O C T O B E R 1, 1 9 6 1

Reprint f rom SOUTHERN LUMBERMEN

October 1, 1961

Wind-Powered Solar-Heated members and covered with one-inch­thick shiplap lumber and 36-pound black roofing felt. The floor is of one­inch-thick, home-sawed oak lumber covered with hardboard. The south side of the dryer is enclosed with four storm windows of single-strength glass, giv-

Lumber Dryer By CURTIS L. JOHNSON, Engineering Aid ing a glass area of about 37 square feet:

Forest Products Laboratory,1 Forest Service, U.S. Department of Agriculture Note that the north and south walls slope outward from top to bottom at

Introduction I redesigned it a number of times on a an angle of 221/2° with the vertical. The In my need for drying small amounts trial and error basis. This model con- ventilating slot in the floor (Fig. 3) is

of home-sawed lumber, which I use vinced me that the plan I had in mind covered with hardware cloth to keep in my own workshop and, on occasion, had merit, and I began work October sell to other hobby workers, I decided 24, 1959, on the full-scale unit I have

Editor’s Noteto explore the possibility of using a since used. simple solar dryer for drying the lum- The solar dryer has met my needs and The Forest Products Laboratory, ber more rapidly than was possible by expectations, and this report is pre- as part of its research on wood-

moisture relations, is interested in ordinary air-drying. The area in which sented for the benefit of those who may developing seasoning methods thatI select the trees to be cut and sawed have similar needs or who may wish provide dry lumber rapidly, eco­into lumber is in the vicinity of my to carry on further investigations in this nomically, and with a minimum ofbrother’s farm, which is located 50 field. The dryer was built to meet my degrade. As a part of this research,miles west of the Forest Products Lab- personal needs, and I make no claims studies are being conducted on the oratory at Madison, Wis. My decision for its use on a commercial basis. drying of lumber, using solar to locate the dryer on my brother’s Raymond C. Rietz and Edward C. Peck, energy as the heating medium in farm thus made it necessary that the seasoning specialists of the Laboratory two experimental solar pre-dryers.

Mr. Johnson, an engineering aid atdryer operate with a minimum of con- technical staff, have inspected the dryer the federal laboratory and authortrols and attention. Another require- in operation and have offered sugges- of this article, became interested inment was that the construction cost be tions for improvement. these kilns and adapted the princi­low and that only ready available ma- ples to the design of a kiln thatterials be used to build it. Roman Construction would meet his personal needs. Mr. Baltes, a technician on the Laboratory Figure 1 shows the south side of the Johnson’s report indicates the suc­staff, offered constructive criticism and solar dryer. Figures 2 and 3 show the cess he has had with this kiln. It valuable suggestions. principal dimensions and construction has been prepared to show others

In the summer of 1958, I built a small- details. The dryer has a capacity of how effective his kiln has been and scale model of the dryer I had in mind. about 400 board feet of lumber eight to provide suggestions others may

find helpful in designing such a 1Maintained a t Madison, Wis.. in co-opera-

feet long. The unit is framed with two- kiln to meet their own needs. tion with the University of Wisconsin. by four-inch and two- by six-inch

FIGURE 1-View of solar dryer from the south. Note black metal heat collector behind glass storm sash. Dryer is mounted FIGURE 2.-Sketch showing principal dimensions and construc­

on concrete blocks to permit air to circulate under floor. tion features of the wind-operated solar dryer.

SOUTHERN LUMBERMAN

FIGURE 5.-Results of first experimental run in solar-heated dryer, showing lumber moisture loss during 52 drying

days. FIGURE 3.-Details of heat collector and fan assembly.

transmitted to the internal fan through a friction plate and a rubber roller drive that increases the fan speed by a 3 to 1 ratio. The friction plate permits slip­page during high gusts of wind, thus minimizing possible damage to the in­ternal fan. With the exception of weld­ing and some machine work, the wind­mill and fan were made in my work­shop. The blades of the windmill are of plywood covered with glass cloth. Grease fittings are provided on both windmill and fan.

Operation The solar dryer is simply a device

for capturing a part of the sun's heat and using it to dry lumber. Throughits use, lumber can be reduced to a lower moisture content than by air-dry­ing, and such lumber is suitable for high-quality uses, as in furniture. The dryer wall containing the windows is sloped at an angle of 221/2° with the vertical and faced directly to the south to take the best advantage of the sun's rays. The sun's rays pass through theFIGURE 4.-Loadingdoor propped open to show method of piling lumber. glass, and a portion of the available heat

out rodents. The face of the heat col­lector nearest the glass windows (Figs. 2 and 3) is covered with 26-gage galva­nized flat iron sheets painted dull black. The other face of the collector is covered with 1/8-inch-thick hardboard. The north side of the dryer consists of a large panel that is hinged at the top to permit opening for loading purposes. Figure 4 shows this door in the openposition. A small door that will permit entrance for inspection purposes is lo­cated in the west wall. The dryer is mounted on four six-inch blocks to provide for proper air circulation into and out of the chamber.

The windmill, which provides forced air circulation in the dryer, has a four-bladed, 54-inch-diameter windmill wheel, which is mounted on the roof, driving a 14-inch-diameter centrifugalfan mounted just under the roof of the dryer. The vane on the windmill guides the wheel directly into the wind. The rotary motion of the windmill is

FIGURE 6-Results of second experimental run in solar-heated dryer, showing moisture loss during 220 drying days Curve based on average of six moisture

samples.

.

is absorbed by the black metal face of the heat collector. This plate, in turn, heats the air within the space formed by the two walls of the heat collector, which is open at the top and bottom.

With the fan in operation, outdoor air is drawn through the slot in the floor, passed through and heated in the heat collector. and then forced through the pile of lumber, where it picks up mois­ture from the lumber. The inner hard­board face of the heat exchanger extends only part way to the floor, thus permitting re­circulation of a portion of the air which has passed through the lumber pile. It is assumed that the remaining air escapes through leakage and through the ventilating slot. Some air circula­tion presumably takes place even though the windmill is not operating, due to a natural chimney effect in the heat collector. The windmill will operate in winds as light as four miles per hour. In an average wind, a strong blast of air is forced through the lum­ber. The lumber is piled in the pattern shown in Figure 2 to facilitate air cir­culation.

Careful records were kept of lumber moisture loss during the drying of two experimental runs. It has not been possible to keep records of temperatures inside and outside the dryer or records of amount of sunshine or wind duration

and velocity. Temperatures of 130° and 140° F. have been noted inside the dryer when the outside temperature ranged from 80° to 90° F. In winter, temperatures of 80° to 90° F. have been reached inside the dryer with outside temperatures at about freezing.

The disadvantage of this system of drying is that the operator has no con­trol of either sunshine or wind. The windmill often runs at times when it is probably doing more harm than good. The obvious advantage is that both sunshine and the wind are available at no cost.

Results To date, two runs have been made

in the dryer. The first run consisted of a load of mixed cherry and white oak boards of full one-inch thickness. Both species were partially air-dried before being placed in the dryer. The results of this run are shown in Figure 5. The moisture content of the lumber entering the dryer was determined by cutting small pieces from representative boards and weighing, oven-drying, and weigh­ing them. As the experiment pro­gressed, the boards from which the pieces, called moisture sections, had been cut were placed in representative locations in the dryer and weighed at intervals to determine the drying prog­ress. At the end of the run additional

SOUTHERN LUMBERMAN

pieces of these same boards were cut and oven-dried to determine the final moisture content. As shown in Figure 5, the cherry lumber dried from an average of 15% to an average of 8 per cent in 52 days. The white oak dried from an average of 60 per cent to an average of about 61/2 per cent in the same period.

This experiment was conducted dur­ing the best drying period in this area. The greatest loss of moisture occurred during the first 15-day period. No at­tempt was made to obtain comparable figures for air-drying by the usual out­side piling method for the same period. On removal from the dryer, the lumber was bright, clean, and apparently free of face checking.

The second run consisted of freshly sawed black cherry lumber which was placed in the dryer on November 14. As shown in Figure 6, the moisture content was reduced from about 50 per cent to about 20 per cent in 75 days of comparatively poor drying weather. A final moisture content of about 8% per cent was reached after 20 days of drying. Again, the boards were bright and clean with no evidence of seasoning checks.

I plan to continue these experiments as time and material permits, and to modify the unit as experience dictates.