GB2185857A - Controls for vessels for heating liquids - Google Patents

Abstract

A thermal protective control for an electrical heater of a vessel for heating liquids includes two series connected contact sets 20,21 actuatable via actuating members 25,26 by thermally responsive members 23,24 set to respond to different temperatures of a heater hot spot 17. The higher set thermally responsive member is arranged to be non-resetting on cooling to hold open one of the contact sets. The thermally responsive members 23,24 and the contact sets 20,21 are located on opposite sides of a thermal and electrical insulating barrier 27 with the actuating members 25,26 extending through the barrier 27. The thermally responsive members 23,24 are retained in a socket 28 in the barrier 27 by a cap 44 and are spaced apart by a thermally conductive spacer 29. One of the contacts sets 20,21 is also actuated by a manually operable or steam responsive mechanism. <IMAGE>

Description

SPECIFICATION Vessels for heating liquids This invention concerns control means, for controlling an electrical heater of a vessel for heating liquids.

There are known electrical heaters of a kind in which the heater comprises a head, adapted to be secured to a vessel, a heating element projecting from one side of the head, and control means located at the other side of the head and electrically connected to the heating element, the heating element being arranged to heat part of the head to provide a "hot spot" thereon.

There are known vessels for heating liquids of a kind in which the vessel comprises a body, of moulded plastics material, to which the head of such an electrical heater is secured so that the heating element is located within the body for heating liquid contained in the body.

In order to limit the temperature of the heating element it is known to incorporate, in the control means, a thermally actuated protective device such as a thermostatic switch or dry overload responsive mechanism arranged to open a switch in the control means, for example as disclosed in our Specifications Nos. GB 2102205, 2036541, 2056777 and 2090064.

Such protective devices are arranged to act as a temperature limiters which prevent damage to any part of the vessel and electrical heater and are manually or automatically resettable, after the heater has been allowed to cool down.

However, such control means have the major disadvantage that failure of any part of the protective device can allow the heater to reach dangerously high temperatures before the heater itself burns out or a mains electrical fuse is blown. Whilst the risk of such a failue arising can be made exceedingly small by careful design, manufacture and testing of the components and completed vessels, even that small risk may be unacceptable in vessels of said kind, because rupturing of an overheating heating element may ignite the plastics body of the vessel.

There is a further problem inherent in electrical heaters of said kind, in that the packing density of the components in the control means is already high, and little, if any, space is available for further components. Therefore, in order to reduce the aforementioned risk only extremely small secondary protective devices have been proposed which devices are arranged so that an electrical connection is made or held by a small body of material which melts or softens at a predetermined temperature to cause or allow the connection to be broken. Such small bodies have various disadvantages, such as lack of long term stability, assembly cost, and potentially unreliable operation.

An object of the invention is to enable the aforementioned problems and risk to be avoided or reduced in vessels and heaters of said kinds.

According to the present invention there is provided control means in or for use in a vessel or heater of said kind, the control means including a manually or thermally actuable mechanism and a thermally actuated protective device comprising a switch body supporting first and second switches which are connected electrically in series and disposed at one side of a thermal and electrical insulating barrier; first and second heat responsive members, each of which is constructed to change shape when heated beyond a predetermined temperature, which heat responsive members are located at the opposite side of the barrier so as to be mutually adjacent; first and second actuating members extending through said barrier to enable the first heat responsive member to open the first switch and the second heat responsive member to open the second switch independently of the state of the first switch; and a part of one of said actuating members, or a further actuating member, extending at least partially into said switch body and actuable by said mechanism to actuate one of said switches.

Each switch preferably comprises two contacts each of which are mounted on a respective conductor; and of these four contacts of the two switches preferably at least three thereof are carried by flexible limbs of said conductors, each of which limbs can be displaced by a respective one of the actuating members.

One of the heat responsive members is preferably exposed at an opening in the switch body, and may obscure at least part the other heat responsive member. The heat responsive members may have a thermally conductive member interposed therebetween.

For improved protection against overheating and to reduce long term functional problems the invention further provides control means in or for use in a vessel or heater of said kind, the control means including a thermally actuated protective device comprising a switch body supporting first and second switches which are connected electrically in series and disposed at one side of a thermal and electrical insulating barrier; first and second heat responsive members, each of which is constructed to change shape when heated beyond a predetermined temperature, which heat responsive members are located at the opposite side of the barrier; and first and second actuating members extending through said barrier to enable the first heat responsive member to open the first switch and the second heat responsive member to open the second swich independently of the state of the first switch; wherein barrier provides a socket having a mouth, and provides a mounting for a cap or like retaining member to retain said heat responsive members in the socket; wherein said heat responsive members are spaced apart by a thermally conductive spacer member so that the second heat responsive member is disposed in said socket away from the mouth and cap so as to be thermally insulated except for heat conducted via the first heat responsive member and said thermally conductive spacer member.

The retaining member is preferably apertured to expose the first heat responsive member at said mouth.

One of said switches is preferably a low temperature switch actuable by a reversible change of shape of its heat responsive member when a predetermined temperature is traversed; and the other switch is preferably a high temperature switch which can be opened independently of the low temperature switch by its heat responsive member being heated to a higher temperature to change shape to a state in which it remains thereafter stable at all temperatures below said higher temperature to serve as a lock-out member to hold open the high temperature switch.

One of said actuating members may extend through one of the heat responsive members, and may also extend through the other actuating member.

The heat responsive members are preferably snap acting or otherwise arranged to change shape suddenly, for rapid opening of the switch contacts. One of the heat responsive members, namely the primary member, is preferably arranged to effect said change of shape at a low temperature within the range of from 120"C to 150"C; and the other of the heat responsive members, namely the lock-out member, is preferably arranged to effect said change of shape at a higher temperature which is at least 50"C and is preferably 80" to 150"C above said low temperature.The primary member is preferably constructed to revert automatically to its original shape upon cooling to below said low temperature; whilst the lock-out member, which remains in its changed shape irrespective of temperature after effecting said change of shape, prevents further use of the vessel.

The control means may be arranged to allow the lock-out member to be thrust back into its original shape.

The high temperature switch may also be actuable by a or the member which can be moved by manual actuation or by a mechanism responsive to boiling or liquid in the vessel so that the low temperature switch is actuable only by its heat responsive member.

However, it has been found that such an arrangement of the switches has advantages and disadvantages.

Accordingly there is provided in accordance with the present invention control means or a protective device generally as hereinbefore disclosed, wherein a or the manually actuable member or said mechanism serves to actuate the low temperature switch, and the lock-out member is arranged to actuate only the high temperature switch.

The switches preferably comprise a live input conductor, which carries one switch contact and is connected to a live input terminal; a live output conductor which carries one switch contact and is arranged for connection to the heater, and an intermediate conductor which carries two switch contacts.

In one embodiment the intermediate conductor carries one fixed contact, of one switch, the one movable contact of the other switch.

However, in a preferred embodiment, both of the contacts carried by the intermediate conductor are movable contacts, and the intermediate conductor is preferably mounted at a junction between two independently movable contact carrying limbs of the conductor.

The switch body preferably comprises a body moulding and a thermal insulator moulding constituting the barrier and wherein the body moulding and barrier preferably together define a switch compartment in which the switches are disposed, and provide walling between said compartment and other electrically conductive components which are carried by or pass through the switch body; and preferably provide a further compartment in which a neutral conductor is disposed, which conductor is connected to a neutral terminal and is arranged for connection to the heater.

The invention includes a vessel or heater provided with the aforementioned control means.

The invention will be described further, by way of example, with reference to the accompanying drawings, wherein: Figure 1 shows a barrier of a switch body of a first embodiment of control means of the invention; Figure 2 shows the switch body with the barrier removed to show switches mounted on a body moulding; Figures 3 and 4 show sections of the switch body and protective device corresponding to the lines Ill-Ill and IV-IV in Fig. 2 but with the barrier in position, Fig. 3A showing an enlarged detail of the protective device in an overheated state; and Figure 5 is a cross-sectional diagram of a modified form of the control means.

The control means 1 is constructed for fitting into a head 12 of a heater 13 (Fig. 5) of a vessel having a plastics wall 15, and part of a heating element 16 is secured to part of the head 12 to provide a hot spot 17 in known manner. The control means 1 incorporates a protective device 11, a switch body assembly consisting of a body moulding and barrier moulding 27. The latter provides recesses 40 to receive end portions of the element 16 of the heater so that live and neutral cold tails 18A and 18B of the element (Fig. 2) extend through the barrier into sockets 41 which extend through the body moulding 22; provides abutment 42 to abut the heater's head; and provides a hollow portion 43 which is spaced apart from said abutments 42, carries a retaining member in the form of a metal cap 44 to retain actuating parts of the protective device on the barrier.Said actuating parts include a first bimetal disc 23 which serves as a primary heat responsive member; a second bimetal disc 24 which serves as a lock-out heat responsive member; a hollow actuting member 25 and a rod actuating member 26 slidably located in the member 25 and co-axial therewith, which members 25 and 26 extend slidably through the barrier 27 from a socket recess 28 in the portion 43 in which the discs 23 and 24 are located in tandem, to a switch chamber 30. The barrier provides thermal and electrical insulating between the chamber 30 and the recess 28. The discs 23 and 24 are dished and are spaced apart by ring spacer 29 of a material of high thermal conductivity whereby to allow each disc to change shape so as to reverse its curvature and thereby thrust its central portion towards the barrier 27, without being impeded by the other disc.

In the first embodiment shown in Figs. 1 to 4, the central portion of the disc 24 abuts the member 25, is apertured to allow the member 26 to extend therethrough to abut a central portion of the disc 23, and confronts the hot spot 17 through an opening in the cap. The positions of the discs may be reversed as shown in Fig. 5.

The body moulding 22 provides walling, which includes a peripheral wall 45 and interior walls 46, 27 which extend from the wall 45 to an abutment wall 48, and which abut one side of the barrier to define the switch chamber 30, and neutral and live connector chambers 50 and 51 from which the sockets 41 and recesses 40 extend. The wall 48 provides apertures 52 and a seating 53 for an earth connector and a fastening device to extend through the switch body to the head for earthing the head and clamping the control means to the head.

The switch chamber 30 contains switch means which comprises a primary low temperature switch 20 and a lock-out protective switch 21 and includes of a live output conductor 35, a live input conductor 33 and an intermediate conductor 31.

In the first embodiment, Figs. 1 to 4, the conductor 33 is secured at one end to the body mould 22 by a live input terminal pin 19 which extends through and is partially embedded in the body moulding; and at its other end, on a limb of the conductor, there is a movable contact of the switch 20. The conductor 35 carries a fixed contact of the switch 21 at one end, extends through a confined space between the barrier and the walling (to which the conductor is secured) into the live connector chamber 51, and is secured to the live cold tail 18A. The conductor 31 is of reflex form to provide two limbs 54, 55 which extend from an elbow 56 secured to the body moulding 22.The limb 54 extends transversely of the limb of the conductor 33, carries a second movable contact of the switch 20, and extends beyond the contact to lie alongside one end of a movable member 14 which extends slidably through the moulding 22 for external actuation e.g. by a manually or steam actuable mechanism. The limb 55 passes, and is spaced from, the conductor 33; extends to support a movable contact for the switch 21; and has a central portion which is apertured to permit the second actuating member to extend therethrough towards the conductor 33 and which confronts one end of the first actuating member 25.

The neutral connector chamber 50 houses a neutral conductor 56 secured at its ends to a neutral pin 57, similar to the pin 19, and to the cold tail 18B.

The primary switch can be opened by forwards movement (towards the head) of the member 14 and limb 54; and by reversal of shape (Fig. 3A), from forwardly convex to rearwardly convex, of the front primary bimetal member 23 to move the member 26 and conductor 33 rearwards. The member 23 is set for reversal at about 1300C to 140"C and to revert automatically upon cooling to about 70 to 100"C.

The lock-out switch 21 is openable by reversal of shape of the rear disc member 24 to move the member 25 and limb 55 rearwards.

The rear disc member is set so that its change of shape is irreversible and takes place at about 270 to serve as a lock-out member to hold open the switch 21.

Under normal circumstances, the control means is actuated by a manually operable and/or steam responsive mechanism (not shown) of any suitable or known form. Such known mechanisms are bistable and movable between an "on" position and an "off" position, e.g. by manual operation to switch on the vessel and by automatic operation to switch off the vessel when the liquid has boiled.

The control means is shown in Figs. 3 and 4 in the "on" condition, and the switch 20 is arranged to be opened by forwards movement of the member 14 by the aforementioned mechanism when the latter is moved to the "off" position. The mechanism may possibly be arranged to move the member 14 rearwards when the mechanism is moved to the "off" position, and to accommodate such operation the member 14 may be re-arranged to act on the conductor 33 to pull it rearwards from the conductor 31 to open the switch 20.

Furthermore, the member 14 may be mechanically connected to or part of the member 26, but at increased risk of the switch 20 being made inoperative by failure, e.g. sticking, of either of the members 14 and 26.

The invention is not confined to details of the foregoing example, and many variations are possible, for instance, as disclosed diagrammatically in the modified form shown in Fig. 5.

The device 11 similarly comprises a switch body 22, providing a chamber 30 in which the switches 20 and 21 are mounted: the first bimetal disc 23 which serves as the primary heat responsive member; the second bimetal disc 24, which serves as the lock-out heat responsive member; the hollows actuating member 25 and a rod actuating member 26 slidably located in the member 25 and co-axial therewith, which members 25 and 26 extend slidably through the barrier 27 from the socket recess 28 in which the disc 23 and 24 are located in tandem, to the switch chamber 30.

The discs 23 and 24 are dished and are spaced apart by a ring spacer 29A of a material of high thermal conductivity whereby to allow each disc to change shape so as to reverse its curvature and thereby thrust its central portion towards the barrier 27, without being impeded by the other disc.

In this example, the central portion of the disc 23 abuts the member 25 and is apertured to allow the member 26 to extend therethrough to abut a central portion of the disc 24; and the disc 23 is set to reverse its shape from forwardly convex to rearwardly convex at about 135"C for thrusting the member 25 rearwards to displace the switch blade 31 rearwards and thereby separate the contacts 32 of the switch 20, and is arranged to revert automatically to the forwardly concave shape when the temperature falls below about 60"C to 110"C. The disc 24 in contact with or proximal to the hot spot to react as quickly as possible to serious overheating of the heater in the event of the switch 20 failing to open; is set to reverse its shape at about 290 to displace the member 26 and the switch blade 33 rearwards to separate the contacts 34 of the switch 21; and is arranged to remain in its reversed shape permanently thereafter (unless it is mechanically forced back to its original shape by an externally applied force) so as to hold the switch 21 open.

However, the roles of the switches and the positions of the discs could be reversed to that the disc 23 and switch 20 serve as the lock-out protector and the switch 21 and disc 24 serve as the low-temperature or dry overload protector, as in the first embodiment, and the member 14 may be modified, omitted or arranged to act on the switch 21.

However, the invention is not confined to details or features of the foregoing examples, and many variations are possible within the scope of the invention. For example, other forms of heat responsive members, such as helical coils of "memory metal", may be employed, e.g. in a concentric arrangement; and the actuating members may be connected to the heat responsive members and or respective switch blades e.g. to pull instead of push the switches open. However, irrespective of the form and arrangement of the heat responsive members, they are preferably arranged directly beyond the hot spot so that heat is transferred thereto from the hot spot 17 via a short, low resistance, low heat capacity, conduction and/or radiation path or paths.Instead of being arranged in tandem, the heat responsive members may be disposed side by side behind respective portions of the hot spot, which is of relatively elongate form. In any arrangement thereof, at least a part. of each heat responsive member is preferably within 5mm of the hot spot, or most of each heat responsive members is preferably within 10mm of the hot spot, and these members are preferably located in a recess which is open towards the hot spot. In the second embodiment, the ring 29 may be extended, as indicated partially at 29B, to contact end portions of the elongate hot spot or the retaining member, and the front disc may be apertured for radiation to pass from the shot spot direct to the rear disc.

The spacer member may provide side faces of any suitable form to contact both bimetal discs; need not be a complete ring; and its mass and specific heat may be varied to vary its heat capacity.

Although the devide 11 can take a great number of different forms employing a variety of kinds of heat responsive member and arrangements of such members, some of which are mentioned above, problems arise in achieving a high degree of accuracy of temperature response, in relation to the rate of heating of the heater under abnormal conditions in which the device is intended to operate to protect firstly the element and secondly the vessel by successive operation of the heat responsive members as the temperature of the hot-spot rises.

Furthermore, we have discovered certain secondary problems which can arise especially if the vessel is abused, e.g. the heater is frequently operated with an insufficient level of fluid therein.

To minimise these problems the device 11 is preferably arranged, for example as indicated in Figs. 3, 3A and 4, so that: (a) the front thermally responsive member operates the primary or low temperature switch; has a central portion which abuts or directly confronts the hot spot and reverses shape to move away from the hot spot; is peripherally surrounded by a member providing location and thermal insulation; and is arranged so that in the reversed shape condi tion, its extreme peripheral portion abuts or is proximal to the retaining member and a nearperipheral portion, adjacent the extreme portion, abuts a side face of the spacer member 29 or 29A, so that the major or sole significant thermally conductive path from the spacer member to the head is via said extreme and near peripheral portions and the retaining member; (b) the spacer member is peripherally surrounded by a member providing thermal insulation, and has side faces arranged to contact peripheral or near peripheral portions of the thermally responsive members in the normal conditions and in their reversed shape conditions;; (c) the high temperature or lock-out thermally responsive member is located to the rear of the thermally insulated chamber so that its major or sole significant conductive path to and from the head is via said spacer member.

This preferred arrangement enables the response time of the primary or low temperature member to be minimised whilst increasing the return period during cooling of the heater, due to return of heat stored in the spacer member to this primary member, thus increasing both the switch off response accuracy and sensitivity whilst incresing the cycle time of each switching cycle whereby to reduce overheating of the heater; and, in the event of a failure of the primary switch to open, enables the rate of heating of the high tempeature or lockout thermally responsive member to be substantially maintained at a specific rate when the primary member reverses shape via a short and precisely arranged heat path via the spacer member, said peripheral portions and the retainer member (which itself may be made of very thin metal as it serves only a retaining function and its front part is supported by the barrier to abut or closely confront the hot spot) so that the temperature setting of the lock-out member can be minimised to increase protection of the heater and vessel without increasing the risk of a premature reaction rendering the vessel inoperative.

The device 11 is particularly compact, easy to mass produce accurately, and can be easily incorporated into apparatus of the kind disclosed in our aforesaid patent specifications as a substitute for the dry overload actuating mechanism disclosed therein at minimal extra cost.

Claims (14)

1. Control means in or for use in a vessel or heater of said kind, the control means including a manually or thermally actuable mechanism and a thermally actuated protective device comprising a switch body supporting first and second switches which are connected electrically in series and disposed at one side of a thermal and electrical insulating barrier; first and second heat responsive members, each of which is constructed to change shape when heated beyond a predetermined temperature, which heat responsive members are located at the opposite side of the barrier so as to be mutually adjacent; first and second actuating members extending through said barrier to enable the first heat responsive member to open the first switch and the second heat responsive member to open the second switch independently of the state of the first switch; and a part of one of said actuating members, or a further actuating member, extending at least partially into said switch body and actuable by said mechanism to actuate one of said switches.

2. Control means as claimed in Claim 1, wherein the heat responsive members have a thermally conductive member interposed therebetween.

3. Control means in or for use in a vessel or heater of said kind, the control means including a thermally actuated protective device comprising a switch body supporting first and second switches which are connected electrically in series and disposed at one side of a thermal and electrical insulating barrier; first and second heat responsive members, each of which is constructed to change shape when heated beyond a predetermined temperature, which heat responsive members are located at the opposite side of the barrier; and first and second actuating members extending through said barrier to enable the first heat responsive member to open the first switch and the second heat responsive member to open the second switch independently of the state of the first switch; wherein barrier provides a socket having a mouth, and provides a mounting for a cap or like retaining member to retain said heat responsive members in the socket; wherein said heat responsive members are spaced apart by a thermally conductive spacer member so that the second heat responsive member is disposed in said socket away from the mouth and cap so as to be thermally insulated except for heat conducted via the first heat responsive member and said thermally conductive spacer member.

4. Control means as claimed in Claim 1,2 or 3 wherein each switch comprises two contacts each of which are mounted on a respective conductor; and of these four contacts of the two switches at least three thereof are carried by flexible limbs of said conductors, each of which limbs can be displaced by a respective one of the actuating members.

5. Control means as claimed in Claim 2, 3 or 4 wherein one of the heat responsive members is exposed at an opening in the switch body, and may obscure at least part the other heat responsive member.

6. Control means as claimed in Claim 2, 3, 4 or 5 wherein the retaining member is apertured to expose the first heat responsive member at said mouth.

7. Control means as claimed in any preceding claim wherein one of said switches is a low temperature switch actuable by a reversible change of shape of its heat responsive member when a predetermined temperature is traversed; and the other switch is a high temperature switch which can be opened independently of the low temperature switch by its heat responsive member being heated to a higher temperature to change shape to a state in which it remains thereafter stable at all temperatures below said higher temperature to serve as a lock-out member to hold open the high temperature switch.

8. Control means as claimed in Claim 7 wherein one of the heat responsive members, namely the primary member, is arranged to effect said change of shape at a low temperature within the range of from 1200C to 1500C; and the other of the heat responsive members, namely the lock-out member, is arranged to effect said change of shape at a higher temperature which is at least 50"C above said low temperature.

9. Control means as claimed in Claim 7 or 8 wherein the control means is arranged to allow the lock-out member to be thrust back into its original shape.

10. Control means as claimed in Claim 7, 8 or 9 wherein a or the manually actuable member or said mechanism serves to actuate the low temperature switch, and the lock-out member is arranged to actuate only the high temperature switch.

11. Control means as claimed in any preceding claim wherein the switches comprise a live input conductor, which carried one switch contact and is connected to a live input terminal; a live output conductor which carries one switch contact and is arranged for connection to the heater, and an intermediate conductor which carries two switch contacts; and wherein both of the contacts carried by the intermediate conductor are movable contacts, and the intermediate conductor is mounted at a junction between two independently movable contact carrying limbs of the conductor.

12. Control means as claimed in any preceding claim wherein the switch body comprises a body comprises a body moulding and a thermal insulator moulding constituting the barrier and wherein the body moulding and barrier together define a switch compartment in which the switches are disposed, and pro vide walling between said compartment and other electrically conductive components which are carried by or pass through the switch body.

13. Control means substantially as herein before described with reference to Figs. 1 to 4 or Figs. 1 to 4 as modified by Fig. 5.

14. A heater or vessel incorporating con trol means as claimed in any preceding claim.