Apparatus for drying wood and other solid material

Apparatus for drying wood or other solid material has an elongate cylindrical vessel in which there is an elongate platform on which is placed the material to be dried. The platform divides the interior of the vessel into an upper drying chamber and a lower condensing chamber with restricted communication between the two chambers. In the drying chamber there is a heater and one or more fans for circulating the air and water vapor in the chamber. An air cooling channel of much greater width than height extends below the vessel for its full length and is formed between a lower part of the vessel and a spaced outer wall formed of sheet metal or plastic and preferably heat insulated. One or more fans in the base of a chimney at one end of the cooling channel produce variable and reversible flow of air through the cooling channel.

FIELD OF INVENTION 
The invention relates to apparatus for drying wood and other solid material 
consisting of a vessel for vacuum operation in which there is installed a 
support for cut wood, a heating device and a circulating device for the 
drying medium, a condenser and a cooling air channel of which one wall is 
formed by a part of an outer wall of the vessel. 
BACKGROUND OF THE INVENTION 
Such apparatus is the subject of EP 0 505 586 A1. This apparatus dries wood 
by vacuum and represents a so-called vacuum dryer. This consists of a 
pressure-fight hermetically closable drying chamber with a connected 
vacuum pump, has a support for the cut wood, is provided with a heating 
device and a circulating device for the gaseous drying medium and has, in 
the interior of the vessel, a condenser which is wholly or partially 
formed through a part of the chamber wall which is cooled by a cooling 
channel lying outside the vessel so that that part of the chamber wall 
functions as a condenser. The cooling channel thereby serves to carry off 
the condensation heat, which is produced by condensation of water in the 
form of water vapor upon its condensation on the condenser which is cooled 
to a temperature below the point of condensation. 
In this publication, the vessel has no insulation on the portion below a 
partition of separating the condensation space from the drying space but 
has laterally arranged ventilators which circulate cooling air 
transversely to the axial direction of the vessel. As the length of the 
vessel is usually much greater than its width, it is necessary, in order 
to provide effective cooling, to provide a plurality of blowers arranged 
along a line parallel to the axis of the vessel in order to provide 
effective cooling. The cost and arrangement of so many blowers is 
expensive. 
The energy requirements for driving such a large number of blowers makes 
the cost of drying wood excessive. By reason of the short distance which 
the cooling air travels with the known arrangement of blowers, relatively 
little heat is carded off per volume of cooling air. 
This disadvantage is partially avoided through another arrangement of a 
vacuum dryer described in DE-OS 42 08 913. This publication shows a 
cooling air channel which is defined by the vessel outer wall and the 
foundation, which extends in the axial direction of the vessel and 
contains cool air ventilators in the channel below the vessel. In this 
manner the need of a large number of blowers is avoided. This embodiment 
has, in particular, with a long vessel, the disadvantage of high 
foundation cost. Moreover, because of the relatively great height of the 
channel, which is greater than the diameter of the ventilator, the 
electrical energy required for the ventilator motor is not well utilized 
since an essential part of the conveyed air does not come in contact with 
the vessel wall and therefore does not contribute to cooling. 
However, not only is the condenser to be cooled but also other components, 
in particular the vacuum pump or its parts. The energy for cooling the 
vacuum pump or its parts is not so great because the vacuum pump is not 
constantly in operation but is switched on in particular at the beginning 
of the drying procedure. It is thus in operation when the vessel wall need 
not yet be cooled. 
SUMMARY OF THE INVENTION 
The present invention avoids the disadvantages of the state of the art. It 
is an object of invention with low equipment cost and with limited energy 
requirements to achieve exceptional cooling, in particular of the 
condenser. 
The invention consists therein that the cooling channel extends axially 
over the greater part of the vessel in a lengthwise direction, that the 
air stream in the cooling air channel flows in a direction lengthwise of 
the vessel and that the effective channel width is greater than the 
channel height. 
Through this design there is attained a large heat exchange surface 
relative to the volume of the cooling channel. According to the vessel 
length, one or two ventilators are sufficient. When, for a given channel 
width, the height of the channel and thereby also its cross section, is 
reduced, the velocity of the air in the channel, with equal conveying 
capacity of the ventilators, is correspondingly increased. Now the amount 
of heat which is transferred with a given temperature difference per unit 
time and surface area of a fixed body in a flowing medium (known as the 
heat-transfer coefficient) is to a great extent proportional to the 
velocity of the stream. It follows that when a channel height which is 
small with respect to the channel width (i.e., the length of the arc of 
the channel) is selected, effective cooling of the condenser with less air 
is achieved. However, the height of the channel cannot be reduced to zero 
since the flow resistance of the channel becomes too great without the 
stream velocity further increasing. The amount of air transported per unit 
time, i.e., the heat absorbing capacity of such an amount of air, must be 
sufficient to absorb the required amount of heat before reaching the 
temperature of the vessel. In practical design, according to to the length 
and diameter of the drying vessel, as well as according to the cross 
section of the installed ventilator, the channel height is selected 
between 2 cm and 15 cm. The exact selection of the height of the channel 
within these limits depends primarily on the kind and size of the material 
to be dried, namely, the favorable speed of drying for this material 
through which the necessary cooling capacity is provided. 
In accordance with that intention, a channel height is selected which is so 
small that the air conveyed in the channel reaches a sufficiently high 
velocity and all parts of the air volume come into contact with the vessel 
wall in their travel through the channel, while the channel cross section 
is, however, sufficiently large that the air volume conveyed per unit time 
is sufficient in order to carry away the condensation heat according to 
the speed of drying. 
It is advantageous when the cooling channel in its length, preferably at 
its ends, is widened and these widened portions contain ventilators for 
conveying the cooling air. It is hereby achieved that high-output blowers 
of the usual design can be installed in order to forward the cooling air 
through the very flat channel. 
It is advantageous when the end portions of the cooling channel, which 
extend beyond the ends of the vessel, are closed by a vertical wall in 
which openings provide orifices in which ventilators for forwarding the 
cooling air can be mounted in a simple manner. 
The ventilators can be axial or radial fans or blowers. 
At opposite ends of the vessel, missing surfaces of the cooling channel can 
be replaced by a cover. 
The production of the cooling channel is very simple and inexpensive when 
the outer wall of the cooling channel is formed of sheet metal or plastic. 
It can be advantageous when the outer wall of the cooling channel is heat 
insulated or is formed of heat insulating material. It is hereby achieved 
that the cooling power with ventilators at rest is reduced to a small 
value when in the running drying phase, there occurs little or no 
demisting. 
When, for example, for rapid drying of green wood with high outside 
temperature, an additional condenser with closed cooling circuit is used 
in the drying chamber with the flat heat exchanger of this additional 
condenser in the cooling channel, an additional cooling device can be 
avoided. 
In order to reduce the current requirement for the cooling fans, it is 
advantageous when the cooling channel is connected with a chimney. For 
this, it is advantageous when the cooling channel is connected through 
ventilator openings for fans with a chimney having a height of at least 
three meters and having at its upper end an adjustable damper. With the 
damper open, the ascending air current in the chimney produces an air flow 
through the cooling channel even when the ventilators are not running. The 
strength of this air flow can be controlled by the position of the damper. 
A further function of the chimney is to dampen the noise of the cooling 
ventilators. For this purpose is it advantageous when the chimney is lined 
with sound-absorbing material. The same purpose can, for example, be 
obtained by a brick chimney. Thereby it is possible to operate the drying 
apparatus in the vicinity of residences since commercial drying is not 
interrupted at night, when the cost of electric current is less. 
With especially long vessels it can be advantageous to exhaust the cooling 
air from both ends of the cool air channel and to provide in the region of 
the middle of the vessel an enlarged space in the cool air channel from 
which the used cool air can be exhausted, for example, through one or two 
chimneys. 
The cooling requirements in vacuum drying relate also to the drive of the 
vacuum pump. A high-performance pump requires first a precooling in the 
intake region so that the partial pressure of the steam to be condensed 
can be reduced to the pressure required for the pump intake. Secondly, the 
heat of operation of the pump must be carded off so that a favorable 
operating temperature can be maintained. With larger pumps this is 
achieved by means of an external heat exchanger or through direct cooling 
with expensive fresh water. Thirdly, it is necessary to cool the electric 
motor of the pump in order to reduce wear and increase the life of the 
pump. It is therefore advantageous when the vacuum pump, or a part of the 
vacuum pump, preferably the intake, a heat exchanger and/or the motor are 
accommodating in the cooling channel. The cost of an additional cooling 
device for the vacuum pump is thereby avoided. The intake pipe between the 
drying vessel and the pump is led through the cooling channel, the pump 
being assembled so that the required air stream flows over the pump motor 
and the heat exchanger of the pump cooling circuit is arranged in an 
appropriate place in the cooling channel. An appropriate place is the 
chimney or the widened part of the cooling channel. 
In order on the one hand to control the cooling power of the condenser and 
also the pump heat exchanger and, at the same time, to reduce the cost for 
electrical energy, it is advantageous to use speed regulated ventilators. 
A controlled cooling of two independent systems with only a single blower 
control is possible because normally the two systems are not in operation 
at the same time. The vacuum pump is in operation only before the 
beginning of drying, during the evacuation of the drying chamber for a 
long period of time when the wood is not yet heated and there is no 
condensation. In the further process of drying, the pump comes into 
operation only for a relatively short period of time when the partial 
vacuum in the chamber falls below the desired value on account of a leak 
in the chamber or because of air entrained in the wood. The adjustment of 
the desired value of vapor partial pressure occurs above the condensation 
temperature and requires no operation of the pump. 
When drying with relatively high temperatures and the cooling air along the 
length of the vessel is no longer sufficient to cool the pump, the 
introduction of reversible cooling ventilators is advantageous. By 
reversing the direction of rotation, it is achieved that cool fresh air is 
drawn in adjacent the vacuum pumps or their parts. The drawing in of fresh 
air can also be effected by the chimney. 
The reversability has another advantage. When drying is effected with 
relatively high air partial pressure of the drying medium, spontaneous 
uniform diffusion of the water vapor in the drying chamber through the 
diffusion process is hindered. Wood piles which lie in the cooler 
condenser region on the air inlet side dry somewhat faster than the piles 
on the other side. Through reversing the air flow direction at suitable 
intervals, this effect is eliminated. There are also wood piles in the 
regions of the ends of the drying vessel which are subject to unequal 
drying when, for example, they are brought into the chamber with different 
moisture content.

DESCRIPTION OF PREFERRED EMBODIMENT 
A cylindrical vessel 2 having at one end a hinged door 1 is provided on its 
upper side with heat insulation 3. The uninsulated lower part of the 
vessel has on its outer side a cooling channel 4 of which the width is 
very much greater than its height. This cooling channel is formed on one 
side through the lower noninsulated part of the wall of the vessel 2 and 
on the other side through a bottom or outer wall 5. The outer wall 5 is 
formed of sheet metal or plastic sheeting and is preferably 
heat-insulated. Running on tracks in the interior of the vessel is a 
carriage 6 having a load surface 7 on which the stacks 8 of the wood to be 
dried are supported. The load bearing surface 7 of the carriage 6 forms a 
part of a partition which is extended on both sides of the carriage 6 with 
the stacks of wood 8 through partitions 9 and which divides the interior 
of the vessel 2 into a lower condensation chamber 10 and and upper drying 
chamber 11. The condensation chamber 10 is thus formed by the load bearing 
surface 7, the partitions 9 and the uninsulated lower part of the wall of 
the vessel 2. The remaining part of the inner space of the vessel is the 
drying chamber 11 in which there are one or more ventilators or fans 12 
for circulating air in the chamber. A heating device 13 is provided for 
heating air in the upper drying chamber 11. Through the ventilator or 
ventilators 12, a circular flow of the drying medium in the drying chamber 
is produced. Through the gaps between the wall of the vessel and the load 
bearing surface 7 and the partitions 9, water vapor continually flows from 
the drying chamber 11 into the condensation chamber 10 and is here 
condensed in the form of water on the lower part of the vessel wall. The 
vessel stands on feet 14 which are supported by a suitable foundation (not 
shown). 
The cool air enters the cooling channel 4 at the end of the vessel provided 
with the door 1. The cooling channel extends to the end of the vessel 
opposite the door and terminates in a part 15 with enlarged cross section. 
The cooling channel is here terminated through a wall 16 having openings 
17 in which fans or ventilators 18 are arranged. These forward the warmed 
cooling air into a chimney 19 in which the vacuum pump 20 and a heat 
exchanger 21 of the pump cooling cycle are accommodated. In the upper end 
portion of the chimney, there is an exhaust opening in which a variable 
damper 23 is accommodated.