A method and apparatus for controlling cooling temperature and pressure in wood veneer jet dryers

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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

CA 02864367 2014-09-18 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

CA 02864367 2014-09-18 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 fell 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

CA 02864367 2014-09-18 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-integralderivative (PID) loop. As would be known to one skilled in the art, the PID 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

CA 02864367 2014-09-18 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 and 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 PID 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.