Drying Parawood with Superheated Steam

American Journal of Applied Sciences 4 (4): 215-219, 2007
ISSN 1546-9239
© 2007 Science Publications

Drying Parawood with Superheated Steam

Surachai Bovornsethanan and Somchai Wongwises
Department of Mechanical Engineering, King Mongkut’s University of Technology Thonburi (KMUTT)
Bangmod, Bangkok 10160, Thailand

Abstract: A superheated steam dryer (SSD) was used to dry the parawood. The average moisture
content and temperatures in the dryer vs. elapsed drying time were experimentally investigated. It was
found that the superheated steam could increase the potential of the conventional hot air drying. By
using a SSD at the superheated steam temperature of 110 °C with volumetric flow rate of 125 m3 min¯1
for parawood’s thickness of about 30 mm, the moisture content of the parawood could be reduced from
40% (d.b.) to about 9.75% (d.b.) within 35 hrs while the conventional hot air dryer required the drying
time of approximately 8-16 days. The physical structure of the parawood was also investigated by a
prong test. The quality of dried parawood was not found to be different from those dried with the hot
air drying method.

Keywords: Superheated steam drying, parawood

INTRODUCTION
The convective heat and mass transfer coefficients were

determined from the test conducted with sugar maple

Over the years, the application of superheated
sapwood in a laboratory vacuum kiln. The average air
steam for drying have been studied by a number of
velocity was 2.5 m s ?1 and the dry-bulb temperature
researchers[1-11]. However there are few studies varied between 60 and 66°C. The ambient pressure
applying the superheated steam with the wood drying.
varied from 15 to 11 kPa. The simulation results
To the best knowledge of authors, up to now, there has
elucidated that heat and mass transfer coefficients were
been only two works carried out by Pang and dependent on the moisture content.
Pearson[10] and Defo et al.[11], dealing with the
As described above, information on drying wood
application of superheated steam drying for wood.
with superheated steam is still limited. In order to have

Pang and Pearson[10] conducted an experiment to
fully understanding of the wood drying process using
dry radiate pine timber by using superheated steam
superheated steam, more experimental studies on this
under vacuum and at various pressures. Experiments
issue should be done. In the present study, the main
consisted of two parts. In the first one, softwood timber
concern is to experimentally study the use of
was dried in a superheated steam kiln. Drying rates,
superheated steam in drying parawood. The drying
steam temperature across the stack and wood characteristics in terms of average moisture content vs.
temperature were measured. In the second one, elapsed drying time and temperatures in the dryer vs.
experimental studies were performed to investigate the
elapsed drying time were investigated. The effects of
applications of the superheated steam drying at ultra-
superheated steam temperature, flow delivery and wood
high temperatures. The experimental results showed
thickness on the drying kinetics were investigated and
that the ultra-high temperature drying saved more
compared. The quality of the product after drying was
energy and could reduce the drying time by a factor of
discussed.
5 to 10. The pressurized steam conditioning was proven

to be a method to decrease twist of the dried timer.
MATERIALS AND METHODS
Defo
et al.[11] developed a two-dimensional
mathematical model for vacuum-contact drying of
The experiment was conducted at the Department
wood using superheated steam. The heat and mass
of Mechanical Engineering, KMUTT. Schematic
transfer equations were based on the water potential
diagram of the experimental apparatus is shown in
concept whereas the pressure equation was formulated
Fig. 1. The main principle is to blow superheated steam
considering unsteady-state mass conservation of dry air.
into the temperature-controlled drying chamber.
Corresponding Author:
Surachai Bovornsethanan, Department of Mechanical Engineering, King Mongkut’s University of
Technology Thonburi (KMUTT), Bangmod, Bangkok 10160, Thailand
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Am. J. Applied Sci., 4 (4): 215-219, 2007

Table 1: Detailed information of the dryer and experimental

conditions
Drying chamber size
1.5 m × 1.5 m × 1.5 m
Boiler capacity
Maximum steam production 9 kg hr¯1,

Maximum pressure 9 bar
Effective volume in drying chamber Maximum capacity 1.2 m3
Size of parawood
25 mm × 75 mm × 1000 mm
30
mm
× 75 mm × 1000 mm
50
mm
× 75 mm × 1000 mm
Electrical heater
Maximum power 5 kW.
Drying temperature
110° C, 120 ° C
Motor for blower
1 Hp. 220 volt.
Volumetric flow rate
55 m3 min¯1, 90 m3 min¯1,
induced by blower
125 m3 min¯1

4
3

1
1 Controlled temperature
5
2 Wood bed temperature
7
3 Supplied steam temperature
Fig. 3: Temperature measuring points
6

Detailed information of the dryer is shown in Table 1.
A blower was used for the steam propulsion. Steam
9
sucked in by the blower was fanned through layers of
2
wood, which lowered its temperature. This steam was
mixed with the superheated steam generated from the
boiler and passed through the reheater to make its
10
8
temperature higher than the controlled temperature.
Finally, the mixed steam was blown back into the

drying chamber. Schematic diagram of steam flow
1.Drying chamber
6. Wood supporting frame
directions is shown in Fig. 2. The drying procedure
2.Hood

7. Balance
3.Blower
8. Boiler
continued until the wood’s final moisture content is
4.Blower motor
9. Steam pipe
below 12%. Positions of temperature readings are
5.Exhaust pipe
10. Parawood
shown in Fig. 3.
Fig.
1: Schematic diagram of the experimental
apparatus
RESULTS AND DISCUSSION


Effect of temperature of superheated steam: Figure 4
shows a relationship between wood’s moisture content
and elapsed drying time, at the controlled drying
temperatures of 120°C and 110°C. At the beginning,
superheated steam blown into the drying chamber
which had lower temperature had caused condensation -
some occurred on the wood and some occurred around
the chamber wall. This was evident from the higher
weight of wood which signified the higher moisture
content. After a while, the moisture content of wood
decreased back to the same level as the initial value and
continued decreasing gradually. It was also evident that
the drying rate increased with increasing drying
temperature. At the temperature of 120°C, it took about
25 hrs to reduce moisture content from 38% to 9.7%.

On the contrary, at the temperature of 110°C, it took up

to 35 hrs to reduce the moisture content from 40% to
Fig. 2: Schematic diagram of the steam flow direction
10%. However, more cracks were found after drying at

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Am. J. Applied Sci., 4 (4): 215-219, 2007

45
.
) 40
B
35
30
Temperature 120 C
t ( % D. 25
Temperature 110 C
t
en
n 20
o
15
re C
s
tu 10
Moi 5
0
0
5
10
15
20
25
30
35
40
Elapsed Drying Time (hr.)

Fig. 4: Plot of average moisture content against time

at various temperatures, (Flow rate=125 Fig. 7: Plot of temperature against time for wood
m3/min, thickness=30 mm)
thickness of 25 mm at flow rate of 90 m3/min


80.0
70.0
B
)

60.0
D.
Flow rate 55 m3/min
t

(
%
50.0
Flow rate 90 m3/min
C
o
n
t
en
Flow rate 125 m3/min
40.0
o
i
s
t
u
re
30.0
M
20.0
10.0
0.0
0
5
10
15
20
25
30
35
40
Elapsed Drying Time (hr.)

Fig. 5: Plot of average moisture content against time
at various flow rates, (Temperature=110 °C,
thickness=30 mm)


Fig. 8: Plot of temperature against time for wood
thickness of 25 mm at flow rate of 125 m3/min

chamber, whereas at the temperature of 110°C, the
atmospheric pressure would cause about 70% of
relative humidity.

Effect of flow delivery from the blower: The
temperature of 110°C was thus selected for the next
experiment. The study about effect of volumetric flow
rate was done by varying speed of motor driving
blower. This enabled the volumetric flow rate to be
varied from 55 – 125 m3 min¯1. Results of the
experiment shown in Fig. 5 revealed that, at the blower

delivery of 90 m3 min¯1, the moisture content decreased
Fig. 6: Plot of temperature against time for wood
quicker than at the flow rate of 55 m3 min¯1 and 125 m3
thickness of 25 mm at flow rate of 55 m3/min
min¯1. Figures 6, 7 and 8 also showed that the time

constant value was 2 hrs at the flow rate of 55 m3 min¯1,
higher temperature. This may be because at the 13 hrs at the flow rate of 90 m3 min¯1 and 20 hrs at the
temperature of 120°C, the atmospheric pressure would
flow rate of 125 m3 min¯1. Considered the changing
cause about 50% of relative humidity in the drying
temperature in the drying chamber, it was found that at

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Am. J. Applied Sci., 4 (4): 215-219, 2007



Fig. 9: Plot of moisture content against time at Fig.
11:
Comparison between superheated steam
controlled temperature of 110 °C, flow rate of
drying and hot air drying
55 m3/min for various wood thicknesses


Quality of wood after drying: Besides the crack and
crook, color and hardness of dried parawood were also
considered. Generally, color of parawood might be
changed by fungus and oxidation. The steam drying
could reduce oxidation and keep color of dried wood
similar to fresh wood. Results from Prong Test as
shown in Fig. 10 revealed no bend in the prong, which
means no residual stress from drying.

Comparison between superheated steam drying and
hot air drying: Figure 11 shows a relationship between
average moisture content and elapsed drying time of
superheated steam drying at the temperature of 110 °C,
flow rate of 125 m3 min¯1, compared with hot air drying

at the air temperature of 40 °C, relative humidity of

70%. A significant difference can be clearly seen from
Fig. 10: Photograph of parawood after the prong test
this figure. The hot air temperature gradually increased

while the wood’s moisture content decreased.
higher time constant, wood temperature would be low,
Therefore, longer time was needed in this drying
which resulted in higher quality of dried wood. Hence,
process.
it could be seen that the volumetric flow rate in drying

would better be 125 m3 min¯1.
CONCLUSION


Effect of wood’s thickness: As shown in Fig. 9, at the
It could be concluded from results of the
blower delivery of 125 m3 min¯1, it was found that the
experiment that the most suitable steam flow rate in the
wood of 25mm thickness yielded maximum moisture
drying chamber should not be less than 125 m3 min¯1
content reduction – from 64% to 10% within 26 hrs.
while the most suitable thickness of wood should not be
The wood of 30mm and 50mm thickness yielded
less than 30 mm. The use of superheated steam at the
similar reduction rate. In the wood of 30mm thickness,
temperature of 110°C in atmospheric pressure could
moisture content reduced from 40% to 10% within 31
maintain constant moisture content at about 50%
hrs, whereas in the wood of 50mm thickness, moisture
throughout the drying duration; hence, no stress was
content reduced from 52% to 10% within 36 hrs.
caused in the dried wood. The drying of parawood with
Concerning the physical structure of the dried wood,
superheated steam at the temperature of 110 °C could
the wood of 25mm thickness was found to be more
replace traditionally hot air drying method and could
damaged (bent, crooked and cracked) than the wood of
reduce drying duration from 7-10 days to only 35 hrs.
30-50mm thickness.
Moreover, the color of dried wood also remained

similar to the color of the freshly transformed wood due

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Am. J. Applied Sci., 4 (4): 215-219, 2007

to shorter drying duration and no occurrence of
oxidation.



ACKNOWLEDGEMENTS
6. Shibata, H., 2005. Comparison of drying rate

curves of porous solids in superheated steam to
The authors would like to express their
those in air. Drying Technol., 23: 1419-1434.
appreciation to the Thailand Research Fund (TRF) for
7. Tang, Z., S. Cenkowski and M. Izydorczyk, 2005.
providing financial support in this study. The authors
Thin-layer drying of spent grains in superheated
also express gratitude to Mr. Surawut Kaewkappetch,
steam. J. Food Eng., 67: 457-465.
Mr. Yutakit Maion and Mr. Somram Pannachit, for
8. McCall, J.M. and W.J.M. Douglas, 2006. Use of
their assistance in the experimental work.
superheated steam drying to increase strength and

bulk of papers produced from diverse commercial
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