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The absorption refrigerator is a refrigerator that utilizes a heat source to provide the energy needed to drive the cooling system rather than being dependent on electricity to run a compressor. These refrigerators are popular where electricity is unreliable, costly, or unavailable.
The absorption refrigeration and absorption cooling are attractive when there is a source of inexpensive or waste heat readily available. This cycle uses a refrigerant that is readily soluble in a liquid transport medium. In brief, the condensation, expansion and evaporation processes are identical to those of conventional refrigerators and air conditioners. But instead of the latter's compression process, the absorption cycle's liquid transport medium absorbs the refrigerant vapor upon leaving the evaporator, creating a liquid solution. This solution is then pumped to a higher pressure, and then heat is used to separate the refrigerant from the solution, whereupon the high-pressure refrigerant flows to the condenser to continue the familiar cycle. The equipment used to accomplish the solution-dissolution processes is complex and heavy, but the advantage lies in the low work input requirement to raise the pressure of a liquid solution as compared to that required for compressing a gas. If the heat utilized is otherwise wasted heat, the low operating costs of absorption systems can be quite attractive.
The two most common refrigerants used in absorption systems are ammonia, with water as the transport medium, and lithium bromide in water. However, toxicity issues with ammonia require safeguards, adding to system cost and complexity. Lithium bromide can be corrosive to most common materials, adding to cost and complexity again. Absorption systems are used mostly in large non-vehicular building applications, though occasionally there has been advocacy of their use as vehicle air conditioning system.
In the developing world gas absorption refrigerators running on kerosene are very valuable where there is no reliable electricity supply. Besides the obvious advantages for food preservation, they supply safe storage of vaccines and drugs.
In the developed world, including the United States, absorption refrigerators are most often used for camping and in recreational vehicles, though some are used in remote areas or in off grid homes. The heat source that powers the refrigeration cycle is most often a propane burner, though other fuel gases such as butane and natural gas are sometimes used instead in areas where these are more readily available. Electricity can provide the heat for an absorption refrigerator and many camping versions can be switched to use it. In extremely remote areas, kerosene is used.
The cooling components of absorption refrigerators can be constructed without moving parts and with no reliance on electricity. However, most new units have thermostats, ignition systems, safety pilot controls, and lights that introduce some moving parts and reliance on a nominal amount of electricity -- little enough to be supplied by a dry cell or battery.
These refrigerators have the disadvantage of using a lethal gas, ammonia, although it is contained within a closed system; and if they are being run using an open flame then attention has to be paid to its safety and to maintaining the flame at an appropriate size.
The basic principle of how these refrigerators cool their interior is the same as in conventional compression refrigerators. Both use refrigerant liquids that boil at low temperatures to achieve cooling. A large amount of energy is needed to turn the liquid into gas, and this is absorbed from the surroundings, cooling them. The difference is in how the gas is turned back to liquid to use it again for more cooling. A conventional electric refrigerator uses a pump to compress the gas back to liquid; a gas absorption refrigerator uses another liquid to absorb the gas and then has extra stages to separate the pure refrigerant liquid out again for re-use, this extra part being where the heat source is involved.
Variations exist, but the usual pattern of gas absorption refrigerator uses ammonia as the refrigerant and water as the absorber, with hydrogen gas included to adjust the pressure in the system. The exact mixture and pressure has to be correct to give proper cooling at normal temperatures.
The cooling system consists of four main parts: the evaporator, absorber, generator and condenser. The evaporator is where chilling happens as the liquid ammonia evaporates, absorbing heat from its surroundings to enable the liquid to turn to gas. The ammonia gas passes on to the absorber, where it dissolves in water. The solution flows down to the generator where the external heat source is used to bubble the ammonia out again. This gas is purified by condensing out water vapour before rising to the main condenser. The condenser has big fins on the outside which let air cool it sufficiently for the ammonia to flow out of the bottom as a liquid again, down into the evaporator.
The hydrogen plays an interesting part in making this cycle work. Why does the ammonia which turned to liquid in the condenser then evaporate at essentially the same temperature and pressure in the evaporator? Because the evaporator also contains hydrogen gas, and the partial pressures exerted by the two gases mean that the ammonia liquid will evaporate (Dalton's law applies). The hydrogen is present in the absorber too, but is not absorbed as it does not dissolve in the water.
The ammonia, water and hydrogen system is closed. The water in the absorber is actually a weak solution of ammonia carried out of the generator when the ammonia bubbles out by a "bubble pump" like the pipe of a coffee percolator. There is no other pump in the system, liquids flowing back down under gravity and the gas pressure being the same throughout the system.
There are other gas absorption refrigerators which use only ammonia and water with a heat source applied intermittently, possibly just once a day. These have a pressure valve between the condenser and evaporator so that the generator and condenser are pressurised when the generator is heated. The liquid ammonia is forced slowly through the valve from the high pressure side into the evaporator where it evaporates in the lower pressure. The pressure differential will last for some time after the heat source is removed, so active cooling occurs for longer than the time the heat is applied. When the pressures have equalised, the ammonia solution from the absorber can flow back into the generator ready for the cycle to start again.
Gas absorption cooling principles can be applied at other temperatures, notably for air-conditioning where water may be the refrigerant, evaporated at low pressure and absorbed into a salt solution.
History
The absorption refrigerator was invented by Baltzar von Platen and Carl Munters in 1922, while they were still students at the Royal Institute of Technology in Stockholm, Sweden. Commercial production began in 1923 by the newly formed company AB Arctic, which was bought by Electrolux in 1925.
Bibliography
In Swedish:
- Baltzar von Platen and Carl Munters, Om alstring av kyla (1925)
There are few good explanations on the web of how gas absorption refrigerators work, and some very bad ones. A useful reference is:
- Andrew Delano (1998). Design Analysis of the Einstein Refrigeration Cycle. Retrieved September 13, 2005.
There is another explanation here with a helpful diagram -- both possibly originated from Elecrolux:
- How It Works, Details about the absorption system. Retrieved September 13, 2005.
This goes into great detail:
This is a home-made absorptive refrigerator:
Source
See also
"Gas-absorption refrigerator"