Woodworking – Process – Timber harvesting or processing
Patent
1992-02-26
1993-09-28
Bray, W. Donald
Woodworking
Process
Timber harvesting or processing
144208R, 144208B, 144356, 364478, B27L 100, B27B 100
Patent
active
052479781
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a procedure and an apparatus for controlling a barking process as defined in the introductory part of claim 1.
Since only a low-quality fibre yield may be obtained from the bark of trees, and because the bark requires the use of plenty of chemicals and causes difficulties with impurities and extractives, logs are debarked before defibration. The need for barking depends primarily on the product aimed at, and also on the equipment and process used for the defibration. The production process for bleached softwood sulphate permits the presence of some bark, whereas the production of groundwood generally requires that the logs be completely debarked. In current practice, pulpwood is mainly barked using barking drums.
The barking drum is a rotating steel cylinder, the logs being fed into the cylinder from one end. The drum is somewhat inclined longitudinally. The rotation of the cylinder sets the logs in motion, causing them to be rubbed against each other so as to detach the bark. At the same time, due to the inclination, the logs move forward in the longitudinal direction and are delivered from the other end of the drum. The bark detached from the logs is removed through elongated bark exit slots in the drum shell.
The aim is to remove bark from the logs so as to achieve the desired barking degree. At the same time, some of the wood itself is rubbed off and crushed. This wood is removed with the bark and constitutes a wood loss. Naturally, the aim is to keep this loss as low as possible. Typically, the wood losses during barking vary between 1.5 . . . 5%, depending on the quality of the wood and the manner of operation of the barking lines.
The degree of debarking of the logs in the barking drum increases as a function of the amount of abrasion they are subjected to (FIG. 1).
The barking capacity of the barking drum for different varieties of wood depends on the dimensions and speed of rotation of the cylinder. A drum of given dimensions will bark a given wood variety to a desired degree of purity (degree of debarking) with a given capacity. In practice, the debarkability of logs varies depending on the wood variety, season of the year, temperature and the solids content of the wood.
To eliminate the effect of temperature, logs may be heated so as to thaw any ice in them either before or after barking or by supplying heat energy into the barking drum in the form of water or steam.
To compensate for the effects of the wood variety, the speed of rotation of the drum is varied. For wood with higher bark adhesion strength, a higher speed is used, and vice versa.
To achieve an efficient and economical barking process, it is important that the degree of fullness of the barking drum is correct. For example, in so-called tumble-barking, which means that the ratio of log length to drum diameter is max. 0.7, the optimal degree of fullness is 50 . . . 60%. In this case the rubbing action between the logs is at its strongest and the desired barking degree is reached in a minimum of time. Also, the wood loss occurring during barking is at its minimum. This is shown by investigations made by Piggott and Thompson (article R. R. Piggott; L. A. Thompson: TAPPI Pulping Conference 1986).
In practice, controlling the feed, discharge and degree of fullness of the barking drum on the basis of the operator's observations and control actions leads to fluctuations causing variations in the degree of barking and high wood losses.
The present invention allows automatization of the control of the barking drum and the equipment used in conjunction with it. The invention allows automatic adjustment of the feed capacity of the drum, supply of thawing energy, speed of rotation of the drum and the position of the delivery gate controlling the discharging of the drum. The features characteristic of the invention are presented in the claims.
In the following, the invention is described in detail by the aid of an example by referring to the drawings in which
FIG. 1 represents the degree of barking of the logs as
REFERENCES:
patent: 3807469 (1974-04-01), Schnyder
patent: 4173239 (1979-11-01), Reiche
patent: 4445558 (1984-05-01), Banner et al.
patent: 5019123 (1991-05-01), Clarke-Pounder et al.
Silenius Seppo
Suominen Ari
Bray W. Donald
Kone Ov
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