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mold on 8/4 oak7/6
When drying 8/4 red & white oak in th 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 & 100 DB. Velocity is 200 fpm. Should we run the fans &/or open the vents when the kiln is off?
Just some random thoughts. 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 3 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 3. 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. Ha!
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
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.
It's time to put down the Forest Products schedule book, and figure out for yourself what you are tring 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 3 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 4 or 5 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 tellng 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.
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.
What should be the correct air velocity?
Keep in mind that billions of board feet of lumber have been dried in conventional down-draft pre-driers that A) do not reach 100 or even 90 degrees temp, and B) 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 esential. The closer the depression is to saturation, or 0 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.
TheUS FPL has kiln schedules and not predryer 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 abut entering air temperatures. This is true for pre dryers and kilns.
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.
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
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.
I did developed 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.