Abstrict An improved method and apparatus for regenerating aqueous desiccants
for use in an air conditioning system are disclosed. The disclosed
method and apparatus utilizes a gas-fired desiccant boiler and a
combined desiccant regenerator/interchange heat exchaner. The combined
regenerator/heat exchanger utilizes steam produced from the boiler
to provide heat for partial regeneration. The desiccant boiler has
a liquid/vapor separator chamber and thermosyphon recirculation
to reduce scale and corrosion of the boiler.
Claims What is claimed is:
1. A process for regenerating liquid desiccant used in an air-conditioner
comprising:
providing (a) a first and a second heat exchanger each accommodating
a flow of liquid desiccant therethrough for transferring sensible
energy, (b) a desiccant regenerator and, (c) a desiccant boiler;
removing diluted desiccant from the air conditioner and routing
the desiccant through said first heat exchanger so that the temperature
of the desiccant is raised, routing the diluted desiccant from said
first heat exchanger to said desiccant regenerator wherein the diluted
desiccant is partially regenerated, routing the partially regenerated
desiccant through said second heat exchanger so that the temperature
of the desiccant is raised, routing said partially regenerated desiccant
through said desiccant boiler where said desiccant is fully regenerated
with moisture from said desiccant being vaporized, routing the regenerated
desiccant through said second heat exchanger to lower the temperature
of said desiccant, routing said regenerated desiccant through said
first heat exchanger to lower the temperature of said desiccant,
returning said regenerated desiccant to the air conditioner; and
routing the vaporized moisture from said desiccant boiler to said
desiccant regenerator to supply energy so that the diluted desiccant
is partially regenerated.
2. A liquid desiccant regeneration system for use in an air conditioner
comprising:
first and second heat exchangers constructed such that sensible
heat is transferred from concentrated desiccant to diluted desiccant;
a desiccant regenerator for partially regenerating the liquid desiccant;
a desiccant boiler for completing the regeneration of the liquid
desiccant and vaporizing moisture absorbed in the desiccant;
means for directing diluted desiccant through said first heat exchanger
and into said desiccant regenerator;
means for removing liquid desiccant from said desiccant regenerator
and pumping liquid desiccant through said second heat exchanger
and into said desiccant boiler;
means for removing liquid desiccant from said desiccant boiler
and passing liquid desiccant through said second heat exchanger
and said first heat exchanger; and
means for passing the vaporized moisture through said desiccant
regenerator so that the vaporized moisture is condensed and transfers
heat to the liquid desiccant.
3. The liquid desiccant regeneration system according to claim
2 wherein said desiccant boiler comprises:
inlet means for said liquid desiccant;
a plurality of generally vertical finned tubes connected to said
inlet means;
at least one generally vertical down comer tube connected to said
inlet means;
burner means positioned so that hot gases produced from said burner
means heats said plurality of finned tubes;
a liquid/vapor separator connected to the top of said plurality
of finned tubes and to the top of said at least one down comer tube;
a desiccant outlet connected to a bottom of said liquid/vapor separator;
a vapor outlet connected to a top of said liquid/vapor separator;
said desiccant boiler being operable to heat liquid desiccant as
it flows upward through said finned tubes and downward through said
at least one down comer tube, to vaporize moisture from said desiccant
and remove said moisture through said vapor outlet means, and to
remove regenerated desiccant through said desiccant outlet.
4. The liquid desiccant regeneration system according to claim
2 wherein said first and second heat exchangers and said desiccant
regenerator are combined into one component comprising;
a housing;
a plurality of a first plates having a first flow channel for routing
the vaporized moisture that is produced from said desiccant boiler
from the top of said first plate and out the middle of said first
plate, a second flow channel for routing the hot desiccant from
the desiccant boiler from the middle of said first plate to the
bottom of said first plate;
a plurality of a second plates having a first flow channel for
routing desiccant from the air conditioner from the bottom to the
middle of said second plate and having a second flow channel for
routing partially heated desiccant from the top of said second plate
to an exit at the middle of said second plate, said second channel
having a second branch which exhausts air and moisture at the middle
of said second plate;
said plurality of first and second plates being stacked alternately
in a sealingly engaged manner and being surrounded by said housing.
5. The liquid desiccant regeneration system according to claim
4 wherein said channels of said first plate and said second plate
are formed by corrugations on said first plate and said second plates.
6. The liquid desiccant regeneration system according to claim
4 wherein said plurality of said first and second plates are made
of polysulfone.
7. A liquid desiccant system for use in an air conditioner comprising:
a desiccant boiler for regenerating liquid desiccant and vaporizing
the moisture absorbed by the desiccant;
a combined regenerator heat exchanger having a first flow channel
for routing vaporized moisture, a second flow channel for routing
hot desiccant from said boiler, a third flow channel for routing
diluted desiccant from the air conditioner and a fourth channel
for routing partially heated desiccant, said fourth channel having
a first exit for air and water vapor and a second exit for desiccant;
means for removing the liquid desiccant from the air conditioner
system and pumping the liquid desiccant through said third flow
channel;
means for removing liquid desiccant from said third channel and
routing it to said fourth channel;
piping means for routing the liquid desiccant from the fourth channel
to the boiler;
piping means for routing the regenerated liquid desiccant from
the boiler to said second flow channel;
piping means for routing the vaporized moisture from the boiler
to said first flow channel;
piping means for routing the regenerated liquid desiccant from
the second flow channel to the air conditioner.
8. The liquid desiccant regeneration system according to claim
7 wherein said combined regenerator/heatexchanger comprises:
a housing;
a plurality of first plates having said first flow channel for
routing the vaporized moisture produced from a boiler and having
said second flow channel for routing hot desiccant from the boiler;
a plurality of second plates having said third flow channel for
routing the dilute desiccant from the air conditioner and having
said fourth channel, a first exit for air and water vapor, and a
second exit for the desiccant:
said plurality of first and second plates being stacked alternately
in a sealingly engaged manner and being surrounded by said housing.
9. The liquid desiccant system according to claim 7 wherein said
desiccant boiler comprises:
inlet means for said liquid desiccant;
a plurality of generally vertical finned tubes connected to said
inlet means;
at least one generally vertical down comber tube connected to said
inlet means;
burner means positioned so that hot gases produced from said burner
means heats said plurality of finned tubes;
a liquid/vapor separator connected to a top of said plurality of
finned tubes and to the top of said at least one down comber tube;
and
a desiccant outlet means connected to the bottom of said liquid/vapor
separator;
said desiccant boiler being operable to heat liquid desiccant as
it flows upward through said finned tubes and downward through said
at least one down comer tube, to vaporize moisture from said desiccant
and remove said moisture through said vapor outlet means, and to
remove regenerated desiccant through said desiccant outlet means.
10. The liquid desiccant regeneration system according to claim
8 wherein said channels of said plurality of first and second plates
are formed by corrugations on said first and second plates.
11. The liquid desiccant regeneration system according to claim
8 wherein said plurality of first and second plates are made of
polysulfone.
Description BACKGROUND OF THE INVENTION
This invention relates to air conditioning systems utilizing liquid
desiccants for dehumidifying air and more specifically to an air
conditioning system providing improved regeneration of liquid desiccants.
Devices that use hygroscopic liquids such as lithium chloride (LiCl)
or calcium chloride (CaCl.sub.2) solutions to dehumidify air have
been known for many years. However, the use of these systems has
been limited to specialized applications due to the problem of regenerating
the liquid desiccant. Regeneration usually requires heating the
desiccant to drive off the excess moisture or contacting the desiccant
with a hot gas which absorbs the excess moisture. Generally, air
regenerators are used to regenerate the desiccant. These regenerators
are costly to run especially where waste heat is not available,
as in residential applications. Boiler-type regenerators may also
be expensive, requiring large amounts of corrosion-resistant metals.
If pressurized boilers are employed to provide high efficiency,
costly components are needed and issues of safety become more complex.
SUMMARY OF THE INVENTION
The present invention overcomes the above-mentioned problems by
providing an air conditioning system, having a novel desiccant boiler
and a combined desiccant regenerator and interchange heat exchanger
to produce an effective, economic liquid desiccant regeneration
system. The term "air conditioning system" as used herein
refers to apparatus which dehumidifies air and may also provide
cooling.
Regeneration of the liquid desiccant is accomplished by diverting
a portion of the desiccant flowing through a desiccant conditioner
and heating the desiccant successively in an interchange heat exchanger,
an air desiccant regenerator, a second interchange heat exchanger
(or higher temperature portion of a single interchange heat exchanger),
and a boiler. The heat is provided from concentrated desiccant flowing
back to the desiccant conditioner, from ambient air heated from
condensing steam provided from the boiler, and from energy furnished
to the boiler by, for example, combustion products of a natural
gas-fueled burner.
In a preferred embodiment, several heat exchange subsystems are
combined into a single desiccant regenerator/interchange heat exchanger
unit. The combined unit preferably formed by alternately stacking
two different plates, reduces pumping requirements and permits a
compact air conditioning system while preserving efficient use of
available heat for regeneration of the liquid desiccant.
The liquid desiccant air conditioning system of the invention also
preferably includes a unique gas-fueled boiler employed as the final
heating stage in removing water from and concentrating the liquid
desiccant. The boiler, which preferably includes multiple finned
tubes over which are directed combustion products of a natural gas-fueled
burner, maintains cool wall temperatures by employing natural circulation-that
is, the difference in density of the fluid in several "fired"
tubes and the liquid in the "unfired" tubes drives water
vapor and desiccant solution in the "fired" tubes up into
a top manifold which operates as a liquid/vapor separator. Liquid
desiccant flows down the "unfired" tubes and water vapor
exits at the top through a vent. A portion of the circulating liquid
desiccant that has been concentrated leaves at the bottom of the
separator.
This concentrated liquid desiccant is shunted through the combined
interchange heat exchanger to raise the temperature of the incoming
dilute desiccant and lower its temperature. The cool concentrated
desiccant is returned to the air conditioner. The steam vented in
the boiler is used to partially regenerate the weak desiccant.
It is an object of the present invention to provide a liquid desiccant-based
air conditioning system with improved regeneration of the liquid
desiccant.
Another object of the invention is to utilize effectively the heat
provided to a boiler to regenerate a liquid desiccant, including
utilizing latent heat in the water vapor generated in the boiler.
Still another object of the invention is to provide an improved
liquid desiccant boiler/separator operable at atmospheric pressure
and with natural (thermosyphon), recirculation.
Yet another object of the invention is to provide staged regeneration
of a liquid desiccant with multiple heat exchange and desiccant
regeneration steps performed in a single combined unit.
These and other features and objects of the present invention will
become apparent from the drawings and detailed description of the
invention which follows.
DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a liquid desiccant air conditioning system according
to the invention.
FIG. 2 shows a direct-fired natural circulation desiccant boiler
suitable for use in the air conditioning system of the invention.
FIG. 3 shows a combined desiccant regenerator and interchanger
heat exchanger incorporating several of the elements of the system
illustrated in FIG. 1.
FIG. 4 shows alternating plates used in construction of the combined
desiccant regenerator and interchange heat exchanger of FIG. 3.
DESCRIPTION OF PREFERRED EMBODIMENTS
The present invention provides an efficient and economic means
to regenerate or concentrate aqueous desiccants which are used in
an air conditioning system employed for dehumidification and, optionally,
cooling. A multistage regeneration process is used to recondition
liquid desiccant. A boiler is used to heat the spent liquid desiccant
to remove the moisture as steam. Most of the energy from the steam
is extracted through a combined regenerator heat exchanger which
heats and partially regenerates the diluted desiccant prior to its
entry into the boiler and cools the regenerated desiccant on its
return for reuse in drying air.
FIG. 1 is a schematic diagram of an embodiment of a liquid desiccant
air conditioning system 20 of the invention. The several elements
of the system 20 will now be described with reference to its operation.
Outdoor air, drawn through an inlet duct 22 is evaporatively cooled
by a water spray 24. This cooled air is directed to a desiccant
conditioner 26 to which return air is also directed through a duct
30. In the desiccant conditioner 26 the return air is contacted
with a liquid desiccant solution from a sprayer 28 removing water
from the air. Lithium chloride (LiCl) or calcium chloride (CaCl.sub.2)
are typical of liquid desiccant solutions but other liquid desiccants
may be employed. The dehumidified air can be used directly as supply
air to the space to be dehumidified or it can first be sensibly
cooled through an evaporative cooler 32 or similar device. The evaporative
cooler 32 provides sensible cooling to the supply air. The outdoor
cooling air is exhausted from the conditioner 26 through an exhaust
duct 38.
As the desiccant dehumidifies the air stream, the moisture-absorbing
capability of the desiccant is lessened and it must be regenerated.
A portion of the dilute desiccant from the conditioner 26 is passed
by a pump 42 to a first interchange heat exchanger 44 where the
temperature of the desiccant is raised. Next, the weakened desiccant
is partially concentrated in an air desiccant regenerator 46 in
which heated air from a regeneration air heater 48 contacts the
liquid desiccant. The desiccant then is forced by a pump 50 through
a second interchange heat exchanger 52 and then to a desiccant boiler
56 in which regeneration of the desiccant is completed. The water
vapor generated in the desiccant boiler 56 heats the air passing
through the regeneration air preheater 48. The interchange heat
exchangers 44 and 52 also reduce the temperature of the regenerated
desiccant as it returns along a pipe 60 to the conditioner 26.
FIG. 2 illustrates a preferred boiler 56 which may be used to evaporate
water from the desiccant during regeneration. The boiler 56 operates
on natural circulation, with the density of the fluid (part liquid,
part vapor) in the "fired" tubes 70 being less than the
density of the liquid in the outer "unfired" tubes 74.
This difference creates a thermosyphon effect or natural circulation
of fluid in the tubes 70 and 74. In the embodiment shown in FIG.
2 a porous ceramic burner 80 fueled by natural gas, is used essentially
as a flame holder for combustion of natural gas supplied through
a fuel line 82 and mixed with air in a blower 84. The hot combustion
gases are blown through a combustion chamber formed by a housing
88 enclosing the fired tubes 70 and flow across fins 90 of the
fired tubes 70 thereby heating the tubes 70. Weak desiccant pumped
into the fired tubes 70 through a manifold 94 connected to their
lower ends is heated, and as the desiccant rises in the fired tubes
70 water in the desiccant is vaporized. As the water is vaporized,
a density differential is created between the fluid in the fired
tubes 70 and the unfired tubes 74 connected between the manifold
94 and a liquid/vapor separator 98 outside the combustion chamber
housing 88. This density differential induces a natural flow of
desiccant solution up the fired tubes 70 and down the unfired tubes
72. This natural circulation of the desiccant keeps the inside walls
of the fired tubes 70 coated with desiccant, which reduces or prevents
"hot spots" from forming on the inside of the fired tubes
70. Thus corrosion and scale build-up in the fired tubes 70 are
reduced.
The liquid/vapor separator 98 at the top of the boiler 56 operates
to separate water vapor from the concentrated liquid desiccant.
A portion of the concentrated desiccant is withdrawn from the bottom
of the liquid/vapor separator 98 for return to the desiccant conditioner
26 (see FIG. 1). Water vapor flows out the top of the liquid/vapor
separator 98 and is subsequently condensed to heat air for use in
an earlier regeneration step as discussed in more detail hereinafter
with reference to FIGS. 3 and 4.
In the combined regenerator and interchange heat exchanger illustrated
in FIGS. 3 and 4 four components shown in FIG. 1 namely the two
interchange heat exchangers 44 and 52 the desiccant regenerator
46 and the regeneration air heater 48 are combined into a single
component. This component is best understood by reference to FIGS.
3 and 4. The combined desiccant regenerator/interchange heat exchanger
102 is constructed by alternately stacking two different corrugated
plates (shown in FIG. 4) to form alternating flow channels. As shown
in FIGS. 3 and 4 water vapor or steam from the desiccant boiler
56 is introduced near the top of the regenerator/exchanger 102
in alternate channels (see plate A). The water vapor condenses,
transferring heat to air and weak desiccant entering adjacent channels
near the top of the regenerator/heat exchanger 102 (see plate B).
The upper portion of each plate corresponds to the desiccant regenerator
46 and regeneration air heater 48 of FIG. 1. As the water vapor
condenses the weak desiccant and air mixture is heated and the desiccant
is partially regenerated. Warm air and moisture are exhausted by
a fan 106 to the outdoors. An entrainer 108 is provided to prevent
desiccant from escaping the combined regenerator/exchanger 102.
As the partially regenerated desiccant flows into the middle of
a channel plate B, it is further heated by the hot concentrated
desiccant removed from the liquid/vapor separator 98. Hot concentrated
desiccant from the boiler 56 is introduced at the middle of plate
A while the partially regenerated desiccant which has been heated
is removed from the middle of plate B. The partially regenerated
desiccant is pumped to the desiccant boiler 56. The middle section
of plate A and plate B corresponds to the interchange heat exchanger
52 shown in FIG. 1. Diluted desiccant from the regenerator/heat
exchanger 102 is introduced at the bottom of plate A and is heated
by the hot desiccant from the boiler 56 which has been partially
cooled from traveling through the center portion of plate A. The
heated dilute desiccant from the regenerator/heat exchanger 102
is removed from the center of plate B and pumped to the top of plate
B. The lower portions of plate A and plate B correspond to the interchange
heat exchanger 44 shown in FIG. 1.
In the combined desiccant regenerator and interchange heat exchanger
approximately 20-30 plates are stacked together to produce the component
102 shown in FIG. 3. An important feature of this combined desiccant
regenerator/heat exchanger 102 is that the heat transfer is accomplished
at near atmospheric pressure. This reduces fabrication costs. In
a preferred embodiment the plates are produced from a polymer such
as polysulfone and are connected by an adhesive such as epoxy.
While the foregoing invention has been described with reference
to its preferred embodiments, various alterations and modifications
will occur to those skilled in the art. For example, materials other
than high temperature polymers such as metal can be used to fabricate
the combined desiccant regenerator/heat exchanger. Various liquid
desiccant solutions can be used with the present system. These and
other such alterations and modifications are intended to fall within
the scope of the appended claims. |