Who needs a kiln?

[Stubby]

Ok, so if you've got 70 degrees in the kiln with a breeze and 25 degrees with a breeze outdoors then the kiln would dry maybe three times faster? What difference does humidity make to the equation?

 

Dry wood will absorb moisture from the air in the same way wet wood will give it up to the air .

[openspaceman]

They don't know the weight of the bit wood you're sticking them into though. I assumed they worked on basis of resistance between pins when compared to the control measure (100% moisture?)

 

Yes that's understood; it's the way they express the percentage I was commenting on, nothing to do with functionality or accuracy.

 

The timber trade tends to express moisture content of a piece of wood as a percentage of the dry weight of the wood. For energy purposes we find it more convenient to express the water content as a percentage of the wet weight.

 

So a piece of deal sold for joinery may be 10%mc dwb and 9% wwb but a log of fresh elm will measure 140% on the dwb but 58% wwb. As I said I think moisture meters are likely to be calibrated to show dry weight basis.

[openspaceman]

Ok, so if you've got 70 degrees in the kiln with a breeze and 25 degrees with a breeze outdoors then the kiln would dry maybe three times faster? What difference does humidity make to the equation?

 

It's complex will many dependences; high temperature speeds things up, it speeds up the water migration to the surface and lowers the viscosity and capillary attraction of the water.

 

High temperature also decreases the humidity of air as well as increasing the energy it contains. This is important because unless the wood is receiving energy from solar radiation (as happens in my transparent plastic roofed workshop) the energy supplied to evaporate the water from the wood's surface has to be delivered by the same air that carries the moisture away. The dryer the air the more moisture it can carry away from the wood. In evaporating water from the surface of the wood energy is given up to the water vapour and hence the air is cooled. With forced ventilation the aim is to vent this water laden cool air as saturated as possible and as near to ambient as possible to make use of the initial heat energy in the air and the power used for the airflow.

 

As we have seen at this time of year solar energy can provide both the heat and airflow to season split logs in a few days at no cost. I tested the humidity and temperature in the workshop today at 42C (ambient in shade was 23C) and 20%RH (ambient in house was 44%)

 

Fair enough if you can carry the stock and expense over till the selling season but else come winter and you want dry logs to sell...

[sandspider]

Interesting this.

 

I've got three different wood drying areas at my house. 1) A greenhouse, with windows and door all open. 2) A stack under an overhanging roof on a pallet 3) Just a stack out in the open on pallets. I put similar sized samples of any wood I get into each location. My unscientific research shows that the wood out in the open dries fastest (less heavy, goes greyer, cracks start to show in the end grain). The wood in the greenhouse (temperatures of 45C+ in the sun and humidity of 20% odd vs 23C / 50% humidity odd outside) doesn't seem to dry as fast. I can only assume that even though the windows and door to the GH are open, it still doesn't get as much air movement. So this would suggest that air movement is more important than temperature and humidity? (As I say, non scientific - but I see the same pattern with all wood types I've tried this with).

 

Having said that, it is of course summer, so I imagine when the sun is less strong and there is more wind, wood in the greenhouse might dry more quickly?

[ericw]

Higher temperatures will allow wood to dry faster, as it increases the capacity of the surrounding air to hold moisture. Another way to say this, is it lowers the air's relative humidity.

 

Alternately, putting the wood in a drier environment will also speed up the process, even if the temperature remains unchanged.

 

When you're measuring with a moisture meter, keep in mind that the wood dries from the outside, and there will be a gradient with the outside being drier than the core, so you need to measure deeper than just the surface. Also, keep in mind that pin moisture meters are very affected by temperature (both of the wood being measured, and the surrounding air).

 

If you'd like to play around with how each of those two variables affects the drying process, look up an EMC (equilibrium moisture content) calculator online, and see how changing temperature and relative humidity changes the answer. The lower the EMC, the more effective your drying process will be. (Since you're working with firewood, you don't have to worry about the cracks and splitting that other moisture meter users do).

 

Source: I work for a meter manufacturer.

[cornish wood burner]

ericw

You have missed the third and probably the most important variable. Air movement. Without it the moisture laden air stays adjacent to the wood. A large stack of wood facing the wind will dry quicker and down to a lower MC in a given time, than the sheltered stack downwind. I have found typically 5% difference with a reasonable gap. However some years ago a couple of lorry drivers stacked some of our wood with the rows touching. We found this wood started to go over( rot)after a couple of years presumably because of lack of air movement . It was restacked with a 1.5 M gap and dried better but the damage was done.

[Stubby]

Ok here is what happens to me . I have a couple of wood stores out side and one in the garage . All filled with the same split wood in the early summer/late spring . The wood out side dries out much faster than the wood in the garage but come the winter the wood out side has regained some moisture where as the wood in the garage has now reached the moisture content that the outside wood achieved early on in the year and remains that low as you start to burn it .

[Two Acres]

Well what happens for me Stubby is that I have a woodstore next to the house. That holds about seven cube and is open to the elements on five sides and has an airgap under the roof, so the wood gets lots of airflow. Then I have another stack of split wood (3 cube) stacked under a tarp my Dads place. Theres also my woodpile there. Then I have another stack in the wood itself. From time to time I get given wood so the temptation is to get that straight in my woodstore at the house if theres space. If I do that I cut down on the double handling, but I may be burning three month seasoned wood when I have two year seasoned wood elsewhere. The birch I split three weeks back is now at 29% (middle of log) so no problem. obviously I can't do that with the oak though

[Logsnstuff]

Ok here is what happens to me . I have a couple of wood stores out side and one in the garage . All filled with the same split wood in the early summer/late spring . The wood out side dries out much faster than the wood in the garage but come the winter the wood out side has regained some moisture where as the wood in the garage has now reached the moisture content that the outside wood achieved early on in the year and remains that low as you start to burn it .

 

we found that bags stored under tarps even with good airflow will go from 14% back to 22% with a surface reading by November but bags in polytunnel remain around 15% 16%.

[cornish wood burner]

we found that bags stored under tarps even with good airflow will go from 14% back to 22% with a surface reading by November but bags in polytunnel remain around 15% 16%.

 

As we all know its the moisture laden winter air doing the damage. I try to minimise this by stacking in my shed at the end of the summer. It has poor airflow but if the wood is stacked dry then it stays dry.