Small-Scale Lumber Drying

Transcription

1 Small-Scale Lumber Drying Lumber Drying: How & Why Adding value to sawn lumber Tree School March 24 th, 2012 Scott Leavengood Oregon Wood Innovation Center Oregon State University Outline Small-Scale Scale Technology Air/ shed drying, solar & DH kilns Why dry lumber? How wood dries Common problems Checking, warping, collapse, casehardening 2

2 What is Small-Scale? Low volume (capacity) Low capital investment varies by capacity level of control heat source efficiency desired consistency 3 Drying Technology Air/ Shed Solar Vacuum Radio-Frequency Dry Kiln operational (compartment or progressive) temperature (<120, 180, 211, >212 ) heat and energy source (steam, direct, DH) 4

3 Air Drying Lowest cost and simplest drying technology Minimum MC limited by climate Generally slow, more difficult to estimate drying time Temperatures not high enough to set pitch, kill fungi and insects 5 Dry Kiln Operator s Manual p. 146 Killing 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 may be adequate. [Note: Maximum temperature in table 7-31 is 140 F] 6

4 Air Drying Wood can dry too fast (check, split, warp) in hot, dry weather More difficult to equalize and condition to relieve drying stresses Lumber can become dirty and/or weathered 7 Sorting species thickness length moisture content heartwood/ sapwood grain 8

5 Air Drying More than just a neat looking stack Location gentle slope weed-free (gravel yards best) protection from wind orientation to wind depends on desired drying rate Stacking minimum 12 off ground stickers ¾ - 1 thick stickers at board ends and apart stickers vertically aligned lumber uniform thickness lumber uniform length or step-out t or box pile protective cover & top weight 9 Stacking- Details Prepare site before buying or sawing lumber Foundation of cinder blocks 3 ft. on center 4x6 mudsill on cinder blocks (shim to level) 4x4 bolsters on centers Stickers on bolsters Low grade lumber on first course* Cover w/plywood and top-weight (50 psf) *Due to ground moisture 10

6 Stacking- Details Wider and highest quality on inside Cupped boards w/ cups down Random-length material step-out stacking box piling step-out box piling 11 Green MC = 150% Green MC = 37% Source: Simpson, W.T. and C.A. Hart, Estimates of Air Drying Times for Several Hardwoods and Softwoods, USDA Forest Products Laboratory General Technical Report FPL-GTR-121.

7 Improving Control in Air Drying: Drying Sheds Canbesimple(4 posts and a roof) For added control movable walls for air flow control fans for control of circulation Source:

8 Monitoring MC While Drying: Sample Boards section 1 50g sample board 4.32 kg 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 board 3. Cut 1 sections from each end of the sample board 4. Number the 1 sections 5. Weigh the sections to an accuracy of ± 0.1g and record on sections 6. Weigh the sample board and record on board 7. End coat the sample board 8. Place sample board in stack 9. Dry 1 sections in oven 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 sections 2. 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/( %)] x 100 = 2.98 kg 3. Write sample board ovendry weight on board & return to stack 4. Periodically reweigh sample board to obtain current MC: (current weight/ calculated ovendry weight) 1 X Compare change in MC to maximum safe rate per day for species 16

9 Solar Kilns Next step up in technology from air (shed) Similar to greenhouses (passive solar) Faster than air drying; can obtain lower MC s More control of air flow, temp, and humidity Nighttime humidity increase can reduce drying stresses Low energy costs (fans, thermostats, etc.) Design plans available from numerous sources 17 Solar Kilns How do they work? Solar energy enters through the collector and heats the interior surface (temps. can reach 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 Nighttime increase in RH provides for stress relief Heat is greater as wood MC drops below 20%- cooling effect of evaporation is lessened 18

10 Solar Kiln Design Considerations: Collector area- 100 ft 2 to dry 1 MBF Roof pitch- 42 to 46 Orientation- South Collector construction- glass or fiberglass Collector 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 19 More Design Considerations: Oversize floor dimensions to allow for 12 space around all sides of load Insulate walls and doors with kraft-backed insulation (foil will trap moisture), floor with rigid foam Do not insulate exterior walls (e.g., foam or vapor barrier) allow moisture that gets into walls to escape Paint interior surfaces with 2 coats aluminum paint and one coat flat black Stain exterior finish must allow moisture to escape 20

11 Collector Designs: Material One or more layers of visqueen, plexiglass or corrugated fiberglass Storm windows over box Front - black painted sheet metal Back - hardboard Bottom - vented Top - open Shape Flat or angular 21 OSU Mobile Solar Kiln Demonstration Unit 22

12 3/ / / / x6 cedar 7-14 ~475 BF 80, 67% RH EMC = 12% 34, 71% RH EMC = 14% Oregon City March 2008 Temp = 34 to 82 (avg. 52 ) Humidity = 30% to 100% (avg. 84%) For more info:

13 Dehumidification (DH) Kilns Next 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) Compressor and condenser coils remove water in liquid form vs. venting 25 Dehumidification (DH) Kilns Can be more efficient than conventional steam kilns - venting not used to remove moisture Max. temps. in the range Extra equipment may be needed for conditioning (to relieve drying stresses) 26

14 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 A sample, very small-scale DH Kiln: Drying rate - weigh water in catch pan Condition - spray with water then put back in kiln

15 Why Dry Wood? 30

16 Western redcedar (sapwood) 1 ft 3 piece green would weigh about 68 lbs - of which about 52 lbs would be due to water! Drying- Why? Stability Weight Reduced risk of stain and decay Fastening Finishing Adhesion Conductivity Preservatives Set pitch Kill insects Strength Surfacing 32

17 Drying- How? Control: temperature relative humidity air flow time 33 EMC Equilibrium Moisture Content The moisture content eventually attained in wood exposed to a given level of relative humidity and temperature. R.B. Hoadley, Understanding Wood Temperature ( F) Relative Humidity (%)

18 Wood Structure (150x) (~10x) 35 Wood Fibers/ Tracheids 36

19 How does wood dry? Wood holds water in two ways: free water bound water 38

20 Free Water: Water or water vapor in the cell lumens (pore space) or adhering to the cell walls. (4200x) 39 Bound Water Water chemically held within the cell walls H 2 O H 2 O 40

21 FSP - Fiber Saturation Point that moisture content at which the cell wall is completely saturated with water, but no moisture is present in the cell lumen Panshin & DeZeeuw, Textbook of Wood Technology cell wall lumen 41 Loss of Water 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 free water 42

22 Shrink/Swell: H 2 O Drying H 2 O H 2 O H 2 O Softwood tracheid side view Softwood tracheid end view 44

23 It s a moisture problem Common drying defects Checking End, Surface, Internal Warp Bow, crook, twist, cup Casehardening Collapse Some you can control, others you must simply work around 46

24 Checking Occurs in early stages of drying (~>24% MC) Usually occur in rays Caused by stresses developed due to rapid moisture loss through board ends Wetting after formation drives checks further into board Can be minimized by using high initial RH, end coating (very soon after felling), and/or end-stickering 47 Drying Stresses Moisture movement ~ times faster from end-grain than edge-grain Outer shell dries faster than core 48

25 Ray cells act as a plane of weakness. As wood dries checks occur along these planes. End checking in white oak logs. Checks occur along ray cells. Warping Due to differential rates of shrinkage: Radial/ tangential Juvenile wood/ mature wood Reaction wood/ normal wood 50

26 Radial/ Tangential R T Quartersawn T R Flatsawn 51

27 Types of Warp Bow Crook Twist Cup Juvenile/ Reaction Wood 54

28 Reaction Wood Formed by the tree s reaction to a leaning stress 55 Drying Stresses Moisture movement is approximately times faster from the end-grain than from the edge-grain The outer shell dries faster than the core. 56

29 Collapse Madrone If no checking or honeycomb, collapse can 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. 57 Photomicrograph showing collapsed wood cells Source: Dry Kiln Operator s Manual

30 Casehardening 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 uniformly dry. Source: Dry Kiln Operator s Manual 59 Casehardening Dry-Shell - Tension Early in drying cycle Wet Core - Compression Late in drying cycle Dry Shell - Compression Dry Core - Tension

31 Drying stresses must be relieved in lumber that will be remanufactured. Stresses are relieved by conditioning during drying. Testing for Casehardening Source: Dry Kiln Operator s Manual

32 Summary Small-scale technology Why dry wood? How wood dries Common problems 63 References Understanding Wood: A Craftsman s Guide to Wood Technology. R. Bruce Hoadley. Taunton Press ( Forest Products Society ( The Dry Kiln Operator s Manual The Wood Handbook: Wood as an Engineering Material Independent Sawmill & Woodlot Management magazine ( Woodweb 64

33 To Contact Us: Scott Leavengood Director, Oregon Wood Innovation Center Oregon State University Dept. of Wood Science & Engineering 119 Richardson Hall Corvallis, OR