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