Absorptive refrigeration uses a source of heat to provide the energy needed to drive the cooling process. The most common use is in commercial climate control and cooling of machinery. Absorptive refrigeration is also used to 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 (see Trigeneration).
The basic thermodynamic process is not a conventional thermodynamic cooling process based on Charles' Law. Instead, it is based on evaporation, carrying heat, in the form of faster-moving (hotter) molecules from one material to another material that preferentially absorbs hot molecules.
A familiar example is human sweating. The water in sweat evaporates and is "absorbed" into air, carrying away heat from the body. However, absorptive refrigerators differ in that they regenerate their coolants in a closed cycle, while people need to keep replacing their lost water (evaporated sweat) through drinking.
The classic gas absorption refrigerator sends liquid ammonia into a hydrogen gas. The liquid ammonia evaporates in the presence of hydrogen gas, providing the cooling. The now-gaseous ammonia is sent into a container holding water, which absorbs the ammonia. The water-ammonia solution is then directed past a heater, which boils ammonia gas out of the water-ammonia solution. The ammonia gas is then condensed into a liquid. The liquid ammonia is then sent back through the hydrogen gas, completing the cycle.
A similar system, common in large commercial plants, uses a solution of lithium bromide salt and water. Water under low pressure is evaporated 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 water solution. As shown in the figure below, warm moist air is passed through a sprayed solution of salt water. The spray lowers the humidity. The less humid warm air is then passed through an evaporative cooler which cools and rehumidifies. Humidity is removed from the cooled air with another spray of salt solution. The salt solution is regenerated by heating it under low pressure, causing water to evaporate. The water evaporated from the salt solution is recondensed, and rerouted back to the evaporative cooler.
Process
A single-pressure absorption refrigerator uses three substances: ammonia, hydrogen gas, and water, whereas large industrial units generally use only two, a refrigerant such as ammonia, and an absorbent such as water (with an expansion valve and pump, not described here). Normally, ammonia is a gas at room temperature (with a boiling point of -33 °C), but the system is pressurized to the point that the ammonia is a liquid at room temperature.
The cooling cycle starts at the evaporator, where liquefied anhydrous ammonia enters. (Anhydrous means there is no water in the ammonia, which is critical for exploiting its sub-zero boiling point.) The "evaporator" contains another gas (in this case, hydrogen), whose presence lowers the partial pressure of the ammonia in that part of the system. The total pressure in the system is still the same, but now not all of the pressure is being exerted by ammonia, as much of it is due to the pressure of the hydrogen. Ammonia doesn't react with hydrogen - the hydrogen is there solely to take up space - creating a void that still has the same pressure as the rest of the system, but not in the form of ammonia. Per Dalton's law, the ammonia behaves only in response to the proportion of the pressure represented by the ammonia, as if there was a vacuum and the hydrogen wasn't there. Because a substance's boiling point changes with pressure, the lowered partial pressure of ammonia changes the ammonia's boiling point, bringing it low enough that it can now boil below room temperature, as though it wasn't under the pressure of the system in the first place. When it boils, it takes some heat away with it from the evaporator - which produces the "cold" desired in the refrigerator.
The next step is getting the liquid ammonia back, as now it's a gas and mixed with hydrogen. Getting the hydrogen away is simple, and this is where the "absorber" comes in. Ammonia readily mixes with water, and hydrogen does not. The absorber is simply a downhill flow of tubes in which the mixture of gases flows in contact with water being dripped from above. Once the water reaches the bottom, it's thoroughly mixed with the ammonia, and the hydrogen stays still (though it can flow freely back to the evaporator).
At this point, the ammonia is a liquid mixed with water and still not usable for refrigeration, as the mixture won't boil at a low enough temperature to be a worthwhile refrigerant. It's now necessary to separate the ammonia from the water. This is where the heat from the flame comes in. When the right amount of heat is applied to the mixture, the ammonia bubbles out. This phase is called the "generator". The ammonia isn't quite dry yet - the bubbles contain gas but they're made of water, so the pipe twists and turns and contains a few minor obstacles that pop the bubbles so the gas can move on. The water that results from the bubbles isn't bad - it takes care of another need, and that is the circulation of water through the previous absorption step. Because that water has risen a bit while it was bubbling upwards, the flow of that water falling back down due to gravity can be used for this purpose. The maze that makes the ammonia gas go one way and the bubble water go the other is called the "separator".
The next step is the condenser. The condenser is a sort of heat sink or heat exchanger that cools the hot ammonia gas back down to room temperature. Because of the pressure and the purity of the gas (there is no hydrogen or water here), the ammonia condenses back into a liquid, and at that point, it's suitable as a refrigerant and the cycle starts over again.
