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Solar, Vacuum and Radio-Frequency Drying of Wood

Solar, Vacuum and Radio-Frequency Drying of Wood. Forestry 486 Lecture 3-6. Solar Drying Basics. Characteristics of solar energy:

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Solar, Vacuum and Radio-Frequency Drying of Wood

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  1. Solar, Vacuum and Radio-Frequency Drying of Wood Forestry 486 Lecture 3-6

  2. Solar Drying Basics • Characteristics of solar energy: • Low intensity solar “insolation” (amount of available energy): approx. 428 BTU/hr/ft2 in the upper atmosphere vs. 58.5 BTU/hr/ft2 at surface due to interception by clouds etc. • Intermittent (day vs. night; seasonal variation) • Subject to interruption (cloud cover, smoke, etc.)

  3. Solar Drying Basics • Classes of solar radiation: • Ultraviolet (UV): < 0.38 μm wavelength, approximately 7% of solar radiation • Visible: 0.38-78 μm, 47% • Infrared (IR): > 0.78 μm, 46%. • (lower wavelength, e.g., UV=higher energy light)

  4. Solar drying basics… • Available energy influenced by angle of incidence; determined by position of sun in the sky and the collector tilt angle • A “rule of thumb” for north temperate region is to set collector tilt angle as follows: • Summer, latitude – 10o • Winter, latitude +15o

  5. Types of Solar Collectors used for wood drying • “Hothouse” – not unlike a plant greenhouse • External collectors • Heat transfer medium • Air; low “energy volume” • Liquid; efficient but more complicated • Flat plate collectors • Concentrating collectors, e.g., parabolic mirror with sun tracking capability

  6. Solar Drying • A home-built solar drier can provide quality lumber drying at an accelerated rate • 4/4 hardwood may be dried in 6 weeks of good weather • 8/4 hardwood may be dried in 15 weeks

  7. Greenhouse-design solar dry kiln

  8. Semi-greenhouse design

  9. Solar dry kiln with external collector

  10. Solar drying attributes • Can dry wood 2-3x faster than air drying • Can dry to 7-12% final MC • Can reduce casehardening by shutting off fans and/or vents at night (increase EMC during evening hours) • Largest design I’ve seen was for 50Mbf • Most are used by hobbyists or in developing countries (not a bad idea for tropical regions with greater amount of sunlight)

  11. Solar Drying References • “Improvements in Solar Dry Kiln Design”, available at http://www.fpl.fs.fed.us/documnts/fplrn/fplrn212.pdf • “Constructing and Operating a Small Solar-Heated Lumber Dryer” http://www.fpl.fs.fed.us/documnts/fputr/fputr7.pdf • Chen, P.Y.S. 1981. Design and tests of a 500 bf solar dry kiln. Forest Products Journal 31(3):33-38. • Tschernitz, J.L., and W.T. Simpson. 1986. Design for lumber dry kiln using solar/wood energy in tropical latitudes. Drying Technology 4(4):651-670. • Lumley, T.G., and E.T. Choong. 1979. Technical and economic characteristics of two solar kiln designs. Forest Products Journal 29(7):49-56.

  12. Solar/Dehumidification Drying References • Chen, P.Y.S., W.A. Helmer, and H.N. Rosen. 1984. Energy efficient lumber dry kiln using solar collectors and refrigeration systems. United States Patent No. 4,432,147. • Helmer, W.A., and P.Y.S. Chen. 1985. Computer simulation of a new method to dry lumber using solar energy and absorption refrigeration. Wood and Fiber Science 17(4):464-476.

  13. Additional References on Drying Small Quantities of Lumber • Miceli, C.M., H.N. Rosen, and H.A. Spalt. 1982. Drying small quantities of green hardwoods. U.S. Department of Agriculture, Forest Service, North Central Forest Experiment Station Research Paper NC-228. • Bois, P.J., E.M. Wengert, and R.S. Boone. 1982. A checklist for drying small amounts of lumber. U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. Forest Products UtilizationTechnical Report No. 6.

  14. Vacuum Drying of Lumber • Use of vacuum permits a low EMC condition at lower temperature than conventional • Advantageous for refractory species, thick stock, color maintenance • Time and energy savings possible Russian vacuum lumber driers

  15. Vacuum may be used to dry wood at lower temperatures - Example • An oven set at 102 oC, operating in a room at 20 oC and 50% RH will have a relative humidity in the oven of 1.1% and corresponding EMC of 0.2% • A vacuum oven with an absolute pressure of 0.13 kpa at 50 oC, in the same 20 oC/50%RH room will also have a 1.1% RH and 0.2% EMC. See Siau 1995, p. 13

  16. Vacuum drying… • Various means are used for heat transfer, including: • Convective heat transfer…inefficient and ineffective in a partial vacuum. May use alternating vacuum and atmospheric pressure • Steam-heated platens or electrically-heated conductive blankets in contact with lumber (see fig. 2-27 of DKOM) • Radio-frequency energy (see papers by Avramidis et al. for principles of operation)

  17. See pix from Zwick and Lamb pdfs to illustrate vacuum & vacuum/rf kilns…

  18. RFV References • Avramidis, S., R.L. Zwick, and J.B. Neilson. 1996. Commercial scale RF/V drying of softwood lumber. Part I. Basic kiln design considerations. For. Prod. J. 45(5):44-51. • Avramidis, S. and R.L. Zwick. 1996. Commercial scale RF/V drying of softwood lumber. Part II. Drying characteristics and lumber quality. For. Prod. J. 46(6):27-36. • Avramidis, S. and R.L. Zwick. 1997. Commercial scale RF/V drying of softwood lumber. Part III. Energy consumption and economics. For. Prod. J. 47(1):48-56.

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