Red oak drying problems

      Troubleshooting a problem of varying moisture contents in loads of kiln-dried red oak. March 20, 2001

Q.
I am having problems drying 4/4 and 5/4 red oak in upgraded computerized kilns by Nardi. I find boards throughout the kiln charge with varying moisture content. One end of the board will read 6%, the middle of the board will read 6%, the other end of the board will read 20%. I also find a board that meets my desired percentage, but the board right next to it reads 10% higher.

These kilns have a 50,000 bd ft capacity. I use six test boards of the highest and greenest percentages placed within the loads. I follow the drying schedules to the "t", maintaining the proper depressions, 1200 RPM's of velocity and proper fan reversals. The last charge started at 52%. It took 34 days to reach my desired percentage before equalization and conditioning. I am also finding more honeycombed lumber than usual.

Forum Responses
Have you successfully dried this particular species and thickness in these kilns with the upgraded controls?

What type of airflow are you getting at 1200 RPM?

A 34 day drying cycle sounds pretty reasonable for that entry MC, so I guess your safe MC drop rate is being monitored by daily weighouts of your samples.

Have your sample boards been properly prepared using oven-tested wafers and weighed out on a proper set of scales?

When you say you maintained the proper depression throughout the cycle, is this information Nardi is giving you in a history, or is this data you've obtained with other measuring devices such as hygrometers, psychrometers, etc?



There are two possibilities--first is that you have poor air flow that is causing differences in drying. This is unlikely to cause 6% to 20% MC in the same piece of lumber, however, so I think we can discard this in your case.

Unless your MC measurements are wrong, that leaves us with one other possibility. (Incidentally, you need to use 12 samples and then choose the five wettest pieces and the one driest piece for daily monitoring. Do not use remote MC determination--it is always wrong--but use the weight system, etc.)

The probable cause is bacterial infection. The bacteria usually move into the bottom of the tree, through the roots. As a result, you will notice their presence only in one end of the lumber--the butt end of the log close to the ground. They increase the MC when green, so it is common to see 90 to 100% MC or higher, compared to the normal 75% MC. If you do not have a sample with infected wood, then you will think that you are dry, but there will still be many pieces that are wet, especially on one end. The bacteria also weaken the wood, so now we see a lot of end checking and honeycomb at the infected end of the lumber--surface checking too--even though drying conditions are "normal."

Does the wood smells? Do your eyes sting a little when you enter the kiln when the wood is green? Have you noticed shake (a separation parallel to the rings and not across)? All these are indications of bacteria.

Have you studied the Drying Oak Lumber book?

Gene Wengert, forum technical advisor



From the original questioner:
I have dried lumber for two years with these kilns. I have had no serious problems other than normal warpage, cupping, splitting and a very low percentage of honeycomb in the 5/4 oak.

I stopped wafer samples last year because of the differences between my probe readings and my wafer samples. Maybe I should reconsider that option.

I have also used hygrometers to check my kilns periodically. The readings are very similar to my computer settings, so I assume there is proper humidity. I am now questioning the airflow.

The center of our stickered packages is where most of the problem lies. Each package is 24 coarses high by 68" wide. We use a 3/4" sticker spaced 14" apart. The kilns are 32' wide by 35' deep, 18' high. The lumber is placed in four rows with no spacing between the packages. The tops and sides are baffled, front and back. How can I tell if I'm getting proper circulation through my center packages?



Airflow definitely seems to be an issue. Have you checked the exiting air with a velometer?

Pushing air across 24' of 50+ percent lumber with no plenums can very well result in honeycomb and MC variation.

Does your Nardi give you feedback as to what your inactive conditions are, i.e., what's happening to your RH and temperature across the load?

Think about the principle in which a track kiln works, with the booster coils between the tracks to overcome the temperature drop. And this is usually through a pair of 8' packages.

Twenty-four feet of air-dried oak with no plenum should dry okay. I think the issue is the heavy MC and airflow.



I doubt that airflow will cause such a dramatic difference in final MC. It would also be hard to get extreme differences in airflow without you noticing it (fan out, blockage of plenum, etc.)

I do suggest 3" to 4" spacing between the packs, edge to edge.

Gene Wengert, forum technical advisor



To determine if you are dealing with bacterial infection, see if the wood has an almost gritty appearance inside. Does it seem brittle? Will a piece break when struck against a solid surface? Is it separating at the growth ring, similar to shake but not showing on the surface? Does the wood seem over-dried, but the moisture content is 6% or above?

In green bacterially infected red oak, you will notice a dark brown appearance and an ammonia smell. Even green lumber will be brittle.



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