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W estern Dry Kiln Association May, 20067
MOISTURE SENSING IN THE DRY KILN
Richard Magill and Patrick YoussiSignature Control Systems
Denver, Colorado
Agenda
W h y u se in -k iln mo is tu re se ns in g? How does it work? W
hat variables are involved? W h at k in d of re su lts are ex pe
cted ?
Benefits of Using an In-Kiln Moisture Measurement System
Three key advantages:
Reduced degradeImproved throughputLower energy costs
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W estern Dry Kiln Association May, 20068
SPF Kiln Pack age, Initial MC DistributionTypical kiln package
change in MC, 128 board sample, (kiln study Jan 2006)
How D oes Drying Impact the Distribution?Typical kiln package
change in MC, 128 board sample, (kiln study Jan 2006)
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How D oes In-Kiln Moisture Measurement Reduce Degrade? A clo se
r lo ok at the final MC dis trib utio n e ffe ct of 3% error in
final MC
How D oes In-Kiln Moisture Measurement Reduce Degrade? A clo se
r lo ok at the final MC dis trib utio n e ffe ct of 3% error in
final MC128 board sample, (kiln study Jan 2006)
Controlled to accurate final ave MC: 14% targetPercent
under-dried (over 19% FMC): 3.1% (4 of 128)Percent over-dried
(under 10% FMC): 7.0% (9 of 128)
Probable impac t if under-dried by 3%: 17% avePercent
under-dried (over 19% FMC): 19.5% (25 of 128)Percent over-dried
(under 10% FMC): 0
Probable impac t if over-dried by 3%: 11% avePercent under-dried
(over 19% FMC): 0Percent over-dried (under 10% FMC): 42.2% (54 of
128)
What About Improved Throughput and Reduced Energy Costs?
Frequently see 20% or more reduction in the length of the kiln
cycle becauseunnecessary over-drying is avoided
Allows you to move the next load in sooner improved
throughput
Saves energy due to reduced k iln residence time
After you reach the optimum final MC, if you dont s top drying
you are paying for energy just to cause more degrade!
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How Does In-Kiln Moisture Sensing Work?
1 . Based on d ie lectr ic p ropertiesa . E le ctric al pr op
ertie s of wo od
i. Conductance (ab ility to transmit curren t)i i. Capacitance
(abil ity to s tore energy)
(1) Capacitance measure is the key input(a) Capacitance is
driven by dipoles(b) Water is the major d ipole in wood
What is a Capacitor?
What Impacts the Capacitance?
1. C= A/d2. A=area of the plates3 . D =d is ta nc e b etw ee n
th e p la te s4. =dielectric constant of material between the
plates
a . Typical d ie lect ric constan tsi. air is about 1ii. dry
wood is about 4iii. wa te r is a bou t 80
5. Presence of water drives the capacitance signal6. High
correlation between MC and capacitance
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Sensor Plates Removed from Stack
Top Sensor Plate Inserted in Stack
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Both Sensor Plates Attached
Capacitor Formed In-Kiln Stack
P la ce tw o sta in le ss s te el Pla te s in s ta ck Typic ally
ab ou t 40 inc he s ap art P la tes a re typically about 5-6 square
feet L um b er be tw ee n th e p la te s= die le ctr ic Typically
up to 8 measurement poin ts in one k iln
What Variables Impact the Readings?
Best reference is Torgovnikov, Dielectric Properties of W ood
and W ood-Based Materials
S ign ific ant effe cts- Species and density
W e have separate HW and SW models- Tem perature
Less of an impact under FSP
Lesser Effects- H um idity in the kiln- W ater pockets- Ratio of
la tewood vs earlywood- Even grain direction (not an issue for uswe
are always oriented in the same
direction)
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Hardwood vs Softwood Dielectric Constant (Torgovnikov, Table
5.1).
Dielectric constant as function of temperature and moisture
content (Torgovnikov, Append ix 2) .
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In-Kiln Moisture Measurement
1. Not very accurate early in run (above FSP) due to:a. R am p
up to tem pe ratureb. Ramp up to W BSPc. High moisture
i. n ot a p er fe ct ca pa cito r, ve ry lo ss yii. lots of
running water, free ions, conductive paths, etc .
2. Accuracy improves dramatically from FSP on downa. Conform to
more idea l mode of a capac itor b. Very stable relationship
between CAP and MC in this temperature and
humidity range
Data from in-kiln moisture system (8 channels: Ave MC in
red)
What Kind of Results are Expected?
Comparison of SCS Final MC vs Oven Dry W eight5 kiln runs, 128
board sam ples, (kiln study Jan 2006)
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Questions
If you didnt get all your questions answered here, you can
contact m e at:[email protected]
mailto:[email protected]:[email protected]
