GB2250334A - Waterheating apparatus - Google Patents

Abstract

Waterheating apparatus of the thermal storage type comprises a vessel 1 containing a mass of stored primary hot water 2 which is pumped to and from a boiler 3 by a pump 4. Immersed in the primary hot water 2 is a high efficiency heat exchanger 8 and an auxiliary heat exchanger 7 through both of which mains cold water passes to be instantaneously heated to provide domestic hot water on demand. If such demand is relatively large, the pump 4 is activated by a flow switch 19 or 19', located in the secondary pipework and having a pre-determined flow rate actuation threshold, so as to maintain a sufficiently high flow of primary hot water 2 through the heat exchanger 8. When demand is below the threshold of the flow switch 19 or 19', however, the pump 4 is not activated and the mains cold water is heated primarily during its passage through the auxiliary heat exchanger 7. The arrangement results in lower overall heat losses and reduced pump wear compared to prior proposals. <IMAGE>

Description

Waterheating Apparatus This invention relates to waterheating apparatus of the type comprising a vessel containing, in use, a thermal storage ("primary") medium, for example water, and heating means to heat the thermal storage medium, the storage medium having immersed in it (or otherwise associated with it) a high efficiency heat exchanger through which, on demand, cold ("secondary" ie usable) water, usually at or driven by mains pressure, passes whereby the cold water is instantaneously heated, by heat exchange with the storage medium, 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. The thermal storage medium may also be used to heat a "wet" space heating circuit by being pumped around the circuit.Hereinafter, such apparatus is referred to as "water heating apparatus of the type described". The thermal storage medium may be heated by any suitable heating means, for example directly, or possibly indirectly, by a gas-,oil-,coal- or electricity-fuelled boiler or by one or more electric immersion heaters. The present invention is particularly, but not exclusively, concerned with waterheating apparatus of the type described wherein the thermal storage medium is heated by an external boiler from and to which the medium is circulated by an electric pump.

Waterheating apparatus of the type described is known in the form of, for example, our successful range of FLOWMAX (Registered Trade Mark) units. In those units, in which the high efficiency heat exchanger is in the form of coiled finned tubing immersed in the storage medium, the medium immediately adjacent to the finned tubing is substantially static and continuing, effective heat exchange between the secondary cold water and the thermal storage medium relies mainly on there being natural convection of the storage medium within the vessel, which is usually a cylinder. The heat exchange performance may be significantly increased by pumping the storage medium over the external surface of the finned tubing so as to cause turbulence within, and mixing of, the medium.Similarly, in other known apparatus of the type described, the high efficiency heat exchanger is of the known coaxial tube type comprising an inner, externally finned tube and a coaxial, outer plain tube, the secondary water passing, on demand, through the inner tube and the thermal storage medium being pumped through the annular space between the inner and outer tubes. In the latter arrangement, the heat exchanger could be, for example, of the plate-to-plate type instead of the coaxial tube type.In any case, the thermal storage medium is pumped by the pump that serves also to circulate the storage medium to and from the vessel via the boiler, the pump conventionally being activated, inter alia, in response to any draw-off of secondary hot water as sensed by a flow switch located in the secondary pipework [the pump is, of course, also activated, substantially simultaneously with firing of a gas or oil-fired boiler, in response to the temperature of the thermal storage medium dropping below a predetermined value, typically between about 80 and 90"C, as sensed by a thermostat].Accordingly, even for small, low-rate draw offs as at sinks and wash basins, the pump is automatically activated and thermal storage medium is pumped round the primary circuit, via the boiler, notwithstanding that it might be at its pre-determined temperature; relatively large amounts of energy are thereby wasted, principally through heat loss from the often lengthy primary flow and return pipework that links the boiler to the vessel. The pump is thereby also subjected to increased wear.

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

According to one aspect of the present invention, there is provided an apparatus of the type described in which the thermal storage medium is pumped over or through the high efficiency heat exchanger in response to a demand for secondary hot water, characterised in that activation of the pump in response to such demand occurs only when the demand equals or exceeds a pre-determined, finite limit.

Apparatus of the present invention is based on our realisation that, in the case of relatively small draw offs of secondary hot water (for example when it is desired to fill only a small sink or hand basin) the apparatus is generally quite able to supply the requisite amount of hot water without the need to activate the pump. This, of course, should be contrasted to relatively large draw-offs, for example when it is desired to fill a bath, when it will be necessary to activate the pump so as to ensure that a sufficient amount of thermal energy contained in the thermal storage medium is made available to the heat exchanger.

The aforesaid "demand" for secondary hot water may be expressed, and sensed accordingly, in terms of any of a number of parameters, for example in terms of absolute volume drawn off, volume per unit time of draw off, continuous draw off time or in terms of a demand that results in the temperature of the secondary hot water eventually dropping to a pre-determined lower temperature.

Thus, for example, activation of the pump in response to the predetermined minimum demand may be achieved by using a flow switch located in the secondary water circuit and adapted to sense flow rate or total flow volume (ie. in the nature of an integrating flow meter). The flow switch may be located at such a position in the secondary circuit, and be so set, that it will activate the pump in the event that it senses any demand for secondary hot water from the apparatus equal to above the minimum limit; alternatively the flow switch may be located in a secondary water pipeline that feeds only a selected, relatively high draw off point of use, such as a bath, in which case the flow switch could be responsive simply to flow (regardless of the rate) within that pipeline.

Alternatively, for example, activation of the pump in response to the pre-determined minimum demand may be achieved by using a temperature-sensitive device. For example, a temperature sensitive switch, such as a simple thermostatic switch, could be located in a secondary water pipeline that feeds only a selected, relatively high draw-off point of use, as aforesaid, the switch causing the pump to be activated when it senses the eventual flow of hot water past its location in the pipeline. Alternatively, the pump may be activated in response to reduction of the temperature of secondary water being drawn off to a pre-determined value, as sensed by a temperature sensing device.

In yet a further alternative, activation of the pump could be time responsive, ie be activated only after secondary water has been drawn off for a pre-determined, continuous time period.

Preferably the minimum secondary hot water demand, at or above which the pump is activated, is, or corresponds to, a draw off rate of about ten litres/minute, although the preferred limit in any given case will depend on the characteristics of the apparatus. For that reason, the demand sensor employed is preferably adjustable to suit the apparatus.

In apparatus of the invention in which the high efficiency heat exchanger consists especially of a coaxial tube, plate-to-plate or like heat exchanger, as described above, performance at low rate, but nevertheless sustained, draw offs, for example at a sink or basin, may be improved by including an additional, simple heat exchanger, for example a length of externally finned tubing, located within the thermal storage medium and connected in series with the coaxial tube plate-to-plate or like heat exchanger. Thus, adequate heating of the secondary water will be obtained in respect of such sustained, low rate draw offs, principally via the additional heat exchanger, without the need to activate the pump.

According to a second aspect of the present invention, therefore, there is provided apparatus of the type described characterised in that it includes a first, high efficiency heat exchanger over or through which the thermal storage medium is arranged to be pumped in response to a demand for secondary hot water equal to or exceeding a pre-determined finite limit and a second heat exchanger consisting of a single-walled, preferably externally finned, tube immersed in the thermal storage medium and connected in series with the first heat exchanger whereby the secondary hot water passes sequentially, in either order, through said tube and said first heat exchanger.

Preferably, the high efficiency heat exchanger is a coaxial tube type of heat exchanger (for example a YORCO-AX heat exchanger - Registered Trade Mark of our sister company, IMI Yorkshire Alloys Limited or a heat exchanger of the type described in UK Patent Specification No 1289542) or a plate-to-plate type of heat exchanger, each of which comprises, in mutual heat exchange relationship, a primary circuit through which the thermal storage medium is, when appropriate, pumped, and a secondary circuit through which the secondary water passes on demand, thereby becoming heated.

In another aspect of the invention, the apparatus has immersed in the thermal storage medium a single high efficiency heat exchange unit of the coaxial tube type (eg. a YORCO-AX heat exchanger as referred to above), the secondary water passing, on demand, through the annular space between the two tubes and the thermal storage medium being pumped through the inner tube (preferably in the direction opposite to the direction of the secondary water) when the demand for secondary hot water equals or exceeds a pre-determined finite limit, for example as determined in a manner described above.Thus, in the case of small demands for secondary hot water below said limit, the incoming secondary water will pick up heat principally by heat exchange with thermal storage medium surrounding the outer tube (for which purpose the outer tube, as well as the inner tube, may be externally finned) whereas in the case of a larger demand for secondary hot water, the secondary hot water will pick up heat principally from thermal storage medium being pumped, in response to the larger demand, through the inner tube. This arrangement should be contrasted to the known arrangement described earlier in which the secondary water is passed through the inner tube, the thermal storage medium is passed through the annular space between the two tubes and the pump is activated in response to any demand whatsoever for secondary hot water.

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 one form of apparatus according to the invention.

Referring to the drawing, the apparatus includes a well lagged cylinder 1 filled, in use, with a primary liquid thermal storage medium 2 in the form of water.

The primary water 2 is heated by a boiler 3 through which it is circulated, via flow and return pipework, by a pump 4. The temperature of the primary water 2 is controlled in response to a temperature sensor 5, usually within the region of 80-90"C, although this may be varied as described, for example, in our co-pending UK patent application No 9021248. Expansion and occasional replenishment of the primary water 2 is dealt with by an integral feed and expansion tank 6, although as is well known a remote feed and expansion tank may be used instead.

Within an upper region of the cylinder are located two, serially connected, heat exchangers 7 and 8. The heat exchanger 7 consists simply of a coiled length of externally finned (and preferably internally rifled) tubing, such as INTEGRON tubing - INTEGRON is a Registered Trade Mark of our sister company, IMI Yorkshire Alloys Limited. The heat exchanger 8 likewise comprises a coiled length of externally finned tubing 9, such as INTEGRON tubing, but further comprises a tubular housing 10 closely surrounding the tubing 9. The tubular housing 10 is open at its top and is connected, at its bottom end to the primary water return pipe connected to the boiler 3 via the pump 4. The lower inlet end of the tubing 9 is connected to a cold water mains pipe 11, optionally via a pressure regulator (not shown), and its upper outlet end is connected to the upper, inlet end of the tubing 7.The lower, outlet end of the tubing 7 is connected to secondary hot water distribution pipework 12,13. The pipework 12 may, for example, distribute hot water to relatively small demand points of use such as hand wash basins and sinks whereas the pipework 13 may distribute hot water to relatively high demand points of use such as baths and shower installations. The secondary hot water is fed to the various points of use, through pipework 12, 13 at mains, or reduced mains, pressure via, in known manner, a thermostatic mixing valve 14 where the hot water generated by the apparatus may mix with mains cold water fed to the valve 14 by a branch pipe 15. There is also provided, again in known manner, a sealed expansion vessel 16 for accommodating the thermal expansion of water within the tubing 7 and 9 as it heats up under static conditions.

The cylinder 1 is connected to the flow and return pipework 17, 17 of a wet space heating circuit, the primary water 2 being circulated therearound by a pump 17'' in response to the state of a room thermostat 18.

All of the features so far described, save for the combined heat exchanger 7, 8 arrangement and the way in which they operate are well known to those skilled in the art. Briefly, however, and as will be apparent, when there is a demand for secondary hot water, the incoming mains cold water in the pipe 11 passes through the heat exchanger 7, 8 arrangement and becomes instantaneously heated by virtue of heat exchange with the primary water 2 contained in the cylinder 1. The heated secondary water, if at a temperature greater than the setting of the mixing valve 14 (as it usually will be) then mixes with cold water in the valve 14 and is fed to the point of demand at the temperature (ignoring subsequent heat loss from the distribution pipework 12, 13) set by the valve 14, typically of the order of 60"C.

The primary water 2 is, as already noted, continually reheated to a pre-determined temperature of, say, 80-90"C by the boiler 3. More particularly, when the temperature of the primary water 2 drops below the pre-determined temperature as sensed by the sensor 5, the boiler 3 fires and the pump 4 is simultaneously activated thereby circulating the primary water 2 in the cylinder 1 via the boiler 3. When the primary water 2 reaches the pre-determined temperature, as sensed by the sensor 5, the boiler and pump switch off, and so on.

Reverting to the heat exchangers 7, 8, the use alone of a high efficiency coaxial tube type of heat exchanger 8 in apparatus of the type described is known.

However, in known apparatus1 it is conventional to pump the primary water 2 through the tube 10, and thus over the external surface of the tube 9, as shown by the arrows in the drawing, whenever there is any demand whatsoever for secondary hot water. In known apparatus, this is achieved by providing a flow switch in the secondary pipework which, when it detects flow, however small, in that pipework, activates the pump 4, unless of course the pump 4 is already operating in response to the primary water temperature sensor 5.Whilst there is clearly a need so to activate the pump in response to a heavy demand for hot water, for example when filling a bath, we have appreciated that there is often, in the case of small demands (such as when using a hand basin), no such need and that indeed, in such cases, activation of the pump can lead to very significant heat losses especially through the often lengthy flow and return pipework that connects the cylinder 1 to the boiler 3.

In accordance with one embodiment of the invention, therefore, a flow switch 19 is provided in the secondary pipework intermediate the hot water outlet from the heat exchanger 7 and any secondary pipework branches. In contrast, however, to known apparatus of the type described including a flow switch in the secondary circuit, the flow switch 19 is adapted to activate the pump 4, in response to a demand for hot water, only in the event that the flow rate, at the location of the switch 19, exceeds a predetermined, finite value, for example 10 lr/minute. Thus, if there is a demand for hot water at a point of use, eg a hand basin, served by the 15mm pipework 12, then the pump 4 will not be activated in response to the demand, because the maximum flow rate therethrough will be less than the threshold value set on the flow switch 19.Nevertheless, if there is a sustained demand at a point of use served by the pipework 12, then the apparatus should be able to cope with it by virtue of heat exchange, between the incoming relatively cold secondary water and the substantially static primary hot water 2, surrounding the finned tube 7.

On the other hand, demand at a point of use served by the 22mm pipework 13, eg a bath or shower, will be above the selected threshhold value set on the flow switch 19 which will therefore activate the pump 4 if it is not already operating in response to the sensor 5.

The threshhold value set on the switch 19 is preferably adjustable so that an appropriate value may be selected to suit the installation in question.

In an alternative arrangement, also shown in the drawing, the flow switch 19 may be replaced by a flow switch 19 located in the 22mm pipework 13. The flow switch 19 may then be of a type that is responsive to any flow of whatever magnitude in the pipework 13 and the pump 4 will, of course, be activated in response to a demand for water only at a point of use served by the pipework 13.

In yet another alternative arrangement, the flow switch 19 may be replaced by a temperature sensor which is adapted to activate the pump 4 only when, during a demand for hot water, the temperature of the water passing the sensor drops below a predetermined value thereby indicating that there is a need to enhance the rate of heat exchange within the heat exchanger 8 by pumping primary hot water within the cylinder 1 over the finned tube 9.

It will be appreciated, therefore, that the considerable heat losses that accompany activation of the primary hot water pump 4 in response to small demands for secondary hot water, as occurs in certain known apparatus, may be somewhat reduced by using apparatus of the invention. Further, other things being equal, the pump 4 will be activated less frequently and so pump wear will be reduced.

As already noted, the heat exchanger 8 may comprise any suitable alternative heat exchanger through or over which the primary hot water can be pumped in order to enhance the rate of heat transfer. By way of example, it could simply be a length of, preferably finned, tubing or a plate-to-plate type of heat exchanger which may, albeit less preferably, be located outside the cylinder 1, the primary hot water 2 being pumped from the cylinder 1, through the primary circuit of the plate type heat exchanger, through the boiler 3 and back to the cylinder 1, and the incoming mains cold water passing, on demand for hot water, through the secondary circuit of the plate type heat exchanger and, optionally, through a finned (or even plain) subsidiary heat exchanger, as 7, immersed in the primary water 2 contained in the cylinder 1.

Claims (19)

CLAIMS:

1. Waterheating apparatus of the type described in which the thermal storage medium is pumped over or through the high efficiency heat exchanger in response to a demand for secondary hot water, characterised in that activation of the pump in response to such demand occurs only when the demand equals or exceeds a pre-determined finite limit.

2. Apparatus according to claim 1 wherein said demand is measured in terms of flow rate in the secondary hot water circuit, the apparatus including a flow switch, located in said circuit, adapted to sense the flow rate therein and to cause activation of the pump in the event that the sensed flow rate equals or exceeds a pre-determined finite value.

3. Apparatus according to claim 2 wherein said pre-determined value is within the range of from 5 to 10 litres per minute.

4. Apparatus according to claim 1 wherein said demand is measured in terms of volume flow in the secondary hot water circuit, the apparatus including in the secondary circuit a measuring device to measure said volume and means responsive thereto to cause activation of the pump when said device has measured a continuous secondary hot water demand equal to a pre-determined value.

5. Apparatus according to claim 4 wherein said measuring device is a volume flow meter.

6. Apparatus according to claim 1 wherein the demand for secondary hot water is monitored by a temperature-sensitive device located in the secondary circuit, said device causing activation of the pump in response to reduction of the temperature of the secondary hot water, at the location of and as determined by said device, to a pre-determined value during draw-off of secondary hot water.

7. Apparatus according to claim 6 when said device is a thermostatic switch.

8. Waterheating apparatus of the type described in which the thermal storage medium is pumped over or through the high efficiency heat exchanger in response to a demand for secondary hot water, characterised in that the apparatus includes, in a secondary hot water pipeline that feeds only a relatively high draw-off point of use, for example a bath, detecting means to detect flow of secondary water in said pipeline, said detecting means causing activation of the pump in the event that it detects flow in said pipeline.

9. Apparatus according to claim 8 wherein said detecting means comprises a flow switch.

10. Apparatus according to claim 8 wherein said detecting means comprises a temperature-sensitive device adapted to cause activation of the pump when it detects, by way of a temperature increase, the eventual flow of hot water past its location in said pipeline.

11. Apparatus according to claim 10 when said temperature-senstive device is a thermostatic switch.

12. Apparatus according to any one of claims 1 to 11 wherein said high efficiency heat exchanger is connected in series to an auxiliary heat exchanger comprising a length of tube immersed in, and exposed directly to, the mass of thermal storage medium.

13. Apparatus according to claim 12 wherein said length of tube is externally finned.

14. Apparatus according to anyone of claims 1 to 13 wherein the high efficiency heat exchanger comprises a length of externally finned tubing contained in an envelope, the pump serving during its operation to pump thermal storage medium through the envelope and over said tubing and the cold water passing through said tubing upon a demand for secondary hot water.

15. Apparatus according to any one of claims 1 to 13 wherein the high efficiency heat exchanger comprises a plate-to-plate type of heat exchanger.

16. Apparatus according to claim 14 or claim 15 wherein said high efficiency heat exchanger is immersed in the thermal storage medium contained in the vessel.

17. Apparatus according to any one of claims 1 to 13 wherein said high efficiency heat exchanger is immersed in the thermal storage medium and comprises a length of externally finned tubing contained in an envelope, the pump serving, during its operation, to pump thermal storage medium through said tubing and the cold water passing through the envelope and over said tubing upon a demand for secondary hot water.

18. In or for apparatus of the type described, a vessel for containing thermal storage medium, a high efficiency heat exchanger arranged such that, in use, said medium can be pumped through the primary circuit of the heat exchanger, an auxiliary heat exchanger in the form of a tube located within the vessel and one end of which is connected to one end of

circuit of the heat exchanger, and means to connect the other ends of said tube and

circuit respectively into secondary water supply and delivery pipework.

19. Waterheating apparatus substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawing.