Note: Descriptions are shown in the official language in which they were submitted.
8QS
The present invention relates to a process
and apparatus for drying water-containing materials.
More particularly, the present invention relates to a
process and apparatus for drying materials with in-
ternally generated superheated steam in a drying zone
while preventing ambient air from entering such drying
zone. In one aspect, the present invention relates to
a method and apparatus for drying thin sheet materials
such as wood veneer. In another aspect the invention
relates to a process for drying particulate materials.
Wood veneer is normally dried by passing
large volumes of hot air over the veneer sheets. The
water content of the hot air is usually about five
percent, but may be as high as 30 percent or more in
unusual cases. When overly hot air contacts the veneer,
the surface of the veneer begins drying immediately and
very rapidly dries out completely, becoming overly hot,
` while the interior of the veneer is still relatively cool
and moist. This temperature gradient, and the resulting
moisture gradient in the veneer cause hardening, cracking
and general degradation of the veneer. For this reason,
the wood veneer drying art has resorted to relatively low
drying air temperatures and long drying times.
It is desirable to have a uniform moisture
content in the veneer after drying is completed. In con-
ventional veneer drying the outer surface of the veneer
is overly dry after drying is completed, while the interior
of the veneer remains overly moist. Moreover, sheets of
veneer dried at different times in the same dryer often
have different moisture contents, making it difficult to
glue the veneer uniformly to produce plywood.
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805
A major problem in the veneer drying art has
been disposal of large amounts of hydrocarbonaceous vapors
which are evolved from the veneer during drying. The
hydrocarbonaceous vapors are removed from the veneer by
hot air in the dryer and carried out of the dryer up a
smoke stack along with the air. In the past, after the
hot air has been used in drying, it has simply been dis-
carded up the stack and released into the atmosphere. In
addition, much of the hydrocarbon vapor leaks out of door
seals and cracks in the veneer dryer as "fugitive emissions"
because of the positive pressure created within the dryer.
This has caused severe air pollution problems. Under
present environmental restrictions, it has become neces-
; sary to curb release into the atmosphere of such hydro-
carbonaceous vapors.
Drying of particulate materials such as seeds,
alfalfa, manure, etc., has also been accomplished by con-
tacting the materials with large volumes of hot air.
Typically, air has been taken from the atmosphere and
heated, contacted with the material to be dried, and then
simply released back into the atmosphere. Heat energy
imparted to the air before it is used in drying is thus
lost when the air is released. Air drying of particulates,
like air drying of veneer, has created air pollution
problems as a result of production of various volatile
materials stripped from the drying particulate material
by the hot drying air, which have simply been released
into the atmosphere along with used air.
Conventional air drying of particulates is
relatively uneconomical in that the heat energy imparted
to air used in drying is wasted. Further, it has been
1~3;928~5
found difficult and expensive to remove volatile pol-
lutants from the heated air after use in drying, before
the air is returne~ to the atmosphere.
These problems of pollution control and energy
waste are overcome by the steam drying method and ap-
paratus of the present invention. While superheated
steam has been used previously in batch drying systems
for conditioning and drying heavy lumber and has been
suggested for increased humidity to prevent fires in the
operation of veneer dryers along with reduced air inflow
for heat conservation as discussed by S. E. Corder in
Forest Products Journal, October 1963, pages 449 to 453,
it has not been employed along with means for producing
a negative internal pressure within the drying chamber
in continuous production drying systems such as those
used for drying wood veneer or particulates to prevent
the fugitive emission of pollutants from such chamber
into the plant containing such dryer.
It is an object of the present invention to
provide a method and apparatus for drying water-containing
materials in a more rapid and efficient manner than pos-
sible using conventional drying systems.
It is another object of the present invention
to provide a drying method and apparatus for drying thin
sheets and particulates which employ a drying medium con~
sisting essentially of internally generated superheated
steam and prevent ambient air from entering the drying
zone.
It is a further object of the present invention
to provide a drying method and apparatus which conserves
heat energy loss and reduces atmospheric pollution from
pollutants stripped from a material during drying~
~w~s
It is a further object of the present invention
to provide a drying method and apparatus for drying a
water-containing material to a substantially uniform
water content at the surface and in the interior of the
material r
It is a further object of this invention to
provide a veneer dryer having increaseZ production capa-
city.
It is a further ob~ect of this invention to
provide a veneer dryer method and apparatus which allows
automatic regulation of the internal pressure in a veneer
dryer~
It is a further object of the present invention
- to provide a drying method and apparatus for efficient
drying of water-containing materials and reducing break-
down and degradation of the dried materials.
In one embodiment, th~ present invention re-
lates to a drying method for removing water from a water-
containing material, which includes contacting the material
with a drying gas consisting essentially of steam, at a
temperature higher than 212F., at atmospheric pressure
or lower, vaporizing water in the material, heating the
drying steam to maintain a temperature above 212F., and
discarding a small portion of the drying steam to maintain
the desired pressure.
In another embodiment, the present invention re-
lates to drying apparatus for removing water absorbed in
a material, which includes a container for holding the
material, heating means for providing a heating fluid
consisting essentially of superheated steam in the con-
tainer, means for preventing ambient air from entering
1~ 5
said container, and means for removing a portion of the heating fluid
from the container to maintain a desired pressure therein.
According to a broad aspect of the present invention there is
provided a drying method for removing water from a water containing material
. to provide a dried product which comprises the steps of:
;~ contacting said water containing material, in a drying zone at
drying conditions, with a recycled drying fluid comprising superheated
steam at a first temperature above the boiling temperature of water,
whereby at least a portion of the water in said material is vaporized,
removed from said material, and commingled with said drying fluid to form
an effluent gas stream comprising steam at a second temperature lower than
said first temperature;
removing said effluent gas stream from said drying zone and
heating said effluent gas stream to about said first temperature;
. discarding a minor first portion of said heated effluent gas
. stream and recycling a major second portion of said heated effluent gas
.,i stream to said drying zone to provide said recycled drying fluid;
producing a negative pressure in said drying zone along at
; least a major portion of its length, said negative pressure being below
the ambient pressure of the atmosphere surrounding said drying zone; and
transporting said material through said drying zone along its
length from an inlet to an outlet during drying and recovering said material
from said outlet to provide said dried product.
According to another broad aspect of this invention there is
provided a drying apparatus for removing water absorbed in a material
comprising:
container means for containing said material and including means
for moving said material through inlet and outlet ends of said container
means during drying;
heating means for producing a heating fluid within sai.d container
means from water evaporated from said material, said heating fluid
comprising superheated steam;
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9~805
contacting means for contacting said material with said steam
within said container means to heat said material and vaporize said water
absorbed therein, whereby said water is removed from said material;
means for preventing the entry of subs,antially all ambient
air into said container means through said open ends;
means for producing a negative internal pressure within the
container means along at least the major portion of its length, said
negative pressure being below the ambient pressure of the atmosphere
surrounding said container means; and
means for removing a minor portion of said steam from said
container means.
Other objects and advantages of the present invention will be
apparent from the following detailed description of certain preferred
embodiments thereof and from the attached drawings of which:
. Figure 1 is a top view of a veneer dryer which has been modified
for drying veneer sheets according to the present invention, the flow
of drying air in its former conventional operation being depicted by
solid arrows, and the flow of superheated steam after modification
according to the present invention being shown by outlined arrows;
Figure 2 is a side view of the embodiment shown in Figure 1
with parts broken away for clarity; and
Figure 3 is a schematic representation of another embodiment
of the present invention used for drying particulate materials.
The present invention may be employed for drying a variety of sheet
or particulate moist or wet materials. The invention is particularly useful
in drying organic materials such as wood veneer, seeds, alfalfa, manure, etc.
This invention is particularly adapted for drying of wood veneer
to be used in making plywood. The use of the present invention in drying
veneer sheets can best be understood by reference to Figures 1 and 2 which
show a veneer dryer 10 in accordance with a preferred embodiment of the
present invention. Green or undried sheets of wood veneer are introduced
into the dryer 10 at an inlet end 11 thereof, moving into a green end drying
5a -
s
:. section 12. Sheets pass longitudinally through the dryer
10 from green end drying section 12 into dry end drying
section 14. The dried veneer sheets are ultimately with-
drawn at the outlet end 15 of dryer 10.
According to the present invention, an entrance
: hood 16 is provided over the area where wood veneer sheets
; enter drying section 12. Likewise, an exit hood 18 is
provided over the area where dried veneer sheets exit from
` drying section 14 into a conventional veneer cooler 20,
attached to the outlet of drying section 14.
Hood 16 has a smoke or exhaust stack 21 extend-
ing upward from its top. At the inlet end of green end
drying section 12, the dryer 10 is provided with two
laterally a~tached, vertically extending, heating gas
.. ducts 22 and 24 on opposite sides of drying section 12.
Vertical ducts 22 and 24 lead, respectively, into two
blower fan housings 26 and 28, which are mounted on the
roof of drying unit 10. A conventionally placed exhaust
stack 30, not utilized in the present invention, is
; 20 shown extending upward from the top of green end drying
section 12. Green end drying section 12 has two verti-
cally extending ducts 32 and 34 positioned on opposite
sides thereof at its outlet end near the center of dryer
10 .
Dry end drying section 14 is likewise equipped
with two laterally positioned vertically extending ducts
36 and 38, on opposite sides of its inlet end near the
center of dryer 10. A conventionally placed stack 40,
not utilized in the present invention, is shown extending
upward from the roof of drying section 14. Dry end drying
section 14 is equipped, on opposite sides of the outlet
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8~S
:
end thereof adjacent hood 18, with two laterally posi-
tioned, vertically extending ducts 42 and 44. Ducts 42
and 44 are respectively connected to two blower fan housings
46 and 48, which are mounted on the roof of drying section
14. Another exhaust stack 50, employed to withdraw heat-
ing steam from the outlet end of veneer dryer 10 according
to the present invention, extends upwardly from hood 18.
;~ Referring to Fig. 2, hood 16 and stack 21 are
supported by dryer entrance frame 52 at the inlet end 11.
A plurality of veneer sheets 54 are introduced through
; the inlet end 11 into the drying zone of dryer 10, in
this case such drying zone is a drying chamber 56, which
occupies the bottom part of green end drying section 12.
Vertical duct 22 is provided with an internal
; manifold apparatus (not shown) for introducing superheated
drying steam in a plurality of paths between veneer sheets
54 at the entrance to drying chamber 56. The drying
steam is forced downwardly into duct 22 by a gas impel-
ling means such as a blower fan 57 mounted within fan
housing 26. Vertical duct 24 is likewise equipped with
manifold apparatus (not shown) for directing superheated
drying steam in plural paths between veneer sheets 54.
; Drying steam is likewise forced downwardly through duct
24 by a blower fan within housing 28, identical to blower
fan 57.
Veneer sheets 54 are conveyed through green end
drying chamber 56 between horizontal rows of vertically
and longitudinally spaced conveyor rollers 58, which are
conventional in design, their operation being well known
in the veneer drying art. Drying sections are normally
of the order of 5~ feet in length or more. Drying chamber
S
56 is further e~uipped with heat exchange pipes 60,
through which a suitable heat exchange fluid, such as
boiler steam at 250 p.s.i. pressure, is passed to heat
the drying steam as it passes through chamber 56 be-
tween veneer sheets 54.
In operation of green end drying section 12
according to the present invention (the desired flow
being indicated by outlined arrows~, a drying gas con-
sisting essentially of superheated drying steam at a dry
bulb temperature above 212F. is blown downwardly through
ducts 22 and 24. A minor portion of the drying steam is
blown toward the inlet end 11 of veneer dryer 10 and
passes into hood 16. Being relatively hot and light,
this portion of the superheated steam rises and passes
through hood 16 into stack 21. The small amount of steam
removed via stack 21 may be treated, in any desired
manner, to remove pollutants before it is discharged
into the atmosphere. A major portion of superheated
drying steam blown into ducts 22 and 24 is conveyed into
drying chamber 56 between the plurality of sheets 54 of
wood veneer therein. The superheated drying steam passes
to the right downstream through chamber 56 toward ducts
32 and 34. The heat of the superheated steam passing
through chamber 56 is maintained at the desired tempera-
ture by heat exchange contact with pipes 60 spaced
throughout chamber 56.
At the outlet end of chamber 56, the used
drying steam is withdrawn from chamber 56 into vertical
ducts 32 and 34. The used steam is passed upwardly
through ducts 32 and 34 into a horizontal top duct 62
Duct 62 conveys the steam through a superheated heat
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10~;~8~ 5
exchange chamber 64~ positioned above drying cham~er 56.
As shown in the cutaway portions of Figs~ 1
and 2, heat exchange chamber 64 is equipped with a
plurality of conventional, finned, closely spaced heat
exchange pipes 66, through which a suitable high temper-
ature heat exchange fluid, such as the same pressurized
steam in pipes 60, is passed in order to heat the used
drying steam from duct 62 to the dPsired high tempera-
ture of, for example, about 350~F. The heated drying
steam is withdrawn from heat exchange chamber 64 through
a horizontal duct 68 and returned to fan housings 26 and
28 as recycled steam. The recycled drying steam is then
fed into vertical ducts 22 and 24 as described above.
According to the present invention, the conventional ex-
haust stack 30 is completely blocked off by baffle 69,
so that none of the freshly heated steam passing into
duct 68 from heat exchange chamber 64 will be exhausted
through stack 30.
Gas impeller means, in this case blower fan at
the inlet end of green end drying section 12, including
blower fan 57 in fan housing 26 and the fan tnot shown)
in housing 28 are used to move the steam throughout dry-
ing section 12. These fans produce a slightly higher-
than-ambient pressure stream of superheated steam in
vertical ducts 22 and 24. This slightly positive pressure
is rapidly dissipated after the steam is passed between
the veneer sheets 54 into drying chamber 56 and extends
over only a minor portion of the length of the drying
chamber 56 from its inlet end. Within chamber 56,
suction from the blower fans creates a slightly less-
than-ambient pressure extending from a point about five
_g _
1~19;~8(~S
feet to the right of the inlet end of chamber 56 through-
out the major portion of chamber 56, ducts 32 and 34,
duct 6~, and, to some degree, through heat exchange
chamber 64. As steam passes through heat exchange chamber
64, because of heating its pressure rises from slightly
subatmospheric back to atmospheric, or slightly higher.
A particular advantage of the blower and exchanger ar-
rangement of the present invention is thereby provided,
in that the blower fans in housings 26 and 28 may operate
more efficiently, since they are used for blowing a
higher pressure gas than they would if the steam were
passed through them before reaching the heat exchanger.
The fan~ force out steam at higher than ambient pressure
to insure that no ambient air enters the inlet of drying
chamber 56. The positive pressure at the inlet end of
chamber 56 hlocks air entry.
Operation of the dry end section 14 of veneer
dryer 10 is similar to that of green end section 12.
Drying sectlon 14 includes a drying zone, in this case
drying chamber 70, which contains conventional conveyor
rolls and heat exchange pipes (not shown) like those in
green end drying chamber 56. The flow of superheated
drying steam through drying chamber 70 (shown by outlined
arrows) is to the left toward vertical ducts 36 and 38.
Used drying steam withdrawn from chamber 70 is passed
upwardly through ducts 36 and 38 into a horizontal duct
72, through which the steam is passed into another heat
exchange chamber 74. Heat exchange chamber 74 contains
conventional finned heat exchange tubes 76, through
which a suitable high temperature heating fluid is passed
to heat the drying steam flowing from duct 72 to a higher
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~(~9Z805
temperature o~, for example, about 350~F. After the
desired amount of heating in chamber 74, the resulting
high temperature superheated drying steam is passed into
a horizontal duct 78 and on to the fan housings 44 and
46. According to the present invention, conventionally
placed stack 40 is completely sealed by emplacement of
baffle 80 therein to prevent the escape of steam through
such stack.
Fan housing 46 is equipped with a blower fan
82. Fan housing 48 is equipped with an identical blower
fan (not shown). Heated drying steam conveyed from
duct 78 into housings 46 and 48 is blown by the blower
fans downwardly through vertical ducts 42 and 44 into
dry end drying chamber 70. The drying steam blown into
ducts 42 and 44 is conveyed into chamber 70 through a
manifold apparatus (not shown) in duct 42 and similar
manifold apparatus in duct 44 into a plurality of paths
between the sheets 54 of veneer, to evenly distribute
the steam throughout chamber 70. A minor portion of
the superheated steam directed downward through ducts
42 and 44 is withdrawn from the downstream end of drying
section 14 and passed into hood 18. This small amount
of steam being hot and light, rises and passes upwardly
through stack 50 for exhausting to the atmosphere after
appropriate pollution control treatment.
Previously, veneer dryers have been constructed
to operate by passing hot air over wood veneer sheets in
the manner depicted by the solid arrows in Figs. 1 and 2.
The blowing and heating arrangements, as well as air flow,
3Q in conventional dryers have been essentially opposite to
that employed in the drying method and apparatus of this
92B(~5
invention. Prior art veneer dxyers have not been equipped
with the hoods 16 and 18 or stacks 21 and 50 shown on
dryer 10. Instead, for example, in operation of a green
end drying section, such as section 12, ambient outside
air has been drawn into the inlet end of the dryer and
upward into ducts, such as ducts 22 and ~4, where the
outside air has been mixed with hot air recovered from
the drying chamber, such as chamber 56. A large portion
of the resulting mixture of used hot air and fresh outside
air has then been exhausted from the dryer through a
conventionally placed stack, such as stack 30 in section
12. This creates severe problems due to the loss of heat
energy and the large amount of pollutants emitted into
the atmosphere. The remainder of the air mixture has
then been passed through a heat exchange chamber, such
as chamber 64, and through ducts, such as duct 62 and
ducts 32 and 34, into a drying chamber such as chamber 56.
By operating in accordance with the present
invention and passing used superheated drying steam re-
covered from chamber 54 directly into heat exchangechamber 64, substantially the whole of drying chamber 54
is maintained at less~than-ambient atmospheric pressure
by the suction of the fans. Leakage of any pollutant-
laden drying gas outward through door seals and cracks
in the walls of chamber 56 into the atmosphere around
veneer dryer 10 is thereby prevented. In contrast, con-
ventional veneer dryer operations have been troubled by
"fugitive emission" leakage of pollutant-laden drying air
into the ambient atmosphere from the drying chamber due
to the positive pressure created within such chambers.
Drying conditions other than those specified
, 5
herein for use in carrying out this invention in veneer
dryers such as dryer 10 are known in the veneer drying
art. For example, the lsngth of time to which veneer
sheets are subjected to drying depends upon the moisture
content of the particular wood before drying and also
upon the moisture content desired for the dried veneer.
The exact temperature of the superheated steam above
212~. and the time period employed can be selected to
obtain a desired moisture content in the dried veneer.
Usually, good results are obtained by maintaining the
drying steam at a dry bulb temperature above about 240F.
and a wet bulb temperature of 212F. Particularly good
results may be achieved using drying steam at a dry bulb
temperature between about 350F. and 400F. and a wet
bulb temperature of 212F.
By using a drying gas consisting essentially
of superheated steam at a wet bulb temperature greater
than 212F., I have been able to substantially increase
the production capacity of a conventional veneer drying
unit, e.g., by 50 percent or more, when it has been
modified in accordance with the present invention. In-
creases in production capacity of as much as 75 percent
have been obtained. Although not essential to an under-
standing of the present invention, I believe that such
striking improvement in capacity and efficiency in veneer
drying obtained using the present process and apparatus
is due, at least in part, to the superior ability of
superheated steam to transmit heat to liquid water con-
tained in the veneer sheet~, as compared with conventional
hot air. Not only do the present process and apparatus
substantially decrease the amount of time necessary to
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l~tg~' 5
dry veneer sheets, but the dried veneer product produced
according to the present invention also has a more
uniform moisture content than is found in dried veneer
produced according to conventional methods~
An important aspect of the present drying
process, as used for drying wood veneer, is the continu-
ous recirculation or recycling of a drying gas consisting
essentially of superheated steam, with only a small
portion of the heated steam being continuously withdrawn
from the drying apparatus and discarded. Preferably, the
minor portion of steam which is thus discarded contains
an amount of water substantially equal to the amount of
water evaporated from the veneer sheets and forming ad-
ditional steam when if mixed with the remaining super-
heated drying steam that was previously formed in a
similar manner within the drying chamber. In this way,
the pressure of the system and the steam flow rate are
automatically regulated at a proper level. By exhausting
this portion, the amount of circulating steam i8 kept
constant, so that the pressure is also constant inside
the system.
An important feature of the invention in veneer
dryers is heating the drying steam when it is removed
from drying zones, such as drying chambers 56 and 70,
before it is reintroduced into such zones by the blower
fan or other gas impelling means. The impeller then
provides a positive pressure at the inlet to the dryer
to prevent entry of ambient air, while suction from the
impeller provides a negative pressure within the drying
zone. The system provides automatic bleeding of sub-
stantially all of the moisture which is removed from
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1~2&~S
the veneer sheet within the drying ~one. This small
amount of exhaust steam or gas can easily and inexpen-
sively be treated for pollution control, e. g., removal
of hydrocarbonaceous vapors before being released into
the atmosphere.
The practice of the drying method of this in-
vention as used for drying wood veneer sheets is not re-
stricted to the longitudinal flow veneer dryer apparatus
depicted in Figs. 1 and 2. Substantially any conventional
veneer drying apparatus may be converted to the practive
of the present invention by the alterations therein shown
in Figs. 1 and 2. For example, the method of the present
invention may be utilized in veneer dryers employing cross
flow jet drying systems or other types of conventional
drying apparatus.
The present method and apparatus are also use-
ful in drying particulates or other solids of relatively
small cross sectional diameter. Fig. 3 shows a preferred
embodiment 200 of such a dryer, which may be employed in
drying organic particulate materials, such as seeds. In
dryer 200, wet seeds are fed through conduit 210 into a
drying zone within drying chamber 212 as a water-seeds
sturry. Even distribution of the seed particles within
drying chamber 212 is obtained using a conventional
distributor 214 at the end of conduit 210 within the top
of chamber 212. Superheated drying qteam at a temperature
of 300 DF. iS introduced into drying chamber 212 from
annular steam distributing conduit 216 surrounding conduit
210. The pressure within drying chamber 212 is maintained
at slightly less than the ambient pressure during the
drying operation in a manner hereafter described.
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109Z8~5
The seeds and drying steam pass downwardly
through drying chamber 212. Contact between the seeds
and drying steam is facilitated by a perforated plate
218, located beneath the distributor 214 in chamber
212, so that the seeds strike such plate and are spread
uniformly across the width of the chamber before they
fall through the chamber. The seeds and used steam
are removed from the bottom of chamber 212 through
conduit 220 in admixture. They are passed through
conduit 220 into a conventional cyclone collector 222,
which separates the dried seeds from the steam. The
dried seeds are removed from the bottom of cyclone 222
through conduit 224. The used drying steam, after
separation from the seeds product, is removed from the
top of cyclone 222 through conduit 226.
During startup of the seed drying system 220,
and before charging any wet seeds into the drying
chamber 212, it is generally necessary to begin circu-
lation of steam within the closed loop of the apparatus
with an auxiliary source of water. For this purpose,
a small amount of water is introduced into conduit 226
through a valved conduit 228. The amount of water thus
introduced is sufficient to provide steam for beginning
operation of the system. Once the required amount of
water is introduced, valve 230 is closed off, and no
further auxiliary water is introduced into the system
so that the drying steam is produced within the drying
chamber by water evaporated from the seeds or other
material being dried.
In normal operation, used steam is passed
from cyclone 222 through conduit 226 into steam super-
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s
heater 232. After the drying steam is heated above
212F. to provide superheated steam, it is withdrawn
from heater 232 and passed through conduit 23~ into a
fan blower 236. Blower 236 forces the heated steam
into conduit 238, from which it is passed into distribu-
ting conduit 216 for injection into the drying chamber
212 as describe~ above.
When the superheated drying steam is pro-
vided in contact with wet seeds in drying chamber 212,
part of the heat in the steam is used to vaporize
water in the seeds. Accordingly, the temperature of
the drying steam is reduced within the drying chamber,
from the high temperature level at which the steam is
introduced, to a lower temperature at which the steam
is removed from chamber 212 in conduit 220. In normal
operation, there is a slightly larger amount of steam
leaving chamber 212 via conduit 220 than is introduced
into chamber 212, because of the addition of vaporizecl
water removed from the dried seeds within chamber 212
to the drying steam. Because of the drop in tempera-
ture of the drying steam within chamber 212, and the
evacuating action of the blower 236, the steam within
chamber 212, conduit 220 and collector 222 is at less
than atmospheric pressure. This negative pressure pre-
vents hydrocarbon vapor ancl other pollutants produced
by the seeds during drying from being emitted to the
ambient atmosphere through c:racks in chamber 212,
conduit 220 or collector 222~ The negative pressure
is achieved even though there is a larger amount o~
gaseous water, by weight, removed from the drying
chamber than enters such chamber. This is because
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1(~9~8-~S
the withdrawn steam has a lower temperature.
After the used drying steam is separated
from the dried seed product in cyclone collector 222,
the used steam is heated to a high temperature of, for
example, 350F. in heater 232. For this reason, the
heated steam in conduit 234 is at a relatively higher
pressure than that in conduit 226. However, the steam
in conduit 238 is at atmospheric pressure because such
conduit is provided with an exhaust stack 2~0 which
exhausts a minor portion of the steam to the ambient
atmosphere. Any excess pressure over atmospheric
pressure which tends to be produced in conduit 238 is
reduced to at least atmospheric pressure or lower by
the passage of the minor portion of the drying steam
in conduit 238 out of the system through stack 240.
The small amount of exhausted steam has substantially
the same amount of water as that removed from the seeds
during drying within drying chamber 212. This small
amount of exhaust steam is easily and inexpensively
treatable to remove any pollutants before it is re-
leased.
By recycling drying steam continuously through
the apparatus 200, the only heat energy required to be
introduced into the apparatus via steam heater 232 is
the small amount necessary to vaporize the water con-
tained in the seeds entering chamber 212 and the amount
of heat needed to raise the temperature of the water
vapor thus produced to the temperature of the drying
steam in conduit 234.
The preferred embodiments having been de-
scrib~d, the broad scope of the invention will be
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1~9Z8(~S
apparent to those skilled in the art. Therefore, the
scope of the present invention should only be determined
by the following claims.
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