GB2215134A

GB2215134A - Thermally-sensitive control switch

Info

Publication number: GB2215134A
Application number: GB8901884A
Authority: GB
Inventors: Philip Gordon Binns
Original assignee: Strix Ltd
Current assignee: Strix Ltd
Priority date: 1988-01-28
Filing date: 1989-01-27
Publication date: 1989-09-13
Anticipated expiration: 2009-01-27
Also published as: SG100292G; HK42793A; GB2215134B; GB8901884D0; GB8801907D0

Abstract

A thermally sensitive control 7 for an electric liquid heater (e.g. a kettle) comprises a conventional bimetallic actuator 14 which when exposed to vapour trips a switch 12 via overcentre spring mechanism 11 to disable the heater 2. The actuator 14 is arranged in an upwardly open chamber which communicates with a vapour outlet aperture 16 so as to establish a convection current flow through the chamber, and impinging on the actuator 14. The chamber may be formed by a collar portion of a convection member 17 which also has a hood like section 18 which covers aperture 16. The member 17 is preferably a separate member which engages the upper part 9 of the control body 10, although the chamber and hood sections could also be formed integrally with the control body 10. Conventional boil-dry mechanism is also provided. <IMAGE>

Description

THERMALLY-SENSITrVE CONTROLS This invention relates to thermally-sensitive controls for electric heaters of liquid heating containers such as water beating kettles and ugs, urns, coffee percolators, laboratory equipment and the like, such containers being referred to hereinafter as "containers of the kind described".

The heaters of such containers may be in the form of immersion heaters or similarly constructed heaters mounted externally of the container e.g. by being brazed to the underside of the container base.

The heaters traditionally terminate in so-called cold leads by means of which electrical connections may be made to the heatina element. Thermallysensitive controls for such heaters are well-known which include thermallv-responsive actuating means, such as a bimetallic actuator, mounted in the proximity of the heating element and operable to open switch means of the control so as to interrupt the power supplv to the heater in the event of over-heating consequent upon the heater boiling dry or having been switched on with insufficient liauid therewithin to cover the element. The bimetallic actuator of such a control is typically arranged in thermal contact with a so-called hot return part of the element and connected to the switch means via a mechanical coupling such as a push rod.

Thermally-sensitive controls of this type are also known which comprise a second bimetallic actuator positioned in use adjacent a steam outlet aperture formed in a side or top wall of the associated container so as to he exposed to steam or vapour egressing from the container during boiling. The second actuator is coupled to the same or further switch means of the control ia an over-centre spring mechanism the arranqement being such that the power supply to the element is disabled upon boiling. A control of this type may thus have a body portion mounting or housing the first bimetallic actuator and clamped in use in the proximity of the heater, and a part protecting upwardly to adjacent the steam outlet aperture of the container and mounting the second bimetallic actuator and the associated overcentre spring mechanism.Thermallv sensitive controls having this general configuration are well known and are described, for example, in British patent specification No. 1,470,367.

With such thermally-sensitive controls it is desirable to achieve a fast response time of the steam sensitive bimetallic actuator to ensure that the heater is disabled sufficiently rapidly upon boilinq. In known controls of the general configuration discussed above the steam-sensitive bimetallic actuator is usually located in a partly enclosed chamber defined beneath a hood portion which extends over the steam outlet aperture of the container whereby steam accumulates in the vicinity of the actuator and a portion therefore impinges and condenses thereon. The latent heat released thus actuates the bimetallic actuator thich trips the sprinq mechanism to disable the heater.

With such an arrangement tbere is a limit on the response tine of the bimetallic actuator owing to the time taken for sufficient steam or vapour to accumulate in the region of the actuator, particularly where overall design requirements for the control dictate that the bimetallic actuator cannot be located immediately adiacent the steam outlet aperture of the container. A further problem with known arrangements concerns the amount of condensation collecting beneath the hood portion which can undesirably run down onto other parts of the control.

Viewed from one aspect the invention provides a thermally-sensitive control for an electric heater in or for a container of the kind described, the control comprising a body portion mounted in use to a part of the heater or container, a thermallysensitive actuating means arranged in thermal contact with a part of the element of the heater, means for making electrical contact with the cold leads of the heater for the supply of electrical power to the heater, and electrical switch means actuable to interrupt the power supply to the heater in response to operation of the thermallv-sensitive actuating means consequent upon the element of the heater overheating, the control further comprising a steam sensitive bimetallic actuator coupled to the same or to a further electrical switch means of the control via an overcentre sprinq mechanism mounted on the control and arranged to interrupt the power supply to the heater upon liquid within an associated container boiling, wherein the steam sensitive bimetallic actuator is located in a chamber which communicates with a steam outlet aperture in a wall of the container and which includes an ucwar'lv facing opening, the arrangement be na such that, in use, upon boiling of liauid within the container, steam or vapour egressing from the container via said outlet aperture establishes a convection current which flows upwardly through said opening, the upward flow of steam impinging on at least part of the bimetallic actuator.

Such an arrangement leads to certain advantages over known controls of this general type as discussed above which rely on steam accummulating and condensi-nq in a chamber. Firstly, the response t.me is improved since the convection current carries a continuous flow of steam more auickly to the act11ator. This improvement is of particular significance where overall design requirements for the control preclude the actuator from beinq mounted fnunediately adiacent the steam outlet aperture. Secondly, the reset time for the actuator may be shortened, sInce after boiling has ceased the effect of the convection current is to draw a flow of cold air across the actuator so that it is more quickly returned to below its critical operating temperature than with known arrangements.Thirdlv, the amount of condensation collecting within the chamber is reduced, since a manor part of the steam is discharged upwardly via the opening and as a conseauence of the more rapid response achieved. Fourthly, the amount of condensation collecting on the bimetal is reduced which reduces the tendene of condensation to "mask i.e. insulate the bimetal and thus lengthen the response time and/or the subsequent reset time.

The chamber may communicate with or be at least partly defined by a hood shaped portion having a generally inverted U-shap cross-section which at one end extends over the steam outlet aperture.

The hood shaped portion may be defined by a part of the container, ,by part of a cover member for the control, althouqh preferably it forms part of the control in which case it may be formed inteqrally with the control or form part of a member which is fabricated separately from the control and assembled thereto before the control is mounted to the heater or container. In either case, the control preferably includes a body rart e.g. of plastics extending upwardly from the said body portion and mounting the overcentre spring mechanism, the bimetallic actuator and the associated chamber and hood nortion.

Such body part may be formed integrally with the main body portion, or may be fabricated separately so that the control consists of two principal parts which are adapted to be assembled toqether when the control is mounted te a heater or container.

In a preferred such arranqement in which the chamber and hood portion form part of a separate member the said upwardly facing opening of the chamber may also be used to locate the member on a top portion of said upwardly extending body part of the control with said top portion extending in mating engagement through the opening. In this way, the member may be adapted to be fitted to an existing control without the need substantially to otherwise modify the control.

It is also possible however that the chamber is formed as part of the control, or is defined by part of a cover member for the control.

In a preferred arrangement , the bimetallic actuator is spaced somewhat from the steam aperture in which case the convection current established through the upwardly facing opening is effective to draw a continuous flow of steam from the outlet aperture and along the underside of the hood portion to the actuator.

In a preferred embodiment the bimetallic actuator is in the form of a sheet of bimetal of dished or domed configuration adapted to reverse its curvature, preferably by snap action1 at a certain critical temperature upon exposure to steam or vapour.

It is envisaged that the actuator might be arranged in a generally horizontal plane, in which case an upwardly facing opening in a wall of the chamber above the actuator may he effective to establish a convection current impinging transversely on the actuator.

In a preferred embodIment, however, the actuator is orientated in a qenerally vertical plane so that the convection flow of steam is parallel to the plane of the actuator. In such an embodiment the actuator may extend partly throuqh the upwardly facing opening.

In a preferred such arrangement, a peripheral wall of the chamber closely surrounds the actuator, and the chamber effectively constitutes a chimney in which the vertically orientated actuator is located, the chimney communicating in the preferred embodiment with a forwardly projecting hood portion which extends over the steam outlet aperture.

In the arrangement discussed above in which the hood portion and chamber form part of a separately fabricated ember, the chamber may he at least partly defined by a collar which mainly engages in use over the upwardly extending part of the control and over the vertically arranged bimetal mounted thereto.

The invention extends to a steam convecting member for the preferred control as aforesaid, the member comprising a hood-like portion of inverted U-shaped cross section which opens into an open ended collar adapted in use for mating engagement over an upper part of a thermally sensitive control mounting a bimetallic actuator.

An embodiment of the invention will now be described, by way of example onlv, with reference to the accompanyihg drawings, in which: Figure 1 is a partly schematic cross-section of a water boiling container incorporating a thermally sensitive control in accordance with the invention; FSure 2 is a perspective view of a steam convecting member which forms part of the control shown in Figure 1; and Figures 3a and 3b are respectively underside plan and end vie on an enlarged scale of the member shown in Figure 2.

Referring firstly to Fiqure 1 there is shown part of a water boiling container in the form cf kettle 1 incorporating an electric immersion heater 2 comprising a heatinq element 3 carried by a heater head 4. As is conventional the head 4 is mounted in water tiqht relation in or adiacent an aperture formed in the side wall of the container to form a seal therewith. The element includes a hot return 5 brazed to the head and a pair of cold leads 6 extending through the head as is conventional.

A thermally-sensitive control 7 is mounted adjacent the heater head 4, e.g. by being clamped by means of mounting bolts or studs secured to the head. As is conventional the control 7 includes a thermally-responsive actuating means, such as a bimetallic actuator, mounted in thermal contact with the hot return 5 of the heater and coupled to electrical switch means operable to disable the heater in the event of over-heating. The control includes a connector 8 for mating engagement with a complementary connector of Q power supply lead, the connector 8 including terminal pins (not shown) connected to the cold leads 6 of the element 3 via the switch means. To this end the control 7 incorporates suitable means e.g. leaf spring contacts for making electrical connections with the cold leads 6.The parts of the control described thus far are conventional in controls of this type, and the internal confiquration may be as described in British Patent Specification No. 2,181,598, for example.

The control further comprises a body part 9 extending upwardlv from a main body portion 10 (shown only schematically), the body part 9 mounting an over-centre spring mechanism 11 arranged to co-operate at its lower end with a push rod 12 coupled to a further switch means of the control for disabling the heater, which again may be as described in British Patent Specification No. 2,181,598.

The spring mechanism 11, moves between a first stable condition wherein contacts of the switch means are closed and a second stable condition in which the contacts are open, via an unstable dead centre condition. The details of such a spring mechanism are well known to those skilled in the art, and as is conventional the spring mechanism has associated therewith a push button 13 for manual resetting.

The upwardly extending part 9 of the control mounts a second thermally-responsixre actuator 14 which is vertically orientated and is shown edge on in Figure 1. The actuator 14 is of the general type described in British Patent Specification No. 1,542,257 and is of a generally domed configuration having a cut out portion defining a central tongue.

The actuator 14 is calibrated to reverse its curvature by snap action when exposed to steam or vapour, the tongue of the actuator then engaging an upper part 15 of sprinq mechanism 11 so as to trip the mechanism to its second condition in which the contacts of the switch means of the control are open and thus the heater is disabled.

As shown in Fissure 1 the kettle 1 includes a steam outlet aperture 16 in a peripheral part of its upper wall to permit the egress of steam or vapour during boiling. The control further includes a steam convecting member 17 fitted to the top of the upwardly extending part 9. As shown in Figures 2, 3a and 3b the member 17 includes a hood like portion 18 of inverted U-shaped crosssection which opens into a collar like portion 19. The portion 19 defines an upwardly open chamber in which the bimetallic actuator 14 is located.

In the illustrated embodiment the convecting member is matingly engaged over the top of the upwardly extending part 9 of the control and includes tabs 20 which together with a cut away portion 21 formed in the rear wall of the collar 19 co-operate with corresponding formations on the part 9 of the control so that the convector member 18 is lodged in the position shown in Figure 1 with the hood portion 18 extending over the steam outlet aperture 16.

It will however be appreciated that both the hood portion and chamber surrounding the actuator 5 might instead be formed integrally with the body part 9 of the control.

The collar 19 thus surrounds the actuator which projects partly through the open top end of the collar portion. Accordingly as shown by the arrows in Figure 1 the collar portion 19 effectively acts as a chimney whereby vertical convection currents are established. Thus steam egressing from the container via the aperture 16 Passes firstly along beneath the upper wall of the hood like portion 18 before flowing upwardly out via the opening at the top of the chamber defined by the collar portion 19. The upward steam flow impinqes on the actuator 14 to cause operation of the actuator and of the over-centre spring mechanism as discussed above. The steam then enters a space enclosed by a hollow handle part 20 of the kettle and can escape via outlet ducts 2i.

Of course it is possible to envisage other arrangements falling within the scope of the invention.

For example, instead of forming parts of a separate member 17 as described above, the hood portion and/or the chamber could be defined bv part of the handle 20 (e.g. by beinq formed integrally therewith), the handle effectively constituting a cover for the control.

Claims

CLAIMS:

1. A thermally-sensitive control for an electric heater in or for a container of the kind described, the control comprising a body portion mounted in use to a part of the heater or container, a thermallY sensitive actuating means arranged in thermal contact with a part of the element of the heater, means for making electrical contact with the cold leads of. the heater for the supply of electrical power to the heater, and electrical switch means actuable to interrupt the power supply to the heater in response to operation of the thermally-sensitive actuating means consequent upon the element of the heater overheating, the control further comprising a steam sensitive bimetallic actuator coupled to the same or to a further electrical switch means of the control via an overcentre spring mechanism mounted on the control and arranged to interrupt the power supply to the beater upon liauid within an associated container boiling, wherein the steam sensitive bimetallic actuator is located in a chamber which communicates with a steam outlet aperture in a wall of the container and which includes an upwardly facing opening, the arrangement beinq such that, in use, upon boiling of liquid within the container, steam or vapour eqressing from the container via said outlet aperture establishes a convection current which flows upwardly through said opening, the upward flow of steam impinging on at least part of the bimetallic actuator.

2. A thermally sensitive control as claimed in claim 1 wherein the chamber communicates with or is at least partly defined by a hood shaped portion having a qenerally inverted U-shaped crosssection which at one end extends over the steam outlet aperture.

3. A thermally sensitive control as claimed in claim 2 wherein the hood shaped portion is defined by part of the container.

4. A thermally sensitive control as claimed in claim 2 wherein the hood shaped portion forms part of a cover for the control.

5. A thermally sensitive control as claimed in claim 2 wherein the hood shaped portion is formed integrally with th control or forms part of a member which is fabricated separately from the control and assembled thereto before the control is mounted to the heater or container.

6. A thermallv sensitive control as claimed in claim 5 wherein the control includes a body part, e.q. of plastics extending upwardly from the said body portion and mounting the overcentre spring mechanism, the bimetallic actuator and the associated chamber and hood portion.

7. A thermally sensitive control as claimed in claim 6 wherein the said body part is formed integrally with the main body portion, or fabricated separately.

8. A thermally sensitive control as claimed in claims 6 cr 7 wherein the chamber and hood portion form part of a separate member and the said upwardly facing opening of the chamber is used to locate the member on a top portion of said upwardly extending body part of the control with said top portion extending in mating engagement throuqh the opening.

9. A thermally sensitive control as claimed in any one of claims 1 to 7 wherein the chamber is formed as a part of the control or as part of a cover for the control.

10. A thermally sensitive control as claimed in any preceding claim wherein the bimetallic actuator is spaced from the steam aperture.

11. A thermally sensitive control as claimed in any preceding claim wherein the bimetallic actuator is in the form of a sheet of bimetal of dished or domed comfiguration adapted to reverse its curvature, preferably by snap action, at a certain critical temperature upon exposure to steam or vapour.

12. A thermally sensitive control as claimed in any preceding claim wherein the actuator is arranged in a generally horizontal plane.

13. A thermally sensitive control as claimed in any one of claims 1 to 11 wherein the actuator is orientated in a generally vertical plane.

14. A thermally sensitive control as claimed in claim 1.3 wherein the actuator extends partly through the upwardly facing opening.

15. A thermally sensitive control as claimed in claim 13 or 1 wherein the peripheral wall of the chamber closely surrounds the actuator, and the chamber constitutes a chimney in which the vertically orientated actuator is located.

16. A thermally sensitive control as claimed in claim 15 wherein the chimney communicates with a forwardly projecting hood portion which extends over the steam outlet aperture.

17. . thermally sensitive control as claimed in claim 15 wherein the hood portion and chamber form part of a separately fabricated member and the chamber is at least partly defined by a collar which matinqlv enqages in use over the upwardly extending part of the control and over the vertically arranged bimetal mounted thereto.

18. A steam convecting member for use in a control as claimed in claim 17, the member comprising a hood-like portion of inverted U-shaped cross section which opens, into an open ended collar adapted in use for mating engagement over an upper part of a thermally sensitive control mounting a bimetallic actuator.

19. A thermally sensitive control, substantially as hereinbefore described with reference to the accompanying drawing.

20. A steam convecting member substantially as hereinbefore described with reference to the accompanying drawings.