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
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 100-180 o F
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 220 o F
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 6 Top View 8 8 6 7 8 7 8
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
Humidity Airflow Heat Fans heating Moisture Stickers method from lumber between lumber insulation Steam & water spray Baffles Vents
Steam Heat
venting cools the kiln, requiring more energy in the form of heat to be added, thus increasing energy costs.
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
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 than vented 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 85 o F Low cost kiln structure Maximum temperature may be limited Smaller or Older units: 120 o F Larger or newer units: 160 o F
University of Wisconsin Cooperative Extension PUB FR-396 2007 http://www.dnr.state.wi.us/forestry/publications/pdf/ FR-396-2007.pdf
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
Dries at lower temp Faster drying rate Rapid drying rate Low degrade Excellent color High energy costs Higher labor costs
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 conditions Occurs 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