GB1291763A - Microwave applicator employing a flat multimode cavity for treating webs - Google Patents

Abstract

1291763 Drying by microwaves VARIAN ASSOCIATES 21 Jan 1970 [21 Jan 1969 17 Feb 1969] 2997/70 Heading F4G [Also in Division H5] A microwave applicator for heating and drying flat webs sheets or films 7 comprising a cavity resonator having openings 6 for the passage of the sheet 7 through a thin wide treatment zone in the cavity, the cavity having a pair of parallel broad walls 3 closed at their side edges by narrow walls 4, 5, the opening 6 being in end walls 5 and being aligned in the plane of the treatment zone, and means 9, e.g. magnetron for exciting the cavity in resonance modes whose electric field vector in the treatment zone lies parallel to the broad walls 3 and the treatment zone, wherein the spacing between the broad walls 3 is >¢# and <#, # being the excitation wavelength of the cavity. The magnetron 9 is coupled to the cavity by a waveguide 11 arranged at the corner of a narrow wall 4 with its broad walls perpendicular to the broad walls 3 of cavity so that TE 1,l,m and TM 1,l,m modes may be supported in the cavity. The cavity is made in two halves joined at 12 along the narrow walls 4, 5, a lossy mode damping element (Fig. 4, not shown) made of rubber loaded with iron, carbon or ferrite powders, being provided between the lips of the joint in walls 4 to suppress undesired modes which are generated despite the precautions taken above. Air ducts 8 in end walls 5 direct streams of air in counterflow to the direction of movement of speet 7. By making the length of the cavity sufficiently large for the range of the m or n designators to be greater than 5, preferably greater than 10 mode stirring is facilitated. The broad walls may be periodically reactively loaded. In Fig. 6A, the broad tap and bottom walls 33, 34 are defined by the inner ends of clusters of waveguide sections 43 dimensioned below cut-off at the operating frequency of the cavity. The outer ends of sections 43 are covered by an energy absorbing asbestos plates to avoid back cavity resonances. The asbestos plates are covered by metal, e.g. Al plates to prevent leakage. Alternatively, the outer ends of sections 43 may be open to facilitate ventilation, resistive cards being disposed diagonally in the sections if their lengths are insufficient to prevent leakage. In one example where the waveguide sections 43 are square their transverse dimensions are less than ¢# and their lengths greater or equal to transverse dimensions. The sections 43 may be of circular cross-section. The reactive periodic loading elements concentrate the frequencies of the desired resonance modes into narrow pass bands separated by stop bands thus increasing the mode density and improving the time-averaged impedance match during mode stirring. The reactive loading elements need be placed on one broad wall only. The loading elements may take the form of “# slots in the broad walls, or rods or bores in the broad walls. The walls may be loaded in one direction only by parallel conductive vanes having a height of “# by dielectric vanes, e.g. aluminance ceramic, by forming the walls from sinusoidally corrugated metal sheet whose corrugations have a height of less than “#, by fixing a corrugated dielectric sheet to the broad walls, or by forming the walls with triangular or square wave corrugations.