HEATING TIME SIMULATION FOR FROZEN AND UNFROZEN WOOD

Transcription

1 HEATING TIME SIMULATION FOR FROZEN AND UNFROZEN WOOD H. Peter Steinhgen Deprtment of Forest Products University of Idho Moscow, Idho INTRODUCTION The heting time simultion softwre LOGHEAT1 (Steinhgen et l.) ws developed for peeler logs used in the veneer nd plywood industry. These logs re heted in wter in order to soften the wood for improved veneer cutting. Accurte estimtes of heting times required to rech trget tempertures t given depths re importnt, prticulrly when the logs re frozen during the winter nd mny hours of heting re needed just to thw the ice. This simultion might lso be used for poles tht re heted nd dried prior to being treted with slt preservtives, provided some limittions re tken into ccount. This pper outlines the cpbilities nd limittions of the softwre LOGHEAT1 nd discusses heting time dt generted with it. LOGHEAT1 LOGHEAT1, developed for n IBM-PC, uses finite-difference pproch to nlyze het trnsfer in the rdil direction of debrked, cylindricl log hving length of t lest four dimeters ("long log") nd mximum dimeter of 25 inches. The log is ssumed to be fully immersed in the gitted wter bth which is mintined t constnt temperture. The input required includes indiction of the wood species or specific grvity nd moisture content (considered the min keys to the species effect), log dimeter nd length, initil log temperture--which is ssumed to be uniform nd my be bove or below the freezing point--nd bth temperture (Figure 1). The output tble records the computed tempertures for ech hlf-hour increment into the heting cycle; t ech time level the tempertures re listed for hlf-inch increments long the log rdius. Log energy consumption is lso recorded. Computtions were compred with vilble experimentl dt (Steinhgen et l.). For given temperture within log, the discrepncy between mesured nd computed heting time ws usully within the 1 percent trget, which ppers stisfctory from prcticl point of view. DISCUSSION OF HEATING TIME DATA LOGHEAT1 ws used to generte heting time dt over rnge in prmetric vlues (Tble 1). The tble vlues reflect the lrge effect of log dimeter on heting time. Theoreticlly, if the log dimeter is incresed by fctor of two (e.g., from 1 inches to 2 inches), heting time will increse by fctor of four (squred reltionship), for ny 33

2 constnt rtio of core dimeter to log dimeter. This is not evident from the tble vlues; s the core dimeter ws ssumed to be constnt, the rtio of core dimeter to log dimeter is different for ech different log dimeter. To give nother exmple, temperture histories were computed for two logs (both were grnd fir logs t 27 F initilly, nd were exposed to wter bth t 156 F); only the dimeter ws vried in these computtions, using 12 inches nd 24 inches. As result, fter 2 hours of heting the 12-inch log registered 119 F t rdil depth of 6 inches; the comprble temperture for the 24-inch log ws only 39 F. Frozen logs need much time to thw so tht their totl heting time is significntly longer thn tht of nonfrozen logs. As n exmple, the center of log reched temperture close to 32 F fter bout 15 hours of heting (Figure 2, curve mrked "6"); but it then took the ice nother 15 hours to melt before the temperture rose gin t the log center. If the log's specific grvity is.3 insted of the ssumed tble bse vlue of.4, heting time will be bout five percent smller thn the tble vlue, nd conversely, five percent lrger if the specific grvity is.5. If the bth is nongitted wter, the heting time will be bout 1 percent lrger thn the tble vlue. When the heting medium is ir insted of wter, considerbly more heting time my be needed thn tht recommended by the tble. This is due to difference in the rte of het trnsfer-- 5 to 5 Btu/hr-sqft- F for forced ir versus 5 to Btu/hrsqft F for gitted wter, ccording to Kreith (1965)--which will ffect the surfce temperture history. The ltter, in turn, ffects the internl temperture history. In wter, surfce temperture rises lmost immeditely to the level of the bth temperture (Figure 2, curve mrked ""). In ir, this rise occurs grdully over the whole rnge of moisture loss (Figure 3, curve mrked ""). It is for this reson tht heting of poles in ir followed by drying will tke more time thn suggested by the vlues of Tble 1. CONCLUSIONS The computer softwre LOGHEAT1 simultes log heting in gitted wter. Heting time estimtes vi LOGHEAT1 for nonfrozen s well s frozen logs re stisfctory; dt re presented here over rnge in prmetric vlues. The log dimeter hs considerble effect on heting time. Thwing lso ffects heting time significntly. Heting in ir my be considerbly slower thn heting in wter. LITERATURE CITED Kreith, F Principles of Het Trnsfer. Interntionl Textbook Compny, Scrnton, PA. Second edition. Stnish, M.A., nd F. Kyihn Moisture trnsport in wood prticles during drying. AIChE Symposium Series 8(239):

3 Steinhgen, H.P., H.W. Lee, nd S.P. Loehnertz. Computer progrm LOGHEAT1. Submitted October 1986 for publiction in the Forest Products Journl. Tble 1. Heting time estimtes for "long" logs heted in gitted wter. The logs' specific grvity bsed on dry weight nd green volume is.4. T i is the initil wood temperture, Tf the finl temperture t 5-inch core dimeter, T' the bth temperture, MC the wood moisture content bsed on dry weight, t the heting time, nd D the log dimeter. T. TT MC f co ( F ) ( F) ( F) (%) t (hrs) D (1 in.) (15 in.1 (2 in.)

4 START 'Vlue from Wood Hndbook 'Input moisture content / Input log dimeter nd length/ Input log temperture Input bth temperture Clculte: Therml conductivity Specific het Temperture history for ech node Energy consumption 4, PRINT: Input dt Heting time Temperture for ech node Energy consumption V I STOP 4, NOTE: Recycle through 12 nd 13 until cre temperture is 1 degrees less thn bth temperture. Figure 1. Flow chrt, LOGHEAT1 36

5 12 -T T Distnce from surfce in. 1 in. 2 1/z in. 4 in. O 5Y1 in. 7 in. 9 in HEATING TIME (HO) Figure 2. Trnsient temperture distribution in "long" log heted in gitted wter. Specific grvity ws.32, moisture content 97%, log dimeter 18 inches, log length 13 inches, initil log temperture (T ) -8 F, bth temperture (T ) 13 F. Drying Rte (%/min) or Temperture ( C) :6.,,... c, 4 EXPERIMERTAL DRT/NO RATE X 16 11/MU-ATION O DPERIPENTN. RIRFACE TOP --SIMULATION.EXPERIMENTAL Moisture Content (% dry bsis) Figure 3. Smll wood block (.1 cubic inch) with one-dimensionl (tngentil) moisture movement. Wood species ws Dougls-fir, initil moisture content 128%, ir temperture 23 F, ir velocity 37 FPM. Source: Stnish nd Kyihn,