Note: Descriptions are shown in the official language in which they were submitted.
CA 02868020 2014-10-17
METHOD AND APPARATUS FOR INHIBITING PITCH FORMATION IN
THE WET SEAL EXHAUST DUCT OF A VENEER DRYER
Field of the Invention
The present invention relates generally to apparatus and methods for
drying material and, in particular, to an apparatus and method for controlling
the type
of dryer used to reduce the moisture content of material such as wood veneers,
plasterboard, etc.
Background of the Invention
Single and multiple deck conveyor dryers for reducing the moisture
content of various materials, including rigid and semi-rigid material in sheet
form, such
as, green veneer, wet plasterboard, fiberboard, perlite and bagasse matte and
the like,
wherein the material being
dried is conveyed through a stationary housing on one or a plurality of tiered
conveyors while heated gases are force circulated through the housing or a
part
thereof, are known. The increase in volume of the gas in the dryer incident to
the
evaporation of moisture from the material being dried is typically removed by
one or
more vents or ducts. In some systems, the exhaust is discharged directly to
the
atmosphere.
It has been found that in a typical dryer of this type, if the drying
process is not carefully controlled and optimized, gases will be discharged
through not
only the exhaust stacks, but through the input and output ends of the dryer.
Attempts
have been made to control the inflow and outflow of gases through the input
and
output ends of a veneer drying apparatus. An example of one such attempt to
improve
the drying efficiency, is disclosed in U.S. Pat. No. 4,439,930, which is owned
by the
assignee of the present application. The assignee also owns United States
Patent No.
5,603,168. The present invention is an improvement over the drying apparatus
which
is the subject of that patent.
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As reported therein, it has been found desirable to control the flow of
exhaust
gases from a jet veneer drying apparatus, to not only optimize the drying
efficiency of the
dryer, but to also provide a means for containing and treating the exhaust gas
prior to
discharging to atmosphere. More specifically, it is now considered desirable
to convey the
exhaust from a jet veneer dryer to a volatile organic carbon (V.O.C.)
alienating device such as
a catalytic or thermal oxidizer prior to atmospheric discharge. In order to
optimize the
performance of this equipment it was disclosed as being desirable to maintain
the temperature
of the exhaust gas at or above a minimum operating temperature. However, pitch
build-up,
condensed V.O.C. material, in the exhaust fan duct of the wet end seal
resulted from the lower
exhaust stream temperature inherent to that design, which may also have been
contributed to
by improper setting of the exhaust control by dryer operators and insufficient
maintenance and
cleaning practices. Obviously, pitch build-up represents a fire hazard. A fire
in the exhaust
fan duct would result in costly repairs and downtime.
Sununary of the Invention
The present invention provides a new and improved apparatus and method for
controlling a dryer. In the illustrated embodiment, the invention is applied
to a jet veneer dryer
used to reduce the moisture content of rigid and semi-rigid sheet material,
such as green
veneer, wet plasterboard, fiberboard, perlite and the like, and to heat the
input seal chamber
exhaust gases above the pitch (V.O.C.) condensation temperatures.
In the art of veneer dryers it is known to provide an elongate drying chamber,
including a means for conveying material to be dried from an input end to an
output end. The
drying chamber includes at least two juxtaposed heating units, each heating
unit providing a
means for circulating air within the unit.
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In the illustrated embodiment, the invention forms part of a jet veneer dryer
which includes nozzles in each drying section for directing air into an
impinging relationship
with the material moving through the drying section. An input seal chamber is
located at the
input end of the drying chamber and includes an air seal system for
restricting the outflow of
gases from the drying chamber into the input seal chamber and further includes
an exhaust
passage by which a gas sample is preferably, continuously extracted from the
input seal
chamber and after temperature measurement is heated above the pitch
condensation
temperatures before being exhausted to the main exhaust system.
A main exhaust system including an exhaust fan, communicates with one of the
dryer sections, preferably the dryer section immediately adjacent the input
seal chamber and is
operative to extract gases from the dryer section with which it communicates.
A first
temperature sensor senses the ambient temperature of feed section air which
can easily enter
the input seal chamber. A second temperature sensor monitors the temperature
of the gas
sample extracted via the sample exhaust passage. A flow controller adjusts the
rate of exhaust
flow of the maul exhaust system as a function of the temperature difference
sensed by the first
and second temperature sensors.
A flow controller controls an inlet damper communicating with the main
exhaust fan. The damper is operative to reduce or increase the rate of exhaust
flow through the
main exhaust system as a function of the sensed temperature difference.
In one embodiment of the invention, each drying section includes a heating
unit
for heating the air being circulated within the drying section. Each drying
section includes its
own circulating fan which draws air from an inlet plenum defined within the
drying section
and blows the air through a heating unit which may comprise a steam heated
coil or a gas-fired
burner. The inlet plenum of a given drying section communicates with the inlet
plenum of the
adjacent drying section and, as a result, a path of exhaust flow is
established across the drying
chamber which allows excess exhaust gases to travel from the remote drying
sections, i.e.,
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those near the output end of the drying chamber, and travel towards the first
drying section
where they are exhausted through the main exhaust system. In the preferred
method and
apparatus, virtually all of the excess exhaust gases are exhausted through the
main system i.e.
at a single point.
The input seal chamber includes restricted passages formed in stop-off
members located at the entry point to the input seal chamber. These restricted
passages allow a
controlled amount of ambient air to enter the input chamber. The sampling fan
draws sufficient
gases from the input seal chamber to reduce the pressure within the input seal
chamber to a
level only slightly below atmospheric. As a result, ambient air enters the
input seal section and
is in effect mixed with exhaust gases which bleed from the drying chamber into
the input seal
chamber. The rate of exhaust bleed into the seal chamber (which is a function
of the pressure
build-up within the drying chamber), affects the temperature of gases drawn
from the wet end
seal section by the sampling fan. An increase in temperature of the sampled
gases indicates
that excess exhaust gas is being produced in the drying chamber. According to
one aspect of
the invention, a controller operatively connected to a sampled gas temperature
sensor and an
ambient temperature sensor adjusts the damper of the main exhaust system to
increase the
exhaust flow. Conversely, as the temperature of the sampled gas decrease, the
controller will
reduce the outflow of exhaust gas through the main exhaust system.
The first drying section, i.e., the drying section immediately adjacent to the
input wet end seal section, may differ from the other drying sections in that
it does not include
its own heating unit for heating the circulating air. Instead, the first
drying section in this
embodiment is used to preheat the material entering the drying chamber. The
exhaust gas
drawn from the adjacent drying sections (by the main exhaust system which
communicates
with the first drying section) is circulated around the material traveling
through the first drying
section. In this embodiment, the first drying section becomes a "preheat
section" and the
exhaust gas releases its sensible heat to the incoming material, prior to
being exhausted
through the main exhaust system.
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A reheat subsystem may be provided in order to maintain the temperature of the
gases exhausted by the first drying or preheat section, above a predetermined
minimum. The
present invention contemplates the treatment of exhaust gases by a catalytic,
thermal oxidizer
or other V.O.C. eliminating devices. To optimize performance of this type of
treatment
apparatus, the temperature of exhaust gas can be maintained above a
predetermined level.
According to this embodiment of the invention, the first drying section
includes a means for
receiving heated gas from a remote drying section. In particular, this
embodiment includes at
least three serially connected drying sections. The first drying section
includes a downblast
blower which is connected via a conduit to the plenum of a remote drying
section which is
preferably the third drying section as counted from the input end of the
drying chamber. A
temperature sensor monitors the flow of exhaust gases into the main exhaust
system from the
first drying section. Should the temperature fall below a predetermined
minimum, gases from
the third drying section which are at a higher temperature than the gases in
the first drying
section, are added to the first drying section to increase the overall
temperature of gases
exhausted from the first drying section by the main exhaust system.
The first drying section may include a split inlet plenum. The inlet plenum is
preferably provided with a diagonal baffle which includes a flow restricting
screen. The baffle
provides a positive communication between the inlet plenum of the second
drying section and
the inlet plenum of the first drying section.
An improved cooling section may be provided at the output end of the drying
apparatus. The cooling section cools into the material exiting the drying
chamber by blowing
ambient air around the material as it travels through the section. A control
is provided for
maintaining the pressure within the cooling section at a level greater than
the pressure in the
drying chamber. By operating the cooling section at a slightly higher
pressure, leakage of
exhaust gases from the drying chamber into the cooling section is inhibited.
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An automatic control may maintain the required pressure differential between
the cooling section and the drying chamber. Pressure sensors are disclosed for
monitoring the
pressure in the drying chamber and the pressure in the cooling section. A
controller connected
to the pressure sensors is operatively coupled to a damper for controlling the
flow of cooling
air thereby controlling the pressure within the cooling section. Alternately,
the speed of a
cooling air blower may be adjusted
According to a first embodiment of the present invention there is provided a
veneer dryer. The veneer dryer comprises an elongate drying chamber having an
input end
and an output end and defining a path of movement between the ends, and a
conveyor for
conveying veneer product to be dried along the path of movement through the
chamber. The
chamber includes a plurality of juxtaposed heating units, each heating unit
defining a
circulation path for heated air being substantially transverse to the path of
movement of the
product to be dried and nozzles forming part of each of the heating units for
directing heated
air into an impinging relationship with the path of movement. The veneer dryer
further
comprising an input seal chamber at the input end of the chamber, including an
air seal system
for restricting an out flow of gases from the drying chamber. The seal system
includes an
exhausting passage for extracting a sample of gases inputted to the seal
section. The veneer
dryer further includes an exhaust system adjacent the seal section including
an exhaust fan for
extracting gases from an adjacent heating zone, a first temperature sensor for
sensing an
ambient temperature external to the input seal chamber and a second
temperature sensor for
sensing a temperature of the sample of gases in the exhausting passage. The
veneer dryer
further comprises a flow controller for adjusting the rate of the exhaust flow
as a function of
the difference in temperature sensed by the first and second temperature
sensors and a heater
cooperating with the seal system mounted down stream of the second temperature
sensor for
raising the temperature of the sample of gases in the exhausting passage to a
temperature
greater than the pitch condensation temperature for the volatile organic
components in sample
of gases.
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According to a further embodiment of the present invention there is disclosed
an apparatus for drying sheet material containing pitch. The apparatus
comprises an elongate
drying chamber including means for conveying sheet material to be dried from
an input end to
an output end, at least two adjacent dryer sections each providing a means for
circulating air
within the section and a main exhaust system including an exhaust fan
communicating with
one of the dryer sections and operative to extract exhaust gases from the
dryer section with
which it communicates. The apparatus further includes an input seal section
located at the
input end of the drying chamber and including an air seal system for
restricting an outflow of
gases from the drying chamber into the input seal chamber and further
including means for
providing a restricted flow of ambient air into the input seal section. The
apparatus further
includes a sampling conduit communicating with the input seal section by which
gas samples
are extracted from the input seal section, a first temperature sensor for
sensing a temperature of
the ambient air entering the input seal section, a second temperature sensor
for sensing a
temperature of the gas samples extracted from the input seal section; and an
exhaust controller
for controlling a rate of exhaust flow through the main exhaust system as a
function of a
difference in the temperatures sensed by the first and second temperature
sensors. The
apparatus further includes a heater cooperating with the input seal section
and the sampling
conduit for heating to an elevated exhaust temperature the gas samples,
wherein the elevated
exhaust temperature is greater than a pitch condensation temperature of the
pitch contained in
the gas sample; wherein the second temperature sensor is located downstream of
the heater
along a direction of flow of the gas samples.
According to a further embodiment of the present invention there is disclosed
a
method for operating a dryer. The method comprises the steps of providing a
drying chamber
having a plurality of individual drying sections, cross-communicating fan
inlet plenums of the
drying sections and provides a single point exhaust system communicating with
a first drying
section. The method further comprises controlling a rate of exhaust flow out
of the first drying
section by monitoring a temperature of ambient air drawn into a wet seal
section and
comparing it with a temperature of gases sampled from the wet seal section,
adjusting the rate
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of exhaust flow in the single point exhaust system in order to maintain a
substantially constant
temperature differential between the ambient air temperature and the
temperature of gases
sampled from the wet seal section and heating the gases sampled from the wet
seal section
above a pitch condensation temperature of pitch contained in the gases
sampled.
Brief Description of the Drawings
FIG. 1 is a sectional view of a prior art jet veneer dryer constructed in
accordance with the preferred embodiment of United States Patent No.
5,603,168.
FIG. 2 is a top plan view of the prior art jet veneer dryer shown in FIG. 1;
FIG. 3 is a fragmentary sectional view of the prior art dryer as seen from the
plane indicated by the line 3--3 in FIG. 2;
FIG. 4 is another sectional view of the prior art dryer as seen from the plane
indicated by the line 4--4 in FIG. 2;
FIG. 5 is a sectional view of the prior art dryer as seen from the plane
indicated
by the line 5--5 in FIG. 2;
FIG. 6 is a sectional view of the prior art dryer as seen from the plane
indicated
by the line 6--6 in FIG. 2;
FIG. 7 is a fragmentary, side elevational view of another prior art jet veneer
dryer constructed in accordance with United States Patent No. 5,603,168;
FIG. 8 is a top plan view of the prior art jet veneer dryer shown in FIG. 7;
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FIG. 9 is a sectional view of the prior art dryer as seen from the plane
indicated
by the line 9--9 in FIG. 8;
FIG. 10 is a sectional view of the prior art dryer as seen from the plane 10-
10
in FIG. 8;
FIGS. lla and 1 lb represent a compound sectional view of the prior art dryer
with portions broken away to show interior detail, as seen from the plane
indicated by the line
ha--ha and the plane indicated by the line lib--lib;
FIG. 12 is a sectional view as seen from the plane indicated by the line 12--
12
in FIG. 8; and
FIG. 13 is a fragmentary, sectional view, shown somewhat schematically, as
seen from the plane indicated by the line 13--13 in FIG. 7.
FIG. 14 is the view corresponding to the elevation view of FIG. 5 in an
alternative embodiment incorporating a wet end seal burner assembly mounted in
exhaust gas
heating cooperation between the wet end seal section and the inlet duct into
the sampling fan
feeding the sampling duct and the main exhaust duct.
FIG. 15 is, in plan view, the alternative embodiment of FIG. 14.
FIG. 16 is, in elevation view corresponding to the elevation view of FIG. 1,
the
alternative embodiment of FIG. 14.
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Detailed Description of Embodiments of the Invention
As disclosed in United States Patent No. 5,603,168, FIGS. 1 and 2 illustrate
the
overall construction of a jet veneer dryer. A "jet veneer dryer" is the type
of dryer which is
used to reduce the moisture content of, or dry, sheet material, such as wood
veneers, pulp
board, plasterboard, fiberboard, perlite board, and the like. The material to
be dried is
introduced at a "wet end" 10 of the apparatus, is conveyed through a drying
chamber 12,
ultimately exiting the apparatus at a "dry end" 14.
The illustrated prior art dryer includes a plurality of juxtaposed, drying
sections
16 which, in the illustrated embodiment, are virtually identical. Each drying
section 16 is
considered conventional and includes a drive motor 20 for driving an axial-
type fan 22 which
circulates air within the drying section in a circular path, transverse to the
path of movement of
material through the drying chamber 12.
As moisture is driven from the material passing through the chamber 12, the
volume of gases within the drying chamber 12 increases requiring that the
excess gas be
exhausted. The exhaust of gases from the apparatus are carefully controlled to
ensure efficient
dryer operation with minimum exhaust and to also contain and direct the
required exhaust
gases so that they may be properly treated before being released to the
atmosphere.
Referring also to FIG. 3, a first drying section 16a includes an exhaust
apparatus indicated generally by the reference character 34. Except for the
exhaust system 34
and associated interconnections, the overall construction of the first drying
section 16a is
substantially similar to the other drying sections 16. It includes an axial
fan 22 belt driven by a
drive motor 20'. The drive motor 20' is located at an offset position as
compared to the drive
motors 20 forming part of the other drying sections 16 to accommodate the
exhaust apparatus
34. The first drying section 16a, like the drying sections 16, circulates air
in a circular path,
transverse to the path of movement of material through the drying chamber 12.
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Referring in particular to FIG. 3, the drying sections 16, 16a each include a
circulating fan 22 for re-circulating air in a circular path, transverse to
the path of movement of
material through the section. The fan forces air through a heat source 36
which may be a gas-
fired burner, steam coil, etc. and forces it into conventional jet veneer
dryer nozzles (not
shown) disposed above and below the sheet material passing through the drying
section via a
nozzle inlet chamber 38a. The nozzles are positioned in an impinging
relationship with the
sheet material, such that the heated air is forced to impinge against upper
and lower surfaces of
the material. The air then flows into a fan inlet plenum or receiving channel
38b which
communicates with an input 39 to the circulating fan 22. The nozzle input
chamber 38a and
other chambers/plenums of a given dryer section communicate with the nozzle
input chambers
and other chamber/plenums of the adjacent dryer sections within any zone. (A
typical dryer is
divided into several zones each containing a plurality of drying sections 16.)
However, all fan
inlet plenums 38b within the dryer communicate with each other. In effect the
joined dryer
sections define an elongate, channel like fan inlet plenum that extends the
full length of the
dryer chamber 12.
Immediately upstream and adjacent to the first drying section 16a is a wet
seal
section 40. As seen best in FIG. 4, the wet seal section includes a plurality
of, vertically-
spaced, entrance pinch roll assemblies 42, 44, 46, 48. A series of spaced
apart supporting
pinch roll assemblies 42a, 44a, 46a, 48a are transversely aligned with
respective entrance
pinch roll assemblies 42, 44, 46, 48 and define a path of movement or "deck"
along which
sheet material to be dried is conveyed and supported. It should be understood
that each dryer
section 16 includes a similar arrangement of pinch rollers, or alternately
conveyors, for
supporting and conveying sheet material through the drying chamber 12. It
should also be
understood that the entrance and supporting pinch rollers 42-48, 42a-48a could
also be
replaced by a single support roll or one or more belt conveyors.
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Disposed between each entrance pinch roller assembly is a flow restricting
stop-off 50. Each stop-off 50 seals the gap between vertically adjacent pinch
roll assemblies
and includes upper and lower flanges 50a, 50b, respectively. In particular,
the upper flange 50a
is positioned in close proximity to a lower pinch roller of a pinch roll
assembly, whereas the
lower flange 50b is positioned in close proximity to an upper pinch roll of a
pinch roll
assembly located below the first pinch roll assembly. The air seal established
between the
stop-offs 50 and the respective pinch rolls allows the pinch rolls that
comprise a given pinch
roll assembly to move relative to the stop-off as material enters the nip of
the rollers. The
lower pinch roll for an assembly may be fixed and the upper pinch roll allowed
to move
upwardly as material enters the pinch roll nip. The uppermost and lowermost
pinch rolls are
sealed by angled stop-offs 52. The stop-offs 50, 52 inhibit the flow of
ambient air into the
input end of the dryer. Each stop-off 50 includes a plurality of flow
restricting ports 51a which
allow some ambient air to enter the wet seal section.
Returning to FIG. 1, the disclosed prior art apparatus includes a conventional
material feed section 56 and a chain tightener for adjusting tension in the
deck drive chains
forming part of the apparatus. In the illustrated construction, four levels or
decks of pinch rolls
are provided so that four sheets of material spaced vertically, can be
concurrently fed through
the drying apparatus. It should be understood that the invention is not
limited to a four deck
dryer and may be used with a dryer having any number of decks.
Disposed between a last drying section 16b and the output end 14, is a cooling
section indicated generally by the reference character 70. Ambient air, drawn
through inlet
stacks 72 is directed into impinging contact with the sheet material traveling
through the
cooling section. After circulating around the sheet material, the cooling air
is exhausted
through exhaust stacks 80.
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A conventional drive unit 84 is disposed at the output end of the drying
apparatus and provides the necessary drive for the rolls and/or conveyors
which are used to
transport the sheet materials through the dryer.
All gases exhausted from the drying apparatus are exhausted through the single
point exhaust apparatus indicated generally by the reference character 34. In
the illustrated
embodiment, all exhausting is done at the wet end of the apparatus where the
temperature of
the gases is generally the lowest. It should be understood that as material
travels from the wet
end 10 to the dry end 14 of the apparatus, less and less moisture is driven
off and, hence, the
temperature of air in the fan inlet plenum in the rightmost dryer section 16b
is higher than the
air circulating in the fan inlet plenum of section 16a, if all other process
parameters are kept
constant.
As indicated above, the fan inlet chambers 38b (shown in FIG. 3) of the dryer
sections 16a, 16 cross communicate. Consequently, as exhaust gas develops in a
given drying
section 16, it can travel leftwardly as viewed in FIG. 1, along the cross-
communicating
chambers and/or channels 38a, 38b (shown in FIG. 3.) As a result, the single
point exhaust
system 34 can serve to exhaust all the excess gas generated in the drying
sections 16.
The quantity of gas exhausted through the single point exhaust system 34 is
carefully controlled so that process parameters remain relatively constant and
the efficiency of
the drying process is maximized. In order to achieve this control, the
temperature of gas in the
wet seal section 40 is monitored and compared with an ambient temperature
measured in the
feed section. The temperature of gases in the seal section 40 is a function of
the gas flow from
the drying chamber 12 into the seal section 40. Exhaust gases in the seal
section 40 are
continuously monitored using a sampling arrangement which includes a sampling
fan 100 for
drawing gases from the seal section 40. The sampled gases are conveyed to a
main exhaust
stack 104 through a sampling duct 106. A temperature sensor 110 located in the
sampling duct
continuously monitors the temperature of gases drawn from the seal section 40.
This
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temperature is continuously compared to an ambient temperature which is
monitored by an
ambient temperature sensor 112 located in the feed section 56.
Referring to FIGS. 3 and 4, some of the exhaust gases drawn from the seal
section 40 by the sampling fan 100 are introduced into the wet seal section
from the drying
section 16a. As seen best in FIG. 4, a series of stop offs 114, similar to the
stop offs 50 but
without flow restricting ports (i.e. ports 51a in the stop-offs 50) are
positioned upstream of
drying section pinch roll assemblies 118, 120, 122, 124. Angled stop offs 126,
similar to the
angled stop offs 52, are also used to seal the upper and lowermost pinch
rolls. As indicated
above, the stop offs 50 include apertures or openings 51a to allow ambient
feed section air to
enter the wet end seal section 40 with only a minimum restriction. This
"controlled leakage"
provided by the apertures 51a in the stop offs 50, assures a sufficient
quantity of ambient air
flow into the wet seal section 40 so that the sampling fan 100 draws only the
leakage exhaust
gas from the drying section 16a. Seal section 40 includes a slight negative
pressure at the dryer
chamber entry stop offs 114 and 126. In lieu of, or in addition to the
apertures 50, the stop offs
50, 52 may be positioned a predetermined distance from the pinch rolls so that
an air leakage
gap is defined between the pinch rolls and the stop offs.
Referring in particular to FIG. 5, gases flowing into the wet seal section 40,
move outwardly into receiving channels 128 and move to an upper channel 129
defined in the
wet seal section 40 and are drawn into a centrally positioned fan inlet duct
100a. Arrows 125
indicate the path of gas flow. It has been found that as excess gases are
generated in the drying
chamber 12, they are forced to bleed past the stop offs 114, 126 into the wet
end seal section
40. This increases the temperature of gases being removed by sampling fan 100.
Conversely,
when the drying rate is lower (i.e. the rate at which moisture is being driven
off the material
being conveyed through the drying chamber) and excess gas is not being
generated or is being
overly exhausted by the main exhaust fan 34, the temperature of gas sampled by
the sampling
fan 100 will decrease. By maintaining a fixed temperature differential between
the temperature
sensed by the ambient sensor 112 and the temperature sensed by the sampling
duct sensor 110,
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a relatively constant positive drying pressure and maximum drying efficiency
can be
maintained. When the temperature differential increases indicating that an
insufficient amount
of gases is being exhausted, the rate of exhaust flow through the single point
exhaust system
34 is increased by the controls. Conversely, when the temperature differential
decreases,
indicating excess exhausting, the rate of exhaust flow through the single
point exhaust system
34 is proportionally reduced by the automatic control.
The rate of exhaust flow through the single point exhaust system 34 is
determined by a power-operated inlet damper assembly 132 which dynamically
controls the
inlet conditions to the exhaust system fan 140 (see FIG. 3). However, a
variable speed exhaust
fan could be used as a substitute for, or in combination with, the power-
operated inlet damper
assembly 132 in order to adjust the rate of exhaust flow from the first drying
section 16a to the
main exhaust stack 104.
Turning to FIG. 3, the details of the exhaust flow path are illustrated. The
inlet
to the circulating fan also communicates with an exhaust receiving channel 136
which in turn
communicates with an inlet duct 138 connected to an inlet to an exhaust fan
140. The power-
operated inlet damper 132 is located between the exhaust chamber 136 and the
exhaust fan
inlet and determines the dynamic conditions of the fan inlet and hence, the
rate of exhaust
flow. In normal operation, the exhaust fan 140 is in continuous operation and
continuously
exhausts some gases to the main exhaust duct 104.
The sampling duct 106 as indicated above also merges with the main duct 104
so that the gases drawn from the seal chamber 40 are also exhausted. The
position of the inlet
damper 132 is controlled, preferably by a differential temperature controller,
which adjusts the
position of the damper as a function of the difference in the wet seal section
exhaust
temperature and the feed section ambient temperature. A closed loop feedback
control may be
used so that the position of the inlet damper 132 is continually modulated in
accordance with
the temperature difference monitored.
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Referring to FIGS. 1 and 6, the cooling section 70 includes a provision for
controlling the rate of cooling air such that a pressure is maintained in the
cooling section that
is greater than the pressure in the drying chamber 12. As a result, the flow
of exhaust gas from
the drying chamber 12 to the cooling section 70 is inhibited. As seen best in
FIG. 6, cooling air
flowing from the inlet duct 72 enters an inlet chamber 150. As is
conventional, the cooling air
flows through jet nozzles and around the four levels of sheet material
traveling through the
cooling section and ultimately enters a receiving chamber 152. From the
receiving chamber
152, the cooling air is exhausted through the outlet stacks 80. A damper
assembly 154 is
positioned between the receiving chamber 152 and outlet stacks 80 and controls
the flow rate
of the cooling air. As seen in FIG. 1, pressure sensors 156, 158 are
positioned in the last drying
section 16b and near the entrance to the cooling section, respectively. A
differential pressure
monitor or controller connected to the pressure sensors monitors for manually
or automatically
controlling the position of the damper assembly 154 so that a positive
pressure at the entrance
to the cooling section, as compared to the drying sections 16b, is maintained.
As long as the
pressure sensed by the sensor 158 is greater than the pressure sensed by the
drying section
sensor 156, exhaust gases from the drying chamber 12 will be inhibited from
flowing into the
cooling section. When an automatic control is employed, the position of the
damper assembly
is controlled by an electrically-operated rotary actuator 154a.
In the prior art it has been reported that a Honeywell model 5000 controller
for
controlling the exhaust inlet damper assembly 132 based on the sensed
temperature differential
between the temperature sensors 110, 112, provides satisfactory results. A
Modus monitor or
controller connected to the pressure sensors 156, 158 can directly or through
manual
adjustment, determine the position of the cooling section damper assembly 154.
This
equipment has also been found to provide satisfactory results. It should be
understood that
other types of control may be used to provide the controlling functions for
the exhaust system
34 and the cooling section 70 and the invention should not be limited to the
above-identified
controls.
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FIGS. 7 and 8 illustrate another prior art embodiment of a jet veneer dryer.
To
facilitate the description, components substantially similar to those
components identified in
connection with the description of the FIG. 1 embodiment, will be given like
reference
characters followed by an apostrophe.
The dryer of FIGS. 7 and 8 is similar in construction and operation to the
prior
art veneer dryer shown in FIG. 1 and includes a drying chamber 12' formed by a
plurality of
juxtaposed drying sections 16'. The dryer is adapted to reduce the moisture
content of sheet
material passing through it and like the first embodiment, defines four
vertically-spaced levels
or "decks" on which four vertically spaced sheets of material can concurrently
travel through
the dryer.
As in the first embodiment, the drying efficiency in the dryer is maximized
and
maintained by a single point exhaust system indicated generally by the
reference character 34'.
The single point exhaust system is in fluid communication with a preheat
section 16a'. The rate
at which gases are exhausted to a main exhaust duct 104' from the drying
section 16a' is
determined by the temperature differential sensed between an ambient sensor
112' and the wet
end seal exhaust sensor 110'. Exhaust gases in the wet seal section 40' are
constantly drawn by
an exhaust fan 100' into a sampling duct 106' in which the sensor 110' is
located. The sampling
duct 106' merges with the main exhaust duct 104' so that the sampled gases are
exhausted with
the exhaust gases drawn f in the preheat section 16a'.
Referring also to FIGS. 1 la and 1lb, additional details of the dryer are
illustrated. The wet seal section 40' like the seal section of the first
embodiment, includes a
series of vertically spaced, transversely aligned pinch roll assemblies 42',
44', 46', 48'. The
pinch roll assemblies define four levels or "decks" along which the material
to be dried is
conveyed and supported. The dryer sections 16 and 16a' also include spaced
pinch roll
assemblies, indicated generally by the reference character 160 which support
the material as it
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CA 02868020 2014-10-17
travels through a given section. Nozzles indicated generally by the reference
character 164 are
positioned above and below the path of material and direct air in an impinging
relationship
with upper and lower surfaces of the material.
The entry of ambient air into the wet seal section 40' is controlled by stop
offs
50', 52' which are similar, if not the same, as the stop offs 50, 52 shown in
FIG. 1. Leakage of
exhaust gases from the preheat section 16a is restricted by stop offs 114'
positioned at the inlet
to the first preheat section 16a. Again, the stop offs 50', 52' are similar,
if not identical, to the
stop offs 50, 52 illustrated in FIG. 4 of the first embodiment. The stop offs
50', 52' may include
apertures or other openings 51a' to allow controlled ambient air leakage from
the feed section
56' into the wet seal section 40' (shown in FIG. 9).
The drying sections 16' are similar in function to the drying sections 16a, 16
of
the first embodiment, but differ in detail. Referring to FIG. 10, each drying
section includes a
centrifugal fan 22' for establishing a flow of air in a circular path,
transverse to the path of
movement of the material through the dryer. The drying section 16a'
illustrated in FIG. 10,
differs slightly from the other drying sections 16' in that it does not
include a heat source for
heating the circulating air and its fan inlet plenum 176 is diagonally split
by a baffle 178
(shown in FIGS. 12 and 13).
All of the other drying sections 16' include a source of heat (not shown) such
as
a gas fired burner, steam heater, etc. located in a heating circulation
chamber indicated by the
reference character 180. After traveling through the heating chamber 180, the
heated air enters
a nozzle inlet chamber 38a', travels through the nozzles 160 (shown in FIG.
11b), around the
material traveling through the dryer section, ultimately entering a receiving
chamber 38b' also
termed the fan inlet plenum. The fan inlet plenum 38b' of each drying section
16'
communicates with an inlet 182 of the fan 22'. As seen in FIG. 10, a constant
circulating flow
of air is established in each drying section. The fan inlet plenums 38b'
communicate with the
corresponding plenums in all adjacent drying sections 16'. As a result,
exhaust gas can flow
18
CA 02868020 2014-10-17
axially along the drying chamber 12' from the dry end 14' towards the wet end
10' where it can
be exhausted through the single point exhaust system 34'.
Exhaust gas is drawn from the preheat section 16a' via an exhaust collection
chamber 184 which, as seen in FIGS. 8 and 9, is formed by an isolated
compartment located
adjacent the wet seal section 40' and which opens into a partial plenum 176a
located in the
preheat section 16a'. The chamber 184 includes a baffle 186 which isolates the
chamber 184
from the wet seal section 40'.
Exhaust gas is drawn from the exhaust collection chamber 184 via an elbow
190 which is connected to an inlet of an exhaust blower 140'. A power-operated
damper
assembly 132' is disposed between the inlet to the exhaust blower 140' and the
inlet elbow 190
and controls the dynamic flow into the fan 140' and thereby controls the flow
rate of exhaust
gas out of the exhaust collection chamber 184. As in the first embodiment, the
temperature
differential as measured by the wet seal exhaust temperature sensor 110' and
an ambient sensor
112' is used to control the quantity of gas exhausted by the single point
exhaust system 34'.
The exhaust gas is used in preheat section 16a' to preheat the incoming sheet
material prior to being exhausted. As indicated above, the dryer section 16a'
does not include a
heat source for heating the circulation air in the heating chamber 180.
Instead, the exhaust gas
drawn from the adjacent first drying section 16 is drawn into the preheat
section 16a' and is
circulated through the nozzles 160 and around the sheet material thereby
releasing the sensible
heat contained in the exhaust gas to the incoming sheet material. Baffling
between the drying
section 16a' and the adjacent drying section 16' controls the flow of exhaust
gas between the
sections.
As indicated above, the baffle 178 (shown in FIGS. 12 and 13) diagonally
splits
what would ordinarily be the fan inlet plenum of the preheat section 16a' into
partial plenums
176a, 176b. The plenum 176b also communicates with the fan inlet plenum 38b'
of the
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CA 02868020 2014-10-17
adjacent drying section 16'. The plenum portion 176b communicates with the
inlet to the
preheat section circulating fan 22'. A horizontal baffle plate 188 (shown in
FIG. 12) isolates
the plenum portion 176a from the fan inlet. As a result, the fan 22' of the
preheat section 16a'
primarily draws exhaust gas from the adjacent drying section 16', rather than
recirculate gases
within the preheat section 16a', as indicated by the arrow 179 in FIGS. 12 and
13. As indicated
above, the plenum portion 176a communicates with the exhaust collection
chamber 184 and,
as a result, the exhaust fan 140' draws exhaust from the plenum chamber
portion 176a
whenever it is operating, as indicated by the arrow 181.
The diagonal baffle 178 also includes a screened or restricted port 178a.
Under
some operating conditions, the exhaust fan 140' will exhaust less gas from the
plenum portion
176a than is being delivered by the circulating fan 22' of the preheat section
16a'. Since the
required exhaust is also less than the main fan circulation, the large open
screen port 178a
exists in the diagonal baffle to allow the bypassing of the additional needed
flow. In particular,
the port 178a allows some of the gas to be recirculated into the fan inlet
from the plenum
portion 176b (as indicated by the arrow 183 in FIG. 13). Under optimum
operating conditions,
exhaust gas delivered to the plenum portion 176b moves through the preheat
section in a single
pass and is then delivered to the exhaust collection chamber 184 from where it
is exhausted by
the exhaust fan 140'.
It should be understood, that the exhaust gas drawn from the drying apparatus
by the single point exhaust system 34' is intended to be conveyed to an
exhaust treatment
apparatus which removes or reduces pollutants in the exhaust stream before
releasing the
exhaust to atmosphere.
For some applications, the exhaust will be treated by a catalytic or thermal
oxidizer. In those applications, the exhaust gas communicated to the oxidizer
must be
maintained above a predetermined temperature. In accordance with this
requirement, the
disclosed apparatus provides a means for maintaining the exhaust temperature
above a
CA 02868020 2014-10-17
predetemiined minimum. This is performed by a reheat sub-system indicated
generally by the
reference character 200 in FIG. 7. The reheat subsystem includes a downblast
blower 202
having an inlet connected to a remote drying section 16". The outlet of the
downblast blower
communicates with the circulation chamber 180 in the preheat section 16a'. The
inlet to the
downblast blower is connected to a section 16b' which is at least one removed
from the
adjacent dryer section. An inlet duct 210, including an electrically actuated
inlet damper 214
interconnects the downblast blower 202 with the preheat drying section 16a'.
It should be understood, that the temperature of circulating air in the drying
section 16' that communicates with the downblast blower inlet conduit 210 is
generally at a
higher temperature than the air circulating in the preheat drying section
16a'. The downblast
blower provides a means for adding heated air to the preheat drying section in
the event that
the exhaust gas being exhausted from the preheat drying section 16a' is below
a predetermined
temperature. The temperature of the exhaust gas leaving the preheat drying
section via the
exhaust collection chamber 184, is monitored and is used to control the
position of the reheat
inlet damper 214 so that the exhaust gas leaving the preheat section 16a' is
maintained above a
predetermined minimum. When the temperature falls below the predetermined
minimum, the
inlet damper 214 is opened allowing heated air to mix with the circulating air
in the preheat
section 16a' thus raising the overall temperature of the air in that section
which, as explained
above, is ultimately exhausted through the single point exhaust system 34'.
Returning to FIG. 7, a purge stack 220 is illustrated. The purge stack 220 is
used in dryers that are gas fired which require purging prior to ignition of
the burners. For
applications that require purging of the drying chamber 12', one or more of
the stacks 220 may
be provided. The stack includes a power-operated cap 222 which is closed by a
powered
actuator 224 at the conclusion of the purging cycle. Once the cap 222 is
closed, all gas is
discharged from the drying chamber through the single point exhaust system
34'. Purging
stacks are normally not required for dryers that employ indirect heat
exchangers such as steam
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CA 02868020 2014-10-17
heated coils or in operations which do not require purging of the drying
section 12 prior to
initiation of dryer operation.
In the embodiment of FIGS. 14-16, to facilitate the description, components
substantially similar to those components identified in connection with the
description of the
FIG. 1 embodiment will be given like reference characters followed by a double
apostrophe.
As seen in FIGS. 14-16, the present invention includes wet end seal burner
assembly 230
cooperating with a prior art veneer dryer such as the two embodiments of FIGS.
1-13
described above. Burner assembly 230 provides a wet end heating system which
may be,
without intending to be limiting gas-fired, hot oil, steam, etc. The heating
system is for
boosting, that is elevating or increasing the temperature of the gases flowing
in direction 125"
substantially entirely from receiving channels 128" into wet end seal section
40, thence to inlet
duct 100a" and to sampling fan 100" so as to pass into sampling duct 106" for
exhaust through
main exhaust duct 104". The heating system elevates the temperature of the gas
flow above
the pitch condensation temperatures so as to minimize pitch build-up in the
sampling fan and
duct.
As illustrated in Figure 14, the burner assembly 230 may be located within a
wet end seal section exhaust plenum 41. The wet end seal section exhaust
plenum 41 is
located above the wet end seal section 40" and has an equal width and depth as
the wet end
seal section 40". The wet end seal section exhaust plenum 41 includes an inlet
opening 41a
and an outlet opening 41b. The inlet opening 41a is continuous with the wet
end seal section
40". The outlet opening 41b is located at an uppermost end of the wet end seal
section exhaust
plenum. As illustrated in Figures 14-16, the outlet opening 41b is
horizontally centered within
the roof of the wet end seal section exhaust plenum 41 although it will be
appreciated that
other locations of the outlet opening 41b will be useful as well, such as by
way of non limiting
example the front wall 41c or the side wall 41d of the wet ends section
exhaust plenum 41. It
will be appreciated that horizontally locating outlet opening 41b will assist
in evenly
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CA 02868020 2014-10-17
distributing the airflow from the entire wet end seal section 40" into the
inlet duct 100a".
However, the outlet opening 41b may also be located in the front wall 41c of
the
As illustrated in figure 16, the burner assembly 230 is located within a front
wall 41c of the wet end seal section exhaust plenum 41. The burner assembly
230 is located
proximate to the outlet opening 41b. The burner assembly 230 increases the
temperature of
the air flow as represented by arrows 125" through the wet end seal section
exhaust plenum 41.
It will be appreciated that locating the burner assembly 230 adjacent or
proximate to the outlet
opening 41b will provide the most consistent temperature increase of air flow
entering the
outlet opening 41b. As illustrated in Figures 14 and 15, the burner assembly
230 is vertically
centered within the wet end seal section exhaust plenum and is located below
the outlet
opening 41b. It will be appreciated that it may be necessary to locate the
burner assembly
away from the outlet opening 41b such that the higher air flow velocities
experienced at the
outlet opening 41b do not interrupt or impede the combustion at a direct fired
burner as
illustrated in Figures 14-16. Although a direct fired natural gas burner is
illustrated, it will be
appreciated that an indirect fired burner may also be used as well as other
types of heaters as
are known in the art and as set out above.
Self-correcting programmable logic controller (PLC) setpoint control loops are
used for total automatic control of the equipment, which prevents improper
adjustment of the
setpoint control by the dryer operator. More specifically, the wet end heating
system will be
automatically controlled and set by the PLC, based on the monitoring of the
operating
parameters of the automatic dryer exhaust control system fan.
The PLC may control the burner assembly by controlling the input of natural
gas into the direct fired burner. Specifically, in response to a demand to
increase the
temperature in the inlet duct 100", the PLC will cause the natural gas to be
supplied to the
burner assembly to be increased. It will be appreciated that the burner
assembly 230 may also
include a combustion air fan for use with an indirect fired natural gas
heater. For such
23
CA 02868020 2014-10-17
arrangements, it will be appreciated that the PLC will cause the combustion
air fan to increase the
combustion air supplied to the burner assembly 230 as well. Conversely, in
response to a demand
to decrease the temperature, the PLC will cause the natural gas and the
combustion air, as
required, to be supplied to the burner assembly to be decreased. It will be
appreciated that for other
types of known heaters, the PLC will cause the heat output of the heater to
increase or decrease in
a similar manner as required. For example, the PLC may cause the current to
electric heaters to be
increased or decreased to cause a corresponding increase or decrease in the
heat output of an
electric heater or cause the flow of a heating fluid such as hot oil or steam
to a heat exchanger to
be increased or decreased to cause a corresponding increase or decrease in the
heat output of the
heat exchanger.
As set out above, the burner assembly 230 increases the temperature of the air
flow
represented by arrows 125". It is therefore desirable to measure the
temperature of the airflow
125" at a location prior to the air flow reaching the burner assembly 230.
This will prevent the
burner assembly, from increasing the temperature of the airflow 125" which is
utilized to control
the gas exhausted by the single point exhaust system 34 as previously
described. Accordingly the
embodiment as presently described includes a sampling temperature sensor 110"
for comparison
with the ambient temperature sensor 112" for controlling the exhaust system
fan 140". Although
the sampling temperature sensor 110" is illustrated as being located within
the receiving channels
128" in the Figures 14-16, it will be appreciated that other locations may
also be suitable such as
for example, within the plenum 41 below the burner assembly 230 or to one side
of the burner
assembly 230 such that heat from the burner assembly does not heat any air
moving past the
sampling temperature sensor.
The scope of the claims should not be limited by the embodiments set forth in
the
examples, but should be given the broadest interpretation consistent with the
description as a
whole.
24
