Another form of refrigeration is known as the absorption system, and strangely enough direct heat is applied to the machine in one place in order to abstract heat from it in another. A diagrammatic representation of the first machine of this type (which was invented by Frederick Carré) is shown in Figure 81. Two vessels, A and B, are employed, which are connected by a tube C. The vessel A contains ammonia solution. A lamp, D, is placed under the vessel A. Sufficient heat is produced to vaporize the ammonia, whose boiling point is very low, and distill it out of the water in the solution. It passes through tube C into vessel B. This vessel is surrounded by a tank, E, containing cold water, which condenses the ammonia vapor into liquid ammonia, then the process is reversed.
The circulation of water through tank E is checked and water from pipe F is sprayed on vessel A. This cools the contents of vessel A, producing a partial vacuum. The ammonia in chamber B boils and its vapors pass back into chamber A, where they are reabsorbed by the water in that chamber. The rapid vaporization in chamber B absorbs heat from the immediate surroundings and will freeze water placed in the tank E.
In commercial practice the absorption system is very extensively employed. The ammonia is placed in a large cylinder known as a generator and is heated by steam coils. The ammonia vapor passes through an analyzer which traps any water vapor it may contain, and then it goes through a series of condensing coils, which are cooled by water. In these condensing coils the ammonia vapor is liquefied by the pressure in the generator and collects in a receiver, whence it passes through an expansion valve into the cooling coils in the brine tank. From the cooling coils the ammonia gas passes back through an absorber which reverses the operation of the generator. Here weak aqua ammonia is sprayed on the ammonia gas and absorbs it. The rich ammonia solution is then pumped into the generator.
There are several auxiliary apparatus which are omitted in this brief description in the interest of clarity.
THE COMPRESSION SYSTEM
FIG. 82.—DIAGRAMMATIC VIEW OF A COMPRESSION MACHINE
One more type of refrigerating machine remains to be described and that is the compression type. This was invented by Jacob Perkins in 1834, but was not developed into a commercial machine until 1850. This machine is the most important of them all. In some respects it is like the absorption process, but in place of adding direct heat a compressing machine is employed. Figure 82 illustrates the system diagrammatically. At A is the compressor which compresses ammonia gas. The gas which is heated by the compressor is then cooled and liquefied in the condenser B. Thence it passes into a brine tank C, where it expands and absorbs heat. From this point it is drawn back into the compressor, thus completing the cycle. In other words, the ammonia must absorb as much heat from the brine as is taken out of it at the condenser.