The main objectives of this report were 1) review an existing heat transfer model (FP LogconTM) developed to estimate required conditioning times for commercial log species according to diameter and environmental conditions (mainly ambient yard and water spray temperatures) and 2) compare theoretical conditioning behaviour with real data supplied by plywood mills and previous experiments on log conditioning. The main findings were:
Logcon(TM) underestimated actual log heating rates in larger diameter logs (>15”), at least when ambient temperature is above freezing and warmer water (160°C) is used. It predicts an exponential increase in core conditioning time with diameter and a much greater diameter effect in freezing conditions. Review of past measured heating rate data and mill data from water heating facilities point to a more linear relationship with diameter for logs up to 28”, and heavy confounding effects introducing enormous variability into the data.
Contrary to theory, mill data of recorded ribbon average and end (core) temperatures from two different mills and conditioning systems (water spray or wet steam) at different times of the year show no correlation between wood temperatures and log diameters. This suggests significant deviations from model due to factors like location in pile and the vat (front, middle or back).
Sampled ribbon temperature data in Summer showed a full edge-to-core temperature inversion by the time of peeling, with ribbon coolest at leading end (log periphery) and hottest at core (Summer sample). Whole ribbons were within optimal temperature window for Douglas fir.
Ribbon temperature profiles taken in winter with deficient water heating at the time showed highest temperature in the mid-length, colder cores, and almost no ribbons within the optimal temperature window. Short conditioning times, insufficient heat energy delivered by the water recirculation system combined with heat losses from outside conditioning chambers are the main causes of cold wood at peeling time.
Mill data suggest that the shorter period of log heating via wet steam followed by hold equalization may be significantly more effective, particularly in freezing conditions. However this mill recorded ribbon average rather than trailing end (core) temperatures making it difficult to ascertain if there were any frozen cores in the severe Winter samples. Nevertheless the higher and much more consistent ribbon temperature averages across season (summer, mild and severe winter conditions) associated with far more consistent veneer production indices seasonally (reported separately) at the mill suggest the significant advantages of that mill’s wet steam facility.
Further work is recommended to modify and update FPLogconTM by collecting real-time log core heating rate data under warm water and wet steam practices in a wider range of environmental conditions. Measured log cooling data is also required.
Given the suspected ‘door-cooling’ effect further collection of lathe shift records should aim to identify any patterns associated with where in the vat logs came from if possible, to determine whether better insulation of entrances would be warranted.