GB1235004A - Microwave heating - Google Patents

Abstract

1,235,004. Microwave heating. VARIAN ASSOCIATES. 11 Oct., 1968 [13 Oct., 1967], No. 48442/68. Heading H5H. In order to ensure coupling to a large number of resonator modes a microwave heater comprises a multimode resonator cavity 12, a coupling hole 51 adjacent a corner 56 of a boundary wall for introducing microwave energy of a selected frequency to the resonator, the proximate edge of the coupling hole 51 being not further than from said corner, where # is the free space wavelength at said selected frequency, a single waveguide 52 so constructed and oriented at the coupling hole relative to the resonator cavity that the energy propagated thereby has an electric field component in a direction which lies parallel to a line intersecting the angle of corner 56; and means e.g. mode stirrers 88, for shifting the frequencies at which the various resonator mode patterns occur about said selected frequency. As shown, the coupling hole 51 is triangular, its apex being formed in the corner of boundary wall 18 and its sides extending a distance #/2 from the corner. This arrangement provides time averaged uniform heating. The coupling hole 51 may be displaced from the adjacent corner providing it does not extend to points further than # along the boundary wall edges. The waveguide 52 used may propagate TE or TM waves. To obtain non-uniform heating additional coupling holes may be provided in other corners, and these additional coupling holes may be arranged in the manner disclosed in copending Specification 1,233,030. The oven shown is a conveyer type oven, the entry and exit tunnels 25, 27 comprising hollow tubular members containing a lossy material e.g. water, ethylene glycol, glycerol, or any low molecular weight monohydric alcohol, to prevent escape of energy. To enable maintainance to be easily carried out two hinged doors 33 are provided which close on to respective triangular-section compressible conductive ribs 37 surrounding the door oyenings. Air is directed through a lower part of the chamber 12 beneath a microwave transparent polypropylene panel 97 from an aluminium air plenum 41 and is exhausted through a plenum 43. The dimensions of tubes 42, 44 joining the plenums to the chamber 12 are chosen to ensure that they do not propagate energy. Three mode stirrers 88 are provided, each being rotated by an external electric motor 93. The microwave source 74 may be mounted directly on the wall of the resonator at the coupling hole.