WO2002097841A1

WO2002097841A1 - Non-retriggerable thermo-protector with active safety

Info

Publication number: WO2002097841A1
Application number: PCT/IB2002/001632
Authority: WO
Inventors: Fausto Fioroni
Original assignee: Thermowatt SpA
Current assignee: Thermowatt SpA
Priority date: 2001-05-28
Filing date: 2002-05-08
Publication date: 2002-12-05
Anticipated expiration: 2003-11-28
Also published as: ITAN20010023A1

Abstract

An improvement of the safety devices used for thermostats is described. In particular, it is applicable to immersion thermostats for water boilers. An elastic element (6), pre-stressed by a pressing organ (10), is provided with an expansion capacity and elastic force large enough to force the opening of the pair of electric safety contacts in the case in which the sensors and actuators (2, 3) providing for the proper functioning of said thermo-protector suffer a failure. The main advantage of the invention is that it provides thermo-protectors with so-called 'active' safety.

Description

NON-RETRIGGERABLE THERMO-PROTECTOR WITH ACTIVE SAFETY

DESCRIPTION

This invention relates to a non-retriggerable thermo-protector with active safety. In several applications and in particular in the field of water heating in electric water boilers electro-mechanical devices called thermostats are used. These provide both for the setting of the heating temperature of the water and for the important task of limiting the temperature: in the first case these devices are called regulation thermostats, while in the second case they are called safety thermostats or thermo-protectors.

Regulation thermostats provide for the operating of the elements that heat the water until the water reaches the operating temperature required by the user as well as for maintaining this temperature, while the thermo-protectors take care of limiting any rise in the temperature ascribable to a malfunction of the regulation thermostat in case said increase in the temperature exceeds a safety threshold temperature capable of damaging the thermostat itself. An extremely widespread type of regulation thermostats envisages the use of a tube with a high coefficient of thermal expansion as a sensor. This tube is usually made of brass and is covered by a sheath that in turn is immersed in the water to be heated and bearing on the inside a metal rod positioned coaxially and marked by an extremely low degree of thermal expansion. This rod is generally made of INVAR.

The brass tube and the INVAR rod are joined together externally at their extremities. The differing thermal expansion coefficients of the brass and INVAR cause the elongation or shrinkage of said brass tube depending on the temperatures of the water. These deformations cause, in turn, an equivalent shift of the INVAR rod along its longitudinal axis. This shift is conveyed to a bi-stable micro switch by means of a rocking component located inside the body of the regulation thermostat.

By means of this type of thermostat it is possible both to stop automatically the supply of electric current to the heating elements when the water in the water boiler has reached the temperature required by the user and to maintain this temperature automatically by switching on and off the electric power supply at intervals.

Another widespread type of thermostat is the one called "clickson" in which the sensors/actuators are composed of "bimetallic" disks obtained from a sheet having two layers made of different metals and hence having also an adequate difference in the thermal expansion coefficient; these disks are then provided with a bulge which abruptly inverts its concavity when a given temperature is reached and hence is deformed owing to the difference in the thermal expansion of the two layers.

"Clickson" thermostats can perform both the regulation and safety functions described above. In the first case, the deformation of said bulged bimetallic disks is reversible depending on the variations in the temperature. The bimetallic disks used in the safety thermostats are built so that said deformation is not reversible to prevent a new closing of the electric contacts as the temperature of the water decreases.

Usually the safety thermo-protectors are "bipolar", i.e. they isolate completely the electric heating elements by disconnecting both poles from the electric network through the opening of one pair of electric contacts. This pair of electric contacts will be hereinafter referred to as "safety contacts".

The previous document IT 947 921 is well known. It explains how to fit into a single electro- mechanic device both a regulation thermostat and a thermo-protector (the latter is called "third safety" in technical jargon). In said previous document, with regard to the regulation thermostat, the device forming the subject matter of the claim envisages the use of the aforementioned brass tube as a sensor, while the function of actuator is performed a transmission unit composed of the aforesaid INVAR rod connected to a rocker and to a bistable micro-switch; as regards the safety thermostat, instead, the aforesaid non-reversible deformation bimetallic disk is used as a sensor. The disk, together with a plain pusher, opens the safety contacts. A first inconvenience attached to a thermo-protector as per said patent lies in the difficulty and the lack of precision of the control of the maximum temperature to be limited: the thermostat is positioned on the bottom of the water boiler and it detects the temperature of the lower layers of the water therein contained, but it must also limit the maximum temperature in the upper layers. It is hence very difficult to build an adequately sensitive device and to calibrate it.

Another inconvenience lies in the possibility of retriggering the bimetallic disk if it is faulty and hence in the impairment of its safety function which must prevent any new closing of the safety contacts; in fact, if the thermo-protector has been triggered, it means that there has been a failure of the regulation thermostat and hence its further operating must be stopped. A great step ahead was made with the prior document EP 0 651 412 which illustrates how to use the brass tube and the INVA rod as a single sensor both for the regulation and for the limitation of the temperature. With regard to the part pertaining to the regulation thermostat, the device forming the subject matter of the claim of the latter prior document envisages that the function of sensor is performed by the brass tube and that of actuator by the transmission unit formed by the INVAR rod - rocker - bi-stable micro switch already known, while as regards the safety thermostat the sensor element is the brass tube and the actuator element is the INVAR rod which is capable of inverting abruptly the concavity of a non-bimetallic bulged disk, i.e. thermally inert, made of an adequately elastic metallic material; the latter item, in turn, lifts a cursor which finally opens the safety contacts. However, the device described in said latter prior document has an inconvenience consisting in the fact that, if there is a failure, the INVAR rod breaks away from the brass tube and the safety thermostat is not triggered. In fact, in this case the brass tube does not shift the INVAR rod even if it stretches and hence the INVAR rod fails to trigger the bulged disk. It is known to all that any safety device must provide "active safety", i.e. in case of its failure, it assumes a state that provides for the safety of the controlled equipment and in particular for the automatic opening of the safety contacts. This inconvenience is addressed only in part by a variant specified in said document by which a bimetallic disk is coupled to the non-bimetallic disk so that the pair of contacts can open if one of the two disks is triggered. The main purpose of this invention is to eliminate the inconvenience of document EP 0 651 412 above; that is to say, to assure the triggering of the thermo-protector even in the case in which, owing to a mechanical failure, the INVAR rod breaks away from the brass tube acting as sensor device.

Another goal is that of creating a thermo-protector acting both as a regulation thermostat and as an independent constituent part. These and other goals are attained by means of a device as per the following description, drawing and claimed features hereto attached and forming an integral part of the description itself.

A description of a preferred embodiment is illustrated below by way of example and in no way exhaustive. Figure 1 shows a partial and sectional view of an immersion thermostat comprising a thermo- protector as per this invention in its normal configuration, i.e. at normal operating temperatures and in the absence of mechanical breaks in the device.

Figure 2 shows a partial and sectional view of the same thermostat illustrated in the figure above in the configuration assumed if, in the presence of any failure, the INVAR rod is no longer connected to the brass tube.

With reference to figures 1 and 2, with 1 is indicated an immersion thermostat comprising a thermo-protector; said thermostat comprises a tube 2 made of a material with a high thermal expansion coefficient - in this case brass - to be inserted in a sheath immersed in the liquid of a tank not illustrated like that of a water boiler in particular. Rod 3 made of a material with a low thermal expansion is positioned inside said tube 2; the material of the rod can be either LNVAR or the like.

Tube 2 and rod 3 are joined together in a known manner. The end of rod 3 located inside thermostat 1 is connected to a mechanism 4 which, in turn, is connected to an electrical connection 5. An elastic element 6, made of synthetic rubber, metal or other material, is inserted inside rod 3 in a cavity 7 obtained inside the body 8 of the thermostat 1; said elastic element 6 must have adequate elastic features to assure the performance of the action described below. On top of the elastic element 6 there is a thermally inert, bulged disk 9 - that is to say, it does not deform independently owing to the variations in the temperature - made of an adequately elastic metal material; said disk is bulged and the concavity is directed towards elastic element 6 when the safety system formed by the thermo-protector is inactive.

Said bulged disk 9 must be elastic and thin enough to exploit the capacity of inverting abruptly the concavity of its bulge when stimulated mechanically while maintaining unaltered the impressed bulge in the absence of mechanical stimuli. This disk is inserted in rod 3. A pressing organ 10 is screwed on rod 3 on top of the disk. It provides for the calibration of the safety system and exerts, when required, pressure on the elastic element 6. On top of said pressing organ 10, it too inserted inside of rod 3, there is a cylindrical element 11 having a section diametrical to C. Its external perimeter rests on the external perimeter of the bulged disk 9. The operating of the device forming the subject matter of this invention is described by following the references specified in the figures.

When the rod 3 is stimulated to stretch by the elongation of tube 2 owing to an increase in the temperature of the liquid in which the tube is immersed, the pressing organ 10 presses against the bulged disk 9 until it overcomes the elastic resistance of the elastic element 6 and triggers the disk thus changing the direction of the concavity of the bulge of the bulged disk 9.

The triggering of the disk above leads to the raising of the cylindrical element 11 and hence of mechanism 4 and the opening of the safety contacts not illustrated in the figure thus stopping the supply of electricity to the elements heating the water. Hitherto the device and the single components herein described operate exactly like those described in EP 0 651 412.

In addition, according to this invention, compared to the description and claims in EP 0 651 412, the elastic element 6 is kept in normal conditions, i.e. pressed by the pressing organ 10 screwed on the rod 3; the exerted pressure is strong enough to provide for yet another compression action in the case in which the rod 3 must be pressed on the bulged disk 9 to invert the concavity of the bulge in case the maximum temperature is exceeded.

If, for any reason at all, the bond between tube 2 and rod 3 fails and hence said rod 3 is no longer held by the tube 2, the elastic element 6, as illustrated in fig. 2, is no longer pressed and hence relaxes thus pushing the bulged disk 9 which, in turn, without changing its concavity, presses against the cylindrical element 11 and as a consequence raises the mechanism 4 and opens the safety contacts belonging to the thermo-protector; the result is that the electrical power supply to the elements heating the water is interrupted.

In order to make this happen, it is necessary that the elastic element 6 is pre-stressed by the pressing organ 10 and has an adequate elastic rigidity once it is released from the rod 3 so as to enable it to shift the cylindrical element 11 to an adequate extent to force it towards the opening of the safety contacts; the necessary shift is usually slightly more than 0.65 mm.

Clearly, the opening of the pair of contacts according to the procedures described above is carried irreversibly as soon as, opening to a break, the rod 3 is no longer attached to the tube 2; therefore, the thermo-protector provides for active safety in the sense that, in the case of the breaking of the kinematic connection between tube 2 and rod 3 providing for the opening of the pair of contacts of the thermo-protector in case the maximum temperature is exceeded, the opening of the safety contacts takes place immediately and naturally owing to the decompression of the elastic element 6.

Of course, a thermo-protector as per the invention can be obtained both together with a regulation thermostat as illustrated in the attached figures and already described in the aforementioned documents and, instead, when used as a device independent from a regulation unit, with tube 2 and rod 3 solely for the purpose of controlling the maximum temperature. After describing the invention as per the main variant, it must be pointed out that the illustrated sensors and actuators, that is to say tube 2 and rod 3 and even the pressing organ 10 can be replaced with equivalent means which, in normal operating conditions, keep the elastic element 6 pressed and provide for the opening of the thermo-protector's safety contacts by exerting on the convex face of the bulged disk 9 a degree of deformation large enough to cause the abrupt inversion of the bulge, while, in case of the failure of said sensors and actuators, the opening of the pair of electric contacts is obtained by the fact that the break releases the elastic element 6 from the compression. The elastic element 6 can hence push the bulged disk 9 towards the cylindrical element 11 far enough to obtain the required opening of the contacts.

An example of means equivalent to tube 2 and rod 3 can be a sensor/actuator composed of a known wax bellows - bulb - capillary system. More precisely, a partially expanded wax bellows can be mounted coaxially compared to rod 3 instead of the pressing organ 10. In case the threshold temperature detected by the bulb connected to the bellows is exceeded, the bulb expands and, as it does so, it presses further against the bulge of the bulged disk 9 thus causing it to trigger; instead, in case of the breaking of the capillary, the bellows is emptied and, as it does so, it contracts thus providing for the decompression of the elastic element 6 which pushes the bulged disk 9 towards the cylindrical element 11. According to another variant, there is nothing that prevents the device, as already in the case of EP 0 651 412, from envisaging the presence of a bimetallic disk between the elastic element 6 and the bulged disk 9. This variant operates as described already above: the opening of the safety contacts occurs with the normal operating of the thermo-protector by way of the abrupt inversion of the bulge of the bulged disk 9 and/or of the bimetallic disk and, instead, by way of the slackening of the elastic element 6 if, owing to a mechanical failure, it is no longer pressed. The operating is the same also if the bulged disk 9, according to yet another variant, is of the bimetallic type: the opening of the safety contacts occurs with the normal functioning of the thermo-protector by way of the abrupt inversion of the bulge of the bulged disk 9 owing to the increase in its temperature and/or owing to the pressure exerted on it by the pressing organ 10 and, instead, always by way of the slackening of the elastic element 6 if, owing to a mechanical failure, it is released from the pressure.

The device, as illustrated, is to be considered as an improvement of EP 0 651 412 which adds to the advantages specified in this document also that of providing active safety in case of a mechanical failure of the thermo-protector by requiring, at least in the case of most of the variants described above, only simple modifications to the parts already envisaged and described in EP 0 651 412.

Claims

Claim 1 Non-retriggerable thermo-protector with active safety of the type composed of

- transmission means (11, 4) providing for the opening of the electric safety contacts

- a bi-stable trigger means (9), comprising elastic parts, to maintain its shape constantly if no external forces are exerted on it and to disconnect the aforesaid electric connections by shifting the aforesaid transmission means (11, 4),

- a pressing organ (10) to exert a deforming force on said bi-stable element (9),

- sensors and actuators (2, 3) to exert a force on said pressing organ (10), characterized by the fact that - an elastic element (6) is also envisaged; it is subject to pre-stressing by said pressing organ (10), although it is possible to exert a further compression in case the threshold temperature is exceeded,

- said pre-stressing and the elastic rigidity of the elastic element (6) itself being adequate to allow the elastic element (6) to expand to an adequate extent capable of forcing said bi-stable triggering means (9) against the aforesaid means (11, 4) so that the aforesaid electric safety contacts can be opened if, for any reason at all and in particular owing to the failure of the sensors and actuators (2, 3), the pressure exerted by the pressing organ (10) fails. Claim 2 Thermo-protector as per claim 1, characterized by the fact that said bi-stable triggering means (9) comprises a thin bulged disk (9) made of an adequately elastic metallic material capable of maintaining the impressed bulge. Claim 3 Thermo-protector as per claim 2, characterized by the fact that said bulged disk (9) is made of a thermally inert metallic material.

Claim 4 Thermo-protector as per claim 2, characterized by the fact that said bulged disk (9) is of the bimetallic sheet type and is capable of abruptly and irreversibly inverting the direction of its bulge owing to the variation of the temperature. Claim 5 Thermo-protector as per claim 2, characterized by the fact that said bulged disk (9) is made of a thermally inert metallic material and that between said bulged disk (9) and said elastic element (6) there is a bulged disk made of a bimetallic sheet capable of abruptly and irreversibly inverting the direction of its bulge owing to the variation of the temperature. Claim 6 Thermo-protector as per any one of the previous claims, characterized by the fact that the expansion necessary to force the opening of the relevant pair of electric contacts by means of the appropriate devices amount to slightly more than 0.65 mm.

Claim 7 Thermo-protector as per one or more of the previous claims, characterized by the fact that said sensors and actuators (2, 3) are composed of a tube (2) made of a material with a high thermal expansion coefficient and in particular brass and of a material with a low thermal expansion coefficient and in particular INVAR

Claim 8 Thermo-protector as per one or more of the previous claims, characterized by the fact that said sensors and actuators (2, 3) consist in the known wax bellows - bulb - capillary system. Claim 9 Thermo-protector as per one or more of the previous claims, characterized by the fact that said thermo-protector is incorporated in a regulation thermostat since the aforesaid sensors and actuators (2, 3) are the same used by said regulation thermostat.