Chapter 2 Kiln Types and Features

Chapter 2 Kiln Types and Features Classification systems 43 A lumber dry kiln consists of one or more chambers Operational techniques 43 designed to provide and control the environmental con- Compartment kilns 43 ditions of heat, humidity, and air circulation necessary Progressive kilns 48 for the proper drying of wood. As the development of Temperatures of operation 48 the modern dry kiln has progressed, a number of de- Low-temperature kilns 49 sign modifications have been explored in relation to the Conventional-temperature kilns 49 mechanism of heat supply, arrangement and type of Elevated-temperature kilns 49 fans, control of relative humidity or wet-bulb temper- High-temperature kilns 49 ature, and use of various materials for construction of Type of heating and energy source 49 the chamber. Steam 49 Direct fire 49 The design of a kiln has an important bearing on its Electricity 50 operation and drying efficiency. A properly designed Hot water and hot oil 50 and operated kiln will dry most species of lumber or Solar 50 other wood products to any specified moisture content General construction features 50 between 3 and 19 percent in a reasonably short time Construction materials 50 without appreciable losses caused by drying defects. Aluminum 50 Concrete block, poured concrete, and brick 50 Wood and plywood 51 Classification Systems Foundations and floors 51 Dry kilns can be classified in a number of different Heating systems 51 ways. In this manual, we have chosen a system that Indirect heating 52 classifies by (1) operational techniques, (2) tempera- Direct heating 53 tures of operation, and (3) type of heating and energy Steam traps and control valves 54 source. Other possible classifications might include fan steam traps 54 arrangement and method of loading the kiln. Control valves 56 Air-circulation systems 56 Kiln fans 56 Operational Techniques Baffles 58 Plenum chamber 59 Classification by operational techniques distinguishes Venting and humidification systems 60 between the more common compartment-type kiln and Venting 60 the less common progressive-type kiln. Humidification 60 Equipment to control drying conditions 61 Compartment Kilns Automatic control equipment 61 Semiautomatic control systems 61 Compartment-type kilns (figs. 2-1 to 2-8) are designed Fully automatic control systems 64 for a batch process in which the kiln is completely Zone control 65 loaded or charged with lumber in one operation, and Manual control equipment 66 the lumber remains stationary during the entire dry- Temperature-measuring devices 66 ing cycle. Temperature and relative humidity are kept Humidity-measuring devices 66 as uniform as possible throughout the kiln, and they Specialized drying approaches and kiln types 66 can be closely controlled over a wide range of tempera- Dehumidification kilns 66 ture and humidity. Temperature and relative humidity Predryers 68 Solar dry kilns 69 Vacuum drying 70 Chapter 2 was revised by R. Sidney Boone, Literature cited 73 Research Forest Products Technologist, and Sources of additional information 73 William T. Simpson, Supervisory Research Table 73 Forest Products Technologist. 43 Figure 2-1—Some plans for location of fans and baffles in dry kilns. (ML88 5604) are changed as the wood dries based on a schedule that that are rolled into and out of the kiln on tracks. The takes into account the moisture content and/or the dry- majority of the softwood lumber in the United States is ing rate of the stock being dried. Drying schedules vary dried in track-loaded kilns. The other method of load- by species, thickness, grade, and end use of material as ing involves moving stacks or packages of lumber di- discussed in detail in chapter 7. All modern dry kilns rectly into and out of the kiln with a lift truck. These use some type of forced-air circulation system, with air are generally called package-loaded kilns, although they moving through the load perpendicular to the length of are frequently called side-loaded kilns in the western the lumber and parallel to the stickers. Although some softwood region. The majority of the hardwood lumber cross-circulation kilns (airflow parallel to the length of in the United States is dried in package-loaded kilns. the lumber and perpendicular to the stickers) can still be found, kilns have not been built using this technique Track-loaded kilns commonly have one or two sets for several decades. The natural draft circulation sys- of tracks and occasionally three sets, and are known tem, which took advantage of the principle that heated as single-, double-, or triple-track kilns, respectively air rises, is now considered inefficient and is of historic (figs. 2-2 to 2-5). The width of the stack of lumber per interest only (Rasmussen 1961). A more detailed dis- track is typically 6 to 9 feet. In kilns more than one cussion of the different types of air circulation systems track wide, some provision for reheating the air is made can be found later in this chapter under the heading before it passses through the next stack of lumber. The General Construction Features. length of a track kiln is usually some multiple of the lengths of the lumber being dried correlated with the Compartment kilns can be classified by the method of amount of lumber production required. Kiln lengths loading. Perhaps the largest number of kilns are of the vary from about 40 to 120 ft; those used for hardwood track-loaded type. The lumber is stacked on kiln trucks drying are typically 40 to 66 ft long and those used for 44 Figure 2.2—Lineshaft, double-track, compartment kiln with alternately opposing fans. Vents are over fan shaft between fans. Vent on high-pressure side of fans be- comes fresh air inlet when direction of circulation is reversed. (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 parallel to stickers. (ML88 5594) 45 kiln require a minimum amount of time. Kiln trucks loaded with green lumber are pushed into the kiln im- mediately after the dried lumber is removed from the kiln. 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 the kiln will protect the top courses of freshly sawn lumber from degrading in the sun as a result of uncontrolled drying and from rain or snow. Figure 2-8 shows a kiln with protective cover at both the dry and green ends. Frequently cited advantages of track kilns include short downtime for loading and unloading and more uniform drying primarily because of narrower load widths. Dis- advantages include greater building cost, because track Figure 2-4—Double-tray-loaded aluminum pre- kilns require more land area than package kilns espe- fabricated kiln with doors at both ends of kiln. cially if kiln has tracks at both ends, and the added (MC88 9017) expense of track and kiln trucks. softwood, typically 66 to 120 ft long. Lumber-holding Package-loaded kilns are generally smaller than track- capacity can vary from around 25,000 fbm (4/4 basis) loaded kilns and have a different configuration for to 220,000 fbm (8/4 basis). loading the lumber (figs. 2-6, 2-7). Large doors per- mit the stickered and stacked lumber to be loaded into Track kilns may have doors at one end or, more com- the kiln with a lift truck. Most package kilns are de- monly, at both ends so that unloading and loading the signed to hold 24 ft of lumber from front to back of Figure 2-5—Direct-fired, double-track-loaded high temperature kiln in which hot products of combustion are discharged directly into the airstream circulating within the kiln. (ML88 5605) 46 Figure 2-6—Package-loaded kiln with fans connected directly to motors. (ML88 5598) Figure 2-7—Lift truck delivering package of stickered lumber to package-loaded kiln. (MC88 9024) 47 Figure 2-8—Track-loaded, concrete block kiln with doors and protective cover at both ends of kiln. (MC88 9023) kiln, although some are designed for a depth of 16 ft of and the initial and final moisture content of lumber be- lumber. Since airflow in package kilns is from front to ing dried. Because of the relatively continuous move- back, or vice versa, the length of air travel through the ment required in this approach, progressive kilns are load is also 24 ft. No provision is generally made for re- usually of the track-loading type. As with compartment heating the air as it passes through the load. Lumber- kilns, the early models relied on natural draft circula- holding capacity of package kilns varies from around tion, but forced circulation using either internal fans or 25,000 to 90,000 fbm (4/4 basis). Some frequently cited external blowers soon became the preferred method of advantages of package kilns include lower building cost air circulation. and use of less land area. Disadvantages include long downtime for loading and unloading and generally less Progressive kilns lack flexibility in drying kiln charges uniform drying if initial wood moisture content is above that vary in species, dimension, or moisture content. 25 percent. Using shorter air-travel distances and hav- They do not provide the close control of conditions re- ing all lumber at about the same moisture content in- quired by most hardwood operations or the speed of crease drying uniformity. If starting moisture content is drying required by most softwood operations.

Load more