Problems with solar kiln start-up with very wet oak

Hello,

I have just finished my solar kiln and have dried a fist load of 6/4 white oak from 16.5 to 12 percent in 6 days without any problem.

But now i loaded the kiln with very wet 6/4 and 8/4 oak (white - upland) , problems arise...

The lumber is very wet because the logs were sprayed with water for conservation at the sawmill. Oven-dry test shows 83 to 90% MC. The wettest lumber is also the thickest (8/4) and is on the bottom.

I cut three kiln samples : 2 quarter sawn on the north side, lower part of the stack, one flat sawn in the top part, south.

The flat sawn kiln sample on the south side dries well. It is hard to keep it from drying to fast : I covered the entire collector with a white sheet.

But during the first 3 days the surface of the kiln samples on the north side became soaking wet.

The only thing i can think of is condensation, because the RH being around 85% and temp 30 to 33°C, the dew point is just a few degrees lower. The wood was certainly not that warm in the beginning. That said, the south kiln sample always looked good.

The moisture loss of the south sample is around 1,2% per day wile the southern samples lose like 0,4%.

I thought i had to improve air velocity for more homogeneous drying. So today, I cranked up the fan speed (frequency modulator) and partialy closed up all bolster spaces.

The result was that the nothern kiln samples look dry at the end of the day but the dry rate is :
1,7 on the south (flatsawn), and 0,5 on the North (quartersawn).

So this was no solution.

My questions are :

Is it normal to have this great difference between quarter sawn and flat sawn lumber,

and what about the difference in dry rate between the stack sides ? My stack is 4 ft wide.

Also, At 85% RH, 30°C, i turn on the fan just a few hours, and the dry rate is already to high... How is it possible to run a conventional kiln 24/7 and respect safe dry rate of 1% per day ?

Did anyone notice moisture drying from the warmer lumber, condensing on the colder boards ?

Any suggestions how to successfully dry this precious and very wet load ?

Frans

GeneWengert-WoodDoc

Is it normal to have this great difference between quarter sawn and flat sawn lumber...NO. Only about 15% difference in speed.

and what about the difference in dry rate between the stack sides ? My stack is 4 ft wide. VERY UNUSUAL, and so you have some air flow problems. What design did you use?

Also, At 85% RH, 30°C, i turn on the fan just a few hours, and the dry rate is already to high... How is it possible to run a conventional kiln 24/7 and respect safe dry rate of 1% per day ? I suspect RH is incorrect measurement.

Did anyone notice moisture drying from the warmer lumber, condensing on the colder boards ? NO. Something else is happening.

Any suggestions how to successfully dry this precious and very wet load ? A solar kiln based on my design will dry 4/4 oak but for thicker, the collector is too large.

You do not mention vent opening. Normally vents are closed the first week for oak. Also, your drying of the previous load seems off. Maybe the vents were open too far or the fan was running too short or too long.

Frans

Member

Thank you for your response.

you have some air flow problems. What design did you use?
My design is based on your design, with however a fixed absorber plate. I made this modification to be able to use a kiln truck, and to be able to go inside the kiln both sides. See photograph

I installed manual vents as well as a simple automated system with an extractor fan coupled with a hygrostat. This system seemed to work very well for the first load.

I suspect RH is incorrect measurement.

I have checked the RH of my hygrostat and hygrometer before with a wet bulb. I'll check it again tomorrow.

A solar kiln based on my design will dry 4/4 oak but for thicker, the collector is too large.
To ratio should be ok and I covered the entire collector with a white plastic screen 2 1/2 inch above the collector to vent heat...

You do not mention vent opening. Normally vents are closed the first week for oak.
For this wet load i didn't vent more than opening the door to measure and inspect kiln samples.

On the photograph you can see how the stack is loaded. There are quite some bolster openings which i closed with 6/4 pieces with 3/4 sticker beneath, just like there were a board. With very thin polythene i tried to check air flow on the north side. I baffled on the south to obtain something even.

Today i inversed the rotation of the fan. It is not efficient to have it turn in inverse direction, but just a try...

I use one big three phase industrial fan (some one gave it to me). Initially i feared it would not spread, but as the air flow bumps in to the collector, it spreads very well. I checked this with a simple anemometer, and polythene sheets.

I don't really understand why my first load seems off. I measured with professional pinn and pinn-less meters. The kiln was vented above 50% RH initially and at 40% the last few days. It gradually reached a max of 53°C. The wood and kiln stayed quite warm over night (I have the temperature sensor of the pinn meter in the wood.) And even during one cloudy day.

This morning again something special : The sample that lost 0.5% during the day, lost 0.9% during the night wile fan was off. RH meter shows 85 to 90%, lots of condensate on the collector and water on the floor... temp drop about 8 °C overnight in the kiln, 15°C drop exterior.

Voila...

Frans

View higher quality, full size image (2285 X 1714)

GeneWengert-WoodDoc

All moisture meters do not work above 30% MC. Readings are totally inaccurate.

Vents are kept closed the first week...maybe cracked open a tiny amount at the end of the week. I do not understand why you vent at 40% RH. If you have automatic vents, vent at about 87%RH, not lower.

Based on your comments, a short course in drying would really be helpful for you as it seems that there are many points of uncertainty.

Frans

Member

Mr Wengert :

I'am sorry if i am not clear when i write.
When i said :
I don't really understand why my first load seems off. I measured with professional pinn and pinn-less meters. The kiln was vented above 50% RH initially and at 40% the last few days. It gradually reached a max of 53°C.

I was talking about the FIRST load. The load of air dried oak which i dried from 16.5 to 12 %... and to me everything seems fine

The actual load is my second load and very wet. I only work with kiln samples for the moment. I've read a lot of info, am reading "drying of hardwood lumber" now.
By the way, i am interested in your book about drying oak lumber, but i don't know how i can obtain the book in France. ?? e-book would be fine.

Well, i might have found the solution to the weird kiln behavior.

Today i inversed the Fan. It makes a lot less noise. When i checked with my anemometer what kind of wind speed i got at a given place, i was surprised : much more !

I'm confused, i think the fan was mounted in the wrong way !

I can now clearly feel a gentle airflow between the planks wile first i had some doubts.

I weighted the samples... North : -0.5 and South : -0.4

Here is a photograph of a fan blade, could anyone tell if this is the suction or the blow side ?

Thanks,

Frans

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GeneWengert-WoodDoc

Counter clockwise will blow away, while cw will blow toward.

I now understand your use of RH and moisture meters.

Frans

Member

To make sure i understand well :

The fan grille you see on the photograph is the North side

We agree the fan should blow towards the collector (south) away from the grille.

So does this mean ,given the fan blade shape, that the fan is mounted back to front ?

As it is a three phase fan, it can turn either left or right.

Thanks,

Frans

Kelvin

Hi
I've found it to be very difficult to dry any oak let alone thick stock from green in either my solar or conventional kiln. I switched to always air drying to at least 20%mc first. This allows other wood to cycle through the kiln as air drying is about as fast as oak should dry anyhow 2% per day max. A load of honey combed oak is all it takes to loose a lot of $$$. In fact here in the summer oak air dries too fast so we wrap in landscape fabric to slow it down. From 80% mc to 7% mc is a long time in the kiln. As gene has mentioned most damage happens to green lumber above 40%mc. It's pretty safe to dry by the time it's 20% mc. Makes my life easier!
Kp

Frans

Member

Hi Kelvin,

Thank you for these wise words.
Seems we have to learn the hard way !
Landscape fabric : is that what is also called geotextile ?

Things are going better now since I reverse the rotation of the fan every hour. This really makes the difference ! I really think it's because the load is so soaking wet. I measured 6 to max 8 percent RH difference between one side and the other of the stack.

I build a solar kiln because they are said to be able to dry green lumber. As the air drying practices in the sawmills here are quite far from perfect, it thought i could get better lumber if i solar dried it my self green from the saw.

But i think Kelvin is right about air drying :
If you ad strain on the kiln equipment (acid vapor , condensate) ; energy and risk of serious damage to the wood, all due to the fact the lumber is green, i wonder if it all pays for the quality loss due to air drying.

This sayed i should not have had his lumber sawn during the hottest part of the year. And air drying should be well "conducted" too.

I'll post my further experiences and the outcome of this load here, for if it could be helpful to anyone else.

Frans

GeneWengert-WoodDoc

If air drying can be done correctly, then quality losses will be very small, so,AD followed by KD is the best financial decision.

Lawrence Roe

Hi I saw and dry as a hobby ,but heating and air conditioning is what I do. The fan blade pictured is not a good picture but the correct design airflow direction is to the grill pictured. also this type blade is usualy used with a specific Venture that was engenired for the blade the shape and tollerances and blade position is important.

Scott Smith

[X] View Member Profile Name: Scott Smith Member

Frans, from the photo's it appears that the spacing between your boards varies from level to level.

Since air will take the path of least resistance, it is important to baffle the opening so that you're forcing all of the airflow through the stacks. I think that your problem may be related to inconsistent airflor through your lumber, since a wet surface on some lumber and dry surfaces on others indicates inadequate air flow where it's wet.

It appears that the distance between the stack and the front of the kiln (plenum area) is adequate; how much space exists between the back of the stack and the back of the kiln? If it is too tight, this too would hinder airflow through the rear plenum down to the lower portions of the stack, and it would be exacerbated by the larger openings between the boards higher up in the stack.

frans

Member

Thanks guys for your help !

Laurence , I'm sorry for this bad photograph, but it was difficult to get a better view as the kiln is loaded ! The fan was used before to cool a cooling radiator. I do not know if there was an other venture. I've never seen it in its original installation. I reverse the rotation every hour and that really makes the difference, even if the fan does not move as much air when it turns in the wrong way. The fan blades are made by Multi-wing. They also make symmetric blades able to give good airflow in both directions. But as long as this works...

Scott,
After having my problems, I partially closed off all the bolster spaces so that there's only 2 times the sticker space left.

But maybe you talk about the spaces between the slaps. Those are indeed very irregular, and quite bigger near the top. Almost all oak wood is sawn in slaps here (to avoid loss from ripping), so this will almost always be the case. Do you think it's necessary to close of the space between the slaps at,lets say, each row of bolsters ? i haven't seen this done anywhere.

Frans.