GB2111754A - Electric immersion heater - Google Patents

Abstract

An electric immersion heater (not shown) for an electric kettle or like container includes a thermally-sensitive electrical control 1 mounted to the heater head. The control 1 includes two movable switch contacts 18 mounted at the free ends 22 of conductive U- shaped leaf springs 20 connected to respective terminal pins and engageable with stationary contacts (not shown) connected to the heating element such that in normal operation power is supplied to the heater via the switch contacts. A bimetallic actuator 28 is mounted with a lower portion 31 thereof in good thermal contact with the heating element (not shown) such that upon the element over heating, an upper bridge or margin portion 32 of the actuator 28 spaced above a free end of a torque 37 engages by snap action a movable link member 23 which in turn engages the leaf springs 20 and breaks the contacts, thereby deenergising the heater. By virtue of the low disposition of the portion 31 of the actuator 28, a low profile heating element may be used enabling boiling of small quantities of liquid in an associated container. <IMAGE>

Description

SPECIFICATION Electric immersion heater This invention relates to electric immersion heaters for containers such as electric kettles, jugs, coffee percolators, pans, urns, laboratory equipment and the like.

Such immersion heaters conventionally comprise a heating element carried by a head which is adapted to be secured in a water-tight manner in or adjacent an opening in a container wall. The heating elements of such heaters terminate in so-called cold leads by means of which electrical connections are made to the heater. An intermediate part of the element or so called hot-return is generally secured to the head e.g. by brazing. Such immersion heaters are referred to hereinafter as "of the kind described".

Electric immersion heaters of the kind described are well known which include thermally-sensitive controls mounted to or adjacent the element head which controls include electrical switch means actuable upon the element overheating after for example having been switched-on-dry, i.e. with insufficient water in the container to coverthe element, to interrupt the power supply to the heater. A common form of a such a control includes a thermally responsive actuating means for the electrical switch means in the form snap-acting bimetallic actuator which is mounted within the control so as to extend face-to-face with the element head so as to be in good thermal contact with the hot-return of the element.A common form of actuator comprises a stressed sheet of bimetal having an aperture defining a tongue free at one end, a margin or bridge portion of the bimetal sheet extending transversely to and being spaced from the free end of the tongue; such an actuator has a generally dished configuration and will snap between two oppositely dished configurations at a particular temperature and is termed hereinafter as "an actuator of the kind described". Examples of actuator of the kind described are disclosed in British Patent Specifications Nos. 657,434 and 1542252.

An example of an immersion heater and a control therefor incorporating so-called switch on dry protection is described in British Patent Specification No. 1470366. In this arrangement the switch-means comprises a single set of contacts comprising a stationary contact electrically connected to a respective cold lead of the element and a movable contact mounted towards the free end of an elongate conductive resilient strip which extends upwardly from a live or neutral terminal pin of the control or connection to a source of electrical power. During normal operation, the resilient strip urges the movable contact against the stationary contact and electrical power can be supplied to the heater.A bimetallic actuator of the kind described is mounted by means of its tongue so that the portion of the actuator adjacent the root of the tongue lies against and conforms in shape to a protrusion formed in the head opposite the hot return of the element. In the event of overheating of the element, the actuator undergoes snap action and the bridge or margin portion thereof engages and moves away from the element head a push rod which in turn engages and displaces the free end of the resilient strip. This causes the movable contact to be displaced away from the stationary contact thereby interrupting the power supply to the heater.However a disadvantage with this arrangement is in that the bimetallic actuator and correspondingly the hot-return of the element must be positioned high up on the element head relative to the base of an associated container so that the bridge or margin portion of the actuator lies in the same general horizontal plane as the free end of the resilient strip which must to relatively long to provide a spring of suitable rate to be operated by the actuator. Therefore, since the hotreturn must always be immersed during operation of the heater, relatively small quantities of liquid cannot be boiled. Thus, where only small quantities of liquid are required to be heated, energy is wasted.

A further example of such an immersion heater and control is described in British Patent Specification No. 2042269 wherein the bimetallic actuator and the hot-return have been lowered to same extent and lie below the cold-leads of the element. In this arrangement the necessarily relatively long resilient strip on which the movable contact of the switch means is mounted extends downwardly from a live or neutral terminal pin of the control and the free end thereof lies in the lowermost portion of the control.

The bimetallic actuator is again mounted by means of its tongue and is positioned so that the bridge or margin portion of the actuator is lowermost and is adapted to engage a slideable member which in turn is adapted for engagement with the free end of the resilient strip. However, the major portion of the actuator which lies adjacent the head in thermal contact with the hot-return of the element is disposed above the bridge portion of the actuator and this imposes a restriction on the lowest possible disposition of the hot return on the head relative to the base of the container. Thus, a problem still exists concerning the boiling of small quantities of liquid.

According to the invention there is provided an electric immersion heater of the kind described and a thermally-sensitive control mounted or mountable to the head thereof, therebeing electrical switch means comprising at least one set of breakable contacts, said or each set including a stationary contact and a movable contact mounted towards a free end of a generally 'U' shaped leaf spring of resilient conductive strip material, wherein, in normal operation, the leaf spring urge the movable contact against the stationary contact and electrical power can be supplied to the heater via the switch means, and a bimetallic actuator of the kind described coupled to said or each contact set by means of a movable link member adapted for engagement with the or each leaf spring wherein the actuator is actuable upon the element overheating to move the link member which displaces the movable contact of the or each contact set away from the stationary contact to open the switch means and deenergise the heater, said actuator being mounted so as to lie face-to-face with the heater head with the bridge or margin portion of the actuator lying in use above the tongue thereof.

The 'U' shaped leaf spring or springs permit a sufficient length of resilient conductive strip to be used to accommodate the necessary displacement of the movable contact or contacts away from the stationary contact or contacts without the resilient strip extending either to an uppermost or lowermost part of the control.

Thus the restrictions previously imposed on the positioning of the bridge portion of the bimetallic actuator either towards the top or towards the bottom of the control are relaxed and the actuator may be mounted in the lower part of the control with the bridge or margin portion above the tongue so that the portion of the actuator adjacent the tongue root adapted to be in good thermal contact with the part of the head opposite the hot-return of the element may be disposed adjacent the lowermost part of the element head.

A heater in accordance with the invention therefore permits a lower positioning of the hot-return on the element head and thus the element may have a lower profile as compared with known heaters which in turn permits smaller quantities of liquid to be boiled with resulting energy saving.

In a preferred embodiment, the height of the hot return relative to the container base has been reduced by 8mm as compared with that previously available.

In the case of movable contacts mounted on simple elongate strips used heretofore, it is well known that as the contacts are broken the pivotal movement of the strip about its mounting point causes the movable contacts to move laterally or slide across the surface of the stationary contacts to some extent. This is advantageous in that the contacts are to a limited degree self cleaning and corrosion which tends to build up on the surface of the contacts is discouraged. The use of a 'U' shaped leaf spring in accordance with the invention increase this relative lateral movement of the contacts since during displacement of the free ends thereof the leaf springs are caused to both pivot and twist about their mounting point. Therefore the relative contact movement is effectively in two directions and the self cleaning effect on the contact surfaces is improved.

Preferably, the bridge or margin portion of the bimetallic actuator is adapted for direct engagement with the link member which is slidably mounted within the control and is movable in response to operation of the actuator so as to engage the or each leaf spring at or adjacent the free end or ends thereof.

Preferably, the control includes terminal pins adapted for engagement with a separate femalesocket connector for connecting the heater to a source of electrical power. In this embodiment, the or each leaf spring may include an apertured base portion which surrounds and is directly secured to a respective base portion of a live or neutral terminal pin. Such an itself to the assembly arrangement lends of the pins and the leaf springs to a body portion of the control in an automated operation whereby costs may be reduced.

In a preferred embodiment there are two sets of breakable contacts each comprising a stationary contact mounted directly to a respective cold lead of the heater and a movable contact mounted towards the free end of a respective leaf spring. This arrangement is advantageous in that the need for spring clips or the like mounted to the control for making electrical connections to the heater is avoided.

The bimetallic actuator may be mounted either directly to the heater head or to the control. In a preferred embodiment, however,the actuator is mounted by means of a stainless steel bridge member which is clamped between the heater head and a body portion of the control.

A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 7 is an exploded perspective view of a control taken from the side of the control which in use is secured to the head of an immersion heater.

Figure 2 is an exploded perspective view of the control illustrated in Figure 1 taken from the opposite side of the control and showing the immersion heater head.

Figure 3 is a perspective view similar to Figure 2 with certain parts of the control removed.

Figure 4 is a cross-sectional view taken from one side of the control of Figures 1 to 3 mounted to the head of an immersion heater showing the overcentre spring mechanism in the deactivated position.

Figure 5 is a view similar to Figure 4 but showing the overcentre mechanism in the operational position.

Figure 6 is an elevational view, partly broken away, of the end of the control adapted to be secured to the immersion heater head.

Figure 7 is a cross-section view taken along line 7-7 in Figure 6 showing the switch contacts open.

Figure 8 is a view similar to Figure 7 but showing the switch contacts closed.

Figure 9 is a view similarto Figures 7 and 8 but showing one contact open and one closed.

Figure 10 is a plan view of the over centre spring for the control illustrated in Figures 1 to 9.

Referring firstly to Figures 1 to 5, a thermallysensitive control 1 is mounted on the head 2 of an electric immersion heater. The cup-like head 2 is retained in a water-tight manner in an opening formed in the wall 4 of an associated container by means of an externally threaded retaining ring 3 which urges a peripheral flange 9 of the head 2 against the inside of the wall 4 compressing a resilient O-ring seal 5 therebetween as shown in Figure 4. The immersion heater head 2 carries a tube 6 for the egress of steam from the interior of the container which tube is brazed to the head and extends, in use, above water level within the container. The immersion heater comprises two cold leads 7 and a hot return 8 which passes underneath the cold leads 7 and is brazed to the head 2 so as to be in good thermal contact therewith. The low position of the hot return 8 enables the container to boil small quantities of liquid if desired.

The control 1 comprises a body portion 10 of thermo-plastic material, mounted snugly within the head 2 by means of three bolts 11 which engage threaded female studs 12 brazed to the inside of the head, and a cover member 13 which is secured to the body portion 10 by means of a single centre fixing bolt 14. The body portion 10 includes a horizontal shelf member 15 which lies close to the central longitudinal axial plane of the control 1 and defines at the side of the body portion remote from the head, upper and lower segmental volumes of the control lying respectively above and below the central horizontal axial plane of the control. Three terminal pins 16 are secured to and extend through the body portion 10 and include a live pin, a neutral pin and an earth pin for connection to a female socket connector to supply mains electrical power to the heating element.The end portions of the pins 16, which extend parallel to and are disposed below the shelf member 15, are arranged in a triangular relationship with the central or earth pin lying below the live and neutral pins. The cover member 15 defines a shroud 17 which surrounds the pins 16 and is shaped so as to snugly receive a standard socket connector.

The live and neutral pins are each connected to a respective cold lead 7 via a pair of breakable contacts, each pair including a movable contact 18 and a stationary contact 19 mounted directly to a respective cold lead 7. The stationary contacts 19 may be mounted to the cold leads 7 by means of cylindrical spring clips to which the contacts 19 are secured and which are simply pushed over the ends of the cold leads. Alternatively, the stationary contacts 19 might be mounted to the cold leads 7 by means of interference-fitted sleeves. The movable contacts 18 are mounted towards the free ends 22 of respective generally 'U'-shaped leaf springs 20 formed from resilient metallic strip material.The leaf springs 20 extend upwardly adjacent the side of the body portion 10 facing the head from the bases of the respective live and neutral terminal pins and are secured thereto by means of apertured base portions 21 of the leaf springs 20 which surround the base portions of the pins. During normal operation of the heater, the leaf springs 20 urge the movable contacts 18 against the respective stationary contacts 19 and electrical power may be supplied from the live and neutral terminal pins to the element. The earth pin, by virtue of its disposition towards the outer periphery of the control, is conveniently electrically connected with the head 2 by means of a metallic spring clip 29 which is secured to the base of the pin and directly engages the inside of the head 2.

A generally 'U' shaped link member 23 of moulded plastics material lies in a horizontal plane ofthe control slightly below the switch contacts 18, 19 and includes a transverse arm 24 disposed between the body portion 10 and the head 2, beneath the contacts, and two spaced longitudinal arms 25. The longitudinal arms 25 extend through the body portion 10 and are slidably mounted in axially directed channels 26 formed on either side of the body portion 10 close to the outer periphery thereof.

The transverse arm 24 of the link member 23 includes two stepped abutments 27 at either end thereof adjacent the longitudinal arms 25 which abutments 27 are adapted to engage respective free ends 22 of the leaf springs 20 whereby movement of the link member 23 in a direction away from the head 2 causes the free ends 22 of the springs 20 and the respective movable contacts 18to be displaced in a direction away from the head 2 thereby opening the contacts and interrupting the power supply to the heater.

During displacement of the free ends of the U-shaped leaf springs, the springs are caused to both pivot and twist about their mountings. This causes an increased relative lateral movement be tween the contacts as they are opened or closed which results in the surfaces of the contacts being self cleaning.

In addition, two vertical abutment arms 90 extend upwardly from the link member transverse arm 24 which are provided with abutment surfaces 91 adapted for engagement with the uppermost portions of the respective leaf springs 20. Upon dis placement of the link member 24 away from the head, the abutment surfaces 91 engage the leaf springs slightly after engagement thereof by the abutments 27 and urge the contacts further apart.

A first thermally responsive actuating means comprises a snap-action bimetalic actuator 28 of the sort described in British Patent Specification No.

1,542,252. The bimetallic actuator 28 is mounted on a stainless steel bridge member 30 as described in more detail below and overlies the bases of the terminal pins 16. A lower peripheral portion 31 of the actuator 28 is held in good thermal contact with wedge-shaped protrusion 43 formed on the inside of the head 2 towards the bottom thereof adjacent to the point where the hot return 8 of the element is welded thereto, and an upper bridge portion 32 of the actuator 28 is adapted for engagement with a central abutment surface 33 formed on the 'U'shaped link member 23. The actuator 28 is calibrated with a nominal operating temperature of 140"C so as to undergo snap action when the element overheats.

Thus, when the element overheats after for example having been switched on dry, the actuator 28 reverses its curvature by snap-action whereupon the bridge portion 32 engages and moves rightwardly, i.e. away from the head, the link member 23 which in turn pushes the movable contacts 18 away from the stationary contacts 19 as shown in Figure 7 so as to cut off the power supply to the heating element.

An important safety feature of the preferred control arises from the fact that the 'U'-shaped link member 23 in addition to being slidable in a substantially axial direction is pivotable to a limited extent in its horizontal plane about a vertical axis.

Such pivotal movement is accomodated by the outer edges of the longitudinal arms 25 of the link member 23 being contoured within the channels 26 so as to define curved surfaces 34 which abut the outer wall of respective channels 26 therebeing a lateral clearance between the major part of the edges of the arms 25 and the channels 26. Thus, in the event of either pair of contacts 18, 19 becoming welded together after an extended period of use, upon operation of the actuator 28 engagement of the bridge portion 32 thereof with the abutment surface 33 of the link member 23 will cause the link member 23 to pivot about the welded-together pair of contacts and open the remaining pair as shown in Figure 9 thereby de-energising the heater.This additional safety feature is particlarly significant where the container forms part of an appliance which in general use is likely to be left unattended for extended periods of time such as, for example, an early morning automatic tea maker.

The stainless steel resilient bridge member 30 includes a pair of spaced lateral flanges 35 projecting from each side thereof which fit snugly within respective generally rectangular apertures 36 formed at either side of the body portion 10 and through which the lowermost two of the body portion mounting bolts 11 extend. As the bolts 11 are tightened, the flanges 36 bear directly against the respective female studs 12 of the head and are firmly clamped between the heads of the respective volts 11 and the studs 12 within the apertures 36. In this way, the stainless steel flanges 35 act as spacers between the heads of the mounting bolts 11 and the ends of the studs 12 and prevent undue stresses being imparted to the plastic body portion as is bolted to the head.Thus, the body portion may be tightly clamped to the head without the possibility of inadvertently damaging the plastic by overtightening the lowermost bolts.

The bimetallic actuator 28 is mounted to the bridge member 30 by means of its tongue 37 which passes between two folded-over ears 38 of the bridge member 30 and overa folded-up ramp 39 thereof which enters an aperture in the tongue 37.

The portion 41 of the bridge member 30 which carries the bimetallic actuator 28 is longitudinally offset from the side arms 42 of the bridge member so as to provide necessary clearance between the periphery of the actuator and the bridge member for the actuator to reverse its curvature by snap-action and to permit the necessary movement of the bridge portion 32 of the actuator. In addition, as seen most clearly in Figure 4, the portion 41 which carries the ears 38 and the ramp 39 is at an acute angle to the vertical wall of the head 2 so as to ensure that lower peripheral portion 31 of the actuator 28 is in intimate contact with the protrusion 43. The actuator 28 is mounted on the bridge member 30 by lifting the bridge portion 32 over the ears 38 while sliding the tongue 37 between the ears 38 and over the ramp 39.

Once the ramp 39 enters the tongue aperture 40, the actuator is retained in place. However, should an actuator malfunction during testing of the control, it may conveniently be replaced simply by deforming the free end of the tongue clear of the end of the ramp and there is no need to replace any of the other parts of the control.

The stainless steel bridge member 30, having a relatively low thermal conductivity and heat capacity, minimises heat-loss from the actuator 28 and as such the actuator reacts quickly to overheating of the element.

In the illustrated embodiment an overcentre spring mechanism 44 is disposed above the shelf member 15 on the side of the body portion remote from the element heat i.e. above the axial horizontal plane of the control. The mechanism 44 includes a lever member 45 having a generaly semi-cylindrical profile so as to fit snugly within the upper segmental space of the control. The lever member 45 is pivotably mounted to the body portion by means of two knife edges 46 formed integrally with the member 45 which engage in respective pivot notches 51 formed respectively in spaced arms 47 extending from the body portion 10 in a direction parallel to the shelf member 15.An overcentre spring 48 extends between notches 49 formed at the free ends of the arms 47 and notches 50 formed in the lever member 45 in the end thereof remote from the knife edge 46 and biases the knife edges 46 into engagement with the pivot notches 51. As illustrated in Figure 10, the wire spring 48 is a combined double spring and includes two U-shaped portions 80,81 disposed in back-to-back relation. Each U-shaped portion interconnects a pair of generally parallel arms 82,83 one of which arm 83 is common to both U-shaped portions. The spring imparts a symmetrical biasing force on the lever member 45 and is sufficiently low rate to adequately accommodate the pivotal movement of the member 45.The lever member 45 is pivotable between a first stable position as shown in Figure 5 wherein a lower surface of the lever member 45 engages stops 53 formed on the free ends of the arms 47 and a second stable position shown in Figure 4. In moving from the first stable position to the second, the lever member 45 passes through an unstable dead centre position in which the spring notches 49, 50 and the knife edges 46 all lie in a common horizontal plane.

A bimetallic actuator 56 similar to the actuator 28, but having a nominal operating temperature of 80"C is located below the lever member 45 and lies at an acute angle to the shelf member 15 between the arms 47 which engage the outer periphery of the actuator 56 and serve to locate the actuator laterally.

The base of the actuator 56 rests on support members 57 of the body portion and is retained longitudinally by a projection 58 at the end of the shelf member which abuts the bridge portion of the actuator 56. A small cut-out 100 is provided on the outer periphery of the actuator and is engaged by a small projection on the control to prevent rotation of the actuator. During assembly, the actuator is simply dropped into which extend over the bridge portion of the actuator 56 thereby clamping it to the shelf member 15.

Steam or vapor resulting from liquid within the container boiling is transmitted to the actuator 56 via an aperture 60 formed in the body portion 10. The steam tube 6 is sealed to the aperture 60 by means of a silicone rubber sleeve 61 one end of which is stretched over the end of the tube 6 and the other end of which is pushed into the aperture 60. The upper wall of the aperture 60 is upwardly tapered at the side adjacent the head.

Upon liquid within the container boiling, steam or vapour impinges on the steam sensitive bimetallic actuator 56 which then reverses its curvature by snap-action so that its tongue 62 engages and bears upwardly against a post 63 extending downwardly from the lever member 45 thus tripping the lever member 45 from its first stable position to its second stable position. The lever member 45 includes two downwardly projecting legs 64 which extend either side of the shelf member 15 and terminate within notches 65 formed in the longitudinal arms 25 of the 'U'-shaped link member 23. A strengthening bridge 66 extends between the legs 64 and passes between the body portion arms 47 and shelf member 15.

Thus, upon the lever member 45 being tripped from its first stable position to its second stable position the legs 64 thereof engage the ends of the notches 65 and pull the link member 23 rightwardly away from the element head and open the switch contacts as described above. The effect is that when the liquid within the container boils the actuator 56 trips the lever member 45 from its first to its second position thereby opening the switch contacts and deenergising the heating element.

As described above, the steam or vapour is shielded from the electrical components on the side of the body portion facing the head by means of the resilient sleeve 61. On the lower side of the body portion, the terminal pins 16 are shielded from steam and from condensed liquid by the shelf member 15 and by the cover member shroud 17.

Liquid condensing on the shelf member 15 runs off the member at each side into a perhipheral passage defined between the shroud 17 and the outer wall of the cover member 13. An aperture 67 is provided at the base of the cover member to allow condensed liquid within the peripheral passage to drain out of the control. The shroud forming portion of the cover member co-operates with the body portion to form a capillary seal 101 therebetween to prevent liquid seeping into contact with the terminal pins.

A knob 68 is pivotably mounted above the lever member 45 for manually resetting the control after the overcentre mechanism has been tripped as a result of liquid within the container boiling. The knob 68 is mounted by means of a flange 70 thereof which is held between a shoulder 69 projecting from the body portion and part of the cover member. When the knob 68 is pressed downwardly, the lever member 45 is returned to its first position thus allowing the movable contacts 18to move into contact with the respective stationary contacts 19 and the electrical power supply to the element can be resumed.

Upon movement of the link member in response to operation of the switch-on-dry actuator 28, the lever member is tripped slightly after the contacts are opened. Thus, if an attempt is made to energise the heater with no liquid in the container by manually holding the knob in the closed position, the actuator 28 will cycle causing the contacts to repeatedly open and close thus preventing serious overheating of the element.

The embodiment of the control illustrated herein is particularly suitable for sale ready-mounted to the immersion heater a replacement element for userfitting. However, the illustrated control may serve solely as a switch-on-dry protector unit simply by omitting the over-centre mechanism and associated actuator.

Claims (7)

1. An electric immersion heater of the kind described and a thermally-sensitive control mounted or mountable to the head thereof, there being electrical switch means comprising at least one set of breakable contacts, said or each set including a stationary contact and a movable contact mounted towards a free end of a generally 'U' shaped leaf spring of resilient conductive strip material, wherein, in normal operation, the leaf spring urges the movable contact against the stationary contact and electrical power can be supplied to the heater via the switch means, and a bimetallic actuator of the kind described coupled to said or each contact set by means of a movable link member adapted for engagement with the or each leaf spring wherein the actuator is actuable upon the element overheating to move the link member which displaces the movable contact of the or each contact set away from the stationary contact to open the switch means deenergise the heater, said actuator being mounted so as to lie face-to-face with the heater head with the bridge or margin portion of the actuator lying in use above the tongue thereof.

2. An electric immersion heater as claimed in claim 1 wherein the bridge or margin portion of the bimetallic actuator is adapted for direct engagement with the link member which is slidably mounted within the control and is movable in response to operation of the actuator so as to engage the or each leaf spring at or adjacent the free end or ends thereof.

3. An electric immersion heater as claimed in claim 1 or 2 wherein the control includes terminal pins adapted for engagement with a separate female-socket connect, and the or each leaf spring includes an apertured based portion which surrounds and is secured to the base of a respective terminal pin.

4. An electric immersion heater as claimed in any of claims 1 to 3 including two sets of contacts each set comprising a stationary contact mounted directly to a respective cold lead of the heater and a movable contact mounted towards the free end of a respective leaf spring.

5. An electric immersion heater as claimed in any preceding claim wherein the bimetallic actuator is mounted by means of a stainless steel bridge member which is clamped between the heater head and a body portion of the control.

6. An electric immersion heater, substantially as herein described with reference to the accompanying drawings.

7. A container including an electric immersion heater as claimed in any preceding claim.