Absorptive refrigeration
Absorptive refrigeration is a process that uses a source of heat to pump heat. The most common home use is in a propane refrigerator. Absorptive refrigeration is also used by absorptive chillers that air-condition buildings using the waste heat from a gas turbine or water heater. The process is very efficient, since the gas turbine produces electricity, hot water and air-conditioning, using heat twice - once to power the turbine, and again to heat water or run the chiller.
The continuous absorption type of cooling unit is operated by the application of a limited amount of heat furnished by gas, electricity or kerosene. No moving parts are employed.
Theory of operation
The basic idea is that one substance (A) absorbs hot molecules from another substance (B), cooling (B). (A) is purified of the molecules (usually by distillation), the hot molecules are cooled in a radiator, forming (A) and (B) and the process repeats.
The most common example of absorptive cooling is sweating. When people sweat, they are absorptively cooled by the air when it carries away hot water molecules.
Standard home refrigerator
The unit consists of four parts: the boiler, condenser, evaporator and absorber.
The unit can be run on either electricity, kerosene or gas. When the unit operates on kerosene or gas, the heat is supplied by a burner which is fitted underneath the central tube (A) and when the unit operates on electricity, the heat is supplied by a heating element inserted in the pocket (B).
The unit charge consists of a quantity of ammonia, water and hydrogen at a sufficient pressure to condense ammonia at the room temperature for which the unit is designed.
When heat is supplied to the boiler system, bubbles of ammonia gas are produced which rise and carry with them quantities of weak ammonia solution through the siphon pump - C. This weak solution passes into the tube (D), while the ammonia vapor passes into the vapor pipe (E), and on to the water separator. Here any water vapor is condensed and runs back to the boiler system, leaving the dry ammonia vapor to pass to the condenser.
Air circulating over the fins of the condenser removes heat from the ammonia vapor to cause it to condense to liquid ammonia in which state it flows into the evaporator.
The evaporator is supplied with hydrogen. The hydrogen passes across the surface of the ammonia and lowers the ammonia vapor pressure (absorbs it) sufficiently to allow the liquid ammonia to evaporate. The evaporation of the ammonia extracts heat from the evaporator, which in turn extracts heat from the food storage space, as described above, thereby lowering the temperature inside the refrigerator.
The mixture of ammonia and hydrogen vapor passes from the evaporator to the absorber.
Entering the upper portion of the absorber is a continuous trickle of weak ammonia solution fed by gravity from the tube (D). This weak solution, flowing down through the absorber, comes in contact with the mixed ammonia and hydrogen gases which readily absorbs the ammonia from the mixture, leaving the hydrogen free to rise through the absorber coil and to return to the evaporator. The hydrogen thus circulates continuously between the absorber and the evaporator.
The strong ammonia solution produced in the absorber flows down to the absorber vessel and from there to the boiler system, thus completing the cycle of operation.
The liquid circulation of the unit is purely gravitational.
Heat is generated in the absorber by the process of absorption. This heat must be dissipated into the surrounding air. Heat must also be dissipated from the condenser in order to cool the ammonia vapor sufficiently for it to liquefy. Free air circulation is therefore necessary over the absorber and condenser.
The whole unit operates by the heat applied to the boiler system and it is of paramount importance that this heat is kept within the necessary limits and is properly applied.
Other forms
A similar system uses lithium bromide solution and water. Water is evaporated under low pressure from the coils that are being chilled. The water is absorbed by a lithium bromide/water solution. The water is driven off the lithium bromide solution using heat.
Another variant uses air, water, and a salt solution. Warm air is passed through a sprayed solution of salt water. The spray absorbs humidity from the air. The air is then passed through an evaporative cooler. Humidity is removed from the cooled air with another spray of salt solution. The salt solution is regenerated by heating it under low pressure. The water evaporated from the salt solution is recondensed, and rerouted back to the evaporative cooler.