Modelling air-drying of wooden poles Jarl-Gunnar Salin 1 Jarl Gunnar - - PowerPoint PPT Presentation

Modelling air-drying of wooden poles

Jarl-Gunnar Salin1 Jarl Gunnar Salin Peder Gjerdrum2

1Romensvägen 12 A, Esbo, Finland jarlgunnar.salin@welho.com 2The Norwegian Forest and Landscape Institute Aas Norway

The Norwegian Forest and Landscape Institute, Aas, Norway peder.gjerdrum@skogoglandskap.no

Subject of investigation: Preserved wooden poles used for power and tele- used for power and tele communication lines, etc. Before the creosote preservation these poles preservation these poles have to be dried to a MC below the FSP every below the FSP every- where in the pole.

Air-drying Air drying

In the present case air-drying outdoors is used. This method has both benefits and drawbacks. One drawback is One drawback is the long drying ti d d time and a good way to determine the drying end point is thus p important.

End point determination End point determination

• Sampling possible only from the pole ends, but

p g p y p , these are not reliable and a sample shortens the pole pole.

• Resistance meters are difficult to use as the

sapwood/heartwood borderline depth is not accurately known. y

• Advanced methods like X-ray, CT-scanning etc.

are too expensive in this small scale operation are too expensive in this small scale operation.

• One possibility is to use simulation models and

this alternative has now been investigated.

Simulation model Simulation model

The model consists of two parts: 1 A model for moisture migration in a cylindrical

• 1. A model for moisture migration in a cylindrical
• solid. Sapwood and heartwood have to be

considered as two different materials considered as two different materials.

• 2. A model for the interaction with the surround-

ing climate. The climate is defined by data from a nearby weather station (temperature, from a nearby weather station (temperature, RH, wind speed, rain).

Internal moisture migration Internal moisture migration

35% MC 130% MC

Internal moisture migration Internal moisture migration

⎞ ⎛ ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ ∂ ∂ ∂ ∂ = ∂ ∂ r u Dr r r t u 1

Fick’s equation in cylindrical coordinates

⎠ ⎝ ∂ ∂ ∂ r r r t

MC is not an adequate potential for describing MC is not an adequate potential for describing flow across the heartwood/sapwood border. The equilibrium MC pairs in these two materials have equilibrium MC-pairs in these two materials have to be determined in order to define a replacing potential potential.

Heartwood and sapwood mutual equilibrium MC values.

38 40

%

34 36 38

MC, %

30 32 34

artwood

26 28 30

Hea

26 25 50 75 100 125 150

Sapwood MC % Sapwood MC, %

Internal model part Internal model part

Moisture migration potential used in the model: 1 In sapwood: MC (as normally)

• 1. In sapwood: MC (as normally)
• 2. In heartwood: The MC in sapwood that is in

ilib i i h h l MC i h d equilibrium with the actual MC in heartwood (linear relationship assumed). Th fi l i t l d l i d t d i The final internal model is an updated version

• f an old model for kiln drying of logs and

contains thus no further adjustable parameters.

External model part External model part

• Daily average climate data are obtained from a

y g nearby weather station (temperature, RH, wind speed and rain) speed and rain).

• The main problem is to determine the relation

b h l i l i d d d h between the meteorological wind speed and the external heat (and mass) transfer coefficient, h, in the stack of poles.

Attempt:

67

h

Attempt: dj t bl t i d d

67 ,

w a h ⋅ =

a = adjustable parameter, w = wind speed

Tuning of the model Tuning of the model

• The model contains only one adjustable para-

meter, i.e. the factor a in the wind speed equation

• The factor was determined by weighing 31 poles

The factor was determined by weighing 31 poles in a test stack during one summer period. In this a the MC de elopment co ld be follo ed and way the MC development could be followed and compared to model predictions for different a- values Test stack

Test poles Test poles

Pole Diameter Butt Colour Stack length, m class diameter, mm code layer 12 12 Medium S 261 366 Yellow G Top U d 12 11 Stout Medium 366 305 Green Red Under Under 9 Light 239 Blue Under

Results Results

• In the first analysis the influence of rain was
• neglected. The rain is assumed to flow off the

poles without absorption. p p

• It turned out that the uppermost pole layer has a

27% higher al e than lo er la ers This is ~27% higher a-value than lower layers. This is probably due to the influence of sunshine and a higher air velocity above the top layer. Thus the top layer and the lower layers are simulated p y y separately.

• The results are presented for the different classes:
• The results are presented for the different classes:

Simulation results best fit Simulation results – best fit

Blue class Blue class

100 120

%

M d l MC 80 100

ntent, %

Model MC

• Obs. MC

40 60

ure con

20 40

Moistu

2008- 02 22 2008- 04 12 2008- 06 01 2008- 07 21 2008- 09 09 2008- 10 29 2008- 12 18 02-22 04-12 06-01 07-21 09-09 10-29 12-18

Simulation results worst fit Simulation results – worst fit

Yello class Yellow class

100 120

%

80 100

ntent, %

Model MC

• Obs. MC

40 60

ure con

20 40

Moistu

2008- 02 22 2008- 04 12 2008- 06 01 2008- 07 21 2008- 09 09 2008- 10 29 2008- 12 18 02-22 04-12 06-01 07-21 09-09 10-29 12-18

Further analysis Further analysis

• In the next step it was assumed that a certain

p amount of the rain hitting a pole is absorbed and has to be evaporated in the drying process. This p y g p model has thus two adjustable parameters. Further a 24 hour sine-variation was super- p imposed on the daily average temperature.

• Again it turned out that the top layer differs from
• Again it turned out that the top layer differs from

the rest as a higher amount of rain is absorbed. I th d l t i th l t i hi i

• In the model tuning process the last weighing in

December with already frozen poles was now i l d d included.

New simulation best fit New simulation – best fit

Green class

120

%

80 100

ntent,

40 60

ure con

20 40

Moistu

2008- 02-22 2008- 04-12 2008- 06-01 2008- 07-21 2008- 09-09 2008- 10-29 2008- 12-18

M

02 22 04 12 06 01 07 21 09 09 10 29 12 18

New simulation worst fit New simulation – worst fit

Yellow class

100 120

%

80 100

• ntent,

40 60

ure co

20

Moist

2008- 02-22 2008- 04-12 2008- 06-01 2008- 07-21 2008- 09-09 2008- 10-29 2008- 12-18

Conclusions Conclusions

• The model seems to capture the main features of

the pole drying process, despite only one (or two) p y g p , p y ( ) adjustable parameters. It is thus believed that the model can be used as an additional valuable tool model can be used as an additional valuable tool in the determination of the drying end point.

• The factory is interested in the MC development

in the innermost part of the sapwood, i.e. the p p point when all free water has been removed.

Example of model use Example of model use

120 140

Sapwood max MC Butt Sapwood max MC 1/4

100

ent, %

Sapwood max MC 1/4 Sapwood max MC Half Sapwood max MC 3/4

60 80

re conte

Sapwood max MC Top Pole average MC

40 60

Moistur

20

M

2008-03-25 2008-05-25 2008-07-25 2008-09-24 2008-11-24