The aerodynamics study of timber drying kilns in order to reduce the drying time and to increase the quality of wood products. Bedelean Bogdan, Alexandru Stefan Sova Daniela Transilvania University of Brasov, ROMANIA Faculty of Wood Industry EDG seminar in Oslo, 21 May, 2008
CONTENT • Actual situation; • The factors that influence the aerodynamics of drying kilns; • Solutions for improving the aerodynamics of the drying kilns; • Conclusions; • Research in progress and perspectives.
• Actual situation. In Romania over 3.7 mil. cubic meters (softwoods and hardwoods) are dried annually. From this amount, 60 – 65 % is dried up to 8 -10 %. With all the progress achieved in the field of wood drying, from technical, constructive and automatic operation perspectives, the companies are confronted still with great problems regarding: � the drying times are still very high. This fact involve an increase of energy costs; � the quality of the final dried material is lower; � a lack in the complete training of the wood drying operators.
The authors have observed that the actual drying kilns have a deficient aerodynamics which amplify the actual difficulties. The map of Romania
The deficiency consists in the fact that the air percentage that participates to drying is low (40 – 50 %) and the by-pass is large. With the increase in the by-pass, the drying time is increasing. Bypass, % Drying time, increase % 10 % 4.3 % 20 % 8.5 % 50 % 20.3 % Riley, S., (2005). “ Baffled about baffling! Why is it so important to baffle my drying stack” Ensis Wood Processing Newsletter, Issue N0.36 . The stacking method and the position of the stacks in the drying kiln influence the by-pass value.
• The factors which influence the aerodynamics of drying kilns. The by-pass is caused by the fact that the passive channels oppose a lower aerodynamic resistance comparative to that of active channels.
Details about the location of passive channels Superior passive channels Lateral passive channels Horizontal passive channels Vertical passive channels
In order to reduce the by-pass value, some kilns-construction companies have elaborated top and, more rarely, lateral baffles. Top baffles Lateral baffles
For the vertical and horizontal passive channels there are not developed yet baffles Based on measurements we have observed that the present baffles positioning mode is not sufficient. Within the National Centre for Systems Engineering with Complex Fluids , the three present positioning variants of the top baffles were numerically investigated. The following analysis criteria were established: the maximum amount and the uniformity of the air flux through active channels.
The three baffles positioning variants which were studied: The central location The location only at one end The location at both ends
With help of Fluent program the flow of the air flux in the drying kiln was numerically analyzed.
� The rate and the velocity of the air flux through the active channels were determined with Fluent and Tecplot programs. � It has been observed that the present baffles positioning mode ensures a slow increase in the air percentage, but it causes a non- uniformity of the air flux through active channels. Air percentage 80.0% Air uniformity 70% 70% 68.0% 67.5% 70.0% 60.6% 69% 69% 60.0% 53.1% 68% 50.0% 67% 40.0% 66% 65% 30.0% 65% 65% 20.0% 64% 10.0% 63% 0.0% 62% Without baffles Baffle at one end Baffle at middle Baffles at both ends Without baffles Baffle at one end Baffle at middle Baffles at both ends
• Solutions for improving the aerodynamics of the drying kilns. The minimization of the material stack resistance upon the air flux can be achieved by diminishing the negative effects of the material edges. Based on the fluid mechanics recommendations, the variant of attaching an aerodynamic profile was chosen. The present active channel The proposed active channel
Using the numerical analysis we have compared our solution with the present one. The analysis domain was chosen with help of a periodicity band The Preprocessing stage was performed with GAMBIT, and the Processing and Postprocessing stages with FLUENT.
The numerical results have shown that the proposed solution will reduce with 25 % the resistance caused by the wooden material edges. This decrease is due to the fact that: The recirculation zones that were formed at the entrance and exit of the air flux, in and from the active channel, are eliminated. Without aerodynamic profile With aerodynamic profile
With aerodynamic profile Without aerodynamic profile It will ensure a gradually transition from the large plenum surface to the active channels surface.
The aerodynamic profiles attachment will be performed by means of a spacers assembly made of aluminum. Spacers assembly
The general aspect of a material stack piled up with the proposed spacers assembly zoom
Using this type of spacer assembly, the unparallel positioning of the individual spacers which can create supplementary local resistances, and therefore the decrease of the air rate that goes through the active channels, is avoided. 25 The pressure in active channel II , Pa 20 15 10 5 0 0 5 10 15 20 25 30 35 The value of X, Degrees 3 The air flow in active channel II , kg/s 2.5 2 1.5 1 0.5 0 0 5 10 15 20 25 30 35 The value of X, Degrees
Also, the spacers assembly will ensure: � a perfect geometry of the stacks; � an increase of productivity at the stacking operation, too; � a perfect alignment of stickers at the ends of stack and in the vertical plane. The vertical alignment of the spacers assembly elements
In order to obtain a maximum percentage and a uniform air flux we have established that all superior passive channels from the drying kiln must be obstructed.
The numerical results have shown that this solution ensures an increase of the air percentage and also its uniformity within the material stacks comparative to the existent variants. Without baffles Without baffles Baffles for all top passive channels 76.0% Baffles for all top passive channels 74.4% 74% 74.0% 72% 72% 72.0% 70% 68% 70.0% 68.0% 66% 65% 68.0% 64% 66.0% 62% 64.0% 60% Without baffles Baffles for all top passive Without baffles Baffles for all top passive channels channels Air percentage Uniformity
Since this variant presupposes the increase of the number of top baffles, the authors have created a new type of baffle which will have a large action zone.
The created top baffle is an accordion type and it offers the advantage of having a variable working position and a minimum retire position. D1
Since through the lateral passive channels an important amount of air flux is also wasted we have elaborated for these ones a folding type deflector, too. The D2 detail D2 Top view
The numerical results have shown that an amount of the available air flux is wasted through the horizontal passive channels too and that these represent a non-uniformity generating source. Bolster The passive channels between bolsters
For the obstruction of the passive channels between the bolsters, rubber bands can be used.
The numerical results have shown that through the obstruction or elimination of the passive channels between bolsters both percentage and uniformity of the air flux that participates to drying increase significantly. Without baffles Baffles for all passive channels Without baffles Baffles for all passive channels 120.0% 120% 100% 95.6% 100.0% 100% 80.0% 80% 68.0% 65% 60% 60.0% 40% 40.0% 20% 20.0% 0% 0.0% Without baffles Baffles for all passive channels Without baffles Baffles for all passive channels Air percentage Uniformity
In our country many companies have decided to purchase drying kilns of high capacity being not aware that they cannot load them to the maximum capacity. The numerical results have demonstrated that the uncovering of the drying capacity leads to a non-uniformity of the air flux through the material stacks.
In order to eliminate this inconvenience, a stack-type deflector has been elaborated, which can ensure a uniformity of the air flux through active channels. If all passive channels in the drying kiln are obstructed, then this solution will ensure a maximum and uniform participation of the air flux through the material stacks.
Conclusions - By use of the new spacer type, the aerodynamic resistance of the material stack will decrease with 25 % and a uniform flow of the air flux through the active channels is ensured. - Through the obstruction of all passive channels the air flux will participate in maximum percentage and it will be uniform in the active channels. - By use of the stack-type baffles, the negative effects of the partial loading of the drying kiln will be eliminated. - All these achievements of the authors team will lead to the acquirement of a uniformly dried material within a minimum drying time.
Research in progress and perspectives - The presented solutions are about to be put in practice and to be finally tested. - For the testing of the proposed solutions a monitoring system of air velocity, temperature and relative humidity will be used. - The influence of the proposed solutions upon the drying time and the quality of the dried material will be determined.
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