Methods for size classification of wood chips

摘要

Methods for size classification of wood chips were analysed in an international round robin using 13 conventional wood chip samplesand two specially prepared standard samples, one from wood chips and one from hog fuel. The true size distribution of these two samples(according to length, width and height) had been determined stereometrically (reference method) using a digital calliper gauge and byweighing each of the about 7000 wood particles per sample. Five different horizontal and three rotary screening devices were tested usingfive different screen hole diameters (3.15, 8, 16, 45, 63 mm, round holes). These systems are compared to a commercially availablecontinuously measuring image analysis equipment.The results show that among the devices of a measuring principle—horizontal and rotary screening—the results are quite comparable,while there is a severe incompatibility when distributions are determined by different measuring principles. Highest conformity with thereference values is given for measurements with an image analysis system, whereas for all machines with horizontal screens the medianvalue of the size distribution only reached between one-third to half of the reference median value for the particle length distribution.These deviations can be attributed to a higher particle misplacement, which is particularly found in the larger fractions. Such differencesdecrease when the particle’s shape is more roundish (i.e. sphericity closer to one). The median values of length distributions fromscreenings with a rotary classifier are between the measurements from an image analysis and horizontal screening devices.