GB1215369A - Oven for heating sealed packages - Google Patents

Abstract

1,215,369. Microwave oven. REJLERS INGENJORSBYRA A.B. 25 June, 1968 [26 June, 1967], No. 30302/68. Divided out of 1,194,594. Heading H5H. [Also in Division B8] A microwave oven for heating food &c. in tightly sealed packages 5a, 5b, which packages have at least one wall which will bulge on rise of internal pressure, has at least one device which detects bulging when the package is in a predetermined position and either switches off or reduces the microwave heating supply or supplies cooling air to the package. In the oven shown packages 5a, 5b are placed on a conveyer 3, which consists of two or more parallel spaced narrow bands or cords, and are introduced into the multizone oven by an electromagnetically operated stop 10. The packages 5a, 5b are arrested individually by a stop in heating zone 1 and heated therein either for a set time or until foil cover 50 bulges, the bulge interrupting a beam of light which normally illuminates a photo-cell 6. The photo-cell thus releases the stop and the package moves to the next heating zone where it is tested by further photo-cells for tightness. In the final heating zone 2, the leading package continues its movement until it interrupts the light beam of a photo-cell 7 which actuates a solenoid 9h to raise armatures 9a, 9f and lift the package from the conveyer belt. A pair of spaced temperature sensors 9d, 9e on the armature engage the bottom of the packet and measure the temperature at spaced points. Heating continues until the temperature at both points reaches the desired temperature. A photo-cell 6 interrupts or reduces heating or supplies cooling air by way of nozzle 12 should the packet bulge during this heating stage. When the final temperature is attained solenoid 9h is de-energized and the packet is lowered to belt 3 which conveys it out of the oven. The temperature sensors 9d, 9e may be thermocouples, thermistors, e.g. wires, layers or heat sensitive semi-conductor diodes or transistors. Alternatively a temperature responsive elongated resistive strip may be used in place of the two sensors, its means resistance determining the final temperature. A radiation pyrometer sensitive to infra-red may be used or the packets may have strips of thermo-colour, i.e. portions which change colour at a specified temperature, which are detected by photo-cells. Softenable portions of the packet made, for example of wax may be detected optically, mechanically, or electrically. The bulge detector may be mechanical, pneumatic, or capacitive. A bolometer may be used, the packet either interrupting the radiation beam or an air jet directed to the bolometer. Packets which do not bulge are rejected. Unbulged, or bulged packets may be marked, and the marked packets sensed photo-electrically. Food in unsealed packets may be cooked, a height detector at the oven entrance rendering the bulge detectors inoperative for this procedure.