Lumber Drying Processes and Techniques

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1 Lumber Drying Processes and Techniques Daniel Cassens Professor of Forest Products Purdue University West Lafayette, IN

2 Reasons for Drying Wood Strength Weight Preservation Stability

3 Moisture Related Problems in Finished Products Raised Panel Door

4 Moisture Related Problems in Finished Products Miter Joint

5 Moisture Related Problems in Finished Products Checking

6 Moisture Related Problems in Finished Products Checking

7 Moisture Related Problems in Finished Products Checking

8 Moisture Related Problems in Finished Products Stress

9 Moisture Related Problems in Finished Products Stress

10 Moisture Related Problems in Finished Products Planking

11 Moisture Related Problems in Finished Products Sunken Glue Joint

12 Moisture Related Problems in Finished Products Sunken Glue Joint

13 Moisture Related Problems in Finished Products Expansion

14 Moisture Related Problems in Finished Products Shrinkage (from South East Asia)

15 Moisture Related Problems in Finished Products Veneer Checking

16 Moisture Related Problems in Finished Products Veneer Checking

17 Dimensional Change of Wood Effect of Relative Humidity R.H. (%) EMC (%) Width (in.) Approximate Change Fractional Equivalent / / /4

18 Dimensional Change of Wood Effect of Relative Humidity

19 Dimensional Change of Wood Factory Conditions

20 Dimensional Change of Wood Grain Direction

21 Dimension Change of Wood Grain Direction

22 Dimension Change of Wood

23 Moisture Content Weight of the water contained in the wood, expressed as a percentage of the oven-dried wood. A. Average B. Shell C. Core

24 Moisture Content Sample technique to determine MC of shell and core

25 Calculating Moisture Content OriginalWeight Oven Dry Weight M. C. x Oven Dry Weight 100

26 Moisture Meter

27 Relative Humidity Ratio of the amount of water vapor present in the air to that which the air would hold at saturation at the same temperature.

28 Instrumentation

29 Equilibrium Moisture Content The moisture content at which wood neither gains nor loses moisture when surrounded by air at a given relative humidity and temperature.

30 Equilibrium wood moisture content for a given relative humidity and temperature o F Relative Humidity Wood Moisture Content (percent) The equilibrium moisture content (EMC) of wood is that moisture content which wood will eventually equalize to given a fixed temperature and relative humidity. If the temperature and relative humidity change the moisture content of the wood will change. At an average temperature of 60 o F and 70% relative humidity wood will reach a moisture content of 13.3%. Twelve to 15% is the typical moisture content of air-dried lumber or lumber stored in unheated sheds. If the average temperature is about 70 o F and the relative humidity of 35% the moisture content of wood is about 6.9%. These are typical inside conditions for houses and temperature/humidity controlled commercial buildings. Small, inexpensive sensors are available at building supply stores to determine relative humidity and temperature. Source Wood Handbook

31 Average EMC by Location and Season Average Relative Humidity and Moisture Content of Interior Woodwork for Selected Locations and Seasons Indianapolis New Orleans Las Vegas West Coast Jan. July Jan. July Jan. July Jan. July Average Relative Humidity Average Moisture Content of Interior

32 How Water Is Held in Wood Free Water Moisture that is held in the cell cavities of the wood, not bound in the cell wall

33 How Water is Held in Wood Bound Water Moisture that is bound by adsorption forces within the cell wall; that is, the water in wood below the fiber saturation point.

34 Fiber Saturation Point The stage in the drying or wetting of wood at which the cell walls are saturated with water and the cell cavities are free from water. It is usually taken as approximately 30 percent moisture content.

35 Drying Stages

36 Air Drying and Stacking Techniques

37 Air Drying and Stacking Techniques

38 Air Drying and Stacking Techniques

39 Air Drying

40 Kiln Drying Steam

41 Example: Steam Kiln Schedules Kiln schedules for 4/4 northern or upland red oak and yellow-poplar

42 Kiln Drying Dehumidification

43 Kiln Drying Solar 43

44 Reconstituted Products

45 Lumber Storage

46 Lumber Storage Typical temperature increase values to maintain desired EMC in a heated shed. 1 Desired EMC Amount above outside temperature Percent ( o F) Drying Eastern Hardwood Lumber. Agr. Hbk. No. 528, USDA Forest Products Laboratory. 104 pp.