Question about the physics of wood drying

[Vg3200p]

My question is. Let's say you have a pile of wood at 40% moisture. Then let's say you throw some 10% wood in there with it. As that 40% wood dries does the moisture try to equalize within the stack/pile? As in that 10% wood will absorb some of thar? Or does each piece dry individually? Or does a stack/pile kinda dry as a whole?

 

[Ashful]

Assuming good air flow, i think all will try to reach their equilibrium MC% for your climate and time of year. That is likely above 10%, closer to 15% for most places in the US where wood stoves are warranted.

 

[EbS-P]

think of drying as a process where each water molecule bounces around and some probably of crossing the out of the wood at the wood air boundary. Same thing happens for water in the air it hits that wood/air boundary. The higher the concentration of water the more collisions with the boundary but each collision has the same chance of making it across.

When wood reaches it’s equilibrium point of dryness it looses as much water as it absorbs. Really humid conditions it will absorb really dry it will loose some but the daily RH cycle is much faster than the drying rate so think about the average humidity on the time scale of weeks.

So if you had really dry wood wood that was drier than the equilibrium point it would absorb water. If It is in close proximity to wet wood there is a good chance that the water it is absorbing came from the wet wood. But, as pointed out the equilibrium point is about 13%mc for many locations. Arid climates can get down to single digits.

But all this is kinda mute as the only way to get really dry wood is a kiln. And we would probably agree that it’s not a good idea to to stack kiln dried wood outside with wet wood. There are better ways to dry wet wood like a solar kiln. I had decent results using a dehumidifier but, it came at a cost.

 

[stoveliker]

Initially the 10% wood will be the driest place near the wet wood, and will absorb some moisture.
But when the 40% wood is dry (enough for burning), all will have equilibrated as Ashful said.

 

[Vg3200p]

Thanks makes sense. So to dumb it down the wood would dry somewhat as a stack? The 10% in this hypothetical case would absorb some moisture of the 40% as it dries. Now what about wood tgat hasn't reached the Mc equilibrium? If you changed the 10% wood to 25% would it still absorb some of the moisture of the 40% wood? Or would it not absorb because is hasn't reached equilibrium?

 

[stoveliker]

What ebs-p tried to say is that you can't really say "this water came from there".
The low MC wood will absorb some (if it's below the equilibrium MC for the RH of the air surrounding it). ANd likely some of that will come from the neighboring pieces.

If the neighboring wet pieces shed a lot of water into the air, the RH of the air near the drier pieces is relatively high, and thus the drier pieces will absorb some more to reach that equilibrium MC for the higher RH.
However, if you have decent air flow, the wet wood might shed a lot of water, but the wind blows that wet air away, and the drier wood won't see the "very humid" air that is very humid due to the wet wood.

Basically, if you make an air sealed box, then yes, it'll all equilibrate to the MC based on the total water content of wet and dry wood that you start with.
But you don't have a sealed box. Hence all will equilibrate to the MC pertaining to the (average, see ebs-p) MC of the overall air in the town where the drying is happening.

 

[Vg3200p]

Ok thanks I understand better now.

 

[stoveliker]

Ok thanks I understand better now.

Good.

And (tongue in cheek):
lol, go claim that PhD.
There are still scientists that study wood drying.
See e.g. https://physics.aps.org/articles/v13/182

 

[EbS-P]

Drying happens right at the wood air interface. So each piece dries individually but the surroundings affect the environment at that interface. I Do think you could look at it piece by piece, inch by inch or smaller but I don’t think that level of detail is warranted because we see it as a stack of wood. We know the outside will probably be drier than the middle because the outside got more sun and air.

 

[Vg3200p]

Wow very interesting article. Would never have guessed there was so much to wood drying lol

 

[EbS-P]

Wonder how many paint drying jokes they have made!

 

[stoveliker]

Lol. Or what jokes their funding agency made at the coffee corner. ..

Off topic, but there have been papers where they dumped materials in different liquor and they became superconducting (if I remember correctly because of the water in the liquor, not the ethanol). We were all joking at the time that they needed their stock replenished, wrote a proposal, and bought 25 bottles of liquor of which they used one shot glass each for their experiment. Genius.


These wood drying folks needed firewood, so they found a way to dry it at taxpayers expense.

 

[ozarkoak]

One thing I wonder is how it works when wood gets wet while seasoning. What I wonder if its possible for wood to dry faster if it actually gets wet due to Van der Waals force. Due to the wet outer shell pulling water from deeper inside the wood.

​

 

[Ashful]

.. Van der Waals force.

Mere use of that name is going to trigger some pretty serious college exam PTSD in this forum. Last night, there was a collective dream of people showing up to a final exam, for a class they forgot to attend.

 

[Vg3200p]

Just looked up van Der walls on Google and just looking at the equation made my head hurt. Didn't even bother trying to read about it

 

[stoveliker]

One thing I wonder is how it works when wood gets wet while seasoning. What I wonder if its possible for wood to dry faster if it actually gets wet due to Van der Waals force. Due to the wet outer shell pulling water from deeper inside the wood.

​

Capillary forces are far dominant over van der Waals forces. And I think osmotically driven water movement as well.

 

[Split]

I would keep the dry with the dry and the wet with the wet. I put about 2-3 cords in my shed now that it's below 20%. Left the rest covered outside. Sometimes I wonder if all that dry wood in the shed will actually suck moisture in. I can be a wet winter here. On nice days I open the shed doors and let s breeze in.

 

[switepine]

Desert Wood Burning or "Toto, I do not think we are in Northern Minnesota anymore"
Sooooo..... that said now while I figure out my Woodstove, part of my problem is wood that is TOO DRY! Huh? In northern Minnesota that was never a problem. Now I am learning my Beautiful Dead and down bone hard solid Oak I have been putting up has basically a 0% moisture content and it is causing massive creosote build up. There seems to be no free lunch anywhere. So some of my wood may reach 8-10% if I spend a lot of time checking so my question is how do I remedy the too dry/too green/ too wet wood issue? Spray it down? Leave it uncovered? Mix with hard to find greener wood? if I use a 0% MC (moisture content) piece of oak and a 25% MC.....any other desert rats out there that have hit this snag?

 

[stoveliker]

Creosote is a consequence of water in your exhaust that is allowed to condense by having the chimney too cold.

Too dry wood is not a cause of creosote. Burning dry wood produces water (and CO2). Incompletely burning dry wood results in the above plus soot etc.
So the creosote issue is due to your chimney not being kept hot enough, and due to the fire being damped too much (, smoldering).

 

[Split]

Dry wood causing creosote? Is that possible?

 

[stoveliker]

Regarding too dry wood, there may be an issue with how well it can be controlled (see the boiler forum).
But for a stove, I don't think it can be too dry. As long as the stove offers enough control so that the fire doesn't run away on you.

Creosote: no. that is operation or system issues, not the wood being too dry

 

[stoveliker]

Sawdust bricks are dry as well. No issues.

I also think that even in the desert, the equilibrium moisture content is not zero percent, so your measurement is likely off. But that is not a critical issue.

 

[switepine]

Creosote is a consequence of water in your exhaust that is allowed to condense by having the chimney too cold.

Too dry wood is not a cause of creosote. Burning dry wood produces water (and CO2). Incompletely burning dry wood results in the above plus soot etc.
So the creosote issue is due to your chimney not being kept hot enough, and due to the fire being damped too much (, smoldering).

Well there is no debating the fact that there is plenty of talk about wood being too dry.

Regarding too dry wood, there may be an issue with how well it can be controlled (see the boiler forum).
But for a stove, I don't think it can be too dry. As long as the stove offers enough control so that the fire doesn't run away on you.

Creosote: no. that is operation or system issues, not the wood being too dry

Well ok. I have heard from you and you have not addressed the "IMPORTANT" part of my post as I mentioned previously so perhaps others might want to weigh in? There is NO debating the fact that soem experts say MC can be too low and a mixture may be the answer. Thanks

 

[stoveliker]

I thought I was explicit enough: no. Too dry wood is not the cause of creosote. And too dry wood is not an issue at least for wood stoves as long as you can control the fire.

I believe @MMH is in a very dry climate too.

 

[switepine]

Sawdust bricks are dry as well. No issues.

I also think that even in the desert, the equilibrium moisture content is not zero percent, so your measurement is likely off. But that is not a critical issue.

I am using a moisture content device that has a good reputation and Zero is always Zero. No debating that thanks. You obviously do not live in the desert