small-scale lumber dryinglumber...
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Small-Scale Lumber Drying
Small-Scale Lumber DryingLumber DryingLumber Drying
Lumber Drying: How & WhyAdding value to sawn lumber
Tree SchoolMarch 24th 2012March 24th, 2012
Scott LeavengoodOregon Wood Innovation CenterOregon State University
Scott LeavengoodOregon Wood Innovation CenterOregon State University
Outline
Small-Scale TechnologySmall Scale TechnologyAir/ shed drying, solar & DH kilns
Why dry lumber?How wood driesCommon problems
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Common problemsChecking, warping, collapse, casehardening
What is “Small-Scale”?
Low volume (capacity)Low volume (capacity)Low capital investment
varies by • capacity• level of control
h
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• heat source• efficiency• desired consistency
Drying TechnologyAir/ Shed/SolarVacuumRadio-FrequencyDry Kiln
operational (compartment or progressive)
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temperature (<120°, 180°, 211°, >212°)heat and energy source (steam, direct, DH)
Air Drying
Lowest cost and simplest dryingLowest cost and simplest drying technologyMinimum MC limited by climateGenerally slow, more difficult to estimate drying time
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y gTemperatures not high enough to set pitch, kill fungi and insects
Dry Kiln Operator’s Manual p 146Dry Kiln Operator’s Manual p. 146Killing fungi – “A temperature of 110° F stops the growth of these organisms but does not kill them. Tests show that a temperature of 150° F or higher for at least 24 h should kill all stain and decay fungi. As long as the wood is kept below 20 percent moisture content, new stain and decay will not start.”
Killing insects – [specific to powderpost beetles] “To sterilize, use an EMC that is within 2 percent above or below the moisture content of the wood. If the wood has less than 8 percent moisture content, a temperature above 140° F and a relative humidity somewhat below 60 percent should give satisfactory results, using the times given in table 7-31 for the 130° F temperature. Exact data on temperatures and times required to kill other insects are not available but the higher temperature schedule of table 7-31
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to kill other insects are not available, but the higher temperature schedule of table 7-31 may be adequate.” [Note: Maximum temperature in table 7-31 is 140° F]
Air Drying
Wood can dry too fast (check splitWood can dry too fast (check, split, warp) in hot, dry weatherMore difficult to equalize and condition to relieve drying stressesLumber can become dirty and/or
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yweathered
SortingSorting
speciesspeciesthicknesslengthmoisture contentheartwood/ sapwood
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heartwood/ sapwoodgrain
Air DryingMore than just a ‘neat looking’ stack
Location Stackinggentle slopeweed-free (gravel yards best)protection from windorientation to wind depends on desired drying rate
Stackingminimum 12” off groundstickers ¾” - 1” thickstickers at board ends and 12”-24” apartstickers vertically alignedlumber uniform thicknesslumber uniform length or
t t b il
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• step-out or box pileprotective cover & top weight
Stacking- DetailsPrepare site before buying or sawing lumberFoundation of cinder blocks 3 ft. on center4x6 mudsill on cinder blocks (shim to level)4x4 bolsters on 12”-24” centersStickers on bolstersLow grade lumber on first course*Cover w/plywood and top-weight (50 psf)
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Cover w/plywood and top weight (50 psf)
*Due to ground moisture
Stacking- Details
Wider and higheststep-out
Wider and highest quality on insideCupped boards w/ cups downRandom-length
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gmaterial
step-out stackingbox piling box piling
Green MC = 150% Green MC = 37%
Source: Simpson, W.T. and C.A. Hart, 2000. Estimates of Air Drying Times for Several Hardwoods and Softwoods, USDA Forest Products Laboratory General Technical Report FPL-GTR-121.
Improving Control in Air Drying: Drying Sheds
Can be simple (4Can be simple (4 posts and a roof)For added control
movable walls for air flow controlfans for control of
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circulation
Source: http://www.shadedri.com/drying_shed.html
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Monitoring MC While Drying:Sample Boards
1”1”
30”30”
1”1”
5050
sectio
n 1
sectio
n 1
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50g50g
sample boardsample board4.32 kg4.32 kg
1. While stacking, cut samples from material representative of the MC of the material being dried
wettest lumber highest risk of degrade (most recently cut, thickest and widest, quartersawn, slowest drying species, etc.)
2. Cut a 30” sample board a minimum of 12” in from end of a board3. Cut 1” sections from each end of the sample board4. Number the 1” sections5. Weigh the sections to an accuracy of ± 0.1g and record on sections6. Weigh the sample board and record on board7. End coat the sample board8. Place sample board in stack9. Dry 1” sections in oven 215-218° F for ~24 hrs.10. Weigh the 1” sections and record (keep drying and weighing until weight
stabilizes = ovendry weight)11. Calculate MC = (weight before drying/ ovendry weight) – 1 X 100
Example: if section 1 weighed 50g before drying and 35g after drying (ovendry), MC = (50/35) -1 x 100 = ~43%
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1. Calculate average MC from 1” sections2. Calculate ovendry weight of sample board as: [wet weight/(100 + % MC)] X 100
Example: if sample board weighed 4.32 kg and average MC = 45%ovendry weight = [4.32 kg/(100 + 45%)] x 100 = 2.98 kg
3. Write sample board ovendry weight on board & return to stack4. Periodically reweigh sample board to obtain current MC:
(current weight/ calculated ovendry weight) – 1 X 1005. Compare change in MC to “maximum safe rate per day” for species
Solar Kilns“Next step up” in technology from air (shed)p p gy ( )Similar to greenhouses (passive solar) Faster than air drying; can obtain lower MC’s More control of air flow, temp, and humidityNighttime humidity increase can reduce drying stresses
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Low energy costs (fans, thermostats, etc.)Design plans available from numerous sources
Solar KilnsHow do they work?
• Solar energy enters through the collector and heats the interior surface (temps. can reach 130° - 150° F)• Heat circulates through the stack via natural convection and fans• Baffles help to direct air through the stack • Heat causes water evaporation and an increase in RH in the kiln• Vents in the rear wall of the kiln are opened to exhaust the moist air and allow fresh air in
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fresh air in• Nighttime increase in RH provides for stress relief• Heat is greater as wood MC drops below 20%- cooling effect of evaporation is lessened
Solar KilnDesign Considerations:
Collector area- 100 ft2 to dry 1 MBFCollector area 100 ft to dry 1 MBFRoof pitch- 42° to 46°Orientation- South Collector construction- glass or fiberglass
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fiberglassCollector area = Approx. 1 ft.2 of collector area per 10 BF of lumber to be dried- 100 ft.2 to dry 1 MBF. For faster drying, increase the collector area (trade off is heat loss at night and in cold weather)Roof pitch- approximately equal to the latitude of the kiln’s location- for Oregon, 42° to 46° will work well. (can increase to 55° to improve winter performance)Collector construction- clear fiberglass, corrugated fiberglass, storm windows, recycled tempered glass, and number of layers
More Design Considerations:
“Oversize” floor dimensions to allow for 12” space around all sides of loadInsulate walls and doors with kraft-backed insulation (foil will trap moisture), floor with rigid foamDo not insulate exterior walls (e.g., foam or vapor barrier) – allow moisture that gets into walls to escape
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walls to escapePaint interior surfaces with 2 coats aluminum paint and one coat flat blackStain exterior – finish must allow moisture to escape
Shape
Collector Designs:Collector Designs:Material
Flat or “angular”One or more layers of visqueen, plexiglass or corrugated fiberglassStorm windows
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Storm windows over box
• Front - black painted sheet metal
• Back - hardboard• Bottom - vented• Top - open
OSU Mobile Solar Kiln Demonstration Unit
22http://owic.oregonstate.edu/solarkiln/
80°, 67% RHEMC = 12%
3/25 30-51°3/26 36-45°3/27 34-47°3/28 35-47°
1x6 cedar7’-14’~475 BF
34°, 71% RHEMC = 14%
Temp = 34° to 82° (avg. 52°)Humidity = 30% to 100% (avg. 84%)
Oregon CityMarch 2008
For more info:
http://owic.oregonstate.edu/solarkiln/www.sawmillmag.com
Dehumidification (DH) Kilns“Next step up” in cost and technologyNext step up in cost and technology from solar kilns (better control of temp. and RH)Design can be as simple as a garage but must be well insulated and watertight (see solar kiln design)
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( g )Compressor and condenser coils remove water in liquid form vs. venting
Dehumidification (DH) Kilns
Can be more efficient thanCan be more efficient than conventional steam kilns - venting not used to remove moisture Max. temps. in the 120° - 160°range
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Extra equipment may be needed for conditioning (to relieve drying stresses)
A sample, very small-scale DH Kiln:Capacity ~300 BF in 8’ lengths (56 2x4’s; 37 1x12’s)Total cost ~$245 (1984)Dries to 7% MC (red oak in 60 days)Temp. range 105°- 115°
Drying rate - weigh water in catch panCondition - spray with water then put back in kiln
A sample, very small-scale DH Kiln:
http://owic.oregonstate.edu/pubs/dhkilns.pdf
Why Dry Wood?
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Western redcedar (sapwood) – 1 ft3 piece green would weigh about 68 lbs - of which about 52 lbs would be due to water!
Drying- Why?Drying- Why?
Stability ConductivityyWeightReduced risk of stain and decayFasteningFinishing
yPreservativesSet pitchKill insectsStrengthS f i
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FinishingAdhesion
Surfacing
Drying- How?
Control:Control:temperaturerelative humidityair flowtime
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EMC – Equilibrium Moisture Content
“The moisture content eventually attained in ywood exposed to a given level of relative humidity and temperature.” R.B. Hoadley, Understanding Wood
Temperature(°F)
Relative Humidity (%)
30 60 90
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30 60 90
50 6.3 11.2 20.9
80 6.1 10.8 20.2
110 5.6 10.0 19.1
Wood Structure
35(150x)(150x) (~10x)(~10x)
Wood Fibers/ Tracheids
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How does wood dry?
Wood holds water in two ways:Wood holds water in two ways:free water
bound water
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Free Water:Water or water vapor in the cell lumens (pore space) or adhering to the cell walls.
39(4200x)(4200x)
Bound WaterWater “chemically” held within
H2O
ythe cell walls
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H2O
FSP - Fiber Saturation Point
“that moisture content at which thethat moisture content at which the cell wall is completely saturated with water, but no moisture is present in the cell lumen”
cell wallcell wallPanshin & DeZeeuw, Textbook of Wood Technology
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lumenlumen
Loss of Water
Beginning at the green state:Beginning at the green state:free water is lost first until wood reaches FSP (~30% MC) below FSP = loss of bound water = shrinkage will begin
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free waterfree water
Shrink/Swell:
H2O
H2O
Drying H2O
H2O
Softwood tracheid – side view Softwood tracheid – end view
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It’s a moisture problem...It’s a moisture problem...
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Common drying defectsCheckingChecking
End, Surface, InternalWarp
Bow, crook, twist, cupCasehardening
Collapse
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CollapseSome you can control, others you
must simply work around
CheckingOccurs in early stages of drying (~>24% y g y g (MC)Usually occur in raysCaused by stresses developed due to rapid moisture loss through board endsWetting after formation drives checks f h i b d
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further into boardCan be minimized by using high initial RH, end coating (very soon after felling), and/or end-stickering
Drying StressesMoisture movement ~10-15 times fasterMoisture movement 10 15 times faster from end-grain than edge-grainOuter shell dries faster than core
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Ray cells act as aplane of weakness.
As wood dries checksoccur along theseoccur along these
planes.
End checking in white oak logs. Checks occur along ray cells.
Warping
Due to differential rates ofDue to differential rates of shrinkage:
Radial/ tangentialJuvenile wood/ mature woodReaction wood/ normal wood
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Radial/ TangentialR
Quartersawn
T
T
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R
Types of Warp
Bow
Crook
Twist
Cup
Juvenile/ Reaction Wood
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Reaction WoodFormed by the tree’s ‘reaction’ to aFormed by the tree s reaction to a leaning stress
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Drying Stresses• Moisture movement is approximately 10-
15 times faster from the end grain than15 times faster from the end-grain than from the edge-grain
• The outer shell dries faster than the core.
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Collapse
Madrone
If no checking or honeycomb, collapse can be recovered by
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be recovered by steaming. For cedar, 4 to 8 hours at 212° F –bring the moisture content up 3 – 4 %.
After recovery can take the MC down again without it collapsing.
Source: Dry Kiln Operator’s Manual
Photomicrograph showing collapsed wood cells
Casehardening
“A diti f t d t i“A condition of stress and set in wood in which the outer fibers are under compressive stress and the inner fibers under tensile stress, the stresses remain when the wood is
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Source: Dry Kiln Operator’s Manual
uniformly dry.”
Casehardening
Dry-Shell - TensionDry-Shell - Tension
Wet Core Wet Core -- CompressionCompressionEarly in dryingcycle
Dry Shell - Compression
Dry Core Dry Core -- TensionTension
Late in dryingcycle
Drying stresses must be relieved in lumber that will be remanufactured.
Stresses are relieved byconditioning during drying.
Testing for Casehardening
Source: Dry Kiln Operator’s Manual
Summary
Small-scale technologySmall scale technologyWhy dry wood?How wood driesCommon problems
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ReferencesUnderstanding Wood: A Craftsman’s Guide to Wood Technology. R. Bruce Hoadley. Taunton Press (http://www.taunton.com/index.asp)Forest Products Society (http://www.forestprod.org/)
The Dry Kiln Operator’s ManualThe Wood Handbook: Wood as an Engineering Mate ial
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MaterialIndependent Sawmill & Woodlot Managementmagazine (http://www.sawmillmag.com/)Woodweb – http://www.woodweb.com
To Contact Us:Scott LeavengoodDi t O W d I ti C tDirector, Oregon Wood Innovation [email protected]://owic.oregonstate.edu
Oregon State University
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Dept. of Wood Science & Engineering119 Richardson HallCorvallis, OR 97331-5751
www.orforestdirectory.com