Field of the Invention

This invention generally concerns improvements relating to temperature control, and more particularly though not exclusively concerns the control of storage type immersion heaters as for example in hot water tanks. Background of the Invention

Conventional thermostats for the control of the temperature of water within a storage tank have typically hitherto been of two main types, namely:

(i) a rod type of thermostat, in which the differential expansion of two metal components is used to operate the contacts of a switching means; and

(ii) a bimetal based device, wherein a stressed bimetal form is constrained to change shape at a predetermined temperature and this shape change is arranged to operate the contacts of a switching means.

In both the above cases the temperature at which the operating means functions may be arranged to be adjusted in order to allow the temperature of the

stored water to be varied, for example to allow use in areas of high water hardness where a lower setting is desirable to prevent excessive scaling of the heating element. In particular it should be noted that the temperature at which the stored water is controlled is determined by the temperature at which the contacts of the switching means are arranged to open, and that a swing in the temperature of the water is necessary to cause said contacts to reclose and maintain the water at or close to a desired temperature. This swing may be undesirable and there are a number of known means available to reduce it such as, for example, positioning a heater close to the temperature sensing element so that the heater causes the opening of the contacts at a lower water temperature than would otherwise be the case. The heater is then switched off (possibly by the same contacts), so that the contacts will reclose at the same water temperature as if the heater were not provided. Thus it can be seen that the swing in water temperature is reduced, since the high temperature (switch off) is reduced while the low temperature (switch on) remains unchanged.

It is possible, particularly with bimetal based devices, that the heating effect required for reducing the swing in water temperature may be achieved by arranging for the supply current for the storage tank

heater to be passed through the bimetal thus heating it by the direct effect of the electrical resistance of the bimetal. Notwithstanding the apparent benefits and simplicity of such a construction, hitherto this has been considered undesirable and every effort has been made to utilise bimetal of as low an electrical resistance as possible. Summary of the Invention

In contrast to the prior art as described above, the present invention proposes to make use of this heating effect to almost completely eliminate any swing in the temperature of the stored water. It is a feature of this invention that the temperature of the stored water will be determined by the switch on (low) temperature of the controlling device substantially independently of its switch off (high) temperature. It is a further feature of this invention that the temperature of the stored water will approach its final value at a progressively reducing rate, thus eliminating the overshooting of the desired temperature associated with conventional controls of the first type mentioned above which control at their switch off (high) temperature. This second effect is similar to proportional control, common in more sophisticated temperature controllers.

In accordance with the present invention in one

of its aspects therefore there is provided a system comprising means for storing a volume of liquid, electrically powered heating means for heating said liquid, and thermostatic control means associated with said heating means for enabling the temperature of the stored liquid to be determined, and wherein the thermostatic control means includes a thermally- responsive switch-actuating element arranged to be subjected to heating by the flow therethrough of electrical current for powering said heating means, the said thermally-responsive switch-actuating element is arranged in heat transfer relationship with the volume of liquid to be heated but not directly subject to the temperature thereof, and the said thermally- responsive switch-actuating element being formed so as to be rapidly heated through its ON (low) to OFF (high) temperature differential range by the flow therethrough of the heating means power current whereby the temperature of the volume of liquid will in operation of the system stabilize around the ON (low) temperature end of the said differential range.

The invention also extends to a thermostatic control device for use in a system as above-described and wherein the device includes a thermally-responsive switch-actuating element (e.g. a bimetal) formed to develop significant electrical (I^Rt) heating when

subjected to a predetermined current flow therethrough commensurate with the current loading of the heating means and wherein the switching differential of the switch-actuating element is such as to be readily exceeded by the temperature rise caused by said electrical heating.

In application of the present invention to a domestic or industrial hot water storage and supply system for example, a bimetallic switch might be accommodated in an air space within a cover enclosing the power supply terminals provided in a head portion of an immersion heater associated with a water storage cylinder. The bimetallic switch will thus be subject to the temperature of the air in such air space, rather than to the temperature of the water in the storage cylinder, but nonetheless will be in heat transfer relationship, via the air in the air space, with the water. On switch-on from cold, the current to the immersion heating element flows through the bimetallic element of the switch thereby causing self- heating thereof until such time as the bimetal reaches its switching temperature and the switch switches off; in this time period a certain amount of heat will be injected into the water in the storage cylinder and the water temperature will rise. The bimetal temperature will now begin to fall, given that the

bimetal is in a relatively cool air space. In due course the bimetal will cause the switch to switch on again and the cycle will be repeated with yet a further pulse of heat being injected into the water. This cyclical process will be repeated as the water temperature rises towards the low (switch-on) temperature of the bimetal. When the water temperature is at or about the bimetal switch-on temperature, the effect of the through current when the immersion heater is switched on will be to cause it quickly to heat to its off temperature so that only a small pulse of heat will be injected into the water, and the bimetallic switch will then remain off until such time as the bimetal cools again to its on temperature. The temperature of the air in the air space will track the water temperature and the situation will eventually be reached where the water temperature substantially stabilizes at or about the switch-on temperature of the bimetal. Although the invention is described herein with particular reference to heating and controlling the temperature of a volume of liquid, it will readily be appreciated that the nature of the matter being heated is immaterial. Thus, in an alternative application of the invention, the volume being heated might for example be the seat of an automotive vehicle.

The function and operation of the present invention will become clear from consideration of the following description of a possible embodiment given with reference to the accompanying drawings. Description of the Drawings

Figure 1 shows the outline shape of an exemplary bimetal element;

Figures 2 and 3 are plan and side elevation views, respectively, of an exemplary switching device incorporating a bimetal element as in Figure 1; and

Figure 4 is a graphical representation of the function of the device to be described. Detailed Description of the Embodiment

It is well known to those skilled in the art of bimetal thermal cut-outs and thermostats that the passage of electric current through the bimetal of the device causes a temperature rise, and that this temperature rise may be readily calculated from the form and resistivity of the bimetal. The present invention in the described embodiment proposes to make novel use of an existing product of Otter Controls

Ltd. which is shown in Figures 1, 2 and 3. These show a thermal cut-out comprising a rectangular sheet of bimetal 1, attached by welding to a brass mounting plate 2, to which electrical connection may be made at terminal 3. Within the rectangular bimetal sheet 1 is

a U-shaped cut-out 4 defining a central limb 5 on which is secured a silver faced contact 6. A further brass plate 7 is disposed such that a similar contact 8 mounted on it is in alignment with the first mentioned contact 6. The first and second brass plates 2 and 7 are mounted with appropriate spacing from each other by means of an insulating switch body component 10, and the brass plate 7 has a terminal 9 to allow a second electrical connection to be made to it.

The rectangular sheet of bimetal 1 is deformed into a generally spherical dished surface, with the high expansion component being on the concave side 11, such that the central limb 5 is forced out of the plane of the sheet and the contact 6 is forced into firm contact with the contact 8. As is described in

British Patent No. 657434 (Eric Hardman Taylor), such a configuration of bimetal will, on heating, suddenly reverse its curvature at a certain temperature so as to adopt the position shown dotted at 13 in Figure 3 and will force apart the two contacts 6 and 8. This certain temperature is the switch off (high) temperature. There is also provided a stop 12 on the opposite side of the bimetal 1 from the contact 6 and generally in line with it, which is arranged to limit the movement of the abovementioned central limb 5. As

the temperature of the bimetal falls, at a certain point the bimetal will revert to its original curvature thus forcing the contacts 6 and 8 together again. This temperature is the switch on (low) temperature. The difference in temperature between the high and low values is termed the differential of the switch. It is known that adjusting the position of the stop 12 (for example by means of a screw shown diagrammatically at 14) will adjust the low temperature setting of the device without affecting its high temperature setting.

Consider a device as described, where heating is provided by the passage of an electrical current through the bimetal. Suppose for the purpose of illustration that the temperature rise which is induced in the bimetal by the passage of the particular value of current is 30°C, and that the time in which this temperature rise occurs is 90 seconds. Suppose furthermore that the value of the differential is 20°C, and the switch on (low) temperature of the device is 70°C. Now consider this device in thermally responsive relation as aforesaid with a body of water whose temperature is to be controlled and with a heater for the water connected in series with the device so that the electric current to the heater flows through and is controlled by the device.

Referring now to Fig. 4, on switching on from cold at the position of the graph designated by reference 15, the water temperature will start to rise, as will the temperature of the bimetal. However, because of the heating effect of the current on the bimetal, the bimetal temperature will exceed that of the water by approximately 30°C as indicated by reference 16. Thus, when the water temperature reaches 60°C, the bimetal temperature will be at around 90°C as indicated by reference 17 which is its switch off temperature. The device will therefore switch off the supply to the heater, and the water and the device will start to cool. However, because the device is in thermal contact with the air space in the heating element head which is cooled by the still cool water, it will rapidly cool to below 70°C as indicated by reference 18 and will switch on before the water has cooled by a significant amount as indicated by reference 19. After about 90 seconds, the bimetal will once more have reached its switch off point as indicated by reference 20, the water will have heated by some amount as indicated by reference 21, and the cycle will be repeated. This cycling will continue until the water reaches 70°C as indicated by reference 22, at which point the device will not switch on until the water itself has cooled to slightly below 70°C.

When the device switches on, the water will be heated to slightly above 70°C whereupon the device will switch off and will remain switched off until the water cools again. Thus the water will be maintained at a temperature very close to the switch on value of the device, which will only cycle sufficiently to maintain this temperature.

In order for the device to effect control in this way, it is only necessary for the temperature rise caused in the device by the current being controlled to exceed the differential of the device. The amount by which it exceeds will determine the temperature at which the device starts to cycle, as the temperature of the heated liquid approaches the desired value. As was noted above, the switch on temperature of the device may be adjusted by adjusting the position of the stop 12. This will have the effect of also varying the differential, but provided the above conditions are met control will be achieved and only the cycle rate will vary. It should be noted that varying degrees of sensitivity to current can be achieved by using bimetals of differing electrical resistivity whilst keeping the form of the device otherwise constant. Thus a range of devices may be constructed, suitable for the control of loads of differing power.

This invention is not limited to use of the device described in the above and any thermostatic device which is responsive to current heating may be used in the same way. In addition, the invention may be used to control the temperature of any substance, water being mentioned in the above only by way of an example. The essence of the present invention resides in the provision of a thermostatic switching device having a differential less than the temperature rise caused by the passage of the current being controlled, and the use of such a device to control the temperature of any substance within close limits of the switch on temperature of the device.

While the invention has been particularly described in the foregoing with reference to electrically-heated hot water storage and supply systems, it is to be appreciated that the invention is not restricted to such an application. For example, the invention could be used with a hot water jug as a simmer control, in which case a bimetallic or other thermostatic switch sensitive to load current flow to be heated thereby might for example be secured to the body of a plastics water heating container. In another application, rather than heating a volume of liquid the application could be for the heating of non-liquid matter, such as for example the bulk of an

automotive vehicle seat.