Chip specifications vary by region, mill efficiency levels and tree species. They generally fall into four categories:
- Accepts fall within the limits set by the mill for thickness, width and length and do not contain contaminants.
- Fines fall below a mill’s size specifications.
- Pins are too narrow.
- Overs exceed a mill’s size specifications.
In addition, mills will have a limit for the allowable bark content on the chip. Bark content can vary based on the debarking mechanism used:
- A Ring Debarker removes bark from one log at a time; a ring of knives rotates around the length of the log. This method is best used for smooth, straight and large logs.
- A Drum Debarker removes bark from multiple stems by rotating and rolling logs against each other and knocking the bark off of the log. This method is usually the lowest cost method and works well for any size log except for those with excessive crook or sweep.
- A Chain Flail Debarker removes bark from multiple stems by “whipping” a series of chains against the logs. This method is best for smaller diameter logs and works well against logs with sweep or crook. Broken chains can be an issue and be costly to maintain.
- A Rosser Head Debarker removes bark from one log at a time; a mechanism turns the log and a Rosser debarking head is moved against the length of the log.
The amount of bark content and volume loss from the gross roundwood will depend on each mill’s acceptable limits and debarking/chipping ability. Commonalities do exist within each region, however.
US South and Pacific Northwest
These two regions have similar averages. Conifer species on average have a 12 percent overall volume loss, with generally 8 to 9 percent loss from debarking and another 3 to 4 percent from chip screening of fines, pins and overs. In the Pacific Northwest, during the spring, these percentages will decrease as fir trees shed their bark. This does not occur with hemlock and pine species, however. For hardwoods, both regions have an average of 15 percent overall volume loss, with 12 to 13 percent loss from debarking and 2 to 3 percent from chip screening. In the US South, the bark content allowance is between 0.5 to 1.0 percent. In the Pacific Northwest, 0.5 is the maximum allowance at most mills.
In this region, conifer species generally have a 16 percent overall volume loss, with 12 percent on average from debarking and another 3 to 4 percent from chip screening. For hardwoods, overall loss is slightly lower at 14 percent. Debarking contributes to 12 percent of the loss with another 2 percent on average through chip screening. Losses can be higher during extremely cold weather and lower during warmer periods. For bark content, 1.0 percent is the maximum allowance at most mills.
In this region, conifer species will have an overall volume loss of 18 to 22 percent, generally with 13 to 17 percent from debarking and another 3 to 5 percent from chip screening. For hardwoods, overall loss is slightly lower at 15 to 18 percent with debarking contributing to 10 to 13 percent of the volume loss and chip screening contributing another 3 to 5 percent. Aspen, a prevalent species in the region, will have an overall volume loss of 18 to 20 percent. For bark content, 0.5 percent is the average allowance at most mills, but can go as high as 5% depending on the end-product that is being manufactured.