Salamon, Marian

Western Dry Kiln Clubs

Western Dry Kiln Clubs. Meeting (23rd : 1972)

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COMPARISON OF KILN SCHEDULES FOR DRYING SPRUCE*Marian SalamonDepartment of the Environment Canadian Forestry ServiceWestern Forest Products LaboratoryVancouver, British ColumbiaSpruce (Picea sp.) is Canada's principal softwood and its cut ranksthe highest among Canadian woods, accounting for more than 39 percentof all lumber marketed (1). White spruce [Picea glauca (Moench) Voss]and Engelmann spruce (Picea engelmannii Parry) intergrade in the inter-ior of British Columbia, and their respective woods cannot be distin-guished. Drying schedules have been developed for these species (2, 3,4, 5, 6, 7, 8, 9), but associated grade and strength changes for kiln-dried and air-dried lumber processed over a wide range of scheduleshave not been reported.The objective of this study was to dry matched spruce chargesunder different schedules in a small experimental kiln to about 12%moisture content (M. C.) and evaluate the efficiency of the schedules onthe bases of drying rate, degrade and possible strength reduction.Materials and MethodFour shipments of B. C. Interior spruce, each from a differentmill, containing 36 to 40 pieces of 2-inch lumber in random lengths upto 22 feet were obtained. Three shipments consisted of 2 x 8-inch com-mon lumber, while one was obtained in widths from 6 to 12 inches.All shipments were covered with a polyethylene sheet and sprayedwith water to prevent their drying until further processing. They werecut into 3-foot-long pieces, taking a section for initial M. C. determin-ation from both ends of each 3-foot-long piece. The average of the twosections per piece was used to estimate the initial M. C. of a board.Basic specific gravity was determined on a cross section of each greenpiece. A total of 18 charges were assembled, each containing 36 to 40pieces except for the air-dried parcel, which contained 116 pieces.The charges were randomly assigned to one of six temperature-classi-fication schedules: conventional, low-high, low-variable high, constanthigh, superheated steam or air-drying. The exact schedule used foreach charge is presented in Table 1.An air-dried charge was stickered in a dry shed and kept until16 to 17% M. C. was reached, after which the boards were cut up intorough test specimens and conditioned in a humidity cabinet regulated to12% EMC. The final M.C.s were obtained after at least 12 months ofconditioning.To monitor drying rates in the 3-foot prefabricated, steam-heatedaluminum kiln described previously (10), a print-weight balance wasadded. Through a further modification, air-circulation rates werevaried from 250 to 900 fpm.Because the lumber dried in the experimental kiln was only 3-feetlong, it was not practical to grade it under a standard lumber gradingrule. With the assistance of qualified lumber graders, however, amethod of quality assessment was devised, based on the presence ofseasoning checks, splits, warp, loose knots and honeycomb. Table 2lists these seasoning defects and their potential effect on lumber grade.Each piece was graded twice, initially in the rough green stateand finally in the dried surfaced condition. After drying, the lumberof each charge was divided into three categories (no or slight degrade,medium degrade and severe degrade),_ based on the number and sever-ity of seasoning defects. Only severe degrade can generally be equated"'Published by permission of the Forest Products Research Society58Table 1.	 Drying Schedules of Charges.Schedule class. Conventional Low-high temperatureAir velocity (fpm)*Schedule steps DB°F250	 250WB°F	 hr.	 D13°F	 WB°F hr.900-250DB°F	 WB°F	 hr.250DB°F	 WB°F	 hr.900-250DB°F	 WB°F	 hr.900-500DB°F	 WB°F hr.170 160	 12	 168	 155 6 170	 160	 6 170	 160	 12 170	 160 6 170 160 6180 160	 12	 180	 155 8 180	 160	 6 180	 160	 12 180	 160 6 180 160 6190 160	 24	 188	 155 24 190	 160	 24 180	 160	 7 190	 160 6 190 160 6200 160	 14	 200	 155 20 200	 160	 14 232	 205	 16 232	 205 24 232 205 22180 160	 9	 175	 155 3 180	 170	 4 216	 208	 3180 170	 3	 175	 165 3Charge number 1-3	 4 5-6 7-8 9-10 11Drying time 74 64 54 50 42 40Schedule class. Low-varying high temperature Const. high temperature Superheated steam Air dryingAir velocity (fpm)* 900-250	 900-250 250 250 naturalSchedule steps DB°F WB°F	 hr.	 DB°F	 WB°F hr. DB°F	 WB°F	 hr. DB°F	 WB°F	 hr. DB°F	 WB°F- 212	 2	 170	 160 6 232	 205	 41.5 230	 212	 40- 2	 180	 160 5170 160	 6	 190	 160 6180 160	 8	 200	 160 6190 160	 3	 244	 200 1/4200 160	 3	 232	 200 3 1/2232 205	 4	 255	 200 1/2235 205	 2	 242	 200 4246 205	 4	 258	 200 3/4252 205	 5	 252	 200 3256 205	 1	 256	 200 3260 205	 6	 262	 200 4265 205	 2Charge number 12	 13 14-15 16-17 18Drying time 48 42 41.5 40 1 year*The setting is underlined at which air velocity was changed to lower level.Table 2. Quality Evaluation Applied to 3-Foot-Long S2S Dried Lumber.Type of DefectSurface checksmall	 Less than 4 inches	 Slight or medium in combinationmedium	 More than 4 and less than	 Slight, medium in combination,10 inches	 severe when through piecelarge	 Longer than 10 inches	 Severe in combination, or severewhen through pieceEnd splitshort	 Half width of piece	 Slight or medium in combinationmedium	 Equal to width of piece	 Medium or severe in combinationlong	 More than width of piece	 SevereRing separation(shake)	 When aggravated by drying, treatas split or surface checkLoose grainlight	 Less than 1/16-inch deep	 Slight or medium in combinationmedium	 1/16 to 1/8-inch deep 	 Medium or severe in combinationheavy	 More than 1/8-inch deep	 SevereLoose knots or holes(regardless of size)WarpHoneycombEncased knot	 SlightStar checked knot with checkextended over edge 	 MediumStar checked knot with checkextended over edge .?..1"	 SevereCrook	 Bow‘,1/2"	 Slight1/2-7/8"	 1/2-7/8"	 Mediumj•7/8"	 ?/7/8"	 SevereCup	 Twist..S1/8"	 -3/8"	 Slight1/8-1/4"	 3/8-1/2"	 Mediumj1/4" ?.,1/2"	 SevereFew checks, closed	 MediumSeveral checks, closed	 Severewith a reduction in grade of full-length pieces of dimension lumber.Within a week after completion of kiln drying, rough mechanicaltest specimens were cut from the dried boards, which then were placedinto a 12% EMC controlled-humidity cabinet to equalize for at least sixmonths. After the equalization period, both air-dried and kiln-driedspecimens were processed to final size and tested in static bending andcompression parallel to grain following the appropriate ASTM procedure(11, secondary method).Results and Discussion Drying rate A comparison of drying rates achieved with different schedules isrealistic only when initial M. C. s are similar and final M. C. s are like-wise nearly equal, because the drying rate is faster at high M. C. andmuch slower below the fiber saturation point. Although some variationdoes exist in initial M.G. between charges (Table 3), the average initialM. C. s for various classes of schedules (Figure 1) are approximatelyequal, with the exception of the superheated steam charges. Likewise,average final M. C. s for various schedules (Figure 1) are similar,ranging from 10.6 to 13. 1% with the exceptions of the low-varying highschedule (8.0%) and the superheated steam schedule (16.4%). Prior tocalculating drying rates in these two instances, drying times were ad-justed to the average final M. C. for all charges of 11.4%. These ad-justments were achieved by extrapolating or interpolating the relation-ship between M. C. and drying time to conform to 11.4% M. C.As shown in Table 3, three conventional charges (no. 1-3) took74 hours to dry and the other (no. 4) took 64 hours. Their averagedrying rate was 0.40%/hour at 250 fpm air-circulation rate. The chargeswhich were dried at conventional temperatures, but with circulationrates of 900 and 250 fpm (no. 5 and 6), both took 54 hours to dry at anaverage rate of 0.53%/hour. On the basis of the average conventionaldrying rate, this represents an increase of 33% (Figure 1). This em-phasizes the gains to be made by employing increased circulation rates,at least during the first part of the schedule, and verifies results pre-sented previously (7, 9). Faster circulation rates reduce the build-upof a humid insulating-boundary layer immediately above a wood surfaceand increase the heat transfer rate by maintaining a more uniformtemperature across a load.The two low-high temperature charges at 250 fpm air-circulationrate (no. 7 and 8) dried in 51 and 49 hours, respectively, with a cor-responding average drying rate of 0.56%/hour to the average final M. G.of 13. 1%. The other two low-high temperature charges employing cir-culation rates of 900 and 250 fpm (no. 9 and 10) were dried in 40 to 44hours with an average drying rate of 0. 73%/hour. The low-high temper-ature schedule employing somewhat higher air-circulation rates of 900and 500 fpm (no. 11) increased the drying rate to 0.82%o/hours, an in-crease of 105% over the conventional drying rate (Figure 1).The two low-varying temperature charges (no. 12 and 13) driedin 48 and 42 hours, the first including 2 hours of steaming and 2 hoursof cooling at the beginning of the schedule. The average rate of0.78%/hour \A as based on the time needed for the lumber to reach 11.4%final M. C. , the average of the other charges. It is interesting to notethat the higher circulation rates of 900 and 500 fpm used for charge 11resulted in a faster drying rate than that obtained in charges 12 and 13,which employed very high temperatures.The two constant high-temperature charges (no. 14 and 15) driedin 43 and 40 hours, respectively, at 250 fpm with an average drying rate610.9I--IIJ fx 0.8eln° 0.7r0 1 0.61110■11■1•111111111•••■•■••••■COVcr.2a_ — —5-- (f) 0.5ce to0 s0.4 0.4 _w(.2 0	 _0.3 Z.6 Cr' 01 LPo-.0— 0.2=t1-t t010.1 V-:0.0.7-_ _ ....,Schedule Classification 	 Cony. Cony. Low-Hi Low-Hi Low-Hi Low-Voni	 Const- SuperTemp. Temp. Temp. Hi	 Hi Temp. HeatCirculation	 Rate	 (f.p.m.) 	 250 900-250 250 900-250 900-500 900-250	 250 250Charge	 No. 	 1 - 4 5-6 7-8 9-10 II 12-13	 14-15 16-17Figure 1. Drying rates for different schedules. Numbers in parentheses represent initial and final M. C. s.Table 3. Drying Parameters of Each Charge.Charge Dimension Drying Drying Initial M. C.	 91 Final M. C. % Schedule Air circ. Degrade evaluation 'cno. in. time,hoursrate, %M. C. /hr.min. ave. max. min. ave. max. S. D. class. f. p. m. no or	 medium	 severeslight1 2 x 6 74 0.. 30 25.6 34.0 56.8 8. 3 11.6 15.3 2.59 Conventional 250 100.0 -2 2 x 8 74 0. 33 34.8 7. 9 10. 7 14.7 1. 44 250 97. 2 2. 83 2 x 8 74 0.55 30. 5 52. 4 146. 4 7. 6 11. 4 16. 0 2. 40 250 92.5 2. 5 5. 04 2 x 8 64 0. 43 28.5 40.2 82. 3 9. 1 12. 7 20.2 2.61 250 95.0 2.5 2.55 2 x 8 54 0.52 27.6 39.7 99.7 8. 3 11. 5 13.7 1. 27 Conventional 900-250 95.0 5.06 2 x 6 54 0.54 29.4 42. 7 74. 9 10.0 13. 3 19. 8 2. 62 900-250 92.5 7. 57 2 x 8 51 0.41 28.6 34.0 55.0 7. 8 13. 2 20.3 2. 32 Low-hi temp. 250 97. 2 2. 88 2 x 8 49 0. 71 32.8 48. 1 170.4 7. 3 13. 1 21. 9 2. 33 250 97.5 2.59 2 x 8 40 0.73 31.5 39.4 93.1 5.5 10.1 13.5 1.94 Low-hi temp. 900-250 95.0 2.5 2.510 2 x 6-12 44 0.73 30.6 42.9 133.2 6.4 11.0 19.0 2.99 900-250 90.0 10.011 2 x 8 40 0.82 29.3 43.7 86.6 7. 3 10.9 24.0 2.87 Low-hi temp. 900-500 95.0 5. 012 2 x 6-12 48 0.74 29.4 43.2 76. 1 4. 1 7.6 15.2 2.57 Low-varying hi temp. 900-250 67.5 5.0 27.513 2 x 6-12 42 0.82 28.9 42.9 121.3 4.4 8.5 14.6 2.78 900-250 60.0 7.5 32.514 2 x 6 43 0.53 29.0 32.6 44.6 7. 0 10.0 12. 9 1.67 Constant hi temp. 250 77.8 22. 215 2 x 8 40 0. 92 34.6 50.3 165. 2 7. 3 13.5 34.7 5. 19 250 67.5 22.5 10. 016 2 x 8 40 0. 80 32.4 47.5 171.4 6. 8 15.4 49.2 6. 49 Superheated steam 250 37.5 22.5 40. 017 2 x 8 40 0. 95 36.5 55.3 138.9 8. 9 17.4 43.6 8.31 250 25.0 27.5 47. 518 2 x 8 23.4 42.5 173.4 9. 7 12.5 13. 3 1.09 Air dried N. A.Dried for at least one year.N. A. -,- not applicableof 0.72%/hour. The superheated-steam drying charges were dried in40 hours, but the final average M. C. was an unsatisfactorily high 16.4%.The drying rate averaged 0.61%/hour when drying time was extrapolatedto reach a final M. C. of 11.4%.Final moisture content distributionThe final M. C. of charges averaged 11.5% and ranged between7. 6% and 17.4%. The average standard deviation was 2.05, with a rangeof 1.09 (air-dried charge) to 8.31 (a superheated, high-temperaturerun).Quality of drying Table 3 also shows the quality evaluation of the 17 kiln charges.In conventionally dried charges utilizing the lowest air-circulation rateof 250 fpm, a fall-down ranging from zero to 5.0% was indicated fromthe percentage of severe degrade obtained. The combined air-circula-tion rate of 900 and 250 fpm with conventional temperatures showed amaximum severe degrade of 7.5%.Fall-down generally became excessive only in charges 12 to 17.In these instances, moderately high temperatures (230 to 232°F) wereemployed throughout the schedule, or very high temperatures (up to265°F) were used for a shorter period of time.Extensive honeycomb was noted after cross cutting the boardsdried at the low-varying high temperature schedules of charges 12 and13. The constant high temperature and the superheated-steam schedulesalso produced severe honeycomb in many boards, resulting in unaccept-able quality.Strength comparisonAll specimens were stored long enough to attain constant EMC,which varied between 10.5 and 13.5%, depending on severity of dryingcondition. Those charges exposed to higher temperatures attained thelower EMCs. No attempt was made to adjust individual strength valuesto a common MC, since the EMC obtained reflect the actual moisturelevels expected under service conditions.Each strength value obtained in testing was adjusted to the aver-age specific gravity of 0.341 based on all test pieces. The followingequation was used (12):S	 G nS'	 G'where S is the strength value determined by testing;S' is the strength value adjusted to the average specific gravityof 0. 341;G is basic specific gravity of a test piece;G' is O. 341;= 1.50 for MOR;1.25 for MOE and maximum crushing stressThe adjusted values were averaged by each schedule type and theresults presented in Table 4. These mean values were compared tothe air-dry controls, using Dunnett's test as described by Steel andTorrie (13). The low-varying high temperature treatment gave modulus-of-rupture values which were 8.2% lower than the respective air-drycontrol. This difference was significant at the 5% level, but no signifi-cant difference could be detected arriong any MOE or maximum crushing-stress values. It should be noted that the test samples were cut in sucha way as to avoid honeycomb. The actual strength of low-varying high,constant high-temperature dried and superheated steam-treated speci-mens might have been lower had the "tire board with included honey-comb been tested.64Table 4. Average Strength Properties of Spruce Dried Under Different Schedules.Static bending	 Compr. parallel tograinCharge no.	 Schedule	 Modulus of	 Modulus of	 Max. crushingrupture	 elasticity 	 strength No. tests	 psi	 No. tests	 103 psi	 No. tests	 psi1 - 6	 Conventional,all velocities	 139	 9958	 139	 1482	 135	 55787 - 11	 Low-hi temp.all velocities	 124	 10066	 124	 1513	 141	 556812 - 13	 Low-varying hi temp. 	 51	 9557*	 51	 1461	 53	 552914 - 15	 Const, -hi temp.	 54	 10006	 54	 1426	 52	 553616 - 17	 Superheated steam	 41	 10166	 41	 1489	 48	 540118	 Air dried	 78	 10387	 78	 1458	 81	 5476Significantly different from the air-dried value at the 5% level.Conclusions Using a small experimental kiln, several interior spruce chargeswere successfully dried using conventional kiln temperatures and an air-velocity of 250 fpm. The drying rate was increased by about one-thirdwhen a higher circulation was employed in the first half of the scheduleor when higher temperatures were introduced during the latter part ofthe drying schedule. If both higher circulations and temperatures wereintroduced at the appropriate times in a schedule, drying rates wereincreased by 75 to 105% without inducing excessive degrade in lumber.However, constant high temperature (max. 232°F), low-varying hightemperatures (max. 265°F) and superheated-steam drying schedulesproduced an excessive amount of honeycomb, therefore precluding theiruse.Final M. C. distribution was satisfactory for air dried, conven-tional and low-high temperature schedules, but other kiln schedulesresulted in increased M. C. ranges and high, unacceptable standarddeviations.There was no evidence that modulus of rupture, modulus of elas-ticity or maximum crushing strength of clear samples cut from speci-mens were affected by drying conditions, except in one instance whereexposure to low-varying high temperatures (max. 265°F) caused astatistically significant reduction of 8.2% in modulus of rupture.References 1. Canada Year Book. 1970-71. Dominion Bureau of Statistics,Ottawa, Ont.2. Archer, C. F. 1952. Kiln-drying schedules for British Columbiawoods. Can. Dep. Resources Develop. , Forest Br. , Mimeogr.V-1012, Vancouver, B. C.3. Cech, M. Y. and D. R. Huffman. 1971. High-temperature kilndrying of spruce joists. Forest Prod. J. 21(10):55-60.4. Jenkins, J. H. and F. W. Guernsey. 1954. The kiln-drying ofBritish Columbia lumber. Can. Dep. Northern Aff. Nat. Resources,Forest. Br. Bull. 111, Ottawa, Ont.5. Millet, R. S. 1961. Kiln-drying of lumber in eastern Canada.Can. Dep. Forest. , Tech. Note No. 21, Ottawa, Ont.6. Salamon, M. and S. McIntyre. 1969. Manipulation of air velocitypermits drying time savings. Cmn. Forest Ind. 89(9):49-53, 55-57.7. Salamon, M. 1970. More kiln capacity.: Kiln schedule to meetthe specifications of the new standard for western white' sprucedimension lumber. B. C. Lumberman 54(2):32-34, 50, 52.8. Salamon, M. and S. McIntyre. 1970. Combination schedule im-proves drying of western white spruce lumber. Forest Prod. J.20(7):41-42..9. Salamon,	 1970: White spruce .kiln schedule employs mixed airvelocities. Can. rarest Ind. 90(9):.481.-50.10. Guernsey, F. W 1,957. High temperature drying of BritishColumbia softwoods:. Forest Prod.	 7(10):368-371.11. Amer. Soc.`-Testing Mater. 1965. Standard methods of testingsmall clearspecimens of timber. Secondary Methods. g A. S. T. M.Designation;_ D143-52. p. 89-98. In: Book of ASTM Standards,Part 16, Philadelphia, Penn.12. U. S. Forest Prod. Lab. 1944. Design of wood aircraft structures.ANC Bull. 1:8,■ Artriy-Navi-Civil Comm: on Aircraft Design Cri-teria, Aeran. Board, Washington, D.' C.13. Steel, R. G.	 and J.	 Torrie: 1960.: Principles and proceduresof statistics. McGraw-Hill, Ne York.662

Comparison of kiln schedules for drying spruce

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Comparison of kiln schedules for drying spruce

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