New Timber Kiln Designs for Promoting Uniform Airflows Within the Wood Stack

Abstract

Two geometrically innovative timber kilns have been designed, constructed, and subsequently tested for airflow uniformity within the stacked wood. The roofs of both kiln configurations were barrel-shaped rather than the traditional peak or flat-top shaped. The first configuration employed outward-swinging overhead baffles, which is the arrangement found within most commercial kilns. The second configuration had inward-swinging overhead baffles and contoured right-angled bends to the plenum chambers from the ceiling space, where the fans were located. Velocity measurements down the height of the timber stack showed that the uppermost packs in the kiln with outward-swinging baffles were starved of airflow. Flow visualizations indicated that the blocking action of a recirculation zone or a boundary-layer separation zone in the inlet plenum chamber was responsible for the reduction in flow through these packs. A similar phenomenon was shown to occur in a traditional kiln with a peak-shaped roof and outward-swinging baffles. The velocity range in the contoured kiln with inward-swinging baffles was reduced by a factor of three, and no recirculation zones appeared in the plenum chambers. Thus, the shape of the roof has very little influence on the uniformity of flow within the timber stack, although significant savings in pressure losses through the kiln are possible by employing a barrel-shaped roof. Airflow uniformity is improved by contouring of the inward edges of the right-angled bends from the ceiling space to the plenum chambers. The contoured configuration is currently operating commercially at a number of sites in New Zealand.