The absorption type of air-conditioning equipment works on two basic principles: a salt solution absorbs water vapor and the evaporation of water causes cooling. In this particular discussion, the absorption cooling cycle is appropriate since it is used in gas-fired air conditioners.
Most gas-fueled air-conditioning equipment use a solution of lithium bromide (LiBr) in water. Lithium bromide is a colorless, salt like compound that dissolves in water, even to a greater extent than does common salt.
A solution of lithium bromide and water can absorb still more water. Note that, in Fig. 17-1, a tank of absorbing solution (tank B) is connected with a tank of water (tank A). The air in the system is almost completely evacuated. The partial vacuum aids the evaporation process. Water vapor is drawn from the evaporator to the absorber. Evaporation of the water in the evaporator causes the water remaining in it to cool about 10°F (5.5°C). The evaporator effect in the evaporator is greatly hastened if the water is sprayed through several shower-bath sprinkler heads. A coil of pipe through which a material such as water passes can be placed within the shower of evaporating water. The water entering the coil of pipe at 55°F (12.8°C) will be cooled to about 45°F (7.2°C).
Since the absorber (B), shown in Fig. 17-1, continually receives water, it would soon overflow if the excess water that comes to it as water vapor was not removed. To avoid overflow, the solution that has absorbed water is pumped to a generator (C).
In the generator, the solution is heated directly by a natural gas flame. A steam coil may heat it indirectly. The steam is made in a gas-fueled boiler.
When the solution is heated, some of the water evaporates and passes into the condenser (D). The concentrated solution, that remains is sprayed back into the absorber (B). Here, it again absorbs water vapor that comes from the evaporator.
Water vapor in the condenser (D) is cooled by a separate coil of pipe through which water passes. The condensed water is returned to the evaporator (A).
Careful engineering is needed to make the system work well and economically. Attention must be given to temperatures, pressures, and heat transfer in all parts of the system. Practical machines with very few moving parts have now been developed.
Absorption units may also use ammonia as the refrigerant. In such system, heat from natural gas is used to boil an ammonia-water solution. The operation of the lithium-bromide cycle discussed earlier generally applies. In a system using ammonia, the temperature of the evaporator can go below the freezing point of water. Ammonia is referred to as R-717.