iDRY Vacuum Kilns

Sponsors:

My kiln is not cycling air as expected

Started by Everest123, January 12, 2020, 01:35:27 PM

Previous topic - Next topic

0 Members and 1 Guest are viewing this topic.

GeneWengert-WoodDoc

Something does not add up.  If the kiln is 110 F and @10% RH (these conditions would be drying the wood to 2% MC), the wood has to be drying like crazy.  Yet, it is not.  If the wood was drying fast, it will be using the heat in the air and this will lower the temperature in the kiln.  So, your measurements of temperature and humidity tell us that the wood is not drying.  If it was drying, the humidity would be higher and the temperature lower.

So, is this hot, dry air you have being blown through the lumber stack?  Are the fans blowing toward the front?  Can you stand on the rear side of the pile and feel significant air flowing through?

How certain are you about the MC of the wood?

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

Everest123

Quote from: GeneWengert-WoodDoc on April 12, 2020, 08:42:48 PM
Something does not add up.  If the kiln is 110 F and @10% RH (these conditions would be drying the wood to 2% MC), the wood has to be drying like crazy.  Yet, it is not.  If the wood was drying fast, it will be using the heat in the air and this will lower the temperature in the kiln.  So, your measurements of temperature and humidity tell us that the wood is not drying.  If it was drying, the humidity would be higher and the temperature lower.

So, is this hot, dry air you have being blown through the lumber stack?  Are the fans blowing toward the front?  Can you stand on the rear side of the pile and feel significant air flowing through?

How certain are you about the MC of the wood?
I'm using a pinless moisture meter that I calibrate before every use.  It is a Dr. Meter MD918 - This one to be exact.  https://www.amazon.com/gp/product/B017D6U2DS/.  Yes there is *some* airflow between the boards, you can feel air moving, but I will admit it's not a lot.  I would in no way call it "significant".   I can detect it, but that's about it.  I did turn my fans to "low". I could turn them back up.  I've also considered parking a few box fans along the back / lower side of the kiln to encourage airflow. I would think putting them up against the wood stack would help.  I have struggled with airflow issues on this kiln for quite some time. 
Originally I placed the fan baffle too far away from the back wall.  That caused absolutely mayhem with airflow.  Everything was reversed.  Air was getting sucked in the bottom vents!  So I boxed them off significantly as show below.  



 


I was *very* surprised to find that that amount of baffling was insufficient.  I had to add foam pieces to FURTHER restrict airflow.  With that baffling I wasn't pulling air in the bottom anymore but it wasn't going out in a measurable way either.

So I added foam as shown in this "professional" drawing.  Sorry no picture.  Note there is no maybe a 1.5" gap between the back wall and the fan box!!!  Very little space!



 
THAT solved my airflow issue with the vents open.  Hot / moist air out the bottom and cool air in the top.  Not a lot, but you can definitely feel it moving.
With the vents closed and the baffling fully in place as above though, I think I'm not getting the right cyclical air movement. I need.  There is an extremely pronounced temperature gradient in the kiln. Down on the floor it is MUCH cooler than up by the top.  30+ degrees difference.  I'm guessing that's the issue here.  

Everest123

@GeneWengert-WoodDoc what do you think about this?  I would really value your input. :)

farmfromkansas

Can't see any fans, what do you have?  And did you block above the baffle between the rafters? or is there a gap between the baffle and the glazing? All gaps have to be closed around baffle for your fans to make air flow.
Most everything I enjoy doing turns out to be work

Everest123

Quote from: farmfromkansas on April 15, 2020, 09:43:07 AM
Can't see any fans, what do you have?  And did you block above the baffle between the rafters? or is there a gap between the baffle and the glazing? All gaps have to be closed around baffle for your fans to make air flow.
There are 4 60w metal fans inside that box, but of course, it's impossible to see them.  They are fully boxed against the clear roofing and there isn't much air leaking.  When the vents are open, air flows much more readily.  One issue I have observed is that there is quite a lot of air leaking *out* of the kiln across the front where the air flows down from the fans.  You can see that in the pictures below.  It was extremely hard to make an airtight seal with the plastic roofing.  Maybe I should use some expandable foam to do a better job sealing that?  Thoughts?
 

 

 

farmfromkansas

Hard to tell from a picture, but if there are gaps between plastic and that foam, think I would take the plastic loose and put a bead of caulk on the foam and then put the plastic back down. Hope you used screws.
Most everything I enjoy doing turns out to be work

GeneWengert-WoodDoc

The fans should be at the top of the roof, but about 12 to 18" away from the rear wall blowing to the south.  There would be a solid piece (of plywood?) around them as a baffle.  This fan baffle would reach from the roof to the top of lumber pule, be vertical, reach from end to end and have a hole cut in it for each fan.  So, the air cannot get to the front of the kiln without going through the fans.  The fans blow into the large space on the front between the lumber pile and the roof.  The lumber pile in the front is about 12" from the south wall, so the air then goes down and through the pile.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Everest123

Quote from: GeneWengert-WoodDoc on April 15, 2020, 11:13:21 PM
The fans should be at the top of the roof, but about 12 to 18" away from the rear wall blowing to the south.  There would be a solid piece (of plywood?) around them as a baffle.  This fan baffle would reach from the roof to the top of lumber pule, be vertical, reach from end to end and have a hole cut in it for each fan.  So, the air cannot get to the front of the kiln without going through the fans.  The fans blow into the large space on the front between the lumber pile and the roof.  The lumber pile in the front is about 12" from the south wall, so the air then goes down and through the pile.
I'll post some better pictures tomorrow.  I know all that for sure.  I use a large tarp vs. plywood to go down on the lumber pile, but the fans are fully baffled.  The challenge was that with 18" of space between the bottom of that baffle and the back wall, there was still a circular airflow in the kiln vs. air coming in the top and out the bottom.  This weekend I will check the status of the lumber again and post a comprehensive set of photos.  I realize you all are trying to help but flying a little blind.  Thank you so much!

Everest123

Quote from: farmfromkansas on April 15, 2020, 07:25:59 PM
Hard to tell from a picture, but if there are gaps between plastic and that foam, think I would take the plastic loose and put a bead of caulk on the foam and then put the plastic back down. Hope you used screws.
I did.  Caulking will be trivial to apply.  

GeneWengert-WoodDoc

The fan baffle is vertical, not angled.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Everest123

So today with the kiln at 115F inside at 10% humidity, the lumber is measuring a little under 10%.  8% and 9.5-10% for the ash and poplar respectively. Below is a detailed walkthrough of this kiln so hopefully we can figure out why I'm not getting good drying efficiency.

I did a detailed photos shoot with various measurements today.  One thing to point out - this is a LARGE kiln.  Larger than I actually planned for initially.  As it came together there were quite a few "on the fly" design changes that expanded the total size.  Overall the building is 18' x 8'.  The solar collector is a single layer of poly clear roofing.  The interior is all painted flat black.   Circulation is provided by four 12' metal fans with louvers running at full speed.  You can just see them in this picture.  NOTE they are way too far forward in the structure.  More to follow on that.



 

Here's a show of the kiln running in the solar collection area.  Note that I have a large black tarp on top of the lumber piles to force airflow down the front.



 

(Hypothesis #1 -  this is a big building and that's not enough fan power??).

I pretty quickly figured out that the fan being so far forward screwed up the air circulation.  So to rectify that I boxed in the back side with plywood to effective "seal the fans" closer to the top vents on the back of the kiln.  See below for how I accomplished that.  UPDATE - I actually note in this photo that there is a clear "leak" above the fans that needs to be sealed off.  Easily done tomorrow with a 1x1.  That might actually be contributing to my airflow problems.

   

 

The thing you'll immediately notice - I almost *completely* boxed in those fans.  Why did I do that?  My original job left about a foot gap between that box and the back wall.  This was to in effect "move the fans back" closer to the original 18" called for in the VT plans.  With a 1' gap the air circulation was very poor in terms of air flowing OUT the bottom vents (they are closed along the bottom in this photo) and IN the top vents which you can't see but they are up high in the fan box.  Only when I closed it off almost completely as you see here did I get noticeable hot / moist air flowing out the bottom vents.

I bought both a bunch of meters to help me figure all this out.  An IR thermostat, a pinless moisture meter, a pin moisture meter, an airflow meter, and a magnet mount thermostat / temperature monitor.  Today the air temperature was around 55 degrees, but sunny conditions.  Conditions in the kiln around 3pm were 115 degrees and 10% humidity.  Here's the monitor.



 

For good measure I took some measurements around the kiln with my IR thermometer.  I'm measuring surfaces which I suppose are warming from sun hitting the interior of the kiln.  So the measurements ranged from 119 (lumber on the bottom of the pile) to 146 degrees (the black tarp).    Here's a shot from the back of the kiln measuring the tarp temperature and the top board in my lumber pile.



 

 

Based upon everything I've learned, at this kiln temperature and humidity, that lumber should be drying like CRAZY.  So I went ahead and measured it.  My pinless moisture meter was acclimated in the kiln, operating with fresh batteries, and I calibrated it just before I took these measurements.



 

Encouraging!!  That is a 3/4 poplar plank.  I was measuring this at 10% last weekend. So this was positive.  I have absolutely no idea why the pin meter wouldn't give me a measurement, but I've found once lumber gets under around 12% it get flaky.  Maybe a bad meter?

Next the Ash.



 

Discouraging.  This literally hasn't changed moisture content at all in the last week.  Not one bit.  I move the meter around a bit and did find a more dry reading.



 

So 9% isn't bad.  And I was able to coax a reading out of my pin meter, but REALLY pushing it into the good (with the grain).  FYI, that wood is HARD.  I couldn't get those pins into that wood any deeper than what you see here.

So with all that data I started to take airflow measurements.  Note that all 4 fans were running full blast during these measurements.  First I measured the airflow UP through the gap between the back wall and the fan box.  As expected air was circulating up, meaning air IS going through the stack.



 

I'm getting 2.4 meters per second of airflow, so air is moving up.  Not a lot, but it is moving.

Next I measured in various parts of the stack.  Note that I can sense air moving through the whole stack, but it's not a lot.  The ends in particular were not able to spin up my meter.  But there was a tiny bit of air moving.  



  

I got anything from .7 to .9 meters per second of airflow through the stack.  Again the very ends were nearly still.  

So there you go.  That's all the data I can think to gather.  

My conclusions after thinking about this a lot is that there is absolutely no reason this lumber shouldn't be drying FAST in these conditions.  One of two things may be happening:

1) This lumber is already crazy dry and my method of measuring it is poor.  My pinless meter is not to be trusted and the pin meter is probably right that this stuff is DRY.

2) The pinless meter is right and the wood is hovering around 9-10%, which can only be explained by a lack of brisk airflow.  Air this hot and dry circulating more aggressively through this stack SHOULD be drying it, which means that there is not enough airflow!

Short of some wisdom from ppl like @GeneWengert-WoodDoc that give me a better idea, I'm going to drop some box fans on he back of the stack to pull air through the stack towards the back.   I'm also going to seal that gap above the fan baffle that's visible in the photo.  That might let me open up the fan box a little bit from the back wall by creating a more significant airflow. That's literally the only thing that I can think to do at this point. Otherwise I'm stumped.  

Any ideas guys?  Am I on the right track here?

-Jeff

GeneWengert-WoodDoc

Air flow speed does not affect the speed of drying under 20% MC.  Obviously, you need some flow, but not much.

The vents are almost always closed most of the way.  We want to recirculate the air in the dryer.

The fans should be vertical.
Are all the fans blowing southward?
Is there about 18" space between the edge of the pile and the south wall?

Sealing the space where the air would be going upward toward the roof and the through the fans is incorrect.  We want maybe 90% of the air, even more, to be recirculated with only a little vent air out and in.

Efficiency will be improved with a second layer of clear material, but that is not the issue right now.

At 10% RH, the wood will dry to about 2% MC.  So, 1) the RH measurement is incorrect, which seems unlikely due to the
heat, you MC measurement is incorrect, or 2) you are running the fans too long during the day with the vents open so that much of the time the humidity is higher.  Notethat 10% MC is equivalent to about 55% RH, so that is the humidity of the air in the pile when the fans are on if the wood is no longer drying. 3) the MC measurement is off.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Everest123

Quote from: GeneWengert-WoodDoc on April 18, 2020, 11:07:42 PM
Air flow speed does not affect the speed of drying under 20% MC.  Obviously, you need some flow, but not much.

The vents are almost always closed most of the way.  We want to recirculate the air in the dryer.

The fans should be vertical.
Are all the fans blowing southward?
Is there about 18" space between the edge of the pile and the south wall?
Okay, that is good to know re: airflow.  I'm glad I asked!  The vents are fully closed at this point.
The fans do not hang perfectly vertically, the fans are perpendicular to the solar collector roof line.  If they were vertical they would effectively be blowing air directly into the clear roofing.  Right now they move air down along the inside of the roof.  The photo in my previous post shows their position and relative angle.  That said, the baffle is entirely hinged, so I can remove the boxing and let it hang perfectly vertically if that's the best way to do it.  It just doesn't seem right to me, particularly with all the framing for the roof, it's going to blow right into the roof support and bats. Seems messy.  But hey Gene say the word and I'll redo it :)
The fans flow straight south with airflow moving parallel to the roof line towards the front of the stack as I mentioned above.

I'm going to come clean - there is absolutely NOT 18" of space between the pile and the south wall.  Not even close.  It's more like 4-6".

My vents are closed completely at this point, although the kiln is by no means perfectly sealed.  So there is some nominal amount of air exchanging.  I did apply caulking to the "front" edge of the roofing, so all that leaking air is now stopped.  I'm sure air is coming in the rear "high" roof line though.

So based upon Gene's posting, I see a few issues:

1) The fans aren't vertical.  I'm obviously not an expert (in the LEAST!!) but I can't fathom why this would matter.  If the objective is moving air down the roof line and towards the front of the stack, being parallel to the roof line in terms of airflow seems optimal to me. . . .but. . .obviously I realize that could be completely wrong!  

2) The spacing of my pile is too close to the south side of the kiln. Much too close, it's not even close to 18".

I'm heading up to the farm now and will have most of the day to reconfigure things, so let me know.  Thanks so much for all the wisdom you share in this forum!!

-Jeff

Everest123

I'm concerned y'all have given up on me!  I so hope not.  And I am very sorry for tagging people in my previous posts. I did not realize that was rude and I thank the person who kindly pointed that out to me.

Here's an update.

I spent several hours sealing up around my fan baffle which dramatically improved my air flow by about double.  As a result I now no longer have any noticeable thermocline in the kiln when it is running hot.  Maybe a few degrees based upon the sun but it is not noticeable like it was before.

I did have to leave the very tight seal on the fan box to maintain circular air flow but it is moving very nicely now.

unfortunately my meter is showing me exactly the same moisture content in my lumber even though the kiln has been hovering between 10% humidity when it's hot at about 120.  My temperature and humidity sensor in the kiln show that it has never gone above 55% humidity in there and usually hovers around 40 when the kiln is in hot but drops rapidly to 10 as soon as it warms up.  

My pin meter continues to tell me that that lumber is extremely dry. I can barely get a reading on it and most places. 

So my theory now is that I have a bad moisture meter.  As an experiment I took readings with both my pinless and my pin meter on some Maple that has been drying outside under a roof but no walls for about a year.  It's 3/4 boards.  The pin meter says 14.5 to 15. Which is about what I would expect.  The pinless meter says 11%.  Which I absolutely do not believe.

Sooooooo.....I I'm going to order another pinless meter just to see what's going on here but I suspect I may have a massively over dried load of wood now in my kiln.

If that Ash and Poplar really is under 5% is it ruined?  The meter pics below show the air dried maple maple readings.  That pinless meter can't be right....?  Right? 



 

 

-Jeff

doc henderson

did you set the pinless for maple?  I think airdry for a year under a roof, should be about 11 or 12 %.  I have a Wagner pinless.  do you have any friends/contacts near you with a meter so they could check it against your meter.  do you plan to sell the wood in the kiln, or use it personally?  I would not be afraid to use it, at least try to machine it. for sure do not burn it.   bon_fire  
Timber king 2000, 277c track loader, PJ 32 foot gooseneck, 1976 F700 state dump truck, JD 850 tractor.  2007 Chevy 3500HD dually, home built log splitter 18 horse 28 gpm with 5 inch cylinder and 32 inch split range with conveyor powered by a 12 volt tarp motor

YellowHammer

The best meter you can use is your oven (or microwave) and a cheap food scale.  That will tell you exactly what your wood is at and will also help you calibrate your meters.  

The oven dry method is very easy to use but a little intimidating, and I have a tutorial spreadsheet that I give to folks as a starter thing that is basically fill in the blanks.  

If you want I'll upload it.  
YellowHammerisms:

Take steps to save steps.

If it won't roll, its not a log; it's still a tree.  Sawmills cut logs, not trees.

Kiln drying wood: When the cookies are burned, they're burned, and you can't fix them.

Sawing is fun for the first couple million boards.

Be smarter than the sawdust

Everest123

Quote from: YellowHammer on April 25, 2020, 11:18:37 AM
The best meter you can use is your oven (or microwave) and a cheap food scale.  That will tell you exactly what your wood is at and will also help you calibrate your meters.  

The oven dry method is very easy to use but a little intimidating, and I have a tutorial spreadsheet that I give to folks as a starter thing that is basically fill in the blanks.  

If you want I'll upload it.  
Please do!  I've been intimidated to try!

Everest123

Quote from: doc henderson on April 25, 2020, 10:42:01 AM
did you set the pinless for maple?  I think airdry for a year under a roof, should be about 11 or 12 %.  I have a Wagner pinless.  do you have any friends/contacts near you with a meter so they could check it against your meter.  do you plan to sell the wood in the kiln, or use it personally?  I would not be afraid to use it, at least try to machine it. for sure do not burn it.   bon_fire  
I don't know anyone around here that I could ask for help or who might have a meter. The pin meter doesn't have a calibration option other that "wood" or "building".  I've been using "wood".  Could someone recommend a high quality meter?  I'm not averse to spending a little money to get this ironed out.

GeneWengert-WoodDoc

The pin meter cannot read well under about 6.5% MC.

It would be difficult to go under 5% MC in a kiln like yours.  So, I would not think that you over-dried the hardwoods.

If you are going to get a new meter, get a Delmhorst J-2000 with the 26 ES probe and 1/2" insulated needles.  $500. We know that this meter is as good as you can get.  Lignomat is also very good.  Others are not so good.  

The Wagner or Delmhorst pinless is good for dry lumber but is not well suited for use during drying.  It has the advantage of measuring to 4% MC.🏴
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

YellowHammer

Here is my oven dry worksheet.  There are instructions on the sheet, and its pretty self explanatory, but ask me any questions.  As numbers are entered, the graph will automatically update.  This sheet is an example, and already has been filled with real life numbers, but that is so you can see how it works.

Just substitute your numbers.

YellowHammerisms:

Take steps to save steps.

If it won't roll, its not a log; it's still a tree.  Sawmills cut logs, not trees.

Kiln drying wood: When the cookies are burned, they're burned, and you can't fix them.

Sawing is fun for the first couple million boards.

Be smarter than the sawdust

Everest123

Stupid question!  Will this work with wood that's been in my kiln for quite some time already?  Also, I'm looking to buy a top quality moisture meter and I'm curious if the Wagner Orion 910 qualifies?  It's nearly $400 and gets great reviews everywhere online.  $375 on Amazon.  I'm tempted....

doc henderson

the oven dry method gives you a direct measure of the water weight in the wood, but of course you sacrifice the wood.  you can compare the direct measurement with what you are getting with whatever meter.  so it can be in a kiln, barn swimming pool for a year and you will get the low tech measure of the MC.  I have a Wagner but I think it was several hundred.  may be a newer model.  I like the one I have.  after doing the oven dry method, you can do daily kiln measurement to follow the daily MC losses if you are doing picky woods like oak.
Timber king 2000, 277c track loader, PJ 32 foot gooseneck, 1976 F700 state dump truck, JD 850 tractor.  2007 Chevy 3500HD dually, home built log splitter 18 horse 28 gpm with 5 inch cylinder and 32 inch split range with conveyor powered by a 12 volt tarp motor

GeneWengert-WoodDoc

Everest,
It depends on what information you want from your meter.  Are you checking the final MC?  Are you measuring the MC during the kiln run?  The different types of meters (pin or pinless) have different uses.  The better meters are over $250.  Check Drying Hardwood Lumber for a comparison.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

jwoods86

The Orion 910 is a great wood moisture meter and qualifies. 

Everest123

Well, I went with the J2000x and it seems very easy to use and measures about what I would expect.  I'm tossing my cheaper meters in the garbage.  I am also declaring success with the kiln.  After about 2.5 months or running (still cool here) the wood in the kiln is measuring 6.4 to 7.5.  Air dried maple from about 10 months ago is measuring 11.5% and Black Walnut but about two months ago, 15% yo 17% depending on where I measure it.  Bottom line, everything seems about right.  

This Ash and Poplar is ready to go!

The only oddity about my kiln is how much I had to box in my fans.  To within an inch of the back of the kiln.  I suspect that's due to the fact that my fan baffles are much too far forward, and overall the kiln is much larger than I originally designed.  It just impacted the airflow in funny ways.  BUT, it works now!  

Thanks for all the help everyone.

-Jeff

Thank You Sponsors!