Wood & Water
Most problems encountered with wood or wood products, involve water or moisture content.
Wood is HYGROSCOPIC-- Meaning it will take on or give off moisture to equalize with its environment.
Wood has basically 3 types of water in it:
A. water of constitution -- to remove it is to destroy the wood
B. free water -- water just taking up space in cell lumen or other voids
C. bound water -- in cell wall, physiochemical bonds with cellulose chains
EQUILIBRIUM MOISTURE CONTENT (EMC) -- for a given temperature and relative humidity -- a given piece of wood will always take on the same moisture content. See Table A.1. on Page 508 of Textbook.
What happens when you remove water from wood or let the wood increase in moisture content?
If we start with green wood and slowly dry it, we find that all that we are removing is free water which is just occupying space and with its removal nothing happens to the wood. When we reach the FSP, we start to remove bound water. Now things start to happen with the wood. As we remove more and more water, the wood get stronger at an exponential rate and shrinks at a lineal rate.
If we start with dry wood and add moisture to it, we find that the wood will lose strength and swell until the FSP is reached then nothing more happens.
Chapter Three revisited:
How are microfibrils constructed?
Cellulose chains grouped together --
Crystalline region -- tightly bonded and encased in hemicellulose -- no access to water
Amorphous region-- free hydrogen bonding sites with hydroxyl groups -- where bound water is located
The percentage of crystalline to amorphous helps determine the FSP.
As the water molecules enter or leave the amorphous region, the cellulose chains either move apart or draw closer together. This causes the microfibril to increase or decrease in size which causes the cell to change size which in turn causes the wood to swell or shrink in size. Also, as the cellulose chains come closer, they bond on each other causing the wood to get stronger. As water molecules are added, these bonds are broken and the wood becomes weaker.
As Chapter three stated: cell walls are made of 4 layers -- primary, S1, S2 and S3. The S2 layer is the thickest and most influential.
How are the microfibrils
arranged in the S2 layer?
10 to 30 degrees to the longitudinal
Therefore, what are the shrinkage and swelling rates along the principle axes of wood?
Longitudinal 0.1- 0.3 %
Radial 3 to 4 %
Tangential 6 - 8% sometimes up to 14%
See Table A.2. on Page 509-511 in Textbook for shrinkage values for various domestic woods.
When the principle axes of the wood does not align with the geometric axes of the wood, warpage occurs.
of Wood Products
starts drying from the surface first.
As the surface dries, moisture moves from the interior outward.
moves in two ways: mass movement of
liquid water in cell lumens; and
diffusion of individual molecules both
bound water or water vapor in cell
Diffusion does not take
place in a cell until it is below the FSP.
MOISTURE MOVES 12 TO 15 TIMES FASTER ALONG THE GRAIN THAN ACROSS IT.
surface of the wood can dry below the
FSP while the interior may still be well above the FSP.
A moisture gradient is set up
across the wood. The
steepness of the gradient is determined by
the speed at which the surface dries.
A good drying program
will compromise between speed
of drying and flatness of moisture gradient.
steeper the gradient the more likelihood of drying defects.
Sometimes the degrade caused by fast drying will exceed (moneywise) the
savings in drying time.
the surface dries below the FSP, it wants to shrink but the interior of the wood
is still above the FSP and does not want to shrink.
This causes a stress build up: the surface portion is placed in tension
and the core is in compression.
the stress builds up, two bad things can happen. If the tension exceeds the strength of the surface, you can
have splits or cracks in the surface called checks.
If the compression exceeds the strength of the core, you get collapse.
the wood survives the initial stress build up, the surface will "SET"
and as the core continues to dry, the stresses are reversed.
Now the surface is put into compression and the core is in tension as the
core tries to shrink. As the stress builds up, you can have splits in the core
The higher the specific gravity, the slower the drying rate.
higher the air speed, the faster the drying rate.
The lower the relative humidity, the faster the drying rate.
The lower the atmospheric pressure, the faster the drying rate.
you are air drying lumber, fence posts, or
other wood products, there are some generalities to keep in mind.
should be stacked on solid, level
supports that are 18-24 inches above
level, well-drained ground with the weeds
kept mowed. This way the air
will keep the ground moisture from
influencing the relative humidity
around the wood.
stacks should be covered to prevent
rewetting by rain and exposure to
direct sun. Excessive alternate
wetting and drying will cause extreme checking
and splitting. Direct sunlight
will cause too rapid a moisture loss.
you are drying one stack or more and you are placing the stacks in a straight
line, then the stacks should be placed facing the prevailing wind.
This way the maximum air flow through the
stack(s) will be realized.
If you are designing the air drying yard for multiple rows, then the stack should be placed parallel the prevailing wind.
If the wind is hitting the stack face, the air flow is slowed going through the stack. The wind blowing over the stack will stay at the same rate and tumble into the area behind the stack mixing with the air coming through the stack causing mild turbulence. Thus, there will be less air flowing through the second stack and consequently even less through the third stack.
If the wind is blowing parallel the stacks, the flow of the air will cause a Venturi Effect and draw the moist air out of the stack and it will be taken away with the wind.
A dry kiln is basically a closed chamber where you can control the temperature, relative humidity, wind speed, volume, and direction.
controlling the conditions around the wood, you can develop the best moisture
See Tables 8.6 and 8.7 on Page 193 in the Textbook
reduce moisture content below attainable levels by air drying
dry wood faster (less inventory cost)
with high temperature, can kill insects and fungi
because of the above, it has a higher market value.
denser the wood and the more refractory the wood, the longer it takes to dry the
wood and therefore, energy is required to dry it. In some woods, 70 percent of the energy required to produce
finished lumber from trees is tied up in drying.
DH kilns usually operate at a lower temperature than normal kilns; therefore, a cheaper and less insulated chamber is required. No boiler is needed, so that great capital cost is eliminated.
speaking, if you are going to dry less than 1 MMbf per year, DH is more
economical than normal steam kilns. Over
5 MMbf per year, steam kilns are more economical.
kilns run on electricity.
kilns take as long or longer to dry wood as a conventional kiln.
kilns are cheaper to buy and install but the electric bill makes them more
expensive to operate.
appear to be less drying defects in DH kiln dried material than conventional
solar collectors to concentrate the suns energy to heat the wood.
May not be able to dry below air drying moisture content.
The only energy needed for these kilns is the electricity to run the fans.
In this country, the kiln should face South and the angle of the solar collector should be the same as the degrees of latitude so as to maximize the exposure to the sun's rays.
There are many solar kiln designs. The following is just one example.
VACUUM Drying - by lowering the
atmospheric pressure, you lower the boiling point of water and increase the
diffusion rate. Less heat energy is
required but a chamber with a vacuum seal is required.
RADIO-FREQUENCY Drying - the wood is dead stacked on a metal platen and
another metal platen is placed on top. Microwave
energy is transmitted between the platens.
Heats the wood internally.
(RFV) drying - a combination of the
in oil - normally used in treating
cylinders prior to treating green hardwoods.
- heats wood and drives off a lot
of free water. Used in treating cylinders prior to treating green softwood