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Kiln theory and knowledge
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1. How Lumber Dries
How does wood dry and how does your kiln performance effect this?
In this section we will introduce you to some of the concepts of kiln drying in a very concentrated form. The subject is far too great to try to be too specific with detailed explanations for this web-page. However, there is much literature available (both in books and on-line) on the subject and we encourage everyone to educate themselves in the art of drying lumber. It is an exciting subject. We are always interested in improving our ability to advise you and value your input, so we welcome opinions and suggestions. We will always be available with suggestions, guidelines, and explanations to our customers.
First, it is important to understand that there is a general relationship between how fast the lumber is dried and the quality you get as a result of doing so; however, on some species slow drying can be fatal to the final outcome through stain and mold.
Drying lumber is in it’s nature a controlled removal of water from the lumber at a speed which does not damage the lumber by checking, collapse, splitting, etc. while trying to maximize production through the equipment. This is a place where the lumber company will loose profit if it cannot dry lumber with minimum degrade and still ensure adequate production through the kilns.
When we are drying lumber we have some control parameters that we can utilize:
Chamber temperature (dry bulb)
Relative humidity (wet bulb)
Main fan operation (either interval operation or variable speed)
- Drying defects and their probable causes.
- Normally you can adjust your drying program/schedule by the defect/s you see on the kiln dried lumber. See a list of corresponding drying defects and their probable causes.
- Safe rate of MC% drop per day versus drying gradient.
- Safe rate of MC% per day.
- Each specie and thickness has an approximate rate at which it can lose water (moisture) per day at a safe rate. The “safe rate” is different from specie to specie and from thickness to thickness. Most kiln drying manuals will focus on the “safe rate” for maximizing the drying quality and production.
- Safe drying gradient.
- Safe rates are a good measure, however; we do look at the safe rate of drying per day, but we prefer to focus on the maximum safe drying gradient. In kilns with adequate air velocity and venting it is possible to use drying gradient, whereas with older kilns the kiln operator must sometimes focus on the safe rate even if he has to exceed the safe drying gradient.
- The climate in a kiln has a corresponding equilibrium moisture content (see graph). This means that by controlling the climate you can control the moisture content on the surface of the lumber. Since lumber shrinks during the drying process, the surface which dries first will try to shrink more than the core with the higher MC%. The difference in the inside and outside MC% (the EMC% of the air) is called the drying gradient.
- When operating a kiln with more than adequate air flow and venting, you can control the climate in the kiln to create the exact EMC in the air. This also enables you to control the surface MC% of the lumber and thereby the drying gradient. So it is important to have a kiln with sufficient heat, vent and air flow capacity.
| Example: |
With a drying gradient of 2, and core MC% of 24%, the EMC should be 12% (24/2). |
The maximum drying gradient allowed is different depending on the specie and thickness. Slow drying and harder species; such as oaks, have a low drying gradient, whereas faster drying and softer species; such as pines, have a high drying gradient. On softer species the wood can flex and stretch more, which allows for greater shrinkage difference between the core and the shell, therefore the drying gradient can be higher than on softer species.
- The difference between pulling moisture out and forcing moisture out of the lumber.
- Normally when you have well air dried lumber in a kiln you are trying to force the moisture out through the drier outside shell, in contrast to drying fresh sawn lumber. Here we try to pull the moisture out without over drying (case-hardening) the surface.
This aspect is much more important than it sounds.
Let us explain: Just as you know from a wet wick or sponge , once they dry completely they are actually hard to get wet again. You must dip them in water and squeeze, thereby forcing the water/moisture back into the fabric. This is nearly the same thing that happens to lumber that is over dried. Once the shell of a board has been over dried, either through air drying or drying too fast in the beginning of the drying process it becomes much harder to get the moisture out through the dry shell. This will normally mean a higher temperature is required to force the moisture out.
- The best possible drying quality is achieved
by starting the drying process from fresh sawn lumber and keeping the temperature as low as possible. The lower temperature also makes the wood slightly stronger and thereby the chance of degrading is further reduced. This low temperature approach to drying can have many additional advantages.
We hope this information meets with your expectation and approval. We always welcome your opinion and suggestions. Please feel free to email us anytime.
Thank you for your continued interest.
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