Electric heating – Metal heating – Of cylinders
Patent
1988-02-10
1990-03-13
Leung, Philip H.
Electric heating
Metal heating
Of cylinders
219 1055F, 34 1, H05B 672
Patent
active
049084860
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a microwave drier used for the processing of various materials for example crops after harvesting, such as grains, beans, fruit, rice and the like. The present invention is also useful for the processing of minerals and by-products of the mining industry, peat moss, molasses and sugar by products, cotton, wool, tea, sand, sewerage and any other products which require partial or complete moisture reduction. The present invention can also be used for sterilisation and the eradication of insect pests within a material to be treated. In particular, the present invention relates to the design of a resonant cavity of a microwave drier which can utilise, simultaneously two or more continuous microwave sources to effect the drying/heating of a load placed within said cavity without causing any adverse effects to either microwave source.
In previous attempts to dry/heat material by the use of microwave energy directed into a resonant cavity, problems of `spot` drying arose, this was because the penetration of the microwaves into the material was not consistent throughout the material, thus leaving alternating hot and cold `spots`. This effect was caused by destructive and constructive interference and the antinodes and nodes of the resonance within the cavity. In addition these prior art microwave driers utilised pulsed microwave sources beamed into a square or rectangular resonant cavity. The maximum number of modes throughout the cavity was dependent on the overall size of that cavity. The provision of rotating turntables or field/mode stirrers improved uniformity of heating within the cavity by moving the material to be processed through the cavity or continually altering the mode pattern within the cavity.
The use of stirrers within a cavity causes problems when simultaneous use of several continuous microwave sources is desired in order to increase energy density within the cavity. Stirrers disturb the modal pattern such that a stationary nodal/antinodal pattern is not established within the cavity, resulting in the tendency for the magnetron to cross couple with likely destructive results. The use of a small cavity to increase energy density causes a reduction in the number of available modes resulting in uneven heating.
One of the basic problems of the rectangular or square-shaped cavity is that the nodes or hot spots in the mode pattern within the cavity are evenly spaced throughout the cavity producing regularly spaced hot spots. Most items to be processed within a rectangular cavity do not fully fill the cavity and hence cannot make full use of all the nodes within the cavity, resulting in uneven heating of the item.
The present invention seeks to substantially alleviate the above problems by providing a microwave drying system which utilises one or more controlled continuous microwave sources to allow more uniform penetration of the load material stationary within or passing through a processing zone within the cavity. In particular, the present invention seeks to provide a resonant cavity which, by its particular choice of design shape, size and dimensions, and placement of at least one microwave source provides a more efficient method of microwave drying/heating and provides a greater number of modes within the useable processing zone.
CHAMBER DESIGN CRITERIA
The geometric shape of a cavity is preferably such as to create as many modes of resonance as possible within the processing zone and across the frequency band utilised. Additionally, if more than one microwave source is to be utilised then preferable conditions for continuous and simultaneous operation of several microwave sources should be maintained.
The preferable conditions for continuous and simultaneous operation of several microwave sources within the one resonant cavity include: microwave source, are not reflected to the same or any other microwave source except after many multiple reflections from the walls of the chamber with (i) consequential passage through the load material placed within or travell
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Leung Philip H.
Nearctic Research Centre
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