The Ultimate Guide to Lumber Moisture Control – Part 2: Kiln Drying

Part 2 of a 4-part series – As seen in the November issue of Timber Processing.

Through all the phases of a lumber mill, controlling moisture is key to producing a quality end product. Ironically, despite being where moisture is altered the most, kilns are also often the most overlooked.

West Fraser’s Thermal Energy Supervisor Martin Andres has been with the company for more than 42 years, and knows the importance of kilns as well as anyone. “With wets, you will obviously get claims,” Andres said. “Even though we don’t get a claim for over dries, it degrades the wood and you’ll end up with more end and surface checking. The lumber shrinks just that much more so you end up with skip on your wood.”

Understanding how wood dries is crucial to developing effective drying schedules. Finding the right mix of temperature control, air flow and relative humidity improves the quality of the boards sent to the finishing stage. By finding the correct mix, mills can lessen problems such as wet claims, checks, warping, and unnecessary transportation costs due to excess water weight.

Previous Methods of Kiln Moisture Measurement

Previous methods of measuring included weight sensors that were used to chart the drying process of sample boards. Using that data, it was possible to develop calculations for estimating moisture content. However, this provided more of an educated guess than reliable, repeatable results.

As more advanced technology produced more reliable methods, the process improved. Two types of metering sensors took readings directly from the lumber charges. The first in-kiln sensors developed detected the resistance between two probes inserted into the edge of boards. The data was collected by a handheld meter. Resistance values were extrapolated to indicate moisture content.

While this was a vast improvement, giving more accurate data from inside the kiln, there were limitations. First, the data came from a single representative board. While several probe sets could be used, the number of data collection points was limited. The second limitation was probe placement. Driven into the edge of the board, not the face, the probes could not measure at the optimal point for moisture release.

Used primarily in batch kilns, the probes were inserted before charge placement. Hot checks at various stages of the drying schedule were used to gather data for calculation. Real-time measurement could not be gathered, particularly during the early development of this type of sensor. Additionally, hot checks required shutting down the kiln and bringing it to human-friendly temperature levels. In a direct-fired kiln, this also meant venting noxious fumes. After measurement was complete, the kiln was restarted, resulting in lost time and wasted energy.

Drying Innovations

Other problems occurred with hot checks, primarily that there were less people to do them. As mills looked to reduce costs, many experienced kiln operators were transferred to other parts of the mill, or given added responsibilities that diverted their attention. With this loss of knowledge, technology was needed to pick up the slack.

Initially, wired radio frequency (RF) sensors were developed for batch kilns. These sensors use RF to determine capacitance, another method of determining moisture. Plates are non-invasively inserted between the stickers. With a plate near the top of the stack and the other near the bottom, a more representative value of the entire charge package is possible. Instead of a limited number of data collection points, moisture content value of up to 17 cubic feet, per sensing unit, are tracked. Cables connected the sensor plates to computer-based monitoring hardware, and software, outside the kiln. The data can be displayed, allowing the operator to manually make changes, or integrating with the kiln controls for automation. Especially with southern yellow pine, the variability of guessing when to shut down was removed.

With the development of continuous dry kilns (CDKs), static sensors were no longer feasible, leading to the development of wireless RF sensors. With no cables to tie them down, the sensors travel with the charge. These sensors also integrate with the kiln controls for either manual or automated operation. Up to 100 sensors, monitoring moisture and temperature sets can be used per kiln, meaning the operator gets a more complete view of what’s going on inside the kiln, and can adjust the schedule accordingly.

“At a particular division, we were in need of replacing aged control equipment so we evaluated the [RF sensors] to ensure they would provide the right moisture content,” Andres said. “So far we have experienced time savings and consistent results using the moisture sensing system. We have not had wet lumber since it was installed.”

With the data capturing system integrated with the controls, there is another potential benefit. Using kiln systems connected to each other via network or Internet, multiple kilns can be monitored and controlled from one location, also helping to offset the lack of operators.

According to Tony Nadeau, Sechoir MEC’s Dry Kiln Automation and Control Director, “We worked with a customer that bought kilns for two different sites. We installed KilnScout systems for each kiln with eight sensors per system. MEC’s sensor system was modified so the customer was able to monitor both sites from one location. The RF sensors and data collection units allowed the mill to monitor and modify the drying schedule for each kiln.”

What’s Next

Despite providing several benefits including additional throughput, grade recovery and remote monitoring, in-kiln sensors are continuously being improved to keep up with the needs of modern lumber mills. The most pressing need for these mills is up-time. The sensors are now being built with Internet of Things (IoT) predictive maintenance features that actively notify the operator when there are upcoming issues, such as low battery strength. This allows action to be taken, before issues, and loss of data is prevented. Additionally, software has been built so that the kiln no longer needs to be relied upon as the sole manager of moisture – it can be monitored from the beginning of the process – from sawmill to the kiln.

<<Read Part 1: Introduction

Read Part 3: Planer>>