AMMONIA-WATER (OR PRACTICAL) VAPOUR ABSORPTION SYSTEM (NH3 – H2O)

In the NH₃ − H₂O vapour absorption system, ammonia is the refrigerant and water is the absorber. An analyser, a rectifier and two heat exchangers are added to the simple vapour absorption system. The complete schematic diagram of ammonia-water absorption system is shown in Fig. 19.24.

The functions of various accessories are:

1. Analyser: In the simple vapour absorption system, some water is also varporised along with ammonia and flows into the condenser. These water vapours further move and enter into the expansion valve, where they freeze and choke the pipeline. The analyser is used to remove these water vapours. The analyser may be an integral part of the generator or as a separate piece of equipment. It consists of a series of trays mounted above the generator. The strong solution from the absorber and the aqua from the rectifier (or dehydrator) are introduced at the top of the analyser and flows downward on the trays and into the generator. In this way, considerable liquid surface area is exposed to the vapour rising from the generator. The vapours are cooled and most of the water vapours condense, so that mainly ammonia vapours leave the top of the analyser. This also reduces the external heat required in the generator since aqua is heated by the vapour.
2. Rectifier or Dehydrators: The function of the rectifier is to completely remove the water vapours still left in the analyser. The rectifier is a closed type vapour cooler of the double pipe type, shell and coil, or shell and tube type. Its function is to further cool the ammonia vapours leaving the analyser so that the remaining water vapours are condensed. The condensate from the rectifier is returned to the top of the analyser by a drip return pipe.
3. Heat exchangers: The heat exchanger-1 provided between the pump and the generator is used to cool the weak hot solution returning from the generator to the absorber. The heat removed from the weak solution raises the temperature of the strong solution leaving the pump and going to analyser and generator. This operation reduces the heat supplied to the generator and the amount of cooling required for the absorber. Figure 19.24 Ammonia-water vapour absorption systemThe heat exchanger-2 provided between the condenser and the evaporator may be called liquid sub-cooler. The liquid refrigerant leaving the condenser is sub cooled by the low temperature ammonia vapour from the evaporator. This sub-cooled liquid is now passed to the expansion valve and then to the evaporator.Net refrigerating effect, R_E = Heat absorbed in the evaporatorTotal energy supplied to the system,W_t = Work done by the pump + Heat supplied in generator