MC based or time based drying schedules? Calculation of the influence on the final MC variation
2010 (English)In: Proceedings of COST E53, COST , 2010, , 109-114 p.Conference paper (Refereed)
The quality of dried softwood timber depends of course on the drying process performance and in batch kilns especially on the drying schedule. In the Nordic countries the drying schedule is almost always a time based schedule, i.e. the target climate is defined as a function of drying time. The basic idea is that the average properties of each batch (same wood species and board dimension) are almost constant and when a suitable drying schedule has been found, then it can be used repeatedly without changes. In continental Europe the target climate is very often defined as a function of the actual average MC of the batch. The basic idea here is to adapt the schedule according to the real properties of each batch. This is a correct approach, but the problem is to measure the average MC accurately during the process. This is normally done by pairs of electrodes driven into a limited number of boards for electric resistance measurements. Each board has however individual properties and the measured MC differs normally from the average MC of the batch. In addition the MC value based on the electric resistance is not fully reliable. This introduces a stochastic element in the performance of the kiln control system. The aim of this paper is to analyse the accuracy of the MC based schedule control and compare it to the time based schedule approach. The analysis is performed using a simulation program for the drying process. Virtual boards with different properties (initial MC, heartwood content, density and ‘dryability’) are generated using a random number generator. It has been taken into account that the properties mentioned are not statistically independent. In this way a batch of timber is generated and the drying of each board is simulated in order to get the individual final MC and further the MC standard deviation for the whole batch. The MC based drying schedule is applied according to the calculated MC of a few randomly selected boards. The influence on the final MC variation, by the number of boards selected for MC estimation, can thus be determined, as well as the influence of some other features. The results are then compared to the corresponding results for a similar kiln using a time based drying schedule.
Place, publisher, year, edition, pages
COST , 2010. , 109-114 p.
IdentifiersURN: urn:nbn:se:ri:diva-11944Local ID: 11349OAI: oai:DiVA.org:ri-11944DiVA: diva2:969766
Quality Control for Wood and Wood Products - COST E53 The First Conference