gyrfalcon said:
BK, could you maybe distill all this into a paragraph of practical recommendation?
Since BK asked me to challenge his work, I'll see if I can light two fires with one match, and provide my own summary of BK's work, for debate, discussion, etc.
I'll present a quick summary, followed by an explanation.
Summary:
Indoor drying has the potential to dry green wood to the 20% dry-basis moisture content (which is what standard moisture meters read, and what most consider the threshold for convenient clean burning) in approximately 6 weeks, at the expense of about 5% less net energy out of the wood than if it were dried outdoors.
Detail
First I'll present my analysis of drying time, then net energy loss.
Drying Time
I'm simply eyeballing the red, dry-basis line on the drying chart, and guesstimating where it will cross the 20% line. Given BK's analytical chops, I'm sure he can fit a curve to that data and tell us at what point it crosses the 20% level in nothing flat. BK? We can also wait and see when it gets to that point.
Net Energy
KWillets Noted:
KWillets said:
I got curious and looked up the heat of vaporization for water -- 970 btu/lb., and found a table of cord weight and "excess moisture" (above 20%) for red oak. From these figures I get a total of about 1.13M btu to get a cord down to 20% MC, compared to 24M btu energy content. So drying wood this way takes about 4.7% of the btu/cord, which isn't huge. But you're still evaporating 1165 lbs. of water in your house.
So the bottom line is it takes about 5% of the energy available in the wood to dry it to 20% moisture content. This energy has to come from somewhere--outdoors it comes from the sun, indoors it comes from whatever you are using to heat your house. This may not be perceptible, but it is a fact--barring extreme measures, the water evaporated from the wood will leave the house as vapor, and the energy required to vaporize it leaves along with it.
Evaporative cooling may not be intuitive for wood, but it certainly is for humans. This effect is why we sweat, and why the air feels so cold after swimming, showering, etc. It takes energy to change water from a liquid to a vapor, and that lost energy cools off you, your room, etc. In hot and dry Arizona, evaporative (or "swamp") coolers are the traditional way to cool during the summer--air is circulated over a damp pad, and back into the house. As it passes over the pad, it evaporates water and in the process becomes cooler and more humid. It is quite effective.
In the case of drying wood indoors, drying the wood turns the water in the wood into vapor, which cools the house. Regardless of where or how many times that vapor condenses, as long as it eventually leaves the house in vapor form, the energy required to evaporate it is lost.
An example of this is super-efficient direct vent gas furnaces. To get the last bit of efficiency, they condense the water in the already cool flue gas, and drain it out as a liquid, thereby reclaiming the heat of vaporization. In the case of indoor wood drying, I can think of no efficient way of condensing the water removed from the wood out of the air before that air leaves the house envelope--even if it condenses in the walls, it eventually leaves as vapor, taking its heat with it. Unless of course the 1165 lbs of water/cord stays in your walls until Spring, in which case you have a whole other set of problems.
Finally, my analysis correctly ignores all the factors that are shared by indoor and outdoor drying. For example, both cases require the same amount of energy to bring wood brought inside up to room temperature and evaporate whatever surface moisture happens to be on it.
Okay, that's it for the analysis. On a personal note, I enjoyed pondering this, and it helped distract me from the pain of my workout. In school, I was known for caring more about a theory's consequences in concrete application than tweaking it's abstract equations. I consider the math and theory a very valuable means to the end of applicability, and get little enjoyment out of them for their own sake. This was called "picking the low-hanging fruit", and was either an asset or an annoyance, depending on one's priorities.
Anyway, I hope this is useful, and invite questions, comments, concerns, challenges, etc.
My thanks to BK for his fascinating, rigorous, and educational experiment! :coolsmile:
