IE20100096A1 - Heat recovery system - Google Patents

Abstract

A flue gas heat recovery system having a liquid thermal store, circulated through a heat exchanger to recover heat from flue gases from a boiler or the like. in aspects of the invention, liquid from the thermal store may be used to pre-heat water entering a waterheating boiler, to may be fed to an underfloor heating system. <Figure 1>

Description

The invention relates to methods and apparatus for recovery of heat from flue gases of boilers, particularly domestic hot water and heating systems.

Background Heat exchangers for recovery of waste heat from flue gases are known. These are used to recover heat from flue gases (e.g. from a boiler fuelled by gas, oil or some other combustible fuel), primarily for use in the production of hot water.

Current forms of such flue gas heat recovery systems and storage for hot water utilise an indirect connection, with the primary water circulating within and through the flue gas heat recovery heat exchanger being used to heat a store of water via a second heat exchanger (typically a coil within the store, through which the primary water circulates). This prewarmed water is then used as the inlet to a substantially conventional boiler where it is further heated to the desired temperature, to emerge as domestic hot water.

Inventive Concept The present invention concerns the direct connection of a thermal store, in the form of e.g. a cylinder (or tank) of a primary heat recovery liquid such as water, to a boiler flue gas heat recovery system. The liquid comprising the thermal store is, itself, circulated through the flue gas heat recovery heat exchanger, thereby raising the temperature of the thermal store as heat is recovered from the flue gases.

Heated liquid (the primary heat recovery liquid) from the thermal store may then be used to ‘instantaneously” pre-heat (e.g. via a heat exchanger, .and preferably a plate heat exchanger) incoming domestic cold water feeding a hot water boiler, especially a socalled “combination boiler”. Where the boiler is a combination boiler, the domestic cold water feed usually comprises cold water at mains pressure. ft 1 Ο Ο ο 9 6 The thermal store may also be heated by auxiliary heat sources such as solar panels, wood burning stoves, heat pumps and the like.

Amongst the advantages of such a configuration are that: The use of a primary thermal store allows higher storage temperatures to be safely achieved. Furthermore, the heat store can be open-vented, with the overall installation still providing mains-pressure domestic hot water, thereby avoiding the use of an unvented (and therefore pressurised) storage system, requiring greater engineering complexity and the need to meet the applicable regulatory requirements. Such an openvented thermal store may be maintained in a full condition by the provision of a feed and expansion (“F & E”) tank. By use of a thermal store in the form of a cylinder, or tank, such direct connection allows the colder water from base of thermal store to circulated to flue heat recovery system, thereby maximising the temperature difference between this primary circuit and the flue gases, thereby ensuring maximum efficiency of recovery system. The liquid in the thermal store can be non-potable comprising e.g. water with corrosion inhibitors, or could be a liquid with a high thermal heat capacity such water with dissolved solutes, oil, or another non-aqueous liquid.

Summary of the Invention Accordingly, the invention provides a flue gas heat recovery system comprising: a first heat exchanger configured to recover heat from boiler flue gases; a thermal store comprising a tank to hold a primary heat recovery liquid; and a first circulation means to circulate said heat recovery liquid through said first heat exchanger.

In a first subsidiary aspect, the system further comprises: a second heat exchanger configured to transfer heat from said primary heat recovery liquid to water entering a boiler; and a second circulation means to circulate said heat recovery liquid through said second heat exchanger. Preferably, said second circulation means is configured to withdraw primary heat recovery liquid from a top region of said tank.

In a second subsidiary aspect, the system further comprises: a second circulation means configured to circulate said heat recovery liquid through an underfloor heating system.

IE 1 0 0 0 96 In any aspect of the invention, it is preferable that said tank is vented.

Also included within the scope of the invention is a flue gas heat recovery system substantially as described herein, with reference to, and as illustrated by any appropriate combination of the accompanying drawings.

Brief Description of the Drawings The invention will be described with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of a flue gas heat recovery system, pre-heating a water feed to a boiler; Figure 2 is a schematic diagram of a flue gas heat recovery system with a boiler by-pass diverter; Figure 3 is a schematic diagram of a flue gas heat recovery system augmented by solar capture; Figure 4 is a schematic diagram of a flue gas heat recovery system providing heat to an underfloor heating system; and Figure 5 is a schematic diagram of an alternative arrangement of a flue gas heat recovery system augmented by solar capture.

Description of Preferred Embodiments Figures 1,2 and 3 illustrate, schematically, embodiments of the invention.

Figure 1 shows a heating system, generally indicated by 1. The system comprises a thermal store in the form of a tank 2 containing a primary heat recovery liquid 3. For domestic installations, the tank 2 would typically hold of the order of 200 litres, although tanks as small as 50 litres would also provide a useful heat store for smaller installations. For larger installations, and also those using solar coliection to heat the primary heat IE 1 0 0 0 96 recovery liquid 3 (described below), then tank volumes of up to 500 litres, or even up to 1000 litres are preferred.

A first circulation means 4 in the form of a pump is used to circulate the primary heat recovery liquid 3 through a first heat exchanger 5 configured to transfer heat from flue gases emitted by a boiler 6. Although illustrated with the inlet connection entering at the base of the heat exchanger 5, the connection between the fluid path and the heat exchanger may be made at any location on the unit. The system may be configured to recover heat from the flue gases of boilers such as gas-, oil- or solid fuel-fired central heating or hot water boilers, from wood-burning stoves, and from gas-, oil-, or solid fuel10 fired cooking ranges such as those sold in the UK under the registered trademarks AGA and Rayburn. The system is most efficiently arranged such that the circulation means 4 draws water from the base region of the tank 2, which would generally be at a lower temperature than other regions, thereby giving the greatest temperature difference between the heat recovery liquid and the flue gases, leading to the most effective heat transfer. After passing through the heat exchanger 5, the heated heat recovery liquid 3 is then returned to the tank 2. The liquid may be returned to the top of the tank 2 (which would generally be a higher temperature), but it is preferred that it is returned at a middle region of the tank 2, as illustrated. Returning the heated liquid at this point reduces the risk of temperature stratification of the primary heat recovery liquid 3 in the tank 2. In other embodiments of the invention the first circulation means 4 may be provided by arranging the tank 2 and flue gas heat exchanger 5 such that flow occurs by passive circulation.

A second circulation means 7 in the form of a pump is provided to circulate the heated primary heat recovery liquid 3 through a second heat exchanger 8, configured to transfer heat to a cold water feed 9 to be used as input to the boiler 6 for the production of hot water. If required, the cold water feed 9 may be at “mains pressure”, thereby providing high pressure hot water for use by the consumer. The second heat exchanger is preferably in the form of a plate heat exchanger, having good heat transfer characteristics in a compact volume. Control apparatus is preferably provided to activate the second circulation means 7 only when there is a demand for hot water in the system, thereby reducing energy costs and wear on the pump.

The thermal store tank 2 may be open-vented, thereby allowing a relatively simple construction. A header tank 10 is provided to maintain liquid in the thermal store tank 2 IE 1 Ο 0 0 96 via a top-up pipe 16. A typical arrangement of such a header tank would include a float valve arrangement II fed by a water supply. An expansion, or pressure relief, connection 12 may usefully be provided between the thermal store tank 2 and the header tank 10.

Additionally, inlet 13 and outlet 14 ports may be provided in the thermal store tank to allow auxiliary heat sources such as heat pumps and wood burning stoves to be used to augment the heat input into the thermal store. Electric heaters may also be mounted in the tank 2 to provide auxiliary heating of the heat recovery liquid if required. Such heaters might usefully he powered by e.g. domestic wind turbines.

Figure 2 shows the configuration of Figure 1 with the additional of a diverting/mixing valve 15 to allow the water flow to be diverted past the boiler in the event that sufficient heat input is provided to the cold water 9 from the thermal store.

Figure 3 shows an alternative embodiment in which cold water from a storage tank 17 (rather than mains pressure hot water) is pre-heated by the system before entering the boiler. A thermostatic mixing valve 18 is provided to mix cold water with the hot water emerging from the boiler 6 so that the water temperature is not too high, especially when water is first drawn from the system.

Also in this embodiment, a further heat capture system, in the form of a solar panel 19, is used to augment the heat stored in the thermal store. Heat recovery liquid 3 from the thermal store tank 2 is circulated through the solar panel 19 by circulation means such as a pump 20. Heat recovery liquid 3 is drawn from a an outlet port 14 in the base region of the tank 2 and circulated through the solar panel 19 before being returned to an upper region of the tank through an inlet port 13. A non-return valve 21 is provided to prevent back-flow of heated heat recovery liquid into the solar panel 19 when the pump 20 is not operational.

Temperature sensors 22, 23 and 24 may be provided in the thermal store tank 2 and solar panel 19, to enable a control system (not illustrated) to optimise the circulation of heat recovery liquid 3 though the solar panel 19 dependent on the prevailing environmental conditions.

Also illustrated, schematically, in Figure 3 is a central heating system 25, also supplied by the boiler 6.

IE 1 0 0 0 96 Figure 4 illustrates, schematically, a heat recovery system, generally indicated by 1, ofthe same basic configuration as described for Figure 2. In addition, the system is configured such that heat recovery liquid 3 from thermal storage tank 2 may be circulated through an underfloor heating system by means of an additional circulation means such as a pump. A mixing valve 27 is provided to allow the temperature of water being fed to the underfloor heating system to be regulated. The heat recovery system is particularly appropriate for this type of application, as underfloor heating systems function well with consistent but relatively low-level heat input that can be provided by such a heat recovery system. Furthermore, the relatively continuous heat dissipation into the underfloor heating system 25 (as opposed to the more sporadic heat dissipation for the generation of domestic hot water) reduces the temperature of the primary heat recovery liquid 3 in the thermal store, thereby increasing the efficiency of heat recovery by the flue gas heat exchanger 5.

Figure 5 illustrates, schematically, a heat recovery system configured as that shown in Figure 3, except using an alternative mode of supplying auxiliary heat to the heat store 2.

In this configuration, heat from an auxiliary source such as a solar panel 19, as illustrated, or from a wood-burning stove, heat pump or the like is transferred to the primary heat recovery liquid 3 by passing a second heat transfer liquid, in thermal contact with the auxiliary heat source, through a coil 28, or other heat exchange unit located in thermal contact (e.g. immersed within) the primary heat recovery liquid 3.

Claims (6)

CLAIMS:

1. A flue gas heat recovery system comprising: a first heat exchanger configured to recover heat from boiler flue gases; a thermal store comprising a tank to hold a primary heat recovery liquid; and a first circulation means to circulate said heat recovery liquid through said first heat exchanger.

2. A system according to claiml, further comprising: a second heat exchanger configured to transfer heat from said primary heat recovery liquid to water entering a boiler; and a second circulation means to circulate said heat recovery liquid through said second heat exchanger.

3. A system according to claim 2 wherein said second circulation means is configured to withdraw primary heat recovery liquid from a top region of said tank.

4. A system according to claim 1, further comprising: a second circulation means configured to circulate said heat recovery liquid through an underfloor heating system.

5. A system according to any preceding claim wherein said tank is vented.

6. A flue gas heat recovery system substantially as described herein, with reference to, and as illustrated by any appropriate combination of the accompanying drawings.