US3680219A - Process for steam straightening and kiln drying lumber - Google Patents

Process for steam straightening and kiln drying lumber Download PDF

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US3680219A
US3680219A US74484A US3680219DA US3680219A US 3680219 A US3680219 A US 3680219A US 74484 A US74484 A US 74484A US 3680219D A US3680219D A US 3680219DA US 3680219 A US3680219 A US 3680219A
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lumber
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steam
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    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F26—DRYING
    • F26B—DRYING SOLID MATERIALS OR OBJECTS BY REMOVING LIQUID THEREFROM
    • F26B7/00—Drying solid materials or objects by processes using a combination of processes not covered by a single one of groups F26B3/00 and F26B5/00
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F26—DRYING
    • F26B—DRYING SOLID MATERIALS OR OBJECTS BY REMOVING LIQUID THEREFROM
    • F26B21/00—Arrangements or duct systems, e.g. in combination with pallet boxes, for supplying and controlling air or gases for drying solid materials or objects
    • F26B21/06—Controlling, e.g. regulating, parameters of gas supply
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F26—DRYING
    • F26B—DRYING SOLID MATERIALS OR OBJECTS BY REMOVING LIQUID THEREFROM
    • F26B2210/00—Drying processes and machines for solid objects characterised by the specific requirements of the drying good
    • F26B2210/16—Wood, e.g. lumber, timber

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  • the objective of the instant invention is the development of a broadly applicable process suitable for lumber cut from small southern pine logs aswell as from veneer cores.
  • the invention relates to the drying of southern pine studs to IO-percent moisture content-- quickly and warp-free.
  • Southern pine can be bent and will retain its bend if it is first steamed for a short time. Conversely, a bent piece can be steam straightened.
  • the high-temperature drying schedule consumed less than one-fourth the time and only about one-half the energy required by the low-temperature schedule.
  • racking device may be adapted for drying and straightening lumber in batches or for drying and straightening lumber continuously as it moves under total mechanical restraint through a heated zone,
  • step (c) steaming the dried pieces of lumber from step (c) still secured within the racking means for about three hours at the highest possible dry-bulb temperature at which a IO-degree wet-bulb depression can be maintained (for example, a dry-bulb temperature of 195 F. and a wet-bulb temperature of C.) with air circulation maintained at about 1,000 ft./minute,
  • step (d) cooling the steamed pieces of lumber from step (d) for some period of time before removal from the racking means, for example 48 hours, and
  • EXAMPLE The sources of eight-foot 2 by 4s were logs six to eight inches in diameter, and cores residual from steamed veneer logs.
  • the Kiln schedules were low-temperatured long schedule with studs conventionally stacked, and a high-temperature 24-hour schedule with studs restrained against warp. There were three replications of kiln charges. There were 24 replications of studs in each kiln charge. The total number of studs evaluated was therefore 288, i.e., (two sources) (two schedules) (three replications of kiln charges)(24 replications of studs).
  • a 48-stud sample was randomly divided into two groups of 24-one to be dried at high temperature and the other at low temperature. While one of the two groups was being dried, the other was wrapped in polyethylene and stored at 35 F; this storage period generally was seven days or less.
  • the 24 rough green 2 by 4s in each charge were surfaced to a net dimension of 1-7/8 by four inches and double-end trimmed to length. (100 inches in the case of the studs from cores and 96 inches for the 2 by 4s cut from small logs).
  • the green moisture content of each 2 by 4 was then estimated from four increment cores drawn 10 and 20 inches from each end, and the green weight of the entire stud recorded.
  • Kiln schedules The 24-hour high temperature schedule was simple.
  • the green lumber was clamped rigidly in aluminum frames, in virtually total mechanical restraint against crook, bow, and twist. Still in frames, the studs were wheeled into the pre-heated kiln and dried for 21 hours at a dry-bulb temperature of 240 F. and a wet-bulb temperature of 160 F. Then, for the last three hours they were steamed at a dry-bulb temperature of 195 F. and a wet-bulb temperature of 185 F.
  • Lumber dried at low temperature was not restrained in clamps, but was piled on sticks in the conventional manner.
  • the low-temperature schedule (approximately l 10 hours in duration) was patterned after a conventional schedule as follows. Air velocity was 500 f.p.rn. with fan reversal every 75 minutes.
  • Warp and grade evaluations Promptly on removal from the cooled load, the moisture content of each stud was computed by evaluating two 55-inch borings, one taken 10 inches from one end and a second taken 20 inches from the other end; from these data, the ovendry weight of each 2 by 4 was calculated as well as a corrected initial green moisture content. Crook, bow, and twist were measured and the lumber piled on sticks in a room held at about SO-percent relative humidity and 72 F.
  • each stud was promptly reweighed, remeasured for crook, bow, and twist, and graded by an inspector from the Southern Pine Inspection Bureau.
  • the 2 by 4s were then transferred to a room held at 81 F. dry bulb and 78 F. wet-bulb temperature (87- percent relative humidity) where they were individually and freely suspended from hooks placed in one end. After 20 days of exposure they were removed, weighed, and evaluated for crook, bow, and twist.
  • the high-temperature schedule took less than one-fourth the time and about one-half the energy required for the low-temperature schedule (Table I).
  • Average percent moisture content of studs at each stage was: 1 10.4, (2) value not observed, (3) 9.3, (4) 14.1, (5) 3.4.
  • Each entry in the table shows the average value for 72 studs followed by the standard deviation in parentheses; range is shown in the line below the foregoing entries.
  • the two upper grades require a minimum of four rings per inch; a stud with less than this number of rings can qualify for Stud grade if it meets straightness standards somewhat more stringent than those for higher grades.
  • the 288 studs averaged 7.7 rings per inch (observed on one end only of each stud-the end exhibiting most rings per inch). Cores averaged 8.3 and small logs 7.2. Distribution was as follows:
  • Knots occupying more than four-ninths the cross section at edge of wide face or more than two-third the cross section along the centerline of the wide face eliminated a few more of the pieces from consideration for SPIB. Stud grade.
  • a first means defining a plurality of rigid, elongated, bar-shaped members arranged in spaced, parallel-tolumber array to form a lower course of bar-shaped members less than half the thickness of the lumber to be dried and steam straightened;
  • second means defining a plurality of rigid elongated, bar-shaped members arranged in spaced, parallel transverse-to-lumber array to form a middle course of bar-shaped members; said middle course of bar-shaped members located atop and transverse the lower course of bar-shaped members to provide space for air circulation between lumber courses;
  • third means defining a plurality of rigid, elongated,
  • M is the applied mobar-shaped members arranged in spaced, parallel-tolumber array located atop the middle course to form an upper course of bar-shaped members less than one-half the thickness of the lumber to be dried and steam straightened; the courses secured one to the other at the cross-over intersections thereby to form a rectangular grid; the so-formed grid adapted to accommodate between the members of the lower or upper course one width dimension of a sized piece of lumber, any two immediately adjacent lower or upper course members thus serving to restrain a piece of sized lumber in the horizontal plane, the said piece of lumber being simultaneously supported by the middle course portions of bar-shaped members located directly beneath, clamping means provided across the grid and adapted to facilitate the stacking of a plurality of unitary components in vertical array; the middle course of barshaped members of any one component forming the support for the sized lumber pieces in that particular component, the lower or upper course of bar-shaped members forming width dimension lateral restraints for the sized pieces of lumber, middle course bar-shaped members
  • Strength evaluation-All 288 studs were then stacked on sticks and allowed to equilibrate for at least two weeks at about 72 F. dry-bulb and 60 F. wet-bulb temperature (SO-percent relative humidity). They were then weighed, trimmed to 66-inch length, and evaluated in edgewise bending for modulus of elasticity (MOE), proportional limit (PL), and modulus of rupture (MOR). Midspan deflections were measured with a taut wire and scale. Readings to failure were taken at load intervals of pounds to establish the deflection curve. Just prior to test, a one-inch cross sectional slice was removed 15 inches from one end. From this slice, specific gravity and moisture content at test were determined. From a calculated ovendry weight for each stud, it was then also possible to calculate the moisture content when planed and graded, and when removed from the high-humidity and low-humidity cycles.
  • MOE modulus of elasticity
  • PL proportional limit
  • MOR modulus of rupture
  • Range 1 Each average value shown represents data from 72 studs. Average moisture content at test was 7.6 percent with range from 6.1 to 0.1; per u For specific gravity yell s, 3 For all three properties, v:
  • shear modulus (G) was assumed to equal one-sixteenth the MOE.
  • Strength properties-Studs cut from very small logs and from veneer cores contain considerable juvenile wood of low specific gravity, as well as substantial numbers of defects such as knots and cross grain; these defects cause great variability in strength properties.

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  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
  • General Engineering & Computer Science (AREA)
  • Chemical And Physical Treatments For Wood And The Like (AREA)
  • Drying Of Solid Materials (AREA)

Abstract

A process for steam straightening and kiln drying lumber, the lumber being held under total restraint with respect to warping during the process. The lumber is steam straightened and kiln dried to a moisture content of nine percent using a cross-circulation air velocity of 1000 f.p.m. at dry and wet bulb temperatures of 240* F and 160* F for a first interval followed by a second interval at 195* F and 185* F. The lumber is cooled prior to being released from restraint.

Description

United States Patent Koch [451 Aug. 1, 1972 [54] PROCESS FOR STEAM 1,054,597 2/1913 Moore ..34/13.8 STRAIGHTENING AND KILN DRYING 1,212,583 1 1 917 Tanner ..34/13.8 LUMBER 1,276,979 8/1918 Sidman ..34/17 [72] inventor: Peter KochAlexandria, La 1,328,662 1/1920 Fish, Jr. ..34/l3.4 [73] Assignee: The United States of America as Primary Examiner-Frederick L. Matteson represented by the Secretary of Assistant Examiner-W. C. Anderson Agr culture Attorney-R. Hoffman and W. Bier 22 F1 d: Se t. 22 1970 l 1 p 57 ABSTRACT [21] App1.No.: 74,484
A process for steam straightening and kiln drying lumber, the lumber being held under total restraint [52] US. Cl ..34/ 13.4, 34/] 3.8 with respect to warping during the process [51 ll' ll. Cl. ..F26b 7/00 The lumber is steam straightened and kiln dried to a [58] Field 01 Search ..34/ 13.4, 13.8, 17, 218 moisture content of nine percent using a cr0ss circu|a tion air velocity of 1000 f.p.m. at dry and wet bulb [56] References Cmd temperatures of 240 F and 160 F for a first interval UNITED STATES PATENTS followed by a second interval at 195.F and 185 F. The lumber is cooled prior to being released from 217,022 7/1879 Robbins ..34/ 17 restraint. R15,316 3/1922 Weiss ..34/13.4
528,496 10/1894 Williams ...'...34/l7 1 Claim, No Drawings PROCESS FOR STEAM STRAIGHTENING AND KlLN DRYING LUMBER A non-exclusive, irrevocable, royalty-free license in the invention herein described, throughout the world for all purposes of the United States Government, with the power to grant sublicenses for such purposes, is hereby granted to the Government of the United States of America.
The objective of the instant invention is the development of a broadly applicable process suitable for lumber cut from small southern pine logs aswell as from veneer cores. The invention relates to the drying of southern pine studs to IO-percent moisture content-- quickly and warp-free.
The invention depends upon the following set of facts:
a. Southern pine can be bent and will retain its bend if it is first steamed for a short time. Conversely, a bent piece can be steam straightened.
b. Water leaves southern pine readily at temperatures above the boiling point.
High temperatures and large wet-bulb depressions accelerate drying. If exposure time is short,
strength loss in the wood is minor at temperatures up to 240 F.
. During the early stages of drying, water leaves the wood at a rate approximately proportional to the velocity of the circulating air.
. In kiln-drying, energy costs are proportional to total drying time.
f. Case hardening and internal stresses in kiln-dried coniferous wood can be eliminated by several hours of steaming at dry-bulb temperatures just below the boiling point.
Processing results as listed below have been noted.
Studs from the veneer cores were stronger and of higher grade than those cut from small logs.
The high-temperature drying schedule consumed less than one-fourth the time and only about one-half the energy required by the low-temperature schedule.
Eight-foot 2 by 4s dried at high temperature under total mechanical restraint and then planed were not casehardened and had substantially less warp than those stacked conventionally and dried at low temperature. Conparative values for S48 studs dried at high and low temperatures were as follows: crook, 0.12 vs. 0.23 inch; bow, 0.21 vs. 0.29; and twist, 0.09 vs. 0.24.
When lumber dried on both schedules was exposed to extremely humid or extremely dry atmospheres, warp in the steam-straightened 2 by 4s remained less than that measured on conventionally-dried studs.
Because warp was less in the studs dried under restraint at high temperature, they graded substantially higher (91 percent in SPlB grades 1, 2, and Stud) than those dried conventionally (59 percent in SPlB grades I, 2, and Stud).
Reduction in modulus of elasticity, proportional limit, modulus of rupture, and toughness caused by the high-temperature schedule did not prove statistically significant (0.05 level).
Stress relief following the 21-hour, 240 F, high-temperature schedule probably can be achieved in less than three hours if dry-and wet-bulb temperatures of 205 and 200 F (instead of 195 and 185 F) can be maintained during the conditioning period. Achievement of these high temperatures, however, require reduction of condensation on interior surfaces of the kiln walls via internally heating to about 212 F.
Generally, the process which is the subject of this invention consists of the following steps:
a. forming to'size individual pieces of lumber,
b. securing the sized pieces of lumber from step (a) within a rigid racking means which racking means is adapted to impose upon the sized pieces of lumber total restraint against warping in both the width and the thickness dimensions. The racking device may be adapted for drying and straightening lumber in batches or for drying and straightening lumber continuously as it moves under total mechanical restraint through a heated zone,
at approximately 240 F. dry-bulb temperature, kiln drying and steam straightening the sized pieces of lumber while they are under complete mechanical restraint and subjected to circulation velocity of the air steam mixture of about 1,000 feet per minute; the schedule to be varied according to board thickness and species. For example, southern pine boards two inches thick attain about 10 percent average moisture content after 21 hours in a circulating atmosphere held at 240 F. dry-bulb temperature and F. wet-bulb temperature. One-inch thick southern pine boards require only 9 or 10 hours in the same atmosphere to attain 10 percent average moisture content,
. steaming the dried pieces of lumber from step (c) still secured within the racking means for about three hours at the highest possible dry-bulb temperature at which a IO-degree wet-bulb depression can be maintained (for example, a dry-bulb temperature of 195 F. and a wet-bulb temperature of C.) with air circulation maintained at about 1,000 ft./minute,
. cooling the steamed pieces of lumber from step (d) for some period of time before removal from the racking means, for example 48 hours, and
f. removing the kiln dried, steam straightened pieces of lumber from the racking means.
EXAMPLE The sources of eight-foot 2 by 4s were logs six to eight inches in diameter, and cores residual from steamed veneer logs. The Kiln schedules were low-temperatured long schedule with studs conventionally stacked, and a high-temperature 24-hour schedule with studs restrained against warp. There were three replications of kiln charges. There were 24 replications of studs in each kiln charge. The total number of studs evaluated was therefore 288, i.e., (two sources) (two schedules) (three replications of kiln charges)(24 replications of studs).
To fill the requirements for three replications of each kiln run, three 48-stud samples were randomly drawn during three different weeks from a plywood operation that converts veneer cores into 2 by 4's; another three samples, comprised of 48 studs each, were similarly drawn from a mill while logs six to eight inches in diameter were being sawn. Only one rough green stud was taken from a veneer core or log; pith was visible in most studs. The logs in the mill were sawn as they came from the woods. The logs yielding veneer cores, however, were steamed for about 18 hours prior to peeling.
Each sample of studs came direct from the saw and was promptly wrapped in polyethylene for immediate transportto the test drying site.
A 48-stud sample was randomly divided into two groups of 24-one to be dried at high temperature and the other at low temperature. While one of the two groups was being dried, the other was wrapped in polyethylene and stored at 35 F; this storage period generally was seven days or less.
Immediately prior-to kiln drying, the 24 rough green 2 by 4s in each charge were surfaced to a net dimension of 1-7/8 by four inches and double-end trimmed to length. (100 inches in the case of the studs from cores and 96 inches for the 2 by 4s cut from small logs). The green moisture content of each 2 by 4 was then estimated from four increment cores drawn 10 and 20 inches from each end, and the green weight of the entire stud recorded.
Kiln schedules.--The 24-hour high temperature schedule was simple. The green lumber was clamped rigidly in aluminum frames, in virtually total mechanical restraint against crook, bow, and twist. Still in frames, the studs were wheeled into the pre-heated kiln and dried for 21 hours at a dry-bulb temperature of 240 F. and a wet-bulb temperature of 160 F. Then, for the last three hours they were steamed at a dry-bulb temperature of 195 F. and a wet-bulb temperature of 185 F. Throughout the 24 hours, air was cross-circulated at 1,000 f.p.m.; direction of air flow was reversed every 75 minutes-Weight of charge together with energy consumption for heat, humidity control (steam spray) and fan were continuously charted against time. With the schedule completed, the studs were wheeled from the kiln and cooled 48 hours in an atmosphere that ranged from 70 to 80 F. and 40 to 60-percent relative humidity.
Lumber dried at low temperature was not restrained in clamps, but was piled on sticks in the conventional manner. The low-temperature schedule (approximately l 10 hours in duration) was patterned after a conventional schedule as follows. Air velocity was 500 f.p.rn. with fan reversal every 75 minutes.
Moisture Dry-bulb Wet-bulb Wet-bulb Relacontent Temp. deprestemp. tive at start sion humiof step dity E.m.c.
(percent) (F.) (percent) and all components of energy consumption were recorded against time. On completion of the schedule, the lumber was wheeled from the kiln and left on sticks to cool for 48 hours warp and weight measurements were taken.
Warp and grade evaluations.Promptly on removal from the cooled load, the moisture content of each stud was computed by evaluating two 55-inch borings, one taken 10 inches from one end and a second taken 20 inches from the other end; from these data, the ovendry weight of each 2 by 4 was calculated as well as a corrected initial green moisture content. Crook, bow, and twist were measured and the lumber piled on sticks in a room held at about SO-percent relative humidity and 72 F.
After several weeks the lumber was weighed, doubleend trimmed to 96-inch length, measured for warp, and surfaced on all four sides by a machine equipped with a crook reducer between feed table and planer; the crook reducing cutterhead was set to remove 3/1 6-inch from the concave edge of each stud. Final planed dimensions were l-% by 3-9/ 16 inches. After planing each stud was promptly reweighed, remeasured for crook, bow, and twist, and graded by an inspector from the Southern Pine Inspection Bureau.
The 2 by 4s were then transferred to a room held at 81 F. dry bulb and 78 F. wet-bulb temperature (87- percent relative humidity) where they were individually and freely suspended from hooks placed in one end. After 20 days of exposure they were removed, weighed, and evaluated for crook, bow, and twist.
Next, they were placed singly on shelves in a kiln held at 130 F. (wet bulb uncontrolled but near F.) After 20 days of drying, they were removed and weighed, and crook, bow, and twist were again measured.
A comparison of basic characteristics of 2 by 4s cut from steamed veneer cores and those from small logs or saplings is set forth below. With the exception of specific gravity, the average values shown in the two columns following differ significantly at the 0.05 level.
From small From cores logs Green moisture content, percent Average 75. l 92.9 Standard deviation 33.0 37.1 Range 28.0 27.6
184.7 172.2 Specific gravity (basisof overdry volume and weight) Average 0.51 0.5 1 Standard deviation 0.070 0.085 Range 0.37 0.35
0.70 0.81 Heartwood content (average of both ends), percent Average 50.4 34.2 .Standard deviation 31.1 29.4 Range 0-100 0-100 Growth rate (average of both ends),
percent Average 7.6 6.4 Standard deviation 5.0 4.4 Range 2.0 1.5
A comparison of the high-temperature and the lowtemperature schedules is shown below.
Moisture content 48 hours after discharge from kiln High-temperature Low-temperature schedule schedule (percent) Studs from veneer cores Average 9. Standard deviation 3 Range Studs from small logs Average Standard deviation Range 5. -21.2
Prong tests on the dry studs showed that neither schedule caused case hardening.
In general, the high-temperature schedule took less than one-fourth the time and about one-half the energy required for the low-temperature schedule (Table I).
TABLE 1 Time and energy expended to Kiln Dry each charge of 24 southern pine studs cut from veneer cores and small logs High temperature Low temperature From From small From From small Expenditure cores logs cores logs Time, hours 24 24 102 113 Energy, kilowatthours Heat 442 500 700 790 Humidity control 54 61 295 290 Fan 55 57 107 119 Total 551 618 1102 1199 Each figure is the average for three charges "Supplied by electric resistance-type heating coils.
Steam for humidification was provided by electric immersion heaters in a water bath.
Warp. Studs dried under restraint at high temperature warped significantly less than those dried at low temperature with conventional stick placement (Tables 11 and Ill). The differences were significant at all stages of manufacture.
TABLE II Warp in 8-foot Southern Pine 2 by 4s at Various Stages manufacture; studs were cut from veneer cores and small logs, then kin-dried on two different schedules Kiln schedule Source Sequence and time Low High of measurement temperature temperature veneer 1 small and type of warp unrestrained restrained cores logs Inch 1. 48 hrs out of kiln Crook 0.262 0.147 0.195 0.213 Bow .255 .213 .237 .232 Twist .3 l 3 .122 .195 .240 2. Just before planing Crook .307 .173 .241 .238 Bow .323 .242 .288 .277 Twist .363 .145 .222 .287 3. Just after planing Crook .232 .118 .175 .175 Bow .285 .207 .253 .238 Twist .235 .087 .135 .187
4. After humid cycle Crook .167 .103 .137 .133
Bow .182 .150 .173 .158
Twist .140 .075 .102 .113 5. After dry cycle Crook .347 .227 .283 .290
Bow .520 .423 .462 .482
Twist .440 .232 .283 .388
Values having an asterisk between then are significantly different at the 0.05 level; no interactions were significant at this level.
Average percent moisture content of studs at each stage was: 1 10.4, (2) value not observed, (3) 9.3, (4) 14.1, (5) 3.4.
Data from cores and small logs pooled.
Data from both kiln schedules pooled.
TABLE HI Warp in southern pine studs cut from small logs and steamed veneer cores kiln-dried at high temperature under restraint and at low temperature with conventional sticks High temperature, Low temperature, Sequence and time restrained unrestrained of measurement From From small From From small and type of warp cores logs cores logs Inch 1. 48 hours out of kiln Crook 0.14(0.10) 0.16(0.11) 0.25(0.27) 0.27(0.27)
0.03-0.53 0.03-0.50 0.05-1.57 0.04-1.80 Bow .22(.l3) .21(.13) .26(.23) .25(.13)
.03-.65 .04-.57 .06-1 .82 .06-.61 Twist '.1l(.07) .14(.11) .28(.23) .34(.22) .00-.3S .01-.58 .02-1.04 .02-1.03 2. Just prior to planin Crook .17(.10) .l7(.l2) .3l(.33) .30(.29) .05-.57 .05-.57 .05-1.97 .05-1.90 Bow .24(.11) .24(.l4) .33(.25) .32(.l5)
.06-.65 .09-.66 .10-1 .81 .09-.7 Twist .13(.08) .16(.12) .31(.24) .41(.25) .02-.36 .02-.64 .05-1.24 .04-1. 17 3. Just after planing Crook .12(.07) .12(.09) .23(.29) .23(.28) .05-.40 .04-.47 .04-l.83 .05-1.93 Bow .20(.13) .21 (.1 3) .30(.28) .27(.16)
.04-.62 .05-.65 .05( 1.94) .06-.78 Twist .08(.O6) .l0(.08) .20(.16) .27(.17)
.00-.26 .00-.34 .00-.77 .01.80 4. After humid cycle Crook .11(.06) .l0(.05) .17(.19) .l7(.l9) .05-.37 .04-.29 .05-l.22 .04-1.38 Bow .15 (.09) .l5(.10) .20(.18) .17(.09)
.04-.45 .04-.57 .04-1.26 .05-.56 Twist .07(.03) .08(.05) .13(.10) .l5(.10)
.01-. 17 .01-.25 02-.40 .01-.50 5. After dry cycle Crook 2l(.17) .24(.25) .36(.47) .34(.39) .05-1.02 .05-1.27 .06-2.94 .07-2.50 Bow .40(.30) .45(.3 .52(.55) .51(.38)
' .06-l.21 .07-1.54 06-3.48 .06-1.8l Twist 20(.15) .27(.24) .37(.31) .5 l(.34)
Each entry in the table shows the average value for 72 studs followed by the standard deviation in parentheses; range is shown in the line below the foregoing entries.
Since warp measured immediately after planing largely determines the mill grade and selling price of studs, it is evident that the wood dried under restraint at high temperature had greater value than that dried at low temperature. Average values follows.
Warp High temperature, restrained Low temperature,
(inch) unrestrained (inch) Crook 0.12 0.23 Bow .21 .29 Twist .09 .24
When studs are incorporated in buildings, they are frequently exposed to high humidities for a few weeks before the building is roofed and the heating (or air conditioning) system activated. The planed studs exposed 20 days to high humidity simulated this situation;
average warp after exposure was least in wood dried at high temperature.
Warp High temperature, restrained Low temperature,
(inch) unrestrained (inch) Crook 0.23 0.35
Bow .42 .52
Twist .23 .44
On average, studs cut from cores twisted significantly less than those cut from small logs when measured just after planing and after the dry cycle. When data from both kiln schedules were pooled, average values were as follows:
Twist in studs Twist in studs Time of measurement from veneer cores from small logs (inch) nc Just after planing 0.14 0.19 After dry cycle .28 .39
Other differences in warp between studs from cores and from small logs were not significant at the 0.05 level (Table II).
Grade. -By SPIB rules (Southern Pine Inspection Bureau 1968), each of the three top grades and Stud grade is limited to certain maximum warp. For 8-foot 2 by 4s, these allowable distortions are as follows:
Grade Crook Bow Twist (Inches) (Inches) (Inches) No. 1 0.281 0.844 0.375 No. 2 .375 1.125 .500 Stud grade .188 .563 .250 No. 3 .563 1.688 .750
The two upper grades require a minimum of four rings per inch; a stud with less than this number of rings can qualify for Stud grade if it meets straightness standards somewhat more stringent than those for higher grades.
The 288 studs averaged 7.7 rings per inch (observed on one end only of each stud-the end exhibiting most rings per inch). Cores averaged 8.3 and small logs 7.2. Distribution was as follows:
Rings per inch From cores From small logs (pct. of 144-stud total) 1 0.0 0.0 2 2.1 2.1 3 12.5 17.4 4 10.4 22.9 5 12.5 9.0 6 9.7 12.5 7 11.8 4.2 8 4.9 5.5 9 6.9 1.4 10 2.8 3.5 More than 10 26.4 21.5 Total 100.0 100.0
Thus, 14.6 percent of studs from cores and 19.5 percent of studs from small logs had less than four rings per inch; these open-grain pieces could at best qualify for SPIB Stud grade, and then only if crook graded less than 3/l6-inch' (Southern Pine Inspection Bureau 1968,pp. 12,64, 121).
Several of the studs from both cores and small logs contained readily identifiable compression wood, and these pieces could not be admitted to SPIB Stud grade regardless of how well warp was controlled.
Knots occupying more than four-ninths the cross section at edge of wide face or more than two-third the cross section along the centerline of the wide face eliminated a few more of the pieces from consideration for SPIB. Stud grade.
In general, studs dried under restraint at high temperature graded substantially higher after planing than those dried at low temperature (Table IV). Because 8- foot 2 by 4s of Stud grade or better have approximately twice the value of studs in number 3 and 4 grades, the following tabulation is of particular significance.
Grade High temperature (number) (pct.)
Low temperature (number) (pct.)
No. 1, No. 2, and
131 91 S9 Stud No. 3 and No. 4 13 9 59 41 Total 144 144 100 Totaling both schedules, cores yielded more No. 1 Common and less No. 3 and 4 Common than small logs.
TABLE IV Grade Distribution of Studs Immediately Following Planing High temperature Low temperature Grade From From small From From small cores logs cores logs No. No No. No. No. 1 Common 31 17 22 11 No. 2 Common 21 l7 l8 19 Stud grade 17 28 6 9 No. 3 Common 1 6 20 23 No. 4 Common 2 4 6 Total 72 72 72 72 Southern Pine Inspection Bureau 1968). Had crook of 0.38 inch.
\vVarp was within Stud grade limitations on both of these pieces, but both were downgraded to No. 4 because of readily identifiable compression wood.
Three of these 10 studs were within Stud grade limitations on warp.
The higher grade yield in studs cut from cores was particularly evident with the high-temperature schedule; studs from cores yielded only 4.2 percent in grades 3 and 4, while studs from small logs yielded 13.9 percent in grades 3 and 4.
The 24-hour schedule caused considerable resin exudation to the surface of the rough, dry 2 by 4s, but' planing removed all traces of resin and discoloration.
All of the studs dried on the 24-hour schedule developed some end checks that ranged from 1.5 to 2.1 inches in depth; 29 percent of the studs had few checks; 23 percent had a moderate number; and in 48 percent of the studs dried at high temperature end checks were numerous. In no case, however, were the end checks a cause for degrade.
Set forth below is a description of the particular racking component employed for restraining sized pieces of lumber during the kiln drying and steam straightening process which process of itself is the subject of this invention.
A first means defining a plurality of rigid, elongated, bar-shaped members arranged in spaced, parallel-tolumber array to form a lower course of bar-shaped members less than half the thickness of the lumber to be dried and steam straightened; second means defining a plurality of rigid elongated, bar-shaped members arranged in spaced, parallel transverse-to-lumber array to form a middle course of bar-shaped members; said middle course of bar-shaped members located atop and transverse the lower course of bar-shaped members to provide space for air circulation between lumber courses; third means defining a plurality of rigid, elongated,
- where f is the calculated stress, M is the applied mobar-shaped members arranged in spaced, parallel-tolumber array located atop the middle course to form an upper course of bar-shaped members less than one-half the thickness of the lumber to be dried and steam straightened; the courses secured one to the other at the cross-over intersections thereby to form a rectangular grid; the so-formed grid adapted to accommodate between the members of the lower or upper course one width dimension of a sized piece of lumber, any two immediately adjacent lower or upper course members thus serving to restrain a piece of sized lumber in the horizontal plane, the said piece of lumber being simultaneously supported by the middle course portions of bar-shaped members located directly beneath, clamping means provided across the grid and adapted to facilitate the stacking of a plurality of unitary components in vertical array; the middle course of barshaped members of any one component forming the support for the sized lumber pieces in that particular component, the lower or upper course of bar-shaped members forming width dimension lateral restraints for the sized pieces of lumber, middle course bar-shaped members of those components immediately above and below any one specific component forming a thickness dimension vertical restraint for the sized pieces of lumber.
Strength evaluation-All 288 studs were then stacked on sticks and allowed to equilibrate for at least two weeks at about 72 F. dry-bulb and 60 F. wet-bulb temperature (SO-percent relative humidity). They were then weighed, trimmed to 66-inch length, and evaluated in edgewise bending for modulus of elasticity (MOE), proportional limit (PL), and modulus of rupture (MOR). Midspan deflections were measured with a taut wire and scale. Readings to failure were taken at load intervals of pounds to establish the deflection curve. Just prior to test, a one-inch cross sectional slice was removed 15 inches from one end. From this slice, specific gravity and moisture content at test were determined. From a calculated ovendry weight for each stud, it was then also possible to calculate the moisture content when planed and graded, and when removed from the high-humidity and low-humidity cycles.
A pair of 2- by 2- by 28-cm. clear specimens were then cut from the two trim ends of each stud and evaluated for toughness according to the procedure specified in American Society for Testing and Materials Standard Dl43-52, par 74. Moisture content and specific gravity of each specimen was determined from a one-inch slice removed immediately after each test.
Stress at proportional limit and modulus of rupture (MOR) were calculated from the standard flexure formula:
ment, 0 is the distance from the neutral axis to the outer face of the beam, and l is the moment of inertia of the cross section. The MOEs in tables 5 and 6 were calculated from the deflection formula:
A cross-sectional area, square inches G shear modulus, pounds per square inch This formula accounts for deflections caused by both Table 6 shows the average, standard deviation, and
range of strength values in each category of stud tested full length.
TABLE VI Comparison of strength properties of southern pine studs cut from small logs or veneer cores when kiln-dried 24 hours at temperatures not exceeding 240 F. or about 100 hours at temperatures not exceeding 180 FA High temperature Low temperature Property 3 From cores From small logs From cores From small log Modulus of elasticity, p.s.i.:
' Average 1, 624, 000 l, 457, 000 1, 630, 000 1, 510, 000 Standard deviation 303, 000 433, 000 408, 000 400, 000 Range 812, 000-2, 686, 000 594, 000-2, 828, 000 650, (100-2, 602, 000 684, 000-2, 353, one
Proportional limit, p.s.i.: 1
Average 5, 050 4, 050 5, 3110 4, 740 1, 850 l, 770 2, 100 1,050 1, 400-9, 130 1, 000-0, 600 1, 180-10, 000 1, 100-10, 220
(i, 080 (i, 520 7, 540 (i, 500
Standard deviation.
Range 1 Each average value shown represents data from 72 studs. Average moisture content at test was 7.6 percent with range from 6.1 to 0.1; per u For specific gravity yell s, 3 For all three properties, v:
e 'lithlo 5. res for studs from eores were significantly higher (0.5 level) than values [or studs cut [l'lHllSlllltll logs; the kiln schedules, however, (lid not significantly ((1.05 level) nll'cct values. interactions were not significant.
bending and shear stresses; the shear modulus (G) was assumed to equal one-sixteenth the MOE.
At conclusion of the strength tests, a chemical indicator was used to ascertain the amount of heartwood showing on each end of each stud.
Strength properties-Studs cut from very small logs and from veneer cores contain considerable juvenile wood of low specific gravity, as well as substantial numbers of defects such as knots and cross grain; these defects cause great variability in strength properties.
By analysis of variance, the strength properties of modulus of elasticity, proportional limit, modulus of rupture, and toughness did not differ significantly (0.05 level) between the two drying treatments; in all four strength properties, however, the studs cut from veneer cores were significantly stronger (0.05 level) than those cut from small logs (table 5).
TABLE V Strength Properties of 8-foot Southern Pine 2 by 4s cut from Veneer Cores and Small Values having an asterisk between them are significantly different at the 0.05 level; no interactions were significant at this level.
All properties measured on full-length studs at 7.6 percent moisture content, except for toughness, which was measured on small clear specimens at 8.6-percent moisture content.
TABLE Vll' Comparison of toughness and specific gravity of elearwood taken from southern pine studs cut from small logs or veneer cores when kilndried 24 hours at temperatures not exceeding 240 F. or about 100 hours at temperatures not exceeding 180 F.
High temperature Low temperature From From From small From small Property cores logs cores logs Toughness, inch-pounds:
Avera e 191 200 183 Standard deviation. 77 63 Range 59-355 79-368 61-338 Specific gravity (basis of volume at test and ovendry weight):
Average 0. 52 0. 52 0. 63 0. 52 Standard deviation. 06 .08 08 .10 Range .41-. 70 38-. 72 40-. 86 37-.
Each value shown represents data from 72 studs (two replications per stud). Average moisture content when tested for toughness was 8.6 percent with standard deviation of 0.5 and range from 7.1 to 11.4 percent.
lclaim: l.-An improved process for kiln drying and steam straightening sized permeable lumber comprising the following sequential steps:
a. forming to size individual pieces of lumber. b. securing the sized pieces of lumber within a rigid racking means which racking means is adapted to impose upon the sized pieces of lumber total restraint against warping in both the width and the thickness dimensions, 0. kiln drying and steam straightening the sized pieces of lumber at a dry-bulb temperature of at least about 240 F. and employing an air circulation velocity of at least about 1,000 feet per minute toattain an average moisture content of ten percent, d. steaming the said pieces of lumber from step (c) still under restraint in the racking means for at least about three hours at the highest possible drybulb temperature at which a ten degree wet-bulb depression can be maintained with continued air circulation at about 1,000 feet per minute, e. cooling the said pieces of lumber from step (d) while still under restraint, and
removing the finished, kiln-dried, steam straightened pieces of lumber from the racking means.

Claims (1)

1. An improved process for kiln drying and steam straightening sized permeable lumber comprising the following sequential steps: a. forming to size individual pieces of lumber. b. securing the sized pieces of lumber within a rigid racking means which racking means is adapted to impose upon the sized pieces of lumber total restraint against warping in both the width and the thickness dimensions, c. kiln drying and steam straightening the sized pieces of lumber at a dry-bulb temperature of at least about 240* F. and employing an air circulation velocity of at least about 1,000 feet per minute to attain an average moisture content of ten percent, d. steaming the said pieces of lumber from step (c) still under restraint in the racking means for at least about three hours at the highest possible dry-bulb temperature at which a ten degree wet-bulb depression can be maintained with continued air circulation at about 1,000 feet per minute, e. cooling the said pieces of lumber from step (d) while still under restraint, and f. removing the finished, kiln-dried, steam straightened pieces of lumber from the racking means.
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Cited By (19)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3875685A (en) * 1974-07-23 1975-04-08 Peter Koch Continuous kiln apparatus for producing warp-free lumber
US4106215A (en) * 1976-07-14 1978-08-15 The United States Of America As Represented By The Secretary Of Agriculture Wood impingement dryer
US4663860A (en) * 1984-02-21 1987-05-12 Weyerhaeuser Company Vertical progressive lumber dryer
FR2660424A1 (en) * 1990-04-03 1991-10-04 Electricite De France Method and installation for drying damp solid wood with a high throughput
US5414944A (en) * 1993-11-03 1995-05-16 Culp; George Method and apparatus for decreasing separation about a splitter plate in a kiln system
US5416985A (en) * 1993-09-23 1995-05-23 Culp; George Center bridging panel for drying green lumber in a kiln chamber
US5437109A (en) * 1993-09-23 1995-08-01 Culp; George Aerodynamic surfacing for improved air circulation through a kiln for drying lumber
US5488785A (en) * 1993-09-23 1996-02-06 Culp; George Controlled upper row airflow method and apparatus
FR2786426A1 (en) * 1998-11-27 2000-06-02 Arimpex Sarl Method of thermal treatment of ligneous-cellulose (wood) material with elimination of oxygen in the gaseous phase
FR2786424A1 (en) * 1998-11-27 2000-06-02 Arimpex Sarl Thermal treatment of wood materials to improve resistance to external attack, dimensional stability and mechanical resistance
FR2786425A1 (en) * 1998-11-27 2000-06-02 Arimpex Sarl Thermal treatment of wood materials with cooling stage to improve physical and chemical properties, especially mechanical strength
US20020000050A1 (en) * 2000-02-24 2002-01-03 Ekhard Goldack Apparatus for and process of drying articles
US6370792B1 (en) * 2000-09-01 2002-04-16 George R. Culp Structure and methods for introducing heated ari into a kiln chamber
US6467190B2 (en) 2000-03-22 2002-10-22 George R. Gulp Drying kiln
US20030079364A1 (en) * 2000-03-24 2003-05-01 Luigi Franzoni Vertical kiln
US20110056087A1 (en) * 2009-09-04 2011-03-10 Tinsley Douglas M Dual Path Kiln Improvement
US7963048B2 (en) * 2005-05-23 2011-06-21 Pollard Levi A Dual path kiln
US7987614B2 (en) * 2004-04-12 2011-08-02 Erickson Robert W Restraining device for reducing warp in lumber during drying
US10619921B2 (en) 2018-01-29 2020-04-14 Norev Dpk, Llc Dual path kiln and method of operating a dual path kiln to continuously dry lumber

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US1212583A (en) * 1916-02-24 1917-01-16 Daniel R Tanner Method of and apparatus for preventing warping and twisting of lumber during kilning.
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US1328662A (en) * 1919-12-17 1920-01-20 Jr Frederick K Fish Process of drying lumber

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US528496A (en) * 1894-10-30 Method of drying articles
US1054597A (en) * 1912-08-05 1913-02-25 Moore Dry Kiln Co L Method of treating lumber.
US1212583A (en) * 1916-02-24 1917-01-16 Daniel R Tanner Method of and apparatus for preventing warping and twisting of lumber during kilning.
US1276979A (en) * 1917-12-14 1918-08-27 Edward D Sidman Lumber-curing kiln.
US1328662A (en) * 1919-12-17 1920-01-20 Jr Frederick K Fish Process of drying lumber

Cited By (23)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3875685A (en) * 1974-07-23 1975-04-08 Peter Koch Continuous kiln apparatus for producing warp-free lumber
US4106215A (en) * 1976-07-14 1978-08-15 The United States Of America As Represented By The Secretary Of Agriculture Wood impingement dryer
US4663860A (en) * 1984-02-21 1987-05-12 Weyerhaeuser Company Vertical progressive lumber dryer
FR2660424A1 (en) * 1990-04-03 1991-10-04 Electricite De France Method and installation for drying damp solid wood with a high throughput
US5416985A (en) * 1993-09-23 1995-05-23 Culp; George Center bridging panel for drying green lumber in a kiln chamber
US5437109A (en) * 1993-09-23 1995-08-01 Culp; George Aerodynamic surfacing for improved air circulation through a kiln for drying lumber
US5488785A (en) * 1993-09-23 1996-02-06 Culp; George Controlled upper row airflow method and apparatus
US5414944A (en) * 1993-11-03 1995-05-16 Culp; George Method and apparatus for decreasing separation about a splitter plate in a kiln system
FR2786425A1 (en) * 1998-11-27 2000-06-02 Arimpex Sarl Thermal treatment of wood materials with cooling stage to improve physical and chemical properties, especially mechanical strength
FR2786426A1 (en) * 1998-11-27 2000-06-02 Arimpex Sarl Method of thermal treatment of ligneous-cellulose (wood) material with elimination of oxygen in the gaseous phase
FR2786424A1 (en) * 1998-11-27 2000-06-02 Arimpex Sarl Thermal treatment of wood materials to improve resistance to external attack, dimensional stability and mechanical resistance
US20020000050A1 (en) * 2000-02-24 2002-01-03 Ekhard Goldack Apparatus for and process of drying articles
US6467190B2 (en) 2000-03-22 2002-10-22 George R. Gulp Drying kiln
US6652274B2 (en) 2000-03-22 2003-11-25 George R. Culp Kiln and kiln-related structures, and associated methods
US6751885B2 (en) * 2000-03-24 2004-06-22 Cefla Soc. Coop. A.R.L. Vertical kiln
US20030079364A1 (en) * 2000-03-24 2003-05-01 Luigi Franzoni Vertical kiln
US6370792B1 (en) * 2000-09-01 2002-04-16 George R. Culp Structure and methods for introducing heated ari into a kiln chamber
US7987614B2 (en) * 2004-04-12 2011-08-02 Erickson Robert W Restraining device for reducing warp in lumber during drying
US7963048B2 (en) * 2005-05-23 2011-06-21 Pollard Levi A Dual path kiln
US20110056087A1 (en) * 2009-09-04 2011-03-10 Tinsley Douglas M Dual Path Kiln Improvement
US8201501B2 (en) 2009-09-04 2012-06-19 Tinsley Douglas M Dual path kiln improvement
US8342102B2 (en) 2009-09-04 2013-01-01 Douglas M Tinsley Dual path kiln improvement
US10619921B2 (en) 2018-01-29 2020-04-14 Norev Dpk, Llc Dual path kiln and method of operating a dual path kiln to continuously dry lumber

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