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With a neat sketch explain Double effect Li-Br vapour absorption refrigeration system

Subject:- Refrigeration and Air Conditioning

Topic:- Design of air conditioning systems

Difficulty:- Low

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A single effect absorption system is not suited to utilize a heat source at a temperature higher than a certain point. Single-effect systems have been found to give best results up to a heat source temperature of 105°C.

COP decreases as the heat source temperature increases beyond a point since the absorption system is not a reversible refrigerating machine. This is because a certain degree of irreversibility is involved due to mixing process of refrigerant and absorbent.

A double effect system consists of two heat exchangers and two generators (high and low pressure). It is an absorption system with improved efficiency which employs a second generator which uses water vapour from first generator to provide its heat supply.

Since evaporator pressure is maintained very low, therefore refrigerant water evaporates after absorbing heat from the air to be conditioned, and the vapours are absorbed by strong Li-Br solution which is sprayed in absorber.

This absorption makes the solution weak. The weak solution is passed through heat exchangers where it absorbs heat from the strong solution coming from generators.

The weak solution then goes to first generator where it is heated up. This leads to evaporation of some portion of water thereby making the solution strong.

The strong LiBr solution from first generator passes through a heat exchanger where it transfers heat to weak LiBr solution now going to first generator.

This strong LiBr solution is now throttled to lower pressure to that of second generator causing the solution to develop flash vapour which is liquefied at the condenser and the remaining strong solution is passed back to absorber via another heat exchanger.

The strong LiBr solution that passes to second generator is heated, thereby condensing water vapour driven off by first generator which is also sent to the condenser.

The condensate from condenser is sent to evaporator thus completing the cycle.

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