PDF Publication Title:
Text from PDF Page: 047
By the time that most of the water has left the larger cells, approximately one-third of the water will have evaporated from the lumber. Green white oak, for example, might have an average green MC of about 65%, so when the MC has dropped to about 44% or so you’d know you’re beginning to evaporate water from the smaller pores. The drying rate is going to slow down. When water migrates more slowly from the interior than it evaporates, the wood surface starts drying out and shrinkage and stresses can start to occur. STRESSES IN THE OUTER WOOD SURFACE. When a board starts to dry, the wood fibers on the surface drop below the FSP and come into equilibrium with the EMC conditions quickly. Continuing to dry the lumber “locks in” the original dimension of the wood near the surface; because of the moisture content drop the surface layer would normally shrink, but the wetter, swollen wood in the core prevents it from doing so. Because the shell is restrained from shrinking it goes into tension, this is known as tension set; surface checks open up when the tensile stresses exceed the strength of the wood. DEFECTS ARE CREATED DURING EARLY DRYING. Fast drying rates increase the moisture difference between the core and the surface and serve to increase the tension stress at the surface. For green wood especially, drying defects will occur if the tensile stress exceeds the wood strength. Surface checks can open quickly in green lumber, so air drying yard managers and kiln operators need to monitor the drying progress to ensure that drying doesn’t proceed too quickly. Slower drying can be accomplished by three things: 1) lower air speeds, 2) high relative humidities, and 3) low temperatures. Drying rates increase with air flow rates, and this is why low initial air speeds are essential for quality lumber drying operations. High RHs and low temperatures produce higher EMC conditions, resulting in slower drying. When wood is exposed to both heat and humidity in a kiln, the combination weakens wood more than exposure to just one or the other–hot moisture (steam) is used to plasticize pieces of ash so they can be bent to make Windsor chair backs, for example. For this reason, kiln schedules start at low temperatures to keep wet lumber as strong as possible and kiln schedules are advanced to higher temperatures only when the lumber is drier and stronger. Weak wood is more likely to develop surface and end checks. Drying will proceed most quickly at the edges of the stacks, and on air drying yards you will notice that uncovered top boards and lumber packs stored at different locations will dry at different rates. Controlling the drying rate of wet lumber is very important because checks can form readily during periods of rapid drying (such as high wind conditions). Even transporting uncovered green lumber from one location to another can be enough to initiate checks in species that are especially prone to checking (such as oaks). Checks commonly form on end grain or adjacent to rays, so they will be most noticeable in flatsawn lumber. One problem with surface checks is that they can easily become more severe as drying progresses. Checks are sharp- pointed separations between wood cells, and it takes only a little more energy (shrinkage) to make them deeper. Lumber that’s cut with a circular saw or a dull saw is more likely to tear the wood than shear it cleanly, and this promotes surface checking as well; this is one reason why some firms that dry oak will presurface the lumber with a planer after sawing. WATER EVAPORATES FROM SMALLER AND SMALLER PORES. As it dries, a moisture gradient forms within the lumber. Drying progresses from a dry surface towards the wet core, and in due course the rate of water transport from the interior of the lumber will slow down as the supply of easily-transported, easily- evaporable free water in large pores dwindles. The remaining free water is in smaller pores that give up their water less readily, and more heat is typically used to encourage this water to evaporate at a reasonable rate. 47PDF Image | HARDWOOD DRY KILN OPERATION A MANUAL FOR OPERATORS OF SMALL DRY KILNS
PDF Search Title:
HARDWOOD DRY KILN OPERATION A MANUAL FOR OPERATORS OF SMALL DRY KILNSOriginal File Name Searched:
FOR128.pdfDIY PDF Search: Google It | Yahoo | Bing
5,000 BF Shipping Container Lumber Dry Kiln For Quality Lumber The 5,000 BF container kiln consists of one 40 foot high-cube aluminum shipping container... More Info
Shipping Container Lumber Dry Kilns by Global Energy Global Energy designed and developed the container kiln back in 1991. The purpose is to give access to portable sawmill owners, furniture makers, and small business the value added profit of dry kiln lumber and quality hardwoods... More Info
Vacuum Kiln Conversion Kit for Lumber and Wood Dry Kilns Convert your existing conventional dry kiln into a fast drying vacuum kiln. Similar to vacuum bagging in the boat building and aircraft industry, we have come up with a proprietary process which allows you to build a very simple vacuum kiln at a fraction of the price, and without the intensive conventional metal chamber structure... More Info
Vacuum Pump Cart System for Bagging Clamping Wood Drying and more Vacuum Cart with 2HP Pump and Dual Pistons with multiple multiplex vacuum ports and liquid reservoir... More Info
Vacuum Bagging Basics Vacuum bagging is a method of clamping, which has traditionally been used in the composites industry, but can also be used for vacuum drying materials, including wood products... More Info
| CONTACT TEL: 608-238-6001 Email: greg@globalmicroturbine.com | RSS | AMP |