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. 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 2 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 3 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