Moisture content control by shrinkage behaviour of the wood first - PowerPoint PPT Presentation

Moisture content control by shrinkage behaviour of the wood – first experimental results – Wolfgang F. Gard & Michel Riepen Delft University of Technology, Faculty of Civil Engineering and Geosciences Timber Structures and Wood Technology Delft, The Netherlands COST E53, Bled, Slovenia, 2009 w.f.gard@tudelft.nl

Content • Introduction • Drying trials: material and method • Results • Conclusion 2 COST E53, Bled, Slovenia, 2009 w.f.gard@tudelft.nl

I ntroduction Why would you like to control a drying process ? • To obtain the final moisture content • To avoid cracks, distortions and discoloration 4 COST E53, Bled, Slovenia, 2009 w.f.gard@tudelft.nl

I ntroduction How do you control a drying process ? • Moisture content measurements • Moisture evaporation rate (drying rate) • Strain measurements Along the cross-section of the timber !!!!!!!!! 5 COST E53, Bled, Slovenia, 2009 w.f.gard@tudelft.nl

I ntroduction Common inline measurement systems for moisture content in a kiln ? Electrical resistance probes • • Exceptionally: weighing the stack • ????????? Prediction of water evaporation and moisture content by drying models !!!!!! 6 COST E53, Bled, Slovenia, 2009 w.f.gard@tudelft.nl

I ntroduction Limitations of electrical resistance measurements ? • Moisture content above fibre saturation • Temperature above 80°C 7 COST E53, Bled, Slovenia, 2009 w.f.gard@tudelft.nl

I ntroduction What about shrinkage ? There is also a relationship between shrinkage and moisture content. 8 COST E53, Bled, Slovenia, 2009 w.f.gard@tudelft.nl

Drying trial Material • Sitka Spruce from Scotland/UK • 2500mm x 100mm x 50mm • Initial moisture content 40% - 170% (50% of all boards higher 90%) • Final moisture content required 15%±1 • 63 boards per trial 10 COST E53, Bled, Slovenia, 2009 w.f.gard@tudelft.nl

Drying trial Sensors 6 moisture probes in the timber 2 temperature sensors in the timber 2 relative humidity sensors in the kiln 2 temperature sensors in the kiln 2 shrinkage sensors at the timber stack 2 load cells for weighing the stack 11 COST E53, Bled, Slovenia, 2009 w.f.gard@tudelft.nl

Drying trial Timber stack • Top load 500 kg/m 2 , to avoid deformation during drying • Measuring online the decrease of thickness with a displacement sensor (LVDT) • Measuring the decrease of the mass with a load cell 12 COST E53, Bled, Slovenia, 2009 w.f.gard@tudelft.nl

Drying trial Drying schedule 120 T wet 100 T dry steam drying r.H. 80 60 cooling 40 pre-drying conditioning 20 warming up 0 time 14 COST E53, Bled, Slovenia, 2009 w.f.gard@tudelft.nl

Results 140.0 120.0 100.0 T [°C], RH [%], MC [%] 80.0 60.0 40.0 20.0 0.0 0 24 48 72 96 120 144 168 Time [h] Tair RH T wet T wood MC MC above FSP 16 COST E53, Bled, Slovenia, 2009 w.f.gard@tudelft.nl

Results 0.00 490 -2.50 470 Thickness decrease [mm] mass [kg] -5.00 450 -7.50 430 displacement [mm] In this period timber -10.00 410 mass [kg] is above FSP, but shows linear -12.50 390 thickness decrease proportional with -15.00 370 mass decrease, due to evaporation. -17.50 350 -20.00 330 -22.50 310 -25.00 290 0 24 48 72 96 120 144 168 time [h] 17 COST E53, Bled, Slovenia, 2009 w.f.gard@tudelft.nl

Results Shrinkage parameters total thickness of the boards 350mm in the • beginning of the drying total thickness decrease 17.7mm total • shrinkage 5.0% final moisture content 14.5% (oven dry) • assumed fibre saturation point 30% • derived shrinkage coefficient 0.50% per % MC • change 18 COST E53, Bled, Slovenia, 2009 w.f.gard@tudelft.nl

Results Sitka spruce on industrial scale 120 0 100 -3 Thickness decrease [mm] T [°C], RH and MC [%] 80 -6 60 -9 40 -12 20 -15 0 -18 0 24 48 72 96 Time [h] Tair T wet T wood MC wood MC shrink Thickness decrease 20 COST E53, Bled, Slovenia, 2009 w.f.gard@tudelft.nl

Conclusion of the first results With a displacement sensor we get information on the • drying rate, which is especially in the early drying stages of Sitka very important for the drying quality. Displacement measurements are reliable at high • temperatures (> 100°C). There is a direct linear relation between thickness • decrease and mass decrease due to drying. 22 COST E53, Bled, Slovenia, 2009 w.f.gard@tudelft.nl

Thank you very much for your attention ! COST E53, Bled, Slovenia, 2009 w.f.gard@tudelft.nl