}^^A Au

DRY KILN

U.S. Dl'-T. CF AGRlCULTURt

Í I P R /> R Y

APR 2 8 1961

j CURRENT SERIAL RECOPOS

OPERATOR'S

MANUAL

AGRICULTURE

HANDBOOK

NO. 188

U.S. DEPARTMENT OF AGRICULTURE . FOREST SERVICE
DRY KILN OPERATOR'S

MANUAL

booth Thr«j{h Airicullurll Ptoireu

Agriculture Handbook No. 188

by

EDMUND F. RASMUSSEN,

Engineer

FOREST PRODUCTS LABORATORY,

FOREST SERVICE,

U.S. DEPARTMENT OF AGRICULTURE

{Maintained at Madison, Wisconsin,

in cooperation with the

University of Wisconsin)

1961

March

For sale by the Superintendent ot Documents, U.S. Government Printing Office, Wastiington 25, D.C.

Price $1
ACKNOWLEDGMENT

In the writing of this manual, the author received much A^alued assistance

and information from other staff members of the Forest Products Laboratory,

other units of the Forest Service, and companies and individuals active in the

lumber industry. The very magnitude of this help makes individual acknowledgment to all who contributed impractical.

Special acknowledgment is made to Ra}Tnond C. Rietz for overall planning

and supervision of the project; and to Edward C. Peck, John M. McMillen,

Harvey H. Smith, and A. C. Knauss for their helpful assistance, respectively,

in preparing the sections of the manuscript on storage of wood, kiln schedules,

kiln samples, and drpng defects. (Rietz, Peck, and McMillen are members

of the Forest Products Laboratory staff; Smith is a technologist with the

Pacific Southwest Forest and Range Experiment Station ; and Knauss is on the

staff of the Pacific Northwest Forest and Range Experiment Station.)

In addition, the author gratefully acknowledges the contribution of information, advice, and guidance received from the many manufacturers of drj^

kilns, control instruments, and auxiliary equipment, and from consulting

engineers and dry kün operators.

n
CONTENTS

Page

Introduction 1

Chapter 1. Properties of wood related to drying 3

Hardwoods and softwoods 6

Gross structural features of wood 6

Wood-moisture relations 8

How wood dries 10

Specific gravity and weight of wood 12

Shrinkage of wood 15

Electrical properties of wood 19

Color of wood 19

Literature cited 21

Chapter 2. Kiln types and features 23

Compartment kilns 24

Progressive kuns 32

General construction features 33

Equipment to control drying conditions 43

Literature cited 47

Chapter 3. Auxiliary kiln equipment 49

Equipment for determining moisture content 49

Equipment for determining temperatures 57

Equipment for determining air movement 61

Chapter 4. Inspection and maintenance of dry

kuns and equipment 63

The kiln structure 64

Recording-controlling instruments 67

Heating systems 73

Humidifying systems 75

Venting systems 75

Air-circulation systems 76

KUn trucks 77

Prevention of corrosion 77

Housekeeping and maintenance around dry kilns. 77

Locating troubles in kiln maintenance and

operation 77

Chapter 5. Stacking lumber and other items for

kiln-drying 79

Sorting 7g

Stickering the lumber 82

Mechanical stacking and unstacking equipment. _ 86

Box puing random-length lumber 88

Stacking lumber for various types of dry kilns... 92

Kiln sample pockets 96

Page

Chapter 5. Stacking lumber and other items for

kün drying—Continued

Cover boards on loads stacked for kiln-drying 96

Weights and restraining devices 96

Chapter 6. Kün samples 99

Variability of material 99

Number of kiln samples 100

Selection of kün samples 101

Preparation of kün samples 101

Placing samples in kün charges 103

Using kün samples during drying 104

Intermediate moisture content tests 105

Final moisture content and stress tests 105

Recording drying data 106

Literature cited 111

Chapter 7. Loading the dry kün 113

Loading natural-circulation compartment küns__ 113

Loading forced-circulation compartment kUns 113

Chapters. Kün schedules and drying time 117

Hardwood schedules 117

Softwood schedules 129

Sterüizing, equalizing, and conditioning treats

ments 141

Kün-drying time 144

Literature cited 145

Chapter 9. Drying defects 147

Effect of temperature on strength of wood 147

Defects occurring during drying 147

Effects of drying defects on machining 153

Chapter 10. Operating a dry kün 159

Compartment kün operation 159

Operation of a progressive dry kiln 166

Pulling a charge from a dry kün 166

Operating nues for safety 166

Prevention of fire in dry kuns 167

Chapter 11. Storage of wood 169

Log storage IQQ

Lumber storage igg

Literature cited 175

Glossary J77

Index __ _ 2g7

m
INTRODUCTION

Tlie modern dry kiln is a unique product of

research and development. It represents the

only practical means no^Y in wide use for rapid,

high-volume seasoning of lumber to conditions

essential for maximum serviceability in housing,

farm structures, furniture, vehicles, ships and

boats, sporting goods, and many other wood products. In its development. Forest Service research

begun more than half a centuiy ago has been

a major factor.

A well-desigiied drj^ kiln, properly operated,

can in a few days or weeks transform green

lumber fresh from the forest into a dry, stable

material essential for successful industrial enterprises in today's highly competitive markets.

The more critical the seasoning requirements,

the more firmly the dry kiln becomes established

as an integral part of the lumber mill, the millwork plant, the furniture factory. For many

wood products, kiln-dried lumber is indispensable.

Well-seasoned lumber has many advantages for

producers and users alike. Removal of excess

moisture reduces weight and, thereby, shipping

and handling costs. Proper seasoning confines

shrinking and swelling to inconsequential

amounts under all but extreme conditions of use.

Properly seasoned wood can be cut to precise

dimensions and machined more easily and efficiently; wood parts are more readily and securely fitted and fastened together with nails,

screws, bolts, and adhesives; warping, splitting,

checking, and other harmful effects of uncontrolled drying are largely eliminated; paint,

varnish, and other finishes are more effectively

applied and maintained; and decay hazards are

eliminated if the wood is subsequently treated

or protected from excessive moisture regain.

EiRcient kiln-drying of lumber is therefore

of key importance in the utilization of our forest resources. On the one hand, it helps to assure continued markets for wood products by increasing their service life and contributing to

consumer satisfaction. On the other, it in effect

helps to conserve supplies of wood, and thereby

our timber resources, by extending service life

and usefulness of the product. Both are essential in attaining wise use of timber, which has

long been an accepted tenet of United States

forest conservation policy.

Because adequate supplies of well-seasoned

lumber are essential alike to producers and consumers of wood products, research in the drying

of wood has been conducted by the Forest Products Laboratory ever since it was founded in

1910. This program includes l)oth fundamental

research on the physical processes by which wood

gives off and takes on moisture and applied research on means of accelerating and controlling

these processes.

The fruits of much of the Laboratory's research

have been directly applied in the design, construction, and operation of dry kilns. United

States patents obtained by the Laboratory are

available for puljlic use. Industry uses basic drying schedules that have been develoi)ed for many

of the commercially important wood species.

Demonstrations conducted from time to time by

Laboratory personnel attract hundreds of kiln

operators. The purpose of these demonstrations

is to show how research findings can be put to

use to achieve fast, high-quality kiln-di-ying of

lumber.

This manual presents knowledge of kiln-drying principles that will be of most direct usefulness to owners and operators of dry kilns.

Unless stated otherwise, the techniques described

can be applied in the cliying of lumber, dimension stock, cooperage stock, and many special

items such as gunstock, bowling pin, and shoelast blanks. This manual is also intended as a

text on the tlieory and practice of kiln-drying,

and it should prove of value to students of wood

technology, dry kiln manufacturers, and producers of wood products in general.

The full benefits of modern diy kilns can be

gained only when certain prerequisites are observed. Mill management must recognize the

importance of efficient operation to quality of

product, and operators must be well trained and

encouraged to apply the best techniques. Quality

should not be sacrificed for quantity in the production of kiln-dried stock best suited for the

products being made. Also, adequate attention

must be paid to proper maintenance of kilns and

auxiliaiy equipment.

Insofar as possible, the use of proprietary terms

has been avoided in this manual. Where necessary to describe a given type of equipment or
AGRICULTURE HANíDBOOK NO. 188, U.S. DEFT. OF AGRICULTURE

procedure, however, such use was authorized by

the owner of the proprietary interest. The use

of these proprietary terms, or the ilkistration of

proprietary products, does not constitute an endorsement by the Forest Products Laboratory

or the U.S. Department of Agriculture.

Certain dry kihis and equipment described

and illustrated herein may have features protected under the patent laws of the United States.

The use of such descriptions and illustrations by

the Laboratory cannot be construed as giving or

implying any protection to others from legal action for infringement of patent rights.

Terms used in this manual to describe dry kilns

and their component parts, drying characteristics of wood, and kiln operational procedures are

generally accepted and used throughout the industry. There are, however, some exceptions.

For the purpose of clarification, and to help an

inexperienced kiln operator to become familiar

with terms used in kiln drying, a glossary of

terms is included (p. 177).
CHAPTER 1. PROPERTIES OF WOOD

RELATED TO DRYING

Considerably more than 100 commercially important species of trees grow in the United States.

The wood they produce varies greatly in its drying characteristics. Since there are also many

variations in other characteristics of the wood

of these species, it is necessary for the kiln

operator to have a sound working knowledge of

these characteristics to do an effective job of

kiln-drying. This chapter is concerned with

such variability.

The most commonly used commercial names

for lumber and the corresponding species names

accepted by the Forest Service (4)' for the trees

1 Italic numbers in parentheses refer to Literature

Cited, p. 21.

from which it is cut are given in the following

list. The list was adapted from the Standard

Nomenclature of Domestic Hardwoods and Softwoods adopted in 1952 by the American Society

for Testing Materials. While the commonly

used lumber names are generally satisfactory for

the buying and selling of lumber, they sometimes

cover lumber from a number of species that differ

in their moisture content when gi'een, their

shrinkage, or their seasoning characteristics.

Therefore, in the tables and indexes of drying

schedules given elsewhere in this manual, the

woods are arranged alphabetically by the common species names accepted by the Forest Service.

HARDWOODS

Common tree name

red alder Almis rubra

apple Malus spp.

Commercial name for lumber t:n-mmon tree name Botanical name

Alder:

Red alder

Apple

Ash: „

Black ash' black ash Fraxinus nigra

Oregon ash Oregon ash P- lalifoha

Pumpkin ash pumpkin ash t. pro}unda

fblue ash F. quadrangulata

White ash _ - ^greenash F. pennsylvamca

[white ash F. americana

/bigtooth aspen Populus grandidenlala

Iquaking aspen P- trenndoides

„ ,, fAmerican basswood Tilia americana

Basswood3 (white basswood T. heterophylla

Beech beech Fagus grandifoha

gray birch Betula populifolia

paper birch B. papyrifera

<river birch B. nigra

sweet birch B. lenta

.yellow birch B. alleghaniensis

boxelder Acer negnndo

fOhio buckeye Aescvlus glabra

\yellow buckeye A. octandra

butternut Jnglans cinérea

black cherry Prunus scrolina

American chestnut Castanea dentata

(balsam poplar Populus balsamifera

black cottonwood P. trichocarpa

eastern cottonwood P. deltoides

plains cottonwood P. sargenta

swamp cottonwood P. heterophylla

Cucumber cucumbertree Magnolia acuminata

-p, j /flowering dogwood Cornus florida

ijogwooa \Pacific dogwood C. nullallii

Black ash is known commercially in some consuming centers as brown ash, and is also sometimes designated as such in specifications.

' Aspen lumber is sometimes designated as popple.

' For some commercial uses where a white appearance is a requirement, the sapwood of American basswood (Tilia americana) is specified under the designation "white basswood." This coramerciai-use designation should not be confused with the species (T, heterophylla) having the common name white l^iasswood.

' The principal lumloer species is yellow birch. It may be designated either sap birch (all sapwood) or red Ijirch (all heartwood) or it may lie unselected.

Sweet birch is sold without distinction from yeUow birch. Paper birch is a softer wood used principally for turnings and novelties and is widely known as

white birch. The remaining birches are of minor commercial importance.

3

Aspen '

Birch«

Box elder..

Buckeye. _

ButternutCherry

Chestnut--
AGRíIiCULTURE HANDBOO'K NO. 188, U.S. DEPT. OF AGRICULTURE

Commercial name for lumber

HARDWOODS—Continued

Common tree name Botanical name

Elder, see Box elder

Elm:

Rock elm_

Soft elm=_

Icedar elm Ulmus crassifolia

rock elm U. thowasii

September elm U. serótina

winged elm U. alata

fAmerican elm U. americana

\slippery elm U. rubra

Gum " sweetgum Liquidambar styraciflua

„ ,, fhackberry Celtis occidenialis

Hackberry (sugarberry C. laevigata

(meckernut hickory Carya tomentosa

pignut hickory C. glabra

sand hickory C. pallida

shagbark hickory C. ovata

shellbark hickory C. laciniosa

Holly American holly Hex opaca

Ironwood eastern hophornbeam Ostrya virginiana

/black locust Robinia pseudoacacia

IjQcust \honeylocust Glediisia triacanthos

Madrona Pacific madrone Arbutus ynenziesii

,, ,. /southern magnolia Magnolia grandiflora

Magnolia \sweetbay :.- M. virginiana

Maple :

„ , , g /black maple Acer nigrum

üara maple \sugar maple A. saccharum

Oregon maple bigleaf maple A. macrophyUum

„. ,8 /red maple A. rubnnn

boit maple \silver maple A. saccharinum

Mulberry red mulberry Morus rubra

Myrtle, see Oregon myrtle

Oak:

Red oak_

White oak_

black oak Quercus velutina

blackjack oak Q- inarilandica

California black oak Q. kelloggii

cherrybark oak Q. falcata var. pagodaefolia

laurei oak Q- laurifolia

northern pin oak Q. ellipsoidalis

northern red oak Q. rubra

Nuttall oak 0- nutíallii

pin oak Q- palu^tris

scarlet oak Q- coccinea

Shumard oak Q. shumardii

southern red oak Q. falcata

turkey oak 0- laevis

willow oak Q. phellos

Arizona white oak Q. arizonica

blue oak Q. douglasii

bur oak Q. macrocarpa

California white oak Q. lobata

chestnut oak Q. prinus

chinkapin oak Q. -muehlenbergii

Emory oak Q. emoryi

Gambel oak Q. gambelii

Mexican blue oak Q. oblongifolia

live oak Q. virginiana

Oregon white oak

overcup oak

post oak Q.

swamp chestnut oak Q.

swamp white oak Q.

white oak Q.

garryana

lyrata

stellata

michauxii

bicolor

alba

Oregon myrtle California-laurel Umbelhdaria californica

Osage orange Osage-orange Madura pomifera

' Soft elm lumber is sometimes designated as white elm. A special type oí slowly grown material is sometimes designated commercially as gray elm. Slippery elm is called red elm is some localities, although that term is also used for two other elms.

» Usually designated either as red gum or as sap gum, a.s the case may be, or as gum or sweetgum when not selected for color. (For black gum, see túpelo,

footnote 9.)

' The Impossibility of distinguishing between hickory and pecan lumber for accurate species identification is recognized. Three of the four major Carta

species in the pecan group (which see) have the word "hickory" in their name.

» When hard maple or soft maple is specified to be white, the specification generally is interpreted as being a requirement for sapwood, although it sometimes

may take on the special meaning of being all sapwood with a minimum of natural color.
DRY KILN OPERATOR'S MANUAL

HARDWOODS—Continued

Commercial name Jor lumber Common tree name Botanical name

(bitternut hickory Carya cordiformis

nutmeg hickorj' C. myristicaeformü

water hickory C. aquatica

pecan C. illinoensis

Persimmon common persimmon Diospyros virginiana

Poplar yellow-poplar Liriodendron tulipifera

Sassafras sassafras Sassafras albidum

Silverbell Carolina silverbell Halesia Carolina

Sycamore American sycamore Platanus occidentaUs

Tanoak tanoak Lithocarpus densiflorus

{black túpelo Nyssa sylvatica

Ogeechee túpelo N. ogeche

swamp túpelo N. silvática var. biflora

water túpelo N. aquatica

Walnut black walnut Juglans nigra

("black willow Salix nigra

Willow \peachleaf willow S. amygdaloides

SOFTWOODS

Cedar:

Alaska cedar Alaska-cedar Chamaecyparis nootkatensis

Incense cedar. _. incense-cedar Lihocedrus decurrens

Port Orford cedar Port-Orford-cedar Chamaecy paris lawsoniana

Eastern red cedar_ _ _ /eastern redcedar Juniperus virginiana

' " Isouthern redcedar J. silicicola

Western red cedar western redcedar _ _ Thuja plicaia

Northern white cedar _ northern white-cedar. ._ _ T. occidentaUs

Southern white cedar Atlantic white-cedar Chamaecyparis thyoides

Cypress'» fbaldcypress Taxodium distichum

„. '" """ Ipondcypress T. distichum var. niitans Fir:

Balsam fir" . . . _ /balsam fir Abies balsamea

IFraser fir A. fraseri

Douglas fir 12 fDouglas-iir Pseudotsuga menziessii

T.T , , „ I P. menziesii vSíT. qlauca

Noble fir noble fir Abies procera

(California red fir A. magnifica

grand ñr A. grandis

noble fir A. procera

Pacific silver fir A. amabilis

subalpine fir .4. lasiocarpa

nemiocK: ^^*'^' A. concolor

Eastern hemlock ___ _ /Carolina hemlock Tsuga caroliniana

Mr,nr,+a;^ 1,0 1 1 " " " \eastern hemlock T. canadensis

wlTonZtf?\ mountain hemlock T. mertensiana

Juniper: western hemlock T. heterophylla

falligator juniper Juniperus deppeana

Western juniper . _______ J Rocky Mountain juniper J. scopulorum

Utah juniper J. osteosperma

Larch: Lwestern juniper /. occidentaUs

Western larch western larch. Pine: western larch Larix occidentaUs

L^dglîde pTne".:: ::::::::": lodgepoTe"¿ine P'""'"' bacana

Norway pine red pfne P. contorta Ponderosa ninp icu ^lue P. resinosa

Sugar pine ponderosa pine P. ponderosa

Idaho white'Dine sugar pme p ¡ambertiana

Northern Spine western white pme P. montícola

er Tarnte pine eastern white pme P. strobus

Longleaf yellow pine 13 ._ _ _ Jlongleaf pme p palustris

. . .,, \slashpine P. elliottii

n,... '^ -PossiMity Of distin^hing between Wao. túpelo (blacken.), swa.p túpelo, and water túpelo lumber for accurate species identification is recog-

''^Vif'7 *''" '"''''*'' " "' ^"'""' "'"^ ^^P""' ^""^ ^^""^^ ^^P--- ^'^•^ ^^P«- '^ fr«^--tly classified and sold separately Iron, the

,.„,„•" S.'i!^ ^/ specified either as Coast Ree on Donslns fir nr »c Tr,i.„,

"'^Äb^^^^^Ä^SÄ^ °a\^P¿^^l^^tt'- ^-^'- ^- >>- " ^^^ P-ticnlar type is not so specified or is not

low pme lumber are that not only mi

' pine lumber Is sometime" dfsf¿a?ed as^pitcfptae to tte"xportT/adef" '" '°°"'' ^' P" '^''' ''^'^ °°* '<^="« than one^hiid sum-'

l'TÎ, ^"iuLiBii iiie graue specincations,

P ;'^T 1'''^' ■■«l^irements for longleat yelli and P. ^lufU:î^tV^'^IL^TJStZtu^^^^^^^^^ T'y -"fî it '^S produced from trees of the

merwood. Longleaf yellow pine lumber Is sS^lttoes íesf¿^aled as°n tcS'l,?!*?^' ?Í^'lSSLH!ll''^° ^'^ ^^'^"«l is P" inch

botanical species of Pinus elliottii
6 AGRICULTURE HANDBOOK NO. 188, U.S. DEFT. OF AGRICULTURE

Commercial name for lumber

Southern j'ellow pine.

Redwood

Spruce:

s 0FTW 0 oDS—Continued

Common tree name Botanical name

loblolly pine Pinus taeda

longleaf pine P. palustris

pitch pine P- rigida

shortleaf pine P- echinata

slash pine P- elliottii

.Virginia pine P. virginiana

redwood Sequoia sempervirens

[black spruce Picea mariana

Eastern spruce < red spruce P- ruhens

[white spruce P- glauca

Tp , /blue spruce P. pungens

Engelmann spruce (Engelmann spruce P. engelmannii

Sitka spruce Sitka spruce P- siichensis

Tamarack tamarack Larix laricina

Yew:

Pacific yew Pacific yew Taxus brevifolia

Hardwoods and Softwoods

Trees are divided into two classes, hardwoods and softwoods. The hardwoods, such as

birch, elm, majjle, and the oaks, have broad leaves.

Some softwoods or conifers, such as the cedars,

have scalelike leaves, while othei'S, such as the

pines, Douglas-fir, and the spruces, have needlelike leaves.

The terms "hardwood" and "softwood" are not

directly associated with the hardness or softness

of the wood. In fact, such hardwood trees as

cottonwood and yellow-poplar have softer wood

than such softwoods as longleaf pine and Douglas-fir.

Gross Structural Features of Wood

A cross section of an oak tree is shown in

figure 1. Thé outer part of the tree is the outer

bark (C). Just beneath is a layer called the

inner bark (B), and beneath it is the cambium

(A).

Sapwood

Sapwood (fig. 1, D) plays an important part

in a tree's living processes. In general, only the

last few outside layers of the sapwood are alive.

The rest of the sapwood carries moisture from

the roots to the leaves and stores food for the

tree. It usually has a higher moisture content

than the heartwood.

Heartwood

During the life of a tree, sapwood gradually

changes into heartwood (fig. 1, E). As it does

so, it becomes less permeable. Since moisture

movement is thus retarded considerably, heartwood dries more slowly than sapwood. Heartwood usually surface checks and honeycombs

more readily than sapwood, hence requires milder

drying conditions. It is usually darker in color

and also more resistant to decay than sapwood.

Since the color change may require several years,

a band of heartwood may have the same color

as the sapwood next to it, yet not dry so easily

because it is less permeable.

M—88620-^

FIGURE 1.—Cross section of aa oak tree : A, Cambium

layer (microscopic) is inside of inner bark and forms

wood and bark cells. B, Inner bark is moist and

soft, carries prepared food from leaves to all growing

parts of tree. 0, Outer bark or corky layer is composed of dry dead tissue, gives general protection

against external injuries. D, Sapwood is the lightcolored wood beneath the bark ; it carries sap from

roots to leaves. E, Heartwood (inactive) is formed

by a gradual change in the sapwood ; it gives the tree

strength. F, Pith is the soft tissue about which the

first wood growth takes place in the newly formed

twigs. G, Wood rays are strips of cells that extend

radially within the tree and serve primarily to store

and transport food.
DRY KILN OPERATOR'S MAIOJAL

PUh

The pith of a tree (fig. 1, F) is laid down

by the growing tip. It is usually very small and

of no practical importance in the drying of wood.

The juvenile wood immediately around the pith,

however, differs in some ways from most of the

wood in the tree, and it is almost impossible to

dry such wood without drying defects.

Annual Growth Rings

Diameter growth of a tree in temperate climates is represented by rings that usually can

be easily seen on the end of a log as concentric

circles about the pith. The closer the annual

rings are to the pith, the greater their degree

of curvature. Each annual growth ring is composed of springwood and summerwood (in some

places known as early wood and late wood). The

inner part, which is formed early in the growing

season, is called springwood, and the outer part,

formed later, is called summerwood. Wlien lumber is cut from a log, the annual rings are cut

across in one direction or another and form a

pattern on the broad faces of the boards. Typical patterns are shown in figure 2.

Wood Rays

Wood rays appear as ribbonlike strands on

the face of a quartersawed board (fig. 2, Ä).

Because they are relatively weak, surface, end,

ZM—554—F

FiGTJEE 2.—A, A quartersawed board shows the edge of

the annual rings on its broad face; B, a plainsawed

board shows the side of the rings.

and honeycomb checks usually occur in or next

to them. Such woods as oak and beech, which

have large rays, require special care during the

early stages oï drying in order to minimize surface and end checking.

Grain and Texture

The physical characteristics of various woods

that have some bearing on drying are loosely

termed, "grain" and "texture." Several terms are

used in describing grain. The terms "finegrained" and "coarse-grained" refer to ring pattern, either the prominence of the summerwood

band or the width of the rings. Wlien used in

connection with wood cells, grain refere only to

tlie direction of the cells or fibers. In straightgrained wood, the fibers rim generally parallel to

the length of the board, and in cross-grained wood

they run at an angle. The terms "end grain" and

"side grain" are also commonly used in discussing

moisture loss and seasoning defects. A cross section of a log or board has an end-grain surface.

Any other section, radial, tangential, or intermediate, has a side-grain surface.

Texture usually refers to the diameter of individual cells. Fine-textured wood has small

cells; coarse-textured wood has large cells. If

all the cells of a softwood or all the pores of

a hardwood are of approximately the same size,

the wood is usually called "uniform-textured."

Coarse-textured woods usually diy faster than

fine-textured woods. Unifonn-textured woods in

general are less likely to develop drying defects

than are nonuniform-textured woods. The word

"texture" should not be used in describing hardness of wood.

Variations in Structure

Lumber commonly contains variations in wood

structure, such as knots, spiral grain, tension

wood, and compression wood.

Cross grain in lumber may result either from

the way m which the log is sawed (diagonal

gram) or from spiral grain that occurred in the

growing tree. ^Vhen spiral grain alternately

runs in one direction and another in successive

groups of growth rings, interlocked grain results. Lumber containing diagonal, spiral, or

interlocked grain shrinks more in length than

does straight-grained lumber. Such lumber may

bow, crook, and twist during drying.

Knots are sections of tree branches appearinoin boards. Because of shrinkage, some kinds

ot knots may drop out during drying; more often,

however, they are loosened or checked durinodrying and drop out of the boards during han*^

dlmg or machining. These are called "incased"

knots; they result from dead branches around

which trunk wood later grew. Intergrown knots
8 AGRiICULTURE HANÜBOOK NO. 18 8, U.S. DEFT. OF AGRICULTURE

caused by living branches intergrown with trunk

wood, are much less likely to drop out of dried

lumber.

Compression wood occurs in softwoods mainly

on the lower side of leaning trees but sometimes

in other parts of the tree trunk. Because this

wood shrinks more longitudinally than normal

wood, boards that contain compression wood may

bow, crook, and twist during drying. If this

warping is restrained, the compression wood may

rupture and form cross-breaks in the lumber.

Tension wood occurs in hardwoods, mainly on

the upper side of leaning trees but sometimes in

other parts of the trunk. Lumber containing

tliis wood will shrink more longitudinally than

will normal wood, causing warp during drying.

Further information on wood structure is available elsewhere {2, 9).

Wood-Moisture Relations

All wood in growing trees contains considerable quantities of water, commonly called sap.

Although sap contains some materials in solution, from the drying standpoint it is considered

to be plain water. Most of this water should be

removed in order to obtain satisfactory service

from the wood in use. All wood gains or loses

moisture in an attempt to reach a state of balance with the conditions under which it is stored

or used. This state of balance depends upon

the relative humidity and temperature of the

surrounding air. Therefore, some knowledge of

wood-moisture relations is helpful in understanding what happens to wood during drying, storage, fabrication, and use.

Water is held by wood in two ways, as "free

water" in the cell cavities and as "bound water"

in the cell walls. The free water does not affect

the properties of wood other than weight. The

bound water, however, does affect many properties of wood, and is more difficult to remove in

drying.

Moisture Content

The amount of moisture in wood is spoken of

as its moisture content, and is expressed as a

percentage of the ovendry weight of the wood.

The universally accepted method of determining the moisture content of wood is the ovendrying method, or oven test. This method, described in chapter 6, is used to calculate moisture

content from weight values obtained from a section of wood before and after it has been ovendried. Other methods of determining the moisture content of wood are described in chapter 3.

The amount of water in green wood varies

greatly. One wood may contain as little as 30

percent; another may contain 200 percent or

more. Large variations may occur not only between species but also within the same species

and even in the same tree. Sapwood usually contains more water than heartwood. In some species the butt logs of the tree may contain more

water than the top logs. One example of this

is redwood.

Contrary to a popular belief, the amount of

water in green wood does not vary greatly with

the season of the year in which the trees are

cut. Some species of wood contain zones of unusually high moisture content, commonly called

water pockets. This water moves out very slowly

during the drying process. Woods such as eastern white pine, noble fir, western hemlock, and

sweetgum frequently contain water pockets.

Moisture content values for green wood of various species are given in table 1.

The Fiber Saturation Point.—This is defined

as the point at which the cell walls are saturated,

but no free water remains in the cell cavities.

Moisture content of the individual cell walls at

the fiber saturation point is approximately 30 percent for all woods. The term "fiber saturation

point" pertains to the moisture content of the

walls of a cell and not to the whole piece of

wood. For example, if the average moisture content of a whole piece of wood is at 30 percent,

some of the cell cavities will contain free water.

The fiber saturation point is important in the

drying of wood for the following reasons: (1)

more heat is required to move water from a cell

wall than from a cavity; (2) a wood cell will

not shrink until it reaches the fiber saturation

point; and (3) large changes in the physical

and mechanical properties of wood begin to take

place at the fiber saturation point.

Equilibrium Moisture Content.—Any piece of

wood gives off or takes on moisture until the

amount it contains is in balance with that in

the surrounding atmosphere. The amount of

moisture in the wood at the point of balance is

called the equilibrium moisture content. Assuming constant temperature, the ultimate moisture content that a given piece of wood will attain depends entirely upon the relative humidity

of the air surrounding it. This relationship is

illustrated in figure 3, which shows, for example,

that wood kept in air constantly at 141° F. and

65 percent relative humidity will eventually come

to equilibrium at a moisture content of about 10

percent. Equilibrium moisture content is designated by the letters EMC.

To assure proper control of drying conditions in a kiln, both temperature and relative humidity must be accurately measured. Two thermometers are used for this purpose. One measures temperature in the usual way. The bulb

of the other is kept wet with a wicklike cover,

from which water evaporates at a rate deter-
DRY KILN OPERATOR'S MANTTAl,

TABLE 1.—Average moisture content of green wood

SOFTWOODS

Moisture content '

Species

Heartwood

Sapwood

Mixed

heartwood

and

sapwood

RfllHpvnrpRS

Percent

121

32

Percent

171

166

Percent

Cedar:

Âtlantic white- 35

TT.íictprn rprlpptiiir 33

Tni^pTisp- 40 213

55

Port-Orford- 50

58

37

34

30

98

249

115

154

112

"Wpq+prn rpHfPiiflr

Douglas-fir:

doíLst tvne

Intermediate type

Rocky Mountain type..

Fir:

Balsam 117

California red 108

Grand 91

34

55

136

115

164

Noble

Pacific silver

Subalpine- 47

White 98

97

85

54

160

119

170

119

Hemlock:

Eastern

Western

Larch, western

Pine:

Eastern white 68

Lodgepole 41

40

32

33

31

32

98

62

86

120

148

134

110

106

122

219

148

210

Ponderosa

Red

Southern yellow:

Loblolly

Longleaf

Shortleaf

Sugar-.

Western white

Redwood:

Old-growth.-

Second-growth 127

Spruce:

Black 34

51

41

49

128

173

142

Engelmann

Sitka

Tamarack ..

TABLE 1.—Average moisture content

of green wood—Contitiued

HARDWOODS—Continued

HARDWOODS

Alder, red 97

Apple.. 81 74

95

Ash:

Black

Green. . .. 58

44

113

133

72

White 46

95

81

55

Aspen

Basswood, American

Beech, American

Moisture content '

Species

Heartwood

Sapwood

Mixed

heartwood

and

sapwood

Birch:

Paper

Percent

89

75

74

Percent

72

70

72

Percent

Sweet

Yellow

Buckeye, yellow. 141

Butternut 104

Cherry, black 58

Chestnut, American. 120

Chinkapin, golden _ Î34

Cottonwood, black 162 146

Dogwood, flowering. 62

Elm:

American. 95

66

44

61

80

70

71

69

68

97

92

61

57

65

54

52

49

52

50

62

Cedar

Rock

Hackberry

Hickory:

Bitternut

Mockernut

Pignut.

Red

Sand . .

WaterHollv, American . 82

Hophornbeam, eastern- 52

Laurel, California 65

Locust, black. 40

Madrone, Pacific . 81

Magnolia.- 80

58

65

76

104

97

72

75

Maple :

Silver (soft)

Sugar (hard)

Oak:

California black

Live- 50

Northern red 80

83

79

81

64

82

69

75

66

81

78

74

Southern red .

Southern swamp

Water-.

White

Willow

Osage-orange 31

Persimmon, common .. 58

Sweetgum. 79

114

137

Sycamore, American 130

Tanoak .. .

89

Tupelo:

Black 87

101

150

90

115

108

116

73

Swamp

Water

Walnut, black

Willow, black 139 Yellow-poplar 83 106

' Based on ovendry weight.
10 AGMCTILTURE HAN'DBOO'K NO. 188, U.S. DEFT. OF AGRICULTURE

10 ZO 30 40 60 60 10 80

/RELATIVE HUMIPIT Y IN ATMOSPHERE (PERCENT)

90 100

ZM-87E87-F

FiGUBE 3.—Relation of tlie equilibrium moisture content of wood to the relative humidity of the surrounding

atmosphere at three temperatures.

mined by the dryness of the air around it. This

evaporation has a cooling effect that increases

as evaporation speeds up. Tlius, tlie dryer the

air, the greater tlie cooling effect, and the lower

the temperature recorded by the wet-bulb thermometer. The difference between the dry-bulb

and wet-bulb temperatures—called the wet-bulb

depression—is thus a measure of the relative humidity in the air.

In kiln-drying, it is more con\'enient to convert wet-bulb depressions at given diy-bulb temperatures directly into EMC values. The EMC

values for various dry-bulb temperatures and wetbulb depressions commonly used in kiln drying

are shown in table 2.

For example, assume that the dry-bulb temperature in a kiln is 150° F. and the wet-bulb

temperature 130°. The wet-bulb depression, that

is, the diffei-ence between these two temperatures,

is 20°. Wet-bulb depression values are shown

across the top of table 2, and dry-bulb temperatures on the extreme left of the table. To ñnd the

EMC at the assumed conditions, (1) locate the

20° wet-bulb depression column, (2) follow this

column downward until it intersects the 150° drybulb temperature line, and (3) read the EMC

value, 8 percent, given at this intersection in italic

type.

How Wood Dries

Water in wood normally moves from zones of

higher to zones of lower moisture content (i).

This fact supports the familiar statement that

"wood dries from the outside in," which means

that the surface of the wood must be drier than

the interior if moisture is to be removed. In

drying, the surface fibers of the heartwood of

most species attain moisture equilibrium with the

surrounding atmosphere almost as soon as drying

begins, and at this time a moisture gradient begins to develop. The surface fibers of sapwood

also tend to reach a state of balance with the

surrounding drying atmosphere early in the drying process, if the air circulation is fast enough

to evaporate the water as rapidly as it comes to

the surface of the wood. If the air circulation

is too slow, howeAer, a longer time is required for

the surfaces of sapwood to attain equilibrium.

To reduce drying time, the initial wet-bulb depression in the kiln should be as large as possible

without causing serious end or surface checking.

Moisture in wood moves as liquid or vapor

through several kinds of passageways. These

consist of the cavities of fibers and vessels, wood

ray cells, pit chambers and their pit membrane
DRY KILN" OPERATOR'S MANTTAL 11

TABLE 2.—Relative humidity ' and equilibrium moisture content ^ values occurring at various dry-bulb

temperatures and wet-bulb depressions

Temperature

dry bulb

(°F.)

30.

35.

40-

46.

50.

55.

60.

65.

70.

75.

85..

90..

95..

100.

105.

110-,

115,.

120..

125..

130..

140..

150..

160..

170..

180..

190..

200..

210..

Wet-bulb depression (°F,)

78

IS. 9

81

W.

83

17.6

85

IS. S

86

19.0

88

19. S

89

19.9

90

m. S

90

m. 6

98

91

¡1.0

92

11,1

92

SI. S

92

ILS

93

21. S

93

21. i

93

114

93

il. 4

94

SI. S

94

21

94

21.0

95

i.r

95

'.2

95

i9.«

95

Í9.

96

18.

I

18.5

96

IS.l

96

17. r

67

/2.9

72

IS. 9

75

78

15.6

80

;fi.5

82

ÍS.9

83

17.4

84

/7.

86

18.2

86

18.5

87

;8.7

88

18.8

89

/S. 9

89

19.0

89

M.O

90

/9.0

90

19.0

90

/9.0

91

19.0

91

;«.9

91

i8.«

92

IS. 6

92

Í8. 4

93

IS.l

93

ir. 7

94

Í7.5

94

16.9

94

;6. í

94

16.0

57

ÍO.

63

Ji.9

68

12.9

72

M. 7

74

14-4

76

íí. !

78

Î5. e

80

16.1

81

íe.5

82

16. S

83

Í7.0

84

27.2

85

17. S

85

/7.4

86

17.5

87

Í7.í

87

Í7. 5

88

/7.5

17.

I

/7.

89

17.2

S

2 S.

90

IS. 6

90

;«,

91

25.«

91

25.5

92

15.2

92

14.9

92

24. e

46

9.0

54

10. S

60

22.«

64

IS. O

68

2«. 7

70

23.4

73

13.9

75

77

119

78

25.«

79

25.5

80

25.7

81

15.9

82

16.1

83

2S. 2

83

ÍS.

84

16.2

85

2«. 2

85

16.

86

2ff.2

86

2S.0

87

15.8

87

25.4

88

15.2

89

24.8

89

24.5

90

14.2

90

24. O

90

IS.

36

7..Í

45

8.8

52

9.9

58

20.7

62

11.5

65

2«. «

68

2«. 7

70

IS.

72

2S.7

74

14.0

75

24.?

76

24.5

78

24.7

79

14.9

80

16.0

80

25.2

81

25.2

82

15.1

82

25.2

83

15.0

83

24.9

84

24. S

85

14.5

86

24.2

86

13.

87

2.3.7

88

23.4

88

13.2

88

23.0

27

5.7

37

7-4

45

8.e

51

9.6

56

20.3

60

22.0

63

11.6

66

2«. 2

68

IS.S

70

22.9

72

1S.2

73

2.3.5

74

2.3.7

75

13.9

77

23.9

77

14-0

78

79

u-/

80

14.1

80

24.0

81

14.0

82

23.8

82

23.7

83

2.3.4

84

13. S

85

12.9

85

22.7

86

22.4

86

11. 2

17

3.9

28

6.0

37

7.4

44

8.5

50

9.4

54

20.2

58

20.7

61

11.2

64

22.í

66

12.0

68

22.3

70

IS.S

71

22.8

72

22.9

73

13.1

74

23.

75

13.

76

23.4

77

23.4

77

13. 4

78

23.4

79

13.2

80

23 0

81

22.7

82

:4

83

22.2

84

IS.O

84

22.8

85

22.7

6

1.6

19

'S. 5

29

S. 2

37

7.5

44

8.5

49

9.3

53

9.9

56

10.4

59

20.9

62

22.2

64

22.5

66

11.8

68

IS.O

69

22.2

70

22.

71

IS. 6

73

22. S

74

22.7

74

22.7

75

22.7

76

22.7

77

IS.S

78

•4

79

IS.l

80

22.«

81

11.6

82

22,

82

22.2

83

11.1

10 11 12 13

11

2.9

22

5.0

31

e.5

38

7.0

44

8.4

48

9.2

52

9.7

55

10.1

58

20.5

61

10.9

63

22.2

65

11-4

66

22.0

68

22.8

69

11.

70

22.0

71

IS.l

72

22.2

73

.2

73

73

!. 2

75

22.9

76

11.8

77

22.5

78

11.

79

"•/

80

20.9

80

10.8

81

20.0

57

10.1

59

20.5

61

20.7

63

11.0

65

22.

66

11.3

67

Í/.4

68

22.5

69

11.5

70

22.5

71

11.5

73

n.4

74

22

75

11.0

76

20.8

77

10.6

78

20.5

79

20.3

79

10.0

1.9

19

4.2

27

5.7

34

6.8

39

7.0

44

8.3

48

8.8

51

9.3

54

9.7

56

10.0

58

20.2

60

20.5

62

10. e

63

?0.

65

10.8

66

20.9

67

22.0

68

11.0

69

22.0

70

10.9

72

20.8

73

20.0

74

10.4

75

20.2

76

10.0

77

9.8

78

9.7

21

4.8

28

0.0

34

6.9

39

7.7

44

8.3

47

8.7

50

9.1

53

9.5

55

9.7

57

20.0

59

10.1

60

20.3

62

10.

63

20.4

65

20.5

65

10. S

67

20.5

68

10.

70

20.3

71

20.2

72

9.9

73

9.7

75

9.6

75

9.4

76

9.2

14 15 16 17

10

61

10.0

62

10.0

63

10.0

64

20.0

66

20.0

68

9.9

69

9.7

70

9.6

72

9.4

73

9.2

74

9.1

75

9.0

55

9.4

57

9.5

58

9.«

60

1.7

61

1.7

62

9.7

64

.9. S

66

9.5

67

9.4

69

,0.2

70

9.0

71

«.9

72

Í. 8

73

î. 7

18 19

8.7

51

19

53

9.0

55

9.2

56

9.3

58

9.4

59

9.4

60

9,

62

9.

64

9.2

65

9.

67

9.0

6S

1.8

69

'.0

70

9

'.5

17

4.1

24

5.

29

0.0

34

6.7

38

7.2

41

7.0

44

s.o

46

'.2

49

8.6

50

8.7

52

8.8

54

8.

55

9.0

57

9.0

58

9.0

60

9.0

62

8.

64

20

S. 4

52

8.0

53

8.7

55

8.7

56

8.7

58

8.7

60

S. 6

62

8.5

63

8.4

65

8.1

66

7.9

67

7.7

68

7.6

28

5.6

32

6.3

36

'.7

39

7.f

42

7.5

44

7.8

46

7.9

48

8.1

50

8. S

51

8.3

53

8,

54

«.3

56

8.4

58

8.3

60

8.2

62

8.0

63

7.8

65

7.7

66

7.5

67

7.

13

3.0

19

4.3

24

5.1

29

.5.8

33

0.3

36

6.8

39

7.2

41

7.4

44

7.0

46

7.7

48

.S

49

7.9

51

8.0

52

8.0

54

8.0

57

8.0

58

7.9

60

7.8

62

7.0

63

7.4

64

7.-2

65

7.2

30

;.o

34

0,

37

«.S

39

•.0

42

44

?.5

45

7.0

47

48

7.7

50

7.8

53

7.8

55

7.«

57

22 23 24

6

.4

12

2.9

18

4.1

23

5.0

28

5.6

31

11

34

«4

37

6

40

'? 9

42

7.2

43

7.3

45

7.4

47

7.5

48

7.0

51

2

0.4

9

2,

15

3.5

20

4-5

25

5.2

29

5.7

32

0. 2

35

6.4

37

;. ?

40

-.8

41

7.0

43

7.2

45

'. ?

47

.3

49

25

1.5

12

2.9

18

4.0

23

4.8

26

S. S

30

5.7

33

0.2

35

.4

38

0.0

40

0.7

41

6.8

43

7.0

45

7.0

47

7.2

49

7

3

0.7

10

2.

15

3.5

20

4.3

24

4.9

28

5.3

30

5.7

34

6.1

36

.3

38

0.5

40

26

2.7

12

,3 0

18

3.9

22

.0

26

5.2

28

5.4

31

5. 7

' Relative humidity values In roman type

E qulllbrium moisture content values In italic type.

43

34

6.0

36

0.2

38

0. 3

39

0.5

41

i 6

44

0. 0

46

;. 7

49

.7

51

0 s

52

4

54

0.2

55

6'. 0

57

28 29 30 32 34 36 38 40 46 50

6. Ils. i

4

0.9

10

2.

15

3,

19

4-2

23

i. 8

26

5.2

29

5.4

32

.5

34

,5.9

36

0. 2

38

0.2

40

6.4

43

0 4

45

6.4

6. 8\5. S

31

5.4

33

.5,0

35

5,8

37

',9

40

,'.0

42

'. 0

44

0,0

47

0. 0

48

5.8

50

,5,7

52

5.0

53

24

i. 0

26

;. 8

29

5.2

31

5

33

.5.5

35

5.0

38

5.8

41

5.8

43

5.8

45

5,7

47

5,7

49

S. S

51

5,4

52

5,4 5,3

23

;, 5

26

.7

28

5.0

30

5.

32

5.3

35

5.4

38

,5,4

41

S. S

43

,5,5

45

5

46

5,3

48

5.2

50

.5.2

5

.3

10

S. 3

13

¡.2

17

î. 0

20

;.o

23

4,3

25

4.6

27

4,3

29

4,9

32

,5,2

36

5. S

38

5,2

40

,5.2

42

S. S

44

5.0

46

.9

47

1

0.4

6

2.5

10

2.4

14

3.2

17

3.5

20

'.9

22

4.2

24

4.4

26

.0

30

4,8

33

,9

35

4,9

38

:, 9

40

.8

42

,8

43

, 7

45

4.8\4.6

2

0

7

2,0

11

2-4

14

,3,0

17

3,4

19

3.7

22

4.0

24

4,2

27

4,4

30

4.5

33

4,0

35

4,0

38

,0

39

4-5

41

4-5

43

4

0-7

2

11

2,5

14

,9

17

3

19

3,0

21

,3,8

25

4-1

1

4^2

31

4.3

33

-4

35

4-4

37

4-4

39

4-3

41

,2

4

2,2

8

2,7

10

2.

13

2.7

15

3.0

19

?,4

23

,3,0

25

3,7

28

,3,7

30

3.8

32

3.8

34

3,8

36

7

2

0,4

6

1.1

8

2,0

10

2,0

14

2.6

21

î,«

24

1,2

26

3,3

28

3,3

30

3.3

32

3.2

openings, resm ducts of certain softwoods, other

intercellular spaces, and transitory cell-wall passageways {2). Most of the moisture lost by wood

during drying moves through cell cavities and

the small openings in the cell walls. Moisture

moves in these passageways in any direction,

longitudinally as well as laterally. Lighter

woods in general dry more rapidly than do the

heavier woods.

Forces That Move Water

When wood is drying, several forces may be

acting simultaneously to move water. These

forces are as follows :

1. Capillary action, which causes free water to

flow, for the most part, through cell cavities and

small openings in the cell wall.

2. Differences in relative humidity in the wood

that cause water vapor to move through various

passageways by diffusion.

3. Differences in moisture content that move

the bound water through the small passageways

m the cell wall by diffusion.

When green wood starts to dry, evaporation

of water from the surface cells sets up capillary

forces that exert a pull on the free water in the

zones of wood beneath the surface, and a flow

results. This is similar to the movement of water
12 AGRÍICULTURB HANDBOOK NO. 188, U.S. DEFT. OF AGRICULTURE

in a wick. Much of the free water in sapwood

moves in this manner.

Movement of moisture by diffusion results from

differences in relative humidity and moisture content between the surface and the interior, or between any two zones of the wood. Moisture in

wood moves to the surface by simultaneous diffusion of vapor and bound water. In comparison

with capillary movement, diffusion is a slow

process.

Longitudinal diffusion is about 10 to 15 times

faster than lateral—that is, radial or tangential

—diffusion. Radial diffusion, perpendicular to

the groAvth rings, is somewhat faster than tangential diffusion, parallel to the rings. Although

longitudinal diffusion is 10 to 15 times as fast as

lateral diffusion, it is of practical importance only

when short items are being dried.

Most of the moisture is removed from wood by

lateral diffusion during drying. The rate of lateral diffusion depends to a large extent upon the

permeability of the cell walls and upon their

thickness. Thus permeable woods dry faster than

impermeable woods. Generally, the rate of diffusion decreases rapidly as the specific gravity

of the wood increases.

Moisture diffuses more rapidly in sapwood

than in heartwood. This is probably due to the

fact that extractives plug the small cell wall

openings in the heartwood.

Because moisture moves more freely in sapwood than heartwood, both by diffusion and by

capillary flow, sapwood generally dries faster

than heartwood under the same drying conditions. The heartwood of some species, however,

may reach the final desired moisture content in a

shorter drying time than the sapwood, because

its initial moisture content may be much loAver

than that of the sapwood.

Factors That Influence Drying Rate

The rate at which moisture moves in wood is

dependent upon the relative humidity of the surrounding air, the steepness of the moisture gradient, and the temperature of the wood. The

lower the relative humidity, the greater the

capillary flow. Low relative humidity also stimulates diffusion by lowering the moisture content,

at the surface, thereby steepening the moisture

gradient. If the relative humidity is too low

during the early stages of the drying of green

wood, excessive end and surface checking may

result. The higher the temperature of the Avood,

the faster will be the rate at which the moisture

moves from the wetter interior to the drier surfaces. If the temperature is too high, however,

collapse, honeycombing, or reduction in strength

may result.

Specific Gravity and Weight of Wood

Specific gravity is one of the physical properties of wood that are guides to ease of drymg.

In general, the heavier the wood, the slower the

drying rate and the greater the likelihood of developing defects during drying. Specific gravity

is defined as the ratio of the weight of a body to

the weight of an equal volume of water. Usually

the specific gravity of wood is based on the volume of the wood when green and its weight when

ovendry. Thus, if the specific gravity of a specimen of green wood is 0.5, the ovendry weight of

the wood substance in a cubic foot of the green

wood is one-half the weight of a cubic foot of

water. The higher the specific gravity of wood,

the greater is the amount of ovendry wood found

in a unit volume of green wood. Thus, at the

same moisture content the woods of higher specific gravity contain more water. The specific

gravity of a number of species is given in table 3.

Weight of wood depends on its specific gravity

and its moisture content. Calculated weights are

given in table 4. The values for weights per

thousand board feet in columns 2 and 3 apply to

a thousand feet, surface measure, of boards exactly 1 inch thick (actual board feet) and not to

a thousand board feet, lumber scale. These

weights were determined by multiplying the

weight per cubic foot at the given moisture contents by 83.3, the number of cubic feet in a thousand board feet. Since these weights are based

on actual board feet—a thousand lineal feet of

lumber exactly 1 inch thick and 12 inches wide—

they must be adjusted upward for rough lumber

and downward for dressed. For example, the adjustment factor for a 1- by 8-inch board dressed to

2%2 inch in thickness and 71/2 inches in width

is arrived at as follows:

26/ V71/ /32 A l_2 ^-^^=0.7324

Values in columns 2 and 3 of table 4 multiplied

by 0.7324 give the weights of a thousand board

feet of dressed 1- by 8-inch lumber. Similarly,

for any dressed size a factor can be worked out

and the weight per thousand board feet calculated. The adjustment for rough lumber is made

in a like manner, the actual rough size being

divided by the nominal size (9).

The weight per thousand board feet (actual)

at any moisture content below 30 percent can be

determined by applying the correction factors in

column 4 of table 4. For example, the weight

of a thousand board feet of coast type Douglasfir at a moisture content of 21 percent would be

14.3 pounds more than that shown in column 2;
DRY KILN OPERATOR'S MAJSTUAL

TABLE 3.—Specific gravity of wood

SOFTWOODS

13

Species

Average

specific

gravity '

Species

Average

specific

gravity '

Baldcypress

Cedar:

AlaskaAtlantic whiteEastern redcedar

IncenseNorthern whitePort-OrfordWestern redcedar

Douglas-fir:

Coast type

Intermediate type

Rocky Mountain type.

Fir:

Balsam

CaUfornia red

Grand

Noble

Pacific silver

Subalpine

White

Hemlock:

Eastern

Western

0. 42

. 42

.31

. 44

.35

. 29

. 40

.31

.45

. 41

.40

. 34

. 37

. 37

. 35

. 35

. 31

. 35

. 38

. 38

Larch, western

Pine:

Eastern white

Lodgepole

Ponderosa

Red

Southern yellow:

Loblolly

Longleaf

Shortleaf

Sugar

Western white . _

Redwood:

Old-growth

Second-growth..

Spruce:

Black

Engelmann

Red

White

Sitka

Tamarack

HARDWOODS

Alder, red

Apple

Ash:

Black

Green

White

Aspen

Basswood, American.

Beech, American

Birch:

Paper

Sweet

Yellow

Buckeye, yellow

Butternut

Cherry, black

Chestnut, American.

Chinkapin, golden

Cottonwood, black. _

Dogwood, flowering-.

Elm:

American

Cedar

Rock

Slippery

Hackberry

Hickory

Hickory, pecan:

Bitternut

Nutmeg

Pecan

Water

0.37

. 61

.45

. 53

. 55

. 35

. 32

. 56

.48

.60

. 55

.33

.36

.47

.40

. 42

.32

.64

. 46

. 59

. 57

.48

.49

.65

.60

. 56

. 60

. 61

51

34

38

38

40

47

. 54

. 46

. 35

. 36

.38

. 30

. 38

. 32

.38

. 37

. 37

. 49

Hickory, true:

Mockernut

Pignut

Shagbark

Shellbark

Holly, American

Hophornbeam, eastern.

Laurel, California

Lemonwood

Locust, black

Madrone, Pacific

Magnolia

Maple:

Silver (soft)

Sugar (hard)

Oak:

California black

Live

Red 2

Water

White

Wülow

Osage-orange

Persimmon, common..

Sweetgum

Sycamore, American..

Tanoak

Tupelo:

Black

Water

Walnut, black

Willow, black

Yellow-poplar

. 64

. 66

. 64

. 62

. 50

. 63

. 51

.82

. 66

. 58

. 46

. 44

. 56

. 51

, 81

. 54

. 56

. 60

. 56

. 76

. 64

. 46

. 46

. 56

. 46

. 46

. 51

. 34

. 40

' Based on weight when ovendiy and volume when green.

• Average of northern and southern red oak.

555316 0—61 2
14 AGRICULTURE HANDBOOK NO. 18 8, U.S. DEPT. OF AGRICULTURE

TABLE 4.—Calculated weights of wood

SOFTWOODS

Weight per 1,000 board feet

(actual board feet)

Correction factor

per 1,000 board

feet for each 1

Species

At 20 percent

moisture

content

At 8 percent

moisture

content

percent change

in moisture

content '

Baldcvpress

Pounds

2, 790

2,700

2,020

2,870

2, 190

1,880

2, 580

2, 010

2,930

2,710

2, 620

2,270

2, 420

2,420

2, 320

2, 390

1,940

2,270

2,480

2, 530

3,350

2, 190

2,490

2, 440

2, 680

3, 080

3, 550

3, 030

2, 240

2,380

2, 450

2,460

2,070

2,400

3,205

Pounds

2,620

2,530

1,920

2,680

2,020

1,730

2,410

1,870

2,760

2,570

2,430

2, 200

2,270

2,275

2, 180

2,275

1,775

2, 150

2,330

2,390

3, 180

2,020

2,350

2,290

2, 530

2,930

3, 360

2, 880

2,075

2,260

2,280

2,320

1,930

2,260

3,025

Pounds

14.3

Cedar:

Alaska- 14.3

Atlantic white- - 8.6

Eastern redcedar . - - 15.7

Incense- - - 14.3

Northern white- - 12.9

Port-Orford- 14.3

11.4

Douglas-fir:

Coast type . _. 14.3

Intermediate tvpe 11.4

Rockv Mountain tvDe -- - - 15.7

Fir:

Balsam 5.7

12.6

Grand __ ---_ -_ 12.1

Tsoble 11.4

Pacific silver _ _ _ 9.3

Subalpine - 13.6

White - 10.0

Eastern ____ _ ___ ___ 12.9

Western - __ __ 11.4

Larch, western _ _ _ - _ _ 14.3

Pine:

Eastern white - 14.3

11.4

Ponderosa - . - ____ _ 12.9

12.9

Southern yellow:

12.9

Longleaf - ___- - - 15.7

12.9

Sugar ____ _ 13.8

10.0

Redwood, old-erowth _ _ _ 14 3

Black _.- _ . -- 11.4

11.4

Sitka - n.4

15.0

HARDWOODS

Alder, red

Apple

Ash:

Black

Green

White

Aspen

Basswood, American

Beech, American

Birch:

Paper

Sweet

Yellow

See footnote at end of table.

2, 450

4,090

3,000

3,470

3, 630

2,310

2, 200

3,750

3,280

4,000

3,680

2,330

3,970

2,860

3,280

3,460

2, 190

2, 125

3, 600

3, 180

3,830

3, 530

10.0

10.0

11.4

15.7

14.3

10.0

6.4

12.9

8.6

14 3

12.9
DRY KILN OPERATOR'S MAI>rUAL

TABLE 4.—Calculated weights of wood—Continued

HARDWOODS—Continued

15

Species

Buckeye, yellow

Butternut

Cherry, black

Chestnut, American._.

Chinkapin, golden

Cottonwood, black

Dogwood, flowering .

Elm:

American

Cedar

Rock

Hackberry

Hickory

Holly, American

Hophornbeam, eastern

Laurel, Califoruia

Locust, black

Madrone, Pacific

Magnolia

Maple:

Silver (soft)

Sugar (hard)

Oak:

Northern red

Southern red

White

Osage-orange

Persimmon, common. _

Sweetgum

Sycamore, American..

Tupelo:

Black

Water

Walnut, black

Willow, black

Yellow-poplar

Weight per 1,000 board feet

(actual board feet)

At 20 percent

moisture

content

Pounds

2, 160

2, 340

3, 090

2, 600

2, 750

2,080

4, 340

3,060

3,890

3, 770

3, 190

4, 340

3, 380

4, 240

3, 370

4, 220

3, 860

3, 020

2, 880

3, 770

3, 730

3, 480

3, 970

4, 910

4, 290

3,080

3, 030

3, 080

2, 980

3, 320

2, 340

2, 580

At 8 percent

moisture

content

Founds

2,070

2, 200

2,900

2,460

2,610

1, 980

4, 225

2, 960

3,720

3,560

3,020

4, 200

3, 240

4,070

3, 180

3, 890

3, 720

2,870

2, 730

3, 620

3, 540

3, 340

3, 800

4, 575

4, 140

2, 960

2,890

2,960

2,830

3, 080

2, 240

2,430

Correction factor

per 1,000 board

feet for each 1

percent change

in moisture

content '

Pounds

7. 1

11.4

15. 7

11.4

11.4

8.6

9.3

8. 6

14. 3

17. 1

14. 3

11.4

11.

14.

15.

27.

11.

12.

12. 9

12. 9

15.7

11.4

14. 3

27. 9

12.9

10.0

11. 4

10. 0

12. 9

20. 0

8. 6

12. 9

1 Applies only to moisture values of 30 percent or less.

at a moisture content of 7 percent, 14.3 pounds

less than that shown in column 3.

Shrinkage of Wood

When wood is dried below the fiber saturation

point, it shrinks. "V^Hien it has dried to 15 percent moisture content, about one-half of the total

possible shrinkage has taken place; when dried

to 8 percent moisture content, nearly threefourths of the maximum possible amount has

taken place. Figure 4 indicates how Douglas-fir

shrinks with loss of moisture. While these curves

are not straight, shrinkage is generally thought

of as a straight line relationship (5, 6).

Average Shrinkage Values

Table 5 gives average shrinkage values for

various species of wood. These values are given

in percentages of the green dimension. Wood

shrinks about IV2 or 2 times as much parallel to

the annual growth rings (tangentially) as it does

across the rings (radially). The shrinkage along

the grain (longitudinally) is very little (O.ÜO percent or less) in normal wood. The combined

effects of radial and tangential shrinkage are

shown in figure 5.

Shrinkage values in table 5 can be converted

into useful units of measurement. Each 3 percent

of shrinkage, either radially or tangentially, is

roughly equivalent to a decrease in width or
16 AGRICULTURE HANÜBOO'K NO. 18 8, U.S. DEPT. OF AGRICULTURE

TABLE 5.—Shrinkage values of wood, based on its dimensions when green

SOFTWOODS

Species

Dried to 20 percent

moisture content '

Radial

slirinkage

Tangential

shrinkage

Dried to 6 percent

moisture content ^

Radial

shrinkage

Tangential

shrinkage

Dried to 0 percent

moisture content

Radial

shrinkage

Tangential

shrinkage

Baldcypress

Cedar:

AlaskaAtlantic whiteEastern redcedar

IncenseNorthern whitePort-OrfordWestern redcedar

Douglas-fir:

Coast type

Intermediate type

Rocky Mountain type

Fir:

Balsam

California red

Grand

Noble

Pacific silver

Subalpine

White

Hemlock:

Eastern

Western

Larch, western

Pine:

Eastern white

Lodgepole

Ponderosa

Red

Southern yellow:

Loblolly

Longleaf

Shortleaf

Sugar

Western white

Redwood:

Old-growth

Second-growth

Spruce:

Black

Engelmann

Red

Sitka

Tamarack

Yew, Pacific

Percent

1. :

.9

1.0

1.0

1. 1

.7

1. 5

1.7

1.4

1.2

1.0

1.3

1. 1

1. 5

1.5

.9

1. 1

1.0

1.4

1. 4

.8

1.5

1.3

1.5

1. 6

1.7

1.5

1.0

1. 4

.7

1.4

1. 1

1.3

1.4

1.2

1. 3

Percent

2. 1

2.0

1. 8

1.6

1.7

1.6

2. 3

1.7

2. 6

2.5

2. 1

2.5

2.4

2.3

2.6

2.7

2.0

2.2

2. 1

2.4

2. 5

2,5

2. 6

1.9

2. 5

2.3

2.2

2. 6

2. 5

2.5

1. 8

Percent

3.0

2.2

2. 3

2.5

2. 6

1.8

3.7

1. 9

4.0

3.3

2. 9

2. 3

3.2

2. 7

3. 6

3.7

2. 1

2. 6

2.4

3.4

3. 4

1.8

3.6

3. 1

3.7

3. 8

4. 1

3. 5

2.3

3. 3

2. 1

1. 8

3. 3

2.7

3. 0

3. 4

3. 0

3. 2

Percent

5.0

4.8

4. 3

3.8

4. 2

3.9

5.5

4.0

6.2

6. 1

5.0

5.5

5. 8

6. 0

6. 6

7. 8

5. 9

5.7

5. 4

6.3

6. 5

4.8

5.4

5.0

5. 8

5.9

6.0

6. 2

4. 5

5.9

3. 5

3. 9

5. 4

5. 3

Percent

3. 8

2.8

2.9

3. 1

3.3

2.2

4. 6

2.4

5.0

4. 1

3. 6

2.9

4.0

3.4

4.5

4.6

2.6

3.2

3.0

4.3

4.2

2.3

4.5

3.9

4.6

4.8

5. 1

4. 4

2.9

4. 1

2.6

2.2

4. 1

3.4

3.8

4.3

3. 7

4. 0

Percent

HARDWOODS

Alder, red

Apple

Ash:

Black

Green

White

Aspen :

Bigtooth

Quaking

Basswood, American

See footnotes on following page.

1. 5

2. 0

1.7

1. 5

1. 6

1. 1

1.2

2. 2

2. 4

3.5

2. 6

2.4

2. 6

2. 6

2. 2

3. 1

3. 5

4. 7

4. 0

3. 7

3. 8

2. 6

2. 8

5. 3

5. 8

8. 4

6. 2

5. 7

6. 2

6. 3

5. 4

7. 4

4. 4

5.9

5.0

4. 6

4. 8

6. 6
DRY KILN OPERATOR'S MANUAL

TABLE ^.-Shrinkage values of wood, based on its dimensions when green-Coxninned

HARDWOODS—Continued

17

Species

Beech, American

Birch:

Paper

Sweet

Yellow

Buckeye, yellow

Butternut

Catalpa, northern

Cherry, black

Chestnut, American

Cottonwood, black

Dogwood, flowering

Elm:

American

Rock

Slippery

Hackberry

Hickory:

Mockernut

Pignut

Shagbark

Shellbark

Holly, American

Hophornbeam, eastern.

Laurel, CaliforniaLocust, black

Madrone, Pacific

Magnolia, southern

Mahogany

Maple :

Black

Red

Silver

Sugar

Oak:

Black

Bur

California black. _

Chestnut

Live

Oregon white

Pin

Northern red

Scarlet

Southern red

Swamp chestnut..

Swamp red

Water

White

Willow

Persimmon, common__.

Sweetgum

Sycamore, American. .

Tupelo:

Black

Water

Walnut, black

Willow, black

Yellow-poplar

Dried to 20 percent

moisture content '

V

Radial

siirinkage

Percent

1. 7

2. 1

2.2

2. 4

L2

L 1

.8

1. 2

1. 1

1. 2

2. 5

1. 4

1. 6

1. 6

1, 6

2. 6

2.4

2. 3

2. 5

1. 6

2. 8

1.0

1. 5

1. 9

1. 8

1. 2

Tangential

shrinkage

6

3

0

6

5

5

2

1.8

2. 2

1. 4

1.4

1. 3

1. 5

1. 5

1.7

1.8

1. 4

1. 8

1.7

2. 6

1. 7

1.7

1.5

1.4

1.8

. 9

1.3

Percent

3.7

2. 9

2. 8

3. 1

2.7

2. 1

1. 6

2. 4

2. 2

2. 9

3.9

3. 2

2. 7

3. 0

3.0

4 2

3. 3

3. 3

2.8

2. 2

1. 7

3. 1

2.7

2. 4

3. 2

3. 2

2,9

2.2

3. 2

3.2

3.0

3.2

2. 7

3.2

2.9

3. 6

3. 5

3. 1

3.0

3. 2

3. 7

3.3

2. 5

2.6

2. 5

2.6

2.7

2.4

Dried to 6 percent

moisture content ^

Radial

shrinkage

Percent

4. 1

5.0

5.2

5. 8

2.9

2.7

2. 0

3. 0

2. 7

2. 9

5. 9

3. 4

3. 8

3. 9

3. 8

6.

5.

5.

6,

3,

6. 8

2. 3

3. 7

4. 5

4. 3

2. 9

Tangential

shrinkage

3. 8

3. 2

2. 4

3. 9

3. 6

3. 5

2.9

4. 4

5. 3

3. 4

3.4

3.2

3.7

3.6

4.2

4.4

3.4

42

4.0

6.3

4.2

4 1

3.5

3.4

4. 4

2. 1

3.2

Percent

6. 9

6. 8

7. 4

6. 5

5. 1

3. 9

6. 9

9. 4

Dried to 0 percent

moisture content

Radial

shrinkage

7. 6

6. 5

7. 1

7. 1

8. 8

9. 2

8. 0

0. 1

7. 9

8. 0

6. 8

5. 8

9. 9

5. 3

4 0

7. 4

6. 6

5. 8

7. 6

7.

7.

5.

7.

7.

7.

7.

6.

7. 8

7. 0

8. 6

8. 5

7.4

7. 2

7. 7

9.0

7.9

6. 1

6. 2

6. 1

6.2

6. 5

5.7

Percent

5. 1

Tangential

shrinkage

2. 5

3. 7

3. 4

3. 6

7. 4

4 2

4 8

4. 9

4. 8

7. 8

7. 2

7. 0

7. 6

4 8

8. 5

2. 9

4. 6

5. 6

5. 4

3. 6

4 8

4. 0

3. 0

4. 9

4 5

4. 4

3. 6

5. 5

6. 6

4 2

4 3

4. 0

4 6

4 5

5 2

5 5

4 2

5 3

5 0

7 9

5 2

5 1

4 4

4 2

0 5

2 6

4 0

Percent

11. 0

8. 6

8. 5

9. 2

8. 1

6. 4

4 9

7. 1

6. 7

8. 6

11. 8

9. 5

8. 1

8.9

8. 9

11. 0

11. 5

10. 0

12. 6

9. 9

10. 0

8. 5

7. 2

12. 4

6. 6

5. 0

9. 2

8. 2

7. 2

9. 5

9. 7

8. 8

6. 6

9. 7

9. 5

9. 0

9. 5

8. 2

9. 7

8. 7

10. 8

10 6

9 3

9 0

9 6

11 2

9 9

7 6

7 7

7 6

7 8

8 1

7 1

' These shrinkage values have been taken as H of the shrinkage to the oveudry condition as given in the last 2 columns of this table.

' These shrinkage values have been taken as ii, of the shrinkage to the ovendry condition as given in the last 2 columns ol this table.
18 AGRICULTURE HANIDBOOK NO. 188, U.S. DEPT. OF AGRICULTURiE

I'I

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ZM-61223-P

FIGURE 4.—Typical relation of moisture content to shrinkage of Douglas-fir. Although the curves are not straight

lines, for practical shrinliage calculations they may be considered as such.

This shrinkage is for 1 inch. For an 8-inchwide flat-sawed board, this value would become

8xyi6 = %6 or 1/2 inch.

Knowing the total shrinkage of a species at

0 percent moisture content, the percentage of

shrinkage at any moisture content below 30 percent can be calculated. Since shrinkage curves

are reasonably close to straight lines from 30 percent (the fiber saturation point) to 0 percent

moisture content, each 1 percent change in moisture content below 30 percent is equal to %o '^^

the total shrinkage. For example, the total tangential shrinkage of western hemlock dried to

0 percent moisture content is 7.9 percent (table 5).

At 25 percent moisture content its tangential

shrinkage would be %o of Y.9 percent, or about

1.3 percent; at 12 percent its tangential shrinkage would be i%o of 7.9, or about 4.7 percent.

Shrinkage Variability

Shrinkage differs not only with respect to the

length, width, and thickness of a piece and among

species, but even in material cut from the same

species and from the same tree. The overall

shrinkage of wood also is influenced by drying

conditions. Generally, the higher the temperature and relative humidity during the initial

stages of drying, the greater the shrinkage.

ZM-12494-F

FIGURE 5.—Characteristic shrinkage and distortion of

flats, squares, and rounds as affected by the direction

of annual growth rings. The dimensional changes

shown are somewhat exaggerated.

thickness of %2 inch per inch. For example, the

tangential shrinkage of western hemlock at 6 percent moisture content is 6.3 percent (table 5).

Therefore, the shrinkage per inch of width in a

flat-sawed board would be

6.3 ^Vi2^%2 or V16 ii^ch (approximate)
DRY KILN OPERATOR'S MANUAL 19

On the average, hardwoods shrink more than

softwoods. Although, in general, the woods of

high specific gravity have the greater shrinkage,

there are exceptions. Basswood, a light wood,

has a high shrinkage, while black locust, a heavy

wood, has a moderate shrinkage. The amount of

shrinkage and the difference between radial and

tangential shrinkage have a direct bearing on the

seasoning defects that occur during drying.

Effects of Shrinkage

Shrinkage is responsible for decrease in dimension, loss of footage, warping, end and surface

checking, sphtting, honeycombing, collapse, and

casehardening. All of these are discussed in detail in chapter 9, "Drying Defects."

Electrical Properties of Wood

The most important electrical properties of

wood are its dielectric properties and its resistance to the passage of an electric current. These

properties are utilized in electric meters designed

to determine the moisture content of wood {3, 7).

The direct current electrical resistance of wood

varies greatly with moisture content (table 6),

especially below the fiber saturation point, increasing very greatly as the moisture content decreases. It also varies with species, is greater

across the grain than along it, and is affected by

temperature. The resistance is not greatly influenced by differences in specific gravity. Commercial moisture meters of the resistance type

are often calibrated for one species of wood, but

correction tables for other species are supplied

by the manufacturers. Since these meters are

usually calibrated for use at 70° F., the readings

require correction if used at other temperature

(ñg.6).

Meters that use the dielectric properties of

wood, dielectric constant and power factor, are

known as radio frequency power-loss moisture

meters. With this type of instrument, electrodes

are pressed against the wood, and radio frequency

power is applied. The electrodes do not mar or

damage the wood. The amount of power absorbed depends upon the wood's moisture content.

The meter is calibrated directly in terms of moisture content for one species, and values from zero

to the fiber saturation point are indicated on its

dial. Correction tables for other species are provided by the instrument makers.

Color of Wood

As a tree grows, the white or straw-colored

sapwood gradually changes to heartwood, and

with this change there is usually a marked change

in color (5, 9). Holly, basswood, cottonwood,

/// /

/ /

/

/

/ /

y /

r /

/ /

y

/

/

' / /

y / /

/ ^y / y

y

/

/

/ /

/ /

^ y

^

y'

A /

y

y

X y

y y

y '' y

y y

" y ^

y /\ 1 i 1

"'~-ACTUAL UOISTUHE CONTENT

OF WOOD (PERCENT)

1 1 1

^

-^

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TEMPERATURE i'F.I

M-li:.522-F

FiGUUE 0.—Temperature correction chart for resistancetype UKjisture meters.

and magnolia, however, are examples of hardwoods in which the wood undergoes little or no

change in color. Spruces and true firs are examples of softwoods that do not change color materially.

The temperatures used in kihi-diying tend to

darken wood. Dry kiln operators are usually

little concerned with color changes that might

take place in heartwood during drying, but they

are concerned with those that occur in sapwoocl.

Beneficial color changes may be brought about

by steaming green wood before drying it. Walnut, for example, is steamed in \'ats to darken the

sapwood before it is dried. At one time sap gum

was steamed to produce a salmon color that the

using industries preferred and paid a premium

to get. European beech is often steamed before

it is dried in order to improve its color appearance.

Chemical stain can occur when green sapwood

of some species is kiln-dried. The sapwood of

hickorj' tends to turn pinkish when kiln-dried;

very low initial temperatures must be used if its

whiteness is to be preserved. Paper birch sapwood turns brownish Avhen kiln-dried, and many

plants prefer air-drying for that reason.

Perhaps one of the most troublesome color

changes in wood is brown stain that occurs in

ponderosa pine, the true white pines, and some

other softwoods. It is usually most conspicuous

at or near the surface, but it may be present

throughout the wood. The stain is most pronounced on end- and flat-grained surfaces. The
20 AGRICULTURE HANDBOOK NO. 188, U.S. DEPT. OF AGRICULTURE

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DRY KILN OPERATOR'S MAJSTUAL 21

occurrence and control of brown stain are discussed in chapter 9.

For interior woodwork and furniture that is to

receive natural finish, a type of discoloration

called sticker stain can become a serious problem.

Sticker stain generally occurs during air-diying

and appears after planing as blackish or brownish streaks across the faces of boards where they

were in contact with the stickers. In redwood,

however, the sticker markings are light colored.

Although it is generally limited to sapwood, it

occasionally occurs in heartwood. It is a chemical stain and should not be confused with sap

(blue) stain that is caused by fungi.

Literature Cited

(1) BAKER, WILLIAM J.

1956. HOW WOOD DEiEs. U.S. Forest Serv. Forest

Prod. Lab. Rpt. 1642, 9 pp. [Processed.]

(2) BROWN, H. P., PANSHIN, A. J., AND FORSAITH, C. C.

1949. TEXTBOOK OF WOOD TECHNOLOGY, VOL. i.

STRUCTURE, IDENTIFICATION, DEFECTS, AND

USES OF THE COMMERCIAL WOODS OF THE

UNITED STATES. 652 pp., illus. New York.

(3) .JAMES, W. L.

1954. ELECTRICAL MOISTURE METERS FOR WOOD.

U.S. Forest Serv. Forest Prod. Lab. Rpt.

1660, 18 pp., illus. [Proce.ssed.]

(4) LITTLE, ELBERT L., JR.

1953. CHECK LIST OF NATIVE AND NATURALIZED

TREES . . , U.S. Dept, Agr., Agr. Handbook 41, 472 pp.

(5) PECK, E. C.

1953. SHRINKINC AND SWELLINî: OF WOOD IN USE.

U.S. Forest Serv. Forest Prod. Lab. Rpt.

736, 18 pp., illus. [Processed.]

(6) PECK, EDWARD C.

1957. HOW WOOD SHRINKS AND SWELLS. Forest

Prod. ,Jour. 7: 23.5-244, illus.

(7) ToMBACH, HAROLD.

1954. THE ELECTRIC MOISTITRE METER FOR WOOD.

South. Lumberman 188 (2347): 63-64,

66-68, 70, illus.

(8) U.S. FOREST SERV. FOREST PRODUCTS LABORATORY.

1956. WOOD: COLORS AND KINDS. U.S. Dept. Agr.,

Agr. Handbook 101, 36 pp., illus.

(9)

1955. WOOD HANDBOOK. U.S. Dept. Agr., Agr.

Handbook 72, 528 pp., illus.
CHAPTER 2. KILN TYPES AND FEATURES

A dry kiln consists of one or more chambers,

rooms, or tunnels in which air can be circulated

around the wood being dried. The temperature

and relative humidity of this air can be manually or automatically maintained by controlling

the dry- and wet-bulb temperatures.

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 15 percent in a reasonably short time without appreciable losses due to seasoning defects.

Kilns designed to dry wood are classified in this

manual as either compartment or progressive

kilns, on the basis of their construction and operation. Both classes include natural-circulation

and forced-circulation kilns of the ventilated

type.

Compartment kilns are loaded fully at one

time; the entire charge remains stationary

throughout the drying period, and temperature

and relative humidity are kept as uniform as possible throughout the kiln. The dry- and wetbulb temperatures are changed from time to time

as the wood dries. Compartment kilns liave

doors at one or both ends or along tlie sides. A

few kilns have removable roofs through which

the lumber is put in and taken out.

In a progressive kiln, the charge consists of a

number of truckloads of wood, each at a different stage of drying. Progressive kilns have doors

at both ends. The wettest stock is on the truckload nearest the loading, or "green," end of tlie

kiln; the driest near the unloading, or "dry,"

end. Whenever a truckload of dry lumber is removed from the unloading end of the kiln, the

remaining trucks are movecl toward the unloading

end, and a truckload of green or partially dry

lumber is rolled into the kiln at the loading end.

The drying conditions usually are less severe at

the loading than at the unloading end. This is

accomplished mainly by putting more heating

coils at the unloading end of the kiln, where a

higher dry-bulb tem23ei-ature is desired.

Most dry kilns are of the track type. The wood

is stacked on trucks that are rolled into and out

of the kiln on tracks (fig. 7). Kilns of the trackFiGUBE 7.—Lumber stacked on trucks of a track-type kiln.

ZM-105938—F
24 AGRICULTURE HAJ^DBOOK NO. 18 8, U.S. DEPT. OF AGRICULTURE

less type are loaded and unloaded with lift

trucks or overhead cranes and are commonly

called package-loaded kilns.

Compartment Kilns

ISatural-Cirdilation Compartment Kilns

In a natural-circulation compartment kiln

(fig. 8), the air circulates through the loads of

lumber by natural means; the air heated by the

coils rises and that cooled by the wood falls.

Fresh, cool air enters the kiln through fresh-air

ducts that usually extend the full length of the

kiln below the heating coils. Hot, moist air. is

removed from the kiln through roof or wall ventilators.

During the first stages of drying green or partially air-dried stock in a single-track, end-piled

kiln (fig. 8), the heated air moves upward from

the heating coils through the spaces between the

loads and the kiln walls. "\^nien this air reaches

the ceiling of the kiln, it begins to move downward through flues or openings built into the

loads, cooling as it falls. The downward movement continues until the air reaches the heating

coils, where it is reheated, mixed with fresh air

that enters the kiln through the fresh-air ducts,

and recirculated in the kiln. During this stage

of drying, the side coils are turned on and the

center coils oiï.

Wlien the stock has dried to an average moisture content of about 10 to 20 percent, the side

coils are turned off and the center coils on. This

reverses the direction of air movement; the heated

air moves upward through the load and downward along the side walls. This reversal of air

circulation tends to reduce the moisture content

of the boards in the center and lower parts of the

load, which usually dry more slowly during the

initial stages of drying.

End-piled, natural-circulation kilns can be constructed with one or more loading tracks. In

multiple-track kilns of this type, additional heating coils are placed below and between the tracks.

In a cross-piled, natural-circulation kiln, the

loads of lumber are separated so that air can

circulate between them as well as between the

j^

ZM-59503-ir

FiGUEE 8.—A nfitural-circulation, steam-heated, single-track, end-piled compartment kiln. Vents for exlaausting

moist, hot air may be on the roof or, as shown, in the walls. Arrows indicate air movement during the early

stages of drying, when the side coils are on and the center coils off.
DRY KILN OPERATOR'S MANUAL. 25

ends of the loads and the kihi walls. Flues should

be provided in the loads of lumber to facilitate

circulation.

Forced-Circulation Compartment Kilns

Air is circulated within all types of kilns to

(1) carry heat from the steam coils or other

types of heaters to the lumber, (2) mix the air,

and (3) carry away the moisture evaporated

from the wood. To perform these functions more

efficiently than is possible in a natural-circulation

kiln a number of forced-circulation compartment

kilns have been developed that differ mfunly m

the arrangement of fans or blowers. Forcedcirculation compartment kilns, on this basis, are

of two general types—external-blower and internal-fan. ^ n j.

Exterrwl-Bloioer Runs.—Some types of external-blower compartment kilns are shown m hgures 9,10, and 11. In the type shown m figure 9,

the air ducts are located below track level and

run the length of the kiln. There are two returnair ducts, one on each side of a supply duct. A

blower located at one end of the kiln blows the

air through the heating coils and into the supply

duct located directly under tlie center of the

loads of lumber. Along the top of this supply

duct is a series of slots with adjustable air scoops

that pick up the air and direct it through the slots

and thence into an A-shaped flue built into each

load of lumber.

From this A flue, the air moves outward to

each side of the load, then downward. JMost of

the air leaving the load of lumber enters the

return-air ducts through slots, while the rest is

drawn under the load and mixed with the air

being blown into the A flue from the supply

duct. The air that enters the return-air ducts

travels back to the blower and coils to be reheated and again blown into the kiln. The top

of the A flue in each load of lumber is closed to

prevent the air from short circuiting over the

loads.

Hot, moist air is expelled from the kiln through

roof vents, while fresh air enters the kiln through

doors in the blower housing. Steam spray is injected into the kiln to maintain the desired wetbulb temperature. The steam sprays are usually

located in the return-air ducts between the heating coils and the blower.

ZM-r,9501-F

FIGURE 9.—External-blower, single-track, compartment kiln with A-shaped flue in the load. Air moves through the

load in only one direction during the drying period. Throughout the structure, air is under slight pressure :

hence structure can be vented on the leaving-air side of the load or on the low-pressure side of the blower.
26 AGBICUI/rURE HANDBOOK NO. 188, U.S. DEPT. OF AGRICULTURE

ZM-59502-F

FIGURE 10.—External-blower, double-track, compartment dry kiln with A-sbaped flues in the loads. Air circulates in

same direction at all times. Vent is located in entering-air supply duct.

In the type of double-track blower kiln shown

in figure 10, the blower is located below track

level at the end of the kiln. There are two supply ducts and three return-air ducts. The center

return-air duct is sealed near the blower end of

the kiln so that the returning air in this duct

travels to the opposite end of the kiln and returns

to the blower through the two return-air ducts

located on either side of the kiln.

The blower forces air into a main supply duct,

which is divided just inside the kiln into two

ducts that run under the center of each track.

Along the tops of these ducts are entering-air

ports. The air is blown upward through these

ports into an A flue built into each load, moves

across the load, and then downward through

slots in the floor to the return-air ducts. Only

part of the air leaving the load enters the returnair ducts; the rest is drawn under the load,

mixed with the air entering the A flues from the

entering-air ports, and recirculated through the

lumber. The air in the return ducts passes

through heating units, then through the blower,

and back into the main supply duct. The amount

of air coming from the entering-air ports is regulated by adjusting a relief air slot located at the

closed end of the supply ducts.

Hot, moist air is discharged from the kiln

through an adjustable vent damper located on

the floor of the main supply duct. Fresh-air

openings are located in the blower housing.

Steam spray is injected into the kiln, when required, through nozzles located in the blower

housing on the low-pressure side of the blower.

A trackless type of external-blower compartment kiln is shown in figure 11. The packages

of lumber are loaded into the kiln with a forklift truck. A space of 12 to 18 inches is left between the tiers of packages. The blower is located at one end of the kiln above the drying

compartment. Heat and spray units are located

in the blower housing. A main supply-air duct

connected to the blower is located between the

top of the drying compartment and the kiln roof.

Transverse supply-air ducts are connected to the

bottom of the main suj)ply-air duct directly over

alternate spaces between the tiers of packages.

Return-air ducts, which connect to the blower,

are located in the kiln walls.

The heated air moves from the blower through

the main supply-air duct, and is discharged

through nozzles downward into the space between

alternate tiers of packages. From this space the

air moves across the boards of the stickered packages. It then passes into the return-air ducts in

the walls and back to the blower, to be reheated

and blown back into the kiln.

Internal-Fan Kilns.—Fans of internal-fan compartment kilns can be placed in various locations

in relation to the kiln charge (fig. 12). When

natural-circulation kilns are remodeled, the fans

are usually placed in the pit below track level.

Most new kilns, however, are built with overhead

fans.
DRY KILN OPERATOR'S MAISTUAL 27

;iii?y-.««Tjr»»gijS^«r^jf"* "^'

ZM-10594G-P

FIGURE 11.—External-blower compartment kiln loaded with packaged lumber arranged In tiers. Heated air enters the

ehamber through nozzles in ceiling.
28 AGRICULTURE HAN^DBOOK NO. 188, U.S. DEPT. OF AGRICULTUHIE

NO. I NO. 2 NO. 3

NO. 4 NO. 5 NO 6

NO. 7 NO. 8 NO. 9

M-116599-F

PiGUEE 12.—Some plans for location of fans above, below, or alongside lumber charges in internal-fan kilns.

Internal-fan kilns with direct-connected fan

motors are shown in figures 13, 14, and 15. Figure 13 shows a cross-piled, direct-connected, internal-fan, reversible-circulation kiln, the fans of

which are located below track level. This design

is frequently used when cross-piled, natural-circulation kilns are remodeled. A solid floor is

constructed over the fans and the coils between

the outside rails. Catwalks, gratings of metal or

wood, are usually installed between the outside

rails and the walls.

Air is blown across the heating coils, which

extend the full length of the kiln. It then moves

upward through the catwalk into the space between the kiln wall and the ends of the loads of

lumber. Ceiling baffles prevent the air from

short circuiting over the tops of the loads. The

boards in each course of lumber should be placed

at least 2 inches apart, so that the air can move

through the load. A steam spray line usually

extends the length of the kiln below track level.

Roof vents permit hot, moist air to leave the kiln

and cool, dry air to enter. Kiln conditions usually are automatically controlled. Circulation

can be reversed manually or automatically by

means of switches that change the direction of

rotation of the fans.

The kiln shown in figure 14 is a double-track,

internal-fan, reversible-circulation kiln with endpiled loads. The fans and coils are above the
DRY KILN OPERATOR'S MAIvTUAL 29

ZM-59505-F

FIGURE 13.-Compartment kiln witH internal fans directly connected to individual motors Open-piled loads placed

crosswise in kiln. Air flows against stickers and through spaces between edges oí boards. ^ ent on high-pre^suie

side of fan becomes fresh-air inlet when direction of circulation is reversed.

AIR MOTOR FOR AUTOMATIC

VENTILATION AND HUMIDITY

CONTROL

AUTOMATIC

VENTILATORS

STEAM

'SPRAY LINE

CONTROL BULBS

HINGED BAFFLE

BOOSTER COIL

M-105906-F

FIGURE 14.—Double-track compartment kiln with internal fans directly connected to motors. Lumber piles are

loaded endwise, and boards are stacked edge to edge. Air flows parallel to stickers.

555315 O—61 3
30 AGRICUI/rURE HANDBOOK NO. 188, U.S. DEPT. OF AGRICULTURE

-AUTOMATIC VENTILATORS-

, HEATING COIL Ss.

STEAM

SPRAY LINE

CONTROLLER

DRY BULB CONTROLLER

DRY BULB

ZM-84700-F

FiGUBE 15.—Package-loaded compartment dry kiln with internal fans directly connected to motors.

loads. Hinged baffles prevent the air from short

circuiting over tlie top of tlie loads, and walkways

or baffles on the kiln floor prevent short circuiting

under the loads. The air moves through the

heating coils, which extend the length of the

kiln, and then downward in the space between

the kiln wall and die load. It then moves across

the loads and upward to return to the fans. The

illustration shows booster coils located between

the tracks to raise tlio temperature of the air

after it has passed through the first track load

of lumber. A steam spray line extending the

length of the kiln is also shown. Hot, moist air

leaves the kiln and cool, dry air enters it through

the roof vents. Kiln conditions usually are automatically controlled.

A package-loaded, trackless kiln with directconnected internal fans and reversible circulation

is shown in figure 15. This kiln operates in the

same way as the internal-fan, end-piled kiln

shown in figure 14. Baffles that extend from the

bottom to the top of the drying chamber on each

wall are needed to prevent the short circuiting

of air past the ends of the packages. Baffles are

also required between the false ceiling (fan

floor) and the top of the lumber charge.

Short-shaft, internal-fan kilns have shafts of

varying lengths connecting the fans to the motors,

which are located outside the kiln. The motors

can be connected directly or with belts to the

fan shafts. The fans can be located alongside,

above, or below the loads. Baffles prevent the

air from short circuiting around the loads. Kiln

conditions usually are automatically controlled.

A short-shaft, internal-fan kiln design is illustrated in figure 16. The heating system is an

ordinary house furnace using gas, oil, coal, or

wood for fuel. The fans in this kiln should not

be reversed.

A double-track, long-shaft, internal-fan kiln

with reversible circulation is shown in figure 17.

It has alternate right-hand and left-hand fans

mounted on the shaft. This arrangement, together with the fan baffles, forces the air across

the kihi and prevents lengthwise drifting. Baffles between the fan floor and the top of the load,
DRY KILN OPERATOR'S MANUAL 31

LOUV£R£D OPENING

WITH HING£D COVER.

SCALE

0 12 3 4 5 6 FEET

SECTION C-C

ZM-82ie7-P

FiGXJBE 16.—Internal-fan, short-shaft, end-piled compartment kiln with furnace for heating air.

AUTOMATIC VENTS

REVERSIBLE FAN

FAN MOTOR

HEATING COILS

LUMBER STACKED

EDGE TO EDGE '

HEATING COILS

PLENUM CHAMBER

ZM-10I5908-F

FIGURE 17.—Long-shaft, double-track, compartment kiln with alternately opposing internal fans. Vents are over

fan shaft between fans. Vent on high-pressure side of fan becumeb fresh-air inlet when direction o^' circulation

is reversed.
32 AGRICULTURE HANDBOOK NO. 188, U.S. DEPT. OF AGRICULTURE

together with a combination walkway and bafHe

on the kibi floor, prevent tlie air from sliort circuiting below and above the loads. Eoof ventilators permit hot, moist air to escape from the

kiln, and cold, dry air to enter it. Kiln conditions usually are automatically controlled. A

spray line supplies steam for control of relative

humidity, ancl a booster coil reduces the temperature drop between the two tracks of lumber.

A package-loaded, trackless kiln of the longshaft, internal-fan, reversible-circulation type is

shown in figure 18. The fan-baffling system is

the same as that used in the double-track,

shaft kiln shown in figure 17.

long;-

Progressive Kilns

Like compartment kilns, progressive kilns can

be classified into two broad tyj^es, natural-circulation and forced-circulation kilns.

M-106138-F

FIGURE 18.—Loading a trackless compartment kiln with packages of lumber.
DRY KILN OPERATOR'S MAJSTUAL 33

^Natural-Circulation Progressive Kilns

The henting system in natural-ciiciilatioii progressive kilns is designed to produce higher drybulb temi^eratures at the dry end of the chamber

than at the green end (fig. 19). Fresh air enters

the kiln through a duct located on the floor of the

pit at the dry end. It rises through the heating

coils and moves upward through flues or openings

in the loads of lumber in the dry end of the kiln.

The air then moves along the tops of the loads

of lumber toward the green end, flows downward

through the loads into the pit, and then returns

to the dry end of the kiln to í)egin a new cycle.

Moisture is added to the kiln atmosphere, when

necessary, by means of steam sprays. The hot,

moist air is expelled from the kiln through A'entilators. These kilns are designed for both crosspiled and end-piled loads.

Forced-Circulation Progressive Kilns

Fans or blowers are used to circulate heated

air through the loads in forced-circulation progressive kilns.

External-Blower Kiln.—A progressive kiln of

the external-blower type, sometimes called a

"wind-tunnel" dryer, is shown in figure 20. In

this kihi, air is moved through a heating coil

and blown into the dry end of the chamber by

an external blower. The air travels the entire

length of the chamber, becoming cooler as it

passes through the loads of lumber, and is discharged from the kiln at the green end of the

chamber. Loads of cross-piled lumber progress

from the green end to the dry end of the liiln.

When dry, they are removed from the kiln on a

transfer way. The air is prevented from short

circuiting around the loads of lumber by hinged

or canvas bailies suspended from the ceiling and

swung in from both walls.

Internal-Fan Kilns.—Internal-fan, progressive

kilns are usually similar to the internal-fan compartment kiln shown in figure 17. To be operated progressively, the internal-fan kiln must

have split heating systems so that the temperature in each zone can be controlled independently. The temperature at the green end is held

lower than that at the dry end!

General Construction Features

Dry kilns are constructed of many different

kinds of materials (^),^ including wood, brick,

1 Italic numbers in parentheses refer to Literature

Cited, p. 47.

FIGURE 19.—Natural-circulation progressive kiln wi ZM-67flS9-F

t.eM,nanat.e.resU:irl^;^^S:^.rarJrit^^irïï>f '^^'^'^ '''-''' ^"^ «^
34 AGRICULTURE HANDBOOK NO. 188, U.S. DEPT. OF AGRICULTURE

GREEN END OF KILN

ZM-105911-F

FIGURE 20.—External-blower progressive kiln with cross-piled loads. Air drawn into heater from the outside is

blown through the loads of lumber. Cool, moist air is discharged at the green end of the dryer.

tile, concrete, cement blocks with gravel or lightweight aggregate, asbestos-cement boards, and

sheet metal. Kiln size varies considerably. Most

kilns are large enough to accommodate singleor double-loading tracks; some have as many as

four tracks. In general, the height of a kiln with

underload fans is about 12 feet from track level

to ceiling. Kilns equipped with overload fans

are about 17 feet high from track to ceiling.

Trackless, package-loaded kilns may have a drying chamber as high as 28 feet.

The width of a kiln depends primarily on the

number of tracks installed and the method used

to pile the lumber. A single-track kiln with

end-piled lumber is usually 12 to 16 feet wide,

while a single-track kiln with cross-piled lumber

is generally 18 to 22 feet wide. Kilns for endpiled lumber are usually built about 10 feet

wider for each additional track. Package-loaded,

trackless kilns may be as wide as 38 feet. Kiln

lengths may range from 20 to 225 feet.

Heat is usually supplied by heat exchangers,

such as steam coils, unit heaters, or smoke pipes.

In kilns of the forced-circulation type, the heating sj'stems may be above the loads, below the

track level, or on the side of the kiln, depending upon the location of the fans or blowers.

In natural-circulation kilns the heating system

must always be located below track level.

Kilns in which heating coils and fans are below the tracks usually are constructed with the

tracks about 4 to 6 feet above the pit floor,

depending on the size of the fans and coils. The

floor must be high enough above grade level

to prevent water from seeping into the kiln and

interfering with wet-bulb temperature control;

kilns built on low ground near a body of water

are especially exposed to this hazard. The control or instrument room is usually attached to

one end or wall, below track level. If the fan

or blower motors are located outside the kiln,

they also are generally housed in this room.

If the coils and fans are located above the

load, the control room is frequently built at one

end of the kiln on the level of the fan and heating compartment.

Most kilns are constructed with ventilators to

expel hot, moist air and draw in cool air. The

ventilators are generally located on the roof, but

in some external-blower kilns a ventilator stack

and a fresh-air intake are attached to the blower

housing. Ventilators can be either manually or

automatically operated.

A kiln may be built with doors at one or both

ends, or on one side, depending on which layout is most convenient for handling the lumber.

When the doors are located at both ends, the

tracks are usually installed with a slight downgrade from the front (loading) to the back (unloading) end, so that the trucks can be moved

more easily. This is especially important when

trucks are moved by manpower.

Heating System

Heat is required in a dry kiln for four purposes: (1) to warm the wood and the water in

the wood; (2) to evaporate moisture from the

wood; (3) to warm the fresh air entering the
DRY KILN OPERATOR'S MANUAL 35

kiln; and (4) to replace the heat lost by radiation through the kibi structure. Sources of heat

are of two general classes—direct and indirect.

Dh'ect Heating.—In direct-heated kilns the

hot gases produced by burning gas, oil, sawdust, or other fuels are mixed with the air that

is circulated through the loads of lumber. In

some direct-heated kilns, these products are

forced into ducts and then discharged into the

drying chamber. In other types of direct-heated

kilns,''the open flames of gas or oil burners heat

metal radiating surfaces (heat exchangers). Fans

mounted directly over the heat exchanger move

the hot air through the loads of lumber (fig. 21).

Indirect Heating.—in indirect-heatecl kilns

various sources of heat—hot gases and air, electricity, and steam—are used. The heat exchangers can be located either inside or outside the

drying chamber. The exchangers, in tnrn, heat

the air being circulated through the loads of

lumber.

Steam is commonly used at various pressures

as a source of heat in dry kilns. Since the temperature of steam varies with its pressure, more

radiating surface is required to obtain a given

temperature if the kiln is operated at low steam

pressures than if it is operated at high steam

pressures.

Radiating Surfaces

Several types of radiating surfaces are used

in diy kilns, including pipe coils, unit heaters,

hot gas pipes, and electric strip heaters.

Pipe coils used in steam-lieated kilns are of

several types—plain-header coils, single-returnbend header coils, multiple-return-bend header

coils, booster coils, and ceiling coils.

The plain header coils, both horizontal and

vertical (fig. 22), are frequently used as heating units. They may be of any "leng-th, although

short coils are more efficient and produce more

uniform temperatures along their length than

do long coils. They consist of varying numbers (runs) of pipe connected directly to a feed

header at one end and attached to shoi't springer

pipes at the other end. The springer pipes are

m turn connected to a drain header. The coil

pipes are usually from 1 to 2 inches in diameter

and the springer pipes are usually of smaller

diameter. This provides flexibility in the

springer section, so that unequal thermal expansion of the runs will not crack the headers.

The coils should be installed with a downward

pitch from the feed header to the springer section. The pitch usually varies from Vs to Vi.

inch per foot of coil "length, depending upon

the steam pressure used and the space available

for the coils.

^VENT

GAS JET HEAT EXCHANGER

ZM-105m2

FIGURE 21.—Double-track, end-piled, internal-fan compartment kiln heated directly with natural gas.
36 AGBICULTURK HAJN^DBOOK NO. 18 8, U.S. DEPT. OF AGEICTILTURE

DRAIN HEADERDRAIN HEADER

ZM-60739-P

TiauRE 22.—A, Horizontal, and B, vertical plain header coils.

The runs can be of either plain or fin-type

pipe. Fin pipe is ph\in pipe or copper tubing

equipped with thin metal strips or disks (fig. 23).

These fins increase the radiating surface of the

pipe about 5 to 8 times.

Header coils with a single-return bend (figs.

23 and 24) are highly efficient if they are the

proper size to produce uniform temperatures

along their entire length. The feed and drain

headers are of various sizes, so that almost any

number of runs of pipe of either fin (fig. 23)

or plain (fig. 24) pipe can be used. Although

pipes vary in size, they generally are from 1

to 2 inches in diameter. The runs should pitch

downward from the feed to the return-bend

header and from the bend to the drain header

about i/s to 1/4 inch per foot of length, depending upon the steam pressure used and the coil

space available.

Vertical coils located between the tracks of a

multiple-track, end-piled, forced-circulation kiln

are called booster coils (figs. 14 and 17). As the

ZM-106142-P

FIGURE 23.—Heating coil made with fin pipe.
DRY KILN OPERATOR'S MANTJAL 37

-FEED HEADER

'DRAIN HEADER

-FEED HEADER

-DRAIN HEADER

B

ZM-60456-F

PiGUKE 24.-Horizoiital single-return-bend header coils: A, Header coil fitted ^ith 180° return-bend elbows; B, coil

fitted with 90° elbows and connecting nipples.

term "booster" implies, tliese coils raise the temperature of the air after it has passed through

a trackload of lumber.

Unit heaters are generally used where concentrated radiation is desired. They are made up

of rather small-diameter fin pipe running vertically or horizontally. They are generally used

in blower-type kilns and in some internal-fan

kilns.

Metal pipes of various diameters, usually

larger than coil pipes, are used to cany the

products of combustion from burners or furnaces into or through the drying chamber.

Strip heaters are used in electrically heated

kilns. They are of various sizes and are so located that heat is distributed uniformly throughout the kiln.

Steam Traps and Control Valves

Steam traps and control valves are used to conserve steam and regulate its flow into heating

coils.

Steam Traps.—As steam loses heat, it condenses and forms water. If this condensate is

allowed to remain in the coils, they become

waterlogged and cause nonuniform temperatures.

Therefore, the condensate must be removed as

fast as it collects. Automatic valves called steam

traps are installed in the drain lines to remove

condensate without loss of steam. Another function of steam traps is to release trapped air

mixed with the steam. Steam traps should be

installed below the coils. A bypass line around

the trap allows removal for repair or replacement witliout shutting off the coil. All coila

should be individually trapped to preA^ent the

condensate from short circuiting from one coil

to another. The return line to the boiler must

be large enough to handle peak loads of condensate.

Steam traps generally used on dry kilns are of

three types: thermostatic, gravity, and impulse.

A typical thermostatic trap is shown in figure

25. A bellows that expands or contracts with

changes in temperature is attached to a valve

stem and valve. As the bellows expands or contracts, it closes or opens this valve. When the

heating system is first turned on, the coils and

trap are cold and contain air and water. At

this point, the bellows are contracted and the

valve is open. As steam enters the heating system, it displaces the water and air and forces

them through the open valve. When all the air

and water have been discharged, the trap is filled

with live steam. By then the trap temperature

has increased enough to cause the bellows to expand, closing the valve and preventing loss of
38 AGBICTJliTURE HANDBOOK NO. 188, U.S. DEPT. OF AGRICULTURE

THLRM05TA TVALVEf1^ J^ t^i^i

VALVÍ 5EAT

INLET—-

TEST OUTLET

Z1I-60333-F

FIGURE 25.—Thermostatically controlled steam trap.

steam through the trap outlet. After the valve

is closed, condensate again begins to accumulât«

and cool the bellows. This contracts the bellows

enough to open the discharge valve, and the cycle

is repeated.

The gravity-type traps often used on dry-kiln

heating systems are of open-bucket or invertedbucket design. iVn inverted-bucket type of

gravity trap is sho^Yn in figure 26. As steam

condenses in the heating system, the condensate

flows into the trap. When the trap is filled, the

condensate discharges tlirough the outlet pipe.

As soon as the system is free of condensate,

steam enters the inverted bucket. The pressure

of steam causes the bucket to rise against the

valve arm until the valve closes the discharge

port. Air trapped in the bucket escapes through

a vent at the top of the bucket and accumulates

in the top of the trap. Condensate again begins to flow into the trap, displacing the steam

in the bucket. This reduces the buoyancy of the

bucket until it again rests on the bottom of the

trap. The discharge valve then opens and allows

the condensate to be discharged. Since the air

JL5T OUTLETVALVE SEATFULCRUM PO/AÍTSVAL V£-

OUTLET

CAP

-BUCKET STRAP

-AIR VENT

BUCHET

UCKET WEIGHT

INLET

ZM-60331-Î'

FIGURE 26.—Inverted-bucket steam trap.

in the top of the trap escapes before the condensate does, air binding is kept to a minirnum.

An open-bucket (open-top float) trap is shown

in figure 27. When condensate enters the trap,

it, together with any scale or dirt it contains,

is diverted downward by the splash plate into

the bottom of the trap. As condensate accumulates in the lower part of the trap, the bucket

is floated upward until the valve contacts the

valve seat. The condensate eventually overflows

into the bucket. The bucket loses buoyancy and

begins to sink, allowing more condensate to flow

into it. As the bucket sinks, the projection on

the guide post is lowered until it contacts the

collar at the bottom of the valve stem, pulling

the valve clear of the valve seat. Condensate is

discharged from the discharge tube through the

valve opening and trap discharge outlet.

"V^Hien the weight of condensate in the bucket

is reduced sufiiciently, the bucket rises, seating

the valve firmly against the valve seat, and the

flow of condensate from the trap is stopped.

Since the valve is closed while the discharge tube

is still full of condensate, the valve is water

sealed while closed, and cannot blow live steam.

Condensate again flows into the trap, and the

cycle is repeated. A thermostatic air vent, when

open, permits air to escape from the trap without

loss of steam.

A trap of the impulse type is shown in figure

28. The flow of condensate through this trap

is controlled by differences in pressure between

the inlet chamber and the control chamber. When

the steam is off and the trap is filled with air,

the pressure is the same in the inlet as in the

control chamber, and the control valve rests

firmly against the valve seat. When condensate

enters the trap, the pressure in the inlet chamber becomes greater than that in the control chamber. The pressure on the under side of the control disk lifts the control valve free of the valve

seat, and air and condensate pass through the

valve opening into the discharge line.

The control cylinder has a reverse taper that

adjusts the flow of condensate around the control

disk and into the control chamber, until the

pressures above and below the disk are balanced.

Then the temperature of the condensate increases

because of the hot steam behind it. The hot

condensate entering the lower pressure control

chamber flashes into steam, which increases in

volume and retards the flow of condensate

through the control-valve orifice. When the

downward pressure on the upper surface of the

valve and valve disk exceeds the upward pressure on the rim of the vah^e disk, the valve is

forced downward, shutting off the flow of condensate thi-ough the main orifice. The temperature in the control chamber then drops, and the

cycle is repeated.
DRY KILN OPERATOR'S MAI^UAIJ 39

SPLASH PLATE

INLET

COLLAR

IMPACT PLUG

DISCHARGE PORT

CONTINUOUS

THERMOSTATIC AIR VENT

PROJECTION

OUTLET

VALVE AND VALVE SEAT

DISCHARGE TUBE

VALVE STEM

OPENING IN GUIDE POST

GUIDE POST ATTACHED

TO BUCKET

• DRAIN PLUG

"M-115533-F

FiGUBE 27.—Open-bucket steam trap.

Control Valves.—Both manually and automatically operated valves are used to control the

flow of steam into the coils. Pressure regulators

and reducing valves are also used to control the

pressure of the steam.

Hand valves of the globe type are usually installed on the feed and drain lines of the heating

coils. The hand valves on the feed lines are installed between the automatically operated valves

and the heating coils in automatically controlled

kilns. In manually controlled kilns, they are

installed between the main feed line and the heating coils. Hand valves in the drain lines are installed between the coils and the traps. Additional hand valves are also installed m the bypass line around the traps and sometimes in the

return lines to the boiler room. These hand

valves should be either wide open or tightly

closed.

Pressure-reducing valves or regulators should

be installed in the main feed line between the

boiler and the kiln. Several reducing valves

may be needed in a large battery of kilns to

compensate for pressure drops in the main feed

line. Valves of this type prevent the pressure

from going above a set maximum. A pressure

gage should be installed on the low-pressure side

of the reducing valve. An automatic pressure

regulator of a type used on dry kilns is shown

in figure 29.

Automatic valves, operated by compressed air

or electricity, are recommended for control of

drying conditions in a kiln. They are part of

the automatic control system and, in steam-heated

kilns, are installed between the main steam-supply line and the heating coils and between the

main line and the steam-spray lines. A steam

piping arrangement for a dry kiln is shown in

figure 30.

Spraying Systems

Steam or water sprays are commonly used as

a means of humidification in the dry kiln to control the wet-bulb temperatures during the drying, equalizing, and conditioning stages of the

seasoning operation.

Steam Sprays.—Steam-spray lines are usually

installed in steam-heated kilns and sometimes in

furnace-type kilns. As a rule, the spray lines

are made of plain pipe along which spray holes

or jets are spaced. The spray lines may extend
40 AGRICULTURE HANDBOOK NO. 188, U.S. DE,PT. OF AGRICULTURE

CONTROL CYtrNDER

CONTROL ORIFICE

VALVE

INLET CHAMBER

CONTROL CHAMBER

CONTROL 'DISK

VALVE SEAT

ZM-90210-F

FIGURE 28.—Impulse steam trap.

the length of the kiln, they may run across the

kiln, or they may be located in the housing of

external blowers. They are usually located near

the fans or blower, so that the spray is mixed

with the circulating air before it reaches the

lumber. In natural-circulation kilns, the spray

lines are usually located below track level. Steam

should never spray directly on lumber, heating

coils, rails, or fans. Wet steam at low pressures is more satisfactory than dry steam for

control purposes.

Water Sprays.—In plants that do not have a

source of steam for wet-bulb temperature control, water sprays are sometimes used for humidification. The water should be injected into

the kiln in the form of a fine mist. Usually the

mist is vaporized by spraying it directly onto

the heat exchangers. In some kilns, water sprays

ai'e used in conjunction with steam sprays.

Venting Systems

Some means of removing excess moisture from

a kiln is necessaiy to obtain the desired wetbulb depi-ession during drying. This is accomplished by means of vents and fresh-air intakes.

The size and number of openings required depends upon the material being dried (^). More

venting will be required for fast-drying woods

of high moisture content than for slow-drying

woods of low moisture content. It is better to

provide too much venting area than not enough,

since the amount of venting can be reduced by

adjusting the vent dampere or caps. Although

venting can be controlled manually, automatic

control is preferred.

Venting Natural-Oirculation Kilns.—In natural-circulation kilns, the hot, moist air is vented

through roof or wall vents (fig. 8, p. 24). Sliding dampers control the amount of venting

through wall vents, and vent caps control roof

venting. Fresh-air ducts, located as shown in

figure 8, are also required. The amount of fresh

air entering the kiln can be controlled by opening or closing the fresh-air doors. In old natural-circulation kilns of the progressive type, the

vents are usually located at the green or loading

end of the kiln (fig. 19, p. 33), but on newer

kilns of this type they are spaced along the
DRY KILN OPERATOR'S MANUAL 41

ZM-106108-F

FIGURE 29.—A, Air-aperated steam-pressure regulator ; B, pressure-control valve on steam line.
42 AGRICULTURE HAJSPDBOOK NO. 18 8, U.S. DEPT. OP AGRICULTURE

ZM-106143-F

FIGURE 30.—Manual and, at right, automatic air-operated valves on steam piping for dry kiln.

attached to the diaphragm heads of automatic valves.

Copper air tubes are

length of the drying chamber, as in compartment

kilns.

Venting Force.d-Circidation Kilns.—In forcedcirculation kilns, hot, moist air is exhausted

through vents located on the high-pressure side

of the fans or blower, while fresh air enters the

kiln through openings on the low-pressure side.

For the best and most economical operation at

any one time, both the intake and exliau.st Agents

should be either open or closed. In some internal-fan kilns the vents are located on the

roofs, one line of vents being on each side of the

kiln (fig. 14, p. 29). Long-shaft, internal-fan

kilns with tlie fans above the loads usually have

the vents installed directly over the fan shaft

(fig. 17, p. 31). If additional venting is needed,

manually operated auxiliary vents can be installed (fig. 31).

In external-blower kilns, the vents can be located in the blower housing, in the floor of the

supply duct (fig. 10, p. 26), or on the roof (fig.

9, p. 25). Fresh air usually enters blower kilns

through fresh-air doors located in the blower

housing on the low-pressure side of the blower

(figs. 9 and 10).

Air-Circulation Systems

Air can be circulated in dry kilns by natural

or mechanical means. The air velocities through

the loads in natural-circulation kihis are low,

usually less than 30 feet per minute. In forcedcirculation kilns, the air velocities may vary

from 70 to 400 feet per minute, depending upon

the tj'pe, size, speed, number, and location of the

blowers or fans. Generally speaking, the higher

the air velocity through the load, the faster will

be the drying (S). In most forced-circulation,

internal-fan kilns, and in some external-blower

kilns, the direction of air circulation can be reversed manually or automatically. Some kilns

are equipped with variable-speed fan motors to

vary the velocity of the air passing through the

loads of lumber.

Natural-Circulation Kilns.—In natural-circulation kilns, air movement depends on the fact

that warm air rises and cool air falls. To take

advantage of natural air movement, the heating

system in a natural-circulation kiln must be properly designed and located.

The heating coils in natural-circulation kilns

should be located about 11/2 feet below track level,

and about the same distance above the pit floor.

Usually one or more coils are placed on each

side of the kiln, directly below the space between

the kiln wall and the lumber. This space should

be at least 2 feet wide. Other coils are located

below the loads of lumber (fig. 8). A space of

at least 2 feet should be provided between the

top of the load and the ceiling of the kiln. Load

baffles should not be installed, because they inter-
DRY KILN OPERATOR'S MANTJAL 43

ZM-105942-F

FIGURE 31.—Roof vents on overhead long-shaft, internal-fan, compartment kiln. Vent linkage is connected to diaphragm

of air-operated valve. Row of auxiliary vents on side of roof is manually operated and opened only during

stages of kiln ran, when the main roof vents are inadequate.

fere with air movement. Obstructions at track

level, such as solid floors or walks, will interfere with air circulation. Walkways, if needed,

should be of the gratino- type.

Forced-Circulation Kihvs.—In forced-circulation kilns, air is circulated by fans or blowers.

The air may be moved in any direction, depending upon the kiln type and method of loading.

Some types of external-blower compartment

kihis are shown in figures 9, 10, and 11. For

uniform delivery of air to the loads, the supply

ducts may be tapered or equipped with adjustable air scoops, or relief air slots (fig. 10). Ordinarily no baffles are required other than the closures at the top of the A flues. The space between

the side walls and the loads should be large enough

to permit unrestricted flow of air to the return-air

ducts without causing back pressures. To make

it easier to inspect the lumber as it dries, and to

expedite removal of samples for examination and

weighing, it is recommended that this space be

not less than 18 inches wide.

The size and number of fans required in internal-fan compartment kilns will depend upon

the size of the kiln and the air velocities desired.

The plenum chambers should be wide enough so

that the static pressure built up in them is sufficient to insure imiform air flow across the loads

from bottom to top. For best drying results,

the higher the loads, the wider should be the

plenum chamber.

When fans are located on the side of the kiln,

the distance between the fans and the loads should

be great enough to assure that the air is uniformly distributed over the sides of the loads.

Fan and load baflies should be so arranged that

all the air flows through the loads of lumber and

does not bypass them. Usually, baffles are placed

at the top and bottom of the loads (figs. 14 and

17). The air in the cross-piled, internal-fan kiln

shown in figure 13 moves against the stickers.

Therefore a space of at least 2 inches should be

left between the edges of all boards, so that the

air can move through the loads.

Usually the air m a forced-circulation, progressive kiln is circulated by the same method

used in a compartment kiln. One type of progressive kiln (fig. 20, p. 31) has a large blower

or fan located at one end to force the air lengthwise through the drying chamber. In this kiln,

side and top baffles should be installed at each

truckload of lumber to cut down short circuiting

of the air.

Equipment To Control

Drying Conditions

Though drying conditions in a kiln are best

controlled by automatic equipment (i), manual

control is occasionally used.
44 AGRICULTURE HAJSTDBOOK NO. 18 8, U.S. DEPT. OF AGRICULTURE

Automatic Control Equipment

Most dry kilns are equipped with instruments

(thermostats) that automatically control and record dry- and wet-bulb temperatures. Some

thermostats consist of control bulbs connected to

bellows or pressure springs by capillary tubes.

Changes in temperature at the control bullss result

in pressure changes in the closed systems that

directly or indirectly actuate motor valves either

by air pressure or electrically. One motor valve

controls steam flow into the heating coils, another controls the flow of steam or water used

for humidification, and a third controls the opening and closing of the vents. In one type of

thermostat, the motor valves are indirectly actuated by an electronic circuit.

The thermostat may control either the dry- or

the wet-bulb temperature. To control the wetbulb temperature, the bulb is provided with a

suitable wick and water supply. The control

bulbs should be so located in the kiln that they

are exposed to ample air circulation.

The direct-acting thermostat is called a selfcontained type, while the indirect-acting thermostat is known as an auxiliary-operated type because it actuates a power relay to a motor valve

rather than directly controls a valve.

Self-Contained Theivnostats.—Self-contained

thermostats combine in a single unit a valve and

a filled system consisting of the bulb, the capillary connecting tube, and the motor valve. The

bulb of the tube system is placed in the kiln.

Temperature variations at the bulb change the

pressure inside the bulb, causing corresponding

pressure changes on a bellows diaphragm in the

motor valve. Movement of the diaphragm produces movement of the valve. The valve is usually balanced so that it moves easily. Constant

counter-pressure is provided by an adjustable

spring or sliding weight that tends to keep the

valve open. The instrument is set for desired

temperatures by changing the tension of the

spring or the position of the weight. The correct setting is a matter of judgment.

The principal advantages of the self-contained

thermostat are that no auxiliary source of power

is required for its operation, and its initial cost

is comparatively small. An important disadvantage is its relatively slow response to changes in

temperature. In acldition, fluctuations in the temperature of the operating room where the motor

vah^e is located may upset the balance of the instrument and result in incorrect tempeature control.

Auxiliary-Oferated, Thermostats.—Auxiliaryoperated thermostats are made in a number of

different types and styles. Electricity or compressed air is commonly used to power them.

Most auxiliary-operated thermostats in dry kiln

service are air operated, and record as well as

control kiln temperatures ; they are therefore

called recorder-controllers.

In these instruments the control bulb and capillary tube are connected to a helical or other type

of pressure spring (fig. 32). Temperature

changes at the control bulb cause pressure

changes in the spring that expand or contract

it. This movement operates an air or electric

relay that regulates power to a motor valve on

the heat, spraj', or vent system (ñg. 33). It

also operates the recording pen.

Most recorder-controllers used in dry kilns are

equipped with a wet bulb and one or more dry

bulbs. In instruments with more than one dry

bulb, the bulbs may function either separately

or together in a single system. Instruments in

which the dry bulbs function separately are used

to control the temperature in kilns equipped with

split heating systems; these are usually called

multizone controllers. "V\nien two dry bulbs are

connected to a common capillary tube that transmits pressure changes to a single pressure spring,

the instrument is known as a dual-bulb controller. Instruments of this type are used in

reversible-circulation kilns, with the dual dry

bulbs located directly opposite each other on each

side wall. Since the bulb on the entering-air side

is at the higher temperature, it will act as the

controlling bulb. It sometimes takes several minutes after the air circulation reverses for the

controlling bulb to become effective, because of

the time it takes for the pressure throughout

the system to become equalized. This lag in

control whenever the direction of air circulation is reversed causes a slight break in an otherwise smooth temperature record. Figure 34

shows an installation layout for a two-zone, dualbulb control system.

The electronic type of auxiliary-operated thermostat controls and records temperatures by

means of thermocouples and an electronic potentiometer. The electromotive force generated by

the effect of temperature on the thermocouples

located in the kiln is automatically balanced by

the instrument. The balancing mechanism is continuously receptive to changes in electromotive

force. Changes in the balancing mechanism

change the position of the recorder pen and

temperature indicator. Coupled to the recording unit is the control unit which operates either

electric or air-actuated control valves.

The thermocouples are placed in different parts

of the kiln so temperatures can be easily read

at any location by means of a switching mechanism. Control can be obtained in any desired

zone merely by switching to the thermocouple

located in that zone. A separate instrument with

a thermocouple, wick, and water supply is needed
DRY KILN OPEKATOR'S MANUAL

PEN-ARM MOVEMENT

45

MEASURING ELEMENT

(HELICAL SPRING)

CONNECTING

TUBING

ZM-90209-F

FiQUBE 32.—Schematic drawing of an auxiliary-operated thermostat

to control and record the wet-bulb temperature

in the kiln.

Manual Control Equipment

Although automatic control of kiln-drying conditions is desirable, manual control is possible.

For successful manual control of drying conditions, the dry- and wet-bulb temperatures must

be known. If these temperatures differ from

those desired, the valves that regulate the flow

of steam and spray into the kiln must be adjusted until the desired thermometer readings are

obtained.

Temperature-Measuring Devices.—The temperature-measuring devices used in dry kilns are

of two classes, indicating and recording. Glassstemmed indicating thermometers are frequently

used. The most satisfactory glass-stemmed thermometers have the graduations etched on the

555315 0—61 4

stem. Figure 35 shows such a thermometer encased in a metal sleeve that protects it—but also

causes it to be somewhat sluggish. Thermometers with separate scales stamped on an attached

metal strip are not very satisfactory, since any

shifting of the strip with relation to the thermometer tube will cause incorrect readings. An

indicating thermometer of the pressure-spring

type is shown in figure 36. The temperature at

the bulb, which is placed in the dry kiln, is indicated by the needle on the gage.

Glass-stemmed thermometers of the maximum

type are also used to obtain dry-bulb temperatures. Maximum thermometers show the highest temperature to which they have been exposed and after each reading must be shaken

down like clinical thermometers. Care should

be taken, in using maximum thermometers, to

allow enough time for the mercury to reach a

peak temperature.
46 AGRŒCTILTURE HAÍ^DBOOK NO. 18 8, U.S. DEPT. OF AGRXCULTtIK'E

VENT-CONTROL

VALVE

TEMPERATURE

AND HUMIDITY

CONTROLLER'

HEAT-CONTROL

VALVE

OVERFLOW

DRAIN

DUAL DRY BULB ON

OPPOSITE WALL

WATER

SUPPLY

MAIN

SUPPLY

SPRAY-CONTROL

VALVE

FIGURE 33.—An installation layout for automatic control equipment used on dry kilns.

ZM-87474-F

MAIN

STEAM SUPPLY

,AUTOMATIC STEAM

SPRAY VALVE TEMPERA TURE- CONTROL BUL BS,

^\\V^^\V^V\\\\\V\\\\VVSV^V\VVV\\\\\\\\^^^^^

AUTOMATIC

HEAT-VALVE

(ZONE I)

AUTOMATIC

HEAT-VALVE

(ZONE 2)

^ nti auLa -^^. ItMt^tHAlUHt-CONTROL BULBS ~^_ ^

ZONE I -H K- ZONE 2

FIGURE 34.—Installation layout for control bulbs in kiln with two-zone heating system.

ZM-105913-F
DRY KILN OPE'RATOR'S MANUAL 47

ZM-90208-F

FIGURE 35.—Etched-glass thermometer in metal protecting case.

ZM-STéTS-F

FIGURE 36.- -A pressure-spring type of indicating

thermometer.

Eecording thermometers used in dry kilns are

almost always of the pressure-spring type, in

which the bulb is connected to the instrument by

a capillary tube. Figure 37 shows the internal

mechanisms of a recording thermometer. Pressure changes produced in the bulb are transmitted through the capillary tube to the pressure

spring. The pressure changes cause the free

outer end of the spring to move back and forth.

The movement is transmitted through the connecting link to the pen arm, which records the

temperature on the chart. The clock movement

rotates the chart.

Literature Cited

(1) KiMBALL, K. E.

1956. DEVICES TH.^T MEASURE AND CONTROL TEMPERATURES IN DRY KILNS. U.S. ForOSt

Serv. Forest Prod. Lab. Rpt. 1654, 12 pp.,

illus. [Processed.]

(2) TEESDALE, L. V.

1956. DRY-KILN BUILDING MATERIALS AND CONSTRUCTION. U.S. Forest Serv. Forest Prod.

Lab. Rpt. 1646, 10 pp., illus. [Processed.]

(3) TORGESOX, O. W.

1953. CIRCULATION OF AIR IN A LUMBER DRY KILN.

U.S. Forest Serv. Forest Prod. Lab. Rpt.

1678, 6 pp. [Processed.]

C4)

1954. FUNCTION AND CALCULATION OF VENTILATION

IN DRYING COMPARTMENTS. U.S. FoiCSt

Serv. Forest Prod. Lab. Rpt. 1265, 3 pp.,

illus. [Processed.]
48 AGRiECTILTURE HANDBOOK NO. 188, U.S. DEFT. OF AGEICXJIiTURE

FiouEE 37.—Pressure-spring recording thermometer: A, Clock; B, pressure spring; C, connecting link; D, pen arm;

E, chart ; and F, capillary tube.
CHAPTER 3. AUXILIARY KILN EQUIPMENT

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 and velocity of air in the kiln in order to

maintain uniform conditions for fast drying.

Equipment for Determining

Moisture Content

Such items as balances, scales, drying ovens,

saws, and electric moisture meters are used in

determining the moisture content of wood. Distillation equipment is used for accurate determination of moisture content of woods that hold

relatively large amounts of oil.

Balances and Scales

Many types of balances and scales can be used

for determining the moisture content of wood.

Most types, if properly used, give quick and

sufficiently accurate results.

Triple-Beam Balance.—One of the most commonly used types of balances for weighing small

moisture sections is the triple-beam balance

shown in figure 38. This balance has a central

beam and poise with a 100-gram capacity, a rear

beam of 10-gram capacity, and a front beam of

1-gram capacity. Auxiliary weights hung on the

end of the central beam increase the weighing

capacity. The maximum capacity of the balance shown is 1,111 grams. Its weighing accuriGTJBE 38.—Triple-beam balance with auxiliary weights. ZM-64322—F

49
50 AGRICULTURE HAOT)BOOK NO. 18 8, U.S. DEFT. OF AGRICU1,TURE

racy, when properly balanced, is within ±0.01

gram.

Pan-Type Balunce.—A pan-type balance, commonly called a torsion balance, is also used to

weigh small moisture sections (fig. 39). This

balance also has a weighing accuracy, when

properly adjusted, of ±0.01 gram. Auxiliary

weights are used for weighing beyond the capacity of the beams. This balance, because of

the added manipulation of the loose weights, is

slower to use than one of the triple-beam type.

Self-Calculating Balance.—To calculate moisture content, it is necessary to know the original

and the ovendry weight of the wood sections.

The loss in weight is divided by the ovendry

weight. Self-calculating balances, similar to the

one shown in the upper part of figure 40, have

been developed to speed up these calculations or

to eliminate them entirely. As shown in the

lower part of figure 40, the moisture readings

can be estimated to the nearest 0.5 percent when

the values are less than 10 percent, and to the

nearest 1.0 percent when the values are more

than 10 percent. A prescribed sequence of operating steps, supplied by the manufacturer,

must be followed in carrying out a moisture

content determination with this balance.

Platform and Indicating Scales.—Scales of the

platform type witli a capacity of about 35 to 40

pounds are commonly used to weigh kiln samples (fig. 41). For simplifying moisture content calculations, they should be graduated in

decimals of a pound rather than in ounces.

When a considerable number of kiln samples

are w^eighed daily, an indicating type of scale

(fig. 42) is useful, since the indicator can be

read directly to the nearest graduation, 1 gram

or 0.01 pound.

Self-Calculating Scale.—Another type of scale

used to determine the daily or current moisture

content of kiln samples is the moisture guide

(fig. 43). It has a movable weight on the long

arm of a graduated beam. Attached to the

short arm of the beam is a semicircular plate

graduated in terms of percentage of moisture

content. Above this plate is a movable indicator arm with a hook.

If this moisture guide is to be used with reasonable accuracy, certain procedures must be

followed :

(1) Immediately after the moisture content

sections have been cut and weighed, hang the

kiln sample from the hook on the movable indicator arm, with the indicator set at zero. Move

the sliding weight on the long beam to a point

that brings the beam in balance. Eecord the

value of the balancing point on the kiln sample,

and place the sample in the kiln with the truckload of lumber it represents.

(2) Ovendry the moisture content sections to

constant weight and calculate their moisture content values.

ï^

FIGURE 39.—Pan or torsion balance with double beam and auxiliary weights.

ZM-64321-F
DRY KILN OPERATOR'S MAHSTUAL 51

ZM-90343-F

FIGURE 40.—Self-calculating moisture-content balance. Top, Triple-beam balance with special scale on specimen pan

used to calculate moisture content after ovendrying. Bottom, Specimen pan is carried on revolving indicator,

which, when properly used, indicates moisture content directly on scale.
52 AGRICULTURE HANDBOOK NO. 188, U.S. DEFT. OF AGRICULTURE

ZM-e4323-P

FiGUEE 41.—Platform scale with capacity of 36 pounds and accuracy of 0.01 pound.

FIGURE 42.—Indicating scale.

ZM-60557-F
DRY KILN OPERATOR'S MANUAL. 53

"-';r'

ZM-89T80-P

FiGUBE 43.—Self-calculating scale or guide for determining moisture content of kiln samples.

(3) When the moisture content of the moisture sections has been obtained, remove the saniple from the kiln, hang it on the movable indicator hook with the indicator set at zero, and

move the sliding weight on the long beam to

the setting determined in step 1. Then place

metal weights, such as washers or lead slugs, on

the end of the kiln sample until the long beam

balances.

(4) With added metal weights in place, set

the movable indicator arm to the moisture content value of the sections determined in step 2,

and move the sliding weight on the long beam

until balance is again obtained. Erase or cross

out the previously recorded value on the kiln

sample and record the new balance value. This

new value will be the setting of the sliding

weight on the long beam used for all subsequent

moisture determinations.

(5) Remove the metal weights from the sample. With the sliding weight set at the new

value obtained in step 4, move the indicator arm

until the long beam balances. The current moisture content of the kiln sample can then be read

on the semicircular plate.

(6) Subsequent moisture content values of the

samples are obtained by setting the sliding

weight on the long beam at the new balance

value obtained in step 4, hanging the kiln sample on the movable indicator hook, and moving

the indicator arm until the long beam is balanced. The current moisture content is read on

the semicircular plate.

Saws

HandsaAvs are not recommended for the cutting of moisture sections. Table, swing, portable, and band saws are generally used. A band

saw (fig. 44) is particularly suitable for slotting

and slicing small sections for moisture-distribution and casehardening tests. Saws should be

sharp, properly set, and provided with suitable

safety devices.

Drying Ovens

Several kinds of ovens are used for the drying

of moisture sections. Drying ovens should be

large enough to provide adequate open spaces

between the sections of wood being dried. The

temperature of the oven should be controlled

with a thermostat or other means so it will not

rise above the desired setting (212° to 220° F.).

Excessive temperature will char the sections and

may also start fires. The oven should have ventilators on the top or sides and bottom to allow

the evaporating moisture to escape.

Electrically Heated Ovens.—Ovens commonly

used in kiln-drying work are electrically heated

(fig. 45). Clean the contact points of the thermostat from time to time to prevent them from

sticking. Check the thermostat setting when the

oven is empty, using a thermometer inserted in

the top of the oven. Some ovens contain fans

to circulate the air and speed up drying (fig.
54 AGRICULTURE HAJSTDBOO-K NO. 188, U.S. DEPT. OF AGRICULTURE

ZM-SOIOI-F

FiGUEE 44.—Bandsaw being used to prepare kiln sample. A saw of this type is best suited for cutting casebardening

test specimens.
DRY KILN OPERATOR'S MAJ^JTJAL 55

ZM-116694-F

FiQTjBE 45.—Natural-circulation electric oven for drying moisture sections.

46). Ovens of this type are recommended if

large numbers of moisture sections are dried

continually.

Steam-Heated Ovens.—Steam-heated drying

ovens are satisfactory if a suitable supply of

steam is continuously available. Ovens of this

type are usually homemade and may be equipped

for either natural- or forced-air circulation. The

temperature in the oven is usually regulated or

controlled by a reducing valve on the steam

feed line. The reducing valve is adjusted to

maintain the desired temperature in accordance

with a thermometer inserted through the top of

the oven. Set the temperature -when the oven

is empty. Shelves for the moisture sections

should be made of perforated metal or large

mesh, heavy wire. Provide ventilators to remove

the moisture-laden air.
56 AGRICULTURE HANDBOOK NO. 188, U.S. DEPT. OF AGRICULTURE

i

ZM-90189-F

FiGUBE 46.—Forced-air-circulation electric drying oven for drying large quantities of moisture sections.

Electric Moisture Meters

Electric moisture meters,^ if properly used,

provide a rapid, convenient, and for most purposes a sufficiently accurate means of determining moisture content when it is less than 30 percent. They are frequently used to segregate wet

from dry boards as a preliminary to kiln-drying

and to determine the moisture content of air- or

kiln-dried stock. Electrical moisture meters are

usually provided with temperature and species

correction data. There are two types of meters,

power loss and resistance.

Poiver-Loss-Type Moisture Meters.—A moisture meter of the power-loss type is shown in

figure 47. The surface-contact electrodes vary

in design according to the material on which

they are to be used. The instrument shown has

eight spring-cushioned contact points equally

1 JAMES, W. L. ELECTRICAL MOISTURE METERS FOR WOOD.

U.S. Forest Serv. Forest Prod. Lab. Rpt. 1660. [Processed.] 1958.

FiGUBE 47.—A radio-frequency power-loss-type moisture

meter.
DRY KILN OPERATOR'S MANUAL, 57

spaced on the circumference of a circle. The

range of these meters is from 0 to about 25 percent moisture content.

Resistance-Type Moisture Meters.—The electrodes of resistance-type moisture meters consist

of needles or contact pins that are driven into

the wood to be tested. The accurate range of

most meters of this type is between 7 and 25

percent moisture content, although some instruments have scales that read above the fiber saturation point. A meter of this type is shown in

figure 48. Special electrodes have been developed to be used with resistance-type meters for

measuring the moisture content of veneer and

thick lumber or poles. A two-pin electrode for

use on thick lumber, timbers, and poles is shown

in figure 49. This electrode is provided with a

depth-indicating scale. To determine the average moisture content of thick planks that have a

normal drying gradient, the pins are driven to a

depth that is about 20 percent or Vs of the thickness of the item. To get an estimate of the average moisture content of a pole, the electrodes

are driven to about 15 percent of the pole diameter at the location of the test. In another type of

two-pin electrode, the shanks of the pins are insulated so that moisture-content readings can be

obtained on stock with a Avet surface.

Distillation Equipment

Some woods contain a high percentage of

volatile compounds or have been impregnated

with oily preservatives. These \'olatiles will be

driven off in the ovendrying process, resulting in

an incorrect moisture-content value. Distillation

equipment sliould be used for determining the

moisture content of such woods.^

Equipment for Determining

Temperatures

It frequently becomes necessary to check temperatures in a dry kiln to determine the causes

for nonuniform drying and the differences in

temperature between the areas around the control bulbs and other areas of the kiln. These

temperature measurements are usually made on

the entering-air side of the loads, although at

JICillLLEN, J. M. METHODS OF DETER^II^'ING THE MOISTURE CONTENT OF WOOD. LT.S. Foi'est Sci'v. Forest Prod.

Lab. Rpt. 1049. [Processed] 1956.

FIGURE 48.—A resistance-type moisture meter.

ZM—86262—F
58 AGRICULTURE HANDBOOK NO. 188, U.S. DEPT. OF AGRICULTURE

ZM-90105-r

FIGURE 49.—A special electrode connected to a resistance meter for making moisture tests on thick lumber, timbers,

and poles.

times leaving-air temperatures are simultaneously obtained so that the tempei-ature drop

across the load can be determined. Etchedstem glass thermometers, hygrometers, pressurespring thermometers, and thermoelectric thermometers are used for this purpose.

Etched-Stem Thermometers

Mercury-in-glass thermometers with the temperature scale etched on the glass stem are frequently used to check dry kiln temperature. They

should be accurate to ±1° F. These thermometers should be placed in the kiln at the locations

to be checked, and not moved when temperature

readings are taken. Obviously, it is necessary

for the kiln operator to go into the kiln to make

these temperature readings. If the wet-bulb

temperature is above 120°, he should wear protective clothing and a face mask to provide

cooled air.

The temperature survey of the kiln can be

quickly made if several thermometers are placed

at the different zones in the kiln where temperature checks are desired. The possibility of breaking glass thermometers is reduced by putting

them in metal slieaths (fig. 35, p. 47). The

sheathed thermometer is suitable for making drybulb measurements, but if wet-bulb temperatures

are also being measured, the sheath must be removed so that a wick can be applied directly over

the mercury well or bulb of the thermometer.

Sometimes dry kiln operators make hygrometers out of etched-stem thermometers by mounting them on a metal plate, frame, or other suitable support. A water cistern is also attached,

and the hygrometer is placed in the kiln where

it is desired to check temperatures. Such a combination is shown in figure 50, A.

Maximum thermometers are also used for

checking kiln temperatures. By mounting two

maximum thermometers on a frame and supplying one with a wick and a water cistern (fig.

50, B), both the maximum wet- and dry-bulb

readings can be obtained.

Hygrometers

Hygrometers of the type shown in figure 51

are sometimes used to check kiln temperatures.

These hygrometers are cheaper than etched-stem

thermometers. There is always, however, the

possibility that the thermometers may move on

the metal graduated bases and so give inaccurate temperature readings.

Pressure-Spring Thermom,eters

Pressure-spring thermometers, sometimes called

dial thermometers, are used to indicate or record

kiln temperatures. An indicating pressurespring thermometer is shown in figure 36. A

description of these thermometers and their operation is given on page 47. The advantage of

the recording thermometer over the indicating

type is that a continuous record is made of the
DRY KILN OPERATOR'S MANUAL, 59

&>*'x. (..^''- -'

ZM-86250-F, ZM-90337-F

FIGURE 50.—A, Wet- and dry-bulb hygrometer made from

two etched-stem glass thermometers; B, wet- and drybulb hygrometer with maximum thermometer.

ZM-90339-F

FiGUEB 51.—Wet- and dry-bulb hygrometer with

thermometer calibration on metal.

temperature conditions at the location of the

bulb. Sometimes the recording pressure-spring

thermometer is used to check dry kiln control

instruments or to record the temperatures in

parts of the dry kiln or duct systems other than

where the control-instrument bulbs are located.

The indicating or recording pressure-spring thermometers can be obtained with various lengths

of capillary tubing. It is necessary to check

them for calibration occasionally.

Thermoelectric Thermometers

The thermoelectric thermometer used for measuring kiln temperatures consists of a thermocouple, lead wires, and a potentiometer. The

thermocouple is usually made from copper and

copper-eonstantan wires that are soldered or fused

together to make the junction. Both wires act
60 AGRICULTURE HANDBOOK NO. 188, U.S. DEPT. OF AGRICULTURE

COPPER LEAD-

.^(^

POTENTIOMETER

NONCONDUCTING PANEL

JACK5

ZM-90341-P

COPPER-.

FiQUBE 52.—Method of attaching thermocouple cable to jack box and potentiometer.

THÍRM0C0UPLL5

THERMOCOUPLE

BRA/iCH LEAD5

WIRE5 LEADING,

TO SWITCHTHERMOCOUPLE

CABLE

CONSTITUENT UNITS TAPCD TOGETHER

THERMOCOUPLE 7

PAIRING

THERMOCOUPLE /

PAIRING

UNIT TAPED FROM THERMOCOUPLES

TO SWITCH

B

1RES L LADING

TO SWITCH

—THERMOCOUPLE I

'0N5TANTAN

'0N5TANTAN

'HERMOCOUPLE Z

ZM-60455-P

FIGURE 53.—A, A 50-foot thermocouple cable ; B, section of a typical unit of main cable.

as lead wires back to the potentiometer. One of

the advantages of the thermoelectric thermometer is that any number of thermocuples can be

installed in the dry kiln.

If several thermocouples are used, the wires are

run to a switclibox that is connected to the potentiometer (fig. 52). With a thermocouple cable containing several lead wires the tliermocouples can be installed rapidly and easily. A

50-foot cable with thermocouples spaced at 10-

foot intervals is shown in figure 53, A and B.

The wire size best suited for dry kiln work is

about 20 gage. Thermocouples should be shielded

from direct radiation.

An indicating potentiometer can be either portable or stationary. A portable type is shown

in figure 54. Kecording-controUing potentiometers are permanently mounted on a panel board

(fig. 55). The instrument can be switched to _86246-F

any thermocouple at any time to measure the ^^^^^ 54.-Portable potentiometer used^'for '

temperature at that location. measuring temperatures.
DRY KILN OPERATOR'S MAITOAL 61

FIGURE 55.—Indicating-recording and controlling potentiometer in dry kiln operating room.

ZM-99177-F

Equipment for Determining

Air Movement

Since the direction and rate of air flow are

important in the operation of a dry kiln, means

of determining these factors are necessary. The

direction of flow may be determined by smoke

tests, while the rate may be measured by anemometers.

Smoke-Making Devices

A special chemical smoke generator has been

devised for use in dry kilns (fig. 56). It consists of two small bottles and a few pieces of

tubing. One bottle contains concentrated hydrochloric acid and the other strong ammonia water.

A^Hien air is blown through the bottles, the fumes

of the two chemicals mix and produce a dense

smoke that drifts with the air current. The vapor of titanium tetrachloride also produces _ a

fireless smoke when exposed to a current of air.

Caution: A mask and rubber gloves and apron

should be worn when handling these chemicals,

to avoid injury to the eyes, skin, or clothing.

The smoke of burning tobacco, punk sticks,

rope, or cloth is sometimes used to detect the dis\

Con centrafed

'Hc/drochloric

Acid ¿IS.

Concentrated

Ammonia

ZM-8-132-F

FiQUEE 56.—A smoke-making device for testing air circulation in dry kilns. Alnicst any kind of bottle.s can be used,

but to avoid bulkiness comparatively narrow ones are desirable. Common ink bottles are shown in the drawing.

A box with handle makes a convenient container. Two pieces of bent-glass tubing, a cork, and a rubber tube

(A) complete the apparatus. Short lengths of glass and rubber tubing may be used instead of the bent glass.

Air is blown by mouth through tube A, so it should be long enough to allow the operator to hold the box at arm's

length. Some operators prefer to fit the end of the rubber tube with a syringe bulb, and this is almost a necessity

when a mask is worn.

555315 0—61. 5
62 AGRICULTURE HANDBOOK NO. 18 8, U.S. DEPT. OF AGRICULTURE

rection of air flow. The use of these materials

is not recommended. They produce only a small

quantity of smoke and embody a fire hazard.

Also, the smoke is hot and tends to rise; therefore the true direction of the circulation may not

be known until the smoke has cooled to the temperature of the surrounding air.

Anemometers

Several types of anemometers are used to determine the velocity of air in dry kilns. One

type (fig. 57) consists of a disk fan mounted on

pivot bearings and provided with a revolution

counter. It is customary to let the fan run for

several minutes and note the number of feet indicated by the comiter. The air-velocity measurement in feet per minute is obtained by dividing the number of feet indicated by the number

of minutes the fan has operated. If the fan is

not set perpendicular to the direction of air flow,

the reading may be less than it should be.

Another type of anemometer, called a hot-wire

air meter (fig. 58), is also used to determine air

velocities. The wire in the probe is heated with

electricity. The amount of cooling of the hot

wire is proportional to the velocity of the air.

Velocities are indicated directly on a scale calibrated in feet per minute.

In another type, the air enters the instrument

through a shutter, jet, or port, and velocity is

read directly in feet per minute on a dial. An

instrument of this type is shown in figure 59.

ZM-89244-F

FIGURE 58.—Hot-wire air meter.

ZM-86248-P

FiGUEE 57.—Fan-type anemometer.

ZM-86243-

FiQUBE 59.—A type of air velocity meter.
CHAPTER 4. INSPECTION AND MAINTENANCE OF

DRY KILNS AND EQUIPMENT

Adequate kiln maintenance is as essential as

good design and construction are to efficient dry

kiln operation. It can only be accomplished

through regular, frequent inspections of the kiln

and auxiliary equipment. If inspections reveal

the necessity for repairs or replacements, they

should be made as soon as possible to avoid many

seasoning difficulties.

Regular, systematic inspections should cover

such items as the kiln structure; doors; tracks;

control equipment; heating, spraying, and venting system; trucks; lumber-handling equipment;

and general housekeeping. To make sure tliat

inspections are thorough, the operator should

use a checklist on which the condition of the

kiln structure and the equipment can be noted.

A checklist that can be made to fit any specific

kiln installation follows:

KILN INSPECTION CHECK LIST

(Where maintenance or replacement is recommended, indicate kiln number.)

I. AIR CIRCULATION SYSTEM

1. Fans and Motors, present condition:

Condition of electrical connections and switches : What maintenance or replacement is recommended :

2. Shafts and bearings, present condition :

Are motor and shaft bearings properly lubricated : What maintenance or replacement is recommended :

3. Fan baßes, present condition:

What maintenance or replacement is recommended :

4. Load baffles, present condition :

Can load baffles be improved : What maintenance or replacement is recommended :

5. Air passageways

Are air passageways open and unobstructed :

Could air movement be improved : What maintenance or replacement is recommended :

II. HEATING'AND'HUMYDïFYîNG'SY^STè'M

1. Feed lines and headers, present condition:

Are they properly insulated : What maintenance or replacement is recommended :

2. Heating coils, present condition:

Are all pipes open to full now of steam :

Condition of supports: What maintenance or replacement is recommended :

3. Traps, present condition:

Are traps in best possible location : What maintenance or replacement is recommended :

4. Hand valves and automatic control valves, present condition :

Are hand valves provided for blowing out coils :

Are hand valves provided for shutting off individual coils :

Are check valves working properly:

What maintenance or replacement is recommended :

5. Spray lines, present condition: ll\^l\llll\lllllll[lllllll[\l[ll"ll\

Are spray holes or nozzles open: ._ /_______

Does condensate from spray line drip on lumloer: I_".II^.I"I] [l.\\l[.\[l[.[

What maintenance or replacement is recommended: IIIIII_I'.IIII"I"mi]'"II'I'

6. Vents, present condition: ---I--I-'^""II]-''I"II" _ _ _ _ _ ""

Do vents open and close properly : ' "

What maintenance or replacement is recommended'; II--IIII----]]-]-I'-III]'I]II'II!

63
64 AGRICULTURE HANDBOOK NO. 188, U.S. DEPT. OF AGRICULTURE

in. CONTROL SYSTEM

1. Recorder-controller, present condition :

Is recorder-controller properly calibrated :

Are capillary tubes protected:

Are bulbs properly located and mounted for accurate readings of kiln conditions: What maintenance or replacement is recommended :

2. Water supply

Is water supply line to wet bulb open :

Is wet-bulb water pan clean:

Is drain line from water pan open :

Is wet-bulb wick replaced regularly : What maintenance or replacement is recommended :

3. Air supply

Is supply adequate, clean, and uninterrupted :

Is compressor in good condition : Are water and grease traps in good condition : What maintenance or replacement is recommended :

IV. BUILDÍNG

1. Doors, present condition:

What maintenance or replacement is recommended :

2. Walls, present condition :

Is protective coating adequate : What other maintenance is recommended:

3. Ceilings, present condition :

Is protective coating adequate : What other maintenance is recommended :

4. Floors and walkways, present condition:

What maintenance or replacement is recommended :

5. Rails and supports, present condition:

What maintenance or replacement is recommended:

V. GENERAL'CÖNDYTIONS'OF YARD "AND'KÏLNS

Are yard tracks and transfer in good condition:

Are kiln trucks in good condition:

What maintenance or replacement is recommended :

Are kilns and surrounding area neat and clean:

The Kiln Structure

Dry kilns are relatively short-lived compared

to ordinary buildings, regardless of the materials

used in their construction. They not only have

to withstand weather but also extreme internal

variations in relative humidity and temperature

and frequently the corrosive action of vapors

coming from "the woods being dried.

Generally the doors of a dry kiln are very

large. To make them serviceable, yet as light

as possible for easy handling, they are usually

constructed of lightweight materials with a minimum of bracing.

W alls. Roofs, and Ceilings

Dry kilns can be constructed of many materials, such as wood, concrete, concrete blocks with

various types of aggregates, brick, tile, asbestoscement boards, and sheet metal. Combinations

of these materials are also used.^ Wood is per1 TEESDALE, L. V. DRY KILN BUILDING MATERIALS AND

CONSTRUCTION. U.S. Forest Serv. Forest Prod. Lab. Rpt.

1646. [Processed.] 1956.

haps the cheapest material to use, and it has good

insulating qualities. However, it does shrink

and swell with changes in moisture content

caused by varying drying conditions, and this

eventually leads to splits in the wood members,

bad cracks at the joints, and loose boards. Wood

also deteriorates when exposed continuously to

high temperatures. These factors increase maintenance problems and reduce the life of wood

kilns much below that of kilns constructed of

other materials.

Other building materials also have disadvantages. Their expansion and contraction due to

temperature changes cause cracks, and their poor

insulating and vapor resistance qualities lead to

excessive loss of heat and vapor. Tables 7 and 8

list coefScients of heat transmission for a number

of building materials.

Results of Poor Maintenance.—Cracks will

eventually develop in walls, roofs, and ceilings

of permanent kiln structures. If cracks are not

sealed when small, they will increase in size,

which leads to excessive heat and vapor losses

and premature failure of the entire structure.
DRY KILN OPERATOR'S MAOSFUAL 65

TABLE 7.—Coeßcient of heat transmission for some common dry kiln wall constructions

WOOD FRAME WALLS

Outside

covering

Sheathing material Insulation Inside covering Coefficient of

transmission

Bevel siding

Do

2^^2-inch shiplap

do

None 2532-inch shiplap . 0. 245

2-inch blanket

3}á-inch blanket. _

do

do

. 087

Do . .do . 057

Do ^4-inch fiberboard

do

None - __ - do 191

Do 2-inch blanket. .. do . 080

Do do -- 3%-inch blanket do . 054

Do 2^^2-inch shiplap

do

do

None _- %-inch cement asbestos board

do .

. 320

Do 2-inch blanket . 095

Do 3%-inch blanket . do . 061

CRIB WALLS

Crib or laminated wall, 6 inches thick, no covering

Crib or laminated wall, 6 inches thick, outside covered with drop siding

. 125

. 114

MASONRY WALLS

Poured concrete

Poured concrete

Concrete block.

, 8-inch ... . 700

, 12-inch. _ __ _ . 570

12 inches thick, of erravel or crushed stone aggregai

ageregate

e . . .. . 490

Concrete block. 12 inches thick, of cinder 380

Concrete block,

Brick waU, 13 íE

12 inches thick, of slag or

ches thick _ ._

burned-clay aggregate-.. . 340

360

Table 8.—Coefficient oj heat transmission for a

number oj structural combinations oj dry

kiln roojs

Structural combination

Roof sheathing, ^^'32 inch and composition

roofing

Roof sheathing, 1% inches and composition

roofing

Roof sheathing, ^2 inch, vapor barrier,

IK-inch sheathing, and composition roofing

Roof sheathing, '^'32 inch, vapor barrier, 2

layers of Yi-inch fiberboard

Ceiling, ^Yii inch, 2-inch blanket insulation,

ventilated attic under roof

Ceiling, "-y-^i inch, 3%-inoh blanket insulation, ventilated attic under roof

Laminated or crib roof, 6 inches thick, and

composition roof

Laminated or crib roof, 8 inches thick,"and

composition roof

Concrete slab, 4 inches thick with gravel

aggregate composition roof

Concrete slab, 4 inches thick with gravel

aggregate, 2 layers of Yi-mch fiberboard

or l}^-mch cork composition roofing

Concrete slab, 3 inches thick, expanded

slag aggregate, and composition roofing

Concrete slab, 6 inches thick, expanded

slag aggregate, and composition roofing..

Corrugated asbestos-cement board, vapor

barrier, 5 inches vermiculite concrete (1

to 6 mixture), composition roofing

Coefliicient

of transmission

0. 602

. 380

. 278

. 173

. 108

. 065

. 125

. 096

1. 010

. 195

. 530

. 326

. 159

Large cracks may also cause cold zones in the

kiln that slow up drying and permit mold and

stain to develop in the lumber located in those

zones. Excessively large holes are sometimes

made in the kiln structure for steam lines (fig.

60), from which there is an excessive loss of heat

and vapor. Lack of a suitable kiln paint on the

interior walls, ceiling, and metal items will lead

to corrosion of the metal, the rapid deterioration

of some kinds of mortar, and loss of heat and

vapor.

Corrective Measures.—If corrective measures

are taken promptly to maintain the kiln walls,

roofs, and ceilings, the repair and operating costs

will be reduced and the life of the kiln prolonged.

Some good maintenance practices for kiln structures are as follows:

(1) Regardless of the type of material of

which the kiln is constructed, coat the inside surfaces with a vapor- and corrosion-resistant material before the kiln is used and Avhenever required

thereafter. Usually recoating is necessary each

year or two. Suitable coatings can be obtained

from dry kiln manufacturers.' Never put vaporresistant coatings on the exterior surfaces of dry

kilns, though a water-repellent coating can be

applied if desired.

(2) As soon as possible, seal cracks that develop in the structure as a result of repeated

^ U.S. FOREST PRODUCTS; LABORATORY, LIST OF DRY KILN

MANUFACTUKER.S AND CONSULTANTS IN THE UNITED STATES.

U.S. Forest Serv. Forest Prod. Lab. Rpt. 1031. [Processed.] 1955.
66 AGRICULTURE HANDBOOK NO. 188, U.S. DEPT. OF AGRICULTURE

FIGURE 60.—Excessively large hole cut through kiln wall for steam-feed lines.

expansion and contraction of the building material. If the cracks are small, a coating of kiln

paint may be sufficient, but fill larger cracks with

mastic, mortar, or cement. Coat the mortar or

cement fillers with a kiln paint after they have

set.

(3) Cracks that develop because of settling of

the structure can be temporarily repaired in the

same manner as expansion and contraction cracks.

To reduce future maintenance costs, however, determine the cause of the settling and correct it

as soon as possible.

(4) Openings in the kiln structure for steam

lines, tubing, fan shafts, and the like should be

as small as possible. Insert sleeves in the openings, and plug the space not occupied by pipe

with mastic, asbestos, or similar material.

(5) Promptly caulk with a nonhardening filler

any open joints and splits that occur in wood

dry kilns. Refasten all loosened boards as soon

as possible.

(6) Use noncorrosive metal fastenings if possible.

(7) Immediately repair or replace supporting

members of the structure that fail.

Doors

Doors are usually the weakest and probably

the most troublesome part of a kiln structure.

They are often damaged when they are opened or

closed carelessly or when an improperly blocked

truckload of lumber rolls into them. It is difficult to make a large door that is strong, lightweight, easy to handle, well insulated, and resistant to corrosion. The special composition

doors now available have most of the desired

properties. Lightweight wood doors that are

properly designed have also proved fairly satisfactory, although their life is usually shorter

than that of metal doors.

Results of Poor Maintenance.—Doors, door

stops, hangers, rollers, and roller tracks that are

poorly maintained cause excessive losses of heat

and vapor and are difficult to open and close.

Lower temperatures occur near damaged or

poorly fitted doors, and drying is slower in that

zone.^ Neglect of doors and door equipment may

also create a hazard to workmen.

Corrective Measures.

(1) Immediately repair or replace damaged

door hangers, rollers, and roller tracks.

(2) Lubricate parts in accordance with the

manufacturer's recommendations.

(3) Coat with a kiln paint inside faces of

doors that are likely to corrode, and recoat whenever necessary. Repair or replace badly corroded doors.

3 RASMUSSEN, E. F. NEED FOR UNIFORMITY OF TEMPERATURE IN A FORCED-AIK-CIRCULATION, VENTILATED, COMPARTMENT DRY KILN. U.S. Forest Serv. Forest Prod. Lab.

Rpt. 1669. [Processed.] 1956.
DRY KILN OPERATOR'S MAOSTUAL 67

(4) Make poorly fitting doors tighter by attaching a strip of heavy felting, canvas, or special rubber gasket to the door frame. Some

kinds of stripping material deteriorate rapidly,

and frequent replacement may be necessary.

(5) Block wheels of standing loaded kiln

trucks so that the trucks cannot roll into the

kiln door.

(6) Eepair or replace damaged doors as soon

as possible. If repairs or replacements must be

delayed, install heavy canvas, thin plywood, or

other suitable material temporarily over the door

opening.

Floors

The floor in a dry kiln is ordinarily constructed

of lumber or concrete, though occasionally it

may be dirt, gravel, crushed stone, or sand. All

types of floors require maintenance.

Results of Poor Maintenance.—Poorly maintained floors increase the likelihood of accidents

and may result in nonuniform drying conditions

and damage to heating, humidifying, and aircirculation systems located below track level.

Neglected floors may also result in loose rails or

weakened supports, causing loaded kiln trucks

to leave the track or tiers of packaged lumber to

collapse. Both may cause additional damage to

the kiln and be a work hazard to the men. If

the floors of pits constructed below ground level

in areas where the ground water table is high

are not kept in good condition, water may seep in.

If this occurs, the drying time will be extended

because of excessively high relative humidities.

Mold and stain may also develop on green lumber in the initial stages of drying.

Corrective Measures.

(1) As soon as possible, seal bad cracks or holes

that develop in a floor built over underload fan

systems. This will prevent the forced air from

short circuiting the lumber being dried.

(2) Cover with gratings the openings in the

floor of a forced-circulation kihi that has fans

or ducts located below the loads of lumber.

Gratings prevent debris from falling into and

damaging the heating or air-circulating equipment. They also provide a safe, substantial walkway for workers.

(3) The floors of kilns in areas where water

seepage is likely should be constructed of waterproof material. Promptly seal all cracks developing in these floors.

(4) Cover dirt, gravel, or sand floors, particularly in forced-air-circulation kilns, with lumber,

blacktop, or concrete to prevent abrasive material

from being blown into the loads of lumber, fans,

and fan motors. A lumber, blacktop, or concrete

covering is also easier to keep clean.

(5) As needed, paint all of the metal parts in

a floor with a corrosive-resistant paint.

(6) Eeplace badly corroded metal floors or

gratings, not only to insure better air circulation

through the loads of lumber but as a safety

measure.

Rails and Rail Supports

Generally, rails and rail supports in kilns with

fans or blowers located above or on the sides or

ends of the drying compartment do not gi\e

much trouble, since the rails are usually well

supported and anchored. Weak rails or rail supports in natural-circulation kilns and in forcedcirculation kilns, where the fans or the air-supply

ducts are located below track level, may collapse

or spread under heavy loads.

Results of Poor Maintenance.—Rails or rail

supports weakened by corrosion, or improperly

supported rails, eventually collapse under the

weight of the loads of lumber. This can seriously damage kiln equipment, workmen may be

injured, and drying time will be lost. If rail

fastenings fail, the rails may spread, causing

similar damage.

Corrective Measures.

(1) Place additional supports under sagging

rails or adjacent to track supports that show

signs of failure.

(2) Immediately replace or tighten broken or

loose rail fastenings.

(3) Promptly realine spread rails and securely

fasten them to the rail supports.

(4) Leave a break in the rails under the doors,

to minimize rail corrosion caused by condensate

dripping from the doors.

(5) As needed, apply corrosive-resistant paints

to the rails, metal rail supports, and rail fastenings.

(6) Replace rails or rail supports so badly corroded as to be unsafe.

Recording-Controlling Instruments

Accurate control of both the dry- and wetbulb temperature is essential for efiicient kiln

operation. The most common and the best

method of control is by the use of automatic

recording-controlling instruments. While these

instruments are usually efiicient, they do at times

give trouble. Some of the trouble is associated

with improper location of the bulbs. Once the

bulbs are properly located, troubles may arise

from improper calibration, faulty flow of water

to the Avet-bulb pan, and dirty wet-bulb wicks.

The efficiency of air-operated instruments may

be seriously impaired by oil, water, and dirt iii

the compressed air.

Improperly Located Control Bulbs

Dry bulbs that are improperly located may

result in very high temperatures that increase
68 AGRICULTURE HANDBOOK NO. 188, U.S. DEPT. OF AGRICULTURE

seasoning losses, or in very low temperatures that

prolong the drying time and result in mold and

stain.''

In figure 61, the dry and wet bulbs are

mounted so close to the kiln wall that the circulating air does not contact them properly, and

erratic control of drying conditions can be expected. For example, if the dry bulb is on a

wall adjoining a kiln that is operating at much

higher temperatures, the temperature at the bulb

may be higher than the set point on the control

instrument because of radiation of heat from the

wall. When that occurs, the actual kiln temperature will be lower than the set point.

Improperly located control bulbs are also

shown in figures 62 and 63. The bulbs shown in

figure 62 are in a bypass duct near ñoor level,

between tracks in a double-track kiln. The dry

bulb is too close to the wet bulb, too close to one

side of the duct, and mounted parallel to the

' See footnote 3, p. 66.

ZM-59895-F

FIGURE 61.- -Control bulbs mounted too close to kiln

wall.

direction of air flow. Water from the wet-bulb

wick dampened the dry bulb and caused erratic

dry-bulb temperature control. The location of

the bulbs shown in figure 63 exposes them to

damage, and the planks at track level shield them

to some extent from the circulating air. Here

also the dry bulb Avas dampened because the

water supply to the porous sleeve covering the

wet bulb was excessive and the bulbs were too

close together.

Never locate the dry bulb directly below the

wet bulb. If the wet-bulb water pan were to

overflow, water would fall on the dry bulb.

A dry bulb may be improperly placed in relation to temperature difl^erences within the kiln.

Only a small zone is measured and controlled by

each bulb. If considerable longitudinal or vertical temperature differences persist after all practical measures to insure uniformity have been

taken, move the dry bulb to the hottest zone on

the entering air side of the load. If this cannot

be done, use auxiliary thermometers to fuid the

difference in temperature between the hottest

zone and the control bulb location. Then, underset the instrument's dry-bulb control indicator by

this amount.

Proper Location of Dry Bulbs

Place control bulbs in the entering-air stream

at right angles to the direction of air flow and

as far from the wall as practical (fig. 64). When

bulbs are in this position, the kiln must be loaded

carefully to avoid damaging them. The hazard

can be minimized by mounting the control bulbs

on a hinged extension arm so they can be swung

toward the wall when loading the kiln. Be careful not to bend the capillary tube too sharply,

because a sharp bend could rupture or collapse it.

The bulbs can be mounted horizontally, and

parallel or perpendicular to the kiln wall. In

figure 64 they are placed parallel to the kiln wall,

and in figure 65 they are placed perpendicular.

When mounted parallel to the wall, the bulbs

can be placed end to end as shown in figure 61.

Place them high enough above the floor so that

they will not be accidentally bumped by someone

walking through the kiln, and away from heatradiating surfaces and spray lines.

"V\^ien a reversible-circulation kiln is equipped

with a single dry bulb, place the bulb in a bypass

duct so that it will control the entering-air temperatures regardless of the direction of air flow.

The duct can be located above or below the loads

in most internal-fan kilns, and where it will not

interfere with loading and unloading. The bypass principle is shown in figures 66, 67, and 68.

Make the duct large enough to allow free flow of

air and easy access to the bulbs, and flare it at

each end to increase the velocity pf the air flowing

through it. Use exterior type plywood or other
DRY KILN OPERATOR'S MANUAL 69

M-10980S-P

FIGURE 62.—Control bulbs improperly mounted in a bypass duct at tracic level. Arrow indicates direction of airflow.

Dry bulb is mounted too close to wet bulb, too close to side of duct, and parallel to flow of air.

material that does not radiate heat. If a long

bypass duct is installed, provide a trapdoor beside

or below the control bulbs so they can be examined and removed for calibration from time to

time.

When dual dry bulbs are used to control kiln

temperature, place them on the side walls, directly opposite each other (fig. 69).

In some types of kilns, such as external-blower

kilns, it is impractical to locate the control bulb

in the entering-air stream. In such instances,

Çlace it where it controls leaving-air temperature,

'o compensate for the temperature drop across

the load, underset the dry-bulb temperature control indicator the amount of the temperature

drop.

Location and Care of Wet Bulb

Improper location and care of the wet bulb

results in poor control of the wet-bulb temperature. The wet bulb must be located so that air

circulates around it at all times. When the kiln

is operating, there must be a constant supply of

clean water to the water pan to keep the wick

wet. A dry or partially dry wick will cause an

actual wet-bulb temperature in the kiln lower

than that recorded or indicated on the instrument. Therefore, occasional cleaning of the water

pan and supply line is recommended. The flow

of water to the water pan is usually controlled

by a needle valve, which needs to be regulated

from time to time. If the flow of water is too

rapid, its temperature when reaching the bulb

may be too low.

In some kilns equipped with overhead coil systems, condensate from the drain end of the coils

is used to supply water to the wet-bulb wick.

The condensate is piped from the drain line

through a coiled copper tube for cooling, and

then to the water pan. A needle valve regulates

the flow.

Equip the water pan with an overflow line that

has its discharge end outside the kiln, and regulate the water supply so that a very slow drip,

not a steady flow, appears as discharge. The

overflow line must be kept open to pi^event water

spilling over the top of the pan into the kiln.

If the kiln is shut down for a day or more, shut

off the water supply, and if during this time the

temperature in the kiln is likely to drop below

freezing, drain the water lines and water pan.

After a kiln is shut down, the wick needs to be

replaced.

A dirty or badly encrusted wick affects w^etbulb control. The wet-bulb w-ick should be made

of highly absorbent cloth. Replace it frequently

with a new or laundered one.
70 AGRICULTURE HANDBOOK NO. 18 8, U.S. DEPT. OF AGRICULTURE

M-109804-r

FiGUBE 63.—Control bulbs improperly mounted on side wall of kiln. Bulbs are too close together and exposed to

damage. Planks at track level shield them from circulating air.

The wet bulb is usually plated to minimize

corrosion. T\nien the plating has "worn off so

that corrosion fouls the wick, have the bulb replated by the instrument manufacturer.

The wet-bulb temperature in a dry kiln is

usually lower than the dry-bulb temperature; at

no time can it be higher. If the reading is

higher, the instrument is out of calibration.

Sensitivity and Loose Linkage in

Recorder-Controllers

Some recording-conti-olling instruments lack

the degree of sensitivity necessary for the desired

control of drying conditions. Sometimes this is

due to faulty manufacture, but more often to

loose, worn linkage.

Lack of sensitivity can be detected by comparing the temperature cycles shown on the instrument chart with those obtained at the control

bulb by another thermometer of known satisfactory sensitivity. If the range of temperature

recorded on the instrument is one-third or more

of that shown by the check thermometer, the instrument can be considered sufficiently sensitive.

If it is less than one-third, repair or replace the

instrument linkage.

Care of Recording-Controlling

Instruments

The period of reliable performance of control

instruments can be greatly increased by proper

care. The parts of a recorder-controller are precision built and can be easily damaged. However, they are well protected against injury and

dust, and will give trouble-free service for many

years if the case is not left open long at a time.

Replace broken cover glass immediately. Never

use compressed air, brushes, or cloth to clean off

dust that may settle within the instrument case.

Instrument cleaning and repairs require special

tools, skills, and equipment. Such work should

be done at the manufacturer's plant or by an

authorized serviceman.
DRY KILN OPERATOR'S MANUAL 71

ZM-59899-F

FiGUKE 64.—Control bulbs placed on kiln wall so that

the bulbs project well into and at right angles to the

entering-air stream.

ZM-59969-F

FIGURE 65.—Control bulbs mounted horizontally,

perpendicular to the kiln wall.

and

The only part of the control instrument that

requires lubrication is the clock, and this should

not be done too frequently. Never lubricate the

pivot points on the linkage arms.

The air flowing into an air-operated instrument must be free of oil and moisture. For this

reason, the air is passed through a filter dripwell

or trap before entering the instrument. The

trapped oil or moisture is blown from the dripwell or trap at least once daily by opening a

blowoff valve. Usually the elements in filters

must be replaced once a year.

Calibration of Recording-Controlling

instruments

When instruments are out of calibration, the

actual drying conditions within the kiln differ

from those recorded on the chart, and serious

kiln-drying defects or increased drying time may

result. Because a new instrument may be jarred

during shipment, calibrate it at the time of inDffr BUL

AIR FLOW

TOP OF LOAD

M-109441—F

FIGURE 66.—Single dry bulb so liwated that it will control kiln temperature on entering air. Bulb is located

above the load in a single-track kilu.
72 AGRICULTURE HANDBOOK NO. 188, U.S. DEPT. OF AGRICULTURE

TOP OF LOAD

M-109439—F

FIGURE 67.—Single dry bulb so IcK-ated that it will control kiln temperature on entering air. Bulb is located

above the loads in a double-track kiln.

WALK»/»- BETWEEN LOADS

BAFFLES BETWEEN LOADS

M-10fU40-F

FIGURE 68.—Single dry bulb so located that it will control kiln temperature on entering air. Bulb is located

below the loads in a double-track kiln.

stallation. Thereafter, check it for accuracy frequently by using glass-stemmed thermometers or

other temperature-measuring devices.

Recalibration of fin instrument found to be in

error is not difficult, but it must be carefully

done. The equipment required includes a water

container and an accurate temperature-measuring

device (a glass-stemmed thermometer is commonly used). Since the difference in height between the bulbs and the instrument case affects

the recorded temperatures, calibrate the instrument with the bulbs at about the same height

above or below the instrument case where they

will be in service. Two men are needed—one

stationed at the water container and one at the

instrument.

The procedure is as follows:

(1) Fill a container at least 14 inches deep

with water heated to about 200° F., and place it

near the bulbs.

-BAFFLE BAFFLE-

\

M-lOggOB-F

FIGURE 69.—Dual-bulb control system showing control

bulbs located on opposite walls.

(2) Remove the bulbs from their fastenings

and completely submerge them in the hot water.

If the dry and wet bulbs are located together

in the kiln, calibrate them together. Avoid sharp

bends in the tubing. The bulbs should not touch

the sides or bottoms of the container. Usually

in a dual-bulb system (fig. 69), only one dry

bulb need be calibrated. If there has been a difference in the recorded temperature upon reversal

of air circulation during kiln operation, check

each bulb separately. The man stationed at the

container gently and constantly agitates the

water during calibration.

(3) After abovit 10 minutes, the man stationed

at the container takes a temperature reading of

the hot water with the thermometer. He then

calls this reading to the man at the instrument,

who records it, together with the corresponding

temperature indicated by the instrument pen.

(4) Make a check at every 20° or 30° F. drop

in the temperature of the water as it gradually

cools. If cool water is added to the hot water to

reduce calibration time, let 5 or 10 minutes elapse

before temperatures are taken, so that the temperature change is reflected at the instrument.

Make the periodic check readings until the water

temperature drops to about 100°.

(5) If the indicated temperatures on the instrument chart are consistently lower or higher

than the water temperatures by a constant

amount, adjust the recording-pen arms upward

or downward by that amount. This is done by

adjusting a small screw located on the pen arm
DRY KILN OPERATOR'S MANUAL 73

or on the pen-arm pivot. If the differences between the indicated temperatures and the water

temperatures are not constant, have a trained

serviceman make the adjustment. A correction

chart can be made so that the instrument can be

used until the adjustment is made.

(6) The next step is the adjustment of the

control setting indicator. This must be done with

the air or electricity on. Lower the setting indicator to a temperature below that indicated by

the pen on the chart and then move it slowly

upward until the motor valve it controls begins

to open. Eecord the temperature shown by the

setting indicator. Then move the setting indicator slowly downward until the motor valve

begins to close, and record the indicated temperature. If the average of the two recorded

temperatures is different than the temperature

indicated by the pen, move the control indicator—by means of adjustment screws on the indicator—upward or downward by the amoimt of

difference.

Some kiln operators prefer not to adjust the

instrument pens or control indicators. Instead,

they list the calibration data on a small card,

which they place inside the face of the instrument. These data are used as a guide for setting

the instrument in subsequent control of drying

conditions.

When dry kilns are provided with double-end

heating systems (fig. 34) with the coils at opposite ends controlled by separate dry bulbs, each

bulb must be calibrated. Many kiln operators

prefer to have a gap of about 5° F. between the

two recorded dry^ulb temperatures, so that temperature fluctuations at each end of the kiln are

readily seen on the chart. After calibration,

therefore, the pen arm of one dry bulb is intentionally adjusted upward or downward the desired amount. The amount of underset or overset should be noted on a card attached to the

instrument, so that anyone reading the chart wall

know that the temperature recorded by that pen

does not show the actual kiln temperature.

Dry kiln technicians should be familiar with

the manufacturer's instructions for the care and

maintenance of recorder-controllers. If the instrument should fail, however, call in an authorized serviceman to locate and correct the trouble.

Heating Systems

A correctly designed and properly maintained

heatmg system produces uniform drying conditions m a kiln. Unfortunately, the maintenance

of heating systems is often neglected and the

consequent nonuniform drying conditions cause

kiln degrade, extended drying time, nonuniform

moisture in the lumber, and. increased dryinocost. On the other hand, frequent inspection

and prompt corrective action can minimize, if not

eliminate, many ill effects.

Steam Heating System Problems and

Corrective Measures

hnproperly Insulated Feed Lines.—Insulate

all main feed lines from the boiler to the kiln to

reduce steam consumption. Radiation from some

feed lines within the kiln, running from the control valves to the coil headers or to the spray

line (fig. 34), may produce excessively high temperatures in certain zones. If this occurs, the

lines should be insulated. The insulation on

many steam feed lines is either improperly installed or damaged. Replace deteriorated or

damaged insulation as soon as possible.

Leaking Pipes.—Leaking pipes within the kiln,

caused by corrosion or mechanical damage, increase steam consumption and may upset the wetbulb temperature. Repair or replace them, "\\1ien

necessary clean all pipes and fittings and coat

tliem with a corrosion-resistant material that will

not affect heat transfer.

Sagging or Distorted Pipes.—Feed-line and coil

supports frequently fail, causing the pipes to become distorted and to sag. Condensate and scale

accumulate in the sagged pipes and eventually

plug them. Straighten or replace sagging and

distorted pipes. Protect pipe supports against

corrosion, and reinforce or replace them when

examination shows they are failing.

Debris.—Carelessness and poor housekeeping

frequently cause debris to accumulate on underload heating systems. This not only reduces the

efficiency of the heating system but is a fire

hazard. Such debris consists of stickers, kunber, bark, knots, sawdust, and the like. Move

trucks carefully in loading and unloading operations so that boards, stickers, and bolsters are

not dislodged. Seal cracks around doors and

screen air intakes and vents to minimize infiltration of sawdust and other foreign matter. In

spite of all precautions, however, debris will accumulate; remove it at frequent intei-vals.

Defective Redticing Valves and Pressure Regulators.—Fluctuations in steam pressure caused

by faulty reducing valves and regulators result

in nonuniform drying conditions. If adjustment

does not correct the condition, repair or replace

the defective parts.

Faulty Pressure Gages.—The pressure gages

used in conjunction with the reducing valves and

regulators occasionally go out of calibration;

therefore, recalibrate them at intervals against a

gage known to be accurate, or replace them.

Faulty Motor Valres.—Motor valves may leak

01' fail to open or close properly. If an airoperated motor valve fails to open, the trouble

is usually associated with a leak in the air
74 AGRICULTURE HANDBOOK NO. 188, U.S. DEPT. OF AGRICULTURE

supply line, a damaged diaphragm, or a packing

nut that is too tight. If the valve fails to

clo.se, tlie trouble may be due to improper adjustment of the compression spring. Failure of

electrically operated valves may be associated

with power failure, damaged wiring, or the

motor. A valve that leaks because of worn parts

or the presence of scale on the seat can usually

be detected by a slow, continuous rise in temperature above the set point. A valve that fails

to open can be detected by a slow drop in temperature below the set point when the instrument is calling for heat.

Repair or replace faulty valves. Keep a spare

motor valve on hand for replacement purposes,

and also spare motor valve parts, including diaphragms. If leaks occur around the valve-stem

packing nut, tighten the nut or replace the packing.

Hand-Operated Valves.—Hand valves are used

extensively on steam heating systems. These

valves, which are usually of the globe type,

should be operated wide open or completely

closed. If leaks occur around the A^ilve-stem

packing nut, follow the procedure outlined under

faulty motor valves. Keep replacement valves and

spare parts on hand.

Faulty Steam Traps.—Consult kiln manufacturers, engineers, and steam-trap manufacturers

on trap installations to minimize failures in the

trapping system. The following summary will

assist the operator in locating and correcting

trap troubles.

The failure of a steam trap to discharge may

be due to (1) excessive operating pressures, (2)

no water coming to the trap, (3) a plugged

bucket vent, (4) a trap filled with dirt, (5) worn

or defective parts, or (6) excessive back pressures in the return line. Excessive operating

pressures may be caused by the failure of the

reducing valve or pressure regulator, by inaccurate readings on the pressure gage, by back pressures in the return line from the trap to the

boiler caused by a discharge line that is too small

or is plugged, or by the raising of steam pressures without changing the valve orifice in the

trap. When condensate fails to reach the trap,

the failure may be due to a plugged line, to a

closed valve in the line between the coils and the

trap, or to open or leaking bypass valves that

allow the condensate to flow around the trap.

Dirt in the condensate may plug the bucket vent.

Trouble from dirt can be minimized by installing a strainer ahead of the trap and cleaning

it at frequent intervals. A strainer will also

prevent the trap body from becoming filled with

dirt. Install blowoff valves on all traps, and

blow out the traps for a short period each day

the kiln is in operation.

When the discharge of water from a trap is

continuous, the trap is too small or it has an

orifice too small for the steam pressure used.

These difficulties can be prevented by installing

a trap that has an orifice of proper size and is

large enough to handle the peak condensate load,

which will usually occur during the warmup

period.

If the trap blows live steam, the discharge

valve may not be seating or the trap may have

lost its prime. A badly worn valve seat or dirt

lodged between the valve and valve seat will

cause improper seating of the valve. A trap that

loses its prime is usually subjected to sudden or

frequent drops in steam pressure. If this occurs

frequently, install a good check valve ahead of

the trap and below the coil being drained. Maintaining fairly constant steam pressures will also

minimize this trouble.

Worn or defective trap parts may cause complete failure. Some parts can be easily replaced

on the job with very little, if any, loss in operating time. Replacement is even simpler if a bypass line has been installed around the trap.

When a defective trap cannot be repaired on the

job, replace with a new or reconditioned one.

Repair the defective trap at the first opportunity.

Annual cleaning and overhaul of all traps is

recommended.

Trap failure can be detected by observing discharge from the trap, obtaining temperatures on

the supply and discharge sides, or by listening

to its action. The discharge action of most traps

can be observed from test outlets. These should

be opened frequently. If steam discharges continuously from a correctly sized trap, it is not

functioning properly; determine the cause and

correct it. Do not confuse flash steam with live

steam. Flash steam is due to pressure changes

and appears for only short periods of time when

the trap is discharging, but live steam generally

appears in a continuous flow.

Temperatures obtained on the supply and discharge lines of the trap are useful in checking

trap performance. A spot test pyrometer can be

used to obtain these temperatures. If the supply-line temperature is about 10° to 15° F. above

the discharge-line temperature, the trap is working satisfactorily. If there is no difference in

temperature between the inlet and the outlet pipes

connected to the trap, it is blowing live steam or

has insufficient capacity. A large difference in

temperature is an indication that the trap is water-logged.

On some types, trap action can be checked by

listening to the operation. Place the bit end of

a screwdriver firmly against the top of the trap

and then put one ear firmly against the end of

the handle. If the valve action is plainly heard,
DRY KILN OPERATOR'S MAOSPUAL 75

the trap is working properly; if movement of

condensate inside the trap is heard, the discharge

is blowing through.

Indirect-Heated Furnace Kilns

In indirect-heated kilns with gas, oil, coal, or

wood furnaces, the products of combustion flow

through a system of heat exchangers located

within the kiln. If excessively hot exchangers

are too close to the lumber being dried, seasoning defects may develop, and the lumber may

catch fire. Checks of the temperature throughout the kiln will help find hot spots in the heat

exchanger. To elimmate hot zones, modify, relocate, or insulate the heat exchanger.

Keep heat exchangers free of debris to minimize the possibility of fire. Fly ash deposited

in heat exchangers may cause nonuniform temperatures; therefore, disconnect the exchangers

and clean them occasionally. Compressed air is

frequently used for this purpose. Since heat

exchangers deteriorate rapidly because of the

high temperatures, inspect them frequently and

replace them if they are defective. Badly deteriorated heat exchangers are likely to cause fires.

Direct-Heated Furnace Kilns

Because the products of combustion in directheated furnace kilns are discharged directly into

the drying chamber, both the kiln and the lumber are exposed to the danger of fire from sparks

and flames. Therefore, carefully control the rate

of burning of the fuel, and install spark arrestors

when necessary. Ducts carrying the products of

combustion are subject to deterioration. Inspect

them frequently and repair or replace them when

they are unsafe. Caution: The products of combustion are likely to contain toxic gases; never

enter the kiln during operation or brief shutdown periods unless you are wearing a self-contamed breathing apparatus or the kiln has been

aired out by opening the large doors.

Furnace-type dry kilns (fig. 21) that are heated

by gas or oil burners located within the kiln are

particularly susceptible to fires and often contain toxic gases. Keep the burners free of deposits that may catch fire and blow into the

lumber. Repair leaking gas or oil feed lines

immediately. Do not allow debris to gather

around burners or on plates heated by the burners. Keep the thermostats that automatically

control the operation of the burners in good

repair. ^

Electrically Heated Kilns

All wires leading to the heating units should

be well insulated to avoid short circuits that

could be the cause of fires or injuries to workers.

The electrical system should be installed and

checked by qualified electricians. Inspect the

electric circuits frequently. If damaged circuits

or heating units are found, have a qualified serviceman immediately repair or replace them. Do

not permit debris to accumulate.

Humidifying Systems

Steam Spray

Steam sprays are used to supply moisture to

the kiln atmosphere when required to maintain

the desired relative humidity. Exhaust steam,

if available, is preferable to live steam for this

purpose. Hand or motor valves controlling the

flow of steam spray into a kiln are subject to the

same failures as those in steam heating systems.

Follow the inspection and maintenance procedures discussed on pages 73-74. A flow of steam

or condensate from the steam spray line when

the valves are closed indicates leakage. A falling wet-bulb temperature when the control instrument is calling for steam spray indicates that

the steam spray motor valve has failed to open.

Repair or replace defective valves immediately.

The steam spray lines usually pitch downward from the feed end. Usually a small drain

line discharging outside the kiln is provided to

drain off the condensate that collects at the low

end. Keep this drain line open. Inspect the

steam spray line itself periodically to see that

the discharge holes or nozzles are open and that

the pipe has not been bent or turned so that the

spray discharges onto the lumber or the instrument control bulbs.

Water Spray

Occasionally water-spray lines are installed in

kilns to supply moisture when required for humidification. Inspect the valves frequently that

control the flow of water into the spray line and,

if they are defective, repair or replace them immediately. Open plugged spray holes or nozzles, and repair or replace damaged lines.

Venting Systems

Most kilns are provided with ventilators for

exhausting hot, moist air from the kiln and taking in fresh air. Excessive venting increases

heating and humidification requirements, and it

should be avoided by proper adjustment and

maintenance of the venting system.
76 AGRICULTURE HANDBOOK NO. 188, U.S. DEPT. OF AGRICULTURE

Vent Operation and Care

Although vents can be manually or automatically operated, automatic ones are recommended.

To prevent excessive venting, frequently inspect

the system and keep it in good repair. Following are a number of items to observe in the inspection and maintenance of vents :

(1) Keep the linkage system connecting two

or more vent caps or dampers lubricated, and

inspect it periodically for damage. Straighten,

repair, or replace bent or broken rods, chains,

and levers.

(2) Inspect the vent caps or dampers when

they are in a closed position. If some are partially open, adjust the linkage so that the caps

or dampers fit tightly. This adjustment can he

made quickly and easily on most kilns.

(3) Install gaskets around vent openings if

there is excessive leakage when the vent caps are

closed.

(4) Avoid overventing. Adjust the linkage so

that the caps or dampers are open just wide

enough to obtain the desired venting.

(5) Examine air lines or electric circuits connecting the vent mechanism to the control instrument for air leaks and short circuits.

(6) The compressed air used to operate the

vent mechanism should be dry. Water in the air

supply line may freeze the motor valve during

winter weather. If dry compressed air cannot

be obtained, protect the air supply line against

freezing.

(7) Use screen inserts in the ventilators to

minimize the fire hazard associated with venting. These will keep sawdust, other debris, and

sparks from entering the kiln. Remove and clean

the screens periodically.

(8) Keep the fresh air doors, dampers, and

ducts in natural-circulation kilns in good repair

and free from obstructions.

Air-Circulation Systems

The uniform circulation of air in a kiln is dependent on well-maintained air-circulating equipment.

ISatural-Cirdilation Systems

Because air movement is comparatively slow

in natural-circulation kilns, the air passageways

from the heating coils into and around the loads

of lumber must be unobstructed. Walkway gratings also should be free of planks, debris, and

other obstructions.

Internal-Fan Systems

Any failure or damage to the component parts

of the air-circulation system in internal-fan kilns

extends drying time and may also result in nonuniform drying. Therefore, the maintenance and

care of the component parts of the air-circulation system is essential.

Insi)ection and Maintenance.—The items to be

checked in the periodical inspection of the aircirculation system and some of the maintenance

procedures include the following:

(1) Fan Motors.—Lubricate fan motors in accordance with the manufacturer's instructions.

Replace leaky bearing seals.

Keep windings and armatures free of dust.

Dry compressed air may be used for blowing out

dust.

Keep motor mounts and anchor bolts tight.

Protect from the weather fan motors located

outside the kiln.

Properly ventilate a control room to avoid

overheating fan motors located there. Fan motors located in the kiln must be designed for

high temperatures and high relative humidities.

Protect fan motors against overloading. Relays should be set to kick out under small overload.

Repair or replace damaged or badly worn

motors.

Have a qualified electrician inspect all elements

of the electrical circuits periodically and keep

them in good condition.

(2) Fan Shafts.—Lubricate shaft bearings according to the manufacturer's instructions and

replace leaking oil seals.

Keep bearing supports tight and alined with

the shaft. Misalinement may overload the fan

motor and damage the fan shaft and bearings.

Keep friction bearings tight.

Replace damaged or badly worn bearings.

Replace or repair badly worn keys or keyways.

Keep shaft couplings tight.

Replace damaged fan shafts.

(3) Pulleys and Belts.—Keep pulleys tight on

the shafts.

Replace badly worn or damaged pulleys to

prevent excessive belt wear or belt slippage.

Tighten belts according to manufacturer's recommendations. Do not overtighten.

Replace badly stretched or damaged belts.

(4) Fans.—Repair minor damage to fans; replace badly damaged fans.

Keep fans tight on fan shafts.

See that the clearance between the tips of fan

blades and the fan rings conforms to the manufacturer's recommendations.

(5) Fan Baffles and Fan Floor.—Repair or

replace damaged fan baffles or fan floors.

Keep anchor bolts in fan baffles tight to minimize vibration and possible damage to fans.

(6) Load Baffles.—Repair or replace damaged

load baffles.

Lubricate baffle hinges.
DRY KILN OPERATOR'S MANUAL 77

Maintain pulleys and cables on hinged baffle

systems in good condition.

(7) Oil Lines, Connections, and Bearings.—

Leaking oil lines, connections, or bearings increase safety and fire hazards, and the oil may

stain the lumber. Make a systematic inspection

for oil leaks and tighten loose connections. Repair or replace damaged lines.

Caution: Exercise extreme care when fans

must be inspected while they are running. Serious injuries have resulted from carelessness during the inspection of moving fans.

External-Blotver Systems

The inspection and maintenance of internal-fan

kilns is also applicable to external-blower kilns.

Other maintenance items include:

(1) Repair or replace leaky blower housingpanels.

(2) Repair or I'eplace damaged supply- and

return-air ducts to insure maximum air circulation through the loads of lumber and to avoid

short circuiting.

(3) Lispect air scoops in the supply-air duct

slots for damage and maladjustment so that air

delivery remains uniform throughout the length

of the duct. Adjust, repair, or replace them when

necessary.

(4) Remove any debris covering the supply-air

slots or ports.

(5) Repair or replace badly worn or damaged

reversing dampers.

(6) Keep fresh-air doors in the blower housing in good repair.

(7) Repair damaged vent dampers.

Kiln Trucks

Frequent inspection and proper maintenance of

kiln trucks can minimize down-time and accidents. Proper lubrication will help extend truck

life. Recommended maintenance procedures are :

(1) Repair or replace damaged truck frames,

axles, and bearings promptly.

(2) Keep_bolts or rivets in truck frames tiglit.

(3) Repair or replace damaged metal or wood

cross supports.

(4) Provide enough kiln trucks so that no

truck is loaded over its capacity.

Prevention of Corrosion

Since much of the metal in a dry kiln will

corrode, frequent inspection of metal parts is

essential. Remove rust and coat the affected surface with a suitable protective paint. Such

paints can bé obtained from dry kiln manufacturers.

555315 0—61 6

Housekeeping and Maintenance

Around Dry Kilns

Good housekeeping around dry kilns is essential. To minimize the possibility of injuries,

damage to equipment, derailment of kiln trucks,

and fires, keep the dry kiln, the operating room,

and the surrounding area clean and free of safety

and fire hazards. The following are good housekeeping practices :

(1) Lumediately pick up stickers falling from

loads of lumber and place them in conveniently

located sticker racks.

(2) Pick up lumber falling from loads and

repile it on the loads or return it to the storage

area.

(3) Collect and remove sawdust and other

debris that sifts or drops into the kiln.

(4) Keep walkways in a kiln free of debris.

(5) Push back into the load of lumber any

lioards that project into walkways, to prevent

injuries to workmen. Projecting boards also

cause nonuniform air velocities through the loads

of lumber in some tj^pes of kilns.

(6) Stop oil or grease leaks around bearings,

fans, lilowers. and motors, and wipe up spilled

oil or grease as soon as possible. LTse drip pans

to catch oil or gi'ease dripping from bearings.

Place oily or greasy rags in closed containers.

(7) Keep control rooms and operating pits

clean and well ventilated at all times.

(8) Keep transfère, tracks, and tramways on

the loading and unloading ends of dry kilns in

good alinement and repair.

(9) Inspect stairwa3's and ladders frequently

and replace Aveak members at once.

Locating Troubles in Kiln

Maintenance and Operation

To assist the dry kiln operator in rapidly finding the causes of poor drying, the more common

sources of trouble are summarized.

If the dry-bulb temperature does not reach the

set point in a reasonable length of time, the

causes may be—

(1) Steam pressure is too low.

(2) Insufficient radiation.

(3) Heating coil is damaged, waterlogged, airbound, or plugged.

(4) Hand valves on feed or drain lines are

closed or only partially open.

(5) Automatic motor valve fails to open.

(6) Steam trap is defective.

(7) Valves are open on bypass line around

steam trap.

(8) Excessive back pressures in return line to

boiler.
78 AGRICULTURE HANDBOOK NO. 188, U.S. DEPT. OF AGRICULTURE

(9) Excessive venting.

(10) Excessive leakage from kiln structure and

around doors.

If dry-bulb temperature continues to climb

above the set point, the causes may be—

(1) Leaking hand valves on feed lines.

(2) Leaking automatic motor valve.

(3) Motor valve remains open.

(4) Heat transfer through a common wall

from an adjacent kiln.

If the wet-bulb temperature fails to reach the

set point in a reasonable length of time, the

causes may be—

(1) Insufficient steam entering the spray line

because—

(a) Automatic motor valve fails to

open.

(b) Hand valve on feed line is closed or

only partially open.

(c) Holes or nozzles in spray line are

plugged.

(2) Excessive leakage of heat and vapor from

kiln structure or around doors.

(3) Excessive venting.

If the wet-bulb temperature continues to rise

above the set point, the causes may be—

(1) Leaking motor valve.

(2) Motor valve remains open.

(3) Water seeping into kiln.

(4) Leaking steam or water lines in kiln.

(5) Valve in bypass line around motor valve

is open.

(6) Insufficient venting.

If the lumber is not uniformly dried or has

excessive degrade associated with hot or cold

zones within the kiln—

( 1 ) Hot zones may be caused by :

(a) Excessive radiation due to uninsulated feed lines or headers.

(b) Higher than average air velocities

across radiating surfaces due to faulty

stacking and inadequate baffling.

(c) Leakage of heat through a damaged

wall common to two kilns.

(2) Cold zones may be caused by:

(a) Infiltration of colder air through

cracks in the kiln wall or around doors.

(b) Damaged fans or fan motors.

(c) Short- circuiting of the air because

of faulty stacking or inadequate baffling.

(d) Condensate from the drain end of

plain-header coils is not draining properly.

(e) Down drafts through the ventilators.

Incorrect recording of dry- and wet-bulb temperatures may be caused by—

(1) Control instrument out of calibration or

damaged.

(2) Improper air circulation over control

bulbs.

(3) Control bulbs exposed to direct radiation

from heating coils and feed lines or heat from

steam spray.

(4) Water on the dry bulb.

(5) Wet bulb wick dirty, dry, or made of improper cloth.

(6) Water flowing at either too fast or too

slow a rate to the wet bulb water pan.

(7) No wick on the wet-bulb.

(8) Wick placed on the dry bulb instead of

on the w^et bulb.

(9) Wrong recorder chart.
CHAPTER 5. STACKING LUMBER AND OTHER ITEMS

FOR KILN-DRYING

Much of the degrade and waste that occurs

during kihi-drying results from poor stacking.

Well-stacked lumber dries faster, more uniformly,^ and with less warp, and fewer stickers

break and deform. Stacking procedures vary

among plants because of différences in plant

layout, material to be dried, and types of kilns

and stacking equipment. Several principles of

good stacking, however, apply at all plants.

Sorting

Sorting the stock simplifies stacking and also

aids in getting material of like drying characteristics into a kiln charge. Sorting, therefore,

should be as complete as is practical. It is

planned to accomplish various purposes.

Sorting for Species

Some species of wood have markedly different

drying characteristics than others. For example, the time required to kiln-dry green 4/4 red

oak to a final moisture content of 7 percent is

two to three times that required to kiln-dry

green 4/4 hard maple; moreover, a much milder

drying schedule must be used for the oak. If

these two species were in the same kiln charge,

the hard maple would have to be dried under the

oak schedule, and the cost of drying the maple

would become excessive. On the other hand, the

drying characteristics of hard maple and yellow

birch do not differ greatly, and these species, if

of the same thickness ancl moisture content, can

be dried together economically. ^Vhenever possible, therefore, a kiln charge should be composed

of a single species or of species that are closely

similar in drying characteristics.

Sorting for Moisture Content

It is not desirable to mix air-dried, partially

air-dried, and green stock in the same kiln charge.

The wetter stock requires milder initial drying

conditions and a longer drying time than the

drier stock. For example, 4/4 red oak air-dried

to a moisture content of 25 percent can be kiln-

^ RASMüSSEN, E. F. NEED KOR UNIFORMITY OF TEMPERATURE IN A FORCED-AIR-CIRCULATION, VENTILATED, COMPARTMENT DRY KILN. U.S. Forest Serv. Fore.st Prod. Lab

Rpt. 1669, 5 pp. [Processed.] 1956.

dried to a moisture content of 7 percent in about

one-quarter of the time that is required to kilndry green 4/4 red oak to the same final moisture content. The dried stock should be kilndried separately.

Because of the many variables involved, specific recommendations cannot be made as to the

maximum allowable difference in initial moisture

content between the driest and wettest stock in

a kiln charge. This difference must be determined by each kiln operator on the basis of production needs and the quality of drying desired.

In general, the difference in moisture content

between the driest and wettest stock composing

a kiln charge .should be smaller (1) for air-dried

or ¡martially air-dried stock than for green stock,

(2) for shorter drying times than for longer

times, and (3) for a narrow range in the desired

final moisture content.

Electrical moisture meters are frequently used

for sorting.

Sorting for Heartivood and Sapwood

In some species, sapwood and heartwood differ

considerably in green moisture content (table

1, p. 9). In addition, sapwood generally dries

more rapidly and with fewer defects than does

heai'twood. Because of these differences, it is

advantageous to separate sapwood from heartwood where large quantities of lumber are handled.

Sorting for Grain

A plainsawed board (fig. 2, p. 7) generally

dries faster than a quartersawed board, but it

is more susceptible to such drying defects as

surface and end checks and honeycomb. Therefore, plants producing large amounts of quartersawed lumber can segregate it and dry it under

more severe kiln conditions, thereby shortening

the drying time.

Sorting for Grade

The upper grades of lumber, both hardwood

and softwood, are generally used in products

that require higher strength, closer control of

final moisture content, and better appearance

than the lower grades. Therefore, the higher

grade material is usually sorted out and kiln79
AGRICULTURE HANDBOOK NO. 18 8, U.S. DEPT. OF AGRICULTURE

M-115B49, M-87929-F

FIGURE 70.—A, Stacking lumber of different thicknesses on the same kiln truck results in cupping of the lumber and

deformation and breakage of sticker.?. B, Lumber of uniform thickness remains flat during drying, and stickers

stay straight.
DRY KTLN OPERATOR'S MANUAL 81

dried according to schedules different from those

used for the lower grades.

Sorting for Thickness

Sorting for thickness is essential. Stock of

uniform thickness simplifies stacking and drying. It also reduces warping in the lumber and

breakage and deformation of the stickers.

The thicker the material, the longer the drying

time and the milder the drying conditions required. Cupping of the lumber and deformation

and breaking of the stickei-s resulting from stackintr material of various thicknesses are shown

in^figure 70, A. The cupping and twisting m

the thinner boards is caused by a lack of contact between the boards and the stickers, and it

resuhs in a nonuniform flow of air across the

load. Damaged stickers often have to be discarded. . . A truckload of dried boards of uniform thickness is shown in figure 70, B. Air movement

between the courses of lumber was uniform. The

dried lumber is flat and the stickers are straight.

Misent ií¿Hi5er.—Lumber that is misent is

hkely to vary considerably in thickness across

the width and along the length of the piece.

Miscut lumber is not only diíRcult to stack, it is

impossible to hold the thinner part, of the boards

flat. The thicker parts dry more slowly and

may develop more defects. Some plants find it

economical to skip-dress such stock to a uniform

thickness before stacking.

Sorting for Length

For good stacking, one of the best and

easiest methods is to pile lumber of a single

length on kiln trucks (fig. 70, B) or m packages If the stickers are well supported and m

crSod alinement, such stacking results in flatter

and straighter kiln-dried stock. Overhanging

ends of the longer boards m a truckload ot

mixed-length stock are likely to warp during

drving. .„ , . , .,

Stacking lumber of uniform length on a kiln

truck or in packages is a common practice among

producers of softwoods, and some of the larger

manufacturers follow the same practice with

hardwoods. :\Iost of the hardwood producers,

liowever. either box-pile the lumber or stack

various segregated lengths on a kiln truck, using the "sFep-back" or "step-out" method. Figure 71 illustrates a combination of step-back

and step-out stacking. Step-back stacking generally resuhs in less déformât ion of the lumber.

M-75793-P

FIGURE 71. Scale model showing "step-back" stacking in lower part of load and "step-out" stacking in upper part.

Courses of thick dunnage, located as shown, will minimize warp. Sample pockets and kiln samples are shown.
82 AGRICULTURE HANDBOOK NO. 188, U.S. DEPT. OF AGRICULTURE

Courses of thick dunnage put a solid "floor"

under each of the upper sections of the load and

thereby help prevent warping (fig. 71).

Stickering the Lumber

Proper selection and use of stickers greatly

reduces warp in lumber and assures fast and

uniform drying, and it reduces sticker breakage

and deformation. Among important considerations of stickering are the size of the stickers

and their placement in the pile, the species and

grades of wood used, and sticker supports, guides,

and racks.

Sticker Material

Many stickers are required in a kiln-drying

operation, and reiDlacement is costly. Practical

measures for obtaining long sticker life therefore pay off. Many plants find it worthwhile to

make stickers from clear, strai<;ht-irrained stock

rather than from low-gi-ade stock. Tlie initial

cost of such stickers may be higher, but their

longer service life generally offsets tliis. Straightgrained stickers made from the harder woods

stay straighter and last longer than irregulargrained stickers made from softer woods.

Species such as hickory, hard maple, beech,

oak, Douglas-fir, and larch make good stickers.

Usually, however, the species being dried at the

plant are used for stickers.

Moisture Content of Stickers

The lumber from which stickers are made

should be kiln-dried. There is less chance of

sticker stain, which is a discoloration of the

material where it contacts the sticker during drying. In addition, stickers made from kiln-dried

lumber will distort and change thickness less

during use.

Sticker Size

The sizes of stickers vary considerably, from

14 inch to 2 inches in thickness and from %

inch to 4 inches or more in width. The usual

range is from % inch to li/4 inches in thickness

and from 1 to 2 inches in width. Sticker length

is governed by the width of the load or package,

thickness ancl width by the kind and size of

lumber being dried.

ßticker Width.—Wide stickers slow up the

drying of lumber at areas of contact, ancl may

leave it at a higher final moisture content in

these areas. Stickers used for liardw^oods are

usually 114 inches in width and should not exceed 11/2 inches. Stickers for softwoods average about 2 inches in width, but widths up to 4

inches may be used for the softer species, such

as sugar, ponderosa, and white pine.

Sticke?-- Thickness.—Stickers used for lumber

and dimension are generally 3^ to 1 inch thick.

Some plants, however, use stickers as thin as

^2 inch. The thinner stickers make it possible

to load more lumber on a kiln truck and, in

forced-circulation kilns, make air velocity faster

and more uniform across the loads. They break

ancl deform more readily, however, and sagging

boards are more likely to obstruct air movement.

Regardless of the thickness used, all stickers in

a kiln charge should be surfaced to uniform

thickness.

Load Supports

Unless the load of lumber is properly supported, sagging and distortion m the lower

courses Avill result (fig. 72, A). Lumber piled

as in figure 72, B, will not sag because the load

supports are directly under the tiers of stickers.

The thinner the material being dried, the

greater the number of load supports required.

Sometimes a bottom course of thick dunnage is

used instead of additional supports. Generally,

the maximum spacing of load supports for 1-

inch hardwood lumber is about 2 feet and for

1-inch softwood lumber about 4 feet. In thicker

lumber the distance between load supports usually can be increased, but it is better to use

too many load supports than too few.

Sticker Location, Spacing, and

Alinement

Good location, spacing, and alinement of

stickers reduces warping and minimizes end

checking and splitting in some species of lumber.

Location of Stickers.—If possible, place the

end stickers flush with the ends of the boards

(fig. 71). Warping of the ends of the boards

will be reduced and also end drying will be retarded to some extent, thus helping to minimize

end checking and splitting. Other tiers of stickers should be close enough to control warping.

Spacing of Stickers.—Sticker spacing is governed by the stock's tendency to warp, its thickness, and its resistance to crushing. A spacing of 3

to 4 feet is satisfactory for softwoods. A 2-foot

spacing is satisfactory for most 4/4 hardwood

stock, but some species, such as sweetgum and

the elms, require closer spacing, as little as 15

inches, to prevent warping. Sticker all hardwood stock less than 1 inch thick with this

closer spacing. Softer hardwoods, such as aspen

and basswood, may require closer than 2-foot

spacing to prevent crushing of the stock. Also,

to avoid crushing of stickers between the bottom

courses of heavy loads, the sticker spacing may

need to be reduced.
DRY KILN OPERATOR'S MANUAL 83

M-115545, M-115696-F

FIGURE 72.—A, An insufficient number of load supports, improperly placed, caused this truckload of hardwo<jd lumber

to sag badly. B, Good load supports on truckload of hardwood lumber.
84 AGRICULTURE HANDBOOK NO. 188, U.S. DEPT. OF AGRICULTURE

Sticker Bridging.—AVhen cross supports cannot be located under each tier of stickers, the

weight carried by the unsupported tier can be

distributed to some extent to adjacent cross supports by bridging. Bridging is an A arrangement of extra stickers starting at the supported

tiers of stickers at the bottom of the load, extending progressively inward and upward until

they intersect the tier of unsupported stickers.

The inward offset of the bridging stickers at each

successive course of lumber should be no greater

than the width of the stickers.

Alinement of Stickers.—For fullest control of

warp in lumber during drying, the tiers of stickers should be alined vertically. (Figs. 71 and

72, B). Misalined stickers (fig. 72, J.), particularly in stacks of green lumber, invariably

cause nonuniform distribution of weight and result in sharp kinks in the lumber where the

stickers contact it. The thinner the lumber, the

greater the possibility of kinking. Much waste

results from incorrectly alined stickers, particularly in lumber 1 inch or less in thickness.

Auxiliary Tiers of Stichers in Packages of

Lumber.—Packages of lumber are commonly

transported around dry kilns with forklift trucks

and straddle carriers. When lifted by this kind

of equipment, lumber in the lower courses of a

package has a tendency to sag and stickers at the

ends may be lost. This trouble can be largely

avoided by locating short tiers of stickers above

the forks (fig. 73) or the carrier bunks. The

number of stickers needed in these extra tiers

depends upon the thickness of the lumber and

the weight of the package. Usually stickers are

interlaid between the bottom 6 to 10 courses of

lumber.

Sticker Guides

The use of sticker guides insures good spacing

and alinement.

Sticker guides vary considerably in design and

initial cost. Two types of guides are shown in

figures 74 and 75. These guides are pivoted on

Ijoth sides so that they can be pulled free of

um^efmn'îiÉJ y*W» o*«^**^

M-115553-F

FIGURE 73.—Package of lumber raised by forklift truck. Short tiers of stickers above point of contact with forks

reduce sag in the lower courses of lumber and help prevent dislodging of end stickers.
DRY KILN OPERATOR'S MANUAL 85

M-116546-F

FiGUKE 74.—Combinatton metal and wood sticker guide for stacking lumber on kiln trucks.

the stacked loads of lumber. A simple locking

device holds the guides in place while the lumber is stacked. Vertical channel irons or wood

strips equal in length to the height of the load

are positioned along each guide at points corresponding to the desired sticker spacings. The

stickers, cut about 2 inches longer than the desired width of the load, are held in place by the

guide channels or strips. Sticker racks located

adjacent to, or attached to, the guide hold a

supply of stickers.

A sticker guide of another design is shown in

figure 76. This guide consists of a rectangular

frame made of 2- by 12-inch lumber attached by

means of sleeves to vertical pipes. Cleats nailed

on the inside of the frame hold the stickers in

place. The frame is lifted when required by

means of a small electrically opei'ated hoist attached to the ceiling of the stacking shed. Wlien

a load is completed, the frame is lifted clear of

the top courses of lumber. The load is moved out

of the stacking shed and empty kiln trucks put in

its place. The frame is lowered over the empty

truck, and the stacking of a new load is started.

A supply of stickers can be kept on each side of

the stacking area. Boards too long for the

frame are trimmed off with a portable powersaw.

Portable sticker guides for stacking packao;es

of lumber are shown in figure 77. Guides like

these are used on one side of the package only.

Care of Stickers

Large plants invest considerable money in

sticker-handling equipment, such as conveyors of

various types, to reduce handling costs. This

ec^uipment, if properly designed, may also reduce

sticker breakage. Smaller plants can construct

racks or other devices, at fairly low cost, that

will reduce sticker losses and possibly handling

costs. A metal-roofed sticker rack mounted on

kiln trucks is shown in figure 78. This arrange-
86 AGRICULTURE HANDBOOK NO. 18 8, U.S. DEPT. OF AGEICUX.TURE

M-91974-F

FIGURE 75.—All wood sticker guide device for stacting lumber on kiln trucks.

ment protects the stickers to some extent from

the rain and sun and facilitates stacking and unstacking.

There are many other types of low-cost sticker

racks that will reduce sticker losses and handling

costs. For example, a simple rack can be constructed on a pallet and carried aroimd the plant

by forklift truck or straddle carrier.

Stickers should be stored under cover to keep

them reasonably dry and free of stain and decay.

Mechanical Stacking and Unstacking

Equipment

Many plants use semiautomatic and automatic

equipment for the stacking and unstacking of

lumber. Properly designed equipment will ser^^e

well if correctly operated. Several types of lumber stackers and unstackers are available.

Mechanical Stackers

With one type of semiautomatic stacker for

lumber of uniform length, a solid package of

lumber is placed on a tilting breakdown hoist.

from which the lumber slides onto a horizontal

table equipped with chain conveyors where the

courses are assembled. The kiln trucks are placed

on a hydraulic lift controlled by the stackerman,

and elevated to the desired height; the conveyor

is started, and a course of lumber is moved onto

the truck. The hydraulic lift is lowered a distance equal to the thickness of the boards, stickers

are placed in stationary guides located on each

side of the lift, and another course of lumber

is moved into position.

Sticker magazines are used in conjunction with

automatic stackers. A set of these magazines is

located over the load of lumber. One end of a

sticker is automatically dropped from each magazine onto the load. The other end of the sticker

is forced from the magazine by the weight of the

next course of lumber as it is moved onto the

load.

Lumber Unstackers

With one type of lumber unstacker in common

use, the load of dried lumber is placed on a tilting hydraulic lift. The lift is raised and tilted,

and the topmost course of lumber moves by

gravity to the dry chain. The stickers slide down

a ramp to a sticker bin or conveyor. The lift is

then raised to the next course of lumber and the

cycle x'epeated.
DRY KlUSr OPERATOR'S MANTJAL 87

M-87027-F

FiGUBE 76.—Box frame sticker guide of 2- by 12-ineli lumber can be raised or lowered by means of a small, electrically

operated hoist attached to metal beam shown directly above head of workman. Frame insures good side and end

aliuement of the load.
88 AGRLCÜLTUKE HANDBOOK NO. 188, U.S. DEPT. OF AGRICULTURE

M-11B641-P

FiGUKE 77.—Portable sticker guide for stacking packages of lumber.

Box Piling Random-Length Lumber

At many plants, particularly those drying

harchvoods, segregation for length is not practical. The box-piling method of stacking is recommended for these plants. Random-length

lumber that is box piled will dry straighter,

flatter, and more uniformly, and sticker losses

"will be minimized.

Three methods of box piling are in general use.

For purposes of description they are termed

methods I, II, and III.

Method I

In box piling by method I, the length of the

outside boards in each course is equal to the full

length of the pile. Thus, in figure 7'.), boards

Nos. 1 and 7 in all courses are as long as the

pile. Other full-length boards, when available,

are usually placed near the center of the courses,

such as boards -i in courses A and B of figure

79. The shorter boards in the same course are

alternately placed with one end even with one

end of the load or the other. The shorter boards

in all couises in the same tier of boards are all

])laced with one end even with the same end of

the load. For example, in figure 79 all the evennumbered short boards, with the exception of

board 6, course D, are placed even with the front

end and all odd-numbered boards even with the

rear end of the load. Occasionally two narrow,

short boards, such as boards 5 and 6 of course

D, are placed over a wdder board, such as board

.5 of course C. Also, two or more short boards

can sometimes be laid end to end in the same

tier of lumber. For example, 6- and 8-foot

boards could be laid end to end in a load of lumber 14 to 16 feet long.

The column effect obtained insures that all

boards are well supported and held down, and

warp, particularly cup and bow, is thereby lessened, along with sticker deformation and break-
DRY KILN OPERATOR'S MANUAL 89

M-91975-F

FIGURE 78.—Rack for transporting stickers on a kiln truck.

age. The unsupported ends of the short boards

within the load may warp to some extent.

If enough full-length boards are not available

for placement on the sides of the load in occasional courses, shorter boards, laid end to end if

short enough, can be used. Wlien this is done,

place filler blocks in any gaps between the

stickers above and below the course of lumber,

particularly when the gaps occur at the ends of

the load (fig. 80). These blocks will keep the

ends and sides of the loads from sagging and

will also reduce sticker breakage. The blocks

should be of the same thickness as the lumber

being dried.

Method II

Method II box piling, as illustrated in figure

81, is used for random-width lumber. This

method of piling prevents the building of definitely vertical tiers, except the outer ones. The

short boards in the interior of each course are

alternately placed at opposite ends of the load

as by method I ; this results in some stickers

and board ends being unsupported. In such

cases, the restraint exerted by the stickers is not

sufficient to prevent warping of the lumber. Also,

stickers may deform and break.

Method III

In method III box piling of lumber of random

length and width, the length of the outside boards

in each course is equal to the length of the load

or package. All of the shorter boards are laid

with one end even with the same end of the load.

Whenever possible, however, two or more short

boards together equaling the length of the load
90 AGRICULTURE HANDBOOK NO. 188, U.S. DEPT. OF AGRICULTURE

COURSE A

SECTION B-B

FiGUEE 79.—Method I box piling for random-length lumber.

M-116678-I'
DRY KILN OPERATOR'S MANUAL 91

M-115643-P

FiGUBE 80.—Bos-piled loads ';^'itll short boards on the outside and hlix-ks behveen unsupiwrted stickers.

FIGURE 81.—Method II box piling, used for lumber of random length and width.

M-115SS5-P
92 AGRICULTURE HANDBOOK NO. 188, U.S. DEPT. OF AGRICULTURE

are kid end to end. This method of stacking

results in large empty spaces at one end of the

load. Because of these spaces, lumber warping

and sticker damage may be heavy. Lumber at

that end of the load may diy too fast and increase surface and end checking.

Stacking Lumber for Various Types

of Dry Kilns

Stacking procedures must conform to the type

of kiln used, especially its air-circulation system. The direction of airflow—whether up,

down, across, or lengthwise through the load—

must be considered.- Uniform drying depends

on uniform air movement through the loads, for

which correct stacking is essential.

Stacking for ISatural-Cirdilation Kilns

The principal direction of airflow through the

loads of inaterial being dried in a natural-circulation kiln is vertical, mostly downward. Air

velocity is low, usually 15 to 25 feet per minute.

The vertical movement of air through the loads

is obtained by building flues in the piles (fig.

8, p. 24). Usually, these flues are 3 to 5 inches

wide and are spaced about 18 to 21: inches apart.

The sides of the flues need not be perfectly

alined vertically, since boards projecting into the

flues will not deflect the slowly moving air to a

great extent. The flues, however, must be open

from the bottom to the top of the load.

Siacking Lumber.—One frequently used method of stacking lumber for natural-circulation

kilns (fig. 82) has a large central flue; in addition, all boards in each course are spaced 1 inch

or more apart. The flue is generally 8 to 12

inches wide and need not be tapered : the top is

left open.

>SfacIi'Í7Tg Special Items.—Special items such

as furniture scpares, gunstock blanks, barrel

staves, and other shoi't-length stock must be piled

with special care to permit adequate circulation

of air around the pieces for uniform drying.

The shape and size of the item often dictates

how it must be piled. Following are a few suggestions and cautions.

1. In any load, be sure to include vertical

flue.

2. Space the pieces in aiiy course so that air

can circulate between them.

3. Keep area of contact between pieces in adjacent courses as small as possible, thereby improving the chance of uniform diying.

2 TORGESON, O. W. EFFECT OF PILING METHODS ON AIR

CIRCULATION IN A LUMBER DRY KILN. U.S. ForeSt SeVV.

Forest Prod. Lah. Rpt. Iß.'i.S, 5 pp. fProoesseri.l 19.50,

4. Don't pile as much stock as possible on the

kiln truck at the sacrifice of adequate air circulation; this merely prolongs kiln-drying time

and invites nonuniform drying.

Two ideas for piling special items are shown

in figures 83 and 84. Figure 83 shows a kiln

truckload of barrel staves piled on edge. Unless too loosely piled, the pieces will stay reasonably upright, allowing air to circulate among

them. Figure 84 shows a method of stacking

short-length material, such as barrelhead stock,

that works reasonably well provided the pieces

do not overlap more than 2 inches.

Stacking for Forced-Circulation Kilns

The air-circulation systems in forced-circulation kilns may move the air vertically, transversely, or longitudinally through the material

being dried. Therefore, the method of stacking

the material depends on the direction of airflow.

Because of the higher air velocities produced in

these kilns, boards projecting from the sides of

the loads can act as scoops and should be avoided.

Exter7ial-Blower Kilns.—In some externalblower kilns, the air is blown upward through

the loads (figs. 9 and 10, pp. 25, 26) and A flues

are therefore required (fig. 85). This flue is

usually 12 to 14 inches wide at the bottom and

should taper to about 1 to 2 inches at the top.

If boards project into the flue, air will not circulate evenly and drying will be nonuniform

throughout the load. To avoid this, two Ashaped forms, one at each end of the load, are

used as guides. The edges of boards adjacent

to the flue are laid snugly against these forms,

thereby giving uniform taper to the flue from

bottom to top. To prevent air from short circuiting over the top of the load, the top of the

flue is covered with a course of plywood or dunnage.

Interiml-Fan Kilns.—The fans in some kilns

are located below the loads and blow the air upward through the vertically stacked lumber (fig.

86). Tension springs on the chain ties attached

to the bottom and top of the vertical beams take

up slack as the lumber shrinks, thereby preventing load distortion and holding stickers in place.

In cross-piled, internal-fan kilns, the air moves

from end to end of the loads, perpendicular to

the stickers. A space of at least II/2 inches must

therefore be left between the edges of all boards

in each course (fig. 13, p. 29). If these spaces

are not provided, the air cannot move through

the loads.

The movement of air through end-piled kilns

with fans located above or below the loads is

across the loads and parallel with the stickers

(figs. 14 and 17). In these kilns, the edges of
DRY KILN OPERATOR'S MANUAL 93

1M-115554-F

FiGUEE 82.—Lumber being stacked for drying in a natural-circulation kiln. The large central flue allows free vertical

circulation of air.

any boards projecting beyond the side of the

load into the plenum chamber (the space between the side of the load and the kiln wall)

will act as air scoops and uniform air circulation across the loads will not be obtained. Also,

overhanging ends of boards at the ends of the

loads should be avoided. The openings left between such loads when they are placed end to

end in the kilns permit large amounts of air to

short circuit through them. In loads stacked for

transverse circulation, boards can be placed edge

to edge in each course. A load of softwood lumber stacked for kilns of this type is shown in

figure 70, Z?, p. 80.

55.J315 0—61 7

Stacking of special items for drying in internal-fan kilns is illustrated in figures 87, 88,

89, and 90.

Multiple-Course Stacking of Lumber

It is poor practice to stack loads of lumber

with two layers of boards laid face to face in a

course. The boards in contact with each other

will dry more slowly and will be more susceptible to stain and warping then if in single courses.

AVhen stacking misent lumber, place the thicker

boards on the outer edges of the load ; this will

avoid the need for doubliitg up boards on the
94 AGRICULTURE HANDBOOK NO. 188, U.S. DEPT. OF AGRICULTURE

M-98S47-F

FIGURE 83.—Kiln trnckload of .staves .stacked for a natural-circulation kiln. The small size of the vertical flues

( note hand ) and the topping of the load reduce circulation and extend drying time.

M-11565ÍÍ-F

FIGURE 84.—Barrelheading stacked for a natural-circulation kiln. This load vs'as built with adequately sized flues,

but the rather large overlap could slow the drying of the ends.
DRY KlUSr OPERATOR'S MAJSTUAL 95

M-116548-P

FIGURE 85.—Lumber stacked with an A flue for an

external-blower type kiln.

M-115544-F

FIGURE 86.—Vertically stacked lumber for intemal-fan

kilns with vertical air circulation.

M-115551-r

FIGURE 87.—Good stacking of hardwood squares on kiln

trucks for drying in an end-piled dry kiln with forced

transverse circulation.

M—115666—F

FIGURE 88.—Bundles of softwood squares stacked on

pallets and placed on kiln trucks for drying in an endpiled kiln with forced transverse circulation.
96 AGRICULTURE HANDBOOK NO. 188, U.S. DEPT. OF AGRICULTURE

M-11B547-F

FIGURE 89.—Barrel staves stacked for drying in an endpiled kiln with forced transverse circulation.

outer edges to prevent crowning of the load. If

additional cross support is required at the bottom of the load to prevent sagging of the lumber, a course of dry 2- or 3-inch material is

preferable to a multiple course of the unseasoned

lumber being stacked.

Kiln Sample Pockets

Kiln samples are placed in pockets built into

the loads or packages of lumber at the time of

stacking (fig. 91). The stackermen should be

told where to provide sample pockets in the load

(page 103). Since the kiln samples are usually

longer than the space between tiers of stickers,

the sticker or stickers immediately above the

sample should be shortened the width of the

sample. Otherwise these stickers will bear on

the sample and make it difficult or impossible to

remove for periodic weighing and examination,

especially if the sample cups.

Cover Boards on Loads Stacked

for Kiln-Drying

Loads or packages of material stacked for kilndrying are often air-dried first or held on the

tracks at the loading end of the kiln for several

days before going in the kiln. Such stock should

be protected from the elements. Portable roofs

made from such materials as roofing paper, plywood, or corrugated metal are often used for this

I^urpose (fig. 92). An open shed at the loading

end of a kiln is an effective means of protecting

stock from rain and sun before it goes into the

kiln.

Weights and Restraining Devices

Weights placed on top of a load or restraining devices that exert pressure are frequently

used to reduce warping of the top courses. Short

lengths of rail are sometimes used for this purpose. The weights should be placed directly over

the tiers of stickers. Restraining devices sometimes used consist of wire rope and tension

springs attached to each end of light I-beams

that extend across the load directly over the

stickers and about 6 inches beyond each edge.

The tension spring is pulled into heavy tension and hooked into a sticker opening about 5

to 6 feet below the top of the load. Occasionally, as the lumber shrinks during drying, the

hooks are pulled from the load and reinserted at

a lower point, thus taking up slack in the rope.

Turnbuckles or load binders are occasionally used

instead of springs.

M-11B642-F

FIGURE 90.—Bowling-pin blanks stacked f»r kiln-drying

in an end-piled kiln with forced transverse circulation.

Rack holds blanks in place. Air circulation is parallel

to the stickers.
DRY KILN OPERATOR'S MANUAL 97

1H-75804-P

FiGTTEE 91.—Box pile of random-length lumber showing sample pockets and kiln samples.
98 AGRICULTURE HANDBOOK NO. 188 U.S. DEPT. OF AGRICULTURE

rxo.K. 92.-Con:u.ated asbestos-cement board -fs^ eover^^Hn truckloads of lumber stored out of door;i7to
CHAPTER 6. KILN SAMPLES

The temperature and relative humidity schedules used in a dry kiln affect the drying stresses

that develop in the wood (eh. 8). Such stresses

follow a well-defined pattern closely associated

with the changing moisture content of the wood.

Therefore, moisture content, which can be determined easily at any stage of drying is a logical

basis for applying a drying schedule.

Since it is impractical to make moisture determinations on all the lumber in a kiln charge,

samples are cut from boards representative of the

material being dried. These samples, called kiln

samples, are so placed in the charge that they

can be removed easily for weighing, examination,

and testing periodically during drying. Thus,

the moisture content of the charge can be estimated and the progress of drying charted.

The handling of kiln samples requires much

of an operator's time, and sometimes additional

manpower is needed. Also, some material is lost

when kiln samples are taken. These disadvantages, however, are more than offset by several

advantages. The selection, preparation, placing,

and weighing of kiln samples, if properly done,

provide information that enables a kiln operator

to (1) reduce kiln degrade, (2) obtain better control of the desired final moisture content, (3)

reduce drying time and improve quality, (4) develop time schedules, and (5) locate sources of

trouble that affect kiln performance. All of

these advantages add up to lower drying costs

and more uniformly dried, stress-free material.

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 kiln samples;

how to use samples during drying; how to make

intermediate moisture determinations; final test

procedures; and the recording and plotting of

drying data.

Variability of Material

In order that full use be made of known drying techniques and equipment and that good drying be assured in the shortest time, each kiln

charge should consist of material having about

the same drying characteristics. In order to do

this and to select representative kiln samples,

the operator must consider certain variables in

wood that affect drying: (1) Species; (2) thickness; (3) moisture content; (4) heartwood and

sapwood; (5) grain (plainsawed or quartersawed) ; and (6) final moisture content.

Species

Woods of the many species that grow in this

countiy liave a wide range of physical properties (-5).^ Several physical properties influence

the ease of drying; among them are specific

gravity, shrinkage, moisture diffusion, and

strength perpendicular to the grain. Such woods

as basswood, yellow-poplar, and the pines are

relatively easy to dry with few or no serious drying defects. Others, such as the oaks, black

wahuit, and redwood, are more likel}' to check,

honeycomb, and collapse during kiln-drying. For

this reason, dry only one species in a kiln at a

time or, at most, a iew that have similar drying

characteristics.

Thickness

When lumber dries, the moisture evaporates

from all surfaces, but principally from the broad

faces. Thickness, therefore, is tlie most critical

dimension. The thicker the stock, the longer

the drying time and tlie more difficult the drying

job. Lumber of different thicknesses cannot be

dried in the same kiln cliarge without either

prolonging the drying time or risking drying

degrade.

Badly misent lumber needs to be watched for.

Even nominal 1-inch-thick lumber may, if badly

sawn, vary in thickness from % to IV2 inches

in the same board. The thinner parts will dry

more rapidly than the thicker ones. Dress such

lumber to a more uniform thickness or segregate it from well-cut material.

Moisture Content

The extent to which lumber has been dried

before it is put in the kiln must also be considered, because moisture content governs the

drying conditions that can be used. If all the

free water has already been removed, more severe drying conditions can be used in the initial

stages of kiln-drying with little or no dano-er

of producing the usual drj'ing defects (ch. 9).

Further, a uniform initial nioisture content makes

drying to a uniform final moisture content much

1 Italic numbers in parentlieses refer to Literature Cited, p. m.

99
100 AGRICULTURE HANDBOOK NO. 18 8, U.S. DEPT. OF AGRICULTURE

faster. If the boards vary considerably in initial

moisture content, a longer equalizing time may

be required at the end of the run.

Heartwood and Sapwood

Sapwood usually dries considerably faster than

heartwood. Resins, tannins, oils, and other extractives retard the movement of moisture in the

heartwood. Tj'loses and other obstructions maj'

be present in the pores of the heartwood of some

species, notably white oak and the locusts. Sometimes it is practical to segregate the heartwood

and sapwood boards. The green moisture content of sapwood is usually higher than that of

heartwood, particularly in the softwoods. For

these reasons, heartwood lumber miiy not reacli

the desired final moisture content as soon as sapwood, or vice versa. One or the other may therefore be overdried unless an equalizing treatment

is used.

Grain

Quartersawed boards generally dry more slowly

than plainsawed, but they are less susceptible to

surface checking. Therefore, a more severe cliying schedule can be used on quartersawed boards

to reduce drying time. For this reason, in the

drying of such items as vertical-grained flooring

strips it is advantageous to segi'egate flat- from

vertical-grained material.

Final Moisture Content

As a rule, two or more classes of material

should not be put in the same kiln charge if they

are to have different final moisture content values. If the stock to be dried to the highest final

moisture content reaches that value at about the

same time that the rest of the charge becomes

adequately dried, no harm will result. If, however, this stock reaches the desired moisture content first, it must be removed from the kiln to

prevent overdrying. Its removal leaves empty

spaces in the kiln that can disrupt air circulation through the remainder of the charge, with

resultant nonuniform drying and prolonged drying time.

Drying Mixed Charges

Ideally, segregation of lumber is based on all

of the factors that affect drying rate and diying

quality. Since this is frequently not possible

or practical, a kiln operator must be guided in

his selection of kiln samples primarily by the

drying rate of the most critical, slowest drying

material. That is, he must select the largest

number of samples from the slowest drying material. Some samples of the fastest drying material are also needed, however, since these will

determine the time when the equalizing treatment should be started (ch. 8).

Number of Kiln Samples

The number of kiln samples needed for any

kiln charge depends upon the condition and drying characteristics of the wood being dried, the

performance of the dry kiln, and the final use

intended for the material.

Drying a Charge by a Prescribed Kiln

Schedule

By far the most important purpose of kiln

samples is to enable a kiln operator to dry a

charge of lumber in accordance with a predetermined schedule. Such schedules generally call

for changes in drying conditions that are based

on the moisture content of the stock during various stages of drying.

Because of the many variables that affect drying

results, the specific number of kiln samples required for each kiln charge must be determined

through experience. A rule of thumb is to use

at least four samples in charges of 20,000 board

feet or less. For charges of 100,000 board feet

or more, 10 to 12 kiln samples per charge are

usually satisfactory. Use more samples, however,

when (1) drying a charge of material composed

of diñ'erent species, thicknesses, moisture content

levels, grain, or mixed heartwood and sapwood;

(2) drying a kind of wood not previously handled ; ( 3 ) cliying costly special items, such as gunstock, bowling-pin, and shoe-last blanks; (4)

obtaining drying data for use in modifying a

drying schedule or in developing a time schedule;

and (5) when the performance of the dry kiln

is unknown or erratic.

Developing a Time-Temperature

Schedule

At plants where certain items are regularly

dried in sufficient quantity, the operator can utilize kiln samples to develop time-temperature

schedules for subsequent charges of the same

item dried to the same moisture content in the

same kiln. This may involve extra sampling work

to measure the full range of variables, but after

sufficient information and experience have been

obtained, kiln sampling can be dispensed with

for future charges of the item. Obtain drying

data on at least eight charges of the item before

deciding on a time schedule.

Time schedules are generally used in the drying of softwoods. It is possible, however, to develop satisfactory time schedules for some of the

more easily dried hardwoods. Some samples

should, of course, be used occasionally to check

the performance of the kiln and the final moisture
DRY KILN OPERATOR'S MANUAL 101

content of the stock. Some operators using time

schedules for drying soft\Yoods employ kiln samples to determine when to equalize, condition, and

shut off the kiln.

Checking Kiln Performance

Studies of kiln performance show that drybulb temperature and rate of air circulation

throughout a kiln may vary considerably and

affect the time and quality of drying (3). Variations in temperature and circulation can be determined with testing equipment (pages 57-62)

(i). If such equipment is not available to the

operator, kiln samples can be used to check kiln

performance. Cut all samples for this purpose

from the same board or from boards having

much the same drying characteristics. Place

them near the top and bottom and on both sides

of each load, at intervals of 10 to 16 feet along

the length of the kiln.

Kiln samples that dry slowly indicate zones of

low temperature or low air circulation, and those

that dry rapidly indicate zones of high temperature or high rates of air circulation. If the drying rates of the samples vary greatly, ihid and

correct the causes (4). Differences in drying

rates between the samples on the entering- and

leaving-air sides of the loads will assist the operator in determining how often to reverse the airnow. The greater the difference in drying rate

between the entering- and leaving-air samples,

the more frequently should the direction of air

circulation be reversed.

Selection of Kiln Samples

Boards from which kiln samples are to be cut

should be selected while the lumber is being

stacked for drying. Tliis can be done by a

trained stackerman. The operator cuts kiln samples from those boards most representative of the

heavier, wetter, and thicker stock of the charge

and containing a comparatively high percentage

of heartwood. Usually one kiln sample is cut

from each sample board to assure a representative group of kiln samples (fig. 93). In randomwidth material, kiln samples are cut from boards

about 7 to 9 inches wide.

Some kiln samples are also cut from boards

that represent the drier and faster drying stock.

Such boards will generally be flatsawed, narrow,

or scant in thickness, will contain a high percentage of sapwood, or be drier than the rest at the

time the lumber is stacked for kiln-drying. These

kiln samples are used during the final stages of

drying (ch. 8).

For studies of kiln performance, two or more

matched samples are cut from the same board

(fig. 93).

\

MOISTURE SECTIONS^

A-l \

'2

^ USABLE LENGTH'"'^ KILN ~SÀMPLÈ^^ USABLE "LENGTH ^

I'i^ ^30"OR MORE -^ U/"

ONE SAMPLE PER BOARD

<M0ISTURE SECTIONS

\~20"0R MORE-\ ^50"0R MORE -J \r- 30"0R MORE—-I I— / " ' / "

TWO SAMPLES PER BOARD

M-10900S-F

FiGUEE 93.—Method of cutting and numbering liiln samples and moisture content sections.

Preparation of Kiln Samples

Knots, bark, pitch, and even a small amount of

decay should preferably not be included in the

parts of boards cut for kiln samples, except when

drying lumber of common grades. The moisture

sections that are cut from each end of a kiln

sample (fig. 93) must be of clear, sound wood.

Completely remove any bark jn-esent on the moisture sections and kiln sample before they are

weighed. The bark interferes with moisture content determinations and the rate of drying of the

kiln samples.

Cutting the Moisture Sections and Kiln

Samples

Mark the moisture sections and kiln samples

for identification, as shown in figure 93, before

they are cut. Usable lengths of lumber can be

salvaged from each end of the board when the

moisture sections and kiln samples are cut. If

no usable lengths would be left, cut the moisture

sections and kiln samples about 20 inches or more

back from the ends of the boards to eliminate the

effects of end drying.

With certain exceptions, moisture sections are

cut not less than 1 inch along the grain and

across the. full width of the board. It may be

necessary to cut moisture sections less than 1 inch

along the grain, particularly for obtaining quick

moisture determinations and when using a selfcalculating moisture content scale. To minimize

errors, take extra precautions in cutting, handling, and weighing these thinner sections. In

dimension stock 1 inch square or less in cross

section, moisture sections are cut 2 inches or more

in length along the grain. Cut the moisture sections on a sharp, cool-running saw and weigh

them immediately. If it is necessary to cut a

number of sections at a time before weighing
102 AGRICULTURE HANDBOOK NO. 188, U.S. DEPT. OF AGRICULTURE

them, wrap each one separately in ahiminum foil

to prevent drying.

Keeping saw, scales, and oven close together

in a well-lighted, draft-free area is helpful in

obtaining accurate moisture content determinations. If the sample boards must be cut some

distance from the scale, cut long pieces from the

boards and take them to the weighing ai-ea,

where the moisture sections and kiln samples can

be cut from them with a handsaw. In such cases,

trim and discard a section at least 1 inch long

from one end of each piece.

Determining Moisture Content and

Ovendry Weight of Samples

The moisture content of a kiln sample is determined from the moisture sections cut from each

end of the kiln sample. The average moisture

content of these two sections and the weight of

the kiln sample at the time of cutting are used

to calculate the ovendry weight of the kiln sample. This calculated ovendry weight and the subsequent weights of the kiln sample obtained at

intervals during the drying—called current

weights—are used to compute the moisture content at those times.

Weighing Moisture Sections.—After the moisture sections are cut, rapidly remove all bark,

loose splinters, and sawdust adhering to them,

and weigh them immediately. Weigh each section to 14 of 1 percent of its weight ; it is necessary to use a scale capable of weighing within

this degree of precision. Obtain the weights in

grams, instead of grains or ounces, so that calculations will be simplified by using the decimal

system. A triple-beam balance (fig. 38, p. 49)

is a convenient type to use for weighing moisture

sections. Other types of balances are illustrated

in figures 39 and 40.

To remain accurate, the knife-edge bearing

surfaces of a triple-beam balance must be kept

free of dirt, oil, grease, and corrosion. Protect

the scale with a dustproof cover when it is not

in use, and check its accuracy and sensitivity at

least once a year against standard weights. The

scale should be balanced on zero before each

series of weighings.

To save weighing and calculating time, the

two moisture sections cut from each kiln sample

can be weighed together. This, however, does

not give the ditïerence in moisture content usually

present between the two moisture sections. After

weighing them^ mark the weight on each section

and, when weighings are completed, enter the

weight on tabulation paper or a data form drawn

for the purpose.

Weighing KiZn Samples.—After the kiln samples are cut, remove all bark, loose splinters, and

sawdust adhering to them and apply a good end

coating. Many effective end coatings are available {2). They should be used as recommended

by the manufacturer. Immediately after end

coating, weigh the kiln samples on a scale or

balance that is sensitive to 0.01 pound or approximately 5 grams, and that has a capacity of about

35 pounds (fig. 41, p. 52). The weights should

be in either the metric system or in pounds and

hundredths of a pound, but not in pounds and

ounces. Mark the weight on the kiln sample and

also record it on a data form.

Usually, the weight of the end coating can be

disregarded. In drying some special items, however, it may have to be considered. Wlien that

is necessary, weigh the kiln sample before and

after it is end coated, the difference being the

weight of the coating. Eecord that weight and

subtract it from all subsequent weights of sample

obtained during drying.

Ovendrying Moisture Sections.—After weighing them, dry the moisture sections until moisture-free in an oven maintained at 214° to 221° F.

(101° to 105° C). This usually takes 24 to 48

hours. To test whether they are thoroughly dry,

weigh a few sections, replace them in the oven

for about 3 or 4 hours, and reweigh them. If

they have lost no weight, the entire group of

sections can be assumed to be moisture-free. Electric ovens suitable for ovendrying are illustrated

in figures 45 and 46, pages 55 and 56.

Open-pile the moisture sections in the oven to

permit air to circulate around each (fig. 45).

Avoid excessively high temperatures or prolonged

drying, because they cause destructive distillation

and oxidation of the wood. Never place newly

cut moisture sections in the oven with sections

already partly dry: the drier sections would

temporarily absorb moisture from the newly cut

sections, and this would prolong the drying time.

Newly cut moisture sections, once weighed, need

not be put in the drying oven immediately. Kiln

operators short on drying-oven capacity frequently place them on radiators until oven space

is available. This reduces ovendrying time.

Weighing Ovendry Moisture Sections.—It is

essential that moisture sections be rapidly weighed

immediately after they are removed from the

oven. Weighing is done as described for freshly

cut moisture sections.

Calculating Moisture Content of Sections.—

Moisture content of the moisture sections is

calculated by dividing the weight of the water

removed by the ovendry weight of the section

and multiplying the quotient by 100. Since the

weight of the water equals the original weight of

the section minus its ovendry weight, the formula

for this calculation is :

Moisture content in percent=

original weight—ovendry weight

ovendry weight XlOO (1)
DRY KILN OPERATOR'S MANUAL 103

Example: Calculate the average moisture content of two moisture sections (fig. 93, top) when:

Green weight of moisture section ai

=98.55 grams

Ovendry weight of moisture section ai

=59.20 grams

Green weight of moistirre section a2

=86.92 grams

Ovendry weight of moisture section a2

=55.02 grams

Wanted: The average moisture content of

moisture sections aj and a2.

Two methods of calculating average moisture

content in percent can be used.

Method 1

Moisture content of section ai

98.55-59.20

59.20 X 100=66.5 percent

Moisture content of section &2

86.92—55.02

55.02 X 100 =58.0 percent

The average moisture content of moisture

sections ai and SL^ is :

66.5 +58.0 =62.2 percent

Method 2

If the sections are weighed together, the combined green weight of sections ai and a2 is 185.47

grams and their combined ovendry weight is

114.22 grams. Then:

Average moisture content=

185.47-114.22

114.22 XI00=62.4 percent

Whue the average moisture content of moisture

sections ai and &2 calcidated by method 2 results

in a slightly higher value than that obtained by

method 1, the calculated ovendry weight of the

kiln sample using either method will be the same

when corrected to the nearest 0.01 pound.

It is sometimes convenient, when making

slide-rule or machine calculations, to use a shortcut method of calculatmg moisture content by

the formula :

Moisture content=(——,—^^^^ ,

\ovendry weight 0

1 xioo

Substituting the weights for moisture section ai

in this formida :

Moisture content in percent of section ai =

(^|||-l')xi00=(1.6647-l)X100=66.5

Calculating Ovendry Weight of Kiln Sample.—

The moisture content of a kiln sample at the time

of cutting and weighing is assumed to be the same

as the average of the moisture content values of

the two moisture sections cut from each end of

the sample. Knowing this value and the weight

of the sample at the time the sections were cut,

the ovendry weight of the kün sample can be

calculated by using the following formula:

(2)

Ovendry weight of sample =

Original weight of kiln sample y,^^

100 + moisture content of sample

in percent

Example: Calculate the ovendry weight of kUn

sample A-1 (fig. 93, top), the original weight of

which is 4.46 pounds, using the average moisture

content calculated for moisture sections ai and

a2, 62.2 percent:

Ovendry weight of sample^

4.46

100 + 62.2 X100 =0.02749X100 = 2.75 pounds

Placing Samples in Kiln Charges

After kiln samples are cut, end coated, and

weighed, they are placed in sample pockets built

into the loads or packages of lumber during the

stacking operation (fig. 94). Since the kiln samZM-69968-P

FIGURE 94.—Placement of three kiln samples in sample

pockets built in the side of an end-piled load of lumber. The pockets should be deep enough so that the

kiln samples do not project beyond the edge of the

load.
104 AGRICULTURE HANDBOOK NO. 18 8, U.S. DEPT. OF AGRICULTURE

pies are representative of the stock being dried,

they must at all times be exposed to the same

drying conditions or they will give a false indication of the moisture content of the charge. For

example, considerable moisture may be lost from

kiln truckloads or packages of lumber that are

not loaded into the kiln for several days or weeks

after the samples are cut and weighed. The kiln

samples representing this stock must be in the

loads or packages during this period. Since kiln

samples may dry faster than the lumber in the

central parts of a load, it may be desirable to

shield the sample pocket to slow up air circulation at that point.

Locate the sample pockets at places in the loads

where the samples can be easily removed and

replaced. In end-piled forced-circulation kilns

the locating of sample pockets is relatively easy,

because there is usually a walkway alongside

the loads. In a double-track kiln, the pockets

are placed in the sides of the loads nearest the

walls. In natural-circulation kilns and most

kilns of the external-blower type, however, the

entering-air side of the loads is usually not accessible, and the kiln samples have to l3e placed

on the leaving-air side.

In cross-piled dry kilns of both natural- and

forced-circulation types, there is seldom suiRcient

room between the ends of the loads and the walls

to allow the operator to walk into the kiln.

Only the load nearest the door is accessible.

Samples can be placed in the ends of this load,

or in the side facing the door. These locations

are not the most deseirablej but there is usually

no other choice. In such kilns, the operator can

get a good idea of how the dryness of the samples

compares with that of the stock in other parts

of the charge by making a thorough check of the

final moisture content and quality of the kilndried stock. With this information he may, when

drying future charges, be better able to judge

from the moisture content of the samples when

to change drying conditions or pull the charge.

If a mixed kiln charge is being dried, place the

samples representing each type of material in the

truckloads or packages containing that material.

For example, if 4/4 and 6/4 pine lumber are being

dried in the same charge, put the 4/4 samples

with the 4/4 lumber and the 6/4 samples with

the 6/4 lumber.

Some operators of unlighted kilns use small

colored-glass reflectors or reflective tape on the

edge of the sample or the edges of boards above

or below the sample pocket. These i-eflectors are

easily found with a flashlight. To guard against

replacing samples in the wrong pocket after

weighing them, put the number or letter of each

sample on the edge of the board immediately

above or below its pocket.

Using Kiln Samples During Drying

As drying progresses, the drying conditions in

the kiln are changed on the basis of the moisture

content of the samples at various times during

the run. How frequently the samples must be

weighed will depend on the rate of moisture loss;

the more rapid the loss, the more frequently thev

must be weighed. Immediatel}^ after they are

weighed, they must be returned to their pockets.

Calculating Current Moisture Content

of Sample

To calculate the current moisture content of a

sample, two weights are required; the current

weight and the calculated ovendry weight. The

formula used is as follows :

Current moisture content=

Current weight—calculated

ovendry weight

Calculated ovendry weight

Thus, if the calculated ovendry weight of the

sample is 2.75 pounds and its current weight

4.14 pounds, then:

4 ]^4 2 75

Current moisture content=^—ö~7~^—

X 100=0.5054X100=50.5 percent

After another day of drying, this sample may

weigh 3.85 pounds. The current moisture content

of the sample will then be :

3.85-2.75

2.75 X 100 =0.400X100=40.0 percent

If a slide rule or machine is used in calculating

the current moisture content, the following shortcut formula can be used :

-0xioo

Current moisture content=

/ Current weight

\Calculated ovendry weight

Substituting the above values in this formula:

(III-Ax 100 = (1.400-1) XIOO

=0.400X100=40.0 percent

Use of Samples toFollowKiln Schedules

Kiln schedules provide for changes in kün

conditions as drying progresses. If the Forest

Products Laboratory schedules are used, it is

recommended that drving conditions be changed

when the average moisture content of the wettest

50 percent of the kiln samples equals a given

moisture content in the schedule. Sometimes an
DRY KILN OPERATOR'S MANUAL 105

operator may change drying conditions according

to the wettest one-third of the samples or the

average moisture content of a smaller group of

samples that may be distinctly wetter or more

difficult to dry than the others. These are called

the controlling samples. The moisture content

of the driest sample determines when equalizing

of the kiln charge should be started.

Intermediate Moisture Content Tests

If the moisture content of the moisture sections does not truly represent that of the kiln

sample, the calculated ovendry weight of the

kiln sample will be wrong. This may mislead the

operator into changing kiln conditions at the

wrong time, with such serious consequences as

prolonged drying time, excessive amounts of drying defects, or nonuniformly dried stock. For

example, if water pockets are present in the

moisture sections but not in the sample, the calculated ovendry weight of the sample will be

too low and its current moisture content too high.

Conversely, if water pockets are present in the

sample and not in the moisture sections, the calculated ovendry weight of the sample will be too

high and its current moisture content too low.

These potential ill effects may be avoided by

making intermediate moisture content determinations.

When To Make Intermediate Tests

When the calculated moisture content of one

or a few kiln samples is much higher than that

of the other samples, or if their rate of drying

appears to be much slower than the average rate,

make a check moisture content test on those samples to obtain a better estimate of their calculated ovendry weight. The best time for making

these intermediate determinations is when the

average moisture content of most of the samples

is at about 20 percent. The intermediate determinations can be made on all of the samples in

the charge if the operator wants a precise estimate of the final moisture content.

How To Make Intermediate Tests

Trim a section about 5 inches long off one end

of the kiln sample. Then cut a 1-inch-wide

moisture section from the newly exposed end of

the sample, weigh it immediately, and ovendry

it. Coat the freshly cut end of the shortened

sample and weigh it immediately. The new

weight of the sample is the new "original" weight

used in forniula (2). After weighmg the sample, place it in its pocket in the kiln charge. As

soon as the moisture section has reached constant

weight in the oven, weigh it and calculate its

moisture content with formula (1). Substitute

the new moisture content value, together with the

new original weight of the sample, in formula

(2) to obtain a new calculated ovendry weight.

Use the new calculated ovendry weight in formula (3) to obtain the current moisture content

of the sample in all subsequent weighings.

A moisture content test may be necessary near

the end of the run to check the moisture content

of the samples before starting equalizing and

conditioning treatments.

Final Moisture Content and

Stress Tests

After the lumber has been dried to the desired

final moisture content, the drying stresses relieved by a conditioning treatment, and the charge

pulled from the kiln, the final moisture content

and stress tests can be made on the samples. Tests

of boards selected at random from the charge

can also be made.

Cut three 1-inch sections, several inches in from

an end, from the samples and selected boards.

Use one of these sections to determine the average moisture content by formula (1). Use the

second section to determine the distribution of

moisture in the section by cutting it into a core

and shell (fig. 95). Weigh the core and shell

separately ; ovendry and reweigh them ; and calculate their moisture content by formula (1).

Use the thii*d' section for a stress test to deter-

^THICKNESS OF KILN SAMPLE

-m\ W /4 THICKNESS

M-100006-F

FiGUEE 95.—Method of cutting section for determination

of distribution of moisture in sljell and core.
106 AGRICULTURE HANDBOOK NO. 188, U.S. DEPT. OF AGRICULTURE

mine the eflfectiveness of the conditioning treatment in relieving casehardening (fig. 96).

Recording Drying Data

Properly recorded and evaluated kiln sample

data assist the operator to (1) modify the drying

schedule on subsequent charges to obtain faster

drying without sacrificing quality; (2) develop

time schedules for certain classes of material;

(3) determine the effect of seasonal weather conditions on drying time; and (4) check the performance of the dry kiln and determine the

causes for nonuniform drying and seasoning degrade.

u

/

\

A_

NOT CASEHARDENED

SLIGHTLY CASEHARDENED

The kinds of data to be recorded will varj^ with

the drying job. On a test run in a new kiln, on

a new class of material, or on a new or modified

schedule more drying data are required than are

ordinarily necessary. Likewise, the more precise

the drying job, the more data needed.

The data can include such items as species,

gi'ade, origin of material, date of cutting, kind

of grain (flat or quartered), percentage of sapwood, rings per inch, moisture content, thickness,

date drying was started, the drying schedule

used, drying time, drying defects, method and

length of storage before and after drying, and

shipping date.

CASEHARDENED

REVERSE CASEHARDENED

NOT CASEHARDENEDSLIGHTLY CASEHARDENED

CASÍHARDÍNED REVERSE CiSEHARDENED

M-60300-F

FIGURE 96.—Method of cutting specimens for casehardening tests. Material tha.t is less thaji 1% inches thick is cut

into three prongs, and the middle prong is removed ; material that is l^^ inches thick or thicker is cut into six

nrongs, and the second and fifth prongs are removed.
DRY KILN OPERATOR'S MANUAL 107

Kiln sample data should be recorded on suitable forms, such as those supplied by dry kiln

manufacturers. Many kiln operators develop

their own data forms, or modify available forms

to fit their specific needs. Forms for recording

drying data on 3 of 10 samples in an expenmental charge of 4/4 soft maple are shown in figures

97, 98, and 99. , , n • , ^

Prehminary kiln sample data should mclude

moisture section and kiln sample numbers and

other data shown in figure 97. Origin of material, grain, rings per inch, defects, and other data

can be added if required.

Drying data obtained for each sample durmg

the kiln run are entered on a kiln sample record

form (fig. 98). Other data, including kiln number, board footage in the charge, species and

thickness, and date drying was started, can be

entered as required. Some of the data recorded

on the kiln sample preliminary data form (fig.

97) can also be entered on the sample record

form. Also recorded are tlie data covering the

intermediate moisture determinations and the

data for the moisture regained during tlie conditioning treatment. If a record of the weight of

the end coating used on the kiln samples is required, it also would be entered on this form.

Moisture and St7vss Data.—The data for tlie

final moisture and stress determinations can be

recorded on a form like the one shown in figure

99. The degree of casehardening present in the

stock is noted in this form. Supplemental moisture data, such as those obtained with moisture

meters, should also be recorded.

KILN SAMPLE

PRELIMINARY DATA FORM

Species ÓO/^T M^PLS

Grade _ A/f/XSO

Grain

Sheet No.

Origin W/<SCOAf^//V

/

M/X£'0

Nominal thickness

Date /0/30/5/

-^

Remarks: fí/t/?7-/A¿,iy é4/fí û/?/£'û

Moisture content sections : Kiln sample Rings:

per :

inch :

/4¿

Sample :

No. :

Section

No.

/a ^ncf

/Ó

Green :Ovendry:

or :weight :

initial: :

weight: ;

Gm. : Gm.

225.20J6ß.O^

Moisture:

content :

Percent :

Green :

or :

initial:

weight:

Lb. :

796 \

Calculated

ovendry

weight

Lb.

Defects

/ J6.i^ • A/OA/^

2

2a a/fcl

'/9S3¥\/47.¥f 32.5 4,93 S24 : /â

3

3a aftd

: 36 /99.0¥\/35Sé ¥4>.5 /0.¥3 . 7/2 \ /5 A/oNe

M—1155315-F

FIGURE 97.—Data form filled in with preliminary data on 3 of 10 kiln samples representtng a charge of % soft

maple. The 2 moisture sections cut from each sample were weighed together.
108 AGBICULTURE HANDBOOK NO. 188, U.S. DEPT. OF AGRICULTURE

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DRY KILN OPERATOR'S MANUAL

MOISTURE AM) STRESS RECORD

109

Kiln No. ^

Date /^/9/s/

Run started /öj

After conditioning

äjjjf^^

Sample; Shell Core : Average Casehardening

No. :

/

Wt. :0.D.

: wt.

Gta. : Gffl. !

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9/ \

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M-116534-F

FiGTTBE 99.-—Form for recording final moisture content and stress data for three kiln samples.

Plotting of Data

Plots of drying data show at a glance the time

required to reach a certain moisture content.

A plot of the average moisture content of the

controlling samples in a kiln charge of 2-inch

black túpelo is shown in figure 100. The curve

shows the moisture loss to be fast and steady.

Curves plotted from data obtained from each

sample are useful for checking kiln performance

and the reliability of the moisture content values

of the samples. For example, if the moisture

loss data on samples in the same zones in a kiln

consistently indicate, on several charges, a slower

or faster drying rate than the other samples in

the charge, it is evidence of a cold or a hot zone

in that location. The source of trouble can usuallj be found and corrected. On the other hand,

ii it is known, or an investigation shows, that the

cause is not associated with a cold or hot zone

in the kiln, it can be suspected that the calculated

ovendry weight of this sample is inaccurate and

an intermediate moisture content determination

is needed.

Drying data obtained for an experimental kiln

charge of green 4/4 northern red oak are plotted

in figure 101. The drying conditions during the

run are plotted, along with the moisture loss of

the controlling samples and the driest and wettest samples. The effect of changes in drying

555315 0—61 8

conditions on the rate of moisture loss from the

samples is apparent. Also shown is the effect of

the equalizing and conditioning treatments in reducing the difference in average moisture con-

//o

/oo

90

80

70

60

50

40

30

BO

10

:iXi===-

4 5 6

TIM£ (DAYS)

10

M-109007-F

FIGURE 100.—Moisture content-time curve for 2-inch

black túpelo, showing a fast, steady loss of moisture.
lio AGBICULTFRE HANDBOOK NO. 188, U.S. DEPT. OF AGEICXILTURE

200

"t

I

I

160

120

80

40

I

^

^DRY BULB

WET BULB

^-.

EMC

EQUALIZING

STARTED ~

CONDITIONING

STARTED

SPECIES Green Northern Red Oak

SIZE (INCHES)- - ^/* Random Width

GRADE No. 2 and Better

GRAIN 4//>g</

12 16 20

DRYING TIME (DAYS)

M-11B577-F

FIGURE 101.—Chart showing kiln-drying schedule and moisture content at various times during drying of % northern

red oak.
DRY KILN OPERATOR'S MAJTOAL 111

tent between the driest and wettest samples in

the kihi charge.

Literature Cited

(1) KiMBALL, K. E.

1955. TESTING COMMERCIAL KILNS FOR UNIFORMITY

OF DRYING CONDITIONS. U.S. Forest Serv.

Forest Prod. Lab. Rpt. 1643, 9 pp., illus.

[Processed.]

(2) McMiLLEN, J. M.

1956. COATINGS FOR THE PREVENTION OF END

CHECKS IN LOGS AND LUMBER. U.S. ForsSt

Serv. Forest Prod. Lab. Rpt. 1435, 13 pp.,

iUus. [Processed.]

(3) RASMUSSEN, E. F.

1956. NEED FOR UNIFORMITY OF TEMPERATURE

IN A FORCED-AIR-CIRCULATION, VENTILATED,

COMPARTMENT DRY KILN. U.S. ForeSt

Serv. Forest Prod. Lab. Rpt. 1669, 6 pp.

[Processed.]

(4)

1955. ANALYSIS OF PROBLEMS RELATING TO UNIFORMITY OF KILN CONTROL. U.S. Forest

Serv. Forest Prod. Lab. Rpt. 1645, 9 pp.,

illus. [Processed.]

(5) U.S. Forest Service Forest Products Laboratory.

1955. WOOD HANDBOOK. U.S. Dept. Agr. Handbook 72, 528 pp., illus.

CHAPTER 7. LOADING THE DRY KILN

Basically, all kilns are designed to be loaded

in a specific way. As terms used throughout this

manual indicate, "end-piled" kilns are loaded

with the lumber running lengthwise in the kiln,

and "cross-piled" kilns are loaded crosswise.

These two basic loading patterns are directly

associated with the direction of airflow in the

kiln or the plant's method of handling the

lumber.

Overloading and underloading affect the quality of drying achieved in a given kiln. A capacity load assumes not only that the lumber is

properly piled in the kiln but that the loads or

packages of lumber are of lengths that provide

suitable overall dimensions. That is, the spaces

between truckloads and between the charge and

the walls and ceiling are those called for by the

kiln designer. If these spacings are materially

altered through underloading or overloading, air

circulation is changed, with consequent effects on

drying time and quality.

This does not mean that a particular dry kiln

can be loaded only with certain lengths of lumber. It does mean, however, that a kiln operator

must consider the effects on air circulation imposed by overloading and underloading, and plan

his loading patterns to best advantage, deviating

as little as possible from the oA^erall charge dimensions best suited for his kiln. "Wlien circumstances are such that the loading pattern must be

seriously changed, he should exercise special care

to keep air circulation as uniform as possible by

adding auxiliary load baffles.

Loading Natural-Circulation

Compartment Kilns

The air velocities through loads of lumber in

a natural-circulation kiln are very low, usually

not more than 25 feet per minute; hence short

circuiting through voids is small and has little

if any effect on drying time or uniformity. To

fet maximum benefits from the low air velocities,

owever, certain loading procedures must be followed.

For uniform air circulation in well-designed,

natural-circulation, end-piled kilns (fig. 8, p. 24),

spaces between the sides of the load and the walls

should be at least 18 inches, and preferably 2 feet

wide, while the clearance between the top of the

load and the ceiling should be about 2 feet.

Smaller openings, caused by overloading, limit

the volume of air flowing through, reduce its

velocity, and prolong drying time. Spaces left

between the ends of the kiln and the load have

no appreciable effect on air circulation.

In natural-circulation kilns of the cross-piled

type, the spaces between the kiln walls and ceiling and the loads should be the same as for endpiled kilns of this type. In addition, a space of

at least 8 inches, and preferably 1 foot, should

be provided between the loads and also between

the ends of the kiln and the loads.

Load baffles should not be used in a naturalcirculation kiln. Neither should solid obstructions, such as plank walkways, be placed at track

level; walkways should be of the open-grating

type. Flues built into the loads to permit air

movement through the loads must be open at

the top.

Loading Forced-Circulation

Compartment Kilns

The higher the air velocity in a dry kiln, the

greater is the possibility of short circuiting. In

designing forced-circulation kilns, therefore,, enpfineers g-ive careful consideration to the'sizes of

air ducts, plenum chambers, load flues, and

other passages through which air must flow at

given rates. ~\^nien these carefully calculated controls are upset in any way—and overloading and

underloading are two of the most common ways

—drying time and quality are both altered unless adequate compensatory steps are taken. Xaturally, the particular design of a kiln determines to what extent this is so. Loading characteristics of several basic types of forcedcirculation kilns are reviewed here.

Loading External-Blower Kilns

In some types of external-blower kilns, air

moves upward from ducts at or below floor level

(figs. 9, 10, pp. 2.5, 26). Air velocity is quite high

at floor level, but drops off as the air moves upward

and laterally through the truckloads of lumber.

Both air velocity and air volume are carefully calculated to provide suitable drying rates throughout the kiln when the chamber is loaded to its rated

capacity. Proper loading is therefore necessary

for efficient operation, and truckloads of stock

must be properly located in the kiln.

The external-blower kiln shown in figure 11

was especially designed for package loading, and

movement of air is downward through and between the tiers of packages.

Eoid-Piled Kilns.—In the loading of externalblower kilns of the end-piled type, whether of

single- or double-track design, the A flues in the

loads of luml:)er must be centered over the slots

or portholes in the air ducts. Avoid spaces between the ends of the loads and between the

loads and the ends of the kiln that will short

circuit the air excessively.

If the kiln cannot be fully charged, use one

of two loading methods that help insure uniform

airflow through the loads. By method 1, the

113
114 AGRICULTURE HANDBOOK NO. 188, U.S. DEPT. OF AGRICULTURE

truckloads of lumber are built low enough so

that the full number of trucks is positioned along

the track. Thus, short circuiting of air through

open areas of the kiln will be avoided. By

method 2, full-size loads of lumber are placed

in the kiln, thereby "shorting" one or both

tracks of loads. Cover boards are laid over the

entering- and return-air slots or ports in parts

of the ducts that have no truckloads of lumber

above them, thus preventing air from short circuiting through these empty areas of the kiln.

Method 1 is usually preferred. In double-track

kilns, most effective use of method 2 requires

that the same number of kiln trucks be put on

each track. If a track is not fully occupied,

push the loads to one end of the kiln, preferably

to the end without doors.

Cross-PiUd Kilns.—The A flues in the loads

of lumber in cross-piled, external-blower kilns

must be centered over the supply air ducts so

that air can rise directly into the A flues. Improper placement upsets air circulation, disrupts

drying schedules, and leads to nonuniform drying.

Package-Loaded Kilns.—External-blower kilns

designed for package loading (fig. 11, p. 27)

have certain definite loading requirements. Since

air circulation is downward from overhead ducts,

packages must be built and stacked one on another with this direction of airflow in mind.

Leave a space of about 12 to 18 inches between the

tiers of packages, and locate each alternate space

directly below the air supply nozzles. Partially

block off the bolster or spacer openings between

packages of lumber in each tier, to hold to a

minimum the short circuiting of air through

them. Short circuiting of air around the ends

of the packages of lumber can be largely prevented with vertical baffles that extend from the

bottom to the top of the tiers at each end of the

entering-air space.

Loading Internal-Fan Kilns

The air velocities obtained in most internalfan kilns make proper loading of topmost importance because of the greater tendency of the

air to short circuit. The loading procedures to

be used for each of the several types vary to some

extent.

Cross-Piled Kilns.—In cross-piled, track-type,

internal-fan kilns, the air movement is either parallel to the stickers or at right angles to them

(fig. 13, p. 29).

Maximum efficiency is obtained in cross-piled

kilns with air movement parallel to stickers by

leaving a space of about 3 inches between the

loads and installing load baffles at each end of

the kiln.

In kilns with air movement at right angles to

the stickers, spaces are provided between the

edges of all boards in each course. The space

between the sides of the loads of the kiln charge

should be as small as possible. Install top load

baffles as illustrated in figure 13, between the

tops of the truckloads of lumber and the ceiling.

If the kiln charge is short one or more truckloads, baffle off the empty part of the kiln.

End-Piled Kilns.—In end-piled, single-track,

internal-fan kilns, in which the air moves parallel to the stickers, the distance between the loads

of lumber and top load baffles should not exceed 4 inches. Butt the ends of the truckloads

of lumber snugly together. If this is not possible because excessively long boards overhang

from the ends of the loads or because of stepback or step-out loads, block the voids so caused.

If the kiln charge lacks one or more truckloads

of lumber, push the entire charge to one end of

the kiln and close the empty area by solid baffles

extending from the track level to the kiln ceiling, the fan floor, or the top load baffle. If the

kiln has doors on one end only, push a charge

that is short one or more truckloads toward the

closed end rather than the door end of the kiln.

More care is required in loading end-piled, internal-fan kilns having two or more tracks (fig.

14, p. 29, and fig. 17, p. 31) than single-track

kilns with the same type of loading. Short circuiting through voids in a charge of lumber in a

single-track kiln can be controlled with solid

baffles. In a multiple-track kihi, however, a sohd

baffle blocking a space in one trackload of lumber may reduce airflow through some of the

lumber on the other tracks.

A method of baffling voids in a double-track,

end-piled, internal-fan kiln is illustrated in figure 102. Track 1 has three truckloads of lumber, one of which is a short load, and track 2 is

fully loaded. The void spaces on both tracks

between kiln end D and the loads are small,

about 1 foot wide. A temporary solid baffle extending from the kiln floor to the fan floor can

be installed in this opening if desired, but since

the opening is quite small the value of a baffle

here is questionable. The larger voids between

the short and long loads on track 1 and between

kiln end C and the ends of the loads are blocked

off by temporary baffles to prevent excessive short

circuiting of the air. The baffles shown on track

1, however, should not be solid. A solid baffle

here would block off track 2 from air circulating

through the loads from side A to side 5, and the

lumber on this track would be shorted of air and

dry more slowly than the rest of the charge.

Slotted or perforated plywood baffles have been

used in a situation like this. Snow fence has
D-BY KILN OPE'RATOR'S MANTJAL 115

SIDE B

SOLID BAFFLE

TRACK 2

END D

Ö

--SLOTTED OR

PERFORATED BAFFLE

SIDE A

SLOTTED OR

PERFORATED BAFFLE

TRACK I

END C

PLAN

SLOTTED BAFFLE

FAN FLOOR

SeCTION A-A

M-llB57ft-P

FiotTBE 102.—A method of baffling voids in a charge of

lumber in a double-track, end-piled, internal-fan kiln.

also been used successfully. Perforated baffles

do not provide the same resistance to airflow as

a load of lumber, but they reduce short circuiting

considerably. The space on end C, track 2, is

is blocked off with a temporary solid baffle.

The low load illustrated on track 2 (section

A-A) would produce a large void permitting

excessive short circuiting of air if it were not

baffled. The slotted or perforated baffle shown

between the load baffle and the top of the low

load permits air to move across the loads on both

tracks in both directions of air flow with very little

short circuiting.

If a charge in a double-track kiln is short

two truckloads of lumber, load each track one

truckload short. Butt the truckloads on each

track together. Load both tracks as closely as

possible to the same end of the kiln, so that most

of the space occurs at the opposite end. Then,

with both tracks evenly loaded, two solid, temporary baffles can be used to block off the space

on each track.

Placing Packaged Lumber on Kiln Trucks.—

At many plants operating track-type kilns, lumber is stacked in packages to reduce handling

costs, and the packages are placed on kiln trucks

as shown in figure 103. The sides of adjacent

packages should be spaced 3 or 4 inches apart.

If this is not done, the sticker openings between

courses of lumber will not line up as drying

progresses and the circulating air may be blocked

off, because wood does not shrink uniformly in

thickness during drying. A strip of lumber

nailed to the ends of the spacers between packages of lumber is also shown in figure 103. Blocking of the openings produced by the spacers

(usually 4 inches high) is recommended in

forced-circulation kilns, since large volumes of

air can short circuit through these spaces.

Package Loading of Intevnal-Fam, Kilns.—

Careful placement and baffling of the packages of

lumber in internal-fan kilns (fig. 15, p. 30 and

fig. 18, p. 32) are essential in order to prevent

excessive short circuiting. Short- circuiting is

more critical in this type of Iriln than in the

track type because of the generally longer distance air must travel from the entering-air to the

leaving-air side of the charge. The greater the

capacity of these kilns, usually the more difficult

it becomes to prevent short circuiting.

Often an extra tier of packages is added to a

package-loaded kiln to increase its output. This

makes the space between the outside tier of packages and the kiln wall or door (the plenum chambers) narrower than the manufacturer recommends and causes nonuniform air circulation

through the charge.

The spaces between adjacent tiers of packages

are sometimes made smaller or larger than recommended. If they are too small, circulation

through the sticker openings may be stopped.

If they are too large, a considerable volume of

air will short circuit through these openings over

or around the ends of the tiers of packages.

The installation of additional top and side load

baffles reduces short circuiting over or around

the ends of the tiers of packages and increases

kiln efficiency. Temporary solid or slotted baffles

may be required when large voids occur in a kiln

charge that is short one or more packages or in

which the tiers of packages are incomplete. Never

use a solid baffle, however, to block off a void

space if it will affect airflow through lumber on

either side of the void space.

Because of the generally long distance air

travels across a charge of lumber in packageloaded kilns, spacer openings between packages

should be covered as shown in figure 103 to prevent short circuiting through them.
116 AGRICULTURE HANDBOOK NO. 18 8, U.S. DEPT. OF AGRICULTURE

M-100902-F

FiGTJKE 103.—Packages of lumber properly spaced side to side on kiln tracks for drying in an internal-fan kiln.

Loading Progressive Kilns

Good practices for loading progressive dry

kilns (fig. 19, p. 33 and fig. 20, p. 34) are the

same as those for compartment kilns. If the

kiln is of the natural-circulation, cross- or endpiled type, the suggestions made herein for compartment kilns of this type are recommended.

The correct loading and baffling of the externalblower, progressive kiln shown in figure 20 is the

same as that of the package-loaded, internal-fan

compartment kiln. The loading procedures for

internal-fan, cross-piled or end-piled progressive

kilns should conform to those of the corresponding compartment-type, internal-fan kilns given

herein.
CHAPTER 8. KILN SCHEDULES AND DRYING TIME

A kiln schedule is a carefully worked out set

of dry-bulb and wet-bulb temperatures which

the operator can use to dry a specific wood product at a satisfactory rate without causing objectionable drying defects. Schedules may be

classified as general and special. General schedules cover the entire range of drying conditions

normally used in dry kihis. One of the general

schedules will dry any wood product with reasonable economy. Special schedules are those developed to attain certain diying objectives; for example, to reduce drying time, dry chemically

treated wood properly, or maintain maximum

strength for special uses. Because of the many

variabilities in the character of the wood, type

and condition of the kiln, quality of drying required, and cost considerations, no schedule,

either general or special, can be considered ideal.

The schedules in this cliapter are presented as

guides for the kiln operator in developing schedules best suited to his operations.

In addition to drying schedules, information is

given on sterilizing, equalizing, and conditioning

treatments, and drying time.

Rapid drying is achieved in kilns by the use of

high temperatures and low relative humidities.

ALsuse of these may lead to drying defects. These

defects are caused by stresses that develop in the

wood as it dries. Drying stresses are related to

the average moisture content of the stock. That

is why drying schedules are often based upon the

moisture content of the stock. Since moisture

content decreases as drying progresses, it is also

possible to base kiln schedules on drying time.

Hardwoods generally take a long time to dry,

and many of their uses are critical as to drying

defects and moisture content. Hence, their schedules are usually based on moisture content. The

development of drying stresses in hardwoods usually makes it possible to lower relative humidity

and raise temperature independently to obtain the

fastest drying rate {U)} Such moisture content

schedules can be changed to time schedules when

very similar stock of the same species is dried

repeatedly in the same kiln. When the type of

stock or operating procedures are changed, the

operator should return to the moisture content

schedules.

Satisfactory time schedules have been worked

out by industry for repeatedly drying softwood

1 Italic numbers in parentheses refer to Literature

Cited, p. 145.

items of a very unifomi character in the same

type of kiln. AJí operator inexperienced in drying softwoods, however, may get better results

by using moisture content schedules. Also, an

experienced operator may be able to save time

by using moisture content schedules, when drying

time is unduly long.

The schedules given in this chapter are designed for use in modern forced-air kilns with

air velocities of between 200 and 400 feet per

minute through the load. Modifications ai'e given

for kilns operated with lower air velocities. Other

modifications may be necessaiy in kilns with

higher air velocities.

The selection of a kiln schedule should be based

on an understanding between the operator and

management as to what standards of drying are

desired, such as drying defects allowable, final

average moisture content, degree of moisture uniformity, and fuial stress condition.

The general schedules are conservative enough

to produce stock with a minimum of drying defects in a reasonably short time. The operator

should not make the schedules more conservative

unless he is drying abnormal stock or kiln performance is below standard. With properly

maintained kilns, it is possible to modify the

general schedules to shorten the time.

Hardwood Schedules

General Hardwood Schedules

Extensive pilot testing and widespread commercial use have demonstrated that the general

schedules for hardwoods developed by the Forest

Products Laboratory and presented here are satisfactory for the drying of 2-inch and thinner

hardwood lumber and numerous other products.

They form a base from which an operator can

develop the most economical schedule for a specific type of kiln. Related information on application and modification of schedules is also presented together with suggestions for drying thick

hardwoocfs.

Moisture Content Basù.—The successful control of drying defects in hardwoods depends

upon the proper regulation of temperature and

relative humidity in the drying procedure.

At the start of drying, a fairly low temperature

is required to prevent collapse and honeycombing.

Relative humidity must be kept high to keep surface and end checking at a reasonable minimum.

117
118 AGRICULTURE HANDBOOK NO. 188, U.S. DEFT. OP AGRICULTURE

Even at these mild conditions, the lumber will lose

moisture rapidly. To maintain a fast drying rate,

relative humidity must be lowered and temperature raised as soon as the moisture content and

stress condition of the wood will permit. Eelative

humidity can be dropped gradually after the wood

has lost about one-third of its moisture content

when green. The temperature can be raised gradually from the time the average moisture content

reaches 30 percent. When the moisture content

at midthickness reaches 30 percent, the temperature can be raised drastically. Therefore, the

principles of efficient diying require that hardwood schedules be based on moisture content of

stock. Plenty of kiln samples should be used.

The recommended operating procedure is to take

the average moisture content of the wetter half

of the kiln samples—called the controlling samples—as the factor that determines when to

change drying conditions (see ch. 6).

Material Consideratians.—The general schedules are for hardwoods that are to be dried from

the green condition. They can be modified to

apply to previously air-dried material. The

schedules are also set up to handle the more

difficult-to-drj^ types of wood within a species—

for example, flat-grain heartwood. Because of

the difference in the moisture content of sapwood and heartwood in many species, most of the

kiln samples should be taken from the wettest

heartwood and their moisture content used in

applying the drying schedules. Modifications

are suggested later in this chapter for stock that

is all sapwood.

Opération Consideratíoiis.—The general schedules are designed for full-time operation in modern forced-circulation compartment kilns that

have the control bulbs properly located on the

entering-air side of the loads. (See ch. 4, p. 68,

for proper location of control bulbs, especially the

procedures to use if the control bulb is not located

in the hottest zone. ) The schedules must be modified if the bulbs are located on the leaving-air side ;

otherwise, the wood on the entering-air side will

be subject to excessively severe drying conditions

(ch.4).

Also, wet-bulb depressions in the schedules

must be modified if air velocity through the load

is less than 200 feet per minute. In externalblower kilns, initial wet-bulb depressions can be

1"^ or 2° F. greater than those recommended in

tlie schedule. In natural-circulation kilns, they

can be increased as much as 4°. As drying progresses, the wet-bulb depressions should be gradually brought into accord with the schedules. If

air velocity through the load is more than 400

feet per minute, it may be necessary to use

slightly smaller initial wet-bulb depressions than

those shown in the schedule.

Recommended and Suggested Schedules.—

Schedules for dry-bulb temperatures and wetbulb depressions are given in tables 9 and 10.

Together, the dry-bulb temperature and the wetbulb depression determine the relative humidity

and the wood equilibrium moisture content (table

2, p. 11).

Table 9 gives 14 temperature schedules ranging

from a very mild schedule, Tl, to a severe schedule, T14. Initial temperatures, in all cases, are

maintained until the average moisture content of

the controlling samples reaches 30 percent.

Table 10 gives the wet-bulb depression schedules for six moisture content classes. These

classes are related to the green moisture content

of the wood (table 11). Another moisture content class, "H," is discussed on page 12'5. There

are eight numbered wet-bulb depression schedules,

No. 1 being the mildest and No. 8 the most severe. The wet-bulb temperature to be set on the

recorder-controller is obtained by subtracting the

wet-bulb depression from the dry-bulb temperature.

Table 12 is an index of recommended schedules

for 4/4 to 8/4 hardwood lumber and other items.

"While the same schedule is given for 4/4, 5/4,

and 6/4 stock, these thicknesses obviously will

have different drying times and should be dried

separately. For drying 6/4 stock of such refractory species as oak, the use of the schedule indicated for 8/4 stock may be desirable.

Kiln-drying hardwoods thicker than 8/4 from

the green condition is usually impractical because

of long kiln time. The best practice for this

material is to end coat and air-dry it under a

roof before kiln-drying it. Table 13 is an index

of suggested schedules for 10/4 and thicker hardwood lumber. They should lae considered as tentative until tested commercially.

Assembly of a Drying Schedule.—Using a form

like that shown in table 14, work up a drying

schedule as follows:

(1) From table 12, find the schedule code numbers for the material to be dried. The example

worked out in table 14 is for 4/4 sugar maple

that calls for code numbers T8-C3. Place the

code numbers in the space provided at the top of

the form.

(2) Since the first change in drying conditions

involves the wet-bulb depression, write down the

wet-bulb depression step nos. 1, 2, 3, 4, 5, and

6, in column 2.

(3) In column 3 of the form write the moisture content values corresponding to these steps

from the appropriate moisture content class of

table 10. In this example the class is "C" and

the values are : above 40, 40, 35, 30, 25, and 20,

respectively.

(4) In columns 5 of the fonn write down the

wet-bulb depression values corresponding to the
DRY KILN OPERATOR'S MANTJAL 119

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120 AGRICULTURE HANDBOOK NO. 188, U.S. DEPT. OF AGRICULTURE

TABLE 11.—Moisture content classes for various green moisture content values

Green moisture content

Moisture

content

class

Green moisture content

Moisture

content

class

TTT% +r» ACl

Percent

A

B

C

PeTcerU

80 to 100 D

40 to 60 100 to 120 _____ E

fiO to 80 Above 120 F

TABLE 12.—Code number index of schedules ^ recommended for kiln-drying hardwood 4/4 to 8/4 lumber

and other products

Species

Lumber

4/4, 6/4, and

6/4 stock

Temperature

Wetbulb

depression

8/4 stock

Temperature

Wetbulb

depression

Other products

Name

Temperature

Wetbulb

depression

Alder, red

For darker color

For lighter color

Apple

Ash, black

Ash, green, Oregon, white _

Aspen

Basswood

Beech

Birch, paper.

Birch, yeUow

Buckeye, yellow

Butternut

Cherry, black

Chestnut

Cottonwood, normal

Cottonwood, wet streak

Dogwood

Elm, American and slippery.

Elm, rock

Hackberry

Hickory.

HoUy

Hophornbeam (ironwood).

Lauan, dense

Lauan, light and medium.

Laurel, California

(Oregon Myrtle)

Locust, black

Madrone

Magnolia

Mahogany

Maple, bigleaf, red, silver_

Maple, sugar (hard)

Oak, California black _

Oak, red

Oak, white.

TIO

Til

T5

T6

T8

T8

T12

T12

T8

TIO

T8

TIO

TIO

T8

TIO

TIO

T8

T6

T6

T6

T8

T8

T6

T6

T8

Til

T6

T6

T4

TIO

T6

T8

T8

T4

T4

T4

D4

D3

D5

C3

D4

B4

E7

E7

C2

C4

C4

F4

E4

B4

E4

F5

D5

C3

D4

B3

C4

D3

D4

B3

C2

D4

A4

A3

B2

D4

C4

D4

C3

E2

D2

I C2

T8

T3

T5

T5

TIO

TIO

T5

T8

T5

T8

T8

T5

T8

T8

T6

T3

T5

T3

T6

T6

T4

T3

T7

TIO

T5

T3

T3

T8

T4

T6

T5

T3

T3

T3

D3

C2

D3

B3

E6

E6

Cl

C3

C3

F3

E3

B3

E3

F4

C4

C2

D3

B2

C3

Dl

C3

Bl

B2

D3

A3

Al

Bl

D3

C3

C3

C2

El

Dl

Cl

'1-inch squares

.2-inoh squares

'1-inch squares

^2-inch squares

'1-inch squares

.2-inch squares

T8

T5

TIO

T8

T8

T5

Shuttles.

'White handles—'Small.

—Large _

Pink handles —Small _ —Large.

T3

Tl

Tl

T8

T8

{Bowling pins (end coated).

1-inch squares

2-inch squares

T3

T8

T5

C3

02

C6

C4

C5

04

B2

D2

02

Dl

01

A3

04

03
DRY KILN OPERATOR'S MANUAL 121

TABLE l2.--Code number index of schedules ' recommended for kiln-drying hardwood 4/4 to 8/4 lumber

and other products—Continued

Species

Lumber

4/4, 5/4, and

6/4 stock

Temperature

Wetbulb

depression

8/4 stock

Temperature

Wetbulb

depression

Other products

Name

Temperature

Wetbulb

depression

Oak, southern lowland

Osage-orange

Pecan

Persirnmon

Sweetgum (sap gum) _

Sweetgum (red gum).

Sycamore

Tanoak

Tupelo, black

Tupelo, swamp

Tupelo, water

Walnut, black

WiUow, black

Yellow-poplar

T2

T6

T8

T6

T12

T8

T6

T4

T12

TIO

T6

T6

TIO

Til

Cl

A2

D8

C3

F5

C4

D2

Cl

E5

E3

H2

D4

F4

D4

T3

T6

T3

Til

T5

T3

T3

TU

T8

Al

Dl

C2

D4

C3

Dl

Bl

D3

D2

íGolf club heads

\Shuttles

ri-inch squares ..

\2-inch squares..

T3

T3

T12

TU

C2

B2

F6

D5

T3

T8

TIO

D3

F3

D3

Gunstock blai.ks- T3 D4

' Schedules are given In tables 9 and 10.

TABLE 13.—Code number index of kiln schedules suggested for drying thick hardwoods ^

Schedules for— ^

Species 10/4 stock 12/4 stock 16/4 stock

TemperatureWet-bulb

depression

Temperature

Wet-bulb

depression

TemperatureWet-bulb

depression

Alder, red. _... ... T6

T5

T8

T5

T5

T6

T4

T5

T3

T6

T9

T4

T5

T3

T3

T3

TU

T5

T3

TU

T3

T9

C3

B3

E5

B3

B2

E3

D3

D2

B2

C3

03

B3

02

B2

01

Bl

D3

02

Dl

D3

D3

03

T6

T3

T8

T3

T3

T5

T3

T3

T3

T5

T7

T3

T3

T3

T3

T3

T9

T5

T3

T9

T3

T7

03

B2

D5

B2

B2

D2

02

02

Bl

02

02

B2

B2

Al 3

01

Bl

03

B2

01

02

02

02

Ash, white. . . ._ T3

T7

T3

T3

Al

Aspen. 04

Birch, yellow. . Al

Cherry Al

Cottonwood..

Cottonwood, wet streak

Elm, American. ..

Elm, rock ... . T3

T3

T5

T3

Al

Hackberry Bl

Lauan (medium and light) 01

Mahogany Bl

Maple, bigleaf, red, silver. . _ _.

Maple, sugar (hard) ... T3 Al '

Oak, red....

Oak, white

Sweetgum (sap gum)

Sweetgum (red gum)..

Sycamore T3

T7

Bl

02 Tupelo, black.

Walnut, black

Yellow-poplar. .. ... . _. T5 02

' A good end coating should be applied to all stock hi most cases.

' For squares, use a wet-bulb depression number one unit higher than the one suggested for lumber. Thus, lor 3- by 3-lnch birch use T3-B3

' Alter passing 30 percent moisture content, gradually shut to wet-bulb depression schedule B2
122 AGRICULTURE HAN'DBOOK NO. 18 8, U.S. I>EPT. OF AGRICULTURE

steps from the appropriate wet-bulb depression

schedule number from table 10. The number is 3

in this example, and the wet-bulb depression

values are 5, 7, 11, 19, 35, and 50, respectively.

TABLE 14.—Method of assembly of kiln-drying

schedule for green 4/4 sugar maple

Schedule Code No. T8-C3

/80

Ci

u

So

is

£6

O a

£1 a

Moisture content

at start of

step

-? 2

Q

X! 0

â

^ E

1 1

2

3

4

5

6

6

Percent

Above 40

40

35

30

25

20

15

130

130

130

140

150

160

180

5

7

11

19

35

50

50

°F.

125

1 123

1.. 119

2 121

3 115

4 110

5 130

(5) Write down the temperature step numbers

in column 1 of the form. Since dry-bulb temperature changes are not made until the average

moisture content of the controlling samples

reaches 30 percent, repeat temperature step number 1 as often as necessary. In this example it

is repeated 3 times. The moisture content at the

start of temperature step 5 is 15 percent (table

9). Therefore, in filling out the schedule form

it is necessary to repeat wet-bulb depression step

6 as shown m table 14.

(6) In column 4 of the form write down the

dry-bulb temperature that corresponds to the

temperature step number in table 9. If step 1

is repeated, the initial dry-bulb temperature must

be repeated as shown in table 14.

(7) Subtract the wet-bulb depression from the

dry-bulb temperature in each step to obtain the

corresponding wet-bulb temperature. These values are entered in column 6 of the form.

Columns for relative humidity and equilibrium

moisture content can be added at the right of

the table, if desired. The values can be obtained

from table 2. The T8-C3 schedule for 4/4 sugar

maple and a drying curve obtained in a pilot test

kiln run are illustrated in figure 104.

In the final stages of drying, wet-bulb temperature does not need to be under exact control.

When the wet-bulb depression schedule calls for

a 50° wet-bulb depression while the dry-bulb temperature is still fairly low, discontinue control of

/60

MO

120

lOO

80

80

ct 60

^ 40

20

DRY-BULB

TEMPERATURE-!.

^WET-BULB

TEMPERATURE

^^

CONDITIONING'^

PERIOD

^EQUALIZING

PERIOD -

SCHEDULE T8-C3 \

MOISTURE CONTENT

AT START OF STEP

DRY

BULB

WET-BULB

DEPRESSION

WET

BULB

PERCENT •F •F •F

ABOVE 40' 130 S 125

40 130 7 123

35 130 II 119

30 140 19 III

25 150 35 IIS

20 160 50 no

15 180 50 130

EOUALIZINC IBO 28 152

CONDI TIONING 180 8 172

^MOISTURE CONTENT OF

CONTROLLING SAMPLES

'^O 2 4 6 8 10

DRYING TIME (DAYS!

M-91938-F

FIGUäE 104.—Kiln schedule and drying curve for 4/4

sugar maple.

the wet-bulb temperature by closing the steam

spray hand valve and opening the vents. Do not

raise the dry-bulb temperature, unless you are

deliberately modifying the schedule.

Uniformity of moisture content and relief of

drying stresses are achieved by equalizing and

conditioning treatments near the end of drying

as described on pages 142 to 144.

Examples of Assembled Schedules.—Some general schedules for 4/4 and 8/4 hardwoods, assembled from tables 9 and 10, are illustrated in

table 15. A study of these will be helpful when

assembling schedules for other types of material.

The schedules listed in tables 9 and 10 may

be conservative for some types of dry kilns and

for some drying requirements. As an operator

gains experience, he should, when possible,

modify them to reduce drying time. Schedule

modifications are discussed later in this chapter.
DRY KILN OPERATOR'S MANUAL

TABLE 15.~Examples of general kiln schedules for drying lumber of certain hardwood species

OAK, SOUTHERN LOWLAND

Schedule T2-C1 ^^ schedule established

123

Moisture

content at

4/4, 5/4, 6/4 stock 8/4 stock

start of

step Dry-bulb

temperature

Wet-bulb

depression

Wet-bulb

temperature

Dry-bulb

temperature

Wet-bulb

depression

Wet-bulb

temperature

Percmt

Above 40

40

°F.

100

100

100

no

120

130

150

°F.

3

4

6

10

25

50

50

97

96

94

100

95

C)

°F. °F. °F.

SÄ

30

25

20

15

OAK, WHITE

Schedule T4-C2 Schedule T3-C1

Above 40 no 4 106 no 3 107

40 no 5 105 no 4 1Ü6

35 no 8 102 no 6 104

30 120 14 106 120 10 no

25 130 30 100 130 25 105

20 140 50 (') 140 50 (')

15 180 50 (') 160 50 «

MAHOGANY

Schedule T6-C4 Schedule T4-C3

Above 40 120 7 113 110 5 105

40 120 10 110 no 7 103

35 120 15 105 no 11 99

30 130 25 105 120 19 101

25 140 40 100 130 35 95

20 150 50 (') 140 50 (')

15 180 50 (') 180 50 (')

ASH, WHITE; CHERRY

Schedule T8-B4 Schedule T5-B3

Above 35 130 7 123 120 5 115

35 130 10 120 120 7 113

30 140 15 125 130 11 119

25 150 25 125 140 19 121

20 160 40 120 150 35 115

15 180 50 n 160 50 (')

MAGNOLIA

Schedule T10-D4 Schedule T8-D3

Above 50 140 7 133 130 5 125 50 140 10 130 130 7 1?3 40 140 15 125 130 11 ^^9 35 140 25 115 130 19 111 30 150 40 no 140 35 105 25 160 50 (') 150 50 20 170 50 (') 160 50 15 180 50 w 180 50

* Close control of wet-bulb temperature not necessary.
124 AGRICULTURE HAKDBOOK NO. 188, U.S. DEFT. OF AGRICULTURE

TABLE 15.—Examples of general kiln schedules for drying lumber of certain hardwood species—Continued

TUPELO, BLACK

Schedule T12-E5 Schedule T11-D3

Moisture 4/4, 5/4, 6/4 stock 8/4 stock

content at

start of

step Dry-bulb Wet-bulb Wet-bulb Dry-bulb Wet-bulb Wet-bulb

temperature depression temperature temperature depression temperature

Percent "F. "F. "F. "F. "F. 'F.

Above 60 160 10 150 150 5 145

60 160 14 146 150 5 145

50 160 20 140 150 7 143

40 160 35 125 150 11 139

35 160 50 (') 150 19 131

30 170 50 (') 160 35 125

25 170 50 (0 160 50 (')

20 180 50 (0 170 50 (')

15 180 50 (') 180 50 ( )

ASPEN, BASSWOOD

Schedule T12-E7 Schedule T10-E6

Above 60 160 20 140 140 15 125

60 160 30 130 140 20 120

50 160 40 120 140 30 110

40 160 50 140 50

35 160 50 140 50

30 170 50 150 50

26 170 50 160 60 (')

20 180 50 170 50

15 180 50 180 50

' Close control oí wet-bulb temperature not necessary.

Use of Schedules for Air-Di-ied Stock.—The

general schedules for green hardwoods are also

recommended for the kihi-drying of previously

air-dried stock or stock that has gone through a

predrier. Prepare most of the representative

kiln samples from the wettest and slowest drying

material, but have at least one sample from the

driest and fastest drying stock (see chapter 6).

Determine the average moisture content of the

wettest half of the samples before the kiln is

started. Both the temperature step and the wetbulb depression step of the recommended schedule corresponding to that moisture content should

be entered directly if the stock has not undergone

surface wetting or been exposed for a considerable period to high relative humidity just before

it was placed in the kiln. For stock that has regained surface moisture, follow the recommended

temperature schedule, but use a wet-bulb depression of 8" to 10° F. for 12 to 24 hours before

changing to the wet-bulb depression of the schedule. This change can be abrui:)t for 6/4 and

thinner stock but should be gradual for 8/4 and

thicker stock.

The kiln-drying conditions for 4/4 air-dried

black cherry that has regained surface moisture

before entering the kiln are shown in figure 105.

Air-dried stock should not be steamed at the

start of kiln-drying. Steaming may cause surface checks to open during subsequent drying

and thereafter remain open. It also may increase

W'arping.

Modifications of General Hardwood

Schedules

Once a kiln operator has dried a certain species and item by one of the general kiln schedules without incurring defects or excessive degrade, he should consider modification of the

schedule to reduce drying time. Perhaps the material can stand a more severe schedule without

developing serious defects, or the dried product

does not need to be defect free. The operator

should try to develop the fastest drying schedules

consistent with acceptable amounts and types of

defects. Schedule modification should be done

in a systematic manner, for wliich good records

will be helpful. It must be recognized, however,

that modification satisfactory for material from

one source and dried in one kiln may not be
I>RY KILN OPERATOR'S MANTTAL 125

lOU 1 1

"^DRY-BULB

TEMPERATURE 1 CONDITIONING

Dcoinn

I

\

JEOUAL

' PER

IZING

OD

L

- T

1

-WET-t

EMPERt

lULB

nuRE100

~~ SCHEDULE re-84, MODIFIED

80

UOISTURE CONTENT

AT START OF STEP

DRY

BULB

WET-BULB

DEPRESSION

WET

BULB

PERCENT 'F •F •F

ABOVE BO ISO 10 140

20 160 40 120

IS ISO SO 130

80

EQUAUZINS ISO 28 IS2

CONDITIONING 180 8 172

^ 40

1S 20

1

_ MOISI URE CC

CONTR

INTENT

OLLING

OF

SAMPL

"^ "~\\

0

DRYING TIME (DAYS)

M-90675-F

FiGTJEE 105.—Kiln schedule and drying curve for airdried 4/4 black cherry that had regained surface moisture before entering the kiln.

satisfactory for material from another source

and dried at another plant.

Kiln schedule modifications required by factors

of kiln operation or performance are dealt with

in chapter 10.

The first move in systematic schedule modification is to shift from one wet-bulb depression

schedule to another; the second is to shift temperature schedules; and the third is to modify

certain steps within the schedule.

Shifting Wet-bulb Depression Schedules.—The

moisture content classes (table 11, p. 120) are set

up so that a species of wood can be classified in

accordance with the gTeen moisture content of

its heartwood. The moisture content limits of

the classes were chosen on a conservative basis.

Thus the first modification that a kiln operator

should try is to shift to a higher moisture class

letter, particularly if the green moisture content

is near the upper end of the values in the class.

555315 0—61 9

For example, 4/4 northern red oak at 95 percent

moisture content has been successfully dried in

pilot tests on the E2 instead of the D2 schedule,

with a saving of 4 or 5 days in drying time.

This modification is especially useful when the

wood to be dried is mostly sapwood.

The next modification that should be tried is

to shift to the next higher wet-bulb depression

schedule number. This modification may cause

minor surface and end checks that are generally

of little concern for many uses. A drastic change

in wet-bulb depression may cause severe surface

and end checks.

H Wet-Bulb Depression Schedules.—A special

moisture content class, designated as H, has been

devised to permit more intensive use of the principles that the first change in wet-bulb depression

can be made at the time one-third of the green

moisture is gone and that additional increases in

wet-bulb depressions can be made soon after. This

is particularly useful in drying woods with a

green moisture content in excess of 140 percent,

but may also be appued with some advantage to

stock with a green moisture content of 100 percent or more. The H schedules are given in

table 16. The wet-bulb depressions for each

wet-bulb depression schedule number are the same

as those in table 10, p. 119.

To set up a specific H schedule, find the moisture content for the first change in wet-bulb depression by taking two-thirds of the average

green moisture content of the controlling samples. If, for example, their average green moisture content is 168 percent, the first change point

is 112 percent. For convenience, this is rounded

to 110. Subsequent changes in wet-bulb depression are made after each 10 percent loss in moisture. An H schedule developed for 6/4 water

túpelo heartwood is shown in figure 106. In

view of the long drying time in this particular

case, the stock probably should have been airdried before being kiln-dried. The principle is,

of course, applicable to other woods that dry

more readily.

Shifting Temperature Schedules.—Temperature is critical in preventing collapse and honeycombing, two defects that may not show up until

later in the drying process. Until the kiln operator has gained experience in drying a material,

he should adhere to the recommended temperature schedule number. The general temperature

schedules will safely dry the most defect-susceptible material commonly encountered in commercial drying. If the material being dried is

almost all sapwood or is relatively free of natural

characteristics that contribute to seasoning defects, increasing the temperature (T) number

by 1 or 2 so as to get a 10° F. greater initial

temperature generally is permissible. For example, 9/4 all-sapwood sugar maple free of path-
126 AGRilCULTURE HANDBOOK NO. 18 8, U.S. DEPT. OF AGRICULTURE

TABLE 16.—H-type wet-bulb depression schedules for hardwoods

Wet-bulb

depression

step No.

Moisture content at start of

step

Wet-bulb depressions for wet-bulb depression schedule No. —

1

2

3

4

5

Percent

Green (G.)

2/3 G

2/3 G.-IO

2/3 G.-20

2/3 G.-30

2/3 G.-40

3

4

6

10

25

50

4

5

8

14

30

50

5

7

11

19

35

50

7

10

15

25

40

50

' F.

10

14

20

35

50

50

• F.

15

20

30

50

50

50

20

30

40

50

50

50

25

35

50

50

50

50

/eo

160

S^ 120 Í

100

80

160

I

ISO

80

40

1—

-c~ni mi TIONII

?IOD

/<;-

PEI

1

- c

TEA

RY- L

4P£Rt

JULB

%TURt-^

EQUAL

PER

IZING —

100

"L

r-WET - BULB

JTEMPERATURE

1 1 1

SCHEDULE T6 - H2

-

-

MOISTURE CONTEUT

AT START OF STEP

oRy

BULB

WET-BULB

DEPRESSION

WET

BULB

PERCENT 'F •F 'F

\

ABOVE 110 120 1 116

110 120 5 115

\

-

lOO 120 a 112

90 120 14 106

80 120 30 90

\

TO 120 40 80

30 130 50 80

\

25 140 50 90

20 ISO 50 100

\

15 180 50 130

EOUALIZINO 180 35 145

CONDITIONINO IBO 9 171

\

1 1 1 1 1 1

^<. CONTROLLING SAMPLES

^

-^

10 20

DRYING

30 40

TIME (DAYS)

50 60

M-92565-F

FIGURE 106.—Kiln conditions and drying curve for 1%-

inch water túpelo heartwood, based on liiln schedule

T6-H2.

ological heartwood and mineral streak has been

dried on a T7 temperature schedule instead of

the recommended T5 schedule. The milder T5

schedule would be used for drying a charge of

sugar maple that had a considerable amount of

heartwood or mineral streak.

Changes 'Within the Schedules.—The only significant change that can be made within a wetbulb depression schedule would be a more rapid

reduction of wet-bulb temperature during the

intermediate stages of drying. Limited studies

on drying stress reversal show that this is possible for some woods and may be possible for

more. Some examples are given below under

the heading of "Special Hardwood Schedules,"

but until research establishes which woods can

be subjected to this modification, proceed with

caution. The logical approach is to increase the

wet-bulb depressions in steps 3 and 4 of table

10, p. 119. Dry several charges on the modified

schedule before making further modification. If

an objectionable amount of checking occurs, ease

back the wet-bulb depression to the previously

satisfactory schedule.

Three types of temperature changes within the

T schedules (table 9, p. 119) can be considered.

One is using a temperature in the initial stage

of drying that is intermediate between steps 1

and 2 of' the recommended schedule. For some

slow-drying woods, such as 4/4 red oak, the use

of an initial temperature of 115° F. instead of

110° until the stock reaches 30 percent moisture

content may be satisfactory. Another type of

change is to increase the dry-bulb temperature

during the intermediate stages of drying. This is

to be avoided unless it has been shown to be

satisfactory by special research. A third type of

change is to increase the temperature during the

last stages of drying. After the average moisture

content of the controlling samples has reached

15 percent, temperatures up to 200° can be used

without damaging the wood, except for very

slight strength reductions.

Wlaen modifications are being made in the temperature schedules, use the recommended wet-bulb

depressions or those found satisfactory through

modification.
DRY KILN OPEBATOR'S MANUAL 127

Special Hardwood Schedules

While the general hardwood schedules, with

minor modifications such as any kiln operator

should make, will do a good job of drying most

species, the special schedules developed by research are advantageous in some cases. A few

examples follow.

Maximum Strength Schedules.—Exposure of

wood to temperatures above 150° F. can cause

some permanent reductions in strength. At kiln

temperatures of 200" or less, the exposure would

have to be prolonged to cause excessive strength

reductions. Thus, the general kiln schedules and

proper operating procedures produce kiln-dried

wood with so little strength reduction that the

loss is insignificant for most uses of the wood.

However, when the wood is to be used for products requiring high strength per unit of weight,

such as aircraft, ladders, and sporting goods,

somewhat lower temperatures throughout the

drying process should be used. The special temperature schedules set up to dry aircraft lumber

are given in Military Spec. MIL-W-6109 {18).

Alternate Schedules for Soni-e Species.—Special schedules have been developed at the Forest

Products Laboratory for hickory, black túpelo

(black gum), swamp túpelo, and water túpelo.

Some examples of special schedules are given in

table 17. As schedules become more severe, the

range of quality of material on which they can be

used becomes more restricted. The literature on

the subject should be consulted (i, 9, 10., IB.,

16, 17) before trying these schedules, especially

those for water túpelo. Some of the heartwood

of this species may be difficult to distinguish from

sapwood (9).

TABLE 17.—Special kiln schedules for certain

hardwood species

HICKORY, ALL SPECIES

Temperature schedule for—

Moisture

content at

start of step

4/4 stock 6/4 stock 8/4 stock

Dry

bulb

Wet

bulb

Dry

bulb

Wet

bulb

Dry

bulb

Wet

bulb

Percent

Above 50

50

40

35

30

25

20

15

T.

130

130

130

130

150

150

180

180

"F.

125

123

114

114

112

100

130

130

"F.

120

120

125

130

140

140

180

180

"F.

115

113

114

97

104

104

130

130

°F.

120

120

120

125

130

140

150

180

"F.

117

116

113

114

97

90

100

130

TABLE 17.—Special kiln ■■schedules for certain

hardwood species—Continued

TUPELO, BLACK, HEART- OR SAPWOOD

Temperature schedule for—

Moisture

content at

start of step

4/4 stock 6/4 stock 8/4 stock

Dry

bulb

Wet

bulb

Dry

bulb

Wet

bulb

Dry

bulb

Wet

bulb

Percent "F.

160

180

180

180

180

200

"F.

147

162

162

152

152

150

°F. °F. °F.

160

160

180

180

180

200

OF.

154

70 154

50 172

40 172

30 159

20 150

TUPELO, SWAMP; HEARTWOOD 1

140

140

140

140

140

150

160

170

180

135

133

129

121

105

100

110

120

130

60

50

40

35

30

25

20

15

TUPELO, SWAMP, SAPWOOD

Above 60

60

50

40

35

30

25

20

15

160

160

160

160

160

170

170

180

180

150

146

146

146

110

120

120

130

130

160

160

160

160

160

170

170

180

180

150

150

140

125

125

120

120

130

130

TUPELO, WATER; HEARTWOOD

Above 70 130

130

130

130

130

130

140

150

160

180

123

120

115

105

90

80

90

100

110

130

120

120

120

120

120

120

130

140

150

180

2 116

80

80

80

80

80

80

90

100

130

70

60

50

40

35

30

25

20

15

TUPELO, WATER, SAPWOOD

Above 70 160

160

160

160

160

160

170

170

180

180

150

150

146

146

146

110

120

120

130

130

140

140

140

140

140

140

150

160

170

180

133

130

125

115

100

90

100

110

120

130

70

60

50

40

35

30

25

20

15

' Use 6/4 schedule for 4/4 stock. A more severe schedule Is possible but it

has not been specifically tested.

' See figure 106 lor changes between 110 and 70 percent moisture content on

the H2 schedule.
128 AGRICULTURE HANDBOOK NO. 188, U.S. DEPT. OF AGRICULTURE

Schedules for Thin Stock.—While thinner lumber dries faster than 4/4 lumber on the 4/4

schedule, thei'e are unusual possibilities for

greatly accelerating the drying of thin stock by

special schedules. Such proved to be the case

in an experiment at the Forest Products Laboratory with y2-inch nonhern red oak (7). Wliile

the commercial drying time for 4/4 to 5/4 red

oak to be i-esawn to % or V2 inch is from 18 to

30 days, the Yo-inch test material was dried in

only 36 hours. The schedule is shown in table 18.

Schedules for Specific Products.—Special

schedules have been developed for such hardwood

products as bowling-pin, shoe-last, shuttle, and

gunstock blanks, and tight cooperage and handle

stock. Although general schedules are listed for

some of these in table 12, p. 120, it would be

advisable to examine the literature before attempting to dry them (i, 8, 10, 13). A special

schedule for bowling-pin blanks is shown in figure

TABLE 18.—Kiln schedule for

red oak

Yi-tnch northern

180

160

CiINDI T 10

PERI

NING

0D<

UJ 140 ft:

-7^

¿

'^ ion

Uj TEMPERATURE-"\

^

1

\

100 /

1 1 EQUALIZING-^

/ L PERIOD

^ WET-BULB

TEMPERATURE

KILN SCHEDULE

MOISTURE CONTENT

AT START OF STEP

DRY

BULB

WET-BULB

DEPRESSION

WET

BULB

PERCENT •F •F •F

ABOVE 19 110 5 105

80 29 110 7 103

26 no 10 100

;^ 22 110 14 96

> IB 120 25 95

t^ 14 165 52 113

0:60 EQUALIZING 165 29 136

t s.

CONDITIONING 165 8 157

1 1 1 1 1 1

'^40

8

\

V,

^MC

1

"^^^ ¿ CONTROLLING SAMPLES

1 1 1 1

^

^

0 L_ 1_ L L_ L 1

16 24

DRYING TIME (DAYS)

32 40

M-87200-F

Moisture

content at

Temperature Time in

experimental

start of step

Dry-bulb Wet-bulb

kiln

PtTcenl

Above 70

70

20

170

170

200

"F.

165

123

150

Hn.

0-7

7-24

24-1 36

FIGURE 107.—Kiln schedule and drying curve for maple

bowling-pin blanks, regulation size, end coated but not

side coated.

1 Includes an equalizing treatment of 4 hours.

107. This schedule also may be suitable for No.

11 shoe-last blanks, though they have been successfully dried on the general T3-A3 schedule.

Schedules for Chetnwally Treated Wood.—One

purpose of treating wood with chemicals is to

assist in drying the wood without seasoning defects. This is called chemical seasoning. Other

purposes are to help preserve the wood or to retard fire. Chemically treated materials generally

require modified drying procedures.

A hygroscopic chemical is used in chemical

seasoning to retain moisture in the surface layers,

so that tliey do not check. The relative humidity

in the kiln at the start of kiln-drying must be

low enough to prevent surface condensation,

leaching of the chemical, and possible corrosion

of metal within the kiln. If the relative humidity is too low, however, excessive checking

may result. One chemical seasoning agent is

composed largely of salt (sodium chloride) ha-ving, at kibi-drymg temperatures, an equilibrium

relative humidity of about 75 percent. This requires an initial wet-bulb depression of at least

9° F., and preferably slightly more, if the salttreated stock is green. Hold this rather small

initial depression until a larger one is called for

by the recommended schedule. If the stock has

been air-dried to 25 percent moisture content or

less, initial wet-bulb depressions of 14° or more

should be satisfactory. The recommended drybulb temperature schedules can be used, but limit

the maximum temperature to 150°.

The same general considerations apply in the

drying of preservative-treated or fire-retardanttreated wood. The chemicals used for preservative treatment are not vei-y hygroscopic, so little

or no adjustment in the recommended wet-bulb

depression schedules need be made. Generally,

preservative-treated wood dries more rapidly than

untreated wood with the same average moisture

content.

Fire-retarding chemicals generally are more

hygroscopic than preservative chemicals and are

used in much higher amounts. Follow the adjustments of the wet-bulb depression schedule, as suggested for chemically seasoned material.
DRY KILN OPERATOR'S MANUAL 129

Chemicals accelerate the strength-reducing effect of prolonged exposure of moist wood to high

temperatures, some faster and more seriously

than others. Some chemicals decompose at normal kiln-drying temperatures, dissipating the

gaseous products, so that the chemical treatment

becomes ineffective. Before attempting to kilndry treated wood, obtain drying-temperature recommendations from the treating company or

manufacturer of the chemical.

Schedules for Foreign Woods

Drying studies made at the Forest Products

Laboratory on foreign woods, though limited,

indicate that the general principles involved in

drying native woods are applicable, but that some

of the foreign woods may be considerably more

difficult to dry without seasoning defects. Information on schedules for foreign woods may be

available from several sources, including dry

kiln manufacturers. State laboratories, and forestry schools, as well as the Forest Products

Laboratory. Because of confusion in the common names of foreign woods, the botanical name

of the wood and its origin, and also a sample of

the wood if available, should always be sent along

with requests for information.

Softwood Schedules

Softwood Moisture Content Schedules

The softwood moisture content schedules presented in this manual are derived from Forest

Products Laboratory research results and a study

of current industry practice. These schedules

can be used with the aid of kiln sample procedures (ch. 6) to dry softwoods with a minimum

of seasoning defects. Because softwoods are generally easy to dry, however, it may be more practical to use the commercial time schedules described in a later section.

Moisture Content Ba-ds.—As in the drying of

hardwoods, there is a relationship between the

moisture content of the stock and the drying

conditions it can withstand. Although the stress

patterns developing in softwoods as drying progresses differ from those in hardwoods, the surlace zones do become stressed in tension during

the first stages of drying and ultimately become

stressed in compression. Stress reversal generally does not occur, however, until the stock

reaches a moisture content somewhere between

20 and 15 percent. Therefore, wet-bulb depressions should not be drastically increased until

the stock reaches this moisture content. Gradual

changes in wet-bulb depression can be made early

in drying, however, in accordance with the moisture content of the stock. The temperature and

moisture content relationships that cause collapse

and honeycombing in hardwoods affect softwoods

similarly.

Material Considerations.—Tlie difference between sapwood and heartwood moisture content

is considerable in many softwoods (table 1, p. 9).

Generally, the heartwood is more susceptible to

drj'ing defects, so most of the schedules are based

on the moisture content of the heartwood. In

some situations, however, the heartwood dries to

a safe moisture content value before the sapwood

is dry enough to stand drastic increases in wetbulb depression. In these cases, the schedules

are based on the moisture content of the sapwood

or of a mixture of sapwood and heartwood. In

working out a time schedule for a species in which

both sapwood and heartwood are present in quantity, select kiln samples from both types of wood.

So-called "sinker" stock can be a problem when

drying softwoods. Sinker stock is wood containing so much water and so little air in the cell

cavities that the logs sink in water. This wood

dries slowly and is subject to collapse if too high

a temperature is used during the initial stages

of drying. It is desirable to sort green softwoods

into different moisture content classes, if practical, and to dry each class separately. "^^Hien it is

necessary to dry them together, each should be

represented by kiln samples.

The softwood moisture content schedules are

set up for drying green wood, but they can be

applied to partially air-dried material.

Operation Considerations.-—The schedules are

designed for full-time operation in modern

forced-circulation compartment kilns. The scheduled drying conditions are for the entering air.

If it is necessary to locate the dry bulb in the

leaving air (see ch. 4, p. 69), adjust the control

instrument setting to compensate for the temperature drop across the load. Modify the schedules if the air velocity through the load is less

than 200 feet per minute, as described for drying

hardwoods, p. 124.

Moisture Content Schedules.—The softwood

moisture content schedules are given in tables 19

and 20. These schedules are the same as the

general kiln schedules for hardwoods, except for

a few important differences. Wet-bulb depressions of 40° F. or more are avoided until the

controlling moisture content reaches 15 percent.

Changes in wet-bulb depression between 15° and

35° are made gradually, 5° at a time. For the

drying of lower grade stock, final wet-bulb depressions generally do not exceed 30°, and in

some cases they do not exceed 20°. The main

features of moisture content kiln schedules of this

type were discussed under hardwood schedules

earlier in this chapter. LTnder the moisture content method of operation, the initial temperature

is maintained until the controlling kiln samples

have an average moisture content of 30 percent.
130 AGRiLCULTURE HANDBOOK NO. 188, U.S. DEPT. OF AGRICULTURE oo

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DRY KIIiN OPERATOR'S MAOTTAL 131

Table 21 is an index of recommended schedules

for 4/4, 6/4, and 8/4 softwood lumber, of both

upper and lower grades. The schedules for

lower grade stock generally call for lower final

temperatures and smaller final wet-bulb depressions to reduce loosening of knots and hold planer

splitting to a minimum.

Table 22 is an index of suggested schedules

for 10/4 and thicker softwoods. The drying time

may be too long for ordinary commercial operations, but the schedules are suitable for special

cases where thick stock of the upper grades is to

be dried.

Instructions for assembling a softwood moisTABLE 21.—Code number index of moisture-content schedules ^ recommended for kiln-drying 4-/4, 6/4, (I'nd

8/4 softwood lumber

Species

Lower grades ^ Upper grades 2

4/4 stock 6/4 stock 8/4 stock 4/4 stock 6/4 stock 8/4 stock

BaldcvDress T12-E3_._ T11-D2

Cedar:

Alaska- T12-A3__- T11-A2

Atlantic white- T12-A4___ T11-A3

Eastern redcedar T5-A4 T5-A3

Incense- Tll-B5___ T10-B4

Northern white- T12~B4.. T11-B3

Port-Orford- T11-B4. _ T10-B3

Western redcedar:

Light- .. - - T9-A6. T10-B5.__ T10-B3

Heavy T5-F4 T5-F3

Douglas-fir:

Coast region ... T7-A4. _ T7-A4ä.._

T9-A4 *

Tll-A4.__ T10-A3

Inland region T9-A4<

Fir:

Balsam .. . T12-E5_.. T10-E4

California red T12-E5 T10-E4

Grand.- T12-E5 T10-E4

Noble T12-A5___

T12-B5

Tll-A4_._ T10-A3

Pacific silver. . _ T10-B3

Subalpine _ .. T12-B5 T12-B4

White . T9-D6 T9-D5.___ T12-E5__-

T12-C4

Tll-E5___ T10-E4

Hemlock:

Eastern ._ T11-C3

Western... . . TU-ES' T11-E5-

T7-C5'-..

T9-C4

T12-C5.__

T9-B4

T11-C5

Tll-C5__.

T7-C4

T11-C4

Larch... .. ... T7-C5 * T7-C3

Pine:

Eastern white:

Regular. . . T9-C5_.. T10-C4

Antibrown-stain.. T7-E6 T7-E6 T7-E5

Jack T9-C4 T9-C3

Lodgepole . T5-C5 T10-C4 T9-C3

Ponderosa:

Heartwood T9-A6

T11-C7

T7-A6 T5-A5

Sapwood.. T9-C6

T7-E6

T7-C6 T7-C5

Antibrown-stain T7-E5 Red T12-B4 T11-B3

Southern yellow. T12-C5_.. T13-C6_._

T5-E6

T5-F6

T9-C5

T12-C5___

T5-E6

T5-F6

T7-C5

T12-C5 Sugar:

Light T9-E7 T7-E6 T5-E5 Heavy T5-F5 Western white:

Regular T9-C6 T7-C6*._. T7-C4

Water core T9-E6

Redwood:

Light T&-D6 T5-D4 Heavy T4-F5

T11-B4

T3-F5 T3-F4

T10-B3

Spruce;

Eastern (black, red, white)

Englemann T7-B6

T7-A5.

T.5-B5 T5-B5^__ T9 E5 T7-E4

Tn-B3

T10-B3

Sitka T12-B5__

T11-B3

TT2-B4"] Tamarack

Schedules are given in tables 19 and 20.

> MlZ^i^^^X^h\7^^¿on2pF!'^^''' ^""^ ^°^' "''^" ^'^'^"^ ^"^'^'^^ "''''""' '^''''''' *°P ^°'^ ''"'^'^' ^""^ tight-knotted panelling.

< Maximum wet-bulb depression 20° F.
132 AGRiliCIILTURE HAJSTDBOO'K NO. 18 8, U.S. DEFT. OF AGRICITLTURE

TABLE 22.—Code number index of moisture-content kiln schedules ' suggested for drying thick softwood

lumber ^

Species

Thickness of lumber—

10/4 12/4 16/4

Baldcypress-- . . __ T8-A4

T7-A3

T5-F3

T7-A3

T7-A2

T5-A1

T8-A4

T8-A3

T8-A4

T5-A2

T8-A4

T8-A3

T8-A4

T7-A3

T10-C4

T7-A4

T7-A3

T10-C4

T7-C4

T5-C4

T5-A2

T7-A4

T5-B2

T7-A3

T8-A4

Cedar:

Atlantic white- . _. . _ _ T7-A3

Incense- .._ T5-F3

Northern white-- . T7-A3

Western redcedar (light). _ . _ T7-A2

Douglas-fir, coast region _ . _ .^ _..- T5-A1

T8-A4

T5-A1

Fir:

Balsam

California red_ T8-A3

Grand-- _ T8-A3

Noble T5-A2

White T8-A4

Hemlock:

Eastern- T8-A2

Western - _ - - - T8-A3

Larch, western _ _ -_ _ ^ _ T7-A2

Pine:

Eastern white

Ponderosa- •

T8-C3

T7-A4

T5-C2

Red T7-A3

Southern yellow _ - . ^ _ T10-C4--_

Western white ^ -- --- ---_ T5-C3

Redwood (light) _ _ _ T5-C3

Spruce :

Eastern (black, red, white)- - _ - _-- T5-A2

Engelmann _ _ - T7-A3

Sitka___ - T5-B2

Tamarack -- - _ T7-A3

' Schedules are given In tables 19 and 20.

' Upper grades, including clears, selects, and factory lumber.

ture content schedule are the same as those given

for hardwoods. Schedules for a few softwood

species have been assembled in table 23.

Kiln-Drying Air-Dried Softwoods.—Since preliminary air-drying is uncommon for softwoods

that are to be kiln-dried, recommended schedules

for kiln-drying air-dried stock have not been

worked out. The following steps are suggested

for the assembly of such a schedule.

(1) Determine the moisture content of representative samples of slow- and fast-drying material (ch. 6) and use the average moisture content of the wettest half of the samples as the

controlling moisture content.

(2) Use the temperature step of the recommended schedule corresponding to that moisture

content (table 19).

(3) If the controlling moisture content is aboA^e

40 percent, dry the material as green stock.

(4) If the controlling moisture content is 40

percent or less, change the wet-bulb depression

as follows:

(a) Use a depression of 10° to 15° F. for

the initial 8 to 16 hours.

(b) After this period, if the controlling

moisture content is between 15 and 25 percent, change the wet-bulb depression to

20° F.

(c) Use a depression of 30° F. or more

after the stock reaches 15 percent moisture

content.

An example of a schedule for 4/4 eastern

spruce previously air-dried to 24 percent moisture

content is given in table 24.

Modifying Softwood Moisture Content Schedules.—The principles described above for hardwood schedule modification generally can be applied to softwoods.

Commercial Softwood Time Schedules

In general, the softwood mills of the West and

South use time schedules to dry both upper- and

lower-grade items. The drying conditions are

changed at convenient time intervals, such as

every 12 or 24 hours. These schedules have been

developed from experimental work and mill experience. They are being continuously modified

at individual mills to allow for differences in

material, kiln type, and kiln performance.
DRY KILN 0PE:RAT0R'.S MAJÍTJAL 133

TABLE 2'^.—Examples of moisture-content scliedules for drying upper-grade lumber of certain softwood

species

EASTERN WHITE PINE, ANTIBROWN-STAIN

Schedule T7-E6 Schedule T7-E6 Schedule T7-E5

4/4 stock 6/4 stock 8/4 stock

Moisture

content at

start of Dry-bulb Wet-bulb Wet-bulb Dry-bulb Wet-bulb Wet-bulb Dry-bulb Wet-bulb Wet-bulb

step temper- depres- temper- temper- depres- temper- temper- depres- temperature sion ature ature sion ature ature sion ature

Percent "F. 'F. "F. "F. "F. °F. "F. "f. "F.

Above 60 130 15 115 130 15 115 130 10 120

60 130 20 110 130 20 110 130 14 116

50 130 25 105 130 25 105 130 20 110

40 130 30 100 130 30 100 130 25 105

35 130 35 95 130 35 95 130 30 100

30 140 35 105 140 35 105 140 30 110

25 150 35 115 150 35 115 150 35 115

20 160 35 125 160 35 125 160 35 125

15 160 50 (}) 160 50 (•) 160 50 (')

Schedule T13-C6

SOUTHERN YELLOW PINE

Schedule T12-C5 Schedule T12-C5

Above 40 170 15 155 160 10 150 160 10 150

40 170 20 150 160 14 146 160 14 146

35 170 25 145 160 20 140 160 20 140

30 180 30 150 170 25 145 170 25 145

25 180 35 145 170 30 140 170 30 140

20 190 35 155 180 35 145 180 35 145

15 190 50 (') 180 50 (') 180 50 (')

Schedule T12-E5

WHITE FIR

Schedule T11-E5 Schedule T10-E4

Above 60 160 10 150 150 10 140 140 7 133

60 160 14 146 150 14 136 140 10 130

50 160 20 140 150 20 130 140 15 125

40 160 25 135 150 25 125 140 20 120

35 160 30 130 150 30 120 140 25 115

30 170 35 135 160 35 125 150 30 120

25 170 35 135 160 35 125 160 35 125 20 180 35 145 170 35 135 170 35 135 15 180 50 « 180 50 (') 180 60 (>)

' Close control of wet-bulb temperature not necessary. Steam spray shut ofl.

TABLE 24.—General kiln schedule TII-B4 applied to 414 upper-grade eastern spruce previously air-dried to 24 percent moisture content

Moisture Dry-bulb Wet-bulb Wet-bulb Time content at temper- depres- temperstart of ature sion ature

step

HT. Percent "F. "F. °F. 0-8 24 160 15 145

(') 20 170 20 150

15 180 50 {')

I Change to moisture-content method oí operation.

Close control of wet-bulb temperature not necessary.

The information given here was obtained by

studying the time schedules and drying procedures of a large number of the average and

better-quality producing softwood mills. There

was a wide diversity in schedules used successfully

for the same item. Average time schedules were

developed from them.

These time schedules, like the moisture content

schedules, are intended only as guides from which

an operator can make up the best schedules for

his specific drying requirements and type of kiln.

Time schedules are very dependent upon rates of

air circulation and kiln performance, since these

affect drying rate greatly. The schedules in this
134 AGRICULTURE HANDBOOK NO. 188, U.S. DEPT. OF AGRICULTURE

section are based on the performance of singletrack or of booster-coil-equipped double-track,

internal-fan kilns with air velocities of between

200 and 400 feet per minute through the loads.

Most such kilns have wide plenum chambers and

other design features that insure uniform air distribution. The schedules should be modified for

other types of kilns.

Time Basis.—Changes are made in kiln conditions at a stated time interval, usually every 12

or 24 hours. The 12-hour interval is generally

used for stock that dries in 2 to 5 days. It is

convenient at larger mills having kiln operators

on duty at all times. At smaller mills where an

operator is on duty for only 8 hovirs each day,

drying conditions can be modified for changes

at 8-, 16-, and 24-hour intervals.

Material and Oferation Considérations.—The

material and operation considerations discussed

under "Softwood Moisture Content Schedules,"

p. 129, apply to time scliedules. Small variations

in material and operations at individual mills or

in individual kilns at the same mill may require

adjustment of the time scliedules. Major differences in initial moisture content or in drying rate

of different types of stock require changing to entirely different schedules.

Kün Samples.-—Although kiln samples are not

necessary for operating kilns on a time basis,

many operators find them helpful. They can be

used to tell when to start equalizing or condi-.

tioning treatments or when to shut the kiln off.

This is especially helpful when loads of lumber

with different average moisture content values

are to be dried in the same kiln.

TÍ7ne Schedules.—Softwood time schedules are

shown in tables 25 and 26. The temperature and

wet-bulb depression schedules are set up separately. Contrary to the method of changing drying conditions for hardwoods, temperature and

wet-bulb depression are changed simultaneously

in drying most softwood items. The two sets of

time intervals, A and B shown in tables 25 and

26, are arranged in 12- and 24-hour periods, respectively. Longer time intervals are indicated

by other letters as explained in footnotes.

An index of commercial time schedules for

4/4, 6/4, and 8/4 softwood lumber is given in

TABLE 25.--Avera ge commercial time-temperature schedules for softwoods

Time intervals ' Dry-bulb temperatures for temperature schedule No. —

A B SI S2 S3 S4 S5 S6 S7 S8 89 SIO Sll S12

HT. HT. ° F. ' F. ° F. "> F. ° F. » F. 0 p ° F. ^F 0 p 0 p

° F.

0-12 0-24 115 120 125 130 135 140 145 150 155 160 165 170

12-24 24-48 120 125 130 135 140 145 150 155 160 165 170 175

24-36 48-72 125 130 135 140 145 150 155 160 165 170 175 180

36-48 72-96 1.30 135 140 145 150 155 160 165 170 180 180 190

48-60 96-120 140 140 150 150 160 160 170 170 180 180 190 190

60-72 120-144 150 150 160 160 170 170 180 180 180 190 190 200

72-Final 144-Final 160 160 160 160 170 180 180 180 190 190 200 200

I Additional time intervals used in a few schedules are; 0=48 hours on each step; D=72 hours on each step; and E=96 hours on each step.

TABLE 26.—Average commercial time wet-bulb depression schedules for softwoods

Time intervals ' Wet-bulb depressions for wet-bulb depression schedule No. —

A B Kl K2 K3 K4 K5 K6 K7 K8

Hr.

0-12

12-24

24-36

36-48

48-60

60-72

72-Final

HT.

0-24

24-48

48-72

72-96

96-120

120-144

144^Final

" F.

3

4

6

9

12

18

25

° F.

4

5

7

10

15

20

30

° F.

5

7

10

15

20

25

30

° F.

7

10

15

20

25

30

35

° F.

10

15

20

25

30

35

50

" F.

15

20

25

30

30

35

50

°F.

17

23

30

30

35

43

50

" F.

20

25

30

35

43

50

50

' Additional time intervals used in a few schedules are: C=48 hours on each step; D=72 hours on each step; and E=96 hours on each step.
I>EY KILN OPERATOR'S MANUAL 135

table 27. The table also gives the maximum wetbulb depression for the lower grades and the

average total kiln time for both lower and upper

grades. Total kiln time for the upper grade

lumber generally includes a short conditioning

period. Because the schedules were developed

from a wide diversity of actual schedules, the

times given are for guidance purposes only. The

actual time required for individual kiln charges

may vary considerably from those given. Any

changeover from an operating schedule to one

of these should be done gradually. If at the end

of a kiln run, the average moisture content and

degree of moisture uniformity do not meet consumer requirements, modify the drying or equalizing time, or both, on subsequent charges accordingly.

Assembly of a Time Schedule.—The procedure

for assembling a time schedule for softwoods is

similar to that given for hardwoods earlier in

this chapter. A form similar to table 14, p. 122',

is used, except that the third column is headed

"Time" instead of "Moisture content at start of

step." All times are shown in hours. If the

time interval letter (A or B) is the same for

both temperature and wet-bulb depression schedules, the procedure is simple. When this is not

the case, use the A values in setting up the time

column, and repeat either the temperature or the

wet-bulb depression step numbers and values, as

the case may require.

Exmnples of Assembled Time Schedules.—

Table 28 shows some examples of assembled time

schedules for several softwoods. A few of the

schedules do not fit into the general time-schedule scheme.

Modification of Schedule for Variations in

Initial Moisture Content.—There is no standard

procedure for modifying time schedules to allow

for variations in the initial moisture content of

stock. The most common practice is to use the

same drying conditions and the same hourly intervals for the initial and intermediate drying

steps, and then to lengthen or shorten the final

step to obtain the desired final average moisture

content. At some mills, however, when the stock

is placed in the kiln quite wet in the wintertime,

the initial step is prolonged or is preceded by a

milder step. In view of the late reversal of softwood drying stresses, this practice favors reduction in kiln degrade.

The nonuniformity in final moisture content

that can occur in two charges of softwood placed

in a kiln at different initial moisture content and

dried by the same time schedule is illustrated in

table 29. This shows the necessity for checking

final moisture content of lumber and modifying

time schedules to compensate for differences in

initial moisture content. An equalizing treatment

can also be used to reduce differences in final

moisture content.

Lumber from trees that have been dead for

some time, such as insect-killed trees, is likely to

be lower in moisture content and therefore require less drying time than lumber from live

trees. It may check early in drying, however,

unless the initial drying conditions are mild.

Softwood Schedules for Special Purposes

A number of softwood items require special

drying schedules, and a few are discussed here.

Also discussed are suggestions for reducing brown

stain, setting pitch, retaining desirable oils, and

drying chemically treated wood.

Broion Stain Control.—Brown stain is a discoloration of wood that can occur during kilndrying as a result of a change in the color of

substances normally present in green softwoods.

It can be a considerable problem in the drying

of sugar pine, eastern and western white pine,

and ponderosa pine. This stain also occurs in

some other softwoods, such as western hemlock

and Sitka spruce.

Brown stain can be severe when high dry- and

wet-bulb temperatures are used at the start of

drying. If it is a problem, the initial dry-bulb

temperature should not exceed 130° F. Use as

large a wet-bulb depression as the stock will tolerate without excessive surface and end checking.

The wet-bulb temperature should not exceed

120° F. throughout the drying. During the

equalizing and conditioning treatments, however,

higher wet-bulb temperatures are required. Use

enough venting to obtain the desired wet-bulb

depression. See chapter 9, p. 152, for other suggestions for reducing the development of brown

stain.

Setting Pitch, Retaining Cedar Oil.—Kiln

schedules can be modified either to retain oil in

wood, as in the drying of eastern redcedar used

for cedar chests, or to set pitch that might exude

later from pine and cause paint and finishing

problems {2). High temperature in the presence

of moisture and steam causes volatile material

to vaporize. Therefore, when drying eastern

redcedar, avoid high temperatures and do not

condition the stock unless it is to be resawed or

is surfaced unequally.

On the other hand, to set pitch it is desirable

to drive off the volatile turpentine and other

solvents normally present. This can be done

most easily at the start of drying by using a

high temperature but, if brown stain is a prob-
136 AGRICULTURE HANDBOOK NO. 188, U.S. DEPT. OF AGRICULTURE

TABLE 27.—Code number index of average commercial

Species

Cedar:

IncensePort-OrfordWestern redcedar: '

Light

Medium

Douglas-fir :

Coast type

Inland type

Douglas-fir—

Western larch

Fir:

Noble

White

Hemlock, western

Larch, western

Rne:

Eastern white

Lodgepole

Ponderosa:

Calif., Oregon

Inland, SouthwestSouthern yellow

Sugar:

Light

Heavy

Western white:

Regular

Water core

Redwood, light

Spruce :

Engelmann

Sitka

Lower grades

4/4 stock

Code number

AS5-AK7-..

BS5-BK4_

AS5-AK4_

AS6-AK5_

AS7-AK6

AS8-AK5

BS5-BK6

BS6-BK5

AS7-AK7...

AS5-AK7_-.

BS11-BK5..

AS6-AK7 '_

AS7-AK6.

AS6-BK6_

Maximum

wet-bulb

depression

F.

40

20

22

28

25

30

30

30

AS5-BK7.

AS4-BK5_

Total

time

Hr.

90

52

63

61

60

77

72

139

48

60

67

55

58

102

37

48

6/4 stock

Code number

BS10-BK8-

AS5-BK7_

BS4-BK6.

BS5-BK7

Maximum

wet-bulb

depression

• F.

33

28

Total

time

Hr.

96

100

101

154

1 Schedules indicated are given in tables 25 and 26. Refer to footnote 1 of these tables for C, D, and E time Intervals.

2 Lower grades Include commons, dimension, and box; upper-grades include clears, selects, shop and factory.

' Average moisture content values: Ught, below 80 percent; medium, about 90 percent.

' 5/4 stock.

•Maximum dry-bulb temperature 146° F.
DRY KILN OPERATOR'S MAJSPÜAL

time schedules for 4/4, 6/4, (índ 8/4 softwood ' lumber

137

Lower grades ^ Upper grades ^

8/4 stock 4/4 stock 6/4 stock 8/4 stock

Code number

Maximum

wet-bulb

depression

Total

time Code number

Total

time Code number

Total

time Code number

Total

time

° F. Hr.

AS5-AK7

BS8-AK5

AS6-AK6

BS4-BK7

AS7-AK4

HT.

90

129

50

203

104

HT. Hr.

CS6-BK6 '_-_ 345

BS2-BK6^__

BS7-BK4

296

BS7-AK4 150

AS7-BK6

AS5-BK5

25

20

20

75

55

94

BS7-BK3 200

AS6-BK6 103 AS6-CK5

AS7-AK4

BS9-BK7

BS8-BK5

BS2-BK4

204

145

144

195

220

BS-10-BK8 - 96

109

120

224

AS7-AK7

AS9-AK5

BS6-BK5

BS6-BK5

AS4-AK5 6___

AS5-AK6''___

AS5-AK6«,__

AS11-BK6...

AS3-BK8

BS3-BK8'___

AS5-BK7

72

123

112

139

63

86

74

96

144

264

76

ÀS6-AK5"

BS11-BK5__.

72

BS11-BK6___

BS3-BK6

BS7-BK4

40

25

204

BS7-BK4 224

AS4-BK6 6_.-

BS3-BK6«___

AS10-AK4

137

137

BS4-BK5''-..

BS2-BK5«__.

AS10-AK4

192

BS2-BK5 200 AS11-BK6_..

30 174

100

BS1-BK6'___ 228 BS2-BK7'___

BS1-BK8'___

AS4-CK8'.*._

264

316

124

BS1-BK8'___ 360

BS4-DK6i._. 22 204 CS6-CK6>_.. 276

BSl-CK8i-_.

AS5-AK6 8.__

BS8-BK5

208

57

144

CS1-EK6'-,. 370 'ESÏ-EK5'_'I" '¿16

AS6-AK6 25 54

BS7-BK4i_._ 200 BS6-BK4 244

• Dry-bulb temperature generally does not exceed 160° F.; wet-buib depression does not exceed 36°

than edited for'tathe'^ve^Se s'íhedTe''^"'' '^"'"^ "^^^ ^ dry-bulb temperature 5° or 10° F. lower in step 1 or steps 1 and 2, and 5° or 10° higher to steps 4 to 7

' Maximum wet-bulb depression 25° F.
138

TABLE 28.-

AGRiICULTURE HANDBOOK NO. 188,, U.S. DEPT. OF AGRICULTURE

-Examples of commercial time schedules for drying lumber of certain softwood species

DOUGLAS-FIR, COAST TYPE, LOWER GRADES

Schedule BS5-BK4 No schedule established Schedule BS7-AK4 ^

4/4 stock 6/4 stock 8/4 stock

Time

Dry-bulb

temperature

Wet-bulb

depression

Wet-bulb

temperature

Dry-bulb

temperature

Wet-bulb

depression

Wet-bulb

temperature

Dry-bulb

temperature

Wet-bulb

depression

Wet-bulb

temperature

HT.

0- 12

12- 24

24- 36

36- 48

48- 60

60- 72

72- 75

o p

135

135

140

140

145

145

° F.

7

7

10

10

15

15

o p

128

128

130

130

130

130

o p 0 p ° F. ° F.

145

145

150

150

155

155

160

o p^

7

10

15

20

25

25

25

0 p

138

135

135

130

130

130

135

0- 24

24- 48

48- 72

72- 96

96-120

120-144

144-168

168-192

192-240

240-288

288-336

336-384

384-440

Schedule AS7-AK7

WHITE FIR, UPPER GRADES

Schedule BS9-BK7 Schedule ASfr-AK5

0- 12 145 17 128 155 17 138 140 10 130

12- 24 150 23 127 155 17 138 145 15 130

24- 36 155 30 125 160 23 137 150 20 130

36- 48 160 30 130 160 23 137 155 25 130

48- 60 170 35 135 165 30 135 160 30 130

60- 72 180 43 137 165 30 135 170 35 135

72- 96

96-120

120-144

170

180

180

30

35

43

140

145

137

SOUTHERN YELLOW PINE, UPPER GRADES

Schedule AS11-BK6 Schedule AS10-AK4 Schedule AS10-AK4

0- 12 165 15 150 160 7 153 160 7 153

12- 24 170 15 155 165 10 155 165 10 155

24- 36 175 20 155 170 15 155 170 15 155

36- 48 180 20 160 180 20 160 180 20 160

48- 60 190 25 165 180 25 155 180 25 155

60- 72 190 25 165 190 30 160 190 30 160

72- 96 200 30 170 190 35 155 190 35 155

96-120 190 35 155 190

190

35

35

155

120 144 155

WESTERN REDCEDAR, HEAVY-TYPE,^ UPPER GRADES

Special schedule Special schedule for 5/4 ' No scufdule established

100

110

120

130

140

150

160

170

170

170

170

170

6

13

20

34

44

50

50

50

50

50

50

50

94

97

100

96

96

100

110

120

120

120

120

120

110

114

117

121

125

125

130

130

134

138

143

147

157

10

12

15

17

20

20

24

24

27

30

33

36

45

100

102

102

104

105

105

106

106

107

108

110

111

112

See footnotes at end of table.
DRY KILN OPERATOR'S MAJSTOAL 139

TABLE 28.~Examples of commercial time schedules for drying lumber of certain softwood .s^Ji-CTe.s—Continued

REDWOOD, LOWER GRADE

Special schedule No schedule established No schedule established

4/4 stock 6/4 stock 8/4 stock

Time

Dry-bulb

temperature

Wet-bulb

depression

Wet-bulb

temperature

Dry-bulb

temperature

Wet-bulb

depression

Wet-bulb

temperature '

Dry-bulb

temperature

Wet-bulb

depression

Wet-bulb

temperature

Hr.

0-120

° F.

110

o p

10

o p

100

" F. ° F. ° F. ° F. ° F. ° F.

REDWOOD, UPPER GRADES, HEAVY-TYPE

Special schedule No schedule established No schedule established

0-144

144-192

192-240

240 288

110

115

120

125

130

135

140

145

155

170

15

20

20

25

25

25

25

30

30

40

95

95

100

100

105

110

115

115

125

130

288-336

^*^fî "^ÄA

384-432

432-480

480-504

504 588

' Maximum wet-biilb depression 25° F.

' Stock about 120 percent average moisture content. Î 5/4 "sinker" stock, at about 160 percent average moisture content, has been dried in about 600 hours using steps of about 5 days each with conditions

similar to those oí steps Nos. 1, 3, 6, 7, and 8 of this schedule.

TABLE 29.—Variation in final moisture content of lumber from tioo kiln charges of a softwood uñth dißerent

average initial moisture content when dried by the same time schedule '

Moisture

content

Pieces in each final moisture content

class Moisture

content

Pieces in each final moisture content

class

Run 1 Run 2 Run 1 Run 2

Percent

7

8

Number

161

76

40

21

4

4

Number

3

13

59

75

63

46

Percent

13

14

15

16

17

18

Number

2

Number

30

29

9 7

10 21

11

12 11

' Source: Central Calliomia Dry Kllr Club.

lern, the best compromise is to use an antibrown

stain schedule at the start of drying and finish

up with 160° or higher. The temperature of 160°

is probably satisfactory for setting pitch in 4/4

stock, but the final temperature for thicker stock

should be 170° or higher.

Chemically Treated Wood.—The procedures

given for drying chemically treated hardwoods,

p. 128, also apply to treated softwood lumber.

Unique problems arise in the treatment and

drying of plywood. During treatment with

waterborne chemicals, the cross-banded construction of plywood prevents normal swelling of the

face plies, and compression set occurs. The

amount of set can be kept low by having the

treating temperature as low as possible. At the

start of drying, use a comparatively high temperature with a relative humidity that is as high
140 AGRaCULTTJBE HANDBOOK NO. 188, U.S. MPT. OF AGRICULTURE

as possible without causing leaching. Some checking must be expected when the surface plies are

of rotary-cut veneer, because knife checks are

often present in this veneer.

Maxhniim-Strength Sched.ules.—For drying

ladder stock, oar stock, aircraft lumber, and other

stock where it is necessary to presei-ve maximum

strength, use comparatively low temperature

schedules. Special schedules for drying aircraft

lumber are given in Militay Specification MILW-6109 (18).

Width of Stock.—The softwood moisture content and time schedules given in tables 21, 22,

27, and 28 are set up for random-width stock,

which generally is 6 inches or more wide. They

are also satisfactory for wide flat-grain stock,

but drying time is someAvhat longer than that

for average-width stock. Some examples of

schedules for narrow stock are given in table 30.

In most cases, narrow stock can stand more severe schedules and dry faster than average-width

stock. At mills with large volume, sorting to

width results in a considerable reduction in kiln

time. If both narrow and wide material are included in the same kiln charge, use an equalizing

treatment.

Bevel Sidimg, Venetian Blinds, Resawed Paoieling.—Softwood lumber that is to be resawed into

bevel siding, venetian-blind slats, or paneling

should be properly equalized and conditioned to

TABLE 30.—Bepresentative drying schedules for

narrow softwood stock

4/4 UPPER GRADE DOUGLAS-FIR, COAST TYPE

Time Drv-bulb Wet-bulb Wet-bulb

temperature depression temperature

Hr. » F. o p c p^

0- 12 184 31 153

12- 24 188 34 154

24- 36 193 39 154

36- 48 194 42 152

48- 54 195 39 156

8/4 UPPER GRADE DOUQLAS-FIB, COAST TYPE

0- 24 150 6 144

24- 48 154 12 142

48- 72 162 20 142

72- 96 171 29 142

96-120 182 37 145

120-168 188 42 146

4/4 UPPER GRADE WESTERN HEMLOCK

0- 12 176 23 153

12- 24 178 25 153

24- 36 183 27 156

36- 48 185 27 158

48- 60 185 30 155

60- 72 187 32 155

72- 91 190 33 157

obtain a uniform moisture content over the cross

section and relieve drying stresses. Before equalizing, use the final wet-bulb depression given in

the schedules to get the stock down to a low average moisture content as soon as possible.

Bundled Short-Length Stock.—Most of the

drying of bundled short-length items takes place

from the end-grain surfaces. Because some of

these items do not end or surface check readily,

kiln schedules for them can be rather severe.

Other items, however, require low dry-bulb temperatures to avoid collapse.

Since western redcedar shingles produced from

wet stock logged in low areas may collapse, they

are dried with an initial dry-bulb temperature of

about 95° F. This temperature is gradually increased over a 10- to 14-day period to 150° or

higher. Shingles produced from stock at a relatively low moisture content can be started at 150°

or higher and finished at 180°. In both cases there

is no control of wet-bulb temperature and the vents

are kept open.

Incense-cedar pencil material is usually dried

from a green to a partially dry condition in the

form of 3-inch planks or squares and then cut

into slats and graded. These slats are coated

with a small amount of wax, bundled, and

treated with a water-soluble dye. Because the

treatment generally is a full-cell treatment in

which all the cell cavities become filled with

liquid, the slats may collapse under severe kikdrying conditions. Use low-temperature, highhumidity conditions at the start of drying, and

gradually make them more severe as drying progresses. Drying times are long, 23 to 30 days.

Ponderosa pine squares, 4/4, 5/4, and 6/4 in

cross section and 24 to 36 inches long, are dried

in bundles about 5 inches square. Use constant

kiln temperatures of 140° F. on the dry bulb

and 110° on the wet bulb. Drying time is 13 or

14 days. Similar drying conditions can be used

on other short, rough items of pine or other easily

dried softwoods.

Timlers, Poles, Crossties.—It is not customary

to kiln-dry large timbers, poles, crossties, and

other large items because of the long drying

times required. They are either air-dried or used

green. Before they are given preservative treatment, they are conditioned by steam, or a Boulton

treatment, or air-dried. Materials of this size do

not need to be completely free from seasoning

defects.

The kiln-drying of small to medium-sized

Douglas-fir timbers is aided by a chemical preseasoning treatment. Chemical treatment consists of a dry-spread application of urea, 40 to

60 pounds per thousand board feet, followed by

solid piling until the urea is absorbed. This

treatment results in some surface browning of the

wood.
DRY KILN OPERATOR'S MANUAL 141

The schedules for all sizes of urea-treated material in this class start with a dry-bulb temperature of 140° F. and a 7° or 8° wet-bulb

depression. Gradually increase the temperature

to 160° or 170° and the wet-bulb depressions to

15° or 30°. The larger the timber, the milder

the drying conditions in the intermediate and

final stages. Approximate drying time is 6 days

for 21/2- by 12-inch stock, 8 days for 4- by 6-inch,

16 days for 6- by 8-inch, and 32 days for 8- by

10-inch. Final average moisture content is about

19 percent. A minor amount of checking may

occur.

Tank Stock.—Tank stock can be kiln-dried

by the schedules used for upper grade lumber

of the same thickness. Since the stock is used

in contact with water or aqueous solutions, it

should not be dried lower than 15 to 20 percent

moisture content. Therefore, put it on an equilibrium moisture content condition of about 12

percent when the driest material reaches about

16 percent, and hold that condition until the

wettest material reaches about 20 percent.

Veneer, Battery Shims.—Although softwood

veneer is usually dried in mechanical veneer

dryers, forced-circulation kilns are sometimes

used. Douglas-fir sheathing-grade veneer of

%6-iii^ch thickness is dried in a kiln stacked 4

sheets per course. Initial drying conditions are

160° F. dry-bulb and 145° wet-bulb temperatures.

The final dry-bulb temperature is 190° and the

wet-bulb 140°. Drying time to 7 percent moisture content is 72 to 96 hours. Ponderosa and

sugar pine veneer can be kiln-dried on special

racks in 4 to 6 hours.

Douglas-fir and Port-Orford-cedar battery

shims are stacked 8 to 10 per course for kiln-drying. Each shim is 6% inches wide and 0.06 to

0.09 inch thick. A constant dry-bulb temperature

of 140° F. is used, and the wet-bulb temperature

IS gradually dropped from 134° to 128°. Drying

time for Douglas-fir is 4 days. For Port-Orfordcedar, it is 3 days for the sapwood and 5 days

for the heartwood.

_ Knotty Pine Stock.—Th^ moisture content or

time schedules for lower grade lumber are generally satisfactory for preventing excessive checking or loosening of knots during the first stages

of drymg. Drying time, however, must be prolonged to reach a final moisture content of 7

or 8 percent. Somewhat lower relative humidities may be needed to obtain this moisture content without an undue lengthening of the drying time. Use a final temperature of 160° F. or

higher to "set" the pitch. Conditioning to relieve

drying stresses is desirable. Some tight-knotted

paneling stock is dried in accordance with schedules for upper grade lumber.

Roof Decking.—'Wesi&vn redcedar, Engehnann

spruce, whit© fir, and other softwood species have

B55315 O—61' 10

been successfully used as 4- by 5-inch, double

tongue-and-groove roof decking. The industry

practice is to dry the outer 1/2 inch to 15 percent moisture content or less in from 7 to 10

days under a schedule that allows a small amount

of surface checking. Table 31 shows typical

kiln schedules for this kind of material.

TABLE 31. -Time schedules for kiln drying 4- f>y

ô-inch roof decking

WHITE FIR

Time Dry-bulb Wet-bulb Wet-bulb

temperature depression temperature

HT. "F. "F. "F.

0- 24 150 10 140

24- 48 155 15 140

48- 72 160 15 145

72- 96 165 15 150

96-192 170 15 155

ENOELMANN SPRUCE

WESTERN REDCEDAR

0- 48 130 10 120

48- 72 135 15 120

72- 96 140 15 125

96-120 145 20 125

120-144 150 25 125

144-168 155 30 125

168-192 160 30 130

192-216 165 30 135

216-240 170 30 140

Sterilizing, Equalizing, and

Conditioning Treatments

Sterilizing Treatments

A kiln sterilizing treatment can be used to stop

the growth of excessive mold on the surface of

wood under certain conditions (ch. 9). The dry

kiln can also be used to sterilize wood that has

been infected with stain or decay fungi, or attacked by wood-destroying insects.

Mold.—Mold can develop on green lumber in

a kiln operating at temperatures up to 120° F.

Although the mold generally does not penetrate

the wood enough to cause serious stain during

kiln-drying, it can fill up the air spaces in a load

of lumber and seriously interfere with air circulation. Not only does this slow up the drying of

the load as a whole, but the wood under the

mold may be subject to honeycombing when the

temperature is increased later in the drying

process.
142 AGBIiCULTURE HAJSÍDBOOK NO. 188, U.S. DEPT. OF AGRICULTURE

To sterilize for mold, steam the kiln charge at

or near 100 percent relative humidity at a drybulb temperature of 130° F. or higher for 1 hour

after all parts of the kiln have come up to that

temperature. After such steaming, reset the control instrument to the scheduled conditions. Infrequently, two sterilizing treatments may be required about a day apart to stop the development of mold. If the growth is not heavy enough

to stop air circulation, do not use the sterilizing

treatment.

Fungus Stain and Decay.—The temperatures

normally used at the start of kiln-drying are usually high enough to stop the growth of stain or

decay organisms that may have infected green

wood during storage or air-drying. A temperature of 110° F. stops the growth of these organisms, but does not kill them. Tests show that

a temperature of 150° will kill major wooddestroying fungi in green wood in 75 minutes.

Kiln schedules that have a dry-bulb temperature

of 150° or higher for at least 24 hours should

kill all stain and decay ñmgi. As long as the

wood is kept below 20 percent moisture content,

new stain and decay wiU not start..

Insects.—Both softwoods and hardwoods are attacked by a number of wood-boring insects,

whether the wood is green or dry (4). Imported

wood or air-dried wood that has been stored for

a long time should be examined for evidence of

insects. If they are found, a sterilizing treatment should be given immediately.

Lyctus beetles and their eggs and larvae are

killed by subjecting the wood to the conditions

given in table 32. These conditions include allowances for heating the lumber to the center, for

"cold spots" in the kiln and additional time for

a factor of safety. To sterilize, use an equilibrium moisture content that is within 2 percent

above or below the moisture content of the wood.

If the wood has less than 8 percent moisture content, a temperature above 140° F., a relative humidity somewhat below 60 percent, and the times

given in table 32 for 130° should be satisfactory.

Exact data on temperatures and times required to

kill other insects are not available. Presumably,

the higher temperatures with the corresponding

treatment time given in table 32 would be

adequate.

Normal kiln-drying or sterilization will not

prevent future infestation by insects. Give the

wood a chemical treatment if there is danger

of infestation during storage or use (^, 6).

Equalizing and Conditioning Treatments

Frequently the moisture content varies considerably among boards in a kiln charge during

the final stage of drying. Such variation may

cause serious trouble during storage, fabrication,

or use. Also, satisfactory relief of drying stresses

(casehardening) of all boards in a charge cannot

be obtained if the moisture content varies too

much. Therefore, use an equalizing treatment to

overcome excessive variation in moisture content

near the end of drying.

TABLE 32.—Schedule for killing Lyctus powder-post

beetles and their eggs

Kiln setting

Relative

humidity

EquiUbrium

moisture

content

Thickness

of

lumber

Total

time

Drybulb

temperature

Wetbulb

depression

after

kiln

reaches

set conditions

° F.

140

130

125

° F.

7

16

15

Pet.

82

60

61

Pel.

13. 8

9.4

9. 7

In.

f 1

\ 2

I 3

f 1

i 2

I 3

{ Ï

Hr.

3

5

7

10

12

14

46

48

50

Source: Leaflet 13, British Forest Products Research

Laboratory (S).

If the boards are to be resawed, ripped into

thin strips, or machined nonuniformly, use a

conditioning treatment. Such a treatment accomplishes two things: it relieves drying stresses,

and it produces a more uniform moisture content throughout the thickness of the boards.

Drying stresses and nonuniformity of moisture

can result in serious deformation during fabrication and use (ch. 9).

The procedures for eq^ualizing and conditioning discussed here require the use of an adequate number of properly selected kiln_ samples

(ch. 6). These procedures will give satisfactory

results on lumber that is to be dried to final

average moisture content values of from 5 to

11 percent. Table 33 contains basic information

on the moisture content of the kiln samples and

the kiln equilibriiun moisture content (EMC)

conditions for these treatments. Wet-bulb depression values required to obtain desired EMC

conditions are given in table 2, p. 11. Use of

table 2 is explained in chapter 10, p. 164.

Equalizing Treatment.—The procedure for

equalizing a kiln charge of lumber, using table

33, is as follows:

(1) Start equalizing when the driest kiln sample in the charge has reached an average moisture

content 2 percent below the desired final average

moisture content. If, for example, the desired

final average moisture content is 8 percent, equalizing would be started when the driest kiln

sample reaches 6 percent.
DRY KlUSr OPEBATOR'S MANUAL 143

TABLE 33.—Kiln sample moisture content and equilibrium moisture content values for equalizing and

conditioning a charge of lumber

Equalizing Conditioning equilibrium moisture content values

Desired final average moisture content Moisture content Equilibrium mois- Moisture content

of driest sample ture content dur- of wettest sample For softwoods For hardwoods

at start ing this step at end

Percent Percent Percent Percent Percent Percent

5 3 3 5 8 9

6 4 4 6 9 10

7 5 5 7 10 11

8 6 6 8 11 12

9 7 7 9 12 13

10 8 8 10 13 14

11 9 9 11 14 15

(2) As soon as the driest sample reaches the

moisture value stated in step 1, establish an

equalizing EMC in the kiln equal to that value.

In the example given in (1), the equalizing EMC

would be 6 percent. During equalizing, use as

high a dry-bulb temperature as the drying schedule permits.

(3) Continue equalizing until the wettest sample reaches the desired final average moisture

content. In the example given in step 1, the

wettest sample would be dried to 8 percent.

If the equalizing treatment is to be followed by

a conditioning treatment, it may at times be necessary to lower the temperature to obtain the desired conditioning equilibrium moisture content

condition. When this is necessary, begin lowering the temperature 12 to 24 hours prior to the

start of conditioning. Also lower the wet-bulb

temperature to maintain the desired equalizing

equihbrium moisture content (ch. 10).

Conditioning Treatment.—The conditioning

treatment, whether or not preceded by an equalizing treatment, should not be started until the

average moisture content of the wettest sample

reaches the desired final average moisture content.

The procedure for conditioning a kiln charge of

lumber is as follows:

(1) The conditioning temperature is the same

as the final step of the drying schedule or the

highest temperature at which the conditioning

equihbrium moisture content (EMC) can be controlled. For softwoods set the wet-bulb temperature so the conditioning EMC will be 3 percent

above the desired final average moisture content.

For hardwoods the conditioning EMC is 4 percent above the desired final average moisture

content. The wet-bulb depression that will give

the desired conditioning EMC is obtained from

table 2, p. 11. If, at the desired conditioning

temperature, a wet-bulb depression value is not

shown for the desired EMC, choose the wet-bulb

depression value for the nearest higher EMÇ

given for that temperature.

EXAMPLE.—Assume that this case involves a

hardwood, a final desired moisture content of 8

percent, and a conditioning temperature of 170° F.

The conditioning EMC from table 33 is 12

percent. At 170°, an 8° wet-bulb depression will

give an EMC of 12.4 percent (table 2). If the

material was a softwood, the conditioning EMC

would be 11 percent and the wet-biüb depression

10°.

(2) Continue conditioning until satisfactory

stress relief is attained.

The time required for conditioning varies considerably, depending upon species and thickness

of the lumber, the type of kiln used, and kiln

performance. Hardwoods generally require 16

to 24 hours for 4^/4 stock and up to 48 hours

for 8/4. The 4/4 thickness of some softwood

species can be conditioned in as short a time as

4 hours.

The least time necessary to get stress relief

in a certain thickness of a particular species is

obtained by prong tests, as described in chapter

6, figure 96, p. 106, and chapter 10, p. 164. It is

advisable to hold conditioning time to a minimum

so as to decrease steam consumption and avoid excessive moisture pickup in low-density woods.

If average moisture content determinations are

made immediately after the conditioning treatment, the moisture content obtained will be about

I to 11/2 percent above the desired value because

of the surface moisture regain. After cooling,

the average moisture content of the lumber should

be close to that desired.

The general equilibrium moisture content rule

stated in step 1 of the conditioning procedure

does not apply to moisture content values above

II percent. Conditioning is hard to accomplish

at the higher values, but if the kiln will main-
144 AGKLCULTURE HAKDBOOK NO. 188, U.S. DEPT. OF AGRICULTURE

tain a very high EMC at temperatures of 140°

F. or above, the required conditioning EMC values for obtaining up to about 14 percent average

moisture content can be calculated (i-?).

Kiln-Drying Time

An operator needs some prior idea of the time

required for lumber to dry. Table 34 gives the

TABLE 34.—Approximate kiln-drying periods for

1-inch lumber '—Continued

TABLE 34.- -Approximate kiln-drying periods for

1-inch lumber '

SOFTWOODS

Species

Baldcypress

Cedar :

AlaskaAtlantic whiteEastern redcedar

IncenseNorthern whitePort-OrfordWestern redcedar

Douglas-fir :

Coast type

Intermediate type

Rocky Mountain type

Fir:

Balsam

California red

Grand

Noble

Pacific silver

Subalpine

White

Hemlock :

Eastern

Western

Larch, western

Pine:

Eastern white

Lodgepole

Ponderosa

Red

Southern yellow:

Loblolly

Longleaf

Shortleaf

Sugar:

Light

Heavy

Western white

Redwood:

Light

Heavy

Spruce:

Eastern, black, red, whiteEngelmann

Sitka

Tamarack

Time required to kiln-dry

1-inch stock from—

20 to

6 percent

moisture

content

Days

4- 8

2- 3

2- 3

3- 5

5- 7

Green to

6 percent ^

moisture

content

Days

10-20

4- 6

8-10

6- 8

3- 6

8-10

4- 8

10-15

2- 4

4- 7

4- 7

3- 5

3- 5

3- 5

3- 5

3- 5

3- 5

3- 5

3- 5

3- 5

3- 5

4- 6

3- 5

3- 6

6- 8

3- 5

3- 5

3- 5

3- 4

5-10

3- 5

10-14

20-24

4- 6

3- 5

4- 7

3- 5

HARDWOODS

Time required to kiln-dry

1-inch stock from—

Species

20 to

6 percent

moisture

content

Green to

6 percent ^

moisture

content

Alder, red

Days

3- 5

4- 7

5- 7

4- 7

3- 5

3- 5

5- 8

Days

6-10

Apple 10-15

Ash:

Black 10-14

White --- 11-15

Aspen _ - 6-10

Basswood, American

Beech, American

Birch:

6-10

12-15

3- 5

Yellow - - 5- 8

5- 8

5- 8

5- 7

4- 8

7-12

4- 8

5- 8

4- 6

5- 8

4- 6

4-12

5- 8

5- 8

5- 7

5- 8

8-11

4- 6

4- 7

4- 6

5- 8

6-10

11-15

Buckeve vellow 12-16

Butternut

Cherry, black

Chestnut, American

Chinkapin, golden

Cottonwood -- - -

10-15

10-14

8-12

22-28

8-12

Dogwood, flowering

Elm:

American

Rock - -

12-16

10-15

13-17

Hackberry 7-11

Hickory 7-15

Holly, American 12-16

Hophornbeam, eastern

Laurel, California

Locust, black

Madrone, Pacific

Magnolia, __

12-16

10-15

12-16

15-20

10-15

Mahogany

Maple:

Red, silver (soft)

12-15

7-13

Sugar (hard) - - - 11-15

Oak:

California black

Live

25-35

30-40

Red 5-10

6-12

5- 8

5- 8

8-12

5- 7

4- 7

7-12

4- 6

5- 7

5- 8

5- 8

3- 6

16-28

White 20-30

Osage-orange

Persimmon, common

Sweetgum:

Heartwood

12-16

12-16

15-25

Sapwood

Sycamore, American

Tanoak.. _ ..

10-15

6-12

24-30

Tupelo:

Black_ ^ _ _ _ .

6-10

Water . _

6-12

Walnut, black 10-16

Willow, black 12-16

Yellow-poplar 6-10

I Because of the many factors affecting drying rate and tlie lack of specific

data covering eacli case, wide variation from these values must be ^'¡jf^^tea^

They onlv present a general idea of average drying periods and shoula not oe

used as time schedules. Some of the drying times shown were outaineu

from commercial lúln operators. , ^

' Some softwoods are usually dried to about 10 percent moisture contení,

but the drying times shown will still apply.
DRY KILN OPEBATOR'S MANTJAL 145

approximate time required to kiln-dry 1-inch

lumber from 20 to 6 percent moisture content and

from green to 6 percent moisture content. The

time required to dry a given material varies with

its character, the type of kiln, and the kiln

schedule used. Drying times for some softwoods

are also given in table 27, p. 136. These are actual average values obtained at the mills studied.

Wliile softwoods are generally kiln-dried from

the green condition, the lower grade stock is dried

only to a moisture content of about 17 percent.

Shop-grade and clear-grade softwoods are usually

kiln-dried to 9 or 10 percent. Hardwoods may

be kiln-dried from either the green or the airdried condition. They are generally dried to

lower moisture content values, such as 6 to 8

percent (12).

The drying periods given in table 34 apply

only to 1-inch lumber. For thicker stock, the

increase in drying time is theoretically nearly

proportional to the square of the thickness, but

commercial experience indicates it is somewhat

less than this. A set of factors for estimating

commercial drying times of various thicknesses of

hardwoods has been published (5).

Literature Cited

(1) BAKER, G., AND MCMILLEN, J. M.

1955. SEASONING BEECH LUMBER. U.S. Forest

Serv. Northeast. Forest Exp. Sta. Beech

Utü. Ser. 11, 22 pp., Ulus.

(2) BROWNE, F. L., AND RIETZ, R. C.

1953. EXUDATION OF PITCH AND OILS IN WOOD.

U.S. Forest Serv. Forest Prod. Lab.

Rpt. 1735, 11 pp., illus. [Processed.]

(3) [GT. BRIT.] DEPARTMENT OF SCIENTIFIC AND INDUSTRIAL RESEARCH.

1952. THE KILN STERILIZATION OF LYCTUS-INFESTED

TIMBER. Forest Prod. Res. Lab. Leaflet

13, 4 pp. Rev.

(4) HATFIELD, I.

1949. WOOD-BORING BEETLES THAT DAMAGE FURNITURE AND INTERIOR WOODWORK. Forest

Prod. Res. Soc. Proc. 3: 235-249.

(5)

(6)

(7)

(8)

(9)

(10)

(11)

(12)

(13)

(14)

(15)

(16)

(17)

(18)

HiGGINS, N. C.

[n.d.] A RULE-OF-THUMB METHOD FOR ESTIMATING

DRYING TIME. Mich. State Univ. Coop.

Ext. Serv. Forest Prod. Rpt. 8023, 5 pp.

[Processed.]

JOHNSTON, H. R., SMITH, R. H., AND ST. GEORGE, R. A.

1955. PREVENTION AND CONTROL OF LYCTUSpowDER-posT BEETLES. South. Lumberman 190 (2375) : 72-3.

KIMBALL, K. E.

1955. DRYING SCHEDULE FOR THIN RED OAK LUMBER.

South. Lumberman 191 (2393): 252-54.

McMlLLEN, J. M.

1951. HOW CAN BLACK WALNUT GUNSTOCK BLANKS

BE KILN DRIED WITHOUT HONEYCOMBING?

U.S. Forest Serv. Forest Prod. Lab.

Rpt. 1769-13, 2 pp., illus. [Processed.]

1953. KILN DRYING WATER AND SWAMP TUPELO.

Jour. Forest Prod. Res. Soc. 3(5) : 189-196.

1956. SEASONING HICKORY LUMBER AND HANDLE

BLANKS. U.S. Forest Serv. Southeast.

Forest Expt. Sta. Hicliory Task Force

Rpt. 4, 36 pp., illus.

U.S.

Rpt.

U.S.

Rpt.

1958. STRESSES IN WOOD DURING DRYING.

Forest Serv. Forest Prod. Lab.

1652, 33 pp., illus. [Processed.]

PECK, E. C.

1955. MOISTURE CONTENT OF WOOD IN USE.

Forest Serv. Forest Prod. Lab.

1655, 5 pp., illus. [Processed.]

RASMUSSEN, E. F., AND McMlLLEN, J. M.

1956. SEASONING OF WHITE OAK FOR TIGHT COOPERAGE. U.S. Forest Serv. Forest Prod.

Lab. Rpt. 1784, 15 pp., iUus. [Processed.]

RIETZ, R. C.

1950. ACCELERATING THE KILN DRYING OF HARDWOODS. South. Lumberman 181 (2262):

43-44, 54.

SMITH, H. H.

1947. SEASONING OF ASPEN. U.S. Forest Serv.

Lake States Forest Expt. Sta. Aspen

Rpt. 5, 19 pp., illus. [Processed.]

1954. SEASONING CALIFORNIA HARDWOODS: TANOAK,

MADRONE, GOLDEN CHINKAPIN, AND CALIFORNIA LAUREL. U.S. Forest Serv. Calif.

Forest and Range Expt. Sta., Tech. Paper

5, 15 pp., illus. [Processed.]

ToRGESON, O. W.

1946. ACCELERATED-TYPE OF KILN DRYING SCHEDULE

FOR 1- AND 2-INCH BLACKGUM. South.

Lumberman 173 (2177) : 206-208.

U.S. DEPARTMENT OF DEFENSE.

1950. WOOD, METHOD OF KILN DRYING. Mil.

Specif. MIL-W-6109. 11 April 1950.

CHAPTER 9. DRYING DEFECTS

Many features of wood affect its utility when

it is processed into lumber and special wood products. These include knots, ring shake, bark and

mineral streaks, pitch pockets, compression and

tension wood, and spiral or interlocked grain

that form in a tree and directly influence the

grade and value of each individual board. Ordinary processes of lumber manufacturing may

remove some of these natural features by trimming and thus improve the quality and value of

the remaining piece.

Defects that reduce the grade and value of

lumber often develop during logging, sawmilling, drying, finishing, and mechanical handling.

A principal objective is to accomplish drying

with as little development of defects as possible

consistent with economy. The degree of care

to exercise in controlling the development of defects depends on the final use of the material.

Before adopting a drying procedure to curtail

development of a specific drying defect, the kiln

operator should determine whether the procedure

will induce other defects that may lower the value

of the material. He should always modify the

drying procedures to hold down losses from all

defects.

EflFect of Temperature on

Strength of Wood

High temperatures reduce the strength of wood,

particularly if it has a high moisture content;

the higher the temperature and moisture content and the longer the drying time, the greater

the reduction in strength.

For most uses of wood, some reduction in

strength is not important. Temperatures up to

160° F. reduce the strength of wood very little

during the normal drying cycle. Where high

strength and toughness are desired, however, temperatures in excess of 160° should not be used.

Wood for aircraft, ladders, tennis rackets, bats,

gunstocks, bowling-pin blanks, cooperage stock,

and similar products is in this category.

The effect of temperature on the strength of

wood was considered in preparing the drying

schedules given in chapter 8. When using them,

therefore, the kiln operator need have no concern

about the effect of heat on the strength properties of wood he is drying.

Defects Occurring During Drying

Drying defects fall into three classes, based on

their causes: (1) shrinkage, (2) fungi, and (3)

chemicals in the wood. Defects associated with

shrinkage are usually increased when excessively

high dry-bulb temperatures or large wet-bulb depressions are used during critical stages of drying. Fungal defects generally occur when low

temperatures and high relative humidities are

used in drying wet wood. Chemical stains that

occur in wood during drying are due mainly to

the effect of heat on extractiA'es in the wood.

Defects Associated With Shrinkage

Many defects are associated with tlie shrinking

of wood as it dries. Knowing where, when, and

why they occur will enable an operator to take

action to keep them at a minimum. Kiln-drying

is frequently blamed for defects that have occurred during air-drying, but most defects can

occur during either process. In kiln-drying they

can be kept down by modifying drying conditions, and in the air-drying by altering piling

procedures.^

Surface Checks.—Surface checks are failures

that usually occur in the wood rays on the flatsawed faces of lumber or other wood items (fig.

108). They can also occur in resin ducts and in

mineral streaks. They rarely appear on the edges

of flat-sawed material 6/4 inches or less in thickness, but do appear on the edges of thicker flatsawed and quarter-sawed material. These failures usually take place early in the drying process, but in some softwoods the danger persists beyond the initial stages of drying. They develop

because the surfaces of the lumber get too dry

as a result of excessively low relative humidity.

Thick, wide, flat-sawed lumber is more prone to

surface check than thin, narrow stock.

Many surface checks, particularly those in the

hardwoods, close during the drying process. In

products requiring highly finished wood surfaces,

such as some types of interior trim, cabinets, and

furniture, closed surface checks are undesirable.

They may open to some extent during use because

1 PECK, E. C. AIK DRYING OF LUMBER. U.S. Forest Serv.

Forest Prod. Lab. Rpt. 1657, 21 pp., illus. 1956. [Processed. ]

147
148 AGRICULTURE HANDBOOK NO. ,188, U.S. DEPT. OF AGRICULTURE

Vx

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"ST" M

all species of wood as the thickness and width of

the stock increases. For this reason, the endgrain surfaces of thick and wide stock and of

larger special items, such as squares, gunstocks,

and bowling-pin blanks, should be end coated.^

To be most effective, end coatings should be applied to freshly cut, unchecked ends of the green

wood.

End Splits.—End splits usually result from

the extension of end checks. Therefore, if excessive extension of end checks can be avoided,

end splitting is less likely to occur. The placing

of tiers of stickers at the extreme ends of the

boards being dried helps reduce end splitting.

Collapse.—Collapse is a severe distortion or

flattening of wood cells. A slight amount of

this defect may be difiicult, if not impossible, to

detect. When severe, it often shows up as grooves

or corrugations on the wood surfaces (fig. 110).

Collapse may be caused by (1) compressive

drying stresses on the interior parts of the wood

that exceed the compressive strength of the wood,

or (2) liquid tension in cell cavities that are

completely filled with water. Both of these con2 McMlXUEN, J. M. COATINGS FOB THE PREVENTION OF

END CHECKS IN LOGS AND LUMBEB. U.S. TorCSt ScrV.

Prod. Lab. Rpt. 1435, 13 pp., iUus. 1956. [Processed.]

M-22623-F

FiGTJBE 108.—Surface checking in Douglas-fir dimension

lumber.

of fluctuations in atmospheric conditions. Superficial surface checks that will be removed during machining need cause no concern. In such

Eroducts as bowling pins, tennis rackets, tool

andles, and certain structural members, surface

checks either closed or open will increase splitting tendencies during use. Products whose value

would be unaffected by closed surface checks include flooring and some types of furniture.

Material that has surface checked during airdrying should not be wetted or exposed to very

high relative humidities before or during kilndrying. Such treatments frequently lengthen,

widen, and deepen checks. Neither should materials be wetted that have open checks after kilndrying, because subsequent exposure to plant

conditions will dry out the wetted surface and

enlarge the checks.

Ev2 Checks.—^End checks (fig. 109), Hke surface checks, generally occur in the wood rays,

but on end-grain surfaces. They also occur in

the initial stage of drying, and can be minimized

by using a higher relative humidity. Endchecked stock should not be wetted or subjected

to very high relative humidities prior to, during,

or after drying.

The tendency to end check becomes greater in

M-3610-F

FIGURE 109.—End checks In oak plank.
DRY KILN OPERATOR'S MAJTOAL 149

:' '.ifill

M-111997-P

FiGTJEE HO.—Severe collapse in western redcedar.

ditions occur early in the drying process. Collapse is not usually risible on the wood surface,

however, until later in the process. This defect

is generally associated with excessively high drybulb temperatures in the early stages of drying,

and these temperatures should be lowered on subsequent kiln charges if collapse occurs.

Collapse is a serious defect and should be

avoided if possible. The use of special drying

schedules planned to diminish its occurrence in

susceptible woods is recommended. Some woods

susceptible to this defect are generally air-dried

before being kiln-dried.

licnieyG&nib.—Honeycomb (fig. Ill) is an internal void caused by tensile failure across the

grain, and it usually occurs in the wood rays.

It is produced by the use of excessively high

temperatures for too long a time while free

water is still present in the cell cavities. Wliile

the wood may not actually fail until midway or

later in the drying process, the reduction in its

strength may begin at any stage of the process if

temperatures are excessively high. Therefore,

honeycombing can be held to a minimum by avoiding excessively high dry-bulb temperatures until

all the free water has been evaporated from the

entire piece.

Deep surface and end checks that have closed

tightly on the surfaces of the stock, but remain

open below the surface, are often called honeycomb. These failures are also sometimes called

bottleneck checks.

Honeycomb can result in heavy footage losses.

Unfortunately, in many cases, it cannot be detected at the surface of the lumber and is not

found until the wood is being machined. Severely honeycombed stock, however, frequently

has a corrugated appearance on the surface.

Honeycomb is frequently associated with collapsed wood in zones of high moisture concentration.

Ring Failure.—Ring failure (fig. 112) occurs

parallel to the annual ring either within the ring

or between rings. In appearance it is similar

to shake, which takes place in standing trees or

in trees when they are felled. Frequently the

failure involves several annual rings, starting in

one and breaking across to others along the wood

rays. It can occur as a failure in the end grain

in the initial stages of drying, and extend in

depth and length as drying continues. The failure can also occur internally, because of casehardening and a weakening of the bond between

annual rings when high temperatures are used.

Ring failure can be kept to a minimum by end

coatmg stock and by using higher initial relative humidities and low dry-bulb temperature

schedules.

Boxed-Heart Splits.—A boxed-heart. split is

shown in figure 113. These splits start in the

initial stages of diying and become increasingly

worse as the wood becomes drier. The difference

between tangential and radial shrinkage of the

wood surrounding the pith causes such severe

stresses in the faces of the piece that the wood is

split. It is virtually impossible to prevent this

defect.

Warp.—As wood dries it may become distorted

in shape and form because of the differences in

radial, tangential, and longitudinal shrinkage

(ch. 1, p. 15). "Warping is also aggravated by

irregular or distorted grain and the presence of

abnormal types of wood. The various types of

warp are illustrated in figure 114.

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FioTjEE 111.—Honeycombing in oai.

M-U293-F
150 AGRIiCULTUEE HANDBOOK NO. 188, U.S. DEFT. OF AGRICULTURE

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FiGUBE 112.—Two ring failures (vertical breaks) and a check (horizontal break) in a 3- by 8-inch Sitka spruce.

Cup (fig. 114) is a distortion of a board in

which there is a de^aation ñatwise from a straight

line across the width of the board. It begins to

appear fairly early in the drying process and

becomes progressively worse as drying continues.

Cup is caused by greater shrinkage parallel to

than across the annual rings. In general, the

greater the difference between tangential and raM-115582-F

FIGURE 113.—Boxed-heart split in red oak.

dial shrinkage, the greater the degree of cup.

A ñat-sawed board cut near the bark tends to

cup less than a similar board cut near the pith.

Flat-sawed boards cup toward the face that was

closest to the bark. Cup may result in excessive

losses during machining. It can be reduced to

some extent by modifying the drying schedule

and avoiding overdrying, and to some extent by

air-drying the stock before kiln-drying it. The

best method of controlling cup, however, is by

good piling or stacking practices (ch. 5, p. 82).

The thinner the lumber, the greater is its tendency

to cup.

Bow (fig. 114) is a deviation ñatwise from a

straight line drawn from end to end of a board.

This defect is associated with longitudinal shrinkage in wood adjacent to the pith of the tree,

compression or tension wood that occurs in leaning trees, and cross grain in wood. It can be

controlled by the same procedures used to diminish cupping.

Crook (fig. 114) is a deviation edgewise from

a straight line drawn from end to end of a

board. The causes of crook are the same as

those for bow. It is more difficult to prevent

than cup or bow.

Twist is the turning or winding of the edges of

a board so that the four corners of any face are

no longer in the same plane (fig. 114). It occurs

in wood containing spiral, wavy, diagonal, distorted, or interlocked grain. Lumber containing

these grain characteristics can sometimes be dried

reasonably flat by using proper piling procedures.

Diamonding is a form of warp found in squares

(fig. 114) ; the cross section assumes a diamond

shape in drying. This defect is the result of
DRY KILN OPERATOR'S MAJJTTAIi 151

TWIST

POINT OF GREATEST

DEFLECTION

DIAMOND CUP

FiGUEE 114.—Various types of warp.

differences between radial and tangential shrinkage in squares in which the annual rings run

diagonally from corner to corner. It can be controlled to some extent by air-drying the squares

before kiln-drying them.

Checked Knots.—Checked knots are often considered defects. The checks appear on the end

grain of knots in the wood rays (fig. 115). They

are the result of differences in shrinkage parallel

to and across the annual rings within the knots.

They occur in the initial stages of drying and

are aggravated by using too low a relative humidity. Knot checking can be controlled by

using higher relative humidities and by drying

to a higher moisture content, but is almost impossible to prevent.

Loose Knots.—Encased knots (dead or black

knots) invariably loosen during drying (ñg. 116).

M-169B8-r

FiQTJBE 115.—Checked knot in sugar pine.

M-16268-P

FIGURE 116.—Loose knot In southern yellow pine.
152 AGRICULTURE HANDBOOK NO. 18 8, U.S. DEPT. OF AGRICULTURE

This is due to the fact that they are not grown

to the surrounding wood, but are held in place

only by bark and pitch. Thev shrink considerably in both directions of the face of the lumber,

i.e., across the width and along the length, whereas the board shrinks considerably in width but

very little in length. Consequently, the dried

knot is smaller than the knothole, and frequently

falls out of the lumber during handling and

machining. Nothing can be done to prevent the

loosening of dead knots during drying. Fewer

dead knots will fall out during machining, however, if the lumber is not dried to a low moisture

content before it is machined.

Casehardening.—Casehardening is the inevitable result of the drying stresses associated with

shrinkage—the stresses persisting when the wood

is uniformly dry. Whether or not it is considered to be a defect depends on the final use

of the dried material. Casehardened lumber is

difficult to machine. Casehardening can be relieved in a compartment dry kiln by a conditioning treatment (ch. 8, p. 143, and ch. 10, p. 164).

Defects Associated With Fungal Attack

Three defects are associated with fungal attack

—sap stain, decay, and mold. All, as discussed

earlier, can occur under favorable temperature

and moisture conditions during kiln-drying.

Sap stain, commonly called blue stain, is considered a defect for many uses of lumber, and

the sapwood of many species of wood are susceptible to it (fig. 117). This defect takes place

in the early stages of drying. It is caused by

fungi that depend for growth upon proper food,

moisture, air, and temperature. If any of these

factors are unfavorable, the stain will not occur.

The fungi grow most rapidly between temperatures of 75° and 85° F. in wood that has a

moisture content of 20 percent or more.

Sap stain can be materially reduced and quite

frequently eliminated by the rapid drying of

green lumber, fast air-drying, or kiln-drying at

temperatures of 150° F. or higher. The kiln conditions necessary to control sap stain are described

in chapter 8, p. 142. If green lumber must be

held in solid piles under conditions conducive

to attack by sap stain fungi before it is dried,

it should be treated with a toxic chemical solution.

Defects Associated With Chemicals in

the Wood

Extractives in wood undergo chemical changes

during drying that may cause discolorations—

or chemical stains, as they are generally called.

If pitch fails to harden or set during drying,

trouble may result, particularly with products to

which sealers, varnishes, and paints are applied.

M-92499-F

FIGURE 117.—Sap stain in southern yellow pine. Sap

stain color ranges from bluish gray to black.

Broion Stain.—Brown stain (fig. 118) is a

chemical stain that occurs in many softwoods,

though principally in sugar, ponderosa, and

white pine. Its color varies from light to very

dark brown. While it affects appearance, it does

not impair strength. It is believed to be caused by

chemical reactions that take place in the watersoluble extractives as they are concentrated and

deposited during drying. The stain may develop

within the piece as well as on its surfaces. Planing may remove some of the stain, but it may

expose zones of darker stain.

The stain can be reduced considerably by (1)

cvitting the logs into lumber as soon as possible

after the tree is felled; (2) drying the lumber

without delay; and (3) using an initial dry-bulb

temperature that does not exceed 130° F., with

a relative humidity as low as the stock will tolerate without excessive surface and end checking,

and a wet-bulb temperature that does not exceed

120° throughout the drying.
DRY KILN OPERATOR'S MAJSTUAL 153

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M-374-F

FiQUBE 118.—Chemical brown stain in wliite pine.

Sticker Marking.—Sticker marking (fig. 119)

occurs in many woods during air- and kiln-drying. Tliese discolorations, which vary in color,

may occur on and beneath the surface of the

board under the sticker or appear as narrow,

dark streaks on the surface at the edges of the

sticker. Sometimes they can be removed by

surfacing or sanding. Sticker marking is believed to be associated with concentrations of,

and chemical changes in, the extractive substances during drying. No means of preventing

the discoloration is known. Certain seasoning

procedures will, however, lessen sticker staining.

These include the use of dry, narrow stickers or

of grooved stickers to reduce the contact area, and

starting the drying of the green lumber as soon

as possible.

Other Color Ch-anges.—Other color changes

that take place during the drying of some species of wood appear to be associated with chemical action. One example is "pinking" in hickory

—a pink coloration that has no effect on the

strength of the wood, but is considered to be a

defect for some uses. Pinking can be prevented

to a large extent by using a special drying schedule (ch. 8, p. 127). Tliis schedule, though effective in controlling the discoloration, extends the

drying time considerably.

Soft Pitch.—All of the pines, Douglas-fir, the

spruces, and western larch have resin canals or

pockets. As these woods dry, some of the volatile substances in the pitch evaporate, causing it

to harden somewhat. If the pitch is not hardened suißciently, it will ooze or bleed to the surface of the wood during use (fig. 120). Pitch

can be thoroughly set by using a temperature of

160° F. or higher.

Effects of Drying Defects

on Machining

Lumber can be damaged during machining if

it contains certain drying defects. Such damage

includes planer splitting, broken knots, knotholes,

chipped and torn grain, raised grain, and warp.

Precautions taken during drying can help diminish or avoid such damag-e.

Planer Splitting

A long split often develops when cupped lumber is flattened as it passes through the planer.

End splits of course aggravate planer splitting.

This type of split, also called a roller split or

check, lowers the grade and value of the lumber and causes waste. Not only does the amount

of cupping increase as the wood dries, but the

wood becomes stiffer and is more likely to split

when flattened.

Splitting on the planer can be reduced by taking steps to minimize cupping and end splitting

through good stacking practices. Planing at

moisture content levels of 12 percent and higher

will also reduce splitting. For example, softwood construction lumber is frequently dried to

a moisture content of 12 to 18 percent. On the

^,:»*¿W«<íí»*'<">^'

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FIGURE 119.—Sticker marking in liard maple.

M-20977-F
154 AGRICULTURE HANDBOOK NO. 18 8, U.S. DEFT. OF AGRICULTURE

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FIGURE 120.—Pitch exudation in a painted board.

other hand, in the upper grades of both hardwood and softwood lumber, drying must be carried to moisture levels of 10 percent or less to

meet end-use requirements. If, in so doing, the

lumber becomes cupped, splitting cannot be easily avoided during planing. Splitting on the

planer can also be lessened somewhat by relieving

casehardening stresses and raising the moisture

content of the surfaces of the lumber.

Broken Knots and Knotholes

In most grades of lumber it is desirable that^

the knots in the surfaced board be smooth, intact, and unbroken. Knots check and loosen as

drying proceeds, and they become more brittle

as the moisture content decreases. The knots

are severely hammered as well as cut while in

the planer. This hammering breaks checked knots

and knocks out loose ones. Knotholes and broken knots lower the grade of the board.

In much of the softwood lumber industry,

knotty grades of construction lumber are dried

to final moisture content levels ranging from 12

to 18 percent to permit better machining of the

knots. At such a moisture content, the sound

knots are not severely checked and the encased

knots are fairly tight. Therefore, common

grades of softwood lumber are separated from

finish grades and dried by different schedules,

the finish grades being dried to a moisture content of about 8 to 10 percent.

Encased knots in some species of wood are held

in place largely by a deposit of pitch between

the knots and the laoarcl proper. This deposit is

of appreciable thickness, and if it is removed,

the knots fall out of the board. Old-growth

Douglas-fir, for example, characteristically has

many encased or black knots. If temperatures

of 160° to 180° F. are used to kiln-dry such

lumber, the resin becomes so soft that it runs out

from around the encased knots, which drop out.

At a drying temperature of 125° or lower, relatively little resin will soften and run out.

Chipped and Torn Grain

During the machining of dry lumber, wood

may be chipped and torn from scattered areas on

the machined surface (fig. 121). The occurrence of chipped and torn grain is influenced

largely by the operating condition of the machine; the sharpness and setting of the knife;

the rate of feed into the machine; and the slope

of the grain in the wood, including grain variations. To some extent, however, the susceptibility of lumber to chipping and tearing is affected

t-sea*»-*

;.. ':s .... j.

M-S4986-r

FIGURE 121.—Chipi)ed grain in elm.
DRY KILN OPERATOR'S MANUAL 155

by the moisture content of the wood layer being

removed. Lumber of extremely low surface moisture content—5 percent and less—chips and tea,rs

more during machining than if the surface moisture content is 8 percent or higher. Consequently

kiln operators can, to some extent, help prevent

this trouble by avoiding overdrying and by increasing the surface moisture content with a

conditioning treatment.

Raised Grain

Eaised grain (fig. 122) occurs primarily when

lumber is not uniformly dried to a low enough

moisture content at the time of machining. Generally, raised grain does not develop in wood

that is machined while at a moisture content of

12 percent and less. When wood is machined at

a higher moisture content, the action of the knives

forces the summerwood bands into the springwood bands on the flat-grained surface. Subsequently, the compressed springwood recovers and

lifts the bands of summerwood above the surface.

The uneven surface produced usually reduces the

grade and usefulness of the finished product.

Casehardening

Whether casehardening is considered a defect

or not depends upon the manner in which the

lumber is sawed or machined during fabrication.

The more common fabrication difficulties that occur in the use of casehardened lumber are end

checking, planer splitting, and warping.

End Checking.—End checks will frequently develop in the core of a freshly crosscut casehardened board that is exposed to low atmospheric

relative humidity, even though the average moisture content of the board is fairly low. The tensile stresses present in the core, coupled with additional stresses brought about by end drying, exceed the strength of the wood, which therefore

checks. These checks may develop into splits.

Planer Splits.—Splits can occur in relatively

flat casehardened boards that are being surfaced.

The splits are caused by the internal drying

stresses in the boards, coupled with the forces

applied by the machine knives. As mentioned

M-97880-F'

FIGURE 122.—Raised grain in Douglas-fir.

M-111992-F

FiGUEE 123.—Distortion caused by casehardening. Left

to right—resawed stock ; heavy machining on one

face ; and grooved.

before, planer splits due to casehardening will

be appreciably reduced by a conditioning treatment.

Warping.—If during any sawing or machining operation on a casehardened board the transverse or longitudinal stresses are unbalanced, distortion will result. Eesawing may cause cupping

or bowing. The concave faces will be toward the

saAv (fig. 123). Ripsawing may result in crook,

the concave edges usually being those along the

saw cut. In planing, the depth of cut is not

likely to be the same on both faces; so if the

board is casehardened, it will cup, and the concave face will be the one most heavily cut (fig.

123). "Wlien casehardened stock is edgegrooved,

the lips of the groove may pinch inwards (fig.

123). A tongue or spline inserted into such a

groove may break the lips. "WHien machining

casehardened stock to patterns, as in the manufacture of molding and trim, or in routing and

carving operations where unequal cuts are taken

from the faces and edges, cupping usually results. Any warping of casehardened stock that

is due to sawing or machining is a source of

trouble in fabrication and gluing.

The relief of casehardening by a conditioning

treatnient is strongly recommended for lumber

that is to be used in furniture, architectural

woodwork, sash and door stock, and other products, \yhich may require sawing and machining

operations that may unbalance the casehardening

stresses.
156 AGRIiCULTURE HANDBOOK NO. 18 8, U.S. DEPT. OF AGRICULTURE

Drying Defects of Major Concern in

Commercial Woods

All woods are subject to drying defects. In

attempting to liold drying defects to a minimum,

the final use of the stock should be considered,

since some defects can be tolerated for certain uses.

It is the kiln operator's responsibility to develop

and use drying procedures that fit the specific requirements. Use of procedures to obtain defectfree kiln-dried stock when the final use permits

some defects would, of course, be poor practice.

The operator must also keep in mind that procedures to reduce or prevent one defect may aggravate or cause others. Therefore, he must follow a course of action that will result in the

smallest amount of degrade.

A list of commercial woods and the major defects occurring in drying them follows. Certain defects are excluded for various reasons.

For example, boxed-heart split is not listed because this defect is very likely to occur in boxedheart material of all species. Casehardening is

excluded; it is not always considered a defect.

All types of warp are excluded; warp is controlled iDy stacking procedures rather than by

drying schedules. Other defects associated with

shrinkage are omitted because their severity is

related more to thickness of material than to

species.

SOFTWOODS

Species Major drying defects

Baldcypress End checks. Water pockets.

Cedar:

Alaska- Failure to set resin.

Eastern red- Checking in and around knots.

Excessive loss of aromatic

oils.

Incense- Collapse. Surface checking in

flat-sawed 12/4 or thicker

stock.

Port-Orford- Failure to set resin.

Western redcedar Collapse and honeycomb, particularly in sinker stock.

Douglas-fir: coastal, in- Loose and checked knots. Endtermediate, and check penetration resulting

mountain types. in internal failures in thick

stock. Ring failure in quarter-sawed lumber.

Fir:

California red End and surface checks. Water

pockets.

White End and surface checks. Water

pockets.

Hemlock:

Eastern End and surface checks. Opening of ring shakes.

Western Water pockets. Chemical

stain.

Larch, western End and surface checks.

SOFTWOODS—Continued

Species Major drying defects

Pine:

Eastern white Brown stain. Water pocket.

End and surface checks in

8/4 and thicker stock.

Lodgepole Checked knots. Checks and

end splits in 8/4 and thicker

stock.

Ponderosa Brown stain. Checked knots.

End and surface checks in

8/4 and thicker stock.

Southern yellow End and surface checks in 8/4

and thicker stock.

Sugar: light and sinker. Brown stain. End and surface

checks in 8/4 and thicker

stock.

Western white Brown stain. End and surface

checks in 8/4 and thicker

stock.

Redwood:

Light and medium End and surface checks in 8/4

and thicker stock.

Sinker Collapse and honeycomb.

HARDWOODS

Alder, red Sticker marking.

Ash:

Black Surface checks, particularly in

6/4 and thicker stock.

White End and surface checks.

Honeycomb, particularly in

6/4 and thicker stock.

Aspen Collapse in "wet wood." Water

pocket.

Basswood, American Brownish chemical stain.

Beech, American End and surface checks.

Birch;

Paper End checks. Brownish chemical stain.

Yellow:

normal End and surface checks.

mineral streaked Collapse and honevcomb.

Cherry, black Honeycomb in 8/4 or thicker

stock; surface checks.

Chinkapin, golden End and surface checks. Collapse.

Cottonwood, eastern Water pocket.

Dogwood, flowering End checks. Chemical stain.

Sticker stain.

Elm, American End and surface checks in 8/4

and thicker stock.

Hackberry End checks.

Hickory End checks. Chemical stain

(pinking).

Laurel, California End checks.

Locust, black End and surface checks.

Madrone, Pacific End and surface checks.

Mahogany End and surface checks in

thick stock.

Maple, sugar:

normal End checks in 8/4 and thicker

stock. Honeycomb, usually

associated with end-check

penetration, in thicker stock.

Chemical stain.

mineral streaked Checking, collapse, and honeycomb.
DRY KILN OPERATOR'S MANTTAL 157

HAEDWOODS-Continued

f\ u. Species Major drying defecta

California black End and surface checks.

Honeycomb.

Red End and surface checks.

Hone3'comb, particularly in

8/4 and thicker stock.

White End and surface checks.

Hone}^comb, particularly in

8/4 and thicker stock.

Persimmon, common End and surface checks.

Chemical stain.

Sweetgum (heartwood)___ Honeycomb, collapse, and water pocket.

Sycamore (heartwood) Surface checks and honeycomb.

HARDWOODS—Continued

Species Major drying defects

Tanoak End and surface checks.

Honeycomb.

Tupelo, water End checks. Honeycomb and

collapse, particularly in 8/4

and thicker heartwood stock.

Walnut, black End checks. Honeycomb, usually associated with endcheck penetration in 6/4 and

thicker stock.

Willow, black End checks; honeycomb.

Yellow-poplar End checks in 10/4 and thicker

stock.

555315 O—61- -11

CHAPTER 10. OPERATING A DRY KILN

A dry kiln, no matter how well equipped with

controls, is only as efficient as the operator who

runs it. In the final analysis, it is the operator's

judgment that determines whether a charge of

lumber will go through the kiln in minimum

time and emerge uniformly dried to the desired

moisture content and free of undesired stresses

and defects. It is the operator who selects, prepares, and uses kiln samples, who decides what

drying schedules to use, who manipulates the

controls as drying progresses, who applies and

modifies drying scliedules to suit the stock in the

kiln, who applies equalizing and conditioning

treatments, and who makes the final necessary

determinations of drying quality. Some of the

basic techniques are discussed elsewhere in this

manual.

In this chapter, certain aspects of kiln operation are presented to guide the operator in exercising good judgment as to when and how to

reach the necessary decisions he needs to make

while a charge is being dried. Such judgment,

of course, must be predicated upon adequate

knowledge of wood and how it dries as well as

on the characteristics and capabilities of the kiln

being operated.

Compartment Kiln Operation

A compartment kiln is loaded fully at one

time, and the entire charge remains in the kiln

until all the Imnber has been dried to the desired moisture content. For best results, the drybulb temperature of the entering air must be

uniform throughout the kiln.

The two types of drying schedules used in

compartment kilns, those based on the moisture

content of the stock and those based on time, are

discussed in chapter 8. Kiln samples are not

required when a time schedule is followed,

though many operators use them to check the

performance of their kilns and the final moisture

content of the stock. A drying schedule based

on moisture content—which requires kiln samples—is generally used when drying hardwoods.

Kiln Samples

The selection, preparation, and use of kiln samples are important in kiln operation, and their

neglect can result in erroneous moisture content

values that may cause increased kiln degrade or

drying time and improperly dried stock. Instructions in chapter 6 on kiln samples should

be followed as closely as possible.

Selecting a Drying Schedule

The drying schedule is selected by the operator after careful consideration of the stock to be

dried, as described in chapter 6, p. 99. This consideration includes species, thickness, moisture

content, heartwood or sapwood, quarter-sawed or

flat-sawed, and density. Selection of a drying

schedule is obviously simplified when the charge

consists of one species of one thickness and a

fairly uniform moisture content, and is either all

heartwood or all sapwood and entirely quartersawed or flat-sawed.

Schedules for Homogeneous Charges.—For

charges that consist entirely of one class of stock,

a schedule recommended for that class in chapter

8 should be used to start with. After experience

is gained with that particular class, the schedule

can be modified, as discussed in chapter 8.

Schedules for Mixed Charges.—In selecting a

drying schedule for a mixed charge of lumber,

consider the drying characteristics of all the stock.

Mixed charges are usually dried according to

moisture content schedules. The following examples and suggestions should be helpful in selecting a drying schedule for mixed charges. In

all of these examples, the schedules recommended

in chapter 8 are used.

Example 1.—If a charge of lumber is composed

of the same species and moisture content but of

varying thicknesses, use the schedule recommended for the thickest stock. For example, if

the kiln charge is composed of 6/4 and 8/4 sugar

maple, follow the drying schedule T5-C2 rather

than T8-C3. If the charge is composed of 4/4,

5/4, and 6/4 sugar maple, schedule T8-C3 could

be used, and the changes in drying conditions

would be based on the kiln samples having the

highest moisture content.

Example 2.—If two or more species of the

same thickness and moisture content are dried

together, use the schedule recommended for the

species that is the most difficult to dry. Make

every effort to mix species that require much the

same drying schedule and about the same drying

time. Examples are 4/4 white ash and 4/4 black

cherry. Both call for the same drying schedule,

T8-B4. Several species have approximately the

159
160 AGiRiIiCULTURE HANDBOOK NO. 188, U.S. DEFT. OF AGRICTJLTTJEE

same drying characteristics. These include 4/4

yellow birch, schedule T8-C4; 4/4 black cherry,

T8-B4; and 4/4 sugar maple, T8-C3. They

have the same temperature schedule—T8—but

their wet-bulb depression schedules are different. Since the mildest drying condition is recommended, use the C3 wet-bulb depression schedule. The penalty in drying time or drying degrade connected with the mixing of species with

widely différent drying characteristics, such as

green 4/4 red oak and 4/4 sugar maple, would be

severe. The oak requires a much milder drying

schedule, T'J-D2, than the sugar maple, T8-C3.

Also, the drying time required for the oak is almost twice that required for the maple. Avoid

such mixtures.

Example 3.—Another example of a mixed

charge is tAvo species or more of the same thickness but of varying moisture content, such as a

mixture of green 4/4 black cherry and air-dried

4/4 sugar maple having an average moisture

content of 25 percent. Green black cherry calls

for schedule T8-B4 and green sugar maple for

schedule T8-C3. The air-dry sugar maple with

a moisture content of 25 percent calls for initial

drying conditions of 150° F. dry-bulb temperature (step 3 of T8 schedule) ancl a wet-bulb depression of 35° (step 5 of C3 schedule), whereas

the T8-B4 schedule for the green black cherry

calls for an initial dry-bulb temperature of 130°

and a wet-bulb depression of 7°. To avoid damage to the green cherry, use the milder T8-B4

schedule.

These examples of mixed kiln charges are only

a few of the many that may occur in commercial drying. However, they serve to illustrate

two points, (1) that the drjáng of mixed charges

usually results in either excessive drying time,

nonuniform final moisture content, or-excessive

drying degrade on some of the stock, and (2)

that the selection of the drying schedule depends

on the drying characteristics of the item in the

charge that is most difficult to dry.

An operator must exercise good judgment in

weighing the quality against the quantity of kilndried stock produced and also consider the total

value of the various classes of stock composing

the kiln charge. In some cases it may be economical to choose the faster schedule and take

some kiln degrade.

Starting the Kiln

Steam-Heated Kihvs.—Drying defects and drying time can be reduced if the proper starting

procedures are used. These vary somewhat between kilns equipped with automatic control instruments and those manually operated.

Autom/itically Controlled Kilns.—Generally the

starting procedure for automatically controlled

kilns is as follows:

(1) Set the dry- and wet-bulb control indicators or pointers of the control instrument at the

initial temperatures called for by the drying

schedule.

(2) Keep the hand valve on the steam-spray

feed line closed during the warmup period to

avoid excessive steam consumption and condensation of vapor on the lumber. If there is no

hand valve on the steam-spray feed line, set the

wet-bulb temperature indicator to the lowest temperature on the instrument chart to prevent opening of the automatic spray valve. An alternate

procedure for adjusting the wet-bulb indicator

is described in item 12.

(3) If the kiln is of the forced-circulation

type, open the small inspection doors or leave

the main door partly open. Start the fans or

blower. Close the doors after the fans have operated for several minutes.

(4) Open the hand valve on the main steam

supply line.

(5) Open the hand valves on the feed lines

to all the heating coils.

(6) Open the hand valves between all the heating coils and steam traps and in the return lines

from the steam traps to the boiler.

(7) If the control instrument is air operated,

open the blowoff valve on the air filter located

on the air supply line to the control instrument

until the air is free of water and oil.

(8) Open the main air supply valves to the

control instrument and to the air-operated valves

on the heat and spray lines. If the control system is electrically operated, turn on the power

switches.

(9) Blow all the steam traps to the atmosphere

for a short time to remove scale and dirt from

them.

(10) Just before the dry-bulb temperature

reaches that desired, open the hand valve on the

steam-spray feed line, or reset the wet-bulb indicator on the control instrument to the recommended wet-bulb temperature.

(11) If the kiln is equipped with auxiliary

vents, keep them closed during the warmup period until the wet-bulb temperature reaches the

set point.

(12) Some operators, when warming up a kiln

charge of green wood susceptible to checking,

prefer to bring the wet-bulb temperature up in

easy stages rather than follow the procedure

given in steps 2 and 10. For example, if the

initial drying conditions call for a wet-bulb depression of 4° F., they try to maintain this depression during the warmup period. This is done
DRY KILN OPERATOR'S MANUAL 161

by opening the hand valve on the steam-spray

line for short periods of time, or by gradually

raising the wet-bulb indicator if it was initially

set at the lowest point on the chart. Under this

procedure the recorded temperatures on the control instrument must be checked at frequent intervals during the warmup period.

Manually Oferated Kilns.—A manually operated kiln is started as outlined in steps 2, 3, 4,

5, 6, 9, 10, and 11 above. Manual operation requires the use of auxiliary temperature-indicating devices.

Furnace-Type Kilns.—Both direct- and indirect-heated kilns of the furnace type vary widely

in the design of the heating and the air-circulation systems. Kilns of this type with automatic

control of temperatures are generally started as

follows :

(1) Set the dry- and wet-bulb control indicators or pointers of the control instrument at

the initial temperatures called for by the drying

schedule.

(2) If the kiln is of the forced-air-circulation

type, open the small inspection doors or leave the

main door partly open. Start the fans or blowers. Close the doors after the fans have operated for several minutes.

(3) Start the furnace according to the instructions given by the furnace manufacturer.

(4) If the kiln is equipped with auxiliai-y

vents, keep them closed during the warmup period.

(5) If the kiln is equipped with steam or

water sprays, keep them shut off until the drybulb temperature has almost reached the initial

setting. In warming up a charge of wood susceptible to checking, the operator may prefer to

raise the wet-bulb temperature in easy stages by

following the procedure outlined above for a

steam-heated kiln.

Spray During Warmup

Many dry-kiln operators use both heat and

steam spray to "warm up" a charge of lumber.

This procedure will reduce to some extent the

time required for warmup, but the potential ill

effects connected with it may more than offset

the gain in time.

When both the heating and steam-spray systems are on during warmup, a large quantity of

steam is used. The steam consumption may exceed the boiler output and thereby affect the drying conditions in other dry kilns already operating on schedule.

With the steam spray on, condensing of the

excess moisture in the air on the cold lumber,

cold kiln walls and ceilings, and cold metal surfaces within the kiln has several effects. It allows

very little if any drying of the lumber during

the warmup period; instead, the lumber will

sometimes pick up considerable moisture. It may

cause water stain on the lumber and more rapid

deterioration of the kiln structure. Checks may

widen and deepen in surface-checked lumber.

Time l\eeded to Warm Up Charge

The time required to attain the desired drying

conditions for a charge may vary from 1 to 24

hours or more. Warmup time is lengthened if

(1) the temperature of the lumber and the kiln

structure is low; (2) the temperature of the outside air is low; (3) the initial moisture content

of the lumber is high; (4) heat losses through

the kiln walls, vents, roof, and around the kiln

doors are excessive; (5) if some of the heating

coils are inactive; and (6) if the boiler output

is too low or the heating system is not functioning properly.

Long warmup periods will, of course, be more

costly with short drying cycles than with long

ones. For example, a Avarmup period of 12 hours

will increase the overall drying costs more on a

3-day drying cycle than on a 20-day drying cycle.

Operating Kiln After Warmup

Beduction of Radiation.—About 1 hour after

the kiln reaches the desired drjáng conditions,

start to reduce radiating surface, steam pressure,

or both. This is done by closing the hand valves

in the feed and drain lines to some of the heating

coils or by adjustmg the steam pressure regiilator.

The usual procedure in reducing radiation is to

cut off the larger heating coils firet and gradually work down to the smallest coil that will

maintain the desired dry-bulb temperature. This

procedure should be followed unless past experience has shown how much radiation is required

to maintain the desired temperature. Note on

the recorder chart when and what coils are turned

off and the steam pressure used. These notations

will be helpful in the drying of subsequent kiln

charges of similar material.

Control of Dry-Bulb Temperature.—Variations

in dry-bulb temperature on the entering-air side

of the loads are the greatest source of poor control of drying conditions. These variations are

sometimes associated with faulty kiln design, but

they are usually attributable to poor maintenance

(chs. 2 and 4). The most common cause of temperature variations is heat radiation in excess of

that needed. Excessive radiation results in large

temperature cycles and waterlogging or air binding of the active heating coils, and these in turn
162 AGMCULTURE HANDBOOK NO. 188, U.S. DEFT. OF AGRICULTURE

cause excessive temperature variations along the

length of the coils. To reduce these effects, operate the kiln on the smallest amount of radiation

and the lowest steam pressure necessary to maintain the desired dry-bulb temperature at any

stage of drying.

If the subdivision of the coils is not fine

enough and the steam pressure cannot be regulated, the kiln may have to be operated at a drybulb temperature slightly below the desired one

in order to obtain a nearly constant flow of steam.

In that event, the wet-bulb temperature will also

have to be adjusted slightly downward so as to

obtain the desired wet-bulb depression.

In a manually operated kiln, temperatures can

be controlled by reducing or increasing the radiating surface and by regulating steam pressures

through the use of reducing valves. Though

globe valves are sometimes used for regulating

steam pressure in the coils, they are not recommended. A gate valve should never be operated

partly opened (throttled), because this results in

"wiredrawing." Wiredrawing eventually erodes

the entire gate, making the valve unfit for use.

Control of Wet-Bulb Tempe7'atu7-e.—Generally,

poor control of wet-bulb temperature is associated with inadequate kiln maintenance (ch. 4).

Quite frequently, however, the use of a highpressure steam spray causes wide variations in

both the dry- and wet-bulb temperatures. The

use of wet low-pressure steam should overcome

this difficulty. If the reduction in pressure does

not have the desired effect, desuperheaters may

have to be installed on the steam-spray lines.

Some kiln operators inject water into the steamspray line. The flow of water should not be excessive and may be controlled by a needle valve.

To make this possible, the water pressure must

be greater than the steam pressure. Ordinarily

water is used to saturate the steam spray only

during the equalizing and conditioning treatments.

Proper venting is also required to obtain good

control of wet-bulb temperatures. In kilns

equipped with automatic vents, such control is

attained by good maintenance and operation of

the vent system (ch. 4). In kilns with manual

vent control, the operator should regularly observe the wet-bulb temperatures and open or close

the vents when required. Excessive venting will

add to steam consumption and favor the development of drying defects. On the other hand, operating the kiln for extended periods with insufficient venting and at wet-bulb temperatures above

those called for by the schedule will prolong

drying time.

Some drying schedules call for low wet-bulb

temperatures at certain stages of drying. An example of this is the schedule for green 4/4 red

oak (T4-D2). When this stock reaches a moisture content of 25 percent, the recommended wetbulb temperature is 80° F. Occasionally, in some

areas and at certain times of the year, the wetbulb temperature of the outside air may be above

80°. When this occurs, the wet-bulb,temperature

in the kiln cannot be reduced to 80° regardless

of the amount of venting, and the kiln must be

operated at the lowest wet-bulb temperature attainable. Do not raise the dry-bulb temperature

above that called for in the schedule.

Full-Time or Part-Tims Operation.—Full-time

operation is the usual practice in industry, but

some plants, particularly secondary producers,

operate kilns part time. Under full-time operation, drying is uninterrupted from the start to

the finish of the process. In part-time operation,

the kiln is usually shut down during night hours,

over the weekend, and on holidays. Part-time

operation is generally practiced when fuel and

power costs are high and kiln capacity is greater

than required.

Most woods, particularly air-dried stock can

be dried under part-time operation. However,

equalizing and conditioning treatments must usually be given under full-time operation in order

to be successful. On green, refractory hardwoods,

part-time operation during the initial stages of

drying may result in excessive defects such as

checking, because of a more rapid drop in the wetbulb than in the dry-bulb temperature during the

off-time period. When this occurs, operate the

kiln on a full-time basis until the danger of checking is past. Keep the vents closed during the offtime period to reduce heat losses from the kihi.

The Drying Process

After the kiln has been started, the lumber is

dried in accordance with the schedule selected

(ch. 8). Near the end of the drying process,

equalizing and conditioning treatments are applied as required, depending upon the final use

of the lumber. Final tests should be made on the

stock before it is pulled from the kiln.

Operation on a Moisture Content Schedule.-^

A moisture content schedule requires changes m

drying conditions based on the average moisture

content of the controlling kiln samples (ch. 6,

p. 99). Operation on a moisture content schedule is best illustrated by examples.

Example 1.—A charge of green 4/4 sugar

maple lumber is to be kiln-dried. Six kiln samples are used so that the average moisture content of the three wettest samples will govern the

drying conditions. The drying schedule for this

charge is given in table 35, and the schedule is

applied as follows :

Since the lumber is green, the initial moisture

content will be above 40 percent. Therefore, the

initial drying conditions will be those given for

this moisture content.
DRY KILN OPERATOR'S MANUAL 163

TABLE 35.—Moisture content schedule for 4/4

sugar maple {T8~CS)

Tem- Wet- Moisture Dry- Wet- Wetperature bulb de- content bulb bulb de- bulb

step pression at start temper- pres- temstep of step ature sion perature

No. No. Percent op "F. op

1 1 Above 40 130 5 125

1 2 40 130 7 123

1 3 35 130 11 119

2 4 30 140 19 121

3 5 25 150 35 115

4 6 20 160 50 110

5 6 15 180 50 130

Subsequent changes in drying conditions are

made when the average moisture content of the

controlling samples reaches the values given in

the schedule. For example, when the average

moisture content of the three wettest samples is

less than 30 percent but more than 25 percent,

the dry-bulb temperature is 140° F. and the wetbulb temperature is 121°. Because the drying

rate of the kiln samples may vary from day to

day, the same three samples may not be the wettest during all stages of drying. Therefore, the

moisture content of all the samples in the charge

should be calculated each time they are weighed

(ch. 6, p. 104). The last drying conditions are

maintained until the desired moisture content has

been reached.

Occasionally the controlling kiln samples lose

more moisture between weighings than the interval given in the schedule. When this occurs, a

step in the schedule can be skipped. For example, if the kiln is operating at 130° F. and 119°

dry- and wet-bulb temperatures, respectively, and

the next weighing indicates that the average

moisture content of the controlling kiln samples

is 24 percent, the drying conditions should be

set at a 150° dry-bulb and a 115° wet-bulb temperature, rather than at 140° and 121°. In some

mstances, even two steps can be skipped.

As soon as the final moisture content is reached,

the kiln is shut off unless equalizing and conditioning treatments are required (ch. 8, p. 142).

Example 2.—A charge of partially air-dried

4/4 sugar maple is to be kiln-dried. Eight kiln

samples are to be used. Therefore, drying conditions will be governed by the average moisture

content of the four wettest samples. The drying

schedule will be the same as that used in example 1, and the procedure will be as follows:

If the initial moisture content of the four wettest samples averages more than 40 percent, the

initial drying conditions will be those listed for

this moisture content. Subsequent drying procedures will be the same as for example 1.

If the average moisture content of the four

wettest samples is 34 percent, the initial drying

conditions will be 130° F. and 119°. Subsequent

drying conditions will be as given in the schedule.

If, however, the stock has regained moisture

just before entering the kiln, modify the drying

procedure to conform to the recommendations

given for air-dried hardwoods, chapter 8, p. 124.

Operation on a Time Schedul-e.—Under a time

schedule, the drying conditions are changed at

predetermined peri(xls of time. A time schedule

for 4/4 white fir (AS7-AK7) is given in table 36.

The kiln is started at 145° F. dry-bulb and

128° wet-bulb temperature and these temperatures are maintained for 12 hours.

TABLE 36.—Time schedule for J^jJ,. whiteßr

ÍAS7-AK7)

Tem- Wet- Drv- Wet- Wetperature bulb de- Time buib bulb de- bulb

step pression temper- pression temperstep ature ature

No. No. HTS. °F. "F. °F.

1 1 0-12 145 17 128

2 2 12-24 150 23 127

3 3 24-36 155 30 125

4 4 36-48 160 30 130

5 5 48-60 170 35 135

6 6 60-72 180 43 137

After 12 hours of drying, the kiln temperatures

are changed to those of the second temperature

step; further changes are made every 12 hours

as shown.

After a total drying time of 72 hours, the

charge is pulled from the kiln if equalizing and

conditioning treatments are not required.

Kiln samples are sometimes used in conjunction with time schedules to obtain information on

kiln performance. They are particularly useful

in determining final moisture content. If the

samples indicate a consistently high or low moisture content, the duration of the final temperature step should be changed to produce the desired final moisture content.

Final Wet-Bulb Tem,perature.—The control of

wet-bulb temperature is not critical during the

final stage of drying even though the maximum

wet-bulb depression given in the moisture content

schedules (table 10, p. 119) is 50° F. The hand

valve on the steam-spray feed line should be

closed. In kilns equipped with automatic vent

control, the wet-bulb temperature indicator on the

control instrument should be set to obtain a 50°

wet-bulb depression. If the vents are manually

operated, they should be manipulated to maintain

a wet-bulb depression of at least 50°.
164 AGBXCULTURE HANDBOOK NO. 18 8, U.S. DEPT. OF AGRICULTURE'

Intermediate Moisture Content.—Near the final

stage of drying, the moisture content of the stock

must be known within fairly close limits, or the

stock may be dried to a diiierent moisture value

than desired. Furthermore, equalizing and conditioning treatments, if required, will not be successful if the moisture content of the kiln samples

is in error. The original calculated ovendry

weights of kiln samples cut from green or partially dried stock are sometimes in error. To

minimize trouble of this nature, an intermediate

moisture content determination of the kiln samples is made as the final stage of drying is approached. The purpose and method of making

such a determination is described in detail in

chapter 6, p. 105.

Intermittent Steaming.—Some kiln operators

steam charges of lumber at or near saturation at

intervals throughout the drying process, believing that it speeds up moisture movement and

reduces drying defects. On the contrary, such

steaming is likely to increase drying time. It

can also result in more severe end and surface

checking, possible internal failures, and increased

warping.

Sterilizing Treatment

The growth of mold and fungal stains can be

stopped by a sterilizing treatment as described

in chapters, p. 141.

Equalizing, Conditioning Treatments

Good moisture quality and stress-free stock

can be obtained by equalizing and conditioning

treatments described in chapter 8, p. 142. To get

good results from them, the following details

should be observed.

Equilihrhim Moisture Content Table.—In order

to apply the equalizing and conditioning procedures, an operator must know how to determine the wet-bulb depression needed to give the

required equilibrium moisture content (EMC)

condition. Equilibrium moisture content values

are given in table 2, p. 11. In the example presented here, however, the use of this table is the

reverse of the explanation given in chapter 1.

Assume, for example, that a dry kiln is operating

at a dry-bulb temperature of 170° F. and it is

desired to know what wet-bulb temperature is

needed in order to obtain an EMC of 6 percent.

The dry-bulb temperature of 170° is found in

the lefthand column. In the line to the right

of this temperature, the EMC of 6 percent is

found (in italic type) in the column indicating

a wet-bulb depression of 29°. Tlierefore, to obtain an EMC of 6 percent at a dry-bulb temperature of 170°, a wet-bulb temperature of 170°

minus 29°, or 141°, would be used.

General Considerations.—

(1) The recommended procedures for equalizing and conditioning a charge of lumber will

produce good results in a compartment-type Mln

that is performing satisfactorily, but one factor

must not be overlooked; the control instrument

must be in calibration (ch. 4, p. 71). If because

of poor calibration the wet-bulb depression in the

kiln is different than the recommended setting

on the instrument, the EMC condition in the kiln

will not be correct and the treatments may not

give satisfactory results.

(2) An equalizing treatment is not necessary

if the driest and wettest kiln samples at the end

of the drying process have moisture contenta

within the permitted range.

(3) Some operators prefer drying the driest

samples in the kiln charge to a moisture content

1 percent lower than the value recommended

in table 33, p. 143, before starting equalization.

This might reduce equalizing time, and it might

even eliminate the equalizing treatment.

(4) If the recommended EMC value for conditioning at a specific temperature cannot be

found in table 2, use the next highest value given

in the table for that temperature. For example,

it is desired to condition a charge of lumber at

170° F. with an EMC condition of 11 percent.

Referring to table 2, no wet-bulb depression is

given for an EMC condition of 11 percent at a

temperature of 170°. Use the next highest

value—11.3 percent. The wet-bulb depression for

the 11.3 percent EMC condition is 10°.

Conditioning Time.—High dry-bulb temperatures coupled with high EMC conditions hasten

the deterioration of dry kiln buildings and metal

within the kiln. Also, large quantities of steam

are required during the conditioning treatment.

Therefore, do not extend the treatment beyond

the time required for the relief of casehardening.

Conditioning time depends upon the degree of

stress in the lumber, its species, thickness, and

moisture content, and the kiln's performance. It

may vary from 4 hours for 1-inch softwoods to

48 hours or more for the thicker hardwoods. The

minimum time required is determined by making

prong tests at intervals when it is believed that

casehardening has been nearly relieved. Information so collected will be a guide for estimating

the conditioning time for later charges of similar stock. The prong test is described in chapter 6, p. 105.

Wlien air-dried lumber is kiln-dried, the conditioning time varies from charge to charge, because the degree of casehardening in the air-dried

material varies. Casehardening tests made on

air-dried stock at the time the kiln samples are

prepared will give an idea of the time required

for conditioning.
DRY KILN OPERATOR'S MANTJAL 165

Conditioning Temperature.—The higher the

dry-bulb temperature used during the conditioning treatment, the faster will be the relief of

casehardening. Generally, the required conditioning EMC can be obtained at a dry-bulb temperature of about 180° F. in most well-kept kilns

operated on low steam pressure and equipped

with a desuperheater on the spray line or with

auxiliary water sprays. Sometimes it is impossible, however, to obtain at very high temperatures the rather high EMC conditions that are

required.

If the required EMC cannot be obtained at a

dry-bulb temperature of about 180° F., the temperature will have to be reduced. In such instances, lower the setting on the control instrument 12 to 24 hours before conditioning is started.

For example, assume the kiln is operating on a

dry-bulb temperature of 180° and the temperature must be reduced to 170° to obtain the desired EMC. Twelve to twenty-four hours before

treatment is started, set the control instrument at

a dry-bulb temperature of 170°.

Casehardening Relief at High EMC.—To reduce the time required for conditioning, some kiln

operators use an EMC considerably higher than

that recommended for the treatment. This approach may be satisfactory if the treatment is

not continued for too long a time. If the treatment is too long, reverse casehardening—a condition equally as serious as casehardening—will

result. No satisfactory method of relieving reverse casehardening has been established. In

many instances the use of very high equilibrium

moisture contents during conditioning gives only

superficial relief of the drying stresses. Therefore, to obtain good conditioning without incurring risk of reverse casehardening, the conditioning treatment should be given at the recommended EMC (table 33, p. 143).

Moisture Content, Casehardening Tests

Kiln samples are generally used for final moisture and stress tests to make sure that the lumber

has been dried to the desired moisture content

and is free of casehardening. Other boards from

the kiln charge can also be used.

Method of Testing.—To properly interpret the

casehardening test sections, certain information

concerning the final moisture content and moisture gradient is essential. The method of cutting

sections for such tests is given in chapter 6, p.

105. One section should be weighed immediately

after cutting, placed in a drying oven, dried to

constant weight, reweighed, and its moisture content calculated (ch. 6). This calculation will

give the average moisture content of the kiln

sample or board from which the section is cut.

If this test is made immediately after the conditioning treatment, the moisture content obtained will be about 1 to li/o percent high, because surface moisture has been regained during

the conditioning treatment. If, however, the

test is made after the stock has cooled for about

24 hours, the regained surface moisture will have

evaporated.

A second section should be cut as shown in

figure 95, p. 105, to get an outer shell having a

thickness of about one-fourth the total thickness

of the stock, and a core. The core and shell are

weighed separately as quickly as possible after

cutting, placed in a drying oven, dried to constant weight, reweighed after drying, and their

moisture content calculated. These tests will

give the moisture content of the shell and core.

Section 3 should be cut into prongs (fig. 96, p.

106). If the stock is less than 6/4 inches in thickness, three prongs of equal thickness are cut and

the center prong removed; if the stock is 6/4

inches or greater in thickness, six prongs of

equal thickness are cut and the prongs next to

the outside prongs are removed.

Evaluation of Casehardening.—When you

judge that casehardening has been relieved, shut

off the kiln and remove and test some of the kiln

samples or other boards. If, at the time of sawing, the outer prongs of the test section turn

away from the saw a distance about equal to

the thickness of the prong or slightly more, the

stock is usually free of casehardening and the

charge can be pulled. If. however, the outer

prongs remain straight or pinch the saw, the

stock is casehardened ; continue the conditioning

treatment until tests show satisfactoiy relief.

After the preliminary evaluation is made,

room-dry the test sections for about 24 hours.

The following conditions may then be observed.

(1) The outer prongs have turned in considerably. This indicates that the stock is still casehardened; lengthen the conditioning time for

subsequent charges of the same material.

(2) The outer prongs are straight. This indicates that the lumber is free of casehardening;

use the same conditioning time for subsequent

charges of the same material.

(3) The outer prongs have turned out considerably. This indicates that the stock is reverse

casehardened. Shorten the conditioning time for

subsequent charges of similar material.

Modifying Schedules for Compartment

Kilns

Perhaps one of the greatest causes for excessive

drying cost is blindly following a recommended

kiln-drying schedule. No drying schedule will

produce the best drying results on a specific item
166 AQBICULTURE HANDBOOK NO. 18 8, U.S. DEFT. OF AGRICULTURE

or species of wood in all types of kilns and under all types of conditions. To obtain the best

results, some modification of the recommended

schedules is usually necessary.

Before modifications can be made, the recommended schedule must be tried. By observing

(1) the type and severity of drying defects, their

time of occurrence, and their effect on degrade;

(2) the drying time required; and (3) the final

moisture content, information is obtained by

which schedules can be modified. Schedule modifications are described in chapter 8, p. 124.

Drying Defects.—Defects associated with kilndrying are described in chapter 9. Most of these

defects can be detected by periodic examination

of kiln samples during the drying process.

Knowledge of drying defects, such as their

cause, time of occurrence, and how they can be

minimized, should help a kiln operator to develop a drying schedule that will give the fastest

and best d.rying.

Drying Time

The time required to dry a charge of lumber

depends on the characteristics of the wood, the

type of dry kiln, and the drying schedule used.

Ordinarily an operator has no control over the

type of material being dried. He may, however,

be able to improve kiln performance, thereby reducing drying time to some extent. The greatest

reduction in drying time can generally be accomplished through schedule modifications (ch. 8,

p. 124).

Operation of a Progressive Dry Kiln

A progressive kiln can be used with fair to

good results if the stock does not have to be

dried to close moisture content limits and relieved of casehardening. Generally speaking,

this type of kiln should be used only to dry softwoods and well air-dried hardwoods.

Because of the design of a progressive kiln,

the drying schedules recommended in chapter 8

cannot be completely followed.

Application of the following points will simplify the. operation of progressive kilns and assure better drying of lumber:

(1) Arrange the heating system so that the

dry-bulb temperature gradually increases from

the green to the dry end. This temperature difference is usually .50° to 60° F. in kilns 100

feet or more in length. In shorter kilns it is

usually smaller.

(2) Provide thermostatic control of the heating system, so that lower or higher temperatures

can be obtained as needed.

(3) Provide thermostatic control of the wetbulb temperature, so that the wet-bulb de^Dression

at the green end can be changed when necessary.

(4) Do not move the stock into zones of more

severe drying conditions until it has been dried to

a moisture content low enough to withstand those

conditions without developing excessive seasoning defects. To determine when to move the

stock, weigh and examine the kiln samples in

each load at intervals during drying.

(5) Do not pull a truckload of lumber from

the kiln until it has been dried to the desired

moisture content.

(6) Do not attempt to equalize or condition

lumber within the kihi. This will upset drying

conditions throughout the kiln, and the drying

of most of the charge will be extended excessively. Some progressive kiln installations

have auxiliary chambers in which these treatments are given. When lumber has dried to the

desired moisture content in these kilns, it is

transferred to the auxiliary chamber and there

equalized and conditioned in accordance with the

procedures outlined in chapter 8, p. 142.

Pulling a Charge From a Dry Kiln

After lumber has been kiln-dried, it is usually

cooled before it is machined. Some operators

cool it in the kiln, and others remove it from

the kiln and place it in cooling sheds. Either

method is satisfactory. However, lumber kilndried to a low moisture content should never be

stored outdoors or exposed to conditions of high

relative humidity for extended periods of time,

regardless of whether it is stickered or solid piled

(ch. 11, p. 172).

Operating Rules for Safety

Working in or around dry kilns is not hazardous if ordinary precautions are taken. Carelessness may lead to serious or fatal injuries, however. Care must be exercised at all times, and

observance of the safety rules given here will

help prevent accidents :

(1) Shut off the heat, spray, and fans before

entering a kiln in which a charge of lumber is

being dried. If the kiln has been operating at

high temperatures, it should be cooled to some

extent by opening the doors and ventilators before it is entered.

(2) If it is necessary to enter a kiln that has

the heat, spray, and fans on, station someone

outside the door to give assistance if it is needed.

If a guard cannot be stationed at the door when

a person enters the kiln, leave the door open

and hang a sign on it reading "Man Inside Kiln,

Do iSTot Close Door."

Never enter kiln when the wet-bulb temperature is 120° F. or more without wearing protective clothing that covers the head and body.

This temperature limit applies to a person in good
DRY KXLN OPERATOR'S MAJSTUAL 167

health. Anyone afflicted with heart or respiratory aihnents should never enter dry kilns when

the wet-bulb temperature is 110° or more. The

critical dry-bulb temperature depends on the individual. If the kiln atmosphere feels too hot, do

not enter the kiln.

(3) Equip all small access doors in the kiln

with a latch that can be operated from both sides.

Repair faulty latches immediately. Never use

props to holcl a door closed. The wrong person

may use these at the wrong time. Set up an

emergency signal that can be used if a man is

accidentally locked in. A signal rapped on kiln

pipes will carry a considerable distance.

(4) Exercise care in opening and closing large

kiln doors. If they are too heavy for one man

to handle, use two or more men to open them.

(5) Door carriers that are worn or poorly

maintained may slip from the door or jump the

track. Keep them in good repair.

(6) Be on guard against falling or protruding

objects when in a kiln.

(7) Shut OÎÏ the fans when they are to be

inspected or lubricated and lock the fan switch

in an "off" position. If the fan switch is not

provided with a lock, place a sign on the switchbox reading "Do Not Start Fans."

(8) The fan floor of an overhead fan system

may be oily. Be on guard against slipping.

(9) Install good open-type walkways over

track level openings in natural-circulation or underload fan kilns. Keep these in good repair.

(10) Install guards around shafts and pulleys.

(11) If truckloads of lumber are pushed into

and out of the kiln by men, make sure the men

have good footing and that they will not be

crushed between loads of lumber.

(12) If loaded kiln trucks are moved into and

out of a dry kiln by cable, stay clear of the

cables when they are under tension.

(13) Always carry a flashlight when entering

a kiln, whether or not it is equipped with lights.

(14) Keep the dry kilns and the area around

them free of debris to reduce the tripping hazard.

Prevention of Fire in Dry Kilns

Fires in dry kilns are usually caused by carelessness, improper maintenance, and poor housekeeping. Suggestions that will minimize the possibility of fires follow :

(1) Never smoke in a dry kiln.

(2) Use care with welding and cutting torches.

AVhen they are used for repair work in a kiln,

have a fire extinguisher available for immediate

use.

(3) Keep all electrical circuits in good condition.

(4) Keep all moving parts well lubricated. A

hot bearing may cause a fire.

(5) Never run uninsulated steam pipes

through, or allow them to contact, flammable material.

(6) Install an automatic water sprinkler system in the kilns.

(7) Keep the kiln and the surrounding area

free of all debris.

A fire in a dry kiln should be put out promptly.

Instruct the watchmen to check on the kilns at

regular intervals to detect fires that may occur

when the regular crew is off duty. Be sure they

know what action to take in the event of fire.

The following will at least reduce the spread

of a fire in a kiln, if not extinguish it completely ;

(1) Keep the kiln doors closed.

(2) Close the ventilators.

(3) If the kiln is of the natural-circulation

type, close the fresh-air supply doors.

(4) If the kiln is of the forced-circulation

type, shut off the fans or blower.

(5) Saturate the air in the kiln with steam

by opening the hand valve in the bypass line

around the steam-spray control valve, or set up

the wet-bulb indicator on the control instrument.

CHAPTER 11. STORAGE OF WOOD

Kiln-drying is only one step—although a critically important one—in the harvesting, handling,

and processing of wood. 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,

he needs to know that important sources of difficulty can be the methods used to store logs and

lumber before they reach the kiln, and those used

to store kiln-dried stock and finished products.

The proper storage of wood, from log to finished product, is essential if quality is to be

maintained. Any lowering of quality before the

wood is dried will be reñected in the quality of

the kiln-dried stock. Similarly, good storage is

necessary for kiln-dried stock and finished products because their quality may be improved or

lowered depending on storage conditions.

Log Storage

Logs need to be stored under conditions that

will prevent occurrence of defects associated with

shrinkage or attacks by fungi and insects. Defects associated with shrinkage are at a minimum during periods of precipitation and high

relative humidity. Fungi and insects are inactive at low temperatures.

Logs should be sawed into lumber as soon as

possible after they are taken from the woods,

particularly during warm weather. If prompt

sawing is not practical, two general types of log

storage—pondmg and cold decking—provide reasonable protection for woods-run logs. For highquality logs, other precautions can be taken.

Pond Storage

A log submerged in water is protected completely from drymg defects and from insect and

fungus attack. Of course, some parts of most

logs stored in ponds are above water (fig. 124).

The parts above the water may develop drying

defects, such as end checking, and they are exposed to attack by insects and stain and decay

fungi if stored too long during warm weather.

Frequently logs stored in ponds are banded together (fig. 125). This method increases the

log-holding capacity of the pond, prevents sinker

logs from sinking, and submerges more logs in

each bundle when some sinkers are included.

Some logs, however, may be completely above

water and subject to deterioration during warm

weather.

Cold-Deck Storage

Logs in cold decks (fig. 126) may end check

and be attacked by fungi and insects, especially

during warm weather, when higher temperatures

speed up drying and fungus and insect activity.

Occasionally logs in cold-deck storage are

sprayed with water to keep them wet, especially

during warm, dry weather, thereby lessening the

development of end and heart checks and end

splits. End checks and end splits in logs can

also be reduced by applying a good coating to

the ends of the logs (fig. 127) (i?).^ Losses

caused by stain and decay fungi can be reduced

and often prevented by the application of chemical solutions to the exposed wood (5).

If chemical changes occur in the sap of logs

during storage, they may cause stains during

subsequent drying. Some of the pines are particularly susceptible to a stain that ranges from

light to dark brown in color. This stain, however, can be lessened considerably by the practice

of "hot logging,"—that is, by sawing the logs

into lumber without delay—and promptly drying the lumber (6). Treatment of the logs or

lumber with the usual antistain solutions does not

prevent chemical stains.

Lumber Storage

There are five types of lumber storage. These

are (1) outdoors; (2) in an open shed; (3) in a

closed, unheated shed; (4) in a closed, heated

shed; and (5) in a conditioned shed. The desirable type of storage to use depends on the

moisture content of the lumber and on weather

conditions during storage.

Outdoor Storage

All kinds of lumber can be stored outdoors.

The storage time, however, varies considerably

for different classes of lumber. Also, certain

classes of lumber must be protected from wetting. The storage area should be open and well

drained, and kept free of weeds and debris that

1 Italic numbers in parentheses refer to Literature

Cited, p. 176.

169
170 AGRICULTURE HANDBOOK NO. 188, U.S. DEPT. OF AGRICULTURE

M-99041-F

FIGURE 124.—Logs stored In a pond. Parts of the logs above the waterline may sustain drying defects and be attacked

by insects and micro-organisms if stored too long in warm seasons.

M-108918-F

FIGURE 125.—Logs banded together in log pond. Some logs are completely submerged, others entirely out of the water.
DRY KILN OPERATOR'S MAMJAL 171

M-74441-F

FIGURE 126.—Cold-decked logs. These logs are subject to checking and insect and fungus attack.

restrict air movement along the surface of the

ground, harbor fungi and insects, and create a

fire hazard when dry. The ground, particularly

along runways for lumber-handling equipment,

should be surfaced with gravel, crushed rock,

blacktop, or concrete. Surfacing or paving permits vehicles to operate efficiently in all sorts of

weather, and tends to restrict weed growth. The

method used to pile lumber on an area is governed by the species involved, its moisture content, and the degree of drying desired during the

storage period.

Green Lumber.—To lessen drying defects and

obtain as much drying as possible, thereby reducing kiln-drying time, the kiln operator should

have knowledge of air-drying procedures {¡i).

Briefly, these include (1) piling the lumber

course by course on dry stickers properly spaced

to minimize warping, (2) providing good pile

bottoms or foundations, (3) laying out the yard

with adequate spacing between piles and rows of

piles, (4) constructing firm piles or packages,

and (5) providing good pile roofs.

If green lumber must be stored in solid piles

for more than 24 hours, particularly during

warm weather, dip it in an antistain solution (5).

Green lumber properly piled and protected on a

good site will lose moisture quite rapidly with a

M;-2161-F

FiGUEE 127.—End checking was prevented in the

half of this sweetgum log by coating it.

right
172 AGBIiCULTURE HANDBOOK NO. 18 8, U.S. DEPT. OF AGRICULTURE

minimum of defects and can remain outdoors

indefinitely without excessive deterioration.

Partly Dried Lumber.—If the moisture content of the lumbel- is above 20 percent, or if further drying is desired, store it as green lumber.

Lumber that is below 20 percent can be solid

piled, if no additional drying is desired. The

piles, however, must be fully pi-otected against

infiltration of water. Water that penetrates a

lumber pile is not readily evaporated, and it is

likely to cause stain and decay. Lumber surfaces

that are alternately wetted and dried are likely

to check.

Kiln-Dried Luviber.—Lumber kiln-dried to a

moisture content of 12 percent or less can be

stored outdoors in dry weather in stickered or

solid piles for a short time. Extended storage

will result in excessive moisture regain. Figure

128 shows the change in moisture content of

southern yellow pine during yard storage in

solid piles in inland Louisiana. If the lumber

had been piled on stickere, its moisture content

would have risen to the maximum of about 131/2

percent in a much shorter time. During the

warm, dry season in areas such as the arid Southwest and in parts of Idaho, Montana, Nevada,

Oregon, and Washington, the outside storage period can be extended considerably without serious effects. It is advisable, however, to cover

piles of high-grade lumber with a roof.

Open Shed Storage

Open sheds provide excellent protection for

green and partially dried lumber. Lumber that

has been kiln-dried to a low moisture content

can also be stored in open sheds for varying

periods of time, depending on the weather conditions.

The shed should be located on an open, welldrained area. It should be large enough to permit rapid handling of the lumber, and have a

floor of gravel, crushed rock, blacktop, or concrete firm enough to support the piles of lumber

and the weight of loaded lumber-handling equipment. Use enough pile supports to prevent the

lumber from warping, and provide at least 6

inches of clearance between the floor and the

bottom of the piles. The roof should overhang

far enough beyond the piles of lumber to protect them from driving rains and snow.

Green Lumber.—Green lumber can be stored

for long periods in open sheds without danger of

serious deterioration, provided it is stickered.

Such sheds protect the lumber from the sun,

rain, and snow, thereby keeping end and surface

checks and splits at a minimum. To obtain

rapid air-drying in open sheds, provide adequate

spaces between the sides and ends of the stickered

piles of lumber.

Partly Dried Lumber.—Open sheds afford excellent protection to partly dried lumber. Pile

240 270 300 330

1931

TIME (DAYS)

390 420 450 480 540

ZM-20463-F

riGURE 128.—Change in average moisture content of kiln-dried southern yellow pine 1- by 4rineh flooring and 1- by

8-inch dressed boards during storage in solid piles within sheds, and in the yard with a protective roof.
DRY KILN OPERATOR'S MAJSTJAL 173

the lumber on dry stickers if it has a moisture

content higher than 20 percent. If it has a

moisture content below 20 percent, it can be

solid piled unless further drying is desired, in

which case it should be sticker piled.

Kiln-Dried Lumber.—Kiln-dried lumber is afforded good protection from sun, rain, and melting snow when stored in an open shed. An open

shed will not, however, prevent the lumber from

absorbing moisture during periods of high relative humidity, especially when air temperatures

are high. Therefore, limit the storage time during warm, humid weather. The lumber piles can

be either solid or stickered. Figure 128 illustrates the effect of long-time storage in an open

shed on the average moisture content of solidpiled kiln-dried lumber. Moisture regain would

have been more rapid in stickered piles.

Closed Shed Storage

Closed sheds, of course, protect lumber from

the elements. The shed should be provided with

reasonably tight-fitting doors. Sometimes closed

sheds are provided with ventilators. The need

for ventilators will depend upon the moisture

content of the material being stored and the tightness of the structure. Closed sheds can be used

for the storage of green or partly dried lumber,

but they are used primarily for air-dried or kilndried stock.

Green Lumber.—Green lumber is sometimes

stored in closed sheds in stickered piles. This

type of storage will, however, retard drying. If

rapid drying is desired, provide intake and exhaust vents and fans to circulate air through the

piles of lumber. The solar heat absorbed through

the roof of a closed shed will raise the temperature within it, particularly near the roof. Therefore, if green lumber that is susceptible to checking must be piled so that the tops of the piles

are close to the roof, cover them with a layer

of dunnage.

Partly Dried Lumber.—Partly dried lumber

that is properly piled can be stored in a closed

shed without developing drying defects. Pile the

lumber on stickers if it has a moisture content

of 20 percent or more; if its moisture content is

below 20 percent, it can be solid piled if no further drying is desired. If further drying is desired, sticker the lumber and install fans to move

the air through the piles. High shed temperatures

should not cause checking or splitting in partly

dried lumber, since these defects usually occur

during the initial stages of drying.

Kiln-Dried ZwmJer'.—Kiln-dried lumber in a

closed storage shed, though ñilly protected, can,

of course, pick up moisture during periods of

high relative humidity, and in such weather the

storage period must be reduced. In arid or semiarid areas, kiln-dried lumber can be stored indefinitely during hot, dry weather. The effect of

long-time storage in a closed shed on average

moisture content of kiln-dried lumber is shown

in figure 128. With this method of storage, as

much moisture is regained as in an open shed.

Stickering the pile would have hastened the

moisture regain.

Moisture Regain in Unheated Sheds

Lumber dried to a moisture content of 10 percent or less, and items manufactured from it,

will regain moisture if stored for extended periods under conditions of high relative humidity.

Excessive regain of moisture frequently results

in (1) swelling of whole pieces or of certain

parts, such as the ends of the pieces; (2) warping

of items, as of glued-up panels for example;

(3) wood or glue line failures in solid-piled

items where the moisture regain is confined to

the ends.

During fabrication and use, lumber and items

that have absorbed excessive moisture during

storage, may (1) end check and split when the

high moisture content surfaces are exposed to

low relative humidities in heated buildings;

(2) shrink excessively; (3) warp; (i) suffer extension of end splits; (5) open at glue joints;

and (6) give difficulty in matching material like

dimension stock or stock run to pattern.

Closed, Heated Shed Storage

Storage in closed, heated sheds provides excellent protection for lumber and special products

that will be used inside heated buildings. Stock

generally stored in heated sheds includes hardwood dimension, furniture stock, interior trim,

flooring, cabinet material, bowling-pin and shoelast blanks, and similar items.

A heated shed should be reasonably tight, but

insulation generally is not required. Unit heaters are commonly used. Since the temperature

normally required in heated sheds is only 10° to

20° F. above the outside temperature, the amount

of heat required is not great. Circulation of the

air in the shed is desirable. Ventilators may or

may not be needed, depending on the moisture

content of the stock being stored, the temperature

maintained, and the tightness of the structure.

The shed should be located on a well-drained

site. Its floor should be of gravel, crushed rock,

blacktop, or concrete, and sufficiently firm to support the piles of lumber and loaded lumberhandling equipment.

555315 0—61- -12
174 AGRICULTURE HANDBOOK NO. 188, U.S. DEPT. OF AGRICULTURE

Green Luniber.—Green lumber is not ordinarily

stored in heated sheds, because the higher temperatures within the shed may cause end and

surface checks and splits. Occasionally, however,

it is sticker-piled in such sheds for drying. AVlien

this is done, the shed, if tightly constructed, must

be provided with ventilators and the air circulated with fans.

Partly Dried Lumber.—Partly dried lumber

can be stored in heated sheds for further drying.

Stickering is necessary. If further drying is not

desired, store this lumber outdoors or in an open

shed.

Kün-Dried Lumber.—Closed, heated sheds are

excellent for the storage of lumber kiln-dried to

a moisture content of 12 percent or less. The desired moisture content of the lumber can be

attained or maintained by regulating, manually

or with a hygrostat (5), the dry-bulb temperature in the shed. For example, assume that the

outdoor temperature is 30° F. and the relative

humidity is 75 percent. This point is located on

the chart in figure 129 by a dashed line that

intersects the 30° temperature line at a relative

humidity of about 75 percent. The corresponding wood equilibrium moisture content at these

conditions is 15 percent (heavy line running

slightly upward to the right side of the chart).

If no moisture is added to this air and its temperature is increased to 45°, the relative humidity

becomes about 41 percent (located by the curved

dash line downward to the right, parallel to the

adjacent vapor pressure lines until it intersects

the 45° temperature line) ; the corresponding

equilibrium moisture content of the lumber will

be about 8 percent (dashed approximately horizontal line), a reduction of about 7 percent. The

chart shows that if this same air—that is, air

with the same vapor pressure—is heated to a

temperature of 60°, the relative humidity will be

reduced to about 25 percent and the equilibrium

moisture content to about 5 percent.

Figure 130 shows moisture content data obtained on ponderosa pine during extended storage

in a heated shed. The shed temperature was controlled by a hygrostat that maintained the equilibrium moisture content at about 8 percent. Thus

the fluctuations in the outdoor relative humidity

had little effect on the moisture content of the

stored lumber.

Conditioned Storage Sheds

Kiln-dried lumber can be held at any desired

moisture content in a storage shed in which temperature and relative humidity are controlled

(1). This type of storage is generally used only

for special material, such as airplane propeller

stock, which requires very close control of moisture content.

100

30 40 50

TEMPERN TURE (°F.)

60 70 80

ZM-44797-F

FIGURE 129.—Relationship between the temperature, relative humidity, and vapor pressure of air and the equilibrium

moisture content of wood.
DRY KILN OPERATOR'S MANUAL 175

ZM-29829-P

FIGURE 130.—Moisture content of 1-inch shiplap and 2-inch joists of ponderosa pine stored in a closed, heated shed

maintained at an equilibrium moisture content of about 8 percent for 18 months. The curves marked INDOORS

and OUTDOORS are the indoor and outdoor equilibrium moisture content values.
176 AGRIiCULTTJRE HANDBOOK NO. 18 8, U.S. DEPT. OF AGEICULTURE

Literature Cited

(1) DUNLAP, M. E.

1958. METHODS OF CONTROLLING HUMIDITY IN WOODWORKING PLANTS. U.S. Forest Serv. Forest

Prod. Lab. Rpt. 1612, 4 pp., illus. [ProcCSSGCi I

(2) McMlLLEN, J. M.

1956. COATINGS FOR THE PREVENTION OF END CHECKS

IN LOGS AND LUMBER. U.S. Forest Serv.

Forest Prod. Lab. Rpt. 1435, 13 pp., illus.

[Processed.]

(3) MATHEWSON, J. S.

1953. ELECTRICALLY OPERATED WOOD ELEMENT HTGROSTATS FOR CONTROL OF MOISTURE FLUCTUATIONS IN LUMBER STORED IN CLOSED

SHEDS. U.S. Forest Serv. Forest Prod.

Lab. Rpt. 1140, 3 pp., illus. [Processed.

(4) PECK, E. C.

1956. AiK DRYING OF LUMBER. U.S. Forest Serv.

Forest Prod. Lab. Rpt. 1657, 21 pp., illus.

[Processed.]

(5) SCHBFFBR, T. C.

1958. CONTROL OF DECAY AND SAP STAIN IN LOGS

AND GREEN LUMBER. U.S. Forest Serv.

Forest Prod. Lab. Rpt. 2107, 13 pp., illus.

[Processed.]

(6) U.S. FOREST SERVICE FOREST PRODUCTS LABORATORY.

1951. CHEMICAL BROWN STAIN IN PINE. Forest

Prod. Lab. Tech. Note 254, 3 pp. [Processed.]
GLOSSARY

This glossary includes the generally accepted

definitions of a limited number of terms currently

used in wood seasoning literature. It also includes

closely related terms that are not fully defined in

their special application to present-day seasoning

in most dictionaries or glossaries.

Common abbreviations:

B.t.u. =British thermal unit

C.O.D. Wt. =Calculated ovendry weight

C. Wt. =Current weight

D.B. =Dry bulb

EMC =Equilibrium moisture content

F.S.P. =Fiber saturation point

G. Wt. = Green weight

M.C. =Moisture content

O.D. Wt. =Ovendry weight

R.H. =Relative humidity

Sp. gr. =Specific gravity

W.B. -:Wet bulb

W.B.D. =Wet bulb depression

Wt. =Weight

Air binding.—The presence of air, generally

in pockets, in steam cous and traps, which

interferes with the normal flow of steam and

condensate.

Air drying.—(See Drying, air.)

Air, entering.—Heated air just as it enters the

kün loads of lumber.

Air, leaving.—Air just after it leaves the kiln

loads of lumber. It is usually at a lower

temperature than the entering air.

Air, short circuiting of.—The movement of air

through other than desired channels. Usually

results when a Idln charge is improperly

loaded and/or baffled.

Air travel, length of. —The distance between the

entering- and leaving-air sides of the kiln

charge.

Air velocity.—The speed at which air moves,

generally measured in feet per minute.

Air volume. —The total amount of air occupying

or moving through a given space, generally

measured in cubic feet.

Annual growth ring. —The growth layer put on

a tree each year in temperate climates, or each

growing season in other climates; each ring

includes springwood and summerwood.

Baffle.—A piece of canvas, metal, or wood used

for deflecting, checking, or otherwise regulating the flow of air.

Bark.—Outer layer of a tree, consisting of a

thin, living inner part, and a dry, dead outer

part that is generally resistant to moisture

movement.

Bastard sawn.—Lumber in which the annual

growth rings make angles of 30° to 60° with

the surface of the piece.

Bow.—The distortion in a board that deviates

from flatness lengthwise but not across its

faces.

Boxed heart.—The term used when the pith falls

entirely within the outer faces of a piece of

wood anywhere in its length. Also called

boxed pith.

British thermal unit.—(B.t.u.) The amount of

heat necessary to raise 1 pound of water 1° F.

in temperature.

Bulb.—The temperature-sensitive part of a thermostatic control system.

Control.—The bulbous part of the controlling

system, located in the kün, which contains

the temperature-sensitive liquid or gas.

Controlling dry.—The bulb that controls the

dry-bulb temperature.

Controlling wet.—A bulb, kept completeh' covered at all times with a clean, water-saturated wick or porous sleeve, which automatically controls the wet-bulb temperature.

Double-end control.—Control bulbs, usually

located in each longitudinal half of the kiln,

which control kiln temperatures for their

respective zone, independent of each other.

Dual control.—Two bulbs of a Y-shaped control system. They are usually located on

each kUn wall directly opposite each other

and control the temperature of the entering

air regardless of the direction of air movement.

Recorder.—The temperature-sensitive part of a

system that records but does not control

kiln conditions.

Recorder-controller.—A bulb attached by

means of a capülarj' tube to a recordingcontrolling instrument.

Cambium.—The one-cell-thick laj'er of tissue between the bark and wood that repeatedly subdivides to form new wood and bark cells.

Capillary action.—The combination of solidliquid adhesion and surface tension by which

a liquid is elevated in a vertical tube or moved

through a cellular structure.

Casehardening.—A condition of stress and set in

wood in which the outer fibers are under compressive stress and the inner fibers under tensile stress, the stresses persisting when the

wood is uniformly dry.

Casehardening, reverse.—A final stress and set

condition (in lumber and other wood items)

in which the outer fibers are under a tensile

stress and the inner fibers are under a compressive stress as a result of overconditioning.

177
178 AGRICULTURE HANDBOOK NO. 188, U.S. DEPT. OF AGRICULTURE

Cell.—A general term for the minute units of wood

structure, including wood fibers, vessel segments, and other elements of diverse structure

and function, having distinct cell walls and

cell cavities.

Chamber, plenum. ^A chamber on the pressure

side of a fan or blower in which the air is

maintained under pressure.

Charge.—(See Kiln charge.)

Chart, recorder.—A sheet, usually circular, on

which a graphic record of kiln temperatures

is transcribed.

Check.—A lengthwise separation of the wood that

usually extends across the rings of annual

growth and parallel to the wood rays, resulting

from drying stresses.

Surface check.—A check starting on a sidegrain surface and extending into the interior

of a board.

End check.—A check starting on an end-grain

surface and extending along the length of a

board.

Internal check.—Checks originating in the interior of a piece of wood or extensions of

surface and end checks.

Circulation, air. —The movement of air within a

kiln by either natural or mechanical means.

Direction of.—The direction of movement of

air through the kiln charge, expressed as

longitudinal, transverse, or vertical.

Forced.—The movement of air within a kiln by

mechanical means.

Longitudinal. —Air movement through the kUn

charge to be expressed as front to rear or

rear to front.

Natural.—The movement of air within a kiln

by natural means.

Reversible.—Capable of change in the direction

of air movement.

Transverse.—Air movement through the kiln

charge from wall to wall to be expressed as

right to left or left to right.

Vertical.—Air movement through the kiln

charge from top to bottom or bottom to

top.

Coil header (or manifold).—A pipe fitting to

which a number of pipes are connected on

one side.

Coil, intermittent operation of.—The alternate

opening and closing of the valve controlling

steam flow into the cod.

Coil, pipe.—One or more runs of pipes, the function of which is to heat the air in the kiln.

Booster. —A supplementary cod, usually located

between tracks of a multiple-track kiln,

used to add heat to air that has already

moved across a trackload of lumber.

Ceiling.—A cod placed near the kün ceding to

warm the ceiling and roof, thus preventing

moisture condensation.

Plain header (horizontal or vertical).—A coil

consisting of a supply and discharge header

at opposite ends with the pipes running

from one to the other.

Single-retum-bend header (horizontal or

vertical).—A coil with the discharge header

usually located under or on the side of the

supply header, the pipes running from the

supply header to a 180° bend and back to

the discharge header.

Multiple-return-bend header.—A coU usually

with the discharge header located below

the supply header, the pipes running back

and forth with a 180° elbow at the bends.

Double-end.—Cods usually extending half the

length of the kün from both ends and

usually operating as separate units.

Coil radiating surface.—The entire uninsulated

surface area of a heating cod.

Collapse.—The severe distortion or flattening of

single cells or rows of cells in wood during

drying, often evidenced by a caved-in or

corrugated appearance of the surface of the

piece.

Compression failure.—Rupture of the wood

structure resulting from excessive compression along the grain. It may develop as a

result of bending in the living tree or during

felling. In surfaced lumber, compression

fadures appear as fine wrinkles across the

face of the piece.

Compression wood.—Abnormal wood formed on

the lower side of branches and inclined trunks

of softwood trees. It has relatively wide,

eccentric growth rings with little or no demarcation between springwood and summerwood

and more than normal amounts of summerwood. Compression wood shrinks more than

normal wood longitudinally, causing bow,

crook, and twist.

Condensate.—Water formed by the cooling of

steam.

Conditioning.—(See Stresses, relief of.)

Conditioning treatment.—A controlled high

temperature-high relative humidity condition

used in a dry kün after the final stage of

drying to bring about a uniform moisture

distribution in the boards and to relieve

drying stresses.

Conduction, heat. —Transmission of heat through

or by means of a conductor.

Controller.—An instrument that automatically

controls kün temperatures.

Convection, heat.—Transfer of heat from heating COÜS to lumber by means of air.

Course, lumber. —A single layer of lumber.

Crook.—A distortion of a board in which the

edges deviate from a straight line from end

to end of the board.
DRY KILN OPERATOR'S MANUAL 179

Cup.—A distortion of a board in which there is

deviation from flatness across the width of the

board.

Cycle, heating.—The time intervening between

successive openings of a control valve.

Cycle, temperature.—The time between the

maximum and minimum temperatures during

a heating cycle.

Decay.—The decomposition of wood substance

by fungi.

Advanced (or typical) decay. —The older stage

of decay in which the destruction is readily

recognized because the wood has become

punky, soft and spongy, stringy, ringshaked, pitted, or crumbly. Decided discoloration or bleaching of the rotted wood

is often apparent.

Incipient decay.—The early stage of decay that

has not proceeded far enough to soften or

othenvise perceptibly impair the hardness

of the wood. It is usually accompanied by

a slight discoloration or bleaching of the

wood.

Defects, drying.—Any irregularity occurring in or

on wood, as a result of drying, that may lower

its strength, durability, or utility value.

Degrade, kiln.—A drop in lumber grade that

results from kiln drying.

Density.—The weight of a body per unit volume.

Depression, wet-bulb. —The difference between

the dry- and wet-bulb temperatures.

Dew point. —The temperature at which steam or

water vapor begins to condense.

Diamonding. —A form of warp in which the cross

section assumes a diamond shape.

Diffuse-porous wood.—A hardwood in which the

pores tend to be uniform in size and distribution throughout each annual ring or to

decrease in size slightly and gradually toward

the outer border of the ring.

Diffusion. —Spontaneous movement of heat, moisture, or a gas throughout a body or space.

Movement is from high points to low points

of temperature or concentration.

Dry-bulb temperature. —The temperature of the

air indicated by any type of thermometer not

affected by the water vapor content or relative humidity of the air.

Drying, air. —Process of drying lumber by natural

conditions in a yard or in an open unheated

shed.

Drying in transit.—The partial or complete kilndrying of Imnber by a drying facility located

between the shipping and fabrication points.

Drying, precision kiln. —Process of drying wood

in which controlled procedures are followed

in order to obtain a stress-free product that

has a desired moisture content and has

suffered a minimum loss in strength.

Drying rate. —The amount of moisture lost from

the lumber per unit of time.

Duct, air.—A rectangular, square, or circular

passageway to conduct air.

End coating.—A coating of moisture-resistant

material applied to the end-grain surface to

retard end drying of green wood or to reduce

moisture changes in dried wood to a minimum.

Equalization,—Bringing the pieces of lumber in

a kiln charge to a nearly uniform moisture

content.

Equilibrium moisture content (EMC).—The

moisture content at which wood neither gains

nor loses moisture when surrounded by air

at a given relative humidity and temperature.

Evaporation.—The changing from the liquid to

the vapor form.

Extractives.—Substances in wood, not an integral

part of the cellular structure, that can be removed by solution in hot or cold water, ether,

benzene, or other solvents that do not react

chemically with wood substance.

Fiber saturation point.—The stage in the drying

or wetting of wood at which the cell walls are

saturated with water and the cell cavities are

free from water. It is usually taken as approximately 30 percent moisture content, based on

weight when ovendry.

Fiber, wood.—A comparatively long (Ks or less

to }i inch), narrow, tapering hardwood cell

closed at both ends.

Flat-sawed.—Lumber sawed in a plane approximately perpendicular to a radius of the log.

(See Orain).

Fluctuation, steam pressure.—Variation of

steam pressure.

Flue, "A".—A vertical wedge-shaped space provided in the transverse center and extending

the length of a kiln load of lumber, usually 15

inches wide at the bottom and tapering to a

point 1 or 2 courses down from the top of the

load.

Flue, vertical.—A vertical space, usually 6 inches

or less in width and extending the length and

height of a kiln truckload or package of

lumber.

Grain.—^The general direction of" the fibers in

wood or lumber. When used with qualifying

adjectives it has special meanings concerning

the direction of the fibers or the direction or

size of the growth rings.

Under fiber direction the specific terms are:

Cross grain.—-Grain deviating from a line parallel to the sides of the piece.

Diagonal grain.—-A form of cross grain resulting from sawing at an angle with the bark

of the log.

Interlocked grain.—A form of spiral grain in

which the fiber direction gradually alternates from right-hand to left-hand spiral

and back again in adjacent groups of

annual rings.
180 AGBIiCULTURE HANDBOOK NO. 188, U.S. DEFT. OP AGRICULTURE

Spiral grain.—A form of cross grain resulting

from the fibers, during growth, taking a

spiral course about the trunk of the tree

instead of the normal vertical course.

Straight grain.—Grain parallel to the sides of

the piece.

Under growth ring direction or ring pattern, the

specific terms are:

Coarse grain. —Wood in which the growth rings

are wide or have major differences in density and color between springwood and

summerwood.

Edge grain (or vertical grain).—The grain in

lumber produced by quartersawing so that

the edges of the growth rings are exposed

on the widest faces of the piece, and the

rings form angles of 45° to 90° with the

widest faces.

Fine grain.—Wood in which the growth rings

are narrow and inconspicuous.

Flat grain.—The grain in lumber produced by

flat sawing so that the tangential faces of

the growth rings are exposed on the widest

faces of the piece and the rings form angles

of less than 45° with the widest faces.

Green lumber (or grass green).—Lumber cut

from freshly felled trees.

Hardwoods.—Woods produced by one of the botanical groups of trees that have broad leaves

in contrast to the needles or scalelike leaves

of the conifers or softwoods. The term has

no reference to the actual hardness of the

wood.

Heartwood.—The wood extending from the pith

to the sapwood, the cells of which no longer

participate in. the life processes of the tree.

Heartwood may be infiltrated with gums,

resins, and other materials that usually make

it darker and more decay resistant than sapwood.

Honeycombing.-—Checks, often not visible at the

surface, that occur in the interior of a piece

of wood, usually along the wood rays. (See

Ring failure.)

Humidity, absolute.—The weight of water vapor

per unit volimie of space.

Humidity, relative.—^Ratio of the amount of

water vapor present in the air to that which

the air would hold at saturation at the same

temperature. It is usually considered on the

basis of the weight of the vapor, but for accirracy it should be considered on the basis of

vapor pressures.

Hygroscopicity.—^The property of a substance

which permits it to adsorb and retain moisture.

Hysteresis.—The tendency of wood exposed to

any specified temperature and relative humidity conditions to reach equilibrium at a

lower moisture content when absorbing moisture from a drier condition than when losing

moistm^e from a wetter condition.

Kiln.—^A. heated chamber for drying lumber, veneer, and other wood products in which temperature and relative humidities are controlled.

Compartment.—A dry kiln in which the total

charge of lumber is dried as a single unit.

At any given time, the temperature and

relative humidity are imiform throughout

the kün.

Progressive.—A dry kiln in which the total

charge of lumber is not dried as a single

unit but as several units, such as kiln truckloads, that move progressively through the

kiln- The temperature is lower and the

relative humidity higher at the entering

end (green end) than at the discharge end

(dry end).

Automatically controlled.—A dry kiln in

which drying conditions are controlled by

the action of thermostats.

Forced-circulation.—A dry kiln in which the

air is circulated by mechanical means.

Manually controlled.—A dry kiln in which

drying conditions are controlled by the

manual operation of valves and ventilators.

Multiple-track. —A dry kiln equipped with two

01' more tracks.

Natural-circulation.—A dry kiln in which air

circulation depends on the power of gravity

and the varying density of air with changes

in its temperature and moistiu"e content.

Reversible circulation.—A dry kiln in which

the direction of air circulation can be reversed at desired intervals.

Single-track.—A dry kiln equipped with one

track.

Kiln charge.—The total amount of lumber or

wood items in a dry kün.

Kiln charge, mixed.—Same as kiln charge but

composed of more than one species or thickness of lumber or wood items.

Kiln-drying.—Process of drying lumber in a dry

kiln.

Kiln leakage.—The undesirable loss of heat and

vapor from a kiln through badly fitted doors

and ventUators or through cracks in the walls

and roof.

Kiln run. —The term applied to the drying of a

single charge of lumber.

Kiln sample.—A section 30 inches or more in

length cut from a sample board and placed

in the kiln charge so that it can be removed

for examination, weighing, and testing.

Controlling.—Some of the wettest samples used

to control the drying. The number depends on the total number of samples used

and the composition of the kiln charge.
DRY FTT.Isr OPERATOR'S MA2SFUAL 181

Driest.—The kiln sample containing the least

amount of moistm-e.

Fastest drying. —The kiln sample that loses the

largest amoimt of moisture in a given

period of time.

Pocket. —A space provided for the kUn sample

in the kiln truckloads of limiber.

Slowest drying.—The kiln sample that loses

the smallest amount of moisture in a given

period of time.

Weight, current. —The weight of a kün sample

at given times during the drying process.

Weight, final.—The weight of a kiln sample

after the completion of the drying.

Weight, green (or initial, or original). —The

weight of a kün sample prior to kiln drying

regardless of its moisture content.

Wettest.—The kiln sample containing the

largest amount of moisture.

Knot.—That part of a branch which has become

overgrown by the body of a tree. The shape

of the knot depends on the angle at which

the branch is cut.

Loading, cross-pued.—Lumber piled on kiln

trucks and placed in a dry kiln with the long

axis of the load perpendicular to the length of

the kiln.

Loading, end-pUed.—Lmnber piled on kiln

trucks and placed in a dry kiln with the long

axis of the load parallel to the length of the

kiln.

Longitudinal.—Generally, the direction along

the length of the grain of wood. A longitudinal section may be a plane either tangential or radial to the growth rings.

Lumber, kiln-dry. Lumber that has been dried

in a dry kiln to a specified moisture condition.

Lumber, shipping-dry.—Lumber that has been

partially air- or kiln-dried to an average

moisture content of approximately 30 percent.

Lumber storage room.—A room maintained

within specified equilibrium moisture content limits so that lumber stored in it will not

gain or lose moisture beyond fijxed limits.

Meter, moisture.—An instrument used for rapid

determination of the moisture content in

wood by electrical means.

Mineral streak.—An olive to greenish-black or

brown discoloration of undetermined cause

in hardwoods, particularly hard maples;

commonly associated with bird pecks and

other injuries; occurs in streaks usually containing accumulations of mineral matter.

Moisture content of wood.—Weight of the water

contained in the wood, expressed as a percentage of the weight of the ovendry wood.

Average. —The percentage of moisture content of a single piece or the sum of the

rnoisture contents of a number of pieces

divided by their number.

Core.—The moisture content of the inside part

of a moisture section remaining after a

shell % the thickness of the section has

been removed.

Determination of.—The testing of lumber to

determine the amount of moisture present.

This is usually expressed in terms of percent of the ovendry weight.

Final.—The moisture content of the wood at

the end of kiln-drying.

Green.—The moisture content of wood in the

living tree.

Initial.—The moisture content of the wood at

the start of kiln-drying.

Shell.—The moisture content of the outer

one-fourth of the thickness of a moisture

section.

Moisture distribution.—The variation of moisture content throughout a piece of wood,

usually from face to face but sometimes from

end to end, or from edge to edge.

Moisture gradient.—A condition existing during

drying in which the moisture content uniformly decreases from the inside toward the

surface of a piece of wood. Also a term used

specilicaU}^ to denote the slope of the moisture content distribution curve.

Moisture gradient, reverse.—A condition following moisture regain in which the moisture content is higher at the surface than inside the

wood.

Moisture range.—The difference in moisture content between the driest and wettest boards or

samples.

Moisture section.—A cross section, 1 inch in

length along the grain, cut from a kiln or

random sample and used to determine moisture content.

Moisture section, initial weight of.—The weight

of a moisture section immediately after being

cut from a kiln sample or board.

Moisture section, ovendry weight of.—The

weight of a moisture section after being ovendried to a constant weight.

Mold.—A fungus growth on lumber taking place

mainly at or near the surface and, therefore,

not typically resulting in deep discolorations.

They are usually ash green to deep green in

color, although black is common.

Old growth.—Timber in or from a mature, naturally established forest. When the trees have

grown during most if not all of their individual

lives in active competition with their companions for sunlight and moisture, the timber

is usually straight and relatively free of knots.

Ovendry.—A term applied to wood dried to constant weight in an oven maintained at temperatures of from 214° to 221° F.

Pervious wood.—A wood through which moisture

moves readily.
182 AGBIiCULTURE HANDBOOK NO. 188, U.S. DEPT. OF AGRICULTURE

Piling, box.—The flat piling of random length

boards on kiln trucks so that the ends of the

completed load are in vertical alinement. The

longest boards are placed on the outside of

the load and the shorter boards are alternately

placed with one end even with one end of the

load or the other.

Piling, edge.—Piling lumber so that the broad

face of the board is vertical.

Piling, flat.—Piling lumber so that the broad face

of the board is horizontal.

Pit.—A relativel}" unthickened part of a wood cell

wall where a thin membrane may permit

liquids to readily pass from one cell to

another. A "bordered" pit has an overhanging rim that is not present in a "simple" pit.

Pitch.—The mixture of rosin and turpentine or

other volatiles produced in the resin canals

of pines and other conifers. Term also applied

to mixtures of nonvolatile liquids or noncrystalline solids and volatile oils in other

species.

Pitch pocket.—An opening, extending parallel to

the growth rings, that contains or has contained pitch.

Pitch streak.—A well-defined streaky accumulation of pitch in the wood of certain softwoods.

Pith.—The small, soft core occurring in the structural center of a tree trunk, branch, twig, or

log.

Plainsawed.—Another term for flat-sawed or flatgrained lumber.

Pore.—-The cross section of a specialized hardwood

cell known as a vessel. (See T"e.sse/s.)

Quartersawed.—Lumber sawed so the wide faces

are approximatelj" at right angles to the annual growth rings. (See Grain.)

Radial.—Coincident with or generally parallel to

a radius of the tree from the pith to the bark.

A radial section is a lengthwise section in a

plane that passes through the pith.

Radiation.—Heating coils or elements with a dry

kiln.

Balanced.—Construction and arrangement so

as to insure equal radiating surface and

uniform temperatures throughout the kiln.

Direct.—The transmission of heat energy to a

body or substance by direct heat ra3's from

the heating system.

Excessive.—A greater amount of radiation than

required.

Flexible.—The arrangement of the heating system into small coils equipped with hand

valves that, when opened or closed, permit

rapid adjustment of the radiating surface

to meet the required needs.

Raised grain.—A roughened condition of the surface of dressed lumber in which the hard

summerwood is raised above the softer springwood but not torn loose from it.

Rate of growth.—The rate at which a tree has

laid on wood, measured radially in the tree

trunk or in the radial direction in lumber.

The unit of measure in use is the number of

annual growth rings per inch.

Rays, wood.—Strips of cells extending radially

within a tree and varying in height from a few

cells in some species to 4 inches or more in

oak. The rays serve primarily to store food

and transport it horizontally in the tree.

Resin canal (or duct).—Intercellular passages

that contain and transmit resinous materials.

They extend vertically or radially in a tree.

Ring failure (or separation).—A separation of

the wood during seasoning. Occurs along the

grain and parallel to the annual rings, either

within or between rings; called honeycomb

and ring check in some localities. (See

Shake.)

Sample board.—A board from which one or more

kiln samples will be ciit, or a board taken from

a kiln truckload during drying for the purpose

of cutting a moisture section.

Sample.—(See Kiln sample.)

Sap.—The moisture in unseasoned wood and all

extractives it holds in solution.

Sapwood.—The layer of wood near the outside

of the log, usually lighter in color than the

heartwood, that is actively involved in the

life processes of the tree.

Seasoning.—-Removing moisture from green wood,

and in some cases relief of stresses, in order

to improve its serviceability.

Second growth.—Timber that has grown after

the removal, whether by cutting, fire, wind,

or other agency, of all or a large part of the

previous stand.

Set.—A localized semipermanent deformation in

wood caused by internal tensile or compressive

stresses.

Compression set.—Set, occurring during compression, that tends to give the wood a

smaller than normal dimension after drying, usually found in the interior of wood

items during the last stage of drying hut

sometimes in the outer layers after overconditioning or rewetting. Also caused by

external restraint during rewetting of dried

wood.

Tension set.—^Set. occurring during tension,

that tends to give the wood a larger than

normal dimension after drying, usually

occurring in the outer layers during the

first stages. Also caused by external restraint during drying of wet wood.

Shake.—A separation along the grain, the greater

part of which occurs between the rings of

annual growth. Found in stiunps and ends

of freshly cut logs and green lumber. (See

Ring failure.)
DRY KILN OPERATOR'S MANTJAl. 183

Shrinkage.—The contraction of wood caused by

drying the material below the fiber satiu-ation

point.

Longitudinal.—Shrinkage along the grain.

Radial.—Shrinkage across the grain, in a radial

direction.

Tangential.—^Shrinkage across the grain, in a

tangential direction.

Softwood.—Wood produced by one of the botanical group of trees that, in most species, have

needle or scalehke leaves.

Specific gravity.—The ratio of the ovendry weight

of a piece of wood to the weight of an equal

volume of water (39° F.). In the field of

seasoning, specific gravity values are usually

based on the volume the wood has when

green.

Split.—A lengthwise separation of the wood, due

to the tearing apart of the wood parallel to

the wood rays.

Spray line.—A plain pipe of varying sizes and

lengths and drilled with holes of various sizes

and spacing through which steam is injected

into the kiln.

Springwood (early wood).—The part of the annual growth ring that is formed during the

early part of the season's growth. It is usually less dense and weaker mechanically than

summerwood.

Stain.—^A discoloration in wood that may be

caused by such diverse agencies as microorganisms, metal, or chemicals. The term

also applies to materials used to impart color

to wood.

Blue stain (sap stain).—A bluish or grayish discoloration of the sapwood caused by the

growth of certain dark-colored fungi on the

surface and in the interior of the wood,

made possible by the same conditions that

favor the growth of other fungi.

Chemical stain.—A general term including aU

stains that are due to color changés of the

chemicals normally present in the wood,

such as pinking of hickory and browning

of some softwoods, particularly the pines.

Chemical stain, brown.—A chemical discoloration of wood, which can occur dm-ing the

air-drying or kün-drying of several softwood species, caused by the concentration

and modification of extractives.

Iron-tannate stain.—A surface stain, bluishblack in color, on oak and other tanninbearing woods following contact of the wet

wood with iron, or with water in which

iron is dissolved.

Mineral stain.—An olive to greenish-black or

brown discoloration in hardwoods, particularly maple, caused by bird peck or other

injury and fovmd either in mass discoloration or mineral streaks. The mineral

associated with such streaks is frequently

calcium oxalate, which has a tendency to

duU machining knives.

Sticker stain.—A gray to blue or brown chemical stain occurring on and beneath the surface of boards where they are m contact

with stickers, (also fungus sap stain when

found only in the sticker area).

Water stain.—^A yellowish to blackish surface

discoloration caused by water that dripped

onto the wood during seasoning.

Weather stain.—A very thin grayish-brown

surface discoloration on lumber exposed a

long time to the weather.

Steam.—The vapor into which water is converted

when heated.

Exhaust.—Steam which has already passed

through a steam engine or machine.

Flash.—The reevaporation of hot water produced by the excess heat, when the water

is discharged to a lower pressure.

Live.—Steam obtained directly from the boiler.

Superheated.—Steam at a temperature higher

than the saturation temperatm-e corresponding to the pressure.

Steam binding.—The presence of steam in the

drain line between the heating coil and trap

which temporarily prevents the drainage of

condensate and air from the coU.

Sticker.—A wooden strip placed between the

courses of lumber in a kiln load and at right

angles to the long axis of the boards, to

permit air circulation.

Alinement.—The placing of stickers in a pUe,

package, or truckload of lumber so that

they form vertical tiers.

Spacing.—The distance between stickers measured from center to center.

Stress, drying.—An internal force, exerted by

either of two adjacent parts of a piece of wood

upon the other during drying, caused by

uneven drying and shrinking, and influenced

by set.

Tensile stress.—The stress found in the outer

layers of wood during the early stages of

drying when they are trying to shrink but

are restrained by the still-wet interior

region ; also the stress in the interior layers

later in drying as they try to shrink and are

restrained by the set outer shell.

Compressive stress.—The stress found in the

interior region of wood during the early

stages of drying, caused by the shrinking

of the outer shell; also the stress in the outer

layer later in drying caused by the shrinking of the interior.

Stress-free.—Containing no drying stresses.

Stress section.—A cross section of a sample that

is cut into prongs of equal thicknesses, from

face to face.
184 AGBIiCüLTURE HANDBOOK NO. 18 8, U.S. DEPT. OF AGRICULTURE

Stresses, relief of.—The result of a conditioning

treatment, following the final stage of drying,

which causes a redistribution of moisture

and a relief of the sets.

Summerwood (late wood).—The part of the

annual growth ring that is formed during the

latter part of the growing season. It is

usually denser and stronger mechanically than

springwood.

Tangential.—Coincident with or generally parallel to a tangent at the circumference of a

tree or log, or growth rings. A tangential

section is a longitudinal section through a tree

perpendicular to a radius.

Temperature.—Degree of hotness or coldness.

Cold zone.—The lowest entering-air dry-bulb

temperature in the kiln.

Drop across the load.—The reduction in the

dry-bulb temperature of the air as it flows

through the load and is cooled by evaporating moisture from the load of lumber.

Dry bulb.—The temperature of the kiln air.

Hot zone.—The highest entering-air dry-bidb

temperature in the kiln.

Longitudinal variation of.—The range of

entering-air dry-bulb temperatures in a

tdln measured along its length.

Wet bulb.—The temperatures indicated by

any temperature measuring device, the

sensitive element of which is covered by a

smooth, clean, soft, water-saturated cloth

(wet-bulb wick or porous sleeve).

Temperature gradient, longitudinal.—A term

used to denote longitudinal temperature

differences within a dry kiln.

Tension wood.—A type of wood found in leaning

trees of some hardwood species, characterized

by the presence of fibers technically known

as "gelatinous" and b,y excessive longitudinal

shrinkage. Tension wood fibers tend to

"pull out" on sawed and planed surfaces,

giving so-called ' ' fuzzy grain. ' ' Tension wood

causes crook and bow and may collapse.

Because of slower than normal drying, tenßion

wood zones may remain wet when the surrounding wood is dry.

Texture.—A term referring to the size of wood

cells. Thus, "fine-textured" wood has small

cells; "coarse-textured" large cells. Where

all the cells of a softwood, or all the pores of a

hardwood, are approximately the same size,

as seen on the cross section, the wood can be

called "uniform textured." The term is

sometimes erroneously used in combination

with soft or hard.

Tracheids.—The elongated cells that make up

the greater part of the wood of the softw^oods ;

frequently referred to as fibers.

Transverse.—The directions in wood at right

angles to the wood fibers or across the grain,

including radial and tangential directions.

A transverse or cross section is a section

through a tree or timber at right angles to

the pith. It has an end-grain surface.

Treatment, equalization.—A controlled temperature and relative humidity condition used

in a dry-kiln at the end of drying to stop the

drying of the driest boards while allowing the

wettest boards to continue drying, thus

reducing the moisture range between boards.

Treatment, steaming.—Spraying steam directly

into the kün to attain a condition at or near

saturation in the initial stages of kiln-drying

to retard the growth of mold. Also used to

increase the rate of heating cold lumber.

Sometimes used needlessly during other

stages of drying to restore surface moisture,

and often used without proper control to

partially relieve stresses at the end of drying.

Twist.—A form of warp caused by the turning or

winding of the edges of a board so that the

four corners of any face are no longer in the

same plane.

Tyloses.—Extensions of parenchyma cells into

the pores or vessels of some hardwoods,

notably white oak and black locust, prior to

or during heartwood formation. They tend

to prevent or greatly retard moisture movement through the vessels.

Vapor barrier.—A material with a high resistance

to vapor movement, such as asphalt-impregnated paper, that is used in combination with

insulation to control condensation.

Vapor pressure.—The pressure of a confined body

of vapor. The pressure of a given saturated

vapor is a function of temperature only.

Ventilator (or vent). — An opening in the kiln

roof or wall, or in the blower duct work, that

can be opened or closed in order to maintain

the desired relative humidity condition \vithin

the kiln.

Automatic control.—A ventUator that is

opened or closed by a thermostat.

Linkage.—The adjustable, pivoted rods connecting the vent cover to an air valve or to

a hand-operated lever which facilitates the

opening and closing of the vents.

Manual control.—A ventilator that is opened

or closed by hand.

Vessels.—Wood cells in hardwoods of comparatively large diameter that have open ends and

are set one above the other so as to form

continuous tubes. The openings of the vessels

on the surface of a piece of wood are usually

referred to as pores.

Virgin growth.—The original growth of mature

trees.
DRY KILN OPERATOR'S MANUAL 185

Wane.—Presence of bark or the lack of wood

from any cause on 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).—Moistiu"e that is bound by adsorption forces within

the cell wall; that is, the water in wood

below the fiber saturation point.

Water, free.—Moisture that is held in the cell

cavities of the wood, not bound in the cell

wall.

Water pocket.—An area of unusually high moisture content of various sizes and shapes found

in lumber.

Waterlogging.—The presence of water in steaai

coils, which interferes with the normal flow of

steam and seriously affects the heating efficiency of the coil.

Wet-bulb temperature.—(See Temperature.)

Wood.—The hard material between the pith

and the bark in the stems and branches of

trees, made up of a varietj^ of organized hollow

cells and consisting chemically of cellulose,

hemicelluloses, lignin, and extractives.

Wood substance.—The extractive-free solid

material of which the cell walls of ovendrj^

wood are composed, having essentially the

same specific gravity in all species.

INDEX

Page

Air circulation systems, kiln 76

external blower, maintenance 77

internal fan, maintenance 76

natural, maintenance 76

Air-dried lumber

initial steaming of 124

kiln schedules for 124

Air, short circuiting of 113-116

Alder, red

drying defects 156

drying schedules 120

drying time 144

moisture content 9

names 3

shrinkage 16

specific gravity 13

weights 14

Annual growth rings 7

springwood 7

summerwood 7

Antistain solutions 169, 171

Apple

drying schedules 120

drying time 144

moisture content 9

names 3

shrinkage 16

specific gravity 13

weights 14

Ash, black

drying defects 156

drying schedules 120

drying time 144

moisture content 9

names 3

shrinkage 16

specific gravity 13

weights 14

Ash, brown

names 3

Ash, green

drying schedules 120

moisture content 9

names 3

shrinkage 16

specific gravity 13

weights 14

Ash, Oregon

drying schedules 120

names _ _ _ _ 3

Ash, pumpkin

names _ 3

Ash, white

drying defects 156

drying schedules "_'Í20, 123

drying time '144

moisture content ll.l 9

names _ _ _ "" 3

shrinkage '/.//_ 16

specific gravity '_'_'_'_" 13

weights l'_ ' " '' j4

Aspen

drying defects 156

drying schedules '_.'._. 120, 124

drying time ' 144

moisture content '__ 9

Page

Aspen—Continued

names_- 3

shrinkage 16

specific gravity 13

weights 14

Aspen, bigtooth

names 3

shrinkage, 16

Aspen, quaking

drying schedules 120

names 3

shrinkage 16

Automatic controls

thermostats 44

valves, control 39

Balances 49

Baldeypress

drying defects 156

drying schedules 131

drying time 144

moisture content 9

names 5

shrinkage 16

specific gravity 13

weights 14

Balsam fir

names 5

Barrel heading, staves, stacking 93, 94

Basswood

drying defects 156

drying schedules 120, 124

drying time 144

moisture content 9

names 3

shrinkage 16

specific gravity 13

weights 14

Battery shims, drying schedules for 141

Bearings, fan 77

Beech

drying defects 156

drying schedules 120

drying time 144

moisture content 9

names 3

shrinkage 17

specific gravity 13

weights 14

Beech, American (See: Beech)

Belts, fan 76

Bevel siding 146

Birch

names 3

Birch, paper

drying defects 156

drying schedules 120

drying time 144

moisture content 9

names 3

shrinkage 17

specific gravity '" 13

weights "_ ' 24

Birch, red

names _ _ 3

Birch, sap

names 3

187
188 AGHICULTURE HANDBOOK NO. 188, U.S. DEPT. OF AGRICULTURE

Page

Birch, sweet

moisture content 9

names 3

shrinkage 17

specific gravity 13

weights 14

Birch, white

names 3

Birch, yellow

drying defects 156

drying schedules 120

drying time 144

moisture content 9

names 3

shrinkage 17

specific gravity 13

weights 14

Blowers, external 77

Blue stain 152

Board, flat-sawed 7

cupping tendency 150

Board, plainsawed 7

Board, quartersawed 7

Bow 8, 150

Bowling-pin blanks

end coating 148

drying schedules 120

Boxed-heart splits 149

Box piling 88

Brown stain 19, 135, 152

Buckeye

drying schedules 120

drying time 144

moisture content 9

names 3

shrinkage 17

specific gravity 13

weights 15

Bundled stock, drying of 140

Butternut

drying schedules 120

drying time 144

moisture content 9

names 3

shrinkage 17

specific gravity 13

weights 15

Casehardening

cause 152

end checking of 155

planer splits in 155

relief 143, 152

test

evaluation 165

method 105

warping 155

Casehardening, effects of on

planing 154, 155

resawing 155

routing 155

Cedar, Alaskadrying defects 156

drying schedules 131

drying time 144

moisture content 9

names 5

shrinkage 16

specific gravity 13

weights 14

Cedar, Atlantic whitekiln-drying time 144

moisture content 9

moisture content drying schedules 131, 132

Page

Cedar, Atlantic white—Continued

names 5

shrinkage ig

specific gravity 13

weights 14

Cedar, eastern reddrying defects 156

drying schedules 131

drying time 144

moisture content 9

names 5

shrinkage 16

specific gravity 13

weights 14

Cedar, incensedrying defects 156

drying schedules, moisture content 131, 132

drying schedules, time 136

drying time 144

moisture content 9

names 5

pencil slats, drying of 140

shrinkage 16

specific gravity 13

weights 14

Cedar, northern whitedrying schedules, moisture content 131, 132

drying time 144

moisture content 9

names 5

shrinkage 16

specific gravity 13

weights 14

Cedar, Port-Orforddrying defects 1 156

drying schedules, moisture content 131

drying schedules, time 13fi

battery shims 141

drying time

"battery shims 141

1-inch lumber 144

moisture content 9

names 5

shrinkage 16

specific gravity 13

weights 14

Cedar, Western reddrying defects 156

drying schedules, moisture content 131, 132

drying schedules, time 136,138

drying time 144

moisture content 9

names 5

roof decking, drying 141

shingles, drying 140

shrinkage 16

specific gravity 13

weights 14

Checked knots (See: Knots)

Checks

bottle-neck . . . 149

end 148,155

surface 147

Chemical stains 152

brown stain 135, 152

sticker stain 153

Cherry, black

drying defects 156

drying schedules 120, 121, 123

drying time 144

moisture content ^

names 3

shrinkage 1^
INDEX 189

Page

Cherry, black—Continued

specific gravity 13

weights '^^

Chestnut

drying schedules J-^J

drying time 1**

moisture content ^

names ^

shrinkage ' '

specific gravity 1^

weights '^

Chinkapin, golden

drying defects 1^°

drying time l'*4

moisture content 9

specific gravity 13

weights 1^

Circulation systems, kiln 7o

Coarse grain 7

Code number index tables, drying schedules. 120, 131, 136

Coils, heating 35

booster 36

plain header 35

single-return-bend header 36

two-zone 44

Cold zones in kilns 65, 66, 78

CoUapse 148

Compartment kilns 24, 159

Compression wood 8

Conditioning treatment 143, 164

Control bulbs

dry 67

location 68

wet 68

Controls, automatic 43

Controls,"manual, temperature 45

Corrosion in kilns 77

Cottonwood

drjang defects 156

drying schedules 120, 121

drying time 144

moisture content 9

names 3

shrinkage 17

specific gravity 13

weights 15

Crook 8, 150

Crossties, drying of 140

Cucumbertree 3

Cup 150

Cypress

drying schedule 131, 132

names 1 5

Decay, stopping of 142

Decking, roof 141

Defects 147

brown stain 152

pinking in hickory 153

soft pitch 152

sapstain 153

sticker stain 147

types 153

Defects, machining 154

broken knots, knotholes 154

chipped, torn grain 153

planer splitting 155

raised grain 147

Defects, shrinkage 149

boxed-heart splits 149

casehardening

cause-- -- - ...- 152

relief 143, 164

test for 105

Page

Defects, shrinkage—Continued

checked knots 151

collapse 148

end checks, end splits 148

honeycomb 149

loose knots 151

ringfallure 149

surface checks 147

Depression schedules, wet-bulb

chemically treated wood 139

H-type 125

shifting wet-bulb 125

Diamonding 150

Dogwood

drying defects 156

drying schedules 120

drying time 144

moisture content 9

names 3

shrinkage 17

specific gravity 13

weights 15

Doors, kiln 66

Douglas-fir

drying defects 156

drying schedules

battery shims 141

mixed charge with western larch 136

moisture content 132

time 136, 138, 140

drying time

"battery shims 141

1-inch lumber 144

moisture content 9

names 5

shrinkage 16

specific gravity 13

weights 14

Dry-bulb temperature control 161

Dry bulbs 67

Dry kilns

compartment kiln 24

external-blower 25

internal-fan 26

construction features 33

air circulating systems 34, 42

air velocities 42

controls, automatic 43

manual 45

heating systems 34

control valves 39

radiating surfaces 35

steam traps 37

spraying systems 39

venting systems 34, 40

construction materials 33

heat transmission, coefficients of, tables 65

external-blower compartment 25

loading of 113

housekeeping and maintenance 77

inspection check list 63

internal-fan compartment 26

loading of 114

cross-piled type 114

natural-circulation compartment 24

loading of 113

progressive 23, 32

Dry kUns, operation of 159

Drying data

plotting 109

recording 106

555315 O—61- -13
190 AGHIiCULTURE HANDBOOK NO. 188, U.S. DEFT. OF AGRICULTURE

Page

Drying defects, kiln 147

chemicals in wood 152

effects of, on machining 153

broken knots and knotholes 154

casehardening 155

chipped and torn grain 154

planer splitting 153

raised grain 155

effect of temperature on strength 147

fungus attacks 152

shrinkage 147

species 156

Drying defects, species characteristics 156

Drying procedure 162

applying moisture content schedule 162

air-dried stock 163

controlling kiln samples 162

examples 162

skipping steps 163

applying time schedule 163

final wet-bulb temperature 163

intermediate moisture content 164

intermittent steaming 164

Drying rate, factors influencing 10

Drying schedules, general

hardwoods 117

softwoods -- 129

Drying time

1-inch lumber -- 144

reduction of 166

Elder, box, names 3

Electrical moisture meters 56, 79

Elm, American

drying defects 156

drying schedules 120

drying time 144

moisture content 9

names 4

shrinkage 17

specific gravity 13

weights 15

Elm, cedar

moisture content 9

names 4

specific gravity 13

weights 15

Elm, gray 4

Elm, red 4

Elm, rock

drying schedules 120

drying time 144

moisture content 9

names .-- 4

shrinkage 17

specific gravity 13

weights 15

Elm, slippery

drying schedules 120

names 4

shrinkage 17

specific gravity 13

Elm, soft, names 4

Elm, white 4

End coatings

kiln samples 102

logs in storage 169

thick stock 148

End grain 7

EquaUzing treatments 142, 164

Equilibrium moisture content 8

table for 11

equalizing and conditioning lumber

high conditioning EMC 165

use of EMC value tables 143, 164

values, EMC, table 143

Page

Equipment, temperature 57

hygrometer 58

thermometers

etched stem 58

potentiometers 59

pressure spring 58

thermocouples and cable 59

Equipment, determining air movement 61

anemometers 62

hot-wire air meter 62

smoke 61

velometer 62

Equipment, moisture content 49

balances 49

pan-type 50

self-calculating 50

triple-beam 49

distillation 57

drying ovens 53

electrical moisture meters 56

saws 53

scales

platform and indicating 50

self-calculating 50

Fans, internal

gratings for 67

maintenance 76

Fiber saturation point 8

Final moisture content 105, 165

Fir, balsam

drying schedules 131, 132

drying time 144

moisture content 9

names 5

shrinkage 16

specific gravity 13

weights 14

Fir, California red

drying defects 156

drying schedules 131, 132

drying time 144

moisture content 9

names 5

shrinkage 16

specific gravity 13

weights 14

Fir, eastern (See: Fir, balsam)

Fir, grand

drying schedules 131, 132

drying time 144

moisture content 9

names S

shrinkage 16

specific gravity 13

weights 14

Fir, noble

drying schedules, moisture content 131, 132

drying schedules, time 1^^

drying time 144

moisture content 9

names 5

shrinkage 16

specific gravity J^

weights 14

Fir, Pacific silver

drying schedules 1^|

drying time 1'*^

moisture content "

names ^

shrinkage 1°

specific gravity J*

weights 1*

Fir, subalpine

drying schedules J^J

drying time 1**

moisture content ^
INDEX 191

Page

Fir, subalpine—Continued

names ^

shrinkage }°

specific gravity J^

weights '^'*

Fir, white

drying defects ^o"

drying schedules

moisture content í^L, íóroof decking -- 141

time 136, 138

drying time l^jl

moisture content 9

names ^

shrinkage j^

specific gravity 13

weights l"!

Fire in kiln

danger of '^

prevention . 1 167

Fh-e-retardant-treated stock, kiln drying

schedules 128

Floors, kiln

maintenance 66

types 67

water seepage through 67

Foreign woods, drying of 129

Fresh-air ducts "10

Gage, pressure 39

General drying schedules

hardwoods 117

softwoods 129

Glossary of terms , 177

Golf club head blanks, drying schedule 121

Grain 7

Green lumber, storage of 170-174

Growth rings, annual 7

Gum

black 5, 157

red and sap (See also; Sweetgum) 4,157

Gunstock blanks

drying schedules 121

end coating 148

Hackberry

drying defects 156

drying schedules 120,121

drying time 144

moisture content 9

names 4

shrinkage 17

specific gravity 13

weights 15

Handle stock, drying schedules 120

Hardwoods

gross structural features 6

leaves 6

names 3

Hardwood drying schedules (See also: individual

species; Tables, hardwood drying schedules) 117

general schedules

assembled schedules, examples of 123, 124

assembly of a schedule 122

changes within 126

code number index, species, tables 120, 121

for air-dried stock 124

material considerations 118

modifications of 124

H wet-bulb 125

shifting wet-bulb 117

moisture content basis 118

operation considerations 118

recommended and suggested schedules 118

shifting temperature 125

Page

Hardwood drying schedules—Continued

special schedules 127

alternate 127

chemically treated wood 128

foreign woods 129

hickory 127

maximum strength 127

specific products 128

thin stock 128

Heading, barrel, stacking for kiln-drying 92

Heartwood

characteristics 6

considerations, for kiln samples 100

Heating systems 73

direct-heated hot air 75

electrically heated 75

indirect-heated hot air 75

steam-heated 73

debris 73

defective reducing valves and pressure regulators. 73

faulty motor valves 73

faulty pressure gages 73

faulty steam traps 74

hand-operated valves 74

improperly insulated feed lines 73

leaking pipes 73

sagging or distorted pipes 73

Hemlock, eastern

drying defects 156

drving schedules, moisture content 131, 132

drying schedules, time 136

drying time 144

moisture content 9

names 5

shrinkage 16

specific gravity 13

weights 14

Hemlock, mountain

names 5

Hemlock, western

drying defects 156

drying schedules, moisture content 131, 132

drying schedules, time 136, 140

drying time 144

moisture content 9

names 5

shrinkage 16

specific gravity 13

weights 14

Hickory

drying defects 156

drying schedules

general, lumber, handle stock 120

special 127

drying time 144

moisture content 9

names 4

pinking 120, 153

shrinkage 17

specific gravity 13

weights 15

Holly

drying schedules 120

drying time 144

moisture content 9

names 4

shrinkage 17

specific gravity 13

weights 15

Honeycomb 149

Hophornbeam, eastern

drying schedules 120

drying time 144
192 AGBIiCULTURE HANDBOOK NO. 18 8, U.S. DEPT. OF AGRICULTURE

Page

Hophornbeam, eastern—Continued

moisture content 9

names 4

shrinkage 17

specific gravity 13

weights 15

Hot zones in kilns 75, 78

Humidifying systems 75

defective valves 75

condensate drain line 75

maintenance 75

sprays 75

Hygrometers 58

Improper control of drying conditions, causes of 77

dry-bulb temperature fails to reach set-point 77

dry-bulb temperature overrides set-point 78

dry- and wet-bulb temperatures, incorrect reading

of 78

hot and cold zones, causes of 78

wet-bulb temperature fails to reach set-point 78

wet-bulb temperature overrides set-point 78

Insects, control of 142

Interlocked grain 7

Intermittent steaming 164

Intermediate moisture content tests 105, 164

Ironwood

drying schedules 120

names 4

Juniper, western, names 5

Juvenüe wood 7

Kiln, air-circulation systems 76

Kiln building materials, heat transmission coefficient- 65

Kün-dried lumber, storage of 172-174

Kiln-drying

stacking lumber for 79

time for 1-inch lumber 144

Kiln-drying schedules

hardwood drying schedules 117

softwood drying schedules 129

KUn inspection and maintenance

air-circulation systems 76

corrosion, prevention of 77

doors 66

floors 67

heating systems 73

housekeeping 77

humidifying systems 75

inspection checklist 63

rails, supports 67

recording-controlling instruments

calibration of 71

care of 70

location, care of dry bulbs 68

location, care of wet bulbs 69

sensitivity 70

trucks 77

venting systems 75

walls, roofs, ceilings 64

Kiln operation 159

fuU, part time 162

locating troubles in 77

Kiln samples

control 104, 162

determining moisture content of 102

final tests on 105

intermediate moisture content test of 105

moisture content of sections 102

need for 99

number required 100, 159

ovendry weight 103

ovendrying sections 102

placement of 103

plotting data 109

preparation of 101

recording data 106

Page

Kiln samples—Continued

selection of 101, 159

use of 104

weighing 102, 104

Kiln schedules 117

definition of 117

hardwood schedules 117

kiln-drying time 144

softwood schedules 129

moisture content 131, 132

time-temperature 136

Kiln, starting of 160

Kiln warmup time 161

Knots

broken 154

checked 151

encased 7

intergrown 7

knotholes 154

Knotty pine, drying of 141

Larch, western

drying defects 156

drying schedules

mixed charge with Douglas-fir 136

moisture content 131, 132

time, code number index 136

drying time 144

moisture content 9

names 5

shrinkage 16

specific gravity 13

weights 14

Laurel, California

drying defects 156

drying schedules 120

drying time 144

moisture content 9

names 4

shrinkage 17

specific gravity 13

weights 15

Lemonwood, specific gravity of 13

Lift trucks 84

Load baffles 114, 115

Loading dry kilns

forced-circulation

external-blower 113

internal-fan 114

natural-circulation, compartment 113

progressive, all types 116

Locust

drying defects 156

drying schedules 120

drying time 144

moisture content 9

names 4

shrinkage 1'^

specific gravity 13

weights 15

Log storage

cold-deck 169

antistain solutions 169,171

end coating 169

water sprays 169

pond 169

banded logs 169

Loose knots 151

Lumber, drying characteristics of

plainsawed 79

quartersawed '9

Lumber storage 169

closed, heated shed IJ^

closed shed 1'^

conditioned shed IJ*

moisture regain in unheated sheds 1'^
INDEX 193

Page

Lumber storage—Continued

open shed 172

outdoor - 169

Lyctus powderpost beetles, kiln schedules for killing.- 142

Madrone

drying defects 156

drying schedules — 120

drying time 144

moisture content 9

names 4

shrinkage 17

specific gravity 13

weights 15

Magnoha

drying schedules 120, 123

drying time 144

moisture content 9

names 4

shrinkage 17

specific gravity 13

weights 15

Mahogany

drying defects 156

drying schedules 120, 123

drying time 144

Manual temperature controls 45

Maple, black

names 4

shrinkage 17

Maple, hard

drying defects 156

drying schedules

bowling-pin blanks 120

lumber 120, 121, 122

drying time 144

moisture content 9

names 4

shrinkage 17

specific gravity 13

weights 15

Maple, Oregon 4

Maple, red

drying schedules 120

drying time 144

names 4

shrinkage 17

Maple, silver

drying schedules 120, 121

drying time 144

moisture content 9

names 4

shrinkage 17

specific gravity 13

weights 15

Maple, soft

drying schedules 120, 121

drying time 144

moisture content 9

names 4

specific gravity 13

weights 15

Maple, sugar

drying defects 156

drying schedules 120, 121, 122, 159, 160

drying time 144

moisture content 9

names 4

shrinkage 17

specific gravity 13

weights 15

Maple, white 4

Moisture content

heartwood 8

sapwood 8

wood 8

species (table) — 9

Page

Moisture content tests

final - 105

interpretation of case-hardening tests 165

intermediate 105, 164

Moisture meters 56, 79

Moisture movement 10

Mold, control of 141

Motors, fan, maintenance 76

Mulberry 4

Myrtle, Oregon

drying schedule 120

names 4

Oak, black

names 4

shrinkage 17

Oak, bur

names 4

shrinkage 17

Oak, California black

drying defects 157

drying schedules 120

drying time 144

moisture content 9

names 4

shrinkage 17

specific gravity 13

Oak, chestnut

names 4

shrinkage 17

Oak, live

drying time 144

moisture content 9

names 4

shrinkage 17

specific gravity 13

Oak, northern red

drying defects 157

drying schedules 120, 121, 128

drying time 144

moisture content 9

names 4

shrinkage 17

weights 15

Oak, Oregon white

names 4

shrinkage 17

Oak, pin

names 4

shrinkage 17

Oak, red

drying defects 157

drying schedules 120, 121

drying time 144

names 4

specific gravity 13

Oak, scarlet

names 4

shrinkage 17

Oak, southern lowland

drying schedules 121, 123

Oak, southern red

drying defects 157

drying schedules 120, 121

drying time 144

moisture content 9

names 4

shrinkage 17

weights '__'_ "" 15

Oak, swamp chestnut

names __ 4

shrinkage 17

Oak, water

moisture content 9

shrinkage . . _ 17

specific gravity 13
194 AGRiliCULTURE HANDBOOK NO. 188, U.S. DEPT. OF AGRICULTURE

Page

Oak, white

drying defects 157

drying schedules 120, 121, 123

drying time 144

moisture content 9

names 4

shrinkage 1'

specific gravity 13

weights 15

Oak, willow

moisture content 9

names 4

shrinkage 17

specific gravity 13

Oil lines, fan, maintenance 76

Operation, dry kiln

casehardening tests 165

equalizing, conditioning treatments 164

fire prevention 167

intermediate moisture content 164

intermittent steaming 164

kiln samples 159

modifying schedules 165

moisture content schedules 162

pulling a kiln charge 166

safety rules 166

schedules 159, 162, 165

starting the kün 160

sterilizing treatment 164

time schedules 163

warmup, operation after 161

warmup, spray during 161

warmup time 161

Osage-orange

drying schedules 121

drying time 144

moisture content 9

names 4

specific gravity 13

weights 15

Partly dried lumber, storage of 172, 173, 174

Paneling, resawed, drying of __ 140

Patented equipment, restrictions on use of __ 2

Pecan

drying schedules .- 121

names -- 5

Pencil slats, incense-cedar, drying of __ 140

Persimmon

drying defects 157

drying schedules -- 121

drying time 144

moisture content 9

names -- 5

shrinkage.-. 17

specific gravity _- 13

weights -- 15

Pine, eastern white

drying defects 156

drying schedules

antibrown stain 131, 133, 135

moisture content 131, 132, 133

time 136

drying time _- 144

moisture content 9

names 5

shrinkage 16

specific gravity 13

weights 14

Pine, Idaho white 5

Pine, jack 5

Pine, loblolly

drying defects 156

drying schedules 131, 132, 136

drying time 144

moisture content 9

Page

Pine, loblolly—Continued

names 6

shrinkage 16

specific gravity 13

weights 14

Pine, lodgepole

drying defects 156

drying schedules 131, 136

drying time 144

moisture content 9

names 5

shrinkage 16

specific gravity 13

weights 14

Pine, longleaf

drying defects 156

drying schedules 131, 132, 136

drying time 144

moisture content 9

names 5

shrinkage 16

specific gravity 13

weights 14

Pine, northern white 5

Pine, Norway 5

Pine, ponderosa

drying defects 156

drying schedules

antibrown stain 131

heartwood, sapwood 131

moisture content 131, 132

time 136, 140

drying time 144

moisture content 9

names 5

shrinkage 16

specific gravity 13

weights 14

Pine, red

drying schedules 131, 132

drying time 144

moisture content 9

names 5

shrinkage 16

specific gravity 13

weights 14

Pine, shortleaf

drying defects 156

drying schedules 131, 132, 136

drying time 144

moisture content 9

names 6

shrinkage 16

specific gravity 13

weights 14

Pine, southern yellow (See also: individual species)

drying defects 156

drying schedules, moisture content 131, 132, 133

drying schedules, time 136, 138

drying time 144

names 6

Pine, sugar

drying defects 156

drying schedules 131, 136

drying time 144

moisture content ^

names 5

shrinkage 16

specific gravity 13

weights 14

Pine, western white „

drying defects 15°

drying schedules 131, 132, 13o

drying time 14*

moisture content .
INDEX 195

Page

Pine, western white—Continued

names 5

shrinkage 16

specific gravity 13

weights 14

Pinking, hickory 120, 153

Pith, tree 7

Plainsawed board 7

Planer splitting 153

Poles, drying of 140

Poplar

drying schedules 121

drying time 144

moisture content 9

names 5

Popple 3

Preservative-treated stock, schedules 128

Prevention of fires (See: Fire in kün)

Progressive kiln, operation of 166

Proprietary terms 1

Pulleys, fan 76

PuUing the kiln charge 166

Quartersawed board 7

Radiation 161

Rails, supports, maintenance of 67

Random-length lumber, piling 88

Rays, wood 7

Recording-controlling instruments 67

cahbration 71

cleaning 70

dry bulbs, location of 67, 68

loose linkage 70

sensitivity 70

wet bulbs, location of 69

Relative humidity, table 11

Red gum (See: Sweetgum)

Redwood

drying defects 156

drying schedules 131, 132, 136, 139

drying time 144

moisture content 9

names 6

shrinkage 16

specific gravity 13

weights 14

Resawed paneling, drying of 140

Ring failure '_ 149, 182

Roller spht 153

Roof decking, drying of 141

Safety in kiln operations 166

Sample pockets 103

Sap stain 152

Sapwood 6

drying of ""'////. 100

moisture content, table 9

Sassafras 5

Scales, sample 102

Schedules, kün 117

adjustment of 'Í24, 165

avoiding drying defects 166

modification of 124, 165

Shafts, fan ~ 7g

Shingles, drying of 140

Shoe-last blanks 128

Shrinkage, wood 15

Shuttle blanks, drying schedules 128

Side grain 7

Silverbell _ ._"" '_ 5

"Sinker" stock ll\l"\lVilll"l"[\[[yi^, 139

»oft pitch ^53

Softwoods

difference from hardwoods _ . .. 6

names '_ 5

Page

Softwood drying schedules (See also: Tables, softwood

drying schedules) 129

general moisture content 131, 132

air-dried lumber 132

examples 133

modification of 132

schedules 131, 132

special schedules

bevel siding 140

brown-stain control 135

bundled stock 140

cedar oil retention 135

chemically treated wood 139

crossties 140

knotty pine 141

pitch setting 135

poles 140

resawed paneling 140

roof decking 141

tank stock 141

thin stock 141

timbers 140

veneer 141

Venetian blinds 140

wide stock 140

time-temperature 136

examples 138, 139

kiln samples 134

modification of 135

Southern yellow pine (See: Pine, southern yellow)

Species (See individual species)

names 3

hardwoods 3

softwoods 5

Special grain 7

Splits 148, 149

Spray lines

steam 75

water 75

Spray, use of 161, 162

Springwood 7

Spruce, black

drying schedules 131, 132

drying time 144

moisture content 9

names 6

shrinkage 16

specific gravity 13

weights 14

Spruce, eastern

drying schedules 131, 132

drying time 144

names 6

Spruce, Engelmann

drying schedules 131, 132, 136

drying time I44

moisture content 9

names 6

shrinkage 16

specific gravity 13

weights 14

Spruce, red

drying schedules 131, 132

drying time 144

names 6

shrinkage ig

specific gravity 13

Spruce, Sitka

drying schedules 31^ 132, 136

drying time 144

moisture content 9

names Q
196 AGRÜCULTÜRE HANDBOOK NO. 18 8, U.S. DEPT. OF AGRICULTURE

Page

Spruce, Sitka—Continued

shrinkage 16

specific gravity 13

weights 14

Spruce, white

drying schedules 131, 132

drying time 144

names 6

specific gravity 13

Squares, drying schedules for 120, 121, 140

Stacking for kiln-drying 79

box piling 88

cover boards 96

in forced-circulation kiln 92

in natural-circulation kiln 92

kiln sample pockets 96

multiple-course stacking 93

sorting 79

stickers 82

care of 85

guides 84

load supports 82

material 82

moisture content of 82

racks for 85

size 82

spacing 82

Stain

blue 152

brown 19, 135, 152

sticker 21, 153

Starting the kiln 160

Staves, stacking of 93

Steaming, initial 161

Steam traps 74

Sterilizing 141

Stickers 82

Storage of wood 169

logs... 169

lumber

outdoor 169

shed 172

Stress tests, final 105, 165

Structure, wood 6

Summerwood 7

Sweetgum

drying defects 157

drying schedules 121

drying time 144

moisture content 9

names 4

shrinkage 17

specific gravity 13

weights 15

Sycamore

drying schedules 121

drying time 144

moisture content 9

names 5

shrinkage 17

specific gravity 13

weights 15

Tables, hardwood drying schedules

general

assembled 122, 123

code number index 120, 121

H-type 125, 126

moisture content classes 119

temperature 119

wet-bulb 119

special

hickory 127

killing beetles 142

túpelo 127

Page

Tables, softwood drying schedules

moisture content

assembled 133

code number index 131, 132

moisture content and temperature 130

moisture content and wet-bulb 130

time

assembled 138, 140, 141

code number index 136

time and temperature 134

time and wet-bulb 134

Tamarack

drying schedules 131, 132

drying time 144

moisture content 9

names 6

stirinkage 16

specific gravity 13

weights 14

Tank stock, drying of 141

Tanoak

drying defects 157

drying schedules 121

moisture content 9

names 5

specific gravity 13

Temperature

automatic control of 44

equipment 57

manual control of 45

wet-bulb. 78, 163

Temperature schedules, hardwood 119

Tension wood 8

Texture of wood 7

Thermometers 57

Timbers, drying of 140

Time schedules, softwood 134, 136, 138

Treatments, equalizing and conditioning

conditioning 143, 164

conditioning EMC ]lj

control of.. - 1"*

EMC values

table.. 11

use of --- 164

equalizing 142, Jo*

extended conditioning time. 1°^

final moisture content range 1°^

temperature reduction l"*"

Treatment, sterilizing ,„ ,„,

insects... —- 42, 64

mold 141,164

stain and decay l**^' '^^^

Trucks, kiln ''

Tupelo, black

drying schedules 121, 124, l¿i

drying time 1 ^

moisture content ï

names Í

shrinkage - :i

specific gravity |^

weights

Tupelo, swamp .,-

drying schedules 121, 124, í¿I

moisture content Ï

names

Tupelo, water .._

drying defects --- jqi

drying schedules 121,12'*, |*'

drying time 1?

moisture content , ^

names ^2

shrinkage ji

specific gravity H

weights
INDEX 197

Fase

Twist 7, 150

Urea-treated Douglas-fir, schedules for 139

Unit heaters 37

Valves, control

automatic -5^. ¡y

hand 39, 74

pressure-reducing 39, 73

Veneer, battery shims, schedules for 141

Venetian bUnds, drying of 140

Venting systems

operation of 76

regulation of 76, 162

Walnut

drying defects 157

drying schedules 121

drying time 144

moisture content 9

names 5

shrinkage 17

specific gravity 13

weights 15

Warp

bow --- 7, 150

control of 82, 150

crook 8, 150

cup 150

diamonding 150

twist 7, 150

Water, free and bound 8

Water pockets 8

Western redcedar

names 5

roof decking, drying of 141

Wet bulb (See: Control bulbs)

depression 8

temperature, final 163

Wicks, control-bulb 69

Willow

drying defects 157

drying schedules 121

drying time 141

moisture content 9

Page

Willow—Continued

names 5

shrinkage 17

specific gravity 13

weights 15

Wood

chemical stains in 19, 152

color of 19

electrical properties of 19

moisture content of, table 9

physical characteristics of

gross structural features 6

variation in structure 7

shrinkage of

calculation of 15

effects of 18, 19

longitudinal, radial and tangential 16

table 16

specific gravity 13

storage of 169

weights of 14

Wood, drying

factors affecting

grain 100

heartwood, sapwood 100

moisture content 99

species 99

thickness 99

moisture movement during 10

Wood-moisture relations 8

Woods, foreign, drying of 129

Yellow-poplar

drying defects 157

drying schedules 121

drying time 144

moisture content 9

names 5

shrinkage 17

specific gravity 13

weights 15

Yew, Pacific

names 6

shrinkage of 16

U.S. GOVERNMENT PRINTING OFFICE; 1961 O S5S31S