Hey folks I have a few questions about fan reversal times...
I have six 30,000 foot kilns and two 25,000 kilns. For the most part I dry Red and White oaks and Hard and Soft Maples. My fans reverse every four hours.
I've seen some MC variations in the dried red oak that I get out of my two smaller kilns. The six bigger kilns are a different brand & design and I don't have the MC variations that I see on the two smaller kilns.
I could have sworn that I read somewhere that lowering the fan reversal times to three hours will help control the MC varations. My varying lumber final MC's are between 6% and 10% sometimes, while my bigger kilns put out 6, 7 and 8% range.
I was wondering how often your fans change direction and if there's any advantages to changing it for different species.
I understand that equalizing a little longer will smooth out my MC variations, but I'm on a strict time schedule and every day counts. More time in the kiln is the answer but If I can smooth things out by tweaking the fan reversal times, I'll try it.
FYI, the two kiln's I'm seeing the MC variations in are made by SII and the six kilns that put out more consistant numbers are made by Irvington Moore which is now USDR.
Thanks guys...
-Denny
I don't think a shorter time interval is going to help. Have you contacted SII? BTW Welcome to the forum.
Dennis thats what I'm thinking too.
I have found the paragraph that I had in mind concerning fan reversal times. Its in Dr. Wengert's book "Drying Hardwood Lumber" on page 106, and it deals with controlling warp/cup, not with uneven MC numbers.
"Do not use long fan reversal cycles(over 2 h), which expose the lumber on the exiting-air side of the load to high humiity for a long time, thereby increasing MC."
I haven't contacted SII.
Thanks for the welcoming again, I've been quiet way too long.
Denny
Seems you would be in a good position to experiment with changing the fan reversal times and evaluating the results. That way you can hone in on the better outcome for the smaller kilns.
Most things written in books are not real specific for a particular kiln design, species, and stacking.
But needs some tweaking of the general information and numbers given for best results. Your experience comparing large kilns to the small ones is a good example, IMO.
my fans reverse every 3 hours. i'm wondering about lack of fan movement giving you some dead spots or reduced airflow. towards the end of a cycle fan direction shouldn't play as big of role as would be earlier in a cycle.
the fact the kilns are of differant manufacter makes me point more towards airflow instead of direction.
good luck and welcome !
Yes, it is uniform airflow. The size of the kiln doesn't matter. It's fans, fan placement and baffles.
At high MCs you will find more even MCs with a two hour reversal, than four or six. But, as drying goes on, this difference will disappear. So, frequent reversal does not change MC uniformity. Reversal of two hours is important for very high MCs, to avoid the cycling between wet and dry when the fans reverse.
Further, at 25% MC or drier, air flow no longer controls drying rates, so we can lower the air flow greatly, plus we could,actually extend the time between reversal with no difference in drying.
So, what is causing the MC variation? Major factors are
- - incoming MC variation, which can be very high for AD lumber
- - uneven heating throughout the kiln, which could be the result of veritable air flow, as air flow does affect how much heat we get off the heating coils. But, often the uneven heating coil temperature is the main issue
- - uneven venting
Of course, kiln loading could be poor causing blocked air flow passages. Although variable air flow and reversal times are not important for air dried low MC lumber, we still do need some air flow to get the heat and make the vents work. In the winter, we also need air flow to heat the floor and concrete, uninsulated walls. But again, it is not reversal time, but uniformity at high MCs.
Ok?
Thanks for all the replies everyone... I do have some more follow up questions and comments but I'll hold off till I have more time and I have processed all the info in this thread.
FYI, I just loaded one of my SII kilns with green red oak yesterday & my high sample was 92%. I ususally get a few days of air drying, but this charge got stickered and then went right into the kiln.
Thanks again everyone...
-Denny
Can you determine where the boards with high MC are located in the kiln/stack? Do all the kilns have the same width of stacks? Have you checked the air velocity/temperature at various points in each kiln?
Good points, Jim. The answer is among them.
I reverse my fans twice a day once at beginning of shift and back
Again at end We also stick our wand in the packs in each kiln and in both first packs in and then the last.if we find one end has higher numbers than the other we run our fans in the appropriate direction .which ever end gets the heat first usually dries slightly faster
reversing airflow direction only influences which side of the stack dries faster. Can you tell if the center is where the high MC is? Or the top compared to the bottom, or one end?
I would hang a hygrometer and record readings from top, bottom and ends. Someplace isn't getting the heat.
With oak and other check-prone species, long fan reversals are not good. The reason is that the one side of the load gets used to a dry condition and then when the fans finally reverse, the conditions become quite a bit more humid (with green lumber). Frequent reversals (2 hours) minimize this shock. The problem with variable conditions in the extreme is that checks will worsen...grow bigger and go deeper.