Radio-frequency-vacuum drying

X Boiling point + Moisture content ° Pressure---------------------Power 

Wood temperature measured by fiber optic probes inserted into four specimens located on the top, middle, and bottom rows of the upper packs, and at the top row of the lower packs, indicates the boiling point for the first 35 hours of the drying run. This period coincides with the fiber saturation point and the power reversal of the kiln, where the capability of the wood load to absorb electromagnetic power drops dramatically. From then on, the wood s temperature can be raised to maintain a reasonable drying rate without risk of wood damage (Smith et al., 1994). 

The end-point detection system integrated in the control circuit allows the average moisture content and the time to complete drying to be monitored over the entire drying cycle. Provided with an access to the Internet or an Intranet, the control system secures remote supervising of kiln operation. Once the target moisture content is reached, the kiln automatically stops and unloads the kiln charge. 

As applied to the drying of solid wood, the RFV technology has the following production advantages (Anon., 1999)  

The drying rate is up to 20 times higher than that in a conventional kiln. An added benefit is no stickering of the lumber packages. 

---

Harris JM (1993) Wood quahty forest management and utilization. In Walker JCF, Primary wood processing principles and practice (1st edit). Chapman and Hall, London, 560-83 Harris PJ (2005) Non-cellulosic polysaccharides in plant cell walls. In Entwistle KM and Walker JCF (eds). The HemiceUuloses Workshop 2005, Wood Technology Research Centre, University of Canterbury, Christchurch, New Zealand, 13-35 Harris RA and Taras MA (1984) Comparison of moisture content distrihution, stress distrihution, and shrinkage of red oak lumber dried by a radio-frequency/vacuum drying process and a conventional kUn. Forest Products Journal, 34 ) 44-54 Hartler N and Stade Y (1962) [Penetration and diffusion in sulphate cooking (in Swedish)]. 

Anastasios Koumoutsakos, Stavros Avramidis and Savvas G. Hatzikiriakos, Radio Frequency Vacuum Drying of Wood. I. Mathematical Model, Drying Technology, 2001, 19(1), 65-84. 

Smith, W.B. and Smith, A., 1994. Radio-frequency/vacuum drying of red oak Energy quality value, in Proceedings of the Fourth International lUFRO Wood Drying Symposium, Rotorua, New Zealand, pp. 263-270. 

Figure 24.2 Radio-frequency-vacuum drying characteristics for 8/4" in red oak squares (Smith et al., 1994).

Smith WB, Smith A, Neauhauser EF. Radio-frequency/vacuum drying of red oak energy, quality, value. Proc. 4 lUFRO Inti. Wood Drying Conference, Rotorua, New Zealand, 1994, pp 263-270. 

Table 24.1 compares drying time and electricity consumption for several wood species dried in the radio-frequency-vacuum wood kiln (Anon., 1999). 

Thermal drying caused by the vaporization of the liquid results as heat is supplied to the wet feedstock. As noted earlier, heat may be supplied by convection (direct dryers), conduction (contact or indirect), radiation, or volumetrically by placing the wet material in a microwave or radio frequency (RF) electromagnetic field. Over 85% of industrial dryers are of the convection type, with hot air or direct combustion gases as the drying medium. Over 99% of the application involves removal of water. All modes except the dielectric (microwave and radio frequency) supply heat at the boundaries of the drying object so that the heat must then diffuse into the solid primarily by conduction. The liquid must travel to the boundary of the material before it is transported away by the carrier gas (or by application of vacuum for nonconvective dryers). 

Germanium internal reflection elements (IRE, 50x20x3, 45 aperture) were polished twice with 0.3 urn alumina polish, rinsed with distilled water, rinsed with ethanol, and then cleaned in a radio frequency plasma discharge. The IRE s were pulled vertically (dip coated) from the polymer solutions at 3mm/min. Each coated IRE was dried in a vacuum oven at 60 C for at least 4 hours, and was stored under vacuum until use. 

---