Solar Kiln build

[bwstout]

Started my solar kiln build this weekend with the help of my snl he had to do most of the work. ;D
It is a 6'x12' 8' rear wall and 2' frt wall. I bought one gable fan that is 1050 cfm should I put 2 in it?



  

[btulloh]

Probably need at least 3 of those to create enough air flow.  Better to have too much fan than not enough.

How do load that?

[doc henderson]

will it be insulated.  I assume the top will lift up to load?  I like the idea of a gable fan if it has the adjustable temp on and off switch.  could add a couple more fans but wire them in with the thermostat on the gable fan.

[bwstout]

ok thanks the. To load it the top will raise and the front will lift off or swing out like small doors. Have Kubota with forks to load it. Anyway that is the plans now. 

[bwstout]

Yes sir it will be insulated I built a torsion box for the floor with insulation in it and have foam for the walls I have still to put the outside walls on will put the insulation in before they are installed. 

[btulloh]

Very nice.  You will certainly like having a kiln.  

You may want to check out the plans for the Va Tech kiln design to help understand the baffling , venting, and air flow that makes the system work.  When all that works right, the solar kilns are very easy to operate and achieve good results.  Also look through some of the solar kiln builds here on the forum.  

Looking good.  

[bwstout]

Thank you have read most of them and watch as many videos as I can but there is just some of the math that I cannot fully understand and that has been my weak point all through school and now at my age it just can't get it.  The how much air flow or fans needed for the 6x12 kiln.

[doc henderson]

the solar kilns are very forgiving.  the airflow "amount" is important but also you are forcing it through the piles and want it to be uniform across all the length of the kiln.  so not so much more total, but having fans pushing along the length of the kiln so the wood is uniformly dry.  not just in the center.  follow the plans and it should work well.  looks good!  I think @YellowHammer  uses the 16 dollar box fans and replaces them every few years.  you can change the 3 speeds as needed.

[btulloh]

Not to worry.  The math can get you there, but it doesn't have to go through the math department if you don't want.  i did the math on mine, but it's not absolute under the best of circumstances because of inefficiencies and unknowns.  For my kiln, which is slightly larger than yours I figured out I needed about 3500 cfm to get the air flow right.  That gives me enough air flow for quick drying requirements likeSYP, but then I can turn off some fans for things like oak.  Probably best just to have plenty of fans (CFM) and then adjust down as needed.  It's easier to put fans in at the build stage than add them later.  Or at least have provisions to add them if needed.

If you want to try the math, it's a matter of calculating the area of the air flow through the stack.  Then convert the CFM of the fans to Linear Feet per Minute.  The tricky part is figuring out the losses that happen from loss of CFM due to static pressure and losses due to leaks around the ends of the stack.  Ughhhhh.

Or skip the math and just add fans so you have plenty of CFM.  

It is extremely important to have the air flow in the right direction and the venting correct.  The venting is a bit counter-intuitive, but if you take your cues from the VT design, plus some of the advice you find here on the forum, it will all work out in the end.  The help I got from Dr. Gene and Yellowhammer put me on the right track.  All I had to do was listen, and trust the force.   :D

I never really understood some of that as I was building mine, but I followed the design principles in the VT design.  After I started using it is when it started to sink in.  As long as you get the air flow and venting and seal the stack properly, it is pretty easy to get excellent and predictable results.

[bwstout]

I had first thought about the box fans but then I read one thread where the blades melted and then I thought about some of the 100 plus degree weather with high humidity we have here in east Texas and decided to buy the gable fans. Sometime I find myself in front of the refrigerator wondering if I was opening it or closing it and new that I would forget to open it up if it was not in use and melt the fans.  ;D  I will get 2 more. I have some red oak that I quarter sawed that shows some very pretty tiger striping that I want to use to rebuild the kitchen cabinets for my this year. 

[bwstout]

thank you Mr. Btulloh I will add extra fans and look again at the plans for locating the vents.

[btulloh]

i evaluated the box fans, but then went another way for a variety of reasons. It seems like the newer versions have poor blade design and don't produce the rated CFM when there's static pressure.  That and the melting thing.  Just my choice.  Plenty of people are using the box fans, but I'm glad I went another route.  I don't know how gable fans react to static pressure, but I do know they are not subject to that in their normal use.  Static pressure can de-rate the CFM of a given fan by a lot - or a little.  It just depends on the blade design.

[bwstout]

Ok so is there a way to check the static pressure before I install the fans?

[doc henderson]

oak can be tough to dry and requires a slow roll.   may need some of the clear solar collector cover in part with shade cloth.  if you use the fan with a thermostat to control all the fans, and leave the fans on, it should protect them ( fans will run above a certain temp).  and leave the vents open when empty, or just keep it full all the time! 8)  I think the fan blades melt when closed up and no fans running.  

[doc henderson]

I think the only motivation to use the plastic box fans is cost.  in the caustic humid environment of the kiln, the galvanized meatal fans will prob. not see the normal service life.

[bwstout]

thanks the red oak has been air drying for about 8 months now.

here are the spec for the fans
[th]Series Name[/th]

N/A

[th]Blade Material[/th]

Galvanized steel

[th]Housing Material[/th]

Plastic

[th]Diameter (Inches)[/th]

18

[th]Depth (Inches)[/th]

17.75

[th]Weight (lbs.)[/th]

8.77

[th]Power Source[/th]

Electric

[th]Automatic Thermostat[/th]

Yes

[th]Automatic Humidistat[/th]

No

[th]Maximum CFM[/th]

1620

[th]Maximum Ventilated Area (Sq. Feet)[/th]

2300

[th]Thermal Overload Protection[/th]

Yes

[th][/th]


[th][/th]


[th][/th]


[th][/th]


[th][/th]


[th][/th]

[btulloh]

Yes, but it's not easy and probably best to sort of wing it.  I tend to over-analyze things, because that's my nature and background.  Most people just wing it and come out OK.

I hate to get into the weeds here, but here I go anyway!  The air flow through the stack is created by having a high-pressure area created by the fans on the output side.   Technically, you're making a plenum, which creates the high pressure zone. This makes for a low pressure area on the other side of the stack, and naturally the air tries to balance that out, causing air to flow through the stack.  It's a bit different than sitting a stack of lumber out in the open and just pointing fans at the stack.  This is a good thing, because to tends to make the air flow uniform through the stack, which would be hard to do just by blowing fans directly at the stack.  Static pressure is result of blowing the fans into an enclosed space to create this high pressure.

I can just hear the eyes glazing over out there, but to me it's helpful to understand this a little bit.

Okay - out of the weeds now.  Everybody relax.   :D

[btulloh]

here are the spec for the fans

They never give any specs about static pressure on these fans because it's not a factor in their normal application.  Also true for most types fans you might use.  

I fear I've opened pandora's box, but it's better not to under estimate the CFM needed, and it's more difficult to add fans after the build.  Most everybody just sort of chooses some affordable fans and things tend to work out in the end.  Like I said, I tend to over-analyze.   

[doc henderson]

I think the gable fans will last longer and agree that many would work, but the number may add efficiency ect.  so yo are trying to gain say 5% more efficient.  may save time and or money in the long run!  quality of the lumber in the end is the most important factor.  you are way ahead of me on the build.  good job!

if these are air dried, should be ok.  most defects occur at the higher moisture contents.  have you already mentioned the MC of the air dried oak?

[bwstout]

I have not measured it but will check it this week and see what it is we are suppose to get another round of heavy rain tomorrow. The stack is cover on the top but blowing rain will get it wet.

[bwstout]

Ok the weeds are way above my head I will buy two more fans just to be on the safe side and add them

[btulloh]

Sounds like a plan.  You should be fine.  Sorry about the weeds.  :D

One rough guesstimate on how well a certain fan may perform is just by looking at the blade area vs. the open area.  If blades occupy a large portion of the space, they should be ok.  The gable fans I have around here (in gables) look pretty good in that regard and I would guess they'd be fine.  Yours probably are fine too.  People tend to stay away from them mainly because they cost a lot more than the basic box fans.  The box fans I see out there these days tend to have minimal blade area and pitch, probably to allow them to function with lower-powered motors and sell for $20 retail.  The older box fans seemed to be a little more robust.  Progress.  ?? Or just the nature of things these days.

[bwstout]

thanks for your help, I read your thread again on your build what size are your vents 

[farmfromkansas]

I have read on this forum, posts by the Wood Doctor, that if you are loading with air dried wood, the fans are not so important.  Fans are REALLY important if you load with green lumber.

[YellowHammer]

The key to sizing or quantity of fans is to make sure you have the airflow correct for the species you are drying, as well as its initial moisture content. Too much airflow will crack some wood, too little will prolong the drying process and cause sticker stain and mold.  So the same airflow that will dry pine correctly,650 fpm, will crack oak which likes 150 fpm, or so.  

So too much airflow is sometimes worse than too little.  Which is worse?  Cracked white oak, or sticker stained white oak?  

Anyway, Gene did a great job in the design of the Va Tech design, and as long at that basic design is followed, as far as sizing, plenum areas, collector ratio, and fan sizing, it will work sweet.  If you make some changes, then sometimes problems can occur.

I have a hand-held anemometer that I use to check the true air velocity out of the stack on my kilns.

Plastic fans melt at about the same temperature as the plastic covering of the roof aperture.  So, when they melt, other plastic things are melting, also.  I have never had a problem with that during normal operation.  However, when there is a power outage, or the fans are not turned on, then the hot air will stagnate in the upper cornice of the kiln, and bad things will happen.  That would be another reason to install and run more then one fan's it would provide an operating spare if its companion malfunctions.