GB2225097A

GB2225097A - Water heating apparatus

Info

Publication number: GB2225097A
Application number: GB8923395A
Authority: GB
Inventors: Richard Martyn Griffiths
Original assignee: IMI RANGE Ltd
Current assignee: IMI RANGE Ltd
Priority date: 1988-11-17
Filing date: 1989-10-17
Publication date: 1990-05-23
Anticipated expiration: 2009-10-17
Also published as: GB2225097B; GB8923395D0; GB8826885D0

Abstract

Hot water is generated using the water-jacketed tube heater type of apparatus in which, on demand, mains cold water passes through a high efficiency heat exchanger 4, 4' immersed in a heated thermal storage medium 2, such as water, contained in a cylinder 1 whereby the cold water becomes heated and is conveyed at high pressure to a point of use such as a sink, bath or shower. Substantially the whole of the thermal storage medium 2 is continually maintained, by means of a controller 13 and one or more electric immersion heaters 10, 10', at least one of which extends into the lower regions of the cylinder 1, at about 90 DEG C in the off-peak (night-time) electricity tariff period and at about 60 DEG C in the peak (day-time) tariff period. The controller 13 receives a signal from a temperature sensor 12 located in a upper region of the cylinder 1. Maintaining the specified temperatures in the medium, particularly that in the peak period, reduces the time taken to obtain hot water through the heat exchanger 4, 4'. <IMAGE>

Description

Hot Water Generating and Supply Apparatus This invention relates to hot water generating and supply apparatus of the type comprising a vessel containing, in use, a thermal storage medium, for example water, the storage medium having immersed in it a high efficiency heat exchanger through which, on demand, cold water, usually at or driven by mains pressure, passes whereby the cold water is instantaneously heated and is fed, usually via a thermostatic or other type of mixing valve, to a point of use such as a sink, bath or shower installation. Hereinafter, such apparatus is referred to as "apparatus of the type described".The thermal storage medium may be heated by any suitable means, for example directly or indirectly by a gas, oil or coal fired boiler but the present invention is especially, though not exclusively, concerned with apparatus of the type described wherein the storage medium is electrically heated by, for example, an electric immersion heater and the remainder of this specification refers to such electrical heating.

In the last decade or so in the United Kingdom there has been a significant move, especially in domestic premises that do not have a mains gas supply, towards heating the domestic hot water by an electric immersion heater, utilising, so far as possible, off peak electricity, such as "Economy 7". Conventionally, such a system is a vented low pressure cistern-fed system wherein the hot water is generated and stored in a copper hot water cylinder equipped with a single, top mounted, dual element 3kW immersion heater or, in more modern installations, a pair of single element 3kW immersion heaters, one extending horizontally into a lower region of the cylinder and the other extending horizontally into an upper region of the cylinder.Taking the latter case, the lower element serves to heat substantially all of the water in the cylinder to a pre-set temperature, typically 60-65"C, in response to a thermostat that forms part of the immersion heater unit. On the other hand, the upper element serves to heat, again to a pre-set temperature typically of 60-65"C as determined by its thermostat, only water in the upper part of the cylinder which, because of stratification, does not mix significantly with the water below it.Usually, operation of both immersion heaters is controlled by a dual channel time switch adapted to permit operation of the lower beater, in response to its thermostat, only during an off-peak night-time electricity tariff period whereas the time switch permits operation of the upper beater, in response to its thermostat, only during the remaining peak (or so-called "boost") daytime period. An example of a typical time switch is the 'MICRO 7' manufactured and sold by our sister company, IMI Pactrol Limited.

More recently, high pressure unvented domestic hot water storage systems have been introduced onto the UK market. An example of this is our TRIBUNE system which is in many respects similar to the low pressure cistern fed system referred to above but wherein the domestic hot water is fed to points of use by mains cold water pressure rather than by cistern water pressure. However, the hot water may be heated in precisely the same manner as for low pressure systems.

In the case of both low pressure and high pressure electrically heated systems utilising off peak electricity, the Electricity Boards currently recommend for reasons of economy that, where possible, at least 90% of the total electrical energy used for heating the domestic hot water should be at the off peak rate. On average, this can be achieved, although of course whether it will be achieved in any given case depends largely upon the demand for hot water during the peak period.

Even more recently, apparatus of the type described has been introduced onto the UK market. An example of such apparatus is our FLOWMAX system. The thermal storage medium may be electrically heated, preferably in the manner described above, that is to say by deriving benefit from off peak electricity tariffs and thus using, for example, upper and lower immersion heaters, controlled as aforesaid, with the high efficiency heat exchanger located above the level of the upper immersion heater.However, because the thermal storage medium is not usually consumed, it may be heated to a temperature well in excess of 60-65"C, say of the order of 90 C, this has the advantage that much more energy can be stored up during the off peak night-time tariff period ready for use the following day whereby the Electricity Boards' above recommendation may, on average, be readily met.

However, during continued daytime consumption of domestic hot water, the temperature of the thermal storage medium gradually decreases and, depending on the demand, it may become necessary to boost, again to, say, about 90"C, the temperature in the upper part of the vessel (which typically is an upright copper cylinder) by means of the upper immersion heater. We have, however, surprisingly discovered in those circumstances that, during passage of cold water through the heat exchanger, quite strong convection currents are set up which cause the water in the upper part of the vessel, ie. the water immediately surrounding the heat exchanger, to mix with the much cooler water below it.In other words, stratification is essentially destroyed, and the temperature of the water surrounding the coil is rapidly and significantly reduced and indeed may reduce to such an extent that the domestic hot water in turn reduces to an unacceptably low temperature until such time as the boost heater has been able sufficiently to raise the temperature of the storage medium adjacent to the heat exchanger. This may, cf course, entail the use of considerable amounts of peak tariff electricity and detracts from the Electricity Boards' above recommendation.

It is an object of the present invention to address this problem which, whilst associated particularly with apparatus of the type described in which the thermal storage medium is electrically heated by lower and upper immersion heaters powered respectively by off-peak and peak tariff electricity, could also be associated with such apparatus in which the storage medium is adapted to be heated by other sources of energy supplied at different tariffs depending on the time of day.

According to a first aspect of the present invention, therefore, there is provided hot water generating and supply apparatus of the type described characterised in that it includes heating means for heating substantially all of the thermal storage medium, for example water, temperature sensing means for sensing, at the location of the sensing means, the temperature of said medium, and control means responsive to the temperature sensed by said sensing means and arranged to control the operation of said heating means such that, during a first pre-determined time period, substantially all of said thermal storage medium is heated to about a first pre-determined temperature and that, during a second pre-determined time period, substantially all of said thermal storage medium is heated to about a second pre-determined temperature being a temperature below, and preferably significantly below, said first temperature. Preferably, the first and second pre-determined temperature differ by between 100 and 50"C, more preferably between 20 and 30"C.

Preferably, one and the same heating means serves to heat the thermal storage medium during both of the first and second time periods and is electrical, for example one, or two or more parallel-wired, AC mains immersion heaters; advantageously, at least from the manufacturing cost point of view, the heating means consists of a single immersion heater unit rated at 3kW or higher and preferably extending horizontally within a lower region, preferably adjacent to the base, of the vessel, for example a cylinder, containing the thermal storage medium.In this connection, unless all of the thermal storage medium is circulated within the vessel by, for example, a pump (which, optionally, may be included in apparatus of the invention), there will (as in conventional hot water cylinders) usually be a small quantity of thermal storage medium below the, or the lowermost, heater that does not become significantly heated, ie there will usually always be a stratum of relatively cold water in the basal part of the vessel. Further, there may (again as in conventional hot water cylinders) otherwise be a small temperature gradient in the storage medium such that the medium at the top of the vessel will, under steady state conditions, be slightly higher than that of the medium lower down in the vessel, again a consequence of stratification.This is why we refer above to "substantially" all of the liquid thermal storage medium being heated to "about" a pre-determined temperature.

Preferably, the temperature sensing means consists of a single temperature sensor that is operative at least over a range of temperatures that includes the first and second temperatures, and is, like the heat exchanger, preferably located internally of, or in external contact with, an upper part of the vessel. However, there may, for example, be a pair of temperature sensors operative over ranges that include respectively the first and second temperatures. The or each temperature sensor is preferably a low hysteresis sensor, for example a thermistor.

In the case of 'Economy 7' electrically heated apparatus of the invention, the first pre-determined time period will correspond to all, or selected part(s) of, the off peak tariff period and the second pre-determined period will correspond to all, or selected part(s) of, the remaining peak tariff period.

Preferably, the first pre-determined temperature (in the case where the thermal storage medium is water, which it almost invariably will be) is between about 80 C and 95"C, the water preferably being heated to, and continually maintained at, about 90"C. The second pre-determined temperature will then preferably be between about 450C and 65"C. Depending on the efficiency of the heat exchanger used, the water is, during said second time period, preferably heated to, and continually maintained at, about 60"C.

In a preferred embodiment the control means is preferably an electronic time switch of generally known type but adapted to enable it to control operation of the heating means according to the time period/temperature requirements stated above.

According to a second aspect of the invention there is provided a vessel, for example a cylinder, for containing, in use, a liquid thermal storage medium, heat exchange means located within, preferably, an upper region of the vessel and having an inlet and an outlet for connection to cold water feed and hot water supply pipework respectively, at least one electric immersion heater located within a lower region of the vessel, and a temperature sensor located within, for example in a pocket, or externally of, an upper region of the vessel.

According to yet a further aspect of the present invention, there is provided a controller for use in apparatus of the invention, said controller being adapted to control operation of the heating of the thermal storage medium of the apparatus in accordance with the time period/temperature requirements stated above.

One embodiment of the invention will now be described in more detail, by way of example only, with reference to the accompanying drawing which is a diaglalr, of such embodiment.

Referring to the drawing, the apparatus includes a well lagged cylinder 1 containing, in use, a liquid thermal storage medium 2, such as water. The top of the cylinder 1 is provided with an open-topped condenser tube 3 which serves as a vent and to condense water vapour which can then run back into the cylinder 1. The cylinder 1 may, as and when necessary, be topped up with water through the open top of the tube 3. Alternatively, expansion and replenishment of the thermal storage medium 2 may be dealt with by an integral or remote feed and expansion tank.

Within an upper region of the cylinder 1 is located a pair of high efficiency, finned tube heat exchange coils 4,4' made for example of INTEGRON tube (a registered trade mark of our sister company, IMI Yorkshire Alloys Limited). The adjacent inlets of the two coils 4,4' are connected in parallel to a cold water mains pipe 5, optionally via a pressure regulating valve (not shown), and their adjacent outlets are connected in parallel to the domestic hot water supply pipe 6 which distributes hot water to the various points of use such as sinks, baths, basins and showers via a thermostatic mixer valve 7 where the hot water generated within the coils 4,4' may mix with mains cold water fed to the valve 7 by a pipe 8. There is also provided, in known manner, a sealed expansion vessel 9 for accommodating expansion of water within the coils 4,4' as it heats up under static conditions.An electric immersion heater unit 10 is located in the lower part of the cylinder 1. Usually this will be rated at 3kW, but higher rated units may be used if higher recovery rates are required.

A pocket 11 is formed in a upper region of the cylinder 1 adjacent to the heat exchange coils 4,4' for receiving a temperature sensor 12, for example a thermistor. The temperature sensor 12 preferably has a low hysteresis characteristic, ie is capable, under steady state conditions, of maintaining the temperature of the thermal storage medium, at least adjacent to the location of the sensor, to within about 2"C of the desired value under steady state conditions.

Operation of the immersion heater unit 10 is controlled by a electronic controller designated generally by reference numeral 13. The controller 13 is a modified form of IMI Pactrol Limited's known MICRO 7 controller and comprises five major sections, namely a 24 hour solid state clock 14, a temperature set point controller 15, a comparator 16, an AND gate 17 and output relays 18.

The clock 14 serves to produce "on" or "off" output signals, as the case may be, in accordance with a user-programmed schedule or alternatively in accordance with a schedule programmed into it during manufacture.

In particular, it will produce an "on" signal during all or part of an off-peak electricity tariff period, which in the case of teleswitched 'Economy 7' is typically between 1.00 am and 8.00 am. During such period, the controller 15 is adapted to feed a temperature set point signal to the comparator 16, being a signal which corresponds to the first pre-determined temperature referred to earlier, say of the order of 90"C.

The temperature sensor 12 also feeds a signal to the comparator 16 which in turn feeds an 'on' signal to the AND gate 17 if the temperature sensed by the sensor 12 is below the desired set temperature of about 90"C and an "off" signal if the temperature is at or above the set temperature. The clock 14 also feeds an "on" or "off" signal, as the case may be, to the AND gate 17. If both the comparator 16 and the clock 14 simultaneously feed "on" signals to the AND gate 17, the output relays 8 switch on the immersion heater 10 until such time as the sensor 12 senses that the water in the cylinder 1 adjacent to the sensor 12 has reached about the set temperature, whereupon the comparator 16 feeds an "off" signal to the AND gate 17 and the relays 18 switch off the heater 10, and so on.During the off-peak period, therefore, substantially all of the water within cylinder 1 is heated to, and is continually maintained at, about 90"C.

On the other hand, during the peak period, which is typically from 8 am to 1.00 am, the controller 15 is adapted to feed a temperature set point signal to the comparator 16, being a signal which corresponds to the second, lower pre-determined temperature referred to earlier, say of the order of 60to. During the peak period (or one of more selected parts of it), therefore, substantially all of the water within the cylinder 1 is continually maintained at about 60"C. Accordingly, the problem caused by convection currents in prior electrically heated apparatus of the type described will not arise to any significant extent because substantially all of the water in the cylinder is at about the same temperature.Whilst in accordance with the embodiment decribed above, a much larger volume of water than in prior apparatus of the type described is heated to an elevated temperature during peak periods or parts thereof, the amount of peak electricity utilised will usually not, other things being equal, greatly exceed the amount of electricity used during that period in such prior apparatus because the controlled temperature is significantly lower, say 60"C as against 90"C, whilst still allowing the apparatus to function adequately.

In the above specific description, we have referred to clock 14 producing "on" or "off" signals, as the case may be, in accordance with a user-programmed or factory-programmed schedule. Usually, but depending on the hot water demand pattern, the clock 14 will produce an "on" signal throughout the off-peak tariff period or at least during the last two to three hours of that period (say between 5.00 am and 8.00 am). During peak periods, however, the frequency and duration of such "on" periods will depend on the hot water requirements of the user and thus it is preferable for the user to be able to programme the clock accordingly.

As well as addressing the problem of convection currents associated with prior electrically heated apparatus of the kind described, apparatus of the invention has the advantage that only one, as opposed to two, immersion heater unit is required (although a plurality of parallel-wired heater units may, if desired, be used, again to increase recovery rates, as is indicated in dashed lines by reference numeral l0'),thereby reducing manufacturing costs although any saving here will to some extent be off-set by the need in the embodiment illustrated, to modify a conventional electronic controller.

In an alternative embodiment (not illustrated), the heating means may comprise, for example, a conventional dual element electric immersion heater located in a lower region of the cylinder 1, the elements having operatively associated with them respective, conventional thermostats having set points respectively corresponding to the first and second pre-determined temperatures and being powered via, for example, a conventional dual channel controller.

Reverting to the drawing, the domestic hot water pipe 6 may be provided with a flow switch 19 which causes the heater unit 10 to be switched on (if it is not already on) in immediate response to a demand for hot water thereby improving the performance of the apparatus.

Finally, the control means, in conjunction with the temperature sensing means, of apparatus of the invention may be arranged to ensure that during service the temperature of the thermal storage medium will not fall below a pre-determined lower temperature, for example of the order of 30 to 400C, at least not for any significant period of time, thereby providing precautions against frost especially when the apparatus is installed in an out-building. Alternatively or in addition, the control means may be provided with a standby switch which the user could actuate when, for example, going on holiday and which would cause the thermal storage medium to be maintained, during a substantial period of non-use of the apparatus, at a lower temperature, say likewise of the order of 30 to 40"C thereby reducing the amount of energy utilised during such a period but avoiding, on the user's return, the inconvenience of having to wait a relatively long time for the storage medium to reach its normal operating temperature.

Claims

CLAIMS:

1. Hot water generating and supply apparatus of the type described characterised in that it includes heating means for heating substantially all of the thermal storage medium, temperature sensing means for sensing, at the location of said sensing means, the temperature of said medium, and control means responsive to the temperature sensed by the said sensing means and arranged to control the operation of said heating means such that, during a first pre-determined time period, substantially all of said thermal storage medium is heated to about a first pre-determined temperature and that, during a second pre-determined period, substantially all of said thermal storage medium is heated to about a second pre-determined temperature, being a temperature significantly below said first temperature.

2. Apparatus according to claim 1 wherein said first and second pre-determined temperatures differ by between 10 C and 50"C.

3. Apparatus according to claim 2 wherein said temperatures differ by between 200C and 30"C.

4. Apparatus according to any one of claims 1 to 3 wherein said heating means is electrical.

5. Apparatus according to claim 4 wherein said heating means comprises an electric immersion heater unit located in a lower region of the vessel, preferably adjacent to the base thereof.

6. Apparatus according to claim 5 wherein said heating means additionally comprises one or more additional electric immersion heater units, preferably located generally above said unit, and energisable simultaneously therewith.

7. Apparatus according to any one of claims 1 to 6 wherein the temperature sensing means comprises one or more thermistors.

8. Apparatus according to any one of claims 1 to 7 wherein the first pre-determined temperature is between 80"C and 95"C and the second pre-determined temperature is between 450C and 65"C.

9. Apparatus according to any one of claims 1 to 8 wherein said first pre-determined time period corresponds to all, or one or more selected parts, of an off-peak energy tariff period and said second pre-determined time period corresponds to all, or one or more selected parts, of a peak energy tariff period.

10. Apparatus of the type described substantially as hereinbefore described with reference to the accompanying drawing.

11. A method of generating and supplying hot water where there is utilised apparatus as claimed in any one of claims 1 to 10.

12. A vessel, for use in apparatus according to any one of claims 1 to 10, for containing, in use, a liquid thermal storage medium such as water, heat exchange means located within the vessel and having an inlet and an outlet for connection to cold water feed and hot water supply pipework respectively, heating means comprising solely one or more simultaneously energisable electric immersion heater elements located within a lower region of the vessel, preferably adjacent to the base thereof, and means for location of a temperature sensor in or at an upper region of the vessel.

13. An electric or electronic controller, for use as the control means in apparatus according to any one of claims 1 to 10, said controller being adapted to control operation of the heating of the thermal storage medium in accordance with the time period/temperature requirements set out in claim 1.

14. A controller as claimed in claim 13 substantially as hereinbefore described with reference to the accompanying drawing.