Alexandrou, Christos

Arganbright, Donald G.

Western Dry Kiln Clubs

Western Dry Kiln Clubs. Meeting (31st : 1980)

Undetermined

ScholarsArchive@OSU

THE DEVELOPMENT OF CONTINUOUS-CHANGE SCHEDULES1Donald G. Arganbright	Christos AlexandrouUniversity of California	 University of CaliforniaForest Products Laboratory	 Forest Products LaboratoryRichmond, California	 Richmond, CaliforniaIntroductionSoftwood and hardwood lumber has, for the most part, largelybeen dried using empirically derived schedules, i.e., on a trialand error basis. These schedules are applied to the kiln throughmanual-set controller-recorders, which are non-reactive. That is,they do not respond automatically, schedule wise, to changes inthe drying characteristics of the charge. Such schedules representa combination of discrete changes in dry and wet-bulb temperaturesand can be termed step-form type schedules. In hardwood dryingwhere the use of kiln samples is much more common, the schedulesmight be viewed as semi-reactive since the predetermined step-formschedules are changed in response to the periodic measurement ofthe kiln sample's moisture content. In recent years there hasbeen an increased interest in using schedules that continuallychange with respect to time or continuous change schedules.The ideal case would consist of a fully reactive controlsystem capable of sensing the charge drying rate and/or quality andwhich would change the schedule in a continuous fashion. The varioustypes of possible lumber drying schedules are summarized in Table 1.Cam controllers which are presently used in a number of kilnsare non-reactive but permit the schedule to be changed on a con-tinuous basis. A problem that we have observed when operationsfirst begin using cam controllers and which is true for any othercontinuous change control system is how to develop a suitableschedule.TABLE 1General Classification or Types of Drying SchedulesA. Non-reactive or time schedules1. Discontinuous change - manual set step form2. Continuous change - automatic, ramp form, etc. (cam orprogrammable)B. Semi--reactive-moisture content based schedules1. Discontinuous change - manual set step form2. Partial continuous change - Kil-mo-trol, kiln scan, etc.C. Reactive1. Continuous change - schedule based upon change in chargeweight, moisture content,AT, stress development, etc.1This research was partially supported by McIntire-Stennis funds.92ObjectivesThe general or overall objective of this research is todetermine if kiln drying efficiency can be significantly improvedthrough the use of automatic continuous change or ramp—form schedulesrather than manual set discontinuous step—form schedules.The specific objectives are:1. To identify problems with developing and using continuous—change type schedules.2. To develop a rational basis for developing continuous—changeschedules from previously known manual set step—form schedulesand,3. To experimentally establish if continuous—change schedules leadto improved drying efficiency as expressed by total drying time,final moisture content variation and, drying stress and defectdevelopment.This paper will only discuss the second of these objectives, thatis the methodology we suggest kiln operators use in initiallydeveloping continuous—change schedules from an already availablemanual set step—form schedule.Methods for Developing Continuous Change SchedulesThe easiest method to convert a step—form schedule into acontinuous—change schedule is to simply alter the step—form scheduleusing linear changes in temperature with respect to time. Thisgives a ramp—form type of schedule. This approach is shown forthe dry bulb portion of a schedule in Figure 1, with the wet bulbtemperature changes in Figure 2.Three different approaches for making linear changes becomeapparent and these are to alter a schedule by using the1. maximum temperature point of a time step2. mid—points of a time step or3. the minimum pointsThis method is most easily understood by examining Figs. 1 and 2again. Unfortunately, the use of any three of these methods doesnot necessarily insure that the new schedule is going to give thesame end result as the original step—form schedule.It seems intuitively clear, for example, that a schedulebased upon straight line connected maximum dry bulb temperaturesand minimum wet bulb temperatures would yield the most severe newschedule. Conversely, using minimum dry bulb temperatures andmaximum wet bulb temperatures gives the least severe temperatures.The kiln operator or anyone else working with schedules is hencefaced with a most difficult dilemma, which is how to design a newschedule that gives the same or better result than the originalschedule.Drying EffortFortunately a method is now available that lets us numericallyrepresent the severity of a schedule or any portion of it. Thismethod developed by Bramhall (1975) involves expressing scheduleseverity in terms of a factor called drying effort. For a givenspecies, grade and thickness combination, any two schedules having93the same calculated drying effort will dry from the same initialmoisture content to the same final moisture content in the sameamount of time.More exactly, the things one needs to know to calculatedrying effort are:1. The dry bulb temperatures used2. The wet bulb temperatures used3. The length of time each setting is used4. The saturation vapor pressure of water in millibars at differenttemperatures.The dry and wet bulb temperatures and time present no problembut the saturation vapor pressure of water is somewhat more complex.Data on the saturation vapor pressure of water can be found insteam tables or handbooks of physics or chemistry. The saturationvapor pressure of water in millibars is:saturation vapor pressure of watersaturation vapor pressure =  at some given temperature (in Hg) (millibars)	 29.92 (in Hg)Bramhall (1975) gives this data in tabular form.Drying effort for a given step in a schedule is then:Saturation vapor	 Saturation vapor	 Length of timeDrying effort _ pressure at the	 pressure at the	 x in hours these(millibar—hr)	 dry bulb tempera— wet bulb tempera—	 settings wereture (mbar)	 ture	 (mbar)	 usedThe total drying effort for an entire schedule is simply the sumof the drying efforts for each step change or time interval duringwhich the dry and wet bulb were constant or:Total drying effort =f( DBT saturation	 WBT saturation)(mbar—hr)	 vapor pressure	 vapor pressureApplication to Several Different SchedulesThis technique and the concept of linear schedule changesusing maximum, mid—point and minimum values were applied to threedifferent schedules which varied not only in total drying time butin the dry and wet bulb temperatures used.That is the drying effort was originally calculated for theoriginal step—form schedule and then again three more times whenthe schedules were altered to a continuous change basis usinglinearly changing temperatures. The following three temperaturecombinations were used:1. Maximum dry and wet bulb temperatures at a given time2. Mid—point dry and mid—point wet bulb temperatures3. Minimum dry and wet bulb temperaturesThe results for a schedule (Fig. 3) for 12/4 ponderosa pine shopand better (Knight, 1970) are given in Table 2. Note that thetotal drying effort for the manual set step—form schedule is39,528 millibar—hr. When this schedule is converted to a contin-uously changing linear schedule using the maximum dry and wetbulb temperatures, the drying effort is 34,510 mbar—hr or 12.7%(1000)X Time94less. This altered schedule is, therefore, approximately 13% lesssevere and would lead to considerable undrying. The drying effortsfor mid-point and minimum dry and wet bulb temperature combinationswere 38,800 and 38,300 mbar-hr, respectively, or less severe by1.8 and 3.1%.The results for a shorter schedule (Fig. 4) 4"x6" white firdecking (Knight, 1970) are presented in Table 3. Drying effortfor the manual set step-form schedule was in this case approximately49,100 mbar-hr. Note that although this schedule is projectedto be 336 hours shorter in total drying time than the previous12/4 pine schedule, it employs almost 10,000 more drying effortunits. This is a result largely of the lower diffusivity andgreater thickness of the white fir. The drying efforts for themaximum, mid-point and minimum temperature schedule alterationswere 44,760, 45,891 and 44,604 mbar-hr, respectively. All of theseschedules would be less severe than the step-form schedule withthe mid-point type schedule having the lowest difference or 6.6%.A similar approach was used for 8/4 Douglas-fir dimensionlumber (Rasmussen, 1961) (Table 4 and Fig. 5). In this case, thedrying effort for the step-form schedule was 6,656 mbar-hr. Theuse of maximum and mid-point temperatures lead to greater dryingefforts than the original schedule, just the opposite of the previoustwo examples. Use of the minimum temperatures had, as before, lesstotal drying effort. The mid-point method again had the lowestdifference being only +1.0%.Based upon thee analyses we conclude that the use of themid-point temperatures is the overall most accurate method fordeveloping new continuous-change schedules from previously knownstep-form time schedules. Anticipated differences in total dryingeffort from the manual schedule ranged from +1.0 to -6.6%, whichwould appear to be acceptable. It should be pointed out that withsome practice these schedules could be adjusted so that they hadequal or nearly equal drying efforts as the original schedules.Experimental ProceduresIt was decided to test the validity of these conclusionsusing 8/4 Douglas-fir lumber. Four different kiln charges weredried in a small expeimental micro-kiln; two charges using themanual set step-form schedule of Figure 5 and two charges withthe linearly changing mid-point temperatures (Fig. 5). A total of24, 20-inch long end-coated samples were dried in each charge.The samples in each charge were end-matched from an original setof 24 green mixed heart-sapwood stud grade pieces.The micro-kiln used has both a conventional manual setcontroller and a cam-type controller for continuously changingschedules. The entire 24 sample boards were suspended from anexternally mounted load cell which permitted the charge weight tobe determined on a continuous basis. After drying all 24 pieceswere oven-dried to constant mass to obtain an accurate oven drymass and hence an accurate assessment of final moisture content.Results and DiscussionThe results of these'four kiln runs are summarized in Table 5.95TABLE 2Calculated Drying Efforts for 12/4 Ponderosa PineSelect	 (total	 drying time	 552 hours)Shop andType of ScheduleChange fromTotal Drying Effort	 Step-Form(millibar-hr)	 ScheduleManual-step form 39,528Continuous1.	 Maximum dry andwet-bulb temperatures 34,510 -12.7%2.	 Midpoint dry andwet-bulb temperatures 38,803 -1.83%3.	 Minimum dry andwet-bulb temperatures 38,300 -3.11TABLE 3Calculated drying efforts for 4" x 6" white fir decking(total drying time 216 hrs)Type of ScheduleTotal Drying Effort(millibar-hr)Change fromStep-FormScheduleManual-step form 49,114Continuous1.	 Maximum dry andwet-bulb temperatures 44,760 -8.9%2.	 Midpoint dry andwet-bulb temperatures 45,891 -6.6%3.	 Minimum dry andwet-bulb temperatures 44,604 -9.18%TABLE 4Drying Efforts for 8/4 Douglas-fir Dimension(total drying time 72 hrs)Type of ScheduleTotal Drying Effort(millibar-hr)Change fromStep-FormScheduleManual-step form 6,656Continuous1.	 Maximum dry andwet-bulb temperatures 7,238 +8.7%2.	 Midpoint dry andwet-bulb temperatures 6,724 +1.0%3.	 Minimum dry andwet-bulb temperatures 6,296 -5.4%96TABLE 5Comparison of Results for Charges of 8/4 Douglas-fir Driedby a Manual Set Step-form Schedule and a Continuous ChangeSchedule Using Mid-Point Dry and Wet Bulb TemperaturesInitialMoistureDrying Timeto	 12% M.C.Drying Effortto	 12% M.C.Run No. Type of Schedule Content (ii)	 (hr) (mbar-hr)1 Manual step-form 40.5 68 6,0722 Manual step-form 39.3 80 8,142Average 39.9 74 7,1073 Continuous-change 40.0 72 6,4384 Continuous-change 40.7 76 6,953Average 40.4 74 6,695The initial moisture contents of the four runs were essentiallyequal. Drying times to a final moisture content of 12% were obtainedfrom plots of the drying curve for each run. While total dryingtimes for the individual changes varied somewhat the average totaldrying time for the step-form and continuous change schedule wereboth 74 hours as the drying effort theory predicts it should be.In actuality the step-form schedule employed sightly more totaldrying effort (7,107 mbar-hr) than the continuous change runs(6,695 mbar-hr) but this was not considered significant.Summary and Conclusions1. Continuous change kiln schedules can be developed from step-form schedules without trial and error using the Bramhalldrying effort method.2. Use of linearly changing dry and wet bulb mid-point tempera-tures would appear to give the least difference in anticipateddrying as indicated by any difference in drying effort fromthat of the step-form schedule.3. For three schedules varying significantly in total dryingtime and temperatures used this difference ranged from +1.0 to-6.6%.4. These conclusions were verified using drying tests on fourdifferent charges of 8/4 Douglas-fir lumber.Literature CitedBramhall, G. 1975. Calculating kiln schedule changes. Proc. 26thAnn. Mtg., Western Dry Kiln Clubs, pp. 99-103.Rasmussen, E. F. 1961. Dry kiln operators manual. USDA ForestService Agric. Handbook No. 188.Knight, E. 1970. Kiln-drying western softwoods. Bull. No. 7004.Moore Dry Kiln Co., Portland, Oregon.970	 12	 24	 36	 48	 60	 72	 84TIME ( HOURS )Figure 1. Typical step-form dry bulb temperature schedule withsuperimposed ramp-form schedules using maximum, mid-pointand minimum temperatures.98m 13012	 24	 36	 48	 60	 72	 84TIME (HOURS)Figure 2. Wet bulb temperature schedule indicating three types oframp-form schedule changes.184 -MAX176 -MID/ MAX168 -LL MID• 160 - MIN0152 -81tc144MINwa.	 136- /.W /S- 128 -120112',- -10 I I I	 111111I	 I	 I	 110 48 96	 144	 192	 240	 288	 384	 432	 480	 576336	 528TIME (HRS)Figure 3.	 Step-form and mid-point continuous change schedules for12/4 ponderosa pine shop and better lumber.99MAXMIDMIN—"11111,""MID180175/	 // /2II1—,L7/170LI1/1- ///	 ,,/	 ////'-77-*--,/	 ./0165Wm-.//.,//////// /	 / i	 ./////- MAX\\1.7x	 160CC/•,,/\\/ / /	 •MID/W	 155i-4 //■\/'/ /150 '// / MIN//145 •/140135 7'130 I I	 I	 I	 I	 I ill I I0	 20 40 60 80 100 120 140 160 180 200 220TIME (HRS)Figure 4. Step-form and maximum and minimum mid-point continuouschange schedules for 4" x 6" white fir decking.165MAX■ MID„.-- ----	 -	 MIN■■-- --	■ 	 --■ ■■ 	 ■----■■■ ■■ 	 ■--■--	 .----..-150145140190185-	 MAX1601558	 16 24 32 40 48 56 64 72 80TIME (HRS)Figure 5. Step-form and mid-point continuous change schedule for8/4 Douglas-fir dimension lumber.100135130125120

The development of continuous-change schedules

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The development of continuous-change schedules

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