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What is Refrigeration?

Refrigeration is the transfer of heat from a lower temperature region to a higher temperature one.
Devices that produce refrigeration are called refrigerators, and the cycles on which they operate are called refrigeration cycles.
In nature, heat flows in the direction of decreasing temperature, that is, from high-temperature regions to low-temperature ones.
This heat-transfer process occurs in nature without requiring any devices.
The reverse process, however, cannot occur by itself.
The transfer of heat from a low-temperature region to a high-temperature one requires special devices called refrigerators.

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Types of Refrigeration System

Non-cyclic refrigeration: In this type of system, Ice or Subliming Dry Ice (Frozen Carbon Dioxide) is used to absorb the heat from a substance one wants cooled.
Cyclic refrigeration: This consists of a refrigeration cycle, where heat is removed from a low-temperature space or source and rejected to a high-temperature sink with the help of external work.

Cyclic refrigeration can be classified as:

Vapor cycle: Refrigerant change phase during heat transfer from lower temperature to higher temperature.
Gas cycle: Refrigerant remain same phase during heat transfer from lower temperature to higher temperature.

Vapor cycle refrigeration can further be classified as:

Vapor-compression refrigeration
Vapor-absorption refrigeration

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Performance of a Refrigeration System

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Refrigerator Vs Heat Pump

Refrigerators and heat pumps are essentially the same devices; they differ in their objectives only.

The objective of a refrigerator is to maintain the refrigerated space at a low temperature by removing heat from it.
This is accomplished by absorbing heat from the working space and releasing it to the warm surrounding environment.

The objective of a heat pump, however, is to maintain a heated space at a high temperature.
This is accomplished by absorbing heat from a low-temperature surrounding environment, and supplying this heat to a warmer medium (Working space) such as a house.

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Relation between COP of Heat Pump and Refrigerator

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Reversed Carnot Cycle

The reversed Carnot cycle cannot be approximated in actual devices and is not a realistic model for refrigeration cycles.
However, the reversed Carnot cycle can serve as a standard against which actual refrigeration cycles are compared.

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Vapor Compression Refrigeration- Working

Compressor: The low-pressure and temperature vapor refrigerant from the evaporator is drawn into the compressor through the inlet or suction valve A, where it is compressed isentropically to high pressure and temperature. This high-pressure and temperature vapor refrigerant is discharged into the condenser through the delivery or discharge valve B.
Condenser: The condenser or cooler consists of coils of pipe in which the high-pressure and temperature vapor refrigerant is cooled and condensed. The refrigerant, while passing through the condenser, gives up its latent heat to the surrounding condensing medium which is normally air or water.

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Receiver: The condensed liquid refrigerant from the condenser is stored in a vessel known as a receiver from where it is supplied to the evaporator through the expansion valve or refrigerant control valve.
Expansion Valve: It is also called a throttle valve or refrigerant control valve. The function of the expansion 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 refrigerants evaporate as they pass through the expansion valve, but the greater portion is vaporized in the evaporator at low pressure and temperature.

Vapor Compression Refrigeration- Working

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Evaporator: An evaporator consists of coils of pipe in which the liquid-vapor refrigerant at low pressure and temperature is evaporated and changed into vapor refrigerant at low pressure and temperature. In evaporating, the liquid-vapor refrigerant absorbs its latent heat of vaporization from the medium (air, water, or brine) which is to be cooled.

Vapor Compression Refrigeration- Working

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p-h diagram

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Vapor Compression Cycle

1-2: Isentropic compression in a compressor

2-3: Constant-pressure heat rejection in a condenser

3-4: Throttling in an expansion device (Isenthalpic)

4-1: Constant-pressure heat absorption in an evaporator

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Types of Vapor Compression Cycle

Cycle with dry saturated vapor after compression
Cycle with wet vapor after compression
Cycle with superheated vapor after compression
Cycle with superheated vapor before compression
Cycle with undercooling or subcooling of refrigerant

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Cycle with dry saturated vapor after compression

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Cycle with wet vapor after compression

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Cycle with superheated vapor after compression

Reason of Superheating after compression,

Superheating increases the refrigerating effect and the amount of work done in the compressor.
Since the increase in refrigerating effect is less as compared to the increase in work done, therefore, the net effect of superheating is to have a low coefficient of performance.
Moreover, in practice, the compressor can not deal with the mixture of liquid and vapor refrigerant. Thereby, it is supplied into the compressor at dry vapor state and comes out in a superheated state.

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Cycle with superheated vapor before compression

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Cycle with undercooling or subcooling of refrigerant

Reason of Undercooling,

Subcooling increases the refrigerating effect, thereby, increases COP

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Advantages and Disadvantages of Vapor Compression Cycle

Advantages:

Smaller Size
Less running cost
Can be employed over a large range of temperature
High coefficient of performance

Disadvantages:

High initial cost
The prevention of leakage of the refrigerant is the major problem in vapor compression systems.

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Simple Vapor Absorption Refrigeration

Here the compressor is replaced by an absorber, a pump, a generator, and a pressure-reducing valve.
The low-pressure ammonia vapor leaving the evaporator enters the absorber where it is absorbed by the cold water in the absorber.
The cold water can absorb very large quantities of ammonia vapor and the solution, thus formed, is known as aqua ammonia.
The absorption of ammonia vapor in water lowers the pressure in the absorber which in turn draws more ammonia vapor from the evaporator.
The absorption of ammonia in the water is an exothermic reaction, thus some cooling arrangements are provided to keep the solution cold because at higher temperatures water absorbs less ammonia vapor.

The method utilizes the fact that the solubility of ammonia in water at low temperatures and pressures is higher than it is at higher temperatures and pressures

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The strong solution thus formed is pumped to the generator by the liquid pump.
The pump increases the pressure of the solution up to 10 bar.
The solution in the generator is heated by some external source.
During the heating process, the ammonia vapor is released from the solution at high pressure due to the lower solubility of ammonia at high-temperature water and leaves behind the hot weak ammonia solution.
This weak ammonia solution flows back to the absorber at low pressure after passing through the pressure-reducing valve.
The high-pressure ammonia vapor enters the condenser and then passes through the expansion valve and evaporator before finally coming back to the absorber.

It is economically attractive when there is a source of inexpensive thermal energy at a temperature of 100 to 200⁰C

Simple Vapor Absorption Refrigeration

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Difference between Vapor Compression and Vapor Absorption Refrigeration

Aspect	Vapor Absorption System	Vapor Compression System
Energy Input	Vapor absorption system takes in low grade energy such as waste heat from furnace, exhaust heat or solar heat for its operations.	Vapor compression system takes in high grade such as electrical or mechanical energy for its operation of compressor used in the cycle.
Moving part	It uses a small pump as moving part, which is run by a small motor.	It uses a compressor driven by an electric motor or engine.
Evaporator pressure	It can operate with reduced evaporator pressure, with little decrease in refrigerant capacity.	The refrigerant capacity decreases with lowered evaporator pressure.
Load variation	The performance of vapor absorption system does not change with load variation	The performance of vapor compressing system is very poor at partial load.
Evaporator exit	In vapor absorption system, the liquid refrigerant leaving the evaporator does not put any bad effect on the system except to reduce the refrigerant effect.	In a vapor compression system, it is desirable to superheat vapor before leaving the evaporator, so no liquid can enter the compressor.

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Difference between Vapor Compression and Vapor Absorption Refrigeration

Aspect	Vapor Absorption System	Vapor Compression System
Lowest temperature	Since water is used as refrigerant, thus the lowest temperature attained is above 0°C.	With cascading, the temperature can be lowered up to -150°C or even less temperature.
Coefficient of Performance	The COP of the system is poor.	The COP of the system is excellent.
Capacity	It can built in capacities well above 1000 TR.	For a single compression system, it is not possible to have a system with more than 1000 TR capacity.
Refrigerant	Water or ammonia is used as the refrigerant.	Chlorofluorocarbon, hydrofluorocarbon, and hydrochlorofluorocarbon are used in most of the systems.
Damage	Liquid traces of refrigerant present in piping at the exit of the evaporator constitute no danger.	Liquid traces in the suction line may damage the compressor.
Charging of refrigerant	Difficult	Simple
Leakage of refrigerant	No chance as there is no compressor	More Chances

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