Slab Wood Fired Kiln

[Arbonaut]

I know mill owners who primarily cut ties and ship them. Whatever the local scavengers don't take of the slabs gets burned in a bin. I'd say 90% goes down the tubes.

Other mill operators I know cater to the retail market and furiture shops or woodworkers and such. I think they're making more money, but have to kiln dry the lumber. They use electric elements to create the heat.

Is it feasible to burn the slab lumber in a boiler setup with water to air exchangers in the kiln room in conjuction with fans and ventilation/drains to dry the lumber?

 

[Kapriel]

Think of it this way.....

Would you heat your house with slab wood, even if you had tons of it ?:msp_unsure:

If you do and don't mind all the issues with it as a fuel your good to go.

On the other hand, if you had a cheap fuel such as Natural Gas or even good seasoned oak.

Which would you use ?

If time and maintenance is an issue you would probably go with Natural Gas
if it's available.

If only wood is available, I would use something like seasoned oak.
It will take millions of Btu's to dry your wood but sometimes wood is the only fuel available.

What are you trying to do, and what do you have available for fuel options ?

 

[4b316]

kiln

A friend of mine built his kiln to hold about 2 piles of lumber,42 wide 40 high and however long and runs it with a outside Central boiler.Had it for about 4 years now and just burns slabs off the bandmill,mainly pine.Has a moulding machine and does floors,trim and such.Had a electric one before that and kept getting a mountain of slabwood,now he can burn his excess but still has a chipper come in once a year to do slabs

 

[MHouse1028]

anytime you can use wood over natural gas makes sense.. depending on the size of the kiln i think it could easily be done with wood heat..sure a lil extra work but who the heck would want to pay for that natural gas...convert a old indoor forced hot air wood furnace to do it you can pick them up cheap off craigslist thats where i got mine complete with a blower for 150.00..it heats my garage and a inlaw apartment all winter long i just built a roof and three walls and haven't used any oil in 2 years and all the mess stays outside

 

[Can8ianTimber]

I am planing on convertin my kiln to a stove heated kiln soon. It is pretty much free heat since I don't really get anything from my waste products. I am planing on using an air stove Waterless Wood Furnace . from what I have heard they run much cleaner than outdoor wood boilers and it costs about 1/2 as much. I talked to the guy that builds them and they have done lots of them for kilns.
I have used anything from houshold dehumidifiers to my nyle kiln unit and they don't work as good as heat and venting from what I have found. That is my 2 cents.

 

[Mahindra123]

Volumes have been written on the art of drying wood in a kiln. It is not a matter of raising the heat to a high level, keeping it there, and then just pulling it out when your moisture meter says it is at 10% moisture level or whatever satisfies you. The secret is controlled heat. A slow temperature rise to prevent warpage, then a steady and controlled heat, and finally a slow reduction in heat. Some skilled kiln owners even add moisture in the final process to stabilize some necessary moisture throughout the entire board evenly to the board's moisture content to prevent further warpage and checking. Using slabwood is perhaps the wisest and almost free source of heat. Heat is heat! Hot air produced from a woodburner is a dry heat and if used in your home for a heat source, the air gets too dry and you must add moisture with some sort of humidification. It is hard to control it effectively. A woodboiler is not as drying but the heat produced causes the wood to give up it's moisture in a efficient manner too but it will take longer but the heat is more controllable in my opinion. There is nothing that will dry a stack of stickered wood in a week - minimum of at least 3 weeks. Dehumidifiers are good but I tried that route once with a load of red oak in the kiln. The acids in the red oak ate out the aluminum coils in the first load!!! So much for that idea. Anything that can produce heat or dehuminification will do the job. To do it inexpensively as possible - slabwood is great! When you run a load in the kiln, make sure to clear your calendar for the next 3 weeks and checking your meters or gauges every 6 - 8 hours and stoking the fire.

Two excellent publications that I use as my bible in kiln drying wood are "Dry Kiln Operator's Manaual" and "Dry Kiln Schedules for Commercial Woods". Both publications come out of Madison, WI and try their website - Forest Products Society for possibly ordering those publications. As far as drying wood - leave it air dry if you don't understand the labor and process of kiln drying.

My statements are in no way pointed at any of the other posters. My words are strictly just that - my words plus experience with failures in trying to learn.

Good Luck!

 

[Dad2FourWI]

Sorry for jumping in... I am not the original poster...

but WOW! Mahindra123, great comments and information!!!!

I see that you are a Wisconsonite too... if you air-dry, how long does it take??? and what seasons can I air-dry in for best results??

Many thanks,
-Dad2Four

 

[Mahindra123]

I... if you air-dry, how long does it take??? and what seasons can I air-dry in for best results??

Thanks Dad2Four. Actually, I prefer air dried over kiln dried. I air dry outside but with a roof overhead and the sides open for airflow. When winter hits and the temps drop below 40 degrees, I cover the pile with a tarp even with a roof overhead to keep the snow from accumulating between the stickers. Negligible drying in those temps and below freezing. Anything I saw for myself is at the 1 1/8" mark and try to keep the boards no wider than 6". Occasionally with high temps in the kiln, the wood gets a very hard surface on the face of the board and I've heard the term sintering to describe it. That wood is difficult to plane and when sawed with the best of tablesaw blades, it chips easily. I never think of even looking at the boards for the 1st complete year. In that 1st year, I tear the stack down and try to invert the whole pile where what was on top is now on the bottom etc and move the stickers close to where they were but not in the exact same spot. You probably heard this 100 times before but always place each sticker directly over the sticker below it. A moisture meter is your best tool to tell you when it is dry. At the start, I lay aside some shorter boards, mark them with a number and weigh them and then distribute them in the spaces made by the stickers. A few in the lower areas, few in the middle, a few on the south side facing the sun - you get the idea. Occasionally I check the piles progress of the drying cycle by weighing each test piece, check it with the moisture meter, and record the numbers. You would be surprised with the variance of the boards from the top of the stack versus the bottom and from the sunny side to the shaded side.
In Wisc, the best drying occurs between mid April thru November but in the other months, the moisture from the middle of the board is slowly migrating to the outside. For 1" boards, you might look closely and test them for acceptable humidity. Anything over that thickness goes into the woodboiler heated kiln. If you are shipping out the wood to customers, air drying is not an exact science and if they want to use the wood immediately - perhaps then consider a kiln.

Perhaps more than you needed or wanted to know and these are only tips learned from my experiences. Best of Luck!

 

[Dave Boyt]

Mahindra,

What final moisture content do you reach with your air drying? I hadn't thought about restacking and inverting the pile & moving the stickers-- makes sense. Would also you rotate the boards and/or turn them over as well? A vacuum kiln takes 4/4 red oak from green off the mill to 8% moisture content in three days. I know some people who have one but they say it is very tricky to operate, and they have $50k invested in it.

 

[Mahindra123]

What final moisture content do you reach with your air drying? I hadn't thought about restacking and inverting the pile & moving the stickers-- makes sense. Would also you rotate the boards and/or turn them over as well? A vacuum kiln takes 4/4 red oak from green off the mill to 8% moisture content in three days. I know some people who have one but they say it is very tricky to operate, and they have $50k invested in it.

My goal is 14% to 16% and I do get it most of the time. As for flipping each board over, I do it to knock off the sawdust that stuck to the wood when it was green but what the heck....you already have that board in your hand so why not? Those vacuum kiln #'s are fantastic to say the least!!!!! I'd have to work my butt off just to keep it going.

 

[Can8ianTimber]

I like all this kiln talk. We don't get into this much on this forum. I typically cut 2.5 - 3" thick slabs and air dry 18months before kiln drying 2-3 months. So far that has worked really well for me. The thing I like about the airstove is that the damper is thermostat controled. Once the temp reaches where you have it set it cuts off the air and cools down the fire. I am sure it would take some getting used to just like the DH has but like it has been mentioned here earlier it is just one kind of heat verses another.

 

[Mahindra123]

I like all this kiln talk. We don't get into this much on this forum. I typically cut 2.5 - 3" thick slabs and air dry 18months before kiln drying 2-3 months. So far that has worked really well for me. The thing I like about the airstove is that the damper is thermostat controled. Once the temp reaches where you have it set it cuts off the air and cools down the fire. I am sure it would take some getting used to just like the DH has but like it has been mentioned here earlier it is just one kind of heat verses another.

I like the "kiln" talk too! Anytime someone can share an idea with another person into a topic familar to them, is not only enjoyable but open one's mind to ideas never thought of before. I never thought of air drying my wood and then putting them into a kiln to finish off the process - Great idea! Thanks.

In a prior post in this thread, you mentioned that a woodboiler type of heat being dirtier than a wood furnace. I'm not quite up to speed on that perception. If you are burning wood in either type, smoke and ash are the by products of combustion. My woodboiler heats my house with a coil in my forced air furnace plenum, domestic hot water, the hydronic infloor heat in my garage and workshop, and kiln. The boiler itself is setup within 100 ft of all of the applications with insulated tubing buried underground. For my needs, the woodboiler works great. Yes, definitely more costly than a woodfurnace and there is nothing I can say "bad" about them. For me it was a matter of "more bang for the buck" with a woodboiler.

 

[Can8ianTimber]

From what I have heard the boiler design puts out more smoke than a typical stove because the entire firebox is completly surrounded with water. I have been told this makes the fire burn cooler and therefore does not burn as clean. A lot of the gasses in a typical stove are burned off at high temps improving the effiency and reducing emisions. Again this is all hear-say and some research. I have seen a boiler running once and there was a giant pluem of smoke comming off of it. We don't see a lot of them the Portland OR area because our winters are so mild. A lot of this also is determined by the moisture content of the wood that you are burning.

How much smoke does your boiler put off? I love the idea of a boiler but don't want to upset my neighbors with a giant smoke cloud.

 

[Mahindra123]

How much smoke does your boiler put off? I love the idea of a boiler but don't want to upset my neighbors with a giant smoke cloud.

The woodboilers of past were as you described but then came the EPA....as you might guess, they came up with stringent regulations for the woodboiler manufacturers to jump thru. I bought a Central Woodboiler from MN 3 years ago and it was approved at 86-87% efficiency. The next year they brought out, under the same "E-Classic" model, one that is touted at 93% efficient. I don't know what any efficiency is meant by it - sounded good at the time I bought it and I've been very happy with it.
As far as smoke emitting from the stack .....perhaps for the first 5 minutes if the wood is added to an existing fire. If your are gone for a few days and have to light a fire, the smoke/steam billows out for perhaps 30 minutes. Definitely the moisture content of the wood is the defining factor. States like Colorado and on the east coast are now telling you when you can burn your fireplace! And despite that, 65% of the electricity produced in this state comes from coal burning power plants. Forest fires burn countless thousands of acres of land and they choose not to fight them? If you ever figure it out let me know...PLEASE!!!! Any of my neighbors are 1/2 mile away and they tell me they like the smell of burning wood - especially birch. It appears they are only happy if you are using commercial fuels that the lobbyists party for. They don't make much money from me.

Have a good one!

 

[Can8ianTimber]

THat is good to know. I have read about the e class of Central boilers. They sound good but they want somewhere around 10k for one. I can not afford that. I can get one of the air stoves for about 3k and the pipeing is cheaper too. That is my 2 cents. If I actually get out my pocket book and buy one I will post how it works on the forum here. Best of luck to you.

 

[Arbonaut]

I also like Mahindra 123's Posts and all the input here.

To Kapriel, yes I would heat with slabwood if I didn't have to walk around mountains of oak crotch and petrified heart Osage Orange to get to the door on my wood boiler. Slabs are free by the trailerload here. And with the irregular growth in todays forests, lots of trees aren't straight. You used to get a lot of bark and air in a load of slabs, now there is much more inner cambium in the scrap, too. I know a few habitual stove tenders heating with slabs.

I make solid body electric guitars and furniture with hardwoods. It is my understanding that without the temperatures generated in a kiln, the moisture inside the cell will not reach the threshold required for it to evacuate the cell. Regardless of the moisture reading, air drying results in a product different than what is specified for using lumber as a tonewood.

Any input on this?

 

[Dave Boyt]

As I understand it. it isn't the heat so much as it is the humidity. For any given relative humidity, there is a corresponding moisture content for the wood, known as the equilibrium moisture content. Air drying only gets the wood so far-- 12% to 20% depending on whether you live in Arizona or Oregon. Heating the air in the kiln does two things: it speeds up the movement of the moisture through the wood, and it increases the ability of the air to hold moisture (lowering the relative humidity). Without kiln drying of some sort, the wood just won't dry out enough to match the relatively drier air inside a house. Drying the wood too quickly can cause a variety of issues by creating internal stress, which is why kiln schedules are so important. Temperatures around 160 degrees F are also good for killing off any bugs in the wood.

I've worked mostly with dehumidification kilns, but am looking at a solar design that a friend of mine came up with me. He built four chambers, each with about a 4,000 board foot capacity. He can use any three chambers for air drying, and the fourth for kiln drying by simply moving a few panels. The wood only gets moved into a chamber one time. Drying times are slower than with a dehumidification, but use of a wood furnace might speed things up a bit. His info is on timbergreenforestry dot com.

The weight method for calculating moisture content is the weight difference between the sample as taken from the pile, and the same piece oven dry, divided by the oven dry weight. That means that a piece of wood that contains its weight in water has a 100 percent moisture content. I'm sure that made sense to whoever came up with the formula.

 

[Arbonaut]

Understood and points taken.

If moisture could migrate from inside the cell in a place like Arizona enough to render a workable product, what prevents reuptake of moisture to the wood cell, only the finish applied to the wood?

I've always asked the same question of kiln dried product: that if a permanent change was induced on the cellular level in the kiln-like the internal stresses referred to here does that prevent the wood from reaching a higher level of moisture during its shelf life based on relative humidity?

 

[Talltom]

Lots of discussion on drying and MC on Woodweb, for those interested in researching this further. There are also plans for solar kilns. I have been air drying my lumber to date but looking into getting/building a kiln & this is what I have learned from that and other sources:

Given enough time, wood will reach the same MC for a given RH whether kiln or air dried. Kilns can get wood to much lower MC than air drying, and it will then reabsorb water to get to EMC. Will this change the tone of the wood? I don't know, but I have read that some species change color during kiln drying, e.g. air dried walnut is preferred for it's deeper color. It is my understanding that this is due to the steam used at either the beginning or end of the process (or both). Kiln schedule is important to get the MC where you want it on a set schedule. If drying is too aggressive, you get case hardening, where the drier outside wood slows water movement from the inside which remains wetter. This can be resolved given enough time in the kiln, but time is money for kiln operators, so most commercial kilns address prevent it by adding moisture near the end of the process to re-wet the outside and get more consistent MC.

My main purpose for using a kiln would be to kill borers. I have not been able to confirm whether solar kilns get hot enough for long enough to do this. Anyone out there have any insight on this?

 

[Mahindra123]

Rehumidification

If moisture could migrate from inside the cell in a place like Arizona enough to render a workable product, what prevents reuptake of moisture to the wood cell, only the finish applied to the wood? I've always asked the same question of kiln dried product: that if a permanent change was induced on the cellular level in the kiln-like the internal stresses referred to here does that prevent the wood from reaching a higher level of moisture during its shelf life based on relative humidity?

Great Point - Kiln dried wood will acclimate to the surrounding moisture and reabsorb some of the moisture. However, the kiln takes it far below the acceptable humidity for that exact point. Unless you soak that piece in water, just air humidity will not affect acceptable limits.

In a kiln at the heats stated, that heat has a tendency of sealing saps in the wood and creating a seal of sorts to prevent the wood cell from taking on any more moisture. That high of heat is foreign to that piece of lumber and in some respects, destroys the cellular properties of that board. DESTROY is a strong word but if you are familiar with the true 2" thick by 14ft ash plank that were used for ramps to(example here)drive a 4010 John Deere tractor onto the truck. White oak was another popular species. These planks were never kiln dried because of their loss of strength in kiln drying them. With furniture making - that is not an issue except when you look at an overweighted shelf. Kiln dried will retain it's straight length without bowing. Air dried will bend sooner but with the weight removed - goes back to it's original straight length. The kiln dried piece will snap in half sooner than that air dried piece. Albeit, kiln drying is much quicker but more labor intensive in tending to the kiln properly. Air drying is a natural process where you stack it and forget it except the one time of turning, restickering the stack. Remember too, moisture is always attracted to something or an area that is drier. With air drying, you are allowing for this to happen at it's own rateof speed whereas with a kiln, you are using heat to force the board to give up this moisture. Only my thoughts and experience Guys