NZ505539A - A method for wood drying using x-rays to determine moisture content to enable sorting - Google Patents

Abstract

A method for drying pieces of wood including the steps of; determining an x-ray transmission rate for each of the wood pieces, assigning each of the wood pieces a predicted speed of drying grading based on one of more factors including an x-ray transmission rate or attenuation and drying some pieces assigned a predicted faster speed of drying under different drying conditions from some pieces assigned a predicted slower speed of drying.

Description

New Zealand Paient Spedficaiion for Paient Number 505539 Patents Form No. 5 Number 505539 PATENTS ACT 1953 Dated 3 July 2000 COMPLETE SPECIFICATION A METHOD FOR WOOD DRYING TO ACHIEVE MORE CONSISTENT END MOISTURE CONTENT We, FLETCHER CHALLENGE FORESTS LIMITED, a New Zealand company, of 3 Rockridge Avenue, Penrose, Auckland, New Zealand do hereby declare this invention to be described in the following statement: INTELLECTUAL PROPERTY OFFICE OF N.Z. 0 2 JUL 2001 i RECEIVED C a r i™ f% 505 bo 9 FIELD OF INVENTION The invention comprises a method for drying wood pieces for use in a sawmill or other timber processing plant, to produce end wood products having a more consistent moisture 5 content.

BACKGROUND After sawing from logs in a saw mill wood pieces are typically dried, commonly by kiln 10 drying. The objective is to produce end wood products having a relatively consistent final moisture content. For example for timber intended for structural applications, drying is carried out with the objective of achieving an average moisture content of about 11-12% mc. However we have observed a relatively wide range of final moisture contents if the moisture content of individual pieces in a batch is assessed after drying. Drying is 15 commonly carried out to reduce the number of pieces with a moisture content after drying which is significantly higher than the upper limit moisture content. However this often results in a significant number of other pieces being over dried. In turn this increased drying time, energy and cost, and can increase the subsequent rejection rate of pieces from the batch for distortion (bow, crook, twist), because some of the pieces have been over 20 dried to a moisture content around 4 to 5% for example.

SUMMARY OF INVENTION It is the object of the invention to provide an improved or at least alternative method for 25 drying wood pieces to achieve a more consistent final moisture content in the wood pieces and reduce rejection due to distortion from over drying of some pieces.

In broad terms in one aspect the invention comprises a method for drying wood pieces including determining an x-ray transmission rate, or attenuation, for each of the wood 30 pieces, assigning each of the wood pieces a predicted speed of drying grading based on one or more factors including the measured x-ray transmission rate, or attenuation, and drying 2 INTELLECTUAL PROPERTY OFFICE OF N.Z. 1 0 DEC 2001 RECEIVED a? Preferably each of the wood pieces is sorted after x-ray assessment into a faster or slower 5 drying group along with other similar pieces and each faster drying piece is dried in a faster drying group with a shorter drying time and each slower drying piece is dried in a slower drying group with a longer drying time (for otherwise similar drying conditions including temperature).

Preferably the method includes carrying out an x-ray assessment of each of the wood pieces and assigning each of the wood pieces a speed of drying grading based on whether the approximate x-ray transmission rate, or attenuation, for the wood piece is above or below a predetermined speed of drying grading cut off value.

In a preferred form the method also includes grading each of the pieces as to stiffness or strength or other factor(s) on which the wood piece may be assessed for end use, and sorting each of the wood pieces into a drying group based on both the predicted speed of drying and the end use for the piece. For example wood may be sorted into a fast drying structural end use group, a slow drying structural end use group, a manufacturing end use 20 group, and an industrial end use group.

DETAILED DESCRIPTION OF INVENTION In the method of the invention wood pieces before drying are sorted based on assessed likely speed of drying. An industrial plant based on the method of the invention may comprise an x-ray source and detector between which each of the wood pieces passes on a conveyor for example. The x-ray transmission rate for each of the pieces is assessed on line. Wood pieces having a lower x-ray transmission rate can be predicted to have a slower speed of drying than wood pieces having a higher transmission rate. We believe that the drying rate of wood is dependent not only on its starting moisture content before drying but also on a range of factors including density, grain angle, grain orientation (flat versus quarter sawn), the presence of knots, and the thickness and width of the wood INTELLECTUAL PROPERTY OFFICE OF N.Z. 1 o DEC 2001 RECEIVED pieces. Another main contributor to drying rate variation between wood pieces is whether individual pieces comprise predominantly heartwood, which has a low moisture content and sapwood, which is almost fully saturated with a very high moisture content. We have found that x-ray assessment is a viable means for assessing likely speed of drying.

Preferably each of the wood pieces is assigned a speed of drying grading relative to an x-ray transmission rate cut off value ie those pieces having a measured transmission rate above the cut off value are assigned to a faster drying group while those pieces having a transmission rate below the cut off value are assigned to a slower drying group.

An online x-ray assessment stage may control operation of a down stream sorting "gate" which directs pieces assessed to have a faster drying rate in one direction of product flow and pieces assessed to have a slower drying rate in another. Alternatively an online assessment stage may control for example a small paint or dye marking gun which applies a grading marking to each of the wood pieces for subsequent manual sorting.

The wood pieces assessed to having a faster drying rate are grouped together and may be dried under different drying conditions, typically including but not limited to shorter drying time. It may also be advantageous to vary other conditions of the drying schedule such as drying temperature to optimise drying conditions The wood pieces assessed to 20 have a slower drying time are grouped together and dried under appropriate conditions typically including but are not necessarily limited to a longer drying time. The objective is that the final moisture content for all of the wood pieces whether faster drying or slower drying will be similar and closer to the target moisture content.

After drying the wood pieces may be rebatched together and the entire batch after x-ray assessment, sorting, and drying according to the method of the invention can be expected to have a more consistent final moisture content than if the batch was not pre-sorted and dried but instead dried as an entire batch together with a commonly used kiln drying schedule. Alternatively the faster drying wood pieces may be directed to one end use 30 application and the slower drying pieces to another end use application or end use grading. 4 In a preferred form the method of the invention includes also sorting each of the pieces according to end use For example either before or after x-ray assessment each of the pieces may be assessed for stiffness or strength or some other factor relevant to end use assessment The wood pieces may then be sorted based on the combined speed of drying 5 and end use assessments For example the wood pieces may be batched for drying into one group of pieces assessed as suitable for structural end use applications and assessed as having a faster drying rate, which are then dried under one set of drying conditions, pieces assessed as suitable for structural end use applications but as having a slower drying rate, which are dried under different drying conditions, and group or groups of pieces assessed 10 as suitable for other applications such as manufacturing or industrial applications where variability in moisture content is not as important and which may not be separated into faster and slower drying groups for drying but instead may be dried together with a conventional drying schedule.

EXPERIMENTAL The following description of experimental work further illustrates the method of the invention.

Sample Preparation A random sample of approximately 120pcs of 100x50mm green lumber were obtained as random from a sawmill.

From each piece the following samples were taken: a 150mm discard, 25mm A block, 600mm A sample, 25mm AB block, 600mm B sample, 25mm BC block, 600mm C sample, 25mm C block, 2400mm sample.

The 25mm blocks used to determine: green moisture content, basic density, and heartwood 30 content (visually) The 600mm samples were used for technology assessment and then kiln dried under standard conditions. The 2400mm lengths had stiffness assessed green and dry (Metrigard E Computer), and stress wave analysis. They were also dried to determine an approximate kiln drying time and also assessed for distortion.

X-Ray Transmission Samples were assessed for x-ray transmission using a lOOmCi Am-241 source. This gave a count rate of approx 17kHz. The measurements were taken at the centre of each sample, regardless of whether knots, resin, pith or other characteristics were present Sorting A simple two-sort system was investigated: samples above a critical value were assigned to one sort, and those below to another.

The 'optimum' value for the 'cut-off between sorts was determined by minimising the total variability of the sorts about their respective means. This was achieved with a critical value of 0.495 for x-ray transmission (l/lo).

No Average MC SD Unsorted dry lumber 121 6.97% 4.24% Low Sort (l/lo<0.495) 44 11 16% 3 60% High Sort (l/lo>0.495) 77 4 58% 2.24% The High Sort lumber corresponds to lumber of low green density. That is lumber which 25 has a low moisture content, high heartwood content. The Low Sort lumber was material of high green density and had a high sapwood content.

The 'cut-off value of 0.495 for x-ray transmission (l/lo) approximately corresponds to a green density of 840 kg/m3.

Total variability about the mean, defined as square of the difference from the mean, was over halved (reduced from 2152 to 938), when considered as two sorts, compared to a non 6 sorted sample. Overall standard deviation of each sort was also reduced compared to the overall sample, indicating a reduced variation.

In the analysis which follows, it is assumed that the drying time has been determined by 5 high moisture content pieces, ie drying is stopped at such a time that the spread of high moisture contents is acceptable. At this stage a large number of pieces have been overdried. Where the low moisture content pieces are sorted out and dried separately according to the method of the invention, the aim is to produce a similar final moisture content range as the high moisture content pieces That is overall moisture content levels 10 may increase but without any increase in the high moisture content pieces Reduction in Drying Time To estimate the drying time reduction, the moisture content of each sample for the different 15 moisture contents was determined from the drying trials carried out at 140/90C.

Drying Time 7 75hr 8.5hr 9.5hr Unsorted lumber 7.0% 6 0% 3.8% Low Sort 11 2% 8.7% 4.5% High Sort 4.6% 4.5% 3.4% In order to dry both lumber sorts to an average of 11.2% moisture content, would require a reduction in drying time for the High Sort. A comparison of drying times for Low Sort (reduced from 11.2% to 4.5% in 1.75hr) would indicate that a reduction in drying time of 25 1.75hr would produce a High Sort with average moisture content approx 11%.

Therefore drying times m this example would become: Low Sort 7.75hr, High Sort 6.0hr. Weighted average drying time (based on numbers in each sort) would be 6.6hr, which is a 15% reduction on the combined drying time of 7.75hr. This will result in dry lumber with 30 an average moisture content of 11%. The amount of overdried material will be reduced.

Reduction in Distortion 7 Distortion is approximately linear with reduction in moisture content below fibre saturation point (approx 30%) Hence an increase in the final moisture content of the High Sort from 4.6% to 11%, would reduce the distortion of this material by 25%. On a weighted average, 5 this corresponds to a reduction in distortion of 16%. The actual rejection rate due to distortion will be directly affected as the distortion levels are reduced.

To estimate the reduction in distortion reject rate, the data for distortion of 2.4m lengths was split into High Sort and Low Sort. The allowable distortion levels of NZS3631 10 (National Timber Grading Rules) was used (15mm maximum Bow, 10mm maximum Spring, and 5mm maximum Twist for 2.4m 100x50mm). The distortion of the High Sort material (low final MC) was then corrected as if the moisture content was 7% higher.

Reject due to Distortion 15 High Sort (HS) 43 of 77pcs, 55.8% Low Sort (LS) 11 of 42pcs, 26 2% Combined (HS, LS) 54 of 121pcs, 44.6% High Sort +7% MC (HS+7%) 31 of 77 pes, 40.3% Combined (HS+7%, LS) 42 of 121 pes, 34 7% The high reject rate of the High Sort (fast drying, corewood) is apparent.

Although the reject rate of this material is significantly reduced at a higher final moisture 25 content, it still has a high reject rate compared to the Low Sort lumber. This will be due to the higher spiral grain of the High Sort lumber which is more from the centre of the log.

This indicates that a reduction in distortion rejection rate of 22% could be achieved (ie 100% before sorting 78% after) Overall distortion will also be reduced. 8

Claims (11)

The foregoing describes the invention including preferred forms thereof. Alterations and modifications as will be obvious to those skilled in the art are intended to be incorporated within the scope hereof as defined in the accompanying claims. 9 CLAIMS 20 f..

1. A method for drying wood pieces including the steps of: determining an x-ray transmission rate for each of the wood pieces, 5 assigning each of the wood pieces a predicted speed of drying grading based on one of more factors including the x-ray transmission rate, or attenuation, and drying some pieces assigned a predicted faster speed of drying under different drying conditions from some pieces assigned a predicted slower speed of drying. 10

2. A method for drying wood pieces according to claim 1 further including the step of sorting some of the wood pieces after x-ray assessment into a faster drying group and some of the wood pieces into a slower drying group.

3. A method for drying wood pieces according to claim 2 further including the step 15 of drying the pieces in the faster drying group with a shorter drying time.

4. A method for drying wood pieces according to claim 2 or claim 3 further including the step of drying the pieces in the slower drying group with a longer drying time.

5. A method for drying wood pieces according to claim 4 further including the step of drying both faster drying pieces in a faster drying group and slower drying pieces in a slower drying group at the same temperature. 10 INTELLECTUAL PROPERTY OFFICE OF N.Z. 1 0 DEC 2001 RECEIVED

6. A method for drying wood pieces according to any one of the preceding claims further including the step of assigning each of the wood pieces a speed of drying grading based on whether the approximate x-ray transmission rate, or attenuation, for the wood pieces is above or below a predetermined speed of drying grading cut off value.

7. A method for drying wood pieces according to any one of the preceding claims further including the step of assessing each of the wood pieces for factors relevant to end use.

8. A method for drying wood pieces according to claim 7 wherein a factor on which each wood piece is graded is stiffness.

9. A method for drying wood pieces according to claim 7 wherein a factor on which each wood piece is graded is strength.

10. A method for drying wood pieces according to any one of claims 7 to 9 further including the step of sorting each of the wood pieces into a drying group based on both the predicted speed of drying and the assessed end use for the piece.

11. A method for drying wood pieces substantially as herein described. 11 INTELLECTUAL PROPERTY OFFICE OF N.Z. 1 0 DEC 2001 RECEIVED