GB2250805A - Water heating apparatus - Google Patents

Abstract

In a thermal storage system, the water in the storage tank 12 is either heated by electric immersion heaters 36, 38 or by a separate boiler 42. When electric immersion heaters are used two are spaced vertically of the tank and each is a unit which projects into the tank from a mounting head 54, 56. The head includes the thermostatic controls enabling easy mounting on the tank and the temperature control is effected by a precision temperature sensor such as a thermistor, and a comparator set to operate as close to boiling point as possible. The heaters are set to operate at the same temperature but are operable independently. The lower heater 38 may be supplied with off peak electricity whilst the upper 36 acts as a booster. When the boiler is used, a precision temperature loop spreads the boiler on-off temperatures to prevent the boiler from switching on and off too frequently. <IMAGE>

Description

Improvements Relating to Water Heating Apparatus This invention relates to water heating apparatus in general, and in particular, although not exclusively, to water heating apparatus known as "thermal storage apparatus".

Thermal storage is a term applied to a form of water heating apparatus, typically for domestic purposes, which comprises a tank holding a quantity of water which forms a store for heat, the water being heated from a heat source such as a boiler or immersion heater to maintain a store temperature, and the heat is taken selectively from the store to provide space heating on the one hand, and to provide hot water for consumption at taps and the like on the other hand.

The heat for the space heating may be provided by circulating water from the store through heating devices such as radiators, and then returned to the store, whilst for the provision of the hot water for consumption, cold water at mains pressure is passed through a heat exchanger coil means inside the tank so as to remove heat by heat exchange, and to emerge as hot water which can be used for any suitable purpose.

As experience is gained with increased utilisation of thermal storage, so certain shortcomings present themselves in terms of commissioning, control, installation and operation, and one area in which more effective control appears to be desirable, is in the area of the thermostatic control which previously has been adopted for the thermal storage systems.

Obviously it is desirable to keep a watch on the temperature of the store in order to ensure that a certain heat capacity is maintained. Thus, it is usual to heat the store up during off-peak periods e.g. during the night, and to utilise the heat as required during the day with provision being made for boosting of the store should additional heat during the day be required. The temperature sensitive devices which are used for the control have tended to be simple inexpensive thermostats which operate on an expansion and contraction basis depending upon temperature, which expansion and contraction is monitored and utilised for the switching on and off of the heating means. These thermostats are however rather unreliable in operation in that they tend to "drift" which means that they operate the controls at different temperatures or settings than the original commissioning settings.Consequently a user may have his store at a higher or lower temperature than was originally intended, which impairs the efficiency of operation. To improve the control, electronic temperature sensing devices suggest themselves and some attempts have been made to utilise electronic sensing devices in thermal storage.

Two forms of heat source for supplying the heat to the store have become established, the first of these being an electrical means e.g. immersion heating means, and the second being an external source means such as a boiler.

When immersion heating means is used, it is known to use at least two electric immersion heaters which are spaced vertically in the store to provide an upper heating means and a lower heating means, the upper heating means being used as the boost heater and the lower heating means being used during off peak periods. This leads to stratification of the heat in the store. That is to say the top of the store tends during the day to remain at a higher temperature than the bottom of the store. By having a heat exchange coil at the top of the store, efficient heat exchange between the incoming cold water and the water in the store, can be effected.

It is usual to heat the entire store during off-peak periods using off-peak electricity by using the lower immersion heater, and to use the upper immersion heater for boost heating during the peak period.

A further problem with using the conventional thermostats which do tend to drift in operation, is that because of the large degree of drift, it is difficult for designers to design the thermal store for operation at a temperature as close to boiling point as possible, which is desirable, because if the store is designed for operation at a temperature close to boiling, and the thermostat drifts in an upwards direction, the water in the store may start to boil, which is undesirable, before the heat source is switched off.

The use of the more sensitive electronic temperature sensitive devices do enable the designer to design for a store operation temperature much closer to boiling point.

The other method of heating the store water is to circulate same through a boiler, and again it is desirable to arrange for the heat exchange coil to be at least largely in the top of the store where hot water is returned from the boiler. It is again important efficiently to control the operation of the boiler because there is only a small difference between the temperature of the boiler flow and the top of the store Utilisation of electronic control is again desirable.

The present invention relates in general to the provision of effective electronic control of the heating of the water in the store, both in the case of electrically heated stores and stores heated by other means such as boilers, but certain aspects of the invention contained herein are applicable on a more general basis.

According to a first aspect of the invention, the water in a thermal store is heated by means of electric immersion heaters spaced vertically within the store, and each of said heaters is electronically controlled by means of a precision temperature sensor such as a thermistor coupled to a control circuit controlling the supply of electrical power to the immersion heater, and wherein the immersion heaters are independently controlled and preferably are controlled so as to operate at the same temperature, which preferably is close to boiling point.

Each control circuit associated with each precision temperature sensor preferably comprises a comparitor which compares the temperature sensed by the temperature sensor and a pre-set temperature, which approaches but is less than the boiling point of water, so that when the temperature sensed is greater than the set temperature, the electrical power is cut off from the immersion heater by means of a relay, and when the sensed temperature is less than the set temperature, the said relay is switched on, the difference between the sensed switch on and switch off temperatures being relatively small, in the order of 30C.

Preferably the lower immersion heater is adapted to be supplied with electricity during off-peak periods, whilst the upper immersion heater is adapted to operate during peak periods to heat the top region of the water in the thermal store.

Where the heat exchanger coil is in two sections namely an upper section and a lower section in the thermal store, the upper immersion heater preferably is located between the upper and lower portions of the coil, and the lower immersion heater is located below the lower portion of the coil.

It is preferred that the or each immersion heater unit comprises a heating leg which projects into the tank, and a head, and said head embodies the control circuit and relay so that it is necessary only to connect the electrical power to the said head to achieve heating and accurate temperature control. In this connection, an immersion heater of this design may be used in other applications apart from thermal storage, because a novel immersion heater is created.

In the case where the thermal storage system is heated by means of a boiler, the boiler is preferably under the control of a precision temperature sensor as described above, and wherein the temperature sensed is compared with a pre-set temperature through a comparitor, but in addition there is a feed back loop across the comparitor in order to spread the output characteristic indeed to provide a hysterisis characteristic on the signal to ensure that the boiler switches on and off at temperatures which are more widely spread than might otherwise be the case, in order to avoid too frequent boiler switch-on and switch-off.

The switch-on and switch-off of the boiler may be means of a relay, and said control circuit of the boiler may provide an output means to set a delay circuit, said delay circuit in turn being connected to a relay controlling a pump which circulates the water from the thermal store to the boiler and back to the store, so that said pump will stop at a predetermined time after the boiler relay is switched to switch off the boiler, so that the pump will overrun the boiler shut-down by a predetermined time, and will therefore circulate some colder water from the thermal store through the boiler to prevent the boiler "kettling".

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings, wherein: Fig. 1 is a sectional view of a thermal storage system according to and embodying aspects of the present invention; Fig. 2 is a sectional elevation taken on the line II-II in Fig. 1 but showing only the immersion heaters and the tank wall; Fig. 3 is a circuit diagram of the temperature control elements of the immersion heater shown in Fig. 2; and Fig. 4 is a control circuit which is adapted in accordance with the invention for the control of a boiler when the thermal storage system is boiler fired.

Referring to the drawings, in Fig. 1 a thermal storage is illustrated diagrammatically, and comprises a tank 10 containing a quantity 12 of water which forms a thermal store. The tank is connected to the go and return lines 14, 16 of a space heating circuit such as a central heating system comprising radiators and a pump for effecting the circulation. Additionally, the heat in the store can be used to provide hot water from domestic outlet taps such as tap 18 in that inside the store is a heat exchanger coil in two sections, namely an upper section 20 and a lower section 22 connected inside the coil through an expansion chamber 24, and cold water can be supplied from the mains as indicated by arrow 26 through input pipe 28, so that the water eventually emerges heated through outlet pipe 30.A branch pipe 32 and a mixing valve 34 enable the bleeding of some cold water into the hot water which flows into pipe 30 so as to control the temperature of the water which eventually flows from the taps 18. This mixing arrangement is in fact quite important insofar as with the embodiment of the invention, it is intended that the water in the store 12 should be heated to a level as near boiling as possible e.g. in the order of 90950C and of course the water which emerges from the taps 18 must not be at this level as it could scald users. It is desirable however to keep the store 12 at a temperature close to boiling point without in fact allowing the water to boil.

It is for this reason that it is desirable to use precision temperature sensors such as thermistors as opposed to the cruder expansion and contraction thermostats.

In full lines in Fig. 1 are shown one form of heating system, namely two immersion heaters 36 and 38, and also shown in Fig. 1 is an alternative heating system for the water in the store which is illustrated in dotted lines by reference 40, and comprises a boiler 42 which is external of the tank 12, and is arranged by connection pipes 44 and 46 and a circulating pump 48 to circulate water from the store 12 through the boiler 42 and back to the store.

The immersion heaters 36 and 38 as shown in Fig. 2 each comprises a heating element 50, 52 and a head casing 54 and 56.

In each of the head casings is contained a control circuit as illustrated in Fig. 3 so that in fact the only connections required to the head 54 or 56 are the electrical mains connections 58, 60 and 62 being live, neutral and earth wires.

Such an immersion heater comprises a novel article, which has particular advantages in that it is a simple matter to connect such an immersion heater to any body of water to be heated, and the immersion heater will control the temperature to which the water is heated on an accurate basis. Housing the components of Fig. 3 in the head 54 and 56 provides an extremely neat and useful unit.

Referring to Fig. 3, the immersion heater 50 or 52 works on the following basis. The wires 58 to 62 are coupled to the power supply module 64 which is electronic in nature and module 64 is in turn connected to the precision temperature sensor 66 which typically may be a thermistor embodied in the element 50 or 52, or indeed in the casing 54 so as to sense the temperature of the water in the store 12. The module 64 is also connected to a temperature setting solid state device 68 by which the desired store temperature is set.

A callibration unit 70 is connected to the precision temperature sensor in order to provide callibration of the temperature sensed by the sensor 66.

Outputs 72 and 74 from the temperature set module 68 and the precision temperature sensor are compared in a comparitor 76 which drives a driver 78 mainly for amplification purposes, which in turn is coupled to a relay 80 having contacts in the power supply connections to the elements 50 and 52.

If the signal produced on line 74 is greater than that provided on line 72, driver 78 switches relay 80 in order to disconnect the power from the element 50 or 52, whereas if the signal from the temperature setting module 68 is greater than that from the precision temperature sensor line 74, relay 80 is switched to cause power to be supplied to the element 50 or 52 and by this means automatic thermostaatic control is achieved for the water in the thermal store. The electronic devices used provide switching on and off over a relatively narrow temperature band of the order of 30C and therefore the temperature of the water in the store can be controlled to be as close to boiling as possible and yet there will not be the danger of the water boiling as exists in relation to the conventional expansion thermostat.

In the arrangmeent shown in Fig. 4, parts of the control circuit described in relation to Fig. 3 are present and carry the same reference numerals, and will not be described further in detail.

In the case of the arrangement of Fig. 4, there is a feedback loop 82 between the output of the driver 76 and the precision temperature sensor input 74 so as to flatten the output characteristic to the driver 78. This means that there will be a greater spread between the switch on and switch off condition, controlled by driver 78, which effects switching of the relay connected to the boiler 42. For example the switch on and switch off temperatures for the relay of boiler 42 may be spread by in the order of 8-100C whereas the spread for the switch temperatures in the electrical arrangement of Fig. 3 may be in the order of 30C.

When the relay to the boiler is switched in order to switch off the boiler (heat store requirement satisfied) the relay also sends a signal on line 84 to a solid state delay timer 86 which after a pre-determined time outputs a singal on line 88 in order to switch a relay controlling the pump 48, line 88 including a driver 90 in order that the signal will be augmented. By this means, the pump 48 stops at a predetermined time after switching off of the boiler 42 to enable some of the water from the store to be continued to be circulated through the boiler after the boiler has been switched off.

The invention provides effective controls for thermal storage, and in the case of the embodiment of Fig. 1, the immersion heaters 36 and 38 can be operated one, 38, on an off-peak basis, and the other 36 during peak periods and as a boost to maintain the store at a predetermined temperature which can be close to boiling point. Each of the immersion heaters 50 and 52 can operate independently, and each preferably is set to operate at the same temperature.

During the day and during peak periods, the immersion heater 36 may be utilised to keep the top portion of the store at the desired temperature, and to maintain stratification within the store so that useful heat will still be outputted for the heating system and domestic water taps.

Claims (23)

1. A method of heating the water in a thermal storage tank comprising using at least two electric immersion heaters which are spaced heightwise of the tank, and controlling each of said heaters by means of a precision temperature sensor such as a thermistor coupled to a control circuit controlling the supply of electrical power to the immersion heater, and wherein the immersion heaters are independently controlled.

2. A method according to Claim 1, wherein the immersion heaters are controlled so as to operate at the same temperature, which preferably is close to boiling point.

3. A method according to Claim 1 or 2, wherein each control is effected by a comparitor which compares the temperature sensed by the temperature sensor and a pre-set temperature, which approaches but is less than the boiling point of water, so that when the temperature sensed is greater than the set temperature, the electrical power is cut off from the immersion heater by means of a relay and when the sensed temperature is less than the set temperature, the said relay is switched on.

4. A method according to Claim 3, wherein the difference between the sensed switch on and switch off temperatures being relatively small, in the order of 30C.

5. A method according to any preceding claim, wherein there are two immersion heaters, an upper one and a lower one, and wherein the lower immersion heater is adapted to be supplied with electricity during off-peak periods whilst the upper immersion heater is adapted to operate during peak periods to heat the top region of the water in the thermal storage tank.

6. A method according to any preceding claim wherein the tank includes a heat exchanger coil through which water may be passed to be heated by the thermal storage water in the tank and wherein the coil is in two sections namely an upper section and a lower section in the thermal store, an upper immersion heater being located between the upper and lower portions of the coil, and a lower immersion heater being located below the lower portion of the coil.

7. A method according to any preceding claim, wherein each immersion heater comprises a heating leg which projects into the thermal storage tank, and a head at the end of the leg connected to the tank, said head embodying the control circuit and relay (when provided) so that it is necessary only to connect the electrical power to the said head to achieve the heating and accurate temperature control.

8. A method of heating the water in a thermal storage tank using a boiler through which water from the storage tank is circulated, wherein the boiler is under the control of a precision temperature sensor, and wherein the temperature sensed is compared with a preset temperature through a comparitor.

9. A method according to Claim 8, wherein a feed back loop is used across the comparitor in order to spread the output characteristic to ensure that the boiler switches on and off at temperatures which are more widely spread than might otherwise be the case, in order to avoid too frequent boiler switch on and switch off.

10. A method according to Claim 9, wherein the boiler switch on and switch off is by means of a relay, and a control circuit of the boiler provides an output means to set a delay circuit, said delay circuit in turn being connected to a relay controlling a pump which circulates the water from the thermal storage tank to the boiler and back to the tank, so that said pump will stop at a predetermined time after the boiler relay is operated to switch off the boiler, so that the pump will overrun the boiler shut-down by a predetermined time, and will therefore circulate some colder water from the thermal store through the boiler to prevent the boiler from "kettling".

11. An electric immersion heater comprising a leg for projecting into a thermal storage tank, and a head at one end of the leg, said head embodying a control circuit and relay for the control of the supply of electrical power to the heater, the arrangement being that it is necessary only to connect the electrical power to said head to achieve accurate temperature control of the heating.

12. A heater according to Claim 11, wherein the control circuit includes a comparitor which is arranged to compare the temperature sensed by the temperature sensor and a preset temperature, to provide a control signal for control of the heater.

13. A heater according to Claim 12, wherein said preset temperature is adjustable.

14. An installation for heating the water in a thermal storage tank comprising at least two electric immersion heaters which are spaced vertically within the tank, means controlling each of said heaters, said means including a precision temperature sensor such as a thermistor coupled to a control circuit controlling the supply of electrical power to the immersion heater, and wherein the immersion heaters are independently controlled.

15 An installation according to Claim 14, wherein the immersion heaters are controlled so as to operate at the same temperature, which preferably is close to boiling point.

16. An installation according to Claim 14 or 15, wherein each control is effected by a comparitor which compares the temperature sensed by the temperature sensor and a pre-set temperature, which approaches but is less than the boiling point of water, so that when the temperature sensed is greater than the set temperature, the electrical power is cut off from the immersion heater by means of a relay and when the sensed temperature is less than the set temperature, the said relay is switched on.

17. An installation according to Claim 16, wherein the difference between the sensed switch on and switch off temperatures being relatively small, in the order of 30C.

18. An installation according to any of the preceding Claims 14 to 17, wherein there are two immersion heaters, an upper one and a lower one, and wherein the lower immersion heater is adapted to be supplied with electricity during off-peak periods whilst the upper immersion heater is adapted to operate during peak periods to heat the top region of the water in the thermal storage tank.

19. An installation according to any of the preceding Claims 14 to 18, wherein the tank includes a heat exchanger coil through which water may be passed to be heated by the thermal storage water in the tank and wherein the coil is in two sections namely an upper section and a lower section in the thermal store, an upper immersion heater being located between the upper and lower portions of the coil, and a lower immersion heater being located below the lower portion of the coil.

20. An installation according to any of the preceding Claims 14 to 19, wherein each immersion heater comprises a heating leg which projects into the thermal storage tank, and a head at the end of the leg connected to the tank, said head embodying the control circuit and relay (when provided) so that it is necessary only to connect the electrical power to the said head to achieve the heating and accurate temperature control.

21. An installation for heating the water in a thermal storage tank using a boiler through which water from the storage tank is circulated, wherein the boiler is under the control of a precision temperature sensor, and wherein the temperature sensed is compared with a preset temperature through a comparitor.

22. An installation according to Claim 21, wherein a feed back loop is used across the comparitor in order to spread the output characteristic to ensure that the boiler switches on and off at temperatures which are more widely spread than might otherwise be the case, in order to avoid too frequent boiler switch on and switch off.

23. An installation according to Claim 22, wherein the boiler switch on and switch off is by means of a relay, and a control circuit of the boiler provides an output means to set a delay circuit, said delay circuit in turn being connected to a relay controlling a pump which circulates the water from the thermal storage tank to the boiler and back to the tank, so that said pump will stop at a predetermined time after the boiler relay is operated to switch off the boiler, so that the pump will overrun the boiler shut-down by a predetermined time, and will therefore circulate some colder water from the thermal store through the boiler to prevent the boiler from "kettling".