Why do we need Li-Br with water in absorption chiller?

To absorb water from  atmosphare and from the evaporated refrigerant.

In a water-lithium bromide vapor absorption refrigeration system, water is used as the refrigerant while lithium bromide (Li Br) is used as the absorbent. In the absorber, the lithium bromide absorbs the water refrigerant, creating a solution of water and lithium bromide. This solution is pumped by the pump to the generator where the solution is heated. The water refrigerant gets vaporized and moves to the condenser where it is cooled while the lithium bromide flows back to the absorber where it further absorbs water coming from the evaporator.

The water-lithium bromide vapor absorption system is used in a number of air conditioning applications. This system is useful for applications where the temperature required is more than 32 degree F.

Li-Br is a salt and salt have a ability to absorb water form the atmosphere. Li-Br  is used in absorption chiller to absorb evaporated refrigerant (water) in the evaporator to maintain vacuum. As you know Neutralization Reaction Acid + Base > salt + water

                                                             Hcl + NaoH  > Nacl + H2O

                                                            HBr + LioH >  Li-Br  + H2O

 Li-Br has strong absorption Power