Method and Apparatus for Drying Particulate Material

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.

FIG. 1 is a schematic vertical cross-sectional diagram of an apparatus according to the present invention;

FIG. 2 is a plan view of the apparatus of FIG. 1;

FIG. 3 is an enlarged fragmentary vertical cross-sectional view of the discharge roller between the first and second conveyor sections.

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.