Troubleshooting Mold on Oak in the Drying Kiln

      A question about a mold problem leads to a discussion about process control and kiln conditions. June 13, 2014

When drying 8/4 red and white oak in the kiln from green we need to run part time in the beginning to keep the losses from getting too high. The problem is that mold develops when the kiln is off, especially during the winter. The settings are 97 WB and 100 DB. The velocity is 200 fpm. Should we run the fans and/or open the vents when the kiln is off?

Forum Responses
(Commercial Kiln Drying Forum)
From Contributor K

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I might be missing the picture here but if you are going to run the fans and vents isn't the kiln running, except for some heat? I've always thought of kiln drying as having three integral parts: airflow, venting, and a heat source. Personally, I'm not a fan of running any of those individually if I can't run/control all three. But, maybe that's just me. I know some other operators who do. Also, will you have issues, especially in the winter, running fans and opening vents with no heat on? I'm not sure where you're located, but here in northwestern PA I would freeze every pipe in my kilns and make fast enemies of my maintenance crew if I even thought of opening vents and running fans without any heat.

From Gene Wengert, forum technical advisor:
When drying thick oak, oftentimes the kiln cannot maintain the correct or desired conditions. At 200 fpm, you do not have enough air flow across the WB to get accurate readings. It is drier than you think and that means damage. We seldom KD thick oak green from the saw but instead use a shed to give us good air drying. In your posting, you give us the settings, but we really need the actual conditions plus the controls need to be accurate. It is like giving the judge a picture of the speed limit sign when one is in court for speeding.

From contributor O:
It's time to put down the Forest Products schedule book and figure out for yourself what you are trying to accomplish. You are trying to move free water from dead-green oak without checking or molding it. Now, with that said, the first question is why do I have to be at 100 degrees? You don't. Depending on the time of year, or where you live, Why not start out at 80, 75, or even 70 degrees DB?

Second question is: why do you have to have a tight three degree depression? With a much lower DB setting, less moisture is mobilized from the wood, and your rates will be more in line with what you desire, so you can increase your WB depression to four or five or even more to keep the mold at bay.

Gene is correct about airflow. You need an accurate read on your wet bulb. And not what the controller is telling you. Always cross-reference your controller with a god hygrometer reading. You've made the environment in the kiln less hostile, so an increase in airflow will become more tolerable, and your settings more reliable. Play the game long enough and you'll find that the schedule book and controller readings can become your two worst enemies.

From Gene Wengert, forum technical advisor:
Keep in mind that billions of board feet of oak are dried every year without issues using the US Forest Products Lab's schedules. Further, these schedules that have been published for over 60 years were reviewed time and time again; most recently in Drying Hardwood Lumber. Of course, they assume that your instrument is giving you the correct readings.

At 200 fpm 24 hours a day and a 3 F depression at 100 F, you should not have checking issues with normal red or white oak. Therefore, we know that your instrument is giving you incorrect values. Shutting off the fans, as you do, certainly slows drying, but while the fans are off, you develop a mold generating factory. Also, monitoring the rate of drying is the ultimate way to control drying, as you are already doing.

Hence, the correct procedure is to get the instrument working properly with adequate air flow (the DB and WB conditions are perfect with the correct air flow). As you may have already discovered, most kilns, except for DH and hot water, do not work well under 100 F, so you cannot use lower temperatures. Also, we know that 90 F is usually considered to be the best for mold generation, so cooler may not be better. If this thick oak is to be common for you, then consider 90 days of warm weather in a shed with good air flow (open walls or partly open walls). This will give you excellent quality and reduce costs as your kiln residence time will be half of the green time.

From the original questioner:
What should be the correct air velocity?

From contributor O:
Keep in mind that billions of board feet of lumber have been dried in conventional down-draft pre-driers that do not reach 100 or even 90 degrees temp, and do not achieve 200 fpm airflow evenly through the loading. So, the contention that the established schedules must be complied with in order to dry lumber successfully would mean that the pre-drying process should be abandoned. While I can appreciate your willing defense of the books, I view the schedules more as a speed limit than as an absolute mark to be hit.

I'm unsure why compensating the three variables seems difficult to accept. If temps are reduced, a depression increase combined with airflow adjustments maintain nearly the same EMC values and keep rates in check in the early stages. As for airflow, close monitoring of the exiting air temps is critical. Again, good hygrometer readings are essential. The closer the depression is to saturation, or zero depression, the higher the potential is for mold and stain. Most automated controls are set for entering air values, the adjustment you make in airflow impacts exiting depression values. Drying green oak is living on the edge of the fine line of a sword. Too much of one thing gives us checking, and the other gives us stain and mold.

From Gene Wengert, forum technical advisor:
TheUS FPL has kiln schedules and not pre-dryer schedules. This discussion was about kiln schedules, not pre dryers. I do not know of any book that suggests monitoring exit air temperatures for hardwoods. As the exit will be milder, we are more concerned about entering air temperatures. This is true for pre-dryers and kilns.

From contributor J:
Would adding a fan to move air over the wet bulb solve the incorrect WB reading with the 200 fpm air flow in the kiln?

From Gene Wengert, forum technical advisor:
Yes indeed. You need only a small fan like is used for stereo equipment. The kiln atmosphere may destroy it rapidly however, so make sure you check it now and then. Clean wicks and pure water are also keys.

From contributor O:
Heat is heat, airflow is airflow, and depression is depression, whether it is in a kiln, pre-drier, fan shed, or air-dry yard. The same principles that apply in the schedule book for kilns can and should be applied to pre-drying. The net result should be the same. The only difference lies in the equipment's difference in efficiency. I guess I'm the only person who monitors exiting air (depression) in the early stages of a green oak charge. If my TDAL drops too low, then I could find complete saturation in my exiting air. That not only means that I have no drying taking place, I'm just pushing water from one side of the kiln to the other, creating a mold/stain environment.

From Gene Wengert, forum technical advisor:
I did develop the first TDAL kiln for lumber back in the late 1970s. (TDAL = dry bulb temperature drop across the load.) We tried the same approach for hardwoods, but the TDAL was so small that the errors in measurement were so large that meaningful and accurate data could not be collected. The small TDAL is due to the slow rate of drying of hardwoods. We also have to deal with the upward flow of air through a pile.

Finally, hardwoods are dried based on the wettest and driest pieces, so knowing the average drying is not enough. So, TDAL for hardwood certainly allows for calculation of the EMC raise across the load, which can be helpful when drying. But, the US FPL schedules already have this incorporated. It is well to remember that the FPL schedules came from industrial drying. They work well so that when trouble is encountered, as here, then we know that there is some other issue besides the schedule.

From contributor M:
I've also learned the hard way about mold on oak. Mold grows when the Relative Humidity (RH) is above 70%, between 32 deg.F and 95 F. It grows especially fast between 77 F and 88 F. Air drying inhibits mold by being below 70% RH, even though the temperature is ideal for mold. Kiln drying inhibits mold by being above 100 F even though the humidity is ideal for mold.

It occurs to me that what you have to be concerned about is that transition period. You must not be below 95 F and above 70% RH. You must either be above 95 F or below 70% RH. If you're going to shut down the kiln, presumably meaning you're going to turn the heat off and allow the temp to drop below 95 F, then you must get the RH below 70%. Actually, near 70% may be ideal since below that could make it dry too fast.

There are a number of possible options. Consider raising the wood temp up to 130 F for long enough to sanitize the wood and kill or inhibit most of the mold. I wouldn't rely on this alone though, without also doing some of the following. You could keep some sort of heat source on, so DB was over 100F and close the vents to minimize heat loss, so then RH and WB would also approach 100. This would use more energy than no heat but perhaps minimize stress in the wood which is important with green oak. When shutting down the kiln, air out the kiln as much as possible with fans to get the humidity down below 70%. Consider a DH if necessary, to get the RH down, perhaps in combination with fans, either at low or high speed, I don't know. Forget trying to kiln dry green oak, and instead air dry down to 30% MC.

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