A thermally regenerative desiccant element comprising micron size
silica gel and held within an expanded web of a fluoroplastic elastomer
is disclosed. The expanded web of silica gel is bonded onto a heat
conductive plate to form the desiccant element. The desiccant elements
are stacked in an arrangement such that the air to be processed
passes in contact with the silica gel. The other side of the plate
is made to come in contact with a stream of humidified cooled air.
The cooled air removes the heat of sorption when the silica gel
sorbs moisture. A solar collector is used to heat ambient air for
regeneration of the silica gel.
What is claimed is:
1. A desiccant element, comprising: micron size silica gel, ground
ammonium bicarbonate and a fluoroplastic powder blended into a paste
and physically worked into a thin sheet by roller reduction, said
ammonium bicarbonate then having been made to decompose by heating
said sheet; and wall means connected to said sheet for conducting
heat away from said sheet and for supporting said sheet on one of
its sides, whereby thermal energy on the other side of said wall
means is conducted to said sheet to drive off moisture held by said
silica gel and regenerate said desiccant element and a portion of
the heat liberated from said silica gel upon the sorption of moisture
is transferred to the other side of said wall means.
2. A thermally regenerative desiccant element, comprising:
(a) a plurality of micron-size silica gel particles;
(b) an expanded elastomer web for holding and distributing said
silica gel particles, said elastomeric web of silica gel particles
being expanded by percolating gas through the web of silica gel
particles, said gas being released by heating a gas producing means,
originally blended together with said silica gel and said elastomer
for producing gas within said web by thermal decomposition, the
released gases having been used to stretch apart the strands of
said elastomer web to form a thin, stringy, net-like structure,
whereby the porosity of said elastomer web is increased; and
(c) wall means connected to said web for carrying said expanded
elastomer web of silica gel for conducting heat away from said silica
gel, heat being liberated from said silica gel upon the sorption
3. The thermally regenerative desiccant element defined in claim
2 wherein said gas producing means is ammonium bicarbonate.
4. The thermally regenerative desiccant element defined in claim
3 wherein said elastomer web of silica gel was formed from a blend
of: micron-size silica gel; ground ammonium bicarbonate in a range
from about 20 to 80 percent by weight; and about 5 percent by weight
fluoroplastic powder in an adequate quantity of a naphtha based
solvent so as to form a paste, said paste having been filtered to
remove any excess solvent and then physically worked by roller reduction
to form a thin sheet, heating said sheet between 100.degree. C.
and 130.degree. C. having the effect of decomposing said ammonium
bicarbonate, the gases of decomposition stretching apart the matrix
formed by the elastomer and silica gel and producing voids without
leaving a solid residue.
Apparatus for dehumidifying air using the moisture sorption property
of silica gel including apparatus for regeneration of silica gel
using solar heating and cross cooling. An article of manufacture
composed of micron-size silica gel in an elastomer web for use as
a desiccant in a thermally regenerative dehumidifier.
BACKGROUND OF THE INVENTION
The air drying properties of sorbents such as silica gel, lithium
chloride, and alumina are well known and have been used in many
industrial applications. However, the applicability of these sorbents,
and in particular silica gel, as a desiccant in an air conditioning
system has been explored only recently.
One of the pioneering systems was the Munters Environmental Control
(MEC) System developed at the Institute of Gas Technology (IGT).
In that system a rotating matrix, consisting of a series of channels
whose walls are made of a sorbent, exchanges moisture and heat with
a stream of air flowing through it. The MEC concept has been known
for about thirty years. The first patent covering this principle
was issued in 1949. Despite efforts by both the U.S. and European
organization over the years, the concept was never successfully
developed to a commercial reality. IGT became interested in the
potential advantages of MEC about thirteen years ago. It applied
modern engineering analysis and computer modeling techniques to
study the system. IGT's wholly owned subsidiary, Gas Developments
Corporation, gained a license for the patent from A. B. Carl Munters
of Sweden, the owner of the patent. The IGT modifications included
an asbestos-wax, heat-transfer wheel and an asbestos-lithium chloride
Another variation, MEC II, used an aluminum heat transfer wheel
to improve heat transfer efficiency. The aluminum wheel, however,
became hygroscopic with time, absorbing water and transferring it
to the conditioned dry air stream. Also, the lithium chlorine on
the drying wheel deteriorated into other compounds that could not
carry out the drying function.
In another variation, MEC III, constructed in 1973 the aluminum
wheel was coated with a proprietary material that reduced the water
carry-over to an acceptable level. The drying wheel used a newly
perfected molecular sieve absorbent material. The new material was
a paper-thin asbestos sheet carrying over 50% molecular sieve. The
sheet was corrugated and formed into a wheel. Later that device
was adapted to use solar heat and a natural gas boost to make up
for the solar heating deficiency.
Pennington (U.S. Pat. No. 2700537) describes a humidity changer
for air conditioning that uses a rotary moisture transferrer packed
with an inert, air-pervious carrier having a rigid space structure,
and impregnated with a liquid sorbent.
Another wheel-type humidifier was built by Cargocaire under the
brand name "HoneyCombe." In that device a wheel core was
made of a non-metallic, non-corrosible, bacteriostatic, inert structure
impregnated with an inorganic, non-granular, crystalline, particle
solid desiccant which transfers water in the vapor phase. The desiccant
was evenly dispersed throughout the microscopic pores of the wheel
structure. The wheel structure consisted of small flutes or tubes
parallel to the axis of flow, allowing laminar air flow to give
the maximum moisture transfer with minimum friction loss. Humid
air passing through the flutes was dried. Simultaneously, a counterflowing
hot reactivation air stream passed through the flutes in the reactivation
sector to remove the moisture picked up by the desiccant thus assuring
continuous controlled drying.
The capacity of such exchangers is generally reduced due to the
large heat effects associated with sorption of water. This is because
at a given humidity the equilibrium capacity of a sorbent decreases
with temperature. There is another problem resulting from this reduced
capacity at higher temperatures. After regeneration the matrix,
in which the hygroscopic salt is held, is left at a high temperature.
During sorption, although the relatively cooler incoming air cools
the matrix down, the heat effect associated with sorption may not
allow the desired level of humidity in the outgoing air. As a remedy,
cooling of the solid during sorption has been suggested to obtain
the desired temperature and humidity levels. This can be accomplished
by using a cross-flow heat exchanger where the cooling and the process
streams are separated by a solid wall; and where the cooling and
process streams flow perpendicular to each other.
Among the common desiccants used for drying air silica gel has
the unique property of showing a sharp decrease in the equilibrium
sorptive capacity with a temperature increase at a given partial
pressure of water over it. Although silica gel can be regenerated
at lower temperatures than other desiccants (such as molecular sieves
or activated alumina), the immediate disadvantage arising from a
sharp decrease in capacity with an increase in temperature is that
in an adiabatic rotary dehumidifier (with alternate sorption and
desorption) the desired humidity level may not be achieved during
sorption. If with the help of cross-cooling, the desired humidity
level can be achieved, silica gel would then become a desirable
desiccant in a rotary dehumidifier. Cross-cooling would delay the
"break-through" time for drying operations thus allowing
a slower speed of rotation for a rotary exchanger than for the corresponding
Alternatively, for a given breakthrough time and process channel
width, channel length would be reduced to cross-cooling. More importantly,
there is an accompanying reduction in volume occupied by the process
channel compared to the adiabatic case. However, part of this advantage
is lost due to the increased volume accompanying the addition of
A quantitative study of a cross-cooling dehumidifier was performed
and reported in Chemical Engineering Science 1974 volume 29 pages
2101 through 2114. That study showed that cross-cooled dehumidifiers
can be smaller and require less power than corresponding adiabatic
exchangers. Those calculations also showed that regeneration temperatures
below 180.degree. F. in conjunction with cross-cooling result in
sufficient dehumidification for air conditioning applications.
A solar powered dehumidifier was proposed by the Energy Research
and Development Authority (ERDA) under its Solar Activated Cooling
(SAC) project. It was recognized that solar energy in the form of
heat could be used to dehydrate the desiccant thus closing the functional
SUMMARY OF THE INVENTION
A cross-cooled solar powered air conditioning system using silica
gel has been developed and found to perform in an extremely effective
manner. Desiccant materials which by nature readily remove water
vapor from the atmosphere are prime candidates for meeting the requirement
for dehumidification of air conditioning loads. Solar energy or
any other low level heat source is used to dehydrate the desiccant
thus closing the functional cycle.
Solar powered dehumidification is an excellent way to use solar
power in the cooling season. The latent-heat load on conventional
air conditioning and heat pump systems is significant, and if removed
by desiccant dehumidification, the consumption of non-solar energy
would be significantly reduced. Dehumidification is required for
the ventillation of hospitals and many other public buildings. It
is a requirement that will tend to grow as building construction
techniques become more and more conserving creating new controlled
ventillation demands. More importantly, solar powered dehumidification
will enhance the cost effectiveness of many solar-powered or conventional
air conditioning devices such as solar absorption or Rankine-cycle
chillers or conventional vapor compression chillers.
The system consists of two identical fixed bed dehumidifiers. One
bed dehumidifies while the other bed is being regenerated. Cross-cooling
is achieved with cooling air flowing through rectangular flow channels.
The process stream flows in perpendicular flow channels which are
lined with paper-like sheets consisting of micron-size silica gel
particles held in a TEFLON web.
The dehumidifier beds are formed from a stack of trays lined with
sheets of micron size silica gel particles held in a web of TEFLON.
In manufacturing the sheets, a paste is formed from TEFLON powder,
silica gel particles, a solvent, and a gas producing means, such
as ammonium bicarbonate. The resulting paste is formed into sheets
by roller reduction. After drying the sheets are heated to decompose
the gas producing means thereby increasing the porosity of the web
of TEFLON and silica gel. The thin sheets of silica gel form individual
elements of a desiccant dehumidifier bed.
Dampers are provided to direct air, heated by solar collectors,
into contact with the silica gel to regenerate or drive off the
moisture sorbed by the silica gel. A solar heat storage system provides
a source of heat when the solar collector is not available. An overall
coefficient of performance of 0.5 to 0.7 and a regeneration temperature
of 60.degree. C. to 82.degree. C. have been predicted. This low
regeneration temperature will also permit the use of waste heat.
Due to the low pressure drop in the flow channels, the electric
coefficient of performance (EER) is expected to be very high (about
The advantages of this dehumidifier are:
(a) the desorption temperature is low since the desiccant is silica
(b) the sorption capacity is high as a result of cross-cooling;
(c) the sorption rates are high due to low compactness of silica
gel particles (specifically, about 40% of the sheet volume is silica
(d) the pressure drop in the channels is low compared to the pressure
drops across a packed bed.
Numerous other advantages and features of the present invention
will become readily apparent from the following detailed description
of the invention and of the embodiment disclosed, from the claims
and from the accompanying drawings.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
FIG. 1 is a representation of the dehumidifier bed before being
connected to the associated duct work;
FIG. 2 is a partial cross view of one of the dehumidifier bed trays
as viewed along line 2--2 of FIG. 1; and
FIG. 3 is an overall view of the major components of an air conditioning
system utilizing the desiccant bed as a dehumidifier.
DETAILED DESCRIPTION OF THE INVENTION
While this invention is susceptible of embodiment in many different
forms, there is shown in the drawings and will herein be described
in detail a preferred embodiment, with the understanding that the
present disclosure is to be considered as an exemplification of
the principles of the invention and is not intended to limit the
invention to the embodiment illustrated.
Silica gel is a regenerative sorbent consisting of amorphous silica.
It has found many applications in the dehumidifying and dehydrating
of air. It also has been extensively studied as a humidity controlling
desiccant. However, it was only recently that silica gel was considered
for use in air conditioning. For many years, it was thought to be
unsuitable for use in air conditioning cycles because of the heat
needed to drive off the moisture sorbed in the silica gel. Other
systems were more efficient thermodynamically (i.e., Rankine cycle).
However, when the price of energy is factored into the analysis,
silica gel regenerated by solar heat is competitive to other cooling
systems. This competitive edge is expected to increase as solar
collectors are perfected and the price of traditional fuels increases.
Detailed calculations have shown that regeneration temperatures
below 82.degree. C. (in conjunction with cross-cooling) result in
sufficient dehumidification for air conditioning applications. In
particular, the low regeneration temperature permits the use of
a flat plate solar collector as a source of heated air. If these
collectors are used for both heating and cooling the "payback
period" is accelerated and the "first cost" of the
investment more easily justified relative to conventional heating
and air conditioning installations.
Silica gel's affinity for water has been described as absorption
by some investigators and adsorption by others. Since the particular
process is not relevant to the understanding of the invention, the
term "sorption" has been used throughout this specification
to indicate the process of removing moisture from an air stream.
Similarly, "desorption" will be used to describe the release
of moisture to the atmosphere when silica gel is heated.
FIG. 3 shows the principal features of the cross-cooled desiccant
cooling system operating in the recirculation mode. Humid air from
a room 10 (i.e., the one to be air conditioned) is drawn by a blower
11. It passes through dampered cuts 12 into desiccant lined flow
channels of a bed 14 (see FIG. 1) of silica gel and exits as a dry,
moderately warm air. This air flow is referred to as a "process
stream." Dampered ducts 16 downstream of the silica gel bed
14 direct the process stream to a chamber 20 where it is sprayed
with water before being admitted to the housing 10. The water spray
removes a portion of the heat of sorption transferred to the process
stream in passing through the desiccant bed 14.
A large portion of the heat evolved during dehumidification is
removed by a stream of ambient air 22 drawn by a blower 24. This
flow is ducted through channels in the bed of silica gel 14 on service.
These cooling channels are perpendicular to those of the process
stream. This second or "heat exchange" flow stream is
partially cooled by adiabatic humidification in passing through
a water spray chamber or recooler 26 before being directed to the
desiccant beds 14 and 40. It is discharged to the ambient air after
passing through the desiccant beds.
FIGS. 1 and 2 show the relationship of these two flow streams.
The process stream flows in the flow channels 30 lined with silica
gel sheets 32; the heat exchange flow stream 34 flows in channels
36 perpendicular to the silica gel lined channels 30. The silica
gel sheet 32 is bonded to the walls or trays 38 dividing the two
One desiccant bed 40 (see also FIG. 1) is shown being regenerated
directly with a solar source 42. The other bed 14 is on service
dehumidifying the process stream. The desiccant bed 40 desorbs (releases
moisture) using air heated by the solar source 42 or storage 46
circulated by the third fan 44. This flow stream is defined as the
"regeneration flow stream." The regeneration air stream
from this bed 40 is discharged to the atmosphere since it has a
high moisture content and therefore cannot be reused. A sensible
heat exchanger or preheater 48 is used to heat ambient air which
replaces the rejected humid air. This improves the utilization of
solar energy and increases the overall efficiency of the cycle.
The cycle may also be improved by recovery of waste heat from other
equipment and systems. Suitable ductwork and dampers join the major
components of the regeneration flow stream. The details of the design
of the solar collector 42 and the heat storage mechanism 46 (i.e.,
water, rocks, salts, etc.) follow principles known to those skilled
in the art.
When both beds have completed desorption and sorption, the sensible
heat stored in the desorbed bed is used to preheat the bed which
has just completed sorption. This is done by using the cross-cooling
blower 24 to push air from the hot bed 40 into the cool bed 14.
Next, the flow of air in the system is rerouted (using appropriate
dampers and duct work) and the functions of the two beds are interchanged.
This process improves the solar energy utilization of the desiccant
beds and adds to the overall efficiency of the cycle.
The dampers and duct arrangements shown in FIG. 3 are representative
of one embodiment of the invention. Other arrangements, including
the use of motorized dampers and automatic controls, may be employed
following methods known to those skilled in the art.
In a specific embodiment of this invention a dehumidifier housing
60.times.60.times.60 centimeters was built (see FIG. 1). It included
space for a bed 14 of eighty trays 50 made of 0.2 millimeters aluminum.
The trays were dimpled to insure that the cooling channels 52 remain
rigid and open. Silica gel sheets 32 approximately 1.5 millimeters
thick were bonded (using Glyptal 1201B--a red enamel manufactured
by General Electric) to the outer surface of each tray. The trays
are then assembled in a vertical stack with the aid of four side
supports 54. Four corner flanges 56 complete the assembly. These
flanges facilitate joining the bed to ductwork and form inlet and
outlet plenums. The total weight of the unit is about 66 kilograms
about half of which is silica gel. Tests have shown that unit performance
improves sharply with an increasing process air dew point. It also
has been demonstrated that if the dew point of the regenerating
air increases, unit performance drops. For the adiabatic case (i.e.,
no cross-cooling), the total moisture cycled is only 5% relative
to the total weight of silica gel in the unit. However, if cross-cooling
is used, the moisture cycled increases to 7.4%. At higher cross-cooling
flow rates, it is possible to increase the moisture cycled to 8.2%.
The important conclusion is that moderate cross-cooling increases
the performance of the silica gel sheets by 50%. It can be concluded
that the performance of a cross-cooled desiccant dehumidifier improves
(a) increasing inlet process air dew point;
(b) increasing process air flow rate;
(c) increasing regeneration temperature;
(d) increasing cross-cooling (heat exchange) stream flow rate;
(e) decreasing regeneration air inlet dew point.
In each case, micron-size silica gel, previously ground ammonium
bicarbonate (20 to 80 percent by weight) and polytetrafluoroethylene
(TFE) power (i.e., TEFLON were blended together with an adequate
quantity of an aliphatic petroleum solvent. The solvent serves as
a processing aid. The resulting paste was filtered to form a cake
and then physically worked by roller reduction to form a sheet of
silica gel held in a web-like structure of TFE. A final reduction
was made to produce desiccant sheets of the required thickness and
size for the dimensions of the dehumidifier housing (see FIG. 1).
The wet sheets were dried overnight (about 12 hours). Finally, the
ammonium bicarbonate was made to decompose by heating the desiccant
sheet at a temperature of from 100.degree. C. to 130.degree. C.
for about 30 minutes. When the sheet was viewed through an electron
microscope, the silica gel particles are shown to be held together
by means of thin strands of TFE. The structure resembles that of
a three dimensional net or web.
The dynamics of water vapor sorption is affected by desiccant sheet
thickness, the particle size of the silica gel used in the preparation
of the desiccant sheets, and by the density or porosity of the desiccant
sheets. Each desiccant sheet is an elastomeric web of micron-size
silica gel. The porosity of the sheet was enhanced by the use of
a gas producing means, such as ammonium bicarbonate. Table I shows
three typical starting compositions that were found to produce good
quality desiccant sheets.
More specifically, Davison Syloid-63 nine micron size particles
were used as the silica gel. Particle sizes from 1 to 1000 microns
should do equally as well. Shell Solv 340 (manufactured by the Shell
Oil Company) was the aliphatic petroleum solvent that was used to
blend the TFE powder with the silica gel. It has a boiling point
between 300.degree. F. and 350.degree. F. One simply adds whatever
solvent necessary to convert the blend of silica gel and ammonium
bicarbonate into a paste. If too much solvent is added, the "paste"
becomes too fluid. If too little solvent is added, the "paste"
becomes too crumbly to be self-adherent. There may or may not be
an excess amount of solvent present in the paste when sheet formation
begins. If there is, then that excess is simply filtered off using
conventional techniques. Alternatively, more solid matter can be
added. All of the solvent is eventually evaporated away. These considerations
are not beyond those skilled in the art.
TFE is a fluorocarbon resin and is available in a wide variety
of dry power and water-base dispersion forms. TFE is sold under
the HALON trademark (Allied Chemical Corporation) the TEFLON trademark
(DuPont Co.) and the FLUON trademark (ICI United States Inc.). Because
TFE resists forming even temporary bonds with other molecules, nothing
sticks to it. Other substances slide over it readily. These properties
and characteristics are of importance in the thermally regenerative
dehumidification process described previously. Temperature resistance
and resistance to deterioration ensure a long maintenance free operating
cycle and greater acceptance in the market place, thus leading to
a greater likelihood of commercial acceptance and a willingness
by the public to convert from conventional dehumidifiers.
Although the desiccant sheets described above were specifically
formed from TFE, the important characteristic and function served
by TFE is the creation of stringy fibers to hold the silica gel
particles together. Consequently, other elastomers of a low density
and similar stiffness may be used.
The ammonium bicarbonate gas producing means improved the porosity
(i.e., addition of voids) of the desiccant sheets. Subsequent heating,
after sheet formation drives off the processing air (ex. Shell Solv
340) an decomposes the ammonium bicarbonate:
One advantage of ammonium bicarbonate is that it produces gas (i.e.,
ammonia, carbon dioxide, and steam) without leaving a residue. This
insures the web-like bonding of the silica gel without restricting
the sorption property of silica gel.
Sheet thickness can be varied from 0.1 millimeters to 10 millimeters
by the rolling operation previously described. It was found that
the density or porosity of the desiccant sheet can be varied by
using various ratios of ammonium bicarbonate. Use of too much ammonium
bicarbonate produces a desiccant sheet that falls apart. Use of
too little ammonium bicarbonate produces a sheet that has insufficient
porosity for rapid adsorption. The particular desiccant sheet density
and porosity used with the dehumidifying apparatus previously described
is dependent upon whether a high sorption rate or a high sorption
capacity is desired.
Porous silica gel sheets can also be made by use of salts of sodium
or potassium bicarbonates. Decomposition of these salts produces
a gas, carbon dioxide, that can form a porous structure. However,
a solid product consisting of carbonates is left behind which adds
to the weight of the sheet and also permits a reaction with the
carbon dioxide found in air, causing a possible deterioration of
the sorption capacity of the sheet due to pore closing. Substances
other than ammonium bicarbonates which also leave no residues are
ammonium carbonate [(NH.sub.4).sub.2 CO.sub.3.H.sub.2 O] Camphor
and Napthalene. This latter group of substances, as well as ammonium
bicarbonate, are the preferred means of forming of the porous web-like
structure of the desiccant sheets. Since all of the products are
a gas, they are best described as a means for producing gas within
the matrix of silica gel and TFE by thermal decomposition.
From the foregoing, it will be observed that numerous variations
and modifications may be effected without departing from the true
spirit and scope of the novel concept of the invention described.
It is to be understood that no limitation with respect to the specific
apparatus illustrated herein is intended or should be inferred.
It is, of course, intended to cover by the appended claims all such
modifications as fall within the scope of the claims.