Using Fans for the initial Air Drying

[Glenn1]

I have 400 bf of walnut and plan on getting green 4/4 in cherry, maple, and anything else that looks good.  My question is about air drying for the initial stage (the kiln will come later).  I don't get too much air circulation due to all the trees on the property.  Scott in NC suggested that I pick up a couple of box fans which is what I have done.  Will air drying with fans give a similar result as conventional air drying or will the fans dry the wood too quickly?  Originally the center of the walnut was + 50% MC and now one month later, it is 33%.  The edges are now 16%.

[beenthere]

How did you measure or calculate those MC readings?

[Glenn1]

I have a Lignomat meter with 12' cables and 6 probes place at various depths. 

[jdonovan]

  Will air drying with fans give a similar result as conventional air drying or will the fans dry the wood too quickly? 

Fans will accelerate the drying. Depending on local conditions temp/humidity/fan speed/air volume, it might create too rapid a drying rate.

Originally the center of the walnut was + 50% MC and now one month later, it is 33%.  The edges are now 16%.

If you are using a standard pin type moisture meter, you can't accurately be measuring 50%. Really above 20-25% is not accurately measurable with most pin meters.

[WDH]

I do it all the time.  With those species that you mentioned, you are good to go.  With the oaks, especially white oak, the fans can hurt you by too rapid drying and the resultant defects like honeycombing. 

So, it is species dependent.  You are OK with most species as long as it is not one of the oaks.  Maple, walnut, and cherry will dry faster with the fans, and in my experience, they tolerate it fine.  In fact, here in the deep South, you need fans on maple IMMEDIATELY after sawing to avoid sticker stain in the spring/summer/fall.  Forum member Yellowhammer and I have talked about this issue a lot.  Sticker stain can be a real problem down here, especially with maple and pecan. 

[GeneWengert-WoodDoc]

I would "second" WDH's comments.  I would add that you do not need to run the fans at night when the RH is high, so use a timer...on about 2 hours after sun rise and off about two hours after sunset.

The fans will be most effective when above 40% MC, so no need to use them a lot after the first couple of weeks or month.

You should achieve under 20% MC easily.  For interior uses, the wood needs to be at around 7% MC, so you will need some additional drying.

[Glenn1]

Thank you!  That is exactly what I needed to know.   8)

[Banjo picker]

I have used fans on tulip poplar as well.  Built a tunnel out of some concrete forms  and put a couple of fans on one end.  It pulled it done pretty fast, I didn't know any better and just left the fans on 24/7.  Banjo

[YellowHammer]

Fan drying certain species of wood is very fast and saves lots of kiln time.  Also, as mentioned, fans are so effective in eliminating sticker stain in certain species of whitewoods that I won't even saw them unless I have at least a couple big fans ready and on standby.  Kudos to WDH for helping me experiment with this, and once you get the feel for it, it's simple, although there is a little bit of a learning curve as to which species of wood can tolerate the effects, but it works exceptionally well in the right situations.

This photo shows a couple big barrel fans sandwiched between pallets of wet maple prior to going into the kiln.  The fans are on "high" and run 24/7 for maple, but I use the speed selector on the fans based on the the species.  As imagined, these fans are putting a lot of air over the wood and evaporating a tremendous amount of water, so that on hot summer months, we'll sit on the downwind side of the stacks to catch the cool breeze.


Same thing, fans placed between stacks of wet poplar.  It's hard to see in this photo, but I have the fans tilted up to cover the upper boards in the stack.


The general idea is that above 40% MC wood will dry faster as the air velocity increases.  Below 20% MC wood will not dry appreciably faster as air velocity increases.  Between 40% and 20% MC the effects of air velocity diminishes.  At higher air velocities drying is more uniform.

So, in other words, immediately after sawing, when wood is very wet, the fans will have the maximum effect in drying, but as the wood dries, the effects of the fans decrease.  So you don't have to keep the fans on the wood very long, maybe two or three weeks at most.  Most of the drying is done the first week, then it tapers off as the the effectivness of the fans diminish. In the hot summer months, I find it is essential to put sticker stain prone wood in front of the fans for at least week, which draws off excess moisture and keeps the wood cool by evaporation, before I put it in the kiln.  Before I started using the fans, I wouldn't saw several species of wood in the dog days of summer because of the problems of sticker stain.

Some hardwoods such as oak will not stand high air velocities without creating significant defects while certain whitewoods and softwoods actually need much higher airflows to force rapid drying and reduce or eliminate sticker stain.  Referencing the published maximum moisture removal rates of woods species will give a good idea which ones will tolerate fast fan drying.

YH

[mrector]

Hey I live just right up the road from you! Anyways I use two cheapo box fans on my lumber after cutting green. Works wonders and I run them 24/7 until they stop losing moisture. Then go into the kiln or stacked for finish air drying. Mostly black walnut and cherry.
Mikey

[WDH]

Black walnut and cherry behave very nicely.

White oak is a delinquent. 

[Glenn1]

 :D    :D    :D

[mesquite buckeye]

Black walnut and cherry behave very nicely.

White oak is a delinquent.

Not delinquent, independent and free thinking. ;D

[WDH]

It is acting slovenly in my kiln right now  .

[Denny]

I'll save everyone some Googling time...

slov·en·ly adjective 

: messy or untidy
: done in a careless way
Full Definition of SLOVENLY
1:  untidy especially in personal appearance
b:  lazily slipshod <slovenly in thought>
2:  characteristic of a sloven <slovenly habits>
— slo·ven·li·ness noun
— slovenly adverb

Examples of SLOVENLY
He dressed in a slovenly manner.
<for the sake of their image, the band members transformed themselves from clean-cut lads to slovenly rockers>
First Known Use of SLOVENLY
circa 1568

And then there's this...
http://slovenly.com/

[WDH]

I pulled the plug on the slovenly white oak.  Going into the sterilization cycle and heat those slovenly acting boards to 150 degrees for 24 hours.  That should teach them a lesson ( and any bugs, too)  .

[YellowHammer]

I pulled the plug on the slovenly white oak.  Going into the sterilization cycle and heat those slovenly acting boards to 150 degrees for 24 hours.  That should teach them a lesson ( and any bugs, too)  .

That will certainly make them sweat. ;D
YH

[Magicman]

Danny's Sauna.   

[OneWithWood]

Danny, will you flood your kiln or somehow add moisture during the last few days?

[WDH]

The 8/4 white oak is still not dry.  I gave up on it.  I have more important fish to fry.  I will dry some sycamore, walnut, and red oak.  The white oak will have to wait on stickers.  8/4 white oak is awful stuff to dry  .

[Den Socling]

You ought to try 12/4!

[OneWithWood]

I have more trouble with the red oaks, especially black oak, than I do with white oak.  One thing I have learned is you cannot rush the oaks.  4/4 is going to take 28-30 days and 8/4 will take 56-60 days.  I go from mill to kiln with very little air drying so your times will vary.

[Denny]

Danny, will you flood your kiln or somehow add moisture during the last few days?

Yeah I've been wondering if you guys condition/de-stress your lumber and how you go about conditioning in a solar or dehum kiln.

[WDH]

OWW,

No, I don't add moisture for conditioning. 

Den,

12/4  ???.  Now I will have nightmares 

[YellowHammer]

Danny, will you flood your kiln or somehow add moisture during the last few days?

Yeah I've been wondering if you guys condition/de-stress your lumber and how you go about conditioning in a solar or dehum kiln.

I tried dumping water into the kiln and other such techniques at the end of a drying cycle because I used to have a lot of stress in my kiln dried lumber, and fought it for a while.  Somewhere along the line I heard about "making the boards sweat" at the end of a drying cycle, which involves increasingly heating the boards to a point where the remaining core moisture is driven out through the case and eventually to the outside of the board without the kiln dehumidifier running so as not to remove the moisture from the atmosphere.  Since the dehumidifier is not operating, the moisture driven out of the boards just stays in the kiln chamber and is recirculated, significantly raising the RH, pretty much the same as if moisture was introduced into the kiln externally for equalization. Coincidentally, this happens naturally in a DH kiln during the sterilization cycle when the temps are driven up significantly over the normal drying schedule maximum allowable temperature and the dehumidifier is turned off to protect the compressor against overheating (at least in my kiln model).   There may be some other factors at work here, but it is very rare for me to have lumber come out of the kiln with any appreciable case to core differential moisture content and very rarely will I have any stress in the boards anymore, and I try to dry them on the fast side of the drying schedules, which normally induces drying stress.  So for me, the high temperature dehumidifier off sterilization cycle serves multiple purposes, and is a step I never skip.  I hadn't thought about it before, but happy, relaxed boards coming out of a sauna is a pretty good description. ;D

As far as a solar kiln, it gets equalized every night.
YH 
     

[WDH]

I do the same thing as Robert.  I run the kiln at 150 degrees for 24 hours, and usually let the load sit for a while with all the vents closed and all the fans running.  I have let the load sit for 24 hours after sterilization, and the temp in the kiln will still be above 130 - 135 degrees with every thing including the fans shut down and the vents closed. 

[OneWithWood]

I do the sweat-em-in-the-sauna thing for most of the hardwoods, but I still flood the floor with about 20 gallons of water before running the temp up for oak.  My kiln chamber is 21'x10', the DH unit is a Nyle 200 and controller is a Honeywell (Woodmizer sold this setup as a DH4000 back in 2002).  I have had good results with the oak doing this, especially the 8/4. 

[Glenn1]

I do the sweat-em-in-the-sauna thing for most of the hardwoods, but I still flood the floor with about 20 gallons of water before running the temp up for oak.  My kiln chamber is 21'x10', the DH unit is a Nyle 200 and controller is a Honeywell (Woodmizer sold this setup as a DH4000 back in 2002).  I have had good results with the oak doing this, especially the 8/4.

How are you containing the water on the floor?  Are you keeping it away from the walls or not worrying about it?

[GeneWengert-WoodDoc]

Air dried lumber has very little, if any, stress.  However, when running the stress test with prongs, if there is a moisture gradient, the prongs will show stress.  The prong test must be made with no gradient.  So, keeping the vents closed with heat may actually be eliminating the gradient, which would have shown stress.

A commercial kiln will use 180 F and very high humidity for 18 - 24 hours.  It is hard to understand how a very mild condition (heat and vents closed) would be just as effective.  For this reason, I suspect that the amount of stress in many instances is very low.  Adding a great deal of humidity will cause the surface to gain moisture and swell (or try to swell) so that even a piece with stress will show no stress, until the moisture on the surface (moisture gradient) is dispersed.

To get a good reading with the prong tests when there is a gradient, put an individual prong in a microwave on high power for 15 seconds.  Then take it out and wait a couple of minutes before reading the results.

[WDH]

A commercial kiln will use 180 F and very high humidity for 18 - 24 hours.  It is hard to understand how a very mild condition (heat and vents closed) would be just as effective. 

Is 150 degrees for 24 hours considered a 'mild condition"?

Another question.  If the wood going into the kiln is already air dried to 15%, will much additional stress develop between drying from 15% to 8%?

[GeneWengert-WoodDoc]

Yes, 150 is mild.  We know that stresses are not removed well at 150 compared to 180.

Virtually all stress develops during the loss of the first 1/3 of the moisture, from green.  So, no stress develops at low MCs.  Short answer...need more?

[WDH]

No.  Mostly what I am drying in the DH kiln has pre-air dried.  I am not getting much stress at all, therefore the question about the "mild" 150 degree sterilization. 

I still hate white oak.  All thicknesses  .

[Peter Drouin]

Thanks for the info guys. It will help in the spring when I can get my kiln together. :D

[OneWithWood]

I do the sweat-em-in-the-sauna thing for most of the hardwoods, but I still flood the floor with about 20 gallons of water before running the temp up for oak.  My kiln chamber is 21'x10', the DH unit is a Nyle 200 and controller is a Honeywell (Woodmizer sold this setup as a DH4000 back in 2002).  I have had good results with the oak doing this, especially the 8/4.

How are you containing the water on the floor?  Are you keeping it away from the walls or not worrying about it?

The kiln is very well sealed along the floor wall boundary and the walls are coated.  With a 3000bf charge of 6-7% wood the water does not stay on the floor long enough for me to worry about it. 

[OneWithWood]

I am quite fond of white oak, all thicknesses.  It brings out my more caring patient side  :D :D :D ;D

[WDH]

 :D :D :D

Actually, I love everything about white oak except for drying it. 

[kantuckid]

I have two stacks of 2x6x10&14' of (mostly SYP) pine on sticks under the open roof on the front of my shop. Each stack is a bit over 1,000BF stacked 3.5' wide.

 I have one 20" box fan on the end of each stack blowing 24/7 into the end.

 In my area, about how many days will these fans serve an effective purpose toward drying this lumber? 
Thanks!

[customsawyer]

I have two stacks of 2x6x10&14' of (mostly SYP) pine on sticks under the open roof on the front of my shop. Each stack is a bit over 1,000BF stacked 3.5' wide.

I have one 20" box fan on the end of each stack blowing 24/7 into the end.

In my area, about how many days will these fans serve an effective purpose toward drying this lumber?
Thanks!

If I am reading it right your fans are blowing against your stickers instead of through your lumber. I don't think your fans are going to help much. Look back at the pictures of YH fans and lumber. Bigger fans will help more and have them moving the air parallel to the stickers. 

[kantuckid]

I see what your say & it's true and I considered that when I placed them but not smart enough huh? ;D I'm gonna move the fans to the open walkway between the stacks and opposite ends from each other, then move them along the stack a bit as they'll be pretty close to the wood-maybe 24" from the stacks. I have these box fans sitting on milk crates to position them above the base. 

Now, back to my question? 

[Southside]

If it's framing lumber you are after I bet they would be under 20% MC in a couple weeks. 

[kantuckid]

I wouldn't doubt that %, but since I'm still waiting on wall logs I have some time. My tractor remains dead so the p[ines I dropped have laid since Early May when the first guy managed to not work on it at all and only been moved bit over a week now to 2nd guy. I can hear the bugs eating as past few days the heats been on in the woods. I've got a bunch of 4x6 pine beams to saw yet if those trees stay OK.
 About half the stacks were in place early April but the last were month later. I moved my fans this afternoon. They are blowing oppositely which also mean they are sucking air through the stack beside them and ones doing a middle the other an end for now. I've about got my shop arranged for dead stacks and planning operation plus T&G-ing. I own too many MC's and got several still in the way of lumber milling. ;D
 

[GeneWengert-WoodDoc]

Fans are used for air drying usually in a shed.

The issue is the operating cost.  Let.  For example, if you have a small fan that uses 12 amps or 1500 watts, or 1.5 kWatts, and you run fit 12 hours a day for 25 days, and your electric useable is around 400 kW.With an electric rate of $,15 per kW, the cost is $60.  Is it worth $60 to have drier lumber in a shorter time? Of course, multiply $60 by the number of fans.

[kantuckid]

There are another few localized factors: Electricity is cheap in E KY, plus I'm using box fans that don't draw 12 amps (more like half that) plus my shops on it's own meter and the shop minimum applies given that this time of year I tend not to be inside as much using that electricity. Once that planer kicks on that meter whirls!
My particular stacks were done in roughly two stages as I logged more trees to finish, so this fan thing sort of catches the top up with the bottom as I'll pull off the top when I walk boards to the planer, etc.. 

[Don P]

This time of year drying faster also reduces stain.

[WDH]

I use air drying fans constantly, especially this time of the year.