Vapour absorption refrigeration

VAPOUR ABSORPTION REFRIGERATION

Principle

The difference between the Vapour Absorption System and Vapour Compression System is in the manner in which external heat is added to the vapourized refrigerant to increase its temperature above atmosphere.

In the Vapour Compression System, the motor-operated Compressor adds heat to the refrigerant by compressing it to high pressure and high temperature.

But, in the Vapour Absorption System, there is no compressor. The compressor is replaced by the combined effects provided by an Absorber, Pump and Generator. The compressor work is replaced by the heat supplied in the generator and pump work. The pump consumes comparatively lesser amount of electric power. The generator is operated by heat energy obtained by burning low cost fuel or any other heat source such as solar energy.

 Absorbent: Absorbent absorbs refrigerant vapour and converts it into liquid. When heated subsequently, it produces vapour. Water has this property and is used as the absorbent.

Refrigerant: Ammonia (NH3) is used as the refrigerant, as it easily dissolves in water and vapourizes when heated subsequently.

Aqua-Ammonia System: The homogenous mixture of Ammonia and Water is called AquaAmmonia. So, the vapour absorption system is also called Aqua-Ammonia Refrigeration System.

Description

 See Fig. 3. The Vapour Absorption Refrigeration System consists of the following parts:

1. Evaporator

2. Absorber

3. Circulation Pump

4. Generator

5. Condenser

6. Expansion Valve

1. Evaporator: It is placed in the Refrigerated Space. An Evaporator consists of coiled tubes. The substance to be cooled is placed in the evaporator. It is the coldest region or space in the refrigerator and serves as the Refrigerated Space or Freezer Compartment.

2. Absorber Tank: Absorber Tank contains absorbent (water). The absorbent should have high affinity for the refrigerant, remain in the liquid phase under operating conditions. It should have a high boiling point and low specific heat. It attracts the refrigerant vapour from the evaporator.

3. Circulation Pump: The Pump circulates the refrigerant to flow into the Generator.

4. Generator:  Heat is added to the Generator from an external source. It may be a gas burner, or solar heater.

5. Condenser: Air or water is used as the cooling medium in the condenser. Air is used for refrigerators and window air conditioners. Water is used for large centralized air conditioning systems.

6. Expansion Valve: The condenser is connected to an Expansion Valve. The pressure of the liquid passing through the expansion valve drops for reuse in the Evaporator.

Working

 The dry LP ammonia vapour from the Evaporator enters the Absorber. The Absorber contains cold water. Dry ammonia vapour is absorbed (dissolved) in the cold water, producing a LP strong (concentrated) ammonia solution. A Circulation Pump pumps the strong ammonia solution from the Absorber to the Generator. Thus, the pressure of the solution is increased.

The high pressure solution from the Pump enters the Generator. In the Generator, the solution is heated by any heat source including solar. Hence, high pressure Ammonia vapour is generated, which enters the Condenser.

Also, the weak ammonia solution available in the Generator contains more amount of water. It goes back to the Absorber and is sprayed at the top of the Absorber. In the Absorber, water absorbs the incoming ammonia vapour and becomes once again a strong solution.

In the Condenser, cold water is circulated around the condenser tubes. Therefore, the Ammonia vapour is condensed to liquid Ammonia. The HP ammonia liquid is then expanded to LPLT in the Expansion Valve. The LPLT Ammonia liquid is passed to the evaporator in the Freezer Compartment.

In the Evaporator, the LPLT liquid absorbs the heat from the evaporator and evaporates. The LP Ammonia vapour from the Evaporator flows again to the Absorber. The cycle is repeated.

 

1. VAPOUR COMPRESSION SYSTEM Vs VAPOUR ABSORPTION SYSTEM