Absorption heat pump systems
Introduction
The absorption refrigeration system is a means of cold production that, like compression refrigeration, takes advantage of the fact that substances absorb heat when changing state, from liquid to gaseous. Just as in the compression system the cycle is done by a compressor "Compressor (machine)"), in the case of absorption, the cycle is physically based on the ability of some substances, such as lithium bromide, to absorb another substance, such as water, in the vapor phase "Vapor (state)"). Another possibility is to use water as the absorbent substance (solvent) and ammonia as the absorbed substance (solute).
The technique was born in 1859, when Ferdinand Carré managed to make ice with the first ammonia-water cycle absorption machine.[1].
Operation
The pair of working fluids normally used is water-lithium bromide, because it allows for greater efficiency.[2] Lithium bromide is used because it has a great capacity to absorb water and because it can be dehydrated by heat.
The operation of the absorption refrigeration cycle is as follows. The water (refrigerant), which moves through a circuit at low pressure, evaporates in a heat exchanger, called evaporator. Evaporation requires heat, which is obtained from an exchanger in which a secondary fluid (normally also water) is cooled, which is carried through a network of pipes to cool the environments or chambers of interest. After the evaporator, the lithium bromide absorbs water vapor in the absorber, producing a dilute or weak solution of bromide in water. This solution passes to the generator, where solvent and solute are separated using heat from an external source; The water goes to the condenser, which is another exchanger where it gives up most of the heat received in the generator, and from there it goes back to the evaporator, through the expansion valve; The bromide, now as a concentrated solution in water, returns to the absorber to close the cycle. In short, heat is released in the absorber when the gas is absorbed, while heat is absorbed in the generator when the gas is released.
Although it does not appear in the figure, a heat exchanger is also usually used, putting the absorber-generator and generator-absorber ducts in contact, without mixing, to preheat the water-lithium bromide solution, before passing to the heater (), while, in turn, the concentrated lithium bromide solution is cooled when it goes to the , since absorption is best carried out at a low temperature. In fact, there must be an exchanger in the absorber to cool it with the cooling tower.