Question
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:
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.
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.
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.
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.
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.
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.
