Why You Should Presort Your Lumber Before Kiln Drying
Gavin Wallace
Isoscan
Kiln drying of softwoods
The aim in kiln drying is to dry each and every piece of timber to the same (known) moisture content:
without moisture gradient across the pieceswithout internal stresses developing in the piecesas quickly (and cheaply) as possible
DryTech 2008
Isoscan
Sweden – typically slow, low temperature108 hrs and 75 oC
Fig. 8a: variation of kiln-dried MC% with green MC
y = -0.0165x + 14.756R2 = 0.096
02468
1012141618
0 20 40 60 80 100 120 140
green MC%
kiln
-drie
d M
C%
Fig. 10: sorted kiln-dried moisture content distribution
0%
5%
10%
15%
20%
25%
5 10 15 20 25 30 35kiln dry MC%
%di
strib
utio
n of
boa
rds
<thres
>=thres
28x127mm sprucekiln-dry MC not much related to green MC
Overall mean MC = 13.95%Std Dev = 1.26%
=> boards remained in equilibrium when drying
Isoscan
A’Asia: boards are roasted:
• 50 mm thick structure grade lumber is dried at about 140oC, for periods around 12 hours
• Need fewer kilns, higher throughput• Board MC’s do not remain in equilibrium in kiln
• Very difficult to get a uniform MC at end of kiln drying
DryTech 2008
Isoscan
Why is this so hard?
Every piece of wood to be dried is different:different initial moisture content different grain orientationdifferent permeability
This means that when we put them all together in a kiln and dry them under the same conditions we can end up with an equally diverse range of final moisture contents, induced stresses and consequent loss of value of the final product.
Isoscan
The solution:
1. What is needed is a means of assigning each piece of timber a “driability” rating i.e. the optimum time and conditions required to dry that piece of timber to its required final moisture content without causing any loss of value
2. Then grouping pieces with reasonably close driability ratings together
3. Putting them in a kiln for the time and under the conditions best suited to that group.
DryTech 2008
Isoscan
1. It has been found that the green density of a piece of timber is a good indicator of the “dryability” of the piece
Gre
en d
ensi
ty
Woo
d
Woo
d
Wat
er
Wat
er2. These two pieces have the same green density but different moisture contents and basic densities.
Woo
d
Woo
d
3. After drying the wood density remains different but the final moisture content is similar.
Isoscan
MoistureContent Sorting
Fig. 6: variation of kiln dry MC w ith green MC
y = 0.0341x + 8.1973R2 = 0.1836
0
5
1015
20
25
0 50 100 150 200
green MC%
kiln
dry
MC
%
Most people would think that drying boards with widely varying MCs will result in big variations in final MCs.
This can be very incorrect
2006 trial: 100 juvenile southern yellow pine boards – very poor correlations
Fig. 7: variation of kiln dry MC w ith w ood SG
y = -15.176x + 19.434
R2 = 0.1822
0
5
10
15
20
25
0.350 0.400 0.450 0.500 0.550 0.600 0.650 0.700
SG
kiln
dry
MC
%
DryTech 2008
Isoscan
Reasons for this:
1. MC is inversely proportional to basic wood density
2. it is much easier to drive out water from lower density wood
3. So, low MC+highSG wood dries at same rate as high MC+low SG wood
Fig.8: variation of green MC w ith SG of boards
y = -382.88x + 296.5R2 = 0.736
0
50
100
150
200
0.350 0.400 0.450 0.500 0.550 0.600 0.650 0.700
SG
gree
n M
C%
Note: basic wood density = SG x density of water e.g. 0.5 x 1000 = 500 kg/m3
Fig. 9: kg of water removed per kg of wood with equal drying
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
1.60
1.80
300 350 400 450 500 550 600 650 700
basic w ood density kg/m3
kg o
f wat
er
Isoscan
In this extreme example, MC and SG combine to give very little variation in green density
Fig. 2: southern yellow pine, 100 boards, Oct. 2006
0%
10%
20%
30%
40%
50%
400 500 600 700 800 900 1000 1100 1200
green density, kg/m 3
%di
strib
utio
n of
boa
rds
southern yellow pine, 100 boards, Oct. 2006
0%5%
10%15%20%25%30%
0.25 0.35 0.45 0.55 0.65 0.75
SG
%di
strib
utio
n of
boa
rds
southern yellow pine, 100 boards, Oct. 2006
0%
5%
10%
15%
20%
0 50 100 150 200
MC%
%di
strib
utio
n of
boa
rds
+ =
DryTech 2008
Isoscan
...and it is this small variation in green density that results in a small variation in dried MC, despite large
variations in basic wood density and MC
Fi g. 3 : r e sul t s of k i l n dr y i ng, 10 0 S YP boa r ds
0
5
10
15
20
25
600 700 800 900 1000 1100 1200
green density
kiln
dry
MC
%
Isoscan
Conclusion: green density is the equivalent of dryability, and
dryability is a measure of the difficulty in drying wood• green density is a good measure of moisture content• and has the advantage that it takes into account the increased
difficulty of removing water from timber of higher basic densitythan lower basic wood density i.e. permeability of water in timber is proportional to its basic wood density
SYP is a hard pine, and most softwood species have much larger variations in green
densities, and sorting by green density for kiln drying of softwoods is very beneficial
DryTech 2008
Isoscan
% distribution
0
5
10
15
20
25
30
400 500 600 700 800 900 1000 1100 1200
density kg/m^3
%white fir
hemlock
doug. fir
loblolly/slashPp. radiata
Isoscan
CHH Fortress, production run 25.9.07
0%
5%
10%
15%
20%
25%
30%
35%
40%
400 600 800 1000 1200 1400
green density, kg/m 3
% d
istr
ibut
ion
20, 25mm, 725 boards
40mm, 495 boards
50mm, 1435 boards
75mm, 17 boards
Intuitive selection - green density distribution may be affected by board dimensions
DryTech 2008
Isoscan
A trial was carried out to determine the
benefits of using green density as a predictor of drying rate.
Measurements gave the
frequency distribution of green density
Frequency graph of Green Density
0
5
10
15
20
25
30
0 200 400 600 800 1000 1200
Green Density (kg/m3)
Freq
uenc
y
Isoscan
Rate of drying by green density group
0
200
400
600
800
1000
1200
0 50 100 150 200
Drying time
Mea
sued
den
sity
LowMediumHigh
The pieces were selected into 3 equal size groups and measured density was plotted against drying time.
DryTech 2008
Isoscan
We then plotted
reduction in
moisture content against drying
time for:•all-in
•<650 kg/m3
•>650 kg/m3
Reduction of Moisture Content % with time
0
20
40
60
80
100
120
140
160
180
0 50 100 150
Drying time
Moi
stur
e co
nten
t %
Green Density< 650Green Density >650All in
Isoscan
Relative drying time of sorts according to Green Density
0
20
40
60
80
100
120
% D
ryin
g tim
es All in<650 kg/m3> 650 kg/m3Wgted average of sorts
Std Dev of moisture contents when batches are at average 12% MC
0.0
2.0
4.0
6.0
8.0
10.0
12.0
14.0
16.0
Std
dev
of m
oist
ure
cont
ent
All in<650 kg/m3> 650 kg/m3
1. Overall drying time is shorter
2. Variability is greatly reduced
DryTech 2008
Isoscan
Trial: 25mm p. radiata boards (not sorted)
Plot of green density versus moisture content after kiln drying
0.0
10.0
20.0
30.0
40.0
50.0
500.0 600.0 700.0 800.0 900.0 1000.0 1100.0 1200.0
Green density kg / m3
Moi
stur
e co
nten
t %
GD < 650 650<GD<800 800<GD<950
GD > 950
6.423.9
all in: 11.73.21.73.2SD %
all in: 21.019.415.213.6mean MC %
Isoscan
Trial: 50mm p. radiata boards (structural)
all in>850<850
3.63.21.5SD %11.013.64.37mean MC %
Graph of green density vs moisture content after kiln drying
0.00
5.00
10.00
15.00
20.00
25.00
30.00
0.0 500.0 1000.0 1500.0
Green density kg/m3
Fina
l moi
stur
e co
nten
t
DryTech 2008
Isoscan
Trial: 50mm p. radiata boards• In A’Asia, 50 mm thick structure grade lumber is dried
at about 140oC, for periods around 12 hours• In the lab. test shown previously, boards were
overdried – target was 12%• Nevertheless, trial demonstrated:
– Drying time of 7¾ hours was about right for higher density group
– Only 6 hours was necessary for lighter group, a reduction of 1¾ hours
– The combined weighted drying time is 6.6 hours, an overall saving of 15%
• Distortion, especially twist, in pine is approximately linear with reduction in MC below 30%; a green density sort significantly reduces loss of timber
Isoscan
Trials: 6m boards 50x100mmFig. 2: variation of green MC w ith green density
y = 0.1787x - 59.213R2 = 0.7985
0
20
40
60
80
100
120
140
160
400 500 600 700 800 900 1000 1100 1200
green density kg/m 3
MC
%
Fig. 8: variation of kiln-dried MC% w ith green density
0
5
10
15
20
25
30
35
400 500 600 700 800 900 1000 1100 1200
green density kg/m 3
kiln
-drie
d M
C%
Boards dried in same charge
[target 14%]
NB: green density is a reasonable indicator of MC
DryTech 2008
Isoscan
Trials: 6m boards 50x100mm
Fig. 10: sorted kiln-dried moisture content distribution
0%
2%
4%
6%
8%
10%
12%
14%
16%
18%
20%
5 10 15 20 25 30 35
kiln dry MC%
%di
strib
utio
n of
boa
rds
<thres
>=thres
all-in
Fig. 11: sorted kiln-dried moisture content vs green density
5
10
15
20
25
30
35
400 500 600 700 800 900 1000 1100 1200
green density kg/m 3
kiln
drie
d M
C%
<thres
>=thres
Two sort about 740 kg/m3
allhighlow
5.25.51.7SD
16.720.013.5mean MC %
Isoscan
Green density sort power savingsReduces overall kiln drying time and power usage:
High green density batch is dried in same time as unsorted chargeLow green density batch can have drying time reduced by 40% or moreLow density batches can be 50% of productionActual 3-sort example (US): saving 19.4%
25%0
90
50%-2070
25%-3060
Production amountChange in drying hoursNew drying hours
himedlowdensity
2-sort more usual – 15% saving typical
DryTech 2008
Isoscan
Other benefits from sorting can exceed power savings:Reduces spread in final moisture content:• Copes with seasonal variations in moisture• Reduces wets – enhances timber value • Accurate prediction of properties
MOE, pinus radiata
y = 0.9939x + 0.9676R2 = 0.9488
3.5
5.5
7.5
9.5
11.5
13.5
15.5
17.5
19.5
3.5 5.5 7.5 9.5 11.5 13.5 15.5 17.5
green MOE, GPa
kiln
dry
(tar
get 1
8%) M
OE,
GPa
kiln MC<20%
kiln MC>20%
Linear (kiln MC<20%)
Isoscan
Additional sort benefits• Reduces over-dries
Reduction in distortion due to over-dryingReduced trim lossPromotes recovery of boards to higher grades Tighter control over shrinkage from drying:
reduction in ‘skip’ - hit or miss when planingimproved timber recovery from tighter cutting tolerances
Tighter control of board properties
DryTech 2008
Isoscan
Tighter control on final moisture content provides better tolerance on board
dimensions
Fig. 10: SYP shrinkage against final kiln MC%
y = -0.1678x + 5.2477R2 = 0.232
0.0
2.0
4.0
6.0
8.0
10.0
12.0
0 5 10 15 20 25
kiln MC%
%sh
rinka
ge
w idth
Linear (w idth)
MC distribution about 14% MC target, 100 boards
0
2
4
6
8
10
12
0 5 10 15 20 25 30
MC%
#boa
rds
4% SD
2% SD
thickness distribution, 47mm at 14% MC target, 100 boards
0
2
4
6
8
10
12
45.5 46 46.5 47 47.5 48 48.5
thickness, mm
#boa
rds
4% SD
2% SD
Isoscan
With that better dimensional
control, you can cut to finer
tolerances, and therefore increase
recovery
An 0.6mm reduction in green cut
dimensions of dried 47x147mm board is a 1.6% improvement in
wood recovery
cumulative distribution, 46mm planed, 100 boards
0
10
20
30
40
50
60
70
80
90
100
45 46 47 48 49
thickness, mm
#boa
rds
4% SD
2% SD
2%, -0.6mm
saving in initial green cut of 0.6mm
targetthickness
planed
DryTech 2008
Isoscan
Measurement of green density:
Gamma ray transmission
Load cells
Impact grader
These are usually combined with acoustics to also measure MOE/MSG of boards
Isoscan
Gamma ray transmission: LDS 200Uses very low energy gamma rays emitted by sources placed beneath the timber chain
Gamma rays are measured continuously by detectors placed above the timber chain
When a board passes between the source and detector some of the gamma rays are absorbed by the timber, so the count rate at the detector goes down
The amount of decrease is proportional to the green density of the timber and the thickness of the timber
Board thickness is measured using laser systems
DryTech 2008
Isoscan
LDS 200
Isoscan
Typical LDS200
calibration, accuracy
<4.5%, limited by
mix of timber in board
(repeatability <1.5%)
LDS 200 installation calibration, pinus radiata
y = 0.9988xR2 = 0.9643
400
500
600
700
800
900
1000
1100
1200
400 500 600 700 800 900 1000 1100 1200
weighed density, kg/m3
LD
S200
gau
ge fi
tted
dens
ity
DryTech 2008
Isoscan
LDS 200 - specifications
Up to 210 pieces per minute at common lug spacingsContinually self calibratingNon contactDoes frozen woodSmall footprintQuick installationManages power interruptions, safety requirements; self contained20 systems installed
Isoscan
Load cell systems•Weigh board while moving
Can have slower speeds
•Contact system
•Poor signal-to-noise
Early systems had poor accuracy
•Several systems on offer, normally aimed at MOE measurements
Dynalyse Precigrader
DryTech 2008
Isoscan
Load cell system – A Grader
Isoscan
Load cell system – Lakeland Steel
DryTech 2008
Isoscan
Impact grader – TIG100
New, simpler to install, smaller footprintGreater sensitivity to mass3 systems installed – IsoscanDetermines both density and MOE
Isoscan
TIG 100 – measure mass by displacement when transversely-moving board is pushed on end
• Force is applied horizontally• Horizontal displacement measured• Measures friction
DryTech 2008
Isoscan
Static CalibrationKiwiLumber 20080408 - High : m(est) vs Weight (error = 0.60Kg, 2.9%)
y = 0.9993xR2 = 0.9938
0
5
10
15
20
25
30
35
40
0 5 10 15 20 25 30 35 40
Weightm
(est
)
Static CalibrationKiwiLumber 20080408 - Med : m(est) vs Weight (error = 0.56Kg, 2.7%)
y = 0.9994xR2 = 0.9946
0
5
10
15
20
25
30
35
40
0 5 10 15 20 25 30 35 40
Weight
m(e
st)
Mass measurement is
novel
TIG 100
Isoscan
TIG 100 – does both density and green velocity to
enable estimation of dry MOE
Dynamic Calibration 4.6% SD
y = 0.9905xR2 = 0.8907
0
200
400
600
800
1000
1200
1400
0 200 400 600 800 1000 1200 1400
weighed density kg/m3
TIG
100
dens
ity k
g/m
3
green velocity, 3.55% SD
y = 0.9806xR2 = 0.895
2000
2500
3000
3500
4000
4500
2000 2500 3000 3500 4000 4500
Hitman velocity m/s
TIG
100
velo
city
m/s
DryTech 2008
Isoscan
If proof testing, it is vital that this is done at the
correct MC
comparison of proof-tested MOE w ith TIG100 prediction
y = 0.9041xR2 = 0.4037
0
2
4
6
8
10
12
14
16
18
0 2 4 6 8 10 12 14 16 18
proof test MOE, GPa
TIG
100
pred
icte
d M
OE,
GPa
over dried boards - no density sort
0
2
4
6
8
10
12
14
16
500 600 700 800 900 1000 1100 1200 1300
green density kg/m3
kiln
-dry
MC
%
In this example, mean kiln-dried MC was 9%, and proof testing results were inflated by 10%.
Boards would be graded too high in MSG!
Better kiln drying control will minimise this risk.
Any questions?
Gavin Wallace
DryTech 2008
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