Water Lithium Bromide Absorption Chiller

Water / lithium bromide system

Lithium bromide is the sorbent. Water has the dual function of the refrigerant and the solvent in the absorber. The lower temperature limit is set by the risk of freezing of the water and the crystallization of lithium bromide.

It can not be used to cool air directly in an air cooler, nor can it be used for water vapor condensation. The volumes of both liquids are too large. This prevents its use in small home air conditioners / heat pumps. So it's essentially a water cooler for medium power. The separation of water and lithium bromide is easy. Since lithium bromide is solid, a distillation column is not necessary to separate water and lithium bromide. The water is simply evaporated, leaving enough water to keep the lithium bromide in solution and avoid crystallization.

At these temperatures, water vapor has a very low density and requires special types of evaporators and condensers. Soldered or sealed PHE's are usually not suitable for handling lower density steam. However, there are usually one or two brine-to-brine heat exchangers that are used to heat the poor solution with the rich solution. Good regeneration is crucial for the economy of the process. Due to the long temperature profile, PHE's are extremely suitable, especially those in bipartite design (see positions 4 and 5 in the picture).

A pure lithium bromide solution is corrosive. It must be passivated with molybdate or chromate solutions. The pH should be kept as high as possible. Oxygen and chlorine should be as low as possible.

It is interesting to know that lithium hydroxide, which is used to increase the pH, is a better sorbent than LiBr. Due to the corrosive nature of LiBr, tests should be performed on the solution before it is used in a copper or steel heat exchanger. We have no long-term experience in BPHE corrosion. With regard to the corrosion mechanisms, pitting and stress corrosion play a role. They are all connected. As the names suggest, not only the materials are important, but also the construction and execution of the unit. 

The stainless steel plates used for soldered PHE's are almost as polished as polished, reducing the risk of pitting. The copper effectively fills in all cracks, thus limiting the risk of crack corrosion. Brazing is an effective stress relieving treatment that eliminates stress corrosion (at least the residual stress dependent part). Thus, most conditions are met to prevent corrosion. To improve the surface wetting and thus also to increase the efficiency in the utilization of the heat transfer surface, a type of cleaning agent or surfactant is added, for example based on octyl alcohol or the like.

Picture 1:  The water / lithium bromide absorption cooler

1. Evaporator: Cold water evaporates under vacuum and cools the cold water. The water vapor goes on in the

2. Absorber that absorbs the water vapor in the poor LiBr solution.

3. The pump promotes the produced rich solution in the

4 & 5. NT and HT regenerators, which warm the rich solution near the boiling point before it enters the

6. HT generator. Part of the water is evaporated off, usually in a gas-fired boiler. The resulting poor solution releases its heat in the HT & NT regenerators. More water evaporates in the intermediate NT generator.

7. separator to be used subsequently as heating medium in

8. NT generator, where out of the poor solution further water at lower temperature / pressure than in the HT stage is expelled. The use of the HT steam to heat the NT stage is common in evaporator systems to improve the economy. The conditions are similar here.

9. Condenser: Both the steam coming directly from the HT stage and the NT steam condense here. The resulting condensate is decompressed to evaporating temperature by means of

10. expansion valve and then enters the evaporator.

What: Alfa Laval