Cell Orientation. Hardwoods and Softwoods. Layers of the Cell Wall. AGENDA and OBJECTIVES. Two cell orientations. Wood Behavior And Drying Methods

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1 Wood Behavior And Drying Methods Tree School Clackamas Community College Oregon City, OR March 21, 2015 Jim Reeb Extension Forestry and Natural Resources Clatsop, Lincoln & Tillamook Extension AGENDA and OBJECTIVES * Different types of wood cells. * Measure and calculate moisture content of wood. * Define fiber saturation point (FSP) shrink/swell. * Anisotropic properties of wood w/ regard to shrinking & swelling. * Define equilibrium moisture content (EMC) and why it s important. * The importance of sticker placement and good stacking practices. * Examine the drying process. * Discuss the types of drying with emphasis on air, shed, solar and dehumidification. Hardwoods and Softwoods Cell Orientation Two cell orientations Longitudinal aligned with the long axis of a trunk or branch From Panshin & DeZeeuw (1980) From Haygreen & Bowyer (1989) Radial aligned with the radius of the trunk or branch From Haygreen & Bowyer (1989) Hardwoods Wide Rays Softwoods Narrow Rays Layers of the Cell Wall (~200X) Aggregate ray (~20X) Longleaf pine latewood (4200X) From: Panshin & DeZeeuw, Textbook of Wood Technology Red Alder Red Oak Port-Orford Cedar Douglas-fir 1

2 Water and Wood Moisture Content How we calculate it for lumber drying M.C., % = weight of water in the wood weight of the wood X 100 Or to calculate: M.C., % = wt. of green wood wt. of ovendry wood X 100 wt. of ovendry wood Source: Understanding Wood by Bruce Hoadley But in Pulp & Paper Industry (Wood chips, hogged fuel, shavings) Example: Wet weight = 150, Oven Dry weight = 100 Moisture Relationships at the Cellular Level Dry-basis MC this is what lumber folks use Free water Wet-basis MC this is what chip folks use Bound water So, if chips are $60 per BD ton and they receive 80 tons by rail, mill will pay $60 X (100%-33%) X 80 = $3,216 to the supplier. Free Water Water and water vapor in the cell lumens or adhering to the cell walls. Bound Water Water chemically held within the cell walls. HOH From Haygreen & Bowyer (1989) 2

3 Fiber Saturation Point (FSP) Shrink/Swell That moisture content at which the cell wall is completely saturated with water, but no moisture is present in the cell lumen. OHOHOHOHOHOHOHO O HO All the free water is gone. FSP ~25 30 %MC Above & at FSP = no shrinkage Below FSP shrinkage occurs Mature wood does not shrink much in length! S2 Layer RELATIVE ENERGY 80% to 15% Energy to evaporate bound water Energy to evaporate free water 1000 Btu/lb 2300 kj/kg and higher Moisture Content, % Shrink & Swell Anisotropic Properties of Wood Differential Shrinkage Red Pine Wood Shrinks Approximately: 8% TANGENTIAL direction 5% RADIAL direction 0.1% LONGITUDINAL direction Quarter sawn lumber will retain its rectangular shape as it shrinks and swells. Does not shrink as much (%) in width. Source: Understanding Wood by Bruce Hoadley 3

4 Shrinkage Values for Western Woods Species Tangential Radial Douglas-fir 7.6% 4.8% Western hemlock 7.8% 4.2% W. red cedar 1 5.0% 2.4% Red alder 7.3% 4.4% Pacific madrone % 5.6% 1 Shrinkage is large for red cedar and madrone tangential more than twice that of radial shrinkage (shrinkage is extreme in madrone). End checking in white oak logs. Dry too fast and checks will occur along the rays. Ray cells act as a plane of weakness. As wood dries checks occur along these planes. DRYING DEFECTS Defects can occur in nature and in manufacture Natural defects Start in live tree Cannot eliminate Harvesting Sawing and stacking Drying Planing and remanufacturing Typical Moisture Gradient in Lumber During Drying Differential Shrinkage of Wood Moisture content initial moisture content Target MC t 1 t 2 t 3 When checks extend deep into a board, it is called honeycomb 0 3/4 inch 1 1/2 inches Time (t) increases from t 1 to t 3 Illustration of lumber 1 1/2 inches thick From: Understanding Wood by Bruce Hoadley Oak board has been planed to show the honeycombing. 4

5 Almost Impossible to Stop Unless use a bulking agent such as PEG End Checks Not Visible Until Sawcut is Made (red alder) After drying After drying and ½ inch trimmed THREE WAYS HONEYCOMB FORMS HONEYCOMB DUE TO SURFACE CHECKS EXTENDING Called Bottleneck Checks Spontaneous failure due to stress Extension of surface check Extension of end check Golden chinkapin HONEYCOMB (Spontaneous) Honeycomb will show when wood is remanufactured Occurs late in drying Shell in compression Core in tension Too hot Wood is weak Core still > 20% MC Wood is weak Compression Tension Compression Oregon White Oak Honeycomb does not appear on the surface of a planed red oak board (lower) but does appear after the board is machined into millwork (upper) 5

6 Dry-Shell - Tension Wet Core - Compression Dry Shell - Compression Dry Core - Tension Casehardening Early in drying Later in drying The stresses remain when the wood is uniformly dry. STRESS (Casehardening) Shell in compression Core in tension Prongs bend inward Condition lumber to relieve the stresses put water back into the lumber Conditioning (stress relief) is an important drying step if the lumber is going to be remanufactured! COLLAPSE Not Steamed Problems in the final product! Recover by steaming (Madrone) Post-Steamed T=200 F, 22 hrs COLLAPSE Capillary forces from free water evaporation Severe collapse in western redcedar Occurs early in drying Start with lower T Not Steamed Example of Collapse Raise humidity Post-Steamed T=200 F, 22 hrs Photomicrograph showing collapsed wood cells Source: Dry Kiln Operator s Manual 6

7 Knot Types KNOTS Tight knot Loose knot Source: Forest Products and Wood Science by John Haygreen and Jim Bowyer Source: Understanding Wood by Bruce Hoadley When tight (intergrown) knots dry = Relationships: Sapwood Heartwood Each Log has Equal Volume of Juvenile Wood Juvenile wood Mature wood 50% Juvenile Wood 14% Juvenile Wood Jozsa and Middleton A discussion of wood quality attributes and their practical implications. Special publication. No. SP 34. FORINTEK Canada Corp. Usually, the smaller the logs, the larger the problem with juvenile wood! 7

8 Reaction Wood Compression and tension wood result from leaning stems. It is a tree s method of straightening up. Compression wood formed on the underside of the lean in softwoods Southern Pine Source: Forest Products and Wood Science by John Haygreen and Jim Boyer CW forms in both earlywood and latewood. Underside of lean TW forms only in earlywood. Characteristics of Compression Wood Source: Understanding Wood by Bruce Hoadley Brash failure in compression wood Compression Wood: Douglas-fir latewood (1430X) Source: Panshin & DeZeeuw, Textbook of Wood Technology Jozsa and Middleton A discussion of wood quality attributes and their practical implications. Special publication. No. SP 34. FORINTEK Canada Corp. Juvenile or Compression Wood Normal Wood Longitudinal shrinkage in normal (mature) wood is almost negligible Radiata pine Longitudinal Shrinkage in Compression Wood Breaks across grain occur or wood warps Hemlock S2 Cell Wall Longitudinal shrinkage of compression and juvenile wood can be as much as twenty times that of normal wood. 8

9 Juvenile and compression wood shrink more longitudinally than that of normal (mature) wood. When normal wood and juvenile or compression wood occur in the same board, one part of the board will try and shrink more than the other and warp occurs (bow or crook). Fuzzy Grain Tension wood can be a problem if the piece is remanufactured or stained Source: Forest Products and Wood Science by John Haygreen and Jim Boyer ~ 1 inch Longitudinal Shrinkage and Fuzzy Grain in Tension Wood GROWTH STRESSES Cells shorten in final stage of maturation Longitudinal stress compression near pith tension near bark Bow and crook immediately after sawing In 8 ft board, split was about 4 ft long. Red alder SPIRAL GRAIN SPIRAL GRAIN Helical orientation of cells Very visible in posts and poles To left in young conifers, then straight or right Hereditary Proper stacking Restraint Warp prone material at bottom of the lumber package J.H. Priestley. Amer J Botany 9

10 Types of Warp Juvenile wood and compression wood are major causes of bow and crook Bow The Key to Drying Lumber Fundamental Rule of Drying Quality depends on the rate of drying Twist most common cause is spiral grain Cup is due to differential shrinkage between radial and tangential surfaces in wood thin, wide flat-sawn pieces are prone to cup always cup toward the bark Crook Twist Cup Dry too slow - can result in stain and decay Dry too fast - can result in checks, splits, honeycomb, collapse, non-uniform MC If not worried about defects, can dry wood in an oven in several hours HEAT NEEDED TO KILL FUNGI IN WOOD IN WOOD Wood Above FSP Heated in Steam Wood Deg F Time/Min Wood Above FSP Heated in Air 90-97% RH Time/Min 35-40% RH Time/Min Insects and eggs F at least 6 hours SETTING PITCH EVAPORATE TURPENTINE AND OTHER SOLVENTS 160 o F FOR 4/4, 170 o F FOR THICKER STOCK RETAINING CEDAR OIL Important if lumber is to be finished or glued Higher temp early in schedule is more effective (WPA, circa 1930) 180 F Opposite of setting pitch Stay under 160ºF Avoid conditioning 10

11 BROWN STAIN Color change of chemicals normally present Use fresh logs, Dry soon after sawing Pine - use 120 F and low relative humidity for first part of schedule (opposite of setting pitch) Hemlock avoid steam spray Misplaced stickers inhibit air flow through the package interior pieces can t dry Stickers and Stacking Poor stacking causes warped lumber Stickers not aligned one above the other Boards will warp while drying and remain warped. One misplaced sticker can adversely affect many boards. Bunk and Sticker Alignment Important Even for Small Operators Stickers Kiln dried Keep dry 5/8 to 7/8 inches thick for steam kilns 1-inch thick for air drying ¾ - 1-inch for solar drying J.E. Reeb and T.D. Brown. Air-and Shed-drying Lumber. EM Wide enough so wood does not fail in compression (at least an inch) Control: Temperature Relative humidity Air flow Time Lumber Drying Fan Deck Reversible Fan Automatic Vents Steam Spray Heating Coils Top Load Baffle Lumber Stack Bottom Load Baffle Booster Coil HIGHER TEMPERATURE AT LOWER MOISTURE CONTENT Strength increases as the wood dries below fiber saturation point Higher temperatures can be used when the wood is stronger Defect-prone woods are started at a low temperature, 100 F to 120 F 11

12 Sort Lumber to Improve Drying Always Species or species group Thickness Sometimes Width Length Sap / Heart Moisture content Wet wood Grain (flatsawn vs. quartersawn vs. mixed grain) Drying- How? Air Solar Vacuum Radio-frequency Dry Kiln operational (compartment or progressive) temperature (<120, 180, 211, >212) heat and energy source (steam, direct, DH) Air Drying Stickered lumber is placed in an open yard. Use a roof - protect from sun and precipitation. Use shade cloth - from drying too fast. Orient the stack so moving air can carry away the moisture. Air Drying - Benefits Inexpensive - no energy costs. Shorten the drying cycle by air drying the lumber from green down to a low moisture content, then continue drying in a kiln to the final desired MC. Controls - shade cloth, end-coating the lumber. Air Drying - Problems Control is less than with other methods of drying. BEWARE OF AIR DRYING 80 F and 15% Relative humidity Wood will equilibrate to MC of 2 3% Can dry too fast - checks, splits, honeycomb. Lumber is susceptible to fungi, mold and insect infestation. Temperatures are usually not high enough to kill these. Lumber is susceptible to chemical reactions and bacteria - both can cause stains. 12

13 Shed Drying Stickered lumber is placed in a shed. Better protection from precipitation, direct sun, dirt and other contaminants than air drying. Shed Drying - Problems Prone to many of the same problems as air drying - Final moisture content is dependent on outside ambient temperature and relative humidity. Often cannot dry to a low enough MC for interior uses. More control - can have one or more walls, thus slow the drying process. Install fans - can circulate air through lumber when conditions are right and off when are not right (fan pre-dryer or shed-fan drying). Solar Drying Lumber is placed in an insulated enclosed chamber - heat source for moisture evaporation comes through solar collector. Moist air can either be removed from the kiln through vents or allowed to condense on the cold collector at night and run out of a drain through the floor. Solar Drying - Benefits Relatively inexpensive to build. More control over the drying process than air drying and lumber is protected from weathering. Can result in very high quality lumber - Conditioning step (night) relieve stress. OSU Solar Kiln Solar Drying - Problems Insulated walls, doors, floor & roof Dependent on the weather eg. amount of sunshine. Drying times are relatively long. Electricity is needed to run fans. 13

14 End view of OSU solar dry kiln Fans Baffle South Energy from the sun OSU Solar Kiln Air Flow Panels are angled at 54 degrees Plexiglass (acrylic) vs. Lexan (polycarbonate) Lumber stack 3/4 stickers separate each layer - spaced apart Size of the Solar Collector For woods that are prone to checking and splitting, a typical safe drying rate is about 3.5% moisture content (MC) loss per day. This is equivalent to an evaporation loss of 100 pounds of water per day per 1000 board feet of lumber. The energy required for this evaporation is (1000 Btu s per pound x 100 pounds =) 100,000 Btu s. Average solar input is 1000 Btu s per square foot of collector, the collector size required is: 100 square feet per 1,000 board feet of lumber For species that can be dried faster, the collector to board foot ratio can be increased safely, while for more degrade prone species (or thicker pieces of moderate degrade prone species) the ratio can be smaller. The ratio required for a species (as calculated above) should not be exceeded in the design due to the risk of quality loss in drying. However, smaller ratios can be used with the only penalty being longer drying times. Dehumidification Kiln Temperatures can reach 160 degrees F. Moisture is removed by condensing on the cold coils of a heat pump dehumidifier. Heat used to evaporate the water is recovered and pumped back into the chamber to do more drying. Considered a closed system but can use vents if need to control the temperature during drying. Dehumidification Kiln Conversion of a Schedule from a Steam-heated Kiln to a Dehumidification Kiln Drying mechanism is the same as for steam drying difference is how heat is supplied to the kiln and how moisture is removed. DH is energy efficient but energy can be expensive - electric. It allows small operators to dry their own wood without the expense and expertise of operating a boiler. Not only small some 100 mbf aluminum DH kilns Water must be disposed of ph about Source: Dry Kiln Operator s Manual 14

15 Kiln Schedules for about 500 Species Dehumidification Kiln Dry 4/4 hardwood in 4 5 weeks Dry 4/4 softwood in 2 3 weeks Can attain temperature of 160 degrees F hot enough to sterilize the lumber. 150 degrees F for 24 hours will kill stain and decay fungi. No new fungi will occur as long as wood is kept below about 20% MC. 20% MC or below lumber needs to be at temperature of 135+ degrees F to kill most insects at least 6 hours. Bailey s 4K Kiln Drying Schedules Three Steps Dry > Remove water at a controlled rate Equalize > Reduce moisture content difference between wettest and driest pieces & between shell and core within pieces Condition > Relieve stresses developed during drying cycle Wet Bulb Temperature Typical Steps in a Drying Schedule for Steam Kilns Dry Bulb Temperature More humid air DRYING Wet Bulb Depression Dryer air Time or Moisture Content EQUALIZING CONDITIONING When Drying Defects Occur For thickness changes At high MC (surface of wood is wet mass flow), time for a given MC change is proportional to thickness Wet Bulb Temperature Dry Bulb Temperature At low MC, time for a given MC change is proportional to thickness 2 Total time is approximately proportional to 1.5 power. Time So Where N = 1, 2, or

16 Effect of thickness Known condition: Thickness = 1 What happens to rate if Thickness = 1.25 = 3 Thickness has a dramatic effect on drying time but it gets worse. We must use milder conditions on thick lumber. METHODS FOR MEASURING MOISTURE CONTENT Meter Measure electrical property, infer MC Oven Use weight loss Compare weight of wood & water to weight of wood Meters are calibrated to this Chemical Extract water and separate from oils Most accurate Vacuum dessicator Example of two types of moisture meters: Wagner TM on right is a dielectric meter without prongs. Lignomat TM is a resistance meter or prong meter. No No Yes Yes No Yes No 7 25% No Resistance vs. Dielectric Advantages and Disadvantages Comparison Criteria Highly sensitive to species Highly sensitive to density Somewhat sensitive to temperature Measures at small spot Measures over a small area Measures at an exact depth Measures over an average depth Best between In-line metering is easy Yes Yes No No Yes No Yes % Yes Bottom Line probably should use both types of meters TYPICAL DRY MCs Product MC, % Dimension <19 Shop, Lam Stock Furniture, flooring 6-8 We try to dry to the moisture content the product will see in service Tighter +/- tolerances at lower MCs Equilibrium Moisture Content Line represents white spruce with FSP around 30%. Although a precise curve cannot be drawn for each species, most will fall within the shaded area. Amount of bound water in wood is determined by the RH of the surrounding atmosphere. Amount of bound water in wood changes (slowly) as the RH changes. EMC = MC where wood is in equilibrium with the RH of its environment. Source: Understanding Wood by Bruce Hoadley 16

17 References References (cont.) OR