Nice video on visually showing how much water is in wet wood

[mbutts]

I found this video while looking for wood gasification videos for engines. Its made by a furnace company in the UK. It is totally amazing how much water is in wet wood as opposed to dried. Really makes you RETHINK burning something less than dry.
http://www.youtube.com/watch?v=-Rp1yBpP3xE&feature=player_embedded
I'm glad I didn't hit the back button on the browser too quickly.

 

[begreen]

That's a stunningly graphic illustration. Starting at about 1:50 in the video is the use of scales to show how much more water is in the wet wood. Also amazing is when this difference is multiplied out. If you are burning about 5.5 cords of wet wood a year, you are sending about 124 cu ft (925 gallons) of water up the chimney! According to the video, dry wood contains ~45% more heating energy than wet.

 

[ecocavalier02]

thats a lot of water to boil. interesting how much water goes up the chimney like that.

 

[Backwoods Savage]

That's a stunningly graphic illustration. Starting at about 1:50 in the video is the use of scales to show how much more water is in the wet wood. Also amazing is when this difference is multiplied out. If you are burning about 5.5 cords of wet wood a year, you are sending about 124 cu ft (925 gallons) of water up the chimney! According to the video, dry wood contains ~45% more heating energy than wet.

Yes, this really shows it with a great illustration. If more folks would realize this they would see just why they are not getting the heat they thought they would. How about that guy who posted on ebay bad mouthing the newer EPA stoves?! No doubt, he does not have a clue.

 

[Clarkbar2311]

What a great illustration. I need to learn more about wood boilers and tieing into a standard HVAC system. Now for some more reading.

 

[Battenkiller]

If you are burning about 5.5 cords of wet wood a year, you are sending about 124 cu ft (925 gallons) of water up the chimney! According to the video, dry wood contains ~45% more heating energy than wet.

According to the video? Well, here I go again... according to BK.


There is a lot of bogus info in that segment. Either the makers of the video are misinformed or they are being deliberately disingenuous. It is a grand example of presenting perfectly valid facts in such a way that you end up drawing some very erroneous conclusions, and it is very unfortunate that this stuff seems so credible when presented in a video this way. This is, after all, exactly how propaganda works.

For starters, no one burns wood that is 50% water by weight (which is exactly what they are showing you by weighing them as they add the water). Wood that is 50% water by weight is precisely 100% MC dry-basis - so far above the readable range on a moisture meter that it could only be measured using the oven-dry method. Even green oak only has a MC of 80% dry-basis. Again, they are mixing metaphors, so to speak. Wood at 100% MC dry-basis simply will not burn in a normal wood stove at all.

Secondly, wood that is 20% water by weight isn't really dry wood at all. It's simply what works best in a wood stove. Plenty of water remaining in that split on the left. Heck... 20% by weight, eh?

The most ridiculous thing is the claim that a given split of wood that contains 50% water by weight has 45% less potential energy than the identical split containing 20% water. That is surely true when you consider the BTUs per pound, but not true at all when you are thinking about wood in volume. Look how much bigger that split is on the left. If they could pour all that water back into the wood instead of into a graduated cylinder, you would see that it would not really swell much at all. The wood on the left has more wood fiber in it... at any moisture content. That is the only reason why it contains 45% more energy.

The reality of the situation it that there is only a very small amount of heat energy lost during the evaporation of that extra water. For that amount of water shown, I calculate roughly 640 BTUs lost out of a potential 17,600 BTUs compared to a 1 kg split at 20% water - less than 4% total potential heat loss. Yes, there are other losses due to less efficient combustion, but they don't come anywhere near close to a 45% loss.

The total amount of heat energy contained in any piece of wood is limited by the amount of dry wood fiber locked into it. In order for a split with 20% water to contain 45% more BTUs than one with 50% water, it has to be substantially bigger. Like maybe 40%, allowing for a small amount of shrinkage between the volume at fiber saturation point (about 28% MC) when wood first begins to shrink and the volume at 20% MC. That's what you are really seeing in the video.


FWIW even if your wood is oven-dried to 0% water in the wood, if you burn 5.5 cord of a decent hardwood like ash or oak you are sending up about (drum roll, please) 1335 gallons of water as a product of combustion alone. Adding in the 20% water found in seasoned wood and you exhaust another 495 gallons, for a grand total of 1830 gallons of water per season. That's 14,650 pounds of water. At 970 BTU/pound of unrecoverable latent heat of condensation, you are losing 1.4 million BTUs up your flue every year... even with good seasoned hardwood and 100% combustion efficiency.

 

[Mad Tom]

BK, could you put that into a video?

 

[BrotherBart]

Or maybe CliffNotes.

 

[Battenkiller]

Or maybe CliffNotes.

Funny guy. Hey, it's not my fault, once my typing finger gets going it don't know when to quit. Lucky I only have one good one, if I had two I'd have to break my posts into multiple entries like Craig does on the Ash Can.

 

[BrotherBart]

Actually Professor I enjoy your posts a lot. Of course I have to temper that with that I think anybody that tries to turn burning very large weeds to heat their house into a science is pissing into a hurricane. Every piece of that weed you put in that stove, every position you place it in, the stove itself, the chimney itself and a whole bunch of other stuff will either heat your house or bite you in the butt.

Not like calculating the nozzle size for the oil burner.

At the moment I have an experiment going. It is twenty degrees outside and I am burning a big ass non-cat "low and slow' with blue natural gas looking flames coming off of just three big splits N/S with a couple of inches between them. No secondary burn BS or raging stove top temps. Just a nice little five hundred degree burn to see what is left in the morning. And since I cleaned the pipe yesterday, a good way to see if it craps it up and how long it heats the joint tonight.

 

[lowroadacres]

I am not real strong on math but the one figure that I would question is the 900 plus gallons of water going up the chimney....

At approximately 10 pounds per gallon that would mean over 9000 pounds of "wood weight" gone to water up the chimney?

At the same time.... I am truly a convert to the properly seasoned wood approach and have been for a while.

 

[Renovation]

According to the video? Well, here I go again...

As someone who perpetually overthinks things (though perhaps not as usefully) I enjoyed your analysis. I'll further ingratiate myself with the forum by encouraging you.

Maybe I think too much for my own good
Some people say so
other people say "Oh no,
the fact is you don't think as much as you should."
Hmmm....
.
--Paul Simon

Your analysis of the exaggerations in that video is enlightening.

But, since the topic of wet wood is now on the table, I'd like to suggest another factor:

Aren't the consequences of wet wood more than just a net energy loss, in that wet wood also causes inefficient burning, and creosote?

In other words, isn't burning wet wood not like a great burn with some energy subtracted, but a lousy burn, that get's less energy out of the wood, even if the energy used to vaporize the extra water was added back in?

Inquiring minds want to know.

 

[Battenkiller]

Always happy to entertain, BB.

Actually, I don't really think anyone can burn well using science. Chaos rules inside that box. My hope is to remove some common misconceptions about wood burning and wood itself. The expectation is that folks might use scientific theory to plan their burning in principle without expecting the stove to go along with it in reality.

I've come to think of my windowless heater as Schrodinger's box. Like the cat in that example, until I open the top and peek in, I don't have a clue whether the cat is dead or alive. It may be burning great, or some random event inside may have changed the whole course of the burn. Schrodinger would say that until I look in, the stove is both burning well and burn g poorly at the same time. I'll leave that to the quantum stove theorists and just look in and adjust it.

 

[Battenkiller]

In other words, isn't burning wet wood not like a great burn with some energy subtracted, but a lousy burn, that get's less energy out of the wood, even if the energy used to vaporize the extra water was added back in?

Actually, burning wet wood is more like being in Hell... only colder.

 

[Renovation]

In other words, isn't burning wet wood not like a great burn with some energy subtracted, but a lousy burn, that get's less energy out of the wood, even if the energy used to vaporize the extra water was added back in?

Actually, burning wet wood is more like being in Hell... only colder.

I'll take that as a yes. :lol:

 

[mbutts]

My point for sharing the video was to help people to realize by visualization that it is better to let you wood dry if possible (the longer the better) and to plan ahead for the next season. When you see the water on the left in the scale segment of the vid, it kind of sends home the notion that high moisture wood HAS more water in it. And the fact that it does not warm your house as good as dry wood.
Ideally, IMO wood should be split and either stored inside or covered to speed up the the drying. Air circulation is important too. Last year due to circumstances I had to get by with a lot of less than dry wood. It sucked. This year, things are more back to normal and things are much toastier inside. I like BK's analogy of wet wood like being in hell only colder.
Here is a nice vid also on drying wood
http://www.youtube.com/watch?v=uTwfXc_rQA0&feature=mfu_in_order&list=UL

 

[Battenkiller]

http://www.youtube.com/watch?v=uTwfXc_rQA0&feature=mfu_in_order&list=UL

OMG! Steve Slatter is drying wood by the stove? I thought he knew something about wood and stoves and such. :roll: :lol:

I've been doing that on a much grander scale for about 20 years in this place. Not only does it help keep the humidity up in here, I end up with a drier product than just about any of you here are burning. Too dry. Have to add semi-seasoned to the mix. I checked last night, and cherry that was cut and split in November is already at 10% MC on the outside and 14% on the inside. Huge black birch splits are 12% on the outside and 24% on the inside. And we haven't even hit the coldest (and driest) part of the winter yet.

Best thing about it - no bugs. The stuff was cut and split after the buggies are all dead or hibernating or whatever the bugs do in the winter. OTOH several splits of both red oak and bitternut hickory I had left over from last year had me a bit started when I brought them in the night before and found carpenter ants pouring out of them in the morning after they woke up.

Unlike Slatter's method, I play a large fan on the back of the stack (and toward the stove), which circulates the dry air thorough the spaces in the stack. Like having a breeze 24/7. Blows away the air that would otherwise be sitting right next to the wood at nearly 100% relative humidity. Speeds the whole process up a bunch... no science required. :cheese:

Just brought in half a cord of big black birch splits that are so dang heavy with water it's practically dripping. No way to check the MC with a meter (maybe I'll oven-dry one and post the result), they're so far above the overload point on my meter it's ridiculous. I'll bet I'll be burning this stuff in February and March, and it will be down to 20% on the inside. I could cut that time in half just by having it split smaller, but I deliberately asked the guy to leave it huge so it won't burn too fast when the things drop below 20% MC.

This is the first time I've ever brought in that much black birch at one time. The smell is unbelievable, like 100 Valley girls are in here chewing Teaberry gum. :coolsmile:

 

[begreen]

Love that smell. Can you accurately weigh a couple of the wettest splits right now and write the weight on them? Then weigh again in March and post the results here together with the moisture reading on the inside after it is resplit. That would be great info and we can calculate the weight of the moisture lost from this.

 

[Battenkiller]

Love that smell. Can you accurately weigh a couple of the wettest splits right now and write the weight on them? Then weigh again in March and post the results here together with the moisture reading on the inside after it is resplit. That would be great info and we can calculate the weight of the moisture lost from this.

BG, I'll do better than that. I'll go out right now and grab a wet one from the stack and do the microwave routine on it. Weighing it right now won't tell me what the actual moisture content is at the beginning. I tried that last year and posted the results. A cherry split that weighed almost 8 pounds got down to a little over 5 pounds in about three weeks. Or something like that, I'll have to do a search. Of course, this is black birch. Much higher in water, and it won't let that water out so rapidly.

 

[Battenkiller]

I'll start another thread on this when I'm done with the MC determination. In the meantime, what I did was to grab a fairly dense and sopping wet medium-size piece and brought it in and took a small slice off the end to get rid of any wood that may have dried while it was outside the last couple days. Then I took a 2-3" slice off the end and weighed it on the old hippie herbal remedy weigh station (triple beam scale). Came out to just a hair under a pound - 445.4 grams.

I placed the split on my old produce scale and it weighs 10 pounds, 8 1/2 ounces. I will leave it right there and weigh it each day and record the weight to get a drying curve established. I will use the info from the smaller sample to evaluate the moisture content of the larger split as it changes over the next few weeks.

I thought this was an exceptionally dense piece of wood, so I got wondering... Remember the old joke from the early 80s?

"What the only kind of wood that doesn't float?" "Natalie."

Well, fresh-cut black birch is another one. Took it into the kitchen, filled the sink and dropped it in. Sank like a rock. The little glass dish in there was put in to disturb the top so you can see that the wood is at the bottom.

I then took it out of the water and went downstairs to chop it into little pieces with my trusty kindlin' meat cleaver (had to make sure I got every little piece). This will allow it to dry much quicker. I want to make sure I go slow with this one since the last time I did this I just wanted to demonstrate the method. This one's for the books. Any unwanted charring of the wood will cause the sample to lose some volatile substances, skewing the result to show a higher initial MC than it really had.

 

[Battenkiller]

It was taking forever to dry it in the MW, so I did it old school - overnight in an oven set at 215ºF. Woke up and weighed the pieces on my triple beam - 282.9 grams. There was no evidence of charring anywhere, so I think the stuff is a good.

445.4 - 282.9 = 162.5 grams water lost = 36.5% water by weight (wet-basis), or 57.4% MC (dry-basis). Not as wet as it looked, but certainly twice as high as any moisture meter's accuracy.

I decided I don't feel like maintaining a thread about this, but I will takes note of the weight of the split every evening, along with the RH in the room. So far, the split has lost almost 6 ounces since last night. I expect it will have lost a half pound by this evening. The RH in the shop was determined using a Taylor sling psychrometer. Dry-bulb temp was 83º, wet-bulb temp was 66º. 17º wet-bulb depression at that temp = 40% RH. Not as low as it will get when all the wood starts drying out. Right now it's acting as an organic humidifier with a wonderful scent.

As my stockpile of wood dries more and colder weather hits, RH down there should drop to about 20-25%, at which point the wood will be fairly well dried out. I''l post the results next month or so, but anyone interested in how it's going is free to PM me about it.

 

[Creekyphil]

The reality of the situation it that there is only a very small amount of heat energy lost during the evaporation of that extra water. For that amount of water shown, I calculate roughly 640 BTUs lost out of a potential 17,600 BTUs compared to a 1 kg split at 20% water - less than 4% total potential heat loss. Yes, there are other losses due to less efficient combustion, but they don’t come anywhere near close to a 45% loss.

Your units are off. For 1 kg split at 50% moisture content, there are 500g water, and 500 g wood fiber. At 20% MC, the same split is 500 g wood fiber, 200g water. It should be 643 BTUs lost to heat of vaporization for the 300 g of water / 8800 BTUs for 500 g of wood fiber, not 1000. That is a 7.3% potential heat loss. Not the 45% from the video, but not 4% either. One nice thing about dimensional analysis is that if your units come out right, you've just double checked your own work.

Regardless, I think that the way to test this would be to burn wood of varying MC in a bomb calorimeter at around 75% efficiency, instead of fooling around with the theory. Similar to what they do to test Calorie content in food. Considering reduced efficiency of combustion, etc, I could easily see that number rising a long way from 7%. I wouldn't be so fast to dismiss the 45% figure.

 

[Battenkiller]

Your units are off.

Regardless, I think that the way to test this would be to burn wood of varying MC in a bomb calorimeter at around 75% efficiency, instead of fooling around with the theory. Similar to what they do to test Calorie content in food. Considering reduced efficiency of combustion, etc, I could easily see that number rising a long way from 7%. I wouldn't be so fast to dismiss the 45% figure.

Heh. I should have included the following disclaimer from another thread:

*It's morning and I haven't gotten the cobwebs out yet and I may have made a mistake somewhere, but I know no one will check out my calculations anyway, so I'm golden.

Well, apparently somebody checks them. :cheese:


I don't write this stuff out on paper and do dimensional analysis on my calcs, just my head and an old TI-85. I'll check out those units and calculations later, right now my head hurts, and for some reason, the whisky ain't helping it. :shut:


The point I was trying to make was that they are trying to absolutely quantify the heat loss using a combination of theory and subtraction. They are subtracting the actual unburnable weight of the water from the split as well as accounting for the latent heat loss. No one I know buys, cuts, or fills their stoves by weight.

Cords are measured in cubic feet, so are fireboxes. There is the same amount of wood fiber in a cubic foot of wet wood as there is in a cubic foot of dry wood. There is only about a 10% volumetric decrease in the wood even if you bring it all the way down to 0% MC in an oven. Yes, most definitely a ton of wood that is 50% water by weight has only half the wood fiber that a ton of oven-dried wood has, but a cord of wet wood has the same amount of fiber as a cord of seasoned wood... it just weighs a lot more. A cord of wet wood has all the BTU making hemi-celluloses and lignin as the same cord when seasoned.

A stove full of wet wood will contain essentially the same amount of potential chemical energy that a stove full of dry wood contains (minus, of course, the latent heat of evaporation loss... which is a fact, not a theory). The makers of the video are using that water weight in their calculations of available heat in the wood, and it is just plain incorrect to mix these things up like they did.


The chart below shows the calculated maximum heat that could be obtained in a pound of wood at moisture contents between 0% and 250%. As you can see, it is a linear function. The more water in the wood, the less heat it is theoretically possible to extract from it through combustion. Even at 200% MC (burned in an incinerator, perhaps?), there is only a 25% theoretical heat loss due to evaporation of contained water Between 20% MC and 40% MC, the difference is negligible.


As far as the bomb calorimeter, what's the point? That's still theory unless we start to heat our homes that way. Hell, we're almost there with all the insulation they add to a stove these days. Instead, though, why not ask a real wood heating scientist to compared the overall heating efficiency of a couple real life stoves in a very tightly-controlled laboratory test, using both green wood and seasoned wood the way the average homeowner would burn them and compare the results?

The columns in tan are seasoned oak at 21% MC, the ones in green are green oak burned at 41% MC (both dry-basis). Not 45% less heat in the room... not remotely close to 45%. In fact, about a 5% heat loss from all the variables that should result in heat loss, including the heat of vaporization. Note that the green oak was even more efficient when burned in an open (AKA "Franklin") stove. On the right is a conventional air-tight stove and way over on the left? A Blaze King catalytic stove... which some folks here will tell you won't even burn green wood at all.

 

[Renovation]

Hey BK,

First off, you rock! :thumbsup:

Could you please help me understand how you calculated overall efficiency? Is that the percent of the theoretical energy content in the wood, based on the amount of wood fiber you estimate to be in the wood, without heat of vaporization subtracted out?

For example, if you decided your wood was 30% water, did you calculate the theoretical energy content of the 70% wood, and compare the actual energy to that? Inquiring minds want to know.

Thank you for over-thinking this! :cheese:

 

[Battenkiller]

Could you please help me understand how you calculated overall efficiency? Is that the percent of the theoretical energy content in the wood, based on the amount of wood fiber you estimate to be in the wood, without heat of vaporization subtracted out?

George, you do realize that I didn't do this study, don't you? I didn't decide anything, Dr. Jay Shelton of Shelton Research, Inc. did all the deciding back in 1986.


The overall efficiency was determined by using information gathered from numerous sensors strategically placed in the flue (see diagram below). This is called the the "stack loss method", and is used throughout the industry, both in the design phase and during testing. It is the custom in North America to use the higher heating value (probably to make the stoves appear more energy efficient). This leads to a lot of confusion as well as yielding a figure that no one is interested in knowing. We want to know how much heat is coming into the room for every ounce of wood we burn. The industry is usually not giving us the correct answer, and we have no sure way of knowing when they are.

Shelton Research used a much more realistic method to determine overall heating efficiency by subtracting all energy loses (chemical energy, sensible heat lost up the flue, and latent heat losses through evaporation). This method correlates highly with heat output testing of wood stoves done in actual calorimeter rooms, so it is considered to be very accurate. The formulas Shelton used were:


Overall efficiency = (wood energy input minus sensible heat loss minus chemical energy loss minus latent heat lost) divided by wood energy input

Combustion efficiency = (wood energy input minus chemical energy loss) divided by wood energy input

Heat transfer efficiency = (wood energy input minus sensible heat loss minus chemical energy loss minus latent heat lost) divided by (wood energy input minus chemical energy loss)


By using the lower heating value (values in the chart I posted above) instead of the higher heating value (usually used in North America) for expressing wood energy, he was able to compute the latent heat loss from unrecondensed water vapor leaving the flue based on the assumption of complete combustion without any effect on the accuracy of the data.

It is all here on page 27:


http://www.arb.ca.gov/research/apr/past/a3-122-32.pdf


Have fun with it... it's very long and detailed, but looking at your profile page, I'm sure you can handle it.


BTW thanks for sharing my curiosity about all this, and for expressing your interest in public here. May I call you Curious George? Just remember that none of this will rock your world when it comes to actually using your stove when you finally decide which one you want. That's art, not science. Guys like Dennis ("Backwoods Savage") could give two shits about all this stuff, and they burn better and smarter than 95% of the young guys with all the "answers" (Hint: I ain't a young guy). This stuff may help you make the best decision on what stove to buy, or how to install the best chimney (be nice to have Shelton's drafting system, eh?), etc. That's all I hope to do by discussing this all here.

And feel free to blast away at my statements. I'd hate to have erroneous statements or calculations I may make go unchallenged. In fact, I welcome as much fire as you can hit me with. Makes me think harder and staves off the Alzheimer's for a while longer.