Managing Airflow in the Pre-Dryer

      Advice on using temporary baffles to keep air flowing through the remaining stacks as some wood is removed. June 13, 2014

Question
We have a situation where we have a varying inventory of white oak available for pre-drying. We have experienced problems that correlate very well to pre-dryer inventory and we believe that when at partial capacity that the airflow is compromised significantly by allowing air to wash around the ends of bundles rather than through them.

What methods are available to fill the voids normally occupied by lumber to force airflow between the layers? Is there anything commercially available or should we just build big boxes to do the job? Any suggestions or comments will be appreciated.

Forum Responses
(Commercial Kiln Drying Forum)
From contributor K:
Baffling in the plentum space or some type of wall at the end of your stacks of lumber may help to some degree. Is there another species you dry that could be pre-dried along with the white oak? If your pre-dryer is running, you might as well utilize as much of it as you can.



From the original questioner:
We do fill in with pecan when it is available. We have baffling in the middle of a row to help distribute airflow but there is nothing at the ends to help us as material is moved out. Below is a rough idea of configuration.

Aisle
Lumber
Lumber
Baffles
Lumber
Lumber
Backwall



From contributor M:
End baffles would be perfect. Seldom do people use them, as the variation is air flow is not believed to be much of an issue if it is just for a few days. I have seen them used from time to time however.


From contributor W:
Check air flow from the exit side from the floor to the top of the pile. If you have fans blowing down between the piles you probably have no air flow near the top and good air flow near the floor. Go ahead and check and make a chart of the readings. You will probably be able to determine where the problem is when you look at the chart.


From Gene Wengert, forum technical advisor:
I agree with Contributor M about end baffles. It is important to use plenum baffles, which I believe you are doing already. A chart of the air flow under different conditions is a good idea. I have yet to see anyone do anything more than end and plenum baffles, even when the air flow is poor.


From contributor K:
An end wall or baffle will work but if it is going to be a week or two until more WO is available, my opinion would be to fill the section with another species.


From contributor M:
Lumber arrangement and baffling is a real challenge and the cause of many different drying issues and inefficiencies. Stager stacking alternating gaps provides a tortuous path for air flow forcing more of the air through the lumber. We incorporate a flexible top baffle tarpolin that sags very close to the lumber compared to a rigid metal baffle that air can work its way around. I mention this because I believe that side baffles are largely ineffective since the air can make its way around it. I have seen bolster gaps covered, plastic stapled to cover lumber gaps and even pallets filling gaps. In the final analysis anything that can be done to insure uniform air flow though the lumber pack and not around it is worthwhile.


From Gene Wengert, forum technical advisor:
The idea of uniform air flow in a warehouse pre-dryer is somewhat overstated. Although the drying rate is slower where the air is slower, after a few weeks, the slow areas begin to catch up to the faster areas. So, we need only get air flow somewhat uniform.

Do not close the 4x4 or bolster spaces. We do need air flow in these locations, but not the typical 400 or 500 fpm. Close them partially. I have seen a few pre-dryers with top baffles, but they do not seem to be effective most of the time. I have seen pre-dryers with plenum baffles and they work well. Sometimes there are two plenum baffles, one high and one about halfway down. End baffles do work well too; I disagree with Contributor M on this point. Stagger stacking can cause loading and unloading problems in a pre-dryer, so it is not often used. (This is when you have loads that are two packs wide and you stagger the packs slightly so that the gap between the packs does not line-up, but is offset.) I prefer rotating the lumber stacks, top to bottom and bottom to top when about half done. I also like two stage pre-dryers. There are a lot of little hints for pre-dryer operation indeed.



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