GB760296A - Improvements in or relating to thermally operated control systems for heating vessels - Google Patents

Abstract

760,296. Automatic temperature control systems. COLLIER, R. Feb. 21, 1955 [Oct. 21, 1954; Dec. 20, 1954], Nos. 30338/54 and 36745/54. Class 38 (4). [Also in Groups XI and XXXVII] A thermostatic control for cooking or heattreating vessels 3 heated by gas or an electric hot-plate 1 with sheathed heating elements comprises a switch with one or more bimetallic elements 8 riveted on a metal plate 9 spring- pressed against the wall of the vessel 3 when placed on the heater, the plate 9 being placed in a central aperture in the heater and protected by cylindrical heat shields 12, 13, 24 from heat from the heater. A transformer 16 supplies low voltage to the temperature-sensitive switch and to a relay 5 operated thereby to control through contacts 6 current supply to the heater. Four bimetal strips 8 for operation at different temperatures are mounted on the copper plate 9, which forms the top wall of a switch housing 12 of insulating material. A contact 10 on the end of each strip 8 moves to and from a contact on a metal screw 11 in the switch housing 12. A multi-position control switch 15 has an " off " position and four positions for connecting one of the bimetal strips 8 into circuit with the heater 1, which is connected to mains supply conductors 4 through a low-voltage relay 5 having high voltage contacts 6 and a low voltage coil 7 fed from a step-down transformer 16. A signal lamp 21 is connected across the coil 7 and indicates activity of the heater. The inner metal heat shield 13 is urged upwardly by a leaf-spring 14; and when a vessel 3 is removed from the hot-plate, a copper tongue 30 forming an extension of the plate 9 and normally below the heater 1 is raised with the plate 9 to contact the heater 1, which is thus maintained at a temperature corresponding to that of whichever bimetal strip 8 is in circuit. A corrugated metal strip forming a heat radiator is attached to each bimetal strip 8 at or adjacent its contact. A single bimetal strip 8 may be used in combination with a screw for adjusting its operative temperature, and this screw may be adjusted from a remote control knob by a flexible shaft. The outer metal heat-reflective shield 24 may have a ring of perforations 26 to allow passage of cooling air. A heat reflector 27 is provided and hot air may escape at 28 around the base of the vessel. An air gap may be formed between the metal plate 9 and the switch housing 12. The switch 6 may entirely cut off current to the heater, or may reduce the power supplied thereto. The switch 15 may have a sixth position (not shown) for by-passing the bimetal strips 8. when continuous full power is required, e.g. for deep frying. For gas-heating, a solenoidoperated gas valve replaces the relay 5.