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1、Chapter 1 Properties of Wood Related to DryingCommercial wood species 1 Hardwoods and softwoods 2 Structural features of wood2 Sapwood and heartwood 4 Pith 4 Annual growth rings 4 Wood rays 4 Grain and texture 5 Color 5 Variations in structure 5 Commercial lumber grades 6 Hardwood lumber grades 6 So
2、ftwood lumber grades 6 Wood-moisture relations 7 Free and bound water 8 Fiber saturation point 8 Equilibrium moisture content 8 How wood dries 9 Forces that move water 9 Factors that influence drying rate10 Lumber thickness 10 Specific gravity and weight of wood10 Shrinkage of wood 11 Average shrink
3、age values 12 Shrinkage variability 12 Drying stresses 12 Electrical properties 13 Thermal properties 15 Specific heat 15 Thermal conductivity 15 Thermal expansion 16 Literature cited 16 Sources of additional information 16 Tables 17 Appendix-Equations for relating temperature, humidity, and moistur
4、e content 39 Wet-bulb temperature and relative humidity 39 Relative humidity and equilibrium moisture content 40 Psychrometric charts 41Chapter 1 was revised by William T. Simpson, Supervisory Research Forest Products Technologist.Lumber drying is one of the most time- and energy- consuming steps in
5、 processing wood products. The anatomical structure of wood limits how rapidly wa- ter can move through and out of wood. In addition, the sensitivity of the structure to stresses set up in dry- ing limits the drying rate; rapid drying causes defects such as surface and internal checks, collapse, spl
6、its, and warp. Drying time and susceptibility to many dry- ing defects increase at a rate that is more than pro- portional to wood thickness. The variability of wood properties further complicates drying. Each species has different properties, and even within species, variability in drying rate and
7、sensitivity to drying defects impose limitations on the development of standard drying pro- cedures. The interactions of wood, water, heat, and stress during drying are complex. The purpose of this chapter is to describe some of the fundamental prop- erties of wood that are relevant to lumber drying
8、. We will discuss commercial wood species, wood structure, lumber grades, water movement in wood, how wood dries, specific gravity and weight of wood, wood shrink- age, stress development during drying, and electrical and thermal properties of wood.Commercial Wood SpeciesMore than 100 commercially i
9、mportant species of trees grow in the United States. A similar number of species are imported into the United States, and the potential for additional imported species grows. The lumber pro- duced from all of these species varies greatly in drying characteristics. The most commonly used commercial n
10、ames for lumber and the corresponding species names accepted by the Forest Service for the trees from which the lumber is cut are given in table 1-1 for domestic species and table 1-2 for tropical species. Table 1-1 was adapted from the standard nomenclature of domestic hardwoods and softwoods devel
11、oped by the Ameri- can Society for Testing and Materials (1981). Tropi- cal species follow the nomenclature used by Chudnoff (1984). While the commonly used lumber names are generally satisfactory for the buying and selling of lum- ber, they sometimes refer to lumber from a number of species that di
12、ffer in green moisture content, shrinkage, or drying characteristics. In the tables and indexes of physical properties and drying schedules given in this and other chapters, the woods are arranged alphabet- ically by the common species names accepted by the Forest Service.1Hardwoods and SoftwoodsTre
13、es can be divided into two classes, hardwoods and softwoods. The hardwoods, such as birch, maple, and oak, have broad leaves. Some softwoods or conifers, such as the cedars, have scalelike leaves, while others, such as pine, Douglas-fir, and spruce, have needlelike leaves.The terms hardwood and soft
14、wood are not directly as- sociated with the hardness or softness of the wood. In fact, such hardwood trees as cottonwood, basswood, and yellow-poplar have softer wood than such soft- woods as longleaf pine and Douglas-fir.Figure 1-1Cross section of a white oak tree trunk. A, Cambium layer (microscop
15、ic) is inside inner bark and forms wood and bark cells. B, Inner bark is moist and soft, and contains living tissue; the inner bark carries prepared food from leaves to all growing parts of tree. C, Outer bark containing corky layers is composed of dry dead tissue; it gives general protection agains
16、t ex- ternal injuries. Inner and outer bark are separated by a bark cambium. D, Sapwood, which contains both living and dead tissues, is the light-colored wood beneath the bark; it carries sap from roots to leaves. E, Heartwood (inactive) is formed by a gradual change in the sap- wood. F, Pith is the soft tissue about which the first wood growth takes place in the newly formed twigs. G, Wood rays connect the various layers from pith to bark for storage and transfer of food. (MC88 9016)Structura
