Absorption refrigeration is a useful cooling technology that has not enjoyed the widespread acceptance and use of compression refrigeration. Absorption refrigeration seems counterintuitive because it is cooling driven by a heat source. Ammonia is commonly used as the refrigerant liquid. This is in contrast to compression refrigeration, where the most common refrigerant is a tetrafluoroethane known as R-134. In compression refrigeration, the R-134 gas absorbs heat in an evaporator coil. It is then compressed and pumped into a condenser coil. Under higher pressure, it releases heat and reverts to a liquid. The heat is exhausted out of the building. However, in an absorption refrigerator, ammonia boiling in the evaporator moves heat from the room being cooled. The heat is exhausted when the ammonia mixes with water and condenses in the absorber. The mixture is heated with a flame and the ammonia is boiled out of the water. It is then collected in the condenser for reuse in the absorber.
Operation of an Absorption Refrigerator
The refrigeration process begins with a mixture of anhydrous liquid ammonia and hydrogen gas. The hydrogen gas lowers the partial pressure of the ammonia, causing its boiling point to fall as well. This hydrogen and ammonia mix goes into water in the evaporator. As it evaporates, the vapors rise up into the absorber. The absorber is basically a set of pipes. Cool water trickles down these pipes, running against the gases as they rise up. The ammonia condenses and falls back down but the hydrogen rises up and is collected for reuse. The refrigerant solution is pumped into the boiler or generator next. The water and ammonia mix is heated to a precise temperature by a flame or electric resistor. The solution boils and the vapors rise up into the condenser. The first part of the condenser is the separator. The separator contains angled pipes which pop any bubbles and cause the water to run back down apart from the ammonia. Then, the ammonia enters a heat exchanger where it cools back down to liquid form. The pure anhydrous liquid ammonia is reused from the start of the cycle.
Applications of Absorption Refrigeration
Absorption refrigeration has uses both large and small. Compression refrigeration is well known for its use in cooling residential buildings. Compression refrigeration is also used in kitchen coolers with volumes between 18 and 25 cubic feet. Absorption refrigeration is less powerful than compression refrigeration and absorption refrigerators for personal use are small. However, compression refrigerators consume an amount of electricity too large to generate economically with solar panels. Because they can operate on propane or other natural gases, absorption refrigerators have the advantage of portability. These smaller units are installed in campers, cabins, caravans, and camp sites. Large-scale absorption chillers are also used in industrial settings. These units use an expansion valve or pump instead of the partial pressure of hydrogen. They may also use lithium bromide instead of ammonia. These units have a wide array of uses, including cooling x-ray machines, printing presses, and injection molding equipment.