Green Slabs directly into DH Kiln

[E-Tex]

I have a customer that has 2000bf of 10/4 slabs (Red Oak, Post Oak and some Pecan)...freshly cut this last weekend.  His initial plan was to Air-dry for a year then take and finish off in a kiln.

He did find a kiln operator that said he can put in his DH kiln GREEN and dry his slabs NOW.  He did say it would take about 3 months and they talked about the dangers of drying too fast.

I told my customer to "Hang-On".....let me get a Second Opinion from the Forum Experts.

Can this be done?  Should this be done?
Can the kiln operator set conditions well enough for a slow drying rate on the Slabs?  Or is it recommended to Air-Dry down to 20% or so first?

Thanks

[longtime lurker]

You got more control of the environment "in kiln" then you do outside air drying so - sure. It's actually the safest way to do it.

Personally I tend to go for a couple months of controlled air drying first just to save on kiln time... it doesnt take a year to get timber below 20% even in the densest, most difficult species at 10/4 thickness.

At all costs I avoid uncontrolled air drying - thats when bad things happen.

[YellowHammer]

Yes, it's actually "safer" in the kiln, if it's set right.  I just wonder how a person can afford to lock up his kiln for 3 months on one load of wood.   

[scsmith42]

Per Drying Hardwood Lumber, the estimated time for drying 10/4 oak from green in a conventional kiln is 77 days (best case).  That's in a kiln that operates 7/24.  [Page 89, Drying Hardwood lumber - estimated time for 4/4 northern red oak is 23 days times 3.38 drying time factor for 10/4 on page 90).

90 days in a solar kiln that operates at best 7/14 seems pretty optimistic.

My own experience with 10/4 oak in a solar kiln is that 5-6 months to dry from green down to about 10% MC is more realistic.

Granted that the next few months are the optimal time for a solar kiln, but 90 days still seems unrealistic.

[YellowHammer]

Post oak is a white oak, and drying 2.5 inch white oak without surface checking in air would be virtually impossible in Alabama without taking heroic measures.  Same thing with 2.5" thick hickory, it will check when air drying.  

On the other hand, drying both in a DH kiln will take at least 1/4 of a year, which is not reasonable for anybody who is trying to make money off a kiln.  

It's for this reason I don't cut 2.5 inch thick white oak or hickory.

[PA_Walnut]

On the other hand, drying both in a DH kiln will take at least 1/4 of a year, which is not reasonable for anybody who is trying to make money off a kiln.  

This is why I need multiple solar kilns. Is anyone actually building them commercially? I could use five right now. 

[50 Acre Jim]

A friend of mine is going to buy an iDrly Vacutherm.   iDRY | Vacutherm  

He tells me with the vacuum kiln he'll be able to put wood in right off the sawmill and it'll dry 1" per week. So a 2-inch slab will dry in 2 weeks. Seems pretty aggressive but if you read about the kiln, it's pretty impressive how it works.

Apparently the vacuum kiln brings the pressure down in the chamber to where water will boil at room temperature. That way there is no degradation of the wood from the high heat and trying to extract the moisture.

I took the following directly from the companies website.

Specifications
Capacity: 1000 – 2000 board feet
Lumber Pile: 13ft long x 42in wide x 55in tall
Power requirement: 208-230V 1PH 60A
Energy Consumption:
Electrically Heated: 6kwh average
Hot water Heated: 1kwh average
Drying Time: 1 week per inch of thickness

[nativewolf]

I see that they don't use plates in that small kiln, wonder how they distribute heat?  Is that only for softwoods?  

Thanks for sharing, that's interesting.  Let us know how that works out for him.

[WDH]

I have never dried green slabs in my kiln and do not plan to even try it.  Like YH says, just too impractical to tie the kiln up for 3 months for one load. 

[boardmaker]

Cool link Jim.

They don't say much about the process.  I suppose they've developed a version of their airdry vac kiln.  Our resident vac kiln guy hasn't been on the forum for a good while or I'd ask him.  

If it does what you say, without the labor of the water plates and restacking, they have something awesome.  Especially in that price range.

Edited to add:  The more I look at it, the more it seems like a smaller version of the AirDry.  There's not a lot of info given...

[E-Tex]

I've been looking into the new I-DRY kiln and talking to VacuTherm.

It's my understanding this is "original" vacuum technology......?  No Plates, but sticker stacked.

They are set to start rolling off the line next week, so I'm told.

I have visited with several forum drying guru's about this unit to get their thoughts.  Looks promising but certainly lots of questions on it operations and results produced.

Time will tell when they get into the hands of a good Kiln Operator.......  I want to know more for sure!

[longtime lurker]

On the other hand, drying both in a DH kiln will take at least 1/4 of a year, which is not reasonable for anybody who is trying to make money off a kiln.  

This is why I need multiple solar kilns. Is anyone actually building them commercially? I could use five right now.

try this lot: they send stuff all around the world, and all I hear is good reports about the product. It's smart solar, not "dumb" passive systems and that comes at a cost but... over a couple years it adds up to a whole lot cheaper.
Solar Kilns : Using the Suns Heat .....Genius! | Solar Kilns

[longtime lurker]

My solution to the tying up kiln time with thick timber was in two steps.

I installed the second DH unit for just that reason. That little Nyle never really shuts down and has enabled me to use the larger low temp conventional to pump out 1-2" material where its quick and cost effective. The L53 gets used for oddball stuff requiring different schedules or slabs or thicker stock.

The other was to think sideways. I've always got some thicker stuff going through somewhere, either as shorts for legs or as big heavy slabs. I fill up any void space on loads of 2" with these. Air dry below fibre saturation point first. Let them dry with the 8/4... pull them from kiln... wrap in plastic. The gradient means that they pull water from the middle to find equilibrium. Let them rest a month to 6 weeks doing this, unwrap them and send them through with another lot of two inch. They dont take up much moisture from the green wood... it aint migrating back to core due to high RH at the start of the cycle, just the outside... and repeat the whole dry/ wrap in plastic thing again. You do that a couple or four times and you end up with a pretty dry core, and no worse a moisture gradient across a 4" thick slab then if it air dried for ten years.

If I was going to really get serious about drying thick stock in quantity I think I'd go DH, with real good seals and an auxilary wood boiler for heating. Crank up the temp with the auxilary... turn on the DH unit for a week and suck some moisture out.... turn it off and let it rest... repeat, repeat, repeat. You'd need the auxilary heat loop because it takes a whole lot of time to get the core of a 4" thick piece of wood to 110 degrees... and you dont want to be doing that with electricity.
Either that or solar but here at least solar can be a bit problematic.

[GeneWengert-WoodDoc]

I would expect Drying time would be closer to 4 months.  Initially, the fans are run part time to avoid drying too fast.  Air drying is difficult as some days, even in GA, we have 30% RH which will damage thick oak in 30 minutes for sure.

I tested the VacuTherm kiln with Jim Parker (present owner's father) in the 1980s.  Awesome results.  The best financial results with a vacuum kiln are obtained when it runs 24/7.  This is becaUse the high capital costs are spread out over many BF of lumber.  Having KD 10/4 oak, etc. is also a gold mine, if you have customers.

[schwanee]

Ive also been looking into the iDry quite a bit and vacutherm told me that you cannot yet dry white oak in it. It is the same technology as the vacdry.

Also, I've tried drying post oak from green in a DH kiln and it didn't go well. Lots of honey combing and checking. From what I have read, it just doesn't go well when you try to rush post oak.

[GeneWengert-WoodDoc]

 The main commercial lumber species in the white oak group are white oak (Quercus alba), chestnut (Q. prinus), post (Q. stellata), overcup (Q. lyrata), swamp chestnut (Q. michauxii), bur (Q. macrocarpa), chinkapin (Q. muehlenbergii), and swamp white (Q. bicolor). The most important western oak species, Oregon white oak (Q. garryana).

The problem with white oak species, and other species like Osage orange and black locust, is that they have pores that are plugged (technical term is occluded).  The water cannot move easily within the wood.  Note that chestnut oak is not plugged like the other white oaks.

[mike_belben]

I have zero experience drying lumber, but i have spent many an hour building a few different waste vegetable oil decanting setups, and many more hours running them.  

 vegetable oil is kinda like wood in that it is hygrosopic.. No matter how long its been settled or how clean it looks, there are water molecules bonded to the oil particles. My 3rd and final setup processed a few thousand gallons of oil.  If i remember right, water boils at 70*f when under 28in (hg) of vacuum.  I heated the oil to 140-160 to thin it out for pumping through stages of filtration loop, and lower the necessary vaccuum, speed thinga up etc.   But it worked about 50x better than just air spraying type evaporation methods .. In a 50 gallon batch id pull about a soda can of pure water out from otherwise new looking and smelling, pre-settled waste oil.  Took about 2 hours, most of which was warmup time.  


Basically im saying vaccuum is an ultra potent tool for tearing water molecules away from the things they like to cling to.  Oil will never be freed of dissolved water by basic "air drying" to put it in perspective.