My kiln is not cycling air as expected

[Everest123]

I've finally got my kiln pretty well loaded up and running but I am noticing that air is not circulating in and out of the building as I would expect.  I built a Virginia Tech solar Kiln and my expectation would be that air would flow out the bottom and in the top vents. But I very clearly have air floating in the bottom so presumably out the top....? I cannot understand why the air would circulate in this fashion. The Kiln appears overall to just be confused.  

What did I do wrong???

-Jeff



 

 

 

 

[Everest123]

Oh and it's worth pointing out that are is actually circulating through the stack inside the Kiln very nicely.  I can clearly feel cool air flowing from between the stacks boards.

[scsmith42]

Typically with a solar kiln you are not trying to exchange all of the air; rather just a small portion of it so that you don't dry too quickly.

[YellowHammer]

Are the fans boxed in a baffle?  Are the sides of the load baffled?  That's a lot of vent space open, I would expect to run at 1/4 that open area, or less.  There also appears to be a lot of space between the suction side of the fans and the upper vents.  So the the fans are sucking air up the plenum.  

Put a smoke generator in there and watch the airflow pattern.  

[Everest123]

The fans are boxed and the sides are baffled.  The Kiln was built much larger than I originally intended and there is a fair amount of open space in the back. Much more than I planned.  I fear I've disrupted the airflow. A smoke generator is an excellent idea!  

I may also put in ducts from the top vents to the fans. That would absolutely force the proper air flow, regardless of anything else.

I'm just surprised that it's not working as designed. I had high hopes it would :(

[rastis]

It looks like there is no control on those vents. I believe you need to restrict the air in and out to keep the RH high in the chamber

[btulloh]

X2 on the vent control.  There needs to be some way to vary the vent openings.

You may be closer to ok than you think, except for the vent control.  The venting, air exchange with the outside, and humidity release does not work intuitively.  The main thing is to have the heated air circulated through the stacks evenly.  It sounds like you're getting air flow, so that's good.  I can't see if you've got blocks under the stack, but that is important.  The main thing is to seal off every air path except for the stack itself.  It's never perfect, especially on the ends of the stack, but the better you close off the openings the better.

One mistake I see sometimes is people leaving a gap above the fan board (between the rafters).  This creates a short-circuit for the air circulation.  I can't see if you've blocked this off above your fan board or not.  

It may be that you just need some vent control.  It may be that you need to move the fan board and the stacks back some.  I think some input from @GeneWengert-WoodDoc could clear some of this up, since he was involved in the original design of the VT design.  Don't despair!  You're either ok or real close!

[Everest123]

Thanks folks. Can you clarify what you mean by vent control?  There are no baffles or anything on the vents themselves to force air one way or the other or to allow me to close them off.  There are also no louvers or anything to make it a one-way flow.  

The fans are on a speed control but I don't think that's what you mean.

I did place blocks in between the rafters above the fans so that is not a short circuit zone. I have placed two-by-fours on the bottom with stickers on top of those.  There are foam blocks keeping air from circuiting underneath the two by fours though. The circulation through the stack I'm quite happy with there is a very noticeable air flow coming through the stack of wood.   I can put your face up and feel cool air coming out telling me the evaporative effect is definitely cooling the air and it's working.

I'm just very surprised that there is air coming in the bottom vents.  if you can clarify what you mean by vent control then that's probably the issue.  I figure if I have no idea what you're talking about them clearly I miss something! 😂

-Jeff

[btulloh]

Sounds like the circulation is a-ok.

The vents need to run from partially open to almost completely closed during various stages of drying.  This is critical to the operation of the kiln.  There are a number of threads on here that explain that, including my thread about building the kiln.  It seems like we all need to have that explained to us when we start using the kiln.  You can search for some threads and/or go to my profile and look up my post about building the kiln.  I would link it for you, but I'm short on time and late for a meeting.  

Your vents look like they are passing way too much air right now.  

.. . .  Sorry - have to run . . .

BT

[btulloh]

Well I had a little reprieve so I'll see if I can find a couple links while I'm killing time . . .


Here's a quote from Yellowhammer about his vents:

I used these, I guess I got them from Home Depot.

I use two up top, two on the bottom.

https://www.homedepot.com/p/Master-Flow-16-in-x-8-in-Plastic-Slider-Foundation-Vent-in-Black-PS168BL/206624404

Here's a link to where they were explaining the operation of the vents to me:

. . . Well ... I can't find it and I'm out of time...  It's been explained on here quite a few times, to me, and to others.  Yellowhammer and Dr. Gene have provided good info on this . . . 

[Everest123]

Thanks @btulloh I appreciate your patience, I know it gets annoying to answer the same questions over and over  . I've done a bit of searching and now I definitely see the concept.  I can easily put sliding covers on all the vents made of plywood giving me the ability to restrict airflow in/out of the kiln.  I will do that ASAP.

I'll keep searching for a good explanation of how they should be used once installed.  I haven't found that yet, but I'm sure I will.

[Everest123]

For posterity

https://forestryforum.com/board/index.php/topic,97731.0.html

[YellowHammer]

The vents are used to control both the temperature and the humidity in the kiln.  It's a balance, the higher the temperature, the more the moisture carrying capacity of the air in the kiln, vs the higher the humidity, the more the air needs to be vented.  So the vents, either the upper, lower, or both are used to maintain a relatively high temperature, while at the same time venting enough to get the moisture laden air outside the kiln.

[btulloh]

Yeah, there's some useful info in that thread from Dr. Gene, but I still can't locate YH's explanation.  Must have been in someone else's thread.

I have three vents top and three at the bottom.  They're 7"x14" with simple sliding doors.  When I first put in a load, I set the vents about 1/2 closed.  Maybe just a bit less on the top vents.  As the lumber dries, I keep closing the vents more and more until the last few days when they're barely cracked.  So even at max venting, I really don't have a huge vent area.  There's only limited air exchange with the outside.  The humidity escapes magically, mostly at night as the kiln cools down.  If the vents are open too far, the heat will never build enough.

The only time my vents are wide open is when the kiln is empty.  This keeps it from getting too hot and damaging the fans or the glazing.

My re-telling of this leaves something to be desired, but maybe it will help.  The kiln tends to be very easy to operate and pretty forgiving.  It is possible to over-dry though, so keep a close eye on the lumber when it's getting close to the target mc.  I overcooked a load of SYP and learned my lesson the hard way.  Fortunately I was still able to use the lumber, but it made for difficult machining.

[YellowHammer]

Keep the fans running when heating, got to keep the air moving, but shutting the vents will increase temperature and also increase humidity.  The unscientific way to dial in a solar kiln is to open the door in the middle of the day.  If you get hit in the face with hot, high humidity amazon rain forest air, then you need to open the vents some because you are trapping too much humidity.  In contrast, if you open the door and it feels like a hot dry breeze, then you are venting too much air and can close the vents to get hotter and absorb more moisture.  

The scientific way is to get an EMC chart from the Web and use the relative humidity and temperature, to determine the true EMC of the air.  It's a good process to understand and will help you get the hang of the thing.  

You will get a "feel" for it after a few loads.  When the humidity inside drops as the wood starts to dry, you'll know it and start closing the vents down.  At the end of a run, I'll have mine almost closed, maybe just a crack open.  You'll be done when you open the doors and the air is very hot, dry, and you back up so it won't burn your face and you see dead bugs on the surface of the wood looking like black pepper.  
 :D
If your sunglasses instantly fog up from the heat moisture coming out, shut the doors because it ain't dry yet.  :D
  
Solar kilns are the Easy Bake ovens of the kiln world, and one of the best ways to dry wood reliably.    

You can't do a whole lot wrong, you may just not be drying at the full capacity of the current solar conditions.  

[Everest123]

Thanks this is very helpful.  Sounds like my concerns about the specific flow of air aren't nearly as critical as I thought.  We just want air exchanging with more at first and less as the lumber gets more dry.

Sounds like air flow is akin to squeeing a sponge, lots of squeeze (airflow) in the beginning, but you can't fully dry a sponge that way, to do that you need HEAT - e.g less air flow.  

The interior space in my kiln is very large, much larger than I planned, so I made the vents larger.  I have 4 on top and 4 on bottom all 12" square.  So that's quite a lot of flow even if the specific vent volume isn't high.  My plan is to use simple foam blocks glued to thin plywood as rotating vent covers.  With a screw on one corner holding it in place with sufficient tension to hold whatever position I put it in.  That way I can use a long sticket to adjust the position. My top vents are quite high up.


For reference, on Saturday the interior of the kiln reached 90 with an outdoor temp of 70 with the vents fully open.  I have foam inserts for the vents but had to take them out because the foam insulation started warping in the heat.  This the advice of "fully open" when empty makes sense.  

So I guess in short, worst case, I am running a an accelerated air drying operation because the fans are running 5 hours a day with vents fully open.  Unlikely to do harm.

Thanks for all the great input!

[btulloh]

Right, you're just air drying at the moment.  No harm except it takes longer.

Here I get the temp to about 125 ish with a fresh load.  As it gets towards the target, the temp goes up to about 140-145.  The temp is increased by the solar gain of the kiln, and it's decreased by the heat required for evaporation.  

A cheap remote temp/humidity sensor helps to understand what's happening and helps you tune things a little for more efficient drying.  After a while you get the feel of the thing and you can achieve that with just using the Yellowhammer method.  Here's a little temp/hum sensor that will work:  Sensor Push  It goes to 140.  You can also find ones that have a higher temperature range.  Lot's of options out there.  Most of  the stuff you find at your local big box top out at 114 or so.  I used a meat thermometer when I first started and took sample readings of the humidity (and air flow) with a Kestrel hand held.

The main thing is to know that even after a couple or few loads, you start to get the feel of it and figure out how to adjust things for max efficiency. Studying up on equilibrium moisture content (emc) can't hurt either.  Drying lumber is a pretty deep subject, I've learned, but it's been rewarding to learn about.

[tacks Y]

Everest, Here is what I did to open and close top vents. Works well and is simple.

 

[GeneWengert-WoodDoc]

Your fans should be no more than 18" from the back wall.  They should be as high as possible.  The further from the wall, the poorer the vents will work.  Ok?

[Everest123]

Your fans should be no more than 18" from the back wall.  They should be as high as possible.  The further from the wall, the poorer the vents will work.  Ok?

Well that's a BIG problem.  My kiln is not set up that way at all.  I'll have to measure, but I'd say more like 48" away.  My carpenter wound up making a few modifications in construction and the dang thing wound up a lot bigger than I wanted.  He pushed the back wall back 24".
So what should I do?  Just give it a whirl for a load or two and see how it goes?  I put a good load of freshly cut poplar in there and with no fans running at all it wasn't wet in the least inside, and the wood is clearly drying out.  So some air must be circulating.  Could it be that the size of my vents (4x12" vents top and bottom for a total of 8 square feet of open vents) is compensating for the poor spacing?  Hey I'd rather be lucky than good.

[GeneWengert-WoodDoc]

You need some method of removal no the moisture from the air in the kiln.  Venting is how we do that.  So, modify your design and add a flat, horizontal, baffle running full length, located at the bottom of the fans running toward the back wall.  Leave about an 18" space from the edge of the baffle and the back wall.  

What will happen is that this will restrict some of the air flow circulating through the pile and up to the fans.  With this restriction, the vents will work well.  To reduce venting, as you often want very very little, the doors you described are essential.

[Everest123]

Thank you!!  This is perfect feedback and an easy thing for me to do.  I'll send along a few photos / updates when I implement this fix.

[Everest123]

Update - I added baffles to the kiln as well as simple vent covers as shown before.  NOTE, I actually had to add more baffling to get within 2" of the back wall to get any real airflow out of the bottom grates.  That REALLY surprised me.  Basically the fans and the top vents are almost totally boxed in.  As shows here there was zero airflow. Crazy!!  It was 72 outside when I took these photos so the kiln is getting quite hot, The black tarp was 141 degrees.  I did go back and paint everything black, and installed a thermostat.  The kiln will now only run if the temp is over 110 inside.

QUESTION - The wood has been in this kiln, operating with poor efficiency for about 8 weeks.  The top boards are around 10.5% and the lowest boards are about 12.5%.  The vents are open around 15% right now.  Temps are quite high in the full sun.  I've read in some other posts that I should close the vents completely.  There are enough leaks AND the vents don't perfectly seal, so there will some air exchange, even with the fans running.  I'd like to get this stack down to 6-8% from top to bottom and try to sterilize the wood to kill off any powerderpost beetles, which I'm afraid are VERY common around here.  They get into sapwood regularly.  

Advice on the vents?  I expect these temps will get much higher in the high heat of summer,  is it possible to get TOO hot?

-Jeff




 

 

 

[GeneWengert-WoodDoc]

Are you running the fans only during daylight...9AM to 7PM?

[Everest123]

Are you running the fans only during daylight...9AM to 7PM?

I have a thermostat installed and only run the fans when the interior of the kiln is over 110.  It usually hits that on days with sun by 11:30 and runs until around 6:30-7:00.  On cloudy or rainy days the fans do not run at all.  I installed a temperature and humidity logger in there this last weekend and on Saturday it was running around 115 in the kiln at about 10% humidity.  That was with the vents completely closed. It does leak a fair amount of air out the front where the slatted clear roofing is attached to the front beam.  I put in the matching plastic pieces and the roofing is screwed down to those but it is not a perfect seal by any means. So there is some air still getting in and out of the kiln obviously.

Ambient air temperature was about 70 so I'm running about 35 or 40 degrees warmer than ambient. Although it was partly cloudy so it would get a lot hotter I think in a full sun situation.

The wood was down 2% from a week ago where I was running between 10.5 and 12.5 percent. The wood is uniformly 10% now.  I have about 1500 b/f of poplar and Ash in there.  They seem to be drying at exactly the same rate.