Local air currents in a progressive kiln vary from time to time. Most of the air starting back in the pit from the green end is diverted into the upper air currents before reaching the dry end, but the dry end will get enough recirculated air to increase the humidity, even though little of it may have come from far back in the kiln. Adding a recirculating duct in the pit insures that whatever recirculation occurs in it will go all the way through and usually increases the longitudinal circulation. The air movement in the duct may be slow unless boosted by putting steam nozzles in the duct pointed in the direction of air movement, which encourage the current of air and supply additional humidity to the dry end.

There is some literature stating that piling in a progressive kiln should be crosswise to accord with the longitudinal air movement, because air can move horizontally through a kiln load of lumber much more freely sidewise of the load than endwise; partly because the distance through is shorter, and partly because the horizontal spaces between layers are entirely open sidewise, but endwise are impeded by the cross stickers. But it is now understood that there will be little of this longitudinal movement through the lumber anyway, and that most of the circulation of air through the lumber is supplied by local circulation. Also, it is obvious that the ascent of air from the heating pipes through the lumber will be more positive if the middle of the load runs lengthwise directly over the pipes. There is less waste of kiln space with short loads, because they can be spaced accordingly, while if cross piled a short load occupies the same space as long lumber, whose length nearly fills the entire width of the kiln. The fact that the questionnaire replies showed more end piling than cross piling in the progressive kilns indicates the modern trend on this point.

Thus far only natural-circulation kilns have been discussed, depending for air movement chiefly upon convection, the tendency of light hot air to rise and of cooler heavier air to sink, when the two are balanced against each other. It is inexpensive circulation, but there is usually not quite enough of it for best results; and forced movement of air through the kilns by mechanical power is not very costly if steam power is used, because the exhaust steam from the engine can be used to heat the kilns and has nearly its full heat efficiency left. The kiln is converted into a forced circulation blower kiln of progresssive type by carrying a large air duct from the green end of the kiln to an outside housing containing the fan and also the steam coils, and usually air outlets and instakes for replacing a portion of the old air with fresh air to maintain ventilation. A steam spray inside the housing for controlling humidity is also usual; and the reconditioned air is led back to the dry end of the kiln through another duct.

It would also be possible to operate a progressive kiln as a closedcirculation kiln, removing the surplus moisture by a pipe or waterspray condenser; but there is no commercial kiln of such a type offered at the present time. A blower kiln of pipe-condenser type was formerly manufactured but has been replaced by a ventilating design. Progressive kilns often do poor work because too much is expected from them in the way of output. They are capable of reasonable

volume at reasonable temperature, but because they will turn out more product at higher heats they are operated that way; and though this may reduce the direct kiln-drying cost, it may also produce additional degrade and makes such kiln-drying rather expensive. If circulation and output are unreasonably slower at moderate temperatures, this merely means that the kiln needs improving. A compartment kiln is intended to be operated on a controlled schedule; a certain temperature and humidity when the lumber is green, changed gradually as it dries. The schedule of a progressive kiln is largely built into the kiln, and control is chiefly control of conditions at the dry end, which determines conditions throughout. If the kiln will not work efficiently with proper dry-end conditions, the kiln is wrong and should be made right; the saving in value on its product will promptly repay the cost.

The leading types of commercial kilns are so well known that it is not necessary to describe them here, and such kilns are usually varied in design to adapt them to the conditions under which they are to operate.

One type of kiln reported by a California lumber manufacturer is worthy of special mention. It is a progressive kiln with local circulation reversed in the two halves of the length. It is a doubletrack, endwise-piled kiln, and at the green end the heating coils are underneath at the sides and the hot air passes up the side, sidewise through the lumber toward the center, and then down. Farther along the steam pipes are in the center, where the hot air passes up, sidewise through the lumber in a reverse direction, and down at the sides. Presumably there are baffle curtains to prevent the air from passing over the loads instead of through them; and presumably also there is a longitudinal general circulation in addition to this vertical-lateral circulation.

SORTING OF MILL PRODUCT FOR DRYING IN PROGRESSIVE

KILNS

It is obvious that the entire line of product passing through any progressive-kiln unit is subjected to the same drying conditions, if these are maintained constantly. It follows that if the conditions of heat, humidity, and air circulation, combined with a regular movement of the line of lumber, produce an adequate drying effect on lumber of a certain thickness, it will produce more complete drying of thinner lumber and incomplete drying of thicker stock. Stated in another way, thin lumber would have to pass through the kiln more rapidly and thick lumber more slowly; but as each load must participate in the general rate of movement of all the stock on the one track, variation in drying time for different kinds of stock is impossible. Where a kiln unit has two lines of track it is possible to operate one track on thicker stock than the other by slowing down the rate of movement on that track and thus giving the lumber a longer time in the kiln. Whether it is practical depends upon whether the kiln conditions are suited fairly to both kinds of stock.

A sawmill battery of progressive kilns usually contains at least four units and often many more than that; this gives an opportunity

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to sort the mill product into classes and to run each class through its own kiln or kilns at the drying rate which suits it. But it is not always easy to have the volume of each class in line with the drying capacity of the kiln or kilns assigned to it. Such a separation would base first on thickness. Stock of a certain thickness might be further subdivided by width, as wide lumber dries slowly. If any of the common lumber is kiln-dried, a separation between it and the same sizes in the upper grades may be made because the upper grades of lumber need kiln-drying to a lower moisture content and usually stand it without much degrade; common grades of lumber do not need such thorough seasoning for their customary uses and because of their greater proportion of knot defects suffer heavy degrade if dried too thoroughly. This is especially the case with inch common lumber, in which the knots loosen more easily than in thicker stock.

Separation of product for kiln-drying usually involves separation by widths and lengths in softwoods, because they are usually sold on such a separation. In some woods a considerable proportion is sold random lengths, and if these are kiln-dried they are usually kiln-dried in like mixture. Even if lumber is piled in separate lengths on kiln bunks it need not mean that the different lengths are kept in separate charges in going through the kiln; the mixed lengths of cars can go through together, if equal in other respects. In the redwood region it has been found advisable to separate wood by texture, rather than by weight and moisture content, inasmuch as texture is an important factor in determining the best drying schedule, based on thickness, in carefully operated kilns in this region.

The matter of sorting the mill product to keep the kilns busy without congestion and delay, and to keep each kiln working on its own proper kind of stock, is an individual problem of each mill: and to a considerable extent the combined kiln capacity determines what portion of the stock shall be sent to the yard for air-drying. The questionnaire replies received indicate that the sorting of stock into separate classes for progressive kilns is kept rather simple at most mills. Besides the necessary sorting into thickness classes, redwood is sorted into light and sinker stock. California pine is sometimes separated by hearts and saps, sometimes into two grade divisions in addition to thickness, and sometimes by thickness only. Northern hardwoods and western pine are separated by thickness only. Douglas fir is separated into two grade classes in addition to thickness, and sometimes with some separation between vertical and slash grain. North Carolina pine is separated into two grade divisions besides thickness, or sometimes by thickness only. Cypress is sorted by thickness and by three grade separations, and 22 per cent of the southern pine replies also show that separation, the rest of the replies for that species dividing equally between separation by two grade classes and thickness and separation by thickness only.

DRYING TIME IN PROGRESSIVE KILNS

Replies on the kiln-drying time in progressive kilns naturally varied in any given species with the efficiency of the kilns and their operating temperatures. The following tabulation is a general

compilation of replies without any attempt at adjusting inconsistencies; it gives a general idea of what is being accomplished:

1 Western pine is the same species as California white pine (western yellow pine), but the replies came from the Inland Empire instead of from California.

STEAM-HEATING EQUIPMENT OF DRY KILNS

The steam pipes under the track in a progressive dry kiln are usually in horizontal rows (sometimes in vertical rows) and start from a large header at one end, running to the other end on a light downward slant and ending in another header. The horizontal pipes usually have a downward bend or elbow from which they run down to the other header to allow for unequal expansion in the several lines. The upper or supply header is usually placed at the dry end of the kiln, because the incoming steam is hottest near that end, and it is usually desired to have the dry end the hottest. But the steam may be supplied from the farther end and be compensated by more pipes near the dry end. In case the pipes are in vertical rows the headers are also vertical. The pipes must always have a fair incline to drain off water of condensation, and this incline is usually in the direction of steam travel, so the flow of water will be in the same direction. At the lowest point in the line there should be a trap for disposing of the water, and several such traps may drain to a pump, or similar automatic device, capable of returning the hot water to the steam-boiler supply against the steam pressure. Some progressive kilns are supplied with exhaust steam from the sawmill engines, at a pressure of 5 to 10 pounds; such low-pressure steam requires more heating pipe than live or high-pressure steam, and this leads to the use of live steam in some cases. It is brought through reducing valves which lower the pressure somewhat and provides a steady supply, unaffected by any variations which may occur in the original pressure. Whatever steam is used, the supply should be such as to require some throttling down at the main supply valve to bring it within control.

It is desirable to have heat for the kilns at such times as the plant engines are not operating, which means the addition of supply of live-steam supply to the supply of exhaust steam; and here it is important that the live steam be brought through reducing valves. which will reduce it to a pressure equivalent to the exhaust steam. The steam traps, air relief valves, etc., work best under a uniform pressure.

Drip from the ceiling is a common fault in progressive as well as compartment kilns, particularly if the roof is not well insulated. The standard remedy is to install steam pipes under the ceiling where the drip occurs and keep steam in them, trapping it to a separate steam trap from that which serves the heating pipes. The heat keeps the ceiling warm and prevents condensation on it and incidentally contributes somewhat to the internal heat of the kiln.

WHY THE FINAL KILN-DRYING STAGE NEEDS MORE HEAT AND LESS HUMIDITY

Kiln-drying literature always specifies high temperature and low relative humidity in the dry end of a progressive kiln, and usually in the final stage of a compartment-kiln schedule, but rarely gives an explanation. This is because as wood gets drier its hygroscopicity or thirst for water increases, and it sets up more resistance to the loss of what moisture remains, which resistance requires increased heat to overcome. At the same time the fact should not be overlooked that in some cases, as, for instance, certain sizes of common grades of Douglas fir, a constant temperature and relative humidity are recommended throughout the kiln run. Softwood schedules usually start with temperature of 135 to 180° F. and with a relative humidity of 85 per cent, which produces an adequate drying rate down to 25 or 30 per cent of moisture in the wood; but at such a stage drying would stop entirely at 14 to 17 per cent of moisture content, because the wood would have arrived at equilib rium moisture content with the air conditions in the kiln. The final stage used generally with softwoods is therefore a temperature of 175 to 210° F. (though some manufacturers of redwood doubt whether it should go above 165°) and a relative humidity of 30 per cent, which would correspond to an equilibrium moisture content in the wood of 314 to 4 per cent; drying is, of course, stopped at 6 or 8 per cent or whatever percentage is desired. Such a stage could be reached on a schedule corresponding to not quite so low an equilibrium moisture content in the wood, but it would take longer, as drying becomes very slow when the wood approaches its condition of balance with the air.

Of the progressive kilns reported, 23 installations are in continuous operation, keeping up steam through the night and over week-ends. Air intakes are located at the dry end in 18, at the green end (and through the recirculating duct to the dry end) in 6, at both ends in 3. and with interior openings distributed along the length of the kiln in 1. In 5 installations there are no air intakes. The air discharge ducts are at the green end in 24 and distributed along the roof in 9. Some kilns are not provided with control dampers on the ducts; and while 18 installations were reported with outlet dampers and 27 with inlet dampers, only 16 mills reported that the dampers were actually used to control conditions. Whether the dampers were left permanently open or permanently closed was not usually stated: presumably open, though sometimes intake dampers are left closed because enough outside air gets in through leakage to give all the ventilation the species being dried requires.