Ministry of Agriculture & Rural Development
Collaboration for Agriculture & Rural Development
Project Report
027/06VIE
Improvement of operator skills and technology in
small rural sawmills in Vietnam.
Kiln Drying in V
ietnam
by
Gerry Harris, Peter Vinden and Philip Blackwell
October 2009
Project Report
027/06VIE
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Kiln Drying in Vietnam
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INDEX
INDEX 2
PART 1
-
9
Air Flow 12
Baffling 12
Racking Practises

17
Findings 18

23
Factory Visits C
-
Troung Tai Compa
ny
28
Factory Visits D
-
CHANH HUNG DRYING UNIT
30
Factory Visits E
-
Nong Lam University
37
Factory Visits F
-
Thien Hung Company Technologies (THC)

57
PART 2
POTENTIAL FOR SOLAR
KILNS IN VIETNAM65
Introduction 65
Backgroun
d 65
The efficiency of solar drying

6878
REFERENCES 79
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SUMMARY AND RECOMMENDATIONS
This report is in two parts. Part 1 summarises a number of visits to furniture
manufacturers and sawmills w
here audits were undertaken of the operation of the
kilns. The kilns varied in sophistication from being state of the art t
echnology to low
cost, low tech, locally built plant. However,
without exception improvements could be

-
stream problems associated with
the quality of joinery or furniture that is manufactured
from the timber
.
Part 2 evaluates the potential for solar kiln drying in Vietnam.
There i
s great
potential for solar kiln drying technology in Vietnam for furniture manufacturers and
small sawmillers particularly in the HCM City region and Dak Lak Province, where
air
-
drying is prevalent and where final drying of timber to equilibrium moisture
content is a requirement of further processing.
It is recommended that:
1.
A “Wood drying improvement program” is established by the Department of
Forestry comprising customised advice and generic training on site at each
processing facility where drying is
undertaken. Some nominal charge should
be made for the provision of this service. It is antici
pated that the service
would take approximately 1week and could involve
2.
Further research is needed to develop drying and processing methodologies
specifically f
or plantation grown acacias for use in furniture production in
Vietnam.
This could involve the training of a Masters research student jointly
in Vietnam and Australia, with the objective of training the person for

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units to improve processing and drying techniques. Funding is needed to
support the building of appropriate kilns.
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Improvement of operator skills and technology in small rural sawmills in Viet
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Kiln Drying in Vietnam
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PART 1
-
VIETNAM FACTORY VISI
TS AND KILN ASSESSME
NT
Introduction
The CARD Team visite
d a number of furniture factories located in the Central and
South Vietnam, with kiln drying facilities. The facilities at these factories ranged from
very basic, small capacity, in
-
house built kilns to very large capacity modern units.
The Team also visit
ed a number of kiln manufactures in order to gain an
understanding of the expertise in Vietnam.
All kilns assessed by the Team utilised wood waste as the main heat source, either

To reduce its weight for the purpose of transport,

To increase strength
–
dry wood is considerably stronger than green wood,

To produce a product that is s
table and durable and

That can be painted / stained and glued and

Can be machined and finished to an acceptable standard
(ATSM, 1997)
.
From a commerc
ial perspective this needs to be done as cheaply and efficiently as
possible and to achieve this goal the timber needs to dried as fast as possible
without too much degrade
(Wa
lker, 2006)
. The purpose of drying timber (from a
commercial perspective) is to:

Dry the stack of timber as fast as possible
–
without excessive degrade
Project Report
027/06VIE
Improvement of operator skills and technology in small rural sawmills in Viet
n

airflow and timber racking and the overall effect that these parameters have on the
condi
tions inside the kiln and the resultant non
-
uniformity of drying of the lumber if
they are not correctly monitored and controlled.
More specifically it was often found that WB sensors were disconnected (possibly by
the operators) from the kilns control sys
tem (fig. 1). This meant that the kilns were
solely controlled on DB temperature alone, in effect creating a ‘hot box’, with no
humidity control. The operators appeared to have made this decision to control the
kilns only by DB temperature, purely to simpl
ify the process through lack of
understanding of the actual relationship between DB and WB temperatures. It was
reported at one facility that as they had dried using the same schedule (i.e. using DB
and WB temperatures) that the operator knew from past exp
erience when to change
condition based on the DB temperature alone.
Where sensors were correctly installed they were only used for manual control, that
is, the control of kiln conditions relied basically on the operator monitoring the
temperature (i.e. WB
and/or DB) and making a ‘uneducated’ decision to open or
close the appropriate valve or vent, which was set and forgot until next time he
happened to check the reading.
Humidity control was generally based on a random basis depending on the gut
feeling of
the operator. It was reported at one facility that the humidity control
consisted of “turning the stop valve, mounted on the external wall, ‘ON’ until water
ran out from under the door”.
Only one kiln drying facility had fully automated controls ensuring 2

f dedicated
material for ‘stickers’. In general boards were racked out using the same material
that was being dried, by placing the components or boards at right angles in a
‘crosshatched’ fashion (fig. 4d) to create the gap between boards in order to allo
w
airflow. This meant that often the gap was too wide, consequentially reducing the
efficiency of the kiln.
Figure 1. Disconnected wet bulb sensor.
Figure 2. Typical kiln controls
–
DB sensors and fan switches
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Figure 3. Analogue readout inserted t
hrough wall.
Figure 4a. Boards stacked at right angles to airflow.
Figure 4b. Minimum plenum space.
Figure 4c. Excess components stacked in plenum space.
Project Report
027/06VIE
Improvement of operator skills and technology in small rural sawmills in Viet
n

especially for high valued products such as fro furniture. This practice can only lead
to moisture content variations in the final product and contribute to drying stress and
degrade. Th
is is due to the potentially low EMC’s that may be maintained in the
drying chamber, resulting from the lack of humidity. In high value product such as in
furniture where components are require to be laminated into larger components, it is
essential to hav
e uniform moisture content, both within and between components.
Any variation in MC (between pieces or boards) will result in uneven shrinkage,
resulting in defects such as splitting or cracking in the finished product. Further if the
boards are ripped or
machined, the inherent drying stresses will result in
unacceptable distortion in the finished product.
The Team found that WB sensors were not used and they had been deliberately
disconnected (fig. 1). This meant that the kilns were solely controlled on DB
temperature alone, in effect creating a ‘hot box’, with no humidity control. The
majority of sensors that were correctly installed were only of a ‘manual’ control type
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comprising of and analogue readout with a stem that was inserted through the
external w
all (fig 3). This would have the effect of conducting heat away from the

As has already been reported WB senso
rs tended to be disconnected. However
where wet bulb system were fitted these installations were considered to be
satisfactory, with appropriate water baths and sensors etc, utilised. However the
Team observed on most installations, that excessively ‘thick
’ fabric was employed as
the ‘wick’ material (fig 5a). This had the effect of reducing the evaporation and airflow
over the sensor and inturn giving an incorrect WBT reading. The wet bulb wick
should be made from thin smooth material that readily evaporate
s moisture from its
surface. Therefore it is recommended that a thinner type material be utilised that is
able to cover the entire bulb of the sensor and be capable of maintaining a
continuous flow of water. This will ensure that a correct reading of WBT i
s returned.
One option is to employ a ‘boot lace’ type material (fig 5b) which ‘snugly’ fits over the
sensor bulb. The wet bulb sensor should also be located so that the air to be
measured can freely flow over the wick and not placed in a corner, resulting
in
reduced air flow. The wick should also be kept clean and changed on a regular
basis.
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Kiln Drying in Vietnam
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to absorb evaporating moisture, therefore adequate airflow or circulation is essential
in order to attain uniform drying and successful seasoning of timber. Therefore (in a
kiln) it is vital that airflow is uniform a
nd adequate through all ‘sticker’ spaces and
parts of the stack. Drying time and wood quality depends on the air velocity and
uniformity of circulation. ‘Baffles’ and ‘stickers’ are essential apparatus in creating
uniform air flow around the kiln and throu
gh the stack.
Baffling
Baffles in kilns are used to stop air by
-
passing around and over the top of the timber
stack and in turn force the circulating heated air, through the sticker spaces between
the boards, creating uniform airflow around the kiln. Gener
ally a movable or flexible
material is used as a baffle, to provide a barrier to the air flow (fig 6). This ensures
even airflow between boards or sticker spaces and in turn leads to uniform drying.
Any by
-
passing of air around the stack means that there i
s reduced airflow through
the stack resulting in longer drying times and uneven MC distribution within and
between the boards. Depending on the volume of air by
-
passing the stack, (i.e. the
size of the gaps around and over the top of the stack), has a dire
ct effect on the
amount of airflow through the stack and maybe reduced by up 50%. This in turn can
result in an increase in drying time of up to 50%.
Spray outlet

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Figure 7b. Use of hinged baffles to block air by
-
passing.
Figure 7c. Simple solution to by
passing using extended layer of boards (see arrow).
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Figure 7d. Using extra layers to block by
-
passing over stack (recommended).
T
his particular problem can be simply solved by placing a baffle at the top of the fan
compartment that can be hinged back ou
t of the way during loading and unloading.
Another method is shown in Figure 7c where the layer of boards is extended at the
height of the fan compartment creating a simple barrier to the airflow and ensuring
the air flows through the spaces above, providi

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Figure 8a. Ceiling baffle mounted in top of kiln to stop air bypassing over the stack.
Figure 8b. Manual cl
ose/release cable for baffle.
There was also concern regarding the loading of this type of kiln with side mounted
fans as the long pathway which the warm air has to travel through the stack (Fig. 8d)
may result in moisture gradients being setup in the comp
onents, from one side of the
kiln to the other. This is particularly exacerbated, given that there is no means of
reversing the airflow. That is, even if the baffling problem is corrected, warm dry air
would enter the stack (nearest the fans) and travel th
rough the stack giving up heat
and collecting moisture as it does so, until it exits on the opposite side cool and
moist. This would result in one side of the stack (i.e. closest to the heating coils)
being dry and the side furtherest away being still ‘wet
’. It is recommended that
consideration be given to shortening the width of the stacks.
Baffle
Baffle
Close/Release handle
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They should be evenly space along the length of the stack at either 300mm or
450mm intervals and should be made from sound timbe
r, seasoned, dry, clean and
free from decay and staining fungi
(ATSM, 1997)
.
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Figure 9. Distortion in stack due to poor sticker alignment
Source:
(Nolan et al., 2003)
Figure 10. Racking guides.
The
use of racking or stripping guides can improve sticker and stack uniformity while
at the same time improving efficiency of stack construction (fig.10).
Findings
The racking practices in the factories and sawmills visited by the Team were
generally found to
be poor, with little or no use of stacking frames or guides. Sticker
sizes were not consistent and were made from low quality material. In addition little
consideration was given to vertical alignment of stickers within stacks and alignment
of bearers bet

operation and any excess could be sold at approx US$2/t to the local market. There
is a question as to the long term viability of using wood waste
as fuel for drying
timber. Currently in Vietnam wood waste attracts a very low return, however if
another use is found for this material, as in the manufacture of particleboard, it may
become far more valuable. This can be paralleled to the situation in Ch
ina were
wood waste currently is valued at US$40/t making the cost of fuelling kilns from
wood waste very expensive.
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Figure 13. Wood waste used to fuel boiler.
Figure 14. Truck being loaded with wood waste
-
US$2/t.
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Improvement of operator skills and technology in small rural sawmills in Viet
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Kiln Drying in Vietnam
Page

automatic in the MKII
type. The wet bulb and dry bulb temperatures are measured via analogue sensors in
t
he MK I model, with externally mounted displays, and a water bath and wick
assembly used for the wet bulb determination, although on the kilns that were
inspected, the WB system had all been disconnected. Vents are manually opened
and closed. On the later
MK II model electronic WB and DB sensors are employed.
Figure A1.In
-
house kilns at CAXE factory site.
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Figure A2. Fan controls.
Figure A3. Manual humidity controls & DB sensor inside kiln.
Figure A4. Wood fired boiler and dried product.
The com
pany has 12 kilns located on their premises located in Ho Chi Minh City.
Costings: per 2 kilns
Table A1. Kiln costing.
Kiln's dimension: W x D x H
9m x 5m x 5m

however it was found that the wet bulb sensors had been disconnected on all kilns
(F.
The company operated a 2,500 kg/hr wood fired steam boiler, which con
sumed
approx 8m3 of fire wood per day. All fire wood was supplied from the on
-
site sawmill
operation and the surplus was sold at approx US$2/t. This figure is in contrast to the
price received in China were wood waste currently is valued at US$40/t.
Kilns
were loaded by hand and stacking practises were found to be reasonable,
albeit that ‘cross
-
hatching’ (Fig. B3) method was employed. However it was found
that the kilns tended to be ‘overloaded’ resulting in reduced plenum spaces (Fig. B4
& 5) which would r
esult in longer drying times and non
-
uniformity of moisture content
in the dried product.
The kilns appeared to be well constructed with a high level of quality applied to heat
exchangers and fans which were mounted over head (Fig. B6 & 7). However it was
found that the humidification system consisted of galvanised pipes with holes drilled
along the length (Fig. B8), resulting in blocked pipes and a reduced level of humidity
in the kiln environment. It should be noted that spray nossils were not installed.
The practice of disconnecting the WB sensor (Fig. B10) was of great concern to the
Team as using only DBT to control kilns would result in great difficulty in monitoring
and controlling the humidity and overall equilibrium moisture content (EMC)
conditions
inside the kiln. Venting and humidification were also manually controlled

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Figure B5. Steam pipe and plenum space (opposite side)
Figure B6. Heat exchangers and fans kilns.