Operation
The machine is a single-pressure absorption refrigerator, similar in design to a gas absorption refrigerator. The refrigeration cycle uses ammonia pressure-equalizing fluid, butane refrigerant, and water absorbing fluid, has no moving parts, and does not require electricity to operate, needing only a heat source, e.g. a small gas burner or electric heating element or even solar energy. In an Einstein refrigerator with the standard working fluids, a water-flow loop serves as an ammonia pump, and the ammonia-flow loop serves as a butane pump. Ammonia and water are suitable choices because ammonia is highly soluble in water and its solubility declines steeply with increasing temperature. Butane is a suitable choice for the refrigerant because it has a suitably low boiling point and is virtually insoluble in water.
On the cold side of a conventional refrigerator, a refrigerant evaporates at a temperature-dependent pressure, Prfr(Tlow). Evaporation absorbs heat from whatever is being cooled, and the vapor flows to a compressor. In an equivalent Einstein refrigerator, the refrigerant liquid evaporates at what is now a partial pressure Prfr(Tlow), mixing with a flow of ammonia vapor to form a gas with a total pressure near the system pressure: Prfr + Pamm = Psys. The mixture flows, not to a pump, but to an ammonia absorber.
On the hot side of a conventional refrigerator, a compressor raises the pressure of the refrigerant vapor, enabling it to condense at a relatively high temperature to deliver heat to an external heat sink. On the hot side of an Einstein refrigerator, an ammonia absorber raises the partial pressure of the refrigerant vapor to accomplish the same result.
The absorber works by removing ammonia vapor by dissolving it in water. As this happens, the gas mixture flows to maintain the nearly constant pressure Psys, and as a consequence, the partial pressure of the refrigerant, Prfr, can approach Psys. At this higher partial pressure, it can condense and deliver heat to an external heat sink, as in a conventional refrigerator.
The condensed refrigerant liquid cannot dissolve in water and, in the case of butane, it will float, making it easy to separate and return to the evaporator. Meanwhile, the ammonia/water solution flows to an ammonia generator, where the heat source that powers the refrigerator raises the temperature of the solution, driving out the ammonia and providing the ammonia vapor that is the other input to the evaporator.
This is the operating principle of the system. Practical implementations include other elements, such as heat exchangers, means for contacting liquids and vapors, and so forth. The Einstein refrigerator has been described as "noiseless, inexpensive to produce and durable".
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