Establishing a kiln drying schedule for Paulownia fortunei lumber ...

World of Sciences Journal; 2013-Apri l-Special Issue ISSN 2307-3071
Establishing a kiln drying schedule for Paulownia fortunei lumber with
thickness of 5 cm
Seyeid Mahmoud Miri Tari 1 * & Mehrab Madhoushi 2
1 Former M.Sc. Student, Dep. of Wood Engineering and Technology, Gorgan University of Agricultural
Sciences and Natural Resources, Golestan, Iran
2 Associate Prof., Dep. of Wood Engineering and Technology, Gorgan University of Agricultural
Sciences and Natural Resources, Golestan, Iran
*: Corresponding Author, Email: Miry.Mahmood@gmail.com
Abstract: In order to establish a local kiln drying schedule based on FPL recommendation for paulownia
wood (Paulownia fortunei) commercial sawn lumber with thickness of 5 cm, two standing trees were
randomly cud down from Shasta-Kola region in Iran. Final moisture content was considered equal to 8±2
for all kiln loads and totally three separate kiln loads were performed. Three different kiln drying
schedules were employed namely, T6E3, T6E4 and T7E4. The severity of warping (bowing, crooking and
twisting) and checks (surface and end checks) in the lumber were measured before and after the drying
process. Finally, quality control methods and also statistic analysis were used to choose the best schedule.
Results showed that drying with schedule T6E3 led to low warping amounts and more smooth final
moisture profile, compared to the other schedules. So, schedule T6E3 can be recommended as an optimum
program for drying of Paulownia lumber at commercial scale. From quality control viewpoint (Q.C), no
undesirable phenomenon was occurred and all defects were inside the acceptable levels, but in mild
drying schedule like T6E3 amount of warp variation after drying and distribution around mean line had
more suitable condition.
Keywords: Kiln Schedule, Paulownia Fortunei, Drying Defects, FPL Recommendation
1. Introduction
The industry of wood products plays a key role in the economy of various countries. In some
countries such as Canada, this industry uses about 1% of the total energy consumed, excluding the pulp
and paper industry (Dincer, 1998). On the other hand, the lumber drying is one of the most important
stages in the primary processing of wood which influence extensively the quality of the final product (Da
Silva et al, 2010). Green (freshly cut) wood may have moisture content as low as 30% to as high as 250%.
To increase the strength and rigidity of wood, as well as to protect it against biological damage, most of
the moisture must be removed (Baronas et al 2001). The most important reasons for drying of wood can
be mentioned as: increase of dimensional stability, improvement of mechanical properties, prevention
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World of Sciences Journal; 2013-Apri l-Special Issue ISSN 2307-3071
from rotting and fungal attacks, preparation for application of some treatments (gluing, penetration by
protective materials, fire retardant), improvement of dye ability and trimming, reducing timber
transportation costs and reducing weight of the manufactured artifacts (Wenger, 2006). Lumber drying is
one of the most time and energy consuming steps in processing wood products. The anatomical structure
of wood limits how rapidly water can move through and out of wood. Hence, rapid drying cause's defects
such as surface and internal checks, collapse, splits, and warp. Furthermore, the variability of wood
properties complicates the drying process. Each species has unique properties, and even within species,
variability in drying rate and sensitivity to drying defects impose limitations on the development of
standard drying procedures. The interactions of wood (species, dimensions), water, drying method and
stress during drying are very complex (Simpson, 1998). The main goals of wood drying programs may
include reduced energy consumption, increased drying rate, better quality, and finally reducing drying
costs (Korkut and Guller 2007).
In Iran, research on wood drying started by Madhoushi (1996) who studied the drying behavior of
Iranian beech (Fagus orienatalis). He could introduce a local kiln drying schedule (T5-D1) based on FPL
recommendations. Their method was used by means of other researchers and some schedules were
introduced for some other Iranian wood species. Table 1 summarize the most important studies and kiln
schedules in this regard.
Table 1. The most important studies on Iranian wood species and suggested kiln drying schedules
Researcher Date
Madhooshi
Tamjidi & Ebrahimi
Ashouri & Ebrahimi
Saadat
Madhooshi et al.
Korkut et al
Suggested
Schedule
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Nominal
Thickness (cm)
Species
1996 T5-D1 5 Beech
1998 T8-E4 5 Alder
1999 T4-D3 2.5 Oak
2000 T3-B1 5 Hornbeam
2006 T4-B2 5 Hornbeam
2007 Mild schedule 5 Red-bud Maple
Korkut and Guller 2007 Mild schedule 5
Rafiei & Ebrahimi
Tazakor Rrezaei &
Ebrahimi
Korkut et al.
Shahverdi et al.
European
Hophornbeam
2007 T4-B2 7.5 Hornbeam
2010 T6-C4 7.5 Beech
2010 Mild schedule 5 Rowan
2012 T5-D2 7 Poplar
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World of Sciences Journal; 2013-Apri l-Special Issue ISSN 2307-3071
Also, studies on kiln schedule of other wood species which are belonging to Mediterranean area have
been done in Turkey (Korkut et al, 2007, Korkut et al, 2010). This shows the importance of research on
drying behavior of wood in this area.
Nowadays, the necessity of conservation and preservation of native forest in Iran and also responding
to required resources for use in the wood-related industries has drawn attention to the plantation of fastgrowing
species. Paulownia is one of these important fast growing species which no schedule for properly
drying of it has ever been introduced in Iran. Many excellent characteristic of paulownia wood besides its
growing rate made it exceptional species for wood industries. In China and some of the other Asian
countries, paulownia wood is used for a variety of applications such as furniture, construction, musical
instrument, shipbuilding, aircraft, packing boxes, coffins, paper, plywood, cabinetmaking, and molding
(Flynn and Holder, 2001; Clad and Pommer, 1980). With considering all of these profits the aim of this
study was establishing a kiln drying schedule for plantation-grown Paulownia fortunei in Iran based on
FPL recommendations in order to continue the previous studies and try to complete the puzzle of kiln
dying schedule for Iranian wood species.
2. Material and Methods
2.1. Preparation of experimental samples
Freshly-cut logs of paulownia (P. fortunei) from two standing tress with approximately of 40–50 cm
diameter, belonging to the educational forest of Gorgan University from Shast-Kola in Iran were selected.
Samples were prepared commercially with a nominal thickness of 5 cm, a length of 150 cm and a width
of 10 cm. Then their end-sections were covered with oil paint immediately in order to prevent formation
of local cracks. A semi-automatic conventional kiln drying (a lab kiln) with 1m 3 capacity was used for
research.
To survey moisture reduction process related to time, in order to change schedule steps, 4 control
samples were set among lumbers in kiln drying. From each side of the control samples, test specimens of
humidity indication with 2.5 cm length were cut. The remaining planks of 75cm were considered as the
control samples. The control samples after cross section covering were weighed and put in the designated
place in the kiln stack.
2.2 Kiln schedules
Three different schedules were used for drying (Tables 2 to 4) based on FPL recommendation. At
first the most moderate schedule (T6-E3) performed due to high moisture content in lumbers. With
reduction of moisture content the severity of drying was increased and two other schedules (T6-E4 and T7-
E4) were performed respectively. The regime of change in dry and wet bulb temperatures is shown in Fig
2.
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World of Sciences Journal; 2013-Apri l-Special Issue ISSN 2307-3071
According to F.P.L proposed method, the beginning point was set based on the initial moisture
content (MC) of kiln load and the program continued until MC reached to 8 ± 2 %. In order to determine
the new condition of schedule at different steps of the work, the control samples, depending on kiln stack
moisture draining rate, were weighed at least once a day. The stack moisture was calculated and the
program’s steps were changed based on the average MC of the wettest half of the control samples
(Simpson, 1991).
Moisture
content (%)
Table2. T6-E3 Kiln Drying Schedule for P. fortunei Lumber with 5 cm Thickness.
Dry bulb
temperature
(°C)
Wet bulb
temperature
(°C)
Wet bulb
temperature
depression
(°C)
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Relative
Humidity (%)
MC > 60 49 46 3 85 18.4
60 49 45 4 79 16
50 49 43 6 68 13.3
40 49 39 10 52 9.8
35 49 30 19 23 5.1
25 54 26 28 18 2.1
Final
moisture
54 26 28 18 2.1
Moisture
content (%)
Table3. T6-E4 Kiln Drying Schedule for P. fortunei Lumber with 5 cm Thickness
Dry bulb
temperature
(°C)
Wet bulb
temperature
(°C)
Wet bulb
temperature
depression
(°C)
Relative
Humidity (%)
MC > 60 49 45 4 79 16
60 54 49 5 74 14.5
50 60 52 8 64 11.9
40 65 51 14 48 9
35 65 43 22 28 5.9
25 65 43 28 28 5.9
Final moisture 65 43 28 28 5.9
Equilibrium
moisture
Content (%)
Equilibrium
moisture
Content (%)
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World of Sciences Journal; 2013-Apri l-Special Issue ISSN 2307-3071
Moisture
content (%)
Table4. T7-E4 Kiln Drying Schedule for P. fortunei Lumber with 5 cm Thickness
Dry bulb
temperature
(°C)
Wet bulb
temperature
(°C)
Wet bulb
temperature
depression
(°C)
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Relative
Humidity (%)
MC > 60 54 50 4 80 16.3
60 60 55 5 76 14.9
50 65 57 8 68 12.7
40 71 57 14 52 9.6
35 71 49 22 33 6.6
25 76 43 28 24 5.2
Final
moisture
76 43 28 24 5.2
Equilibrium
moisture
Content (%)
The kiln stack was 60 cm wide. Furthermore, 2×2 cm 2 stickers from the same species were used in
this study. Air movement speed was also about 2 m.s -1 provided by internal fans, and air was horizontally
circulated in the kiln.
2.3 Drying Defects
Residual stresses intensity in dried lumber was determined using the prong response method. Prong
response test samples with dimensions equal to lumber ’ s full thickness and width, and a length of 20 mm
were cut from each of the dried control samples using a band saw. Then, remaining stresses were
calculated from the equation below (Fuller, 1995):
PR = ˊ
(1)
In this equation PR is prong response of test sample (mm -1 ), x is the distance between outer prong
edges before cutting (mm), ˊ is the distance between outer prong edges after cutting (mm), and l is the
length of each test sample’s prong (Shahverdi et al, 2012.)
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World of Sciences Journal; 2013-Apri l-Special Issue ISSN 2307-3071
(a): Test Sample Preparation (b): Shell and core test
Fig.1. Cutting method of specimens for measurement of Casehardening (a) and MC gradient (b) (Simpson, 1991).
To determine dried wood’s MC gradient, shell and core test was used. First, pieces with dimensions
equal to board’s thickness and width, and a length of 20 mm were cut from each of the dried control
samples. Then, further cuts are made into the shell and core portions, as shown in figure (1-b). The shell
and core are weighed separately and then oven dried so that the moisture content can be calculated
according to equation 2 (Simpson, 1991).
Moisture content in percent (%) = Original weight Ovendry weight/Ovendry weight × 100 (2)
The intensity of warping in dried woods of each kiln stack, including twist, bow and crook was
measured according to DIN-EN 1310 standard. Length and depth of checks measured before and after
drying in each program.
2.4 Quality control methods
To investigation drying defect intensity based on quality control methods, each row of lumbers were
assumed as a quotient and the amount of warp variation before and after drying were measured. After
that, upper and lower control level assessed. Distribution around mean line and being in acceptable level
are two main factors to choose the best programs. In order to show total defect occurred in each program,
C quantitative graph was used. Equation 3 to 5 shows how these data obtained (Ebahimi, 1991).
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World of Sciences Journal; 2013-Apri l-Special Issue ISSN 2307-3071
CL= (3)
UCL= + A2 (4)
LCL =- A2 (5)
Where, is average of means, UCL is Upper Control Level, LCL is Lower Control Level, A2 is
Constant Coefficient and is Average of Ranges.
As mentioned before, C quantitative graph that conform from Poisson distribution was used to show
total defect occurred in each program. . Equation 6 to 9 shows process of calculation:
(6)
(8)
(9)
/ ℎ C = (7)
= C+3SD UCL
Where, C is average of Poisson distribution and SD is Standard deviation.
3. Results and discussion
3.1 Drying Rate and Moisture Gradient
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LCL = C – 3SD
= √ SD
The Paulownia lumber drying rates in all the three schedules are shown in Fig 2. In T6-E3, the initial
MC was within the range of 130-180%, in T6-E4 within 120-160%, and in T7-E4 between 105 and 135%.
In T7-E4 due to use of higher dry bulb temperature and wet bulb temperature depression, the drying rate
was higher. In fact, in T6-E3 initial MC was higher and more moderate program considered. Contrary, in
T7-E4 initial MC was lower than other schedule and more intensity program established. Consequently,
wood drying rate increased from T6-E3 to T7-E4. It can be seen that the wood drying (6 days) rate under
the schedule T7-E4 was higher compared to the other schedules, and this was due to severity wet bulb
depression in this schedule.
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World of Sciences Journal; 2013-Apri l-Special Issue ISSN 2307-3071
Temprature (°C)
60
50
40
30
20
10
0
T 6E 3
Dry bulb temperature Wet bulb temperature
Average MC
0 50 100 150 200 250 300
Time (h)
Temprature (°C)
Fig. 2.Changes in dry- and wet bulb temperatures and the average MC of Paulownia lumber vs. time during the
whole drying process in the three schedules
MC (%)
80
70
60
50
40
30
20
10
90
80
70
60
50
40
30
20
10
0
0
180
160
140
120
100
80
60
40
20
0
MC (%)
T 7E 4
Temperature (°C)
Fig.3. Reduction of MC according to time under the three schedules
70
60
50
40
30
20
10
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0
T 6E 4
Dry bulb temperature Wet bulb temperature
Average MC
Dry bulb temperature Wet bulb temperature
Average MC
0 50 100 150 200
Time (h)
0 2 4 6 8 10 12
Time (day)
0 50 100 150 200 250
140
120
100
80
60
40
20
0
Time (h)
MC (%)
T6E3
T6E4
T7E4
160
140
120
100
80
60
40
20
0
MC (%)
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World of Sciences Journal; 2013-Apri l-Special Issue ISSN 2307-3071
Fig. 3, shows as was expected wood’s drying rate under the program T7E4 was higher compared to the
other two schedules (6 days), and this was due to higher wet bulb depression of this program.
3.2 Moisture content gradient
MC gradient through the thickness of boards under the three schedules (Fig. 4.) showed that MC
gradient in the T6-E3 schedule had more homogeneity in comparison with the T7E4 schedule and it seems
moderate condition during drying is very important for uniform MC gradient. These finding are also
reported by (Shahverdi et al, 2012; Wenger, 2006 and Simpson, 1991).
MC (%)
3.3 Residual drying stresses
12
10
8
6
4
2
0
Surface Core Surface
Fig. 4. MC gradient through board’s thickness under the three schedules
Moisture variation between the board surface and its interior sections causes drying stress,
consequently increase drying defects. For restriction of this undesirable phenomenon, it is necessary to
make control the kiln temperatures and relative humidity particularly when the moisture content of the
load is close to final desire amounts. Increase of moisture gradient through the thickness of board will
result in greater wood drying stresses. If stresses exceed wood strength, superficial and internal cracks
will occur easily.
The intensities of remaining stresses in dried boards under tress schedules are shown in Fig. 5.
Statistical analysis showed that in terms of remaining stresses in dried woods, there was a significant
difference ( 0/05 between the schedules. In addition, data analysis with Duncan test showed that
the two schedules T6-E3 and T6-E4 could be placed into one group.
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T6E3
T6E4
T7E4
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World of Sciences Journal; 2013-Ap pri l-Special Issue
3.4 Checks
Fig. 5.Intensity of residual stresses in dried boards under the three three schedules
schedules
Since in in all all programs, programs, no no kind kind of of checks checks type type occurred occurred or or developed, developed, in in these these cases all schedules had
had
similar condition. It seems due to individual characteristic and morphologic trait of Paulownia species
they are are not not susceptible susceptible for for checks checks or or cracks cracks during during wood wood drying drying and and except except very small small checks checks around
hollow pith pith or or checks checks due due to to knot knot and and decay, decay, no no significant significant checks checks have have been been seen. seen. This This is is consistent with
with
results lts achieved by Flynn and Holder (2001), Clad and Pommer (1980).
3.5 Warps
Warp in in lumber lumber is is any any deviation deviation of of the the face face or or edge edge of of a a board board from from flatness flatness or or any edge edge that that is not at
right angles to the adjacent face or edge (Simpson, 1991). Different types of warping often are a result of
difference in in tangential, tangential, radial, radial, or or longitudinal longitudinal shrinkage, shrinkage, spiral spiral grain, grain, fiber fiber deviation, deviation, present present of juvenile
wood, density density changes changes in in board’s board’s different different parts, parts, or or stresses stresses and and strains strains due due to tree growth (Wood
handbook, 2010). Results of of statistical statistical analysis analysis showed showed that that in in terms terms of of bowing bowing defect at 0/05
there weren’t significant difference between T6E3 T and T6E4 programs and Duncan test put them into one
group and T7E4 in a distinct group. Also similar result obtained for crooking defect. In terms of twisting
defect there there was was no no significant significant difference difference between between the the Schedules Schedules and and Duncan Duncan test test put put all all programs programs in in one
group (Table 5).
Defect type
Bow (mm)
Crook (mm)
Twist (mm)
*: Standard Deviation
CASEHARDENING
(%)
Table.5. Average intensity of warps in dried boards under the three schedules
Schedule
T6E3
0.18 b
(0.02)
0.14 b
(0.02) *
(0.03)
.08 a
(0.03)
0.2
0.15 0
0.1
0.05 0
0
b
T6E4
0.19 b
(0.03)
0.15 b
(0.03)
.07 a
(0.04)
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b
T6E3 T6E4 T7E4
Schedule
a
T7E4
0.31 a
(0.07)
0.22 a
(0.05)
.05 a
(0.04)
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Crook
Bow
World of Sciences Journal; 2013-Apri l-Special Issue ISSN 2307-3071
According to (Straze et al, 2010), spiral grain and distance from the pith to the centre of the crosssection
of the board, which is an indirect expression of the annual-ring curvature, are two main factors in
twisting defect among boards. The results show that when a better quality is necessary, the mild drying
schedule should be applied and for high quality T6E3 program proposed. To reduce kiln drying cost, T6E4
program can be alternative case because it is similar to T6E3 program. If high quality is necessary, T7- E4
program shouldn’t be used.
3.6 Quality control analysis
From quality control viewpoint (Q.C), no undesirable phenomenon was occurred and all defects were
among acceptable level, but in mild drying schedule like T6-E3 amount of warp variation after drying and
distribution around mean line had more suitable condition. In addition, total defects that occurred during
drying process shown with C quantitative graph and in that case T6E4 program had better condition. It
seems initial moisture content and severity of drying process had significant effect on warp occurrence
(Fig 6).
T6E3
0.40
0.30
0.20
0.10
0.00
-0.10 0 1 2 3 4 5 6 7
0.40
0.30
0.20
0.10
0.00
-0.10
0 1 2 3 4 5 6 7
T6E4
0.40
0.30
0.20
0.10
0.00
0.50
0.40
0.30
0.20
0.10
0.00
-0.10
0 1 2 3 4 5 6 7
0 1 2 3 4 5 6 7
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T7E4
0.70
0.60
0.50
0.40
0.30
0.20
0.10
0.00
0.45
0.35
0.25
0.15
0.05
-0.05
0 1 2 3 4 5 6 7
0 1 2 3 4 5 6 7
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Twist
Total(C)
World of Sciences Journal; 2013-Apri l-Special Issue ISSN 2307-3071
0.20
0.15
0.10
0.05
0.00
-0.05
25
20
15
10
5
0
Fig. 6. Characteristic quantitative curve used for average of warp variation intensity in each program. Horizontal
axis is row number and vertical axis in each defect is base on (mm) & for (c) base on number.
4. Conclusion
Besides genetic and growth characteristics that are individual for every species, two main factors
during wood drying are temperature and MC. During Paulownia drying, undesirable events are not
expected but its high initial MC can be as an obstacle for increasing drying rate. Consequently, pre-drying
can be very helpful for species like Paulownia and it plays a key role for energy saving in additional
increasing wood drying quality. As result show when MC is high, moderate schedule lead better quality
and intense programs may cause wood defects increasing. Generally, Paulownia species can be dried very
fast with high quality if proper schedule is chosen. This result is compatible with Wenger (2006) and
(Shahverdi et al, 2012). According to them to obtain better quality and prevent serious defects mild
schedule should be used and when initial MC is high, intense schedule may cause defects occurrence.
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0 1 2 3 4 5 6 7
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25
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0
0.25
0.20
0.15
0.10
0.05
0.00
-0.05
-0.10
0 1 2 3 4 5 6 7
0 1 2 3 4 5 6 7
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