GB2468901A

GB2468901A - Microwave Oven

Info

Publication number: GB2468901A
Application number: GB0905195A
Authority: GB
Inventors: Jan S Przybyla
Original assignee: e2v Technologies Ltd
Current assignee: e2v Technologies Ltd
Priority date: 2009-03-26
Filing date: 2009-03-26
Publication date: 2010-09-29
Also published as: GB0905195D0; WO2010109249A1

Abstract

A microwave oven for processing a stream of material comprises a cavity (1), an input port (2) for introducing microwave radiation into the cavity, a tube (3) or feeding the stream of material through the cavity. Material (4) having a dielectric constant greater than unity is positioned in the region between the exterior of the tube and the interior of the cavity. This enables the same mode patterns to be supported but with a reduced size, enabling an increased power density to be attained with a given power input.

Description

MICROWAVE APPLICATOR

This invention relates to microwave applicators.

The invention especially relates to microwave applicators for processing a stream of material, for example, for processing mineral ores.

Referring to Figures 1 and 2 of the accompanying drawings, which are, respectively, schematic end and side views of a known microwave oven, a stream of material is fed through a tube 3 which extends through the cavity 1 of the microwave oven, having an input port 2 through which microwave radiation is introduced. In order to ensure uniform heating, the microwave oven is multi-mode, which means that more than one pattern of waves is set up in use. To achieve multi-mode operation, the dimensions of the cavity need to be carefully chosen. For example, at S-band (2.45 GHz), the wavelength is 12.2 cm, and a typical microwave oven may have a rectangular cavity of dimensions 35 by 20 by 25 cm (approximate figures only). Several patterns of standing waves may be set up in the oven in use.

The power density delivered to the load is dependent on the dimensions of the load and of the cavity. In the oven shown in Figures 1 and 2, the power density delivered to the load is a function of the diameter of the feed tube and the depth of the cavity in the direction of travel of the material. In order to treat certain ores, a typical average power density is needed of 106 W/m3 is required, but it is not possible to reduce the size of the cavity with the same input power, because a multimode design requires internal dimensions to be related to the wavelength.

The invention provides a microwave applicator for processing a stream of material, comprising a cavity, input port means for introducing microwave radiation into the cavity, a feeder for feeding the stream of material through the cavity, and material having a dielectric constant greater than unity positioned in the region between the exterior of the feeder and the interior of the cavity.

The dielectric material decreases the wavelength of the microwave radiation for a given frequency of exciting radiation enabling the cavity dimensions to be reduced while supporting the same modes and thus the power density in the material to be processed to be increased.

Ways of carrying out the invention will now be described in detail, by way of example, with reference to the accompanying drawing, in which: Figure 1 is an end view in schematic form of a known microwave applicator; Figure 2 is a side view in schematic form of the microwave applicator of Figure 1; Figure 3 is an end view in schematic form of a first microwave applicator in accordance with the invention; Figure 4 is a side view in schematic form of the microwave applicator of Figure 2; and Figure 5 is an end view in schematic form of a second microwave applicator in accordance with the invention.

Referring to Figures 3 and 4, the first microwave applicator has a multimode resonant cavity 1, an input port 2 for the microwave radiation, and a feeder tube 3 for feeding a steam of material to be processed through the cavity. The material is typically a mineral ore, and exposure to microwaves tends to break up the structure of the ore, making subsequent processing easier.

Various standing waves are set up in the microwave applicator and, since this is a multimode cavity, there could be many modes, each corresponding to a different wavelength in a narrow spread around the nominal operating frequency of the source, typically a magnetron. In one version, the nominal operating frequency may be in S-band, for example, at 2.45 GHz, and the various modes may correspond to 2.450 GHz, 2.450 1 GHz etc. It is important to have a minimum power density through which the material passes, but the cavity dimensions are determined by the wavelength of the modes, and so the power density cannot be increased by reducing the dimensions of the processing volume, for example, by reducing the length of tube 3 within the cavity.

According to the invention, there is provided material of high dielectric constant 4 surrounding the tube and filling the cavity 1. Because the wavelength of the microwave radiation will now be reduced, for a given frequency, the dimensions of the microwave applicator can be reduced while supporting the same modes, and providing a corresponding increase of power density.

Of course, the material of high dielectric constant will not be provided in the feeder tube 3 itself, but there will still be a reduction of wavelength in the cavity as a whole, enabling the overall size reduction to be made.

In a version with an operating frequency of 915 MHz, typical dimensions of the cavity would be 5X by 4.5X by 5.5X (Roger Meredith, Engineers' Handbook of Industrial Microwave Heating, Institution of Electrical Engineers, 1998), where X, is the free-space wavelength, which with a dielectric constant of 9 would amount approximately to O.53m by O.48m by O.58m. These dimensions would be approximately one third of the dimensions which would be needed without the high dielectric constant filling, since the wavelength is reduced according to the square root of the dielectric constant. The power density in the feeder tube, for any particular input power, would be increased approximately three-fold, because the length of the feed tube within the cavity is now reduced to a third of the value without the dielectric filling.

A suitable material of high dielectric constant would be alumina: this has a dielectric constant of 8. Low loss material is preferred, although there may be instances where a certain loss is deliberately attained in order to provide radiant heat to the material being processed.

Another example of suitable cavity dimensions is 0.424 m. x 0.446 m. x 0.491 m., with the diameter of the tube 3 being 0.27 m. The length of the cavity (the view in Figure 4) is the shortest dimension, and the load volume (the volume of the tube 3 which passes through the cavity) is 0.0242 m3. For a typical input power of 80kW, the average power density in the load volume is 3.33 x 106 watts! m3.

Of course, variations may be made without departing from the scope of the invention.

Thus, in one modification, the cavity 1 is formed by metallisation applied to the exterior of a solid block of dielectric material. Alternatively, the dielectric material may be in laminated form, for example, a column of blocks (with appropriately shaped openings in the region of the feeder tube 3. While a single input port has been shown, more than one input port may be provided.

The shape of the cavity need not be rectangular, for example, it may be octagonal in end view as seen in Figure 5, as indicated generally by the reference numeral 5. In this second embodiment of the invention, the dielectric material could be in the form of a tube 6 with a rectangular hollow region 7 which accommodates the feeder tube. One wall 8 of the cavity could be laterally movable for tuning purposes. Alternatively, more than one wall could be moved for tuning purposes. Equally, a part of the dielectric material 6 may be movable relative to the remainder of the dielectric material for tuning purposes.

The dielectric does not need to totally fill the cavity, as indeed the cylindrical dielectric 6 does not totally fill the cavity 5 in Figure 5.

Ceramic materials other than alumina may be used as the dielectric material, with different loss characteristics. Indeed, non-ceramic materials may be used as the dielectric material, for example, glass, and the dielectric material could even be a liquid.

The term "microwave" is intended to encompass frequencies in the range of from 108 to 10"Hz.

Claims

CLAIMS1. Microwave applicator for processing a stream of material, comprising a cavity, means for introducing microwave radiation into the cavity, a tube for feeding the stream of material through the cavity, and material having a dielectric constant greater than unity positioned in the region between the exterior of the tube and the interior of the cavity.
2. Microwave applicator as claimed in claim 1, in which the dielectric material is solid.
3. Microwave applicator as claimed in claim 2, in which the dielectric material is a ceramic material
4. Microwave applicator as claimed in claim 3, in which the ceramic material is alumina.
5. Microwave applicator as claimed in claim 2, in which the dielectric material is glass.
6. Microwave applicator as claimed in any one of claims 2 to 5, in which the dielectric material comprises a number of individual portions assembled together.
7. Microwave applicator as claimed in claim 6, in which at least one portion is movable relative to the other portion in order to tune the cavity.
8. Microwave applicator as claimed in any one of claims 2 to 7, in which the feeder tube is supported by the dielectric material.
9. Microwave applicator as claimed in any one of claims 2 to 8, in which the cavity is formed by metallisation on the surface of the dielectric material.
10. Microwave applicator as claimed in any one of claims 2 to 8, in which the cavity surrounds the dielectric material, one or more parts of the cavity wall being movable to tune the cavity.
11. Microwave applicator as claimed in any one of claims 1 to 10, in which the applicator is for processing a stream of mineral ore.
12. Microwave applicator substantially as herein described with reference to the accompanying drawings.