Wood Drying Concepts
Temperature and Humidity
In order for wood to dry, energy must be supplied to the wood (approximately 1000 Btu’s per pound of water evaporated) and the air around the wood must be below 100% relative humidity. The more energy supplied and the lower the humidity, the faster the drying. Wood will also dry faster at higher temperatures.
In order to dry wood to a low moisture content, the average conditions in the dryer must be quite dry. As a general rule of thumb, therefore, when the lumber is below 20% moisture content, the fans are run only when humidities in the dryer are quite low. These conditions will be achieved when the temperature in the dryer has been raised approximately 10 degrees C (18 degrees F) above the morning’s lowest temperature. If the circulating fans are run 24-hous per day, the lumber will not dry below approximately 15% moisture content and electrical energy will be wasted.
Because the dryer is a closed structure, it is possible that the humidity in the dryer can rise to very high levels, especially with species at high moisture contents and that dry rapidly. In these cases drying will slow due to the high inside humidity. Drying can be increased by using a high ventilation rate with outside air (assuming the outside humidity is not too high). If inside humidity is not high, however, high ventilation rates will exhaust heated air before it can do any work (evaporation) and therefore the net effect in this latter case is slower drying.
As the wood becomes quite dry, drying rates will slow naturally. At this point, the higher the temperature the more rapidly wood will dry. High dryer temperatures will also result in lower inside humidities. (Heating air lowers its relative humidity!) Maximum heating is obtained by using minimal venting.
Drying Defects
During drying, lumber can develop a variety of defects if drying is not done properly. Defects include staining from drying too slowly; internal, end and surface checking or cracking from drying too rapidly; and warping from poor stacking, from drying too slowly, or from inherent characteristics in the wood. Texts are available that describe the seasoning properties of each species. Because of the risk of checking, each species has a specific rate of drying (moisture content loss per day) that the species can tolerate safely. Exceeding the rate can substantially increase the risk of defect development; drying substantially below the safe rate can waste time and may increase the risk of stain or warp. The safe rate can be estimated by noting drying times for green lumber with steam dry kiln schedules and dividing this time into the difference between the green moisture content minus the final moisture content. For example, assume a species takes 28 days to dry from 80%MC to 7% MC. The (80-7) /28 gives the estimated safe drying rate of 2.6% MC per day. This safe rate is a daily rate, not the average of several days. The drying rate is measured by the use of kiln samples as discussed in the appendix. Electrical moisture measurements have not yet been perfected for accurate use in kiln drying of green lumber.
Dryer Operation
Summarizing the information, then, given in preceding paragraphs, during the initial stages of drying when the lumber is quite wet, drying rates must be carefully controlled. This control is achieved by design – keeping the collector area to board foot ratio within acceptable limits (or even covering part of a collector that is too large) and by keeping relative humidities high by using minimal venting. Of course, some woods do not require much care in drying as the risk of degrade is small and safe drying rates are high.
As the wood dries, its drying rate will slow by itself. Therefore, partially dried wood can tolerate a bigger collector to board foot ratio and can tolerate higher circulation rates and lower humidities.
As the wood becomes quite dry, higher temperatures and lower humidities are required to achieve rapid drying and low final moisture contents. Therefore, ventilation is minimal and collector area to board foot ratios are as high as possible.
Drying Stresses (Casehardening)
Solar Lumber Dry Kiln Designs
The following section contains a summary of selected published information on solar lumber dry kilns that have been designs, constructed, and tested. In some cases the available information was not complete, but sufficient information existed to copy the design. Many designs (such as the Colorado State University dryer) have been duplicated by many researchers and in many locations. In these cases we have not repeated the information.
The authors of this document encourage anyone building a solar kiln to forward as much design and performance information as possible to us at Virginia Tech, Blacksburg, VA 24061, USA, for inclusion of such information in revisions and updates. The information presented here is based on data collected in 1983 with the assistance of many researchers and developers throughout the world. Our thanks to them.
Solar Designs Index
1 Ames, Iowa, USA
2 Ashland City, Tennessee, USA
3 Baton Rouge, Louisiana, USA
4 Blacksburg, Virginia, USA
5 Brisbane, Queensland, AUSTRALIA
6 Canton, Mississippi, USA
7 Carbondale, Illinois, USA
8 Carbondale, Illinois, USA
9 Curitiba, Parana, BRAZIL
10 Dehra Dun, U.P.,INDIA
11 Dodgeville, Wisconsin, USA
12 Dodgeville, Wisconsin, USA
13 Fort Collins, Colorado, USA
14 Griffith, New South Wales, AUSTRALIA
15 Homer, Alaska, USA
16 Kampala, UGANDA
17 Madison, Wisconsin, USA
18 Madison, Wisconsin, USA
19 North San Juan, California, USA
20 Oxford, ENGLAND
21 Ozone, Arkansas, USA
22 Princeton, West Virginia, USA
23 Reader, West Virginia, USA
24 Rio Piedras, PUERTO RICO
25 San Juan, New Mexico, USA
26 Santarem, Para, BRAZIL
27 Stellenbosch, SOUTH ARFICA
28 Tananarive, MADAGASCAR
29 Thunder Bay, Ontario, CANADA
Notes:
Collector area is roof area or external collector area. Roof angle should be modified as appropriate for latitude. Metric/English conversions are rounded off in most cases.
Dryer No. 1
LOCATION: Ames, Iowa, USA
(Latitude 42 N; longitude 94 W)
STATUS: Operational
TYPE: Semi-greenhouse; only roof is translucent
HEATING SYSTEM: Solar hot air collectors; corrugated fiberglass polyester single sheet for glazing; all surfaces in the dryer painted black
CIRCULATION: Two 20-inch (50 cm) diameter fans with 3-speeds (0.1 horsepower ?) near the roof
VENTILATION: Six vent openings in the north wall, four near the top and two near the bottom.
CAPACITY: 1000 board feet (2.4 cubic meters)
COLLECTOR AREA TO CAPACITY RATIO: 100 square feet per thousand board feet (3.9 square meters per cubic meter)
LOADING: Manually through door on north wall
OVERALL DIMENSIONS: East-west, 11.25-ft (3.4 m); north-south, 6-ft (1.8 m) height, 10-ft (3 m)
CONSTRUCTION: Frame construction with 2 x 4 framing; 2 x 6 floor framing; r-13 fiberglass insulation in the walls with a plastic sheet vapor barrier in walls and floor; aluminum paint on top floor covering of hardboard; ¾-inch (1.9 cm) plywood subfloor; 1-inch (2.5 cm) lumber siding on exterior; ¼-inch (0.6 cm) hardboard on interior walls
STORAGE: None
PERFORMANCE: Drying time for green hardwoods, 1-inch (2.5 cm) thick to 7-8% MC is 4-6 weeks in spring, summer and early fall
REFERENCES:
Article – Prestemon, D. R. 1983. Solar Lumber Drying. Forestry Extension Notes F-347. Iowa State University (Ames).
Present contact: Dean Prestemon
Cooperative Extension Service
Department of Forestry
Iowa State University
Ames, Iowa 50011
USA
Dryer No. 2
LOCATION: Ashland City, Tennessee, USA
(Latitude 36 N; longitude 87 W)
STATUS: Operational
TYPE: Opaque walls, hot bot
HEATING SYSTEM: Black painted walls and roof
CIRCULATION: Natural draft
VENTILATION: Natural draft; intake in north wall connected to wood shop area; exhaust at top of roof
CAPACITY: 4000 board feet (9.4 cubic meters)
CAPACITY TO COLLECTOR AREA RATIO: 30 square feet per thousand board feet (1.2 square meters per cubic meter)
LOADING: Doors in the end for manual loading
OVERALL DIMENSIONS: East-west, 20-ft (6.1 m); north-south, 6-ft (1.8 m); height, 10-ft (3 m)
CONSTRUCTION: Metal pipes to hold lumber also are formed to make a frame; 50-mil (1.3 mm) aluminum arches over pipes and painted black with a tar-like coating material; ends and floor made with ¾-inch (1.9 cm) plywood
STORAGE: None
PERFORMANCE: Drying time for cedar, cherry, walnut, and yellow-poplar in a mixed load with green and air-dried MC’s to a final MC of 6-12% is 60 days in the summer.
REFERENCES:
Present contact - Mr. Charles W. Gooch
Cheatham County Central High School
Ashland City TN 37015
Dryer No. 3
LOCATION: Baton Rouge, Louisiana, USA
(Latitude 30 N; longitude 91 W)
STATUS: Operational; ca. 1979
TYPE: Opaque wall
HEATING SYSTEM: solar hot air collectors; single layer of fiberglass reinforced polyester
CIRCULATION: One 24-inch (60 cm) diameter fan; 1.5 horsepower motor
VENTILATION: Unknown
CAPACITY: 360 board feet (0.85 cubic meters); note – tests conducted with 300 board feet
COLLECTOR AREA TO CAPACITY RATIO: 73 square feet per thousand board feet (2.9 square meters per cubic meter)
LAODING: Manually loaded through a door on the west wall
OVERALL DIMENSIONS: Drying chamber – (1.83 m x 1.83 m x 1.22 m); collector glazing – 3.5-ft x 7.5-ft long (1.1m x 2.3 m)
CONSTRUCTION: Chamber – a concrete block house with prestressed concrete roof, insulated. Collector – no details available; flow from collector can be shut off at night
STORAGE: None
PERFORMANCE: Drying time for ash 8/4-inches (5.1 cm) thick from 51% MC to 14% MC is 19 days
Drying time for ash 4/4-inches (2.5 cm) thick, from 50% MC to 7% MC is 20 days
Drying time for hackberry 8/4-inches (5.1 cm) thick from 81% MC to 14% MC is 20 days
Drying time for red oak 6/4-inches (3.8 cm) thick from 82% MC to 17% MC is 29 days
Drying time for cypress 4/4-inches (2.5 cm) thick from 88% MC to 10% MC is 21 days
Fans ran nearly 24-hours per day
Drying quality was good, but some checking, warp, and casehardening was found.
REFERENCES:
Article – Lumley, T.G. and E.T. Choong. 1981. Solar drying of wood in Louisiana. Agricultural Experiment Station Bulletin No 732. Lousiana State University (Baton Rouge). 55p.
Lumley, T.G. and E.T. Choong. 1979. Technical and economic characteristics of two solar kiln designs. Forest Products J 29(7):49-56.
Dryer No. 4
LOCATION: Blacksburg, Virginia, USA
(Latitude 35 n: longitude 81 W)
STATUS: Experimental; operational since 1978
TYPE: Semi-greenhouse with transparent roof
HEATING SYSTEM: Solar hot air collector; double layer of plastic film (6-mil; 0.15 mm); all surfaces inside dryer painted flat black
CIRCULATION: Three speed, 20-inch, non-reversible, 1/10 horsepower fans; thermostatic on-off control; three fans
VENTILATION: right vents on north wall; spaced evenly along the length and 10-inches (25 cm) from the top or 15-inches (37 cm) from the bottom; total area of 2.5 square feet (0.2 square meters)
CAPACITY: 1500 board feet (3.5 cubic meters)
COLLECTOR AREA TO CAPACITY RATIO: 100 square feet per thousand board feet (3.9 square meters per cubic meter)
LOADING: Doors in north wall or doors in south wall
OVERALL DIMENSIONS: East-west, 17-feet (5.2 m); north-south 6-feet (1.8 m); north wall height, 9-1/2 feet (2.9 m); south wall height, 3-1/3-feet (1.0 m)
CONSTRUCTION: All walls are framed with 2 x 4’s with 3/8-inch plywood on the inside and outside of the framing; roof is framed with 2 x 6’s; floor is framed with 2 x 6’s and ¾-inch plywood; fiberglass insulation (R-20) is used in the walls between the framing members; inside plywood is painted with two coats of aluminum paint to act as a vapor barrier and then flat black paint; access man-doors are in the east and west walls
STORAGE: None
PERFORMANCE: Drying time for black walnut, 4/4-inches (2.5 cm) thick, from 72% MC to 8% MC is 69 days in mid-winter and early spring.
Drying time for yellow-popular, 4/4-inches (2.9 cm) thick, from 50% MC to 9% MC is 28 days in the fall
Drying time for red oak, 4/4-inches (2.9 cm) thick, from 80% MC to 20% MC is 80 days in the winter
Drying quality was excellent with no casehardening or end checks noted.
Dryer No. 5
LOCATION: Brisbane, Queensland, AUSTRALIA
(Latitude 27 S; longitude 153E)
STATUS: production kiln since March 1979 (based on smaller, experimental kiln built in Fiji by D. K. Gough)
TYPE: Greenhouse with 3 transparent walls and roof
HEATING SYSTEM: Solar hot air collectors with a single layer of glass (1/8-inch; 3 mm) on the outside and a single layer of polyvinyl chloride film (6-mil; 0.15 mm) inside; all surfaces inside are painted black.
CIRCULATION: One reversible direction, 1 approximately 40-inch (1.1 m) diameter fan near front wall blowing air horizontally; 1.5 kW motor; air velocity through the lumber of 200 feet per minute (1 m/s)
VENTILATION: Vents in rear wall; manual control.
CAPACITY: 6400 board feet (15 cubic meters)
COLLECTOR AREA TO CAPACITY RATIO: 83 square feet per thousand board feet (3.3 square meters per cubic meter)
LOADING: Lumber on carts which are on tracks; 2 sets of tracks enter kiln through rear wall
OVERALL DIMENSIONS: East-west, 19.4-ft (5.9 m); north-south, 27.2-ft (8.3 m); rear wall (south) height, 14.4-ft (4.4 m)
CONSTRUCTION: Wooden floor; metal frame for glass collectors; wood framed wall and door; concrete hollow block on their sides along lumber piles to provide even air flow
STORAGE: None
PERFORMANCE: Drying time for Sydney blue gum, 6/4-inches (38) mm) thick from 27% MC to 12% MC is 62 days
Drying time for rose mahogany, 8/4-inches (50 mm) thick from 38% MC to 12% MC is 72 days
REFERENCES:
Articles – Gough, D. K. 1981 Timber seasoning in a solar kiln. Technical paper No. 24. Department of Forestry, Queensland. 6p.
Gough, D. K. 1977. The design and operation of a solar timber kiln. Fiji Timbers and Their Uses No. 67 (Department of Forestry, Suva, Fiji). 17p.
Present Contact: David K. Gough
Department of Forestry
PO Box 5
Brisbane
Queensland, AUSTRALIA
Dryer No. 6
LOCATION: Canton, Mississippi, USA
(Latitude 32 N; longitude 90 W)
STATUS: Operational
TYPE: Opaque walls (standard aluminum prefab kiln)
HEATING SYSTEM: Solar hot water collectors (2500 square feet – 232 square meters) with two layers of glazing and an equal area of reflector (10% benefit); commercially manufactured collectors; 5000 gallon hot water storage tank; supplemented with conventional steam heat; each kiln has 648 feet of finned pipe as solar hot water heat exchanger
CIRCULATION: Standard dry kiln
VENTILATION: Standard dry kiln
CAPACITY: 100,000 board feet (240 cubic meters)
CAPACITY TO COLLECTOR AREA RATIO: 25 square feet per thousand board feet (0.98 square meters per cubic meter)
PERFORMANCE: Using a standard kiln schedule for 4/4-inch (2.5 cm) thick hardwoods, solar heat provided 23% of the energy used
REFERENCES:
Article – McCormick, P. O. 1980. solar heating system for kiln drying lumber. Sunworld 4(6):204-207 Little, R. 1978. New energy for old kilns. Wood&Wood Products (March): 69-70
Present contact: Dr. Robert Little
Department of Forestry
University of Tennessee
Knoxville, Tennessee
USA
Dryer No. 7
LOCATION: Carbondale, Illinois, USA
(Latitude 38 N; longitude 89 W)
STATUS: Experimental
TYPE: Opaque walls
HEATING SYSTEM: Solar hot air collectors; two layers of fiberglass polyester glazing; black painted metal absorber made from beverage cans
CIRCULATION: Chamber has one 26-inch (66 cm) diameter fan driven by a 1 horsepower motor; collector has one centrifugal blower driven by 0.5 horsepower motor
VENTILATION: Intake vent on south wall, near roof; exhaust vent on east wall with 260 cubic feet per minute (0.12 cubic meters per second) blower; dampers for vents are electrically operated
CAPACITY: 500 board feet (1.2 cubic meters)
CAPACITY TO COLLECTOR AREA RATIO: 260 square feet per thousand board feet (10 square meters per cubic meter)
LOADING: Manually through side door
OVERALL DIMENSIONS: Chamber, inside dimensions – east-west, 8-ft (2.4 m); north-south, 8-ft (2.4 m); height, 6.5-ft (2.0 m). Collector – east-west, 16-ft (4.9 m) sloped height, 8-ft (2.4 m)
CONSTRUCTION: Frame construction with 2 x 6 framing; 6-inches (15 cm) of fiberglass insulation; 2-inches (5 cm) styrene foam insulation; interior sheathed with 5/16-inch (0.8 cm) plywood; exterior sheathed with ½-inch (1.3 cm plywood and then insulated aluminum siding
STORAGE: None
PERFORMANCE: Drying time for yellow-poplar, 4/4 inch (2.5 cm) thick, from 95% MC to 15% MC is 8 days in summer; from 102% MC to 15% MC is 53 days in winter; from 98% MC to 15% MC in spring is 28 days. Electrical power consumption is 258 kWh in summer and 341 kWh in fall.
REFERENCES:
Article – Rosen, H. N. and P. Y. S. Chen. 1980. Drying lumber in a kiln with external solar collectors. New Process Alternatives In the Forest Products Industries. American Institute of Chemical Engineers Symposium eries 76(200): 82-89.
Chen, P. Y. s. and H. Rosen. 1979. Drying yellow-poplar in a highly efficient solar kiln. Proceedings 30th Annual Western Dry Kiln Clubs (School of Forestry, Oregon State Univ., Corvallis), pp. 23-32 1981. Forest Products J 31(3):
Present contact: Dr. Peter Chen
US Forest Service
Forestry Sciences Laboratory
Southern Illinois University
Carbondale, Illinois 62901
USA
Dryer No. 8
LOCATION: Carbondale, Illinois, USA
(Latitude 38 N; longitude 89 W)
STATUS: Experimental
TYPE: Opaque walls (Identical to #18)
HEATING SYSTEM: As in dryer #18, but with the addition of a 1.5 horsepower electric dehumidifier; storage as added in 1983
PERFORMANCE: Drying time for yellow-popular, 4/4-inch (2.5 cm) thick, from 94% MC to 7% MC is 6 days in summer; from 121% MC to 8% MC is 18 days in winter; from 97% MC to 7% MC is 11 days in spring
Drying time for red oak, 4/4-inch (2.5 cm) thick, from 77% MC to 8% MC averaged 26 days throughout the year
Power consumption in fall is 501 kWh, in summer is 323 kWh, and in winter is 900 kWh
REFERENCES:
Article – Chen, P. Y. S., W. A. Helmer, H. N. Roen, and D. J. Barton. 1982. Experimental solar-dehumidifier kiln for lumber drying. Forest Products J 32 (9):35-41
Present Contact: Dr. Peter Chen
US Forest Service
Forestry Sciences Laboratory
Southern Illinois University
Carbondale, Illinois 62901
USA
Dryer No. 9
LOCATION: Curitiba, Parana, BRAZIL
(Latitude 25 S; longitude 50W)
STATUS: Experimental
TYPE: Greenhouse; three walls and roof transparent
HEATING SYSTEM: Solar hot air collectors; three layers of plastic PVC film with a 1-inch (2 cm) separation; surfaces in the dryer painted black
CIRCULATION: One 18-inch (47 cm) diameter fan powered by a 0.5 horsepower motor.
VENTILATION: Two vents, 10 x 10 inches (25 x 25 cm) on north wall
CAPACITY: 420 board feet (1.0 cubic meter)
CAPACITY TO COLLECTOR AREA RATIO: 230 square feet per thousand board feet (9.1 square meters per cubic meter)
LOADING: manually
OVERALL DIMENSIONS: East-west, 8.9-ft (2.7 m); north-south, 10.8-ft (3.3 m); height, 8.5-ft (2.6 m)
CONSTRUCTION: Frame construction with 2 x 4 framing; floor covered with particleboard ¾-inch (20 cm) thick; south wall covered with 1-inch (2.5 cm) lumber; south wall and floor insulated; in front of lumber pile a cement block wall is built to direct air through lumber more evenly
STORAGE: None
PERFORMANCE: Drying time for Ocotea catharinensis from 90% MC to 12% MC is 23 days in late spring
Solar drying is three times faster than air drying with no quality difference
REFERENCES:
Article – Santini, E. J. ca. 1981. Secagem de Madeira serrada em estufa solar e sua comparacao com os metodos convencionais. Revista Floresta: 5-13.
Present Contact: Dr. Ivan Tomaselli
Universidade Federal do Parana
Caixa Postal 2959
80.000 Curitiba, Parana
BRAZIL
