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Lots of defects after solar kiln drying...

Started by Whitetail_Addict, September 13, 2012, 02:33:44 PM

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Whitetail_Addict

I just pulled a load of 1x6 Red Maple out of the solar kiln (based on VT design).  The top half of the stack had all sorts of bowing, twisting, cupping, etc.  The bottom half of the stack wasn't too bad - I saw a small amount of defect much more like what I would expect.  Temperatures were in the 140s - 150s during the day and 70 - 90 at night.  I had a few hundred pounds of steel and concrete block on top of the stack to weigh it down.  Not enough weight? Too much air movement in the upper half of the stack?  Any ideas why I saw the increased defects?  All of the lumber in the stack had been air drying for a few weeks before going in the kiln.   ???
2010 WoodMizer LT28, John Deere 4520 w/ FEL, 2003 Chevy Silverado 2500HD Duramax Diesel, 2007 Polaris Ranger XP 700, 127 Acres of Northeast hardwoods in New York's Whitetail country

pineywoods

I believe the major part of the problem is in the kiln design. The top of the stack gets hotter than the lower part. I solved that in my kiln by attaching black sheet metal to the bottom of the rafters. Sunshine doesn't touch the lumber. Air blown over the hot sheet metal and then through the stack results in even drying. MY solar kiln has no vents. A small cheap room de-humidifier removes the moisture from the air. Warm dry air circulates through the stack of lumber.
1995 Wood Mizer LT 40, Liquid cooled kawasaki,homebuilt hydraulics. Homebuilt solar dry kiln.  Woodmaster 718 planner, Kubota M4700 with homemade forks and winch, stihl  028, 029, Ms390
100k bd ft club.Charter member of The Grumpy old Men

jimF

Not knowing the exact stacking arrangement, along with the sun exposure, check out the uniformity of airflow through the height of the stack.  If the top of the board experiences a higher velocity than the bottom of the board; cupping, bowing and twisting is just about certain to happen.  I would estimate that cupping, bowing and twisting is the second most costly set of defects in the industry and is caused by differential air flow over the board.  This usually occurs on the top of the stack.  A great deal of research has gone into trying to avoid such defects by trying excessive top weights, banding, springs and more.  All failing to eliminate such defects.

tyb525

the VT design includes a feature like pineywoods described. Sounds like you might have left it out.
LT10G10, Stihl 038 Magnum, many woodworking tools. Currently a farm service applicator, trying to find time to saw!

Whitetail_Addict

You mean the collector? I have a piece of 4 x 10 black sheet metal on top of the stack for that. Of course the rest of the interior is painted flat black. I have two 8" fans mounted in the top of the kiln blowing parallel with the 45 degree sloped roof. The vents in the top are blocked off. I have two 6" fans blowing air out of the kiln through vents on the bottom. Attempting to blow the hot air down the inside of the sloped roof and then pull it through the stack and out the vents with the bottom two fans. I actually do have a dehumidifier set up with a hose run out the rear for the moisture to drip out. It typically won't drip anything during the day when it's hot in there. Lots of water at night though.
Pinewoods I like your idea of mounting the collector on the inside of the rafters. It actually makes more sense to have it there than just flat on top of the lumber stack.
2010 WoodMizer LT28, John Deere 4520 w/ FEL, 2003 Chevy Silverado 2500HD Duramax Diesel, 2007 Polaris Ranger XP 700, 127 Acres of Northeast hardwoods in New York's Whitetail country

pineywoods

White_tail, mounting the collector on the bottom of the rafters buys you 2 things. 50% more collector area that is perpendicular to the sun, makes a bunch more heat, leaves room for weights on top of the stack.
You get quite a bit of convective airflow up the channel between the collector and the glazing. Set up the fans to blow the hot air downward and through the stack. Another hint... Install 2 thermometers, 1 up near the top, another in the airstream exiting the stack. The difference in the 2 readings is in direct proportion to the dryness of the lumber in the stack. When the 2 readings are nearly the same, boards will be VERY dry. Functions as a cheap moisture meter that measures the whole stack rather than just 1 board.
I don't vent any air outside. If you do, it has to be replaced with humid outside air, thus limiting how dry you can get the lumber. We run 3 of the pineywoods kilns locally, and have no trouble drying lumber down to 6% with very little defect.
1995 Wood Mizer LT 40, Liquid cooled kawasaki,homebuilt hydraulics. Homebuilt solar dry kiln.  Woodmaster 718 planner, Kubota M4700 with homemade forks and winch, stihl  028, 029, Ms390
100k bd ft club.Charter member of The Grumpy old Men

Den Socling

Warp is weird. I can warp 12/4 wood in a vacuum kiln with no air flow.

pineywoods

Lest I give the impression that my solar drying is defect free, let me clarify::
I usually air dry first, then kiln. That culls out the really bad boards. I agree with Den, some pieces of wood are going to warp, bow, cup, twist, regardless of what you do. They make good fire wood.
On venting, be aware that I live in the deep south where humidity levels routinely hit 85-90 percent. Filling a kiln enclosure with that kind of stuff can actually, worst case, increase the moisture level in a stack of lumber. Therefore, I keep the same batch of air in the enclosure and depend on the dh unit to pull out the moisture. Works for me, your mileage may vary...
1995 Wood Mizer LT 40, Liquid cooled kawasaki,homebuilt hydraulics. Homebuilt solar dry kiln.  Woodmaster 718 planner, Kubota M4700 with homemade forks and winch, stihl  028, 029, Ms390
100k bd ft club.Charter member of The Grumpy old Men

Tree Feller

Quote from: Whitetail_Addict on September 15, 2012, 09:41:24 PM
I actually do have a dehumidifier set up with a hose run out the rear for the moisture to drip out. It typically won't drip anything during the day when it's hot in there. Lots of water at night though.

That could be the problem. The VT kiln is designed to have the fans shut off at night so that the RH inside can go up, thus conditioning the lumber and relieving the drying stresses introduced during the day. If you are running a dehumidifier at night, you are negating that very important feature. Even if running the DH didn't cause the lumber to warp, it did not allow the nightime moisture to relieve the case hardening.

The fans and DH should shut down about an hour after sunset and come back on around 10:00 a.m. Keeping the vents (both upper and lower) open only an inch or so will also slow down the drying rate. Onced the lumber is dry, close the vents entirely and try to get the temperature high enough to sterilize the lumber...very difficult with a solar kiln.

Are you monitoring the drying rate via remote leads to a moisture meter or by measuring the water coming from the DH?
Cody

Logmaster LM-1 Sawmill
Kioti CK 30 w/ FEL
Stihl MS-290 Chainsaw
48" Logrite Cant Hook
Well equipped, serious, woodworking shop

Ianab

I think you are on to something  TreeFeller.

It's that intermittent drying and re-conditioning cycle that lets the solar kiln work without the fancy controls that a normal kiln needs. Even if conditions are a bit hot and dry for a time during the day, the night cool off and higher humidity counters this, and the wood dries with little degrade.

You should be loosing moisture by venting during the day. The wood should be drying during the day, but the DH is probably not removing water due to the high temp in there. Then comes night and the DH dries the kiln out again, and you don't get that re-conditioning

With a Solar / DH hybrid, things aren't going to work the same. I'd suggest you go back to a venting design. It doesn't matter if you are in a humid climate, heating the air lowers it's R/H, so it can still carry water away from the wood, and it gets vented. Even if you are in a swamp with 95% R/H the system will still work.

Ian
Weekend warrior, Peterson JP test pilot, Dolmar 7900 and Stihl MS310 saws and  the usual collection of power tools :)

Okrafarmer

He that dwelleth in the secret place of the most High shall abide under the shadow of the Almighty. Psalm 91:1

Operating a 2020 Woodmizer LT35 hydraulic for Upcountry Sawmill, Dacusville, SC

Now selling Logrite tools!

Writing fiction and nonfiction! Check my website.

jdonovan

Quote from: Whitetail_Addict on September 13, 2012, 02:33:44 PM
Not enough weight? Too much air movement in the upper half of the stack? 

Not even close. If you want to weight the stacks to control movement you need to be thinking in the multiple tons range. And even then you can get movement. I've seen piles with 3-4 tons of stack weight above the warped board.

For practical purposes the small solar/air dry operation can't effectively weight stacks enough for it to make a difference.

QuoteAny ideas why I saw the increased defects?
You said you did some air dry first...

What was your starting moisture content after air dry?
Charge volume?
Solar collector area?
Drying rate per day?

The VA tech design is setup to be reasonably hard to dry too fast... when people lightly load, or add dehumidifiers the balance can get disrupted and all sorts of defects can be created.

If you alter the design you've got to be MUCH more active in monitoring the kiln to make sure its operating properly.

Den Socling

I think you need to have around 400 lbs per square foot to keep 4/4 flat. 8/4 and 12/4 need much more.

jimF

The solar kiln system has been promoted by the designed advantage of nightly "conditioning".  In reality, it is only an unavoidable non-design attribute.  And it is not actually an advantage.  It promotes re-wetting of the surface which has become dry which promotes large surface checks and bottleneck checks especially if it is a species with large rays.  There are other issues with nightly high RH.
A chapter is devoted to this topic, the benefits and problems of each drying system, in the book recently announced in another thread.  Each system has it's use, but a person must be aware the systems attributes to successfully dry lumber in it.

Tree Feller

Quote from: jimF on September 22, 2012, 05:52:06 PM
The solar kiln system has been promoted by the designed advantage of nightly "conditioning".  In reality, it is only an unavoidable non-design attribute.  And it is not actually an advantage. 

I disagree with that statement and so do Joseph Denig, Eugene M. Wengert and William T. Simpson in the USDA Forest Service Publication "Drying Hardwood Lumber."  All three of those gentlemen mentioned are well known and respected experts in the science of drying hardwood lumber.

Page 118, of the above-mentioned publication, second paragraph under "Drying Process" says:

"At night, as the dryer cools and the humidity rises, water may condense on the walls of the kiln. This is an essential part of the drying process; it relieves stresses in the wood that develop during the day as the wood dried. For this reason, do not run the fans at night. Furthermore, do not use auxiliary heat and do not store solar heat during the day to be used at night unless a water spray or steam system is available for relieving stresses."

The best proof of how well a greenhouse-type solar kiln works is the documented success of drying hardwoods in hundreds of them in operation. The only negative I have been able to find is the drying time involved...6 weeks of good drying weather for 4/4 Red Oak.
Cody

Logmaster LM-1 Sawmill
Kioti CK 30 w/ FEL
Stihl MS-290 Chainsaw
48" Logrite Cant Hook
Well equipped, serious, woodworking shop

Okrafarmer

And the renowned engineer Pineywoods uses his solar kiln in conjunction with  :-X a dehumidifier to speed the process up, reportedly with few complaints. . . .
He that dwelleth in the secret place of the most High shall abide under the shadow of the Almighty. Psalm 91:1

Operating a 2020 Woodmizer LT35 hydraulic for Upcountry Sawmill, Dacusville, SC

Now selling Logrite tools!

Writing fiction and nonfiction! Check my website.

Ianab

A lot is going to depend what species you are drying.

Some a pretty forgiving and you can pretty much dry as fast as your kiln is able to suck the water out. Other species, not so good.

If you try and dry White Oak in a Pine schedule, bad things will happen. But in the solar kiln design, with it's stop start and reconditioning you can dry the oak, even though the heat and humidity are not closely controlled. But using a solar in the day, DH at night, it may be possible to push the drying too fast, and lead to the problems in the original post.

A more forgiving species, it might work fine, and give you a faster drying time.

Ian
Weekend warrior, Peterson JP test pilot, Dolmar 7900 and Stihl MS310 saws and  the usual collection of power tools :)

jimF

I don't want to hawk my book, but in it moisture movement and drying stresses are thoroughly and methodically discussed.  In one chapter how this knowledge relates to various types of drying systems is discussed.  This knowledge was obtained from the years of research I performed at the USDA Forest Products Lab when I was down the hall from Bill (William Simpson).

As for designing the area of the collector, by looking at the VT schematic, you can see that the length of the collector is the full length of the building and the lumber is stacked to fill up the chamber.  The only way to change the area is to change the slope which is confined to a perpendicular orientation to the sun to optimize the amount of radiation received.  This just happens to coincide with slow drying of oak.  Other species are best dried faster to avoid stain and mold.  If you can avoid these two defects, other species will dry fine.  Bill and John Tschernitz designed a solar kiln for use in the tropics which had the collected detached from the kiln and laid out on the ground and was able with this design to increase the collector area. 

Tree Feller

Yes, the VT kiln was designed specifically to dry Red Oak  safely. That's why it's important to adhere to the effective area of the solar panels. As someone else stated, if the kiln is not filled to capacity, the drying rate can be too fast for Oak unless the panels are partially covered to reduce the solar gain.

I'm more concerned with your statement that the nighttime conditioning (wetting in your words) is detrimental to the lumber. In the VT kiln, the top of the charge is covered and the wood sees neither sunlight nor any possible condensation from nighttime cooling. I don't see how the absorption of water vapor in a solar kiln is any different from the steam conditioning used in commercial dry kilns.

I would like to learn, though, if you can put it in layman's terms here.

BTW, I intend to purchase your book as I am always eager to learn more about the science (and art) of drying lumber. To me, it's part of the total package of being a woodworker who takes wood from tree to furniture.
Cody

Logmaster LM-1 Sawmill
Kioti CK 30 w/ FEL
Stihl MS-290 Chainsaw
48" Logrite Cant Hook
Well equipped, serious, woodworking shop

Den Socling

As I have seen Gene Wengert write repeatedly, if you rewet dry wood you are looking for trouble. Wood will  check at the rays. The rewetted wood will swell. Checks will be torn deeper and/or bottlenecks will develop. This is simply a fact. But in these solar kilns, the shell is not bone dry when you first start drying. This is the time you will have the most condensation at night. By the time the shell is bone dry, where would condensation come from?

I think this type of discussion is great. As I told Jim, I hope to make his book a long, long discussion. Gene Wengert wants me to do a favor for him. I think I'll see if he will read Jim's book and join in discussion. WoodWeb is sooooo boring.  :D

pineywoods

Hey, look at this. Real experts confirming what I learned by trial and error and just dumb luck...Getting rid of the moisture was a real head scratcher. We tried a whole bunch of what looked like bright ideas, but just didn't work. I have seen it actually rain in my kiln during the trial=and=error process. The one idea that did work reasonably well was putting a small cheap room de-humidifier inside the enclosure with a hose running outside, and closing off all vents. Now to add fuel to the discussion, the dh units have an adjustable humidity cutoff and a high temp shutoff that shuts it off during the day. Don't really know what those setting are, maybe dumb luck again ? Granted the acid from oak will sooner or later destroy the dh. Planman1954 bought 2 of them at a flea market for 10 bucks, for his kiln. just replace them.
1995 Wood Mizer LT 40, Liquid cooled kawasaki,homebuilt hydraulics. Homebuilt solar dry kiln.  Woodmaster 718 planner, Kubota M4700 with homemade forks and winch, stihl  028, 029, Ms390
100k bd ft club.Charter member of The Grumpy old Men

Ianab

Wetting the wood is probably a bad thing, but I don't think that will occur in a solar kiln design. There is only a limited amount of water left in the air inside the kiln. This can condense out as the temp drops, but this should occur on the walls of the kiln that are cooling off. The wood is the warmest part of the system at this stage, and should be above the dew point?

What it does do is pause the drying for a while. This allows time for more moisture to migrate from the core of the wood to the surface, and reduces that gradient between the core and surface that's going to cause checking or case hardening. Next day the system heats up and removes some more water from the surface. Rinse and repeat each day until the wood is dry.

I'm sure the design and operation can be adjusted for different species, higher temps, more venting, adding a DH unit etc to get faster drying with different species.

Ian
Weekend warrior, Peterson JP test pilot, Dolmar 7900 and Stihl MS310 saws and  the usual collection of power tools :)

Tree Feller

Quote from: Ianab on September 23, 2012, 04:04:42 PM
What it does do is pause the drying for a while. This allows time for more moisture to migrate from the core of the wood to the surface, and reduces that gradient between the core and surface that's going to cause checking or case hardening.

Now, that makes a lot of sense and explains how the wood can be stress-relieved without incurring the surface checks that happen when dry wood is re-wetted.

I'd still like to know how wood can be conditioned with steam without  the re-wetting causing checks. Is it because the wood is not dried any further after the conditioning stage?

This is one of the more interesting conversations I've been involved in related to wood.
Cody

Logmaster LM-1 Sawmill
Kioti CK 30 w/ FEL
Stihl MS-290 Chainsaw
48" Logrite Cant Hook
Well equipped, serious, woodworking shop

jimF

Ianab, yes initially the stack of wood is the warmest point in the kiln in the early evening and the vapor will condense on the walls early.  But there comes a point at night that the RH becomes higher than the wood can resist the absorption.  And in the morning, the sun starts to warm the air up drying up the puddles, increasing the temperature and RH when the wood is the coolest point in the kiln.  With the high RH, the cool wood can gain the moisture it gave off and swell. As Den cites Gene W., the rewetting will cause surface checks to progress further into the board.  This does not occur just once but repeated every 24 hrs for how many weeks?  Little by little the surface checks grow.  Drying stresses are not the only concern during drying.  A dry stiff strong shell is what allows the board to resist cupping, twisting or bowing.  If you rewet the shell it becomes weaker and more pliable, promoting these defects.  Also, is this species prone to stain?

Okrafarmer

So then, it is good to have the dehumidifier in there, to reduce this humidity.
He that dwelleth in the secret place of the most High shall abide under the shadow of the Almighty. Psalm 91:1

Operating a 2020 Woodmizer LT35 hydraulic for Upcountry Sawmill, Dacusville, SC

Now selling Logrite tools!

Writing fiction and nonfiction! Check my website.

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