The dryer described herein (fig 1 & 2) has been designed,constructed, and tested at Virginia Tech. This dryer can hold up to 2,000 board feet of 1 inch thick lumber per charge (or load) and has been designed to dry a charge in approximately one month of moderately sunny weather in the mid latitudes of the U.S. The dryer incorporates a passive solar collector, similiar to a greenhouse. There are four insulated walls and an insulated floor. The roof is clear plastic or fib ergflass ( two layers work better than one) sloped at a 45^ angle to the south(1). Solar energy enters the dryer through the clear roof and is incident on one of the black-painted interir surfaces. The energy is converted to heat and this heat is, in turn, circulated through the lumber pile where it is used to evaporate water. The evaporated water increases the relative humidity (RH) of the air; when the RH is too high, vents on the rear (north) wall can be opened manually to exhaust some of the humid air and bring in fresh, dry air.

At night as the dryer cools, the RH will increase up to 100%. It appears that this high humidity is beneficial, in that it relieves drying stresses that have developed during the previous day. As a result, there are minimal residual stresses ( or case hardening) at the end of drying.

To conserve energy, the fans are run only when the dryer has been heated abouve 65^ in the winter and 75^ in the summer One of the basic disign features of this dryer is to have one square foot of collector (i.e.,1 sq foot of sloped,clear roof) for each 10 board feet of 1" lumber in the dryer. This ratio works well for 1" oak, but may provide too much heat for 2" or thicker oak or too little for pine or other fast drying wood (2). The collector area can be increased ( for easier drying woods than oak) by extending the roof southward, accompanied with a shorter south wall. on the other hand, the collector area could be reduced by covering part of the roof with plywood or other non-transparent material.

One may ask if the dryer would perform better if the south wall were clear. There is a tradeoff in a solar dryer between the area of the collector and the area of insulated, solid walls. The more collector, the more solar energy collected, but in addition the greater the heat losses at night and in cold weather. The collector can use several clear covers and special covers and special covering materials to reduce losses, but this can be quite expensive. The Virginia Tech dryer is designed to control expenses and yet provide a functional, reliable design. The design is such as to dry a charge of lumber every month.

--------------------------------------------------------------------------- (1)/ The optimun roof angle, above the horizontal, is equal to the latitude of the location, but can be increased by 10^ to improve winter performance. (2)/Oak is one of the difficult drying woods because if dried too rapidly it will crack and check. Most other woods can tolerate more rapid drying. Thicker woods must be dried more slowly than thinner woods.

Figures 1 & 2

Construction Details

Drying Lumber

Cutting & Using Samples

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