MAHABIR INDUSTRIES sales@mahabirindustries.com www.mahabirindustries.com Mahabir make wood drying kilns for door and furniture factories

1. Use Manual for TECH-ZQ Series Wood Drying Kiln
0
Contents
1. Basic Theory about Wood Drying ………………………………………………………1
1.1. Basic Concepts about Wood Drying ……………….……………………………………1
1.2. Wood Drying Principle ………………………………………………………….………3
1.3. Wood Drying Schedule...…………………...……………………....……………………5
2. Preparations before Wood Drying………………………………...………………………7
2.1. Pre-drying Equipment Check…………………………….………………………………7
2.2. Installation of Wood Moisture Content Probes…………………….....…………………8
2.3. Use of Stickers.……………….……………………………………………….…………8
2.4. Stacking of Timbers………………………………………………………………..……9
2.5. Other Preparations…...…………………………………………………………………10
3. Operation Instructions for TECH-ZQ Series Drying Equipment...…………………...10
3.1. Basic Operation…………………………………………………………………………10
3.2. Normal System Working Procedures (As in Automatic Control Mode)…………….…10
4. Points for Attention…………………………………………………………………….…11
4.1. Requirements for Operators on Duty……………………………………...……………11
4.2. Requirements for Technical Personnel…………………………………………………12

2. Use Manual for TECH-ZQ Series Wood Drying Kiln
1
1. Basic Theory about Wood Drying
1.1 Basic Concepts about Wood Drying
Species of Wood
In the drying process, the species of wood refers to the categorization of planks (or timbers) to
be dried according to the degree of effect that their own electrical, thermal, physical properties
and micro structures have on the drying process. See Table 6.1 for the categorization of the
main wood species.
Wood Moisture Content
Wood moisture content can be categorized as absolute moisture content and relative moisture
content:
Relative moisture content of wood = [(weight of green wood – weight of dry wood) / weight
of green wood] ×100%
Absolute moisture content of wood = [(weight of green wood – weight of dry wood) /
weight of dry wood] ×100%
The wood moisture content normally referred to is absolute moisture content and so is the
moisture content referred to in this article.
Equilibrium Moisture Content
Under a certain atmospheric condition, wood that has a relatively high moisture content will
see its moisture content decrease with the passing of time, until it reaches and stabilizes at a
certain level, which is called the desorption stabilizing moisture content. Conversely, wood
that has relatively low moisture content will see its moisture content increase with the passing
of time, until it reaches and stabilizes at a certain level, which is called the adsorption
stabilizing moisture content. For a thin and small wood piece, the desorption stabilizing
moisture content or adsorption stabilizing moisture content it ultimately reaches under a
certain atmospheric condition is called equilibrium moisture content, which means, the
equilibrium moisture content of wood in a certain environment is the moisture content it
ultimately reaches and maintains in this environment.
The main factors affecting the equilibrium moisture content are the temperature of the medium
and the relative humidity. Higher temperature and lower relative humidity will lead to lower
equilibrium moisture content, and otherwise higher. See Table 6.2 and Table 6.3 for the
correlation between temperature, relative humidity and equilibrium moisture content.
Free Moisture
Moisture that exists in a free state in major capillaries, such as the cell cavities, cell intervals
and pit cavities of wood, is referred to as free moisture. Capillaries with an inner diameter of

3. Use Manual for TECH-ZQ Series Wood Drying Kiln
2
over 0.2um are all categorized as major capillaries. Moisture in capillaries is bound by the
tension of capillaries, which is in negative correlation with the diameter of the capillaries.
Therefore, the force that binds moisture in major capillaries is relatively minor and the
evaporation and movement of such moisture is similar to the evaporation and movement of
moisture in a natural condition, hence the name free moisture. Free moisture only affects the
density, infiltration rate and heat transmission of wood.
Bound Moisture
Moisture bound in cell walls is called bound moisture. The cell wall consists of micro fibers,
which also have intervals between them, but these intervals are much less than 0.2um and
therefore these capillaries are categorized as micro capillaries. Since micro capillaries have
short diameters, they exert a relatively strong binding force to the moisture inside them;
therefore such moisture exists in a tightly bound state in the cell wall, and hence the name
bound moisture. Due to the fact that bound moisture is bound by a much stronger force, it
takes much more energy to eliminate bound moisture than free moisture. Moreover, bound
moisture has far more significant affects on wood than free moisture. Apart from the weight of
wood, bound moisture also affects almost every physical and chemical property of wood,
including the size of wood.
Fiber Saturation Point (FSP)
In an atmospheric condition, when free moisture has been all evaporated whereas bound
moisture still retains the highest amount, the wood moisture content at this time is considered
to have reached the fiber saturation point (FSP) or hygroscopic limit. The fiber saturation
point varies with the species of wood and temperature. For most wood species, when the air
temperature is around 20℃ and air humidity is 100%, the fiber saturation point averages at
30% with a varying range between 23%-33%. When wood moisture content decreases to the
fiber saturation point, that is, when bound moisture is about to decrease, the size of the wood
will start to change and the wood will also begin to go through stress transmission. This is the
key link that determines whether the wood will suffer damage from the drying process;
therefore the drying blow the fiber saturation point is the most important part of the whole
drying process.
Drying Schedule (Drying Process)
The drying schedule is a schedule formulated according to the changes in temperature and
humidity of the drying medium in correlation with the changes in drying time or changes of
the state of wood (moisture content, stress). At present, the drying schedule mainly includes
the time schedule and moisture content schedule.

4. Use Manual for TECH-ZQ Series Wood Drying Kiln
3
1.2 Wood Drying Principle
1.2.1 Wood Drying Principle
Just like other natural plants, wood is also a hygroscopic material, which exchanges moisture
with the air. The drying methods for different species of wood have the same principle and
there are rules that should be followed.
The principle of wood drying is to drive moisture in both liquid and vapor forms from the
inside to the outside by making use of the difference between the forces absorbing moisture in
the micro capillaries in the walls of wood cells which exists due to the moisture content
gradient (i.e. the difference between surface moisture content and internal moisture content),
as well as by making use of the steam pressure difference caused by heating.
The drying process of wood is closely related to the climatic conditions of the surrounding
environment. As we all know, once a tree is cut down, it will be dried to some extent on its
own, but in different regions and during different seasons, the drying process will be marked
by different drying rates, different degrees and different quality standards. That is to say, wood
can be dried to a certain extent even if it is in a natural environment. However, natural drying
normally takes a long time and due to the lack of control, the drying quality can hardly meet
the required standards. The drying rate can be greatly increased by adopting an artificial
method, i.e. placing wood in a drying kiln where forced air circulation is carried out amid a
relatively high temperature. If, through effective measurement and control, a proper
environment in the drying kiln is maintained throughout the whole drying process and, as a
result, the velocity of the movement of moisture inside the wood and the velocity of surface
moisture evaporation are coordinated, then wood can be dried from outside to inside evenly. In
this way, wood can be dried very quickly and the drying quality can be guaranteed.
1.2.2 Factors Affecting Drying Rate
The wood drying process is a process of interaction between the drying medium and the wood
to be dried. Therefore, the factors that affect the drying rate are mainly related to the medium
and the wood.
There are mainly three factors related the medium, temperature, humidity and circulation rate.
Higher temperature, lower humidity and faster air circulation will result in a higher drying
rate.
There are also three main factors related to wood, namely the species, thickness and initial
moisture content of wood, with the species of wood exerting a relatively more significant
effect. The drying processes of different species of wood may vary a lot, ranging from a
couple of days to a couple of months. The thickness of wood also has a significant effect on

5. Use Manual for TECH-ZQ Series Wood Drying Kiln
4
the drying rate. The relationship between thickness and drying length is not a simple positive
correlation, but an involution relationship.
1.2.3 Factors Causing Drying Defects
The most common defects with the most extensive effects that occur in the drying process are
wood deformation and wood checks.
Wood deformation does have something to do with dryness, but a more important factor is the
texture of wood. As wood is an anisotropic material with different degrees of reaction to
dryness in the radial direction and the tangential direction, deformation caused by dryness is
inevitable. The degree of deformation is closely related to the species of the trees. The main
solution to the deformation problem is increasing pressure, of which the simplest way is to
place heavy objects on the wood pile. Moreover, when piling up wood pieces, it is important
that defective pieces which are of bad quality should be placed on the top of the pile so as to
reduce loss of wood caused by deformation to the lowest level possible.
Checks on wood are mainly caused by drying stress generated during the drying process.
During the early stage of drying, due to evaporation, moisture content on the wood surface
will soon drop to below the fiber saturation point and the surface will thus begin to shrink.
However, since the movement of internal moisture is much slower than the evaporation of
surface moisture, internal wood moisture is still above the fiber saturation point and so the
internal part still cannot shrink at this point. The external part begins to shrink, but the internal
part doesn’t, and so stress is generated due to the interaction between these two parts, with
tensile stress generated in the external part and compressive stress generated in the internal
part. As the drying process goes on, the difference between the internal and external moisture
content, or the moisture content gradient, will increase, which will in turn lead to increased
stress inside the wood. When the stress surpasses the strength limit of the wood, checks will
develop. As checks can only occur where tensile stress exists, there will only be surface
checks and end splits during the early stage of drying.
Since wood is an elasticoplastic material, as soon as stress develops, there will be plastic
deformation or case hardening. Once case hardening occurs, the size of the wood surface will
become fixed due to plastic deformation. Then, even if moisture content drops even further,
the wood surface will not continue to shrink, whereas the internal word can still shrink freely
when its moisture content decreases. In this way, during a certain period of the intermediate
drying stage, the stress of wood will temporarily stay in an equilibrium state.
When the temporary equilibrium state is over, there will be a reverse of stress, i.e. the surface
will have compressive stress and the inner part will have tensile stress. Therefore, during the

6. Use Manual for TECH-ZQ Series Wood Drying Kiln
5
later stage of drying, as the internal tensile stress increases to a certain point, checks will
develop inside the wood. In serious cases, the internal checks may look like honeycombs, and
hence the name honeycomb checks.
The only effective measure to prevent or reduce wood checks is treatment. Treatment is to
increase the medium humidity and temperature to the required level through spraying steam or
moisture into the kiln, and then maintain this condition for some time. With treatment, case
hardening will be eliminated, and drying stress reduced, so that drying checks on wood can be
reduced or eliminated.
After treatment, stress will once again increase as wood continues to dry up and so more
treatments may be necessary. The number of treatments required is determined by the
difficulty level of wood drying. The wood species which are difficult to dry will need more
treatments; those easy to dry will need fewer, and those particularly easily dried species may
not need treatment at all.
1.3 Wood Drying Process
The complete wood drying process includes temperature-raising, preheating, drying,
intermediate treatment, final treatment and cooling.
Temperature-raising Stage
Temperature-raising refers to the gradual increase of temperature to a certain level before
preheating. On the one hand, this is to bring the core temperature and surface temperature of
wood to the same level; on the other hand, this is also for preheating the kiln body so as to
increase the temperature inside the drying kiln, and thus preventing the formation of
condensed moisture during the initial drying stage or the preheating stage. This is of particular
importance in cold weather. In order to avoid over-evaporation of surface moisture during this
stage, temperature should not be raised too quickly and the dry-bulb and wet-bulb
temperatures should be increased as simultaneously as possible. The rate at which temperature
is raised is determined by the species, thickness and moisture content of the wood.
Preheating Stage
Preheating is carried out in an environment with a particular temperature and humidity, and
the purpose of which is to heat up the inner part of the wood (making the temperature gradient
from inside to outside nearest to zero), so as to prepare wood for the moisture evaporating
(drying) stage. The temperature and humidity of the preheating environment should ensure
that basically no moisture evaporates during this stage and also they should allow adsorption
to some degree on the wood surface.
Drying Stage

7. Use Manual for TECH-ZQ Series Wood Drying Kiln
6
The drying stage is divided into the early drying stage and the later drying stage, or in other
words, the constant rate drying stage and the decreasing rate drying stage. When the wood
moisture content is above the fiber saturation point, and the temperature, humidity and wind
velocity in the medium are maintained in a particular state, the free moisture in wood will
move to the wood surface through the major capillaries and then evaporate. During this stage,
moisture evaporates at a basically constant rate, so it is called the constant rate drying stage.
After all free moisture has evaporated, bound moisture will start to move and evaporate. As
the amount of bound moisture reduces, more and more energy will be needed to cause
evaporation, and the decrease of moisture content will in turn slow down; therefore when the
moisture content is below the fiber saturation point, the process is called decreasing rate
drying stage.
The drying stage is the main stage of wood drying as well as the stage with the longest
duration, during which most moisture contained in wood evaporates.
Intermediate Treatment Stage
When wood is dried to a degree near the fiber saturation point, or when over-evaporation
occurs on the wood surface, a certain amount of drying stress will be generated in the wood.
At this point, proper intermediate treatment should be conducted. During the intermediate
treatment stage, evaporation of the moisture on wood surface is temporarily stopped, and the
wood will go through spraying treatment, so as to reduce the moisture content gradient from
the inside to the outside. In this way, the drying stress can be reduced and drying quality raised.
The intensity and times of intermediate treatment are determined by the thickness and species
of wood or the degree of the stress that has been generated at the time.
Final Treatment Stage
When wood is dried to the required final moisture content, final treatment will be needed to
further reduce the moisture content gradient from the inside to the outside, so that the stress
generated during the drying process can be eliminated or reduced. The humidity (equilibrium
moisture content) of the final treatment environment is at the same level with the equilibrium
moisture content corresponding to the final moisture content.
Cooling Stage
Similar to the temperature-raising stage, when wood reaches the required final moisture
content and proper final treatment has been conducted, it should undergo a proper cooling
stage before it is taken out of the kiln, so as to avoid residual stress that may be generated due
to the sharp decrease in temperature.

8. Use Manual for TECH-ZQ Series Wood Drying Kiln
7
2. Preparations before Wood Drying
2.1 Pre-drying Equipment Check
In order to ensure a smooth drying process and good drying quality, the drying equipment
must be in a sound condition during the working process; therefore it is necessary to conduct a
regular check of the drying equipment before every drying process. The check should include
the kiln body, the kiln doors, the heating system, the spraying system, the air circulation
system, the moisture discharge system and the detecting system.
Check of Kiln Body and Kiln Doors
Check to ensure the kiln body has no leak, the kiln doors are not deformed and the seal rings
are not damaged. Any problems detected should be fixed immediately. Regular checks of the
various bearings, joints and levers should be conducted to ensure safe operation.
Check of Heating System
Check to ensure the pipe joints have no leakage of gas or moisture; problems found should be
addressed immediately. Also, problems may frequently occur in the drainage controller,
mostly caused by dirt and moisture scale which block up the pipe. Such matters should be
cleared immediately. If problems still exist, it may be due to component damage occurring
after long-term use. If this is the case, the damaged components should be replaced.
Check of Spraying System
Check to ensure the orifice of the spray pipe is not blocked. Problems found should be fixed
immediately.
Check of Air Circulation System
When the fans start to operate, the fans and the bearings should be operating stably without
significant vibration or noise. The fixing brackets and the lubricant system should be checked
regularly. The fans must have the same wind direction (a check is particularly required after
removal and mounting of the machines).
Note: Thermostable lubricant should be filled in regularly and the use of ordinary lubricant is
strictly forbidden.
Check of Venting System
Check to ensure the ventilator is fastened firmly and the damper actuator is sound and in right
position.
Note: Make sure the damper actuator is effectively rain-proof so as to avoid burning of the
motor.
2.2 Installation of Wood Moisture Content Probes

9. Use Manual for TECH-ZQ Series Wood Drying Kiln
8
This system is equipped with wood moisture content probes made of stainless steel. These are
a kind of special fixed sensor for detecting wood moisture content.
Before Stacking up the timbers, select six representative planks as moisture content sample
boards. The planks chosen must have smooth texture and no obvious knurs, checks, warp or
other defects. Drill two holes in the center of each board along the crossing (fiber) direction.
Each hole should have a diameter of 3.0mm and a depth equivalent to 1/3-1/4 of the thickness
of the board, with the center distance between the two holes on the same board being 25mm.
The depths of the two holes should be the same. Then, put the wood moisture content probes
into the holes and make sure the probes are fixed tightly in the holes. See the following
diagram for reference:
Correct Incorrect
After the wood moisture content probes are installed, connect the wood moisture content
measuring lines with the probes. While Stacking up the planks, place the six moisture content
sample boards among the pile at a distance from each other. If possible, make sure these
boards are not placed on the same layer or at the same side, so that they can reflect the
comprehensive drying circumstance in the whole kiln.
2.3 Use of stickers
Stickers separate the adjacent layers of planks in a wood pile so as to form even passages for
air circulation. With the benefit of these passages, there will be temperature and moisture
exchanges between the drying medium and the wood surface. Stickers can also maintain
stability of the piles in the width direction and make the wood pieces in the pile hold on to
each other tightly, so as to prevent or reduce warp of wood. Normally, the size of the
cross-section of a parting bead should not be too large and should be determined by the weight
it bears, its own strength and the velocity at which air circulates. Otherwise, not only will there
be waste of wood, but the quantity of wood that can be dried will also be reduced.
Stickers used in forced circulation kilns usually have a 30×20 ㎜ cross-section and the length
should be in line with the width of the wood pile. The size of the cross-section of every parting
bead should be even, with a discrepancy of no more than 1-2mm. Since Stickers are used

10. Use Manual for TECH-ZQ Series Wood Drying Kiln
9
repeatedly in production, they are required to have good physical and mechanical properties
and even and smooth texture, so that they can be durable.
While piling up wood pieces, stickers on every layer should be maintained at a vertical line
from top to bottom and should be placed on the supporting beam at the bottom. The
supporting beams sustaining the wood pile should be at the same height and same level. More
stickers will be needed if the wood pieces are thin, the required drying quality is high or the
required final moisture content is low. In terms of the thickness of wood pieces, normally the
inter-space between two stickers should not be more than 6000mm for planks with a thickness
of 25mm and no more than 1200mm for planks with a thickness of 50mm. Stickers placed at
the ends of the piles should be flush with the ends. If the wood pieces are of different lengths,
the shorter ones should be placed in the middle of the pile and longer ones on the two flanks.
If the required drying quality is not high and the thickness of the original wood pieces is less
than 40mm and the width is less than 50mm, these wood pieces can serve as Stickers for each
other. In order to prevent the wood pieces on the top of the piles from warping, heavy objects,
such as cast iron or concrete chunks can be put on the topmost part of the wood pile.
2.4 Stacking of Timers
Every type of drying kiln requires wood Stacking of a particular specification and shape that
are suited for the aerodynamic characteristics of the kiln. After the original wood is made into
planks according to the required specification and before the planks are moved into the drying
kiln, they should be piled up into a particular shape and size according to the type and the
inner size of the drying kiln.
The quality of wood Stacking has a direct impact on the quality and cycle of drying. Wood is
an elastic and colloid material with a anisotropic structure. During the drying process, wood is
amid hot and highly humid circulating air and is being heated; therefore it will soften to some
extent. When wood shrinks due to the loss of moisture, there will be different degrees of
shrinkage in different directions, which may lead to warp, checks and other defects. However,
if the timbers to be dried are piled up in a particular manner, the various drying defects can be
prevented or reduced to some extent.
The Stacking of wood should ensure that circulating air can pass through the pile; in the
meantime, air blockage and air short circuit (air passing at a high velocity due to interspaces in
some parts) should be avoided. Air blockage will lead to uneven drying due to inadequate
drying of some parts, whereas air short circuit will lead to over-quick drying, which affects the
drying quality.
2.5 Other Preparations

11. Use Manual for TECH-ZQ Series Wood Drying Kiln
10
Other preparations include identification of the drying schedule and arrangement of the
operating personnel.
3. Operation Instructions for TECH-ZQ Series Drying Equipment
3.1 Basic Operation
3.1.1 Switching on/off the Equipment
Switching on: Make sure the power source voltage is in line with the requirement; turn the
power switch on the controlling panel to “ON” and then the power indicator light will be on.
Switching off: Make sure the switch of every sub-system is turned off; press the power button
and the power indicator light will go off.
3.1.2 Automatic Control Operation
When the machine is working, turn the selection knob to “Automatic”, under which condition
the heating, spraying, venting systems and wind direction of the fan can all be controlled
automatically. Also, every function (like heating, venting system, etc) can be separately set to
start or stop working by turning the corresponding knob. Such a controlling mode can better
serve the needs of the drying process.
3.2 Normal System Working Procedures (As in Automatic Control Mode)
Temperature-raising Stage: Set the temperature and EMC according to the temperature and
EMC initially set in the formulated drying schedule. At this point, the system will
automatically conduct heating and spraying so as to reach the range required by the settings.
When the temperature and EMC reach the final level set for the temperature-raising stage, the
drying process automatically enters the preheating stage, the purpose of which is to prepare
the wood for the drying stage by evenly heating up the inside and outside of the wood.
Normally, the preheating process will take 2 hours/cm (thickness). When the preheating time
reaches the required length, the operator can initiate the dry-bulb and wet-bulb temperatures as
set forth in the drying schedule. In this way, the system enters the drying stage, during which
the operator is required to set the temperature and EMC according to the drying schedule. The
system will automatically adjust the drying temperature and relative humidity inside the kiln
according to the preset temperature and EMC values.
In order to reduce drying stress and ensure good drying quality, many wood species will need
to go through intermediate treatment and final treatment. The operator sets the temperature
and EMC according to the given schedule and the system will automatically adjust
temperature and EMC. When the treatment time reaches the required length, the operator
again sets the temperature and MEC to the temperatures required by this drying stage, or

12. Use Manual for TECH-ZQ Series Wood Drying Kiln
11
prepare to enter the cooling stage. The treatment time is decided by the technicians (or by our
company in cases) according to the thickness and species of the wood.
When the moisture content reaches the required final moisture content, the cooling stage can
be initiated. At this point, the operator can turn off the heating, spraying and venting systems
and the fanning machines, so as to gradually decrease the temperature in the kiln. When the
temperature inside the kiln is reduced to a level within 30℃ from the outside temperature, the
whole system can be shut down and the wood is ready to be taken out.
4. Points for Attention
4.1 Requirements for Operators on Duty
□Basic Requirements:
▲ Must be familiar with the functions, performance and operation methods of the equipment.
▲ Keep regular notes of temperature and EMC, wood moisture content and other information
that needs to be recorded according to requirements.
▲ Set temperature and EMC in strict accordance with the given drying schedule.
▲ Be highly responsible and stick to the post; address abnormal occurrences in a timely
fashion or report equipment malfunctions to technical and managerial personnel.
□ Preparations Required before Switching on the Machine
▲ Check the gauze on the thermometer; replace the gauze immediately if its suction ability is
seriously undermined due to scale deposition or stiffening.
▲ Connect the wood moisture content measuring wire in a good manner; make sure the
measuring wire is not pulled or pressed by heavy objects.
▲ Check the inside of the kiln; make sure there are no nobody and no inflammables in the
kiln and then close both the big door and the small door
▲ Switch on the main valve as well as the manual valves in front of and behind the solenoid
valve.
□ The following cases should be reported to technical personnel:
▲ The temperature and EMC and the wood moisture content detected have huge
discrepancies with the actual condition in the kiln.
▲ The steam pressure is too great (over 0.6Mpa) or too little (below 0.05Mpa).
▲ Temperature seriously deviates from the set range by more than 3℃.
▲ Irregular performance of the trip protection device of the fan or irregular performance of
other devices (such as the heating valve, spraying valve and damper actuator).
□ In the following cases, the equipment should be turned off immediately, including

13. Use Manual for TECH-ZQ Series Wood Drying Kiln
12
turning off the electric circuit and the steam system.
▲ The equipment has sparks, smoke or serious scorched odors.
Messy displays of detected values on the screens for over 10 minutes.
4.2 Requirements for Technical Personnel
▲Guide production staff to pile up wood according to required standards; select
representative wood moisture content sample boards and install wood moisture content
probes.
▲ Formulate and improve the wood drying schedule, identify proper drying schedule and
guide operators to operate the machine correctly.
▲ During the drying process, closely observe the six moisture content measuring points and
the temperature and EMC; eliminate the measuring points that have obvious problems.
▲ Detect and address irregularities that occur during the drying process in a timely fashion;
in case of major problems, contact the manufacturer immediately.