GB2191569A - Multi-compartment rotary drier - Google Patents

Abstract

A rotary drier comprising a cylinder (3) mounted on an axis which is inclined at a small angle to the horizontal, and is provided with internal lifting blades or paddles (9) the interior of the drier being divided into a plurality of axially extending compartments (16). <IMAGE>

Description

SPECIFICATION Multi-compartment rotary drier This invention relates to rotary driers of the kind including heated cylinders and particularly to those which are provided with internal lifting paddles or blades, which are referred to hereinafter as driers of the type herein defined.

In driers ofthis type the cylinder axis is inclined ata sma l l angle to the horizontal, so that the material to be dried progresses through the cylinder each time it is lifted and dropped by a paddle. An axial current of process air carries away the moisture evaporated from the material being dried.

A majorfactor affecting the design of such a drier is the surface area available for conductive heat transfer between internal surfaces of the drier cylinder and the material to be dried. In the case of tobacco, and other particulate materials of low thermal conductivity, only the layers of material in close proximity to the cylinder surfaces are heated significantly. In order to achieve adequate drying rates without charring, the drier is designed to alternately lift and drop the material, thus presenting fresh faces of material to the hot surface of the drier.

Whilst the material is being lifted by the cylinder paddles, conductive heat transfer takes place.

However, whilst the material is being dropped, the only heat transfer is by radiation, which is not significant at the temperature normally used for tobacco processing.

The present invention therefore seeks to provide a drier having an increased surface area availablefor heattransferto the material.

Accordingly the present invention provides a rotary drier of the type herein defined, in which the interiorofthe cylinder is divided into a pluralityof axially extending compartments. Preferably, the compartments are formed by radially-inwardly extending paddles of a basically conventional type, but which are extended in their radial length so asto joint together at the centre ofthe cylinder, so asto form walls defining compartments of a sector-shaped cross-section between them. These walls may be straight and parallel to the axis ofthe cylinder, or alternatively they may be arranged in a helical spiral.

In this preferred form ofthe invention, auxiliary vanes or paddles may also be provided projecting from the radial paddles so asto provide a greater surface area.

Compared with a conventional rotary drier, the material to be dried is distributed between a number of compartments, thus increasing the ratio of its surface area to volume, and hence the surface for heat transfer. This enables the length of drier required for a given duty to be reduced, resulting in a more compact design. A further advantage is that the dwell time of material within the drier is reduced,this improving process controllability and reducing degradation ofthe material caused by abrasion and breakage each time it is lifted and dropped within the cylinder. By increasing the numberof compartments, a greater heated surface area may be incorporated within a cylinder of given diameter.

Some embodiments ofthe invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows a schematic longitudinal cross-section through a known type of cylindrical drier; Figure2 shows a diametral cross-section through the drier of Figure 1; Figure3showsadiametral cross-sectionthrough a drier in accordance with a first embodiment ofthe invention; Figure 4 shows a diametral cross-section through a drier in accordance with a second embodiment of the invention; and Figure 5shows a schematic longitudinal cross-section through a drier in accordance with the invention.

Referring to Figure 1, this shows a drier comprising a rotatable cylinder 3 having radially-inwardly extending heated paddles 4. The product two be dried is introduced into the inlet 1 and is moved along the length of the cylinder as a consequence ofthe downstream inclination ofthe cylinder and the alternate lifting and dropping ofthe product by the motion of the paddles as they are carried around by the rotation of the cylinder.

Rotation ofthe cylinder is achieved by means of track rings (5) which run on support rollers 6 (Figure 2).

Process air is introduced at the downstream end of the cylinderthrough an inlet8 and leaves atthe upstream end through an exhaust passage 7.

The arrangement ofthe paddles (4) can be seen more clearly in Figure 2, from whips it will be appreciated that the paddles are relatively short in the radial direction, and as the cylinder rotates in the direction of the arrow 'A' any given portion ofthe product is successively moved upwardly around the internal circumference of the cylinder and then dropped back into the interior.

Figure 3 shows a first embodiment of the present invention, in which it will be seen how the "paddles" (9) are extended to the centre of the cylinder so as to divide the interior into sector-shaped compartments (16). The appreciable increase in surface area will be clearfrom this drawing, and also the somewhat different manner of movement if material in the cylinder, since each portion 'P' is restricted to move within its own compartment after its initial introduction into the cylinder.

Similarly, the heated surface area may be increased by the addition of auxiliary paddles (10) projecting from the walls of the compartments (Figure 4). Forexample,the heated perimeter of a conventional 6 foot diameter drier with 16 paddles, each 1 foot long is46.6feet. Forasixcompartment drier of the same diameter, with 4 auxiliary paddles per compartment, each being 6" long, the heated perimeter is 78.8 feet, which corresponds to an increase in heated area of 69%.

The function of the auxiliary paddles is twofold: firstly,to augmentthe heated surface area.

Secondly, the auxiliary paddles assist in retaining material on the heated surfaces for longer than would otherwise occur, which also assists in transferring heated the material. When material eventually falls from the auxiliary paddles, it tends to cascadethrough the air, ratherthan rolling or sliding over the inclined surfaces. This is desirable in order to exposethe material to the air, to allow drying to take piace.

The maximum numberofcompartmentsand auxiliary paddles which may be employed is limited by the nature of the material being processed.

Clearance between adjacent paddle tips and compartment walls must be sufficientto allow material to cascade freely as the drier rotates. For granular, free flowing materials, these clearances may be less than thos required for fibrous or stranded materials.

Figure5 illustrates the mounting and inlet and outletarrangements of a preferred form of drier according tothe invention. Thecylinder(13) has an axially extending hollow shaft (18) mounted on trunnion bearings (12) and rotary unions (11) are used to connect a supply of heating medium (such as steam)tothehollowshaft.Sincethecompartment walls extend to the centre of the cylinder, the steam can then be easily distributed within the body of each compartment wall (14). Thus it will be appreciated that this arrangement also has a considerable advantage over the conventional construction in this respect, since the steam distribution can be centralised.At the inlet end (20) ofthe cylinder, the compartment walls (24) ofthe cylinder are cut away to a shorter radial length as indicated at (22), so that they do not extend to the central shaft (18). In this way an annular opening (24) isformed into which an inlet conveyor (26) extends so that the material to be dried is successively deposited into the open "tray" formed between each pair of adjacent compartment walls (14), and passed from there into the compartment proper.

In the preferred emodiments illustrated, the paddlesandcompartmentwallsarestraightand parallel with the axis ofthe drier. Alternatively, they may be arranged in a helical spiral along the axis of the drier.

The drier cylinder may be mounted on track rings andsupportrollers,orasintheembodimentof Figure 5, carried on an axial shaft supported at each end bytrunnion bearings. In the latter case, thefluid used for heating the cylinder can be conducted to and from the cylinder by rotary unions attached to the central shaft as described above, and the drying cylinder can be supported by the radial walls of the compartments.

Steam or pressurised hot water are suitable heating media forthis type of drier. Other fluids may be used e.g. proprietary heattransferfluids such as Dowtherm (Trade Mark).

Claims (6)

1. A rotary drier comprising a cylinder mounted on an axis which is inclined ata small angle to the horizontal, and is provided with internal lifting blades or paddles the interior of the drier being divided into a plurality of axially extending compartments.

2. A rotary drier according to claim 1 including a plurality of internal blades which extend radially-inwardly from the internal walls so as to join together in the interior ofthe cylinder to form the said compartments.

3. A rotary drier according to claim 2 in which the said blades are planar and extend parallel to the axis ofthe cylinder.

4. A rotary drier according to claim 2 in which each blade forms a helical spiral.

5. A rotary drier according to any preceding claim further comprising a pluralityofauxiliary vanes or paddles which are arranged on the radial paddles.

6. A rotary drier substantially as herein described with reference to the accompanying drawings.