I've heard "you can dry as fast as you can when under 20% MC in a kiln" True?

[Kelvin]

Howdy all,
I've been told by someone working for my kilns manufacture that under 20% MC you can run the kiln full throttle.  I did this on some red oak this week from 15%MC air dried to 7% in 2 days and the sample pieces were really surface checked quite deeply (ruined, which worried me greatly).  I checked the main stack and it looks better, but i haven't pulled it out yet.  (fingers crossed)  Is this true regardless of species? 

Also i've heard that air speed in the kiln under 20%MC doesn't matter.  Then why bother running the circulating fans?  The kiln unit has a pretty strong fan on it so i doubt that the air in the kiln will be significantly more dried in some areas than others.

What do you guys think?
Thanks
kelvin

[beenthere]

Kelvin
Do you really know for sure (absolutely) that the air dried oak was 15% ?

How long was it air drying on stickers?

[Den Socling]

If you are truly under 25% including the cores, you can dry as fast as you want. The problem is that every core in every piece is not going to have the same MC.

When you are moving a lot of water vapor, you need a lot of airflow, about 300cfm or more. When the wood is almost dry and little water is being removed, you can use around 150cfm.

[LeeB]

Kevin, is it possible that the samples were already checked and the further drying just opened them up where you could see them better?

[scsmith42]

If you are truly under 25% including the cores, you can dry as fast as you want. The problem is that every core in every piece is not going to have the same MC.

When you are moving a lot of water vapor, you need a lot of airflow, about 300cfm or more. When the wood is almost dry and little water is being removed, you can use around 150cfm.

+1. 

I'll add that most damage that occurs during the drying process occurs when the wood is above 35% MC, but it doesn't become visible until it's below 25%.

I don't think that the fan on the DH unit will move 150CFM through your stacks, unless you only have a few hundred board feet of wood in the kiln.

[Den Socling]

And checks will close as cores come down. They won't open. I would guess that the cores were still high and going full throttle dried the shell too fast. Been there, done that with white oak. :'(

[LeeB]

Now i learn something. That's why I asked. I don't do any kiln drying, so know nothing about it.

[Kelvin]

Howdy all,
The lumber was air dried for a couple of years, and checked with my moisture meter, a pinless wagner L606 which is fairly good.  This is 4/4 lumber so i would think the meter would catch the core moisture content.  But assuming the particular pieces were too high, are we confirming the idea that you can dry any species as fast as possible as long as its below 20% MC?  The limits on moisture loss in the manuals are refering to lumber that is 20%  MC and above even though it doesn't state that?  I believe for instance that the manual with my kiln says to only remove 1.5% MC in 24 hr period on red oak. 

Also that you do in fact need air flow in the last parts of the drying process?

Oh, i forgot, how about conditioning at the end of a run?  I'm always worried i'm too dry already and the idea of pumping the temp up to 160 degrees seems worrisome, what do you guys do?  And for how long?  This is also not in my manual.  I assume the vents are closed to keep the humidity up?

Thanks for the input guys.
KP

[Meadows Miller]

Gday

Kelvin I dont know about going as quick as you like  ??? but then again the type of kiln you have prob low temp low airflow kiln  is nothing like the kilns i spent about 18 months managing putting out between 45 and 60000 bft per day in hardwood where we where operating at the limits all the time and its easy to stuff up a $100000 charge off aperance grade 2" turn it into $70 odd grands worth of structural grade timber in one hit  ;) Going full tilt in the final stage is a verry tricky jugement to make and it all comes down to how much experiance you have wiith the species inolved and what sort of end product you are drying mixed charges are a real pita to judge  : In Mho going hard in the last stage is a verry risky thing to do to save your self wheter it be a few hours or a couple of days Mate  ;)

Also if you where at 15% Mc Air dry why bother putting it in the kiln for a start as up where you are the Emc Eqalibriam Moisture Content would probobly be between 11 and 15 % depending on the time of year with the kilns i ran the lowest i usualy dryed stuff to was about 9% and that was because the customer speced that Mc drying below emc for a runn of mill stock so unless the customer specs it  ;) I would just dress it and sell it  ;) ;D Or if you are getting inconsitant readings in your airdry yard stock pacs wether that be that the packs have not had consistant airflow or getting dead spots due to yard or stack settup  ;) another option would be to Equalise the Mc in the kiln but you wont have to push it that far down in Mc it wastes alot of energy and ties up kilnspace

With kiln drying it was invented for larger mills so they wernt sitting on stock for the longest part of the drying phase which is the Final dry removing the Bound water under 23 %mc  ;)the air drying yard which i also looked after had instock GOS Mountain Ash inch takes about 12 months to get down to fsp about 23% and 2 inch took two years then a Steam Recondition followed by 4 days to a week in the kiln to push that last bit out we where sitting on about $28 millions worth of airdry stock at any one time

I myself will probobly never build and runn my own kiln as i will be sawing pine mostly and around home here it only takes 1 1/2 to 2 weeks mid summer and 6 to 8 weeks mid winter to get to the 12 to 15% mc

Wth moisture meters you wont get an accurate reading at about 19% to Fsp anything under about 18% can be taken as a true reading  ;)

If you have any questions Mate feel free to ask them here or pm me as theres alot to it to cover in one post  ;) :D :D And the games about the same the world over ;) ;D

Reguards Chris

[Den Socling]

Greetings Chris,

The US uses a lower MC than Australia or NZ. And we use the kiln for sterilization.

For the rest of you, the "Mountain Ash" he refers to is Eucalyptus regnans if I'm not mistaken. As you can see by his schedule, it is nothing like our ash.

Den

[scsmith42]

The limits on moisture loss in the manuals are refering to lumber that is 20%  MC and above even though it doesn't state that?  I believe for instance that the manual with my kiln says to only remove 1.5% MC in 24 hr period on red oak. 

Also that you do in fact need air flow in the last parts of the drying process?

Oh, i forgot, how about conditioning at the end of a run?  I'm always worried i'm too dry already and the idea of pumping the temp up to 160 degrees seems worrisome, what do you guys do?  And for how long?  This is also not in my manual.  I assume the vents are closed to keep the humidity up?

Thanks for the input guys.
KP

Kelvin, all of the drying classes and manuals that I have indicate that you can dry at a much faster rate once the wood is below 25% MC.  Personally I maintain the same rate and don't try to speed it up.

Yes, you need air flow to dry.

One question  - why are you conditioning at 160?  Are you drying pine and need to set the pitch?  If not, 135 should be adequate for conditioning and sterilization.

Scott