Three chemicals are used, ammonia, hydrogen gas, and water. At room temperature ammonia is normally a gas that has a boiling point of -33°C, however, the absorption cooling system is pressurized to the point that the ammonia is held in a liquid state at room temperature.
The evaporator part of the absorption cooling system contains the hydrogen, which lowers the partial pressure of the ammonia, so that not all of the pressure is being exerted by ammonia. The hydrogen is used to fill space created by pressure as the rest of the system, but is not ammonia. The boiling point of the ammonia is now lowered so that it will now boil below room temperature, as though if is wasn't under the pressure of the absorption system. When the ammonia boils, it removes some of the heat from the evaporator, thus producing the desired cool temperature.
The next process is known as the absorption phase and is the separation of the ammonia from the hydrogen then transforms the gas ammonia back into its liquid state. Separating the hydrogen is relatively simple as ammonia readily mixes with water whereas hydrogen does not. The gases flow into the absorber, which is a cascade of tubes where the mixture of gases flows while water drips from mixing with the gases and separating the hydrogen from the ammonia.
At this point, it's now necessary to separate the ammonia from the water. This is achieved by heating the ammonia water mixture until the ammonia evaporates out. This phase is known as the generator. The water is then circulated back through the absorption phase.
The next phase of the process is known as the condenser and is where a heat exchanger cools the ammonia gas to room temperature, reverting back into a liquid state because of the pressure and the absents of hydrogen. The condensed ammonia is now suitable as a refrigerant and the process starts over.