GB2249616A

GB2249616A - Hot water generating and supply apparatus

Info

Publication number: GB2249616A
Application number: GB9122029A
Authority: GB
Inventors: Richard Martyn Griffiths
Original assignee: IMI RANGE Ltd
Current assignee: IMI RANGE Ltd
Priority date: 1990-10-19
Filing date: 1991-10-17
Publication date: 1992-05-13
Anticipated expiration: 2011-10-17
Also published as: GB9122029D0; GB9022776D0; GB2249616B

Abstract

In a thermal storage type of waterheater, the primary hot water (2) is heated to a lower temperature of about 65 DEG C by a first electric immersion heater (10) connected to a 24 hour cheap rate electricity supply and to a higher temperature of about 85 DEG C by a second electric immersion heater (11) connected to a cheap rate electricity supply available, as determined by a teleswitching device (17), only during selected time periods of the day/night. The waterheater is thus able to meet typical demands for domestic hot water whilst at the same time being able to meet the electricity supply authorities' requirements as to the consumption of 24 hour cheap rate electricity relative to teleswitched cheap rate electricity. <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 (or otherwise associated with it for effecting heat exchange therewith) 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".

An example of commercially available apparatus of the type described is our FLOWMAX (Registered Trade Mark) range of products. Generally 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 concerned with apparatus of the type described wherein the storage medium is electrically heated by electric immersion heater means.

In our co-pending UK patent application No 2225097 we describe, inter alia, electrically heated apparatus of the type described that is adapted to take advantage of off-peak (eg. "Economy 7") electricity. There is now a proposal by certain Electricity Boards to provide 24 hour cheap rate electricity for powering selected domestic electrical apparatus, in addition to a night-time cheap rate supply, and it is an object of the present invention to provide apparatus of the type described that will be able to take advantage of this in a reasonably economic manner.

According to a first aspect of the present invention, there is provided hot water generating and supply apparatus of the type described characterised in that it includes electric immersion heater means arranged to heat substantially all of the thermal storage medium, for example water, and thermostat means for controlling operation of the immersion heater means and responsive to the temperature of the thermal storage medium, the electric immersion heater means being connected to a 24 hour cheap rate electricity supply and to a time-switched, for example teleswitched, cheap rate electricity supply and said thermostat means being adapted to control heating of the thermal storage medium to a first set point temperature when the immersion heater means is powered solely by the 24 hour cheap rate supply and to a second set point temperature during periods of availability of the time-switched cheap rate supply, said first temperature being below, and preferably significantly below, said second temperature.

Preferably, the respective set point temperatures, which are preferably adjustable, differ by between 10 and 50 C, more preferably between 200 and 30"C.

Typically, where the thermal storage medium is water as it usually will be, the first, lower set point temperature will be from 45 to 750C, preferably from 60 to 700C, advantageously about 65"C, and the second, higher set point temperature will be from 70 to 950C, preferably 80 to 90"C, advantageously about 85"C.

In one embodiment, the electric immersion heater means may consist of a single, for example 3 kW, heater located in a lower region of the vessel, preferably adjacent to the base thereof, connected in parallel to both said supplies, the thermostat means being adapted to control heating of the thermal storage medium to said first temperature when the immersion heater is powered solely by the 24 hour cheap rate supply (ie when the time-switched cheap rate supply is not available) and automatically adapted to control heating of the medium to said second temperature when the time-switched cheap rate supply is available. In such an embodiment, the thermostat means may comprise first and second thermostatic switches through which the heater is connected to, respectively, the 24 hour cheap rate supply and the time-switched cheap rate supply.

In a preferred embodiment of the present invention, however, the heater means comprises first and second, independently operable, electric immmersion heaters, each arranged to heat substantially all of the thermal storage medium, and the thermostat means comprises first and second thermostatic switches for controlling operation of respectively said first and second heaters, said first thermostatic switch having said first set point temperature and said second thermostatic switch having said second set point temperature, said first heater being connected, via said first switch, to the 24 hour cheap rate electricity supply and said second heater being connected, via said second switch, to the time-swiched, for example teleswitched, cheap rate electricity supply.

Preferably, the first and second heaters comprise mutually separate immersion heater units each rated, for example, at 3kW and preferably extending substantially horizontally within a lower region, preferably adjacent to the base, of the vessel, for example a cylinder, containing the thermal storage medium. Alternatively, the first and second heaters may be comprised in a single unit, eg a so-called "dual element" unit, again preferably extending horizontally within a lower region of the vessel.In any event, 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.

This is why we refer above to "substantially" all of the liquid thermal storage medium being heated. 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.

The first and second thermostatic switches are preferably of the thermo-mechanical type conventionally used with electric immersion heaters, the temperature sensing element preferably being located in a pocket extending adjacent to the element(s) of the heater and actuating an electric switch when the temperature sensed falls below, or reaches, a pre-determined temperature about equal to the set-point temperature. However, as will be appreciated, alternative thermostat means may be employed, for example those utilising one or more thermistors and associated switching circuitry.

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 (or otherwise associated with it for effecting heat exchange with the medium) and having an inlet and an outlet for connection to cold water feed and hot water supply pipework respectively, electric immersion heater means, preferably located within a lower region of the vessel, and having associated therewith first and second thermostatic switches via which the heater means may be connected to respective mains electricity supplies.Preferably, the heater means comprises first and second independently operable heaters which may be mutually separate units or may be comprised in a single unit, associated, respectively, with said first and second thermostatic switches.

In use of the preferred embodiment described above, the first heater means is connected to the proposed 24 hour cheap rate supply whereby the thermal storage medium will be continually reheated at all times to a minimum temperature corresponding to about the first set-point temperature, eg. 65"C under the control of the first thermostat. The second heater means is connected to a cheap rate supply which, however, is proposed to be available only at certain pre-selected times during each 24 hour period. During those times, which may be selected by a teleswitch or other time-of-day dependant switching device, the second heater means also will be operative and can heat the storage medium to about the second set-point temperature, eg 85"C, under the control of the second thermostat.

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 diagram 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 cylinder 1 is provided with an integral feed and expansion tank 3 which serves to accommodate expansion of, and to replenish, the thermal storage medium 2.

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 ate 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.

A pair of conventional 3kW electric immersion heaters 10, 11 including respective, adjustable thermostatic switches (not shown) mounted in their heads and associated with respective temperature sensing elements 10', 11', are mounted in a lower region of the cylinder 1. The set point temperature of the thermostatic switch associated with the heater 10 will normally be adjusted to be of the order of 65"C whereas that for the heater 11 will normally be adjusted to be of the order of 85"C.

The 240v AC domestic supply intended to power, inter alia, the apparatus is shown diagramatically at 12 and comprises a number of conventional fuses or circuit breakers and terminals housed in a so-called consumer unit together with meters and a teleswitch. More particularly, the incoming mains cable 13 is connected in parallel to first and second meters 14 and 15. The meter 14 meters units of electricity utilised by lights and appliances connected to conventional power points which are connected up via the bank of fuses/circuit breakers 16. The meter 14 may, as shown, meter units charged at the standard rate and, in response to the operation of a teleswitching device 17, at a cheap (or "economy") rate.

The meter 15 meters units of electricity utilised by, inter alia, the immersion heaters 10 and 11 and which are to be charged at all times (ie. 24 hours per day) at the cheap rate. The immersion heaters 10 and 11 are wired up to the meter 15 via fuses or circuit breakers 18 and 19 respectively. As can be seen, the power is constantly available to the immersion heater 10, whereas (save as explained below) power is normally available to the immersion heater 11 only during selected time periods in dependance upon operation of the teleswitching device 17; such time periods will, as usual, be determined by the electricity supply authorities. Whilst not relevant to the invention, relatively heavy electricity-consuming apparatus such as night storage heaters would be powered via the teleswitching device 17 and fuses or circuit breakers 20.

A typical time/thermal storage medium 2 temperature profile will be-as follows during use of the apparatus.

On initial commissioning or after the apparatus has been switched off for an extended period of time (by means of isolating switches 21, 22), for example during a holiday period or for the purposes of servicing, the immersion heater 10 would immediately be energised until the temperature of the storage medium 2 reaches about 65"C, ie. the setting of the thermostat that controls operation of the heater 10. At the same time, the immersion heater 11 would become energised in the event that a teleswitched "on" period happened to prevail. In that case, the thermal storage medium 2 will reach a temperature of about 85"C, ie the setting of the thermostat that controls operation of the heater 11, provided that the teleswitched 'on' period prevails for long enough.Once "steady state" conditions have been reached, the immersion heater 10 will operate only if the temperature of the thermal storage medium 2 falls below about 650C and during teleswitched "off" periods the temperature of the thermal storage medium 2 will be continually maintained at about 650C, the heater 10 reheating the medium 2 as required in response to demands for domestic hot water. On the other hand, during teleswitched "on" periods, the heater 11 will operate for so long as the temperature of the medium 2 is below about 85"C.

Generally, it is likely to be the electricity supply authorities' requirements that, on average in relation to , for example, a domestic water heater, at most 40% of its total electricity consumption should be from the "24 hour" cheap rate supply and at least 608 should be from the teleswitched supply.

The following Tables 1 and 2 show theoretical calculations in relation to apparatus of the invention in the case of a typical average domestic hot water consumption of 180 litres over a 24 hour period (Table 1) and a much higher than average consumption of 240 litres, also over a 24 hour period (Table 2). In both cases, it is assumed that the thermal storage medium, at the beginning of each 24 hour cycle (midnight), is at a temperature of about 65"C, that the in-coming mains cold water in pipe 5 is at 100C and that the domestic hot water supplied through pipe 6 is at between about 50 and 60 C (depending on the prevailing temperature of the medium 2 and the setting of the valve 7).It is also assumed that the teleswitched "on" periods will extend for pre-selected periods between midnight and 6am and between midday and 6pm, say for 3 hours and 2 hours respectively in the summer and for 5 hours and 3 hours respectively in the winter.

It is believed that, in practice, the electrical energy consumption will not deviate significantly from the theoretically calculated values and, therefore, that the energy "split" as between the teleswitched and 24 hour cheap rate supplies will, in general, readily meet the authorities' above requirements.

TABLE 1

Timespan Store D.H.W. Notes Energy Input Temp C Draw-off (kWh) (litres) T/Switch 24hr Supply Supp Midnight Start 65 0 200C rise 4.8 to possible.

6 am Finish 85 6 am Start 85 40L 40 litres of to water used at 12 noon Finish 75 temp rise of 50"C Temp drop = 9.5 say 10 12 noon Start 75 40L 40 litres as 2.4 to above.

6 pm Finish 85 6 pm Start 85 lOOL 100 litres - 0.5 to drawn drop Midnight Finish 65 store 23 C; 2"C made up from 24hr supply.

Total: 7.2 0.5 Add heat loss 3.0 of say 3kWh 10.2 0.5 Energy split 95% 5% TABLE 2

Timespan Store D.H.W. Notes Energy Input Temp C Draw-off (kWh) (litres) T/Switch 24hr Supply Supp Midnight Start 65 0 4.8 - to 6 am Finish 85 6 am Start 85 100 Heavy morning - 1.2 to draw-off 12 noon Finish 65 5"C make up required.

12 noon Start 65 40 6.0 to 6 pm Finish 79 6 pm Start 79 100 - 2.5 to Midnight Finish 65 Total: 10.8 3.7 Add heat loss 2.23 0.77 of say 3kWh ~~~~~ ~~~~ 13.03 4.47 Energy split 75% 25% Reverting to the drawing, the immersion heater 11 is also connected, in parallel, to the 24 hour cheap rate supply via a manually operable switch 23. This feature, which is optional, enables the temperature of the thermal storage medium 2 to be boosted up to a temperature of about 850C in the event that there is an exceptional demand for domestic hot water during a day-time teleswitch "off" period.

It will be seen that the "split" electrical energy consumption of the apparatus described above is achieved in a manner that is, in principle, similar to the apparatus in our co-pending UK patent application No 2225097, that is to say by controlling the temperature of the thermal storage medium to higher and lower values depending on the time of day. However, apparatus of the present invention has the advantage that there is no need to provide a dedicated time controller which tends to be fairly expensive. Thus, apparatus of the invention may be wired directly to the user's consumer unit, albeit preferably through isolating switches such as 21 and 22, it being the intention that the apparatus be continously "live" except, perhaps, during periods when the user is on holiday or, of course, when the apparatus requires servicing.

Finally, whilst in the specific embodiment described above,. the high efficiency heat exchanger simply consists of finned tubing located within the cylinder, the heat exchanger could be of an alternative type suitable for location within or outside the cylinder (or other vessel), for example a plate type heat exchanger or coaxial tube type of heat exchanger, such as a YORCO-AX heat exchanger - YORCO-AX is a registered trade mark of IMI Yorkshire Alloys Limited - the heated thermal storage medium being pumped (where the heat exchanger is outside the cylinder, from and back to the cylinder) through the primary circuit of the heat exchanger and the cold water to be heated being fed, usually at mains pressure, through the secondary circuit thereof.

Claims

CLAIMS:

1. Hot water generating and supply apparatus of the type described characterised in that it includes electric immmersion heater means arranged to heat substantially all of the thermal storage medium, for example water, and first and second thermostatic switches responsive to the temperature of said medium for controlling operation of said heater means, said first thermostatic switch having a set point temperature below, and preferably significantly below, the set point temperature of said second thermostatic switch, said heater means being connected to a 24 hour cheap rate electricity supply via said first thermostatic switch and to a time-switched, for example teleswitched, cheap rate electricity supply via said second thermostatic switch.

2. Apparatus according to claim 1 wherein the respective set point temperatures of the first and second thermostatic switches differ by from 10 C to 50"C.

3. Apparatus according to claim 2 wherein said set point temperatures differ by from 20 C to 30 C.

4. Apparatus according to any one of claims 1 to 3 wherein the set point temperature of the first thermostatic switch is from 45"C to 750C.

5. Apparatus according to claim 4 wherein the set point temperature of the first thermostatic switch is from 60"C to 70 C.

6. Apparatus according to claim 5 wherein the set point temperature of the first thermostatic switch is about 65"C.

7. Apparatus according to any one of claims 1 to 6 wherein the set point temperature of the second thermostatic switch is from 70"C to 95"C.

8. Apparatus according to claim 7 wherein the set point temperature of the second thermostatic switch is from 80"C to 900C.

9. Apparatus according to claim 8 wherein the set point temperature of the second thermostatic switch is about 850C.

10. Apparatus according to any one of claims 1 to 9 wherein the heater means comprises a single immersion heater preferably extending substantially horizontally within a lower region of the vessel, the heater being connected, in parallel, to both said electricity supplies via their respective thermostatic switches.

11. Apparatus according to any one of claims 1 to 9 wherein the heater means comprises first and second immersion heaters, preferably extending substantially horizontally within a lower region of the vessel, the first immersion heater being connected to the 24 hour cheap rate supply via said first thermostatic switch and the second immersion heater being connected to the timeswitched cheap rate supply via said second thermostatic switch.

12. Apparatus according to any one of claims 1 to 11 wherein the first and second thermostatic switches are each of a thermo-mechanical type and comprise a temperature sensing element for sensing the temperature of the thermal storage medium and an electric switch responsive to the temperature sensed by the sensing element.

13. Apparatus according to any one of claims 1 to 11 wherein the first and second thermostatic switches are controlled via electronic circuitry by the output of one or more thermistors arranged to sense the temperature of the thermal storage medium.

14. Apparatus according to any one of claims 1 to 13 wherein the said set point temperatures are adjustable.

15. Apparatus according to any one of claims 1 to 14 comprising a manually operable switch for connecting the 24 hour cheap rate supply to the heater means via the second thermostatic switch thereby providing a "boost" heating facility especially during periods in which the time-switched cheap rate electricity supply is "off".

16. A vessel, for example a cylinder, for use in apparatus according to claim 1, having heat exchange means located within an upper region thereof, or otherwise associated with it for effecting heat exchange, in use, with the thermal storage medium, said heat exchange means having an inlet and an outlet for connection to cold water feed and hot water supply pipework respectively, said vessel further having located, preferably in a lower region thereof, electric immersion heater means, said heater means having associated therewith first and second thermostatic switches via which the heater means may be connected to respective mains electricity supplies.

17. A vessel according to claim 16 wherein said heater means comprises first and second immersion heaters associated, respectively, with said first and second thermostatic switches.

18. Apparatus-according to claim 1 or a vessel according to claim 16 substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawing.