What is a vapor absorption refrigeration system

The principal difference in the absorption and vapor compression system is the motivating force that circulates the refrigerant through the system and provides the necessary pressure differential between the evaporation and condensation processes.

The vapor absorption refrigeration system uses heat energy as the motivating force, instead of mechanical energy as in the vapor compression refrigeration system.

Basic absorption refrigeration cycle

  • A simple vapor absorption refrigeration system. It consists of the following components.
  1. Absorber
  2. Absorber pump
  3. Generator
  4. Pressure reducing valve(PRV)
  • All the above four components will replace the compressor of the vapor compression refrigeration system.
  1. Condenser
  2. Receiver
  3. Expansion valve
  4. Evaporator
  • All the above four components are as in vapor compression refrigeration system.
Simple Vapor absorption refrigeration system

Two working fluids are employed in the vapor absorption system such as:

  1. Refrigerant (Ammonia)
  2. Absorbent (Water)

Absorbent:

Coldwater is normally used as an absorbent in the absorber. The water has the ability to absorb very large quantities of ammonia vapor. The solution, thus formed is known as aqua-ammonia.

  1. The flow cycle of the refrigerant is from the condenser to the evaporator, to the absorber, to the generator, and back to the condenser.
  2. The flow cycle of the absorbent is from the absorber to the generator and back to the absorber.

Working principle

  • The low-pressure ammonia vapor leaving the evaporator enters the absorber.
  • There it is absorbed by cold water in the absorber. The absorption of ammonia vapor in water lowers the pressure in the absorber, which tends to draw more ammonia vapor from the evaporator.
  • While absorbing more ammonia vapor, the temperature of the solution (rich solution) raises, and the heat thus formed in the absorber is removed with help of a cooling arrangement.
  • The cooling arrangement is necessary in order to increase the absorption capacity of water because at high temperatures, water absorbs less ammonia vapor.
  • 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.
  • The strong solution of ammonia in the generator is heated by some external source such as gas or steam. The ammonia vapor is driven off the solution at high pressure leaving behind the hot weak ammonia solution in the generator.
  • The weak ammonia solution flows back to the absorber at low pressure after passing through the pressure-reducing valve.
  • The high-pressure ammonia vapor from the generator is condensed in the condenser to high-pressure liquid ammonia.
  • This liquid ammonia is passed to the expansion valve through the receiver and then to the evaporator. Where it absorbs the heat from the refrigerated space causes the cooling effect. This completes a simple vapor absorption refrigeration system.
  • Ammonia-water systems are widely used in domestic refrigerators and also in commercial and industrial applications.
  • The efficiency of this system can be improved by the use of
  1. Analyzer
  2. Rectifier (Reflux condenser)

Analyzer – A distillation column where the water vapor is removed from the mixture leaving the generator.

Rectifier – Where the remaining water vapor and a small amount of ammonia vapor, condenses and drains back to the analyzer.

Vapour absorption system Vs Vapour compression system

SI.No.Vapour compression systemVapour absorption system
1.Wear, noise and maintenance are more due to the compressorQuiet, little wear, and less maintenance
2.Mechanical energy is the motivating forceHeat energy is the motivating force. Solar
energy can be used.
3.The system is used where high-grade mechanical energy is availableThe system can work even with a simple kerosene oil lamp
4.Manufacturing cost is moreSimple and less manufacturing cost
5.Co-efficient of performance is much higher being more than 4.0Co-efficient of performance is much lower
being less than 1.0
6.Poor performance at partial loadsNot affected by load variations.
7.Charging of refrigerant is simpleCharging of the system is difficult
8.Chances of leakage are moreChances of leakage are less
9.Liquid traces in the suction line may damage the compressorLiquid traces in piping at the exit of the evaporator constitute no damage
10.Use ozone-depleting refrigerantsNo ozone-depleting refrigerants

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