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Another difference is that dried product is gradually released from the progressive kiln, while the entire charge of the compartment kiln is removed at one time, requiring more room and outside track to handle it; and at the same time a new charge must also have been assembled outside the kiln ready to put in. Compartment kilns of a like capacity of output therefore take more room for outside trackage, though this is not a grave objection, as sawmill sites usually have space to spare.

In the sorting of stock for the kiln the compartment kiln has a considerable advantage. The mill product does not need sorting into continuous lines as for a progressive kiln, but only into two uniform charges, because any compartment kiln may readily operate on successive charges entirely different from each other and apply to each the proper operating schedule. For this reason a few compartment kilns are a useful accessory to a larger battery of progressive kilns; they can readily handle odd lots of product that do not fit into the regular progressive-kiln operations.

In appearance the compartment kiln may be much like the progressive kiln except that it is not usually built as long; a length that will accommodate six or eight bunks of lumber at a time on two parallel end-piled tracks is a roomy size, but it may be made small enough for only one or two loads of lumber in a charge. The sawmill type is usually made like the progressive type, with doors at both ends, so that the incoming charge will not be in the way of the outgoing charge. Plenty of transfer track is required to make single-end operation convenient.

The steam pipes are usually in a pit under the tracks, though sometimes they are near the ceiling; they are distributed along the entire length of the kiln, and the air outlets are distributed in like manner. This is because the circulation is crosswise instead of endwise, and each section along the length of the kiln has its own zone of circulation. Most of the air recirculates, and such ventilation as is used is restricted to the amount that will carry off the surplus moisture.

In the usual compartment kiln the natural movement of the heated air from the steam pipes is directly upward into the center chimney of the lumber load, whence it spreads out to either side through the horizontal crevices, then downward at the sides of the kiln, and around again. (Fig. 49.) If there are two endwise tracks in the compartment, this movement takes place in each, the downward currents being at both sides and also in the center. In some double-track kilns the movement of heated air is upward in the center, sidewise both ways through the lumber, and down at the sides of the kiln. In such kilns baffle curtains are usually provided to prevent the air current crossing over above the lumber instead of passing through it. Where the air passes directly into the lumber from below its distribution through the central chimney carries it all through the lumber.

Of the questionnaire replies, 18 reported on the direction of circulation in compartment kilns. In 11 the movement of air was up into the lumber from the bottom; in 4 downward (mostly in edgestacked piling); in 3, up one side, sidewise through the lumber, and down on the other side (mostly blower kilns).

Because of the crosswise circulation, endwise piling is considered the best for a compartment kiln, though many are built for crosswise piling. In these such air movement crosswise of the kiln as occurs inside the lumber can occur most freely in the center chimney,

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FIGURE 49.-Conventionalized cross section of common type of compartment kiln with lumber end piled with flaring center chimney, steam pipes below the lumber, air-intake duct in bottom of pit, and outlets in side wall either high up or low down, controlled by dampers not shown. Steam sprays pointing downward at sides. Natural circulation up into lumber and outward to the sides may reverse itself at the beginning of a new charge, and reestablish itself as the lumber gets hot

and the currents radiating from it probably are diverted across the lumber or end wise of the kiln and resume their crosswise movement when they pass out.

Convection, the tendency of heated air to rise and of cooler air to sink and replace it, is usually the chief impulse to circulation in a compartment kiln, and the cycle of air movement is so short that

very good circulation is usually secured. Draft in the exhaust chimney on the portion of the air that is vented to the outside also assists the interior circulation. The drying rate is, however, not always uniform in all parts of the kiln. If lumber dries more slowly at one end than at the other, that end probably does not get so much heat; and it may be because the pipes are supplied with steam at the other end and are therefore hottest there. În remodeling a In kiln this difficulty was overcome by splitting the pipes into two units, supplied with steam from opposite ends of the kiln. Returnbend piping systems tend also to equalize the ends. Lumber will also dry more slowly near the doors if there is much air leakage around them, or much heat leakage through them because they do not have sufficient insulation. Sometimes a compartment kiln will have a local cold spot where lumber does not dry sufficiently; location of additional heating pipe at this place will usually cure it. Stagnant circulation is not a common fault of compartment kilns in general.

A special form of uneven heating not found in progressive kilns sometimes develops in the compartment kilns. This results from the fact that the compartment kiln requires less heat at certain stages of the process than others while the progressive kiln is operated on practically uniform heat consumption. Of course this variation in heat consumption depends to a large extent upon the kind of lumber which is being dried and, according to the experience of the Forest Products Laboratory, uneven heating occurs principally during the beginning of the run when drying green or partially green hardwoods rather than toward the end of the drying period in the case of softwoods. At the beginning of the run in drying certain softwoods a tremendous amount of the heat is necessary to evaporate the large amounts of water from the lumber, and in many instances the steam consumption is undoubtedly much greater at the beginning of the run than at the end, and because high temperatures and high-pressure steam are used difficulties such as those under discussion seem to be avoided. In the case of the green hardwoods, however, the situation is entirely different. The temperatures are actually very low, humidities are high, the rate of evaporation from the lumber is comparatively low. The heat losses from the kiln are also very low, as likewise the steam pressure. This condition is generally aggravated by the use of thermostats of nonthrottling types. Because of the variations of heat distribution in kilns there have been developed various types of heater pipes which have tended toward the more even distribution of heat in the kiln. The header-type coil now usually carries steam pipes which are separated into several units each with separate shut-off valves in addition to the main shut-off. By cutting off the heat from some of these units the remaining units run on more nearly full steam pressure. To overcome certain difficulties experienced in using the header type of heater there has been developed "the return-bend header system." The return-bend header type, which is a cross between the true header system and the true return-bend system, consists of a single bank of pipes which spring from the supply header and double back two or three times and are connected at the

discharge end to a return header. This arrangement tends toward greater longitudinal uniformity of heat.

Compartment kilns are usually provided with some form of steam spray for controlling humidity and for occasional steaming treatments of the lumber, and for an initial steaming to heat the lumber through. Sometimes the apparatus consists of a large pipe above the heating pipes below the track, with fine perforations on each side, and fed from a smaller pipe so that the steam will expand in the large pipe and issue from the spray openings as gentle vapor instead of a jet under full pressure. Sometimes spray nozzles are used somewhere in the line of air movement and pointed in the direction of movement, thus aiding air circulation. It is, of course, possible to secure some control of humidity by the ventilating dampers regulating the amount of damp air discharged from the kiln, but it is easier to set these for liberal ventilation and then control humidity by the use of the spray. At certain stages of drying the lumber may not be an adequate source of humidity, in view of moisture leakage through the doors and walls. In some cases steam sprays are supplied with high-pressure steam and in other cases with low-pressure steam.

There are compartment kilns which vary considerably from the usual type above described, the modifications being intended to improve the circulation. In some types this is accomplished by steam sprays in the return-air cycle, while in others fans or blowers are used. The Tiemann water-spray kiln developed at the Forest Products Laboratory and with patent dedicated to public use employs downward-pointing water sprays at either side which help the air circulation and also control humidity by saturating the air at a determined lower temperature; the air then passes through baffles to trap the entrained mist and then through the heating coils. so that when reheated it has the desired working stage of humidity. It is specially adapted to critical drying of difficult material at a slow rate but is rather expensive to install and not much used in sawmill drying.

A more important development was that of the internal-fan reverse-circulation kiln, also a Forest Products Laboratory development. A shaft driven by an external motor is placed lengthwise in the fan pit, with vane fans space along it, and each is separately boxed in with air ducts leading in from the pit through steam coils at either side and leading out under the center chimney of the lumber load. Sometimes the discharge is on one side of the load and the air movement is sidewise through the load, being prevented from passing over it by baffle curtains; and for double-track arrangement the upward movement is in the middle and downward at the sides of the kiln. Suitable vent connections with outside air, and humidity steam sprays, are also provided.

The distinguishing feature of this kiln is the fact that the fan shaft which reverses the circulation can be reversed. Such kilns are usually operated longest in the most efficient direction, with reversed circulation for a shorter period at the end of the run, thus equalizing the drying effect on the lumber, which is greater where the air enters than where it leaves. Such kilns are in successful use for sawmill

kiln-drying at a number of plants. At one plant one fan blows a current of air endwise in the top of the kiln in addition to the cross circulation of the other fans, the purpose apparently being to unify air conditions throughout the kiln.

PILING LUMBER FOR KILN-DRYING

There is one basic principle to remember in piling lumber for kiln-drying, which is that the lumber will dry out chiefly from the surfaces; as the broad faces are the most extensive surfaces, the air which has the chief evaporative effect must pass in contact with these surfaces; horizontally through the pile if it is piled flatwise. The second principle is that this movement must be chiefly crosswise of the lumber, because the stickers bar endwise circulation between the layers, and the only outlet for air in that direction is in the spaces between boards in the layers and in the spaces between stickers in the chimneys if there are any chimneys in the pile.

It will be seen from the description already given of the usual commercial kilns, both compartment and progressive, that in the majority of cases the heating pipes are directly underneath the lumber below the track, and that the hot air from the pipes rises directly into the bottom of the load. It can pass in this direction only through the spaces between the boards. If the lumber is of even widths, these spaces arrange themselves into vertical tiers throughout the load and the air can rise in these and spread through the load, moving sidewise also between the layers and escaping at the sides of the load. If the boards are of random widths, the spaces between will sometimes hit each other and sometimes be staggered, so that the air currents from beneath must take a tortuous path; and wherever the current impinges directly on the lower side of a board that side will dry more rapidly than the upper surface and tend to cup.

Under such circumstances a flaring center chimney in the pile will permit the air to enter the pile much more freely and divert from this tunnel into the horizontal spaces. The tunnel may taper to a narrow width near the top of the pile and stop short of the top, as the top layers will probably get enough air without it; it is chiefly useful in conducting the upward air into the pile.

At least four kiln manufacturers are now extensively advertising kilns which may be termed of the cross-circulation type. In this type the lumber is flat stacked without spaces between the boards in each layer, and the air is constrained to move horizontally through the lumber. No flues or chimneys whatever are provided. In one external-blower kiln of this general type the air is discharged from a plenum chamber which occupies one entire side of the kiln and after passing horizontally through the lumber is exhausted through a vacuum chamber which occupies the entire length and width of the opposite side wall of the kiln. Another external-blower kiln manufacturer has recently placed on the market a kiln which, in addition to providing cross circulation, also claims to provide for reversible circulation if required. In the two makes of internal-fan kiln of this general type upward vertical movement of the air takes place between the load of lumber and the kiln wall on one side and, simi

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