US 3744147 A
The invention relates to an improved method of preventing bowing, cupping twisting or springing of boards or planks of lumber. During the latter portion of the timber drying schedule i.e. after the lumber has reached its fibre saturation point, the boards or planks are passed between spaced pairs of rollers arranged to hold the lumber in a fixed plane. The lumber is therefore held in this plane while it is setting. Preferably the drying schedule includes intermittent exposure of the lumber to microwave radiation in an electronic kiln dryer.
Description (OCR text may contain errors)
United States Patent 1191 Pless ARTIFICIAL SEASONING OF TIMBER  Inventor: John Henry Pless, 48 Kinnerton St.,
London, S.W.l, England  Filed: Sept. 8, 1969  Appl. No.: 855,880
 Foreign Application Priority Data 9 Sept. 6, l968 Great Britain 42,609/68  US. Cl. 34/1  Int. Cl B01k 5/00  Field of Search 34/1  References Cited UNITED STATES "PATENTS 3,474,544 10/1969 Holden, Jr. et a]. 34/1 11 3,744,147 July 10,; 1973 Primary Examiner-Charles Sukalo Attorney-Francis D. Stephens and Hugo Huettig, Jr;
 I ABSTRACT The invention relates to an improved method of preventing bowing, cupping twisting or springing of boards or planks of lumber. During the latter portion of the timber drying schedule i.e. after the lumber has reached its fibre saturation point, the boards or planks are passed between spaced pairs of rollers arranged to hold the lumber in a fixed plane. The lumber is there fore held in this plane while it is setting. Preferably the drying schedule includes intermittent exposure of the lumber to microwave radiation in an electronic kiln I dryer.
6 Claims, 2 Drawing Figures Pmmimu I 3.144.141
Q I noenlor a IV Allorney ARTIFICIAL SEASONING F TIMBER It is well-known that one of the problems involved in drying lumber (i.e. boards or planks cut from logs of timber) is that differential rates of drying throughout the volume of the lumber can cause cupping, bowing, twisting and springing of the lumber. In conventional kilning processes for drying the lumber, these types of deformation are curbed by placing the boards or planks under heavy weights in the kiln. This means that the lumber at the bottom of a stack is subjected to a higher pressure than lumber at the top of the stack. While this may be satisfactory for the lumber at the bottom, (a normal practice being to put the best pieces of lumber at the bottom of the stack), the process is obviously not entirely satisfactory and it is found that the deformations are not completely removed from lumber at the top of the stack. Moreover, the process is not applicable to more recent high speed kiln drying processes where lumber is continuously moved along a conveyer through high temperature drying installations.
According to the present invention a method of drying boards or planks of lumber in which the lumber is heated to reduce the moisture content to the desired final level, includes: passing the lumber between pairs of rollers arranged to hold the lumber in a fixed plane for at least that part of the drying process after the fibre saturation point has been reached, the unsupported length of lumber between adjacent pairs of rollers being not more than five feet, each pair of the rollers being either fixed or resiliently biased toward one an other and the spacing between the rollers in each pair, or the biasing force urging the rollers together, being such that substantially no compressive force is applied to correctly sawn lumber moving between the rollers, whereby the lumber sets in a straightened condition and cupping, bowing, twisting or springing of the lumber is prevented. I Preferably the rollers are situated within an electronic kiln drying tunnel but they may also be positioned toward the end of a high temperature convection heat drying process where lumber is subjected to alternating treatment with dry air at a temperature above 100 C and with wet steam. The rollers are made of a material which is harder than the lumber passing between them, for example cement, stone, glass, porce- -lain or metal. The spacing between the rollers in each pair is determined by the standard thickness of the lumber being fed through the process, and the number of pairs of rollers will depend on the speed at which the lumber is moving and the distance between each pair of rollers. The width of the rollers depends onthe number of pieces of lumber being fed through side by side and the width of each piece.
If the rollers are fixed, the rollers of each pair are spaced apart just sufficiently for the standard size lumber to pass between them and hence any oversize lumber is compressed between the rollers. If the rollers are resiliently biased toward one another, the pressure between the rollers is kept low enough not to compress the standard size lumber. Excessive pressure must be avoided at low moisture content levels since the lumber has lost some of its elasticity and is setting, and excessive pressure will therefore cause splitting.
The rollers are preferably driven and thereby act to move the lumber during the drying process. The movement may include a forward and backward motion in the electronic kiln or between steam and dry air chambers in a convection process. The distance between the rollers in each pair must then be such that sufficient frictional contact'between the rollers and the lumber occurs to prevent slippage.
One example of the invention will now be described with reference to the accompanying drawings in which FIG. 1 is a side elevational view of a electronic kiln drying process embodying the invention and FIG. 2 is a perspective view of a detail of the electronic kiln shown in FIG. 1.
Referring to these Figures, lumber is fed into a kiln drying tunnel 1 on a conveyer 2. The initial portion of the tunnel comprises a pre-heating chamber 9 in which lumber is dried approximately to its fibre saturation point (around 25 percent moisture content). This chamber is filled with relatively low power microwave radiation to permit dielectric heating of the lumber over a relatively long period. Thereafter the tunnel is split into much shorter sections comprising alternate high power microwave drying chambers 3 and control sections 4. Each microwave chamber has its own waveguide 5 and microwave generator 6. Microwave energy is launched through the roof of each chamber to produce a rapid burst of dielectric heating as the lumber moves through the chamber. The power consumption of electrical energy per cu. ft. varies according to the density and thickness of the lumber and lies between 2 and 15 kilowatts per cubic foot. As much energy as possible is fed into each chamber without damaging the timber. The maximum permitted energy concentration is previously calibrated using lumber having known density, thickness and moisture contents. Above the permitted maximum and steam pressure within the lumber becomes too great and the wood ruptures.
For a 1 inch soft wood having a moisture content of 20 percent the maximum power concentration is 12 kilowatts per cu. ft. for 1 minute. On the other hand, for 3 inch hardwood with a moisture content of percent the maximum power concentration is only 2 to 4 kilowatts per cu. ft. This is because softwood is more permeable to the escaping steam and there is therefore less build-up of pressure. However, sustained exposure to the permitted maximum power concentrations would again cause rupture of the wood, and it is therefore essential that control sections 4 lie between the microwave chambers, the control sections being preferably three times as long as the microwave chambers.
Within these control sections are the control rollers 7. If, for example, 1 inch boards of Perana pipe are being fed through the kiln, the maximum distance between adjacent pairs of rollers should not exceed three feet. On the other hand, for 2 inch boards of Hemlock, the adjacent roller pairs may be up to five feet apart. The purpose of the rollers is to hold the boards or planks in a fixed plane during the drying process. Before entering the kiln, the boards have already been dried to their fibre saturation point and the control rollers are therefore only acting on the boards or planks in the latter stages of the drying schedule. It has been found that these rollers almost completely eliminate any cupping, bowing, twisting or springing of the lumber after it is fed out of the kiln.
Typically, a three foot long reinforced steel tubing roller may have a diameter of 2% inches. Such a roller would be able to withstand pressures of 1,000 tons per square inch and hence any oversize lumber will be squashed between the rollers during its passage through the kiln. Normally, however, the pressure on the lumher is fairly low, the rollers merely acting to hold the lumber straight while it sets.
The control chambers may also include watersprays for spraying the surface of the lumber to control the rate of evaporation. The surface is thus kept moist until the centre of the lumber dries out.
A steam pipe 8 runs the complete length of the tunnel and maintains a high ambient temperature. Since the drying is effected by dielectric heating which drives steam out from the centre of the lumber, the high ambient temperature helps to minimise the amount of power consumed in reducing the moisture content to a given level.
1. A method of drying wooden boards in which the wood is heated to reduce the moisture content to the desired final level, including: passing the boards between fixed pairs of rollers arranged to hold the boards in a fixed plane for at least that part of the drying process after the fibre saturation point of the wood has been reached, the unsupported length of boards between adjacent pairs of rollers being not more than five feet, and the spacing between the rollers in each pair being such that substantially no compressive force is applied to the correctly sawn boards moving between the rollers, whereby the boards set in a straightened condition and cupping, bowing twisting or springing of the boards is substantially prevented.
2. The method of claim 11 in which the boards are altemately passed through heating section between adja cent pairs of rollers to expose said boards to heat and through control sections in which said rollers hold said boards and said boards are exposed to controlled drying conditions.
3. The method of claim 2 in which said boards are exposed for a longer time in said control sections than in said heating sections.
4. The method of claim 3 including the step of spraying the boards with water as they pass through said control sections.
5. The method of claim 4 including the step of maintaining temperatures of C and a relative humidity of at least 35 percent in said control sections.
6. A method of drying boards in which the boards are heated to reduce the moisture content to the desired final level, including: the step of passing the boards between pairs of rollers arranged to hold the boards in a fixed plane during that part of the drying process after the fibre saturation point of the boards has been reached, the unsupported length of boards between adjacent pairs of rollers being not more than 5 feet, each pair of the rollers being resiliently biased toward one another, the biasing force urging the rollers together being such that substantially no compressive force is applied to the correctly sawn boards moving between the rollers, whereby the boards set in a straightened condition and cupping, bowing, twisting or springing of the lumber is substantially prevented.