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timber drying cycle

Bridges to Madison County

These days the average house is drier than it has ever been; this poses some big challenges for the woodworker wanting to make high quality furniture from local wood. Firstly this wood must be dried to that moisture content, then it must be stored at that until made. Without these elements you will not be able to produce high-grade high value work. It is not cranky Cornish wood that is to blame but these missing elements in the processing chain.

What I wanted to find was a good working example of a successful solar kiln in northern latitude, and Jim Birkemeier had three in Wisconsin!

RWP has long had the intention to build a solar kiln to cope in-house with the yawning gap in the processing chain i.e. the lack of good dry local wood. We already operate a five cubic metre dehumidifier but this is too small and we have had problems with degrade. A bigger dehumidifier would add too much cost to the material, using a fair amount of electricity as it does.

I heard about Jim Birkemeier through the Small Woods Association. I thought here at last is someone talking my language; here was a small set up utilising the interest of his local natural capital whilst fostering the resource. Growing the capital for the future. A setup using appropriate technology to get the job done in the least harmful way, and producing high quality added-value products all in-house, but most importantly making a living at it.

I had to know more and Woodland Heritage agreed to sponsor my visit to the USA.

Timbergreen Farm

Timbergreen Farm

I had been talking to Jim about his solar kilns, had looked at plans in detail and figured it would work, but, here’s the rub - although Wisconsin is at 45° and we are at 50° the big difference is Wisconsin is a dry continental climate with ± 2500 hours of sun per year (where wood air dries to 12%) and we are known for our foggy wet miserable climate with only 1600 hours of sun (64%) - where we are hard pushed to air dry wood to less than 25%.

In Wisconsin 4" by 4" oak air dries to 20% in 12 months and then to 10% after one month in the kiln (in optimum conditions) given the wood moisture, sunshine hours and relative humidity. I guess we are looking at twice that time to achieve the result in summer months At all times Jim was attempting to install higher added value into his material so he quarter saws much of it, so gaining further premium. This is made easy in his set-up by the use of a resaw and edger both designed for pallet production. These simple and fairly unsophisticated machines were highly efficient and speeded up the conversion process considerably. After sawing, the boards are stacked straight into one of the kiln chambers, the top 20 or so boards are ratchet-strapped together. This keeps them flat and enables weight to be kept on as the wood shrinks.

Before I went to Wisconsin, after 6 years of struggling to build my own business, I was beginning to feel that I would never make it. The horrors of Foot and Mouth and its long term and far-reaching affects, debt and the normal worries of self employment had taken their toll on my natural optimism. Seeing Jim Birkemeier’s set-up working, hearing his story and witnessing his successes has vindicated my approach and strengthened my resolve. I came back with a refreshed vision and knowing that I have been on the right track.

The four solar kilns in Wisconsin were in a timber building having three or four insulated ‘chambers’. At any one time one of the chambers has doors which slide in place front and back, this becomes the ‘drying chamber’ while the others without doors act as pre-drying rooms. The tunnel effect of the open front and back aids the air drying.

The building is orientated so that one side of the roof faces south and is pitched at the same angle as the degree of latitude (45 in Wisconsin) and is a PVC ‘window’ which acts as the solar collector. The collector is backed with black painted sheet metal, which heats up when the sun shines through the window on it. Cool air from the drying chamber is drawn up into the collector space by pressure differential, and up the collector surface - heating it as it goes. The heated air rises to the apex of the roof/collector space, from where it is drawn down by electrically operated fans and ducted back down into the drying chamber and through the stack of timber to be dried. A proportion is vented out of the chamber and the rest is recycled up into the solar collector to be reheated. All the heat from the whole roof/collector is blown through the wood in the one closed drying chamber.

When the drying cycle is completed, the doors slide across the open sides of the next room (already filled with air dried wood) and by simply changing over the ducts, becomes the drying chamber ready for the next drying cycle. One-way valves keep the heat in the kiln chamber when the fans are not running, thus conserving heat.

110ft solar cycle kiln at Sustainable Woods Co-op.

110ft solar cycle kiln at Sustainable Woods Co-op.

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