In both conventional and dehumidification drying, airflow is essential to transfer the heat needed to warm up the lumber, evaporate water from the wood surface and remove the resulting moisture. Faster airflow means greater energy transfer at the wood surface. It also means faster water removal from the wood surface. How much productivity can be gained from increasing air velocity by 100 ft/min?
The objective of this study was to determine the effect of air velocity on productivity, lumber quality and energy consumption in the drying of spruce-pine-fir (SPF) construction lumber in eastern Canada. The authors initially used the Drytek modeling software to evaluate the effect of air velocity on drying productivity. Modeling studies on balsam fir, jack pine and black spruce demonstrated a positive effect of increased air velocity on drying productivity.
They conducted laboratory tests on 2x4x8-ft lumber from the Lac Saint-Jean, Quebec area. These tests used the same moisture content-driven schedule at four different air velocities, i.e.: 300, 600, 900 and 1200 ft/min.
The study showed that, on the basis of initial moisture content (MC) of 40% and a final MC of 15%, a 100-ft/min increase in air velocity raised productivity by approximately 2%. Gains in drying time were obtained only from the green state down to the fibre saturation point (FSP), which corresponds to 25-30% MC. Higher air velocity did not reduce drying time below FSP. Consequently, the gains obtained from raising air velocity by 100 ft/min are greater when the initial moisture content is higher than 40% than when it is below that level.
Final moisture content variations between and within pieces were comparable at the different air velocity levels, as a result lumber quality was also comparable. A visual assessment of lumber distortion in the piles showed no significant difference.
The specific power consumption of the ventilation system was 0.1, 0.2, 0.6 and 1.0 kWh/kgevaporated water respectively at air velocities of 300, 600, 900 and 1200 ft/min, but this level of specific power consumption is only applicable to the laboratory kiln used in the tests. Preliminary industry data suggest that specific power consumption for black spruce would be 0.06, 0.11, 0.14 and 0.18 kWh/kgevaporated water for the same air velocities in the more efficient industrial kilns. These values will need to be confirmed in the second phase of the study.
Economic calculations on the productivity gains obtained from higher air velocities indicate that annual revenues from a given kiln capacity can be increased. A productivity gain of 2% resulting from a 100-ft/min air velocity increase yields additional revenues of $1/Mbf of dry lumber, assuming a dry/green price differential of $50/Mbf. At a $100/Mbf price differential, the revenue gain becomes $2/Mbf. Additional costs related to high air velocity should however be subtracted from such potential gains. For example, the modification or addition of baffles, or the adjustment of fan blades may lead to higher air velocity at minimal cost to the company; and power consumption will not increase significantly, as the system continues to move the same quantity of air per unit of time. If, on the other hand, a more powerful ventilation system is required, this will involve some capital cost as well as increased power consumption per unit of dry lumber. Mills should take these additional costs into consideration before deciding whether to modify the equipment.
To minimize electrical energy costs when increasing air velocity, producers can also adjust air velocity in relation to the different phases of the drying schedule, given that fan speed can be reduced when the lumber moisture content falls below the fibre saturation point. A previous Forintek study showed that lower fan speed below the FSP level could reduce power consumption with no negative effect on kiln productivity. The current study confirmed that higher air velocity did not result in productivity gains below the FSP level. Mills using high air velocities would therefore generate substantial cost savings by lowering fan speed in the final phases of the cycle. This would require some means to identify when the FSP is reached and the use of variable speed drives for the air circulation system.
As part of this study, we used a software program to model airflow in one of Forintek’s experimental kilns with an actual lumber load. We then compared air velocity on the exit side of the stack according to the model versus actual velocity as measured in the kiln. As the values obtained from the two sources were similar, we believe that our model may prove a very useful tool to simulate the effect of modifying kiln geometry. It will allow producers to assess the effect of modifications such as new or modified baffles or a different roof angle on airflow before they make any decision.
In summary, lumber manufacturers should keep the following points in mind before deciding to modify the airflow system:
Ensure sufficient air space in-between rows with good stickering and stacking practices, as well as proper use of baffles.
Optimize fan blade angle in order to use installed motor power as efficiently as possible.
Ensure that the system can provide sufficient heat energy. With increased airflow, the kiln will require the same amount of heat energy to dry a given load, but over a shorter period of time.
Consider installing a variable-speed drive to reduce airflow below the fibre saturation point, thus reducing energy consumption.
Consider the effect of air velocity on systems based on the temperature drop across the load (TDAL). Adjustments to airflow may result in changes to TDAL measurements and require modifications to the drying schedule.
Further work is needed to finalize the recommendations based on this study. Over the coming year, we will run tests on balsam fir and jack pine to determine potential productivity gains from increased airflow with these species. We will also analyze in more detail how power should best be managed and used under industrial conditions. These follow-up studies will be conducted in collaboration with Hydro Quebec’s Laboratoire des Technologies de l’Énergie (LTE), in Shawinigan, Quebec as part of our joint Électrobois II program.