Absorptive refrigeration

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Absorptive refrigeration utilizes a source of heat to provide the energy needed to drive the cooling process. The most common use is in food refrigeration. Absorptive refrigeration is also used to air-condition buildings utilizing 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 fast-moving (hot) molecules from one material to another material that preferentially absorbs hot molecules.

The most familiar example is human sweating. The water from sweat evaporates and is "absorbed" into cool dry air, carrying away heat in fast-moving water molecules. However, absorptive refrigerators differ in that they regenerate their coolants in a closed cycle, while people drink water recycled outside their bodies.

The classic absorptive home refrigerator cools by evaporating liquid ammonia in a hydrogen environment. The now-gaseous ammonia is then absorbed (dissolved) into water, and then later separated (boiled off from the water) by a small source of heat. This drives off the dissolved ammonia gas which is then condensed into a liquid. The liquid ammonia then enters the hydrogen-charged evaporator to repeat the cycle.

A similar system, common in large commercial plants, uses a solution of lithium bromide salt 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, causing water to evaporate. The water evaporated from the salt solution is recondensed, and rerouted back to the evaporative cooler.