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Title:
HEAT EXCHANGER
Document Type and Number:
WIPO Patent Application WO/2016/120590
Kind Code:
A1
Abstract:
Heat exchange apparatus (100) such as a condensing boiler may comprise a gas heating stage comprising a liquid source and an evaporator heat exchanger (135) configured to heat a first gas stream (e.g. boiler inlet air) flowing from a first gas inlet to a first gas outlet, the evaporator heat exchanger being configured to evaporate liquid from the liquid source (139) to add vapour to the first gas stream, and, a gas cooling stage comprising a condenser heat exchanger (235) configured to cool a second (e.g. different) gas stream(e.g. boiler exhaust gas) flowing from a second gas inlet to a second gas outlet, the condenser heat exchanger being configured to condense vapour present in the second gas stream, and preferably to direct condensed vapour into a or the liquid source (239). The evaporator heat exchanger (135) transfers thermal energy from the liquid source to the first gas stream. The condenser heat exchanger (235) transfers thermal energy from the second gas stream to the liquid source. In each heat exchanger, the liquid source may be a liquid stream that flows in counter-flow to the gas stream, and the same liquid stream may flow through both heat exchangers in an open or closed circuit (140, 142) such that such that the liquid stream acts as a heat (or heat and mass) transfer fluid between the two heat exchangers (135, 235). In this way, thermal energy may be transferred between the two respective gas streams by the same liquid stream. The apparatus thus acts as a gas-to-gas heat exchanger comprising a gas-to-liquid heat exchanger configured to cool a first gas stream and a further gas-to-liquid heat exchanger configured to heat a second gas stream. Each of the evaporator heat exchanger (135) and condenser heat exchanger (235) may comprise a porous structure such as an open-cell foam structure (130).The porous structure may be rotatably mounted and partially submerged in the liquid stream, which may flow in counter-flow to the gas stream above it. The structure may comprise an elongate porous structure rotatable about its lengthwise axis, the liquid and gas streams each flowing parallel with that axis such that the heat exchange process causes a temperature gradient to develop along the length of the porous structure. The porous structure may have an average effective pore diameter of at least 3mm,such large pores being resistant to blocking, such that an annular seal against the circumferential outer wall/perimeter of the porous structure is not required.

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Inventors:
MACNAGHTEN, James (7 Brunel WaySegensworth East,Fareham, Hampshire PO15 5TX, PO15 5TX, GB)
Application Number:
GB2016/050091
Publication Date:
August 04, 2016
Filing Date:
January 15, 2016
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
ENERGY TECHNOLOGIES INSTITUTE LLP (Holywell Building, Holywell WayLoughborough, Leicestershire LE11 3UZ, PO15 5TX, GB)
International Classes:
F28C3/08; F24H8/00; F24H9/00; F28D19/04; F28D21/00; F28F13/00
Foreign References:
GB1504385A1978-03-22
FR2775338A11999-08-27
SU1481580A11989-05-23
SU992998A11983-01-30
GB191002072A1911-01-19
GB2031572A1980-04-23
DE2548096A11977-05-12
Attorney, Agent or Firm:
LOWTHER, Deborah Jane et al. (Cleveland, 10 Fetter Lane, London EC4A 1BR, SE1 9BA, GB)
Download PDF:
Claims:
Claims:

1. A heat exchanger comprising:

a porous structure defining a network of interconnecting voids extending from an entrance region of the porous structure to an exit region of the porous structure to define an inner surface extending through the porous structure between the entrance region and the exit region;

a gas inlet for exposing a region of the inner surface to gas from a gas stream during a first phase of operation; and

a liquid applicator for applying liquid from a liquid stream to the region of the inner surface during a second phase of operation.

2. A heat exchanger according to claim 1, wherein the heat exchanger is configured to: in the first phase of operation expose a first region of the inner surface defining a first path through the porous structure to gas from the gas stream and apply liquid from the liquid stream to a second region of the inner surface, the second region of the inner surface defining a second path through the porous structure extending in parallel to the first path; and

in the second phase of operation to expose the second region of the inner surface to gas from the gas stream and apply liquid from the liquid stream to the first region of the inner surface.

3. A heat exchanger according to claim 1 or claim 2, wherein the porous structure comprises an open-cell foam structure. 4. A heat exchanger according to claim 1 or claim 2, wherein the porous structure comprises a chamber for receiving gas, and particulate material housed in the chamber.

5. A heat exchanger according to any of the preceding claims, wherein the heat exchanger is configured to provide substantially direct heat transfer between liquid deposited on the inner surface and gas from the gas stream.

6. A heat exchanger according to claim 5, wherein the inner surface is a liquid attracting surface.

7. A heat exchanger according to any of claims 1-4, wherein the porous structure is configured to exchange thermal energy with the liquid applied from the liquid stream.

5 8. A heat exchanger according to claim 7, wherein the heat exchanger is configured to provide substantially indirect heat transfer between the liquid applied to the inner surface and gas from the gas stream.

9. A heat exchanger according to claim 8, wherein the inner surface is a liquid repelling 10 surface.

10. A heat exchanger according to any of the preceding claims, wherein the porous structure is configured to rotate relative to at least one of the gas inlet and liquid applicator.

15 11. A heat exchanger according to any of the preceding claims, wherein the liquid applicator comprises a reservoir through which the liquid stream flows and the porous structure extends into the reservoir and is partially submerged in the liquid stream.

12. A heat exchanger according to claim 11, wherein the reservoir includes a drain valve 20 for maintaining a predetermined liquid level in the reservoir.

13. A heat exchanger according to claim 11 or claim 12, wherein the reservoir includes a fresh liquid inlet.

25 14. A heat exchanger according to any of the preceding claims, wherein the liquid applicator comprises a spraying or droplet dispensing device.

15. A heat exchange according to any of the preceding claims, wherein the porous structure is movable relative to the liquid applicator and the heat exchanger further comprises 30 at least one liquid collector for collecting liquid as the porous structure moves past the liquid applicator and releasing water into the porous structure during subsequent movement of the porous structure.

16. A gas-to-gas heat exchanger comprising:

a heat exchanger as defined in accordance with any of the preceding claims; and a further heat exchanger comprising:

a further porous structure defining a network of interconnecting voids extending from an entrance region of the further porous structure to an exit region of the further porous structure to define a further inner surface extending through the further porous structure between the entrance region and the exit region of the further porous structure;

a further gas inlet for exposing a region of the further inner surface to gas from a further gas stream during a third phase of operation; and

a further liquid applicator for applying liquid from the liquid stream to the region of the inner surface during a fourth phase of operation.

17. A gas-to-gas heat exchanger according to claim 16, wherein the first-defined heat exchanger operates as a gas-to-liquid heat exchanger configured to cool the first-defined gas stream and the further heat exchanger operates as a gas-to-liquid heat exchanger configured to heat the further gas stream.

18. A gas-to-gas heat exchanger according to claim 16 or claim 17, wherein the further heat exchanger is configured to:

in the third phase of operation expose a first region of the further inner surface defining a first path through the further porous structure to gas from the further gas stream and apply liquid from the liquid stream to a second region of the further inner surface, the second region of the further inner surface defining a second path through the further porous structure extending in parallel to the first path therethrough; and

in the fourth phase of operation to expose the second region of the further inner surface to gas from the further gas stream and apply liquid from the liquid stream to the first region of the further inner surface.

19. A gas-to-gas heat exchanger according to claim 18, wherein the first and second phases occur at a first pressure and the third and fourth phases occur at a second pressure different to the first pressure.

20. A gas-to-gas heat exchanger according to any of claims 16-19, wherein the heat exchanger further comprises a heat removal stage for cooling the liquid stream as it passes from the first-defined heat exchanger to the further heat exchanger.

21. A condensing boiler comprising an air inlet, a combustor, a primary heat exchanger 5 thermally coupled to a heating circuit, and a gas exhaust, the condensing boiler further comprising a secondary heat exchanger comprising: a condenser heat exchanger stage configured to cool exhaust gas passing from the primary heat exchanger to the gas exhaust and condense vapour present in the exhaust gas; and an evaporator heat exchanger stage configured to pre-heat incoming air passing from the air inlet to the combustor and evaporate 10 liquid to add vapour to the incoming air.

22. A condensing boiler according to claim 21, wherein the secondary heat exchanger is a gas-to-gas heat exchanger as defined in any of claims 16-20.

15 23. A gas heating stage comprising:

a liquid source; and

an evaporator heat exchanger configured to heat a gas stream flowing from a gas inlet to a gas outlet, the evaporator heat exchanger being configured to evaporate liquid from the liquid source to add vapour to the gas stream.

0

24. A gas heating stage according to claim 23, wherein the evaporator heat exchanger is configured to transfer thermal energy from the liquid source to the gas stream.

25. A gas heating stage according to claim 24, wherein the evaporator heat exchanger is a 5 heat exchanger as defined in any of claims 1-15.

26. A gas cooling stage comprising: a condenser heat exchanger configured to cool a gas stream flowing from a gas inlet to a gas outlet, the condenser heat exchanger being configured to condense vapour present in the gas stream.

30

27. A gas cooling stage according to claim 26, wherein the gas cooling stage further comprises a liquid source and the condenser heat exchanger is configured to transfer thermal energy from the gas stream to the liquid source.

28. A gas cooling stage according to claim 27, wherein the condenser heat exchanger is a heat exchanger as defined in any of claims 1-15. 29. Apparatus comprising:

a gas heating stage comprising:

a liquid source;

an evaporator heat exchanger configured to heat a first gas stream flowing from a first gas inlet to a first gas outlet, the evaporator heat exchanger being configured to evaporate liquid from the liquid source to add vapour to the first gas stream; and

a gas cooling stage comprising:

a condenser heat exchanger configured to cool a second gas stream flowing from a second gas inlet to a second gas outlet, the condenser heat exchanger being configured to condense vapour present in the second gas stream.

30. Apparatus according to claim 29, wherein the evaporator heat exchanger is configured to transfer thermal energy from the liquid source to the first gas stream.

31. Apparatus according to claim 30, wherein the condenser heat exchanger is configured to transfer thermal energy from the second gas stream to the liquid source. In one embodiment, the gas cooling stage is configured to direct condensed vapour into the liquid source.

32. Apparatus according to any of claims 29-31, wherein at least one of the evaporator heat exchanger and condenser heat exchanger is a heat exchanger as defined in any of claims

1-15.

33. Apparatus according to claim 32, wherein the evaporator heat exchanger and condenser heat exchanger together form a gas-to-gas heat exchanger as defined in any of claims 16-20.

34. A method of exchanging heat using a heat exchanger comprising a porous structure defining a network of interconnecting voids extending from an entrance region of the porous structure to an exit region of the porous structure to defined an inner surface extending through the porous structure between the entrance region and the exit region, the method comprising: during a first phase of a cycle exposing a first region of the inner surface defining a first path through the porous structure to gas from a gas stream and applying liquid from a liquid stream to a second region of the inner surface, the second region of the inner surface defining a second path through the porous structure extending in parallel to the first path; and during a second phase of the cycle exposing the second region of the inner surface to gas from the gas stream and applying liquid from the liquid stream to the first region of the inner surface.

35. A method of exchanging heat using a heat exchanger according to claim 34, wherein the method comprises using a further heat exchanger comprising a further porous structure defining a network of interconnecting voids extending from an entrance region of the further porous structure to an exit region of the further porous structure to define a further inner surface extending through the further porous structure between the entrance region and the exit region of the further porous structure, the method comprising:

during a third phase of the cycle exposing a first region of the further inner surface defining a first path through the further porous structure to gas from a further gas stream and applying liquid from the liquid stream to a second region of the further inner surface, the second region of the further inner surface defining a second path through the further porous structure extending in parallel to the first path therethrough; and

during a fourth phase of the cycle exposing the second region of the further inner surface to gas from the further gas stream and applying liquid from the liquid stream to the first region of the further inner surface.

36. Apparatus or method substantially as hereinbefore described with reference to any of Figures 3 to 8 of the accompanying drawings.

Description:
TITLE: HEAT EXCHANGER

DESCRIPTION

The present invention relates to a heat exchanger and to a method of exchanging heat using a heat exchanger.

Current gas-to-liquid or gas-to-gas heat exchangers offer a compromise between pressure drop (primarily on the gas side of the circuit), heat transfer and size/cost. For example, diffusion-bonded heat exchangers are very compact and can achieve very good heat transfer with temperature differences between two gases in counterflow approaching 1- 2°C of each other. However, this small temperature difference is only achieved by having a large pressure drop (for example 1 bar pressure drop) in the gases passing through the heat exchanger. Alternative designs of heat exchanger can produce a much smaller pressure drop if the temperature difference between the fluids is allowed to increase to say 20-30°C. Better quality heat exchange (i.e. low pressure drop and small temperature difference) can normally only be achieved by increasing the size of the heat exchanger and reducing the flow rates through the device. This normally adds cost as more material is required for the same amount of heat exchange. Finally if there are corrosive or oxidizing components in one of the flows then it is necessary to use materials that are resistant to these components. The result is that gas-to-liquid or gas-to-gas heat exchanger choice is typically a compromise between size, materials, cost and performance.

It should be understood that the reason that the pressure drop on the gas side of a heat exchange circuit is more significant is that there is a significant energy penalty (loss) associated with this pressure drop unless the gas circuit is at very high pressures. Where a pressure drop occurs on the liquid side of the circuit, the energy penalty (loss) associated with the same pressure drop is normally much lower. Consequently it is possible to get liquid-to-liquid heat exchangers that are both low cost and have small temperature differences between the two flows (e.g. as provided by a plate heat exchanger).

There is a link between pressure drop and heat exchange that is described by Reynold's analogy. This applies only to the flow in a pipe or duct and it broadly predicts that there is a direct correlation between the amount of heat exchange occurring and the amount of pressure drop. It is often thought to apply generally to heat exchange problems, however there is a class of heat exchange where it does not apply: heat exchange in a chaotically packed media, such as a loosely packed bed of particles or a porous foam matrix. In this situation the relationship between pressure drop and heat exchange can be more favourable as the chaotic nature of the structure ensures that there is constant mixing of the fluid as a result of the fluid passing through the structure, even if the flow is not turbulent. This means that it is theoretically possible to get higher quality heat exchange without increasing the size of the heat exchanger.

The problem with a chaotically packed structure is that it is difficult to build a practical heat exchanger that will transfer heat between a liquid and a gas. If the heat exchange is from the gas to the packed structure, then it is difficult to get the heat from this structure to the fluid. For example with open cell metal foams it has been proposed that they can act as heat exchangers with metal pipes bonded into the foam. In this way the heat from a gas can be passed to the metal foam by direct contact and then from the metal foam by conduction to the pipe and then to the fluid. The problem with such a heat exchanger is that the nature of the structure means that the conduction paths are not optimal unless there is a very large quantity of pipes through the foam. At this point the benefits of the foam have been largely lost and the complexity and cost of the heat exchanger has greatly increased. It should be understood that foams, whether metallic, ceramic or polymer are generally quite simple to manufacture. Adding complicated structures to these foam greatly increases the complexity and cost of the manufacturing process.

The present applicant has identified the need for a gas-to-liquid heat exchanger that is relatively compact, has high efficiency, low cost and that can be designed to handle corrosive or oxidising flows.

In accordance with a first aspect of the present invention, there is provided a heat exchanger comprising: a porous structure defining a network of interconnecting voids extending from an entrance region of the porous structure to an exit region of the porous structure to define an inner surface extending through the porous structure between the entrance region and the exit region; a gas inlet for exposing a region of the inner surface to gas (e.g. vapour) from a gas stream (e.g. vapour stream) during a first phase of operation; and a liquid applicator for applying (e.g. depositing) liquid from a liquid stream to the region of the inner surface during a second phase of operation.

In this way, a heat exchanger is provided with a gas/liquid permeable structure defining a substantial surface area over which gas from the gas stream and liquid from the liquid stream are sequentially applied to achieve heat transfer between the gas stream and liquid stream. The present invention allows construction of a gas-to-liquid heat exchanger that is relatively compact, has high efficiency, low cost and that can be designed to handle corrosive or oxidising flows. In one embodiment, the heat exchanger may be a gas-to-liquid heat exchanger configured to cool a gas stream (e.g. condenser heat exchanger). In another embodiment, the heat exchanger may be a gas-to-liquid heat exchanger configured to heat a gas stream (e.g. evaporator heat exchanger). A pair of these heat exchangers can act as a gas-to-gas heat exchanger, where the liquid acts as a heat transfer fluid between the two heat exchangers. While many combinations of fluid and gas are possible there are a significant number of opportunities where the wetting heat transfer fluid is water and the gas is air or principally air, such as air with some products of combustion.

In one embodiment the heat exchanger is configured to operate cyclically between the first and second phases of operation.

In one embodiment, the heat exchanger is configured to: in the first phase of operation expose a first region of the inner surface defining a first path through the porous structure to gas from the gas stream and apply liquid from the liquid stream to a second region of the inner surface, the second region of the inner surface defining a second path through the porous structure extending in parallel to the first path; and in the second phase of operation to expose the second region of the inner surface to gas from the gas stream and apply (e.g. deposit) liquid from the liquid stream to the first region of the inner surface.

In one embodiment, the liquid stream flows in counter-flow to the gas stream.

In one embodiment, the entrance region and exit region are provided on opposed ends of the porous structure.

In one embodiment, the porous structure has an average effective pore diameter of between 1 and 20 mm. For example, the porous structure may have an average effective pore diameter of between 3 and 10mm.

In one embodiment, the interconnecting voids of the porous structure are chaotically or randomly distributed. In this way, the porous structure may encourage mixing of fluid as gas or liquid passes therethrough even if the flow is not turbulent.

In one embodiment, the porous structure comprises an open-cell foam structure. Advantageously, the quality of the heat exchange with open-cell foams with large void fractions can be very high indeed. In one embodiment, the open-cell foam is a polymer foam (e.g. to provide a low cost solution for use in lower temperature heat transfer applications). In another embodiment, the open-cell foam is a metal or ceramic foam (e.g. for use in high temperature applications).

In another embodiment, the porous structure comprises a chamber for receiving gas, and particulate material housed in the chamber. The particulate material may comprises solid particles, mesh and/or fibres packed (e.g. loosely packed) to form a gas-permeable structure.

In one embodiment, the heat exchanger is configured to provide substantially direct heat transfer between liquid deposited on the inner surface and gas from the gas stream.

In one embodiment, the inner surface is a liquid attracting surface (e.g. wettable surface). In the case that the liquid stream comprises water, the inner surface may be a hydrophilic surface.

In one embodiment, the porous structure is configured to exchange thermal energy with the liquid applied from the liquid stream (i.e. the heat exchanger is configured to provide indirect heat transfer).

In one embodiment, the heat exchanger is configured to provide substantially indirect heat transfer between the liquid applied to the inner surface and gas from the gas stream (i.e. with the inner surface acting as a heat transfer material). In this embodiment, the inner surface may be liquid repelling surface (e.g. non-wetting surface). In the case that the liquid stream comprises water, the inner surface may be a hydrophobic surface.

In one embodiment, the porous structure is configured to move (e.g. rotate around an axis of rotation) relative to at least one of (e.g. both) the gas inlet and liquid applicator. In one embodiment, the porous structure is rotatably mounted.

In one embodiment, the porous structure is mounted on a core (e.g. fluid impermeable core).

In the case of a rotatable porous structure, the axis of rotation may extend through the core. The core may rotate with the porous structure.

In one embodiment, the liquid applicator comprises a reservoir through which the liquid stream flows and the porous structure extends into the reservoir and is partially submerged in the liquid stream. In the case of a rotatably mounted porous structure, the porous structure may be configured to rotate (e.g. continuously rotate) whilst partially submerged in the liquid stream.

In one embodiment, the reservoir includes a drain valve for maintaining a predetermined liquid level in the reservoir.

In one embodiment, the drain valve is configured to maintain a predetermined liquid level below the level of the axis of rotation.

In one embodiment, the reservoir includes a fresh liquid inlet. In this way, fresh liquid may be added to the reservoir to dilute build-up of contaminants in the liquid (with excess fluid being removed from the reservoir via the drain valve).

In another embodiment, the liquid applicator may comprise a spraying or droplet dispensing device.

In one embodiment, wherein the porous structure is movable relative to the liquid applicator (e.g. rotatable relative thereto) and the heat exchanger further comprises at least one liquid collector for collecting liquid as the porous structure moves past the liquid applicator and releasing water into the porous structure during subsequent movement of the porous structure. In this way, liquid may be distributed under gravity to improve liquid coverage. The at least one liquid collector may be mounted on the porous structure (e.g. and rotate with the porous structure).

In accordance with a second aspect of the present invention, there is provided a gas-togas heat exchanger comprising: a heat exchanger as defined in accordance with the first aspect of the invention; and a further heat exchanger comprising: a further porous structure defining a network of interconnecting voids extending from an entrance region of the further porous structure to an exit region of the further porous structure to define a further inner surface extending through the further porous structure between the entrance region and the exit region of the further porous structure; a further gas inlet for exposing a region of the further inner surface to gas from a further gas stream during a third phase of operation; and a further liquid applicator for applying (e.g. depositing) liquid from the liquid stream to the region of the inner surface during a fourth phase of operation.

In one embodiment, the first-defined heat exchanger operates as a gas-to-liquid heat exchanger configured to cool the first-defined gas stream (e.g. condenser heat exchanger) and the further heat exchanger operates as a gas-to-liquid heat exchanger configured to heat the further gas stream (e.g. evaporator heat exchanger).

In one embodiment the further heat exchanger is configured to operate cyclically between the third and fourth phases of operation.

In one embodiment, the further heat exchanger is configured to: in the third phase of operation expose a first region of the further inner surface defining a first path through the further porous structure to gas from the further gas stream and apply liquid from the liquid stream to a second region of the further inner surface, the second region of the further inner surface defining a second path through the further porous structure extending in parallel to the first path therethrough; and in the fourth phase of operation to expose the second region of the further inner surface to gas from the further gas stream and apply (e.g. deposit) liquid from the liquid stream to the first region of the further inner surface.

In one embodiment, the first and second phases occur at a first pressure and the third and fourth phases occur at a second pressure different to the first pressure. In one embodiment, a pressure changing device (e.g. pump) may be provided between the first- defined heat exchanger and further heat exchanger to change the liquid pressure between the first and second pressures.

In one embodiment, the second pressure is lower than the first pressure (e.g. substantially lower).

In one embodiment, the heat exchanger further comprises a heat removal stage for cooling the liquid stream as it passes from the first-defined heat exchanger to the further heat exchanger.

In accordance with a third aspect of the present invention, there is provided a condensing boiler comprising an air inlet, a combustor, a primary heat exchanger thermally coupled to a heating circuit, and a gas exhaust, the condensing boiler further comprising a secondary heat exchanger comprising: a condenser heat exchanger stage configured to cool exhaust gas (e.g. vapour) passing from the primary heat exchanger to the gas exhaust and condense vapour present in the exhaust gas; and an evaporator heat exchanger stage configured to pre-heat incoming air passing from the air inlet to the combustor and evaporate liquid to add vapour (e.g. moisture in the case of water) to the incoming air.

In this way, the condensing boiler may be operated to utilise the energy of condensation even during periods in which the water return temperature is above the threshold temperature for condensation to occur in the condensing boiler (with the energy released during condensation being balanced by energy absorbed during evaporation in the evaporator heat exchanger).

In one embodiment, the secondary heat exchanger is a gas-to-gas heat exchanger as defined in the second aspect of the invention. In one embodiment: the first-defined heat exchanger is a condenser heat exchanger configured to cool the first-defined (exhaust) gas stream and condense vapour present in the first-defined (exhaust) gas stream; and the further heat exchanger is an evaporator heat exchanger configured to heat the further (incoming air) gas stream and evaporate liquid from the liquid stream (e.g. liquid disposed on the further inner surface of the further porous structure).

In accordance with a fourth aspect of the present invention, there is provided a gas heating stage comprising: a liquid source; and an evaporator heat exchanger configured to heat a gas stream flowing from a gas inlet to a gas outlet, the evaporator heat exchanger being configured to evaporate liquid from the liquid source to add vapour to the gas stream.

In one embodiment, the evaporator heat exchanger is configured to transfer thermal energy from the liquid source (e.g. liquid stream) to the gas stream.

In one embodiment, the evaporator heat exchanger is a heat exchanger as defined in accordance with the first aspect of the present invention.

In accordance with a fifth aspect of the present invention, there is provided a gas cooling stage comprising: a condenser heat exchanger configured to cool a gas stream flowing from a gas inlet to a gas outlet, the condenser heat exchanger being configured to condense vapour present in the gas stream.

In one embodiment, the gas cooling stage further comprises a liquid source (e.g. liquid stream) and the condenser heat exchanger is configured to transfer thermal energy from the gas stream to the liquid source. In one embodiment, the gas cooling stage is configured to direct condensed vapour into the liquid source.

In one embodiment, the condenser heat exchanger is a heat exchanger as defined in accordance with the first aspect of the present invention.

In accordance with a sixth aspect of the present invention, there is provided apparatus (e.g. gas pre-heating apparatus) comprising: a gas heating stage comprising: a liquid source; an evaporator heat exchanger configured to heat a first (e.g. incoming) gas stream flowing from a first gas inlet to a first gas outlet, the evaporator heat exchanger being configured to evaporate liquid from the liquid source to add vapour to the first gas stream; and a gas cooling stage comprising: a condenser heat exchanger configured to cool a second (e.g. outgoing) gas stream flowing from a second gas inlet to a second gas outlet, the condenser heat exchanger being configured to condense vapour present in the second gas stream.

In one embodiment, the evaporator heat exchanger is configured to transfer thermal energy from the liquid source (e.g. liquid stream) to the first gas stream.

In one embodiment, the condenser heat exchanger is configured to transfer thermal energy from the second gas stream to the liquid source. In one embodiment, the gas cooling stage is configured to direct condensed vapour into the liquid source.

In one embodiment, at least one of the evaporator heat exchanger and condenser heat exchanger is a heat exchanger in accordance with the first aspect of the present invention.

In one embodiment, he evaporator heat exchanger and condenser heat exchanger together form a gas-to-gas heat exchanger in accordance with the second aspect of the present invention.

In accordance with a seventh aspect of the present invention, there is provided a method of exchanging heat using a heat exchanger comprising a porous structure defining a network of interconnecting voids extending from an entrance region of the porous structure to an exit region of the porous structure to defined an inner surface extending through the porous structure between the entrance region and the exit region, the method comprising:

during a first phase of a cycle exposing a first region of the inner surface defining a first path through the porous structure to gas from a gas stream and applying liquid from a liquid stream to a second region of the inner surface, the second region of the inner surface defining a second path through the porous structure extending in parallel to the first path; and during a second phase of the cycle exposing the second region of the inner surface to gas from the gas stream and applying (e.g. depositing) liquid from the liquid stream to the first region of the inner surface.

In one embodiment, the method comprises using a further heat exchanger comprising a further porous structure defining a network of interconnecting voids extending from an entrance region of the further porous structure to an exit region of the further porous structure to define a further inner surface extending through the further porous structure between the entrance region and the exit region of the further porous structure, the method comprising: during a third phase of the cycle exposing a first region of the further inner surface defining a first path through the further porous structure to gas from a further gas stream and applying liquid from the liquid stream to a second region of the further inner surface, the second region of the further inner surface defining a second path through the further porous structure extending in parallel to the first path therethrough; and

during a fourth phase of the cycle exposing the second region of the further inner surface to gas from the further gas stream and applying (e.g. depositing) liquid from the liquid stream to the first region of the further inner surface.

Specific embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a simplified schematic view of a prior art condensing boiler;

Figure 2 A is a graph of moisture per kg of dry air at different saturation temperatures;

Figure 2B is a graph of energy extractable from the gas output of a condensing boiler at different exhaust temperatures; Figure 2C is a graph of heating load for a condensing boiler at different outside temperatures;

Figure 2D is a graph of relative gas boiler load and outside temperature showing how often cold temperatures occur;

Figure 2E is a graph of gas boiler efficiency at different inlet water temperatures;

Figure 3A is a schematic view of a novel condensing boiler including a heat exchanger in accordance with a first embodiment of the present invention;

Figure 3B is a detailed schematic view of the heat exchanger of Figure 3 A;

Figure 3C is a schematic end view of components of the heat exchanger of Figure 3B;

Figure 4 is a schematic view of a heat exchanger in accordance with a second embodiment of the present invention;

Figure 5 A is a schematic view of a heat exchanger in accordance with a third embodiment of the present invention;

Figure 5B is a schematic end view of components of the heat exchanger of Figure

5A;

Figure 6 A is a schematic view of a heat exchanger in accordance with a fourth embodiment of the present invention;

Figure 6B is a schematic end view of components of the heat exchanger of Figure 6A;

Figure 7 is a schematic view of a heat exchanger in accordance with a fifth embodiment of the present invention;

Figure 8 A is a graph of % of energy extracted versus air inlet temperatures for the condensing boiler of the present invention; and

Figure 8B is a graph of exhaust temperature versus air inlet temperature for the same condensing boiler.

Figure 1 shows a simplified overview of a conventional condensing boiler 10 comprising an air inlet 12, a fuel inlet 14, a combustor 16, a combustion chamber 18, a heat exchanger 20 and a gas exhaust 22. A water inlet 30 and a water outlet 32 (connected to a heating circuit 40) convey water to and from heat exchanger 20.

In use, condensing boiler 10 draws in external air via air inlet 12 at approximately ambient pressure and temperature. Natural gas or heating oil received via fuel inlet 14 is added to this air and burnt in combustion chamber 18 by means of combustor 16. This raises the temperature of the air while adding products of combustion to the mixture. The hot gas pass through heat exchanger 20 where this gas is cooled while warming water conveyed between water inlet 30 and water outlet 32 and thereby heat heating circuit 40. Normally heating circuit 40 will involve either heating hot water or central heating circuits. The now cooled gas is then expelled back into the atmosphere via gas exhaust 22. If the water flow from heating circuit 40 can cool the gas to a sufficiently low temperature then some of the moisture in the gas will start to condense out (Figure 2A illustrates the grams of moisture per kg of dry air at different saturation temperatures). As illustrated in Figure 2B, there is significant energy in this condensation and if some of this can be recovered it improves efficiency.

In conventional condensing boilers, condensing normally occurs in the (main) heat exchanger, i.e. in a space that is located close to the high temperatures generated by combustor 16. As the products of combustion are acidic, in addition to water condensation there will also be acid condensation. This proximity to high temperatures means that the condensing section of the heat exchanger must normally be metallic and resist acidic corrosion and are therefore normally made from stainless steel.

Condensation of acidic products can take place at higher temperatures than the water, e.g. 80-90°C, while condensation of water vapour normally occurs below 57°C for gas boilers and 47°C for oil boilers. To achieve condensation of the water it is necessary to have low water inlet temperatures to the heat exchanger or the water will not cool the exhaust sufficiently to ensure that significant condensation takes place.

In a combi-boiler where water is taken from the mains and heated up for domestic hot water (DHW) at the time of use it is possible to have a supply of cold water (normally around 10°C) to ensure that condensing occurs. Where there is a separate hot water tank condensing may occur when the hot water tank is initially cold, but will steadily reduce as the tank gets progressively warmer (note the tank is normally heated to above 65°C). Likewise if there is a radiator system in the house, then condensing will also reduce once the radiators have reached normal operating temperature, such as 70°C or when they are being operated at full power.

Figure 2C shows that as the outside temperature drops the return water temperature in the heating increases, to the point where the boiler is no longer able to condense, even though it is freezing outside. Figure 2D illustrates how often these cold temperatures occur and Figure 2E shows the effect of these temperatures on efficiency. Furthermore, poor installation or incorrect boiler settings can make this situation worse and lead to permanently high water inlet temperatures for example if the water pump is set at too high a flow rate.

It can be seen that if this condensing energy is fully utilised, particularly with gas 5 boilers which have a high water content in the exhaust, then it is possible to increase boiler efficiency by 10-14%.

The present invention will now be described with reference to a condensing boiler and an improved version of a condensing boiler.

Figure 3A shows a novel gas boiler 10' based on the gas boiler 10 of Figure 1 10 (corresponding features are labelled according) and including a secondary (condensing) heat exchanger 100 used to pre-heat the air inlet.

As illustrated in Figures 3B and 3C, heat exchanger 100 comprises an evaporator heat exchanger (EHX) 135, a condenser heat exchanger (CHX) 235, water pipes 140, 142, water pump 160, water supply valve 170 and water drain 171.

15 EHX 135 comprises a rotating assembly 137, casing 138 defining a water reservoir

139, air inlet 110, air outlet 111. EHX 135 is connected to water pipes 140 and 142 such that water enters via pipe 142 and exits via pipe 140. The gas flow entering through the air inlet 110 is normally air drawn in from the atmosphere.

CHX 235 comprises a rotating assembly 237, casing 238 defining a water reservoir 20 239, exhaust inlet 210, exhaust outlet 211. CHX is connected to water pipes 140 and 142 such that water enters via pipe 140 and exits via pipe 142. The gas entering through the exhaust inlet is normally the products of combustion from combustor 16', so principally nitrogen, some oxygen, carbon dioxide and water vapour.

Rotating assemblies 137, 237 each comprise a central rod 131 on bearings (not 25 shown) connected to a drive, such as an electric motor 136. Outer wall 134 is rigidly connected to central rod 131 by lightweight elements 132. The space between the central rod 131 and the outer wall 134 is filled with an open-cell foam 130. Lightweight elements 132 are designed to provide minimal obstruction to the gas path through the foam. Foam 130 could be a reticulated open cell foam made of metal, ceramic or some suitable polymer. 30 If properly constrained, chaotically packed particulate media or fibrous material or some combination thereof could be used as an alternative to foam 130. The key characteristic is to provide a large surface area for heat exchanger and a relatively low pressure drop for the gas passing through the porous media. Both EHX 135 and CHX 235 are installed with their rotating elements horizontal with respect to the ground and at the same height relative to each other. In this way the free water surface 141 in each reservoir 139, 230 is located at approximately the same position in each rotating unit. The free water surface 141 is controlled by the position of water drain 5 171, which connects to the water at a certain height. Consequently, any addition of water to the system raises the free surface height to the drain level with any excess flowing into water drain 171 and out of the system. The water level is configured to be touching or near central 131 for each rotating assembly 137, 237.

Air outlet 111 and exhaust inlet 210 replace air inlet 12 and gas exhaust 220 respectively of gas boiler 10. There will be a closed circuit between air outlet 111 and exhaust inlet 210. Typically there will be a fan (not shown) located before combustor 16 that draws in air via EHX 135 and then exhausts the products of combustion through CHX 235. The fan must be designed to operate in a hot humid environment for long periods of time, potentially with temperatures approaching 100°C for example.

5 Each electric motor 136 may be external to EHX 135 or CHX 235. In one embodiment, a single motor may be mechanically linked to both central rods 131. The electric motor on the water pump 160 might also provide the drive to rotate the central rods. The connection may not even be directly mechanical, for example it could be a magnetic coupling through the wall of EHX 135 or CHX 235. Alternatively the drive might be0 hydraulic and driven by the action of pumping water around the circuit.

Outer wall 134 is impermeable to gas and may require a seal 133 between outer wall 134 and casing 138 to ensure that the flow passes primarily through the foam 130. If the gap between the casing is small and/or the foam is very open cell, then this seal may not be required. Where a seal 133 is installed it must provide an obstruction to gas and water flow5 between outer wall 134 and casing 138. In this way passage through foam 130 always has a significantly lower flow resistance so that the majority of the air/gas and water flow will always pass through the foam 130. Seal 133 may be a single seal or may comprise multiple seals. Seal 133 may be fixed to the casing 138 and seal against outer wall 134, it may be fixed to and rotate with outer wall 134 while sealing against casing 138 or it may float free0 and seal against both outer wall 134 and casing 138.

The water flow in pipe 140 is normally cooler than the water flow in pipe 142. Consequently, it may be preferable to locate both water pump 160 and water supply valve 170 on the cool part of the circuit. Likewise, it may be preferable to locate water drain 171 on the cold side of the rotating assembly 137 that is located in EHX 135. In this way clean cold water can be added to the water circuit via the water supply valve 170 at a point in the circuit where the incoming water is closest in temperature to the water in the circuit. With the water pump 160 operating, clean water will water will enter the circuit via the water supply valve 170 and then pass into CHX 235. As more water is added the clean water will tend to displace the existing water, which will include some products of combustion, and this water will exit via water drain 171. In this way the level of acidity in the water in the circuit is kept low by introducing clean water at regular points of operation. This could happen at start up, during operation or at shut down. The water drain 171 can connect in much the same way that the drain pipe in a normal condensing boiler is connected, which is present for the removal of condensates.

Certain stenching agents are typically added to natural gas to ensure that gas leaks are quickly detected, however the addition of these stanching agents mean that one of the products of combustion is usually sulphuric acid. Consequently in a normal boiler it is important to ensure that all of these condensates drain away as pockets of condensate can become heavily concentrated acids over time. A pocket of condensate will tend to evaporate the water off first, leaving highly concentrated acids.

Therefore, one advantage of the heat exchanger of the present invention is that while it allows condensation to occur, it keeps the acidity levels low as small quantities of water can be continually added to the circuit to dilute the liquid in the circuit and stop any acid build up. Water supply valve 170 may have a sensor (not shown) that activates the valve when the level of water in the system is low, i.e. below a certain pre-set limit. In this way the level of water in the system is maintained between two pre-set levels and there is also a method of keeping the level of acidity in the water low.

A further advantage of the low temperature range of operation is that most or all of the components of this device can be built of materials that are resistant to acids, such as plastics or ceramic materials.

In addition it may be possible to minimise condensation occurring in primary heat exchanger 20'. In this way it may no longer be necessary to build the primary heat exchanger of materials that resist acid, such as stainless steel. Instead lower cost metals may be used.

The exhaust gas from a normal boiler might contain 140 grams of H20 per kg of dry air post combustion (57°C). In this example it is assumed that the exhaust inlet flow 210 is at a temperature where condensation has occurred or it is close to occurring.

If it is assumed that the exhaust from the gas boiler is at 57°C and fully saturated, if the inlet air increases in humidity then this can have no impact on the quantity of air leaving the boiler in the exhaust flow, i.e. if the air entering the boiler is at 50°C and 80% humidity (carrying 70g of moisture per kg dry air) then this can have no effect on the amount of moisture in the exhaust flow at 57°C as it is already saturated.

In order to improve condensing it is necessary to remove significant heat from the exhaust flow to allow this condensation to occur. It can be seen from Figure 2B that the condensing energy is far higher than the sensible heat. Consequently the best way to remove heat from the exhaust is to transfer it to the inlet flow, but in such a way that there is a transfer of both sensible and latent heat of vaporization to the incoming air.

In operation, condensing boiler 10' starts operation by activating the fan to draw air in through EHX 135. Seal 133 should ensure that the majority of the air will pass through foam 130 that is located between free water surface 141 and the outer wall 134. Part of the foam is always submerged within water flowing through water reservoir 139 and the air will not pass through this submerged region. The air passes through the fan and enters gas combustion chamber 18' where gas will be added and combustion carried out by combustor 16'. The hot products of combustion will pass through primary heat exchanger 20' where the majority of the heat is transferred to heating circuit 40'. The now cooled, but still above ambient temperature, exhaust gas leaves primary heat exchanger 20' at or close to condensing temperatures and enters CHX 235 via the exhaust inlet 210. The exhaust gas passes through foam 130 located between free water surface 141 and outer wall 134 in CHX 235. The exhaust gases then leaves via exhaust outlet 211 where it is rejected back to the atmosphere. The exhaust gases are cooled and condensation will occur as the gases pass through foam 130. This condensation may occur on a surface, in which case it will end up in water reservoir 239 or it may occur as droplets in the gas flow, in which case it will be expelled into the atmosphere, where it will normally re-evaporate. This will create a similar effect to the vapour plume normally seen from some boilers exhausts.

At start up of boiler 10', the water pump 160 is activated so that it circulates water so that the water leaves water pipe 140, flows through the rotating assembly 137 in CHX 235 and then exits via pipe 142. From pipe 142 the flow enters EHX 135 and flows through rotating assembly 137 in EHX 135 to pipe 140. In this way a water circuit is created that continually circulates water in an opposite sense to the air circuit. The free water surface 141 is normally configured to be touching or near the central rod 131. Water cannot enter the air or exhaust inlets and/or outlets as, has been previously explained, water drain 171 ensures the free surface cannot rise above a certain level in at least one of the chambers, if not both.

At start up of boiler 10' electric motors 136 start rotating both of the rotating assemblies 137, 237. This ensures that a region of foam 130 in both rotating assemblies is being continually submerged within water and some is being extracted from the water. The result is that the foam that is submerged in the water is continually transferring heat directly between the foam and the water. The foam that is outside the water always has a thin layer of water on the surface of the foam. This water will act to either heat or cool the air/exhaust gas in close proximity to it.

The exhaust gas will exit primary heat exchanger 20' at a temperature that is well above ambient. Consequently this exhaust gas will enter via the exhaust inlet 210 and pass through the wetted foam 130. The exhaust gas will transfer heat to the wetted foam 130 and as the exhaust gas cools, some condensation will occur. As has been explained this condensation may form within the gas flow or on a surface. The exhaust gas will leave the wetted foam 130 at a lower temperature than it enters from exhaust inlet 210 and leave the system via the exhaust outlet 211. This heat exchange process will tend to set up a temperature gradient along the length of the wetted foam 130. As the foam is rotated this heat is transferred to the water that is flowing in the opposite direction. If the mass flows of the air and water are suitably matched then the water temperature will steadily rise as it passes through the foam from water pipe 140 to water pipe 142. Likewise the exhaust gas will drop in temperature as it passes from exhaust inlet 210 to exhaust outlet 211. If sufficient heat exchange area is allowed for this process then the temperatures difference between the exhaust gas and the water at any point along the length of the foam will be small. In this way both the sensible heat of the air and the heat of condensation will be transferred to the water circuit.

Air, at ambient temperature, will enter the EHX 135 via the air inlet 110 and will enter the wetted foam 130. The air will receive heat from the wetted foam and as the air temperature rises, significant evaporation will occur from the surface of the wetted foam. This evaporation uses significant energy and can be used to match much of the cooling required to provide condensing. The air will leave the wetted foam 130 at a temperature and humidity that is close to that of the exhaust gas entering at the exhaust inlet 210 and leave the system via the air outlet 111. This heat exchange process will tend to set up a temperature gradient along the length of the wetted foam 130. As the foam is rotated heat is transferred from the water that is flowing in the opposite direction. If the mass flows of the air and water are suitably matched then the water temperature will steadily fall as it passes through the foam from water pipe 142 to water pipe 140. Likewise the air temperature will rise as it passes from air inlet 110 to air outlet 111. If sufficient heat exchange area is allowed for this process then the temperatures difference between the air and the water at any point along the length of the foam will be small. In this way efficient heat exchange, including heat of evaporation, will be efficiently transferred from the water circuit to the air.

As a result of this process hot humid air will enter the gas condensing boiler. Gas will be added to this hot humid air in combustion chamber 18'. The hot exhaust gas will pass through primary heat exchanger 20' where it is cooled as heat is added to the heating load. Further heat exchange and some condensation will occur in CHX 235 as the exhaust gas is cooled.

However, while the quantity or air in both inlet and outlet flows is roughly constant there will be some disparity in the amount of moisture present in the exhaust flow compared to the inlet flow. Consequently there will be some disparity in the amount of heat that can be condensed and that which can be evaporated from the two flows.

Therefore it is an option to remove additional heat from the liquid circuit between the EHX and the CHX as described below.

Figure 4 shows a heat exchanger 100' based on heat exchanger 100 (corresponding features are labelled accordingly), but with an additional heat exchanger 180 inserted into water pipe 142' where additional heat may be removed from the circuit. This additional heat exchanger 180 may be inserted within the pipe or a separate component that the pipe connects to, for example a plate heat exchanger. There is a separate fluid circuit for removal of the heat, which involves supplying cooled fluid to the heat exchanger to remove heat from the water flow and to raise the temperature of the fluid. The fluid may be water or a mixture of water and anti-freeze or some other suitable heat transfer fluid.

As the heat transfer to the separate fluid is liquid-to-liquid it can be of high quality with low energy penalty. This means that if the fluid is the return circuit from either the hot water or central heating system then it is possible to cool the water flow in pipe 142' to within a few degrees of this temperature. If the flow is from the mains water supply (i.e. a combi-boiler heating hot water) then the flow will be cooled to close to the mains water supply temperature.

Figure 8A shows the approximate efficiency improvement involved between preheating and humidifying inlet air and condensing the water in the exhaust. This increase in efficiency with falling temperatures can be compared to the earlier Figure 2B that showed the efficiency of the condensing boiler fell significantly as the outside temperature falls. For a condensing boiler that is not able to condense (worst case scenario) the present invention may recover approximately 50% of the latent heat of condensation, which would improve the overall boiler efficiency by around 7%.

As illustrated in Figure 8B, inlet icing is unlikely to be an issue as the temperature of the exhaust gas is likely to be significantly above zero, even for very cold inlet temperatures.

As has been previously explained, for condensation to occur it is normally necessary to reduce the exhaust gas temperature below 57°C.

The elements to monitor/control are that:

i. there is sufficient water in the circuit and there is a water sensor (not shown) to detect whether the water level has dropped and needs to be topped up. ii. the water pump is on

iii. the electric motors are operational

The system should automatically take care of itself while the boiler is operated under normal conditions, reaching different equilibrium conditions based upon such factors as the temperature of the heating load, the type of boiler, the air inlet temperature, etc. When the ambient temperature is sub-zero it is preferable that the water flow is adjusted to ensure that the temperature of the exhaust gas leaving the system is above zero. In this way any issues with freezing are avoided.

If there is a risk that the water in EHX 135' or CHX 235' might freeze, for example, where it is located external to the boiler, then it will be necessary to put in some protection. One example could be an additional automatic drain to ensure that the water is always drained down after operation or if the temperature approaches zero degrees. Consequently, where it has been drained down, at start up, it will be necessary to refill the device with water.

Figures 5A and 5B show a further embodiment of heat exchanger 100" based on heat exchanger 100 (corresponding features are labelled accordingly). However, in this embodiment each central rod 131 " has an increased diameter in order to reduce the amount of water in the circuit. The free surface of the water 141 " is lowered so that it is just touching the central rod 131 ". This has the advantage of varying the quantity of air or exhaust gas passing through the foam relative to the quantity of water. For example if the free water surface 141 " is in the middle of CHX 235" or EHX 135" then for equal mass flow the water needs to be travelling at l/900 th the velocity of the air. For equal heat transfer the water needs to be travelling even slower, approximately l/3600 th as water has a higher heat capacity per unit of mass. Consequently there is a risk that the water flow will be a trickle and that some convective flows may occur within the water inside CHX 235"or EHX 135", which will reduce the efficiency of this heat transfer process. By increasing the size of the central rod the water flow rate can be increased and the volume required in CHX 235" or EHX 135" reduced. Figure 5B shows an end view of the same system.

Figures 6 A and 6B shows a yet further embodiment of a heat exchanger 100"' based on heat exchanger 100 illustrating an alternative approach to keep the central rod small. This system has a plurality of circumferentially spaced collector receptacles 300 located at the junction of outer wall 134"' and foam 130" ' to collect water as rotary assemblies 137" ', 237"' pass through water reservoirs 139" ', 239" '. In this way at the top of each rotation the collected water will be released from the collector receptacles 300 to fall back through the foam to the lower level. In this way it is possible to have a large cross-sectional area of foam available for the gas to travel through and a small area of foam for the water to travel through.

In the embodiments of Figures 3, 4 and 5, it is assumed that the pressures on both the evaporator heat exchanger (EHX) and condenser heat exchanger (CHX) are principally the same. Figure 7 shows a different embodiment of a heat exchanger 100" " that is similar to heat exchanger 100, but where the pressures in EHX 135" " and CHX 235 " "are different. Consequently, the casings of both CHX 235" " and EHX 135" " need to be designed for operation at the different pressures. The water pump is replaced with a pressure boosting pump 161 and an additional pressure reducing pump 162 is added. In this way some of the energy required to raise the pressure of the water to a higher pressure can be recovered when the pressure is dropped back to the lower level. Alternatively the pressure reducing pump could be replaced with a pressure reducing nozzle, in which case the energy from lowering the pressure would not be recovered. Suitable non-return or shut- off valves may be added to the water pipes to ensure that a pump failure results in pressure equalisation between the two different pressures. Likewise the drain valve 172 is now a valve that is designed to let water leave the circuit as required but not to allow the pressure to drop unnecessarily.

In this way it is now possible to operate heat exchanger 100"" in a cycle that is operating between two different pressures.

While the present invention has been described in detail with reference to certain preferred embodiments, other embodiments of the invention are possible. Therefore, the scope of the appended claims should not be limited to the description of the preferred embodiments contained herein.

Key features of the invention are summarised again below and such features alone, or in combination, may be combined with the different aspects of the invention as specified above.

The apparatus (e.g. a condensing boiler) may comprise a gas heating stage comprising a liquid source and an evaporator heat exchanger configured to heat a first gas stream (e.g. boiler inlet air) flowing from a first gas inlet to a first gas outlet, the evaporator heat exchanger being configured to evaporate liquid from the liquid source to add vapour to the first gas stream, and, a gas cooling stage comprising a condenser heat exchanger configured to cool a second (e.g. different) gas stream (e.g. boiler exhaust gas) flowing from a second gas inlet to a second gas outlet, the condenser heat exchanger being configured to condense vapour present in the second gas stream, and preferably to direct condensed vapour into a or the liquid source. The evaporator heat exchanger may transfer thermal energy from the liquid source to the first gas stream. The condenser heat exchanger may transfer thermal energy from the second gas stream to the liquid source. In each heat exchanger, the liquid source may be a liquid stream that flows in counter-flow to the gas stream, and the same liquid stream may flow through both heat exchangers in an open or closed circuit such that such that the liquid stream acts as a heat (or heat and mass) transfer fluid between the two heat exchangers. In this way, thermal energy may be transferred between the two respective gas streams by the (same) liquid stream. The apparatus thus acts as a gas-to-gas heat exchanger comprising a gas-to-liquid heat exchanger configured to cool a first gas stream and a further gas-to-liquid heat exchanger configured to heat a second gas stream. Each of the evaporator heat exchanger and condenser heat exchanger may comprise a porous structure such as an open-cell foam structure. The porous structure may be rotatably mounted and partially submerged in the liquid stream, which may flow in counter-flow to the gas stream above it. The structure may comprise an elongate porous structure rotatable about its lengthwise axis, the liquid and gas streams each flowing parallel with that axis such that the heat exchange process causes a temperature gradient to develop along the length of the porous structure. The porous structure may have an average effective pore diameter of at least 3mm, such large pores being resistant to blocking, such that a (annular) seal against the circumferential outer wall/perimeter of the porous structure is not required.

The present invention thus comprises any novel feature or novel combination of features as mentioned above.