WPS 264 Drying Properties

Wood MC and drying

• MC of green wood between 40-200%, dried: 6-20% depending on enviroment

• Below FSP → shrinkage non avoidable

• Should occur under controlled circumstances to limit defects & degradation

Why dry wood

• Ensures that shrinkage occurred before use

• Reduces transport costs

• Obtain a better surface finish before use

• Glues adhere better to dry wood

• Less susceptible for decay (insects, fungi)

• Easier to impregnate when dry

• Burns more efficiently

Rate of drying variables

• RH of surroundings

• Temperature

• Air flow

RH

• Mass of water per volume air divided by mass of water at saturation (AH)

• H = AH/AH₀ × 100% (given)

• RH for # water in air, AH for max # water in air

Drying capacity of air

• Jump from 17g/m³ to 600g/m³ at 20C and 100C respectively

• Drying capacity depends on RH & temperature

RH determination & kiln drying

• Wet/dry bulb thermometer

• Air passes over wet bulb causing water to evaporate, which results in cooling effect. The greater the temperature difference the drier the air.

• RH determined via hygrometric chart or table (easy)

• Depression T & dry bulb temp used to control RH in a kiln

Temperature & kiln drying

• The rate of moisture transfer to the wood surface is increased by enhancing the rate of diffusion through the cell wall

• The absorption capacity of air is increased

Air flow fan speed

• Hot air acts as carrier to distribute heat evenly & air needs to absorb water and transport it away, therefore air velocity affects drying time and wood quality

• Too high fan speed: air takes lower amount of moisture while passing & power consumption is high

• Too low fan speed: air cools and becomes saturated before reaching end of stack → uneven drying

Air flow with position in stack & air flow type

• Difference at front & back of stack. About 50% less at back

• Periodic reversal of airflow prevents uneven drying

• Turbulent (>3m/s): heat transfer more effective, fast drying of green wood, water evaporation from wood surface → used in first drying stage

• Laminar (<1m/s): heat transfer low efficiency, slow drying, water has to diffuse to surface → used in final drying stages

• Depends on permeability of species

Drying process

• Evaporation of moisture from the surface

• Moisture movement from interior to surface

• Happen simultaneously

Stage 1 - Evaporation

• Drying from a moist wood surface

• Determined by heat transfer

• Can be controlled through wet-bulb depression and fan speed

Stage 2 - Mass flow

• Drying above FSP

• Determined by mass flow of water through pits & cavities

• Faster if wood more permeable

Stage 3 - Diffusion

• Below FSP

• Water diffuses through cell walls

• Diffusion rate depends on temp & MC

Moisture transport

• Mass flow of moisture from center depends on permeability of wood

Permeability

• Permeability is a basic characteristic of wood determining the flow of fluids in wood.

• Influenced by pore size, pit aspiration, extractives, MC, etc.

• Permeability is higher in sapwood than in heartwood and higher in longitudinal than in transverse direction.

Permeability & stages

• Permeable wood: stages 1, 2 & 3

• Impermeable wood: stages 1 & 3 only

Permeable wood wet-line

• As MC at the surface drops below FSP air/water interface recedes inside

• Line along water menisci is called wet-line

• MC along wet-line is 40-50% → Too low to sustain liquid flow

• With further evaporation menisci become concave, water level recedes further down the cell causing pressure: water pulls on the cell wall

Permeable wood wet-line 2

• Water cavitates (formation of air bubbles) and flows through pits to the wet-line. Air bubbles cause mass flow

• Wet-line recedes further through pits. Capillary movement of water. Fibers can collapse due to pressure, mostly in low density parts (earlywood)

• Collapse occurs above FSP and precedes shrinkage

• 1.5 – 3 times larger in tangential direction than in radial because of reinforcement by rays

• With severe collapse shrinkage can be up to 50%

• Collapse of fibers can be avoided by drying at low temperatures

Impermeable wood

• Pits blocked by tyloses

• Tyloses: The bladder-like ingrowth of parenchyma cells into the lumen of vessels (mainly) & tracheids through the pits in their wall

• Hard to remove absorbed water, because no pits are present or they are blocked

• Water is removed by diffusion only, no mass flow

• Wood can easily collapse if the cell walls are weak