Brian Bond Virginia Cooperative Extension Virginia Tech
Welcome message from author
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Brian Bond
Virginia Cooperative Extension
Virginia Tech
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
Weight of water = Volume of wood
in cubic feet x Specific gravity of
wood x 62.4 x (Green MC – Final
MC in decimal)
93.75 x 0.56 x 62.4 x (.77 – .07) =
2293 pounds of water per nominal
mbf in red oak from 77 to 7 % MC
Usually:• To drive water out of wood at an acceptable rate of
speed with the maximum obtainable quality
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?
Hardwoods• Main problems
Checks and splits
Warp
Stain
Uneven moisture content
• Dried on a MC basis
• Temperatures 100-180oF
Species Days to dry
Oak 25-30
Hard maple 8-10
Walnut 7-8
Yellow-poplar 5-7
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
Species Days to dry
Southern pine 1-2
White pine 6
Spruce 5
Douglas-fir 7
White pine 4-6
Purpose• Promote uniform air circulation
heat
humidity
• Reduce or eliminate warp
Box-piling
7’
6’8’
6’8’
7’8’
8’
Top View
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
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
Quality
• Straight grained
• Hardwoods
• Density
Moisture Content
• 9-12% MC
• Keep them dry
(out of the rain)
Low cost moisture
loss?
• Air-flow
Yard orientation and direction of wind
• Sun direction
• Drainage
• Yard surface
• Location
• Time on the yard
• Pile covers or roofs
• End coating
• Lumber pile spacing
• Lumber pile width
• Lumber pile covers / roofs
Heat
• heating method
• insulation
Airflow• Fans
• Stickers between lumber
• Baffles
Humidity
• Moisture from lumber
• Steam & water spray
• Vents
Steam Heat
venting cools the kiln, requiring more energy in the form of heat to be added, thus increasing energy costs.
• Heat Transfer
Fin pipe
Control
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
Hot water
• Lower pressure boiler
Advantages
• Good temperature control
• Fuel for heater
• Consistent drying times
• Disadvantages
Less humidity control
Conditioning?
Woodmizer
Direct-fired
Kilns that are heated by blowing hot combustion gases
directly into the kiln environment•Wood, gas or electric fired
•Widely used for softwood dimension lumber
•Large kilns used in high temperature pine drying
•Less capital and operating expense compared to steam kilns
•Used extensively for high-temp drying softwoods• southern pine
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
• 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
• 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
• 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
University of Wisconsin Cooperative Extension
PUB FR-396 2007
http://www.dnr.state.wi.us/forestry/publications/pdf/
FR-396-2007.pdf
Reducing schedule times• May need some type of auxiliary heating
• May require auxiliary heat for start up
•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
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
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
1.0 atm 100ºC (212ºF)
0.5 atm 82 ºC (180ºF)
0.2 atm 60 ºC (140ºF)
0.1 atm 32 ºC (90ºF)
• Dries at lower temp
• Faster drying rate
• Rapid drying rate
• Low degrade
• Excellent color
• High energy costs
• Higher labor costs
• Conduction by direct contact
• Convection using superheated steam
• Convection using hot air (cyclic systems)
• Radio Frequency
5/4 8/4
Radio Frequency 2 4
Vacuum only 13 23
Pre-drying 38 75
Source: Wengert, E.M.
MethodTime (days) for thickness
Drying Times
5/4 8/4
Radio Frequency 152 170
Vacuum only 208 285
Pre-drying 103 168
Source: Wengert, E.M.
Energy Costs
MethodCost ($/MBF) for thickness
Lumber in the basement, attic, barn, etc.
Alan Tomazek
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?
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
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
Generally defects that occur in the drying
process are caused by:
1. Wood failure
2. Warp
3. Uneven moisture content
4. Discoloration
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!
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.
Cure:
End coating
Lower the drying rate
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
Interior graying and other
enzymatic oxidative
discolorations begin in the
stored log or lumber
Factors•Log Age
•Weather
•Temperature
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
Fungal Stains• Sapstain (Blue stain)
Caused by fungus growing on the the sapwood
Fungus feeding off sugars and starches in sapwood
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
Related Documents
