US2834120A - Lumber curing process - Google Patents

Description

May 13, 1958 E. R. 4GREIENHOOD LUMBER CURING PROCESS Filed May 24, 1954 4 Sheets-Sheet 1 4 Inverzivaf:

Eis'za Busse eeizood,

May 13 1958 E. R. GREENHOOD 2,834,120

LUMBER CURING PRocEss Filed may 24, 1954 4 sheets-snaaiv 2 Esa Russe ifeezzood, y 7%, e yfm Hwzegys May 13, 1958 E. R. GREENHOOD 2,834,120

LUMBER CURING PRocEss Filed May 24. 1954 4 sheets-sheet s Eisa llwseZZ weeazanod, w diloafneys 4 Sheets-Sheet 4 My 13, 1958 E. R. GREENHOOD LUMBER CURING PROCESS Filed May 24, 1954 United LUMBER CURNG PRGCESS Elisha Russell Greenhood, Wellesley, Mass.

Application May 24, 1954, Serial No. 431,671

Claims. (Cl. 34-37) The invention relates tothe curing of green or partly dried lumber. The present application is a continuationin-part of the inventors copending application Serial No. 213,106, led February 28, 1951 now abandoned.

The methods commonly used for drying lumber are classified generally as air drying and kiln drying. Air drying is accomplished simply by stacking the lumber in a yard or shed for a long enough period so that the evaporation which takes place under natural conditions reduces the internal moisture of the lumber to a tolerable percentage. Kiln drying, as it is presently practiced, is essentially a process of heating the lumber in a closed kiln to speed up the evaporation of internal moisture. Although the process here described might be referred to as a kiln drying process, inasmuch as a kiln is employed, it differs in several respects from conventional kiln drying, and is more properly termed a curing, rather than a drying, process as it produces effects other than mere moisture removal.

According to the most common method of kiln drying, the lumberis stacked in a kiln, and warm air is circulated through the stack. By controlling the. humidity and temperature of the air the rate of evaporation is controlled. Ordinarily the drying is started at high humidity and low temperature, and the humidity is gradually reduced and the temperature increased during the drying cycle. The purpose of circulation is to maintain uniform conditions of temperature and humidity throughout the kiln, and comparatively low circulation speeds are adequate. It is advisable to use a high humidity at rst to avoid too rapid evaporation at the surface, and in some cases the humidity is kept high enough to prevent evaporation altogether until the lumber has become heated through. These precautions are necessary to prevent case hardening, a condition resulting from too rapid drying of the surface of a board while the interior is still wet. In this condition the surface has shrunk and set under considerable tension. This condition makes a board stili and also gives rise to such defects as checking, cracking and warping. In spite of such precautions', some degree of case hardening frequently occurs, leaving the boards stit, and it is then necessary to destress the boards by restoring moisture to their surfaces. Case hardening also prevents proper drying of the interior of the board, as the internal moisture cannot penetrate the case hardened surface. The optimum temperature and humidity schedule for safe drying varies considerably for diiferent types of wood and for different lots of the same wood, and the control of the process requires skilled judgment, as Well as constant vigilance andfrequent testing by the operator. An error in judgment can result in a very costly loss. i

Conventional kiln drying, although much faster than air drying, is still a slow process, as it takes many days to kiln dry green lumber to 12% moisture content, which is about the average ultimate moisture content for air dried lumber, and several days to reduce the moisture content from 12% to 8%, which is representative of 2,834,120 Patented May 13, 1958 ice 2 the dryness required for lumber to be used for furniture, lindoor trim, and similar purposes.

Both air drying and conventional kiln drying are strictly evaporation processes and accomplish no more than removal of moisture from the lumber. The lumber rcrnains highly hygroscopic. Such lumber must be kept in heated storage until used or, if left outdoors for a considerable period, will regain high moisture content. Furthermore, because of its hygroscopicity and conse quent tendency to shrink and lswell under changing atmospheric conditions, wood is inferior for many purposes to other materials, such as metals and plastics.

The chief object of the invention is to provide a process for curing lumber which is much fasterand more economical than present kiln drying processes, which substantially eliminates the danger of case hardening, with consequent checking, cracking and warping, which can be automatically controlled so that the results do not depend on the skill and experience of the operator, and which renders the lumber substantially less hygroscopic and thus more dimensionally stable than lumber dried by conventional methods, while retaining the original strength and flexibility of the boards.

Another object is to provide inexpensive and efcient apparatus for carrying out the process. The process and apparatus here described are suitable either for curing Y green lumber or for further curing of lumber which has been previously dried by other methods.

Further objects and advantages of this invention will be apparent from the following description of the process in connection with the recommended apparatus shown in the accompanying drawings; but it will be understood that tne structural details of the apparatus herein illustrated and described as a means of carrying-out the improved process, may be varied to suitv particular conditions or installations, without departing from the essence of the invention as defined in the appended claims.

The apparatus for carrying out the process here described is illustrated by drawings, in which Fig. l is a horizontal'section through a kiln equipped with the improved lumber curing apparatus;

Fig. 2 is a longitudinal section on line 2 2 of Fig. 1;

Fig. 3 is a transverse section on line 3-3 of Fig. 1, showing the outer end of the kiln car with the lumber stacked thereon;

Fig. 4 is a transverse section at the inner end of the kiln car, showing a modification;

Fig. 5 is a transverse section of the kiln showing the inner wall of the empty kiln, complemental to the modication of Fig. 4; and

Fig. 6 is a perspective, exploded view of the modified kiln car and its cover members.

ln the form chosen for the purpose in Figs. 1-3, the lumber curing apparatus comprises a relatively small shed or other enclosure having heat-insulated side walls 10, at least one of which is provided with a dampered vent 11 in its upper portion, an insulated end wall 12, and an insulated roof 13, erected on a foundation 14 of concrete, and having a transverse, insulated wall 15 dividing the shed into a kiln compartment 16, and a blower compartment 17 having a subcompartment or control room 13 defined by a partition 19. The oor of the kiln compartment preferably has tracks Ztl for a kiln car 21 movable into and out of the kiln through an insulated door 2'2.

Compartment 17 houses one or more blower units of any standard construction and operation, two such units being shown at 23 and 2li in Fig. 1. The fans of the -blower units (not shown) being driven by motors 25 and 26, respectively, suitably mounted in control room 18 and operatively connected to drive shafts 27 and 28 passing through the partition 19. The respectivev blower units have air intake ports 29 and 30, and outlet- ducts 31 and 32 leading to delivery ports spaced vertically with respect to each other in the center of the wall 15. Said ports preferably have louvres 33 and 34, respectively,

or other air deecting vanes, for dilfusing the air driven into a tunnel (hereinafter described) in the center of the kiln 16.

The apparatus is here illustrated as employing a pair of blowers. It is understood, however, that a single blower of adequate power to produce the necessary volume and velocity of circulation may be centrally mounted so as to discharge into the tunnel through a single duct. The blower or blowers must be of a capacity to deliver air or vapor into the tunnel at relatively high speed, the speed being preferably related to the tunnel size in such a manner as to change the tunnel atmosphere at the rate of about sixty times a minute, for the reasons to be further explained with reference to the process.

A steam pipe 35 is connected to heating coils 37 which are placed in the discharge ducts of the respective blowers to heat the kiln atmosphere entering the tunnel. ond steam pipe 36 opens directly into the ducts to supply steam for humidifying the atmosphere, as may occasionally be required. Steam as supplied through these pipes from any appropriate source (not shown) and the admission of steam to the pipe is regulated by appropriate control valves of conventional type. The respective end portions of the wall have vertical openings 39 through which air forced into the kiln 16 may return to compartment 17 after passing through the lumber stacks, for recirculation through the blower units.

The control room 18 is accessible through a door 40 in the end of the shed, and a control or instrument box is suitably located therein, as indicated diagrammatically at 41. 1t is understood that the valve for the steam supply may be automatically controlled by 'a thermostatic control of any conventional type.

The boards 4S are closely stacked with intervening stickers 46 on the kiln car 21 to provide two spaced stacks 47 and 48, respectively (Fig. 3) on opposite sides of the car, and to dene an intermediate tunnel or mixing chamber 49 (Fig. l). The outer end of said tunnel is closed by a panel 50; its top is closed by a cover 51; and its inner end is closed by panel 52 which has openings registering with the outlets of the ducts 31 and 32, respectively. Hence, air or vapor driven through said ducts is forced into the partially closed tunnel 49, thence forced laterally through all portions of the lumber stacks 47 and 48 to the sides of the closed kiln, and then back to the blower compartment through the ports 39, for recirculation.

It is thus apparent that operation ofthe motors Z5, 26 causes the blower fans to drive vapor drawn into the inlets 29, 30, through the outlets 31, 32 into the tunnel 49, then laterally through all portions of the stacked lumber, and thence from the kiln to the -blower inlets, as a continuous circulating system. The circulating vapor may be heated to desired temperature by regulating the steam supply to the heating coils 37, or cooled by shutting olf the steam pipe 36 and opening the cold air dampers 38. And the circulating vapor may be moistened or humidified by admitting steam through pipe 35, as aforesaid.

The operation is the same with the modiiied apparatus of Figs. 4 to 6, where the lumber is so stacked on the car 21 as to provide a central, longitudinal tunnel which is substantially triangular in cross-section, and the compartment Wall 54 has only one vapor port 53, said port having deflecting vanes 61 (Fig. 5). The tunnel between the lumber stacks 55 and 56 is closed at its inner end by a triangular shaped panel S7 (Fig. 4) having an opening 58 adapted to register with the delivery port 53. The opposite, outer end of the tunnel is closed by a triangular panel 59, and a narrow cover 60 closes the top of the tunnel when applied to the upper ends of said panels after lumber is stacked against the inclined edges thereof. This modeof stacking may be preferable when only one blower outlet is provided near the bottom of the kiln chamber,

A secas indicated in Fig. 5, to ensure substantially uniform lateral air ow through the stacks.

In either stack arrangement, the panels closing the top and ends of the tunnels may be omitted if the lumber stack extends almost to the roof and the end wall of the kiln, so that the incoming atmosphere is substantially conlined to the tunnel and forced laterally through the stacks.

The improved curing process is performed by means of the apparatus just described in the following manner:

The lumber is stacked in the kiln, and the blower is turned on. At the same time suiiicient steam is supplied to the coils 37 to raise the temperature of the atmosphere in the tunnel to F. in about an hour. This period may be termed the heating stage, and the chief pur- 2 pose of the rapid circulation is to carry enough total heat to the lumber so that it becomes heated through. When the sap in the lumber has reached a temperature of about 140 F., the phenomenon of sap removal which is observed during the next stage, or curing stage, begins to occur.

During the next stage, which may be termed the curing stage, the heating of the entering atmosphere by means of heating coils 37 is continued to maintain the temperature of the kiln atmosphere above 140 F. Preferably the thermostat is set for 212 F. to 220 F. and the initial heating rate is continued until the atmosphere in the tunnel reaches this limit. The high velocity circulation is continued throughout the curing stage which may last from about ve hours to about three days, depending on the type and size of the boards to be processed.

When the lumber has been cured to the desired moisture content, the heat is shut otf. The lumber can then be removed from the kiln.

The process has been described as being accomplished in two stages because of the fact that the sap removal phenomenon, which is about to be discussed, begins to be observable when the temperature of the tunnel atmosphere reaches about 140 F. In actual practice the operator merely sets a thermostat or automatic temperature control at 220 and turns on the Iblower system at the beginning of the run. If the capacity of the heating system is such as to raise the temperature of the tunnel atmosphere to 140 F. in about an hour, no further manipulation of the controls is needed. The vent 11 may be opened during the process to remove moisture from the atmosphere more rapidly.

By this process lumber may be cured and dried to any desired degree much more rapidly than by anyconventional kiln drying method, as evidenced by the following table, showing typical curing runs for various types and sizes of lumber:

Final Specie Thickness, Starting Moisture Length inches Moisture Content, of Run,

Content percent i hrs.

1 S-l0 6 l 5-7 1 12-15 24-30 .l5-18 72 1 d0. 48 do 8 48 2 d0 8 48 Boards cured by the process are exible, thoroughly pre-shrunk, and remarkably free from case hardening and other defects, and compare favorably to air dried lumber in structural strength. Furthermore, the boards are highly resistant to reabsorption of moisture. Boards left outdoors for long periods have been found to reabsorb only 2% to 3%, whereas boards dried by conventional methods reabsorb so much moisture that they must be re-dried after similar exposure. When this process is employed, the need for maintaining large and costly indoor storage facilities is thus eliminated. Due to their low hygroscopicity, the boards cured by this process have a high degree of dimensional stability, a quality which is of great value in almost any end use for which wood is a suitable material.

The velocity of circulation of the atmosphere immediately surrounding the lumber employed in this process is considerably higher than the circulation speeds commonly used in conventional kiln drying operations. A velocity suicient to circulate tunnel atmosphere 60 times per minute appears to be about the optimum from the standpoint of curing time and economy. This radical increase in the circulation speed gives rise to effects other than mere evaporation of moisture. While the kiln is in operation a considerable amount of resinous substance is deposited on the walls and floor ofthe kiln, indicating that much of the sugar and resin content of the sap is removed from the lumber along with the moisture. No humidication of the atmosphere is required except that steam may be admitted at the beginning of the operation through pipe 35 if the boards are Very dry on the surface. The same thing can be accomplished by wetting down the boards before the run is started. This initial wetting serves to establish contact of the surface moisture and the inner moisture to facilitate the starting of the sap ow. When the sap removal has started the moisture is drawn to the surface suliciently rapidly to keep the surface wet and prevent case hardening.

Two conditions are necessary for-successful application of the curing process:

(l) The internal temperature of the lumber must be maintained at a temperature above about 140 F. This minimum temperature is not an exact limit but appears to be the point at which sap removal begins to occur to a noticeable degree under average conditions. The temperature may be considerably higher than 220 F., but temperatures in excess of 240 to 250 may cause discoloration of the wood.

(2) The velocity of circulation of the kiln atmosphere over the wood must exceed a certain lower limit, for example, a speed high enough to change the atmosphere immediately surrounding the boards about 40 times per minute. This limit is merely approximate and may very according to the type, size, and condition of the wood, as well as the temperature maintained. The lower limit of circulation speed may be said to be the minimum speed at which noticeable amounts of visible moisture and/or sap solids appear in the kiln atmosphere when the temperature is at approximately 140 F. or over. The process can be used to advantage even below the speed at which sap solids are removed, as the drying time is shorter, as long as the speed of circulation is sufficient to remove internal moisture by dynamic or rnechanical means. The presence of sap solids may be readily determined by observing condensate from the kiln atmosphere on any cool surface. lf the condensate is sticky, this is an indication that removal of sap solids is occurring. From the standpoint of operability of the process there appears to be no upper limit for the circulation speed. It is a practical matter, however, that any advantages resulting from shortened curing time must be weighed against the increase in power requirements, as the power required increases out of proportion to the gain in speed. it is obvious, also, that excessive speeds will result in undue stress on the kiln structure and on the lumber itself. Speeds suiicient to change the atmosphere around the boards at rates or frequencies varying from forty to ninety times per minute have been successfully employed.

The blower capacity for these speed requirements may be calculated on the basis of the volume of the tunnel, as the ends and top of the tunnel are conned so that substantially all of the atmosphere entering the tunnel is discharged through the stacks. For example, for a tunnel 31/2 feet wide by ll feet high by l2 feet long, the volume would be 462 cubic feet. To move this volume sixty times per minute the lower must be -able to deliver 4 27,720 cubic feet per minute (calculated at atmosphere pressure.)

The physical phenomena which occur in the processare not entirely understood. It is believed, however, that the following discussion explains, in part, what occurs:

The heating sets up molecular agitation and consequent internal pressure in the sap, causing the sap to expand and migrate to the surface, and also redces the viscosity so that the sap will flow more readily. Sap ow toward the surface is induced by aerodynamic forces, such as the dynamic pressure differential set up between one side of the board and the other by the rapidly moving air stream. This pressure differential causes the sap to be blown or sucked from the lumber. The friction of the rapidly moving atmosphere may also serve to tearlotf particlesv of sap and moisture from the surface. The high velocity of circulation not only produces these dynamic effects but also serves to carry enough heat to the lumber to maintain the molecular agitation and low viscosity of the sap.

From'the table of representative curing time, it is apparent that this process is much quicker than conventional kiln drying. For example, green lumber can be cured to 12% moisture content in 24-30 hours, an operation which ordinarily takes five or six days by kiln drying. A small kiln can thus handle as muchlumber in the same time as one about ve times as large. The smaller space required and the reduction in handling and storage costs makes the overall cost of the process lower, even though the power and steam lconsumption for the same size kiln may be higher when this process is used.

The resulting product is superior in quality and in resistance to water absorption as previously discussed, and has excellent strength and flexibility.

What is claimed is:

1. The process of curing a stack of boards arranged in spaced courses in a kiln which comprises circulating in a horizontal path a gaseous atmosphere through a horizontally disposed stack at a velocity in excess of 200 feet per minute so as to change the atmosphere immediately surrounding the lumber at a frequency between 40 and times per minute, while supplying heat to the circulating atmosphere at a rate suitable to raise the temperature of the lumber to F. in about an hour, thereafter heating the atmosphere at the same rate to a temperature in the range of 212-220 and maintaining the atmosphere at said temperature while continuing the circulation at said velocity.

2. The process of curing lumber which comprises arranging boards at and horizontally in spaced courses in two laterally spaced stacks in a kiln to form a substantially conined tunnel between the stacks disposed longitudinally of the boards, and blowing a gaseous atmosphere horizontally into the tunnel and thence laterally through the stacks and thence horizontally back to the tunnel at a speed in excess of 200 feet per minute so as to change the atmosphere immediately surrounding the lumber at a frequency between 40 and 90 times per minute, while` heating the atmosphere to maintain it at a temperature of at least 140 F., and continuously recirculating the same atmosphere, including materials extracted from the lumber, through the stacks.

3. The process of curing lumber which comprises continuously recirculating in an unimpeded horizontal path agaseous atmosphere over horizontally disposed lumber at a velocity in excess of 200 feet per minute so as to change the atmosphere immediately surrounding the lumber at a frequency between 40 and 90 times per minute, while maintaining the atmosphere at a temperature in the range between 140 F. and 250 F.

4. A process as described in claim 2, which includes heating the atmosphere to the specified temperature at the region of its discharge into the tunnel.

5. The process of curing lumber, which comprises arranging boards in spaced horizontally disposed courses 7 in two spaced stacks in` a kiln to form a substantially confined tunnel between the stacks disposed longitudinally of the boards, and continuously circulating in a horizontal path and at a velocity which changes the atmosphere immediately surrounding the boards at a frequency between 40 and 90 times per minute a gaseous atmosphere into the tunnel through the stacks and back to the tunnel, While heating the atmosphere to maintain it at a temperature of at least 140 F., all the circulation taking place substantially in planes parallel to the boards.

References Cited in the le of this patent UNITED STATES PATENTS OTHER REFERENCES Thelen: Kiln Drying Handbook, U, S. Dept. oAgriculture, Bulletin No. 1136, May 1929, pp. 72 to 77.

Drying by Means of Superheated Steam and the Kiln Drying of Softwoods, U. S. D. A., Forest Products Laboratory Report 702, revised August 15, 1931.

The drying Rate of Sugar Maple as Affected by Relative Humidity and Air Velocity, U. S. D. A., Forest Products Laboratory Report R1264, December 1940 13 1"' pages.

Why the Drying Time of a Kiln Load of Lumber is Affected by Air Velocity, U. S. D. A., Forest Products Laboratory Report R1269, .lune 1941, 5 pages.

Mettet: The Wind Tunnel Lumber Dryer, West Coast Lumbermans Association, Research Bulletin No. 6, April 1946, pages 1 to 8.

A Small Lumber-Drying Unit Employing a Portable Crop Drier for Heat and Air Circulation, U. S. D. A., Forest Products Laboratory Report R1799, June 1952,

20 21 pages. Pages 4, 5 and 12 relied on.

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