iceman
Minister of Fire
gyrfalcon said:
Lmao I thought this was the one I started last year!
Wow early onset of what is that memory loss called?
Does firewood still dry below 32 degrees?
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Lmao I thought this was the one I started last year!
Wow early onset of what is that memory loss called?
LLigetfa
Minister of Fire
mesuno
Member
For people who are doubting drying below freezing - that is exactly how instant "freeze dried" coffee is made, albeit under carefully controlled and engineered conditions.
What matters is the relative moisture content of the air - provided it is below 100% it still has potential to remove moisture from wood.
Some other factors are worth mentioning - water in wood is stored in two ways, as "free" moisture such as sap and "bound" moisture trapped as part of the structure of cell walls. Ice swells as it forms, potentially rupturing cell structures and freeing more moisture so it is quite possible that a good hard freeze will overall speed up drying.
Also, below freezing temperatures it tends to snow rather than rain. Snow blows off without wetting the wood so firewood can continue to dry even in a snow storm - not possible when it is raining however.
Mike
What matters is the relative moisture content of the air - provided it is below 100% it still has potential to remove moisture from wood.
Some other factors are worth mentioning - water in wood is stored in two ways, as "free" moisture such as sap and "bound" moisture trapped as part of the structure of cell walls. Ice swells as it forms, potentially rupturing cell structures and freeing more moisture so it is quite possible that a good hard freeze will overall speed up drying.
Also, below freezing temperatures it tends to snow rather than rain. Snow blows off without wetting the wood so firewood can continue to dry even in a snow storm - not possible when it is raining however.
Mike
mesuno
Member
Sadly not 
But the vacuum isn't essential to the process, it just speeds it up. The question was whether or not drying happens at sub zero temps.
Interestingly, you should be able to get "0%" moisture content wood by freeze drying - I wonder how well it would burn :D
But the vacuum isn't essential to the process, it just speeds it up. The question was whether or not drying happens at sub zero temps.
Interestingly, you should be able to get "0%" moisture content wood by freeze drying - I wonder how well it would burn :D
Felled a 12" caliper Shagbark Hickory ten days ago cut it up and stacked it in the shade, covered top, not sides. MC 22%
It has not gone above freezing here since then and we have had a major coastal snowstorm.
Today, all the stove- length log ends are visibly checked. MC now 20%
Read my previous post. Properly stacked and covered firewood dries in winter. No vacuum chamber required. Tired of the naysayers and instigators, I am done with this thread.
It has not gone above freezing here since then and we have had a major coastal snowstorm.
Today, all the stove- length log ends are visibly checked. MC now 20%
Read my previous post. Properly stacked and covered firewood dries in winter. No vacuum chamber required. Tired of the naysayers and instigators, I am done with this thread.
I could be wrong but the vacuum process is very important in freeze drying, I dont get it everything dries quicker with heat so why do people seem to think fire wood dries well when it is cold, been burning for over 30 years and have never thought the wood dries very much in the winter.
PapaDave
Minister of Fire
Mrs. Krabappel
Minister of Fire
LOL!PapaDave said:
I hang laundry outside all year, so yup. Sometimes it gets a little crunchy first. Sometimes it dries better in February than it does in August.
Kilks
New Member
Fun thread and I love all of the science! I am a science dork at heart so I had to crank a few numbers and it turns out that ice in air that is 0C or below will always sublimate. For ice to sublimate it has to have a temperature and pressure below the triple point of 0 C and 0.006037 atm (approximately...). Even under the worst case scenario of 0C and 100% relative humidity, each cubic meter of air has 4.8g of water. Convert that to moles and use the ideal gas law (of course assuming water is an ideal gas and under these conditions it would behave pretty much like an ideal gas) and you find that gives a partial pressure of 0.00597atm. Under these conditions water will sublimate. If you drop the temperature to say -25C and 10% relative humidity, each cubic meter of air would have 0.1g of water giving a partial pressure of 0.000113atm, far below the necessary triple point. It is difficult to calculate the exact rate of sublimation due to the large numbers of variables involved, but the conditions are right in terms of pressure and temperature. Studies that have looked closely at the evaporation rate of snow have found wind (flow rate) to be the greatest contributing factor, and found the greatest rates of sublimation closer to 0 C. At -5C they were able to measure snow sublimation rates of 0.35E-6kg/s at 8SLPM flow rate, which would translate to a loss of about 1/2 pound of water sublimated in a week in a windy location.
I found it pretty interesting that ice will always want to sublimate when it is below 0C out, even at 100% relative humidity. That surprised me for sure! I teach chemistry, so next time we go over phase diagrams I will definitely have another cool example for my students - well at least I will think it is cool :cheese:
I found it pretty interesting that ice will always want to sublimate when it is below 0C out, even at 100% relative humidity. That surprised me for sure! I teach chemistry, so next time we go over phase diagrams I will definitely have another cool example for my students - well at least I will think it is cool :cheese:
gyrfalcon
Minister of Fire
Kilks said:
Something missing here, I think. "a loss of about 1/2 pound of water sublimated in a week in a windy location" out of how much what?
Those of us who live in windy northern areas know wood seasons surprisingly fast in the mid-winter months, but our more southerly brethren don't believe us.
Thanks for speaking up.
Kilks
New Member
Just make sure it is completely seasoned (sublimated) before you put it in the stoveBrotherBart said:
Kilks
New Member
For the sample in this study they used a cylinder 7cm diameter 1cm thickness - they calculated the rate in kg/s but I just extended it out to pounds per week to get a better sense of the numbers. Greater surface area of a sample also yields greater sublimation rates, just like evaporation rates.gyrfalcon said:
Yeah, hard to beat a windy winter day for seasoning in my book.
Mrs. Krabappel
Minister of Fire
rphurley
Feeling the Heat
I found a huge Shagbark had fallen in June, 2009. Within a couple of months I had it split and stacked in a single row. I didn't think I would burn it until next winter. It's proximity to the house led me to to burn some of it this season, and sure enough, it is as dry as a bone. If this was White Oak I never would have tried to burn it.5cordayear said:
cozy heat said:ISeeDeadBTUs said:
Well, lets see...mathematically (and making a few assumptions
180 lbs of wood, 2% moisture = 180 lbs wood x 0.02 = 3.6 pounds of water
Heat required to vaporize 3.6 lbs of frozen water =
Ice @ 32F to water @ 32F = 144 btu/lb x 3.6lb = 518 btu
Water @32F to water @ 212 F = 1 btu/lb per degree = 1 x 3.6 x 180 = 648 btu
Water @ 212F to steam @ 212F = 540 btu/lb x 3.6 lb = 1944 btu
So, assuming you had 180 lbs of frozen wood @ 15% MC, you'd expect about 7600 btu/lb or 180 x 7600 = 1.368 million btu. (call it 1.4 million btu for a round number) If that wood had 17% MC at the same starting temp, you'd expect (518 + 648 + 1944) = 3100 btu to be used simply vaporizing the additional water. You'd only get ~ 1.396 million btu out.
That's why I belong to this forum!
ie - the wet wood cost you 3100 btu
For cold wood, It looks like most hardwoods have a specific heat of ~ .287 btu/lb-F, so to raise 180 lbs of wood from 32F to 50F would take 180 x (50F-32F) x .287 = 1446 btu
so warming the wood saved 1446 btu
Overall, on these specific conditions, you'd be better off with dry wood versus warm wood. Though you also have to consider where the warmth for the wood came from in the first place...if it happened to be solar or some other external source, then this is truly a gain. If the heat came from simply being in the same room from the stove - then you've only moved heat from the room into the wood...you haven't really gained anything.
That's why I belong to this forum.
gyrfalcon
Minister of Fire
Kilks said:
OK, thanks very much. Very much appreciate that you did your calculations-- and also uncovered the bit about the sublimation happening at below freezing. That's really interesting, and surprising to us non-geekish types. ;-)
OhioBurner©
Minister of Fire
I havent read through this thread but something I noticed made me think of it. My basement is the dryest it has been since installing a dehumidifier. It is saying 50% right now, if that is accurate. Its about 50 degrees. I spilled a container of water and noticed it beaded on the small peice of carpet I had there. Its been almost 2 weeks and the water is still beaded. I think having wind makes a huge difference as even in ~50 degree temps with 50% RH tiny water droplets arent going anywhere, but outside even in colder temps I am sure they wouldnt last a few hours.
gyrfalcon
Minister of Fire
►►OhioBurner◄◄™ said:
My totally unscientific observation is that wind is at least, if not more, critical to drying wood than heat per se-- especially humid heat. Outdoors in winter (serious winter like we have in the north), I'm thinking from what the poster above says, sun matters not at all.
My first winter burning, I ended up doing the usual thing and buying a nice load of good rock maple in late fall that was ostensibly "seasoned" but was really unburnable, and I stacked it up against the north wall of my barn in complete shade. I lucked into some very burnable firewood, but it only got me through the worst of winter, and by early spring I was flat out unless I started looking seriously at the furniture. I went over and eyed the wood stacked against my barn, hefted a couple pieces and decided to try it. Burned like a beaut.
Apparently, I have ideal winter drying conditions-- well below freezing all but a day here and there during four months of winter, low humidity (except when it snows) and nearly constant wind. Seems only fair, no?
gyrfalcon
Minister of Fire
oldspark said:
Dunno Oldspark. I don't imagine you have high winter humidity there in Iowa, do you? Are you below freezing most of the winter? Try sometime marking a split from your stacks and weighing it on a scale in the fall, then weigh it again in March or April and see what you think. No question, none, that wood dries significantly over the winter where I am, so much so that I'm not yet convinced it doesn't dry faster than it does in summer. What it does in the stove is the bottom line, though.
Things dry better with heat even with high humidity, not sure why people think firewwod is any different than anything else, the wood checks much much more in the summer than in the winter.
Below freezing in winter up here, yes we are in North West Iowa so below zero a fair amount.
Not being a smart ass but the thought of wood drying more in the winter than summer is amusing to me.
Below freezing in winter up here, yes we are in North West Iowa so below zero a fair amount.
Not being a smart ass but the thought of wood drying more in the winter than summer is amusing to me.
Not sure if this is any help to any one but it makes sense to me.
"Snow and ice sublimate, although more slowly, below the melting point temperature. This allows wet cloth to be hung outdoors in freezing weather and retrieved later in a dry state. In freeze-drying the material to be dehydrated is frozen and its water is allowed to sublimate under reduced pressure or vacuum. "
"Snow and ice sublimate, although more slowly, below the melting point temperature. This allows wet cloth to be hung outdoors in freezing weather and retrieved later in a dry state. In freeze-drying the material to be dehydrated is frozen and its water is allowed to sublimate under reduced pressure or vacuum. "
Adios Pantalones
Minister of Fire
Ice sublimates. It happens much slower than water evaporating. Both happen slower the colder it is.
This is fact.
Further- when ice sublimates from the outer layer of wood, redistribution (getting the water in the middle to move out to the edge so that it can sublimate) is verry sloow, so you can't just look at ice sublimating and guess how fast wood will dry. Not even in the same ballpark. Not in the same league. Not the same sport.
Phase diagram is a good starting point, but it doesn't address the kinetics (rate) of the problem, which are the question here.
This is fact.
Further- when ice sublimates from the outer layer of wood, redistribution (getting the water in the middle to move out to the edge so that it can sublimate) is verry sloow, so you can't just look at ice sublimating and guess how fast wood will dry. Not even in the same ballpark. Not in the same league. Not the same sport.
Phase diagram is a good starting point, but it doesn't address the kinetics (rate) of the problem, which are the question here.
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