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is still ongoing. Therefore, further improvements exist with drying times, RFV kiln productivity, and drying costs. In addition to the impressive drying times, quality improvements have also been excellent. Timbers containing heart center (i.e., pith) exhibit no V-checking defects and timber distortion amounted to less than 15% of the pieces. In addition, there is an approximate 30% reduction in shrinkage compared to what is found in conventional kiln drying technologies. The key defect that limits drying times for timbers is internal honeycomb. It is a result of the internal temperature and pressure conditions that are so high; the steam escapes the easiest path. Normally moisture movement for RFV drying is out the ends of the boards but if the internal pressure is too high, wood fibers rupture to allow the steam to escape. Uncontrolled heating causes internal honeycomb, which is a difficult defect to detect externally. Unless identified by the trained eye, it can only be found after resawing the lumber. Power amplifiers found in this new technology guarantees absolute RF power control and eliminate internal honeycomb defects from RFV drying. Internal honeycomb can be controlled during RFV drying by taking into consideration 3 parameters: • Power density (kW/m3) or how much RF power is put in per cubic volume of wood. • Wood permeability where for example, wood that is more "treatable" can accept a higher power density, i.e., white vs. red oak, SYP vs. D.fir. • Lumber cross-section dimensions, where internal temperature and pressure conditions are higher with thicker lumber for the same RF power density and wood permeability. Recent Commercial RFV Kiln Drying Successes – RE-DRYING LUMBER or Q-Sift One of the most significant opportunities uncovered with recent drying investigations is the RFV re-drying of lumber. The length of time for RFV drying is dependent not only on the RF power density, but the amount of water to remove from the kiln charge of wood. For an equivalent power density, removal of minimal percentage in moisture content results in substantially reduced drying times. RF heating is selective only with water content. This fundamentally different aspect of heat transfer can be exploited with an operating strategy discussed at the end of this paper, which can potentially revolutionize how wood is processed and dried in large lumber manufacturing installations. Table 2 is a comparison in drying times for various products at the same power density but with different amounts of water to remove. Recent Commercial RFV Kiln Drying Successes – TRIM ENDS Not requiring stickering of lumber creates an obvious opportunity for drying small pieces of lumber, namely trim ends. In this application, trim ends can be solid-stacked and dried thereby considerably reducing piece-handling operating costs. In addition, the type of drying conditions found in an RFV kiln results in no end-checks as opposed to more conventional methods of drying trim ends. Fingerjoined lumber recovery is therefore considerably higher. The commercial RFV kiln is at a fingerjoining facility where there were no incidents of glue failure or reduced strength characteristics of the Western Dry Kiln Association 40 May, 2000PDF Image | COMMERCIAL RFV KILN DRYING
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