GB2037958A - Heating liquid in a storage tank - Google Patents

Abstract

Method of and apparatus for heating liquid, wherein an inverted funnel 14 is located on a vertical axis in a liquid storage tank 10 and a heating element 12, especially a pipe passing hot water from a solar heating system, is located adjacent the bottom of the tank beneath the inverted funnel, the arrangement being such that in use colder water 20 entrained in the hot water circulatory flow 19 separates out on the funnel cone 16 and flows into the space beneath the funnel through a small peripheral gap 17 defined between the rim 18 of the funnel cone and the side wall of the tank. As shown in Fig. 3 the funnel 24 discharges into a hot water storage compartment 23 within a tank 21, which compartment is designed to separate hot and cold liquid and from which hot liquid may be withdrawn. <IMAGE>

Description

SPECIFICATION Improvements in heat exchange systems This invention relates to heat exchange apparatus providing an improved method of heat exchange in a tank wherein a liquid to be heated is contacted with an immersed heating element, more especially a pipe through which passes a hot second fluid. The invention is especially but not exclusively applicable to the heating of water in a storage tank by contact with an immersed coil which is connected in the circulatory water path of a solar heating system.

Conventionally, hot water derived from a solar heating system is circulated through a water storage tank which constitutes the base supply of hot water for the premises where the solar heating system is installed. The storage tank has a conventional valve controlled cold water input from the water main or from a cold water store.

Within the tank, usually in the bottom half thereof, is located on a vertical axis a helical coil heat exchange element through which circulates hot water, commonly at a temperature of about 70 degrees Centigrade. This hot water is derived from the absorption coil or absorption panel of the solar heating system, and is returned to the absorption means to be reheated after passage through the heat exchange element in the storage tank.

With the above-described arrangement, the water in the storage tank can be heated to a substantially uniform maximum temperature of about 60 degrees Centigrade, when overall heat loss from the tank effectively balances the heat input. At this time, water is being returned from the heat exchanger to the solar heating system at about 60 degrees Centigrade. More generally, as the water in the storage tank becomes hotter, so the differential temperature between the input side and the output side of the heat exchanger becomes smaller, so that the full heating capacity of the solar heating system is no longer being utilised.It will also be appreciated that, at this time, the maximum temperature attainable by the water in the stprage tank is being limited, not only due to heat loss, but also because heat extraction from the heat exchanger is reduced to that determined by the reduced temperature differential of about 1 10 degrees Centigrade.

It is therefore an object of the present invention to overcome or minimise the above-described disadvantage and, more generally, to reduce the increasing fall in the efficiency of heat exchange which tends to occur as a first liquid being heated attains a temperature closer to that of a source providing the heat.

According to a first aspect of the invention, there is provided a heating element which heats a liquid to be heated in the lowermost region of a storage tank beneath an inverted stemmed funnel, whereby the heated liquid is caused to rise through the funnel stem and is then turned back ,downwardly on the outside of the stem in a main circulatory flow which entrains colder Iqiuid in its downward movement, the funnel cone acting as a barrier to said circulatory flow in order to separate the heated liquid from the entrained colder Iqiuid, and the rim of the funnel is narrowly spaced from the side wall of the tank to define a peripheral passage through which the separated colder liquid can pass from the outside face of the funnel cone into the lowermost region of the tank.

Thus, the action of the funnel is to induce a continuous circulation of the liquid in the storage tank, in a manner which causes the colder portion of said liquid to be diverted into the region where the heat exchange element is located. In this way, the temperature differential is maximised between the second liquid and the portion of the first liquid which is being heated at any given time. The temperature differential between the input side and the output side of the heat exchanger is thereby maximised, which, in the case of heat derived from a solar heating system, means that the heating capacity of the system is more fully utilised.

In a preferred method, the heated liquid is circulated through a secondary tank after emerging from the top of the funnel stem and before being turned back downwardly in the main circulatory flow.

According to a second aspect of the invention, there is provided heat exchange apparatus cornprising an inverted stemmed funnel located on a vertical axis within a storage tank containing a liquid to be heated, the rim of the funnel being narrowly spaced from the side wall of the tank, and a heat exchange element located beneath the inverted funnel adjacent the bottom of the tank.

Preferably, the rim of the inverted funnel has an upturned edge.

In a preferred system, the apparatus includes a secondary storage tank mounted within and near the top of the primary first-mentioned storage tank, the upper end of the stem of the inverted funnel opening into the secondary tank toward the top thereof, the secondary tank also communicating with the primary tank through a passage means leading upwardly from the bottom of the secondary tank towards the top of the primary tank.

Thus, the secondary tank is incorporated into the circulatory flow of hot liquid in the primary tank, the passage means acting as a high temperature thermal separator enabling the secondary tank to retain and store the hottest liquid therein while permitting the inverted funnel to continue its function of optimising heat exchange efficiency by acting as a low temperature thermal separator.Thus, in use, heated liquid rises upwardly through the funnel stem into the secondary tank, displacing the coldest liquid from the bottom of this secondary tank (which coldest liquid subsequent to an initial period of operation will be substantially hotter than the bulk of liquid in the primary tank) upwardly through the passage means into the primary tank, wherein this displaced hot liquid returns to the previously described circulatory flow, entraining colder liquid in its downward movement towards the outside face of the inverted funnel cone, whereat the hot liquid and the entrained colder liquid tend to separate, the colder liquid flowing into the space beneath the funnel where the heat exchange element is located.

The provision of the secondary tank ensures that hot liquid is available at the earliest possible time after the heat exchange system is brought into operation. Furthermore, when hot liquid is drawn from this secondary tank, it is replenished by a reverse flow of liquid downwardly through the passage means from the top of the primary tank. This replenishment liquid is thus also relatively hot. The arrangement thereby also enables almost the whole of the hot liquid in the system to be drawn off, when a large supply is required, with minimum dilution by cold liquid which is supplied, in order to replenish the primary tank, directly to the underside of the inverted funnel where the heat exchange element is located.As previously explained, this maximises the efficiency of heat exchange, especially in the case when the primary source of heat is substantially continuous but has a limited capability for heating the second liquid passed through the heat echange element, as for example in the case of a solar power unit, or in the case of a second liquid constituted by the waste supply of an industrial unit.

A practical example of the heat exchange apparatus and the method exchange in accordance with the invention will now be described with reference to the accompanying drawings, in which: Figure 1 shows one simple arrangement of heat exchange apparatus diagrammatically in vertical section; Figure 2 is a horizontal sectional view of a preferred bulk hot water storage arrangement, on the line A-A in Figure 1; Figure 3 is a vertical sectional view of the arrangement, on the line A-A in Figure 2; Figure 4 is a perspective view of the hot water supply tank in the arrangement of Figures 2 and 3; Figure 5 is a diagrammatic view of a thermostatically controlled heat exchange apparatus; Figure 6 shows heat exchange apparatus having dual heating elements; and Figure 7 shows a heat exchanger for heat extraction from industrial liquid waste.

Figure 1 shows a water tank 10. This is assumed to be the conventional hot water supply tank of a domestic water system, having at its bottom a cold water inlet 11 from a cold water storage tankfedfrom the supply main. However, instead of being heated by an electric immersion heater or by circulation through a boiler, it is further assumed that the water in the tank 10 is being heated by heat exchange with hot liquid (usually also water) derived from a solar heating system. This hot liquid is circulated in closed circuit through a solar heat absorption means whereat the liquid is heated and through a heat exchange whereat heat is extracted from the liquid and passed to the water in the storage tank 10.

The present invention concerns this heat exchanger.

A flat heat exchange element 12, such for example as a flat spiral coil, is located at the bottom of the tank. The inlet and outlet 1 3 of the exchange element 12 to the solar heating system are also at the bottom of the tank 10. Immediately above the element 12, an inverted stemmed funnel 14 is mounted inside the tank. This funnel 1 4 is located on a vertical axis, with its stem 1 5 extending upwardly on the centre axis of the tank for at least one half of the height of the tank, and with its cone 1 6 occupying substantially the whole area of the bottom of the tank except for a small peripheral clearance, where a peripheral passage 1 7 is formed between the rim of the cone 16 and the side wall of the tank 10.The rim 17 of the funnel cone 1 6 has a small upturned edge 1 8.

The method of heat exchange is as follows.

Water in the tank 10 is heated at the element 12 beneath the funnel 14. This heated water rises up the inside of the stem 15 of the funnel, and is then turned backwardly at the top of the tank, descending outside the stem, in a region near the side wall of the tank, towards the outside face of the funnel cone 16. In course of this descent, the molecules of heated water entrain molecules of colder water.The mixture of heated and colder water impinges the outside face of the funnel cone 1 6 where, due to their differing densities, the heated water molecules and the colder water molecules are primarily separated, the heated water being diverted inwardly and upwardly again, close to the outside surface of the funnel stem 15, and the colder water running down the outside face of the funnel cone 1 6 and down through the peripheral passage 1 7 into the region beneath the funnel where the heat exchange element 12 is located.

In Figure 1, the path of the heated water is indicated by the dashed lines 1 9 and the entrained cold water path by the dash-dot lines 20.

Thus, in use, the effect of the funnel is to promote a continuous circulation of the water in the tank 10, in a manner which causes the colder portion of the water to be diverted into contact with the heat exchange element. Heat exchange is therefore effected more efficiently, and the maximum possible amount of heat is extracted from the hot liquid circulating in the element 12.

The arrangement also has the advantage that when hot water is drawn from the tank 10, cold water to refill the tank enters at the inlet 11, directly into the region where the heating element 1 2 is located. Heat extraction under this condition is thereby also maximised.

In general, as compared to the conventional heat exchange arrangement previously described, it is possible to maintain a temperature differential between the input and output sides of the heat exchanger which is increased by at least several degrees Centigrade, even when the water in the tank 10 is relatively close to that of the solar heating liquid. In consequence, it is possible to obtain a higher maximum temperature of the water in the tank before the heat loss from the tank increases to a level corresponding to the rate of heat extraction from the heating liquid. For example, a maximum water temperature of about 65 degrees Centigrade may be attained with solar heating liquid entering the exchanger at 70 degrees Centigrade.

Although the above-described example has been described with refernece to a hot water system having a conventional hot water storage tank, the same heat exchange apparatus can be employed in a storage tank supplied direct from the water main. In the latter case, the cold water input to the tank can be controlled by a conventional ballcock valve, but the feed pipe must lead directly into the bottom of the tank where the heat exchange element is located.

Furthermore, the above-described apparatus and method also have applicability in other fields where a first liquid is to be heated by heat exchange with a hot second liquid, especially in cases when, as with the solar heating system above referred to, the capacity for heating the second liquid can, at least for part of the time, exceed the heat exchange capability of the heat exchanger. This can be true, for example, in industrial plants issuing waste hot fluid which is utlised in a heat exchanger before discharge.

Referring now to Figures 2 to 4, there is shown a primary water storage tank 21 having a mains water cold feed 25 controlled by a ballcock 26. A flat heat exchange element 27, for example a flat coil passing liquid heated from a solar power unit, is located at the bottom of the tank 21. A low temperature thermal separator in the form of an inverted stemmed funnel 22 is positioned in the tank above the heat exchange element 27, forming a peripheral passage 37 between the edge of the funnel cone and the wall of the tank.

The cold water feed 25 has an outlet beneath the funnel 22, adjacent the heat exchange element 27. The pipe 24 forming the stem of the inverted funnel 22 extends upwardly through the primary tank 21 to enter the bottom of a hot water supply secondary tank 23 fixed within the primary tank 21 at the top thereof, the bottom of the tank 23 being sealed around the pipe 24. The secondary tank 23 has an outlet pipe 28 for hot water supply, together with a high temperature thermal separator in the form of a double end wall arrangement 33 (see Figure 4), which defines a passage for water flow from the bottom of the secondary tank upwardly back into the primary tank, or vice versa.

In Figure 3, the right-hand part of the drawing shows thermal water currents existing in the water storage system under normal heating-up conditions, when hot water is not being drawn off.

The left-hand part of the Figure shows the thermal water currents when hot water is being drawn off at 28, and the system is being replenished by cold water at 25. The dash lines indicate high temperature thermal currents, the dash-dot lines indicate medium temperature thermal currents and the dash-circle lines indicate low temperature thermal currents.

The action of the low temperature thermal separator constituted by the inverted stemmed funnel to maximise the efficiency of heat exchange is explained above in relation to Figure 1. The same action applies in relation to the funnel 22 in Figures 2 to 4.

The effect of the high temperature thermal separator 33 at the secondary tank 23 is to separate the hottest water from the circulatory flow through and around the funnel 22, and to confine this hottest water in the secondary tank 23 ready for use. It will be noted that, when hot water is drawn off at 28, the secondary tank is replenished by the highest temperature water available in the primary tank, generally in the region thereof referenced 29, this replenishment water flowing downwardly through the passage means 33, contrary to the direction of flow of the high temperature thermal current therethrough during normal heating-up conditions.

The bulk water supply arrangement described with reference to Figures 2 to 4 has the advantages that usable hot water is available in the shortest possible time after the commencement of heating, and that the maximum amount of hot water can be drawn off with minimum dilution by cold water.

Figure 5 shows the heat exchange apparatus of Figure 1, and indicates the minimum vertical separation S which is desirable between the heat exchange element 42 and the bottom of the funnel 43. S has a minimum value of 1 50 mm. If, as indicated in Figure 5, the heat exchange element 42 is an electric heating element, rather than a coil passing heated liquid derived from a solar power source, a pair of thermostats 45 at differing levels of the tank 24 may be employed selectively to control the operation of the heating element 42 according to the amount of hot water required for use, thus achieving increased efficiency and cost saving as compared with the conventional use of two separate electric heating elements at differing levels.

Figure 6 shows the heat exchange apparatus of Figure 1 used in conjunction with two heat sources for alternative use, the first a heat exchange element 52 passing heated liquid from a solar power source and the second a heating element 53 deriving heat from an alterantive source such as a gas or oil or solid fuel boiler. The latter element 53 can be brought into operation when the solar power source is inoperative or providing insufficient energy.

Finally, Figure 7 shows a modification of the arrangement of Figures 2 to 4, applied to heat exchange in a heat recovery sphere with hot fluid waste derived from an industrial plant. The hot waste fluid is passed through the heating coil 75 and a secondary tank 76 for hot water storage and supply is located at the top of the heat recovery sphere 77, in association with a high temperature thermal separator 78. The inverted stemmed funnel constituting the low temperature thermal separator is designated 79, whilst the reference 80 indicates a vent pipe.

Claims (12)

1. Heat exchange apparatus comprising an inverted stemmed funnel located on a vertical axis within a storage tank containing a liquid to be heated, the rim of the funnel being narrowly spaced from the side wall of the tank, and a heat exchange element located beneath the inverted funnel adjacent the bottom of the tank.

2. Apparatus according to claim 1, wherein the heat exchange element is a pipe through which passes a hot second fluid for heating the liquid in the storage tank.

3. Appatatus according to claim 2, wherein the said pipe is connected to a solar heating unit andpasses a liquid which has been heated within said unit.

4. Apparatus according to claim 1 or claim 2 or claim 3, wherein the rim of the inverted funnel has an upturned peripheral edge.

5. Apparatus according to any of claims 1 to 4, including a secondary storage tank mounted within and near the top of the primary firstmentioned storage tank, the upper end of the stem of the inverted funnel opening into the secondary tank towards the top thereof, the secondary tank also communicating with the primary tank through a passage means leading upwardly from the bottom of the secondary tank towards the top of the primary tank.

6. Apparatus according to claim 5, including a cold inlet for the liquid to be heated into the primary storage tank on the underside of the funnel and a hot outlet from the upper part of the secondary storage tank.

7. Apparatus according to any of claims 1 to 6, wherein the heating element is spaced a minimum distance of 150 mm below the level of the rim of the funnel.

8. A method of heat exchange according to which a heating element heats a liquid to be heated in the lowermost region of a storage tank beneath an inverted stemmed funnel, whereby the heated liquid is caused to rise through the funnel stem and is then turned back downwardly on the outside of the stem in a main circulatory flow which entrains colder liquid in its downward movement, the funnel cone acting as a barrier to said circulatory flow in order to separate the heated liquid from the entrained colder liquid, and the rim of the funnel is narrowly spaced from the side wall of the tank to define a peripheral passage through which the separated colder liquid can pass from the outside face of the funnel cone into the lowermost region of the tank.

9. A method according to claim 8, according to which the heated liquid is circulated. through a secondary tank after emerging from the top of the funnel stem and before being turned back downwardly in the main circulatory flow.

10. A method according to claim 9, according to which the heated liquid emerges into the upper region of the secondary tank from the top of the funnel stem and is diverted downwardly through the secondary tank to emerge from the lower region of said secondary tank through a passage means exiting near the top of the main storage tank.

11. Heat exchange apparatus substantially as hereinbefore described with reference to Figure 1 or Figures 2 to 4 or Figure 5 or Figure 6 or Figure 7 of the accompanying drawings.

12. A method of heat exchange substantially as hereinbefore described with reference to Figure 1 or Figures 2 to 4 or Figure 5 or Figure 6 or Figure 7 of the accompanying drawings.