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  • Principles and Practices of Drying Lumber

    Eugene M. Wengert, Extension Specialist Brooks Forest Products Center

    Virginia Polytechnic Institute and State University Blacksburg, Virginia

    Copyright 2006 Lignomat USA Ltd. All Rights Reserved.

  • 2

    Table of Contents Why Dry Wood? ............................................................................................................................5 How Dry Should Wood Be? ..........................................................................................................6 Wood Drying Methods ..................................................................................................................8

    Air Drying..................................................................................................................................8 Forced-Air Air-Drying (or Fan Shed Drying) ...........................................................................9 Low Temperature Warehouse Predrying ...................................................................................9 Low Temperature Dehumidification Drying .............................................................................9 Normal Temperature or Conventional Temperature Kiln Drying .............................................9 Drying Systems........................................................................................................................10 Degrade ....................................................................................................................................10 Air Drying Degrade .................................................................................................................10 Forced-Air Air-Drying Degrade ..............................................................................................11 Predryer Degrade .....................................................................................................................11 DH Degrade .............................................................................................................................11 Kiln Drying Degrade ...............................................................................................................11

    How Wood Dries - Definitions ....................................................................................................12 Definitions Related to Wood and Its Structure........................................................................12 Definitions Related to the Air Around Wood..........................................................................18

    How Wood Dries - Shrinkage, Stresses, Moisture Gradients......................................................20 Stage I ......................................................................................................................................20 Stage II .....................................................................................................................................22 Stage III....................................................................................................................................23 Stage IV ...................................................................................................................................24

    How Wood Dries - Environmental Effects ..................................................................................25 Temperature .............................................................................................................................25 Relative Humidity....................................................................................................................25 Velocity....................................................................................................................................26 Changing the Rate of Drying (or) Changing the Amount of Degrade.....................................26

    The Role of Velocity in Drying ...................................................................................................29 Drying Rate..............................................................................................................................29 Integral Effect ..........................................................................................................................30 Venting Effect..........................................................................................................................30 Heat Effect ...............................................................................................................................31 Electrical Consumption............................................................................................................31 Other Concerns With Velocity ................................................................................................31 Diagnostic Tools ......................................................................................................................32

    Operating a Lumber Dryer...........................................................................................................34 Log Handling and Storage .......................................................................................................34 Stacking ...................................................................................................................................34 Preparing and Selecting Samples.............................................................................................34 Checking the Quality of Incoming Lumber .............................................................................36 Loading a Dryer .......................................................................................................................37 Starting a Dryer........................................................................................................................37 Final Quality Checks ...............................................................................................................38

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    Safety .......................................................................................................................................38 Dry Lumber Storage ................................................................................................................38

    Drying Schedules .........................................................................................................................40 U.S. Forest Products Laboratory Schedules ............................................................................40 Sampling Moisture Content .....................................................................................................41 Modifying kiln schedules.........................................................................................................42 Start-Up Procedures .................................................................................................................44 Equalizing and Conditioning Procedures ................................................................................45

    Computer Automation for Hardwood Lumber Drying................................................................48 Present Options ........................................................................................................................48 Determining Moisture Content ................................................................................................48 Computerized Control..............................................................................................................49 Visual Inspection .....................................................................................................................50 Temperature Drop Across the Load.........................................................................................50

    Drying Degrade............................................................................................................................51 Causes of and Cures for Drying Degrade ................................................................................51 Types of Drying Degrade.........................................................................................................53 Quality Control ........................................................................................................................55

    Analyzing the Lumber .........................................................................................................55 Analyzing the Equipment ....................................................................................................57 Operating Techniques ..........................................................................................................58

    Summary ......................................................................................................................................59

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    List of Tables and Figures Table I Approximate Relationship Between RH and EMC at Room Temperature..............6 Table II Drying Systems and Quality for 4/4 Lumber ............................................................10 Table III Variation in Shrinkage for Different Oak Species ...................................................16 Table IV Change in Width for a 3" Wide Piece When MC Changes...................................17 Table V The Maximum Safe Rate of Drying ..........................................................................22 Table VI Relationship Between Shrinkage, MC, EMC, and RH ..........................................39 Table VII Recommended Kiln Schedule (T4-D2) for 4/4 and 5/4 Upland Red Oak ..........42 Table VIII Modified Kiln Schedule (T3-D2) for 4/4 and 5/4 Upland

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