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  • DRYING EASTERN HARDWOOD LUMBER

    By JOHN M. McMILLEN

    and EUGENE M. WENGERT

    Forest Products Laboratory Forest Service

    U. S. Department of Agriculture

    (Maintained at Madison, Wisconsin, in cooperation with the University of Wisconsin)

    AGRICULTURE HANDBOOK NO. 528 September 1978

    Library of Congress Catalog No. 77-600073

  • MCMILLEN, JOHN M.

    1977. Drying eastern

    and WENGERT, EUGENE M.

    hardwood lumber. U.S. Dep. Agric., Ag- ric. Handb. 528, 104 p.

    Presents recommendations based on recent research and industry practice for drying eastern hardwood lumber, in- cluding dimension items. Accent is on comparing methods for energy-saving management decisions, but practical guid- ance is also given to wood drying personnel. Air drying, accelerated air drying, and kiln drying are covered.

    KEYWORDS: Hardwoods, drying, air drying, kiln drying, forced-air drying, high-temperature drying, steaming, de- humidifier drying, cost accounting, energy saving, degrade reduction.

    ABOUT THE AUTHORS . . .

    JOHN M. MCMILLEN is a forest products technologist who has specialized in the drying of wood for more than three decades at the Forest Products Laboratory.

    EUGENE M. WENGERT, formerly a member of the wood dry- ing group at the Laboratory, is now extension specialist in wood technology at Virginia Polytechnic Institute and State University, Blacksburg, Va.

    For sale by the Superintendent of Documents U.S. Government Printing Office

    Washington, D.C. 20402 Stock Number 001-000-03761-7

    ii

  • PREFACE Interest in broader utilization of hardwoods has been spurred by shortages of softwoods

    and an apparent surplus of hardwoods. However, to greatly increase use of hardwoods will require simplified drying procedures. At the same time, the better grades of hardwoods must be seasoned by methods less wasteful of both material and the energy required to dry it. These facts make this publication desirable.

    The handbook is confined to eastern hardwoods because most of the hardwood timber grows in the Eastern United States. Much of the general methodology will apply to western hardwoods, but some species have special problems and some localities have severe air drying weather; western hardwood users should consult local authorities on specific questions.

    Without supplanting the two existing handbooks on kiln drying and air drying lumber, this handbook combines improvements in practice with new research findings to provide savings in costs and energy. It assembles scattered technology on accelerated air drying and presents new facts and interpretations on air drying. Kiln drying information is concentrated here, and other methods of drying are discussed. An overall guide is also provided to combine various drying procedures most economically for various product requirements.

    This publication should be of the greatest value to operating and managerial personnel responsible for hardwood lumber processing in hardwood mills, custom drying operations, and furniture plants. It should also help individuals dry small quantities of lumber inexpensively without sacrificing quality. Teachers and students should find this handbook helpful in career development programs.

    The authors acknowledge the assistance of Val Mitchell, State Climatologist, University of Wisconsin, Madison in supplying and interpreting some of the climatological information needed for this publication. Other vital contributions were made by many Forest Service colleagues and coworkers in industry. Particular recognition goes to Walton Smith and Paul Bois for photographs, to Kenneth Compton for specific information on steaming of walnut, and to members of the U.S. Forest Products Laboratory research unit on improvements in drying technology. For all these forms of assistance, we are most grateful.

    JOHN M. MCM ILLEN E UGENE M. WE N G E R T

    Use of trade, firm, or corporation names in this publication is for the information and convenience of the reader. Such use does not constitute an official endorsement or approval of any prod- uct or service by the U.S. Department of Agriculture to the exclusion of others that may be suitable.

    iii

  • CONTENTS

    Drying Hardwoods . . . . . . . . . . . . . . . . .

    Chapter I.—Comparison of Drying Methods for Hardwoods . . . . . . . . . . . .

    Basic Drying Concepts . . . . . . . . . . . Methods . . . . . . . . . . . . . . . . . . . . . . . . . . How Dry is Dry Enough . . . . . . . . . . Literature Cited . . . . . . . . . . . . . . . . . .

    Chapter 2.—Stock Preparation and Stacking . . . . . . . . . . . . . . . . . . . . . . . . . . .

    From Logs to Lumber . . . . . . . . . . . . Sorting . . . . . . . . . . . . . . . . . . . . . . . . . . . Stacking . . . . . . . . . . . . . . . . . . . . . . . . . Further Warp Control . . . . . . . . . . . . Literature Cited . . . . . . . . . . . . . . . . . .

    Chapter 3.—Air Drying . . . . . . . . . . . . . Advantages and Limitations of

    Air Drying . . . . . . . . . . . . . . . . . . . . . Utilizing Air Movement . . . . . . . . . . Other Factors Relating to Drying

    Rate and Defects . . . . . . . . . . . . . . . Drying Time and Final Moisture

    Content . . . . . . . . . . . . . . . . . . . . . . . . Deterioration of Lumber While

    Air Drying . . . . . . . . . . . . . . . . . . . . . Air Drying Small Quantities

    of Lumber . . . . . . . . . . . . . . . . . . . . . . A Quick Guide for Improving Air

    Drying Efficiency of Hardwoods Literature Cited . . . . . . . . . . . . . . . . . .

    Chapter 4.—Accelerated Air Drying Research Basis . . . . . . . . . . . . . . . . . . . Types of Dryers and Procedures . . Advantages and Disadvantages of

    Accelerated Air Drying . . . . . . . . . Drying Times . . . . . . . . . . . . . . . . . . . . Literature Cited . . . . . . . . . . . . . . . . . .

    Chapter 5.—Conventional Kiln Drying . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

    Drying Procedures . . . . . . . . . . . . . . . Operational Considerations . . . . . . . Drying Times . . . . . . . . . . . . . . . . . . . . Literature Cited . . . . . . . . . . . . . . . . . .

    Page

    1

    2 2 2 4 4

    5 5 7 8 8

    10

    11

    12 13

    13

    19

    21

    21

    22 22

    23 23 24

    31 31 33

    34 36 52 54 55

    Chapter 6.—High-Temperature Drying . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

    Basic Concepts . . . . . . . . . . . . . . . . . . . Research and Practice to Date . . . Drying Times and Species

    Potentialities . . . . . . . . . . . . . . . . . . . Apparent Process Requirements . Literature Cited . . . . . . . . . . . . . . . . . .

    Chapter 7.—Special Predrying Treatments . . . . . . . . . . . . . . . . . . . . . . . .

    Steaming . . . . . . . . . . . . . . . . . . . . . . . . . Chemical Seasoning . . . . . . . . . . . . . . Polyethylene Glycol Process . . . . . . Surface Treatments to Prevent

    Checking . . . . . . . . . . . . . . . . . . . . . . . Other Pretreatments . . . . . . . . . . . . . Literature Cited . . . . . . . . . . . . . . . . . .

    Chapter 8.—Other Methods of Drying . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

    Heated Room Drying . . . . . . . . . . . . . Dehumidifier Drying . . . . . . . . . . . . . Solar Drying . . . . . . . . . . . . . . . . . . . . . Press Drying . . . . . . . . . . . . . . . . . . . . . High-Frequency and Microwave

    Heating . . . . . . . . . . . . . . . . . . . . . . . . Solvent Seasoning . . . . . . . . . . . . . . . . Minor Special Methods . . . . . . . . . . . Literature Cited . . . . . . . . . . . . . . . . . .

    Chapter 9.—Storage of Dried Lumber . . . . . . . . . . . . . . . . . . . . . . . . . . . .

    Air-Dried Lumber . . . . . . . . . . . . . . . . Kiln-Dried Lumber . . . . . . . . . . . . . . .

    Chapter 10.—Economics and Energy Typical Costs for Various Drying

    Procedures . . . . . . . . . . . . . . . . . . . . . Degrade Costs and Causes . . . . . . . A Cost Accounting Approach . . . . . Energy Considerations . . . . . . . . . . . Literature Cited . . . . . . . . . . . . . . . . . .

    Appendix A.—Species, Thicknesses, Drying Schedules, and Comments . . . . . . . . . . . . . . . . . . . . . . . .

    Page

    56 56 57

    58 59 60

    61 61 65 65

    66 66 67

    68 68 68 70 71

    71 72 72 73

    74 74 74

    76

    76 77 78 81 83

    84

    iv

  • CONTENTS (Con.) Page Page

    Appendix B.—Mixed Drying of Some Appendix D.—Lumber Names, Tree Species by Simplified Species, Seasoning Types, and Schedules . . . . . . . . . . . . . . . . . . . . . . . . 87 Botanical Names Mentioned in

    This Handbook . . . . . . . . . . . . . . . . . . . 92 Appendix C.—Special Kiln Schedules

    for Special Purposes . . . . . . . . . . . . . . 89 Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

    Requests for copies of illustrations contained in this publication should be directed to the Forest Products Laboratory, Forest Service, U.S. Department of Agriculture, P. O. Box 5130, Madi- son, Wis. 53705.

    LIST OF TEXT TABLES Table

    1

    2

    3

    4

    5

    6

    Title

    Expected average interior relative humidities and recommended moisture content values for most wood items for interior uses . . . . . . . . . . . . . . . . . . . . . . .

    Estimated time to air dry green 1- and 2-inch hard- wood lumber to approxi- mately 20 percent average moisture content .

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