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EVAPORATION OF WATER VAPOR FROM CELL WALLS. Wood structure is nonuniform, and it doesn’t dry uniformly either; this means that some parts of the boards will be drier than others. Water will continue to evaporate from smaller and smaller capillaries; water in microcapillaries will move by diffusion through the cell walls to places where it can evaporate. When the free water is gone in any location the only remaining water will be the bound water in the cell walls. This too will evaporate, but additional heat and/or time will be needed to further dry the wood in these places. Wood will begin to shrink when the moisture content drops below the fiber saturation point, but it’s important to note that “moisture content” doesn’t refer to the average moisture content of a piece of lumber–it refers to the moisture content at a specified location. This is why the shell will try to shrink (resulting in tension set) even though the core is wet enough to make the entire board have a moisture content over 30%. When much of the wood is at a moisture content below the FSP (and therefore stronger), it’s safe to start accelerating the drying schedule. The rate of water loss will be noticeably slower, and the dry bulb can be increased at the same time that the RH is lowered. DRYING STRESSES IN DRIER LUMBER. When the core finally dries and shrinks, the stresses reverse! The drying shell will pull the shell inwards in all directions, closing any surface checks; the shell will now be in compression and the core will be in tension. Often those closed-up checks won’t be discovered until boards are planed or stained. This can cause problems for kiln operators, because surface checks are often initiated during transport or air drying before the lumber ever enters the dry kilns–but the kiln operator will still get the blame! DEFECTS CREATED AT THE END OF DRYING. Dry wood is much stronger than moist wood, so new defects are unlikely to occur regardless of the kiln settings (though high temperatures are often avoided to prevent darkening light-colored woods). There’s one important exception to that statement: the defect known as honeycomb becomes visible (Figure 26). This is why preventing surface checks in the initial stage of drying is so important! You probably won’t notice honeycomb until the lumber is cut or machined; this can cause product returns and headaches for the kiln operator. 49PDF Image | HARDWOOD DRY KILN OPERATION A MANUAL FOR OPERATORS OF SMALL DRY KILNS
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