Author Topic: New Solar Kiln (Read 3763 times)

Hokiemill · « **on:** April 24, 2008, 10:11:57 pm »

After starting construction of my kiln a year ago, I finally got it up and running this week. Too many house renovation projects kept me away. It's pretty much the Tech design. I took the solar kiln class in Blacksburg last year.

I have no power on my miniature log lot, so I had to go all-solar with solar gable fans from Home Depot.

Here's the back side.

And here's my first trial load. Mostly oak with some cherry as well. None of it is great lumber (I let the logs sit for too long), but that makes them perfect for this first experimental run.

I still have some finish work to complete, but at least it is operational. I'm going to put together a web site that documents the construction process and covers anything I learn on the operation. I have a temp datalogger outside and a temp/humidity datalogger inside so I can get a good feel for how well it runs and I can include the charted data on the web site.

ironstumper · « **Reply #1 on:** April 24, 2008, 10:32:33 pm »

Good work.I hope mine looks that good when completed.Be sure to post the site when you get it up ok?

deeker · « **Reply #2 on:** April 25, 2008, 01:22:01 am »

Geat pics and story. Waiting for the web site stuff.......

As I am going to build a solar kiln asap. Or hopefully sooner. Keep up the great work!!!

Kevin Davis
Ruff Cutts

Sparty · « **Reply #3 on:** April 25, 2008, 09:13:35 am »

I did a double-take at that picture. At first I thought you put the exhaust vents on the front of the kiln $:-\$ . I thought "oh no, this isn't going to work!" Then I saw that they were your solar panels...guess I should put my glasses on before I checkthe FF. Great job, looks great.

JackLeg · « **Reply #4 on:** April 26, 2008, 08:19:01 pm »

Kiln looks good! I too have just finished one and plan to load the first charge this week. I attended the seminar at VT this past week. Those guys are good! I learned a ton. Now to apply it!

Good luck and keep us posted.

Peterrum · « **Reply #5 on:** April 27, 2008, 04:24:03 am »

Very impressive. I look forward to your site getting up and running as well. Good work.

ironstumper · « **Reply #6 on:** April 27, 2008, 05:06:38 am »

Looks like those vents have sliding covers....And the back doors? Hmmm Are they insulated?..Appreciate the phone number. I will give you a call. My hours are strange thou. Full time job nights. Part time stump grinding business. And a back yard full of logs waiting to be cut....Ah well....just love the smell of sawdust I guess.....

Hokiemill · « **Reply #7 on:** April 28, 2008, 10:12:53 am »

Yes the vents are sliding doors. I'm still undecided on whether that was a good idea. I thought the sliding doors would give me a little better control. A door with hinges at the top would be more difficult to prop open, but it would be better at keeping bad rain out. I have some left over polycarbonate that I plan on using for a small awning off the back to help protect the vent openings from the weather.

The doors are definitely insulated. Here's a picture after insulating, but before the vapor barrier and plywood went on:

The vent openings are just framed in, and the covers are all on the exterior.

JackLeg · « **Reply #8 on:** April 28, 2008, 07:22:16 pm »

According to Dr. Brian Bond, at last week's seminar at VT, when drying oak, he NEVER opens the vents. I asked him what, if anything, he'd do differently. He said the only change he'd make would be to double glaze the kiln by putting a layer of Mylar film under the "rafters" to create an insulated space and prevent more heat loss.

Looks like you're ready to go. I hope to hang my fans tomorrow and load her up on Wednesday with 7/4 Cypress.

Hokiemill · « **Reply #9 on:** April 28, 2008, 08:04:08 pm »

Yeah, I can definitely believe that the vents need to be closed with oak (and with many other woods as well). The first day I had it loaded was a pretty sunny day and the temp got up into the upper 70s/low 80s. I had all the vents all the way open (by accident really) and by mid-day the temp in the kiln was 102F and the humidity was at 34%! The last several days have been all rain around here, but tomorrow I'm heading over there to completely shut down all the vents.

The second layer of glazing may be an addition in the future. I tried to make a couple minor improvements over the kiln that Brian Bond has to increase the heat generation. I used black painted corrugated metal for the "collector" and they are sized to completely fill the horizontal space from one end to the other (the improvement comes from the better fitting panels, not that they are metal). I also used a black tarp as the baffle whereas Bond had a blue tarp hanging. I'm not sure if those minor difference will make much of an improvement but I'm hoping so. Looking into the kiln from the glazing side, there is nothing but black and three fans - lots of collection surface.

Joel Eisner · « **Reply #10 on:** April 29, 2008, 07:05:55 am »

Here is my alteration:

I placed a horizontal baffel from the back wall to the top of the stack. This is used for pine and seems to decrease the drying time by helping minimize the cycling of damp air so dry comes in the top, gets heated, goes through the stack, picks up moisture and exits out the back bottom vents.

JackLeg · « **Reply #11 on:** April 29, 2008, 08:42:12 pm »

I like the idea of the extra baffle. My kiln has rear doors. What do you rest the back of the baffle on? Is it full length of your kiln box?

Chet · « **Reply #12 on:** April 29, 2008, 09:48:10 pm »

When I built mine a few years ago I put in da same extra baffle. I used black plastic and just attached it to the back wall. I use it when drying pine and cedar.

JackLeg · « **Reply #13 on:** April 30, 2008, 10:04:42 pm »

Your kiln load from the end?

Hokiemill · « **Reply #14 on:** May 01, 2008, 10:45:48 am »

Let me see if I can walk through this logic on the baffle. Relative humidity is the real drying force (as long as the air is moving) and added heat just serves to decrease the rh (and kill bugs if you get it high enough). With the baffle in place we're trading off the loss of heat (venting directly vs. recirculating some of the warm air) for the increased loss of moisture and hence lower rh. The collectors work well enough to heat the air that recirculation isn't needed to keep temps up/rh down. This is enhanced by the fact that no moisture is being recirculated. In other words, the loss of moisture outweighs the loss of heat when it comes to the rh.

So, for hardwoods, maybe the baffle can go in place after the wood reaches a moisture content less than 20 or 30%?

Yesterday I checked the kiln and outside temps were around 60F, it was partly sunny, all vents closed; the interior temp was at 105% (on the floor entering the stack) and the rh was still pretty low at 45%. I'm hoping the nighttime conditioning will help offset the too low rh considering the wood is still around 40% moisture content and conventional drying schedules say the rh should be in the 70s and 80s. We'll see.

Jackleg, my kiln has the rear doors and the bottom of my fan deck is about the same height as the top of the doors, so the baffle would be an easy attachment from fandeck to framing above the doors. That would still allow opening of the doors and access to the wood stack. It also helps that I'm a short guy and wouldn't have any problems with accidentally poking my head through the baffle

Joel Eisner · « **Reply #15 on:** May 01, 2008, 03:56:28 pm »

My baffle is also black 6 mil plastic that is nailed to the back wall using a long 1x1 and then a 1x4 is stapled to it and that is secured to the top of the pile.

JackLeg · « **Reply #16 on:** May 02, 2008, 10:19:36 pm »

Thanks, guys. May give that baffle a shot next charge. Joel, is that Chatam County NC or VA?

Hokiemill · « **Reply #17 on:** May 29, 2008, 10:50:18 pm »

Noticed my drying rate has slowed down quite a bit recently. I'm below 30%. I've shut all the vents to just a crack to raise the temps and hopefully get more of that bound water out. Jackleg, how's your kiln charge coming along?

WDH · « **Reply #18 on:** May 29, 2008, 11:26:08 pm »

Hokiemill,

You have been drying that load for over four weeks, so it does not seem that you are drying the wood too fast if you are at 30% moisture.

How much does the kiln hold?

Hokiemill · « **Reply #19 on:** May 30, 2008, 06:54:53 am »

My kiln sample is currently at 20.1 % mc although I think other parts of the load are a bit higher (most of the charge is quarter and riftsawn red oak). We've had a bit of rainy/overcast weather, but I think most of the slowdown is my fault. About two weeks ago I opened up the vents. I think I should have left them nearly closed to keep the temps way up. I closed them back to almost shut yesterday.

If it continues drying at a slower than normal rate it may be that my solar fans just don't move enough air. It's all a learning experience for me. When it's all complete I'll try to chart the temp/humidity vs. the moisture loss on my newly completed website about the kiln (www.solarkilninfo.com).

The kiln has about 480bdft of 4/4 in it. I could go up another layer, maybe two and get the capacity over 500 bdft of 8 ft boards. It will hold 10 ft boards and the capacity goes up over 600 bdft.

Larry · « **Reply #20 on:** May 30, 2008, 08:28:48 am »

I did some trials with fan speed to see if I could improve the drying rate in my solar kiln. My fan was belt driven so it was easy to alter the speed. What I found is drying speed increases as air speed increases until a certain point is reached. After that point is reached higher air speeds do nothing. I never took the time to measure air flow. My test results were just by moisture loss each day in my samples.

Hokiemill · « **Reply #21 on:** May 30, 2008, 07:37:34 pm »

After only one day it is clear that my vent position was the problem. I closed the vents to only slightly open and a day later the sample has dropped almost 1 1/2% moisture content. Learning, learning, learning.

WDH · « **Reply #22 on:** May 30, 2008, 11:58:56 pm »

Man, I need one of those. Figure it all out good so that I can learn from you.....

tnpete · « **Reply #23 on:** June 09, 2008, 01:50:16 pm »

Hello I like your solar kiln.
I am trying to decide on one to build. Would like to be able to have the whole back be doors like yours. That way I can take the loader and sit a load of lumber that I would have stacked in the shed air drying. Into the kiln and not have to stack it again.
Think that would work?
Any help anyone can give me toward getting started would be great .
Pete

Radar67 · « **Reply #24 on:** June 09, 2008, 02:08:32 pm »

Welcome to the forum TNPete. The search feature above will yeild a lot of information on solar kilns. Here is a search I just did...

Use solar for the search string and use Pineywoods for the member. This will gather all the posts by member Pineywoods dealing with solar. He has a lot of info about kilns on here.

Where are you located?

Hokiemill · « **Reply #25 on:** June 09, 2008, 10:07:48 pm »

Pete, welcome to the forum! Those double back doors are perfect for what you want to do.... up to a point. I think it depends on how long your boards will be. If they get too long then hanging those doors may become a problem. I believe that's why the Virginia Tech 2000 bdft kiln uses an end door - more suited to kiln cart and tracks. For 12' boards or shorter, the back doors will make your loading and unloading easy with the loader.

JackLeg · « **Reply #26 on:** June 15, 2008, 02:31:22 pm »

Quote from: tnpete on June 09, 2008, 01:50:16 pm

Hello I like your solar kiln.
I am trying to decide on one to build. Would like to be able to have the whole back be doors like yours. That way I can take the loader and sit a load of lumber that I would have stacked in the shed air drying. Into the kiln and not have to stack it again.
Think that would work?
Any help anyone can give me toward getting started would be great .
Pete

Pete: I built mine with rear doors (like the new VT plan)and I am really pleased with it. They doors are heavy and do sag some, but, since they are closed most of the time, it hasn't been a problem.

I just took out my 2nd charge of cypress lumber. It was 1" boards, 8' long. It dried to <20% in 10 days of sunshine, which is fine for what we do with it.

I have a simple Radio Shack Temp/'RH meter in it. Highest temp I have seen is 136F. The kiln is doing a great job of reducing our drying times. Cypress is very forgiving and loses its moisture quickly. Oak is another matter.

Hokiemill · « **Reply #27 on:** July 01, 2008, 08:24:05 am »

My first load is now officially complete. The sample board weighs out to 7% moisture content. The total time to dry from about 48% to 7% was 59 days but a couple factors go into why it took so long. The main reason is that I was experimenting with the vents and opened them all the way after the load reached about 30% mc. I thought the open vents would allow more moisture to exit, keep the rh down and allow for faster drying. Not entirely true. After noticing the slowdown in drying, I shut the vents to only slightly open and the drying rate increased again. So having higher temps is better than having greater venting in this case. With the vents almost completely closed, my maximum temp was 148f.

The other factor that I feel slowed the drying rate was my lack of end baffles. There was plenty of hot air bypassing my stack by going around the ends. The next load will include some sort of end baffles but I don't know exactly what to use (a new thread for that question).

The final factor in the long time to dry is that most of the lumber was riftsawn and quarter sawn oak which I understand dries slower than plain sawn.

I'm currently searching for logs so I can mill up my next load. Once the second load is complete, I'll drop in with another report.

WDH · « **Reply #28 on:** July 01, 2008, 11:00:47 pm »

Very interesting. I await the second load.

pineywoods · « **Reply #29 on:** July 02, 2008, 09:59:21 am »

Hokie, looks like you have come to the same conclusion I did about the venting. Openin the vents to exhaust hot humid air just replaces that air with cool wet air. I finally closed off all vents and stuck a small room de-humidifier in there to take out the moisture. Made a huge difference.

News:

Author Topic: New Solar Kiln (Read 3763 times)