GB2235115A - Microwave baking ovens - Google Patents

Abstract

Dough is supported in an oven by a conductive conveying band 1, the oven being heated by electrical heating elements 4 located below the band, and by microwave heating employing a tri-plate electrode assembly in which the band 1 forms one electrode of that assembly, the other electrodes being provided respectively by transverse flat bars 6 located above the band, and by the bottom walls (25) of manifold fingers 19 which supply forced convective heating nozzles 16, 17. Infrared heaters 13 are positioned above the band 1 to provide additional heating in the gaps between the flat bars 6. Earthing of the conductive band (1) is made by way of choke assemblies arranged along the respective edges of the band, an air-gap (26) of L-section, Figure 5, constituting two lambda /4 waveguides connected in series. <IMAGE>

Description

BAKING OVENS This invention relates to baking ovens.

One aspect of the invention is concerned with microwave heating of an oven. Microwave heating of ovens is, of course, known but we have devised an arrangement for heating an oven of the kind comprising a conductive conveying band.

According to the first aspect of the invention a baking oven comprises a conductive conveying band, a plurality of electrodes located above the band, and a microwave energy source connected to the electrodes for exciting the electrodes with microwave power.

Previous attempts to use microwave heating in conjunction with a conveying band have employed an electrically non-conductive band, such as a glass fibre band.

The conductive conveying band may be, for example, a strip of metal or a wire mesh.

Preferably the conductive band is arranged as a second electrode of a 'tri-plate' assembly, the first electrode being one of said plurality of electrodes connected to the microwave power source, a third electrode being positioned above said first electrode.

The conductive band and the third electrode are then preferably earthed.

Earthing of the band is preferably provided by means of at least one choke.

A respective choke is preferably arranged to provide earth connections to the opposite edges of the band.

The choke is preferably a quarter wave choke, that is, it has a length which is an odd multiple of a quarter of a wavelength.

Each choke preferably comprises a choke element of angle shape in a transverse section of the oven, and constituted by insulative material, such as an air gap.

Preferably each limb of the L-shape is J/4 long.

The angle of the L is then set in by A/4 from the respective edge of the band, and the inboard limb of the L is bounded by conductive walls, whereby the outwardly directed limb at its free end provides a short circuit connection with the adjacent edge of the band.

According to a second aspect of the invention, a baking oven comprises a conveying band above which are positioned a plurality of electrodes to provide microwave heating of dough etc carried by the band, a second form of heating means being provided at locations between the electrodes.

The second heating means may be infra-red heating means and/or convective gas heating means.

Such heating means may be arranged to direct the heating medium downwards through gaps between the electrodes.

According to a third aspect of the invention in a baking oven a conveying band is heated from above by near infra-red heating means and/or by convective gas heating means, and from below by a black element electrical heating means and/or by a convective gas heating means.

A baking oven in accordance with the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a longitudinal vertical cross-section of a length of the oven, Figure 2 is a cross-section on the line B-B of Figure 1, Figure 3 is a section on the line A-A of Figure 1, Figure 4 is a horizontal cross-section on the stepped line C-C of Figure 1, and Figure 5 is an enlargement of a portion of Figure 2 to show the choke.

As shown in Figure 1, a steel conveyor band 1 extends across an interrupted horizontal support surface provided by the tops 2 of spaced band support units 3. Support units 3 are of oblong-rectangular box shape, with open bottoms, and may be of electrically conductive or insulative material. A set of conventional finned black element electrical heating elements 4 is provided below the band 1, the elements 4 being disposed in the respective gaps 5 between the support units 3 and spaced downwards from the steel bands. The elements 4 provide, in conventional manner, infra-red heating to the underside of band 1.

Above the band 1 is disposed a series of transverse electrodes 6 of flat bar form, each electrode being disposed in a common horizontal plane parallel to the band 1. The width of the electrodes 6 in the longitudinal direction of the band 1 is preferably substantially equal to the corresponding dimension of the support 3 and is preferably substantially in register therewith, as shown in Figure 1.

The vertical clearance between the electrodes 6 and the band 1 is typically 38 mm.

The electrodes 6 are connected at one end in pairs respectively to longitudinal waveguides 7, 8, positioned externally of the insulated oven wall 10 and suspended at their free ends by supports 9, Figure 2.

The ten electrodes shown are connected by waveguides 7, 8 and respective waveguide feeders 11, 12 connected therewith to a 60 kW 915 MHz 60 Hz Magnetron power unit. In the illustrated oven the band is 500 mm wide, and the rating of the microwave heating provided by electrodes 6 is therefore 41.8 kW/m2. Waveguide feeders 11, 12 are suppliedXin known manner, out of phase to minimise power fluctuations.

Disposed above the gaps 5 and in substantial register therewith is a series of eleven NIR (near infra-red) heaters 13 with associated reflectors 14 to provide infra-red radiation shining down on band 1 between electrodes 6. The total energy input from the NIR heaters 13 above the band 1, taken with that of the black element heaters 4 below the band, is 45 kW approximately, giving a rating of 31.4 kW/m2.

The infra-red heating elements of heaters 13 are shielded from the microwaves by respective microwave screens 15 positioned below heaters 13.

Forced convection heating is provided both in the spaces 5 below the band 1 and in the regions between the electrodes 6 above the band 1, respectively by a series of first and second nozzles 16, 17 supplied with blowing air respectively from a duct 18 and from a manifold comprising a series of manifold fingers 19 positioned between the adjacent reflectors 14. The duct 18 and fingers 19 are fed by a feeder duct 20, and the exhaust gases from above and below the band 1 pass through collecting ports 21 leading to a pair of convection collecting ducts 22, Figure 2.

The air to duct 20 is provided by a fan 23 by way of a duct heater 24.

It will be seen that the nozzles 16, 17 are arranged to direct the convection air towards the exposed portions of the band 1. A typical air flow rate is 60 m3/min, heated by heater 24 having an energy input of 54 kW, thus providing the same rating as the NIR/black elements (13, 4) at 31.4 kW/m2.

Each electrode 6 forms part of a respective 'tri-plate' configuration of electrodes, (such a configuration having three electrodes), each electrode 6 constituting a first electrode, a second, earthed, electrode being provided by the band 1 itself, and a third, earthed, electrode being provided by the bottom wall 25 of the respect manifold finger 19 which is in register with the electrode 6.

Earthing of the conductive band 1 is made by way of choke assemblies arranged along the respective edges of the band 1. Reference should be made to Figure 5 which shows how an L-shaped air gap 26 is defined between a cranked metal side-plate 27, and on the one hand the unsupported margin 28 of the band 1, and on the other hand the vertical side wall 29 of band support 3.

The air gap 26 has a horizontal limb 30 of length equal to a quarter wavelength 84 of the microwaves, and a vertical limb 31, similarly of a quarter wavelength 44, to constitute two 44 waveguides connected in series. A flange 32 provides a conductive termination to the waveguide constituted by limb 31, with the result that for the microwaves the open end of limb 30 provides a virtual short circuit between the edge of band 1 and the plate 27, which is earthed.

Thus the air gap 26 in conjunction with the plate 27 and support side wall 29 constitute a choke.

In a modification the gap 26 is filled with a solid electrically insulating material.

The illustrated length of oven can be made of greater length, or more usually will be associated with further lengths or sections of oven provided with modified heating means. Some sections of the oven may not include all of the different heating means provided In the illustrated section, but such sections may still embody some aspects of the present invention.

With regard to the wavelength of the microwave radiation to be employed, this will normally be in one of the two currently permitted ranges of wavelength, which are: (i) 2,450 + approx 100 MHz.

(ii) 880-950 MHz, but it should be appreciated that other wavelengths may in future be used if the regulations change.

Claims (10)

1. A baking oven comprising a conductive conveying band, a plurality of electrodes located above the band, and a microwave energy source connected to the electrodes for exciting the electrodes with microwave power.

2. An oven as claimed in claim 1 in which the conductive band is arranged as a second electrode of a 'tri-plate' assembly, the first electrode being one of said plurality of electrodes connected to the microwave power source, a third electrode being positioned above said first electrode.

3. An oven as claimed in claim 2 in which the conductive band and the third electrode are earthed.

4. An oven as claimed in claim 3 in which earthing of the band is provided by means of at least one choke.

5. An oven as claimed in claim 4 in which a respective choke is arranged to provide earth connections to the opposite edges of the band.

6. An oven as claimed in claim 4 or claim 5 in which the choke is a quarter wave choke, that is, it has a lengtn which is an odd multiple of a quarter of a wavelength.

7. An oven as claimed in any one of claims 4 to 6 in which each choke preferably comprises a choke element of angle shape in a transverse section of the oven, and constituted by insulative material, such as an air gap.

8. A baking oven comprising a conveying band above which are positioned a plurality of electrodes to provide microwave heating of dough etc carried by the band, a second form of heating means being provided at locations between the electrodes.

9. A baking oven in which a conveying band is heated from above by near infrared heating means and/or by convective gas heating means, and from below by a black element electrical heating means and/or by a convective gas heating means.

10. A baking oven substantially as described with reference to the accompanying drawings.