Industrial Wood Drying Airflow Distribution, Internal Heat Exchange and Moisture Content as Input and Feedback to the Process

University dissertation from Luleå tekniska universitet

Abstract: At a sawmill, the most energy demanding process is the wood drying. The drying process also has a great impact on quality parameters determined for the boards in the final sorting and thereby affects the sales value to a great extent. The objective of the work described in this compilation thesis was to find ways to improve the industrial wood drying process through better input, feedback and process itself. As input and feedback to the drying process, ways to improve the accuracy in estimating the moisture content (MC) was investigated. For the drying process itself, an investigation of the airflow distribution in a batch kiln at several different fan speeds was performed. Potential energy savings by introducing a new kiln layout was also investigated. To estimate the green MC, i.e. the MC of boards prior to drying, the average green MC of batches rather than individual boards were considered since large batches are handled in the drying anyway. Two different methods were investigated, one method in whichthe MC of the heartwood was presumed to be constant and a certain relation between the heartwood and sapwood basic density was present. In the other method, the average basic density was estimated as a function of the logs diameters. It was found that both methods worked satisfactory. To improve the accuracy in measuring the MC of the dried boards two approaches were used. The first approach was to combine different measurement techniques, i.e. microwaves and X-rays, to predict the same properties with a multivariate approach. By adding X-ray measurements to microwave measurements, the amount of boards with a predicted MC deviating less than 1% MC from the oven dry reference increased with 7%. The second approach was to investigate the potential of increasing the measurable board area by compensating for the vicinity of the boards edge with aid of a function developed through final element simulations. It was shown that this was possible although the magnitude of the improvement will depend on the actual measurement setup. The airflow distribution in drying was investigated by industrial trials in which the air speed was measured simultaneously at 20 spots throughout the load with the air circulating fans run at a number of different speeds. It was found that the airflow distribution did not change remarkably as a function of the fan speed and once the airflow is measured at a certain fan speed, the effect of changing the fan speed can be estimated using the fan affinity laws. Finally, potential energy savings with a new kiln layout was investigated through drying simulations and associated determinations of the drying air condition in the kiln. It was found that heat savings of roughly 30% could be accomplished in comparison to a kiln with no heat recovery.

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