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Raziskava primarno proučuje kinetiko konvekcijskega sušenja bukovine (Fagus sylvatica L.) v svežem stanju ter učinke procesnih pogojev v nadaljevanju sušilnega procesa. Uporabili smo sveže radialno orientirane preizkušance, debeline od 6 mm do 24 mm (ΔL = 6 mm). Enosmerno konvekcijsko sušenje smo izvedli pri konstantni temperaturi 30 °C in 85 % relativni zračni vlažnosti z variiranjem hitrosti gibanja zraka od 0,6 m/s do 7,6 m/s. Med procesom smo gravimetrično določali lesno vlažnost in vlažnostni gradient. Sušilna hitrost je v svežem stanju lesa splošno naraščala s povečevanjem hitrosti gibanja zraka. V začetnem stanju je v kratkem intervalu faza relativno konstantne sušilne hitrosti dosežena le pri tanjših preizkušancih (≤ 12 mm) pri hitrostih gibanja zraka do 2,5 m/s. Višja hitrost gibanja zraka (> 2,5 m/s) pri sušenju lesa v svežem stanju sicer poveča masni tok vode, a hkrati povzroči velik podpovršinski vlažnostni gradient. Posledično kinetiko sušenja v nadaljevanju determinira značilen notranji difuzijski upor, kar bistveno podaljša celotni proces For determination of suitable process conditions at drying of European beechwood (Fagus sylvatica L.) the unidirectional drying kinetics, with an emphasis of free water removal was researched. We used radially oriented sapwood specimens, having thickness from 6 mm to 24 mm (ΔL = 6 mm). Series of conventional drying processes were carried out in the laboratory tunnel drier at constant climate (T = 30 °C, φ = 85 %) with various air velocities from 0.6 m/s to 7.6 m/s. During the drying, moisture content, moisture gradient and moisture flow were gravimetrically determined at successive time intervals. Drying rate generally increased by rising of the air velocity and decreased by increasing of wood thickness. Relatively constant drying rate was present at thinner specimens (≤ 12 mm) in a short time interval of the initial stage of the process, when low air velocity was used (≤ 2.5 m/s). Increasing of the air velocity (> 2.5 m/s) lead to early irreversible reduction of initial moisture flow and to the transition into the period of significantly falling drying rate, where the internal mass resistance predominated. The diffusion barrier was induced at too fast free water removal which controlled further drying rate and significantly prolonged the process in continuation

Kinetika konvekcijskega sušenja bukovega lesa (Fagus sylvatica L.) je bila raziskana pri stacionarnem postopku sušenja ter v adaptivnih sušilnih pogojih. Uporabili smo radialno orientirane preizkušance, debeline od 6 mm do 24 mm, s tračno žagano in s skobljano sušilno površino. Stacionarno konvekcijsko sušenje lesa smo izvedli v laboratorijskem sušilnem kanalu pri konstantni temperaturi in relativni zračni vlažnosti (T = 30 °C, f = 85%), z variiranjem hitrosti zraka (v) od 0,6 m/s do 7,6 m/s. Med sušenjem smo v zaporednih časovnih intervalih gravimetrično določali povprečno lesno vlažnost (u), vlažnostni gradient in masni tok vode v okolico. V drugem delu raziskave se je kinetika konvekcijskega sušenja lesa uravnavala s trenutno doseženim masnim tokom vode z iterativnim prilagajanjem sušilnega potenciala v zaprto-zančnem računalniškem algoritmu. Sušilna hitrost se je splošno povečevala s hitrostjo gibanja zraka ob površini lesa ter zmanjševala z večanjem debeline preizkušancev. Pri majhnih hitrostih zraka ter manjših debelinah preizkušancev je bila sušilna hitrost pri izločanju proste vode pri preizkušancih s skobljano površino, večja kot pri vzporednih, z žagano površino, kar je posledica razlik v snovni prestopnosti. Povečanje začetne sušilne hitrosti (v > 2,5 m/s) je pri preizkušancih večjih debelin povzročilo neposredno sušenje s padajočo sušilno hitrostjo, ob prevladujočem notranjem snovnem uporu. Adaptivno sušenje je bilo v splošnem hitrejše v primerjavi s konvekcijskim sušenjem pri konstantnih sušilnih pogojih. Pri visoki lesni vlažnosti in izločanju proste vode smo optimalni sušilni potencial dosegli z iterativnim prilagajanjem zračne vlažnosti in hitrosti zraka. Pri sušenju pod točko nasičenja lesnih vlaken in naraščanju notranjega snovnega upora dosežemo optimalni sušilni potencial s kombinacijo zračne vlažnosti in temperature sušenja. Drying kinetics of convectional drying of beechwood (Fagus sylvatica L.) was researched at common stationary drying and at adaptive kiln drying process. We used radially oriented specimens of thickness from 6 mm to 24 mm, having sawn and planed drying surface. Firstly, a series of convectional drying processes were carried out in the laboratory tunnel drier at constant drying conditions of 30 °C, at relative humidity of 85%, varying air velocity (v) from 0.6 m/s to 7.6 m/s. During the drying, moisture content (u) and water mass flow were gravimetrically determined at successive time intervals. In the second part of the research, the real water mass flow and the iteratively adaptive drying potential were used at the computer controlled convective drying. Drying rate generally increased with the air velocity and decreased with the increasing thickness of wood. Low air velocities caused also initially higher drying rate at specimens with planed surface comparing to sawn one, due to differences of the surface mass transfer coefficient. Increasing of the air velocity (v > 2.5 m/s) caused irreversible reduction of initial water mass flow and transition to the period of falling drying rate, where the internal water mass resistance predominates. Too high initial drying rate, especially at greater material thicknesses caused high initial moisture content gradient and significantly prolonged the drying process in continuation. Adaptive drying was generally faster in comparison to convective drying at constant climatic conditions. In the first drying period, at the removal of free water, the optimized drying potential was reached with the iterative variation of the air humidity and the air velocity. Combination of the air humidity and the temperature was successful to optimize the drying potential at the moisture content bellow fibre saturation, where higher internal mass transfer resistance existed.

Changes of pH of sawn wood during conventional drying were studied. Green ashwood boards (Fraxinus excelsior L.), 25.4 mm in thickness, were exposed to 1 month air drying and to kiln drying using 7 schedules, reached by varying temperature (20°C, 30°C and 65°C) and relative air humidity (33% to 90%). Wood samples for pH and moisture content (MC) determination were taken in successive time interval at 3 board`s depths (1/10, 1/3 and 1/2 of thickness).MC was established gravimetrically. pH-value determination of solid wood water system by suspending wood dust into distilled water was used. The lowest used temperature and duration of drying procedure had not significant influence on wood pH. Wood acidification was confirmed at higher drying temperatures (30°C and 65°C), particularly with lower drying rates and prolongation of the procedure. Among all the factors, drying temperature had the greatest impact on the pH-value of wood. Raziskovano je bilo spreminjanje pH-vrednosti lesa med procesom naravnega in komorskega sušenja žaganega lesa. Les velikega jesena (Fraxinus excelsior L.), debeline 25 mm, je bil sušen pod sedmimi konstantnimi sušilnimi režimi, z uporabo 3 temperatur (20°C, 30°C in 65°C) in relativnih zračnih vlažnosti med 33% in 90%. Vzporedno je potekalo zračno sušenje jesenovine v spomladanskih kontinentalnih klimatskih razmerah. Določanje pH vrednosti in lesne vlažnosti je potekalo z odvzemanjem vzorcev v zaporednih časovnih intervalih na 3 globinah žaganic (1/10, 1/3 in 1/2 debeline). Lesna vlažnost je bila določena gravimetrično, pH-vrednost lesa pa z vodno ekstrakcijsko metodo. Vrednosti pH jesenovine se pri sušenju z najnižjo temperaturo niso spremenile. Z višanjem temperature sušenja (30°C in 65°C) se je pH jesenovine značilno zmanjšala, še posebej pri daljšem sušenju in ob uporabi vlažnejših klimatskih razmer v komori. Temperatura sušenja je imela med vsemi dejavniki najbolj izrazit vpliv na pH lesa.

Raziskali smo kinetiko sušenja žaganega lesa invazivnih lesnih vrst, robinije, amerikanskega javorja in navadnega divjega kostanja. Preizkušance debelin 22 mm, 28 mm in 46 mm smo sušili v laboratorijskem sušilnem kanalu, pri temperaturi 40 °C in 60 °C. Dinamične mehanske lastnosti smo med sušenjem analizirali s frekvenčnim odzivom pri vzdolžnem in prečnem nihanju 490 mm dolgih preizkušancev. Intervalno smo gravimetrično določali še povprečno vlažnost in vlažnostni gradient, ter prečno krčenje lesa. Sušenje je potekalo hitreje pri višji temperaturi, debelina preizkušancev pa je na hitrost sušenja vplivala negativno. Najnižji modul elastičnosti, vzdolžni in upogibni, smo pri vseh lesnih vrstah in pogojih sušenja potrdili v svežem stanju. Pri sušenju, se je s padanjem vlažnosti lesa najprej začel povečevati vzdolžni modul elastičnosti. Spremembe so se pojavile že pred krčenjem lesa, pri povprečni vlažnosti nad točko nasičenja lesnih vlaken. Odziv je bil večji pri debelejših sortimentih, kjer je bil prisoten večji vlažnostni gradient. Upogibni modul elastičnosti je bil vselej nižji kot vzdolžni modul elastičnosti. Upogibni modul elastičnosti se pri vlažnosti lesa nad točko nasičenosti značilno ne spreminja, prične pa naraščati s padanjem vlažnosti lesa v higroskopskem območju. We investigated the drying kinetics of sawn wood of invasive wood species, black locust, American maple, and European horse chestnut. Specimens of 22 mm, 28 mm and 46 mm thick were dried in a laboratory convective kiln drier at 40 °C and 60 °C. Dynamic mechanical properties were analysed during drying by the frequency response in the longitudinal and flexural vibrations of 490 mm long specimens. We also determined gravimetrically in short periods the average moisture content (MC) and moisture content gradient, as well as the transverse shrinkage of wood. Drying was faster at higher temperatures, where the thickness of the specimens had a negative effect on the drying rate. The lowest longitudinal (EL) and bending modulus of elasticity (EB), were confirmed in the green state for all wood species and drying conditions. During drying, the reduction of MC of wood first caused an increase of the longitudinal modulus of elasticity. The changes appeared even at an average moisture content above the fibre saturation point (FSP), before shrinking of wood. The change was greater in thicker boards where a higher MC-gradient was present. The EB was always lower than the EL. The bending modulus of elasticity did not change significantly above FSP, but began to increase with the reduction of MC of wood in the hygroscopic range.

Sušenje lesa evkalipta (Eucalyptus nitens) je zahteven postopek, saj je ta vrsta zaradi inherentnih lastnosti zelo dovzetna za sušilne napake. Izvedli smo konvekcijsko komorsko sušenje 27 mm in 47 mm debelih preizkušancev pri temperaturah med 25 °C in 90 °C, pri ravnovesni vlažnosti 16 % ter hitrosti zraka 1 m/s. Kinetiko sušenja ter razvoj sušilnih napak smo kontinuirano spremljali z medicinskim CT-skenerjem z rentgenskimi žarki. Proučevali smo razvoj površnskih razpok ter kolapsa, s pomočjo CT-števila pa smo po Watanabe-jevi metodi določali lesno vlažnost. S CT-skeniranjem smo potrdili, da počasno sušenje pri nizki temperaturi omili obseg in podaljša čas razvoja kolapsa in površinskih razpok. Drying of eucalyptus (Eucalyptus nitens) is a demanding process, as this species is very susceptible to drying defects due to its inherent properties. We performed convectional kiln drying of 27 mm and 47 mm thick specimens at temperatures between 25 °C and 90 °C, at an equilibrium moisture content of 16 % and an air velocity of 1 m/s. Drying kinetics and the development of drying defects were continuously monitored with a medical X-ray CT scanner. We studied the development of surface checking and collapse, and with the help of CT-number we determined the moisture content of wood according to the Watanabe method. CT scans confirmed that slow drying at low temperature reduce the extent and prolongs the development time of internal cell collapse and surface checking.

V diplomski nalogi smo proučevali kinetiko sušenja lesnih sekancev treh domačih lesnih vrst, bukve, breze in smreke, z upoštevanjem dveh različnih velikostih razredov P16 in P31,5. Stacionarno konvekcijsko sušenje sekancev smo izvedli v laboratorijskem sušilnem kanalu z variiranjem temperature sušenja med 40 ⁰C in 80 ⁰C, ter pri hitrosti gibanja zraka v razponu med 1 m/s in 3 m/s. Sušenje smo spremljali z intervalnim tehtanjem nasutja sekancev debeline 1,5 cm, 3 cm in 5 cm, od koder smo izračunali karakteristične čase sušenja, koeficiente difuzivnosti v nasutju sekancev ter koeficiente snovne prestopnosti. V splošnem je najhitreje potekalo sušenje smrekovih sekancev, najpočasneje pa sušenje bukovih sekancev. Hitrost sušenja se je povečevala z višanjem temperature in hitrosti gibanja zraka, ter z manjšanjem debeline nasutja sekancev. Pri vseh pogojih sušenja, vseh drevesnih vrstah ter pri obeh velikostnih razredih smo ugotovili prevladujoč zunanji snovni upor. Koeficienti snovne prestopnosti in koeficienti difuzivnosti so v splošnem naraščali s dviganjem temperature sušenja in večanjem hitrosti gibanja zraka. We studied the drying kinetics of wood chips of three domestic wood species, beech, birch and spruce, taking into account two different chips size classes P16 and P31.5. Stationary convective drying of wood chips was carried out in the laboratory drying tunnel at varying temperature between 40 ⁰C and 80 ⁰C, and the air velocity in the range between 1 m/s and 3 m/s. Drying process of 1.5 cm, 3 cm and 5 cm chips layer was monitored gravimetrically, on the basis of which we calculate the time constant, diffusion coefficient in the chips layer and convection mass transfer coefficient. Generally, the fastest drying was at spruce chips, and the slowest was drying of beach chips. Drying rate increased with the raising of temperature and the air velocity, and decreased with thickness of the chips layer. The prevailing external mass transfer resistance was determined at all drying conditions, in all wood species and all size classes of chips. Convection mass transfer coefficient and diffusion coefficient generally increased with increasing of drying temperature and air velocity.