Mode of operation

Simplified diagram of the absorption chiller’s

set-up and mode of operation

The cooling part of the absorption chiller is comparable to that of compression chillers. A refrigerant compressed to high pressure, in our case ammonia, is condensed in a condenser and releases heat to a recooling circuit and finally mostly to the environment. The liquid refrigerant is then subcooled in an internal heat exchanger which increases the efficiency of the system.

The refrigerant is then expanded from high pressure to low pressure by means of an expansion valve. This lowers the temperature of the liquid refrigerant to the evaporating temperature of the low pressure. At this low temperature, the refrigerant absorbs heat from a refrigerant circuit and evaporates. This produces the useful cooling capacity and the refrigerant is cooled down. The evaporated refrigerant with a still low temperature is superheated in the internal cold/heat exchanger. In case of compression chillers, the compressor would now come into operation compressing the vaporous refrigerant from low pressure to high pressure.

In case of absorption chillers, the ammonia’s feature to dissolve very well in water is taken advantage of. This takes place in the absorber. There, the vaporous ammonia is brought together with the ammonia-poor aqueous solution, where the ammonia dissolves in the water and the ammonia concentration of the solution increases. During this process, heat is released. However, the higher the temperature of the solution, the less ammonia can dissolve in the water. Therefore, the absorber is cooled. This waste heat is also released into the environment via the recooling circuit.

After the absorber there is an ammonia-rich solution which can be pumped from low pressure to high pressure with a simple pump. Due to the incompressibility of a liquid, the power requirement of this pump is only a fraction of that of a comparable refrigeration compressor. Since the solubility of ammonia in water depends only on the pressure and temperature, the ammonia boils out of the liquid again even at high pressure if only the temperature is raised far enough. For this purpose, the rich solution is first preheated in a solution heat exchanger by means of internal heat recovery and is then transferred to the generator. The actual actuating energy of the plant is fed into this generator. With the actuating heat, which for example comes from a CHP, the temperature of the solution is increased until the ammonia evaporates again. In the separator, the vaporous ammonia separates from the then ammonia-poor solution flowing upwards to the cooling part of the absorption chiller. This solution is pre-cooled in the solution heat exchanger and then expanded to low pressure in the expansion valve before being returned to the absorber.