11

Fundamental Rule of Drying“Quality depends on the rate of drying”

• 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

The Key to Drying LumberDrying- Why?

• Stability

• Weight

• Reduced risk of stain and decay

• Fastening

• Finishing

• Adhesion

• Conductivity

• Preservatives

• Set pitch

• Kill insects

• Strength

• Surfacing

IN WOOD

HEAT NEEDED TO KILL FUNGI IN WOOD

Wood Above FSP Heated in Steam

Wood Above FSPHeated in Air

Wood Deg F Time/Min90-97% RH

Time/Min35-40% RH

Time/Min

150 75 100 --

160 -- -- 190

165 -- -- 60

170 30 30 50

180 20 20 --

200 10 -- --

Insects and eggs 135-140°F at least 6 hours

Surface mold

Blue stain

Fungal hyphae

Examples of Stain and Mold – If Dry Too Slow or AllowDry Lumber to Become Wet

SETTING PITCH

EVAPORATE TURPENTINE AND OTHER SOLVENTS 160oF FOR 4/4, 170oF FOR THICKER STOCK

Higher temp early in scheduleis more effective (WPA, circa 1930)

Important if lumber is to be finished or glued

180 F

12

RETAINING CEDAR OIL

Opposite of setting pitchStay under 160ºFAvoid conditioning

BROWN STAIN

•Color change of chemicals normallypresent•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

Stickers and Stacking PRESSURE DIFFERENCE MAKES THE AIR MOVE

• Air moves faster through larger openings

• More volumethrough large openings

LowerPressure

HigherPressure

Stickers not alignedone above the other –Boards will warp while

drying and remain warped.

Poor stacking causeswarped lumber

One misplaced sticker canadversely affect many boards.

Misplaced stickers inhibitair flow through the package

– interior pieces can’t dry

Support Endsof Boards

• Reduces – end split– Cup– Warp

• Within 1 sticker width, if possible

• If too close, they fall out while on forklift

13

Bunk and Sticker AlignmentImportant Even for Small Operators

J.E. Reeb and T.D. Brown. Air-and Shed-drying Lumber. EM8612http://extension.oregonstate.edu/catalog/pdf/em/em8612-e.pdf

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

• Wide enough so wood does not fail in compression (at least an inch)

Lumber Drying

• Control:–Temperature–Relative

humidity–Air flow–Time

Automatic Vents Steam Spray

Top Load Baffle

Bottom Load Baffle

Fan Deck Reversible Fan

Lumber Stack

Booster Coil

Heating Coils

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

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)

14

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 awaythe moisture.

Air Drying - BenefitsInexpensive - no energy costs.

Shorten the drying cycle by air dryingthe lumber from green down to alow moisture content, then continue drying in a kiln to the final desired MC.

Controls - shade cloth,end-coating thelumber.

Air Drying - Problems

Control is less than with other methods of drying.

Lumber is susceptible to fungi, mold and insect infestation. Temperatures are not high enough to kill these.

Lumber is susceptible to chemical reactions and bacteria, both can cause stains.

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.

Air Drying - Problems

Lumber can become “weathered” fromdirt and other contaminants.

Temperatures are not high enough to setthe pitch in highly resinous species.

Final moisture content depends on ambientair temperature and relative humidity.

Shed Drying

Stickered lumber is placed in a shed.

Better protection from precipitation, direct sun,dirt and other contaminants than air drying.

More control - can have one or more walls, thusslow 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).

15

Shed Drying - ProblemsProne to many of the same problems as air drying -

Final moisture content is dependent on outsideambient temperature and relative humidity.

Cannot dry to a low enough MC for interior uses.

Solar Drying

Lumber is placed in an insulated enclosedchamber - heat source for moistureevaporation comes from solar collector.

Moist air can either be removed from the kilnthrough vents or allowed to condenseon the cold collector at night and run outof a drain through the floor.

Solar Drying - Benefits

Relatively inexpensive to build - less than$1,000 to build 500-1000 bf kiln.

More control over the drying process and lumber is protected from outsidecontaminants.

Can result in very high quality lumber -Equalizing step (night) relieve stress.

Solar Drying - Problems

Dependent of the weather eg. amount ofsunshine.

Drying times are relatively long.

Electricity is needed to run fans.

OSU Solar Kiln

Insulated walls, doors, floor & roof

South

Energy fromthe sun

Fans

Lumber stack

Panels are angledat 54 degrees

3/4” stickers separateeach layer - spaced18” - 24” apart

Air Flow

End view of OSU solar dry kiln

Baffle

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OSU Solar KilnFor 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.

Size of the Solar Collector

Dehumidification Kiln

• Temperatures can reach 160 degrees F.

• Moisture is removed by condensing on thecold coils of a heat pump dehumidifier.

• Heat used to evaporate the water is recoveredand pumped back into the chamber to do moredrying.

• Considered a closed system but can use ventsif need to control the temperature during drying.

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 is expensive - electric.

• It allows small operators to dry their own wood withoutthe expense and expertise of operating a boiler.

• Not only small – some 100 mbf aluminum DH kilns

• Water must be disposed of – pH about 3.6 – 6.0

Conversion of a Schedule from a Steam-heatedKiln to a Dehumidification Kiln

Source: Dry Kiln Operator’s Manual http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr57.pdf

Kiln Schedules for about 500 Species

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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 – hotenough to sterilize the lumber.

150 degrees F for 24 hours will kill stain anddecay fungi. No new fungi will occur as longas wood is kept below about 20% MC.

15% MC or below – lumber needs to be at temperature of 140 degrees F to kill mostinsects – 4-6 hours.

Bailey’s 4K Kiln – www.baileys-online.com/kiln.htm

Drying Schedules – Three Steps

� Dry> Remove water at a controlled rate

� Equalize> Reduce moisture content difference between

wettest and driest pieces & between shell andcore within pieces

� Condition> Relieve stresses developed during drying cycle

Typical Steps in a DryingSchedule for Steam Kilns

DRYING

EQUALIZING

CONDITIONING

Dry BulbTemperature

Wet BulbTemperature

Time or Moisture Content

Wet BulbDepression

When Drying Defects Occur

10 1520

25

3020

8

Time or Moisture Content

Dry BulbTemperatureWet Bulb

Temperature

For thickness changesAt high MC (surface of wood is wet – mass flow), tim e

for a given MC change is proportional to thickness

At low MC, time for a given MC change is proportiona l to thickness 2

Total time is approximately proportional to 1.5 pow er.

So –

Where N = 1, 2, or 1.5

TimeTime

ThicknessThickness

2

1

2

1

N

=

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.

TimeTime

1.251.00

1.42

1

1.5

=

=

TimeTime

31

5.22

1

1.5

=

=

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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:

WagnerTM on right is a dielectric meter without prongs.LignomatTM is a resistance meter or prong meter.

Resistance vs. DielectricAdvantages and Disadvantages

Highly sensitive to speciesHighly sensitive to density

Somewhat sensitive to temperatureMeasures at small spot

Measures over a small areaMeasures at an exact depth

Measures over an average depthBest between

In-line metering is easy

Comparison Criteria

NoNoYesYesNoYesNo

7 – 25%No

YesYesNoNoYesNoYes

4.5 – 25%Yes

Bottom Line – probably should use both types of meters

TYPICAL DRY MCs

• We try to dry to the moisture content the product will see in service

• Tighter +/- tolerances at lower MCs

Product MC, %

Dimension <19

Shop, Lam Stock 10 - 12

Furniture, flooring 6 - 8

Equilibrium Moisture Content • Amount of boundwater in wood isdetermined by theRH of the surroundingatmosphere.

• Amount of boundwater in woodchanges (slowly)as the RH changes.

• EMC = MC wherewood is in equilibriumwith the RH of itsenvironment.

Line represents white spruce withFSP around 30%. Although aprecise curve cannot be drawnfor each species, most will fallwithin the shaded area.

Source: Understanding Wood by Bruce Hoadley

References

http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/fplgtr113.htm

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References (cont.)

http://www.fpl.fs.fed.us/documnts/usda/ah188/ah188.htm

http://ir.library.oregonstate.edu/xmlui/bitstream/handle/1957/7623/RC8.pdf?sequence=1

OR