Absorption System

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A few household units, called gas refrigerators, operate on the absorption principle. In such gas refrigerators a strong solution of ammonia in water is heated by a gas flame in a container called a generator, and the ammonia is driven off as a vapor, which passes into a condenser. Changed to a liquid state in the condenser, the ammonia flows to the evaporator as in the compression system. Instead of the gas being inducted into a compressor on exit from the evaporator, however, the ammonia gas is reabsorbed in the partially cooled, weak solution returning from the generator, to form the strong ammonia solution. This process of reabsorption occurs in a container called the absorber, from which the enriched liquid flows back to the generator to complete the cycle.

Increasing use of absorption refrigeration now occurs in refrigeration units for comfort space cooling, for which purpose refrigerant temperatures of 45° to 50° F (7.2° to 10° C) are suitable. In this temperature range, water can be used as a refrigerant with an aqueous salt solution, usually lithium bromide, as the absorbent material. The very cold boiling water from the evaporator is absorbed in concentrated salt solution. This solution is then pumped into the generator, where, at elevated temperature, the surplus water is boiled off to increase the salt concentration of the solution; this solution, after cooling, recirculates back to the absorber to complete the cycle. The system operates at high vacuum at an evaporator pressure of about 1.0 kPa (0.145 psi); the generator and condenser operate at about 10.0 kPa (1.45 psi). The units are usually direct-fired or use steam generated in a boiler.

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