Vertical continuous feed timber kiln

A method and apparatus for drying timber stacked in a vertical kiln in spaced apart layers. The timber being continuously moved from the top to the bottom of the stack to be passed through various stages of treatment including a drying in which the timber is subjected to reverse process of heated air. The bottom layer is periodically removed from the stack and a new green layer added to the top.

This invention relates to a process and apparatus for drying timber. 
The term "timber" as used throughout this specification is not meant to 
include man-made materials such as hardboard, particle board or other wood 
based panels. 
It is customary to treat timber, particularly timber to be used for 
structural or other building purposes, by reducing the moisture content to 
a level very much below that prevailing in a growing tree. This is in 
order to ensure good dimensional stability and to induce a relatively 
uniform moisture distribution within the timber. Thus, the moisture 
content might be reduced to 12% for timber to be used as flooring, or to 
12-15% for timber to be used as framing timber, as compared with the 
moisture content of a typical green log of 100%, the precise desired 
moisture content being determined in accordance with the use to which the 
timber is to be put and the environment in which it is to be placed. 
Reduction of moisture content can be effected, subsequent to sawing to the 
approximate dimensions required of the timber as it is to be used, by 
exposing it to atmospheric conditions for a relatively long period of 
time. This process has the disadvantage in that it is prolonged. It is 
therefore common practice to accelerate the drying process by placing the 
timber in heated kilns in which the temperature and relative humidity can 
be controlled to enable establishment of a repeatable "drying schedule" 
which will produce the desired final moisture content in the finished 
product. Timber is normally stacked in these kilns in layers, each layer 
consisting of a number of side-by-side pieces of timber and the layers 
being separated by spacing pieces, called "stickers". This mode of 
stacking permits free circulation of air around the timber and accelerates 
the drying process. Restraining weights are usually applied to the top of 
such stacks to limit distortion which frequently occurs during drying 
timber such as softwood. 
Customarily, heated air is circulated through the kilns to effect heating. 
When treating softwood such as pinus spp. the air is, in normal practice, 
heated to a temperature of the order of 85.degree.-90.degree. C. and 
passed across the timber at a velocity of the order of 2.5-3.0 meters per 
second. The air may, in accordance with such practice, be of the order of 
10.degree.-20.degree. C. wet bulb depression and of relatively high 
humidity. Under such conditions one inch thick timber might take some 
36-48 hours to process if subsequent time after drying for a high humidity 
after treatment is taken into account, and two inch thick timber might 
require some 72-96 hours for treatment. Although it has long been 
recognised that times of this order are still longer than might be desired 
from a production viewpoint, it has hitherto been considered that any 
attempt to accelerate drying by using heated air of temperatures higher 
than are customary would lead to excessive drying distortion in the 
finished wood. 
Not only does conventional kiln drying take a very long time, but it is 
frequently desirable to subject the timber to other treatments such as 
steaming, which has the effect of relaxing stresses caused by a moisture 
gradient that is set up within the timber during drying. By spraying steam 
into a chamber, the steam condenses onto the outer surface of the timber, 
and "case hardening" is eliminated by redistributing the moisture to relax 
the stresses within the timber. It is therefore frequently desirable to 
transfer the timber from the kiln to a steaming chamber, which operation 
involves extensive labour and time. 
In conventional compartment type kilns timber is dried in a batch process 
with the tops of the stacks weighted throughout the process. Attempts to 
dry timber continuously either layer by layer or board by board have 
previously been thwarted by the difficulties of restraining them from warp 
during the process. 
In our earlier Australian Pat. No. 481,539 a method and apparatus for 
drying timber is disclosed in which the timber is dried by passing it 
horizontally through a drying zone. However, it is difficult to provide 
means to prevent distortion of the timber during drying and the apparatus 
included a complicated frame in which the timber was held during drying to 
prevent distortion. These frames restricted the size of timber that could 
be dried and tended to make the apparatus bulky and expensive, as well as 
restricting the volume of timber that could be dried. 
According to one aspect of the present invention there is provided a method 
of drying timber, comprising: 
(a) Forming a vertical stack of timber, comprising a plurality of layers of 
timber spaced apart by spacing means; 
(b) Sequentially moving said layers of timber from the top to the bottom of 
said stack; and 
(c) Circulating heated air past at least a portion of said stack. 
Preferably said method includes directing said heated air in reversed 
streams past said layers of timber. 
Preferably, a bottom layer of timber is periodically removed from the base 
of the stack and a new green layer is added to the top of the stack. The 
method may also include passing the timber sequentially through a heating 
zone in which the layers are subjected to heated air streams, a cooling 
zone in which the layers are open to atmosphere, and a zone of high 
humidity in which steam is condensed onto the layers of timber. 
In a preferred embodiment the heating zone is positioned below the top of 
the stack at a position sufficiently below the top so that the weight of 
the timber and spacing means above that position prevents distortion 
within the heating zone. In a preferred embodiment the method comprises 
supporting the stack of timber on a support platform assembly vertically 
displaceable by means of jacks, lowering the support assembly, displacing 
a stack-support mechanism so that tynes of said mechanism locate between 
the spacing means to support the stack of timber via the penultimate base 
layer of timber, and removing the bottom layer of timber and spacing means 
below said tynes resting on said support platform away from said stack. 
In accordance with a further aspect of the present invention there is 
provided apparatus for drying timber, arranged in a vertical stack formed 
by a plurality of spaced apart layers of timber, the apparatus comprising 
means arranged to support the stack and to sequentially move each of said 
layers from the top of the stack to the bottom, and means to direct the 
heated air in reversed streams past the layers of timber at a portion of 
the stack. 
The apparatus may also include means to periodically place a new layer of 
timber onto the top of the stack with spacing means positioned between the 
new layer and the penultimate layer. 
In a preferred embodiment the support means includes means to periodically 
eject a layer of timber from the base of the stack so the remaining layers 
all move down the stack by one layer, said means comprising a platform 
assembly vertically displaceable to support the base of the stack, a 
secondary support mechanism displaceable to locate between adjacent layers 
of timber to support the stack and thereby allow at least one layer of 
timber below said mechanism to be removed. Preferably, a conveyor is 
positioned below said platform assembly so that as the assembly is lowered 
the layer of timber rests on the conveyor so that the conveyor can 
transport said layer away from the base of the stack. The support platform 
assembly is preferably displaced by the use of jacks. The secondary 
support mechanism may include a plurality of tynes which are arranged, 
when supporting the stack, to extend between adjacent spacers. 
Preferably, means to circulate heated air comprises at least one fan 
coupled to ducting, the ducting being arranged to direct the air past the 
layers of timber. The ducting also being designed to reverse the direction 
of the stream of hot air so that as the stack passes through the apparatus 
the timber is sequentially subjected to currents of hot air from opposed 
directions. 
The apparatus preferably includes a plurality of zones sequentially spaced 
from the top of the stack and commencing with a preloading zone, a drying 
zone in which the timber is subjected to heated air, a coolng zone in 
which the layers of timber are open to atmosphere, a high humidity zone in 
which layers of timber are subjected to an air, water vapour mixture 
created by injecting steam into the space and finally an ejection zone in 
which the layers of timber are removed from the bottom of the stack.

Apparatus for drying timber as illustrated in FIGS. 1-3 comprises a 
vertical kiln supported by a frame comprising six vertically extending "I" 
girders 10 which connect the floor of the room in which the timber is 
dried to the ceiling and thereby provide a very firm and rigid support for 
the kiln. However, in an alternative arrangement, the whole apparatus 
could be free standing. At an upper end 11 of the girders a platform is 
provided by transversely extending girders 12 and 13, this platform 
constituting the timber in-feed end of the kiln. Adjacent the lower end of 
the girders 10 is a similar platform constituted by cross members 14 and 
this platform constitutes the ejection end 34 of the kiln. Four of the 
vertical girders define a zone 16 in which the timber is stacked during 
drying. The remaining space 17 defined by the framework of girders houses 
fans 20 and air dicts 21, together with a feeding conveyor system 22 and 
ejection means 23. 
The stack 19 of timber to be dried comprises spaced layers of timber, each 
layer comprising a plurality of strips of initially green timber 
approximately four to six meters in length and cut to a substantially 
constant thickness of typically between twenty to fifty millimeters. These 
strips are laid in a side-by-side relationship with their longitudinal 
axis parallel to the longest side of the stack, as shown in FIG. 1. The 
stack is supported at the base by a vertically displaceable support 
platform assembly 140. Each layer is spaced by the use of wooden spacers 
5, or stickers 5, which are laid in a spaced-apart manner on a layer of 
timber transversely to the longitudinal axis of the strips. A plurality of 
stickers are equally spaced on top of the adjacent layer and have the 
effect of spacing the layers of timber strips by about twenty to thirty 
millimeters. The next layer of strips is then laid on top of the spacers 
until the stack comprises a six meter high stack of timber with a layer of 
spacers between each layer producing, in the embodiment illustrated in 
FIGS. 1 to 3, a stack comprising ninety-four layers of timber. 
In FIG. 3 the various operative zones within the stack are illustrated. At 
the top of the stack there is a preloading zone 30 which extends for 
approximately one meter and in which green strips of timber are stacked 
and guided in the stack by the columns 10. The preloading zone 30 merges 
into the drying zone 31 which extends for three meters down the stack and 
terminates in a cooling zone 32 which adjoins a steaming zone 33. The 
cooling zone, including the steaming zone, extends for about one and a 
half meters. The steaming zone merges into an ejection zone approximately 
one meter in length terminating in the support platform assembly 140. 
The arrangement of the drying kiln is such that the bottom layer of dried 
timber is ejected by the ejection means every three to ten minutes, 
depending on the speed at which the apparatus is working and as the bottom 
layer is ejected the whole stack moves down one layer and a new layer is 
added at the top. In this way the timber passes from the top to the bottom 
of the stack to be ejected in a normal operation after about seven hours. 
The timber is dried within the drying zone 31 by a current of hot air which 
is driven by four electrical fans 20 which are connected to ducting 21 to 
drive air heated by a gas burner (not shown) through the drying zone. As 
shown in FIG. 3, which is a section taken along the lines A--A of FIG. 2 
illustrating the fan 20A and its associated duct 21A, the fan drives the 
air up a duct 50 around a corner 51 via baffles 52, along a horizontally 
extending upper pipe 33, through an upper region of the drying zone 31 
past the adjacent layers of timber. The air current is then turned by 
baffles 56 and 57 to continue along a horizontal path in a reverse 
direction through a lower group of layers of timber and finally to be 
reversed yet again in a still lower passageway 60 by baffles 59 and 61 
before returning to the air supply to be reheated. It can be therefore 
seen that within the drying zone 31 the hot air current passes through the 
stack of timber and is constantly reversed to dry the timber from two 
opposite directions. This ensures even efficient drying of the whole stack 
of timber within the drying zone. The ducting associated with each of the 
fans operates in the same manner to ensure that the drying zone operates 
across the whole length of the stack. It is understood that many other 
variations of ducting are acceptable to provide the opposed streams of hot 
air and that subsidiary heating members may be provided within the ducting 
to assist drying of the timber 
The timber, on leaving the drying zone, is then allowed to cool in the 
cooling zone 32 which can either be an area of the stack open to 
atmosphere or one subjected to a forced cool draught. The timber is 
subsequently exposed to conditions which induce stress relaxation and 
moisture redistribution created by introducing steam to a high humidity 
zone 33, to finally reach the ejection end 34 of the kiln. 
At the infeed end 22 of the kiln strips of timber are initially fed into 
the upper platform 13 of the structure via an infeed conveyor 72 
comprising a plurality of spaced-apart rollers 70 until the end of each 
strip abuts a stop at the edge of the framework. A strip is then 
transferred to a conveyor system 71 illustrated in FIG. 3, comprising a 
plurality of spaced-apart belts 77 driven by a pair of pulleys 74 and 75, 
the piece of timber 2 sliding down a slope 101 from the infeed conveyor 
system 72 to land on the belts 77. The belts then transfer each strip 
towards the stacking zone 16 until it moves into abutting contact with 
other strips 110 forming the next layer awaiting transfer to the stack 19. 
The strips are crowded into edge-to-edge abutment against fixed stops by 
the conveyor 77 and then a number of lifting bars 120 extending across the 
frame lift a layer of the boards clear of the stops so that they can be 
automatically transferred to the stack 19. In the meantime a layer of 
spacers 5, or stickers 5, has been positioned on the adjacent layer in the 
stack so that the new layer can be positioned thereon. The stickers may be 
positioned by hand or, alternatively, an automatic device may be used to 
position the stickers on the adjacent layer of timber. Because the strips 
of timber tend to be of varying length the infeed conveyor 72 is provided 
with "kick-off" (not shown) means at each end which are alternatively 
activated by opposite ends of the strips to transfer the strips from the 
rollers 70 to the belts 77. In this way it is ensured that long and short 
strips are uniformly distributed within the layers abutting alternate ends 
of the layer so that the layers are evenly supported in the stack as shown 
in FIG. 1. 
At the other end of the stack the ejection means 23 is provided to 
periodically remove the bottommost layer of timber and stickers 5 from the 
stack. The ejection means comprises the support platform assembly 140 
which extends across the width and length of the stack and is displaceable 
vertically by screwjacks 100 illustrated in FIG. 3 to abut the base of the 
bottom layer 130 of the stack of timber. Although screwjacks are 
illustrated in FIG. 3, other jacking means such as hydraulic or scissor 
jacks may be used. As shown in FIGS. 2, 4 and 5, the support platform 
assembly 140 consists of a spaced pair of longitudinal beams 160 which 
extend along the length of each side of the stack and a plurality of 
spaced transversely extending bridging "I" beams 161 which extend 
transversely from the longitudinal beams 160 so that the "I" beams 161 are 
directly beneath the spacers or stickers 5. An outfeed conveyor 120 
comprising a pair of spaced driving chains 129 (FIG. 5) is arranged to run 
parallel and between two pairs of the transverse "I" beams 161. 
The operation of ejection means is illustrated with reference to FIGS. 4A 
to 4F. During operation of the apparatus the support platform assembly 140 
abuts the bottom layer 130 until a new layer of green timber is added to 
the top of the stack 19 (FIG. 4A). At this stage the whole stack 19 is 
lowered by lowering the support platform assembly 140 by use of the jacks 
100 (FIG. 4B). 
A stack support mechanism 115 comprising a longitudinal beam 112 having 
secured thereto a plurality of spaced-apart tynes 113 (FIG. 1) to extend 
across the width of the stack is arranged to be horizontally driven by a 
conventional driving mechanism 119 in the direction of the arrow A in FIG. 
3, so that the tynes 113 locate between the layers 130 and a layer 131 of 
timber above the lowest layer 130, with each tyne between a pair of 
adjacent stickers (FIG. 4C). The location of the tynes 113 between the 
stickers 5 is shown in detail in FIG. 5. In this position the stack 
support mechanism 115 supports the stack and the platform assembly 140 can 
be lowered, thereby allowing the bottom layer 130 of boards and stickers 
150 to drop onto the chains 129 of the outfeed conveyor 120 which 
transports the dry boards and stickers away from the kiln (FIGS. 4D and 
E). The platform assembly 140 then rises to support the stack 19 and the 
stack support mechanism 115 is removed by moving it horizontally in the 
reverse direction of the arrow A (FIG. 4F). The support platform assembly 
140 then continues to take the weight of the stack until at the upper end 
a new green layer is deposited and the cycle repeats itself with layer 131 
becoming the bottom layer which is to be removed. 
The ejection means described above can also be reversed to load the 
apparatus with timber and stickers. Each layer of timber and stickers can 
be added to the bottom of the apparatus by simple reversal of the 
procedures described above. The layers can be fed in one by one until the 
stack is full. At this point, the operation and direction of the means is 
reversed and the drying, cooling and humidifying takes place in the usual 
manner. Since the bottom layers of timber, if green, would not be dried 
they could be recycled or alternatively the apparatus can be fed with 
timber in a varying state of dryness, the drier timber being positioned at 
the bottom of the stack. 
To operate satisfactorily, it is understood that the apparatus must also 
include a number of control mechanisms to ensure synchronous operation of 
the machine such as limit switches, hydraulic mechanisms, and timing 
mechanisms to ensure that each phase of the operation of the apparatus 
starts and finishes at the correct time before the next phase comes into 
operation. It is understood from the spirit of this invention that the 
apparatus could include any conventional type of control mechanism which 
would be used in an automated plant of the kind described above. The 
control mechanism is also arranged so that the speed of the machine can be 
varied to increase or decrease the frequency in which a bottom layer of 
dried timber is removed and replaced by a new green layer on the top of 
the stack. In a normal situation, the bottom layer of timber is removed 
every three to ten minutes depending on the drying speed that is selected. 
The apparatus may also include means to vary the temperature and velocity 
of the circulating hot air. Furthermore, the velocity, temperature and 
relative humidity of the air and speed of movement of the layers of timber 
may be monitored to allow an operator to vary any one of a number of 
variable parameters as desired. 
To illustrate the operation and advantages of the method and apparatus 
described above there follows hereunder two examples of a drying method 
utilizing an apparatus designed to simulate the conditions which the 
timber would experience when used in a vertical kiln of the kind described 
above. 
EXAMPLE 1 
One hundred and thirty-two Pinus radiata boards, 42 mm.times.102 mm in 
cross section and 3 m long were dried in an apparatus designed to simulate 
the conditions which such material would experience in the vertical 
continuous feed kiln. 
The boards were made up into a stack consisting of six layers of twenty-two 
boards, separated by 25 mm thick stickers on a rigid steel base frame. A 
second steel frame was placed on the stack and hydraulic cylinders 
attached to it were connected to rods passing through the stack and 
attached to the base frame. Hydraulic pressure in the cylinders exerted a 
compressive load on the stack. This load was increased during the 
seasoning cycle in such a manner as to simulate the increasing load 
experienced by an element of the stack in the vertical continuous feed 
kiln as it passed downwards through the kiln. 
In this test the tunnel kiln was equipped with a single natural gas burner 
so that the direction of air flow was not reversible. In order to reverse 
the direction of flow through the stack, midway through the drying the 
stack assembly was removed, turned through 180.degree. about a vertical 
axis and replaced in the kiln. Air velocity through the stack was 10 m/s. 
The temperature of the circulating air reached 120.degree. C. in half an 
hour, 150.degree. C. in two hours and 180.degree. C. in three and 
three-quarter hours. Drying time was four hours. At the end of the drying 
phase the stack assembly was removed from the kiln, the stack allowed to 
cool for one and a half hours and then steamed for one hour. Steaming was 
effected by enclosing the sides and ends of the stack assembly with panels 
containing steam inlets. 
The average initial moisture content of the boards was 67%, with the 
individual pieces varying from 30% to 173%. Final average moisture content 
was 11%. 
The level of distortion in the dried boards was extremely low, much lower 
than that in similar boards dried in conventional processes or in earlier 
tests with a prototype horizontal continuous kiln. 
EXAMPLE 2 
One hundred and eight boards of Pinus radiata were stacked into 6 layers of 
18 boards, but otherwise treated in a similar manner to Example 1. 
The tunnel kiln was made reversible by installing a second, larger gas 
burner; by reversing the fans and changing burners, reversal air flow was 
made three times during the drying phase. After each reversal the 
temperature of the air entering the stack was made approximately equal to 
the temperature of the air exiting the stack before reversal. This was 
done to simulate no reheating between the four passes of air through the 
stack in the vertical kiln design. 
______________________________________ 
Entering Air 
Leaving Air 
Temperature 
Temperature 
______________________________________ 
After heating for 1/2 hr 
174.degree. C. 
131.degree. C. 
Before first reversal 
197.degree. C. 
153.degree. C. 
Before second reversal 
165.degree. C. 
140.degree. C. 
Before third reversal 
126.degree. C. 
118.degree. C. 
Final conditions 
126.degree. C. 
119.degree. C. 
______________________________________ 
After drying for 33/4 hours the stack of timber was cooled for 1/2 hours 
with a forced draught of ambient air and then steamed for one hour. The 
hydraulic loading system was used to increase the load on the stack 
progressively until the stack had cooled for one hour after steaming when 
the load was completely removed. 
The boards dried from an initial average moisture content of 77% with 
extremes of 26% and 165% to a final average value of 15%. 
The level of distortion in the dried boards was again extremely low. 
It can therefore be seen from the method and apparatus described above that 
green timber can be dried efficiently and quickly in a single stack as 
well as being subjected to steaming and cooling, thereby allowing layers 
of completely processed timber to be removed from the base of the stack. 
In other high temperature cooling kilns the timber has to be dried in 
batches and then moved to different areas to be steamed and cooled. 
Furthermore, whenever a batch of timber is loaded or unloaded from such 
kilns much of the heat stored in the kiln to effect drying is lost and 
therefore fluctuation of the heat input is required to build up the 
temperature to the desired level at the start of the drying of each batch 
of timber. 
The continuous feed kiln of the kind described above allows the advantages 
of high temperature drying to be realized without the inefficiencies 
inherent in a batch-type process. The timber is fed layer by layer to the 
top of a high stack and for each layer added to the top a layer is removed 
from the bottom so that the timber progressively moves downwards through 
the stack to be dried, cooled, steamed and then cooled again before being 
removed. Because the layers are continually fed to the stack there is a 
regular heating demand and there is not the high loss of heat that takes 
place every time a complete batch is removed or added to the kiln, as in 
contentional kilns. Furthermore, by stacking the timber layer upon layer 
there is no requirement for weights at the top of the stack to prevent 
distortion. The drying zone can be positioned at a certain distance below 
the top of the stack so that there will be sufficient gravitational force 
caused by the green layers of timber above the drying zone to prevent 
those layers within the drying zone from distorting during drying. Another 
important advantage of the apparatus described above is that the cooling 
and steaming takes place within the one apparatus and there is therefore 
no need to transport the stacks of timber from the kiln to an area for 
cooling and then to a steaming or high humidity chamber. Because stickers 
stay in one stack throughout the whole process and for a much longer time 
in a conventional system, this kiln would require many fewer stickers. The 
reduced requirement would enable more durable material to be used with 
very much lower recurring costs. In the apparatus described herein, as a 
layer of timber is removed from the stack a layer of stickers is provided 
which can be placed at the top of the stack to support the green layer be 
being added to the top. Accordingly, only one set of stickers is required 
for each apparatus. 
The method and apparatus of the present invention also has the advantage 
that because the timer is free-standing in the stack, the thickness of the 
layers of timber can be then varied and the apparatus will work quite 
satisfactorily as long as each layer is of uniform thickness. In a 
preferred embodiment the timber is cut to constant thickness and length 
but the apparatus could be used to dry two different thicknesses of timber 
consecutively as long as each layer is of constant thickness and the 
variable parameters are adjusted as the interface between thicknesses 
progresses down the kiln. 
Accordingly, the method and apparatus of this invention provides a vastly 
improved and more efficient system for drying timber. The apparatus is 
virtually automatic and requires only one supervisory operator. Green 
timber can be dried quickly and efficiently to leave the kiln dried, 
steamed and cooled to the desired condition with a minimum of warpage of 
other distortion.