Publications - Of General Interest |
Subject - Dry Kiln Operator's Manual - USDA Agricultural Handbook AH-188 |
Title: Chapter 01 Properties of Wood Related to Drying hide Abstract: Lumber drying is one of the most time- and energyconsuming steps in processing wood products. The anatomical structure of wood limits how rapidly water can move through and out of wood. In addition, the sensitivity of the structure to stresses set up in drying limits the drying rate; rapid drying causes defects such as surface and internal checks, collapse, splits, and warp. Drying time and susceptibility to many drying defects increase at a rate that is more than proportional 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 sensitivity to drying defects impose limitations on the development of standard drying procedures. The interactions of wood, water, heat, and stress during drying are complex. The purpose of this chapter is to describe some of the fundamental properties of wood that are relevant to lumber drying. We will discuss commercial wood species, wood structure, lumber grades, water movement in wood, how wood dries, specific gravity and weight of wood, wood shrinkage, stress development during drying, and electrical and thermal properties of wood. |
Title: Chapter 02 Kiln Types and Features hide Abstract: A lumber dry kiln consists of one or more chambers designed to provide and control the environmental conditions of heat, humidity, and air circulation necessary for the proper drying of wood. As the development of the modern dry kiln has progressed, a number of design modifications have been explored in relation to the mechanism of heat supply, arrangement and type of fans, control of relative humidity or wet-bulb temperature, and use of various materials for construction of the chamber. The design of a kiln has an important bearing on its operation and drying efficiency. A properly designed and operated kiln will dry most species of lumber or other wood products to any specified moisture content between 3 and 19 percent in a reasonably short time without appreciable losses caused by drying defects. |
Title: Chapter 03 Dry Kiln Auxiliary Equipment hide Abstract: Certain auxiliary equipment is needed to operate a dry kiln in the most economical manner and to obtain good drying results. Drying schedules based upon moisture content cannot be successfully applied unless the moisture content of the stock is known. Therefore, equipment should be available for determining the moisture content of the stock. Equipment should also be available for determining the temperature, humidity, and velocity of air in the kiln to maintain uniform conditions for fast drying. |
Title: Chapter 04 Inspection and Maintenance of Dry Kilns and Equipment hide Abstract: Adequate kiln maintenance is as essential to efficient dry kiln operation as good design and construction. Adequate maintenance can be accomplished only through regular, frequent inspections of the kiln andauxiliary equipment. If inspections reveal the need for repairs or replacements, they should be made as soon as possible to avoid drying problems. Regular, systematic inspections should cover such items as the kiln structure; doors; floor; tracks; control equipment; heating, spraying, and venting system; trucks; lumber-handling equipment; and general housekeeping. To make sure that inspections are thorough, the operator should note the condition of the kiln structure and the equipment on a checklist. The checklist at the end of this chapter can be made to fit any specific kiln installation. |
Title: Chapter 05 Stacking and Loading Lumber for Kiln Drying hide Abstract: Much of the degrade, waste, and moisture content variation that occurs during kiln drying results from poor stacking and loading. Well-stacked lumber and properly loaded and baffled kilns result in faster and more uniform drying, less warp, and less sticker loss. Stacking and loading procedures vary widely for hardwoods and softwoods, differences in plant layouts, type of material to be dried, and types of kilns and stacking equipment. However, certain principles apply to all stacking and loading. The purpose of this chapter is to describe these principles. |
Title: Chapter 06 Kiln Samples hide Abstract: This chapter covers selection and preparation of kiln samples; the number of samples required in a kiln charge; determination of moisture content and ovendry weight of samples; how to use samples during drying; how to make intermediate moisture content estimates; tests for residual drying stresses; and recording and plotting of data. |
Title: Chapter 07 Kiln Schedules hide Abstract: A kiln schedule is a carefully worked-out compromise between the need to dry lumber as fast as possible and, at the same time, to avoid severe drying conditions that ill cause drying defects (ch. 8). It is a series of dry and wet-bulb temperatures that establish the temperatureand relative humidity in the kiln and are applied at various stages of the drying process. Temperatures are chosen to strike this compromise of a satisfactorydrying rate and avoidance of objectionable drying defects. The stresses that develop during drying (ch. 1) constitute the limiting factor that determines the kiln schedule. The schedules must be developed so that the drying stresses do not exceed the strength of the wood at any given temperature and moisture content. Otherwise, the wood will crack either on the surface or internally, or be crushed by forces that collapse the wood cells. Wood generally becomes stronger as moisture content decreases, and, to a lesser extent, it becomes weaker as temperature increases. The net result is that as wood dries, it becomes stronger because of the decreasing moisture content and can tolerate higher drying temperatures and lower relative humidities without racking. This is a fortunate circumstance because as wood dries, its drying rate decreases at any given temperature, and the ability to raise the drying temperature helps maintain a reasonably fast drying rate. Thus, rapid drying is achieved in kilns by the use of temperatures as high as possible and relative humidities as low as possible. For hardwoods, relative humidity can generally be reduced substantially before temperature can be raised substantially. |
Title: Chapter 08 Drying Defects hide Abstract: The success of a company and the livelihood of the dry kiln operator may depend on knowing the causes of defects in lumber and methods to prevent their occurrence. Since some defects are not observed in green lumber and are first noted after the drying operation, they are often called drying defects even though the defects may have started in the tree, log, or green lumber. Defects that develop in dry wood products during machining, gluing, and finishing operations may also be blamed on poor drying practices. A drying defect is any characteristic or blemish in a wood product that occurs during the drying process and reduces the product’s intended value. Drying degrade is a more specific term that implies a drying defect that lowers the grade of lumber. Every year, drying degrade and other drying defects cost the softwood and hardwood lumber industries millions of dollars in lost value and lost volume caused by poor product performance. When unexpected defects appear in dried wood products, their cause is often blamed on the drying operation. The purpose of this chapter is to describe the various types of defects that can occur in dried wood products and to show how these defects are related to the kiln-drying operation. |
Title: Chapter 09 Operating a Dry Kiln hide Abstract: A dry kiln, no matter how well equipped with controls, is only as efficient as the operator who runs it. Despite advances in control technology that give the operator more information than was possible in the past, it is still largely the operator’s judgment that determines whether a charge of lumber will go through the kiln in a minimum time, emerge uniformly dried to the desired moisture content, and be free of undesired drying defects and stresses. The operator determines what kiln schedule to use, and whether it should be a time or moisture content schedule. If kiln samples are to be used, whether manually or automatically weighed or monitored with probes, the kiln operator is still responsible for selecting representative kiln samples. The kiln operator must monitor the progress of drying, whether manually or with the assistance of computer readouts,and apply judgment in deciding if adjustments are necessary during drying. Also, the operator must apply judgment in determining when the lumber has reached final moisture content with a minimum of moisture content variation and apply any necessary equalizing or conditioning treatments. |
Title: Chapter 10 Log and Lumber Storage hide Abstract: Kiln drying is only one step in the harvesting, handling, and processing of wood products. The best results can be obtained in kiln drying, therefore, when adequate attention is paid to related phases of wood processing.Although a dry kiln operator may have no responsibility for these related phases, knowledge of them is required to understand how they interact with drying. Problems that occur in drying, or that are erroneously blamed on drying, are sometimes related to the methods used to store logs and lumber before drying and those used to store kiln-dried lumber and finished products. |
Title: Chapter 11 Energy in Kiln Drying hide Abstract: Drying of materials in general and of wood in particular is energy intensive, primarily because a high amount of energy is required to evaporate water (liquid to gas). Depending upon the type of equipment used to dry wood, the efficiency level of the operation may require one and one-half to four times the energy actually needed to evaporate the water. In addition, green wood to be dried may contain, by weight, as much as wo-thirds water. Wood can be successfully dried in different types of dryers. However, even when optimally operated, the dryers may have different levels of efficiency as an inherent property of their physical design and the materials of construction, and their efficiency may also be affected by environmental factors. Certain practices or maintenance procedures may further reduce dryer efficiency. In this chapter, we discuss energy demand as related to various methods of drying, types of environmental and geographical factors, fuel, and equipment misuse. |
Title: Dry Kiln Operator's Manual--Intro, glossary, index hide Abstract: The purpose of this manual is to describe both the basic and practical aspects of kiln drying lumber. The manual is intended for several types of audiences. First and foremost, it is a practical guide for the kiln operator-a reference manual to turn to when questions arise. It is also intended for mill managers, so that they can see the importance and complexity of lumber drying and thus be able to offer kiln operators the support they need to do their job well. Finally, the manual is intended as a classroom text-either for a short course on lumber drying or for the wood technology curriculum in universities or technical colleges. |