Difficulty with Drying Small Loads

Running a predryer with a quarter charge of green wood is tricky to manage and full of risks. April 24, 2014

Lumber - End Grain Blowout!

100% solid end grain.
Douglas fir, Ponderosa pine, White oak, and Mesquite.

Question
We have a 420,000 board foot pre-dryer that is empty. Does anyone see any problems putting in one charger of 70,000 board foot of green 8/4 red oak? I am concerned with holding the depression.

Forum Responses
(Sawing and Drying Forum)
From Gene Wengert, forum technical advisor:
I would be concerned too.



From contributor T:
I think you have to watch the RH and loss per day very closely. I also wouldn't run the fans all the time and if need be I'd put some 4/4 oak in with it as well to help control the RH. Gene, what do you recommend for temp in the pre-dryer and do you have suggestions for run times of the fans?


From Gene Wengert, forum technical advisor:
Each pre-dryer has characteristics that need to be understood and accounted for before I suggest the conditions. Certainly a key is uniformity of air flow and the performance of the control system. For the fans, I could imagine cases where we need 50% on for the first two weeks. Temperatures should be as low as possible - as low as 85 F. If the equipment will maintain a constant temperature at that level then varying temps are bad. Then use an RH to give a safe drying rate in the fastest drying region.


From Contributor K:
What does holding the depression mean? I am new to these references.


From Gene Wengert, forum technical advisor:
The depression is the difference between the dry bulb and wet bulb temperatures. It can be used to calculate the relative humidity (RH). To hold the depression means to keep the RH as high as required.


From contributor K:
I would be very cautious about attempting to pre-dry a single charge of 8/4 RO in that manner. You will have to control your drying rate with fan run time-maybe full fan for a few days on wet/green then gradually wind it down to as little run time as 60 minutes on/180 minutes off. The RH will be difficult to maintain with not having the unit loaded. This is what you rely on in a pre-drier for moisture. I utilize fan timers. Otherwise it is very labor intensive to do it accurately unless you want to camp out at your pre-dryer. I find that temps near 70 degrees F are much more satisfactory and forgiving on the drivers, boilers and the building itself here in the northeast in winter. Even in the summer seldom go above 80 degrees.