Kiln Drying walnut logs

[Mfrost459]

This week I will be drying some walnut logs. Customer wanted me to flat saw the logs and leave the live edge on the lumber. We milled the logs about 4 month ago and they have been in our air drying shed. Right now the MC is right at 25 to28%. What type of kiln schedule would you use to dry this lumber. Most of the boards are cut at 4/4 there are a few 8/4.

[GeneWengert-WoodDoc]

What type of kiln do you have?
Get a copy of Drying Hardwood Lumner from an internet download.

[Mfrost459]

I have a 20 ft container kiln with a L53 Nyle unit. Also have a Nyle booster heater to help raise up to operatin temp, really don't use the booster much in the summer.

[YellowHammer]

You're still a little in the danger zone, but walnut is pretty forgiving down to 15% then you will really have to push it.  

For 8/4 at 28% I'd be careful, and start at 100/95 for a couple days then drop to 100/90 if you can maintain the cooler temps,  and get a feel for the wood, and see what the actual measured moisture removal rate is.  4/4 can handle 8% and 8/4 can handle half that but I really don't like more than a couple percent per day on 8/4 walnut until it stalls, and it will. So watch it close for awhile. And don't let the 4/4 get away from you, concentrating on the 8/4.  

If the MC is not dropping at those conditions, (it may not drop much, they are conservative settings) then go to 110/98 and see what that does.  I would not start at those conditions at 28% my until I understood the behavior of that particular stack of walnut for a couple days at the more conservative setting.

If the wood is dropping moisture in a reasonable fashion, then just keep it there, wherever  "there" is and cooler is better.  Remember, measure the moisture loss every day.  This is where a bucket under your condensate drain in invaluable, it give you a visual reference of actual moisture removed.  Then when the wood starts to get cranky go up to 120/98 and it will walk down until it stalls.  Ideally, you would like to get both the 4/4 and 8/4 to stall both at lower temps and then you can walk them both down further, somewhat in tandem.

When it stalls, you're going to have to push it hard. 120/95 or even full on, but then you'll have to start playing games with it, pre sterilize, overheating, flooding the floor, even going and getting some chickens ready.  

The main thing is that walnut can be very temperamental and erratic, and it important to start conservatively and double check things constantly until you get down to the safer 18 to 15 percent range.  Then you couldn't hurt it with a hammer.  

With 8/4 walnut at those conditions, I would expect a 3 week cycle, give or take.  So be patient with it, you can't really sticker stain it, so slower is better.  

Just my seat of the pants experience with cranky walnut.  

[GeneWengert-WoodDoc]

If we assume that the outside average conditions are 13% EMC, then th outer fibers of the lumber will be 13% MC.  When we bring the lumber into the kiln, we can set the kiln conditions at 13% EMC and the lumber will dry very slowly...about as fast as it was drying in air drying.  The kiln temperature might be 110 F, give or take, so the higher heat would indeed result in a little faster drying.

So, as the wood is getting quite dry and stronger when under 30% average MC, and drier means stronger, we might feel comfortable starting the kiln and perhaps 12% EMC so we do get a little faster drying.

Now, consider what happens if we start at an EMC higher 13% EMC.  The surface fibers, which are 13% MC will increase in MC and that will also result in swelling,  with the swelling comes stress on the outside...tension stress.  As wood is weak in splitting failure, this swelling will aggravate existing small surface or end checks, driving them deeper.

The same effect occurs when we rewet partially dry lumber with rain, melting snow, etc.  in fact, we do see with thick oak that sometimes there is honeycomb even though the lumber has only been air dried.

[Marshall7199]

YellowHammer, can you explain what you mean by, "start at 100/95 for a couple days then drop to 100/90"?  I dont have a Nyle kiln but a small DIY kiln with a dehumidifier.  

[WDH]

Those are the set points for the dry bulb probe and the wet bulb probe.  A home dehumidifier does not have these probes. 

[Marshall7199]

Thanks WDH.  From what i can tell this is about 85% relative humidity at 100 dry, 95 wet bulb.

[Mfrost459]

I rechecked the boards before putting them in the kiln this week. The MC has dropped more, the boards range from one at 7% and most of the others between 15 and 18%. What is the best way to proceed from this point? Thank so much for your information!!

[WDH]

I would run the compressor full out. 

[YellowHammer]

To save electricity and compressor time, I like 120/90.  This is a 30 drop and will generally cause the compressor to run full time, but as the wood starts to get down, will allow the compressor to turn off now and then.

[WDH]

I have not tried that setting.  I usually go 120/75 and put the hammer down. 

[K-Guy]

@WDH

Running the compressor at 120/75 usually doesn't gain much if anything, at that point it is all up to the wood to give the moisture up.

[WDH]

Yes Sir. 

[Mfrost459]

First let me say, Thank you for all your advice. It is nice to be able to draw on your experience. Since the MC is in this range can I dry4/4/ together with 8/4. Usually only dry the same thickness lumber at one time. Are there times when you can dry 8/4 with 4/4?
Also this won't be a full load in the kiln but I have some White Oak that is the same MC. Could I dry walnut with the white oak to make a full load?

[GeneWengert-WoodDoc]

The rule is that you can mix species and thicknesses, but never mix moisture contents going into the kiln.  The thickest lumber will determine drying time, so the faster drying wood is along for a free ride, so to speak.

[Mfrost459]

Thanks Gene! Looks like I will be loading kiln first thing tomorrow morning.

[Mfrost459]

We ended up loading the kiln with the 8/4 walnut first. Almost a full load so we didn't put any 4/4 in there. Set the kiln at 100/90 and have been getting about 2 1/2 gallons of water out each day. Is this about right? Also when you say the load has stalled, does that mean there will be no water or like a gallon. 
Yellow Hammer also mentioned about flooding with water. When would you need to do this and what is the best method to use?

[YellowHammer]

Is the water coming out at a relatively consistent flow rate per day?
What is your moisture loss rate per day measured in the 8/4?
What is your actual measured moisture content?
How does the wood look?

If the flow rate is consistent, and the moisture loss rate is acceptable, then just keep the settings the way they are.  At some point you'll notice a pretty significant decrease in flow rate, and then bump up the temperature, set for 120/98 which increases the temp and doubles the depression.  You should see a resumption in water flow rate, and hopefully these settings will allow you to walk it down to a finished load.

Get to a 8% case and core, raise the temp to 150, shut off the compressor, spend a day to get to150F, hold for a 24 hours, then let it cool for a day, with the vents closed, and you'll be at a perfect and consistent 7%.  

If you can't get the moisture out, then go to 120/90 or even play with the high pressure safety and do 125/90 and if that doesn't do it, then you have a stalled load, which may only provide a half pint water per day.  

At that point, water on the floor, chickens, etc become more of a strategy.    

[Mfrost459]

Again I am so thankful for all your input! This forum is a tremendous teaching tool. I have learned so much from here. Also really miss the Sawmill Project at Jake's, another great place to get information.

Right now the kiln is set at 100/75 and I am getting 2 1/2 gallons of water out each day. The load consist of 800 BDFT of walnut and one white oak slab. 
 I have a J-2000 Delhorst moisture meter, took 19 readings in the pack and it averaged 14.4% MC. There was a wide range of readings, a few at 8 to 9%, most between 12 to 14% and a few at 18 to 20%. It is normal to have such a wide range of readings? The wood looks about like it did when it went in. Seems like the MC content is about 2% lower than the day before. I will check it tomorrow after noon to make sure.

[YellowHammer]

In walnut it is very normal.  Walnut will have many wet pockets, that if you are lucky, will naturally resolve themselves in the kiln during the drying process. 

A 2% drop per day in 8/4 walnut is very reasonable, and I wouldn't make any changes unless unless the water flow rate changes.