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Saturday, November 30, 2013

Simple Solar Cycle Kilns

Timbergreen Simple Solar Cycle Kilns
This 350 board feet/batch Simple Solar Cycle Kiln costs less than $10.00/batch, is portable (weighs less than 20 pounds), is solar heated and wind powered, uses no electricity, produces superior quality lumber, and is reusable for many years.
For small woodworking businesses anywhere on the planet, this simple, safe, effective lumber kiln quickly and efficiently removes nearly half of the weight of lumber before having to transport it or manufacturing  the wood into high value products.
Using natural wind power to pre dry and solar heat to kiln dry lumber, this design works well at all latitudes.  The warmer and drier the climate, the more effective the SSCK is, but this will work any place the breeze blows and the sun occasionally shines.
The SSCK can be made smaller or larger.  Larger batches are cheaper for materials and labor and ground space, but are slower per board foot.  The lower the humidity and higher the ambient temperature, the larger the kiln size can be.  350 board feet/batch to 1,000 board feet/batch are recommended.  For larger volumes in remote locations, use more individual units.  For large production facilities with power and transportation, our Timber-Tropic Hoop House Solar Cycle Kiln are much more efficient.

The lumber is stacked one time, air dried under a clear poly tarp cover, then kiln dried to the target MC  using solar heat.  Strong nylon straps hold the boards flat while they dry and the ends of the lumber are protected from over-drying.  The daily/nightly heating then equalizing cycle keeps the moisture content of the lumber uniform through the thickness of the lumber so there is no need to “steam condition” the lumber at the end of the heating cycle.
Fresh sawn lumber is usually about half water by weight.  Initially, most of the drying happens by evaporation and simple air movement from the wind dries the lumber.  Mold and Mildew and sticker stain are the concerns in the first cycle of wood drying, if progress is too slow.   Having the pile of lumber well covered by a clear tarp keeps nearly all the rain off of the wood.  The open sides allow the air to flow across the surface of the lumber between the stickers.  Some solar heat is gained from the sun, on the surfaces of the pile. Any difference in drying throughout the pile during the day equalizes every night and cloudy day.
Once the wood pre-dries with the sides open, the side walls are sealed to trap all of the solar energy that strikes the kiln during the kiln drying cycle.  Any evaporated water that condenses on the poly and drains out the bottom.  Some venting of moist air during sunshine may be used to remove some of the water.   Spaces between the edges of the boards allows convection to move the air within the kiln when the sun shines.   The difference in drying within the wood piles equalizes at night and cloudy days.
Specific locations using specific tree species can be handled by variations in the timing of the air drying and kiln drying cycles and the amount of fresh air allowed into the system.  Once you have a few cycles complete, solar lumber drying can be quite foolproof and requires minimal monitoring.  Properly assembled, the SSCK is very storm proof and could be left to work in remote locations for several weeks between visits.
A mixture of species and lumber thickness works well, due to the nightly equalizing of the moisture content  of all pieces.  UV protected 6 mil clear poly tarps cost $.10-.15 per square foot and should last at least 4 years.  Total costs for kiln drying (not counting the labor to stack the piles) is less than $.02 per board foot.  From the backyard to the rainforest of Brazil, this simple idea will work for a small woodworking business.

When stacking a batch, build a flat base of 4X4 cross timber about 18” apart.  I use 6 cross timbers on a 8 or 9 foot long pile.  Thin pieces of lumber are perfect shims.  The whole base can be set up off the ground a foot on blocks if the ground is wet.  Always start with a solid, flat, strong base of cross timbers.
If there is a prevailing wind direction, orient the width of the pile with the wind for flow through ventilation.   The solar gain is all directional in the heat cycle of this process.

A 14 foot wide by 25 foot long poly tarp costs about $55 andcovers 13 layers of lumber 9 feet long and 4 feet wide, totalling about 350 board feet (at least 1” spaces between the boards in a layer).  6 mil thick, UV protected clear greenhouse poly tarp should last 4-6 years if handled with care.  Some holes in the tarp on the bottom or sides of the stack are not a problem, but the top part of the plastic should be water tight to repel rain and snow.

Place the tarp over the cross timbers and roll up the excess on each side so you never step on the poly tarp.  Orient the long dimension of the tarp with the length of the pile.  Most of the length will be extra on the ends of the pile for now, just roll it up to keep it out of your way when you are working.   Protect the tarp for future use and avoid falling on the slippery plastic.  Rolling the tarp onto sticks makes a neat roll with enough weight so the wind won’t blow it around while you work.  
Add the first stickers over each cross timber, on top of the poly tarp.  Carefully stack your lumber, leaving a good space between the edges of the boards so the air can move vertically through the stack.  It is easy to walk around the pile and make a nice square stack of boards.  Stack denser species or thicker lumber at the top of the pile as the heat will rise and dry the upper boards more quickly.

Once your stack reaches the proper number of layers, place four 2X4s across the top of the pile, and connect and stretch the nylon straps around the pile.   Once all four straps are in place, tighten them firmly, and retighten them every week or so.   This helps the straps exert more even pressure to the lumber to hold it flat as it dries.  

Wrap the ends of the tarp across the top of the stack, wrapping the two ends together around a stick – making a strong water tight seal.  An old piece of carpet placed across the ends of the boards at each end of the pile would protect the tarp from being cut by sharp ends of the lumber, and protect the end grain of the lumber from over drying in the sun.  Place an extra 4X4 under the joint wrapping to raise the tarp and help the top of the tarp drain after it rains.   An extra 4X4 over the center two 2X4 cross timbers under the straps helps the tarp to drain.   Tie the ends of the stick back to the pile to secure the joint.

Fresh sawn lumber contains a lot of water – we want to leave the sides of the pile open for a few weeks to let most of the water in the boards simply evaporate and blow out of the stack.  The clear poly tarp keeps the rain off and will let a lot of heat into the lumber to accelerate the pre drying part of the cycle.
Now roll up the extra tarp on the sides of the pile.  Rolling the tarp around a stick adds weight and makes a tight roll.  Roll it outside – in, so water rolls off  (not flow into the roll).  Place the lower roll close to the pile to minimize the water getting in under the lumber.   Try to support the upper side rolls away from the edge to form an overhang to get the rain water to drain away from the piles.  Short stickers inserted in between the top layers can help hold the roll in proper position.
Once the lumber is pre dried with the flow through ventilation, close the sides of the poly tarp by rolling the edges together around a long stick, again outside in to make the water run off.   Tuck in any extra tarp and wrap a strap or rope around the whole pile to secure any floppy and loose tarp.  If excess condensation is collected, consider pre drying with the sides open for a longer time.   Drain holes in the bottom layer may be cut as needed to get rid of water inside the tarp.
The nightly cycle of no added heat and higher humidity - helps the moisture content of the boards throughout the stack to equalize.
Check back in about September for the start of the kiln cycle and any updates.
If by chance you want to dry just one batch of lumber, you can use much less expensive plastic tarps that are not treated with UV protection.   Same idea.

Sunday, September 29, 2013


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