NZ247075A - Water boiler with twin elements has controlled heating rates - Google Patents

Description

247 075 Complete Specification Filed: ...» Class:(C) .fiasco .13 Publication Date: ..1995.; P.O. Journal No:.. luon Patents Form No. 5 NEW ZEALAND PATENTS ACT 1953 COMPLETE SPECIFICATION WATERHEATER WE, IMI SANTON LIMITED, a British company of Somerton Works, Spytty Road, Newport, Gwent NP9 OXU, UNITED KINGDOM hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: [isTZ. PATENT OFFICE ■ - 1 ~ (followed by page la) 14 JUN 1996 RECEIVED 24 7 0 7 'j Waterheater This invention relates to continuous electric waterheaters especially, but not exclusively, to such heaters for storing relatively small quantities of boiling, or near-boiling, water for use in making, for example, hot drinks such as tea and coffee.

In recent years, the use of such heaters has become quite widespread in, for example, offices and factories. Typically they are wall hung and have capacities of from 5 to 15 litres. Generally they comprise a vessel for storing the water, a valved dispensing outlet such as a manually operable tap and, located within and near the bottom of the vessel, a heating means usually consisting of a single electric heating element of, say, 3kW output. In addition they are plumbed into a controlled supply of mains cold water such that water in the vessel is continually replenished and heated to the desired temperature. Such a heater is referred to hereinafter as a waterheater of the type described.

The temperature of the water is controlled by a thermostatic switch which switches on the heating element when the water temperature drops somewhat below a pre-determined value and switches it off when it reaches that value, typically just below 100 °C. In other words, the heating element continuously switches between fully on and off modes during use. This gives rise to a number of problems, firstly that of cavitation noise generated whilst the element is energised; secondly, and more significantly, that of scaling of the element, especially in hard water areas, which as is well known, occurs much more rapidly at high watts densities than at lower ones and which necessitates fairly frequent replacement or (followed by page 2) 247 0 7 3 de-scaling of the element; thirdly, the thermostatic switch undergoes a large number of on/off cycles which, because of the high currents involved (typically around 12 amps), limits its life.

It is an object of the present invention to mitigate the above 5 problems.

According to the present invention, therefore, there is provided a waterheater for storing hot water including a vessel for storing the water, first and second mutually adjacent, independently operable, electric heating elements for heating the water, a manually operable valved outlet 10 through which hot water can be dispensed, means to control, in use, supply of fresh cold water to the vessel, and a control circuit for controlling energisation of the heating elements, wherein the control circuit is adapted to cause simultaneous energisation of the first and "—ond electric heating elements when water in the vessel is, at the location of and as sensed by a 15 temperature sensor, below a first pre-determined temperature whereby said heating elements heat the water up to said first pre-determined temperature and, when the water attains said first pre-determined temperature, to cause de-energisation of the first heating element and pulsing on and off of the second element thereby to heat the water up to a second pre-determined, 20 desired storage, temperature higher than said first pre-determined temperature.

As has already been noted, a waterheater of the invention is especially applicable to the generation and storage of boiling or near-boiling water and, in that context, the first pre-determined temperature 25 may, for example, be between 90 °C and 98 °C, preferably around 96 °C, 14 7 0 7 5 and the second pre-determined temperature 100 °C or fractionally less. Preferably, user-operable means are provided to adjust the second predetermined temperature to a desired value. By way of example, one of the heating elements may be rated nominally at about 2kW and the other at 5 about lkW. When the water is below the first pre-determined temperature (for example, at cold start-up) both elements are simultaneously energised and thus the total power output is about 3kW. However, when the wate r eventually attains the first temperature, the 2kW element is switched off by the control circuit, leaving only the lkW element operative. The latter is 10 then pulsed on and off thereby further heating the water until it reaches the second pre-determined, desired storage temperature. Under steady state conditions, that is to say when there is no demand for hot water, the lkW element is continually pulsed on and off by the control circuit so as to make good heat loss from the vessel whereby the water therein is 15 maintained at about the second pre-determined temperature. During on and off pulsing of the lkW element, the pulse width is preferably controlled by, for example, a ramp and pedestal circuit, which controls the on and off pulse widths in proportion to the difference between the prevailing water temperature sensed by the sensor, which is preferably a thermistor, and the 20 second pre-determined temperature so as to maintain fine control, and maintenance, of the second pre-determined temperature. Preferably, of course, the vessel is thermally insulated so as to minimise heat loss.

The mode of heating utilised in a waterheater of the present invention thus contrasts with known heaters, in particular water boilers, of 25 the type described in that, in the latter, the full heating load, typically 3kW, r^[P^fENf oFFicr 14 JUN 19SS 247 0 7 5 is always switched on if, for whatever reason, the water temperature drops slightly below about boiling point. This exacerbates, in particular, scaling and cavitation noise problems. On the other hand, during substantial periods during the operation of a waterheater of the invention, the power 5 output of the heating means is modulated to relatively low levels - in the embodiment exemplified above, of the order of lkW - at which levels cavitation noise and scaling tend to be somewhat less of a problem. Also, because during those periods a relatively small load is being continually switched, the switch associated with that load tends to have a long life. 10 As in known waterheaters of the type described, water is dispensed from a heater of the invention by means of a valved outlet located preferably near the bottom of the vessel. Preferably, it is located just above the level of the heating elements. Usually, the valve will be a conventional, manually operable tap although it could, for example, be a 15 solenoid valve actuable by the user. When a quantity of hot water is dispensed, fresh cold water is automatically admitted into the vessel. Preferably, the fresh water issues into the vessel near the bottom thereof, in the region of the heating means, directly from the cold water mains, the flow being throttled so as to minimise turbulence of water already 20 contained in the vessel and controlled by a solenoid valve actuable by a float switch. Alternatively, for example, flow of water into the vessel could be controlled by a conventional float valve of the type used to control the water level in cold water cisterns. Depending on the demand for hot water or, in other words, the rate of admittance of fresh water into the vessel, the ^PATENT OFFICEl —< 14 JUN 1998 ! RECEIVED f 24 7 0 7 5 full heating load in the case of relatively high demands or the modulated load only in the case of relatively small demands may be switched on.

An embodiment of a water boiler constructed in accordance with the invention will now be described in detail, by way of example only, with 5 reference to the accompanying drawings in which: FIG 1 is a vertical section through the boiler; and FIG 2 is a block diagram of the control system of the boiler of Fig 1. Referring to Fig 1 of the drawings, the boiler comprises a vessel 1 having a capacity to hold about 7 litres of water 2. The vessel is lagged 10 with foam 3 and is enclosed by a casing 4. Water 2 is admitted into the vessel 1 through a silicone rubber tube 5, the lower, outlet end 6 of which is located adjacent to the base of the vessel 1 and which, in use of the boiler, is connected to a mains cold water supply pipe via a copper tube 7 and solenoid valve 8, the operation of which is controlled by an upper reed 15 switch 9 actuable by an upper magnetic float 9'. The boiler also includes a lower reed switch 10/magnetic float 10', which ensures that the heating elements of the boiler cannot be energised until, at start up, they are completely immersed by the water 2.

The lower region of the vessel 1 has located in it a pair of heating 20 elements 11,12. The element 11 is rated nominally at 2 kW whereas the element 12 is rated nominally at 1 kW. Both elements are powered by mains electricity via a control circuit 13, in response to the output of a temperature sensor in the form of a thermistor 14 which is contained in the bottom of a metallic pocket 15.

N-Z. PATENT OFRCEj ! 14 JUN 1SPS ! 6 247 075 The upper part of the vessel, above th^ normal maximum level of the water 2, communicates with a metallic vent pipe 16 to which is brazed a short length of copper tube housing a temperature sensor 17 that is connected to a manually resettable safety cut-out device 18 which operates to cut off the supply of power to the heating elements 11,12 in the event that the control circuit malfunctions and permits excessive boiling of the water 2. In such an event, steam will be conveyed along the vent pipe 16, which is fitted with a pressure relief valve 19, and the increased temperature sensed by the sensor 17 will cause the device 18 to interrupt the power supply to the heating elements.

Water 2 is dispensed from the boiler from a manually operable tap (not shown but, for example, of the type described in our UK Patent No 2267951) connected to an outlet 20 formed in the side of the vessel 1.

As water is dispensed through the tap, the magnetic float 9' associated with the reed switch 9 falls and consequent operation of the switch 9 causes the solenoid valve 8 to open whereby fresh, cold water flows into the vessel 1 via the tube 5 until the "full" level is again reached, as sensed by the float 9', when the solenoid valve 8 is de-energised and closes. The flow of fresh water into the vessel 1 is throttled to a rate of about 0.5 litre/minute by a restrictor 21 located in the mains water inlet.

The electrical heating control of the boiler will now be described in more detail.

When cold water, at say about 10 °C, is first admitted into the boiler through the tube 5, the water level in the vessel 1 gradually rises and, upon actuation of the switch 10 by the magnetic float 10', the thermistor 14, N.Z~ PATENT OFFICE 14 JUN m | RECEIVED j Z 4 7 0 7 5 sensing a temperature well below the set-point temperature, for example about 100 °C, of the boiler as selected by appropriate adjustment using a knob 22 mounted on the housing of the control circuit 13, causes both the elements 11 and 12 to be energised thus giving a total input of about 3 kW.

Cold water 2, (which continues to be admitted until the "full" level is reached, as sensed by the float 9', when the solenoid valve 8 closes) is thus quite rapidly heated until the thermistor 14 senses a water temperature of about 96 °C, ie about 4 °C lower than the set-point temperature. At that stage, a switch forming part of the control circuit and that is responsive to 10 the output of the thermistor 14 switches off the 2 kW heating element 11. The 1 kW element 12 continues, however, to be energised and further heats the water 2 until its temperature reaches about 97 ° whereafter the input to the 1 kW element is controlled by a ramp and pedestal circuit forming part of the control circuit. During such control, the 1 kW element 15 is pulsed on and off such that the time averaged power input to the 1 kW element increases the temperature of the hot water to, and maintains it at, its set point temperature of about 100 °C.

Under steady state conditions (ie when there is no demand for hot water) the 1 kW element 12 will continue to be pulsed depending on the 20 rate of heat loss from the unit. During such steady state conditions, therefore, it will be seen that the water 2 is maintained at about boiling point solely by intermittent energisation of the 1 KW element 12 as distinct from the full 3 kW load as in the case of known water boilers. This contributes to quieter operation and longer life of, especially, the heating 14 juh irr RECEIVED 247 0 7 5 8 elements relative to known boilers principally because the heater is for much of the time operating at a modulated, relatively low watts density.

When boiling water is dispensed from the tap, the solenoid valve 8 is switched on in response to actuation of the reed switch 9 as described 5 earlier and fresh cold water will be admitted into the lower part of the vessel 1. If the amount of hot water dispensed, and therefore the amount of fresh cold water admitted, is relatively large, the thermistor 14 will more or less immediately sense a drop in temperature to below 96 °C whereupon both heating elements will be switched on, ie there will be a full heating 10 load of 3 kW and all of the water in the vessel 1 will rapidly be heated firstly to about 96 °C by the full load and then to about 100 °C solely by the 1 kW element 12 as described above. On the other hand, if the amount of hot water dispensed is relatively small, the thermistor 14 will more or less immediately sense only a small drop in temperature to one above 96 15 °C, in which case the water will be re-heated to about 100 °C solely by the 1 kW element 12 as described above.

The control circuit of the water boiler illustrated is shown diagrammatically in Figure 2 which will bs self-explanatory to those skilled in the art, as will be the various electronic components required to 20 achieve the heating characteristics of the boiler.

N.Z. PATENT OFFICE! 14 JUN 1996 RECEIVED 9 247 0 7 5

Claims (18)

WHAT WE CLAIM IS:

1. A waterheater for storing hot water including a vessel for storing the water, first and second mutually adjacent, independently operable, electric heating elements for heating the water, a manually operable valved outlet 5 through which hot water can be dispensed, means to control, in use, supply of fresh cold water to the vessel, and a control circuit for controlling energisation of the heating elements, wherein the control circuit is adapted to cause simultaneous energisation of the first and second electric heating elements when water in the vessel is, at the location of and as sensed by a 10 temperature sensor, below a first pre-determined temperature whereby said heating elements heat the water up to said first pre-determined temperature and, when the water attains said first pre-determined temperature, to cause de-energisation of the first heating element and pulsing on and off of the second element thereby to heat the water up to a second pre-determined, 15 desired storage, temperature higher than said first pre-determined temperature.

2. A waterheater according to claim 1 wherein the second heating element is pulsed on and off by pulse width control means which control the on and off pulse widths in proportion to the difference between the 20 prevailing water temperature sensed by the temperature sensor and the second pre-determined temperature.

3. A waterheater according to claim 2 wherein said pulse width control means comprises a ramp and pedestal circuit forming part of the control circuit. N.Z. PATENT OFROE? 14.JUN 1SS5 i ____ I RECEIVED i 247 0 75 10

4. A waterheater according to any one of claims 1 to 3 wherein the first heating element is nominally rated at about 2 kW and the second heating element is nominally rated at about 1 kW.

5. A waterheater according to any one of claims 1 to 4 including user-5 operable means to adjust the second pre-determined temperature to a desired value.

6. A waterheater according to any one of claims 1 to 5 wherein said second pre-determined temperature is set at, or is gettable at, substantially 100 °C. 10

7. A waterheater according to claim 6 wherein said first pre-determined temperature is between 90 and 98 °C.

8. A waterheater according to claim 7 wherein said first pre-determined temperature is substantially 96 °C.

9. A waterheater according to any one of claims 1 to 8 including first 15 level-sensing means and associated electric switch means for ensuring that the heating elements can be energised only when they are fully immersed in water.

10. A waterheater according to claim 9 wherein said first level-sensing means and associated electric switch means comprises a magnetic float co- 20 operable with a reed switch located at a level just above the level of the heating elements.

11. A waterheater according to any one of claims 1 to 10 wherein the supply of fresh cold water to the vessel is controlled by a solenoid valve.

12. A waterheater according to claim 11 wherein operation of the 25 solenoid valve is controlled by a second level-sensing means and 20 24'/ 0 / 3 11 associated electric switch means, the latter causing the solenoid valve to open when the level of the water in the vessel is below a pre-determined maximum level and to close when the water level has risen to the pre-determined maximum level.

13. A waterheater according to claim 12 wherein said second level-sensing means comprises a magnetic float/reed switch.

14. A waterheater according to any one of claims 1 to 13 wherein said heating elements are located adjacent to the base of the vessel and wherein fresh cold water is discharged into the vessel at a location adjacent to the heating elements.

15. A waterheater according to any one of claims 1 to 14 wherein flow of fresh cold water into the vessel is throttled so as to minimise turbulence of water already contained in the vessel.

16. A waterheater according to any one of claims 1 to 15 wherein the temperature sensor is located in the vessel at about the same level as the hot water dispensing outlet.

17. A waterheater according to any one of claims 1 to 16 wherein the dispensing outlet is located just above the level of the heating elements.

18. A waterheater substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings. IMI WATERt^TINOr LIMITED By tlrexr/Attorneys >•— BALDWIN/ SON & CAREY N.Z. PATENT OFFICE 14 JUN 1998 REcavi5~