desiccantLow volume desiccant dehumidifiers, such as dehumidifiers
used for the processing of resin pellets, which provide high dehumidification
performance, simplified mechanical arrangement and high reliability
in a desiccant dryer optimally suited for low volume applications.
What is claimed is:
1. In a dehumidifying apparatus in which a desiccant medium is
cyclically shifted between an operating active process condition,
a regenerating condition, and a cooling condition, the improvement
(1) a rotatable desiccant bed having a plurality of essentially
identical modular desiccant containers being disposed substantially
parallel to a central bed axis, each of said containers containing
a desiccant medium and having a pair of air permeable surfaces situated
on each end of said containers which permit the flow of air from
one side of a container to the other side through the desiccant
and sealing means on each end of each container;
(2) two fixed parallel low friction sealing plates cooperatively
engageable with the sealing means on each end of each desiccant
container in said bed, having a plurality of apertures to provide
a first isolated class of air process containers in which the bed
desiccant is active, a second isolated class of containers in which
the bed desiccant is in a regenerating condition, and a third isolated
class of containers in which the bed desiccant is in a cooling condition
after it has been regenerated and prior to the time it is in active
process such that each desiccant container in said bed only in the
process and regeneration stage has associated with it a pair of
apertures in said sealing plates, and each desiccant container in
said bed in the cooling stage is not associated with any apertures
in said plates, whereby heat dissipates independently of forced
air flow through said desiccant container in the cooling stage,
said sealing plates;
(a) being operatively disposed with respect to connections with
the process and regeneration air flows to define separate process
or regeneration sections in the apparatus for the counter flow of
air through the beds in each respective class of containers; and
(b) being further operatively disposed with respect to said seals
on the ends of each desiccant container so as to provide continuous
sealing of said containers such that rotation of said bed a predetermined
angular segment of a circle permits sealed cyclical process, regeneration,
and cooling conditions to occur; and
(3) motor means for intermittently rotating said bed with respect
to said fixed sealing plates about a predetermined angular segment
around the axis of said bed a predetermined time interval to shift
a desiccant container in the first class of active air process containers
to the regenerating condition, and then to the cooling condition,
and then again to the active condition.
2. The apparatus of claim 1 further comprising means for directing
the flow of regeneration air to the class of regenerating condition
containers in a direction which is countercurrent to the flow of
air through said desiccant containers while said containers are
in the process condition.
3. The apparatus of claim 1 wherein said sealing means comprise
gasket means disposed with respect to the perimeters of each end
of each desiccant container to permit continuous sealing of the
end of said containers with said sealing plates during the rotation
of said desiccant bed between said sealing plates.
4. The apparatus of claim 1 wherein said desiccant containers are
cylindrical in shape and contain two air permeable surfaces which
face said two sealing plates.
5. The apparatus of claim 1 in which said sealing plates are coated
with a polymeric low friction coating of the class of polytetrafluoroethylene
compounds or their equivalent.
6. The apparatus of claim 1 wherein said sealing plates are shot
peened to increase adhesion of a low friction material to be applied
to said sealing plates.
7. The apparatus of claim 1 further comprising a hopper operatively
connected to said desiccant bed by inlet and outlet air flow means.
FIELD OF THE INVENTION
This invention relates to low volume desiccant dehumidifiers such
as dehumidifiers used for the processing of resin pellets
BACKGROUND OF THE PRIOR ART
Desiccant humidifiers or dryers using an absorption/desorption
cycle are well known but are not particularly suitable for low cubic-feet
per minute (CFM) drying applications because the mechanical complexity
of the control apparatus for the absorption/desorption cycle increases
the cost of such apparatus beyond its economic feasibility in a
low volume application.
OBJECTS OF THE INVENTION
It is an object of this invention to provide high dehumidification
performance, simplified mechanical arrangement and high reliability
in a desiccant dryer optimally suited for low volume applications.
The apparatus of the invention will be useful to dry resins for
molding, to dry enclosures for scientific pursuits, and to dry other
critical materials or spaces. It is an object to provide reduced
cost, simplicity, reliability, and improved performance over known
types of comparable desiccant bed dryers that require a complicated
bed disengagement sequence during the bed indexing process. Such
bed dryers tend to be high in cost, require more service and critical
adjustments, and generally perform less than optimally. By contrast,
the apparatus of the present invention provides an integral valve
system that is uncomplicated compared to conventional dryers, which
system also enhances drying performance.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a general outline of a typical dehumidification cycle
and dehumidifier apparatus.
FIG. 2 is an exploded view of the dehumidifying unit.
FIGS. 3A and 3B respectively depict a cutaway of the bed rotation
between plates and the assembly relationship of a desiccant container.
FIGS. 4A and 4B are respectively a side cross section of a desiccant
container and a plan view of the container.
FIG. 5 shows an image of the base and top plates of the invention
in a plan view.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Generally, the apparatus of the invention removes moisture in the
vapor state from a sealed air stream by exposing the air to an adsorbing
media such as a desiccant. The apparatus of the invention includes
a desiccant bed comprising a plurality of essentially identical
modular desiccant containers which at any time are classified in
one of three isolated stages or conditions of a dehumidification
operating cycle, such that as to any individual desiccant container,
all the desiccant in the container is classified in one of the three
Water vapor removal occurs during the process or drying stage of
an operating cycle of the claimed invention. After the adsorption
limit of the desiccant media has been reached, the media is reactivated
by exposing it to a second air stream of an elevated temperature
in the reactivation stage of an operating cycle. During reactivation,
moisture adsorbed during the previous process phase of the cycle
is driven from the media preparing it for further process stages.
A third stage in the cycle permits cooling of the desiccant to eliminate
heat carryover from reactivation to process stage and to reduce
drops in performance experienced during a bed index.
The entire cycle of the claimed apparatus comprising process drying,
reactivation and cooling is carried out on a continuous basis with
respect to individual desiccant containers mounted in a bed. The
bed is intermittently rotated a predetermined segment of a circle
between fixed, low friction sealing plates so that at any one time
a particular desiccant container is in one of the process, reactivation,
or cooling positions of the apparatus's cycle. A constant drying
action is thereby attained with continuous flow of air into the
desiccant bed. The process and reactivation air streams are independent
of each other to prevent contamination of the dry air stream.
Further, the apparatus of the invention provides positive sealing
of the bed desiccant at all times, including bed indexing during
operation of the apparatus of the invention.
FIG. 1 illustrates the arrangement and air flow Pattern of the
dryer of the invention. The dryer is designed with the process and
reactivation air streams flowing in opposite directions, i.e. in
a counter flow relationship thereby promoting performance and energy
In FIG. 1 there is shown a plastic hopper 1 feeding to an extruder
2. The hopper has inlet and outlet orifices 3 and 4 respectively
for a flow of process stage drying air, to which in turn are connected
hoses or ducts to respective connections in the process sections
5 and 6 of the four container desiccant bed 7. The desiccant container
in section 8 of the desiccant bed of the apparatus undergoes reactivation.
A reactivation air stream which is passed through a blower 10 and
heater 11 and passes through a hose or duct connected to reactivation
section 8 of the bed eliminates moisture adsorbed in the section
8 desiccant container during the prior process step of the cycle.
The fourth desiccant container in section 9 of the bed, which previously
was subjected to reactivation is maintained in the cooling step
of a cycle immediately prior to the time that this desiccant container
is once again used in the process step of the cycle. A blower 12
and heater 13 for the two process sections are provided. Filters
may also be provided for the process inlet stream 14 and reactivation
inlet stream 15.
In FIG. 2 there is shown the assembly of the desiccant bed in a
vented cabinet with the desiccant containers not depicted for purpose
of clarity. In the cabinet assembly a first, fixed planar lower
sealing panel 21 is provided. This panel is covered by a first lower
replaceable sealing plate 22 spaced apart by a suitable distance
from a second upper fixed sealing plate 23 removably mounted on
the upper sealing panel 24. Process filter box 25 communicating
with the two desiccant containers in process sections 5 and 6 may
be mounted on the upper sealing panel.
The upper and lower sealing plates provide a low friction surface
upon which seals of the desiccant containers cooperatively engage
and rotate during indexing of the desiccant bed between process,
reactivation, and cooling connections. The upper and lower sealing
plates contain a number of apertures corresponding to the number
of process condition and regeneration condition desiccant containers
desired in a given desiccant bed. Corresponding numbers for process
and regeneration air stream connections are indicated. For example,
the apparatus depicted in FIG. 1 contains two apertures in each
sealing plate to provide connections to process air and one aperture
to provide connection to regeneration air. Desiccant containers
in the cooling condition remain sealed between the sealing plates
with no plate apertures provided for cooling. The combination of
the desiccant container seals with the low friction sealing plates
provides an integrally valved system that allows the desiccant to
be essentially sealed from ambient air during both the process phases
of cycles and during indexing of the desiccant bed. Thus conventional
valving required for bed engagement and disengagement is eliminated
with the present invention.
The sealing plates are coated with a polymeric low friction coating
such as that marketed by DuPont Company, FBFP Department, Teflon.RTM.
Finishes, Wilmington, Delaware 19898 under trademarks Teflon.RTM.,
SilverStone.RTM., and SilverStone SUPRA as described in Product
Bulletins TC-216-894; TC-224-85; TC-225-84 and TC-227-84 with respect
to specifications and applications of the coating through the applications
of primer, intermediate coat and top coat. These low friction coatings
are of the polytetrafluorethylene class of chemical compositions,
of their equivalent. The sealing plate base metal is initially shot
peened to increase adhesion of a base coat of low-friction material
to the base metal. In an example, low friction c coating sold under
the trademark SilverStone.RTM. by DuPont Corporation as referred
to above is applied to the so prepared base metal comprising each
of the upper and lower sealing plates. A primer coat, for example,
DuPont Primer 459-516 is first applied at a thickness of a minimum
of up to about 1.5 mils and baked at 300.degree. F. for six minutes.
An intermediate coat of material, for example, DuPont Intermediate
Coat 456-227 is then applied at a thickness of a minimum of up
to about 1.5 mils and while the intermediate coat is still wet,
a clear top coat, for example, DuPont top coat 456-300 is applied
at a thickness of a minimum of up to 1.5 mils. These coating layers
are all applied in accordance with recommended specifications of
DuPont Company in the Product Bulletins referred to above. The coated
plates are then sintered for ten minutes at 800.degree. to fuse
the top coat to the intermediate coat.
Control of the indexing of the desiccant containers is determined
by an indexing motor 26 connected by sprocket 27 or other suitable
means to the bed shaft assembly comprising shaft 28 and axial spacing
means 29 and 30 conforming to the outer shape of the desiccant containers.
An enclosure cabinet 31 may include removable top cover 32 and side
access panel 33 or other panels determined by application requirements.
Provided on each of lower and upper sealing plates 22 and 23 are
three openings positioned radially on the plates equidistant from
shaft 28. Lower sealing plate 22 contains outlets for process sections
5 and 6 denoted 5o and 6o respectively, and an inlet for process
section 8 denoted 8i. Similarly, upper sealing plate 23 contains
inlets 5i and 6i for process sections 5 and 6 and outlet 8o for
process section 8. The indexing motor intermittently rotates the
bed desiccant between lower sealing plate 22 and upper sealing plate
23 a predetermined angular segment, for example, 90.degree., around
the axis of shaft 28 at a predetermined time interval, depending
upon the particular application, to the various process and regeneration
The FIG. 3A cutaway view of the desiccant bed rotation between
sealing plates 22 and 23 shows further the arrangement of the desiccant
containers of bed 7. Retaining band 34 holds the desiccant containers
in place as the entire shaft, axial spacing means, and four desiccant
containers of the bed rotate as a unit between the stationary sealing
plates to each of the four process sections 568 and 9.
The assembly of a typical desiccant container as seen in FIG. 3B
shows a desiccant container 35 comprising a hollow cylindrical solid
body with seal retaining areas on the perimeters both ends of the
body (only upper retaining area 36 shown) which hold in place upper
bed seal 37 and lower bed seal 38. The bed seals are preferably
hollow o-rings of silicone rubber, the size of which varies with
the size of the desiccant container used. A typical specification
for a 20 CFM unit is as follows:
A desiccant fill hole 39 is provided in the container body which
is closed by a fill hole cover gasket 40 and finally a fill hole
cover 41 after the container is filled with desiccant or other adsorbing
medium. Upper and lower bed seals of each desiccant container in
a desiccant bed serve to continuously seal the desiccant from ambient
air, thereby enabling continuous drying.
FIG. 4A shows a side cross section view of one of the desiccant
containers that comprise the desiccant bed which includes a cylindrical
sectioned solid side 42 having screens or perforated termi 43
44 at each end to maintain the desiccant (not shown) in place. The
container includes seals 37 and 38 around the circumference of each
end of the container which sealingly press against the lower 22
and upper 23 sealing plates to provide a tight seal that prevents
the escape of process or drying air during the times that the desiccant
containers are connected to process and regenerating air streams
as well as during bed indexing.
A plan view of a desiccant container shown in FIG. 4B depicts the
top, a typical desiccant container showing placement of a screen
43 at the end of the container and bed seal 37.
FIG. 5 shows a plan view of replaceable sealing Plate 22 which
is identical in the upper position or lower position. As above-described,
the plate provides two outlet openings, 5o and 6o for the flow of
drying process air and one inlet opening, 8i, for reactivation air,
spaced distance R from shaft 28.
In an exemplary application as a plastics dryer, the apparatus
of the invention has a process CFM of 20 at 10" ESP and a reactivation
CFM of 10. The dryer is equipped with a process heater of 1.75 KW
and reactivation heater of 1.25 KW. Depending on the application,
the dryer may be used for polymeric materials such as ABS, acrylic,
barex, nylon 6/6 polycarbonate, PBT, PET, polysulfone, polyurethane,
noryl, SAN, and vinyls (PVC) that require drying temperatures from
160.degree. to 375.degree. F. and drying times from 1 to 6 hours
at a drying rate of from 25 to 40 pounds per hour.
While we have set forth a description of the preferred embodiment
utilizing specific spatial relationships, other spatial orientations
or modifications are apparent to those skilled in the art and are
intended to be within the scope of the claimed invention. In particular,
dryer apparatuses embodying the claimed invention can be constructed
with any number of desiccant containers in a desiccant bed for each
of process, regeneration, and cooling stages of a cycle, in which
case the low friction sealing plates with the required openings
for process and regeneration connections can be appropriately constructed.