GB2071833A - Vacuum drying apparatus - Google Patents

Abstract

The present invention relates to material drying apparatus particularly for drying heat sensitive materials and foodstuffs, the drying being effected under vacuum conditions. The apparatus includes an elongate vacuum chamber 2 partitioned into a number of drying sections, and material to be dried is fed through the sections from an inlet to a discharge by means of a vertical array of parallel endless bands 5. A first drying section utilises conventional conduction or radiation drying, e.g. hot plates 10, using heating medium such as steam, to remove the bulk of the liquid, while for the final stage of drying microwave heating is employed in a second drying section. The microwave drying means includes two separate microwave input units 12, 13, located at opposite edges of the bands and axially spaced. The units which comprise a plurality of vertically arranged microwave elements face over the parallel bands, and the form and arrangement of the microwave units provides for efficient drying in the microwave section. <IMAGE>

Description

SPECIFICATION Improved vacuum drying apparatus The present invention relates to vacuum drying apparatus.

Vacuum drying apparatus is known comprising an elongate cylindrical vessel having therein a continuous moving band or bands extending from end to end of the vessel, the material to be dried being pumped onto this band in the form of a paste or slurry via nozzle means at the inlet end of this vessel and the dried material falling into a product discharger at the outlet end. Means are provided for creating a vacuum in the vessel and drying is achieved for example by means of heating using circulating hot water or steam.

It is the principal object of this invention to provide a more efficient drying arrangement in this general type of apparatus.

According to the present invention there is provided apparatus for drying material, particularly heat sensitive materials and food stuffs, comprising an elongate vacuum chamber, at least two separate drying sections within the chamber and separated by partition means, an inlet for material to be treated, a discharge for dried material, conveyor means for the passage of the material through the drying sections between the inlet and the discharge, the second one of the drying sections including microwave drying means, said microwave drying means having a pair of microwave input units arranged in opposed relationship on either side of the elongate chamber.

'Preferably but not essentially said microwave input units are relatively axially spaced.

Preferably further, the first drying section includes conduction, radiation or contact heating means.

In a preferred embodiment these heating means comprise steam heating.

The microwave input units preferably comprise a plurality of groups of microwave elements. For example, each group can comprise four elements; each element consisting of one magnatron and one wave guide.

Preferably a microwave mode stirrer device is located between each group and the microwave cavity.

The microwave input units can feed into separate cavity chamber portions of the vessel, and each cavity chamber preferably includes end partitions or choke plates to minimise escape of microwave energy from the cavity. The choke plate will include narrow apertures for the through-passage of the conveyor means, and preferably also includes aperture means for escape of vapour from the cavity. The microwave input units and the mode stirrers can be mounted on a support structure on a side duct of the cavity.

An embodiment of the present invention will now be described by way of example with reference to the accompanying drawings wherein: Figure 1 shows a sectional elevation of a vacuum moving band dryer according to the present invention; Figure 2 shows a plan view of the dryer of Fig. 1; Figure 3 shows an end view through section A-A in Fig. 1; and Figures 4 and 5 show side and end views respectively on a single microwave input unit used in the dryer of Fig. 1.

Referring to the drawings, a vacuum band dryer 1 particularly for low temperature drying of heat sensitive materials and food stuffs such as meat, vegetable and other extracts, beverages and chemicals, comprises a cylindrical vessel 2 divided into flanged sections 2A-2E, closure ends caps 3, 4 and a vertical array of four continuous bands 5 in the vessel 2 and extending from end to end thereof. The bands 5 are trained around end rollers 6, 7 with spring tensioning means 8 connected to rollers 6 in inlet section 2A, and the bands 5 are driven by drive gear 9.

Suitable means (not shown) are provided for creating a suitable vacuum within the vessel 2. The vessel can generally be made of mild steel with the conveyor bands of glassfilled P.T.F.E. or other suitable material. Material is delivered evenly to the bands by any suitable delivery system, such as for example by pumping through nozzles in the form of a paste or slurry.

Initial drying of material is achieved by a vertical array of hot plates 10 located in sections 2A, 2B and engaging the underside of each bands top run, the plates 10 being heated for example by circulating hot water or steam.

In section 2C, microwave heating is utilised rather than the steam or hot water heating of the previous sections. The microwave system 11 comprises a pair of side-by-side microwave input units 1 2,1 3 the peripheral casings 14 of which constitute the casing portion 2C of the cylindrical vessel 2, each unit 1 2,1 3 having end flanges 1 5 to facilitate fitting to the adjacent casing section. Each microwave input unit 12,13 (shown in detail in Figs. 4 and 5) includes a diverging side duct 16, and on an end support structure 1 7 three groups of microwave elements 18 are mounted in vertical array.Each group of microwave elements 1 8 includes two opposed pairs of magnetrons 1 9 which deliver to a mode stirrer 20 via wave guides 31 whence the waves pass to the respective cavity. Each magnetron 1 9 has an output of 1.5 KW of microwave power.

This power is supplied to each magnetron pair from a respective power supply unit 32, each of which units 32 can feed to two magnetrons independently, so that the actual microwave power input to the cavity can be varied by direct switching to suit a particular operating condition up to maximum power output. The groups of microwave elements 1 8 are located horizontally to allow microwaves to transverse of bands 5, and, as best seen in Figs. 2 and 3, an important feature of the present system is that the groups of microwave elements of one unit (1 2,1 3) are oppositely located to but axially spaced from the groups of microwave elements of the other unit.The support structure 1 7 is removably secured to the duct 1 6 by pivotal clamp screws 21, a polypropylene plate 22 being sandwiched between the structure 17 and the duct 16. The cavity of each unit 1 2,1 3 has its ends substantially closed by a partition or choke plates 23 so that escape of microwave energy from the cavity can be minimised: In Fig. 5 the !eft side choke plate has been removed for clarity.

Each choke plate 23 is held between adjoining flanges of adjacent sections as can be seen in Fig. 1, and includes a series of channel element pairs 24,25 defining apertures for through-passage of a respective band 5, the upper channel 24 being of greater depth than the lower one for accommodation of material. Further, the upper channel 24 includes flanged steps 26 for the support of polypropylene band supports 27 within the cavity, the upper run of each band passing over the respective band support 27, and the plate 23 additionally carries vent tubes 28 for vapour release from the cavity. The groups of microwave elements 1 8 can for example provide a microwave energy with a frequency of 2450 M.Hz and a wavelength of 0.122 metres.Drains can be provided in the cavities for the removal of washing fluids, and pivotal irradiation windows can be used for access for the cavities. The materials of construction used in the microwave section will be stainless steel or aluminium with the exception of the band supports, bands themselves and cavity windows giving access to the cavity of the microwaves from the external source, all of which will be in a material transparent to microwaves. In particular the band supports and the windows can be of polypropylene construction. All inspection windows, light glasses and like parts are desirably screened with stainless steel perforated plates with the perforations not exceeding 3 mm diameter and at the maximum pitch which can still permit effective vision.Also, joints at the microwave section or adjacent thereto should use Copandus" type jointing, while other jointing can utilise silicon rubber. By these means it should be possible to maintain microwave leakage to the order of 1 m.w/cm2 measured at a distance of Scms from the external surface. A vacuum of 10 torr can be maintained within the cavities. Fewer or a greater number of conveyor bands could be used Additionally sensing and control apparatus (not shown) can be provided to ensure the following functions.

(a) The microwave power cannot be switched on until the load is in the microwave cavity and (b) that the microwave power is switched off before the load leaves the cavity.

As the machine is designed for continuous operation this will apply particularly at the beginning and end of a production run. At the same time it will be operational on a continuous basis to detect and correct any inadvertent interruptions in load.

Conventional conduction heating using steam or hot water can dry material satisfactorily and efficiently when the material has substantial wetness content and is easily dried, and consequently this heating mode is employed in sections 2A and 2B. However, when the wetness content of the material is markedly reduced heating by the above conduction mode becomes relatively inefficient, so that the provision for drying by the microwave technique as used in section 2C should provide a considerably increased overall efficiency in the drying operation. It is possible for the microwave heating mode to account for 1 /3-2/3 of the overall drying cycle. Microwave equipment can be relatively expensive but by utilising the above dual mode heating system the overall cost of the equipment can be optimised while providing an overail efficient drying system.Further, in the above arrangement tnree groups of microwave elements 1 8 introduce the microwave power from one direction while the other three groups of microwave elements 18 introduce the microwave power from the other side and at an axial spacing. This should provide improved microwave absorption patterns, since the moisture removal profile obtained in the first cavity 1 2 is substantially reversed in the second cavity 13, resulting in improved penetration of the material layers by the microwaves and a uniformly dried product.

The section 2D includes cold plates 29 engaging the bands 5 for cooling of material, while final section 2E has a discharge 30 into which the dried material falls from the bands 5.

Modifications are of course possible in the apparatus. For example, a different array of groups of microwave elements could be used while still providing oppositely introduced microwave power. In particular, the precise number of groups of microwave elements could be adjusted to provide the desired power level. In the above example, the six groups of microwave elements provide no less than 24 microwave elements each of 1 > kw maximum power giving 36 kw total power for the system, so that substantial variation is possible.

Higher or lower maximum power levels could be provided, using substantially the same configuration, to suit either an increased or decreased evaporative load or an increased or decreased number of conveyor bands by varying (a) the number of input units (d) the number of groups of microwave elements (c) the number of elements in each group (d) the power levels of the elements or a combination of any or all of items (a) to (d).

Claims (11)

1. Apparatus for drying material, particularly heat sensitive materials and food stuffs, comprising an elongate vacuum chamber, at least two separate drying sections within the chamber and separated by partition means, an inlet for material to be treated, a discharge for dried material, conveyor means for the passage of the material through the drying sections between the inlet and the discharge, the second one of the drying sections including microwave drying means, said microwave drying means having a pair of microwave input units arranged in opposed relationship on either side of the elongate chamber.

2. Apparatus for drying material, particularly heat sensitive materials and foodstuffs, comprising an elongate vacuum chamber, at least two separate drying sections within the chamber and separated by partition means, an inlet for material to be treated, a discharge for dried material, and conveyor means for the passage of material to be dried through the drying sections between the inlet and the discharge, said conveyor means comprising a plurality of parallel endless bands arranged one above the other, a second one of the drying sections including microwave drying means, said microwave drying means including a pair of microwave input units arranged in opposed relationship on either side of the bands and facing across the bands.

3. Drying apparatus as claimed in claim 1 or 2, wherein said microwave input units are relatively axially spaced.

4. Drrying apparatus as claimed in any one of the preceding claims, wherein the first drying section includes conduction, radiation or contact heating means.

5. Drying apparatus as claimed in claim 4, wherein the heating means comprise steam heating.

6. Drying apparatus as claimed in any one of the preceding claims, wherein the microwave input units comprise a plurality of groups of microwave elements.

7. Drying apparatus as claimed in claim 6, each element consisting of at least one magnetron and one wave guide.

8. Drying apparatus as claimed in claim 6, wherein each element comprises one or more magnetrons firing into a common launch section.

9. Drying apparatus as claimed in claim 6, 7 or 8, wherein a microwave mode stirrer device is located between each group and the microwave cavity.

10. Drying apparatus as claimed in any one of the preceding claims, wherein the microwave input units feed into separate cavity chamber portions of the vessel.

11. Drying apparatus as claimed in claim 9, wherein each cavity chamber includes end partitions of choke plates to minimise escape of microwave energy from the cavity.

1 2. Drying apparatus as claimed in any one of the preceding claims, wherein the microwave section is removably connected to the remaining sections by fastening means.

1 3. Material drying apparatus substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.