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Drying wood in a dehumidifier Kiln intermittently

Started by woodweasel, January 19, 2017, 10:57:49 PM

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woodweasel

Would it hurt a load of wood in a dehumidification Kiln to intermediately stop and start.   I work offshore and I'm gone 20 days in a row.  I  dont want my wife to have to monitor it while I'm gone?

GeneWengert-WoodDoc

When you turn off the kiln, the humidity will increase, and mold and stain can form.  You can probably open the doors.  Is it located outside?
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Den Socling

I've often wondered if some drying is done by capillary action and that process can be stopped if you interrupt drying.

Ianab

I'd think that if the wood was under ~20% mc then it wouldn't be harmed.

Otherwise if you opened the door and left the fans running, it should simply continue to air dry until you turned on the DH again.
Weekend warrior, Peterson JP test pilot, Dolmar 7900 and Stihl MS310 saws and  the usual collection of power tools :)

Den Socling

Capillary drying probably only applies to the high speed stuff I do. Ian is correct for a DH kiln.

GeneWengert-WoodDoc

In the past, drying was interrupted at all moisture contents in operations that ran their boilers only five days a week. It was called part time drying. It did not affect quality.  Similarly, air drying varies from day to day.  So overall, stopping drying is not an issue.  It is only a concern of what happens in or during the time that kiln drying is stopped...the humidity is not out of line. This humidity is within the pile, so fans must run or there must be natural circulation.    Temperature is not important, except as it affects humidity.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Den Socling

I wasn't around during the days of 5 on & 2 off but I bet that drying degrade wasn't a big deal back then.

YellowHammer

Quote from: Ianab on January 20, 2017, 12:28:17 AM
I'd think that if the wood was under ~20% mc then it wouldn't be harmed.

Otherwise if you opened the door and left the fans running, it should simply continue to air dry until you turned on the DH again.

You didn't mention what species of wood you dry, it would make a difference to how I handled it.  I've have done exactly that a few times, when I had to go on unanticipated travel for business.  Open the doors, dump the heat and moisture, open the vents, and turn off any heat sources to let the pack cool down.  Depending on species and moisture levels, keep the airflow going to avoid sticker stain, or turn most of the fans off if its oak.  Bring it down to ambient temps.  I've only done this for relatively short periods of time, a few days.  If I was going away for 20 days, I'd unload the kiln, and put it back in the air-drying yard.  This would prevent damage to the wood, and would also let Mother Nature dry the wood while you are away.
Another option, by Nyle, is a web based control.  You could run the kiln while you are away.
YellowHammerisms:

Take steps to save steps.

If it won't roll, its not a log; it's still a tree.  Sawmills cut logs, not trees.

Kiln drying wood: When the cookies are burned, they're burned, and you can't fix them.

Sawing is fun for the first couple million boards.

Be smarter than the sawdust

Den Socling

That's how all our kilns are. We supply a supervisory computer that is connected to the kiln and the Internet. It has historical logs that show you what the kiln has been doing and, if you want to change something, you click on the control screen and make the change. We connect to all new customers and monitor them at least twice a day for at least three months. You can check your kiln from your phone. That's the way all new kilns should be. This is cheap to do but then DH kilns are dirt cheap.

WDH

I rarely dry loads that go into the kiln over 20% moisture content (pre-air dried), and I have interrupted the drying by shutting the kiln down for a day or two with no ill effects. 
Woodmizer LT40HDD35, John Deere 2155, Kubota M5-111, Kubota L2501, Nyle L53 Dehumidification Kiln, and a passion for all things with leafs, twigs, and bark.  hamsleyhardwood.com

longtime lurker

If you wish to run a kiln hard and dry wood in the shortest possible time with the least possible degrade you require accurate in kiln monitoring and close supervision, coupled with knowledge and experience of what to do and when to do it. And if that means getting up at 3am to change a set point then thats what you got to do.

Mostly I dont want to work that hard. I got enough stress in my life already, so I set things up so I dont have to stress about the kiln.

Look, if you know your charge size and thickness and average intake moisture level of the pack of timber...
And you know what the water removal rate per day of the kiln unit is...
And your timber is below 20% core MC on entering the kiln...
It aint rocket science to figure out how long its going to take to reduce the moisture content to the desired level.

Low temp DH is low temp DH... you dont have the capacity to shift the dry bulb from 150 to 160 to 170 and keep dropping the wet bulb back as it drops below 20% because you cant get that hot in the first place. So dont fret it.

All degrades that happen at high MC's have already occured. Kiln supervision wont change that.
So long as your charge size is correct with regard safe drying rates you wont dry it too quick. Kiln supervision wont change that
When the charge hits the right EMC based on wet/dry bulb the unit will stop cycling so you cant over dry it. Kiln supervision wont change that.
If wood MC's are low before commencing kiln drying and you ramp it up then let it sweat so that the core temp matches shell temp before water removal begins you'll avoid a lot of case hardening issues before they start. Takes a lot of pressure of the reconditioning cycle that may otherwise be required.

Me, I'd be pretty confident that I could toss a load of hardwood at a moisture level below 20% into my DH kiln, shut the door, ramp it up to a temperature around 110° F, hold it there until the core temp catches up, then set for a wet bulb depression of 17°F and walk away with the thing on a timer and get it close enough to cooked with no stress, because I am usually able to guesstimate the finish time of a charge to within about 6 hours on the species I commonly dry. 6 or 12 hours extra on a cycle isnt going to break the bank with electrickery ... removal rates arent that fast that it's critical either.

Low temp DH is the Ford Taurus of the kiln world... you dont need the reflexes of a Formula 1 driver to drive it if you're going down the highway at 55.
The quickest way to make a million dollars with a sawmill is to start with two million.

Den Socling

Yeah I'm always talking about how great vac kilns are but 10 days ago I started a load of 4.5" square White Oak post. I haven't touched a thing since start up. This wood was fresh off the saw. I put the settings in the PLC and walked away. All automatic.

Don P

Could you just set a humidistat to some level controlling fans and leave the vents open, essentially a fan shed?

GeneWengert-WoodDoc

A fan she'd allows much more outside air circulation than kiln vents.  Opening the kiln doors, if the wind will not hurt them, is essential and will help in most cases to make kiln fans only work well.  It will not work for some kilns, like steam kilns or hot water kilns, when below freezing.

A humidistat needs to get humidity within the stack and on the edges, so maybe 100% fans is a reasonable idea with some air flow all the time to assure that the center of the pack will dry ok.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

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