Screw Conveyor for Hog Fuel Boise Cascade Plywood Mill in Moncure, NC

General Description

Boise Cascade Company manufactures a variety of engineered wood products, plywood, lumber and particleboard for the construction industry. The location in Moncure, NC produces plywood. In an effort to decrease waste, wood trimmings are used as fuel for boilers to provide additional electricity for the mill. The wood trimmings are called hog fuel in the wood products industry and consist of every form of wood, from sized chips to long/fibrous strands. The non-uniformity of the hog fuel makes it very difficult to convey effectively using standard screw conveyors with hanger bearings.

One of the inclined screw conveyors at the Moncure mill conveyed hog fuel from several other conveyors to a surge bin. The inclined screw conveyor was a constant source of maintenance and production issues due to having several hanger bearings in the flow path of the hog fuel. The long fibers of the hog fuel matted together and formed a restriction to the flow at the hanger bearing location. The inclined screw conveyor plugged, causing an interruption in production. Sometimes, the hanger support brackets would fail and shut down the system. KWS provided a long-term solution to the problems and eliminated all the downtime and production issues.

Design Parameters

1. Product Type: Hog Fuel (Wood Chips & Fibers)
2. Material Density: 15 to 30 Lbs. per Cubic Foot
3. Capacity: 1,867 Cubic Feet per Hour
4. Duty: 24 Hours per Day, 7 Days per Week

KWS Advantages

Boise Cascade asked engineers from KWS to visit the mill to help solve their problems with the inclined screw conveyor. After reviewing the application, KWS recommended a single piece screw section to span the full length of almost 31-feet. Internal hanger bearings would be eliminated along with any restriction to the flow of hog fuel. KWS also recommended an open-end discharge so the hog fuel would discharge directly into the surge bin. KWS designed and manufactured a 24-inch diameter screw mounted on 10-inch schedule 40 pipe to span the long length with minimal deflection. Since the screw conveyor was located on a 25-degree incline, the flight pitch of the screw was selected for maximum efficiency.

Special Features

One aspect of screw conveyor design that is often overlooked is the bending load on drive and end shafts due to the weight of a 31-foot long screw. The single piece screw section weighed over 2,000-lbs. Both the drive and end shafts must be designed to handle the bending load for the life of the conveyor. Engineers from KWS carefully designed the new screw conveyor and selected 3-15/16-inch diameter shafts. The heavy screw section was supported on the drive end by a pillow block tapered roller bearing to relieve the overhung load on the shaft-mounted reducer. A special torque arm trough end was designed for the application.