Chapter 2Kiln Types and Features

Foundations and floors 51Heating systems 51

Indirect heating 52Direct heating 53

Steam traps and control valves 54steam traps 54Control valves 56

Air-circulation systems 56Kiln fans 56Baffles 58Plenum chamber 59

Venting and humidification systemsVent ing 60Humidification 60

Equipment to control drying conditionsAutomatic control equipment 61

Classification systems 43Operational techniques 43

Compartment kilns 43Progressive kilns 48

Temperatures of operation 48Low-temperature kilns 49Conventional-temperature kilns 49Elevated-temperature kilns 49High-temperature kilns 49

Type of heating and energy source 49Steam 49Direct fire 49Electricity 50Hot water and hot oil 50Solar 50

General construction features 50Construction materials 50

Aluminum 50Concrete block, poured concrete, and brickWood and plywood 51

60Classification by operational techniques distinguishesbetween the more common compartment-type kiln andthe less common progressive-type kiln.

61 Compartment Kilns

Semiautomatic control systems 61Fully automatic control systems 64Zone control 65

Manual control equipment 66Temperature-measuring devices 66Humidity-measuring devices 66

Specialized drying approaches and kiln typesDehumidification kilns 66Predryers 68Solar dry kilns 69Vacuum drying 70

Literature cited 73Sources of additional information 73Table 73

A lumber dry kiln consists of one or more chambersdesigned to provide and control the environmental con-ditions of heat, humidity, and air circulation necessaryfor the proper drying of wood. As the development ofthe modern dry kiln has progressed, a number of de-sign modifications have been explored in relation to themechanism of heat supply, arrangement and type offans, control of relative humidity or wet-bulb temper-ature, and use of various materials for construction ofthe chamber .

The design of a kiln has an important bearing on itsoperation and drying efficiency. A properly designedand operated kiln will dry most species of lumber orother wood products to any specified moisture contentbetween 3 and 19 percent in a reasonably short timewithout appreciable losses caused by drying defects.

50 Classification Systems

Dry kilns can be classified in a number of differentways. In this manual, we have chosen a system thatclassifies by (1) operational techniques, (2) tempera-tures of operation, and (3) type of heating and energysource. Other possible classifications might include fanarrangement and method of loading the kiln.

Operational Techniques

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Compartment-type kilns (figs. 2-1 to 2-8) are designedfor a batch process in which the kiln is completelyloaded or charged with lumber in one operation, andthe lumber remains stationary during the entire dry-ing cycle. Temperature and relative humidity are keptas uniform as possible throughout the kiln, and theycan be closely controlled over a wide range of tempera-ture and humidity. Temperature and relative humidity

Chapter 2 was revised by R. Sidney Boone,Research Forest Products Technologist, andWilliam T. Simpson, Supervisory ResearchForest Products Technologist.

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in dry kilns. (ML88 5604)Figure 2-1—Some plans for location of fans and baffles

are changed as the wood dries based on a schedule thattakes into account the moisture content and/or the dry-ing rate of the stock being dried. Drying schedules varyby species, thickness, grade, and end use of material asdiscussed in detail in chapter 7. All modern dry kilnsuse some type of forced-air circulation system, with airmoving through the load perpendicular to the length ofthe lumber and parallel to the stickers. Although somecross-circulation kilns (airflow parallel to the length ofthe lumber and perpendicular to the stickers) can stillbe found, kilns have not been built using this techniquefor several decades. The natural draft circulation sys-tem, which took advantage of the principle that heatedair rises, is now considered inefficient and is of historicinterest only (Rasmussen 1961). A more detailed dis-cussion of the different types of air circulation systemscan be found later in this chapter under the headingGeneral Construction Features.

Compartment kilns can be classified by the method ofloading. Perhaps the largest number of kilns are of thetrack-loaded type. The lumber is stacked on kiln trucks

that are rolled into and out of the kiln on tracks. Themajority of the softwood lumber in the United States isdried in track-loaded kilns. The other method of load-ing involves moving stacks or packages of lumber di-rectly into and out of the kiln with a lift truck. Theseare generally called package-loaded kilns, although theyare frequently called side-loaded kilns in the westernsoftwood region. The majority of the hardwood lumberin the United States is dried in package-loaded kilns.

Track-loaded kilns commonly have one or two setsof tracks and occasionally three sets, and are knownas single-, double-, or triple-track kilns, respectively(figs. 2-2 to 2-5). The width of the stack of lumber pertrack is typically 6 to 9 feet. In kilns more than onetrack wide, some provision for reheating the air is madebefore it passses through the next stack of lumber. Thelength of a track kiln is usually some multiple of thelengths of the lumber being dried correlated with theamount of lumber production required. Kiln lengthsvary from about 40 to 120 ft; those used for hardwooddrying are typically 40 to 66 ft long and those used for

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Figure 2.2—Lineshaft, double-track, compartment kilnwith alternately opposing fans. Vents are over fan shaftbetween fans. Vent on high-pressure side of fans be-comes fresh air inlet when direction of circulation isreversed. (ML88 5595)

Figure 2-3—Double-track kiln with fans directly con-netted to motors. Lumber stacks are loaded endwise,and boards are stacked edge-to-edge. Air flows parallelto stickers. (ML88 5594)

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Figure 2-4—Double-tray-loaded aluminum pre-fabricated kiln with doors at both ends of kiln.(MC88 9017)

softwood, typically 66 to 120 ft long. Lumber-holdingcapacity can vary from around 25,000 fbm (4/4 basis)to 220,000 fbm (8/4 basis).

Track kilns may have doors at one end or, more com-monly, at both ends so that unloading and loading the

kiln require a minimum amount of time. Kiln trucksloaded with green lumber are pushed into the kiln im-mediately after the dried lumber is removed from thekiln. A covered shed is frequently built over the “dry”end of the kiln to protect the dried lumber from in-clement weather while it is cooling and awaiting fur-ther processing. A cover over the “green” end of thekiln will protect the top courses of freshly sawn lumberfrom degrading in the sun as a result of uncontrolleddrying and from rain or snow. Figure 2-8 shows a kilnwith protective cover at both the dry and green ends.Frequently cited advantages of track kilns include shortdowntime for loading and unloading and more uniformdrying primarily because of narrower load widths. Dis-advantages include greater building cost, because trackkilns require more land area than package kilns espe-cially if kiln has tracks at both ends, and the addedexpense of track and kiln trucks.

Package-loaded kilns are generally smaller than track-loaded kilns and have a different configuration forloading the lumber (figs. 2-6, 2-7). Large doors per-mit the stickered and stacked lumber to be loaded intothe kiln with a lift truck. Most package kilns are de-signed to hold 24 ft of lumber from front to back of

Figure 2-5—Direct-fired, double-track-loaded high tem-perature kiln in which hot products of combustion aredischarged directly into the airstream circulating withinthe kiln. (ML88 5605)

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Figure 2-6—Package-loaded kiln with fans connecteddirectly to motors. (ML88 5598)

Figure 2-7—Lift truck delivering package of stickeredlumber to package-loaded kiln.(MC88 9024)

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Figure 2-8—Track-loaded, concrete block kiln withdoors and protective cover at both ends of kiln.(MC88 9023)

kiln, although some are designed for a depth of 16 ft oflumber. Since airflow in package kilns is from front toback, or vice versa, the length of air travel through theload is also 24 ft. No provision is generally made for re-heating the air as it passes through the load. Lumber-holding capacity of package kilns varies from around25,000 to 90,000 fbm (4/4 basis). Some frequently citedadvantages of package kilns include lower building costand use of less land area. Disadvantages include longdowntime for loading and unloading and generally lessuniform drying if initial wood moisture content is above25 percent. Using shorter air-travel distances and hav-ing all lumber at about the same moisture content in-crease drying uniformity. If starting moisture content isbelow 25 percent, uniformity of final moisture contentof lumber in package kilns is usually little different fromthat of lumber in track kilns.

Progressive Kilns

Progressive-type kilns are designed for a continuousprocess in which the loads of stacked lumber enter thegreen end of the kiln and are moved forward, usually ona daily basis, through progressively more severe dry-ing conditions until exiting the dry end of the kiln.Each move forward is accompanied by the removal ofa completed load from the dry end and the addition ofa fresh green load at the green end. The temperatureincreases and the humidity decreases as charges movefrom one zone to the next along the length of the kiln.The desired schedule effect is obtained in this way. Toachieve the necessary range of drying conditions, pro-gressive kilns vary in length depending on the species

and the initial and final moisture content of lumber be-ing dried. Because of the relatively continuous move-ment required in this approach, progressive kilns areusually of the track-loading type. As with compartmentkilns, the early models relied on natural draft circula-tion, but forced circulation using either internal fans orexternal blowers soon became the preferred method ofair circulation.

Progressive kilns lack flexibility in drying kiln chargesthat vary in species, dimension, or moisture content.They do not provide the close control of conditions re-quired by most hardwood operations or the speed ofdrying required by most softwood operations. For thesereasons, there are relatively few progressive kilns oper-ating in the United States, and no new ones have beenconstructed in several years.

Temperatures of Operation

Most lumber dry kilns are designed to operate withina specified range of temperatures. This range dependslargely on the species to be dried and quality and enduse of final products. Also considered are amount ofproduction expected, source of energy, and limitationsof certain components of the system, such as compres-sors and electric motors. A common classification ofkilns based on maximum operating temperatures is asfollows:

Low-temperature kiln . . . . . . . 120 °FConventional-temperature kiln . . . 180 °FElevated-temperature kiln . . . . . 211 °FHigh-temperature kiln . . . . above 212 °F

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Regardless of the temperatures used, the basic require-ments of controlled heat, humidity, and air circulationapply. Therefore, kilns of different temperature classi-fication differ primarily in terms of the source of heatenergy and the type of materials and equipment used inthe kiln structure.

Low-Temperature Kilns

Low-temperature kilns typically operate in the rangeof 70 to 120 °F, though some may not exceed 110 °F.This classification typically includes fan dryers, predry-ers, shed dryers, and some types of vacuum, dehumidifi-cation, and steam-heated kilns.

Conventional-Temperature Kilns

Conventional-temperature kilns typically operate in therange of 110 to 180 °F. The majority of hardwood lum-ber and sizeable amounts of softwood lumber are driedto final moisture content in kilns operating in this tem-perature range. These include steam-heated kilns andthose designs of dehumidification kilns that operate upto 160 °F. The bulk of the kiln schedules available forthe various species and thicknesses are for kilns operat-ing at “conventional temperature.”

Elevated-Temperature Kilns

Elevated-temperature kilns typically operate in therange of 110 to 211 °F. The final dry-bulb temperaturein a schedule for use in an elevated-temperature kilnis commonly 190 or 200 °F and occasionally as high as210 °F. Many western softwood operations and somesouthern pine operations have kilns operating in thisrange. A few easy-to-dry hardwood species may useelevated temperatures in the final step of the schedule.

High-Temperature Kilns

High-temperature kilns typically operate for most ofthe drying schedule at temperatures above 212 °F, usu-ally in the range of 230 to 280 °F. Perhaps the major-ity of southern pine lumber and increasing amounts ofwestern softwood lumber are dried in high-temperaturekilns. These kilns are more often used for dryingconstruction-grade lumber where some surface check-ing and end splitting are acceptable in the grade, ratherthan upper-grade lumber where these defects are lessacceptable. A very small amount of hardwood lumberis dried at high temperatures.

Type of Heating and Energy Source

The type of heating of lumber dry kilns and the energysource for that heat can be divided into the followingcategories: steam, direct fire (hot air), electricity, hotwater and hot oil, and solar. Heat is required in a drykiln for four purposes: (1) to warm the wood and thewater in the wood; (2) to evaporate moisture from thewood; (3) to replace the heat lost from the kiln struc-ture by conduction or radiation; and (4) in kilns withvents, to warm the fresh air entering the kiln.

Steam

Steam has long been the most widely used heatingmedium for kiln drying of lumber. Steam is movedfrom the boiler into the kiln by pipes, and the heat isthen transferred to the circulating air in the kiln. His-torically, many lumber processing operations requiredsteam for a variety of applications, and it was there-fore natural to include sufficient boiler capacity forkiln-drying operations. With the increasing popular-ity of electrically powered sawmills, the dry kilns arefrequently the principal user of steam at an installation.In the early days of dry kilns, burning of wood wastein the boiler was the standard procedure. As oil andnatural gas became more available and less expensive,most operations switched to these energy sources fortheir boilers. Since the “oil scare” and rising prices ofthe 1970’s, there has been a return to burning of woodwaste to generate steam. A more detailed discussion ofboilers, including such items as sizing and horsepower,can be found in chapter 11. For a more complete dis-cussion of heat transfer surfaces and how temperaturesare achieved and controlled in a kiln, see Heating Sys-tems section later in this chapter.

Direct Fire

Direct-fired heating systems differ from steam heatingsystems in that the heated air for the kiln originatesdirectly from the burning of oil, natural gas, or woodwaste. The heated air produced from the burning ofthe fuel is passed through a mixing or blending cham-ber to control the temperature and volume of air goinginto the kiln (fig. 2-5). Direct-fired systems have beenused extensively for high-temperature drying of soft-woods, especially southern pine. The required temper-atures are easily achieved and controlled, and any dis-coloration of the wood caused by combustion gases is oflittle consequence in most softwood operations. Direct-fired systems are seldom used for drying hardwoods,primarily because these systems do not provide theclose control of relative humidities generally requiredfor proper hardwood drying.

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Electricity

The use of electric power to heat a dry kiln is currentlymost often thought of as related to dehumidificationdrying systems or the type of vacuum drying systemsusing electric energy (radiofrequency, microwave, orelectric resistance blankets). I” dehumidification sys-tems, electricity is used to power the compressor orheat pump and the strip heaters that are frequentlyused to bring the kiln up to a minimum temperaturefor efficient operation of the compressors. For smallkilns drying 500 to 1,000 fbm, designs using electricstrip heaters have been suggested (Rice 1977).

Aluminum

Hot Water and Hot Oil

Some kilns are heated by hot water rather than steam.These systems have much lower drying efficiency andare not commonly found in typical commercial opera-tions. However, hot water heating systems are some-times found in smaller homemade or do-it-yourself in-stallations where steam generation is regarded as eitherimpractical or too expensive.

Few lumber dry kilns in the United States use the hotoil system, although interest in using this system hasincreased since the mid-1980’s, particularly in plantsthat have both particleboard presses and dry kilns.

Solar

In the United States and Canada, use of solar energyto heat a lumber dry kiln is limited to small operationsor hobbyists where drying large quantities of lumberon a tight production schedule is not required. Inter-est in totally solar-heated kilns or solar-assisted kilnsis much higher in tropical countries, especially thosewhere more traditional forms of energy are very expen-sive or are not readily available.

General Construction Features

Construction Materials

Dry kilns are constructed of a number of materials, in-cluding aluminum prefabricated panels, concrete block,poured concrete, brick, wood, and plywood. Variouskinds of vapor barriers are used to restrict movementof water vapor from inside the kiln into the struturalmembers and panels and thus prevent deteriorationof the structure. To have acceptable efficiency, kilnsmust be reasonably well insulated against loss of heatthrough the structure. In addition, doors and otheropenings must fit tightly to minimize loss of heat andhumidity. The choice of building materials is frequentlygoverned by such things as operational temperaturesrequired for the species and thicknesses to be dried, life

expectancy of the kiln, capital investment, insurance,source of energy, and type of heating system.

Many kilns constructed in the last decade use prefab-ricated aluminum panels with fiberglass or some formof rigid foam insulation. The panels are joined togetherand bolted to structural load-bearing members of ei-ther steel or aluminum (figs. 2-4 to 2-7). fill-lengthwall and roof panels are manufactured (prefabricated)in standard dimensions for rapid installation on siteand to give flexibility in kiln size. All connecting jointsshould be designed to minimize heat losses and to allowfor expansion and contraction of the metal with chang-ing temperature. This ability to withstand expansionand contraction without damaging the structure makesaluminum the preferred construction material for kilnsexpected to be operated at high temperatures (above212 °F).

Kiln doors are of similar lightweight, insulated, alu-minum panel construction, mounted in a steel or alu-minum frame, with additional bracing for strength andrigidity. Most doors are moved by hangers, which areconnected to rollers operating on a rail over the dooropening. Some type of flexible gasket is generally usedaround the opening to minimize air infiltration andleakage.

Because aluminum is extremely resistant to corrosion,no special vapor sealants or moisture barriers are re-quired. However, regular inspections are needed toensure that no leaks develop in the joints, and anypunctures or tears in the skin of the panel need to berepaired to prevent moisture from the kiln atmospherepassing through to the insulation and reducing its ef-fectiveness. If a steel supporting structure is used,usual precautions of applying a good paint or sealermust be observed to protect the steel from the cor-rosive atmosphere found in most kiln environments.Particular attention should be paid to locations wherethe steel support structure comes into contact withsources of cold temperature (where condensation willoccur on the steel), such as around doors and the first12 to 18 inches above floor level of the vertical supportcolumns.

Concrete Block, PouredConcrete, and Brick

Concrete block, poured concrete, and brick, which aresometimes known collectively as masonry, have histor-ically been used for construction of low-temperature,conventional-temperature, and elevated-temperaturedry kilns (figs. 2-2, 2-3, 2-8). Concrete block filled withsome type of insulation material, such as vermiculiteor rigid foam, is currently the most common type of

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masonry kiln. Kilns with poured concrete walls are oc-casionally seen, but the use of brick has largely fallenfrom use. Masonry kilns may have either load-bearingor nonload-bearing walls. Where walls are nonloadbearing, the block or brick is laid between structuralsteel members that support the roof beams or trusses.Masonry materials should be of high quality, takinginto consideration such factors as durability, insulat-ing properties, and resistance to moisture, humidity,and temperature fluctuations. A high quality mortarmust also be used. To protect the masonry against hu-midity and condensation and to reduce heat and vaportransmissions, the interior walls and ceiling must begiven one or two coats of a specially formulated heat-and vapor-resistant kiln paint or coating. Some designssuggest an inside coating of lightweight concrete to im-prove insulation and to retard moisture movement intothe concrete block. Such designs also require a vapor-resistant coating. Expansion and contraction of thesemasonry materials during routine kiln operation cancause cracks, which should be sealed promptly to pre-vent further deterioration of the wall and roof. Largelybecause of this expansion and contraction, masonrymaterials are not usually chosen when constructinghigh-temperature kilns.

Roofing materials for masonry kilns are frequently pre-fabricated aluminum panels or a “built-up” roof con-sisting of a layered composite of roofers’ felt, vapor bar-rier, and insulation on top of wood, reinforced concreteslabs, or metal decking.

Kiln doors on newer masonry kilns are frequently thesame type of aluminum prefabricated panel doors asthose used on aluminum prefabricated kilns. Someolder kilns may have doors constructed of insulatedwood panels; however, these doors are heavy and de-teriorate with time.

Wood and Plywood

The use of wood for kiln construction is usually lim-ited to low-temperature applications where inexpen-sive, short-term installations are planned, and wheresmall, possibly homemade facilities are considered ade-quate. Plywood interiors in metal or wooden buildingsare fairly common in dehumidification kilns. Construc-tion for dehumidification kilns requires insulation val-ues of R-20 or more for walls and roof; higher valuesare needed in colder climates. Vapor barriers must beextremely tight for efficient operation, and great caremust be given when installing to ensure proper joints.

Foundations and Floors

Kilns must be built on a firm foundation to preventshifting and settlement. The structural misalignmentand cracks caused by settling of the foundation are‘more serious in kilns than in many other types of con-

struction. Misalignment of kilns throws the track sys-tem out of line in track-loaded kilns, which creates se-rious problems when moving kiln trucks. Misalignmentof kilns with lineshaft fan systems can also cause wearand maintenance problems in the fan system. Settlingof the structure can cause cracks, which cause heat lossand problems in humidity control.

Foundation footings and walls are almost invariablymade of concrete. Their width or bearing area is deter-mined by the character of the soil and by the loads tobe imposed upon them.

Most kiln floors are made of poured concrete, usually6 in thick. Placing some form of insulation under theconcrete floor is an increasingly common practice. Thisreduces heat loss and helps to prevent condensation ofwater on the kiln floor in the early part of the kiln runwhen the relative humidity of the air is high and thefloor may be cold.

In some cases, a thick layer of crushed stone may beused. In package-loaded kilns, which use lift trucks toload and unload lumber, floors made of crushed stoneare difficult to maintain. Uneven floors can cause lum-ber stacks to lean, resulting in poor drying, or to fall,damaging the structure or injuring workers. Anotherdisadvantage of crushed stone is that heat is more read-ily lost to the soil or, alternatively, moisture from thesoil enters the kiln when kiln humidity is low. However,crushed stone does permit rapid drainage when conden-sation and water from melting snow or ice accumulatein the kiln during warmup.

Heating Systems

The drying of lumber requires the removal of largequantities of water from the wood. For example, dry-ing southern pine dimension stock green from the sawto 15 percent moisture content requires the removal of1.92 lb (0.23 gal) of water per board foot (24.9 lb/ft3).Drying l-in-thick red oak lumber green from the sawto 7 percent moisture content requires the removal of1.83 lb (0.22 gal) of water per board foot (22 lb/ft3).Since the heat of evaporation of water is approximately1,000 Btu/lb, great quantities of heat energy must begenerated and transferred to the circulating air and tothe wood in the drying process. This section discussesthe mechanism of heat energy transfer from the gen-erating source into the kiln and types of heat transfersurfaces.

The principal methods of conducting heat into the kilnare (1) indirect, where a hot fluid (commonly steam)flows into the kiln through pipes and radiates heat tothe kiln atmosphere through a suitable radiating sur-face, and (2) direct, where hot gases are discharged di-rectly into the kiln atmosphere.

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Figure 2-9—Headers with heating coils. (ML88 5599)

Indirect Heating

Perhaps the best examples of hot fluids used in indi-rect heating systems are steam, hot water, and hot oil.Steam systems are by far the most common in lumberdrying, though systems using hot water or hot oil areoccasionally found.

Steam.-Steam is used at various pressures. Sincethe temperature of steam varies with different levels ofpressure, more radiating surface is required to main-tain a given heat transfer rate or operating temperaturewith low-pressure steam than with high-pressure steam(see ch. 11 for a more detailed discussion of energy).

Steam is transported from the boiler to one or morekilns through large insulated pipes, often called themain feedline. At the kiln, steam enters one or moredistribution header pipes, from which each bank ofheating pipes originates (fig. 2-9). A condensate headeris located at the opposite end of the bank of pipes.Plain iron pipes were the standard material for radi-ating surfaces for many years, but now finned pipeheating coils are used almost exclusively (figs. 2-10,2-11). Depending on diameter and other factors, finnedpipes are considered to have from four to eight timesthe radiating capacity of conventional black iron pipes.Finned pipes are made of iron, aluminum, or copperpiping, which are wound with thin metal strips or at-tached to discs by welding or pressing to increase theheat transfer surface. Fins are made of various materi-als. Heavy gauge steel is the most rigid and serviceablebut is subject to corrosion, and aluminum is an excel-lent heat conductor but much more subject to dam-age. The heat transfer rate of aluminum fins is twice asgreat as that of steel fins. Copper is an excellent con-ductor but is generally considered too expensive forextensive use in lumber dry kilns and is easily damagedbecause of its softness.

Figure 2-10—Return-bend heating coil made with finpipe. (M 106142)

The return-bend heating system has historically beenthe most common arrangement of steam pipes within akiln (fig. 2-10). In this system, the banks of pipes leavethe distributing header, extend the length of shorterkilns, and return to a discharge (condensate) header.In longer kilns (over 66 ft), a return-bend header is ateach end of the kiln, such that returns meet in the mid-dle of the kiln (fig. 2-10, bottom).

It is now considered better practice to divide the heat-ing coils into banks of shorter length, single-pass coils(fig. 2-11) rather than return-bend coils. These shortbanks can be separately valved and thus produce moreuniform temperatures along their length than do longcoils.

As heat is transferred from steam through the coilsto the kiln atmosphere, the temperature of the steamdrops. It cools to the point of condensation, and water(condensate) begins to gather along the length of thecoil, providing the opportunity for uneven heating inthe kiln. Thus, all horizontal coils should be installedwith a downward pitch varying from 1/8 to 1/4 in perfoot of coil length to allow for drainage of condensate.

In multiple-track kilns where the circulating air passesthrough more than one truckload of lumber, it is goodpractice to install booster or reheat coils between thetracks (figs. 2-1 to 2-3, 2-12). The coils may be ar-ranged either vertically or horizontally and serve tomaintain a more uniform temperature within the kiln.

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Figure 2-11—Horizontal single-pass header coils andenlarged view of coils. (MC88 9027)

Hot water and hot oil—In hot water and hot oilsystems, the liquid is circulated by pumps throughheating coils similar to those used in a steam kiln. Thelower amount of heat available from hot water (whereno latent heat is present) in comparison with steamrequires a greater radiating surface. Maximum temper-ature attainable in the kiln is about 180 °F, which isadequate for many operations. However, few of thesesystems are currently in use in the United States. Hotoil systems work on the principle of pumping heatedoil through the heating coils in the kiln, though tem-peratures considerably higher than hot water can beattained.

Direct Heating

In direct-heated kilns, the hot gases produced by burn-ing gas, oil, or wood waste are discharged directly intothe kiln. These hot gases frequently pass through amixing or blending chamber to control temperature andvolume of air entering the kiln.

Burners commonly have electrically or pneumaticallymodulated fuel valves, which operate in connectionwith the recorder-controller. The fuel and air supply

Figure 2-12—Booster coils. (a) Vertical boosteror reheat coils between loads in track-loaded kiln.(b) Booster coils in horizontal position.(MC88 9032, MC88 9033)

for combustion is regulated to maintain the desiredkiln temperature. Some designs use several burner noz-zles, which can be operated individually or the seriesmodulated over a wide turndown range. Many burnersare designed to utilize wood waste and oil or gas inter-changeably.

In the blending chamber the hot products of combus-tion are mixed with the circulating air, raising its tem-perature to the point where subsequent mixing in thekiln will produce the required temperature as governedby the dry-bulb control mechanism. Temperature-limitswitches on the inlet and discharge ends of the combus-tion chamber shut down burners if they overheat. Thedischarge air is usually limited to a maximum of 425 to450 °F. A centrifugal blower forces the heated air fromthe burner through ducts to a plenum chamber, whichdistributes the air to the circulation fans (fig. 2-5).Most kiln air makes repeated circuits through the lum-

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ber piles, and only a portion is returned to the heatingchamber, usually by means of a collecting plenum run-ning the full length of the kiln. As mentioned earlier,in some designs the heat energy is transmitted from theburner through a heat exchanger to the circulating airto prevent combustion gases from entering the kiln.

Steam Traps and Control Valves

Steam traps and control valves are used to conservesteam and regulate its flow through the heating coils.

Steam Traps

In any steam kiln, large volumes of condensate formas steam cools when heat energy is transferred fromthe coils to the surrounding atmosphere. For every1,000 Btu of heat delivered, approximately 1 lb of wa-ter condenses in the steam lines. This condensate,initially at the temperature of the steam, must havea controlled discharge, otherwise the temperature ofthe coils would drop as they fill with condensate thuspreventing the entry of the higher temperature steam.Steam traps operate like automatic valves to controlthe flow and discharge of steam.

Steam traps are installed in the drain lines to removecondensate without the loss of steam. Another func-tion of steam traps is to release trapped air mixed withthe steam. Steam traps should be installed downstreamfrom and below the coils. For best operation, a strainermust be placed upstream of the trap to remove dirt andoil, and a check valve must be placed downstream ofthe trap to prevent back pressure or reverse condensateflow. A blowdown valve should be provided to peri-odically clean out scale and debris from the line. Allheating coils should be individually trapped to preventthe condensate from short circuiting from one coil toanother. The return line to the boiler must be largeenough to handle peak loads of condensate.

Proper sizing of steam traps for dry kilns is extremelyimportant and is more difficult in a dry kiln operationthan in many other applications of steam traps. It isjust as harmful to oversize a trap as it is to undersize.Undersizing a trap retards the discharge of conden-sate, which results in a slow and waterlogged heatingsystem. Oversizing a trap causes a discharge of somesteam with each discharge of condensate, which inter-feres with efficient operation of the heating system andwastes energy.

Steam traps generally used on dry kilns are of threetypes: mechanical or gravity, thermostatic, and thermo-dynamic.

Figure 2-14—Thermostatically controlled steam trap.(ML88 5596)

The mechanical or gravity-type traps often used on drykiln heating systems are of the inverted bucket or open-bucket design. The inverted bucket design (fig. 2-13)has generally superseded the open-bucket type and isthe most commonly used mechanical trap. As steamcondenses in the heating system, the condensate flowsinto the trap. When the trap is filled, the condensatedischarges through the outlet pipe. As soon as the sys-tem is free of condensate, steam enters the invertedbucket. The pressure of steam causes the bucket torise against the valve arm until the valve closes the dis-charge port. Air trapped in the bucket escapes througha vent in the top of the trap. Condensate again be-gins to flow into the trap, displacing the steam in thebucket. This reduces the buoyancy of the bucket un-til it again rests on the bottom of the trap. The dis-charge valve then opens and allows the condensate tobe discharged. Since the air in the top of the trap escapes before the condensate does, air binding is kept to

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Figure 2-15—Impulse steam trap. (MC88 9038)

a minimum. Because bucket-type traps contain liquidcondensate or water, it is important to provide ade-quate insulation in colder climates to prevent freezingof the water and damage to the trap.

In a typical thermostatic trap (fig. 2-14), a bellows thatexpands or contracts with changes in temperature isattached to a valve stem and valve. As the bellows ex-pands or contracts, it closes or opens the valve. Whenthe heating system is first turned on, the coils and trapare cold and contain air and water. At this point, thebellows are contracted and the valve is open. As steamenters the heating system, it displaces the water and airand forces them through the open valve. When all theair and water have been discharged, the trap is filledwith live steam. By then the trap temperature has in-creased enough to cause the bellows to expand, closingthe valve and preventing loss of steam through the trapoutlet. After the valve is closed, condensate again be-gins to accumulate and cool the bellows. This contractsthe bellows enough to open the discharge valve, and thecycle is repeated.

The third type of trap is the thermodynamic or impulsedesign (fig. 2-15). The flow of condensate through thistrap is controlled by differences in pressure betweenthe inlet chamber and the control chamber. When thesteam is off and the trap is filled with air, the pressure

is the same in the inlet as in the control chamber, andthe control valve rests firmly against the valve seat.When condensate enters the trap, the pressure in theinlet chamber becomes greater than that in the controlchamber. The pressure on the underside of the controldisk lifts the control valve free of the valve seat, and airand condensate pass through the valve opening into thedischarge line.

The control cylinder has a reverse taper that adjuststhe flow of condensate around the control disk and intothe control chamber, until the pressures above and be-low the disk are balanced. The temperature of the con-densate then increases because of the hot steam be-hind it. The hot condensate entering the lower pres-sure control chamber flashes into steam, which increasesin volume and retards the flow of condensate throughthe control-valve orifice. When the downward pres-sure on the upper surface of the valve and valve diskexceeds the upward pressure on the rim of the valvedisk, the valve is forced downward, shutting off the flowof condensate through the main orifice. The tempera-ture in the control chamber then drops, and the cycle isrepeated.

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Figure 2-16—Fans in lineshaft arrangement showingdisc fans and zig-zag baffle-shroud system, which di-

Control Valves

Both manually and automatically operated valves areused to control the flow of steam into the coils. Pres-sure regulators and reducing valves are also used tocontrol the pressure of the steam.

Steam flow is regulated by automatically controlled air-operated or electrically operated control valves coupledto the recorder-controller (see section on Equipmentto Control Drying Conditions). Hand-operated gatevalves are usually installed upstream of the controlvalves for “on-off” control of the steam supply. Handvalves are also advantageous on the feed and drain linesof individual heating-coil banks, especially in hardwooddrying operations. These hand valves enable operatorsto close certain banks for better control at lower tem-peratures, thereby reducing excessive fluctuations intemperature due to overshoot of the dry bulb when allbanks are open. The ability to isolate banks of coilsalso permits damaged or leaking ones to be removedand repaired without disturbing the remainder of theheating system.

rects the air through the lumber in either direction de-pending on fan and motor rotation. (MC88 9022)

Air-Circulation Systems

To dry lumber, air of controlled temperature and hu-midity must be passed uniformly over its surface. Thiscirculating air is the “workhorse” of the dry kiln. Assuch, the air performs two functions: it carries heatto the wood to effect evaporation, and it removes theevaporated water vapor. Effective and uniform circu-lation of air involves several factors: the size, location,and speed of the fans to drive the air; provision for re-versal of air circulation; installation and use of bafflesto direct the air through the load; and placement ofstickers within the load to facilitate the movement ofair across each piece of lumber.

Kiln Fans

In modern kilns, fans can be classified in two broadcategories: internal fan kilns, that is, fans located in-side the kiln itself; and external blower kilns, a systemwhere the fan or blower is located outside the kiln andthe air is conducted into the kiln through ducts.

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Figure 2-17—Control room for battery of lineshaftkilns, showing motor and pulley on lineshaft, recorder-controller, air-operated control valves to headers,

Before discussing different types of fans in these twocategories, it may be helpful to review the followinglaws regarding fans: (1) the volume of air moved variesdirectly with the fan speed in revolutions per minute(rpm), (2) the static pressure varies with the squareof the fan speed, and (3) the horsepower varies as thecube of the fan speed and directly as the air density.For more detailed discussion of fan engineering andpower consumption, see chapter 11.

Internal fans.-For internal fan kilns, there are twoprincipal arrangements of the fans: lineshaft and cross-shaft. In both of these arrangements, the fans are typ-ically placed overhead, with a false ceiling or deckbetween the fans and the load of lumber but not ex-tending beyond the edge of the lumber (figs. 2-1 to 2-3,2-5, 2-6).

In the traditional lineshaft arrangement, a series ofmultibladed disc fans (up to 84 in. in diameter in somelarge softwood kilns) is mounted on a single shaft run-ning the full length of the kiln. The fans are alternatelya left- and right-hand design. They are housed in a zig-zag baffle-shroud system that directs the air across thekiln (figs. 2-2, 2-16). So that air circulation may be re-versed efficiently, the fans are designed to operate in ei-ther direction. The motor, usually 50 to 75 horsepower,

hand-operated valves, and air-motor controlling vents.(MC88 9021)

is generally located in the operating room or controlroom at the end of the kiln (fig. 2-17). This type oflineshaft arrangement provides for moving large vol-umes of air at low speeds (up to 400 ft/min throughthe load) with a minimum of power, and it is particu-larly suited to drying lumber with low initial moisturecontent or a species that needs to be dried slowly.

In a more recent adaptation to the lineshaft arrange-ment, propeller-type fans are mounted on the lineshaft(fig. 2-18). This modification can deliver upwards of800 ft/min through the load, and the propeller-typefans are considerably more efficient per motor horse-power than disc fans. When changing from disc topropeller-type fans in retrofit operations, it may be nec-essary to change the type of bearings used for the shaft.

In the cross-shaft arrangement, fans are mounted onindividual shafts aligned across the width of the kiln(figs. 2-3, 2-5). Each fan is driven by an individual mo-tor (usually about 7.5 hp) either belt driven or directconnected. The motor may be mounted inside or out-side the kiln. Motors mounted inside the kiln must beof special construction to withstand high temperatures,especially in kilns operating above 200 °F. With exter-nally mounted motors, consideration should be givento offering some protection from the weather, particu-

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Figure 2-18—Propeller-type fans mounted on lineshaft.(MC88 9020)

larly in colder climates where freezing of condensed wa-ter vapor on the motor or shaft may present problems.Either multiblade-disc or propeller-type fans are com-monly used for cross-shaft kilns. They can deliver largevolumes of air at speeds considerably higher than thefans found in the traditional lineshaft kilns. With themodern trend to higher air velocities, especially desir-able in the high-temperature kilns, propeller-type fansare becoming increasingly popular. These fans havetwo to six blades, some of which have adjustable pitch;are made of cast aluminum; operate at high revolutionsper minute; and are capable of producing air velocitiesof 1,500 ft/min or more (fig. 2-19).

Traditionally, kilns have been designed such that fanspeeds and thus the velocity of air through the load oflumber do not change during the time of the kiln run.However, for the most efficient drying, higher airspeedsare needed during the early stages of drying when thewood is wet and large quantities of water need to beevaporated. Later in the drying schedule, lower air-speeds are adequate as the wood becomes drier and lessmoisture needs to be evaporated. As electrical energycosts have increased over the last decade, there hasbeen increasing interest in installing control equipmenton fan motors so that fan speeds can be adjusted dur-ing the run, thus saving on energy costs. The amountof savings appears to be higher in softwood drying,

which generally starts with relatively high moisturecontent woods that can be dried rapidly with minimaldrying degrade. In hardwood drying, which uses milderschedules and slower drying, less energy costs are ap-parently saved through reduction in fan speeds in thelater stages of the kiln run. Perhaps the greater advan-tage of variable fan speeds is to provide flexibility inairspeed requirements for those operations that dry anumber of species that differ markedly in airspeed re-quirement, such as pine, maple, and oak. Continuinginterest and research in the area of variable speed fansis expected as electrical energy costs rise and cost ofcontrol equipment becomes more competitive.

External fans.—External blower systems, though notas widely used as internal fan systems, offer anotherapproach to air circulation. These commonly use onlyone motor and blower to move air into the kiln. In thissystem, air is drawn from the discharge side of the loadthrough large ducts to an external centrifugal blower,from which the air is passed over the heater, humidifiedto the proper level, and redistributed in the kiln by an-other set of ducts to the high-pressure side of the load.The disadvantages of this approach are the low air ve-locities caused by the length of the necessary ductworkand the fact that the direction of air circulation is dif-ficult, if not impossible, to reverse. The advantage ofthis approach is that the air circulation system (themajor moving parts of a kiln) is concentrated in an eas-ily accessible place and can be readily serviced.

Baffles

To achieve uniform and, where desired, rapid drying,the properly heated and humidified air must be uni-formly directed to and through the lumber. To do thiseffectively, all alternate flow paths must be blocked sothat airflow over, under, and around the load is pre-vented. The best practical way to do this is by usinghinged baffles. The lack of effective use of baffling isone of the major causes of uneven or too slow drying.Airflow under the load in a track-loaded kiln may beprevented by having baffles hinged to the floor that canbe turned up against the kiln trucks to prevent air by-passing under the load. An alternative is to constructthe floor of the kiln with a trough just wide enough toaccommodate the rails and trucks and high enough sothat the lowest course of lumber just clears the level ofthe floor (fig. 2-2). The use of ceiling-hinged baffles ar-ranged so their lower free edge rests on the top of theload is an effective way of preventing airflow over thetop of the load. As the load shrinks during drying, thebaffles must have the ability to move down to keep con-tact with the load (fig. 2-6). Airflow around the ends ofthe load can be prevented by mounting bifold-hingedbaffles in or near kiln corners, ensuring contact withthe ends or corners of the load. A real effort shouldbe made to construct all kiln loads so that no holes or

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Figure 2-19—Propeller-type fans of cast aluminum in cross-shaft arrangement. (MC88 9019)

gaps occur between stacks because of mixed lumberlengths or stacks of uneven height.

Considerable care must be taken by personnel unload-ing the kiln to make sure all hinged baffles (floor, ceil-ing, and end) have been moved away from load beforestarting to move the load out of the kiln. Failure to doso results in baffles being ripped off or damaged. If baf-fles are damaged, they should be replaced immediatelyso that uniform air circulation can be maintained.

Plenum Chamber

The proper design and use of the plenum chamber orplenum space are necessary for adequate and uniformair circulation in a kiln. The plenum chamber is thespace between the lumber and the wall on either sideof a track-loaded kiln or between the lumber and thedoor or wall in a package-loaded kiln (figs. 2-2, 2-6).This area provides space for the fans to build up slightair pressure before passing through the courses of lum-

ber, thereby improving the uniformity of air distribu-tion through the load. When the fans reverse direction,the positive pressure reverses sides; the other side isalways under slightly negative pressure. The plenumchambers should be wide enough so that the staticpressure built up in them is sufficient to ensure uni-form air flow across the loads from bottom to top. Afrequently heard rule-of-thumb for estimating plenumwidth is that the width of the plenum should be equalto the sum of the sticker openings. Thus, if the sumof the sticker openings from top to bottom on one sideof the load is 60 in, the plenum on that side should beabout 60 in wide. A properly designed and loaded kilnwill have adequate plenum space.

It would be a mistake in loading package kilns to putan extra row of packages in what should be the plenumspace on the door side. This results in improper andnonuniform air circulation, and it is a practice to bestrongly discouraged.

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Venting and Humidification Systems

As mentioned before, drying of lumber requires the re-moval of large quantities of water from the wood. Inconventional kilns, the water is carried from the sur-face of the wood by the air passing over the wood. Toachieve proper drying of lumber, the amount of mois-ture in the kiln atmosphere (humidity) must be pre-cisely controlled. When the humidity inside the kilnis higher than desired, the excess moisture is ventedto the outside atmosphere and replaced with air fromthe outside. When the humidity inside the kiln islower than desired, additional moisture is added tothe kiln atmosphere by a steam spray or water spray-atomization.

Venting

Excess kiln moisture can be vented in one of two ways:(1) by static venting with the fans required for air cir-culation in the kiln or (2) by pressure venting with anadditional fan and ductwork.

In static venting, vents are placed in the roof on theintake and exhaust sides of the fans. When the ventsare opened, fresh air is drawn in on the suction sideof the fan and moist air forced out on the pressureside (figs. 2-1 to 2-3, 2-5). When the direction of rota-tion of the fans is reversed, the flow of air through thevents is also reversed. The size and number of ventsrequired depend on the species to be dried, that is,the amount of water to be removed from the wood.Species with large quantities of water, such as mostpines and poplars, require more ventilation than specieswith lower initial moisture content, such as oak or hardmaple or woods that have been air dried or partiallydried in a predryer. Kilns may have one or two linesof vents running the length of the kiln depending onthe fan arrangement (lineshaft (fig. 2-2) or cross-shaft(figs. 2-3, 2-5, 2-6)). Each line is automatically openedand closed by pneumatically or electrically powered mo-tors activated by the recorder-controller system. Insome cases, an additional row or two of manually oper-ated vents are located on the roof. Opening these ventscan provide additional venting when drying speciesthat require large venting capacities (such as sugarpine or white pine). Static venting is the most commonmethod of venting currently used in dry kilns.

In the pressure or powered venting systems, roof open-ings are replaced with two identical metal ducts placedinside the kiln, running the full length in the zoneabove the fan deck. These ducts vent to the atmo-sphere through louvered openings. Adjustable open-ings along the length of each duct regulate the volumeof air discharged into or withdrawn from the kiln; thusair is distributed uniformly throughout the kiln. A fanunit at the end of each duct acts interchangeably as

intake or exhaust, depending on the direction of aircirculation. When the fans are reversed, the ventingsystem also reverses, with the louvers actuated to openimmediately before the fans start. These systems aredesigned to exhaust moist air that has already passedthrough the lumber and bring in drier air on the pres-sure side of the fan. The greater capital cost of such asystem is claimed to be more than offset by lower main-tenance costs.

The vent system in any kiln exhausts more air volumethan it draws into the kiln to accommodate the expan-sion in volume of cooler incoming air as it is heated tothe higher kiln temperatures. In the case of poweredventing, this is accomplished by the design of the fanblade airfoil. The venting system is regulated by therecorder-controller mechanism as in normal roof vent-ing. In some direct-fired kilns, the centrifugal blowerproduces a type of powered ventilation by venting mois-ture through a damper in the return-air duct to theblower.

When venting is used to control excess moisture inthe air, substantial amounts of heat energy are thrownaway or wasted. This phenomenon has been recognizedfor some time, but the energy crisis of the 1970’s in-creased the interest in developing heat exchangers oreconomizers to use or reclaim some of the energy ex-hausted in vent air (Rosen 1979). By the mid-1980’s,at least one system had been developed that has proveneconomically feasible in western softwood kilns. Theair-to-air heat exchanger has replaced the need for tra-ditional venting; it preheats the incoming or makeupair to the kiln. It seems likely that other systems or im-provements to this system will be forthcoming, and theeffectiveness and savings in energy costs to heat kilnswill increase over the next decade.

Humidification

Control of the wet-bulb temperature or humidity inthe kiln is important during the drying, equalizing, andconditioning stages of the drying operation. Close con-trol of wet-bulb temperatures is especially importantin the early stages of drying hardwood species that areprone to surface checks, such as oak and beech, and tominimize surface and end checking in the upper gradesof softwood species. Close control of wet-bulb tempera-tures is also important during the conditioning phase atthe conclusion of the kiln run of any species requiringthis stress-relief treatment.

As mentioned earlier, when the humidity or the wet-bulb temperature of the kiln atmosphere is lower thandesired, additional moisture is added. In steam-heatedkilns, humidity is usually supplied as steam spray fromthe same source that supplies the heating coils. Steamis ejected through special nozzles on a steam spray

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line located in the airstream adjacent to the circula-tion fans, so the spray is mixed with the circulating airbefore it reaches the lumber (figs. 2-2, 2-6). As withthe heating system, steam spray is regulated by therecorder-controller.

If high-pressure steam is used for heating the kiln andis available for humidification, it should not be useddirectly to humidify the kiln. Use of high-pressuresteam adds a considerable amount of heat to the kilnin addition to increasing the humidity. This may causefluctuation or overshoot of the dry-bulb temperaturesuch that it is difficult or impossible to maintain thewet-bulb depression desired. This may be especiallytroublesome during conditioning when controlling wet-bulb depression is critical and adding large quantitiesof steam is necessary to increase the wet-bulb temper-ature. Steam pressure for the steam spray line should,therefore, be reduced to about 15 lb/in2-gauge by apressure regulator some distance before the line en-ters the kiln. If permitted by safety regulations, theportion of the line between the regulator and the kilnshould be left uninsulated so that the superheat in thesteam can dissipate, and the steam for humidificationwill be cooled to near saturation (250-260 °F). Anotheralternative is to install a desuperheater in the sprayline. This device injects water as a fine spray or mistinto the steam spray line, thereby removing the super-heat and reducing the temperature of the steam to nearsaturation.

In some installations that do not have a source ofsteam for humidification, water sprays are sometimesused. The water should be injected into the kiln in theform of a fine mist. It is highly desirable to heat thespray water since cold water has an appreciable cool-ing effect in the kiln and can cause fluctuation of thedry-bulb temperature and poor control of drying condi-tions. In some kilns, water sprays are used in conjunc-tion with steam sprays, but extra care must be taken toprevent water droplets from falling on the lumber andcreating stains.

For close control of wet-bulb temperatures in direct-fired kilns where no steam is available from the centralboiler, a small boiler may need to be installed to gener-ate the large volumes of low-pressure steam required forproper conditioning of the lumber.

Equipment toControl Drying Conditions

While drying conditions in most commercial dry kilnsare controlled by automatic or semiautomatic con-trollers, manual control is sometimes used in smallerinstallations or home-designed equipment.

Automatic Control Equipment

Automatic systems can be further divided into semiau-tomatic and fully automatic. Semiautomatic systemsrecord and control on set points that are changed fromtime to time during the kiln run by an operator. Infully automatic systems, process control informationis entered at the start of the kiln run, and any neededchanges are made automatically by the equipment dur-ing the kiln run.

Several process control techniques, some using special-ized equipment, are available for use with either thesemiautomatic or the fully automatic control equip-ment. They include zone control (see Zone Controlsection), variable frequency speed control for fans (seeKiln Fans section), and in-kiln moisture meters. In-kilnmoisture meters are generally of two types: (1) the re-sistance meter, in which electrodes (pins) are drivenor screwed into boards in the charge of lumber, and(2) the capacitive admittance meter, in which the elec-trode (a strip of metal) lies flat on the surface of thelumber. The electrode is slipped into the load parallelto the stickers. Electric signals on both systems areconverted to moisture content values and read on ameter. Both types of meters are subject to tempera-ture corrections and are not considered very reliable atmoisture contents above 30 percent. In-kiln resistancemeters are frequently used to monitor moisture contentof drying lumber below 30 percent and may be used tocontrol kiln schedules. Capacitive admittance metersare most commonly used in softwood kilns to monitormoisture content of drying lumber below 30 percentand are frequently used to determine when a charge isfinished. Some capacitive admittance meters are con-nected to the controller to shut the kiln down when apredetermined moisture content is achieved.

Semiautomatic Control Systems

Semiautomatic dry kiln control systems are typicallycharacterized by having a recorder-controller. This in-strument continuously measures and records on a chartthe conditions prevailing in the kiln and controls theheat and humidity to conform to the conditions presetby the kiln operator. As drying progresses, the oper-ator changes the instrument set-points to the desiredconditions in the kiln. This may be done based on timeelapsed since the start of the run or on the currentmoisture content of the wood as measured by a sam-

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pling technique such as weighing sample boards. Thefirst is more typical of a softwood drying operation, andthe latter is more commonly used in drying hardwoods.Once a dry-bulb and a wet-bulb temperature have beenset, the instrument automatically controls the condi-tions until they are reset.

Signals indicating the current conditions in the kiln arereceived at the recorder-controller from sensors locatedin the kiln. There is typically only one wet-bulb tem-perature sensor in a kiln but multiple dry-bulb temper-ature sensors. This is because the wet-bulb tempera-ture is essentially the same throughout the kiln, but thedry-bulb temperature may vary considerably over thelength and height of the kiln. The instrument comparesthese kiln conditions to the instrument set-point condi-tions, Changes in the kiln conditions are made throughsignals to air-operated valves that open or close heatingsystems, valves that open or close vents electrically, andhumidification systems as necessary to bring the kiln toset-point conditions.

For many years, the recorder-controller and its com-panion valve systems worked in an on-off mode; thatis, the controller told the valve to be completely openor completely closed. This method often wastes energyand does not offer as close control of kiln conditions asmay be desired. More recently the use of proportionalvalves and controllers has become the accepted prac-tice in most kiln operations. In this approach valves areopen to varying degrees depending on how far the kilnenvironment deviates from set-point conditions, thusoffering more precise control and saving energy.

Sensors currently used in lumber dry kilns are of threetypes. The traditional sensor used for over 50 years isthe gas-filled or liquid-vapor system. A more recentintroduction is an electric system using a resistancetemperature detector (RTD). A third type of sensoris used to measure equilibrium moisture content (EMC)of the kiln atmosphere. This sensor measures EMC di-rectly by electric resistance measurements across elec-trodes clamped to a small wood specimen or cellulosepad (EMC wafer) mounted in the kiln.

Liquid-vapor or gas-filled systems consist of four mainparts: (1) the temperature-sensing bulbs inside thekiln, (2) the armor-protected capillary tubes connect-ing the bulbs with the recorder-controller, (3) the he-lical movement (Bourdon tubes) inside the recorder-controller that provides the mechanical force to movethe pens on the recorder chart and the air relay por-tion of the controller, and (4) the clock movement thatturns the recording chart.

The dry-bulb and wet-bulb temperature-sensing unitsare connected individually by long capillary tubesto the Bourdon tubes inside the recorder-controller

Figure 2-20—Internal view of three-pen, gas-filledrecorder-controller (Moore type) showing Bourdontubes, air relays, clock, gauges, and dials. (MC88 9018)

(fig. 2-20), which is normally located in the kiln controlroom. The bulbs and capillary and Bourdon tubes aresealed with a volatile liquid (butane) and its vapor. In-creasing kiln temperature causes an increase of pressurein the liquid-vapor system; the capillary tube transmitsthe pressure change into the helical or Bourdon tube,causing it to expand. This movement is transmittedto the pen arm, which moves radially outward on therecording chart to indicate the increase in temperature.When the temperature in the dry kiln decreases, thereverse process takes place.

A typical dry kiln is usually equipped with one wetbulb and two or more pairs of dry bulbs. The wet bulbmeasures the wet-bulb temperature in the kiln result-ing from the cooling effect of evaporation on the moistwick and controls the humidity in the kiln through theinstrument. The paired or dual dry-bulb system (twobulbs connected to a common capillary tube) measuresand controls the temperature of the kiln environmenton the entering-air side of the lumber load in the kiln.The entering-air side of the load will always be the hot-ter side. When the air circulation reverses, the oppositeside becomes hotter, and the bulb on the opposite side

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of the load becomes the controlling bulb. Larger kilnshave two or more pairs of dual-control bulbs to bettercontrol the temperature in different zones of the kiln.

The controlling function of the liquid-vapor instru-ment is a pneumatic system of operating valves thatcontrol the amount of steam entering the dry kiln. In-side the recorder-controller case of a Moore instrument,the capillary tube from each bulb system is divided,with one lead going to the recording function and theother to a second Bourdon tube. Foxboro and Honey-well liquid-vapor instruments do not split this capillarybut achieve the same results using mechanical linkages.As pressure changes within the system, needle-type airvalves are brought into play, thereby accurately con-trolling heat input into the kiln and also controllingventing and spray or humidification.

Although the gas-filled or liquid-vapor control systemhas been time proven to be very dependable and ade-quately accurate, it does have some disadvantages whencompared to the newer electronic recorder-controllers.

Electronic recorder-controllers use platinum RTD-type bulbs for sensors of both dry-bulb and wet-bulbtemperatures and are connected to the instrument by16-gauge, three-conductor lead wire. The recordingfunction of the RTD control system contains an elec-tronic servo module that measures resistance changesof a RTD and positions the pen accordingly on thechart. The instrument contains a separate servo mod-ule for each measuring system (fig. 2-21). For example,a three-pen RTD electronic control system will havethree servo module units, one for the wet bulb and twofor the dry bulbs.

The principal element of the controlling system is ei-ther a modulating or an off-on pneumatic control unit,which tracks the measured variable through movementof the pen linkage. When the measured variable crossesthe set point, the control unit actuates a pneumatic orelectrical relay, which in turn sends an air signal to thecontrol valves, activating them as required.

The heart of the RTD electronic system is the servomodule assembly, which contains an electronic bridgecircuit, balancing amplifier, slide wire, and direct-current balance motor. One of the elements of the elec-tronic bridge circuit is a resistance bulb that senses thedry-bulb or wet-bulb temperature. An external relayswitches in the appropriate resistance bulb when fansreverse, thus assuring measurement of entering airtemperature.

Figure 2-21—RTD sensor and instrument.(M87 0167, M88 132-4)

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The advantages and disadvantages of the liquid-vaporand RTD control systems are as follows:

1. The RTD electronic controller has a fast responsetime, with a nominal period of 4 s for total pentravel over the total chart radius (about 4-3/8 in).This is extremely fast compared to a liquid-vaporcontroller, especially if the capillary system is over50 ft long. The graduations on the recording charton liquid-vapor instruments are nonlinear; the spacesbetween lines are closer nearer the hub and fartherapart on the edge. The graduations on the record-ing charts of the RTD controllers are linear over theentire range, thus making the task of setting andreading temperatures easier.

2. The RTD system is not limited by the length of leadrequired. The control instrument can be mountedat distances of up to 2,000 ft with no loss of accu-racy or response time. Any temperature changescaused by variation in lead length are compensatedfor automatically. By comparison, the liquid-vaporcontrollers are generally limited to capillary lengthsof about 100 ft, and the capillaries may be affectedby temperature changes between the sensing bulbsand the control instrument.

3. Temperature ranges can be easily changed on theRTD electronic controller by simply removing theexisting range card and replacing it with a newrange card. The liquid-vapor system requires remov-ing the instrument from the kiln and returning it tothe manufacturer or repair facility, where the sys-tem has to be refilled and recalibrated with specialequipment.

4. The liquid-vapor instruments are sensitive to bulblocations related to the instrument mounting (higheror lower). If these distances change for any reason,calibration is affected. The RTD system is not af-fected by bulb location, and sensing bulbs can bemoved at any time without affecting the calibrationof the instrument.

5. If any damage occurs to the sensing system of theRTD controller, it can be repaired at the site. Sens-ing bulbs can be replaced in a matter of minutes,and damage to lead wires can be repaired withoutany change in calibration or accuracy of the instru-ment. Liquid-vapor systems require removing theinstrument from the kiln with all capillary lines andbulbs intact and returning the instrument to themanufacturer or repair facility.

6. Perhaps the biggest advantage of the RTD electroniccontroller is the ease of calibration. Unlike the time-consuming two-person operation of using bucketsof hot water or hot oil and an etched stem ther-mometer required for calibrating the liquid-vaporsystem (see ch. 4), the calibration of the RTD con-troller is a very simple one-man operation using a

decade box. A given amount of electrical resistancecan be applied to the instrument for various tem-perature ranges, and a direct readout on the chartindicates either proper or improper calibration. Ad-justments are done very easily at the front of theinstrument by simply adjusting the appropriate link-age. Note that this technique calibrates only theinstrument, not the RTD sensor. The sensor is gen-erally assumed to be accurate. Proper resistance inthe RTD sensor can be checked against an electronicbridge. To check the total system, sensors and in-strument, it is suggested that the sensor(s) be placedin an ice-water slurry (32 °F) and then boiling wa-ter (212 °F) and the respective values read on theinstrument chart.

Fully Automatic Control Systems

In this manual, fully automatic control means the pro-cess control information or other drying schedule in-formation is entered at the start of the kiln run. Anychanges in temperature or humidity are made auto-matically by the controller during the kiln run. Thesechanges may also include determination of final targetmoisture content and shutdown of the kiln. Overridechanges are possible with these systems, but seldomused. This procedure differs from semiautomatic con-trol in which the recorder-controller effectively main-tains preset conditions but does not change set points,which must be changed by the operator. Fully auto-matic systems range from cam-operated controllers,used in some regions for several decades, to controllersbased on load cells that weigh the load or part of it, tothe rather recently introduced computerized controllersthat measure or infer changing lumber moisture contentin the kiln.

Cam controllers represent the earliest attempt at fullyautomatic control. They are a form of time-basedschedule and depend on the assumption that for agiven species, thickness, and grade of lumber, the loadmoisture content and hence the conditions in the kilnwill depend on the length of time drying has been inprogress. Two specially cut cams are required, one tocontrol the dry-bulb temperature, the other to controlthe wet-bulb temperature. Different cams have to becut for different species and thicknesses.

The advantages of cam controllers include the follow-ing: (1) schedules are predetermined and monitoringis minimal; (2) schedules can be ramped or movedsmoothly from one set of conditions to the next ratherthan arranged in steps, which cause abrupt changes inconditions and which may waste energy or put extraloads on the boiler; and (3) cams can be cut to givevery predictable, reproducible results, based on expe-rience in drying a given thickness(es) of given speciesstarting at similar initial moisture contents.

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The disadvantages of cam controllers include the follow-ing: (1) there is no direct link between the controllerand the moisture content of the lumber at any giventime during the kiln run—a load drying more slowlythan usual could easily be shut down while at a mois-ture content higher than desired; conversely, a load dry-ing more rapidly than usual could easily be overdried;(2) a recorder-controller rigged for following cams isnot readily converted to other forms of set-point de-termination; (3) care and experience are necessary tocut accurate cams; and (4) failure to monitor at fre-quent intervals may result in not implementing neces-sary changes in response to unforeseen factors such asboiler shutdown, steam leak, or loss of water to the wetbulb.

Load-cell systems are available that weigh the load ora portion of it and make changes in the schedule as thelumber dries. Selected boards are sampled to determineinitial or “green” weight in the usual way by cutting,weighing, ovendrying, and reweighing moisture sections(ch. 6); these values are averaged or weighted. This in-formation together with details of the schedule to befollowed are preprogrammed into the controller at thestart of the run, and the system takes complete con-trol of the drying operation. The main disadvantages ofthis approach are the problems of sampling and deter-mining reliable initial moisture content values, dryingon the average moisture content of the load or portionsampled, and lacking an indication of board-to-boardvariation in moisture content in the load during dry-ing. A preferred approach would be to use very smallload cells to follow the weight loss of individual sampleboards and to make these data available to the controlsystem by board or in groups of boards.

Since about the middle 1980’s, computerized controllershave been introduced in both softwood and hardwoodoperations. The introduction of desk-top-sized personalcomputers has provided a big boost to computerizedcontrol systems. Computerized control systems canrange from those that are little more than electric camtime-based systems to those that measure the mois-ture content of the wood in certain ranges and infer themoisture content of the wood in other moisture con-tent ranges. Since there is currently no reliably accu-rate method of measuring wood moisture content abovethe fiber saturation point (about 30 percent) exceptby weighing, values above 30 percent are inferred fromcontrolled temperature and relative humidity condi-tions in the kiln. Moisture values below fiber saturationpoint are determined by measuring the electrical resis-tance between metal pins or electrodes driven into theboard. Pins may be of different lengths so that mois-ture contents in the core and near the surface may bemonitored and some idea of gradient may be deter-mined. Some systems not only closely monitor and con-trol temperature and humidity conditions in the kiln

but also make changes in fan speeds and monitor orcontrol energy consumption. One computer may con-trol from 1 to as many as 8 to 10 dry kilns.

Computerized kiln controllers will likely find wider ac-ceptance in the lumber industry in the future. Manycurrent installations have shown that computerizationcan make the operation of dry kilns easier and can re-duce the cost of producing high-quality lumber. Thetechnology is advancing rapidly, and as we learn tosense more variables such as shrinkage, stress, woodtemperature, and moisture content, we will add tothe precision with which computer controllers can drylumber.

Zone Control

Zone control is a process control technique for equal-izing dry-bulb temperatures throughout the kiln andcan be used with either semiautomatic or fully auto-matic control systems. Cool spots in the kiln have longbeen noted for uneven drying, producing lumber that ishigher in moisture content than desired. Hot spots tendto produce lumber that is drier than desired. In zonecontrol, the kiln is divided into several zones, with tem-perature sensors coupled to control valves or dampersin the heating system. Zones typically run along thelength of the kiln; some designs have vertical zones aswell. The number of independently controlled zones canvary from 2 up to 24. Historically, zone control withgas-filled recorder-controllers meant that long (66 ftor longer) kilns were divided into two zones: one zonefor each end, or one zone control for operating the re-heat coils in a double-track kiln and another zone con-trol for operating the overhead heating coils. Comput-erized control with electronic RTD sensors has madeit possible to control a much larger number of zones.With computerized control, paired sensors measure the,temperature drop across the load (TDAL or ∆T) andseek, through their circuits with control equipment, tokeep the drying rate at the same level in all zones. Thistechnique is successful and rather widespread in newerhigh-temperature softwood kilns in both the southernand western United States. It is expected to becomemore common in older remodeled or retrofitted soft-wood kilns and some hardwood operations. However, inconventional-temperature hardwood kilns, the TDAL isusually so small that trying to control using this vari-able is not very promising.

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Manual Control Equipment

Some form of automatic kiln control is commonly usedon commercial kilns in the United States and Canada.However, manual control is possible and is generallyof interest to very small operations, often using homedesigned equipment, and to operations in which a per-son monitors the kiln on nearly a full-time basis. Forsuccessful manual control of drying conditions, the dry-and wet-bulb temperatures must be known. If thesetemperatures differ from those desired, the valves thatregulate the flow of steam (heat) and spray (humidity)into the kiln must be adjusted until the desired temper-ature readings are obtained. The appropriate amountof venting must also be watched and adjusted. To keepthe temperature or temperatures reasonably close tothose desired requires considerable operator time formonitoring and making minor adjustments to valves.

Temperature-Measuring Devices

The temperature-measuring devices commonly usedfor manual control are of two classes, indicating andrecording. Glass-stemmed indicating thermometers arefrequently used. The most satisfactory glass-stemmedthermometers have the graduations etched on the stem.Thermometers with separate scales stamped on an at-tached metal strip are not very satisfactory, since anyshifting of the strip with relation to the thermometertube will result in incorrect readings. Indicating dig-ital thermometers have largely replaced the pressure-spring type mentioned in the earlier edition of thismanual. The sensor for these digital thermometers maybe either a thermocouple or a RTD. Type–T (copper-constantan) thermocouple wire is suggested for mostdry kiln use.

Glass-stemmed indicating thermometers of the maxi-mum type are also used to obtain dry-bulb tempera-tures. Maximum thermometers show the highest tem-perature to which they have been exposed. After eachreading, they must be shaken down like clinical ther-mometers. Care should be taken in using maximumthermometers to allow enough time for the mercury toreach a peak temperature.

Digital thermometers with the capability to be cou-pled to a printer are also available when written recordsof the temperature are desired over a period of time.As with indicating-type thermometers, the sensor ofrecording thermometers can be either a thermocoupleor an RTD.

Humidity-Measuring Devices

To follow standardized kiln schedules with manual con-trol requires a knowledge of the wet-bulb temperatureor the relative humidity of the air circulating in the

kiln. This can be done by using an instrument thatreads relative humidity directly or with wet-bulb ther-mometry, which reads the wet-bulb temperature. Thedifference between the dry-bulb temperature and thewet-bulb temperature is the wet-bulb depression. Byknowing these values and by using a psychrometricchart, relative humidity can be calculated (see appendixto ch. 1). As discussed in some detail in the appendixto chapter 1, wet-bulb sensors must be continuouslywetted and located in a position in the kiln where suffi-cient airflow over the sock or wick will provide adequateevaporation of the water and thereby cooling so thataccurate wet-bulb temperatures can be determined.The wet-bulb wick should be changed after every kilncharge or more frequently if it becomes hard or crustyand is not wicking properly. If kiln conditions are tobe controlled by monitoring relative humidity and dry-bulb temperatures, then a high-quality relative humid-ity sensor should be obtained. Inexpensive sensors ormeters of the type commonly found in hardware storesare not recommended as they do not stay in calibra-tion well and can rather quickly give misleading or er-roneous readings. Wet- and dry-bulb hygrometers aresometimes used for manual control. These provide wet-and dry-bulb temperatures from the same instrumentand are illustrated in chapter 3 under Equipment forDetermining Temperatures.

Specialized Drying Approachesand Kiln Types

Dehumidification Kilns

Dehumidification kilns have been mentioned in severalplaces in this chapter. In many respects, these kilns aresimilar to steam-heated or direct-fired kilns, but theydiffer enough to warrant a separate description. De-humidification kilns have several advantages: a boilermay not be required (except as required for stress reliefor desired for warmup); they are more energy efficient,offering good control in drying refractory species thatrequire a low initial dry-bulb temperature as well ashigh relative humidity; and a low-cost kiln structure isadequate for some applications. Disadvantages are thatdehumidification kilns operate primarily on electricalenergy, which in some regions may be more expensivethan gas, oil, or wood residue (even though these kilnsare more energy efficient than other types of kilns);maximum temperatures are limited to about 160 °Fand in some units to about 120 °F; and, in some cases,there may be concern over chemicals in the condensate.

Air-circulation systems are essentially the same asthose used in steam or direct-fired kilns. The entiredehumidification unit may be located outside the kilnin an equipment room and blowers used to circulate airbetween the dehumidifier and the kiln. Another com-mon arrangement is a split system with the compres-

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Figure 2-22—View of a typical dehumidification kilnand schematic of typical dehumidification drying sys-tem. (ML88 5625)

sor and control panel in a separate equipment roomand the blower and coil cabinet inside the kiln. In somesmaller systems (less than 10,000 fbm), the entire dehu-midification unit may be inside the kiln. Air circulationwithin the drying compartment is provided as in theother types of kilns. While earlier designs typically hadair velocities in the lower range, the industry has grad-ually increased the air velocity to a level comparable tothat used in a conventional-temperature hardwood kiln.A typical dehumidification kiln is shown in figure 2-22.

The major difference between dehumidification kilnsand other types of kilns is the method by which wateris removed from the kiln air. The majority of the wa-ter is condensed on the coils of the dehumidifier andremoved as liquid, rather than being vented to the out-side atmosphere. Many larger dehumidification systems

have provisions for periodically venting excess heat,and some moisture is vented in the process of vent-ing heat, but only a small part of the total moisture inthe air is vented. These two characteristics account forthe greater energy efficiency of dehumidification kilns.First, since little moist air is vented to the outside ofthe kiln, the energy contained in the warm, moist airis not lost. Second, when the moisture in the air con-denses on the cold coils of the dehumidifier, the heat ofvaporization is recovered. Most dehumidification kilnsare built so that this recovered energy is used in dry-ing the lumber. The same approximately 1,000 Btu ofenergy per pound of water required to evaporate thewater from the lumber in the first place is recovered inthis condensation.

Kiln control systems on dehumidification kilns of about5,000 fbm and larger are similar to those of other kilns.They typically use RTD dry- and wet-bulb sensors andrecorder-controllers. Controllers for smaller systemsmay use a timer to control the percentage of time thecompressor operates or a humidistat to activate thecompressor. Most large systems (over 10,000 fbm) usedfor drying hardwood lumber are installed with a boilerfor warmup and conditioning or stress relief. Smallersystems often have electrical resistance heating ele-ments that are used to bring the kiln up to operatingtemperature to the point where the compressor cansupply enough energy to maintain the desired dryingconditions. These heating elements can also be used toattain the higher temperatures often called for near theend of the drying schedule.

Materials of construction vary from wood to masonryto prefabricated aluminum panels. The main criteriaare that the drying compartment be well insulated sothat maximum benefit can be derived from the energyefficiency and that the compartment be both airtightand moisture resistant. For medium to large kilns, in-sulation values of R-20 for walls and R-30 for roofs arerecommended. Slightly lower values may be acceptablein warmer climates. For smaller kilns (10,000 fbm ca-pacity or less) with less compression-generated heat,higher R values are required. Very serviceable and low-cost kilns can be built with simple wood-frame con-struction, in both large and small sizes.

In general, one can expect that drying stresses will bepresent after dehumidification kiln drying, as they areafter drying in other type kilns. When drying lumberfor uses where drying stress must be relieved, specialprovisions must be made if the system is not equippedwith a boiler. A small-capacity electric or gas-firedboiler can be incorporated in the kiln for this purpose.

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Figure 2-23—Typical predryer. (ML88 5600)

It is very important to properly size the compressor forthe thickness and species to be dried in the dehumid-ifier. If the compressor is too small, there is a risk ofstain, increased warp, and checking. If the compressoris too large, humidities in the kiln can cycle excessively,possibly resulting in a lack of heat.

Predryers

Predryers are large low-temperature dryers used to drygreen lumber to a moisture content of around 25 per-cent prior to drying to a lower final moisture content ina kiln. Also called warehouse dryers, these large freespan buildings range in lumber-holding capacity from50,000 to over 1,000,000 fbm and are typically sized atfour times kiln capacity (fig. 2-23). Generally, predryersare forklift loaded, although track loading may be pre-ferred in some cases. Most predryers are preengineeredbuildings of structural steel with 1 to 2 in of rigid foaminsulation between painted steel or aluminum sheath-ing. They commonly have concrete floors. Tempera-ture and relative humidity are controlled with temper-ature set-points typically ranging from 75 to 100 °Fand with relative humidity maintained between 60 and90 percent.

Predryers for controlling air-drying conditions havebeen used successfully for over 25 years by some com-panies in the northern latitudes of the United Stateswhere natural air-drying conditions are unfavorable formany months, from both the standpoint of defect de-velopment and length of air-drying time. However, inrecent years high lumber prices and high interest rateshave produced financial incentives strong enough tointerest lumber producers in other areas, especiallyhardwood producers who had typically air dried theirlumber 60 to 90 days or longer before final drying inthe kiln.

The advantages of predrying over air drying in the yardare brighter lumber, more uniform moisture contentof dried lumber, and reduction of drying defects, all inabout one-third less time. Inventory can be reduced byone-third to one-half, freeing capital and yard space.Several species and thicknesses can be mixed in thesedryers. Thus, lumber of different moisture contents,species, and thicknesses may be in the predryer at thesame time; drier lumber can be moved frequently outto the kilns and newly acquired green lumber can bemoved in. The lumber is usually arranged by blocks ina zone to group similar species, thicknesses, and levels

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of moisture content. Disadvantages of predrying overair drying are largely associated with costs of building,energy, and maintenance.

Predryers are typically heated with steam and finnedcoils. Humidity is controlled by external venting whenhumidities are too high and by using moisture releasedfrom the lumber to maintain humidities as high asrequired. Larger predryers are divided into two andsometimes three zones; conditions are controlled sep-arately in each zone by a recorder-controller similarto those described earlier in this chapter. In some in-stallations, temperature and humidity conditions aremaintained with dehumidification units.

Temperatures are usually sensed by electronic RTDs,and humidity is sensed by wet-bulb thermometry orwith relative humidity sensors using the cellulose pador similar sensor. Placement of the sensors above theload in the rafters is often criticized because the sam-pled air is not as representative of the air entering thestacks as one would like. However, the free-span con-struction of the structure does not provide much choicein where to place the sensors.

Air circulation is usually provided by overhead fans ar-ranged in a row horizontally over the plenum betweenthe two rows of lumber (fig. 2-22). The air is directeddown into the plenum by belt-driven or directly drivenfans, and then it is passed through the stacks of lum-ber. One criticism of predryers has been uneven airdistribution, resulting in uneven drying from top tobottom of the stacks. Various forms of baffle systemshave been suggested to improve distribution and unifor-mity of airflow. Exhaust ventilation should be designedso that it does not direct the humid exhaust air downonto the roof; this has been reported to cause localizeddeterioration of the roof. Rather, exhaust ventilationshould consist of “upblast” units that direct vented airstraight up and way from the building. The makeupair enters through louvers in the walls and can be pre-heated if needed.

The concept of using predryers rather than air dryinghas gained wide acceptance in the hardwood indus-try, though it may not be a technique that works welland is profitable for all operations and installations.The techniques are still evolving, and many changes arelikely to be seen in the next few years.

Solar Dry Kilns

Interest in solar dry kilns was low until the energy con-cerns of the mid-1970’s. The advantage of solar kilnsis the free and often abundant energy available, butthe disadvantage is that there is a cost to collectingfree energy. Free energy is also low-intensity energy,which often limits the operating temperature of a kiln

to approximately 130 °F unless prohibitively expensivespecial solar collectors are used. Despite the cost of col-lecting the energy, another advantage of solar kilns isthat relatively small, simple, and inexpensive kilns arepossible, and this level of technology is often well suitedfor small operations.

The average annual solar energy available on a horizon-tal surface in the United States ranges from 1,000 to2,000 Btu per day per square foot of collector area.Average amounts for several locations are given intable 2-1. Tilting the collector surface perpendicularto the sun maximizes the intensity of the direct solarradiation and minimize losses caused by the reflectionof the direct radiation. The general rule for maximiz-ing solar radiation on a year-round basis is to tilt thecollector at an angle to the ground equal to the lati-tude. If solar radiation is to be maximized in the sum-mer months in locations where latitude and ambienttemperature make winter drying impractical, direct ra-diation can be maximized by reducing the tilt angle toabout 15° less than the latitude. In the northern hemi-sphere, the collector should face directly south.

Solar kilns can operate by direct solar collection (green-house type) or by indirect solar collection where thecollector is isolated in some way from the drying com-partment. They can also operate with solar energyalone or with supplemental energy. The four types ofsolar kilns are as follows:

1. Direct collection (greenhouse)

a. Solar only, which is characterized by wide diur-nal and day-to-day changes in temperature andrelative humidity

b. Solar with supplemental energy, which is charac-terized by the ability to follow a drying scheduleand has large nighttime heat losses because of thelow insulating ability of the transparent cover

2. Indirect collection (isolated drying compartment)

a. Solar only, where the diurnal change in temper-ature and relative humidity can be reduced byenergy storage and reduced heat losses at night

b. Solar with supplemental energy, where sched-uled drying is possible and nighttime losses areminimized

Generalized solar kiln designs are shown in figure 2-24.Possible collector surfaces are south-facing walls, eastand west walls, and a roof. Solar collection is eitherdirect (fig. 2-24a,c) or indirect (fig. 2-24b,d). The col-lector surface is either uninsulated (fig. 2-24a,b) or in-sulated at night (fig. 2-24c,d).

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Figure 2-24—Generalized solar kiln design types.(a) Uninsulated dryer and collector are one unit.Airflow mixes on both sides of absorber panel (Ab).(b) Uninsulated, improved design. Airflow mixes withinchamber. (c) Insulated (ln) externally. Airflow mixeson both sides of Ab, day and night. (d) Insulated (ln)internally. Air flows over the front of the absorberpanel (Ab) when damper (Da) is open. (ML88 5603)

Figure 2-25—Solar kiln design for northern latitudes,showing inexpensive control system. (ML88 5602)

In the simplest uninsulated form (fig. 2-24a), air flowson both sides of the absorber surfaces (Ab). The dry-ing chamber and collector surfaces are one unit as in atrue greenhouse structure. A somewhat improved de-sign (fig. 2-24h) isolates the collector surfaces (R and S)and the outer absorber surface (Ab) from the dryingchamber. The energy absorbed on the absorber surfaceflows through the absorber to its inner surface where itis transferred to the circulating kiln air. Both of thesesystems suffer large nighttime heat losses.

For the insulated designs, two variations are possible.In the simpler of these designs (fig. 2-24c), airflow issimilar to that in figure 2-24a except that diurnal insu-lation (ln) is accomplished by external means such asshutters or blankets. Collector and absorber surfacesare also isolated in the design shown in figure 2-24d,with the drying air acting as the medium for heattransfer. When the dampers (Da) are open, the airflows over the black absorber surface (Ab) and backinto the dryer chamber. When the dampers are closed,nighttime airflow is interrupted, thereby reducing night-time (and cloudy day) heat losses because the absorberhas an insulated back (ln). A more detailed schematicof this type of solar kiln is shown in figure 2-25. Inanother common variation of this insulated-type so-lar kiln, the solar collector is detached from the dryingcompartment, and blowers transfer the heated air fromthe collector to the drying compartment (fig. 2-26).

At present, solar drying is not widely used in theUnited States. The main uses are hobbyists or smallwoodworking shops that do not require large dryingcapacity and that do not wish to make large capital in-vestments in drying equipment.

Vacuum Drying

Vacuum drying of lumber is not a new idea, and, infact, it has been considered since the turn of the cen-tury. However, vacuum drying did not come into useuntil the 1970’s because it was considered uneconomi-cal. The principal attraction of vacuum drying is thatthe lowered boiling temperature of water in a partialvacuum allows free water to be vaporized and removedat temperatures below 212 °F almost as fast as it canat high-temperature drying at above 212 °F at atmos-pheric pressure. Drying rate is therefore increased with-out the dangers of defects that would surely develop insome species during drying above 212 °F. Vacuum dry-ing is essentially high-temperature drying at low tem-peratures. During the early 1970’s, the economic out-look for vacuum drying became more favorable, largelybecause of the increased costs of holding large inven-tories of lumber during long drying processes. This isparticularly true in the drying of thick, refractory, high-

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Figure 2-26—Schematic diagram of solar wood-residuedry kiln. A, furnace room; B, intake air enters col-lector; C, solar blower; D, manifold ducts for solar-heated air; E, internal fans; Fc, differential temper-ature sensor-collector; Fd, differential temperaturesensor–dryer; G, humidifier; H, return-air duct from

value species, which can be safely dried in a vacuumkiln in a small fraction of the time required in a con-ventional kiln.

The main difference between the several types of vac-uum kilns currently on the market is the way in whichheat is transferred to the lumber. Convective heattransfer in a partial vacuum is almost nonexistent. Inone common type of vacuum kiln, there are alternatevacuum and atmospheric pressure cycles. Heat is ap-plied to the lumber convectively at atmospheric pres-

dryer to collector (dampered at night); J, entry pointof intake air; K, exhaust vents; RH1, humidistat for ex-haust vents K: RH2, humidistat for shutting kiln offat high humidity; RH3, humidistats for humidifier G.(ML88 5601)

sure, and then a vacuum cycle is applied to remove wa-ter at low temperature. These cycles are alternatedthroughout the drying. Another common type of vac-uum kiln maintains a vacuum throughout the entiredrying process, and the heat is transferred to the lum-ber by direct contact with steam-heated platens or byelectrically heated conductive blankets that contact thelumber (fig. 2-27). A third type employs high-frequencyelectrical energy to heat the lumber. In all types, wateris removed from the drying chamber by pumps.

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Figure 2-27—Vacuum-kiln type in which heat is sup-plied to the lumber by contact with electrically heatedblankets. (M85 0351-10)

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Literature Cited

Rasmussen, E. F. 1961. Dry kiln operator’s manual.Agric. Handb. No. 188. Washington, DC: U.S. Depart-ment of Agriculture. 197 p,

Rice, W. W. 1977. Dry kiln: A design to season500 board feet. Fine Woodworking. Spring: 39-43,

Rosen, H. N. 1979. Potential for energy recovery fromhumid air streams. Res. Pap. NC-170. St. Paul, MN:U.S. Department of Agriculture, Forest Service, NorthCentral Forest Experiment Station. 10 p.

Sources of Additional Information

Branhell, G.; Wellwood, R. W. 1976. Kiln drying ofwestern Canadian lumber. Western Forest ProductsLaboratory Information Report VP-X-159. Vancou-ver. 112 p.

Cech, M. Y.; Pfaff, F. 1977. Kiln operator’s manual foreastern Canada. Eastern Forest Products LaboratoryReport OPX192E. Ottawa. 189 p.

Knight, E. 1970. Kiln drying western softwoods.Moore-Oregon, Portland. (Out of print.) 77 p.

McMiIlen, J. M.; Wengert, E. M. 1978. Drying easternhardwood lumber. Agric. Handb. No. 528. Washing-ton, DC: U.S. Department of Agriculture. 104 p.

Tschernitz, J. L. 1986. Solar energy for wood dryingusing direct or indirect collection with supplementalheating. Res. Pap. FPL-477. Madison, WI: U.S. De-partment of Agriculture, Forest Service, Forest Prod-ucts Laboratory. 81 p.

Wengert, E. M.; Oliveira, L. C. 1985. Solar heated,lumber dry kiln designs. Blacksburg, VA: Departmentof Forest Products, Virginia Polytechnic Institute andState University. 91 p.

Table 2-1—Annual average of daily solar radiationavailable at various locations in the United States

CitySolar radiation

(Btu/ft2-day)

Fort Worth, TXGrand Junction, COGreensboro, NCIndianapolis, INLexington, KYLittle Rock, ARShreveport, LA

Albuquerque, N MAmes, IAAtlanta, GABoise, IDBoston, MACorvallis, ORDavis. CA

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