Hardwood Lumber Kiln Schedules
by Dr. Eugene Wengert
A kiln schedule is a set of temperatures and humidities that are used during kiln operation. The kiln humidity can be expressed as an EMC, relative humidity, or wet-bulb temperature. The velocity through the load is assumed to be between 275 and 350 fpm, except for the white woods (including maple, ash, basswood, hackberry) where higher velocities (up to 600 fpm) are desired and acceptable. Changes in kiln temperatures and humidities are based on the MC of the lumber being dried, which in turn is determined from properly prepared kiln samples.
Choosing a Kiln Schedule
The schedules are all moisture content based, using the moisture content of the wettest half of the kiln samples. The schedule designations use the popular U.S. Forest Products Laboratory designations. (See the following sections for a discussion of how to develop a schedule from these designations.) A small "m" after the designation indicates that the wet-bulb temperatures should be adjusted to avoid temperatures below 90 F. See the above mentioned publication for details.
Formulating a Kiln Schedule
1. Determine the correct designation, such as T3-D2 for 4/4 red oak.
2. Go to the second letter designation (the MC Class Designation) and find the corresponding MC ranges in the table below. (Note: This letter designation is related to the green MC of the species. The MC ranges are chosen so that the loss of the first one-third of moisture, and the highest risk of checking and staining, will occur in the first step listed.)
3. Using a piece of paper with 6 columns, write down the Schedule Step numbers in the first column on the left. Then, write down the appropriate MC ranges (determined in #2 above) in the second column.
4. Go to the second number designation (the wet-bulb depression designation) and find the corresponding depressions in the table below.
5. In the fourth column, write down the wet-bulb depressions (determined in #4 above), one for each Schedule Step, starting at the top (Schedule Step 1). There may be more depression values than required; ignore the extra depression values in this table.
6. Go to the first number designation (the temperature designation) and find the corresponding temperatures in the table below.
7. In the third column, write down the appropriate dry-bulb temperatures. BE CAREFUL--The first temperature value must be repeated for every MC class (column #2) that is above 30% MC. The second temperature value is written for the 25 to 30% MC class; the third for the 20 to 25% MC class; and so on.
8. Subtract the wet-bulb depression from the dry-bulb temperatures for each schedule step and enter the value in the fifth column.
9. If required, look-up and write down the RH (sixth column) and EMC (seventh column) for each schedule step.
10. Determine the equalization settings and conditioning settings, as required. This procedure is discussed in a following section.
Using a Kiln Schedule
Equalizing and Conditioning
Professor Gene Wengert is Extension Specialist in Wood Processing at the Department of Forestry, University of Wisconsin-Madison
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