Solar Kiln Vent Questions

[Joel Eisner]

Background:
I built the Wood magazine solar kiln but increased its size from 8x12 to 8x14 to fit the 13ft boards that I cut on my Lumbermate 2000.  The fans are attic ventilator fans and I used two of them.  They came with a thermostat and I wired them together.  I have 2 bottom (facing out), back wall vents and two top ones (facing in).  They are 20" vents that close when the fans are off. 

When I loaded the kiln and put the fan baffle down to the wood pile the air circulates inside the kiln and does not open the vents.  To get the mosit air out I would like to have the vents open.  I placed a second horizontal baffle from the top of the pile to the back wall to block the flow of air that was short circuiting the vents.  Now the vents open.

Questions:
I am able to reach upwards of 120+ deg F on top of my pile on a 55 to 60 deg F sunny day.  Is that were I should be taking my temp readings?

Should I remove the horizontal baffle and manually keep the vents open as a means of controling my temps?  The only problem is that the vents would stay open during the night.

Should I skip the thermostat and go with a timer?

Finally,  what is the optimum temp to have the fans cut on/off (i.e., deg above ambient temp)?

Thanks

Joel

[jimF]

I don't see any mention of species your are drying

[Joel Eisner]

It is charged mainly with 4/4 SYP that has been air dried for up to a year.  I plan on drying mostly wood that has been air dried for at least a few months.  The main wood that I cut is SYP, tulip poplar and oak.

Joel

[Joel Eisner]

Anyone have any ideas???   :P

[Den Socling]

Joel,

Air dried SYP and poplar should have low MC and should be impossible to hurt. Oak, of course, can be different.

Normally, you have fans that circulate air through the kiln charge. I didn't visualize your set up while reading your post. Fans don't open the vents unless they are 'powered' vents. In that case, the fans on the vents are in addition to the fans that circulate the air.

I have never run a solar kiln but venting normally goes like this: in 'conventional' kilns, you vent to lower humidity. You need to lower humidity to enhance drying conditions. In DH kilns, you vent to dump humidity and you also vent to dump excess heat.

In a solar kiln, guessing from other experiences, I would vent to dump humidity but would be hesitant to vent to dump heat. At least with oak.

I don't have an answer but I hope I gave you some food for thought.

Den

[Skip]

Try posting your question on woodweb.com in drying forum doctor wengert is good.

[beenthere]

Skip
You're new here, and you are welcome.
Be careful (as a friendly warning) with 'praises' for the Dr. on the 'other' forum, as his antics are not too impressive to several of us members. Many of us have been 'deleted' by asking too many questions that the Doc didn't like, and pointing out his contradictory statements and errors.
You'll pick up on this as you become involved. 
Our members are quite expert in the kiln drying and solar drying 'world'.
Again, welcome. 

[Dan_Shade]

i have a solar kiln, but i'm certainly no expert.

my rules: start with sorta dry wood, try to let the fans shut off at night, towards the end, block the vents off to get the temp up to steralize, and hope for the best!

[WkndCutter]

Joel,

Congrats on the kiln.  I am in the process of planning one myself.  As I understand it powered vents can be used to control humidity and temperature.  If you are using wood that has been air dried prior to placement in the kiln you should not have many problems.  The nice thing about a solar kiln is the natural venting thus no need for powered vents unless you want to set it up that way.  The fans should circulate the air through the pile.  The temperature may depend more on the size of the solar collector, Insulation and outside temperature than on your  natural venting.  Sounds like a nice setup though.  I'm sure it will work fine.

Andy

[Joel Eisner]

The main question that I had is what do people think about the addition of the horizontal baffle as noted in the diagram. 




This allows enough air flow in the top vents that are positioned to open in and bottom vents that are positioned to open out.  This was the vents open due to air flow and close when the fans go off.  There is less internal air circulation but more air exchange with the outside air.

Joel

[WkndCutter]

Joel,

Nice drawing!  I get the point though.  If you add the baffle you have to keep a better track of the inside temperature and humidity.  During normal operation the kiln without a baffle or powered vent will heat up as the sun comes up and heats the kiln.  This heat stays inside the kiln and is used to remove moisture from the wood.  If you put in a baffle and add outside air the kiln will not heat up as much because you would be exhausting the hot air out the backside of your wood pile and sucking in colder outside air in the top.  I would add a baffle to get fresh outside air into the kiln only as a means to keep better control of the inside temperature and humidity.  This will require more extensive monitoring devices but will also make your kiln more effective.  Think about being able to open and close those baffles from outside so you don't have to open the kiln to make the change.  I may make this change to my kiln plans.

Andy

[chet]

Joel,
I set up my kiln exactly as yur diagram shows.     During the early stages of drying I use the extra baffle, with vents opened as necessary. Then when the wood is down to roughly 15 percent I barely crack the vents and remove the extra baffle to finish the load.

[isassi]

Jeff; Is that design you are using available online?? I need to buy a book on this subject.  I keep lots of ideas floating on how this will work, and yeatserday, I really thought up a beauty  :D All of us have seen war movies where POW's are blaced in the black iron boxes, so I was thinking...why not wood...of course, the answer is readily apparent. I am working on a design that the collector side to the sun will draw cool air up the stud pockets (assuming a black collector surface inside, polycarb panels outside) and drop the heated air on the lumber pile. Now since I know less then anyone here about kilns, I was wonder why this can work on the thermal cycle or does it need forced movement? I was thinking only the temperature switched fan would come on to exhaust when air temps reached 125 or so...and maybe I am nuts...so where can I opbtain a reference fro ideas?????

[isassi]

Opps. I was asking Joel about the reference, not Jeff..... :

[Dan_Shade]

there are a lot of sources for solar kiln information online.

the basics of it are:

the roof should point south

the roof angle should be your latitude + 10 degrees.

the capacity in board feet is roof surface area * 10

air dry a bit before sticking it in the kiln

hope for the best!

[Joel Eisner]

joasis,

The plans that I used came with my Norwood sawmill.  It is the plan published in the June 1994 #70 issue of wood magazine.  It is the basic design that you see on woodweb etc.

Joel

[Snag]

Hey Joel,

I am going to be building the same unit in a couple of weeks.  Do you mind if I ask how much your kiln cost to build?  With the current price of plywood I imagine it wasnt cheap.....  Thanks.