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Author Topic: Starting to build the solar kiln  (Read 5656 times)

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Offline btulloh

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Re: Starting to build the solar kiln
« Reply #140 on: September 07, 2017, 05:46:40 pm »
Cool rainy day yesterday.  The kiln had the day off.

Today was nice and bright, and it only got up to around 78f.  Nice day.  We have 3 more of those coming, then Irma comes to town.  It'll be somewhat milder by the time it gets here, tropical storm at worst or maybe just some rain.

Today I had the vents open about 10% and the air out of the stack was 140f @23%rh.  I'm hoping someone will tell me what to think about that.  Yesterday the lumber looked to be about 12%mc on average.  I'm thinking today will get it to the finish line.  I opened up the vents to 50% about 5:30.  We're at about 74f 53% rh outside right now.  I'll let the fans run until the temp gets down below 90.  That's been around 9pm the last couple days.  The vents are open more now, so we'll see.

If all goes well, I'll take this load out tomorrow and put in another load of SYP that's been air dried to 20%.  Film at 11:00.

HM126

Offline YellowHammer

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Re: Starting to build the solar kiln
« Reply #141 on: September 08, 2017, 06:32:47 am »
140 23%RH is an EMC of 4%


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Offline btulloh

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Re: Starting to build the solar kiln
« Reply #142 on: September 08, 2017, 07:18:12 am »
Hmmmm. Thanks.  Where do I find this information?
HM126

Offline btulloh

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Re: Starting to build the solar kiln
« Reply #143 on: September 08, 2017, 07:26:48 pm »
I've still got a little more work to do on the outside of the kiln, but that can wait a few days while I catch up on a lot of things I've neglected around here.  While that's happening, there's another load of SYP in the kiln that should be ready in two or three days.

I'm going to start a new thread on using the kiln.  I'm already starting to find out what I don't know about operating this thing.
HM126

Offline grouch

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Re: Starting to build the solar kiln
« Reply #144 on: September 08, 2017, 09:06:58 pm »
Hmmmm. Thanks.  Where do I find this information?

I want it, too!

I don't see how measuring the RH of air being forced through the stack can tell you the moisture content of the wood. It looks to me like that would be measuring somewhere between the potential for the air to absorb moisture and the amount of moisture available for it to absorb.
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Offline YellowHammer

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Re: Starting to build the solar kiln
« Reply #145 on: September 08, 2017, 10:36:49 pm »
Grouch, you are correct.  The EMC value is the moisture level when given enough time, the wood will have reached an equilibrium with its environment and is no longer gaining or losing moisture.  In some species of wood, such as pine which looses moisture very quickly, the wood will rapidly approach EMC, however, in some species, such as white oak, the drop in moisture content is very slow.  However, given enough time, they will all reach it. 

The important thing about Btullohs number is that he has achieved, during a heating cycle, an EMC value which is lower than the actual wood moisture content, so the wood will dry.  Manipulation of the vents will allow this to be controlled to some degree.     

There are calculators and tables on the web for computing the EMC of wood at or below the fiber saturation point (about 30% moisture content) given the temperature and relative humidity.  Here one:  http://www.csgnetwork.com/emctablecalc.html

The difference between the current moisture content of the wood and the EMC value is used to determine, or in the case of conventional kilns, regulate how much moisture the wood will loose in a certain period of time.  These chamber conditions are manipulated in a predetermined "schedule" to safely dry the wood in a fast but safe manner.  I have printed out an EMC table and have copies at each kiln for quick reference.

For example, in an air conditioned house of 70 F and 40% RH, the EMC value will be 7.7%, so given enough time, all the wood in the house will come to equilibrium at that value.  A good way to do a sanity check on a moisture meter is to check wood moisture values in a stable, air conditioned or heated house, especially where the thermostat displays RH and Temp. 


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Offline grouch

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Re: Starting to build the solar kiln
« Reply #146 on: September 09, 2017, 03:19:22 pm »
Thanks YellowHammer!
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Offline btulloh

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Re: Starting to build the solar kiln
« Reply #147 on: September 09, 2017, 07:06:17 pm »
I'm taking this to a new topic:  "Using the new solar kiln"  http://www.forestryforum.com/board/index.php/topic,97731.0.html

(Grouch, posted a neat way to do these links, but I didn't use it at the time, and now I've forgotten the procedure.  That never happened to me when I was younger. ;D)
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Offline GeneWengert-WoodDoc

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Re: Starting to build the solar kiln
« Reply #148 on: September 09, 2017, 09:52:41 pm »
I agree that a timer is likely most useful.  But, technically, we would not run the fans unless the kiln is 20 F warmer than outside.

MORE TECHNICAL INFO:  in general, the humidity is the key variable as it allows us to consider how fast wetter wood is drying and how low the final MC will be.

Here is info from another posting about air dried wood going into a solar kiln:
The key for your low MC lumber is the average humidity for the day, not a peak value in the afternoon, etc.

The VT kiln is design to achieve 7% final MC with the vents 95% closed, but with a few leaks.  This will indeed happen.

If the kiln building were perfectly sealed and vents were tightly closed, the MC average would never change...water could not get out.  However, if there were a few wetter pieces, they would loose MC and the drier ones would come up in MC.  In truth, the air would heat and get to 100% RH in the afternoon.  The kiln would then cool as the sun goes down, so we would see the moisture in the air condense on the walls, floor and collector.  Hopefully the liquid can drain out of the kiln.  This cycling would occur everyday.  The average RH would be very high, so we would not see much drying initially, but there would be some if we drain the liquid.

For pine we want no drier than 10% MC.  We know that if the vents were wide open and the fans were run 24/7, then the lowest MC in most of the USA would be 11 to 12% MC.  Lower MC if the fans are not run when the kiln and outside temperature are nearly the same.

So, you need to use a setting between these two extremes...VENTS OPEN PRETTY WIDE FOR PINE, BUT NEARLY CLOSED FOR OAK.

Note that you cannot damage well air dried material no matter what you do.  So we run the kiln to achieve the correct final MC.

Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Offline btulloh

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Re: Starting to build the solar kiln
« Reply #149 on: September 12, 2017, 08:06:35 pm »
Thanks again for the help, Gene.



One note to anyone building one if these: I have to say that the lexan twinwall is a good choice for glazing.  I am amazed by how much of thermal barrier that quarter inch gap provides.  When the kiln was at 140F on the air leaving the stack, I put my hand on the outside of the lexan and it did not even feel warm.  It actually felt cooler than the ambient air.  All in all it seems to be a good solution.  Compared to double layers of the corrugated stuff it's a lot easier to install and about the same price. 
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Offline btulloh

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Re: Starting to build the solar kiln
« Reply #150 on: October 05, 2017, 05:36:21 pm »
Last phase of construction on the kiln is complete.  I added a little overhang on the front to keep the weather away from the vents and doors.  Just seemed like a good idea.  This kind of slid to the back burner after I had all the operational part complete, but now it's done.  And done.

 

 

(It's hard to get good light on the north side of a solar kiln.  Does not photograph well.)
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Offline YellowHammer

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Re: Starting to build the solar kiln
« Reply #151 on: October 05, 2017, 05:38:54 pm »
That looks great.  Better than many of the houses in my neighborhood :D
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Offline Crusarius

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Re: Starting to build the solar kiln
« Reply #152 on: October 05, 2017, 05:55:33 pm »
I love it. One day I will have to steel the design :)
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Offline grouch

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Re: Starting to build the solar kiln
« Reply #153 on: October 05, 2017, 07:08:59 pm »
Not being critical, but around here, we generally cut a crescent moon shaped vent for every hole. That looks to be a 4-holer at least, so you might have some more cuttin' to do.

It's a fine looking building, for sure.


 ;D
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Offline btulloh

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Re: Starting to build the solar kiln
« Reply #154 on: October 07, 2017, 09:59:07 am »
Grouch, I like that crescent idea for the vents.  It would generate some comments for sure.  Not easy to install though, all things considered.'
'
Crusarius, you should take a look at vfauto's design.  If I was doing it again, I think I'd frame it the way he did his.  It will take more materials than mine did though, but less fiddling involved.  He must be a builder, because his framing looks well thought out without the little issues to solve at the finishing stage.

I am curious to see how he proceeds from this point.  What kind of glazing, etc.  Some of the details with the glazing and fan mounts and creating the plenum got to be quite fiddly for me.  I want to see how he does all that.
HM126