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Author Topic: Help with my first load drying  (Read 1498 times)

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Offline offrink

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Help with my first load drying
« on: February 12, 2019, 06:45:15 PM »
I have my first load of wood in a kd250m dehydration kiln. Its a medium size load for the kiln. The wood is a 3-3 1/4 thick 36-42 wide basswood from 8-13 long. The moisture content was 68% to start and after 16 days the mc is only at 59%. The dry bulb is at 100 and wet bulb is set to 97. It seems to be very humid in the kiln.

The kiln seems to be keeping the temperature up just fine, the dry bulb is 100 but the wet bulb reading hovers around 80 instead of going up to 97.

Does this sound correct? Seems like the wet bulb temp should be higher and less humid. Anything that I can do to increase the drying speed? Why isnt the wet bulb temp higher?

Thanks

Offline GeneWengert-WoodDoc

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Re: Help with my first load drying
« Reply #1 on: February 12, 2019, 08:44:06 PM »
Moisture has to be leaving the kiln in some manner, or else, as you imply, the humidity would reach 100% RH.  So, are there air leaks or vents that are not tightly closed?  Is the compressor running and lowering the humidity?  Is the floor so cold that it is acting as a dehumidifier, condensing moisture? The answer to all three should be "No!  If the compressor is running, then it is running too much.  If the floor is cold, then insulate it around the perimeter and maybe 2 feet deep with 2 thick rigid insulation board.

You are correct that 80 F wet-bulb is quite severe for some species, especially at high MCs.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Offline offrink

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Re: Help with my first load drying
« Reply #2 on: February 12, 2019, 09:11:07 PM »
The only air leak that I can find is at the bottom of the door to load the kiln. Its icy outside the kiln there. Floor has 4 of ridged foam over concrete. And the compressor has not ran at all. I will see if I can do some additional work to seal those small leaks.

The lower wet bulb temperatures shouldnt keep the wood from drying, should it? Seems like it would increase it significantly.  Is my thinking off?

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Re: Help with my first load drying
« Reply #3 on: February 12, 2019, 09:54:33 PM »
The greater the difference in the dry bulb and wet bulb temps (called the wet bulb depression), the lower the humidity and the greater the drying force on the wood.
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Re: Help with my first load drying
« Reply #4 on: February 12, 2019, 10:13:48 PM »
The greater the difference in the dry bulb and wet bulb temps (called the wet bulb depression), the lower the humidity and the greater the drying force on the wood.
isn't 30 degree depression usually when you are dry in our kilns?
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Offline doc henderson

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Re: Help with my first load drying
« Reply #5 on: February 12, 2019, 10:26:48 PM »
might be due to smaller load and thick slabs therefore decreased surface area to evaporate from.  medium volume with decreased interface to release water.  ? increased airflow and tighter seals.  thinking out loud.  @GeneWengert-WoodDoc 
timberking B 2000, 277c track loader, PJ 32 foot gooseneck, 1976 F700 state dump truck, JD 850 tractor.

Offline offrink

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Re: Help with my first load drying
« Reply #6 on: February 12, 2019, 10:30:31 PM »
might be due to smaller load and thick slabs therefore decreased surface area to evaporate from.  medium volume with decreased interface to release water.  ? increased airflow and tighter seals.  thinking out loud.  @GeneWengert-WoodDoc
This is what Im going to go for. Im going to try to force more air through the slabs and possibly pick up some more moisture. If it does it should show pretty quickly. 
Thanks for the ideas. Ill give an update soon. 

Offline doc henderson

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Re: Help with my first load drying
« Reply #7 on: February 12, 2019, 10:53:44 PM »
the way I understand it.  the dry bulb is the temp.  the wet bulb temp. is cooled by evaporation and is relative to the lower humidity, like getting out of a pool when you are wet and the air is dry.  In theory at 100% humidity the temps would be the same.  the bigger the difference in readings, the more cooling of the wet bulb by evaporate cooling, due to the lower humidity.  you could add water to the air to slow surface water loss that might degrade some species.  This is how I understand it, but I am not the expert.  so I hope the increased air flow and evaporation increases the humidity and narrows the gap in wet and dry bulb reading, but if your loss of water from the kiln exceeds your humidity gains, you may get surface checks ect.
timberking B 2000, 277c track loader, PJ 32 foot gooseneck, 1976 F700 state dump truck, JD 850 tractor.

Offline Southside

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Re: Help with my first load drying
« Reply #8 on: February 13, 2019, 12:00:20 AM »
Thinking of simple potential issues - how close is your wet bulb to your reservoir?   How much wick do you have pulled over your wet bulb?  Is there a chance that your wet bulb is being influenced by the door seal leak or otherwise getting some decreased air flow?  Condensation is not dripping onto the bulb is it?  Just trying to think if it might be a reading error not a function error.  
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Offline YellowHammer

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Re: Help with my first load drying
« Reply #9 on: February 13, 2019, 09:25:40 AM »
You picked a very hard load to dry correctly for your first load.  Very thick, sticker stain prone white wood.  Ouch.  

You didn't say how big the load is.  Is the compressor obeying commands?  As per your settings it shouldn't be running at all, it should be calling for humidity, if the KD250M has that on the display, even if you are not using a humidifier.

Also, have you checked the accuracy of your wet bulb and dry bulb sensors?  An easy way is to remove the wet sock and the two readings should match up.  Then replace the sock and you should see a significant drop.  Also, wet bulb sensor placement is critical, so sometimes its best in a newly commissioned kiln to physically move it around to get optimum airflow over it, and insure its not is a semi dead spot, which will effect the reading significantly.  

There are a couple things working at cross purposes to insure you get a quality load out.

Basswood is classed as a "white wood" as is poplar and several others, and is very prone to sticker stain, therefore should be dried in a relatively high wet bulb depression environment.  For me, I go by the literature recommendations and won't go less than 10% and I prefer 20%.  Your initial settings are 100 and 97.  I'm curious where that came from.  For whitewoods, thats an almost surefire sticker stain value.

However, and I'm simplifying here, the maximum safe drying rate for typical woods of this species, in 4/4 is about 10%-13%, in 8/4 drops to approximately 5%, and in 12/4 drops to about 3%, or less.  You can see the dramatic trend in safe allowable moisture removal rate as a function of thickness.  Its very significant.  There is lots of literature on the subject if you want to get the exact numbers and convert a true kiln schedule for this species and thickness, however, I'd probably move from a conventional schedule and get more toward and "Bright White" type of schedule, which is generally cooler.  Either way, if your daily moisture removal rate gets to the 3%ish number for your 3.25" thick basswood, its going to crack.  So don't go there.

However, and here is the good news, and it is good news, since the kiln doesn't seem to be operating at your 100 and 97 set point, sticker stain shouldn't be a problem yet, assuming the sensors are measuring correctly.  Also, good news is that you've only lost 9% in 16 days which would put you under the max safe drying rate, about 1.8% per day, so structurally, the basswood should be in good shape.  However, and here is another issue, most resistance probes don't really measure well in that high of a range, because, among other things, the wood species calibration curves can sometimes be off.  Another good thing to do with a newly commissioned kiln is to run oven dry samples and cross check this values with the kiln display moisture values.  You may have to make species group adjustments.  I did, very easy.  (Nyle 200 M) With all that being said, and without me having more knowledge of your kiln configuration, you are about where you need to be.

However, I am having trouble reconciling your measured wet bulb and dry bulb readings, vs the moisture of the wood vs th kiln conditions.  So that is an issue, and if the compressor is not running, the moisture is leaving the kiln somehow, or at least condensing out of the air.  What kind of vents are you using?  Passive vent on one side, fan vent on the other?  Which side of the fan baffle is the passive vent mounted?  Is it mounted to act as a check valve or is it inadvertently dumping or sucking air?

IMO, this is definitely a case of going into the kiln and inspecting the wood, to see what effect the kiln settings are having.  Look for end checking, surface checking, sticker stain, discoloration, etc.  Sensors aside, the the best way to tell what the kiln is doing is to look at the wood itself.  It will tell you a lot.  Assuming everything is looking OK, this could be a case of you won the lottery and don't fix anything for now.  According to what you have said, you are within the max allowable safe drying rate for this thickness and species.  You also have the necessary wet bulb depression to avoid sticker stain.  

Hope some of this helps you figure out what is going on.





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Re: Help with my first load drying
« Reply #10 on: February 15, 2019, 02:26:36 PM »
Thank you all again for trying to help me out! I have increased the baffling in the kiln and air circulation is high. I had some minor air leak around the loading door. I temporarily fixed it with masking tape to see if it helped. With the two the kiln is dropping about 1% a day. 

I check the wet bulb sock every reading to ensure it was wet. One of the readings was at just over 80 (80.2 exactly) but the sock was dry. I added more water but I didnt think much about it. Could the wet bulb be giving me that much of an erroneous reading? At the current readings the humidity in the room would be at about 40% and it is WAY higher than that. How would I adjust the reading if it is off?

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Re: Help with my first load drying
« Reply #11 on: February 15, 2019, 03:46:20 PM »
offdrink
It could be that too much air is going over the wet bulb drying it out or the wet probe is too far away from the bottle drying the wick out. I've attached a pic of how it should be. If you have too much air coming over it a baffle ( about a foot square) can be placed about 3-4 in in front of the wet bulb bottle to reduce the airflow and give a better ambient read.
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Re: Help with my first load drying
« Reply #12 on: February 15, 2019, 05:40:10 PM »
Thanks K-Guy. It is a Nyle kiln under the woodmizer name. I may have called you a couple of days ago and talked to you. I will try  out putting a baffle by it. Either way when the wb was dry it should have matched the db. I will have to look into its accuracy more and if I can calibrate it.

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Re: Help with my first load drying
« Reply #13 on: February 15, 2019, 07:34:39 PM »
The dry bulb and wet bulb probe readings should match anytime the sock is dry, thats one way to tell if your wick is dry.  


To troubleshoot, take the sock off, turn the fans off, put both probes in a still area if they aren't already, no wind, a couple inches apart, they should read the same.  If they don't there is a problem.  If they read the same, then start adding variables back to figure out what is going on.  

 
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Offline GeneWengert-WoodDoc

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Re: Help with my first load drying
« Reply #14 on: February 15, 2019, 11:08:01 PM »
You cannot get the wet wick cooler than the actual WB in the kiln except of the water for the WB is in direct contact with the WB itself.  In general, the bulb is about 1" above the water and there is no splashing of the water.  The end of the bulb is where the sensor is.

It is rare that the bulbs are not incalibration.  Therefore, I suspect you have liquid water in contact with the sensor or maybe a cold air leak on the bulb.  Something is wrong with your system.

As YH indicated, if the wick is dry, then you have a dry-bulb and the reading must agree with the regular DB.  In fact, this is a good calibration test.

Are your bulbs designed with three wires?  This design allows the resistance of the wires to be cancelled out.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

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Re: Help with my first load drying
« Reply #15 on: February 18, 2019, 09:54:03 AM »
I did some experiments this weekend. The dry and wet bulb match when the wet bulb is allowed to dry. The wick that keeps the wet bulb wet is actually drying when there is plenty of water in the wb tank. With the compressor running for 24 hours the wet bulb has no change in temp and the mc of the wood is continuing to drop about 0.7-1.0% a day. With a pinless mc meter (goes to 3/4) it is about 20-24% while the kiln probes, 1 1/2 deep are reading 55.5%.

I ended up covering above the wet bulb with a 6x8 piece of cardboard to try to slow down the air.  We shall see if things change. I am wondering if we lost to much moisture in the beginning to make a difference now.

Offline doc henderson

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Re: Help with my first load drying
« Reply #16 on: February 18, 2019, 10:54:05 AM »
offrink, If you learn a ton on your first load, then it is a success, no matter how the wood turns out.  If these are thick slabs, then your pins may be accurate.  that is why you do not want the outer surface drying to fast and creating surface degrade, due to large gradient between inner and outer wood.  This is done by keeping the relative humidity up in the kiln.  it takes time to move water across the grain.
timberking B 2000, 277c track loader, PJ 32 foot gooseneck, 1976 F700 state dump truck, JD 850 tractor.

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Re: Help with my first load drying
« Reply #17 on: February 18, 2019, 11:02:51 AM »
That is why this 3 basswood was chosen. Not high dollar price and it was ok to cut some off the ends if it doesnt dry correctly. If the capacity of the kiln is about 75% more than the load will that cause a low dry bulb temp? We dont have it filled to the max and was thinking that was one reason why the wet bulb is so low. 

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Re: Help with my first load drying
« Reply #18 on: February 18, 2019, 11:15:10 AM »
some might recommend adding moisture to the kiln,  more heat could help water move but will lower the relative humidity.  Is everything sealed up?  some gas companies will com out and test a building for leaks.  It seems if it is well sealed up, then the humidity should rise.  I am a science guy but not experienced with a formal DH kiln, so I think stan or gene would be your best mentors.  @GeneWengert-WoodDoc@K-Guy .  This will let them know that they have been tagged.  How is the wood looking in the kiln?  It may a combo of a small charge and leaks the are relatively large enough to loose the build up of humidity, that is supposed to be the job of the compressor to help control the drying rate.  The thicker slabs have less surface area for the volume of water, and it all has to migrate to the outside.
timberking B 2000, 277c track loader, PJ 32 foot gooseneck, 1976 F700 state dump truck, JD 850 tractor.

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Re: Help with my first load drying
« Reply #19 on: February 18, 2019, 08:21:28 PM »
Glad to hear the DH unit itself is working correctly.
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