Note: Descriptions are shown in the official language in which they were submitted.
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A METHOD AND APPARATUS FOR CONTROLLING COOLING TEMPERATURE AND
PRESSURE IN WOOD VENEER JET DRYERS
Field of the Invention
This invention relates to the field of producing wood veneer and in particular
to a
method and apparatus for controlling the temperature and pressure in the
cooling sections of
wood veneer jet dryers.
Background of the Invention
Applicant is aware of United States Patent No. 5,603,168 which issued to
McMahon, Jr. on February 18, 1997 for a Method and Apparatus for Controlling a
Dryer
wherein it is taught that the cooling section cools into the material exiting
the drying chamber of
the dryer by blowing ambient air around the material as it travels through the
cooling 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. An
automatic control for maintaining the required pressure differential between
the cooling section
and the drying chamber pressure is described. Pressure sensors are disclosed
for monitoring the
pressure in the drying chamber and the pressure in the cooling section. A
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controller was suggested to be connected to the pressure sensors and
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. Applicant is
also aware of United States Patent No. 4,439,930 which issued April 3, 1984 to
McMahon, Jr.
Conventionally, the last structural units (sections), typically one to four,
sections
of veneer jet dryers comprise the cooling zone. They are typically fitted with
vane axial-type
supply air fans and motors delivering outside air to nozzle systems for direct
cooling of the
veneer passing through the heating and cooling sections. It is typically
desirable to utilize the
cooling zone to drop the surface temperature of the veneer to a specified
level. This has
typically been accomplished by turning certain sections of the cooling zone
"on or off' as
necessary to achieve the desired temperature, or to utilize an alternating
current (AC) variable
speed drive on the fan motors to vary the speed of the fans and, thereby, vary
the veneer
temperature. Being that these cooling sections are typically connected
directly, that is, in fluid
communication with the heated sections of the dryer, with only a baffle wall
separating the two,
there has not been the ability to control the flow of cooling zone air into or
out of the dryer.
This has resulted in either "cool" air being pushed into the heated drying
process or heated
process air flowing into the cooling zone specifically when the damper
described in Patent No.
5,603,168 is not present or set too far open.
The present invention contemplates an improved automatic control for
maintaining 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 out of the dryer
thereby controlling
the pressure within the cooling section above dryer pressure. Alternately, the
speed of a cooling
air blower may be adjusted.
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Summary of the Invention
Among its various objects, the present invention provides for automatically
balancing the pressure between an enclosed veneer dryer and its associated
cooling section by
adjusting the pressure in the first cooling section, both up and down, as
needed to inhibit
airflow between the adjacent sections.
Thus, in one aspect of the present invention, the first cooling section, which
is
attached directly to the last heated dryer section, is modified to create a
"pressure seal" for
minimizing both the flow of heated process air from the dryer into the cooling
zone or the flow
of cool air from the cooling zone into the enclosed heated dryer. In one
embodiment the first
cooling section is fitted, in its discharge vent, with a tube-axial extractor
fan and motor
controlled by a frequency drive, conjoined with a modulating, balanced-blade
damper. The
section is mechanically sealed from both the enclosed dryer and second cooling
section by two
sets of baffle-like "stop-offs" that are mounted between the dryer rolls at
the beginning and
end of the section, restricting the movement of air in and out of the first
cooling section. The
stop-offs extend laterally across the veneer flow path and work in conjunction
with the veneer
conveying rolls. They, therefore, only allow restricted leakage or entrance of
air past the
pressure seal section entrance and exit.
Pressure-sensing manifolds are mounted on either side of the stop-offs between
the enclosed dryer and first cooling section and are piped to a pressure
transducer, which
continuously monitors the differential pressure between the heated dryer and
first cooling
section. The signal from the transducer is processed in the dryer programmable
logic
controller (PLC) using a PID loop, described below, with split range control
and a "near zero"
set point, which produces a signal that both modulates the damper through the
first half of the
control range and controls the speed of the tube-axial extractor fan through
the second half of
the control range. The effect of this control is to maintain a slightly higher
pressure in the first
cooling section with a "near zero" pressure differential between the enclosed
dryer and first
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cooling section, that is the "pressure seal" section, under all operating
conditions. The
resulting controlled condition minimizes pitch buildup in the dryer and
cooler, minimizes
volatile organic carbon (VOC) in the cooler vent and improves the drying
process thermal
efficiency.
In an additional embodiment, the cooler section air supply fans are controlled
either by one or individual frequency drives receiving a signal from a
proportional-integral-
derivative (PID) loop in the dryer PLC and having an operator-established
veneer temperature
"set point" and a "process variable" measured by an infrared scanner mounted
at the dry
veneer moisture detector. If reduced cooling is required the air supply fans
slow to satisfy the
temperature set point. This action lowers the pressure in the in the first
cooling section and its
discharge damper closes to again balance the pressure in this the cooler
"seal" and the
extractor fan stops. If increased cooling is required, the air supply fans
increase in speed and
the pressure seal discharge damper modulates to full open at the end of the
first half of the
control range and, as more cooling is required, in the second half of the
control range the
extractor fan begins to increase in speed to satisfy the near-zero pressure
"set point" of the first
cooling section.
The supply and exhaust air for the cooling sections are normally taken from
and
vented to atmosphere, for example above the factory roof, thereby allowing the
cooling zone
of the dryer to have a "net zero" impact on makeup air to the factory.
In summary, the wood veneer dryer according to the present invention may be
characterized in one aspect as including an elongate drying chamber having an
input end and
an output end and defining a path of movement between the ends. A conveyor
conveys
product to be dried along the path of movement through the drying chamber. The
chamber
includes a plurality of juxtaposed heating units sections, each heating unit
defining a
circulation path for heated air, the path being substantially transverse to
the path of movement
of the product to be dried. Nozzles forming part of each of the heating units
direct heated air
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into an impinging relationship with the path of movement. An exhaust system
extracts gases
from an adjacent heating sections. A first pressure sensor senses a pressure
in the output end
of the drying chamber; a cooling section cools the veneer leaving the output
end of the drying
chamber. The cooling section includes pressure controlling means for
maintaining a pressure
in the cooling section that is higher, for example slightly higher than the
pressure in the drying
chamber while maintaining a near-zero pressure differential between the drying
chamber and
the cooling section. A second pressure sensor senses a pressure in the cooling
section
downstream of and adjacent to the output end of the dryer. A flow controller
adjusts the rate
of the exhaust flow as a function of the difference in pressure sensed by the
first and second
pressure sensors.
In one embodiment the flow controller includes a forced air input and a forced
air extractor arranged laterally opposed across the path of movement in the
first cooling
section, and a damper cooperating with the air extractor.
Thus in the present invention, the method for controlling a wood veneer dryer,
may be characterized as including the steps of:
a) providing a drying chamber having at least one drying section and
corresponding upstream input and downstream output ends,
b) providing a cooling section at an output end of the drying chamber;
c) monitoring a first pressure of dryer gases at the output end;
d) comparing the first pressure with a second pressure in the cooling
section;
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e) adjusting a flow rate of cooling air in the cooling section so
that the second
pressure is greater than the first pressure and the pressure differential
between
the first and second pressures is near-zero.
In one embodiment the control is provided by the use of a PID loop using a
split range controller wherein in a first, lower range, that is below the
split, the position of the
cooling section exhaust damper is controlled to control the pressure
differential, and in the
second, upper range, above the split, a forced air mover is also employed in a
graduated
fashion.
Brief Description of the Drawings
With reference to the drawings in which similar characters of reference denote
corresponding parts in each view:
Figure 1 is, in plan view, the wood veneer dryer cooling sections according to
the present invention.
Figure 2 is, in side elevation view, the cooling sections of Figure 1.
Figure 3 is a sectional view along line 3-3 in Figure 2.
Figure 4 is a sectional view along line 4-4 in Figure 1.
Figure 5 is a sectional view along line 5-5 in Figure 2.
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Detailed Description of Embodiments of the Invention
First cooling section 10 is mounted directly to the last, that is most
downstream,
heated dryer section 12. Section 10 is modified to create a pressure seal for
minimizing both
the flow in direction A of heated process air from the dryer air into the
cooling zone
commencing in section 10 or the flow in the opposite direction of cool air
from the cooling
zone into the enclosed heated dryer. In one embodiment first cooling section
10 is fitted, in its
discharge vent 14, with a tube-axial exhaust fan 16 and motor 18 controlled by
a frequency
drive, conjoined with a modulating, balanced-blade damper 20. Section 10 is
mechanically
sealed from both the last dryer section 12 and a downstream second cooling
section 22 by two
sets of stop-offs 24 that are mounted between the dryer rolls 26 in both the
upstream and
downstream ends of section 10, thereby restricting the movement of air into
and out of first
cooling section 10.
Pressure-sensing manifolds (not shown) are mounted on either side of stop-offs
24 between dryer section 12 and first cooling section 10 and are piped to a
pressure transducer
(not shown), which continuously monitors the differential pressure between the
heated dryer
and first cooling section. The signal from the transducer is used for
predictive control and in
particular is processed in a programmable logic controller (PLC) using a
proportional-integral-
derivative (P11)) loop. As would be known to one skilled in the art, the ND
loop automates
what an intelligent operator with a gauge and a control knob would do. The
operator would
read a gauge showing the output measurement of a process, and use the knob to
adjust the
input of the process until the process's output measurement stabilizes at the
desired value on
the gauge. The position of the needle on the gauge is the "process variable"
as used herein.
The desired value on the gauge is referred to as the ''setpoint" herein. The
difference between
the gauge's needle and the setpoint is the "error".
A control loop consists of three parts: measurement by a sensor connected to
the process; decision in a controller element; and, action through an output
device or actuator
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such as the extractor fan and damper herein. As the controller reads the
sensor measurement,
it subtracts this measurement from the setpoint to determine the error. It
then uses the error to
calculate a correction to the process's input variable so that this correction
will remove the
error from the process's output measurement. In a PID loop, correction is
calculated from the
error in three ways: cancel out the current error directly (Proportional), the
amount of time the
error has continued uncorrected (Integral), and anticipate the future error
from the rate of
change of the error over time (Derivative). The sum of the three calculations
constitutes the
output of the PID controller.
In the present invention the PID loop has a split pressure range control and a
near-zero pressure differential set point. The PLC PID loop produces a signal
that both
modulates the actuation of damper 20 and its associated drive motor 28 through
the first half
of the control signal range and controls the speed of the tube-axial extractor
fan 16 through the
second half of the control signal range. The effect of this control is to
maintain a near-zero
pressure differential between the dryer section 12 and first cooling section
10, that is the
pressure seal section, under all operating conditions. The control minimizes
pitch buildup in
the dryer and cooling sections 10, 22 and 30 minimizes volatile organic carbon
(VOC) in the
cooling section vents and improves the drying process thermal efficiency.
In an additional embodiment, the cooling section fans are controlled either by
one or individual frequency drives receiving a signal from a PID loop in the
dryer PLC and
having an operator-established veneer temperature set point arid a process
variable measured
by an infrared scanner (not shown) mounted at the dry veneer moisture detector
(not shown).
If reduced cooling is required the cooling section supply fans slow which
lowers the pressure
in the seal section and damper 20 adjusts toward closed to maintain the
pressure balance in the
seal section 10 and the extractor fan 16 stops. If increased cooling is
required, the cooling
section supply fans increase in speed, damper 20 modulates to full open and,
as more cooling
is required to maintain the veneer temperature setpoint and the extractor fan
16 begins to
increase in speed to meet the cooling section pressure setpoint.
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The first cooling section includes a provision for controlling the rate of
exhausted cooling air such that a pressure is maintained in the cooling
section that is greater
than the pressure in the drying chamber. As a result, the flow of exhaust gas
from the drying
chamber to the cooling section is inhibited. Cooling air flowing from the
inlet duct through
the cooling section supply fan and enters an inlet chamber. As is
conventional, the cooling air
flows through jet nozzles and around the multiple levels of sheet material
traveling through the
cooling section and ultimately enters an exhaust chamber. From the exhaust
chamber, the
cooling air is exhausted through the outlet stacks. A damper assembly is
positioned between
the exhaust chamber and outlet stacks and controls the, flow rate of the
cooling air. Pressure
sensors are positioned in the last drying section and also in the cooling
section near the
entrance to the cooling section. A differential pressure monitor or controller
connected to the
pressure sensors monitors for automatically controlling the position of the
damper assembly so
that a slightly positive pressure at the entrance to the cooling section, as
compared to the
drying sections, is maintained. As long as the pressure sensed by the sensor
is greater than the
pressure sensed by the drying section sensor, exhaust gases from the drying
chamber will be
inhibited from flowing into the cooling section. The position of the damper
assembly is
controlled by an electrically-operated rotary actuator.
The supply and exhaust air for the cooling sections is obtained and vented to
atmosphere, for example above the factory roof, thereby allowing the cooling
zone of the dryer
to have a "net zero" impact on makeup air to the factory.
Cooling section 10 differs from cooling sections 22 and 30 in that cooling
section 10, being the pressure seal section, includes exhaust fan 16 and
damper 20 controlled
by the PD loop. The intake side of cooling sections 10, 22 and 30 each,
however, include
ambient air intakes 32 so as to intake ambient air in direction B from intake
stack 34. A hood
36 may be mounted atop each intake stack 34. Ambient air is drawn down through
intake
ducts 32 by supply fans 38 driven by drive motors 40.
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Ambient air passes through fans 38 downwardly into supply chambers 44 so as to
be
turned in direction C. The ambient cooling air is thereby forced between the
sheets of veneer
passing downstream in direction A on rollers 26 thereby cooling the veneer.
Once the cooling
air has passed between and over the sheets of wood veneer on roller 26, the
now warmed air
is turned in direction D in exhaust chamber 46.
The warmed air then passes through damper 20 and continues upwardly in
direction E
through extractor fan 16 so as to be vented from discharge vent 14 through
outlet stack 48.
In the illustrated embodiment, and in order put the scale of the diagrams into
perspective, a ladder 50 and guard rail 52 are illustrated.
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.
