Where the lumber drying art becomes a science
The new AccuStop™ In-Kiln Moisture Measurement System has been designed to measure the moisture content of a large sample of lumber at various locations in the dry kiln during the drying process.
Measures moisture content of lumber above and below fiber saturation point.
Monitors the drying process continually, providing a real-time feedback used to advance the segments of the drying schedule and achieve desired final moisture content within the charge.
Provides precise moisture measurement for consistent and repeatable results at shutdown.
Reads humidity of large samples, typically between 250 and 350 boards.
Measures full range moisture content within 1% endpoint accuracy.
Fully integrates with new and existing Secovac Multizone Control System and works in conjunction with the TDAL to ensure superior lumber drying results.
The only system that shows the apparent resistance of the lumber between the 2 plates. This information helps determine the real fiber saturation point of the lumber in order to develop more efficient drying schedules for MSR or dense lumber.
Integrates with practically any computerized kiln controller.
Eliminates the need for manual hot checks after each load.
Shuts the kiln down based on actual moisture content within 1% of the target humidity.
Increases premium grade by reducing downgrade due to overdrying.
The dimension of green lumber can be reduced since overdrying is eliminated.
Improved shorten drying schedules that reduce energy consumption.
Reduces the statistical variance when reconditioning is performed.
HOW IT WORKS
The sample is set by inserting two stainless steel plates into a stack of lumber. The plates extend the full width of the stack and all the boards between the plates form part of the sample.
Depending on the width of the boards, a typical sample will have 200-350 boards in it. Depending on the kiln length and system layout normally 4 to 8 such samples are taken for each dry kiln.
A meter located at the exterior of the kiln measures the time-harmonic electromagnetic field between the plates. The reading is converted into a data string and transferred to a controller using a RS485 transfer protocol on the data communication cable.
The data is received at the controller where it is converted to a moisture measurement. This moisture reading is displayed both numerically and graphically on the AccuStop™ LCD screen. It is also made available via a RS232 connection to a computerized kiln controller or PLC that is configurated to receive the AccuStop™ moisture information.
The kiln operator can use this information to run moisture driven drying schedule and to shut the kiln down when the desired moisture content is reached.
The cables should be disconnected from the plates before pushing the charge out of the kiln. Once the charge is out, the plates should be removed and stored so they will be ready to be inserted into the next green charge.
COST OF OVERDRYING
When a mill looks to increase its profitability the cost of overdrying is always at the heart of the discussion. The study of Mr. Peter Garrahan (The Hidden Cost of Drying), a scientist at Forintek, on the Analysis of Degrade Levels in Kiln Drying SPF made possible to establish that the final moisture content of the lumber is the most significant factor affecting value loss due to drying.
On a recent conference by M. Denis Thibeault of Resolute Forest Products on the use of the AccuStop™ system, detailed results show that the precision of the final moisture content when shutting the kiln has a significant impact on dry lumber value.
The following examples are intended to help you to calculate, by yourself, the cost of overdrying lumber at your mill. Let`s suppose, that the actual average moisture content at the planer is 12% and you aim at increasing it to 15%, then you overdrying would be reduced by 3%.
1. Increases in Premium Grade Lumber
On average 1.5% of premium grade lumber is degraded per percentage of humidity of overdrying. For example, an increase of 3% of the average final humidity at the planer will result in 4.5% more of premium grade lumber. For a mill producing 100 millions of FBM per year, this represents an economy of $450,000 ( 4.5% x 100 MMfbm x $100 ) when the difference between the economy and premium lumber class is $100/1,000 fbm.
2. Shorten Drying Schedule
A reduction of 3 hours per percentage of overdrying is expected. For an increase in humidity of 3% and an average drying time of 48 hours, this could represent, on average, an increase in drying capacity of 18.75% ( 3% x 3h / 48h).
3. Energy Savings
On average 46,000 BTU per 1,000 fbm is saved per percentage of humidity increase. For a dry kiln with 250,000 fbm, with an increase of 3% of the final humidity, this represents 34.5 MMBTU per load ( 3% x 46,000 x 250 ). For steam kilns with a burner efficiency at 80%, it represents a saving of 43.125 MMBTU ( 34.5 / 0.80 ) per load.
4. Hot Checks Eliminated
With the AccuStop™ system, it is not necessary to do hot checks. This represents a time savings and a diminution of the safety hazard for the kiln operator inherent with entering in a hot kiln.
5. Increased Productivity at the Planer
Because the lumber is less twisted and brittle, the productivity at the planer is greatly improved.
6. Reduced Target Dimension of Green Lumber
Because the final humidity of the lumber is better controlled and more consistent, the target dimension of green lumber can be reduced in general. The reduction of even a couple of thousands of an inch on the thickness of the lumber will permit to add more lumber to the existing kiln. Also, the length of the schedule will be reduced because the drying time is proportional to the square of the thickness of the material.
7. Smaller Standard Deviation and Humidity Gradient
The AccuStop™ system is the only one who permits to wait that all the humidity readings are below the fiber saturation point before you start the most aggressive segments at the end of the schedule. This has for effect of regrouping the final humidity of the lumber (reduce the standard deviation) and to diminish the humidity gradient between the surface and the earth of the lumber.
Stainless plates are inserted into the stack. Spring clips are then attached to the plates.
Meters are attached to a through-wall fitting and connected to a junction box.
Moisture content of the lumber inside the kiln as displayed by the Secovac control system.