Airflow in Kiln

Mfrost459 · November 06, 2021, 10:30:12 PM

I have a 20 ft insulated high cube kiln with a L200s Nyle kiln unit. My biggest concern is the amount of airflow going through the stack. Right now I am getting about 100 Feet per minute. There are 4, 12 inch fans used to provide airflow. Currently there is about 1700 bdft of air dried white oak in the kiln ( air dried to 20 to 25 % MC). Yesterday I pulled out 1.5 gallons of water in about 36 hours. Today 24 hours later there was practically no water taken out. The dry bulb is set at 120 and wet bulb is 85. Checked the kiln today and the readings are 120 db and 93.4 wb. Last week we dried about 800 BDFT of green poplar and everything went well, lumber came out at 7%mc after about 18 days.
So, after saying all that, what is the proper airflow for the kiln stack? If airflow is my problem I can install 3 more of the fans if needed. Or maybe I need larger fans?
Any help is appreciated, thanks.

Southside · November 06, 2021, 11:47:35 PM

I have 6 fans in a 40' unit, so I think you are fine in that respect. How thick is the oak? At 20% I set my DB to 75 and let her rip, so depending on exactly where your stack is that might be part of the issue. Also, Poplar gives up it's moisture a lot easier than white oak.

YellowHammer · November 07, 2021, 08:02:07 AM

Typically for oak and most hardwood, 150 fpm is optimum and is what Nyle like to see and designs to, however that is a little low for me. I like to stay at about 200 fpm. Also, remember that as wood dries the effect of fans decreases. White oak, being waterproof, can be finicky to dry. The kiln compressor is working fine. You have a nearly 30 degree depression, so the air is very dry.

What is the moisture content now?

Here is one on my little Yellowhammerisms I don't generally pass on, but since its you, I will. I like to generate about a gallon of water per thousand bdft per decade of moisture content of the wood per 24 hours. So for red oak, poplar, etc at 50 percent moisture should give me 5 gallons of water per Mbf per day. 30% moisture and I like to see 3 gallons per MBF per day. At 10% moisture I like to see 1 gallon.

If you aren't getting hardly any water out, depending in your moisture content, the wood may be stalled.

If your airflow or baffling is spot on, then when you had the poplar in, you should have easily gotten 4 gallons a day, because poplar hardly ever stalls. It's hard to make adjustments in kiln performance on white oak or walnut because they can be as finicky as a 2022 Ford pickup.

800 bdft of green poplar taking 18 days seems slow for the ratio of compressor power (Nyle 200) vs the wood load (2hp/800 bdft). Poplar can take 11% per day, so you should have been forced to dial things back pretty hard to keep the kiln from sucking the poplar flat and drying too quick. I can bring 3,000 bdft of semi green poplar to 7% routinely in two weeks (14 days), max, with a Nyle 200.

For poplar, I like to use a much higher airflow, around double at least.

Mfrost459 · November 07, 2021, 08:32:29 PM

This is why I like this forum so much!!! Thanks for the advise.

Robert thanks for sharing the secret decade formula!!! Seems like an easy and quick way to check the loads progress.

The lumber is 8/4.. Seems like I might have about the right airflow for this load but it may be the max these fans will produce. The fans I am running are smaller than the ones that usually come with the L200 unit. The ones I am running are about the size of the ones in the L53 unit. How do you change the amount of airflow for running different types of lumber? If you run Red Oak at 200FPM and Poplar at 400 to 500, how do you make those adjustments?

How much space to you allow from the wall of the kiln to the stack for adequate airflow?

Also, how to you work a load that has stalled? OR, as you say Robert, one that is being cranky.

Southside · November 07, 2021, 08:59:51 PM

Quote from: Mfrost459 on November 07, 2021, 08:32:29 PMAlso, how to you work a load that has stalled?

Shut off the compressor, crank the heat up to 160, and throw 5-10 gallons of water on the floor. Leave it for 24 hours, go find a few chickens, sacrafice them using an approved Santaria ceremony and apply the blood to the kiln door. Turn the compressor back on and you should be good to go. At least that is the process YH taught me.

YellowHammer · November 07, 2021, 10:42:52 PM

I use 6 of the Nyle standard L200 baffle fans, and have them set to breakers in the main panel in multiples to switch them on an off.

18 inches from the side of a stack of wood to the wall is about minimum.

Stalled wood takes patience (I don't have much of that), lots of heat (think of opening up the lumber's pores) and voodoo chickens.

Mfrost459 · November 08, 2021, 07:14:58 PM

Checked the moisture content of the lumber today. It is running from 9% to 12%. One board was at 18% but it was only one that read the high. Any reason why one board would be so out of wack. There was 1.75 gallons of water pulled out in the last 24 hours. Not sure if I should start sacrificing chickens just yet.Guess I will see how much water comes out tomorrow and go from there.

When you raise the temp to 160 with the added water will that help balance out the MC in the load?

Southside · November 08, 2021, 08:10:39 PM

The 18% could be an airflow issue or just a pocket of water. Bacterial infection in Red Oak will do that and thick Walnut is famous for it. The water and heat will help to add moisture back to the exterior surface which "primes the pump" to remove water from the core. When you get the shell too dry and the core still wet you have "case hardened" lumber and since moisture is removed via osmosis the process stops. Think of a bone dry sponge, toss it into a bucket and it will just sit there as the difference in moisture content is so great water can't penetrate throught the sponge exteiror, but get some onto the surface and it eventually fills all the pores. Same thing, just reversed in the kiln.

Mfrost459 · November 09, 2021, 06:59:12 PM

Thanks so much for all your advice. I have learned a lot through this forum and am very great full. Now I just have to learn the proper way to sacrifice the chickens, then I'll have all the essential tools for success.

K-Guy · November 10, 2021, 08:24:50 AM

Quote from: Mfrost459 on November 09, 2021, 06:59:12 PMNow I just have to learn the proper way to sacrifice the chickens, then I'll have all the essential tools for success.

You aren't too far from where Southside is, the high priest of chicken sacrifice for lumber drying, maybe you two will meet and some night he'll show you the secret ritual. smiley_jester :D

Wudman · November 10, 2021, 09:26:39 AM

Quote from: K-Guy on November 10, 2021, 08:24:50 AM
Quote from: Mfrost459 on November 09, 2021, 06:59:12 PMNow I just have to learn the proper way to sacrifice the chickens, then I'll have all the essential tools for success.

You aren't too far from where Southside is, the high priest of chicken sacrifice for lumber drying, maybe you two will meet and some night he'll show you the secret ritual. smiley_jester :D

The rules of chicken sacrifice.....check the kiln before you close the doors......an unintended sacrifice will negate any official ceremonies. Check under your toolbox before you drive offsite.......Relocated chickens will disturb the natural balance of the universe. Beware the rooster that occupies the mill shed. Make eye contact and prepare to defend yourself. A LogRite cant hook is a viable tool for this purpose as is a size 12 Red Wing. Remind him that you are fairly efficient at processing chickens and even he can make a fricassee. The blood sacrifice is supposed to be chicken blood, not yours. Expect a hen to be beside the seat on the Lull. Don't have a heart attack when one jumps by your head as you are getting on the machine. A heart attack adds to the bad mojo. Check all four wheels for a setting hen before moving.....see bullet point above on unintended sacrifices. Don't run a chicken through the edger. See above point on unintended sacrifices. Finally, beware of bomb craters caused by scratching chickens. Rake them out before start-up that day or you will be stumbling around. And.......Dang, those fresh eggs are good.

smiley_turkey_dancing

Wud

YellowHammer · November 10, 2021, 08:12:53 PM

I don't know if it can be done wrong, but I believe the original instructions I gave to @customsawyer when he had some stalled wood was very similar to what I use, and have been proven to be effective for years. It sounds like @ Southside has perfected the technique, as I have seen pictures of chickens everywhere in his operations, and why else would he have them their except as a solution to a possible upcoming lumber drying or saw milling problem?

Its pretty basic. Get some chickens. Sacrifice them. Wipe their blood all over the sides of the kiln, slinging it everywhere, while making all kinds of secret Voodoo symbols. Then hold the dead chickens in the air and start dancing around in your underwear, and chanting. When exhausted, or when sure the Voodoo was taking hold, or when the beer was gone, throw the chickens on the grill and have a good dinner. If someone is squeamish, KFC is a suitable substitute.

The funny thing was during one of Jake's Sycamore Projects, Dr. Gene was giving a kiln drying class, and the subject of stalled wood came up from the audience. After Dr. Gene gave a scientific, although dry and matter of fact answer, Jake breaks in and authoritatively describes how VooDoo chickens will do the same thing just more reliably. :D :D :D Gene listened, looked at him and was virtually speechless because Jake delivered the technique so matter of factly. I'll never forget the look on Gene's face. Eventually, he realized it was a joke, but for a while there I don't know what he thought. I and a few others, including Jake, were just about to blow out from laughing at the whole thing. Good times.

The VooDoo Chicken Kiln Drying Schedule works wonders. You just have to believe, and like dancing.

customsawyer · November 12, 2021, 06:33:50 AM

Still laugh about that moment at the project on a regular basis. I went buy Southsides place on my way home from Richmond. I will say he has plenty of chickens. I can't figure if he has never stalled a load so never had to sacrifice one or if he stalls loads all the time and tries to keep some on hand.

GeneWengert-WoodDoc · November 13, 2021, 08:09:54 AM

That was two years ago, and thanks to your posting today, I finally got the truth...it was a joke. ;) ;)

Back to air flow. Under about 20% MC, and maybe closer to 30% MC for 8/4 oak, air flow is not an issue for drying lumber. We are waiting for the moisture to move from the interior to the surface, which is slower and more difficult than being scrubbed off the surface by air flow. The air flow on the outside does not make moisture on the inside move more quickly. So, it is only temperature and humidity in the air that affects drying speed at low MCs. As low humidity speeds up drying, it also increases the shrinkage which can aggravate small checks, making them worse. So, we elevate the temperature. But temperature increase is impossible with some DH kilns, so drying slows down after an initial MC loss of the moisture at and near the surface.

When drying slows down or seems to stop, if we are able to raise the temperature for a few hours to 130 F, maybe up to six hours, and then returning to the normal temperature,, that will get the moisture moving again.

On the farm, when I was a teenager, I had to go into the chicken coop and collect eggs. The smell was unbelievable from about 60 chickens. So, I learned to hold my breath for about 90 seconds before I came out and breathed clean air. I still remember the smell, so chicken stories bring back unpleasant memories, but I was able to swim under water for huge distances as I could hold my breath so well...I won a contest beating even adults.

Southside · November 13, 2021, 08:28:17 AM

Seriously? It was a joke? Uh oh. :D Anyone know where I can sell a bunch of high priestest candles?

scsmith42 · November 13, 2021, 10:29:19 AM

Love the chicken ala Wengert story! :D

Mfrost, what thickness of stickers are you using? In my 6 fan L200 there is a significant difference in airflow if I use 1" stickers versus 3/4".

Typically when we're milling, if we know that we will be kiln drying the load we will stack using 3/4" stickers on 18" centers.

However, if we're milling wide slabs that will be air dried for a few years, we will go with 1" stickers to allow for less restriction during the air drying process. Also, wood that we don't intend to kiln dry may get stickered with the 1" stickers (and sometimes we end up having to grab a stack of 1" stickered wood and place it in the kiln).

Typically the different thicknesses in stickers can make up to a 30% difference in airflow rates, with the thicker stickers having a lower air flow.

customsawyer · November 14, 2021, 07:50:58 AM

Not to hijack this post but during that project there was several folks that had the same look on their face as Gene had. Most of the looks had something like "what's wrong with these people" type of looks.

Southside · November 14, 2021, 09:31:23 AM

Pretty sure that same look flashed over the crowd when @doc henderson did his little chemical firebug concoction.

doc henderson · November 14, 2021, 02:36:52 PM

WHAT? who does not do a spontaneous combustion demo in a sawmill! bon_fire

wkf94025 · January 26, 2022, 10:29:40 AM

@YellowHammer and @GeneWengert-WoodDoc thanks for your guidance on this. I *really* like @YH's formula, though scratching my head on the ~10 gallons per day I am extracting from each of two kilns, each with ~2,000bf of redwood, most of which is 1x8, some 1x6 and a minority of 6x6. Kilns are Wengert-inspired solar, with a Honeywell residential DH running 24x7 and a few box fans for internal recirc. Does the formula change for softwoods such as redwood, doug fir, et al? Regardless, for a given set of daily solar gain, appliance 24x7 schedule, box fan schedule, and venting the kiln once or twice a day, watching the drip rate coming out of the DH's is a pretty good indicator of "doneness" of the bake, right? And safe to say redwood is one of the less fragile species in terms of case hardening, checking, etc.?

No apparent need for chickens at the moment, but will keep them in mind...

YellowHammer · January 26, 2022, 11:34:50 AM

Yes, the rate changes for softwood, which generally gives up its moisture very quickly and easily. It will come out like a faucet, up to the capacity of moisture removal by the kiln unit. However, you should still be aware of the particular species maximum allowable drying rate, although most times, it's so high, it practially impossible to exceed.

Most times, when the condensate line stops dripping, the wood is dry. However, there is a real possibility of over drying the wood, and then it will have a tendency to chip out when getting planed or even increase a board's tendency to bow, warp and crook. So its best to hit the target moisture level, and not overshoot it.

Another Yellowhammerism "When the cookies are burnt, they are burnt, and you can't unburn them." So dry the wood, but don't over dry the wood.

wkf94025 · January 26, 2022, 01:26:23 PM

Thanks for confirming what I suspected. And is the climate RH% <> wood MC% correlation I've seen in several places now independent of species? I would think so, if you dry it long enough. So for redwood, my MC% goal is low teens. What stable RH% in the kiln over several days would suggest to you that I'm about there? Easy to open doors on both ends and pin check a board, just curious what my nominal RH% target should be for low teens final MC%.