Brian Bond Virginia Cooperative Extension Virginia Tech · Surface checks Cause: too fast drying...
Transcript of Brian Bond Virginia Cooperative Extension Virginia Tech · Surface checks Cause: too fast drying...
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Brian Bond
Virginia Cooperative Extension
Virginia Tech
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Most lumber must be dried prior to use.• Hardwoods 6-8%MC
• Softwoods 12-19% MC
Air-drying will typically not drop below
15% MC
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Usually:• To drive water out of wood at an acceptable rate of
speed with the maximum obtainable quality
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The two most common methods of drying lumber are:• Air drying and then kiln drying
• Kiln drying Are there any limitations?
How much control over conditions?
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Hardwoods• Main problems
Checks and splits
Warp
Stain
Uneven moisture content
• Dried on a MC basis
• Temperatures 100-180oF
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Species Days to dry
Oak 25-30
Hard maple 8-10
Walnut 7-8
Yellow-poplar 5-7
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Softwoods• Dry faster
• Typically dry more uniform
• Main problems Warp
Stain
• Usually dried on time based schedules
• Often requires resin setting in schedule
• Often allows for higher temperature drying 220oF
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Species Days to dry
Southern pine 1-2
White pine 6
Spruce 5
Douglas-fir 7
White pine 4-6
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Purpose• Promote uniform air circulation
heat
humidity
• Reduce or eliminate warp
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Box-piling
7’
6’8’
6’8’
7’8’
8’
Top View
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Thickness• ¾” to 7/8”
• speed and uniformity
• Typical
Thinner stickers increase kiln capacity
Increase air velocity
Make air-flow more uniform
Width• 1-1/4” to 1 1/2 inch
• Too wide – stain
• Too narrow – improper placement
• Strength is a function of width and thickness
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Spacing• 16” to 24” Based on tendency to warp
• Flush or as close as possible to the ends Reduces warp and minimizes splitting
12” on end for multiple length packs
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Quality
• Straight grained
• Hardwoods
• Density
Moisture Content
• 9-12% MC
• Keep them dry
(out of the rain)
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Low cost moisture
loss?
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• Air-flow
Yard orientation and direction of wind
• Sun direction
• Drainage
• Yard surface
• Location
• Time on the yard
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• Pile covers or roofs
• End coating
• Lumber pile spacing
• Lumber pile width
• Lumber pile covers / roofs
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Heat
• heating method
• insulation
Airflow• Fans
• Stickers between lumber
• Baffles
Humidity
• Moisture from lumber
• Steam & water spray
• Vents
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Steam Heat
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venting cools the kiln, requiring more energy in the form of heat to be added, thus increasing energy costs.
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Advantages
• Steam is used to control the
temperature and humidity
• Easy to increase humidity
• Excellent temperature and
humidity control
Disadvantages
• Requires boiler to produce steam
The boilers fired with:
wood waste, natural gas, or fuel oil
• High initial capital cost
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Hot water
• Lower pressure boiler
Advantages
• Good temperature control
• Fuel for heater
• Consistent drying times
• Disadvantages
Less humidity control
Conditioning?
Woodmizer
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Examples:
• Wood stove in building
• Gas furnace heating
Considerations:
• Evenly heat building
• Humidity control
• Stress relief
• Green or air-dried
material
• Can reach higher
temperatures for
sterilization and pitch
setting
Kiln Direct
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• Removes water from air
rather than venting
• Greater energy efficiency
over steam/hot water
conventional kilns
• Small to large sizes
American Woodworker #94 June 2002
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• Water is condensed rather thanvented
• Recovers heat of vaporization• Uses less energy • Low temperatures• Energy efficient (??)
Energy cost can be 50% less than conventional steam kilns
Savings is often offset by the higher cost of electricity
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• Offer good control at low dry bulb temperatures
Compressor will work at 85oF
• Low cost kiln structure
• Maximum temperature may be limited
Smaller or Older units: 120oF
Larger or newer units: 160oF
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University of Wisconsin Cooperative Extension
PUB FR-396 2007
http://www.dnr.state.wi.us/forestry/publications/pdf/
FR-396-2007.pdf
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•Low capital cost•Easy to use
• Requires less knowledge and experience
• BTU’s/day limited by design
•Satisfactory for operations where drying times are not critical
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Advantages
• Easy to construct and
operate
• Low capital cost to
construct and to
operate
• Operation be “off the
grid”
• Stress relive can be
achieved through the
process
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Disadvantages• Variable drying times Available Heat varies:
–Location
–Time of year
–Weather conditions
• Smaller capacities 100-8,000 bd. ft.
• One design is not optimal for a large mix of species
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• Dries at lower temp
• Faster drying rate
• Rapid drying rate
• Low degrade
• Excellent color
• High energy costs
• Higher labor costs
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What kind of kiln do I need?• What is my expected level of production?
• What species do I expect to dry?
• What thickness do I expect to dry?
• Will I air-dry first or dry green?
• How much capital do I have to invest?
• Is cost of drying important?
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All drying results in shrinkage of the wood and some differential shrinkage will occur in the process
• Drying stresses are a result of differential shrinkage
• Greater for faster drying than slower drying
• Can be relieved through a conditioning process
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Differential Shrinkage
• Wood shrinks as it dries
• Wood dries from the surface
• Preventing shrinkage causes tension
• Stresses in a system must be balanced
• Excessive stress will cause failure
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Generally defects that occur in the drying
process are caused by:
1. Wood failure
2. Warp
3. Uneven moisture content
4. Discoloration
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Surface checks Cause: too fast drying conditionsOccurs at the first 1/3 moisture content loss
Cure: Slow the drying rate during the initial stages of drying Use higher humidity's and lower temperatures
Danger: If the lumber is not re-wetted, the checks will close at stress reversal. They are STILL THERE!
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End checks
Cause: moisture moves faster at the ends of the board, which causes stresses to develop at the ends. Occurs at the first 10-20% moisture loss from green.
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Cure:
End coating
Lower the drying rate
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Honeycomb
Cause: Tensile failure across the grain and usually occurs in the wood rays. Greatest risk between 22-28%MC.
Cure: Avoid high temperatures until all the free water has been evaporated from the entire board. Slower initial drying rate
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Interior graying and other
enzymatic oxidative
discolorations begin in the
stored log or lumber
Factors•Log Age
•Weather
•Temperature
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How to prevent• Use only fresh logs Less than 2 weeks
• Stack lumber immediately after sawing 12 hours minimum
• Begin drying as rapidly as possible
• Use high air velocities in kiln• Avoid temperatures over 160 F • Do not over-load kiln• Proper air-flow
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Fungal Stains• Sapstain (Blue stain)
Caused by fungus growing on the the sapwood
Fungus feeding off sugars and starches in sapwood
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Start the drying process immediately after
sawining
Rapid drying
Use of chemicals to prevent fungal growth•Sapstain chemicals, lumber dip
•PQ-8
•StayBriteP
•NeXgen
Chemicals do NOT prevent enzymatic stains
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