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