Compositions – Vaporization – or expansion – refrigeration or heat or energy... – With low-volatile solvent or absorbent
Reexamination Certificate
1998-11-17
2001-01-23
Ogden, Necholus (Department: 1751)
Compositions
Vaporization, or expansion, refrigeration or heat or energy...
With low-volatile solvent or absorbent
C062S112000, C062S476000
Reexamination Certificate
active
06177025
ABSTRACT:
FIELD OF THE INVENTION
This invention is directed to absorption heat pumps which achieve improved efficiency by widening the difference between the high and low temperatures of the working fluid. In particular, the invention is directed to additives which depress the crystallization/precipitation temperature of salt contained in the working fluid.
BACKGROUND OF THE INVENTION
In heat pumps of the absorption type, an absorbent, diluted with an absorbed refrigerant, is heated in a generator to vaporize some of the refrigerant. The refrigerant vapor then flows to a condenser where it is condensed to a liquid by heat exchange with an external cooling fluid maintained at a low temperature by a heat sink. The liquefied refrigerant then flows through a valve to an evaporator which vaporizes the refrigerant (usually at low pressure) to produce refrigeration.
The vaporized refrigerant then flows to an absorber where it is absorbed by concentrated absorbent supplied from the generator. From the absorber, the diluted absorbent passes to the generator where it is concentrated by heating to vaporize some of the refrigerant, and thus repeat the cycle.
Conventional absorption heat pumps typically employ an aqueous solution of lithium bromide as an absorbent and water as a refrigerant. The operating efficiency of these heat pumps increases with the difference between the highest fluid temperature where the solution is dilute in lithium bromide and water is being vaporized, and the lowest fluid temperature where the solution is very concentrated in lithium bromide and water is being absorbed. When operating in a refrigeration/air conditioning mode, the high temperature is fixed by the ambient temperature. When operating in a temperature boosting mode, the high temperatures can reach 98° C., 177° C., and 232° C. for single, double and triple effect machines, respectively. Some of these machines are described in Herold and Radermacher “Absorption Heat Pumps”.
Mechanical Engineering
, August 1989, pp.68-73. Since the high cycle temperature is generally fixed by the application and/or pump type, the efficiency of the cycle can be increased by lowering the low cycle temperature.
As the low cycle temperature is reduced in an air conditioning application, the concentration of lithium bromide must be increased in order to permit the continued absorption of water vapor. As the salt concentration is increased and the temperature is decreased, a solubility limit is approached. If the solubility limit of lithium bromide in water is exceeded, hydrated salt crystals may form which block the flow circulation in the absorber, rendering it useless. Thus, conventional absorption heat pumps use solutions containing about 60-62% salt, and operate at a minimum fluid temperature of about 4-7° C. in air conditioning applications. For heating applications, the salt concentration may be lowered, to prevent freezing of the solution at temperatures down to −25° C. or lower.
Absorption heat pumps have many large-scale uses in industrial air-conditioning and refrigeration, as well as heating and temperature boosting. There is always a need or desire for more efficient heat pumps which maximize the difference between the high and low fluid temperatures at different parts of the cycle.
SUMMARY OF THE INVENTION
The present invention is an absorption heat pump which achieves a greater difference between the high and low fluid temperatures of the circulation fluid, by reducing the minimum fluid temperature to levels not previously contemplated. Additives have been discovered which inhibit the crystallization and precipitation of lithium bromide from water at concentrations of 60-62% lithium bromide and temperatures below about 4° C., without adversely affecting 1) the heat capacity of the solution, 2) the solution rheological properties, 3) the solution diffusion or mass transfer coefficients, or 4) the ability of the solution to absorb water vapor and transfer heat in the process. In an absorption cycle, these additives permit operation at a lower low temperature, thereby improving the efficiency of the cycle. The increased efficiency makes the absorption heat pump more cost effective compared to conventional refrigeration technologies.
The additives for the aqueous lithium bromide solution can reduce the minimum low fluid temperature from about 4-7° C. to about 0° C. or lower. Some of the additives can reduce the minimum low fluid temperature to −5° C. or lower, to −8° C. or lower, or even to −10° C. or lower. The additives can also be used to reduce the minimum low fluid temperature in applications using lower concentrations of lithium bromide in water.
Suitable additives are those which form complexes with lithium and/or bromine ions in aqueous solution. The additives and complexes formed may 1) decrease the crystallization driving force, causing supersaturation, 2) increase the critical supersaturation needed for effective nucleation, and/or 3) decrease the crystal growth rate. Useful additives include compounds which form complexes with the lithium and bromine ions in solution, and which alter the surface energy of crystal embryos formed in solution just prior to nucleation.
With the foregoing in mind, it is a feature and advantage of the invention to provide an aqueous lithium bromide solution useful in absorption heat pumps, which has a wider range of cycle temperature due to a lower minimum temperature for the onset of crystallization.
It is also a feature and advantage of the invention to provide an absorption heat pump having greater efficiency due to a wider range of cycle temperatures and a lower minimum cycle temperature.
The foregoing and other features and advantages will become further apparent from the following detailed description of the presently preferred embodiments, read in conjunction with the examples. The detailed description and examples are merely illustrative rather than limiting, with the scope of the invention being defined by the appended claims and equivalents thereof.
DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS
In accordance with the invention, an absorption heat pump is provided which is operable using an aqueous circulation fluid including a hygroscopic salt as an absorbent material and water as a refrigerant. The preferred hygroscopic salt includes lithium bromide, alone or in combination with one or more additional absorbent materials or aids. The solution should contain about 30-85% by weight total absorbent material, preferably about 40-75% by weight, more preferably about 55-65% by weight. In practice, the maximum absorbent salt concentration will be the concentration at saturation at the lowest temperature experienced by the solution during operation.
The salt concentration must, at minimum, be sufficient to effectively absorb the refrigerant at the lowest cycle temperature. The minimum salt concentration useful for this purpose increases as the solution temperature is lowered, until a low temperature is reached where the minimum and maximum concentrations converge at a saturation level. For a solution employing only lithium bromide as the absorbent and only water as the refrigerant, an optimum salt concentration of about 60-62% yields both absorption and saturation at a minimum operating temperature of about 4-7° C.
The present invention is directed primarily toward lowering the minimum operating temperature of solutions containing lithium bromide and water. to levels below the conventional minimum temperatures. To this end, crystallization-inhibiting compounds of selected types and amounts are added to the aqueous solution of lithium bromide and water. The additives are present in low concentrations, typically up to about 5000 molar ppm, more typically about 200-2000 molar ppm, most typically about 500-1500 molar ppm. The term “molar ppm” is based on the amount of lithium bromide in the solution. For instance, an additive level of 500 molar ppm means the solution contains 500 moles of crystallization-inhibiting additive for every million mole
Dirksen James A.
Ring Terry A.
Ogden Necholus
Pauley Petersen Kinne & Fejer
University of Utah
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