United StatesDepartment ofAgriculture
ForestService
ForestProductsLaboratory
Madison,Wisconsin
AgricultureHandbookNo. 188
Dry KilnOperator'sManual
Dry KilnOperator’sManual
Edited byWilliam T. Simpson,Research Forest Products Technologist
United States Department of AgricultureForest ServiceForest Products Laboratory 1
Madison, Wisconsin
Revised August 1991
Agriculture Handbook 188
1The Forest Products Laboratory is maintained in cooperation with theUniversity of Wisconsin.
This publication reports research involving pesticides. It does not containrecommendations for their use, nor does it imply that the uses discussed herehave been registered. All uses of pesticides must be registered byappropriate State and/or Federal agencies before they can be recommended.
CAUTION, Pesticides can be injurious to humans, domestic animals, desirableplants, and fish or other wildlife-if they are not handled or appliedproperly. Use all pesticides selectively and carefully. Follow recommendedpractices for the disposal of surplus pesticides aand pesticide containers.
Preface Acknowledgments
The purpose of this manual is to describe both the ba-sic and practical aspects of kiln drying lumber. Themanual is intended for several types of audiences.First and foremost, it is a practical guide for the kilnoperator-a reference manual to turn to when questionsarise. It is also intended for mill managers, so that theycan see the importance and complexity of lumber dry-ing and thus be able to offer kiln operators the supportthey need to do their job well. Finally, the manual isintended as a classroom text-either for a short courseon lumber drying or for the wood technology curricu-lum in universities or technical colleges.
This manual is a revision of the 1961 edition byEdmund F. Rasmussen. Forest Service staff whocontributed to that original edition were Raymond C.Rietz, Edward C. Peck, John M. McMillen, Harvey H.Smith, and A.C. Knauss. It is a credit to these menthat the 1961 edition has been in wide use and demandfor the past 28 years. It is also to their credit that eventhough the manual is out of date in many parts, wewere able to use the basic framework of the originaledition to build on.
The Forest Products Laboratory staff involved in thisrevision were William T. Simpson (who wrote the intro-duction and had overall responsibility for coordination),R. Sidney Boone, James C. Ward, and John L. Tscher-nitz. Each person was responsible for revising certainchapters or parts of chapters. This assignment of re-sponsibilities is indicated on the chapter-opening pages.Chapters 5 and 7 of the original manual were combinedin this revision. Chapter 11, Energy in Kiln Drying,is a new chapter and was written by John L. Tscher-nitz. In addition to this assignment of chapters, therewere many formal and informal meetings among us toexchange ideas.
Many people helped in the revision. We visited manymills to make sure we understood current and develop-ing kiln-drying technology as practiced in industry, andwe thank all the people who allowed us to visit. Pro-fessor John L. Hill of the University of New Hampshireprovided the background for the section of chapter 6on the statistical basis for kiln samples. Kiln manufac-turers were also very helpful in spending time with usand providing photographs and schematics of dryingequipment. In particular, we wish to thank Coe Man-ufacturing Company, Hemco (Harvey Engineering andManufacturing Corp.), Irvington–Moore, Nyle Corpo-ration, Uraken Canada, Ltd., and Wagner ElectronicProducts, Inc., for their help. We also thank ProfessorCharles J. Kozlik, retired from Oregon State Univer-sity, for arranging and accompanying several of us on aplant tour in the Northwest.
The use of trade or firm names in this publication is forreader information and does not imply endorsement bythe U.S. Department of Agriculture of any product orservice.
Contents
Page
Introduction vi
1 Properties of wood related to drying 1
2 Kiln types and features 43
3 Dry kiln auxiliary equipment 75
4 Inspection and maintenance of dry kilnsand equipment 87
5 Stacking and loading lumber for kiln drying 103
6 Kiln samples 117
7 Kiln schedules 133
8 Drying defects 179
9 Operating a dry kiln 207
10 Log and lumber storage 219
11 Energy in kiln drying 239
Glossary 257
Index 269
Introduction
The modern dry kiln is a unique product of research,development, and experience. It is the only practicalmeans now in wide use for rapid, high-volume drying oflumber to conditions necessary for maximum service-ability in housing, furniture, millwork, and many otherwood products. As part of our charge to help furtherthe efficient utilization of our nation’s timber resource,Forest Service research and development in lumber dry-ing has made a significant contribution to the technol-ogy. The Forest Products Laboratory (FPL) has beenconducting research in lumber drying since it was es-tablished in 1910. Early work by Harry Tiemann (TheKiln Drying of Lumber: A Practical and TheoreticalTreatise, J.B. Lippincott Company, Philadelphia, PA,1917) at FPL established lumber kiln-drying technologyand the first lumber dry kiln design. Tiemann’s bookcan really he considered the first drying manual. Sev-eral other FPL drying manuals followed before the 1961manual by Rasmussen.
A well-designed and properly operated dry kiln can ina few days or weeks turn green lumber fresh from theforest into a dry, stable material necessary for success-ful industrial enterprises in today’s highly competitivemarkets. The more critical the drying requirements,the more firmly the dry kiln becomes established as anintegral part of the lumber mill, the furniture factory,or the millwork plant. For many wood products, kiln-dried lumber is essential.
Dried lumber has many advantages over green lumberfor producers and consumers alike. Removal of excesswater reduces weight and thus shipping and handlingcosts. Proper drying confines shrinking and swelling ofwood in use to manageable amounts under all but ex-treme conditions of relative humidity. Properly driedlumber can be cut to precise dimensions and machinedmore easily and efficiently; wood parts can he more se-curely fitted and fastened together with nails, screws,bolts, and adhesives; warping, splitting, checking, andother harmful effects of uncontrolled drying are largelyeliminated; paint, varnish, and other finishes are moreeffectively applied and maintained; and decay hazardsare eliminated if the wood is subsequently treated orprotected from excessive moisture regain.
Efficient kiln drying of lumber is therefore of key im-portance in the utilization of our forest resource. Onone hand, it helps to assure continued markets for woodproducts by increasing their service life, improving theirperformance, and contributing to consumer satisfac-
tion. On the other hand, it helps to conserve our forestresource by reducing waste in manufacture and extend-ing service life and usefulness of products. Both areessential in using timber wisely, which has long been anaccepted tenet of forest management policy.
The full benefits of modern kiln-drying technology canbe gained only when certain prerequisites are observed.Mill management must recognize the importance of ef-ficient operation to quality of product, and operatorsmust be well trained and encouraged to apply the besttechniques. Quality should not be sacrificed for quan-tity in the production of kiln-dried lumber. The highvalue of our timber resource makes it uneconomical todo so.
Terms used in this manual to describe dry kilns andtheir components, drying characteristics of wood, andkiln operational procedures are generally accepted andused throughout the industry. For clarification and tohelp the newcomer with common terminology, a glos-sary of terms is included after the last chapter.
vi
Glossary
This glossary includes generally accepted definitionsof a limited number of terms currently used in wood-drying literature. It also includes closely related termsthat are not fully defined in their special application topresent-day drying in most dictionaries or glossaries.
The following abbreviations are used throughout thismanual.
Btu British thermal unit
COD wt Calculated ovendry weight
DB Dry bulb
EMC Equilibrium moisture content
fbm Board feet (foot board measure); althoughit is not used in this manual, MBF is a com-monly used term for thousand board feet
FSP Fiber saturation point
MC Moisture content
OD wt Ovendry weight
RH Relative humidity
sp. gr. Specific gravity
WB Wet bulb
WBD Wet-bulb depression
wt Weight
Absorption, liquid—The taking in or imbibing of aliquid.
Adsorbed water—In context of wood drying, ad-sorbed water is held in wood substance by hygroscopicor molecular attraction. (Syn: bound water)
Air, entering—Heated air just as it enters the kilnloads of lumber.
Air, fresh—Air brought into the dryer to replacevented air.
Air, laminar—In kiln drying, airflow across the lum-ber, parallel to the stickers, which is very smooth andlayered, with no eddies or swirls; generally considereda condition in which velocity is too low to produce anoptimum drying effect.
Air, leaving—Air just after it leaves the kiln loads oflumber. It is usually at a lower temperature than theentering air.
Air, short circuiting of—See Short circuiting of air.
Air, turbulent—In kiln drying, airflow across thelumber, parallel to the stickers, which is not layeredand has fluctuations creating definite eddies and swirls;generally considered preferable to laminar flow for opti-mum drying.
Air binding—The presence of air (generally in pock-ets) in steam coils and traps, which interferes with thenormal flow of steam and condensate.
Air drying—See Drying, air.
Air reversal—Changing the airflow to flow in theopposite direction through a load of drying lumber orproducts.
Air travel, length of—The distance between theentering- and leaving-air sides of the kiln charge.
Air velocity—The speed at which air moves, generallymeasured in feet per minute.
Air volume—The total amount of air occupying ormoving through a given space, generally measured incubic feet.
Anemometer— An instrument for measuring airvelocity.
Annual growth ring—The growth layer put on a treeeach year in temperate climates, or each growing seasonin other climates; each ring includes springwood andsummerwood.
The glossary was compiled by R. Sidney Boone,Research Forest Products Technologist.
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Baffle—A piece of canvas, metal, or wood used for de-flecting, checking, or otherwise regulating the flow ofair.
Load—A rigid or flexible panel placed to minimizethe amount of air short-circuiting over, under, andbetween the kiln loads of lumber or other woodproducts.
Balance—An instrument used for measuring mass orweight and often used in weighing moisture content sec-tions and kiln sample boards.
Bark—Outer layer of it tree, which consists of a thin,living inner part and a dry, dead outer part that is gen-erally resistant to moisture movement.
Bastard sawn—Lumber in which the annual growthrings make angles of 30° to 60° with the surface of thepiece.
Blue stain—See Stain, blue.
Bolster—A piece of wood, generally a nominal 4 in.in cross section, placed between stickered packages oflumber or other wood products to provide space for theentry and exit of the forks of a lift truck.
Bow—The distortion in a board that deviates fromflatness lengthwise but not across its faces.
Boxed heart—The term used when the pith falls en-tirely within the outer faces of a piece of wood any-where in its length. Also called boxed pith.
Bright—A term applied to wood that is free fromdiscolorations.
British thermal unit (Btu)—The amount of heatnecessary to raise 1 lb of water 1 °F in temperature.
Bulb—The temperature-sensitive part of a thermo-static control system.
Control—The sensing part of the controlling system,located in the kiln, which contains a temperature-sensitive liquid, gas, or electronic sensor.
Controlling dry—The bulb that controls the dry-bulbtemperature.
Controlling wet—A bulb, kept completely covered atall times with a clean, water-saturated wick or poroussleeve, which automatically controls the wet-bulbtemperature.
Double-end control—Control bulbs, usually located ineach longitudinal half of the kiln, which control kilntemperatures for their respective zone, independent ofeach other.
Dual control—Two bulbs of a Y-shaped control sys-tem. They are usually located on each kiln wall di-rectly opposite each other and control the tempera-ture of the entering air regardless of the direction ofair movement.
Recorder—The temperature-sensitive part of a sys-tem that records but does not control kiln conditions.
Recorder-controller—A bulb attached by means of acapillary tube to a recording-controlling instrument.
Zone control—A bulb or sensor that controls thetemperature within a zone.
Bypass pipe or duct—A pipe or chamber that per-mits air, steam, or water to be diverted from their reg-ular channels.
Cam—A rotating plate so cut that the edges act as aguide for a pin rolling along the edge. In drying con-trol instruments, the pin is employed to control tem-peratures and/or moisture conditions in the dryingchamber.
Cambium—The one-cell-thick layer of tissue betweenthe bark and wood that repeatedly subdivides to formnew wood and bark cells.
Capacitance, electrical—The property of an electri-cal conductor or configuration of conductors that allowsit to store potential energy in the form of an electricalfield.
Capillary action—The combination of solid-liquid ad-hesion and surface tension by which a liquid is elevatedin a vertical tube or moved through a cellular structure.
Casehardening—A condition of stress and set inwood in which the outer fibers are under compres-sive stress and the inner fibers under tensile stress, thestresses persisting when the wood is uniformly dry.
Casehardening, reverse—A final stress and set con-dition (in lumber and other wood items) in which theouter fibers are under a tensile stress and the innerfibers are under a compressive stress as a result of over-conditioning.
Cell—A general term for the minute units of woodstructure, including wood fibers, vessel segments, andother elements of diverse structure and function, havingdistinct cell walls and cell cavities.
Charge—See Kiln charge.
Chart, recorder—A sheet, usually circular, on whicha graphic record of kiln temperatures is transcribed.
Check—A lengthwise separation of the wood thatusually extends across the rings of annual growth andparallel to the wood rays. Checks result from dryingstresses.
Surface—A check starting on a wide-grain surfaceand extending into the interior of a board.
End—A check starting on an end-grain surface andextending along the length of a board.
Internal—Checks originating in the interior of a pieceof wood or extensions of surface and end checks.
Circulation, air—The movement of air within a kilnby either natural or mechanical means.
Direction of—The direction of movement of airthrough the kiln charge, expressed as longitudinal,transverse, or vertical.
Forced—The movement of air within a kiln by me-chanical means.
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Longitudinal— Air movement through the kiln chargeto be expressed as front to rear or rear to front.
Natural— The movement of air within a kiln by nat-ural means. Reversible. Capable of change in the di-rection of air movement.
Transverse— Air movement through the kiln chargefrom wall to wall to be expressed as right to left orleft to right.
Vertical— Air movement through the kiln charge fromtop to bottom or bottom to top.
Co-generation— The simultaneous generation of elec-tricity and low-pressure steam for on-site use, such asin drying.
Coil header (or manifold)—A pipe fitting to which anumber of pipes are connected on one side.
Coil, intermittent operation of—The alternateopening and closing of the valve that controls steamflow into the coil.
Coil, pipe—One or more runs of pipes, the function ofwhich is to heat the air in the kiln.
Booster— A supplementary coil, usually located be-tween tracks of a multiple-track kiln, used to addheat to air that has already moved across a trackloadof lumber.
Ceiling—A coil placed near the kiln ceiling to warmthe ceiling and roof, thus preventing moisturecondensation.
Double-end—Coils usually extending half the lengthof the kiln from both ends and usually operating asseparate units.
Multiple-return-bend header—A coil usually with thedischarge header located below the supply header,the pipes running back and forth with a 180° elbowat the bends.
Plain header (horizontal or vertical)—A coil consist-ing of a supply and discharge header at opposite endswith the pipes running from one to the other.
Single-return-bend header (horizontal or vertical)—Acoil with the discharge header usually located underor on the side of the supply header, the pipes runningfrom the supply header to a 180° bend and back tothe discharge header.
Coil radiating surface—The entire uninsulated sur-face area of a heating coil.
Collapse— The severe distortion or flattening of singlecells or rows of cells in wood during drying, often evi-denced by a caved-in or corrugated appearance of thesurface of the piece.
Compression failure—Rupture of the wood structureresulting from excessive compression along the grain. Itmay develop as a result of bending in the living tree orduring felling. In surfaced lumber, compression failuresappear as fine wrinkles across the face of the piece.
Compression wood—Abnormal wood formed on thelower side of branches and inclined trunks of softwoodtrees. It has relatively wide, eccentric growth rings withlittle or no demarcation between springwood and sum-merwood and more than normal amounts of summer-wood. Compression wood shrinks more than normalwood longitudinally, causing bow, crook, and twist.
Condensate— Water formed by the cooling of steam.
Conditioning—See Stresses, relief of.
Conditioning treatment—A controlled hightemperature-high relative humidity condition used ina dry kiln after the final stage of drying to bring abouta uniform moisture distribution in the boards and torelieve drying stresses.
Conduction, heat—Transmission of heat through orby means of a conductor.
Controller—An instrument that automatically con-trols kiln temperatures.
Convection, heat—Transfer of heat from heating coilsto lumber by means of air.
Course, lumber—A single layer of lumber.
Crib— A stickered kiln truckload of lumber usuallystacked onto kiln trucks and kiln bunks to make a load6 to 10 ft wide, 10 to 16 ft high, and as long as thelumber being stacked.
Crook—A distortion of a board in which the edges de-viate from a straight line from end to end of the board.
Cup—A distortion of a board in which there is devia-tion from flatness across the width of the board.
Cycle, heating—The time intervening between succes-sive openings of a control valve.
Cycle, temperature—The time between the max-imum and minimum temperatures during a heatingcycle.
Decay—The decomposition of wood substance byfungi.
Advanced (or typical)—The older stage of decay inwhich the destruction is readily recognized becausethe wood has become punky, soft and spongy, stringy,ringshaked, pitted, or crumbly. Decided discolorationor bleaching of the rotted wood is often apparent.
Incipient— The early stage of decay that has not pro-ceeded far enough to soften or otherwise perceptiblyimpair the hardness of the wood. It is usually accom-panied by a slight discoloration or bleaching of thewood.
Defects, drying—Any irregularity occurring in or onwood, as a result of drying, that may lower its strength,durability, or utility value.
Degrade, kiln—A drop in lumber grade that resultsfrom kiln drying.
Dehumidification kiln—See Kiln, dehumidification.
Density—The weight of a body per unit volume.
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Depression, wet-bulb—The difference between thedry- and wet-bulb temperatures.
Desorption—The loss of adsorbed (hygroscopic) mois-ture from wood to the surrounding air.
Desuperheater— A device for removing from steamthe heat in excess of that required for saturation at agiven pressure. In kiln drying, atomized water injectionis often used to eliminate the superheat from the steamemployed for humidification.
Dew point—The temperature at which steam or watervapor begins to condense.
Diamonding—A form of warp in which the cross sec-tion assumes a diamond shape.
Diffuse-porous wood—A hardwood in which thepores tend to be uniform in size and distributionthroughout each annual ring or to decrease in sizeslightly and gradually toward the outer border of thering.
Diffusion— Spontaneous movement of heat, moisture,or gas throughout a body or space. Movement is fromhigh to low points of temperature or concentration.
Direct fired—A method of heating a dry kiln wherethe hot gases produced by burning gas, oil, or woodwaste are discharged directly into the kiln atmosphere.
Dry-bulb temperature—The temperature of the airindicated by any type of thermometer not affected bythe water vapor content or relative humidity of the air.
Drying, air—Process of drying lumber by natural con-ditions in a yard or in an open unheated shed.
Drying, precision kiln—Process of drying wood inwhich controlled procedures are followed in order toobtain a stress-free product that has a desired moisturecontent and has suffered a minimum loss in strength.
Drying in transit—The partial or complete kiln dry-ing of lumber by a drying facility located between theshipping and fabrication points.
Drying rate—The amount of moisture lost from thelumber per unit of time.
Duct, air—A rectangular, square, or circular passage-way to conduct air.
Electrodes—In testing wood for moisture content,devices made of an electrical-conducting material forconnecting wood into the electric circuit of an electricmoisture meter. In high-frequency heating, metal platesused to apply the electric field to the material beingheated.
Insulated—In testing wood for moisture content, spe-cial electrodes for use with resistance-type electricmoisture meters that are coated with an insulatingmaterial to limit or control the point of contact be-tween the electrode and the wood.
End check—See Check, end.
End coating—A coating of moisture-resistant materialapplied to the end-grain surface to retard end drying ofgreen wood or to minimize moisture changes in driedwood.
Equalization— Bringing the pieces of lumber in a kilncharge to nearly uniform moisture content. See Treat-ment, equalization.
Equilibrium moisture content—The moisture con-tent at which wood neither gains nor loses moisturewhen surrounded by air at a given relative humidityand temperature.
Evaporation—The change from the liquid to thevapor form.
Extractives—Substances in wood that are not an in-tegral part of the cellular structure and can be removedby solution in hot or cold water, ether, benzene, orother solvents that do not react chemically with woodsubstance.
Fan
Centrifugal fan—A device for moving air by means ofa rotating wheel or impeller, which gives a centrifugalaction to the air being moved. Frequently used forpressure venting.
Deck fan—Fan mounted with fan impeller horizon-tally oriented in a horizontal panel opening, such as afloor or ceiling opening.
Disk or propeller fan—An axial-flow fan with the airflowing through the impeller parallel to the shaftupon which the impeller is mounted. The impellerblade is designed to deliver about the same volume ofair in either direction of rotation.
Pitch—The angle a fan blade is set with respect tothe axis of the propeller fan shaft.
Shroud—Flanges around the wall opening for a diskfan impeller that give support to the wall and provideprotection for the impeller and personnel.
Fiber, wood—A comparatively long (<1/25 to1/3 in), narrow, tapering hardwood cell closed at bothends.
Fiber saturation point—The stage in the drying orwetting of wood at which the cell walls are saturatedwith water and the cell cavities are free from water.It normally applies to an individual cell or group ofcells, not to whole boards. It is usually taken as ap-proximately 30 percent moisture content, based on theweight of ovendried wood.
Flat-sawed— Lumber sawed in a plane approximatelyperpendicular to a radius of the log. See Grain.
Flitch—A portion of a log sawed on two or more sidesand intended for remanufacture, as into lumber orveneer.
Fluctuation, steam pressure—Variation of steampressure.
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Flue, vertical—A vertical space, usually 6 in or lessin width and extending the length and height of a kilntruckload or package of lumber.
Grain—The general direction of the fibers in wood orlumber. When used with qualifying adjectives, it hasspecial meanings concerning the direction of the fibersor the direction or size of the growth rings. Specificterms for fiber and growth ring direction are as follows:
Cross grain—Grain deviating from a line parallel tothe sides of the piece.
Diagonal grain—A form of cross grain resulting fromsawing at an angle with the bark of the log.
Interlocked grain—A form of spiral grain in which thefiber direction gradually alternates from right-hand toleft-hand spiral and back again in adjacent groups ofannual rings.
Spiral grain—A form of cross grain that results dur-ing the growth of the tree; the fibers take a spiralcourse about the trunk instead of the normal verticalcourse.
Straight grain—Grain parallel to the sides of thepiece.
Coarse grain—wood in which the growth rings arewide or have major differences in density and colorbetween springwood and summerwood.
Edge grain (or vertical grain)—The grain in lumberproduced by quartersawing so that ‘the edges of thegrowth rings are exposed on the widest faces of thepiece, and the rings form angles of 45° to 90° withthe widest faces.
Fine grain—Wood in which the growth rings arenarrow and inconspicuous.
Flat grain—The grain in lumber produced by flatsawing so that the tangential faces of the growthrings are exposed on the widest faces of the piece andthe rings form angles of less than 45° with the widestfaces.
Green lumber (or grass green)—Lumber cut fromfreshly felled trees.
Growth rate—The rate at which a tree has laid onwood, measured radially in the tree trunk or in the ra-dial direction in lumber. The unit of measure in use isthe number of annual growth rings per inch.
Hardwoods—Woods produced by one of the botanicalgroups of trees that have broad leaves in contrast to theneedles or scalelike leaves of the conifers or softwoods.The term has no reference to the actual hardness of thewood.
Heartwood—The wood extending from the pith tothe sapwood, the cells of which no longer participatein the life processes of the tree. Heartwood may be in-filtrated with gums, resins, and other materials thatusually make it darker and more decay resistant thansapwood.
Heat
Conduction—The transfer of heat within a materialor from one material to another in contact with it.
Consumption—In kiln drying, the total heat requiredto heat the wood and the kiln structure and to evap-orate the water from the wood, as well as heat losses,including venting.
Convection—The transfer of heat to or from a mate-rial by reason of the mass movement of a fluid or gasin contact with it. In kiln drying wood, air is gener-ally used as the medium of exchange.
Exchanger—Normally a device for transferring heatfrom one fluid or gas to another without allowingthem to mix. Examples: fin pipes, radiators.
Latent heat of evaporation—The heat requiredto change water into steam without a tempera-ture change and at constant pressure; for exam-ple, water at 212°F and at atmospheric pressurechanges to steam at the same temperature by adding970.3 Btu/lb.
Loss—The amount of heat lost by transmissionthrough the building walls, roof, doors, floor, andvents.
Radiation—The transfer of heat by waves throughspace by reason of a temperature difference existingbetween two bodies. In common terminology in kilndrying, all forms of heat transfer are often lumpedinto one term--radiation of heat.
Sensible heat—n kiln drying, the amount of heatrequired to raise the kiln and lumber to drying tem-perature. (Syn: enthalpy)
Total heat—The latent heat of the water vapor in theair-water vapor mixture plus the sensible heat of themixture.
Transfer coefficient—An experimentally derived num-ber for a particular system that quantifies the rate ofheat exchange between two zones.
High-temperature drying—Use of dry-bulb temper-atures of 212°F or more.
Honeycombing—Checks, often not visible on the sur-face, that occur in the interior of a piece of wood, usu-ally along the wood rays. See Ring failure.
Humidity, absolute—The weight of water vapor perunit volume of space.
Humidity, relative—Ratio of the amount of watervapor present in the air to that which the air wouldhold at saturation at the same temperature. It is usu-ally considered on the basis of the weight of the vapor,but for accuracy it should be considered on the basis ofvapor pressures.
Hygrometer—An instrument for measuring relativehumidity, often consisting of dry-bulb and wet-bulbthermometers.
Hygroscopicity—The property of a substance thatpermits it to adsorb and retain moisture.
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Hysteresis— The tendency of wood exposed to anyspecified temperature and relative humidity conditionsto reach equilibrium at a lower moisture content whenabsorbing moisture from a drier condition than whenlosing moisture from a wetter condition.
Implosion— The caving in of kiln doors and/or wallsbecause of a sudden marked decrease in pressure belowatmospheric within the kiln. Normally occurs at kilnstartup with very cold lumber or when restarting afterfan failure early in a run.
Indirect fired—A method of heating a dry kiln wherea hot fluid (steam, water, or oil) flows into the kilnin pipes and gives off its heat to the kiln atmospherethrough a suitable radiating surface.
Infiltration, cold air—The uncontrolled and inadver-tent entry of cold air into the dryer through cracks inthe walls and ceiling, or leaky doors, or openings otherthan the fresh-air intake.
Juvenile wood—The initial wood formed adjacent tothe pith, characterized often by lower specific gravity,lower strength, higher longitudinal shrinkage, and dif-ferent microstructure than mature wood.
Kiln—A heated chamber for drying lumber, veneer,and other wood products in which temperature andrelative humidity are controlled.
Automatically controlled—A dry kiln in whichdrying conditions are controlled by the action ofthermostats.
Compartment—A dry kiln in which the total chargeof lumber is dried as a single unit. At any given time,the temperature and relative humidity are uniformthroughout the kiln.
Conventional-temperature—A kiln for drying lumberand other wood products typically operated in therange of 110 to 180°F.
Dehumidification—A type of kiln in which themoisture removed from the lumber, or other woodproduct, is condensed out of the circulating air,which is reheated instead of being exhausted to theatmosphere.
Elevated-temperature—A kiln for drying lumber andother wood products typically operated in the rangeof 110 to 211°F.
Forced-circulation—A dry kiln in which the air is cir-culated by mechanical means.
High-temperature—A kiln for drying lumber andother wood products operated at temperatures above212°F.
Low-temperature—A kiln for drying lumber or otherwood products typically operated in the range of 85and 120°F.
Manually controlled—A dry kiln in which dryingconditions are controlled by the manual operationof valves and ventilators.
Multiple-track—A dry kiln equipped with two ormore tracks.
Natural-circulation—A dry kiln in which air circula-tion depends on the power of gravity and the varyingdensity of air with changes in its temperature andmoisture content.
Package-loaded—A trackless compartment kiln fordrying packages of stickered lumber or other woodproducts. The dryer usually has large doors that canbe opened so that the kiln charge can be placed in orremoved from the dryer by forklift trucks.
Progressive—A dry kiln in which the total charge oflumber is not dried as a single unit hut as severalunits, such as kiln truckloads, that move progres-sively through the kiln. The temperature is lowerand the relative humidity higher at the entering end(green end) than at the discharge end (dry end).
Reversible-circulation—A dry kiln in which the di-rection of air circulation can be reversed at desiredintervals.
Side-loaded—See Package-loaded.
Single-track—A dry kiln equipped with one track.
Track-loaded—A compartment kiln for drying stick-ered lumber that is stacked on kiln trucks, which arerolled into and out of the kiln on tracks.
Vacuum—A compartment kiln in which lumber isdried at less than atmospheric pressure either contin-uously or intermittently during the drying cycle.
Kiln charge—The total amount of lumber or wooditems in a dry kiln.
Kiln charge, mixed—Same as kiln charge but com-posed of more than one species or thickness of lumberor wood items.
Kiln drying—Process of drying lumber in a dry kiln.
Kiln leakage—The undesirable loss of heat and vaporfrom a kiln through badly fitted doors and ventilatorsor through cracks in the walls and roof.
Kiln run—The term applied to the drying of a singlecharge of lumber.
Kiln sample—A section 30 in or more in length cutfrom a sample board and placed in the kiln charge sothat it can be removed for examination, weighing, andtesting.
Controlling—Some of the wettest samples used tocontrol the drying. The number depends on the totalnumber of samples used and the composition of thekiln charge.
Driest—The kiln sample with the lowest moisturecontent.
Fastest drying—The kiln sample that loses thelargest amount of moisture in a given period.
Pocket—A space provided for the kiln sample in thekiln packages of lumber.
Slowest drying—The kiln sample that loses the leastamount of moisture in a given period.
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Weight, current—The weight of a kiln sample atgiven times during the drying process.
Weight, final—The weight of a kiln sample after thecompletion of drying.
Weight, green (initial, original)—The weight of a kilnsample prior to kiln drying regardless of its moisturecontent.
Wettest—The kiln sample with the greatest amountof moisture.
Knot—That part of a branch that has become over-grown by the body of a tree. The shape of the knotdepends on the angle at which the branch is cut.
Laminar air—See Air, laminar.
Loading, cross-piled—Lumber piled on kiln trucksand placed in a dry kiln with the long axis of the loadperpendicular to the length of the kiln.
Loading, end-piled—Lumber piled on kiln trucks andplaced in a dry kiln with the long axis of the load par-allel to the length of the kiln.
Longitudinal— Generally, the direction along thelength of the grain of wood. A longitudinal sectionmay be a plane either tangential or radial to the growthrings.
Lumber, kiln-dried—Lumber that has been dried ina dry kiln to a specified moisture condition.
Lumber, shipping-dry—Lumber and other woodproducts that have been air or kiln dried to a suffi-ciently low moisture content to prevent stain, mold,and decay in transit; generally taken to be 25 percentmoisture content or less.
Lumber storage room—A room maintained withinspecified equilibrium moisture content limits so thatlumber stored in it will not gain or lose moisture be-yond fixed limits.
Makeup air—Ambient air that replaces vent air usedto exhaust water vapor being released within the dryer.
Meter, moisture—An instrument used for rapid de-termination of the moisture content in wood by electri-cal means.
Mineral streak—An olive to greenish-black or browndiscoloration of undetermined cause in hardwoods, par-ticularly hard maples; commonly associated with birdpecks and other injuries; occurs in streaks usually con-taining accumulations of mineral matter.
Moisture content of wood—Weight of the watercontained in the wood, expressed as a percentage ofthe weight of the ovendried wood.
Average—The percentage of moisture content of asingle piece of wood or the sum of the moisture con-tents of a number of pieces divided by their number.
Core—The moisture content of the inside part of amoisture section remaining after a shell one-fourththe thickness of the section has been removed.
Determination of—The testing of lumber to deter-mine the amount of moisture present. This is usuallyexpressed in terms of percent of the ovendry weight.
Find—The moisture content of the wood at the endof kiln drying.
Green—The moisture content of wood in the livingtree or freshly sawn wood.
Initial—The moisture content of the wood at thestart of kiln drying.
Shell—The moisture content of the outer one-fourthof the thickness of a moisture section.
Moisture distribution—The variation of moisturecontent throughout a piece of wood, usually from faceto face but sometimes from end to end or from edge toedge.
Moisture gradient—A condition during drying inwhich the moisture content uniformly decreases fromthe inside toward the surface of a piece of wood. Also aterm used specifically to denote the slope of the mois-ture content distribution curve.
Moisture gradient, reverse—A condition followingmoisture regain in which the moisture content is higherat the surface than inside the wood.
Moisture meter—See Meter, moisture.
Moisture range—The difference in moisture contentbetween the driest and wettest boards or samples.
Moisture section—A cross section, 1 in. in lengthalong the grain, cut from a kiln or random sample andused to determine moisture content.
Initial weight of—The weight of a moisture sectionimmediately after being cut from a kiln sample orboard.
Ovendry weight of—The weight of a moisture sectionafter being ovendried to a constant weight.
Mold—A fungal growth on lumber taking place mainlyat or near the surface and, therefore, not typically re-sulting in deep discolorations. Growths are usually ashgreen to deep green in color, although black is common.
Old growth—Timber in or from a mature, naturallyestablished forest. When the trees have grown duringmost if not all of their individual lives in active com-petition with their companions for sunlight and mois-ture, the timber is usually straight and relatively free ofknots.
Ovendry—A term applied to wood dried to constantweight in an oven maintained at temperatures of from214 to 221°F.
Pervious wood—A wood through which moisturemoves readily.
Piling
Box—The flat piling of random-length boards on kilntrucks so that the ends of the completed load are invertical alignment. The longest boards are placed onthe outside of the load, and the shorter boards are
263
alternately placed with one end even with one end ofthe load or the other.
Edge—Piling lumber so that the broad face of theboard is vertical.
Flat—Piling lumber so that the broad face of theboard is horizontal.
Pipe
Condensate—The pipe on the downstream side ofheating coils and steam traps that carries condensateback to the boiler.
Feed—Usually the pipe conducting steam from thecontrol valve to the heating coils.
Fin—A heating pipe with many finlike projectionsthat increase the radiation surface.
Plain—A heating pipe with a smooth outer surface.
Steam-spray—A pipe with numerous holes or nozzlesthrough which steam is ejected to increase the rela-tive humidity in the kiln.
Pit—A relatively unthickened part of a wood cell wallwhere a thin membrane may permit liquids to readilypass from one cell to another. A “bordered” pit has anoverhanging rim that is not present in a “simple” pit.
Pitch—The mixture of rosin and turpentine or othervolatiles produced in the resin canals of pines and otherconifers. Term also applied to mixtures of nonvolatileliquids or noncrystalline solids and volatile oils in otherspecies.
Pocket—An opening, extending parallel to thegrowth rings, that contains or has contained pitch.
Streak—A well-defined streaky accumulation of pitchin the wood of certain softwoods.
Pith—The small, soft core occurring in the structuralcenter of a tree trunk, branch, twig, or log.
Plainsawed— Another term for flat-sawed or flat-grained lumber.
Platen pressdrying—Contact heating of wood be-tween heated metal plates to affect drying while underrestraint.
Plenum chamber—The space between the lumberstack and kiln wall for air circulation on the pressureside of a fan or blower in which the air is maintainedunder pressure.
Pore— The cross section of a specialized hardwood cellknown as a vessel. See Vessels.
Predryer—A type of low-temperature dryer. Stickeredloads or unit packages of lumber or other wood prod-ucts are placed in a large building provided with fans,heating system, and vents such that air of a given tem-perature and humidity can be circulated through thelumber.
Pressdrying, platen—See Platen pressdrying.
Psychrometer—An instrument with both wet-bulband dry-bulb thermometers for determining the amountof water vapor in the atmosphere.
Psychrometric char—A table or graph used to re-late the absolute humidity, relative humidity, and dry-and wet-bulb temperatures.
Quartersawed— Lumber sawed so the wide faces areapproximately at right angles to the annual growthrings. See Grain.
Radial— Coincident with or generally parallel to a ra-dius of the tree from the pith to the bark. A radialsection is a lengthwise section in a plane that passesthrough the pith.
Radiation—A term often used in kiln drying to de-scribe heat transfer from heating coils to lumber. Inthis common use, it is understood to include both con-vection and radiation heat transfer, although the for-mer is the most important in kilns.
Balanced—Construction and arrangement so as toensure equal radiating surface and uniform tempera-tures throughout the kiln.
Excessive—A greater amount of radiation thanrequired.
Flexible—The arrangement of the heating systeminto small coils equipped with hand valves that, whenopened or closed, permit rapid adjustment of the ra-diating surface to meet the required needs.
Raised grain—A roughened condition of the surfaceof dressed lumber in which the hard summerwood israised above the softer springwood but not torn loosefrom it.
Rays, wood—Strips of cells extending radially withina tree and varying in height from a few cells in somespecies to 4 in or more in oak. The rays primarily serveto store food and transport it horizontally in the tree.
Redry—A process in which material that has beendried but is at a moisture content level higher than de-sired is returned to the dryer.
Refractory—In wood, implies difficulty in processingor manufacturing by ordinary methods, difficulty indrying, resistance to the penetration of preservatives, ordifficulty in machining.
Relative humidity—See Humidity, relative.
Resin canal (or duct)—Intercellular passages thatcontain and transmit resinous materials. They extendvertically or radially in a tree.
Ring, annual growth—See Annual growth ring.
Ring failure (or separation)—A separation of thewood during drying. Occurs along the grain and paral-lel to the annual rings, either within or between rings;called honeycomb and ring check in some localities. SeeShake.
Ring-porous wood—Wood in which the pores of theearlywood (springwood) are distinctly larger than thoseof the latewood (summerwood) and form a well-definedzone or growth ring.
264
Sample—See Kiln sample.
Sample board—A board from which one or more kilnsamples will be cut, or a board taken from a kiln truck-load during drying for the purpose of cutting a mois-ture section.
Sap—The moisture in green wood, which contains nu-trients and other extractives in solution.
Sap stain—See Stain, blue.
Sapwood—The layer of wood near the outside of thelog that is actively involved in the life processes of thetree. Usually lighter in color than the heartwood.
Sapwood stain—See Stain, blue.
Seasoning— Removal of moisture from green wood,and in some cases relief of stresses, in order to improveits serviceability. (Syn: drying)
Second growth—Timber that has grown after the re-moval, whether by cutting, fire, wind, or other agency,of all or a large part of the previous stand.
Sensible heat—See Heat, sensible.
Set—A localized semipermanent deformation in woodcaused by internal tensile or compressive stresses.
Compression—Set, occurring during compression,that tends to give the wood a smaller than normaldimension after drying. Usually found in the inte-rior of wood items during the last stage of drying butsometimes in the outer layers after overconditioningor rewetting. Also caused by external restraint duringrewetting of dried wood.
Tension—Set, occurring during tension, that tends togive the wood a larger than normal dimension afterdrying. Usually occurring in the outer layers dur-ing the first stages. Also caused by external restraintduring drying of wet wood.
Shake—A separation along the grain, the greaterpart of which occurs between and within growth rings.Found in stumps and ends of freshly cut logs and greenlumber. See Ring failure.
Short circuiting of air—The movement of airthrough other than desired channels. Usually resultswhen a kiln charge is improperly loaded and/or baffled.
Shrinkage—The contraction of wood caused by dryingthe material below the fiber saturation point.
Longitudinal—Shrinkage along the grain.
Radial—Shrinkage across the grain, in a radialdirection.
Tangential—Shrinkage across the grain, in a tangen-tial direction.
Sinker—A log that sinks or has low buoyancy inwater.
Sinker stock—Green lumber or other green sawmillproducts that will not float in water. Sinker stock maybe sawn from sinker logs that were water-logged duringponding or from freshly cut logs containing wetwood.The green moisture content is abnormally high, and
the lumber tends to dry slowly and is prone to developchecks and honeycomb.
Softwood— Wood produced by one of the botanicalgroups of trees that, in most species, have needle orscalelike leaves.
sorter
Drop—A mechanical lumber-sorting device that sortslumber for thickness, width, and length by droppingthem into separate compartments accordingly.
Edge—A mechanical lumber-sorting device consistingof grooves or slots in which the lumber is placed onedge. Lines of live rolls, arranged under the slots,carry the lumber to the desired bin or compartment.
Tray—A mechanical lumber-sorting device consistingof a series of trays one above the other into which thelumber is ejected by either mechanical or electricalsignaling devices.
Specific gravity—The ratio of the ovendry weight of apiece of wood to the weight of an equal volume of water(39°F). In drying, specific gravity values are usuallybased on the volume of the green wood.
Split—A lengthwise separation of the wood, caused bythe tearing apart of the wood parallel to the wood rays.
Spray line—A plain pipe of varying sizes and lengthsthat is drilled with holes of various sizes and spacingthrough which steam is injected into the kiln.
Springwood (earlywood)—The part of the annualgrowth ring that is formed during the early part of theseason’s growth. It is usually less dense and mechani-cally weaker than summerwood.
Stain—A discoloration in wood that may be causedby such diverse agencies as micro-organisms, metal, orchemicals. The term also applies to materials used toimpart color to wood.
Blue (sap stain, sapwood stain)—A bluish or grayishdiscoloration of the sapwood caused by the growthof certain dark-colored fungi on the surface and inthe interior of the wood, made possible by the sameconditions that favor the growth of other fungi.
Chemical—A general term including all stains thatare due to color changes of the chemicals normallypresent in the wood, such as pinking of hickory andbrowning of some softwoods, particularly the pines.
Chemical, brown—A chemical discoloration of wood,which can occur during the air drying or kiln dryingof several softwood species, caused by the concentra-tion and modification of extractives.
Iron-tannate—A bluish-black surface stain on oakand other tannin-bearing woods following contact ofthe wet wood with iron, or with water in which ironis dissolved.
Mineral—An olive to greenish-black or brown discol-oration in hardwoods, particularly maple, caused bybird peck or other injury and found either in massdiscoloration or mineral streaks. The mineral associ-
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ated with such streaks is frequently calcium oxalate,which has a tendency to dull machining knives.
Sticker—A gray to blue or brown chemical stain oc-curring on and beneath the surface of boards wherethey are in contact with stickers (also fungal sapstain when found only in the sticker area).
Water—A yellowish to blackish surface discolorationcaused by water that dripped onto the wood duringdrying.
Weather—A very thin grayish-brown surface dis-coloration on lumber exposed for a long time to theweather.
Steam—The vapor into which water is converted whenheated.
Exhaust—Steam that has already passed through asteam engine or machine.
Flash—The reevaporation of hot water produced byexcess heat when the water is discharged to a lowerpressure.
Live—Steam obtained directly from the boiler.
Superheated—Steam at a temperature higher thanthe saturation temperature corresponding to thepressure.
Steam binding—The presence of steam in the drainline between the heating coil and trap, which temporar-ily prevents the drainage of condensate and air fromthe coil.
Sticker— A wooden strip placed between the coursesof lumber in a kiln load and at right angles to the longaxis of the boards to permit air circulation.
Alignment—The placing of stickers in a pile, pack-age, or truckload of lumber so that they form verticaltiers.
Spacing—The distance between stickers measuredfrom center to center.
Stress, drying—An internal force, exerted by eitherof two adjacent parts of a piece of wood upon the otherduring drying, caused by uneven drying and shrinkage,and influenced by set.
Tensile—Stress in the outer layers of wood duringthe early stages of drying when the layers are tryingto shrink but are restrained by the still-wet interiorregion; also, the stress in the interior layers later indrying as they try to shrink and are restrained by theset outer shell.
Compressive—Stress found in the interior region ofwood during the early stages of drying, caused by theshrinking of the outer shell; also, stress in the outerlayers later in drying caused by the shrinking of theinterior.
Stress free—Containing no drying stresses.
Stress section—A cross section of a sample that is cutinto prongs of equal thicknesses, from face to face.
Stresses, relief of—The result of a conditioning treat-ment, following the final stage of drying, which causes aredistribution of moisture and a relief of the sets.
Stresses, reversal of—The normal change from ten-sion in the surface and compression in the center tocompression in the surface and tension in the center.
Summerwood (latewood)—The part of the annualgrowth ring that is formed during the latter part of thegrowing season. It is usually denser and mechanicallystronger than springwood.
Surface check—See Check, surface.
Tangential— Coincident with or generally parallel to a.tangent at the circumference of a tree or log, or growthrings. A tangential section is a longitudinal sectionthrough a tree perpendicular to a radius.
Temperature— Degree of hotness or coldness.
Cold zone—The lowest entering-air dry-bulb temper-ature in the kiln.
Drop across the load—The reduction in the dry-bulbtemperature of the air as it flows through the loadand is cooled by evaporating moisture from the loadof lumber.
Dry-bulb—The temperature of the kiln air.
Hot zone—The highest entering-air dry-bulb temper-ature in the kiln.
Longitudinal variation of—The range of entering-airdry-bulb temperatures in a kiln measured along thekiln length.
Wet-bulb—The temperatures indicated by any tem-perature measuring device, the sensitive element ofwhich is covered by a smooth, clean, soft, water-saturated cloth (wet-bulb wick or porous sleeve).
Temperature gradient, longitudinal—A term usedto denote longitudinal temperature differences within adry kiln.
Tension wood—A type of wood found in leaning treesof some hardwood species, characterized by the pres-ence of fibers technically known as “gelatinous” and byexcessive longitudinal shrinkage. Tension wood fiberstend to “pull out” on sawed and planed surfaces, givingso-called fuzzy grain. Tension wood causes crook andbow and may collapse. Because of slower than normaldrying, tension wood zones may remain wet when thesurrounding wood is dry.
Texture—A term referring to the size of wood cells.Thus, “fine-textured” wood has small cells and “coarse-textured,” large cells. Where all the cells of a softwoodor all the pores of a hardwood are approximately thesame size, as seen on the cross section, the wood canbe called “uniform textured.” The term is sometimeserroneously used in combination with soft or hard.
Thermocouple— A temperature-sensing device madeby soldering or fusing two dissimilar metal wires to-gether and connecting the wires to a potentiometer or
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similar device, thereby determining the temperatureat the junction. Copper-constantan thermocouples areusually used in dry kiln work.
Tracheids—The elongated cells that make up thegreater part of the wood of the softwoods; frequentlyreferred to as fibers.
Transverse—The directions in wood at right angles tothe wood fibers or across the grain, including radial andtangential directions. A transverse or cross section is asection through a tree or timber at right angles to thepith. It has an end-grain surface.
Trap, steam—A device that discharges air and con-densate from steam-heating coils but limits the passageof steam.
Treatment, equalization—A controlled temper-ature and relative humidity condition used in a drykiln at the end of drying to stop the drying of the dri-est boards while allowing the wettest boards to con-tinue drying, thus reducing the moisture range betweenboards.
Treatment, steaming—Spraying steam directly intothe kiln to attain a condition at or near saturation inthe initial stages of kiln drying to retard the growthof mold. Also used to increase the rate of heating coldlumber. Sometimes used needlessly during other stagesof drying to restore surface moisture, and often usedwithout proper control to partially relieve stresses atthe end of drying.
Twist—A form of warp caused by the turning or wind-ing of the edges of a board so that the four corners ofany face are no longer in the same plane.
Tyloses—Extensions of parenchyma cells into thepores or vessels of some hardwoods, notably white oakand black locust, prior to or during heartwood form-tion. They tend to prevent or greatly retard moisturemovement through the vessels.
Vacuum kiln—See Kiln, vacuum.
Vapor barrier—A material with a high resistance tovapor movement, such as asphalt-impregnated paper,that is used in combination with insulation to controlcondensation.
Vapor pressure—The pressure exerted by a vaporwhen the rates of condensation and evaporation are inequilibrium between the liquid and vapor state.
Ventilator (or vent)—An opening in the kiln roof orwall, or in the blower duct work, that can be opened orclosed in order to maintain the desired relative humid-ity condition within the kiln.
Automatic control—A ventilator that is opened orclosed by a thermostat.
Linkage—The adjustable, pivoted rods connecting thevent cover to an air valve or to a hand-operated level,which facilitate the opening and closing of the vents.
Manual control—A ventilator that is opened or closedby hand.
Vessels—Wood cells in hardwoods of comparativelylarge diameter that have open ends and are set oneabove the other so as to form continuous tubes. Theopenings of the vessels on the surface of a piece of woodare usually referred to as pores.
Virgin growth—The original growth of mature trees.
Wane—Presence of bark or the lack of wood from anycause on the edge or corner of a piece.
Warp—Any variation from a true or plane surface.Warp includes cup, bow, crook, twist, and diamonding,or any combination thereof.
Water, bound (adsorbed, hygroscopic)—Moisturethat is bound by adsorption forces within the cell wall;that is, the water in wood below the fiber saturationpoint.
Water, free—Moisture that is held in the cell cavitiesof the wood, not bound in the cell wall.
Water box—A water container that is mounted underthe wet bulb and supplies water to the wick.
Water pocket—An area of unusually high moisturecontent in lumber; pockets are of various sizes andshapes. Also called wet pocket.
Waterlogging—The presence of water in steam coils,which interferes with the normal flow of steam and seri-ously affects the heating efficiency of the coil.
Wet-bulb temperature.—See Temperature.
Wetwood—Green wood with an abnormally highmoisture content that generally results from infectionsin living trees by anaerobic bacteria, but may also re-sult from water logging during log ponding. Wetwoodcan occur in both softwoods and hardwoods; the greenlumber is usually difficult to dry without defects. Al-though difficult to recognize, wetwood is often charac-terized by a translucent, water-soaked appearance anda sour or rancid odor.
Wood—The hard material between the pith and thebark in the stems and branches of trees, made up of avariety of organized hollow cells and consisting chemi-cally of cellulose, hemicelluloses, lignin, and extractives.
Wood, reaction—In wood anatomy, wood with moreor less distinctive anatomical characteristics, formedin parts of leaning or crooked stems and in branches.Reaction wood consists of tension wood in hardwoodsand compression wood in softwoods.
Wood, refractory—See Refractory.
Wood, ring-porous—See Ring-porous wood.
Wood substance—The extractive-free solid materialof which the cell walls of ovendried wood are composed.Wood substance has essentially the same specific grav-ity in all species.
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INDEX
Air circulationbaffles, 58
perforated, 115slotted, 115
disk fan, 57external fan, 58forced-air system, 44internal fan, 57
cross shaft, 57lineshaft, 57
kiln fan, 56natural draft, 44plenum chamber, 59plenum width, estimating, 59propeller-type fan, 57
Air movement, equipment for determining, 85Aluminum panels, inspection and maintenance, 88Baffling, 114-116Balance. See also Scale.
electronic top-loading, 75indicating, 78self-calculating, 78triple-beam, 75
Bastard sawn, 4Bending strength. See Modulus of rupture (MOR)Bevel siding, softwood schedules for, 144Biocides, 192Blending chamber. See Mixing chamber.Bourdon tubes, 62Box piling, 110Brown stain, 189, 222
softwood schedules for control, 143Cam controller, 64Capillary tubes, 62Casehardening, 12, 125-127
defects associated with, 199Cedar oil, softwood schedules for retaining, 143Centrifugal blower, 53Color, wood, 5Combustion chamber, 53Compartment kiln, 43-48Compressed air, quality, 91Compression wood, 5, 179Conditioning treatment, 145-147. See also
Kiln operation, conditioning treatment.Construction features, kiln, 50-61
air-circulation system, 56-59control valves. 56heating system, 51-54humidification system, 60
materials, 50steam traps, 54venting system, 60
Construction lumber, 6appearance, 6nonstress-graded, 6stress-graded, 6
Construction materials, kiln, 50aluminum, 50brick, 50concrete block, 50floor, 51foundation, 51plywood, 51poured concrete, 50wood, 51
Convection oven, for drying moisture sections, 79Conventional-temperature kiln, 49Defect, drying, 179
alkaline stain, 197bow, 187boxed-heart splits, 186broken knots, 198checked knots, 186chemical stain, 189, 194, 222chipped grain, 198collapse, 12, 183commercial woods, 1, 6crook, 187cup, 187diamonding, 187discoloration, 189-197
removal, 197end checks, 182heartwood discoloration, 194
chemical, 194fungal, 195
honeycomb, 12, 185loose knots, 186machining, relationship to, 198metallic stain, 197planer splits, 199raised grain, 199residual drying stress, 199ring failure, 186rupture of wood tissue, 180sapwood discoloration, 189
bacterial stain, 193blue stain, 191, 222brown stain. 190
269
chemical, 189fungal stain, 191oxidation, 189-191sticker marks, 194sticker stains, 194
shrinkage, 180splits, 182surface checks, 180torn grain, 198twist, 187uneven moisture content, 188warp, 180, 187water pocket, 188wetwood discoloration, 196
Degrade, drying, 179Dehumidification kiln, 66
advantages, 66disadvantages, 66energy demand, 249
Desuperheater, 61Dry-bulb temperature sensor, 62, 90Drying
benefits, vienergy consumption, 242, 246process in wood, 9rate, factors influencing, 10specialized approaches, 66specialized kiln types, 66-72
Drying conditions, equipment to control, 61-66automatic, 61computerized, 65fully automatic, 64manual, 66semiautomatic, 61zone, 65
Earlywood, 4Economizer, 60Edge grained, 4Electronic servo module, 63Elevated-temperature kiln, 49EMC wafer. See Equilibrium moisture content waferEnergy, 239-255
consumption, 239-246demand, 246-250
air drying followed by kiln drying, 249dehumidification drying, 249forced-air drying, 246-249platen press drying, 250predrying followed by kiln drying, 249solar drying, 249
vacuum drying, 250energy partition, 250fuel costs, 250heat capacity, 240heat of adsorption, 240heat loss, 243-244
dryer, 243steam delivery, 246steam generation, 246
vent air, 244heat transfer, 241latent heat of evaporation , 239, 243, 246overall heat transfer coefficient, 241practical application, 250sensible heat, 245, 246thermal conductivity, 240units, 239
Energy source. See Heating system, kiln.Equalizing treatment, 146-147, 213Equilibrium moisture content. See Moisture
content, equilibrium.Equilibrium moisture content wafer, 62, 69Fans, 56, 96Fiber saturation point, 8Fire prevention, 216Fire retardants, softwood, schedules for lumber
treated with, 144Firsts, 6Flatsawn, 4Flatgrained, 4Fungal stain, 191-193, 195-196
sterilizing treatments, 145Fungicides, 192Grade, lumber
B&BTR, 7C&BTR, 7Factory Select, 7Firsts, 6hardwood, 6KD, 7MC-15, 7S-Dry, 7S-GRN, 7Seconds, 6Select Shop, 7Selects, 6softwood, 6
Grain, 5Growth rings, 4Heartwood, 4Heat exchanger, 60Heating system, kiln, 49, 51-54
direct fire, 49, 53electricity, 50, 71hot oil, 50, 53hot water, 50, 53indirect, 52solar, 50, 69steam, 49, 52. See also Steam heating.
High-temperature kiln, 49Honeycomb, 13, 135, 185Humidity sensor, 60, 62, 66Hygrometer, 84Insects
lumber treatment, 233sterilizing treatment, 146
Instrument, kiln. See Recorder-controller.
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Inspection, dry kilns and equipment. SeeMaintenance, dry kilns and equipment.
Insulation values, 51, 67, 88Interlocked grain, 179Juvenile wood, 5, 179Kiln classification, 43-50
heating and energy source, 49operational techniques, 43temperature of operation, 48
Kiln loading, 114-116package loading, 116track loading, 115
Kiln operation, 207-217conditioning treatments, 146-147, 213-214
stress relief, 214temperature, 214time, 214
cooling a charge, 215drying process, 212-213equalizing treatments, 146-147, 213fire prevention, 216modifying schedules, 138-139, 215moisture content tests, 214part-time, 211safety precautions, 215starting, 209-210stress tests, 214-215temperatures, 211warmup, 210
Kiln performance, 119Kiln samples, 112, 117-131, 207
bolster space, 113controlling samples, 124cutting, 120determining moisture content, 120-122final test, 125intermediate test, 124-125, 213moisture content test, 214number, 119ovendrying, 121placement, 123pockets, 113recording data, 127schedule changes, 124selecting, 120stress test, 125, 214variability of material, 118-119
grain, 119heartwood, 118moisture content, 118sapwood, 118sinker stock, 118species, 118thickness, 118wetwood, 118
weighing, 121Kiln schedules, 133-178, 208
conditioning treatments, 145-147
dehumidification, 145equalizing treatments, 145-147hardwood, 135-141
air-dried lumber, 137alternative schedule, 140assembling a drying schedule, 136-137general, 135high-temperature, 140imported species, 140material considerations, 135maximum strength, 140modification, 138-139moisture content, 135-140predried lumber, 137special, 140-141steam-heated kiln, 135time, 140
homogeneous charge, 208mixed charge, 208modifying, 215selecting, 208softwood, 141-145
air-dried lumber, 142brown-stain control, 143, 190bundled short-length items, 144conventional-temperature, 143fire-retardant-treated, 144high-temperature, 143knotty pine lumber, 145large timbers, 144maximum strength, 144modifying, 142moisture content, 142poles, 144preservative-treated, 144resawed products, 144retaining cedar oil, 143setting pitch, 143special purpose, 143-145tank stock, 145time schedule, 142-145
sterilizing treatments, 145Knots, 186Latewood, 4Load-cell system, 65Log storage, 220-224
dry, 220-223debarked, 222with bark, 221
end coating, 221pond, 223staining, 221, 224submerged, 223-224transpiration drying, 222water sprinkling, 224wet, 223
Low-temperature kiln, 49
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Lumber storage, 225-234chemical treatment, 230-234closed, unheated shed, 229conditioned storage shed, 230equilibrium moisture content, 225green lumber, 226kiln-dried lumber, 226open shed, 228outdoor, 225partly dried lumber, 226pile covers, 228relative humidity, 225staining, 230temperature, 225transit, 234-235
rail, 235ship, 235truck, 234
treating, 231-234Maintenance, dry kilns and equipment, 87-102
air-circulation system, 95belts, 96fans, 96fan baffles, 96fan motors, 96fan shafts, 96load baffle system, 96oil lines, 96pulleys, 96
checklist, 99-101heating system, 92
direct fire, 94steam, 92
humidification system, 95kiln structure, 87
ceilings, 87doors, 89floors, 89rail supports, 89rails, 89roofs, 87walls, 87
kiln trucks, 96problems, locating, 97protective coating, 97recording-controlling instruments, 90
calibration, 91dry-bulb sensors, 90wet-bulb sensors, 90
steam-heated kiln. See Steam-heated kiln,maintenance.
traps, 54, 94valves, 93venting system, 95
Masonry, inspection and maintenance, 88
Maximum strength uses of lumber, 140, 144aircraft, 140, 144ladders, 140, 144sporting goods, 140, 144
Microwave oven, for drying moisture sections, 79Mineral streaks, 179Mixing chamber, 49, 53Modulus of elasticity (MOE), effect of drying
temperature, 180Modulus of rupture (MOR), effect of drying
temperature, 180MOE. See Modulus of elasticity (MOE)Moisture, 7, 8
bound water, 8free water, 8
Moisture content, 7-8classes, 135determining number of kiln samples, 119equilibrium, 8
and relative humidity, 8equipment for determining, 75
balances and scales, 75distillation equipment, 75drying ovens, 79electric moisture meters, 80saws, 79
test, 214Moisture content schedules, 134
hardwood, 135-141operation on, 212softwood, 141-145
Moisture meter, electric, 80dielectric power loss, 81resistance, 81
Moisture section. See Kiln samples.Mold, sterilizing treatment, 145MOR. See Modulus of rupture (MOR).Oven, drying, 79
electrically heated, 79steam-heated, 80
Package-loaded kiln, 46Pencil stock, softwood schedules for, 144Pine squares, softwood schedules for, 144Pitch, softwood schedules for setting, 143Pitch, fan, 58Pitch pockets, 179Pith, 4Plainsawn, 4Platinum RTD-type bulbs, 63Plenum chamber, 59Predryers, 68
advantages, 68disadvantages, 68
Preservatives, softwood schedules for lumbertreated with, 144
Presorting, to control uneven moisture content, 189Progressive kiln, 48
272
Psychrometer, 84Psychrometric chart, 39Quartersawn, 4Reconditioning, removing collapse-related defects, 184Recorder-controller, 62, 90
calibration, 91on-off mode, 62proportional valves, 62
Resistance, electrical, 13-15Ring shake, 179RTD. See Temperature sensors, resistance
temperature detector (RTD).Safety precautions, 215-216Sample boards, 120Sap, 7Sapwood, 4Scale. See also Balance.
self-calculating, 78Schedules, 133-178Seconds, 6Selects, 6Shingles, softwood schedules for, 144Shrinkage, 11
differential, 12and moisture content, 12
Side-loaded kiln, 44Sinker stock. See Wetwood.Slashgrained, 4Solar kiln, 69
absorber surface, 69collector, 69collector surface, 69energy demand, 249greenhouse type, 69
Solar radiation, 73Sorter, lumber, 106Sorting, lumber, 103-108
grade, 104grain, 104heartwood, 104length, 106moisture content, 104sapwood, 104species, 103thickness, 105wetwood, 104
Specific gravity, 10Specific heat, 15Spiral grain, 179Springwood, 4Stackers, 110-112
automatic, 110semiautomatice, 112
Stacking, lumber, 112clamps, 114restraining devices, 113weights, 113
Stains, 189-197Steam-heated kiln
hardwood schedules, 131-141maintenance, 92
feedline insulation, 92pipes, 92pressure gauges, 93regulators, 93traps, 93unions, 92valves, 93
Steam heating, 49, 52back pressure, 54blowdown valve, 54booster coils, 52check valve, 54condensate, 54condensate header, 52control valves, 56
gate, 56hand, 56
discharge header, 52distribution header pipes, 52finned pipe heating coils, 52reheat coils, 52return-bend header, 52return-bend heating system, 52single-pass coils, 52steam traps, 54strainer, 54traps, 54
gravity, 54impulse, 55inspection and maintenance, 93inverted bucket, 54mechanical, 54open bucket, 54thermodynamic, 55thermostatic, 55
Steam spray systems, maintenance, 95Steel components, maintenance, 88Sterilizing treatments, 145Stickering, 106-110
load supports, 107stain, 107, 194sticker guides, 109
Stickers, 107alignment, 109auxiliary, 109care, 110locations, 108material, 107moisture content, 107pinned, 114serrated, 114size, 107spacing, 108
273
Stiffness. See Modulus of elasticity (MOE).Storage, log and lumber, 219-238. See also
Log storage; Lumber storage.Stress, drying, 12-13
hydrostatic tension, 12Stress tests, 125-127, 214-215Summerwood, 4Superheat, 61Temperature
dry-bulb, controlling, 211equipment for determining, 82-84wet-bulb, controlling, 211
Temperature-limit switches, 53Temperature schedules
code numbers, 135, 139, 140shifting, 139steam-heated kilns, 135
Temperature-sensing bulbs, 61Temperature sensors
gas-filled, 62liquid-vapor, 62proper location, 90resistance temperature detector (RTD), 62
Tension wood, 5, 179Texture, 5Thermal conductivity, 15Thermal expansion, 16Thermocouple wire, Type-T, 66, 83Thermometer
dry-bulb, 84electric digital, 66, 83etched-stem, 66, 84glass-stemmed indicating, 66maximum, 66, 84wet-bulb, 84
Time schedules, 134, 140, 142determining number of kiln samples, 119operation on, 212softwood, 142-143
Track-loaded kiln, 44Traps, steam, 54, 93Unstackers, 112Vacuum kiln, 70
electrically heated conductive blankets, 71energy demand, 250high-frequency electrical energy, 50, 71steam-heated platens, 71
Venetian blinds, softwood schedules for, 144Venting system, 60
powered, 60pressure, 60static, 60
Vertical grained, 4Warehouse dryers, 68Water spray systems, maintenance, 95
Wet-bulb depression schedulesH-type, 138hardwoods, 135shifting, 138softwoods, 142steam-heated kilns, 135
Wet-bulb temperature sensor, 62Wetwood, 196
responsible for uneven moisture content, 189susceptible to collapse, 184
Woodcommercial species, 1electrical properties, 13structural features, 2-6
variations, 5thermal properties, 15
conductivity, 15expansion, 16specific heat, 15
weightcalculated, 10dry, 7wet, 7
Wood rays, 4
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