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Drying Hardwood Lumber

Joseph Denig Eugene M. Wengert William T. Simpson

United States Department of Agriculture

Forest Service

Forest Products Laboratory

General Technical Report

FPL−GTR−118

Abstract

Drying Hardwood Lumber focuses on common methods for drying lumber of different thickness, with minimal drying defects, for high quality applications. This manual also includes predrying treatments that, when part of an overall quality-oriented drying system, reduce defects and improve drying quality, especially of oak lumber. Special attention is given to drying white wood, such as hard maple and ash, without sticker shadow or other discoloration. Several special drying methods, such as solar drying, are described, and proper techniques for storing dried lumber are discussed. Suggestions are provided for ways to economize on drying costs by reducing drying time and energy demands when feasible. Each chapter is accompanied by a list of references. Some references are cited in the chapter; others are listed as additional sources of information.

Keywords: drying, hardwood, lumber, warp, kiln

September 2000

Denig, Joseph; Wengert, Eugene M.; Simpson, William T. 2000. Drying hardwood lumber. Gen. Tech. Rep. FPL–GTR–118. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 138 p.

A limited number of free copies of this publication are available to the public from the Forest Products Laboratory, One Gifford Pinchot Drive, Madison, WI 53705–2398. Laboratory publications are sent to hundreds of libraries in the United States and elsewhere.

The Forest Products Laboratory is maintained in cooperation with the University of Wisconsin.

The use of trade or firm names is for information only and does not imply endorsement by the U.S. Department of Agriculture of any product or service.

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Units of Measurement

In this manual, measurements are expressed in both English (inch–pound) and SI units. The following provides SI equivalents for lumber thickness sizes, dimension lumber, board foot volume, and other units.

SI equivalents for lumber thickness sizes

3/4 19 m 4/4 25 m 5/4 32 m 6/4 38 m 8/4 51 m 10/4 64 m 12/4 76 m 14/4 89 m 16/4 102 m

SI equivalents for dimension lumber Nominal (in.) Standard (m)

2 by 4 38 by 89 2 by 6 38 by 102 2 by 10 38 by 165

SI equivalents for other units ft3 0.0283 m3 ft/s 0.305 m/s ft/min 0.005 m/s lb 0.454 kg lb/in2 6.895 kPa lb/ft3 16.0 kg/m3

°F 0.56°C temperature TC = [TF − 32]/1.8 aThe conversion factor for board foot is used to convert gross volumes of lumber. It does not take into account any variation between actual and nominal sizes but rather is based on the volumetric ratio between 1 cubic meter (1 m × 1 m × 1 m) and 1 board foot (1 in. × 12 in. × 1 ft).

Drying Hardwood Lumber

Joseph Denig Associate Professor North Carolina State University, Raleigh, North Carolina

Eugene M. Wengert Professor Emeritus University of Wisconsin, Madison, Wisconsin

William T. Simpson Research Forest Products Technologist Forest Products Laboratory, Madison, Wisconsin

Preface

For hardwood lumber producers, drying is an opportunity to add value to products and to enter new, previously inaccessible markets. For most hardwood users, such as furniture manufacturers, lumber drying is an essential procedure in the manufacturing process. As with any part of the manufacturing process, costs must be controlled. Costs can be magnified by improper drying techniques that cause degrade, resulting in quality losses; mistakes can be made that cause problems in subsequent manufacturing processes; and considerable amounts of energy can be wasted. As hardwood lumber prices escalate, ensuring that the highest yield is obtained from the hardwood resource becomes critical in controlling overall costs. Fortunately, drying techniques and systems are available that can produce a quality hardwood lumber product at minimum cost.

Drying Hardwood Lumber is an update of a previous Forest Service publication, Drying Eastern Hardwood Lumber by John M. McMillen and Eugene M. Wengert. Both publications contain information published by many public laboratories, universities, and associations, as well as that developed at the Forest Products Laboratory and other Forest Service units. The updated version includes much basic information from the original publication and new information relevant to new technology and the changing wood resource.

iv

Contents

Chapter 1 Overview1
Quality Requirements and Cost of Degrade1
Basic Drying Concepts1
Drying Methods3
Moisture Content5
References6
Chapter 2 Drying Mechanisms of Wood7
Wood Characteristics That Affect Drying7
Environmental Factors12
Rate of Drying18
Stages of Drying21
References24
Chapter 3 Stock Preparation and Stacking25
Protection of Logs25
Sawing Procedures25
Protection of Green Lumber26
Prevention of Surface Checks27
Prevention of End Checks27
Color Enhancement Through Steaming28
Other Lumber Pretreatments28
Sorting29
Stacking30
Additional Ways to Control Warp3
References3
Chapter 4 Air Drying35
Advantages and Limitations36
Utilization of Air Movement36
Other Factors That Affect Drying Rate and Degrade37
Drying Time and Final Moisture Content40
Air Drying and Shed Drying Operating Costs43
Quick Guide for Improving Air Drying43
References4
Chapter 5 Drying Sheds45
Open Sheds45
Fan Sheds46
References47
Chapter 6 Accelerated Air Drying and Predrying49
Accelerated Air Drying49
Warehouse Predrying50
References56
Chapter 7 Conventional Kiln Drying57
Dry Kiln Designs57
Dehumidification Drying58
Basic Kiln Operating Philosophy60
Kiln Samples60
Recording of Drying Date71
Basic Hardwood Kiln Schedules72

Page

Tropical Hardwoods78
Kiln Start-Up Procedures79
Equalizing and Conditioning86
Sterilization8
Drying Time8
Operational Considerations90
References91
Chapter 8 Advanced Kiln Drying Procedures93
Modifications to General Hardwood Schedules93
Special Hardwood Schedules94
Alternative Schedules for Some Species96
Kiln Operational Techniques97
References102
Chapter 9 Drying Defects103
Checking103
Shake105
Collapse106
Warp106
Discoloration107
Moisture Content109
Residual Tension set (Casehardening)110
Machining and Gluing Problems110
Insect Damage1
Statistical Process Control112
References114
Chapter 10 Special Drying Methods115
Heated-Room Drying115
Press Drying115
High-Frequency and Microwave Heating115
Solvent Seasoning116
Lesser-Used Drying Methods116
Solar Drying116
References118
Chapter 1 Storage of Dried Lumber19
Air-Dried Lumber119
Kiln-Dried Lumber119
Storage Facilities119
References120
Chapter 12 Economics and Energy121
Economics121
Energy Considerations122
References125

Page Adjustment of Moisture Content of Kiln-Dried Wood...95 Problems Caused by Incorrect Lumber

of Commercial North American Hardwoods126
Appendix B—Portable Electric Moisture Meters128

Appendix A—Lumber, Tree, and Botanical Names Glossary ......................................................................... 131

Chapter 1—Overview

The fundamental reason for drying lumber is to enhance the properties of the wood and thereby make the lumber more valuable. In short, the primary objective when drying hardwood is to produce a useful product, minimizing any quality losses, thereby conserving natural resources and at the same time making a profit. Stated another way, hardwood lumber drying is, or should be, a conservation-oriented, profitable process.

Some advantages of dried lumber over undried or partially dried lumber are as follows:

• Lumber with less than 20% maximum moisture content

(MC) has no risk of developing stain, decay, or mold as a result of fungal activity.

• Dry lumber is typically more than twice as strong and nearly twice as stiff as wet lumber.

• Fasteners driven into dry lumber, including nails and screws, will perform much better than do fasteners in wet lumber, especially if the wet lumber dries after fastening.

• Dry lumber weighs 40% to 50% less than wet, undried lumber. For example, an 18-wheel, flatbed truck can haul about 7,500 board feet of wet lumber, 10,500 board feet of partially dried lumber, and 12,500 board feet of kiln-dried lumber.

• Products made from properly dried lumber will shrink very little or none at all while in service; products made from wet lumber often shrink substantially as the wood dries.

• Gluing, machining, and finishing are much easier to accomplish with dry wood.

• Wood that will be treated with fire retardants or preservatives (such as copper chromium arsenate, CCA) after drying must be at least partially dried to allow for quick penetration of the treating chemicals.

Quality Requirements and Cost of Degrade

Proper drying, aimed at achieving the highest possible quality of the wood, seems to have assumed new importance and gained appreciation in the past decade. For example, the rules of the National Hardwood Lumber Association allows ordinary surface checking in clear cuttings only if the checks surface out at the standard thickness. Furthermore, end splits must be very large before they reduce the grade of lumber. However, an increasing number of companies are insisting on higher quality standards. Customers have become aware of the factors that influence costs. Because the wood raw material often constitutes 75% of total costs, customers now insist on exceptional quality of dried lumber. Furniture and cabinet industries, for example, have found that a 1% increase in yield through better drying can reduce the cost of parts by more than $40/thousand board feet, based on estimates using 1998 cost and values. The importance of correct final MC in reducing rejects in machining and gluing, and even the importance of proper MC in the final product, is now well accepted by the industry, especially with the advent of the affordable, in-line moisture meter that checks the MC of every piece of lumber. In short, lumber drying has entered a new era, one of high quality drying. In a poor drying operation, the costs incurred by loss in quality (perhaps as high as $100/thousand board feet) can easily exceed all other drying costs combined. In a high quality operation, the costs incurred by loss in quality can be considerably lower ($15/thousand board feet), and most of this loss is the result of the inherent quality of the wood and not the drying procedures.

When drying lumber, the key question is “What level of quality does the customer require?” This question, which determines the quality that must be achieved, must be answered before analyzing the correct drying method. Specifically, the major quality factors for dried lumber (Table 1.1) must be considered. Although this is a long list, in most cases the customer is concerned about only a few of these items.

In addition to knowing the level of quality required, the trained kiln operator needs to have the right equipment; to assure that the equipment is operated and maintained properly; to receive properly stacked, good quality lumber; and to have adequate time to do the job correctly.

Basic Drying Concepts

Understanding the fundamental concepts that underlie lumber drying can guide the selection of economical and efficient drying methods that result in high quality products. This knowledge will allow kiln operators, drying practitioners, and drying managers to apply general drying concepts and information to specific situations.

To dry wood, three basic requirements must be met:

1. Energy (heat) must be supplied to evaporate moisture throughout the drying process. Two types of water can be found in wood: free water and bound water. Free water fills the wood cell cavity and is easily evaporated from the wood. Drying green wood with large amounts of free water requires 1,045 Btu/lb (2.4 MJ/kg) of water evaporated (Fig. 1.1). Bound water refers to the water in a wood cell when MC is less than approximately 30%. Bound water is chemically attached to the cell wall; an increasing amount of energy is required to remove a given amount of water as MC decreases. In practical terms, however, lumber drying must be considered on a larger scale, such as a piece of lumber, rather than on the small scale of a cell.

Because a piece of lumber consists of many wood cells, during drying some cells located on the lumber surface have low MC while cells located in the center of the piece have high MC. As a result, the amount of energy required to remove a given amount of water varies only slightly with a change in the average MC of the lumber— approximately 1,100 Btu/lb (2.6 MJ/kg) are required to evaporate water from lumber during drying. Additional energy is required for heat losses (conduction and ventilation) in the dryer.

Proper control of temperature during drying is essential for quality drying. In short, as the temperature increases, drying is more rapid; wood becomes weaker (in the short term), thereby increasing the risk of checking, cracking, honeycomb, collapse, and warp; drying is more uniform throughout the load; wood darkens in color; and insects, insect eggs, and fungi become less active and are killed when the temperature is above 130oF (54oC).

2. The environment surrounding the lumber must be capable of receiving moisture from the wood surface. That is, the relative humidity of the air surrounding the lumber must be below 100%.

Proper control of humidity during drying is also essential for quality drying. The lower the relative humidity (RH), the faster the drying, resulting in flatter, brighter colored lumber. On the other hand, low RH values at the beginning of the drying process can result in excessively fast drying that may cause cracks, splits, and honeycomb in some species and lumber thicknesses.

3. During drying, air movement through a stack of lumber must be adequate to bring energy into the stack, to remove evaporated moisture, and to maintain the desired RH.

Proper control of the air velocity during drying at high and intermediate MCs is essential for quality drying. Inadequate velocity, especially at high lumber MCs, can result in excessively high humidity and slow drying within a stack of lumber, leading to warp and to poor color and staining in some species. Higher velocity at high MCs can result in excessive drying rates, which in turn can lead to checking, cracking, and honeycomb. At low MCs, velocity is not a critical factor in controlling lumber quality or limiting the drying rate.

The combination of these three factors—temperature, RH, and air velocity—determines the rate at which lumber dries. These factors can be manipulated to control the drying process, minimizing defects while drying the lumber as rapidly as possible. For more detailed information on the theory and details of the drying process, wood properties, and causes and cures of specific drying defects, refer to the Dry Kiln

Table 1.1—Quality factors for dried lumber

Correct MC—average and spread within individual piece (shell-core; end to end)

Correct MC—average and spread for entire load No checking on surfaces No checking in interior No checking and splits on ends No warp (cup, bow, side bend (crook), twist) No casehardening Good color Good strength No or minimal fungal stain No or minimal chemical stain Good machinability Good glueability

Figure 1.1—Energy required in drying of wood cell as wood MC changes (after Skaar and Simpson 1968).

Operator’s Manual (Simpson 1991), Air Drying of Lumber (FPL 1999a), Lumber Drying Sourcebook (Wengert and Toennisson 1998), and Drying Oak Lumber (Wengert 1990). Highly technical information is provided in other references (McMillen 1969, Panshin and De Zeeuw 1980, and Stamm 1964).

Drying Methods

A number of methods are in widespread commercial use for drying hardwood lumber:

• Air drying

• Shed air drying

• Forced-air shed drying

• Warehouse predrying

• Low temperature kiln drying

• Conventional electric dehumidification kiln drying

• Conventional steam-heated kiln drying

Combinations of two of these methods are often used in commercial hardwood drying operations to produce a low cost, high quality product (Table 1.2). A typical example is warehouse predrying of lumber followed by conventional drying.

The most popular drying combination for hardwood lumber prior to the 1990s was air drying, which often took up to 6 months, followed by kiln drying. Recently, four factors have necessitated an aggressive search for drying systems that produce a higher quality dried product in, if possible, a shorter amount of time:

• Increasing lumber prices

• Pressure by firms to cut manufacturing inventory, driven by high interest rates and/or operating cash shortages

• The use of lower grade lumber for kiln-dried products

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