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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


    Small-Scale TechnologySmall Scale TechnologyAir/ shed drying, solar & DH kilns

    Why dry lumber?How wood driesCommon problems


    Common problemsChecking, warping, collapse, casehardening

  • What is Small-Scale?

    Low volume (capacity)Low volume (capacity)Low capital investment

    varies by capacity level of control



    heat source efficiency desired consistency

    Drying TechnologyAir/ Shed/SolarVacuumRadio-FrequencyDry Kiln

    operational (compartment or progressive)


    temperature (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


    y gTemperatures not high enough to set pitch, kill fungi and insects

    Dry Kiln Operators Manual p 146Dry Kiln Operators 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


    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




    speciesspeciesthicknesslengthmoisture contentheartwood/ sapwood


    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 - 1 thickstickers at board ends and 12-24 apartstickers vertically alignedlumber uniform thicknesslumber uniform length or

    t t b il


    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)


    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



    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





  • Monitoring MC While Drying:Sample Boards






    n 1


    n 1



    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%


    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 KilnsNext step up in technology from air (shed)p p gy ( )Similar to greenhouses (passive solar) Faster than air drying; can obtain lower MCs More control of air flow, temp, and humidityNighttime humidity increase can reduce drying stresses


    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


    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 46Orientation- South Collector construction- glass or fiberglass


    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 kilns 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


    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 angularOne or more layers of visqueen, plexiglass or corrugated fiberglassStorm windows


    Storm windows over box

    Front - black painted sheet metal

    Back - hardboard Bottom - vented Top - open

    OSU Mobile Solar Kiln Demonstration Unit


  • 80, 67% RHEMC = 12%

    3/25 30-513/26 36-453/27 34-473/28 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:

  • Dehumidification (DH) KilnsNext 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)


    ( 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 - 160range


    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 2x4s; 37 1x12s)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:


    Why Dry Wood?


  • 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 decay


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