GB2153505A - Water heating apparatus - Google Patents

Abstract

In a water heating apparatus of the type comprising a hot water tank from which primary water is circulated through domestic radiators and hot secondary water is produced for taps and the like by passing mains water through a heat exchanger in the tank, the improvement that the heat exchanger comprises a finned coil or coils and where two coils are provided they are spaced vertically in the tank. The coils are individually removable from the tank. <IMAGE>

Description

SPECIFICATION Water heating apparatus This invention relates to water heating apparatus, and in particular is concerned with water heating apparatus of the type (referred to hereinafter and in the claims as "the heating apparatus of the type set forth") having a tank which acts as a thermal store for a body of water (primary water) which is or can be selectively circulated around a flow heating circuit, such as a small bore central heating system in a domestic dwelling. The heat is added to the body of the primary water in the tank by any appropriate heating means, which may be either inside the tank but preferably will be external of the tank and will be a boiler or solar heating means through which the primary water from the tank is circulated and by which the primary water is heated.

Reference is made in the above to "primary" water because the present invention concerns heating apparatus of the type set forth having a secondary water circuit, and that secondary water circuit includes a heat exchange coil which is inside the thermal store tank. The secondary water passes through the coil and receives heat from the heated primary water in the tank which surrounds the coil, and the heated secondary water which issues from the coil is for consumption i.e. at domestic water taps, washing machines, or for general purposes.

It can been seen therefore that the heating circuit through which the primary water flows is in fact a closed circuit in that the water which passes through the heating system is returned to the tank, whereas in the secondary water circuit the cold water, which may be supplied directly from the mains, passes through the heat exchanger coil and receives heat from the primary water in the tank, and then the heated secondary water is consumed, and therefore the secondary water circuit is an "open" circuit.

The above arrangement is in fact somewhat opposite to what is currently used in domestic hot water systems. Currently, the hot water tank receives water from the mains after it has passed through a water break, which is required by law in the United Kingdom, which may be an expansion tank having a flow control valve. Water is drawn directly from the hot water tank for general consumption. The heating system is a separate circuit and the water therein is heated directly by the boiler.

The purpose of the break tank in the supply of mains water to the hot water tank in the conventional system is to ensure that the water in the hot water tank will essentially be at atmospheric pressure, or at least at a low pressure, and domestic hot water tanks in the United Kingdom currently are therefore of the "non-pressure" type. However, nearly all other countries in Europe operate on pressurised supply systems, which involves bringing the water at mains pressure directly to the hot water tank, through appropriate pressure reducing valves and nonreturn valves to prevent back flow or contamination from the tank to or in the mains supply. Such systems require the use of a large volume expansion chamber in order to avoid explosions and burst tanks. The expansion chamber is required for the case where the water in the water tank overheats and expands.The expansion chamber will take up this expansion as related to the maximum possible expansion of the water in the water tank calculated on the volume of the water tank. As the total volume of the tank is relatively large, then a large expansion chamber is required. The expansion chamber comprises a sturdy vessel containing a diaphragm to one side of which is a volume of inert gas, in particular nitrogen, and to the other side of the diaphragm, hydraulic communication is made with the body of water in the water tank.If British domestic hot water tanks were therefore converted to receive the supply direct from the mains, then depending upon the allowable internal pressure in the hot water tank, so the tank would require an expansion chamber as described, a pressure regulating valve in order to step down the mains pressure (the mains pressure would not be allowed to exist in the tank even if it were permissible to use the tank in a pressure system) and filters.

With heating apparatus of the type set forth however, because the mains supply water, the secondary water, passes through the heat exchanger coil to receive its heat, only a relative small volume of the secondary water i.e. that in the coil, will be heated, and therefore the danger of explosion is considerably reduced, because the expansion of a small volume of water is correspondingly small.

The present invention is concerned with the design of the heat exchanger coils for the secondary water, in order to improve the heat transferance, and maintain stratification of the primary water in the thermal store tank, and in accordance with the invention the coil of the heat exchanger of heating apparatus of the type set forth includes at least a portion which is of a high heattransferance design, such as being finned.

Preferably, the heat exchanger is in two spaced coils, and each coil is of finned tubing.

The individual coils preferably are adapted to be removed from the tank for cleaning, repair and descaling.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings, wherein: Figure 1 is a diagrammatic, sectional side elevation of heating apparatus according to one embodiment of the present invention; Figure2 is a diagrammatic, sectional side elevation of heating apparatus according to a second embodiment of the invention; Figure 2a is a diagrammatic side elevation of a coil of the heat exchanger; Figures 3, 4 and 5 indicate methods of controlling the flow of primary water through the heating circuit driven by the apparatus; and Figure 6 shows graphically the performance of the apparatus according to either of the embodiments of the invention shown in Figures 1 and 2.

Referring to the drawings, in Figure 1 is shown a tank 10 which comprises an inner casing 12 of copper or like material, cladding 14 of insulation, such as fibreglass, and an outer case 16 of galvanised mild steel.

The tank 10 defines two cavities, namely a main cavity 18, and an expansion overflow cavity 20. The cavity 18 contains primary water which is circulated around a flow heating circuit through flow and return pipes 22 and 24, the direction of flow being indicated by arrows 26. The primary water in cavity 18 is heated by means of a small (approx. 3 Kw) open flue gas circulator 28 which circulates the primary watertherethrough as indicated by the arrows 30.

Cold water is supplied to the cavity 20 through pipe 32 and float controlled valve 34, and flows from the cavity 20 into cavity 18through down pipe 35.

Reference 36 illustrates an overflow in cavity 20, whilst expansion pipe 38 enables water to flow from the cavity 18 and the cavity 20 when the water in cavity 18 expands.

By drawing primary water from the bottom of cavity 18, and by passing it through the gas boiler 28 to heat same and then injecting the heated water into the top of the cavity 18, the water in cavity 18 becomes hotter at the top than at the bottom, and hence the water becomes "stratified" in that there is a temperature gradient over bands of water levels from bottom to top of the cavity 18. As water is drawn off to the heating system through pipe 22, so the hottest water at the top of the tank is immediately fed into the heating system, adding to efficiency.

Reference numeral 40 indicates a pump for circulating the hot water around the heating system 22, 24.

Finally, in relation to the heating system, there is a bleed pipe 42 from pipe 24 to bleed a variable amount of the returning, cooler water in pipe 24 through a controllable mixing valve 44 back into the line 22 without circulating through the cavity 18, whereby the temperature of the water flowing along pipe 22 can be controlled.

In said cavity 18 is a pipe 46, which is in fact a heat exchange pipe, and this pipe is immersed in the primary water in cavity 18, and is in two groups of coils 46A towards the bottom of the cavity 18, and 46B towards the top of the cavity. The group of coils 46B is in the hottest region of the water in cavity 18, and the opening of the coils 46A, 46B is of particular advantage providing that the secondary water which is in the pipe 46, is heated more strongly in the coils 46B so that when the secondary water is outputted through pipes 48, it is at the correct temperature.The secondary water is inputted into a lower group of coils 46Athrough an inlet pipe 50, which may be supplied directly from the mains through suitable pressure reduction and non-return valves in order to ensure that the secondary water will be at the correct pressure, and that it will not flow back into the mains supply, which could cause contamination. The output pipe 48 is directed to consumer points in the domestic dwelling, such as hot water taps, and a washing machine supply.The secondary water is heated in passing through the two coils 46A and 46B, but the secondary circuit also has a bleed pipe 52 which has a one-way valve 54 leading to a control valve 56 by which a controlled and variable amount of cold water bled from the line 50 is mixed with hot water issuing from the group of coils 46B, to control the temperature of the hot water which is eventually discharged from the domesitctaps.

In the arrangement shown in Figure 1, the expansion cavity 20 is integral with the tank 10, and the boiler 28 is mounted on or adjacent the tank. In the arrangement shown in Figure 2, the system is essentially the same, except that the expansion cavity is replaced by a separate expansion tank 20A which may be remotely located, and the boiler is a balanced flue gas circulator 28A, which again is remotely located. Other components already described in relation to Figure 1 operate in the same manner as described in relation to Figure land are designated with the same reference numerals. An additional pump 30A is required for circulating the primary water through the boiler 28A.

Referring to Figure 2A, this figure illustrates a coil which is similar to each of the respective groups of coils 46A, 46B and comprises a finned coil mounted on a boss, the coil having a straight portion 51 which extends from a water connection 53, and a spirally wound portion 55 which coils round the straight portion 51, and leads to a water connection 57. The portions 51 and 55 are of copper, finned tubing which is a high heat transfer design, and the coils 46A and 46B may be displaced in the tank by the maximum distance. Each of the coils 46A and 46B by virtue of its mounting boss is adapted to be mounted on the tank wall so as to extend horizontally thereof and is adapted to be removed from the tank for cleaning and repair, and therefore the coils are in fact coupled externally of the tank.With the utilisation of at least a portion of finned tube giving high heat transfer characteristics, the spacing between the coils 46A and 46B can be increased because the same heat transfer is achieved as compared with a plain coil, but within a lesser height of the tank, in view of the large heat transfer surface of the finned tubing. It is not necessary that the coils be removable from the tank, and they could be connected internally of the tank. Indeed with the eutilisation of finned tubing, it may be possible to have a single coil for the heat transfer to the secondary water passing through the coil.

Figures 3,4 and 5 show various control systems for the control of the heating circuit 22,24. In the arrangement of Figure 3, the mixing valve 44 is shown as being simply manually controlled by a controller 60, and a pump 40 is controlled by a simple onioff switch 62, which is in turn controlled by a thermostatic radiator valve on the main living room radiator or at any other suitable location.

In the arrangement of Figure 4, the mixing valve 44 is automatically controlled by means of a room sensor 64, the pump 40 again being controlled as described in relation to Figure 3.

Heating can of course be controlled in a more conventional fashion using a time clock and conventional room thermostat 66 and 68 as shown in Figure 5, these two components controlling the pump 40, so that the pump 40 runs during the time that is set by the time switch, and as long as the room thermostat does not detect that the room temperature is above the predetermined level.

The embodiments operate in that the fluctuating demands for heating and hot water are evened out by storing energy produced when demand is low and discharging it when demand is high. An important feature of the design is that the secondary water can be supplied directly from the mains at conventional flow rates without the need for non-return valves, temperature and pressure safety relief valves or expansion vessels.

Typically, the cavity 18 will contain 160 litres of primary water which is maintained at a temperature approximately 80 C at the top of the cavity.

With such an arrangement, and a heating load of 2.5 KW, two 70 litre baths at a flow rate of 15 litres per minute with a 30 minute interval between the two whilst maintaining the hot water draw temperature above 45"C can be drawn, as shown in Figure 6, and during this period the room temperature remains between 20 and 22"C.

Although the apparatus described has particularly good application for small start-up homes, there is no reason why the principle cannot be used for any dwellings.

Claims (8)

1. Heating apparatus of the type set forth wherein the coil of the heat exchanger includes at least a portion of which is of high heattransferance, such as being finned.

2. Apparatus according to claim 1, wherein said portion is copper finned tubing.

3. Apparatus according to Claim 1 or 2, wherein said heat exchanger comprises a coil of finned tubing connected to a mounting boss by which the heat exchanger is mounted on but is removable from the tank.

4. Apparatus according to Claim 3, wherein said heat exchanger includes a second similar coil, the first and second coils being spaced vertically inside the tank and being connected in series outside the wall of the tank containing the water so that water to be heated passes first through the second coil and then through the first coil.

5. Apparatus according to claim 3 or 4, wherein the boiler is external of the tank and returns water heated thereby to the tank at the top of the tank where the said coil orthe first coil is located.

6. Apparatus according to claims 4 or 5, wherein said coil or each of said coils is substantially elongate and lies longitudinally in the tank.

7. Apparatus according to claim 6, wherein the or each coil comprises a straight tube length extending from the boss to the free end of the coil and a return length which spirals around the straight tube length in returning to the boss.

8. Heating apparatus of the type set forth substantially as hereinbefore described with reference to the accompanying drawings.