Four-Zone Preheat Oven for Fiber Cement Trim Board Primer Processing

The customer required a conveyorized preheat module for its facility to serve as the front-end thermal stage of a larger finishing line for fiber cement trim boards. The preheat system was not a stand-alone machine; it had to operate as part of an integrated production cell that also included downstream primer application, a nine-zone cure oven, and ambient / forced-air cooling.

LINE INTEGRATION REQUIREMENT

• The preheat unit had to function as one section of a larger coordinated system.
• Overall machine control had to integrate into the customer's plantwide SCADA environment.
• The customer selected an Allen-Bradley GuardLogix control platform for the overall line.
• Operators can monitor and interact with equipment through the larger system architecture, including mobile tablet-based interfaces, giving the line a facility-integrated operating capability beyond a conventional isolated oven control package.

PRODUCTION REQUIREMENT

• Required production rate: approximately 337 million square feet per year.
• The equipment had to process standard fiber cement trim boards in a continuous-flow production environment.
• Board length to be processed: 12 ft. Board width basis: 144 in.

PROCESS-SEQUENCE REQUIREMENT

• Boards are preheated first, then primer coated, then sent through a downstream nine-zone cure oven, followed by ambient cooling and forced-air cooling.
• The preheat stage had to prepare boards thermally for the downstream coating process.

CONVEYOR / PRODUCT HANDLING REQUIREMENT

• The system had to reliably transport long boards at high speed through the preheat section.
• Required pass height: conveyor carrying surface at 48 in above floor. Clear height above conveyor: 6 in.
• The low pass height created significant packaging and maintenance constraints for the chain return and rail system.

LAYOUT / LENGTH CONSTRAINT

• Available overall conveyorized process length within the oven section: approximately 595.34 in.
• Shipment constraints required the oven to be split into individual zones for transport.

ACCESS / MAINTAINABILITY REQUIREMENT

• The customer required side access so operators could remove boards if product broke or jammed inside the oven.
• The line was expected to run at high speed and high duty cycle, so conveyor wear components had to be maintainable with minimal downtime.

These thermal process requirements were established based on the customer's required production rate, the product geometry, and Precision Quincy's prior testing and application experience with similar board-processing systems.

THROUGHPUT-LINKED THERMAL BASIS

• Required annual production basis: approximately 337 million sq ft/year.
• Conveyor speed required to satisfy the production / length relationship: approximately 72 ft/min.
• Total in-oven dwell time: approximately 29.16 seconds.

ZONE DWELL TIMES

• Zone 1 dwell: 7.03 sec
• Zone 2 dwell: 7.55 sec
• Zone 3 dwell: 7.55 sec
• Zone 4 dwell: 7.03 sec

TEMPERATURE CAPABILITY

• Maximum operating temperature capability: 500°F.
• Temperature uniformity target: ±10°F at the nozzles.

AIRFLOW DELIVERY REQUIREMENTS

• Required airflow pattern: top-and-bottom impingement / vertical-down-and-up airflow in all four zones.
• Top airflow discharges downward; bottom airflow discharges upward; airflow exits the chamber to both sides.
• Supply air delivered through 3/8 in slots. Nominal nozzle discharge velocity: approximately 5,800 FPM. Minimum adjustable velocity: approximately 3,700 FPM.

ZONE AIRFLOW / PRESSURE BASIS

• Recirculation airflow per zone: approximately 37,000 CFM @ 3.6 in. w.c.
• Exhaust airflow per zone: approximately 2,850 CFM @ 1.5 in. w.c.

HEAT INPUT REQUIREMENT

• Heat input per zone: 2.5 MMBtu/hr. Total installed heat input: 10.0 MMBtu/hr across four zones.
• Heat source: natural gas.

To deliver the thermal process requirements that support the customer's production goals, Precision Quincy developed the following equipment concept and architecture for the preheat section.

OVERALL CONCEPT

• Four-zone conveyorized preheat oven designed as the upstream thermal stage of the customer's larger finishing line.
• The system uses a side-mounted heat chamber approach with roof-mounted rotating equipment and service walkway access.

ZONE ARCHITECTURE

• The oven is divided into four independently heated zones. Zone 1 and Zone 4 share one geometry set; Zone 2 and Zone 3 share another.
• Zone heated lengths: Zone 1: 101.25 in; Zone 2: 108.75 in; Zone 3: 108.75 in; Zone 4: 101.25 in.

AIRFLOW / HEATING ARCHITECTURE

• Each zone uses vertical top-and-bottom impingement airflow with discharge exiting toward both sides of the chamber.
• Each zone includes: (1) recirculation fan, (1) natural gas burner, (1) exhaust fan / exhaust connection.
• Recirculation fan per zone: 37,000 CFM @ 3.6 in. w.c.; 40 HP TFC motor; Allen-Bradley PowerFlex 755 VFD; mild steel fan wheel.
• Exhaust fan per zone: 2,850 CFM @ 1.5 in. w.c.; 3 HP motor; mild steel fan wheel.
• Heating per zone: ACCESS Combustion LP30 burner at 2.5 MMBtu/hr; Karl Dungs gas train.

DUCT / CHAMBER CONCEPT

• Supply ductwork is aluminized steel. Supply discharge is through 3/8 in slots sized to deliver the required high-velocity impingement.
• Return path is open to the sides; no enclosed return duct. Chamber style: side monolithic / side-mounted heat chamber.

CONVEYOR ARCHITECTURE

• Seven strands of C2080 high-bar engineering chain running on AR500 wear rails.
• Conveyor speed: approximately 72 ft/min. Drive: 10 HP gearbox.
• Individual pneumatic tensioners per strand with air-proving switch. Automatic chain oiler provided.
• Sprockets use a bearing-supported cartridge-like arrangement for fast changeout under high-duty-cycle operation.

CONSTRUCTION / SHELL ARCHITECTURE

• Wall thickness: 6 in. Tongue-and-groove insulated panels with 6 lb density mineral wool insulation.
• Interior / exterior skins: 20-gauge sheet metal. Structural support: structural steel.
• Exterior finish: Precision Quincy Blue two-part epoxy.

ACCESS / MAINTENANCE ARCHITECTURE

• 4 side access doors opposite the heat chamber. Roof-mounted service areas protected by safety walkway / guard railing.

SAFETY / CONTROLS ARCHITECTURE

• NFPA 86 Class A. Explosion relief in doors and roof.
• Each zone includes heat chamber high limit, work chamber high limit, airflow switches, and Type J control thermocouple.
• Controls built around Allen-Bradley GuardLogix PLC and plantwide SCADA with remote I/O / junction-box architecture. Operators access via mobile tablet-based interfaces.