Popular Woodworking 2006-12 № 159, страница 58

he solar drying kiln is the most cost-effective way for the craftsman to get quality boards for woodworking from green lumber. Today there are many band saw operators cutting boards from trees that grow in abundance in much of America. The solar kiln is the link between this resource and the shop.

A wood kiln is any space used for controlling heat and humidity where lumber is dried. The solar drying kiln harnesses the free energy of the sun. It operates on the regular cycle of day and night to prevent wood stress that can ruin lumber in other systems. The scale of the unit at 1,000 board-feet (bf) capacity, plus the simple structure and affordable cost of running a fan, makes this solar kiln practical for the home craftsman or small professional shop.

The Basics of Drying

Evaporation from the surface of lumber occurs when surrounding air picks up the water. The capacity of air to hold water is dependent on temperature. This is expressed in the term "relative humidity," where humidity is the degree of saturation in a volume of air, and the amount is relative to the temperature. When temperatures fall, moisture from saturated air precipitates. When temperatures rise, the air can hold increased amounts of water, making it effectively dry air. Hold on to this last statement because it is how a kiln works.

To give an example of relative humidity at work, take our homes

(Left) Solar kilns used at The Home Shop in Charlotte, Mich. Two 16' kilns are joined at the middle by a 4'-section for sticker storage. The end wall is opened with the door to the left and the main panel is temporarily leaning against the outside.

The window wall and roof make use of readily available salvaged sliding door glass panels, which are 76" long. Each piece of glass lands on a rafter or stud and is held is place by a batten. The end of the rafter batten has a strap to prevent the glass from sliding.

in winter. Air passes in and out of the normal home every one-half to three hours for a complete air exchange. Outside air at 30° Fahrenheit (F) at a comfortable 60 percent relative humidity comes inside where it is heated to 70° F and is 20 percent relative humidity, because of the capacity of warmer air to hold more moisture. This phenomenon is even more at work at higher temperature ranges. The interval from 70° F to 120° F sees a 10-fold increase in moisture-holding capacity, which is the range in which the solar drying kiln works. The ambient air on a hot sticky day in summer at 100 percent relative humidity with little capacity for drying becomes desert-dry air of 10 percent relative humidity when the sun heats it to 120° F inside the kiln.

Another operating principle is the daily cycle of the sun's energy shutting off at night. This is key to the trouble-free operation of the kiln. The danger inherent in standard hot-air dryers is the stress in wood due to surface shrinkage before core areas can dry. Such stress not only warps lumber, it is the cause of "case hardening," which we encounter when sawing a board that will bind the saw blade. In the extreme, it can tear the core apart in a condition known as "honeycombing."

In the solar kiln, the boards have a chance to "normalize" every day - what I like to think of as smoothing out the differences in moisture levels throughout the wood. Because of this daily cycle, you do not have to monitor the progress of drying to be sure to have good results. Nature does it for you. A word of caution: If you incorporate ways of shortening the drying time by having drying go on 24 hours a day, attention must be given to the impact it will have on boards because that then erases the normalization period.

Building the Solar Kiln

The kilns used in our business of providing wood to craftsmen who make Shaker oval boxes have a capacity of 1,000 bf to 1,500 bf of lumber. To give scale to this amount, 500 bf of dry hardwood

boards weighs about a ton, or a good sized pickup truck load. Each kiln is a building wide enough to hold a 4'-wide stack, and long enough to fit the longest logs you are likely to have sawed. Our boards are 12' long and the kiln is 16', which allows for the fan at the end. You can adapt the size you build to suit your cutting needs.

The construction uses standard 2 x 4s, ¹/2" plywood sides, a ³/4" plywood floor and 4 x 4s and cap blocks or short posts for a foundation. The south wall and roof are made from recycled tempered glass of the kind used in sliding patio doors. These ther-mopane units have a nasty habit of developing air leaks in the seals, which fogs the glass. Replacement is common, and used glass is readily available from those in the window business. The condition

4 x 4 outdoor wood

2 x 4 outdoor wood Cap blocks

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64"

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approx. 16' - depending on glass size

Foundation plan

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