Compositions – Vaporization – or expansion – refrigeration or heat or energy... – With low-volatile solvent or absorbent
Patent
1994-05-06
1998-12-08
Skane, Christine
Compositions
Vaporization, or expansion, refrigeration or heat or energy...
With low-volatile solvent or absorbent
252 67, 252 76, 252 79, 62112, 95230, 95231, C09K 504, C09K 500, F25B 1502
Patent
active
058464506
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to absorption refrigeration and more particularly to absorbents for absorption refrigeration systems. The invention also relates to absorbents for use in heat pumps, dehumidifiers and air conditioning systems which operate according to the absorption principle.
A description of the operation of a typical absorption refrigeration unit is given in the introduction to U.S. Pat. No. 4,311,024.
Absorption refrigerators and heat pumps utilise the latent heat of evaporation required when a solution is concentrated by removal of a solvent. The absorption refrigeration cycle uses two fluids in a totally enclosed system. One fluid is the refrigerant which provides the cooling effect, the other is the absorbent which carries the refrigerant through part of the cycle. The absorbent is commonly a solution containing the refrigerant.
In operation, the refrigerant vapour is generated by heating the absorbent solution. This is then condensed by a cooling air or water stream. The vapour generator and condenser operate at the highest pressure in the system and the condensed refrigerant is expanded into a lower pressure region where it evaporates and absorbs heat from the surroundings, thereby cooling them. The refrigerant vapour then passes back into the contact with the absorbent, forming a refrigerant-rich solution which is returned to the generator again. Commonly, in commercial units the working fluid is a solution of lithium halide in water. Water is the refrigerant and the lithium halide solution is the absorbent.
Absorption refrigeration systems find widespread use in large scale air-conditioning systems. In certain instances adequate air conditioning can be achieved by using concentrated absorbent solutions to provide direct air dehumidification. Hospital operating theatres, for instance, are major users of dehumidification plants which contact incoming air directly with a concentrated solution of lithium chloride or lithium bromide.
By operating an absorption refrigerator "in reverse" it is possible to provide an absorption heat pump.
Many different refrigerant and absorbent pairs have been cited in the literature, but the most important commercial systems are based on either: >+10.degree. C. +10.degree. C. and -60.degree. C.
The lithium bromide-water combination, where the lithium bromide acts as the absorbent, is particularly favoured e.g. for use in air conditioning systems. However, concentrated lithium bromide solutions of interest have a relatively high crystallisation temperature and can solidify or "freeze" inside the refrigeration unit. They are also corrosive to metals. Over the past 40 years a series of elaborate proprietary formulations have been built around the lithium bromide-water system with the objective of enhancing the performance of the working fluid by lowering its crystallisation temperature and reducing its corrosivity to metals. Examples of various formulation additives include other lithium halides, used to lower the crystallisation temperature.
Other compounds such as methanol, alkali metal thiocyanates, alkali metal halides and nitrates are added to reduce the high crystallisation temperatures of lithium bromide solutions that would normally restrict their use as absorption agents in low temperature systems.
U.S. Pat. No. 4,311,024 concerns the use of a lithium bromide absorbent in a refrigeration system. In order to reduce the corrosivity of the lithium bromide towards copper tubing in the system, a nitrate compound and at least one of benzotriazole or tolyltriazole are added.
AU 18362/88 provides an absorbent solution which is a mixture of at least three lithium compounds selected from the group consisting of lithium bromide, lithium iodide, lithium chloride and lithium nitrate. The formulation provides a solution with a high salt concentration yet low crystallisation temperature.
J0 1263-466-A concerns a composition of low crystallisation temperature consisting of lithium bromide and lithium nitrate, or alternatively lithium bromide and zinc nitrate.
M
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