7. In drying refractory hardwoods, such as oak, best results

are obtained at a comparatively low temperature. In more

easily dried hardwoods, such as maple, and some of the more

difficult softwoods, as cypress, the process may be hastened

by a higher temperature but not above the boiling point. In

many of the softwoods, the rate of drying may be very

greatly increased by heating above the boiling point with a

large circulation of vapor at atmospheric pressure.

8. Unequal shrinkage between the exterior and interior

portions of the wood and also unequal chemical changes must

be guarded against by temperatures and humidities suited to

the species in question to prevent subsequent cupping and

warping.

9. The degree of dryness attained should conform to the use

to which the wood is put.

10. Proper piling of the material and weighting to prevent

warping are of great importance.

Requirements in a Satisfactory Dry Kiln

The requirements in a satisfactory dry kiln are:

1. Control of humidity at all times.

2. Ample air circulation at all points.

3. Uniform and proper temperatures.

In order to meet these requirements the United States Forestry Service

has designed a kiln in which the humidity, temperature, and

circulation can be controlled at all times.

Briefly, it consists of a drying chamber with a partition on either

side, making two narrow side chambers open top and bottom.

The steam pipes are in the usual position underneath the material to

be dried.

At the top of the side chambers is a spray; at the bottom are gutters

and an eliminator or set of baffle plates to separate the fine mist

from the air.

The spray accomplishes two things: It induces an increased circulation

and it regulates the humidity. This is done by regulating the

temperature of the spray water.

The air under the heating coil is saturated at whatever temperature

is required. This temperature is the dew point of the air after it

passes up into the drying chamber above the coils. Knowing the

temperature in the drying room and the dew point, the relative

humidity is thus determined.

The relative humidity is simply the ratio of the vapor pressure at the

dew point to the pressure of saturated vapor (see Fig. 30).

[Illustration: Fig. 30. Section through United States

Forestry Service Humidity-controlled Dry Kiln.]

Theory and Description of the Forestry Service Kiln

The humidities and temperatures in the piles of lumber are largely

dependent upon the circulation of air within the kiln. The temperature

and humidity within the kiln, taken alone, are no criterion of the

conditions of drying the pile of lumber if the circulation in any

portion is deficient. It is possible to have an extremely rapid

circulation of air within the dry kiln itself and yet have stagnation

within the individual piles, the air passing chiefly through open

spaces and channels. Wherever stagnation exists or the movement of air

is too sluggish the temperature will drop and the humidity increase,

perhaps to the point of saturation.

When in large kilns the forced circulation is in the opposite

direction from that induced by the cooling of the air by the lumber,

there is always more or less uncertainty as to the movement of the air

through the piles. Even with the boards placed edge-wise, with

stickers running vertically, and with the heating pipes beneath the

lumber, it was found that although the air passed upward through most

of the spaces it was actually descending through others, so that very

unequal drying resulted. While edge piling would at first thought seem

ideal for the freest circulation in an ordinary kiln with steam pipes

below, it in fact produces an indeterminate condition; air columns may

pass downward through some channels as well as upward through others,

and probably stagnate in still others. Nevertheless, edge piling is

greatly superior to flat piling where the heating system is below the

lumber.

From experiments and from study of conditions in commercial kilns the

idea was developed of so arranging the parts of the kiln and the pile

of lumber that advantage might be taken of this cooling of the air to

assist the circulation. That this can be readily accomplished without

doing away with the present features of regulation of humidity by

means of a spray of water is clear from Fig. 30, which shows a

cross-section of the improved humidity-regulated dry kiln.

In the form shown in the sketch a chamber or flue B runs through the

center near the bottom. This flue is only about 6 or 7 feet in height

and, together with the water spray F and the baffle plates DD,

constitutes the humidity-control feature of the kiln. This control of

humidity is affected by the temperature of the water used in the

spray. This spray completely saturates the air in the flue B at

whatever predetermined temperature is required. The baffle plates DD

are to separate all entrained particles of water from the air, so that

it is delivered to the heaters in a saturated condition at the

required temperature. This temperature is, therefore, the dew point of

the air when heated above, and the method of humidity control may

therefore be called the dew-point method. It is a very simple matter

by means of the humidity diagram (see Fig. 93), or by a hygrodeik

(Fig. 94), to determine what dew-point temperature is needed for any

desired humidity above the heaters.

Besides regulating the humidity the spray F also acts as an ejector

and forces circulation of air through the flue B. The heating system H

is concentrated near the outer walls, so as to heat the rising column

of air. The temperature within the drying chamber is controlled by

means of any suitable thermostat, actuating a valve on the main steam

line. The lumber is piled in such a way that the stickers slope

downward toward the sides of the kiln.

M is an auxiliary steam spray pointing downward for use at very high

temperatures. C is a gutter to catch the precipitation and conduct it

back to the pump, the water being recirculated through the sprays. G

is a pipe condenser for use toward the end of the drying operation. K

is a baffle plate for diverting the heated air and at the same time

shielding the under layers of boards from direct radiation of the

steam pipes.

The operation of the kiln is simple. The heated air rises above the

pipes HH and between the piles of lumber. As it comes in contact with

the piles, portions of it are cooled and pass downward and outward

through the layers of boards into the space between the condensers GG.

Here the column of cooled air descends into the spray flue B, where

its velocity is increased by the force of the water spray. It then

passes out from the baffle plates to the heaters and repeats the

cycle.

One of the greatest advantages of this natural circulation method is

that the colder the lumber when placed in the kiln the greater is the

movement produced, under the very conditions which call for the

greatest circulation--just the opposite of the direct-circulation

method. This is a feature of the greatest importance in winter, when

the lumber is put into the kiln in a frozen condition. One truckload

of lumber at 60 per cent moisture may easily contain over 7,000 pounds

of ice.

In the matter of circulation the kiln is, in fact, seldom

regulatory--the colder the lumber the greater the circulation

produced, with the effect increased toward the cooler and wetter

portions of the pile.

Preliminary steaming may be used in connection with this kiln, but

experiments indicate that ordinarily it is not desirable, since the

high humidity which can be secured gives as good results, and being at

as low a temperature as desired, much better results in the case of

certain difficult woods like oak, eucalyptus, etc., are obtained.

This kiln has another advantage in that its operation is entirely

independent of outdoor atmospheric conditions, except that barometric

pressure will effect it slightly.

KILN-DRYING

Remarks

Drying is an essential part of the preparation of wood for

manufacture. For a long time the only drying process used or known was

air-drying, or the exposure of wood to the gradual drying influences

of the open air, and is what has now been termed "preliminary

seasoning." This method is without doubt the most successful and

effective seasoning, because nature performs certain functions in

air-drying that cannot be duplicated by artificial means. Because of

this, hardwoods, as a rule, cannot be successfully kiln-dried green or

direct from the saw.

Within recent years, considerable interest is awakening among wood

users in the operation of kiln-drying. The losses occasioned in

air-drying and in improper kiln-drying, and the necessity for getting

material dry as quickly as possible from the saw, for shipping

purposes and also for manufacturing, are bringing about a realization

of the importance of a technical knowledge of the subject.

The losses which occur in air-drying wood, through checking, warping,

staining, and rotting, are often greater than one would suppose. While

correct statistics of this nature are difficult to obtain, some idea

may be had of the amount of degrading of the better class of lumber.

In the case of one species of soft wood, Western larch, it is commonly

admitted that the best grades fall off sixty to seventy per cent in

air-drying, and it is probable that the same is true in the case of

Southern swamp oaks. In Western yellow pine, the loss is great, and in

the Southern red gum, it is probably as much as thirty per cent. It

may be said that in all species there is some loss in air-drying, but

in some easily dried species such as spruce, hemlock, maple, etc., it

is not so great.

It would hardly be correct to state at the present time that this loss

could be entirely prevented by proper methods of kiln-drying the green

lumber, but it is safe to say that it can be greatly reduced.

It is well where stock is kiln-dried direct from the saw or knife,

after having first been steamed or boiled--as in the case of veneers,

etc.,--to get them into the kiln while they are still warm, as they

are then in good condition for kiln-drying, as the fibres of the wood

are soft and the pores well opened, which will allow of forcing the

evaporation of moisture without much damage being done to the

material.

With softwoods it is a common practice to kiln-dry direct from the

saw. This procedure, however, is ill adapted for the hardwoods, in

which it would produce such warping and checking as would greatly

reduce the value of the product. Therefore, hardwoods, as a rule, are

more or less thoroughly air-dried before being placed in the dry kiln,

where the residue of moisture may be reduced to within three or four

per cent, which is much lower than is possible by air-drying only.

It is probable that for the sake of economy, air-drying will be

eliminated in the drying processes of the future without loss to the

quality of the product, but as yet no method has been discovered

whereby this may be accomplished.

The dry kiln has been, and probably still is, one of the most

troublesome factors arising from the development of the timber

industry. In the earlier days, before power machinery for the

working-up of timber products came into general use, dry kilns were

unheard-of, air-drying or seasoning was then relied upon solely to

furnish the craftsman with dry stock from which to manufacture his

product. Even after machinery had made rapid and startling strides on

its way to perfection, the dry kiln remained practically an unknown

quantity, but gradually, as the industry developed and demand for dry

material increased, the necessity for some more rapid and positive

method of seasoning became apparent, and the subject of artificial

drying began to receive the serious attention of the more progressive

and energetic members of the craft.

Kiln-drying which is an artificial method, originated in the effort to

improve or shorten the process, by subjecting the wood to a high

temperature or to a draught of heated air in a confined space or kiln.

In so doing, time is saved and a certain degree of control over the

drying operation is secured.

The first efforts in the way of artificial drying were confined to

aiding or hastening nature in the seasoning process by exposing the

material to the direct heat from fires built in pits, over which the

lumber was piled in a way to expose it to the heat rays of the fires

below. This, of course, was a primitive, hazardous, and very

unsatisfactory method, to say the least, but it marked the first step

in the evolution of the present-day dry kiln, and in that particular

only is it deserving of mention.

Underlying Principles

In addition to marking the first step in artificial drying, it

illustrated also, in the simplest manner possible, the three

underlying principles governing all drying problems: (1) The

application of heat to evaporate or volatilize the water contained in

the material; (2) with sufficient air in circulation to carry away in

suspension the vapor thus liberated; and (3) with a certain amount of

humidity present to prevent the surface from drying too rapidly while

the heat is allowed to penetrate to the interior. The last performs

two distinct functions: (a) It makes the wood more permeable to the

passage of the moisture from the interior of the wood to the surface,

and (b) it supplies the latent heat necessary to evaporate the

moisture after it reaches the surface. The air circulation is

important in removing the moisture after it has been evaporated by the

heat, and ventilation also serves the purpose of bringing the heat in

contact with the wood. If, however, plain, dry heat is applied to the

wood, the surface will become entirely dry before the interior

moisture is even heated, let alone removed. This condition causes

"case-hardening" or "hollow-horning." So it is very essential that

sufficient humidity be maintained to prevent the surface from drying

too rapidly, while the heat is allowed to penetrate to the interior.

This humidity or moisture is originated by the evaporation from the

drying wood, or by the admission of steam into the dry kiln by the use

of steam spray pipes, and is absolutely necessary in the process of

hastening the drying of wood. With green lumber it keeps the sap near

the surface of the piece in a condition that allows the escape of the

moisture from its interior; or, in other words, it prevents the

outside from drying first, which would close the pores and cause

case-hardening.

The great amount of latent heat necessary to evaporate the water after

it has reached the surface is shown by the fact that the evaporation

of only one pound of water will extract approximately 66 degrees from

1,000 cubic feet of air, allowing the air to drop in temperature from

154 to 84 degrees Fahrenheit. In addition to this amount of heat, the

wood and the water must also be raised to the temperature at which the

drying is to be accomplished.

It matters not what type of dry kiln is used, source or application of

heating medium, these underlying principles remain the same, and must

be the first things considered in the design or selection of the

equipment necessary for producing the three essentials of drying:

Heat, humidity, and circulation.

Although these principles constitute the basis of all drying problems

and must, therefore, be continually carried in mind in the

consideration of them, it is equally necessary to have a comprehensive

understanding of the characteristics of the materials to be dried, and

its action during the drying process. All failures in the past, in the

drying of timber products, can be directly attributed to either the

kiln designer's neglect of these things, or his failure to carry them

fully in mind in the consideration of his problems.

Wood has characteristics very much different from those of other

materials, and what little knowledge we have of it and its properties

has been taken from the accumulated records of experience. The reason

for this imperfect knowledge lies in the fact that wood is not a

homogeneous material like the metals, but a complicated structure, and

so variable that one stick will behave in a manner widely different

from that of another, although it may have been cut from the same

tree.

The great variety of woods often makes the mere distinction of the

kind or species of the tree most difficult. It is not uncommon to find

men of long experience disagree as to the kind of tree a certain piece

of lumber was cut from, and, in some cases, there is even a wide

difference in the appearance and evidently the structure of timber cut

from the same tree.

Objects of Kiln-drying

The objects of kiln-drying wood may be placed under three main

headings: (1) To reduce shipping expenses; (2) to reduce the quantity

necessary to maintain in stock; and (3) to reduce losses in air-drying

and to properly prepare the wood for subsequent use. Item number 2

naturally follows as a consequence of either 1 or 3. The reduction in

weight on account of shipping expenses is of greatest significance

with the Northwestern lumbermen in the case of Douglas fir, redwood,

Western red cedar, sugar pine, bull pine, and other softwoods.

Very rapid methods of rough drying are possible with some of these

species, and are in use. High temperatures are used, and the water is

sometimes boiled off from the wood by heating above 212 degrees

Fahrenheit. These high-temperature methods will not apply to the

majority of hardwoods, however, nor to many of the softwoods.

It must first of all be recognized that the drying of lumber is a

totally different operation from the drying of a fabric or of thin

material. In the latter, it is largely a matter of evaporated

moisture, but wood is not only hygroscopic and attracts moisture from

the air, but its physical behavior is very complex and renders the

extraction of moisture a very complicated process.

An idea of its complexity may be had by mentioning some of the

conditions which must be contended with. Shrinkage is, perhaps, the

most important. This is unequal in different directions, being twice

as great tangentially as radially and fifty times as great radially as

longitudinally. Moreover, shrinkage is often unequal in different

portions of the same piece. The slowness of the transfusion of

moisture through the wood is an important factor. This varies with

different woods and greatly in different directions. Wood becomes soft

and plastic when hot and moist, and will yield more or less to

internal stresses. As some species are practically impervious to air

when wet, this plasticity of the cell walls causes them to collapse as

the water passes outward from the cell cavities. This difficulty has

given much trouble in the case of Western red cedar, and also to some

extent in redwood. The unequal shrinkage causes internal stresses in

the wood as it dries, which results in warping, checking,

case-hardening, and honeycombing. Case-hardening is one of the most

common defects in improperly dried lumber. It is clearly shown by the

cupping of the two halves when a case-hardened board is resawed.

Chemical changes also occur in the wood in drying, especially so at

higher temperatures, rendering it less hygroscopic, but more brittle.

If dried too much or at too high a temperature, the strength and

toughness is seriously reduced.

Conditions of Success

Commercial success in drying therefore requires that the substance be

exposed to the air in the most efficient manner; that the temperature

of the air be as high as the substance will stand without injury, and

that the air change or movement be as rapid as is consistent with

economical installation and operation. Conditions of success therefore

require the observance of the following points, which embody the basic

principles of the process: (1) The timber should be heated through

before drying begins. (2) The air should be very humid at the

beginning of the drying process, and be made drier only gradually. (3)

The temperature of the lumber must be maintained uniformly throughout

the entire pile. (4) Control of the drying process at any given

temperature must be secured by controlling the relative humidity, not

by decreasing the circulation. (5) In general, high temperatures

permit more rapid drying than do lower temperatures. The higher the

temperature of the lumber, the more efficient is the kiln. It is

believed that temperatures as high as the boiling point are not

injurious to most woods, providing all other fundamentally important

features are taken care of. Some species, however, are not able to

stand as high temperatures as others, and (6) the degree of dryness

attained, where strength is the prime requisite, should not exceed

that at which the wood is to be used.

Different Treatment according to Kind

The rapidity with which water may be evaporated, that is, the rate of

drying, depends on the size and shape of the piece and on the

structure of the wood. Thin stock can be dried much faster than thick,

under the same conditions of temperature, circulation, and humidity.

Pine can be dried, as a general thing, in about one third of the time

that would be required for oak of the same thickness, although the

former contains the more water of the two. Quarter-sawn oak usually

requires half again as long as plain oak. Mahogany requires about the

same time as plain oak; ash dries in a little less time, and maple,

according to the purpose for which it is intended, may be dried in one

fifth the time needed for oak, or may require a slightly longer

treatment. For birch, the time required is from one half to two

thirds, and for poplar and basswood, from, one fifth to one third that

required for oak.

All kinds and thicknesses of lumber cannot be dried at the same time

in the same kiln. It is manifest that green and air-dried lumber,

dense and porous lumber, all require different treatment. For

instance, Southern yellow pine when cut green from the log will stand

a very high temperature, say 200 degrees Fahrenheit, and in fact this

high temperature is necessary together with a rapid circulation of air

in order to neutralize the acidity of the pitch which causes the wood

to blue and discolor. This lumber requires to be heated up immediately

and to be kept hot throughout the length of the kiln. Hence the kiln

must not be of such length as to allow of the air being too much

cooled before escaping.

Temperature depends

While it is true that a higher temperature can be carried in the kiln

for drying pine and similar woods, this does not altogether account

for the great difference in drying time, as experience has taught us

that even when both woods are dried in the same kiln, under the same

conditions, pine will still dry much faster, proving thereby that the

structure of the wood itself affects drying.

The aim of all kiln designers should be to dry in the shortest

possible time, without injury to the material. Experience has

demonstrated that high temperatures are very effective in evaporating

water, regardless of the degree of humidity, but great care must be

exercised in using extreme temperatures that the material to be dried

is not damaged by checking, case-hardening, or hollow-horning.

The temperature used should depend upon the species and condition of

the material when entering the kiln. In general, it is advantageous to

have as high a temperature as possible, both for economy of operation

and speed of drying, but the physical properties of the wood will

govern this.

Many species cannot be dried satisfactorily at high temperatures on

account of their peculiar behavior. This is particularly so with green

lumber.

Air-dried wood will stand a relatively higher temperature, as a rule,

than wet or green wood. In drying green wood direct from the saw, it

is usually best to start with a comparatively low temperature, and not

raise the temperature until the wood is nearly dry. For example, green

maple containing about 60 per cent of its dry weight in water should

be started at about 120 degrees Fahrenheit and when it reaches a

dryness of 25 per cent, the temperature may be raised gradually up to

190 degrees.

It is exceedingly important that the material be practically at the

same temperature throughout if perfect drying is to be secured. It

should be the same temperature in the center of a pile or car as on

the outside, and the same in the center of each individual piece of

wood as on its surface. This is the effect obtained by natural

air-drying. The outside atmosphere and breezes (natural air

circulation) are so ample that the heat extracted for drying does not

appreciably change the temperature.

When once the wood has been raised to a high temperature through and

through and especially when the surface has been rendered most

permeable to moisture, drying may proceed as rapidly as it can be

forced by artificial circulation, provided the heat lost from the wood

through vaporization is constantly replaced by the heat of the kiln.

It is evident that to secure an even temperature, a free circulation

of air must be brought in contact with the wood. It is also evident

that in addition to heat and a circulation of air, the air must be

charged with a certain amount of moisture to prevent surface drying or

case-hardening.

There are some twenty-five different makes of dry kilns on the market,

which fulfill to a varying degree the fundamental requirements.

Probably none of them succeed perfectly in fulfilling all.

It is well to have the temperature of a dry kiln controlled by a

thermostat which actuates the valve on the main steam supply pipe. It

is doubly important to maintain a uniform temperature and avoid

fluctuations in the dry kiln, since a change in temperature will

greatly alter the relative humidity.

In artificial drying, temperatures of from 150 to 180 degrees

Fahrenheit are usually employed. Pine, spruce, cypress, cedar, etc.,

are dried fresh from the saw, allowing four days for 1-inch stuff.

Hardwoods, especially oak, ash, maple, birch, sycamore, etc., are

usually air-seasoned for three to six months to allow the first

shrinkage to take place more gradually, and are then exposed to the

above temperatures in the kiln for about six to ten days for 1-inch

stuff, other dimensions in proportion.

Freshly cut poplar and cottonwood are often dried direct from the saw

in a kiln. By employing lower temperatures, 100 to 120 degrees

Fahrenheit, green oak, ash, etc., can be seasoned in dry kilns without

much injury to the material.

Steaming and sweating the wood is sometimes resorted to in order to

prevent checking and case-hardening, but not, as has been frequently

asserted, to enable the material to dry.

Air Circulation

Air circulation is of the utmost importance, since no drying whatever

can take place when it is lacking. The evaporation of moisture

requires heat and this must be supplied by the circulating air.

Moreover, the moisture laden air must be constantly removed and fresh,

drier air substituted. Probably this is the factor which gives more

trouble in commercial operations than anything else, and the one which

causes the greatest number of failures.

It is necessary that the air circulate through every part of the kiln

and that the moving air come in contact with every portion of the

material to be dried. In fact, the humidity is dependent upon the

circulation. If the air stagnates in any portion of the pile, then the

temperature will drop and the humidity rise to a condition of

saturation. Drying will not take place at this portion of the pile and

the material is apt to mould and rot.

The method of piling the material on trucks or in the kiln, is

therefore, of extreme importance. Various methods are in use. Ordinary

flat piling is probably the poorest. Flat piling with open chimney

spaces in the piles is better. But neither method is suitable for a

kiln in which the circulation is mainly vertical.

Edge piling with stickers running vertically is in use in kilns when

the heating coils are beneath. This is much better.

Air being cooled as it comes in contact with a pile of material,

becomes denser, and consequently tends to sink. Unless the material to

be dried is so arranged that the air can pass gradually downward

through the pile as it cools, poor circulation is apt to result.

In edge-piled lumber, with the heating system beneath the piles, the

natural tendency of the cooled air to descend is opposed by the hot

air beneath which tends to rise. An indeterminate condition is thus

brought about, resulting in non-uniform drying. It has been found that

air will rise through some layers and descend through others.

Humidity

Humidity is of prime importance because the rate of drying and

prevention of checking and case-hardening are largely dependent

thereon. It is generally true that the surface of the wood should not

dry more rapidly than the moisture transfuses from the center of the

piece to its surface, otherwise disaster will result. As a sufficient

amount of moisture is removed from the wood to maintain the desired

humidity, it is not good economy to generate moisture in an outside

apparatus and force it into a kiln, unless the moisture in the wood is

not sufficient for this purpose; in that case provision should be made

for adding any additional moisture that may be required.

The rate of evaporation may best be controlled by controlling the

amount of vapor present in the air (relative humidity); it should not

be controlled by reducing the air circulation, since a large

circulation is needed at all times to supply the necessary heat.

The humidity should be graded from 100 per cent at the receiving end

of the kiln, to whatever humidity corresponds with the desired degree

of dryness at the delivery end.

The kiln should be so designed that the proper degree may be

maintained at its every section.

A fresh piece of sapwood will lose weight in boiling water and can

also be dried to quite an extent in steam. This proves conclusively

that a high degree of humidity does not have the detrimental effect on

drying that is commonly attributed to it. In fact, a proper degree of

humidity, especially in the loading or receiving end of a kiln, is

just as necessary to good results in drying as getting the proper

temperature.

Experiments have demonstrated also that injury to stock in the way of

checking, warping, and hollow-horning always develops immediately

after the stock is taken into the kiln, and is due to the degree of

humidity being too low. The receiving end of the kiln should always be

kept moist, where the stock has not been steamed before being put into

the kiln. The reason for this is simple enough. When the air is too

dry it tends to dry the outside of the material first--which is termed

"case-hardening"--and in so doing shrinks and closes up the pores of

the wood. As the stock is moved down the kiln, it absorbs a

continually increasing amount of heat, which tends to drive off the

moisture still present in the center of the stock. The pores on the

outside having been closed up, there is no exit for the vapor or steam

that is being rapidly formed in the center. It must find its way out

some way, and in doing so sets up strains, which result either in

checking, warping, or hollow-horning. If the humidity had been kept

higher, the outside of the material would not have dried so quickly,

and the pores would have remained open for the exit of moisture from

the interior of the wood, and this trouble would have been avoided.

Where the humidity is kept at a high point in the receiving end of the

kiln, a higher rate of temperature may also be carried, and in that

way the drying process is hastened with comparative safety.

It is essential, therefore, to have an ample supply of heat through

the convection currents of the air; but in the case of wood the rate

of evaporation must be controlled, else checking will occur. This can

be done by means of the relative humidity, as stated before. It is

clear now that when the air--or, more properly speaking, the space--is

completely saturated no evaporation can take place at the given

temperature. By reducing the humidity, evaporation takes place more

and more rapidly.

Another bad feature of an insufficient and non-uniform supply of heat

is that each piece of wood will be heated to the evaporating point on

the outer surface, the inside remaining cool until considerable drying

has taken place from the surface. Ordinarily in dry kilns high

humidity and large circulation of air are antitheses to one another.

To obtain the high humidity the circulation is either stopped

altogether or greatly reduced, and to reduce the humidity a greater

circulation is induced by opening the ventilators or otherwise

increasing the draft. This is evidently not good practice, but as a

rule is unavoidable in most dry kilns of present make. The humidity

should be raised to check evaporation without reducing the circulation

if possible.

While thin stock, such as cooperage and box stuff is less inclined to

give trouble by undue checking than 1-inch and thicker, one will find

that any dry kiln will give more uniform results and, at the same

time, be more economical in the use of steam, when the humidity and

temperature is carried at as high a point as possible without injury

to the material to be dried.

Any well-made dry kiln which will fulfill the conditions required as

to circulation and humidity control should work satisfactorily; but

each case must be studied by itself, and the various factors modified

to suit the peculiar conditions of the problem in hand. In every new

case the material should be constantly watched and studied and, if

checking begins, the humidity should be increased until it stops. It

is not reducing the circulation, but adding the necessary moisture to

the air, that should be depended on to prevent checking. For this

purpose it is well to have steam jets in the kiln so that if needed

they are ready at hand.

Kiln-drying

There are two distinct ways of handling material in dry kilns. One way

is to place the load of lumber in a chamber where it remains in the

same place throughout the operation, while the conditions of the

drying medium are varied as the drying progresses. This is the

"apartment" kiln or stationary method. The other is to run the lumber

in at one end of the chamber on a wheeled truck and gradually move it

along until the drying process is completed, when it is taken out at

the opposite end of the kiln. It is the usual custom in these kilns to

maintain one end of the chamber moist and the other end dry. This is

known as the "progressive" type of kiln, and is the one most commonly

used in large operations.

It is, however, the least satisfactory of the two where careful drying

is required, since the conditions cannot be so well regulated and the

temperatures and humidities are apt to change with any change of wind.

The apartment method can be arranged so that it will not require any

more kiln space or any more handling of lumber than the progressive

type. It does, however, require more intelligent operation, since the

conditions in the drying chamber must be changed as the drying

progresses. With the progressive type the conditions, once properly

established, remain the same.

To obtain draft or circulation three methods are in use--by forced

draft or a blower usually placed outside the kiln, by ventilation, and

by internal circulation and condensation. A great many patents have

been taken out on different methods of ventilation, but in actual

operation few kilns work exactly as intended. Frequently the air moves

in the reverse direction for which the ventilators were planned.

Sometimes a condenser is used in connection with the blower and the

air is recirculated. It is also--and more satisfactorily--used with

the gentle internal-gravity currents of air.

Many patents have been taken out for heating systems. The differences

among these, however, have more to do the mechanical construction than

with the process of drying. In general, the heating is either direct

or indirect. In the former steam coils are placed in the chamber with

the lumber, and in the latter the air is heated by either steam coils

or a furnace before it is introduced into the drying chamber.

Moisture is sometimes supplied by means of free steam jets in the kiln

or in the entering air; but more often the moisture evaporated from

the lumber is relied upon to maintain the humidity necessary.

A substance becomes dry by the evaporation of its inherent moisture

into the surrounding space. If this space be confined it soon becomes

saturated and the process stops. Hence, constant change is necessary

in order that the moisture given off may be continually carried away.

In practice, air movement, is therefore absolutely essential to the

process of drying. Heat is merely a useful accessory which serves to

decrease the time of drying by increasing both the rate of evaporation

and the absorbing power of the surrounding space.

It makes no difference whether this space is a vacuum or filled with

air; under either condition it will take up a stated weight of vapor.

From this it appears that the vapor molecules find sufficient space

between the molecules of air. But the converse is not true, for

somewhat less air will be contained in a given space saturated with

vapor than in one devoid of moisture. In other words the air does not

seem to find sufficient space between the molecules of vapor.

If the temperature of the confined space be increased, opportunity

will thereby be provided for the vaporization of more water, but if it

be decreased, its capacity for moisture will be reduced and visible

water will be deposited. The temperature at which this takes place is

known as the "dew-point" and depends upon the initial degree of

saturation of the given space; the less the relative saturation the

lower the dew-point.

Careful piling of the material to be dried, both in the yard and dry

kiln, is essential to good results in drying.

Air-dried material is not dry, and its moisture is too unevenly

distributed to insure good behavior after manufacture.

It is quite a difficult matter to give specific or absolute correct

weights of any species of timber when thoroughly or properly dried, in

order that one may be guided in these kiln operations, as a great deal

depends upon the species of wood to be dried, its density, and upon

the thickness which it has been cut, and its condition when entering

the drying chamber.

Elm will naturally weigh less than beech, and where the wood is

close-grained or compact it will weigh more than coarse-grained wood

of the same species, and, therefore, no set rules can be laid down, as

good judgment only should be used, as the quality of the drying is not

purely one of time. Sometimes the comparatively slow process gives

excellent results, while to rush a lot of stock through the kiln may

be to turn it out so poorly seasoned that it will not give

satisfaction when worked into the finished product. The mistreatment

of the material in this respect results in numerous defects, chief

among which are warping and twisting, checking, case-hardening, and

honeycombing, or, as sometimes called, hollow-horning.

Since the proportion of sap and heartwood varies with size, age,

species, and individual trees, the following figures as regards weight

must be regarded as mere approximations:

POUNDS OF WATER LOST IN DRYING 100 POUNDS OF GREEN WOOD IN THE KILN

=========================================================================

|Sapwood or | Heartwood

|outer part | or interior

=========================================================================

| |

(1) Pine, cedar, spruce, and fir | 45-65 | 16-25

(2) Cypress, extremely variable | 50-65 | 18-60

(3) Poplar, cottonwood, and basswood | 60-65 | 40-60

(4) Oak, beech, ash, maple, birch, elm, hickory,| |

chestnut, walnut, and sycamore | 40-50 | 30-40

=========================================================================

The lighter kinds have the most water in the sapwood; thus sycamore

has more water than hickory, etc.

The efficiency of the drying operations depends a great deal upon the

way in which, the lumber is piled, especially when the humidity is not

regulated. From the theory of drying it is evident that the rate of

evaporation in dry kilns where the humidity is not regulated depends

entirely upon the rate of circulation, other things being equal.

Consequently, those portions of the wood which receive the greatest

amount of air dry the most rapidly, and vice versa. The only way,

therefore, in which anything like uniform drying can take place is

where the lumber is so piled that each portion of it comes in contact

with the same amount of air.

In the Forestry Service kiln (Fig. 30), where the degree of relative

humidity is used to control the rate of drying, the amount of

circulation makes little difference, provided it exceeds a certain

amount. It is desirable to pile the lumber so as to offer as little

frictional resistance as possible and at the same time secure uniform

circulation. If circulation is excessive in any place it simply means

waste of energy but no other injury to the lumber.

The best method of piling is one which permits the heated air to pass

through the pile in a somewhat downward direction. The natural

tendency of the cooled air to descend is thus taken advantage of in

assisting the circulation in the kiln. This is especially important

when cold or green lumber is first introduced into the kiln. But even

when the lumber has become warmed the cooling due to the evaporation

increases the density of the mixture of the air and vapor.

Kiln-drying Gum

The following article was published by the United States Forestry

Service as to the best method of kiln-drying gum:

=Piling.=--Perhaps the most important factor in good kiln-drying,

especially in the case of the gums, is the method of piling. It is our

opinion that proper and very careful piling will greatly reduce the

loss due to warping. A good method of piling is to place the lumber

lengthwise of the kiln and on an incline cross-wise. The warm air

should rise at the higher side of the pile and descend between the

courses of lumber. The reason for this is very simple and the

principle has been applied in the manufacture of the best ice boxes

for some time. The most efficient refrigerators are iced at the side,

the ice compartment opening to the cooling chamber at the top and

bottom. The warm air from above is cooled by melting the ice. It then

becomes denser and settles down into the main chamber. The articles in

the cooling room warm the air as they cool, so it rises to the top and

again comes in contact with the ice, thus completing the cycle. The

rate of this natural circulation is automatically regulated by the

temperature of the articles in the cooling chamber and by the amount

of ice in the icing compartment; hence the efficiency of such a box is

high.

Now let us apply this principle to the drying of lumber. First we must

understand that as long as the lumber is moist and drying, it will

always be cooler than the surrounding air, the amount of this

difference being determined by the rate of drying and the moisture in

the wood. As the lumber dries, its temperature gradually rises until

it is equal to that of the air, when perfect dryness results. With

this fact in mind it is clear that the function of the lumber in a

kiln is exactly analogous to that of the ice in an ice box; that is,

it is the cooling agent. Similarly, the heating pipes in a dry kiln

bring about the same effect as the articles of food in the ice box in

that they serve to heat the air. Therefore, the air will be cooled by

the lumber, causing it to pass downward through the piles. If the

heating units are placed at the sides of the kiln, the action of the

air in a good ice box is duplicated in the kiln. The significant point

in this connection is that, the greener and colder the lumber, the

faster is the circulation. This is a highly desirable feature.

A second vital point is that as the wood becomes gradually drier the

circulation automatically decreases, thus resulting in increased

efficiency, because there is no need for circulation greater than

enough to maintain the humidity of the air as it leaves the lumber

about the same as it enters. Therefore, we advocate either the

longitudinal side-wise inclined pile or edge stacking, the latter

being much preferable when possible. Of course the piles in our kiln

were small and could not be weighted properly, so the best results as

to reducing warping were not obtained.

=Preliminary Steaming.=--Because the fibres of the gums become plastic

while moist and hot without causing defects, it is desirable to heat

the air-dried lumber to about 200 degrees Fahrenheit in saturated

steam at atmospheric pressure in order to reduce the warping. This

treatment also furnishes a means of heating the lumber very rapidly.

It is probably a good way to stop the sap-staining of green lumber, if

it is steamed while green. We have not investigated the other effects

of steaming green gum, however, so we hesitate to recommend it.

Temperatures as high as 210 degrees Fahrenheit were used with no

apparent harm to the material. The best result was obtained with the

temperature of 180 degrees Fahrenheit, after the first preliminary

heating in steam to 200 degrees Fahrenheit. Higher temperatures may be

used with air-dried gum, however.

The best method of humidity control proved to be to reduce the

relative humidity of the air from 100 per cent (saturated steam) very

carefully at first and then more rapidly to 30 per cent in about four

days. If the change is too marked immediately after the steaming

period, checking will invariably result. Under these temperature and

humidity conditions the stock was dried from 15 per cent moisture,

based on the dry wood weight, to 6 per cent in five days' time. The

loss due to checking was about 5 per cent, based on the actual footage

loss, not on commercial grades.

=Final Steaming.=--From time to time during the test runs the material

was resawed to test for case-hardening. The stock dried in five days

showed slight case-hardening, so it was steamed at atmospheric

pressure for 36 minutes near the close of the run, with the result

that when dried off again the stresses were no longer present. The

material from one run was steamed for three hours at atmospheric

pressure and proved very badly case-hardened, but in the reverse

direction. It seems possible that by testing for the amount of

case-hardening one might select a final steaming period which would

eliminate all stresses in the wood.

Kiln-drying of Green Red Gum

The following article was published by the United States Forestry

Service on the kiln-drying of green red gum:

A short time ago fifteen fine, red-gum logs 16 feet long were received

from Sardis, Miss. They were in excellent condition and quite green.

It has been our belief that if the gum could be kiln-dried directly

from the saw, a number of the difficulties in seasoning might be

avoided. Therefore, we have undertaken to find out whether or not such

a thing is feasible. The green logs now at the laboratory are to be

used in this investigation. One run of a preliminary nature has just

been made, the method and results of which I will now tell.

This method was really adapted to the drying of Southern pine, and one

log of the green gum was cut into 1-inch stock and dried with the

pine. The heartwood contained many knots and some checks, although it

was in general of quite good quality. The sapwood was in fine

condition and almost as white as snow.

This material was edge-stacked with one crosser at either end and one

at the center, of the 16-foot board. This is sufficient for the pine,

but was absolutely inadequate for drying green gum. A special

shrinkage take-up was applied at the three points. The results proved

very interesting in spite of the warping which was expected with but

three crossers in 16 feet. The method of circulation described was

used. It is our belief that edge piling is best for this method.

This method of kiln-drying depends on the maintenance of a high

velocity of slightly superheated steam through the lumber. In few

words, the object is to maintain the temperature of the vapor as it

leaves the lumber at slightly above 212 degrees Fahrenheit. In order

to accomplish this result, it is necessary to maintain the high

velocity of circulation. As the wood dries, the superheat may be

increased until a temperature of 225 degrees or 230 degrees Fahrenheit

of the exit air is recorded.

The 1-inch green gum was dried from 20.1 per cent to 11.4 per cent

moisture, based on the dry wood weight in 45 hours. The loss due to

checking was 10 per cent. Nearly every knot in the heartwood was

checked, showing that as the knots could be eliminated in any case,

this loss might not be so great. It was significant that practically

all of the checking occurred in the heartwood. The loss due to warping

was 22 per cent. Of course this was large; but not nearly enough

crossers were used for the gum. It is our opinion that this loss due

to warping can be very much reduced by using at least eight crossers

and providing for taking up of the shrinkage. A feature of this

process which is very important is that the method absolutely prevents

all sap staining.

Another delightful surprise was the manner in which the superheated

steam method of drying changed the color of the sapwood from pure

white to a beautifully uniform, clean-looking, cherry red color which

very closely resembles that of the heartwood. This method is not new

by any means, as several patents have been granted on the steaming of

gum to render the sapwood more nearly the color of the heartwoods. The

method of application in kiln-drying green gum we believe to be new,

however. Other methods for kiln-drying this green stock are to be

tested until the proper process is developed. We expect to have

something interesting to report in the near future.[1]

[Footnote 1: The above test was made at the United States

Forestry Service Laboratory, Madison, Wis.]

SECTION XII

TYPES OF DRY KILNS

DIFFERENT TYPES OF DRY KILNS

Dry kilns as in use to-day are divided into two classes: The "pipe" or

"moist-air" kiln, in which natural draft is relied upon for

circulation and, the "blower" or "hot blast" kiln, in which the

circulation is produced by fans or blowers. Both classes have their

adherents and either one will produce satisfactory results if properly

operated.

The "Blower" or "Hot Blast" Kiln

The blower kiln in its various types has been in use so long that it

is hardly necessary to give to it a lengthy introduction. These kilns

at their inauguration were a wonderful improvement over the old style

"bake-oven" or "sweat box" kiln then employed, both on account of the

improved quality of the material and the rapidity at which it was

dried.

These blower kilns have undergone steady improvement, not only in the

apparatus and equipment, but also in their general design, method of

introducing air, and provision for controlling the temperature and

humidity. With this type of kiln the circulation is always under

absolute control and can be adjusted to suit the conditions, which

necessarily vary with the conditions of the material to be dried and

the quantity to be put through the kiln.

In either the blower or moist-air type of dry kiln, however, it is

absolutely essential, in order to secure satisfactory results, both as

to rapidity in drying and good quality of stock, that the kiln be so

designed that the temperature and humidity, together with circulation,

are always under convenient control. Any dry kiln in which this has

not been carefully considered will not give the desired results.

In the old style blower kiln, while the circulation and temperature

was very largely under the operator's control, it was next to

impossible to produce conditions in the receiving end of the kiln so

that the humidity could be kept at the proper point. In fact, this was

one reason why the natural draft, or so-called moist-air kiln was

developed.

The advent of the moist-air kiln served as an education to kiln

designers and manufacturers, in that it has shown conclusively the

value of a proper degree of humidity in the receiving end of any

progressive dry kiln, and it has been of special benefit also in that

it gave the manufacturers of blower kilns an idea as to how to improve

the design of their type of kiln to overcome the difficulty referred

to in the old style blower kilns. This has now been remedied, and in a

decidedly simple manner, as is usually the case with all things that

possess merit.

It was found that by returning from one third to one half of the moist

air _after_ having passed through the kiln back to the fan room and by

mixing it with the fresh and more or less dry air going into the

drying room, that the humidity could be kept under convenient control.

The amount of air that can be returned from a kiln of this class

depends upon three things: (1) The condition of the material when

entering the drying room; (2) the rapidity with which the material is

to be dried; and (3) the condition of the outside atmosphere. In the

winter season it will be found that a larger proportion of air may be

returned to the drying room than in summer, as the air during the

winter season contains considerably less moisture and as a consequence

is much drier. This is rather a fortunate coincidence, as, when the

kiln is being operated in this manner, it will be much more economical

in its steam consumption.

In the summer season, when the outside atmosphere is saturated to a

much greater extent, it will be found that it is not possible to

return as great a quantity of air to the drying room, although there

have been instances of kilns of this class, which in operation have

had all the air returned and found to give satisfactory results. This

is an unusual condition, however, and can only be accounted for by

some special or peculiar condition surrounding the installation.

In some instances, the desired amount of humidity in a blower type of

kiln is obtained by the addition of a steam spray in the receiving end

of the kiln, much in the same manner as that used in the moist-air

kilns. This method is not as economical as returning the

moisture-laden air from the drying room as explained in the preceding

paragraph.

With the positive circulation that may be obtained in a blower kiln,

and with the conditions of temperature and humidity under convenient

control, this type of kiln has the elements most necessary to produce

satisfactory drying in the quickest possible elapsed time.

It must not be inferred from this, however, that this class of dry

kiln may be installed and satisfactory results obtained regardless of

how it is handled. A great deal of the success of any dry kiln--or any

other apparatus, for that matter--depends upon intelligent operation.

Operation of the "Blower" Dry Kiln

It is essential that the operator be supplied with proper facilities

to keep a record of the material as it is placed into the drying room,

and when it is taken out. An accurate record should be kept of the

temperature every two or three hours, for the different thicknesses

and species of lumber, that he may have some reliable data to guide

him in future cases.

Any man possessing ordinary intelligence can operate dry kilns and

secure satisfactory results, providing he will use good judgment and

follow the basic instructions as outlined below:

1. When cold and before putting into operation, heat the

apparatus slowly until all pipes are hot, then start the fan

or blower, gradually bringing it up to its required speed.

2. See that _all_ steam supply valves are kept wide open,

unless you desire to lengthen the time required to dry the

material.

3. When using exhaust steam, the valve from the header

(which is a separate drip, independent of the trap

connection) must be kept wide open, but must be closed when

live steam is used on that part of the heater.

4. The engines as supplied by the manufacturers are

constructed to operate the fan or blower at a proper speed

with its throttle valve wide open, and with not less than 80

pounds pressure of steam.

5. If the return steam trap does not discharge regularly, it

is important that it be opened and thoroughly cleaned and

the valve seat re-ground.

6. As good air circulation is as essential as the proper

degree of heat, and as the volume of air and its contact

with the material to be dried depends upon the volume

delivered by the fan or blower, it is necessary to maintain

a regular and uniform speed of the engine.

7. Atmospheric openings must always be maintained in the fan

or heater room for fresh air supply.

8. Successful drying cannot be accomplished without ample

and free circulation of air at all times.

If the above instructions are fully carried out, and good judgment

used in the handling and operation of the blower kiln, no difficulties

should be encountered in successfully drying the materials at hand.

The "Pipe" or "Moist-air" Dry Kiln

While in the blower class of dry kiln, the circulation is obtained by

forced draft with the aid of fans or blowers, in the Moist-air kilns

(see Fig. 31); the circulation is obtained by natural draft only,

aided by the manipulation of dampers installed at the receiving end of

the drying room, which lead to vertical flues through a stack to the

outside atmosphere.

The heat in these kilns is obtained by condensing steam in coils of

pipe, which are placed underneath the material to be dried. As the

degree of heat required, and steam pressure govern the amount of

radiation, there are several types of radiating coils. In Fig. 32 will

be seen the Single Row Heating Coils for live or high pressure steam,

which are used when the low temperature is required. Figure 33 shows

the Double (or 2) Row Heating Coils for live or high pressure steam.

This apparatus is used when a medium temperature is required. In Fig.

34 will be seen the Vertical Type Heating Coils which is recommended

where exhaust or low-pressure steam is to be used, or may be used with

live or high-pressure steam when high temperatures are desired.

[Illustration: Fig. 31. Section through a typical Moist-air

Dry Kiln.]

These heating coils are usually installed in sections, which permit

any degree of heat from the minimum to the maximum to be maintained by

the elimination of, or the addition of, any number of heating

sections. This gives a dry kiln for the drying of green softwoods, or

by shutting off a portion of the radiating coils--thus reducing the

temperature--a dry kiln for drying hardwoods, that will not stand the

maximum degree of heat.

[Illustration: Fig. 32. Single Pipe Heating Apparatus for Dry

Kilns, arranged for the Use of Live Steam. For Low

Temperatures.]

[Illustration: Fig. 33. Double Pipe Heating Apparatus for Dry

Kilns, arranged for the Use of Live Steam. For Medium

Temperatures.]

In the Moist-air or Natural Draft type of dry kiln, any degree of

humidity, from clear and dry to a dense fog may be obtained; this is

in fact, the main and most important feature of this type of dry kiln,

and the most essential one in the drying of hardwoods.

It is not generally understood that the length of a kiln has any

effect upon the quantity of material that may be put through it, but

it is a fact nevertheless that long kilns are much more effective, and

produce a better quality of stock in less time than kilns of shorter

length.

Experience has proven that a kiln from 80 to 125 feet in length will

produce the best results, and it should be the practice, where

possible, to keep them within these figures. The reason for this is

that in a long kiln there is a greater drop in temperature between the

discharge end and the green or receiving end of the kiln.

It is very essential that the conditions in the receiving end of the

kiln, as far as the temperature and humidity are concerned, must go

hand in hand.

It has also been found that in a long kiln the desired conditions may

be obtained with higher temperatures than with a shorter kiln;

consequently higher temperatures may be carried in the discharge end

of the kiln, thereby securing greater rapidity in drying. It is not

unusual to find that a temperature of 200 degrees Fahrenheit is

carried in the discharge end of a long dry kiln with safety, without

in any way injuring the quality of the material, although, it would be

better not to exceed 180 degrees in the discharge end, and about 120

degrees in the receiving or green end in order to be on the safe side.

Operation of the "Moist-air" Dry Kiln

To obtain the best results these kilns should be kept in continuous

operation when once started, that is, they should be operated

continuously day and night. When not in operation at night or on

Sundays, and the kiln is used to season green stock direct from the

saw, the large doors at both ends of the kiln should be opened wide,

or the material to be dried will "sap stain."

[Illustration: Fig. 34. Vertical Pipe Heating Apparatus for

Dry Kilns; may be used in Connection with either Live or

Exhaust Steam for High or Low Temperatures.]

It is highly important that the operator attending any drying

apparatus keep a minute and accurate record of the condition of the

material as it is placed into the drying room, and its final condition

when taken out.

Records of the temperature and humidity should be taken frequently and

at stated periods for the different thicknesses and species of

material, in order that he may have reliable data to guide him in

future operations.

The following facts should be taken into consideration when operating

the Moist-air dry kiln:

1. Before any material has been placed in the drying room,

the steam should be turned into the heating or radiating

coils, gradually warming them, and bringing the temperature

in the kiln up to the desired degree.

2. Care should be exercised that there is sufficient

humidity in the receiving or loading end of the kiln, in

order to guard against checking, case-hardening, etc.

Therefore it is essential that the steam spray at the

receiving or loading end of the kiln be properly

manipulated.

3. As the temperature depends principally upon the pressure

of steam carried in the boilers, maintain a steam pressure

of not less than 80 pounds at all times; it may range as

high as 100 pounds. The higher the temperature with its

relatively high humidity the more rapidly the drying will be

accomplished.

4. Since air circulation is as essential as the proper

degree of heat, and as its contact with the material to be

dried depends upon its free circulation, it is necessary

that the dampers for its admittance into, and its exit from,

the drying room be efficiently and properly operated.

Successful drying cannot be accomplished without ample and

free circulation of air at all times during the drying

process.

If the above basic principles are carefully noted and followed out,

and good common sense used in the handling and operation of the kiln

apparatus, no serious difficulties should arise against the successful

drying of the materials at hand.

Choice of Drying Method

At this point naturally arises the question: Which of the two classes

of dry kilns, the "Moist-air" or "Blower" kiln is the better adapted

for my particular needs?

This must be determined entirely by the species of wood to be dried,

its condition when it goes into the kiln, and what kind of finished

product is to be manufactured from it.

Almost any species of hardwood which has been subjected to

air-seasoning for three months or more may be dried rapidly and in the

best possible condition for glue-jointing and fine finishing with a

"Blower" kiln, but green hardwood, direct from the saw, can only be

successfully dried (if at all) in a "Moist-air" kiln.

Most furniture factories have considerable bent stock which must of

necessity be thoroughly steamed before bending. By steaming, the

initial process of the Moist-air kiln has been consummated. Hence, the

Blower kiln is better adapted to the drying of such stock than the

Moist-air kiln would be, as the stock has been thoroughly soaked by

the preliminary steaming, and all that is required is sufficient heat

to volatilize the moisture, and a strong circulation of air to remove

it as it comes to the surface.

The Moist-air kiln is better adapted to the drying of tight cooperage

stock, while the Blower kiln is almost universally used throughout the

slack cooperage industry for the drying of its products.

For the drying of heavy timbers, planks, blocks, carriage stock, etc.,

and for all species of hardwood thicker than one inch, the Moist-air

kiln is undoubtedly the best.

Both types of kilns are equally well adapted to the drying of 1-inch

green Norway and white pine, elm, hemlock, and such woods as are used

in the manufacture of flooring, ceiling, siding, shingles, hoops, tub

and pail stock, etc.

The selection of one or the other for such work is largely matter of

personal opinion.

Kilns of Different Types

All dry kilns as in use to-day are divided as to method of drying into

two classes:

The "Pipe" or "Moist-air" kiln;

The "Blower" or "Hot Blast" kiln;

both of which have been fully explained in a previous article.

The above two classes are again subdivided into five different types

of dry kilns as follows:

The "Progressive" kiln;

The "Apartment" kiln;

The "Pocket" kiln;

The "Tower" kiln;

The "Box" kiln.

The "Progressive" Dry Kiln

Dry kilns constructed so that the material goes in at one end and is

taken out at the opposite end are called Progressive dry kilns, from

the fact that the material gradually progresses through the kiln from

one stage to another while drying (see Fig. 31).

In the operation of the Progressive kiln, the material is first

subjected to a sweating or steaming process at the receiving or

loading end of the kiln with a low temperature and a relative high

humidity. It then gradually progresses through the kiln into higher

temperatures and lower humidities, as well as changes of air

circulation, until it reaches the final stage at the discharge end of

the kiln.

Progressive kilns, in order to produce the most satisfactory results,

especially in the drying of hardwoods or heavy softwood timbers,

should be not less than 100 feet in length (see Fig. 35).

In placing this type of kiln in operation, the following instructions

should be carefully followed:

When steam has been turned into the heating coils, and the kiln is

fairly warm, place the first car of material to be dried in the drying

room--preferably in the morning--about 25 feet from the kiln door on

the receiving or loading end of the kiln, blocking the wheels so that

it will remain stationary.

[Illustration: Fig. 35. Exterior View of Four Progressive Dry

Kilns, each 140 Feet long by 18 Feet wide. Cross-wise piling,

fire-proof construction.]

Five hours later, or about noon, run in the second car and stop it

about five feet from the first one placed in the drying room. Five

hours later, or in the evening push car number two up against the

first car; then run in car number three, stopping it about five feet

from car number two.

On the morning of the second day, push car number three against the

others, and then move them all forward about 25 feet, and then run in

car number four, stopping it about five feet from the car in advance

of it. Five hours later, or about noon, run in car number five and

stop it about five feet from car number four. In the evening or about

five hours later, push these cars against the ones ahead, and run in

loaded car number six, stopping it about five feet from the preceding

car.

On the morning of the third day, move all the cars forward about six

feet; then run in loaded car number seven stop it about four feet from

the car preceding it. Five hours later or about noon push this car

against those in advance of it, and run in loaded car number eight

moving all cars forward about six feet, and continue in this manner

until the full complement of cars have been placed in the kiln. When

the kiln has been filled, remove car number one and push all the

remaining cars forward and run in the next loaded car, and continue in

this manner as long as the kiln is in operation.

As the temperature depends principally upon the pressure of steam,

maintain a steam pressure of not less than 80 pounds at all times; it

may range up to as high as 100 pounds. The higher the temperature with

a relatively higher humidity the more rapidly the drying will be

accomplished.

If the above instructions are carried out, the temperatures,

humidities, and air circulation properly manipulated, there should be

complete success in the handling of this type of dry kiln.

The Progressive type of dry kiln is adapted to such lines of

manufacture that have large quantities of material to kiln-dry where

the species to be dried is of a similiar nature or texture, and does

not vary to any great extent in its thickness, such, for instance, as:

Oak flooring plants;

Maple flooring plants;

Cooperage plants;

Large box plants;

Furniture factories; etc.

In the selection of this kind of dry kiln, consideration should be

given to the question of ground space of sufficient length or

dimension to accommodate a kiln of proper length for successful

drying.

The "Apartment" Dry Kiln

The Apartment system of dry kilns are primarily designed for the

drying of different kinds or sizes of material at the same time, a

separate room or apartment being devoted to each species or size when

the quantity is sufficient (see Fig. 36).

These kilns are sometimes built single or in batteries of two or more,

generally not exceeding 40 or 50 feet in length with doors and

platforms at both ends the same as the Progressive kilns; but in

operation each kiln is entirely filled at one loading and then closed,

and the entire contents dried at one time, then emptied and again

recharged.

Any number of apartments may be built, and each apartment may be

arranged to handle any number of cars, generally about three or four,

or they may be so constructed that the material is piled directly upon

the floor of the drying room.

[Illustration: Fig. 36. Exterior View of Six Apartment Dry

Kilns, each 10 Feet wide by 52 Feet long, End-wise Piling.

They are entirely of fire-proof construction and equipped

with double doors (Hussey asbestos outside and canvas

inside), and are also equipped with humidity and air control

dampers, which may be operated from the outside without

opening the kiln doors, which is a very good feature.]

When cars are used, it is well to have a transfer car at each end of

the kilns, and stub tracks for holding cars of dry material, and for

the loading of the unseasoned stock, as in this manner the kilns may

be kept in full operation at all times.

In this type of dry kiln the material receives the same treatment and

process that it would in a Progressive kiln. The advantages of

Apartment kilns is manifest where certain conditions require the

drying of numerous kinds as well as thicknesses of material at one and

the same time. This method permits of several short drying rooms or

apartments so that it is not necessary to mix hardwoods and softwoods,

or thick and thin material in the same kiln room.

In these small kilns the circulation is under perfect control, so that

the efficiency is equal to that of the more extensive plants, and will

readily appeal to manufacturers whose output calls for the prompt and

constant seasoning of a large variety of small stock, rather than a

large volume of material of uniform size and grade.

Apartment kilns are recommended for industries where conditions

require numerous kinds and thicknesses of material to be dried, such

as:

Furniture factories;

Piano factories;

Interior woodwork mills;

Planing mills; etc.

The "Pocket" Dry Kiln

"Pocket" dry kilns (see Fig. 37) are generally built in batteries of

several pockets. They have the tracks level and the lumber goes in and

out at the same end. Each drying room is entirely filled at one time,

the material is dried and then removed and the kiln again recharged.

The length of "Pocket" kilns ranges generally from 14 feet to about 32

feet.

The interior equipment for this type of dry kiln is arranged very

similiar to that used in the Apartment kiln. The heating or radiating

coils and steam spray jets extend the whole length of the drying room,

and are arranged for the use of either live or exhaust steam, as

desired.

Inasmuch as Pocket kilns have doors at one end only, this feature

eliminates a certain amount of door exposure, which conduces towards

economy in operation.

In operating Pocket kilns, woods of different texture and thickness

should be separated and placed in different drying rooms, and each

kiln adjusted and operated to accommodate the peculiarities of the

species and thickness of the material to be dried.

[Illustration: Fig. 37. Exterior View of Five Pocket Dry

Kilns, built in Two Batteries with the Front of each Set

facing the other, and a Transfer System between. They are

also equipped with the asbestos doors.]

Naturally, the more complex the conditions of manufacturing wood

products in any industry, the more difficult will be the proper

drying of same. Pocket kilns, are, therefore, recommended for

factories having several different kinds and thicknesses of material

to dry in small quantities of each, such as:

Planing mills;

Chair factories;

Furniture factories;

Sash and door factories; etc.

The "Tower" Dry Kiln

The so-called "Tower" dry kiln (see Fig. 38) is designed for the rapid

drying of small stuff in quantities. Although the general form of

construction and the capacity of the individual bins or drying rooms

may vary, the same essential method of operation is common to all.

That is, the material itself, such as wooden novelties, loose staves,

and heading for tubs, kits, and pails, for box stuff, kindling wood,

etc., is dumped directly into the drying rooms from above, or through

the roof, in such quantities as effectually to fill the bin, from

which it is finally removed when dry, through the doors at the bottom.

These dry kilns are usually operated as "Blower" kilns, the heating

apparatus is generally located in a separate room or building adjacent

to the main structure or drying rooms, and arranged so that the hot

air discharged through the inlet duct (see illustration) is thoroughly

distributed beneath a lattice floor upon which rests the material to

be dried. Through this floor the air passes directly upward, between

and around the stock, and finally returns to the fan or heating room.

This return air duct is so arranged that by means of dampers, leading

from each drying room, the air may be returned in any quantity to the

fan room where it is mixed with fresh air and again used. This is one

of the main features of economy of the blower system of drying, as by

the employment of this return air system, considerable saving may be

made in the amount of steam required for drying.

[Illustration: Fig. 38. Exterior and Sectional View of a

Battery of Tower Dry Kilns. This is a "Blower" or "Hot Blast"

type, and shows the arrangement of the fan blower, engine,

etc. This type of dry kin is used principally for the

seasoning of small, loose material.]

The lattice floors in this type of dry kiln are built on an incline,

which arrangement materially lessens the cost, and increases the

convenience with which the dried stock may be removed from the bins or

drying rooms.

In operation, the material is conveyed in cars or trucks on an

overhead trestle--which is inclosed--from which the material to be

dried is dumped directly into the drying rooms or bins, through

hoppers arranged for that purpose thereby creating considerable saving

in the handling of the material to be dried into the kiln. The entire

arrangement thus secures the maximum capacity, with a minimum amount

of floor space, with the least expense. Of course, the higher these

kilns are built, the less relative cost for a given result in the

amount of material dried.

In some instances, these kilns are built less in height and up against

an embankment so that teamloads of material may be run directly onto

the roof of the kilns, and dumped through the hoppers into the drying

rooms or bins, thus again reducing to a minimum the cost of this

handling.

The return air duct plays an important part in both of these methods

of filling, permitting the air to become saturated to the maximum

desired, and utilizing much of the heat contained therein, which would

otherwise escape to the atmosphere.

The "Tower" kiln is especially adapted to factories of the following

class:

Sawmills;

Novelty factories;

Woodenware factories;

Tub and pail factories; etc.

The "Box" Dry Kiln

The "Box" kiln shown in Figure 39 is an exterior view of a kiln of

this type which is 20 feet wide, 19 feet deep, and 14 feet high, which

is the size generally used when the space will permit.

Box kilns are used mostly where only a small quantity of material is

to be dried. They are not equipped with trucks or cars, the material

to be dried being piled upon a raised platform inside the drying

room. This arrangement, therefore, makes them of less cost than the

other types of dry kilns.

They are particularly adapted to any and all species and size of

lumber to be dried in very small quantities.

[Illustration: Fig. 39. Exterior view of the Box Dry Kiln.

This particular kiln is 20 feet wide, 19 feet deep and 14

feet high. Box kilns are used mostly where only a small

amount of kiln-dried lumber of various sizes is required.

They are not equipped with trucks or cars, and therefore cost

less to construct than any other type of dry kiln.]

In these small kilns the circulation is under perfect control, so that

the efficiency is equal to that of the more extensive plants.

These special kilns will readily appeal to manufacturers, whose output

calls for the prompt and constant seasoning of a large variety of

small stock, rather than a large volume material of uniform size and

grade.

SECTION XIII

DRY KILN SPECIALTIES

KILN CARS AND METHOD OF LOADING

Within recent years, the edge-wise piling of lumber (see Figs. 40 and

41), upon kiln cars has met with considerable favor on account of its

many advantages over the older method of flat piling. It has been

proven that lumber stacked edge-wise dries more uniformly and rapidly,

and with practically no warping or twisting of the material, and that

it is finally discharged from the dry kiln in a much better and

brighter condition. This method of piling also considerably increases

the holding and consequent drying capacities of the dry kiln by reason

of the increased carrying capacities of the kiln cars, and the shorter

period of time required for drying the material.

[Illustration: Fig. 40. Car Loaded with Lumber on its Edges

by the Automatic Stacker, to go into the Dry Kiln cross-wise.

Equipped with two edge piling kiln trucks.]

In Figures 42 and 43 are shown different views of the automatic lumber

stacker for edge-wise piling of lumber on kiln cars. Many users of

automatic stackers report that the grade of their lumber is raised to

such an extent that the system would be profitable for this reason

alone, not taking into consideration the added saving in time and

labor, which to anyone's mind should be the most important item.

[Illustration: Fig. 41. Car Loaded with Lumber on its Edges

by the Automatic Stacker, to go into the Dry Kiln end-wise.

The bunks on which the lumber rests are channel steel. The

end sockets are malleable iron and made for I-beam stakes.]

In operation, the lumber is carried to these automatic stackers on

transfer chains or chain conveyors, and passes on to the stacker

table. When the table is covered with boards, the "lumber" lever is

pulled by the operator, which raises a stop, preventing any more

lumber leaving the chain conveyor. The "table" lever then operates the

friction drive and raises the table filled with the boards to a

vertical position. As the table goes up, it raises the latches, which

fall into place behind the piling strips that had been previously laid

on the table. When the table returns to the lower position, a new set

of piling strips are put in place on the table, and the stream of

boards which has been accumulating on the conveyor chain are again

permitted to flow onto the table. As each layer of lumber is added,

the kiln car is forced out against a strong tension. When the car is

loaded, binders are put on over the stakes by means of a powerful

lever arrangement.

[Illustration: Fig. 42. The above illustration shows the

construction of the Automatic Lumber Stacker for edge piling

of lumber to go into the dry kiln end-wise.]

[Illustration: Fig. 43. The above illustration shows the

construction of the Automatic Lumber Stacker for edge piling

of lumber to go into the dry kiln cross-wise.]

[Illustration: Fig. 44. The above illustration shows a

battery of Three Automatic Lumber Stackers.]

[Illustration: Fig. 45. The above illustration shows another

battery of Three Automatic Lumber Stackers.]

[Illustration: Fig. 46. Cars Loaded with Lumber on its Edges

by the Automatic Lumber Stackers.]

After leaving the dry kilns, the loaded car is transferred to the

unstacker (see Fig. 47). Here it is placed on the unstacker car which,

by means of a tension device, holds the load of lumber tight against

the vertical frame of the unstacker. The frame of the unstacker is

triangular and has a series of chains. Each chain has two special

links with projecting lugs. The chains all travel in unison. The lug

links engage a layer of boards, sliding the entire layer vertically,

and the boards, one at a time, fall over the top of the unstacker

frame onto the inclined table, and from there onto conveyor chains

from which they may be delivered to any point desired, depending upon

the length and direction of the chain conveyor.

With these unstackers one man can easily unload a kiln car in twenty

minutes or less.

[Illustration: Fig. 47. The Lumber Unstacker Car, used for

unloading cars of Lumber loaded by the Automatic Stacker.]

[Illustration: Fig. 48. The Lumber Unstacker Car and

Unstacker, used for unloading Lumber loaded by the Automatic

Stacker.]

The experience of many users prove that these edge stacking machines

are not alike. This is important, because there is one feature of edge

stacking that must not be overlooked. Unless each layer of boards is

forced into place by power and held under a strong pressure, much

slack will accumulate in an entire load, and the subsequent handling

of the kiln cars, and the effect of the kiln-drying will loosen up the

load until there is a tendency for the layers to telescope. And unless

the boards are held in place rigidly and with strong pressure they

will have a tendency to warp.

[Illustration: Fig. 49. The above illustration shows method

of loading kiln cars with veneer on its edges by the use of

the Tilting Platform.]

A kiln car of edge-stacked lumber, properly piled, is made up of

alternate solid sheets of lumber and vertical open-air spaces, so that

the hot air and vapors rise naturally and freely through the lumber,

drying both sides of the board evenly. The distribution of the heat

and moisture being even and uniform, the drying process is naturally

quickened, and there is no opportunity or tendency for the lumber to

warp.

In Figure 49 will be seen a method of loading kiln cars with veneer on

edge by the use of a tilting platform. On the right of the

illustration is seen a partially loaded kiln car tilted to an angle of

45 degrees, to facilitate the placing of the veneer on the car. At

the left is a completely loaded car ready to enter the dry kiln.

Gum, poplar, and pine veneers are satisfactorily dried in this manner

in from 8 to 24 hours.

In Figure 50 will be seen method of piling lumber on the flat,

"cross-wise" of the dry kiln when same has three tracks.

[Illustration: Fig. 50. Method of Loading lumber on its Flat,

cross-wise of the Dry Kiln when same has Three Tracks.]

In Figure 51 will be seen another method of piling lumber on the flat,

"cross-wise" of the dry kiln when same has three tracks.

In Figure 52 will be seen method of piling lumber on the flat,

"end-wise" of the dry kiln when same has two tracks.

In Figure 53 will be seen another method of piling lumber on the flat,

"end-wise" of the dry kiln when same has two tracks.

In Figure 54 will be seen method of piling slack or tight barrel

staves "cross-wise" of the kiln when same has three tracks.

In Figure 55 will be seen another method of piling slack or tight

barrel staves "cross-wise" of the dry kiln when same has three tracks.

In Figure 56 will be seen method of piling small tub or pail staves

"cross-wise" of the dry kiln when same has two tracks.

In Figure 57 will be seen method of piling bundled staves "cross-wise"

of the dry kiln when same has two tracks.

[Illustration: Fig. 51. Method of loading Lumber on its Flat,

cross-wise of the Dry Kiln when same has Three Tracks.]

[Illustration: Fig. 52. Method of loading Lumber on its Flat,

end-wise of the Dry Kiln by the Use of the Single-sill or

Dolly Truck.]

[Illustration: Fig. 53. Method of loading Lumber on its Flat,

end-wise of the Dry Kiln by the Use of the Double-sill

Truck.]

[Illustration: Fig. 54. Method of loading Kiln Car with Tight

or Slack Barrel Staves cross-wise of Dry Kiln.]

[Illustration: Fig. 55. Method of loading Kiln Car with Tight

or Slack Barrel Staves cross-wise of Dry Kiln.]

[Illustration: Fig. 56. Method of loading Kiln Car with Tub

or Pail Staves cross-wise of Dry Kiln.]

[Illustration: Fig. 57. Method of loading Kiln Car with

Bundled Staves cross-wise of Dry Kiln.]

In Figure 58 will be seen method of piling shingles "cross-wise" of

dry kiln when same has three tracks.

In Figure 59 will be seen another method of piling shingles

"cross-wise" of the dry kiln when same has three tracks.

[Illustration: Fig. 58. Method of loading Kiln Car with

Shingles cross-wise of Dry Kiln.]

[Illustration: Fig. 59. Method of loading Kiln Car with

Shingles cross-wise of Dry Kiln.]

In Figure 60 will be seen method of piling shingles "end-wise" of the

dry kiln when same has two tracks.

In Figure 61 will be seen a kiln car designed for handling short tub

or pail staves through a dry kiln.

[Illustration: Fig. 60. Car loaded with 100,000 Shingles.

Equipped with four long end-wise piling trucks and to go into

dry kiln end-wise.]

[Illustration: Fig. 61. Kiln Car designed for handling Short

Tub or Pail Staves through a Dry Kiln.]

In Figure 62 will be seen a kiln car designed for short piece stock

through a dry kiln.

In Figure 63 will be seen a type of truck designed for the handling of

stave bolts about a stave mill or through a steam box.

In Figure 64 will be seen another type of truck designed for the

handling of stave bolts about a stave mill or through a steam box.

In Figure 65 will be seen another type of truck designed for the

handling of stave bolts about a stave mill or through a steam box.

In Figure 66 will be seen another type of truck designed for the

handling of stave bolts about a stave mill or through a steam box.

In Figure 67 will be seen another type of truck designed for the

handling of stave bolts about a stave mill or through a steam box.

In Figure 68 will be seen another type of truck designed for the

handling of stave bolts about a stave mill or through a steam box.

In Figure 69 will be seen the Regular 3-rail Transfer Car designed for

the handling of 2-rail kiln cars which have been loaded "end-wise."

In Figure 70 will be seen another type of Regular 3-rail Transfer Car

designed for the handling of 2-rail kiln cars which have been loaded

"end-wise."

In Figure 71 will be seen a Specially-designed 4-rail Transfer Car for

2-rail kiln cars which have been built to accommodate extra long

material to be dried.

In Figure 72 will be seen the Regular 2-rail Transfer Car designed for

the handling of 3-rail kiln cars which have been loaded "cross-wise."

In Figure 73 will be seen another type of Regular 2-rail Transfer Car

designed for the handling of 3-rail kiln cars which have been loaded

"cross-wise."

In Figure 74 will be seen the Regular 2-rail Underslung type of

Transfer Car designed for the handling of 3-rail kiln cars which have

been loaded "cross-wise." Two important features in the construction

of this transfer car make it extremely easy in its operation. It has

extra large wheels, diameter 13-1/2 inches, and being underslung, the

top of its rails are no higher than the other types of transfer cars.

Note the relative size of the wheels in the illustration, yet the car

is only about 10 inches in height.

[Illustration: Fig. 62. Kiln Car Designed for handling Short

Piece Stock through a Dry Kiln.]

[Illustration: Fig. 63. A Stave Bolt Truck.]

[Illustration: Fig. 64. A Stave Bolt Truck.]

[Illustration: Fig. 65. A Stave Bolt Truck.]

[Illustration: Fig. 66. A Stave Bolt Truck.]

[Illustration: Fig. 67. A Stave Bolt Truck.]

[Illustration: Fig. 68. A Stave Bolt Truck.]

[Illustration: Fig. 69. A Regular 3-Rail Transfer Truck.]

[Illustration: Fig. 70. A Regular 3-Rail Transfer Truck.]

[Illustration: Fig. 71. A Special 4-Rail Transfer Truck.]

[Illustration: Fig. 72. A Regular 2-Rail Transfer Truck.]

[Illustration: Fig. 73. A Regular 2-Rail Transfer Truck.]

[Illustration: Fig. 74. A Regular 2-Rail Underslung Transfer

Truck.]

[Illustration: Fig. 75. A Regular 3-Rail Underslung Transfer

Truck.]

In Figure 75 will be seen the Regular 3-rail Underslung type of

Transfer Car designed for the handling of 2-rail kiln cars which have

been loaded "end-wise." This car also has the important features of

large diameter wheels and low rail construction, which make it very

easy in its operation.

[Illustration: Fig. 76. A Special 2-Rail Flexible Transfer

Truck.]

In Figure 76 will be seen the Special 2-rail Flexible type of Transfer

Car designed for the handling of 3-rail kiln cars which have been

loaded "cross-wise." This car is equipped with double the usual number

of wheels, and by making each set of trucks a separate unit (the front

and rear trucks being bolted to a steel beam with malleable iron

connection), a slight up-and-down movement is permitted, which enables

this transfer car to adjust itself to any unevenness in the track,

which is a very good feature.

In Figure 77 will be seen the Regular Transfer Car designed for the

handling of stave bolt trucks.

In Figure 78 will be seen another type of Regular Transfer Car

designed for the handling of stave bolt trucks.

In Figure 79 will be seen a Special Transfer Car designed for the

handling of stave bolt trucks.

[Illustration: Fig. 77. A Regular Transfer Car for handling

Stave Bolt Trucks.]

[Illustration: Fig. 78. A Regular Transfer Car for handling

Stave Bolt Trucks.]

[Illustration: Fig. 79. A Special Transfer Car for handling

Stave Bolt Trucks.]

In Figure 80 will be seen the Regular Channel-iron Kiln Truck designed

for edge piling "cross-wise" of the dry kiln.

In Figure 81 will be seen another type of Regular Channel-iron Kiln

Truck designed for edge piling "cross-wise" of the dry kiln.

[Illustration: Fig. 80. A Regular Channel-iron Kiln Truck.]

[Illustration: Fig. 81. A Regular Channel-iron Kiln Truck.]

In Figure 82 will be seen the Regular Channel-iron Kiln Truck designed

for flat piling "end-wise" of the dry kiln.

[Illustration: Fig. 82. A Regular Channel-iron Kiln Truck.]

[Illustration: Fig. 83. A Regular Channel-iron Kiln Truck.]

[Illustration: Fig. 84. A Regular Single-sill or Dolly Kiln

Truck.]

In Figure 83 will be seen the Regular Channel-iron Kiln Truck with

I-Beam cross-pieces designed for flat piling "end-wise" of the dry

kiln.

In Figure 84 will be seen the Regular Small Dolly Kiln Truck designed

for flat piling "end-wise" of the dry kiln.

Different Types of Kiln Doors

In Figure 85 will be seen the Asbestos-lined Door. The construction of

this kiln door is such that it has no tendency to warp or twist. The

framework is solid and the body is made of thin slats placed so that

the slat on either side covers the open space of the other with

asbestos roofing fabric in between. This makes a comparatively light

and inexpensive door, and one that absolutely holds the heat. These

doors may be built either swinging, hoisting, or sliding.

[Illustration: Fig. 85. An Asbestos-lined Kiln Door of the

Hinge Type.]

In Figure 86 will be seen the Twin Carrier type of door hangers with

doors loaded and rolling clear of the opening.

[Illustration: Fig. 86. Twin Carrier with Kiln Door loaded

and rolling clear of Opening.]

[Illustration: Fig. 87. Twin Carriers for Kiln Doors 18 to 35

Feet wide.]

In Figure 87 will be seen the Twin Carrier for doors 18 to 35 feet

wide, idle on a section of the track.

In Figure 88 will be seen another type of carrier for kiln doors.

In Figure 89 will be seen the preceding type of kiln door carrier in

operation.

In Figure 90 will be seen another type of carrier for kiln doors.

In Figure 91 will be seen kiln doors seated, wood construction,

showing 3-1/2" × 5-3/4" inch-track timbers and trusses, supported on

4-inch by 6-inch jamb posts. "T" rail track, top and side, inclined

shelves on which the kiln door rests. Track timber not trussed on

openings under 12 feet wide.

[Illustration: Fig. 88. Kiln Door Carrier engaged to Door

Ready for lifting.]

In Figure 92 will be seen kiln doors seated, fire-proof construction,

showing 12-inch, channel, steel lintels, 2" × 2" steel angle mullions,

track brackets bolted to the steel lintels and "T" rail track. No

track timbers or trusses used.

[Illustration: Fig. 89. Kiln Door Carrier shown on Doors of

Wood Construction.]

[Illustration: Fig. 90. Kiln Door Construction with Door

Carrier out of Sight.]

[Illustration: Fig. 91. Kiln Door Construction. Doors Seated.

Wood Construction.]

[Illustration: Fig. 92. Kiln Door Construction. Doors Seated.

Fire-proof Construction.]

SECTION XIV

HELPFUL APPLIANCES IN KILN-DRYING

The Humidity Diagram

[Illustration: Fig. 93. The United States Forest Service

Humidity Diagram for determination of Absolute Humidities.

Dew Points and Vapor Pressures; also Relative Humidities by

means of Wet and Dry-Bulb Thermometer, for any temperatures

and change in temperature.]

Some simple means of determining humidities and changes in humidity

brought about by changes in temperature in the dry kiln without the

use of tables is almost a necessity. To meet this requirement the

United States Forestry Service has devised the Humidity Diagram shown

in Figure 93. It differs in several respects from the hydrodeiks now

in use.

The purpose of the humidity diagram is to enable the dry-kiln operator

to determine quickly the humidity conditions and vapor pressure, as

well as the changes which take place with changes of temperature. The

diagram above is adapted to the direct solution of problems of this

character without recourse to tables or mathematical calculations.

The humidity diagram consists of two distinct sets of curves on the

same sheet. One set, the convex curves, is for the determination of

relative humidity of wet-and-dry-bulb hygrometer or psychrometer; the

other, the concave curves, is derived from the vapor pressures and

shows the amount of moisture per cubic foot at relative humidities and

temperatures when read at the dew-point. The latter curves, therefore,

are independent of all variables affecting the wet-bulb readings. They

are proportional to vapor pressures, not to density, and, therefore,

may be followed from one temperature to another with correctness. The

short dashes show the correction (increase or decrease) which is

necessary in the relative humidity, read from the convex curves, with

an increase or decrease from the normal barometric pressure of 30

inches, for which the curves have been plotted. This correction,

except for very low temperatures, is so small that it may usually be

disregarded.

The ordinates, or vertical distances, are relative humidity expressed

in per cent of saturation, from 0 per cent at the bottom to 100 per

cent at the top. The abscissae, or horizontal distances, are

temperatures in degrees Fahrenheit from 30 degrees below zero, at the

left, to 220 degrees above, at the right.

Examples of Use

The application of the humidity diagram can best be understood by

sample problems. These problems also show the wide range of conditions

to which the diagram will apply.

EXAMPLE 1. To find the relative humidity by use of

wet-and-dry-bulb hygrometer or psychrometer:

Place the instrument in a strong circulation of air, or wave

it to and fro. Read the temperature of the dry bulb and the

wet, and subtract. Find on the horizontal line the

temperature shown by the dry-bulb thermometer. Follow the

vertical line from this point till it intersects with the

convex curve marked with the difference between the wet and

dry readings. The horizontal line passing through this

intersection will give the relative humidity.

Example: Dry bulb 70°, wet bulb 62°, difference 8°. Find 70°

on the horizontal line of temperature. Follow up the

vertical line from 70° until it intersects with the convex

curve marked 8°. The horizontal line passing through this

intersection shows the relative humidity to be 64 per cent.

EXAMPLE 2. To find how much water per cubic foot is

contained in the air:

Find the relative humidity as in example 1. Then the nearest

concave curve gives the weight of water in grains per cubic

foot when the air is cooled to the dew-point. Using the same

quantities as in example 1, this will be slightly more than

5 grains.

EXAMPLE 3. To find the amount of water required to saturate

air at a given temperature:

Find on the top line (100 per cent humidity) the given

temperature; the concave curve intersecting at or near this

point gives the number of grains per cubic foot.

(Interpolate, if great accuracy is desired.)

EXAMPLE 4. To find the dew-point:

Obtain the relative humidity as in example 1. Then follow up

parallel to the nearest concave curve until the top

horizontal (indicating 100 per cent relative humidity) is

reached. The temperature on this horizontal line at the

point reached will be the dew-point.

Example: Dry bulb 70°, wet bulb 62°. On the vertical line

for 70° find the intersection with the hygrometer (convex)

curve for 8°. This will be found at nearly 64 per cent

relative humidity. Then follow up parallel with the vapor

pressure (concave) curve marked 5 grains to its intersection

at the top of the chart with the 100 per cent humidity line.

This gives the dew-point as 57°.

EXAMPLE 5. To find the change in the relative humidity

produced by a change in temperature:

Example: The air at 70° Fahr. is found to contain 64 per

cent humidity; what will be its relative humidity if heated

to 150° Fahr.? Starting from the intersection of the

designated humidity and temperature coordinates, follow the

vapor-pressure curve (concave) until it intersects the 150°

temperature ordinate. The horizontal line then reads 6 per

cent relative humidity. The same operation applies to

reductions in temperature. In the above example what is the

humidity at 60°? Following parallel to the same curve in the

opposite direction until it intersects the 60° ordinate

gives 90 per cent; at 57° it becomes 100 per cent, reaching

the dew-point.

EXAMPLE 6. To find the amount of condensation produced by

lowering the temperature:

Example: At 150° the wet bulb reads 132°. How much water

would be condensed if the temperature were lowered to 70°?

The intersection of the hygrometer curve for 18° (150°-132°)

with temperature line for 150° shows a relative humidity of

60 per cent. The vapor-pressure curve (concave) followed up

to the 100 per cent relative humidity line shows 45 grains

per cubic foot at the dew-point, which corresponds to a

temperature of 130°. At 70° it is seen that the air can

contain but 8 grains per cubic foot (saturation).

Consequently, there will be condensed 45 minus 8, or 37

grains per cubic foot of space measured at the dew-point.

EXAMPLE 7. To find the amount of water required to produce

saturation by a given rise in temperature:

Example: Take the values given in example 5. The air at the

dew-point contains slightly over 5 grains per cubic foot. At

150° it is capable of containing 73 grains per cubic foot.

Consequently, 73-5=68 grains of water which can be

evaporated per cubic foot of space at the dew-point when the

temperature is raised to 150°. But the latent heat necessary

to produce evaporation must be supplied in addition to the

heat required to raise the air to 150°.

EXAMPLE 8. To find the amount of water evaporated during a

given change of temperature and humidity:

Example: At 70° suppose the humidity is found to be 64 per

cent and at 150° it is found to be 60 per cent. How much

water has been evaporated per cubic foot of space? At 70°

temperature and 64 per cent humidity there are 5 grains of

water present per cubic foot at the dew-point (example 2).

At 150° and 60 per cent humidity there are 45 grains

present. Therefore, 45-5=40 grains of water which have been

evaporated per cubic foot of space, figuring all volumes at

the dew-point.

EXAMPLE 9. To correct readings of the hygrometer for changes

in barometric pressure:

A change of pressure affects the reading of the wet bulb.

The chart applies at a barometric pressure of 30 inches,

and, except for great accuracy, no correction is generally

necessary.

Find the relative humidity as usual. Then look for the

nearest barometer line (indicated by dashes). At the end of

each barometer line will be found a fraction which

represents the proportion of the relative humidity already

found, which must be added or subtracted for a change in

barometric pressure. If the barometer reading is less than

30 inches, add; if greater than 30 inches, subtract. The

figures given are for a change of 1 inch; for other changes

use proportional amounts. Thus, for a change of 2 inches use

twice the indicated ratio; for half an inch use half, and so

on.

Example: Dry bulb 67°, wet bulb 51°, barometer 28 inches.

The relative humidity is found, by the method given in

example 1, to equal 30 per cent. The barometric line--gives

a value of 3/100H for each inch of change. Since the

barometer is 2 inches below 30, multiply 3/100H by 2, giving

6/100H. The correction will, therefore, be 6/100 of 30,

which equals 1.8. Since the barometer is below 30, this is

to be added, giving a corrected relative humidity of 31.8

per cent.

This has nothing to do with the vapor pressure (concave)

curves, which are independent of barometric pressure, and

consequently does not affect the solution of the previous

problems.

EXAMPLE 10. At what temperature must the condenser be

maintained to produce a given humidity?

Example: Suppose the temperature in the drying room is to be

kept at 150° Fahr., and a humidity of 80 per cent is

desired. If the humidity is in excess of 80 per cent the air

must be cooled to the dew-point corresponding to this

condition (see example 4), which in this case is 141.5°.

Hence, if the condenser cools the air to this dew point the

required condition is obtained when the air is again heated

to the initial temperature.

EXAMPLE 11. Determination of relative humidity by the

dew-point:

The quantity of moisture present and relative humidity for

any given temperature may be determined directly and

accurately by finding the dew-point and applying the concave

(vapor-pressure) curves. This does away with the necessity

for the empirical convex curves and wet-and-dry-bulb

readings. To find the dew-point some form of apparatus,

consisting essentially of a thin glass vessel containing a

thermometer and a volatile liquid, such as ether, may be

used. The vessel is gradually cooled through the evaporation

of the liquid, accelerated by forcing air through a tube

until a haze or dimness, due to condensation from the

surrounding air, first appears upon the brighter outer

surface of the glass. The temperature at which the haze

first appears is the dew-point. Several trials should be

made for this temperature determination, using the average

temperature at which the haze appears and disappears.

To determine the relative humidity of the surrounding air by

means of the dew-point thus determined, find the concave

curve intersecting the top horizontal (100 per cent

relative humidity) line nearest the dew-point temperature.

Follow parallel with this curve till it intersects the

vertical line representing the temperature of the

surrounding air. The horizontal line passing through this

intersection will give the relative humidity.

Example: Temperature of surrounding air is 80; dew-point is

61; relative humidity is 53 per cent.

The dew-point determination is, however, not as convenient

to make as the wet-and-dry-bulb hygrometer readings.

Therefore, the hygrometer (convex) curves are ordinarily

more useful in determining relative humidities.

The Hygrodeik

In Figure 94 will be seen the Hygrodeik. This instrument is used to

determine the amount of moisture in the atmosphere. It is a very

useful instrument, as the readings may be taken direct with accuracy.

To find the relative humidity in the atmosphere, swing the index hand

to the left of the chart, and adjust the sliding pointer to that

degree of the wet-bulb thermometer scale at which the mercury stands.

Then swing the index hand to the right until the sliding pointer

intersects the curved line, which extends downwards to the left from

the degree of the dry-bulb thermometer scale, indicated by the top of

the mercury column in the dry-bulb tube.

At that intersection, the index hand will point to the relative

humidity on scale at bottom of chart (for example see Fig. 94). Should

the temperature indicated by the wet-bulb thermometer be 60 degrees,

and that of the dry-bulb 70 degrees, the index hand will indicate

humidity 55 degrees, when the pointer rests on the intersecting line

of 60 degrees and 80 degrees.

The Recording Hygrometer

In Figure 95 is shown the Recording Hygrometer complete with wet and

dry bulbs, two connecting tubes and two recording pens and special

moistening device for supplying water to the wet bulb.

This equipment is designed particularly for use in connection with dry

rooms and dry kilns and is arranged so that the recording instrument

and the water supply bottle may be installed outside of the dry kiln

or drying room, while the wet and dry bulbs are both installed inside

the room or kiln at the point where it is desired to measure the

humidity. This instrument records on a weekly chart the humidity for

each hour of the day, during the entire week.

[Illustration: Fig. 94. The Hygrodeik.]

The Registering Hygrometer

In Figure 96 is shown the Registering Hygrometer, which consists of

two especially constructed thermometers. The special feature of the

thermometers permits placing the instrument in the dry kiln without

entering the drying room, through a small opening, where it is left

for about 20 minutes.

[Illustration: Fig. 95. The Recording Hygrometer, Complete

with Wet and Dry Bulbs. This instrument records on a weekly

chart the humidity for each hour of the day, during the

entire week.]

The temperature of both the dry and wet bulbs are automatically

recorded, and the outside temperature will not affect the thermometers

when removed from the kiln. From these recorded temperatures, as shown

when the instrument is removed from the kiln, the humidity can be

easily determined from a simple form of chart which is furnished free

by the makers with each instrument.

The Recording Thermometer

[Illustration: Fig. 96. The Registering Hygrometer.]

[Illustration: Fig. 97. The Recording Thermometer.]

In Figure 97 is shown the Recording Thermometer for observing and

recording the temperatures within a dry kiln, and thus obtaining a

check upon its operation. This instrument is constructed to record

automatically, upon a circular chart, the temperatures prevailing

within the drying room at all times of the day and night, and serves

not only as a means of keeping an accurate record of the operation of

the dry kiln, but as a valuable check upon the attendant in charge of

the drying process.

[Illustration: Fig. 98. The Registering Thermometer.]

[Illustration: Fig. 99. The Recording Steam-Pressure Gauge.]

The Registering Thermometer

In Figure 98 is shown the Registering Thermometer, which is a less

expensive instrument than that shown in Figure 97, but by its use the

maximum and minimum temperatures in the drying room during a given

period can be determined.

The Recording Steam Gauge

In Figure 99 is shown the Recording Steam Pressure Gauge, which is

used for accurately recording the steam pressures kept in the boilers.

This instrument may be mounted near the boilers, or may be located at

any distance necessary, giving a true and accurate record of the

fluctuations of the steam pressure that may take place within the

boilers, and is a check upon both the day and night boiler firemen.

The Troemroid Scalometer

In Figure 100 is shown the Troemroid Scalometer. This instrument is a

special scale of extreme accuracy, fitted with agate bearings with

screw adjustment for balancing. The beam is graduated from 0 to 2

ounces, divided into 100 parts, each division representing 1-50th of

an ounce; and by using the pointer attached to the beam weight, the

1-100th part of an ounce can be weighed.

[Illustration: Fig. 100. The Troemroid Scalometer.]

The percentage table No. II has a range from one half of 1 per cent to

30 per cent and is designed for use where extremely fine results are

needed, or where a very small amount of moisture is present. Table

No. III ranges from 30 per cent up to 90 per cent. These instruments

are in three models as described below.

MODEL A. (One cylinder) ranges from 1/2 of 1 per cent to 30

per cent and is to be used for testing moisture contents in

kiln-dried and air-dried lumber.

MODEL B. (Two cylinders) ranges from 1/2 of 1 per cent up to

90 per cent and is to be used for testing the moisture

contents of kiln-dried, air-dried, and green lumber.

MODEL C. (One cylinder) ranges from 30 per cent to 90 per

cent and is applicable to green lumber only.

=Test Samples.=--The green boards and all other boards intended for

testing should be selected from boards of fair average quality. If

air-dried, select one about half way up the height of the pile of

lumber. If kiln-dried, two thirds the height of the kiln car. Do not

remove the kiln car from the kiln until after the test. Three of four

test pieces should be cut from near the middle of the cross-wise

section of the board, and 1/8 to 3/16 inch thick. Remove the

superfluous sawdust and splinters. When the test pieces are placed on

the scale pan, be sure their weight is less than two ounces and more

than 1-3/4 ounces. If necessary, use two or more broken pieces. It is

better if the test pieces can be cut off on a fine band saw.

=Weighing.=--Set the base of the scale on a level surface and accurately

balance the scale beam. Put the test pieces on the scale pan and note

their weight on the lower edge of the beam. Set the indicator point on

the horizontal bar at a number corresponding to this weight, which may

be found on the cylinder at the top of the table.

Dry the test pieces on the Electric Heater (Fig. 101) 30 to 40

minutes, or on the engine cylinder two or three hours. Weigh them at

once and note the weight. Then turn the cylinder up and at the left of

it under the small pointer find the number corresponding to this

weight. The percentage of moisture lost is found directly under

pointer on the horizontal bar first mentioned. The lower portion on

the cylinder Table No. II is an extension of the upper portion, and

is manipulated in the same manner except that the bottom line of

figures is used for the first weight, and the right side of cylinder

for second weight. Turn the cylinder down instead of up when using it.

Examples (Test Pieces)

MODEL A. Table No. II, Kiln-dried or Air-dried Lumber:

If first weight is 90-1/2 and the second weight is 87, the

cylinder table will show the board from which the test

pieces were taken had a moisture content of 3.8 per cent.

MODEL B. Tables No. II and III, Air-dried (also Green and

Kiln-dried) Lumber.

If the first weight on lower cylinder is 97 and the second

weight is 76, the table will show 21.6 per cent of moisture.

MODEL C. Table III, Green Lumber:

If the first weight is 94 and the second weight is 51, the

table shows 45.8 per cent of moisture.

Keep Records of the Moisture Content

=Saw Mills.=--Should test and mark each pile of lumber when first piled

in the yard, and later when sold it should be again tested and the two

records given to the purchaser.

=Factories.=--Should test and mark the lumber when first received, and

if piled in the yard to be kiln-dried later, it should be tested

before going into the dry kiln, and again before being removed, and

these records placed on file for future reference.

Kiln-dried lumber piled in storage rooms (without any heat) will

absorb 7 to 9 per cent of moisture, and even when so stored should be

tested for moisture before being manufactured into the finished

product.

Never work lumber through the factory that has more than 5 or 6 per

cent of moisture or less than 3 per cent.

Dry storage rooms should be provided with heating coils and properly

ventilated.

Oak or any other species of wood that shows 25 or 30 per cent of

moisture when going into the dry kiln, will take longer to dry than it

would if it contained 15 to 20 per cent, therefore the importance of

testing before putting into the kiln as well as when taking it out.

The Electric Heater

In Figure 101 is shown the Electric Heater. This heater is especially

designed to dry quickly the test pieces for use in connection with the

Scalometer (see Fig. 100) without charring them. It may be attached to

any electric light socket of 110 volts direct or alternating current.

A metal rack is provided to hold the test pieces vertically on edge.

[Illustration: Fig. 101. The Electric Heater.]

Turn the test pieces over once or twice while drying.

It will require from 20 minutes to one hour to remove all the moisture

from the test pieces when placed on this heater, depending on whether

they are cut from green, air-dried, or kiln-dried boards.

Test pieces cut from softwoods will dry quicker than those cut from

hardwoods.

When the test pieces fail to show any further loss in weight, they are

then free from all moisture content.

BIBLIOGRAPHY

AMERICAN BLOWER COMPANY, Detroit, Mich.

IMRE, JAMES E., "The Kiln-drying of Gum," The United States

Dept. of Agriculture, Division of Forestry.

NATIONAL DRY KILN COMPANY, Indianapolis, Ind.

PRICHARD, REUBEN P., "The Structure of the Common Woods,"

The United States Dept. of Agriculture, Division of Forestry,

Bulletin No. 3.

ROTH, FILIBERT, "Timber," The United States Dept. of Agriculture,

Division of Forestry, Bulletin No. 10.

STANDARD DRY KILN COMPANY, Indianapolis, Ind.

STURTEVANT COMPANY, B. F., Boston, Mass.

TIEMAN, H. D., "The Effects of Moisture upon the Strength and

Stiffness of Wood," The United States Dept. of Agriculture,

Division of Forestry, Bulletin No. 70.

TIEMAN, H. D., "Principles of Kiln-drying Lumber," The United

States Dept. of Agriculture, Division of Forestry.

TIEMAN, H. D., "The Theory of Drying and its Application, etc.,"

The United States Dept. of Agriculture, Division of Forestry,

Bulletin No. 509.

THE UNITED STATES DEPT. OF AGRICULTURE, DIVISION OF FORESTRY,

"Check List of the Forest Trees of the United States."

THE UNITED STATES DEPT. OF AGRICULTURE, DIVISION OF

FORESTRY, Bulletin No. 37.

VON SCHRENK, HERMAN, "Seasoning of Timbers," The United

States Dept. of Agriculture, Division of Forestry, Bulletin

No. 41.

WAGNER, J. B., "Cooperage," 1910.

GLOSSARY

=Abnormal.= Differing from the usual structure.

=Acuminate.= Tapering at the end.

=Adhesion.= The union of members of different floral whorls.

=Air-seasoning.= The drying of wood in the open air.

=Albumen.= A name applied to the food store laid up outside the

embryo in many seeds; also nitrogenous organic matter found in plants.

=Alburnam.= Sapwood.

=Angiosperms.= Those plants which bear their seeds within a

pericarp.

=Annual rings.= The layers of wood which are added annually to

the tree.

=Apartment kiln.= A drying arrangement of one or more rooms

with openings at each end.

=Arborescent.= A tree in size and habit of growth.

=Baffle plate.= An obstruction to deflect air or other currents.

=Bastard cut.= Tangential cut. Wood of inferior cut.

=Berry.= A fruit whose entire pericarp is succulent.

=Blower kiln.= A drying arrangement in which the air is blown

through heating coils into the drying room.

=Box kiln.= A small square heating room with openings in one end

only.

=Brittleness.= Aptness to break; not tough; fragility.

=Burrow.= A shelter; insect's hole in the wood.

=Calorie.= Unit of heat; amount of heat which raises the

temperature.

=Calyx.= The outer whorl of floral envelopes.

=Capillary.= A tube or vessel extremely fine or minute.

=Case-harden.= A condition in which the pores of the wood are

closed and the outer surface dry, while the inner portion is

still wet or unseasoned.

=Cavity.= A hollow place; a hollow.

=Cell.= One of the minute, elementary structures comprising the

greater part of plant tissue.

=Cellulose.= A primary cell-wall substance.

=Checks.= The small chinks or cracks caused by the rupture of the

wood fibres.

=Cleft.= Opening made by splitting; divided.

=Coarse-grained.= Wood is coarse-grained when the annual rings

are wide or far apart.

=Cohesion.= The union of members of the same floral whorl.

=Contorted.= Twisted together.

=Corolla.= The inner whorl of floral envelopes.

=Cotyledon.= One of the parts of the embryo performing in part the

function of a leaf, but usually serving as a storehouse of food

for the developing plant.

=Crossers.= Narrow wooden strips used to separate the material on

kiln cars.

=Cross-grained.= Wood is cross-grained when its fibres are spiral

or twisted.

=Dapple.= An exaggerated form of mottle.

=Deciduous.= Not persistent; applied to leaves that fall in autumn

and to calyx and corolla when they fall off before the fruit

develops.

=Definite.= Limited or defined.

=Dew-point.= The point at which water is deposited from moisture-laden

air.

=Dicotyledon.= A plant whose embryo has two opposite cotyledons.

=Diffuse.= Widely spreading.

=Disk.= A circular, flat, thin piece or section of the tree.

=Duramen.= Heartwood.

=Embryo.= Applied in botany to the tiny plant within the seed.

=Enchinate.= Beset with prickles.

=Expansion.= An enlargement across the grain or lengthwise of the

wood.

=Fibres.= The thread-like portion of the tissue of wood.

=Fibre-saturation point.= The amount of moisture wood will imbibe,

usually 25 to 30 per cent of its dry-wood weight.

=Figure.= The broad and deep medullary rays as in oak showing

when the timber is cut into boards.

=Filament.= The stalk which supports the anther.

=Fine-grained.= Wood is fine-grained when the annual rings are

close together or narrow.

=Germination.= The sprouting of a seed.

=Girdling.= To make a groove around and through the bark of a

tree, thus killing it.

=Glands.= A secreting surface or structure; a protuberance having

the appearance of such an organ.

=Glaucous.= Covered or whitened with a bloom.

=Grain.= Direction or arrangement of the fibres in wood.

=Grubs.= The larvae of wood-destroying insects.

=Gymnosperms.= Plants bearing naked seeds; without an ovary.

=Habitat.= The geographical range of a plant.

=Heartwood.= The central portion of tree.

=Hollow-horning.= Internal checking.

=Honeycombing.= Internal checking.

=Hot-blast kiln.= A drying arrangement in which the air is blown

through heating coils into the drying room.

=Humidity.= Damp, moist.

=Hygroscopicity.= The property of readily imbibing moisture from

the atmosphere.

=Indefinite.= Applied to petals or other organs when too numerous

to be conveniently counted.

=Indigenous.= Native to the country.

=Involute.= A form of vernation in which the leaf is rolled inward

from its edges.

=Kiln-drying.= Drying or seasoning of wood by artificial heat in an

inclosed room.

=Leaflet.= A single division of a compound leaf.

=Limb.= The spreading portion of the tree.

=Lumen.= Internal space in the spring- and summer-wood fibres.

=Median.= Situated in the middle.

=Medulla.= The pith.

=Medullary rays.= Rays of fundamental tissue which connect the

pith with the bark.

=Membranous.= Thin and rather soft, more or less translucent.

=Midrib.= The central or main rib of a leaf.

=Moist-air kiln.= A drying arrangement in which the heat is taken

from radiating coils located inside the drying room.

=Mottle.= Figure transverse of the fibres, probably caused by the

action of wind upon the tree.

=Non-porous.= Without pores.

=Oblong.= Considerably longer than broad, with flowing outline.

=Obtuse.= Blunt, rounded.

=Oval.= Broadly elliptical.

=Ovary.= The part of the pistil that contains the ovules.

=Parted.= Cleft nearly, but not quite to the base or midrib.

=Parenchyma.= Short cells constituting the pith and pulp of the

tree.

=Pericarp.= The walls of the ripened ovary, the part of the fruit

that encloses the seeds.

=Permeable.= Capable of being penetrated.

=Petal.= One of the leaves of the corolla.

=Pinholes.= Small holes in the wood caused by worms or insects.

=Pistil.= The modified leaf or leaves which bear the ovules; usually

consisting of ovary, style and stigma.

=Plastic.= Elastic, easily bent.