Liquid desiccant air conditioner

Details Liquid desiccant air conditioner Liquid desiccant air conditioner

1998-08-07

2001-04-17

McDermott, Corrine (Department: 3744)

Refrigeration

Atmosphere and sorbent contacting type

C062S094000

Reexamination Certificate

active

06216483

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to room air cooling and dehumidification, and more particularly, to a liquid desiccant air conditioner which is energy efficient, corrosion resistant, and capable of operation with low energy usage.
DESCRIPTION OF THE PRIOR ART
Typical air conditioning units operate on a vapor compression cycle. Over recent years, the phase out of CFC based air conditioning units has been dictated by environmental concerns. One alterative to vapor compression units, is the absorption system. The basic elements include an evaporator, condenser, absorber, pump, heat exchanger, throttle valve and regenerator. In the absorption cycle, an “absorbent” is used to absorb the refrigerant in the vaporized state after leaving the evaporator. The vaporized refrigerant is converted back into the liquid phase in the absorber. Heat released in the absorption process is rejected to cooling water passed through the absorber. A solution of absorbent and refrigerant is pumped to a regenerator, where heat is added and the more volatile refrigerant is separated from the absorbent through distillation. The refrigerant is then communicated to the condenser, expansion valve and evaporator in a conventional manner. A heat exchanger may be used for heat recovery between the absorbent returned to the absorber and the absorbent-refrigerant solution delivered to the regenerator.
Absorption systems currently represent only a small percentage of commercial refrigeration systems because they are generally bulky and inefficient. However, with concerns over CFCs and ever increasing energy costs, the absorption unit has potential to provide efficient cooling by taking advantage of waste heat. This may be provided by combining such an absorption unit with a liquid desiccant dehumidifier.
It is known in the art to dehumidify ambient air using liquid desiccant systems. These devices typically utilize hygroscopic liquids such as lithium bromide (LiBr), lithium chloride (LiCl) or calcium chloride (CaCl
2
) as the desiccant solution. In a desiccant system, the desiccant solution absorbs moisture from ambient air exposed to the solution. As the desiccant solution continues to absorb moisture, it becomes dilute and must be regenerated. In the regeneration process, the desiccant solution is heated to evaporate the excess moisture or the desiccant solution is brought into contact with a hot gas to desorb the excess moisture. In some expedients, air regenerators are used to regenerate the desiccant. These arrangements have relatively high operating costs as energy is required to provide a source of heat and to generate a suitable flow of air. In others, boiler-type regenerators are employed. However, boiler embodiments are expensive, as the corrosive nature of liquid desiccant solutions necessitates the use of costly corrosion resistant metals.
A liquid desiccant dehumidification system in which a liquid desiccant is regenerated with a boiler is described in U.S. Pat. No. 4,939,906 (“the '906 Patent”). The '906 Patent discloses a gas-fired desiccant boiler and a combined desiccant regenerator/interchange heat exchanger, in which 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. Specifically, the overall system is shown in
FIG. 1
, wherein outdoor air is drawn into the system through an inlet duct
22
, and is evaporatively cooled by a water spray
24
. The 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
. The desiccant liquid is disclosed as lithium calcium chloride.
This dehumidified air is then supplied to the space to be dehumidified, or it can be sensibly cooled through an evaporative cooler
32
. The desiccant dehumidifies the air stream, and in the process its moisture-absorbing capability is reduced; this capability is regenerated by passing a portion of the dilute desiccant from the conditioner
26
to a first interchange heat exchanger
44
, wherein the temperature of the desiccant is raised. 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. This desiccant is pumped through a second interchange heat exchanger
52
and thereafter to a desiccant boiler
56
, in which regeneration of the desiccant is completed. The water vapor generated in the desiccant boiler
56
raises the temperature of the air passing through the regeneration air preheater
48
. The interchange heat exchangers
44
,
52
reduce the temperature of the regenerated desiccant as it returns along the pipe
60
to the conditioner
26
.
The boiler
56
is depicted in
FIG. 2
, and 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” tube
74
. A porous ceramic burner
80
facilitates combustion to provide a heat source and 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
. Weak desiccant is pumped into the fired tubes
70
through a manifold
94
which causes water in the desiccant to be vaporized. Accordingly, 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
. In this manner, the natural circulation of desiccant keeps the inside walls of the fired tubes
70
coated with desiccant to thereby reduce or prevent “hot spots” from forming on the inside of the fired tubes
70
to reduce corrosion and scale build up in the fired tubes
70
.
The liquid vapor separator
98
at the top of the boiler
56
separates water vapor from the concentrated liquid desiccant. A portion of the concentrated desiccant is withdrawn from the bottom of the liquid/vapor separator
98
and is returned to the desiccant conditioner
26
. Water vapor flowing out of the top of the liquid/vapor separator
98
is subsequently condensed to heat air for use in an earlier regeneration step shown in
FIGS. 3 and 4
.
The combined regenerator/interchange heat exchanger, depicted in
FIGS. 3 and 4
, comprises two (2) interchange heat exchangers
44
,
52
, the desiccant regenerator
46
and the regeneration air heater
48
. The combined desiccant regenerator/interchange heat exchanger is identified by the reference numeral
102
, and is constructed by alternately stacking two (2) different corrugated plates (see
FIG. 4
) to define alternating flow channels. Water vapor or steam from the desiccant boiler
56
is introduced near the top of the regenerator/exchanger
102
in alternate channels (plate A). This water vapor is condensed, thereby transferring heat to the air and weak desiccant entering adjacent channels near the top of the regenerator/heat exchanger
102
(plate B). The upper portion of each plate corresponds to the desiccant regenerator
46
and regeneration air heater
48
. 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 fan
106
to the outdoors. An entrainer
108
is provided to prevent desiccant from escaping the combined regenerator/exchanger
102
. The partially regenerated desiccant flows into the middle of a channel plate B, and is further heated by the hot concentrated desiccant removed from the liquid/vapor separator
98
. Hot concentrated desiccant from the boiler
56
is intr

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