GB2085564A

GB2085564A - Fluidised bed drier

Info

Publication number: GB2085564A
Application number: GB8031645A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1980-10-01
Filing date: 1980-10-01
Publication date: 1982-04-28

Abstract

A fluidised bed drier comprises a drying trough having a floor through which, in use of the drier, air is passed to form a fluidised bed, the trough having upwardly divergent sidewalls (6, 7) so that the air flow path immediately downstream of the trough floor is of progressively increasing cross section, and means (11, 12) for recirculating part of the air leaving the fluidised bed to a location to enter the bed just downstream of the trough floor. <IMAGE>

Description

SPECIFICATION Improvements in fluidised bed driers The present invention relates to fluidised bed driers and to drying material, particularly, but not exclusively, tea, using such driers.

The establishment and maintenance of fluidisation in a drier handling material at very high input moisture content is not easy, especially where the drier is of the continuous flow (as opposed to batch) type. The difficulties are magnified when the machine is required to produce material at almost bone-dry output moisture content.

For example, in the production of tea, the preliminary withering, cutting and fermenting stages reduce the initial total leaf weight/dry water ratio of 4.2 to almost 3.0, that is, the initial wet basis moisture content of from 420% to 300%. In the final drier, the wet basis moisture content has then to be reduced from 300% to about 3%.

This variation in particle weight of 3:1 between inlet and outlet requires staging of air flow, temperature and grid plate geometry, together with the provision of a substantial length/width ratio. Recirculation of unsaturated exhaust gas from the discharge end of the drier is also desirable.

According to the present invention, there is provided a fluidised bed drier comprising a drying trough having a floor through which in use air is passed to form a fluidised bed, the trough having upwardly divergent side walls so that the air flow path immediately downstream of the trough floor is of progressively increasing cross section.

Preferably the floor and side walls are rectangular and the height and angle of inclination of the side walls is adjustable either by adjustment of the same side walls or by replacing them with ones of different dimensions.

preferably at least part of the air leaving the fluidised bed is recirculated to a location just downstream of the trough floor, e.g. via one or more apertures in the side walls or via one or more gaps between the lower edges of the side walls and the trough floor. The invention also provides a method of drying material, e.g. tea, using a fluidised bed drier according to the present invention and to material dried using the drier or method.

The invention will be further described by way of example with reference to the accompanying drawings, in which: Figure 1 is a simplified perspective view of part of a conventional fluidised bed drier; Figure 2 is a corresponding view of an embodiment of the present invention; and Figure 3 is a scrap sectional view of the embodiment of Figure 1.

The conventional fluidised bed drier shown in Figure 1 comprises a trough 1, in which in use the fluidised bed forms, defined by an elongate rectangular bed plate or grid 2 and vertical side walls 3 and 4. Below the plate 2 is a plenum chamber 5 to which air is supplied from a suitable fan or compressor, not shown. In use, the trough is charged with material to be dried, such as tea, and air is supplied to form the fluidised bed until the material has reached the required degree of dryness.

Figure 2 shows how the drier of Figure 1 may be modified in accordance with the present invention. Here, a pair of flat, divergent rectangular side walls 6, 7 are supported on the plate 2 by a series of optional spacers 8 so that the effective area of the plate 2 is reduced. The side walls 6, 7 are oppositely inclined at the same angle and have two parallel out-turned flanges such as 9 and 10 which engage with the plate 2 and the upper edges of the outer walls respectively. The side walls 6, 7 either by appropriate construction or replacement may be arranged so that their angle of inclination and/or the effective width of the plate 2 may be adjusted as desired. The effective floor plate width may vary along the length of the bed. The side walls 6, 7 may be arranged in sections, each of constant different inclinations, arranged in series uniformly along the length of the bed.

The drier may be fed continuously using a feed conveyor at one end of the bed and a weir discharge at the other.

In one practical form which may be used for drying tea, the drier has a floor use of about 0.9 x 6M and an evaporative capacity of 700 KG of water per hour at a dry material output of 350 Kg an hour. In that particular form, the floor of the trough is flat, although in other embodiments the floor might slope from one end of the drier to the other.

Use of the sloping side walls 6 and 7 confers the following benefits:- 1. The length/width ratio of the bed is increased.

2. The fluidising velocity is raised without, however, increasing the disengagement velocity and the attendant blow-out.

3. The effective percentage free area of the fluidising plate 2 is reduced, in the proportion to the area blanked off by the side walls 6, 7 and, further, the extent of the blanking can be varied down the length of the drier. Thus, for any given floor plate 2, the effective free-area can be adjusted by varying the side wall slope to provide the best practical working conditions, providing striking improvements in design flexibility.

4. The side skirts need not be attached with a sealed joint to the floor plate; a continuous slot, or intermittent slot if through attachments are required, can be provided which now allows the introduction of air (see Figure 3) immediately above the floor plate 2. This confers the great advantage that dust-laden exhaust air can be recirculated as by means of the hood 11 and blower 12 without passing through the floor plate, eliminating both interaction with the fluidising air flow and the necessity of the filtration to avoid clogging of the floor apertures. Instead of being passed through the gaps between the lower edges of the side walls 6, 7 and the floor plate 2, the recirculated air can, of course, be passed through apertures in the side walls.

Claims

1. A fluidised bed drier comprising a drying trough having a floor through which, in use of the drier, air is passed to form a fluidised bed, the trough having upwardly divergent sidewalls so that the air flow path immediately downstream of the trough floor is of progressively increasing cross section, and means for recirculating part of the air leaving the fluidised bed to a location to enter the bed just downstream of the trough floor.

2. A fluidised bed drier according to claim 1, wherein the air recirculating means is arranged to recirculate the air into the fluidised bed through one or more gaps between the lower edges of the side walls and the trough floor.

3. A fluidised bed drier according to claim 1 or 2, wherein the air recirculating means is arranged to recirculate air into the fluidised bed through apertures in the side walls.

4. A fluidised bed drier according to claim 2 or 3, wherein the air recirculating means is arranged to recirculate air into a chamber defined outwardly of the diverging walls, said chamber communicating via said apertures and/or gaps with the fluidised bed.

5. A fluidised bed drier according to any preceding claim, wherein the height and angle of inclination of the side walls is adjustable.

6. A fluidised bed drier according to any preceding claim, wherein the side walls are readily replaceable.

7. A fluidised bed drier according to any preceding claim, wherein each of the side walls is comprised by a plurality of sections arranged longitudinally of the drier.

8. A fluidised bed drier according to any preceding claim, wherein a feed conveyor is provided at one end of the bed operable to feed material continuously to said one end of the bed during operation of the bed with a weir discharge being provided at the other, opposite, end of the bed.

9. A fluidised bed drier constructed and arranged to operate substantially as hereinbefore described with reference to and as illustrated in Figures 2 and 3 of the accompanying drawings.

10. A method of drying tea using a fluidised bed drier in accordance with any preceding claim, wherein tea is fed into one end of the conveyor and drying air is passed through the floor of the drying trough to form a fluidised bed of the tea with the depth of the fluidised bed becoming deeper towards the discharge end of the bed as the moisture is extracted to leave lighter material with the width of the bed being wider at the downstream end due to the upwardly divergent walls.

11. A method according to claim 1, wherein the weight of the material is reduced by substantially 300% during drying due to the removal of moisture therefrom.

12. A method of drying tea substantially as hereinbefore described with reference to Figures 2 and 3 of the accompanying drawings.