A boiling refrigerant in an evaporator absorbs heat. The evaporator pressure must be low for boiling to take place. To produce the low pressure, it is necessary to remove the refrigerant as soon as the boiling refrigerant vaporizes. Vapors can be absorbed quickly by another liquid. However, the other liquid must be able to absorb the vapor when it is cool. It will then release the absorbed heat when it is heated.
Ammonia is one of the refrigerants most commonly used in the absorption-type air-conditioning systems. Ammonia vapors are absorbed quickly by large amounts of cool water. In fact, it can absorb vapor as quickly as a compressor.
High-pressure ammonia can be fed as a pure liquid through a metering device directly into an evaporator. See Fig. 16-11. Refrigeration takes place until the highside liquid ammonia is exhausted or the water in the absorber tank is saturated. Once saturated, it no longer absorbs ammonia. If the ammonia tank and the absorber are large enough, these components can be used as part of an air-conditioning system.
A system can be devised to handle large installations. See Fig. 16-12. In this system, some of the ammonia is removed from the water. This leaves a weak water solution of ammonia. This solution flows by gravity to the absorber.
The water in the absorber absorbs the ammonia. Such absorption continues until ammonia represents 30 percent of the water-ammonia solution.
Such a strong (30 percent) solution of ammonia is called strong aqua. Aqua means water. The strong aqua is pumped up to the generator. The absorber operates at low-side evaporator pressure. That is why the pump is necessary. The generator has a high-side pressure.
Air is driven out of water by heat. Ammonia also can be driven out of water by applying heat. The high temperature ammonia vapor rises and moves to the condenser. Weak condensed liquid flows back to the absorber through the force of gravity. In the condenser, the latent heat is removed from the ammonia vapor. Condensed ammonia liquid flows through the liquid receiver to the evaporator. In the evaporator, the ammonia boils at reduced pressure. Latent heat is absorbed. The liquid ammonia changes into a vapor. In changing to a vapor, the ammonia produces refrigeration. Ammonia is only one refrigerant used for this type of system. Lithium bromide and water also can be made into a refrigerant. Figure 16-13 shows a typical absorber system. Several manufacturers make packaged units for absorber systems.