution at high pressure leaving behind the hot weak ammonia solution in the generator. This weak ammonia solution flows back to the absorber at low pressure after passing through the pressure reducing valve. The high pressure ammonia vapour from the generator is condensed in the condenser to high pressure liquid ammonia. The liquid ammonia is passed to the expansion valve through the receiver and then to the evaporator. Thus the simple absorption cycle is completed.

This system has a least number of moving parts. They are quite in operation and are used both in commercial as well as in domestic installations. In recent years, they have been used extensively in recreation vehicles, camper-trailers and boats as portable units due to their operation from a small gas cylinder.

3. Thermoelectric Refrigeration System:

This system of refrigeration works on the principle of Peltier effect according to which heat energy is evolved at one junction and absorbed at the other one when direct current is passed through a junction of two dissimilar metals like antimony and bismuth. It consists of a number of thermoelectric module assemblies in series joined by copper strips, as shown in Fig. 9.4.

Each thermoelectric module is built up of a large number of thermoelectric couples. When the direct current is passed through the thermoelectric module assemblies in the direction shown in Fig. 9.4, junctions at the top of the assembly are cooled and those at the bottom are heated up.

Thus top junctions abstract heat from the surroundings and produce refrigerating effect and bottom junctions require cooling by water. Module assembly, therefore, abstracts heat from the medium at top and rejects the same to the medium at bottom. If the direction of flow of current is reversed, heat production at the junctions is reversed, i.e., now bottom of the modules will be cooled and top ones get heated up.

If it is an air conditioner, air will get heated up instead of getting cooled. Thus summer and winter operating of air conditioner can easily be obtained just by reversing the direction of flow of current. The main advantages of this system of refrigeration are absence of moving parts and ease of automatic control by controlling the magnitude of current. The only drawback of this system of refrigeration is its very high initial cost.

4. Compound or Multi-Stage Vapour Compression Refrigeration System:

The simple vapour compression refrigeration system in which the low pressure vapour refrigerant from the evaporator is compressed in single stage (or in a single compressor) and then delivered to a condenser at a high pressure. But sometimes, the vapour refrigerant is required to be delivered at a very high pressure as in the case of low temperature refrigerating systems.

In such cases either the vapour refrigerant should be compressed by employing a single stage compressor with a very high pressure ratio between the condenser and evaporator or it should be compressed in two or more compressors placed in series. The compression carried out in two or more compressors is called compound or multi-stage compression.

In vapour compression refrigeration systems, the major operating cost is the energy supplied to the system in the form of mechanical work. Thus any method of increasing coefficient of performance is advantageous so long as it does not involve too heavy increase in other operating costs, as well as initial plant cost and consequent maintenance, Since the coefficient of performance of a refrigeration system is the ratio of refrigerating effect to the compression work, the coefficient of performance can be increased either by increasing the refrigerating effect or by decreasing the compression work.

A little consideration will show that in a vapour compression system, the compression work is largely reduced if the refrigerant is compressed very close to the saturated vapour line. This can be achieved by compressing the refrigerant in more stages with intermediate intercooling. But it is economical only where the pressure ratio is considerable as would be the case when very low evaporator temperatures are desired or when high condenser temperature may be required. The compound compression is generally economical in large plants.

The refrigerating effect can be increased by maintaining the condition of the refrigerant in more liquid state at the entrance to the evaporator. This can be achieved by expanding the refrigerant very close to the saturated liquid line. It may be noted that by sub-cooling the refrigerant and by removing the flashed vapour, as they are during multi-stage expansion, the expansion can be brought close to the liquid line.