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HOW DRYING PROGRESSES Lumber drying doesn’t progress at a constant rate. Think about squeezing out that wet kitchen sponge: it’s easy to get rid of the water in the large open cells and pressing harder will remove water from the smaller cells, but the rate of water loss quickly slows. After all the water in the sponge pores has been removed, all that is left is the water saturating the cellulose itself. This water will evaporate given the right conditions, but the water will leave at a slower rate than squeezing produced. Removing water from a piece of wood is similar; liquid water will evaporate from larger cell pores more readily than it does from small pores, and it will take more energy and more time to evaporate a unit of water from those smaller pores than from the larger ones. Drying the cell wall itself takes even more energy and time to vaporize the bound water and to transport it as vapor to the wood surface where it can be carried away by warm air. Higher temperatures speed up the heat transfer, facilitating vaporization of water below the wood surface; increasing the wet bulb depression (lowering the relative humidity) speeds up the rate at which water can be removed from the surface. You might think of it like this: a pot full of room temperature water gets hotter when you increase the burner heat and steam will form above the surface. Practically no evaporation occurs unless the lid is off, though, because the air inside the pot quickly gets saturated with water vapor. Once you lift the lid, the steam is exposed to a lower relative humidity environment and the water vapor escapes the pot seeking to reach an equilibrium with the less-saturated air. Evaporation goes faster at lower relative humidities. WHY KILN TEMPERATURES GET CHANGED DURING DRYING. Kilns start out at low temperatures because it doesn’t take much energy to evaporate water that’s on (or close to) board surfaces. Air bubbles within the wood cells expand as they’re heated and help push liquid water to the surface at the same time that water is pulled to the surface by wicking to replace the water that evaporates. Driving the so-called “wet line” below the surface too quickly can cause the surface to attempt to shrink before it’s strong enough to resist drying- induced stresses, and defects such as checks will result. As the bulk of the wood starts to dry out and strengthen and the remaining water is contained within small pores and the cell wall, kiln operators can raise the heat in increments to finish drying the wood. Let’s examine how wood dries in more detail. PREHEATING THE LUMBER. Wood starts to dry as soon as its surface is exposed to an EMC condition that’s lower than the MC of its surface (which is essentially saturated in freshly-cut green lumber). Nonetheless, if you’re placing wood in a warmer environment like a kiln, it will have to warm up a bit before drying can really get going; a cold piece of lumber will require more energy for liquid water to begin to change to vapor and evaporate from the surface. WATER EVAPORATES FROM THE SURFACE AND LARGER PORES. The wood surface is wet in the early stages of drying. Water on the surface begins to evaporate, and its loss is fairly rapid. Free water flows from the inside of the wood towards the surface, especially through the lumens of cells with the larger diameters– hardwood pores, and the earlywood pores particularly in ring-porous species. Higher temperatures and lower relative humidities will hasten this evaporation. Internal water vapor pockets begin to form to replace the volume of the water lost to surface evaporation and push more free water to the surface where it can evaporate. During this stage of drying, the rate of water loss is essentially constant. 46PDF Image | HARDWOOD DRY KILN OPERATION A MANUAL FOR OPERATORS OF SMALL DRY KILNS
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