NC State Extension Publications

Introduction

Airflow is a vitally important component in pine dry kilns. Not only is the actual velocity measured in linear feet per minute an important factor, but just as important is the uniformity of that air, top to bottom and end to end. Invariably, the kilns which have direct drive fans (e.g. motors directly inside the kiln) experience a motor failure. As a result, there is less airflow in the vicinity of the lost motor.

One question is: shall I stop the kiln during a charge and repair/replace the motor or wait till the end of the cycle? A second question is: how much wetter will that lumber be in that area where the fan went out?

The Case Study

A typical 2-track steam-heated kiln with center booster coils was charged with green 4/4, 6-10 inch wide pine1 boards. The ending target moisture content was 10% so as to meet the 12% “kiln dried” rule for boards. The kiln had seven 5-foot diameter direct drive fans. Airflow velocity was taken on the exit air side, when all the fans were working.

After the end fan went out, airflow was taken again in that area. This end fan went out after 16 hours into the cycle. The remaining 23 of the cycle was run without this end fan. Figure 1 shows the air velocities.

Table 1. Average moisture content (%), standard deviation (SD), and airflow (linear feet per minute) by bunk location on kiln track No. 1.
Airflow
Fan on 550
Fan off 425
Ave. MC 14.2
SD 2.1
Airflow 650


Ave. MC 11.8
SD 1.9
Airflow 650


Ave. MC 9.5
SD 0.6
Airflow 700


Ave. MC 11.0
​SD 1.2
Airflow 700


Ave. MC 10.6
​SD 1.3
Airflow
Fan on 550
Fan off 400
Ave. MC 13.2
SD 2.5
Airflow 650


Ave. MC 11.1
SD 1.4
Airflow 600


Ave. MC 9.7
​SD 0.7
Airflow 650


Ave. MC 11.1
​SD 0.8
Airflow 600


Ave. MC 11.2
​SD 1.1
Airflow
Fan on 500
Fan off 350
Ave. MC 15.4
SD 2.6
Airflow 500


Ave. MC 11.4
SD 1.4
Airflow 500


Ave. MC 10.9
​SD 1.2
Airflow 500


Ave. MC 11.4
​SD 1.2
Airflow 500


Ave. MC 12.3
​SD 1.2
This fan went out after 13 into the charge cycle.

The kiln schedule combined a low-temperature setting (190°F dry bulb) for 27 hours, followed by 15 hours at 220°F. An equalization period of two hours, set at 220°F dry bulb and 200°F wet bulb (about 7% EMC) then finished out the schedule. Total run time was 44 hours. Because of two other kiln malfunction problems during the run, the kiln was down for a total of eight hours. This time was not included in the actual run time of 44 hours. Had the kiln run continuously without interruption, there would have been less actual run time.

After drying and cooling, moisture contents for 20 to 30 boards from each of 15 separate bunks was determined using an electrical resistance type meter. Moisture content averages and standard deviations (a measure of variability) are shown in Figure 1.

1 The pine was Pinus radiata, a species similar to Southern yellow pine, but with slightly less specific gravity.

Discussion

The area where the fan went out did affect the pine boards. The airflow dropped 125-150 feet per minute. The average moisture content of boards was about 3 percentage points higher than the average moisture content of the boards in the rest of the kiln (14.3% vs. 11%). The moisture content standard deviation was higher (2.1-2.6 vs. 0.6-1.9). Finally, most boards in the area where the fan went out would not meet the 12% moisture rule. Conversely, most boards in the rest of the kiln would meet the rule.

Comments

The question – shall I shut the kiln down and repair the motor(s) still remains. Factors beyond the higher moisture content and variability possibilities may control the decision. Such factors may include: (1) do I have an available motor now that can be quickly installed? or (2) at what stage of drying did the fan(s) go out?

Several thoughts are: During the initial stage of drying, say from green to around 60% moisture content, the rate of drying is highly dependent upon air velocity. If a fan goes out early in the cycle, then one can expect several percent points higher in moisture content, with more variability in the corresponding area. More emphasis should be placed here on changing the motor in this stage.

During the middle part of drying, say from around 60% to 30%, air velocity has less effect on drying rate. If a fan goes out during this stage, then replacing the motor may be a toss-up. As in this case study, a motor went out near the first part of this second stage. Results were higher moisture contents (by 3% points).

During the last stage in drying (say from 30% to dry), air velocity becomes a less significant factor on the drying rate. If a motor goes out during this stage, the need for replacing it is questionable. But keep in mind the moisture content in that area will still be a little higher.

One way to compensate when a motor goes out is to dry a little longer. The extra length of time depends largely upon the time during the cycle when the motor went out. In this particular case study, an additional 6-8 hours would probably be required to get the wetter boards (in the area where the fan went out) down to the target. Had the fan gone out during the first few hours of running, then as much as 12 hours or more may be required to get the wetter boards to the target moisture content. But, the big drawback to this “extra time” procedure is the overdrying of the remainder of the charge, unless an equalizing procedure is used.

If a fan does go out, one of the best, and easiest ways to overcome the problem of wet lumber is to set the kiln conditions on equalization during the final 1/3 stage of drying. This requires raising the wet bulb set point till the EMC is 3% below your target moisture content. In this case study, an EMC of 7% would be desired. Continue drying until you feel comfortable that the wetter boards are down to the target moisture content. This procedure will prevent overdrying, while allowing the wetter boards to continue drying at a reasonable rate. Then, repair the motor!

Author:

Associate Professor and Extension Specialist
Wood & Paper Science

Publication date: July 1, 1988

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