Method and apparatus for drying particulate material

A method and apparatus for drying particulate material such as bark by forming a circulating bed of other material to be dried, which bed, being guided by wires, moves on an endless web around two wire turning rolls. Material to be dried is continuously fed onto the bed and dried material is continuously discharged from the innermost layers of the bed, such that the thickness of the bed remains constant. The material to be dried is discharged by means of a dosing cell roll, which is disposed in the opening between the horizontal section of the first wire loop and the horizontal section of the second wire loop.

BACKGROUND AND SUMMARY OF THE INVENTION 
The present invention relates to a method and apparatus for drying 
pulverous, granular or chip formed material such as bark, in which method 
and apparatus drying is effected by introducing drying gas through a bed 
formed of the material to be dried on an endless, moving wire. 
Conventionally, drying of such materials is effected by means of a conveyor 
belt permeable to gas. Drying gas is blown through the conveyor belt from 
below and the material to be dried is fed to the front part of the belt 
and discharged from the rear end. Having regard to energy consumption and 
the drying process, such drying method is not optimal. 
The object of the present invention is to provide an improved method and 
apparatus for drying particulate material. 
The method of the present invention is mainly characterized in that a 
material to be dried forms a circulating bed, one side of which, i.e., 
where the through-flowing drying gas is discharged, is continuously 
supplied with material to be dried and the other side of which bed, i.e., 
where the drying gas is introduced, has a corresponding amount of dried 
material continuously discharged therefrom. 
The apparatus of the present invention is characterized in that it 
comprises first, second, and third wire loop sections, each with a wire 
turning roll and a wire guide roll. The section supporting the bed of the 
second wire loop is disposed after the section supporting the bed of the 
first wire loop, in the direction of movement of the materials. There is 
an opening between the first and second sections supporting the bed. The 
wire of the third wire loop is guided via the outer wire turning roll of 
the second wire loop, below the first and second wire loops, further to 
the utmost wire turning roll of the first wire loop, and runs partly 
around the wire turning rolls along with the material. The material thus 
lies between the second and third wires and between the first and third 
wires. The feed means is disposed in the substantially straight, 
preferably horizontal, front end of the dryer section of the first wire 
loop, and the discharge means is arranged in the opening between the 
sections of the first and second wire loops, which sections support the 
bed. 
The present invention will be further described in greater detail, with 
reference to the accompanying drawings, which illustrate an example of 
implementation of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS 
A drying apparatus according to an embodiment of the present invention is 
shown in the drawing figures and comprises a conveyor having three endless 
perforated webs, i.e., wire loops, 1, 2 and 3. Inside the wire loop 1 is 
disposed a large wire turning roll 4, a small wire turning roll 5 and 
several wire guide rolls 6. A wire 7 of the wire loop 1 forms, between the 
wire turning rolls, a substantially horizontal section 8 supporting the 
material to be dried. Inside the wire loop 2, there is a large wire 
turning roll 9 and a small wire turning roll 10. A wire 11 of the wire 
loop 2 forms another, substantially horizontal section 12 between the wire 
turning rolls, which section supports the material to be dried. A 
horizontal section 12 of the wire loop 2 is disposed after the horizontal 
section 8 of the wire loop 1 in the direction of movement of the material 
so that an opening 13 is formed between the inner wire turning rolls 5 and 
10 leaving the large wire turning rolls 4 and 9 at opposite ends of the 
conveyor. Inside the wire loop 3 is disposed a plurality of wire guide 
rolls 14, by means of which the wire 15 of loops is guided to run from the 
wire turning roll 9 to the wire turning roll 4 below the wire loops 1 and 
2 so that wire 15 forms a substantially horizontal section 16 supporting 
the material to be dried. Wire 15 is further guided partly around the wire 
turning rolls 4 and 9 along with the material which in this case lies 
between the wires 11 and 15 and between the wires 7 and 15. A feed means 
17 for feeding material to be dried continuously onto the conveyor is 
disposed at the front and upstream end of the horizontal section of the 
wire loop 1. The bed is evened and its height adjusted by means of a 
circulating lug conveyor 18. 
Referring to FIG. 3, a roll 19 serves as a dosing discharge means. Roll 19 
is disposed crosswise to the direction of movement of the material and is 
arranged in the opening 13 between the wire sections 8 and 12. Below roll 
19 is disposed a screw conveyor 20 for transporting dried material 
laterally from the conveyor. 
Referring more particularly to FIG. 3, the discharge means between the wire 
sections 1 and 2 may comprise a discharge roll 19 mounted for rotation on 
a shaft and having longitudinally extending radial ribs 25 
circumferentially spaced one from the other to define cells or 
compartments 24 in the roll 19. Roll 19 is disposed in the gap 13 between 
the wire sections 1 and 2 and prevents the material from falling freely 
through that gap between the rolls 5 and 10. Instead, roll 19 is rotatable 
in a reverse direction from the direction of rotation of wire turning 
rolls 5 and 10. Roll 19 removes in a controlled manner material from the 
overlying bed of material as each compartment 24 is, in turn, filled with 
material when the compartment faces the bed. When the roll 19 rotates, 
e.g., 180.degree., the material within the compartment 24 is emptied from 
the roll into the underlying screw conveyor 20 for transport laterally 
from the conveyor. The amount of material which can be removed can be 
regulated by varying the rotational speed of the roll. However, the 
removal of the material from the conveyor in the manner indicated below is 
a continuous process. It will be appreciated from the foregoing 
description and the following description of the operation that other 
equivalent structure may be provided to remove the under or lower layer of 
the material forming the bed on the upper conveying flights of the 
conveyor as the layered material passes the opening 13 en route from wire 
section 1 to wire section 2. For example, a doctor blade could be used in 
lieu of a rotating roller. 
Hot and dry drying gas is introduced through the inside of wire loop 1 into 
a bed of material 21 forming an endless web thereof guided by the wires 1, 
2 and 3 and circulating around the wire turning rolls 4 and 9. The drying 
gas flows through the wire 1 and the bed and further into a casing 22 of 
the dryer. Humid gas is handled in a manner known per se in order to 
remove water therefrom and to reheat it for further use. 
The dried material removed by roll 19 from below the bed is discharged from 
the dryer by means of screw conveyor 20. Inside the undermost wire loop 3 
is disposed a wire turning roll 23, which is displaceable as indicated in 
broken line so as to discharge the entire bed if necessary. 
The discharge means 19 preferably removes the same amount of material from 
the circulating bed as is fed into it. Thus, the thickness of the bed 
remains substantially constant. The material to be dried circulates three 
to four turns in the dryer and about the conveyor until it is discharged. 
Thus, the bed is formed of several layers, the outermost of which is the 
most humid and the innermost of which is the driest. In this way, a long 
retention time in the drier is achieved for the material to be dried, the 
material moving in counterflow with respect to the drying gas. Because the 
bed moves around the wire turning rolls between the wires, the material 
cannot move and humid and dry material cannot mix. 
In operation, material to be dried is fed into the chute 17 constituting 
the feed means for deposition on the wire section 1 in the form of a layer 
of material extending the width of the wire section 1. By initially not 
rotating roll 19, or covering roll 19 or lowering a doctor blade from a 
position in the path of the material on the bed to a position flush with 
the conveyor surfaces of wires 1 and 2 if roll 19 is not used, the 
material flows continuously from wire section 1 across gap 13 and the roll 
19 or doctor blade onto wire section 2 where it is carried about the 
turning roll 9 by a cooperation of wire sections 2 and 3. The material 
advances along the inside surface of the wire section 3 where it is 
sandwiched between the wire section 3 and wire section 1. The material 
then moves about turning roll 4 for passage below the feed chute 17 where 
the material, being fed continuously, deposits a second layer on the first 
layer. The first and second layers thus are transported about the conveyor 
similarly as the first layer until a third layer is deposited by chute 17 
and so on until the desired number of layers are deposited on the wire 
sections. At that time, the discharge roll 19 is uncovered and rotated in 
the direction indicated by the arrow in FIG. 3. The material of the first 
layer is deposited into the compartments 19 and, upon rotation for 
approximately 180.degree., is released to fall by gravity into discharge 
conveyor 20. The speed of rotation of roll 19 is such that substantially 
all of the first layer along the underside of the multiple layers of 
material forming the bed on the wire section 1 is removed. The theretofore 
second outermost layer of material then becomes the layer of material 
contiguous to wire section 2 beyond roll 19 and which layer of material is 
transported about the conveyor for removal in the same manner the first 
layer is removed. As a consequence, the material makes multiple passes 
about the conveyor before it is located contiguous to the wire sections 
for discharge by roll 19. In this manner, the driest of the material on 
the conveyor is removed from the conveyor while the most humid of the 
material constitutes the outer layer of the material carried about the 
conveyor. By regulating the feed rate of material through chute 17 and 
corresponding it to the discharge rate of material taken from the 
underside of the bed of material transported about the conveyor, it will 
be appreciated that the supply of humid material 2 and the discharge of 
dried material from the endless conveyor may be at the same rate with each 
layer making multiple turns about the conveyor between its initial deposit 
onto and its discharge from the conveyor. 
The present invention is not limited to the embodiment described above, but 
it may vary within the inventive scope defined by the following claims. 
Thus, drying gas may flow through the material in an opposite direction, 
and the points of feeding and discharging may be disposed differently from 
what has been described above. By the term "wire" used anywhere herein, 
there is intended to mean any endless web of perforate or apertured 
material.