WO2015126239A1

WO2015126239A1 - Heat and moisture exchanger

Info

Publication number: WO2015126239A1
Application number: PCT/NL2014/050105
Authority: WO
Inventors: Johannes Antonius Maria Reinders; Mark Hakbijl
Original assignee: Oxycom Beheer B.V.
Priority date: 2014-02-20
Filing date: 2014-02-20
Publication date: 2015-08-27

Abstract

A heat and moisture exchanger comprises a membrane that is selectively permeable to water vapour, having heat conducting fins attached to both its surfaces. The fins on the first surface are arranged in heat conducting relation with the fins on the second surface for enhancing heat transfer through the membrane. In this manner, a first relatively warm, moist air stream flowing over the first surface can transfer heat and moisture through the membrane to a second relatively cooler and drier air stream flowing over the second surface. By additionally providing the membrane with heat conducting fins, greater thermal or sensible heat transfer can be achieved to and through the membrane.

Description

Heat and Moisture Exchanger

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to heat exchangers, in particular to heat exchangers having vapour-permeable membranes separating first and second flow channels from each other while permitting moisture transport. The invention relates specifically to an improved construction that enhances heat and moisture exchange across a membrane.

2. Description of the Related Art

In heating, ventilation and cooling (HVAC) systems, energy may be recovered from a first flow by heat exchange with a second flow. Waste heat recovery systems are well known that are capable of recovering heat from air exiting a building and using this to raise the temperature of fresh air entering the building.

In addition to recovery of what is termed sensible heat, significant energy is associated with moisture vapour in the respective air streams. As will be understood by the skilled person, the specific heat capacity of the air itself is negligible compared to the latent heat of evaporation of the water that can be carried by it. For this reason, during heat exchange, recovery of the latent heat is also of great importance. Heat recovery wheels have been developed that are capable of absorbing moisture from a first flow. The moisture may then be evaporated into a relatively drier second flow. Such a heat recovery wheel is disclosed in US4769053.

Alternatives to heat recovery wheels have been suggested, involving the use of moisture permeable membranes that allow moisture to transfer from one side of the membrane to the other, driven by a difference in absolute humidity. One device is shown in US4040804, which uses folded sheets of water permeable paper. Separators are provided between folds to provide air passages. Another similar device is described in EP777094.

Although existing devices allow recovery of enthalpy, it would be desirable to provide a heat exchanger which provided improved recovery. BRIEF SUMMARY OF THE INVENTION

According to the invention there is provided a heat and moisture exchanger comprising a membrane that is selectively permeable to water vapour, the membrane having first and second opposed surfaces with heat conducting fins attached to both surfaces, the fins on the first surface being arranged in heat conducting relation with the fins on the second surface for enhancing heat transfer through the membrane, such that a first relatively warm, moist air stream flowing over the first surface can transfer heat and moisture through the membrane to a second relatively cooler and drier air stream flowing over the second surface. According to the invention, by additionally providing the membrane with heat conducting fins, greater thermal or sensible heat transfer can be achieved to and through the membrane. In the present context, permeable to water vapour is intended to refer to the fact that water vapour may transport across the membrane while liquid water cannot. As a result of the improved sensible heat transfer due to the conducting fins, the second air stream can be warmed to a temperature closer to that of the first air stream at which it can take in further moisture. In other words, due to its increased temperature, it has a lower relative humidity and can absorb further moisture.

Although the invention may be seen in providing improved moisture transport by increasing the sensible heat transfer, it may also be seen as providing additional moisture recovery in a conventional heat recovery device. Fin-plate heat exchangers provide very efficient heat exchange that can be as high as 120 W/m²K for a flow rate of 2.5 m/s. This is significantly higher than a flat membrane, which may under the same conditions only transfer around 25 W/m²K. By using a permeable membrane according to the invention, additional benefit may be derived due to moisture transport through the membrane, without affecting the overall sensible heat transfer.

According to a preferred embodiment of the invention, the membrane comprises a hydrophilic, non-porous layer of polymeric material. While not wishing to be bound by theory, it is believed that hydrophilic materials of this type move moisture by a process of adsorption and desorption at the respective surfaces, whereby the rates of adsorption and desorption are determined by the relative levels of humidity at these surfaces. Such materials have been found superior to non-hydrophilic materials in terms of moisture transport. In particular, the membrane may comprise a polymer having polar functional groups which can support the hydrophilic activity. These may be provided e.g. on selected blocks of a block-copolymer. Most preferably the polar functional groups are selected from hydroxyl, sulfonyl, sulfonamide, cyano, nitro, amino, carboxylate, amido, ureido, sulfinyl, sulfhydryl, silyl and aryl sulfonyl groups and combinations thereof. The polar water molecule is attracted to the polar functional groups of the polymer and thus adsorbed onto the membrane.

According to a still further preferred embodiment, the membrane comprises a sulfonated block copolymer. Such materials have been found highly suitable for the purpose of moisture transport and allow fine tuning of the permeability properties.

The membrane may be integrated into the exchanger in any suitable manner.

Preferably, the membrane is extremely thin to minimise its thermal resistance. In order to provide sufficient support, the device may further comprise a scaffold and the membrane may be laminated to the scaffold or otherwise supported thereon. In this context, scaffold is intended to include any structure capable of retaining the membrane such that it may perform its function. Most preferably, the scaffold comprises a mesh and the membrane is connected to one surface of the mesh. The mesh may thus be relatively thicker than the membrane and may be of any material that is compatible with the requirements of e.g. strength for supporting the membrane. The mesh or scaffold may be of plastic material and need not have high thermal conductivity if it has an open structure allowing the respective air streams to contact the membrane, such that the thermal and moisture resistance is negligible. Alternatively, the mesh or scaffold may be of conductive material. This may be beneficial in increasing the heat transfer to and from the membrane. The fins may be connected to the mesh or scaffold or alternatively may be integrally formed therewith.

The fins may be made of any material that is adequately conductive for ensuring good heat transfer to and through the membrane. In general, the fins will be formed of metal, most preferably aluminium.

In one preferred embodiment of the invention, the fins may be connected to the first and second surfaces by adhesive. The adhesive should be chosen to minimise obstruction to heat transfer to the membrane. The most effective way of achieving this is to ensure that the adhesive forms a very thin layer. A most preferred construction uses a heat-seal adhesive laminated on the fin. This may be a thin layer of PVC, nitrocellulose or similar thermoplastic polymer, which additionally acts as a primer for the aluminium fins to prevent corrosion.

In a further preferred embodiment, the fins may comprise corrugated strips of conducting material having peaks and troughs, with the fins connected to the respective surfaces of the membrane at the troughs. Although reference is given to the membrane, it is understood that this may also include connection to a scaffold structure supporting the membrane if the requirement of good heat conduction across the membrane is achieved. The strips may have a limited width and when connected to the membrane extend a limited distance in the flow direction. A plurality of strips may be provided, each separated from the next by a gap, whereby heat conduction in the flow direction may be minimised. Each strip may be slightly offset from its neighbour in order to increase turbulence in the flow. The heat and moisture exchanger may be otherwise as described in WO03/091633, the contents of which are incorporated herein by reference in their entirety, with only the membrane being different. In an alternative embodiment, the fins pass through the membrane to extend from both the first and the second surface. The fins may be individual strips that pierce the membrane or may comprise two opposing fins that are joined to each other directly through a hole in the membrane as disclosed in WO03/091648, the contents of which are incorporated herein by reference in their entirety.

According to a further aspect of the invention, the membrane is folded to form one or more flow channels. The membrane may be folded on itself to form a single channel with the first air stream flowing through the single channel and the second air stream flowing over an outside the single channel. Alternatively, two membranes may be folded and joined to each other in the manner disclosed in WO03/091633. In a further alternative, a single membrane may be folded in concertina fashion to produce multiple channels through which first and second air streams can flow. The device may further be provided with inlet and outlet manifolds for guiding the first and second air streams to and from the respective first and second surfaces or into and from the respective flow channels or groups of flow channels.

According to a yet further aspect of the invention, the device may comprise a housing in which the membrane is located, wherein the first and second air streams flow through the housing and over the respective first and second surfaces. The manifolds may form part of an inlet and outlet for the housing.

The device may be embodied for various flow configurations. According to a most preferred configuration, the fins on the first and second surfaces are aligned with each other for flow in counter-flow. This configuration is believed to provide a more efficient overall performance than cross-flow arrangements.

In order to further improve the heat transfer effectiveness, the fins may be provided with boundary layer disrupting formations such as louvers or the like. Other ways of increasing turbulence may be included, including the offsetting of fins relative to each other and the use of crooked or profiled fins.

The invention also relates to a method of heat and moisture exchange, comprising: providing a membrane that is selectively permeable to water vapour, the membrane having first and second opposed surfaces with heat conducting fins attached to both surfaces, the fins on the first surface being arranged in heat conducting relation with the fins on the second surface for enhancing heat transfer through the membrane; passing a first relatively warm, moist air stream over the first surface of the membrane; passing a second relatively cooler and drier air stream over the second surface of the membrane; transferring sensible heat from the first stream to the membrane via the fins on the first surface; transporting moisture and sensible heat through the membrane from the first surface to the second surface; and transferring sensible heat from the membrane via the fins on the second surface to the second stream. BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the invention will be appreciated upon reference to the following drawings of a number of exemplary embodiments, in which: Figure 1 shows a perspective view of part of a heat and moisture exchanger according to a first embodiment of the present invention;

Figure 2 shows another perspective view of part of the exchanger of Figure 1.

Figure 3 shows the exchanger of Figure 1 in full;

Figure 4 shows a cross-sectional view through the exchanger of Figure 3 installed in a housing; and

Figure 5 shows a perspective view of the housing of Figure 4. DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Figure 1 shows a perspective view of a portion of a heat and moisture exchanger 1 according to the present invention. The exchanger 1 comprises a membrane 2 having a first surface 4 and a second surface 6. The membrane 2 is supported on a scaffold structure 8 having a honeycomb configuration. Heat conducting fins 10 are attached to both surfaces 4, 6. Fins 10 each have a base 12 that is adhered to the respective surface 4, 6 in such a way that the bases are directly opposed to each other.

The membrane 2 is a hydrophilic polymer that is permeable to water vapour. In the present embodiment, it is formed of Kraton Nexar™ polymer, a sulfonated block copolymer having a thickness of around 30 microns. It will be understood that other similar membranes may also be used. The scaffold 8 is a polyester mesh having a thickness of around 170 microns. The fins 10 are formed of soft annealed aluminium foil having a thickness of around 70 microns coated with a heat-seal lacquer of nitrocellulose.

In use, a first air stream A flows over the first surface 4. The first air stream A has a temperature Tl and a relative humidity RHl . A second air stream B flows over the second surface 6 with a temperature T2 and relative humidity RH2. At the position of fin 10a located on the first surface 4, the temperature Tl and relative humidity RHl are higher than the temperature T2 and relative humidity RH2 at the location of fin 10b on the second surface 6. As a result, heat is transmitted from the first air stream A to the fin 10a and through the membrane 2 to the fin 10b. Heat is then transmitted from the fin 10b to the second air stream B, causing it to be warmed. By effective heat transfer from the first air stream to the second air stream, the temperature T2 will approach the temperature Tl .

During the heat transfer, moisture in the first air stream A is attracted to the membrane 2, due to its hydrophilic nature. Without wishing to be bound by theory, it is believed that the moisture is adsorbed into the material of the membrane 2 from the first air stream A until it reaches a state of equilibrium with respect to the humidity at the first surface 4. At the second surface 6, the material of the membrane 2 is exposed to a lower humidity RH2 and hence a different equilibrium will be reached. There will be a net transfer of moisture from the membrane 2 to the second air stream B by desorption. According to an important aspect of the invention, due to the presence of the fins 10, enhanced heat transfer occurs. The second air stream thus attains a higher temperature than would otherwise be the case, which in turn leads to the relative humidity RH2 being lower, creating a greater driving force for moisture transport. Figure 2 shows a larger portion of the heat and moisture exchanger 1 of Figure 1. In this view, it can be seen that the fins 10 are formed as corrugated strips 14 having peaks 16 and troughs formed by the bases 12 in contact with membrane 2. The fins 10 are provided with louvers 18 extending through the fin material. At intervals along the strip 14, conduction bridges 20 are provided through the fins 10. Louvers 18 and conduction bridges 20 reduce the effects of heat transfer along the fin 10 in the flow direction.

Figure 3 shows a complete heat and moisture exchanger 1. As can be seen, four strips 14 of fins are provided on each surface 4, 6 of the membrane 2. The gap 22 between adjacent strips 14 also prevents conduction of heat from strip to strip in the flow direction. In this case, the strips 14 are positioned such that the fins 10 in each strip 14 are aligned with each other. It is also possible to slightly offset each strip 14 so that the fins 10 are slightly staggered with respect to each other. This can further enhance heat transfer by increasing turbulence. Figure 4 shows the heat and moisture exchanger 1 of Figure 3 installed in a housing 30 for heat and moisture exchange operation. Housing 30 is provided with a cover 32 that engages with the membrane 2 to seal the exchanger within the housing 30. As can be seen, the exchanger 1 has been folded on itself in concertina fashion into a series of primary and secondary flow channels 34a, 34b for passage of the respective first and second air streams A, B. Spacers 38 assist in maintaining the exchanger 1 in position with the fins 10 acting to support against the spacers 38. These spacers 38 are optional and in an alternative construction may be omitted. The membrane 2 is flexible and can be easily bent into the desired form, as can the aluminium forming the fins 10. It will be understood that in other constructions, the heat exchanger may be less flexible and may be formed into a desired shape by moulding procedures or the like. In certain configurations, it may be desirable to avoid including fins in the folded regions in order to more easily fold the membrane 2.

Figure 5 shows the housing 30 of Figure 4 in perspective view. The housing 30 is provided with first and second coupling manifolds 40, 42 for coupling to the flow channels 34a, 34b. The coupling manifolds 40, 42 are provided with a number of ports 44 that can be selectively opened or pierced according to the flow configuration required. In this manner, the housing 30 can be installed in a number of different

HVAC units in a versatile manner. In the illustrated example, primary inlet ports 46 are formed in second coupling manifold 42 and connect as inlet for first air stream A to the primary flow channels 34A. Outlet of the first air stream A takes place from primary outlet ports 48 formed through the first coupling manifold 40. Secondary inlet ports 50 are formed in the first coupling manifold 40 as inlets to the secondary flow channels 34B with outlet taking place through secondary outlet ports 52 in the second coupling manifold 42.

Thus, the invention has been described by reference to the embodiment discussed above. It will be recognized that this embodiment is susceptible to various

modifications and alternative forms well known to those of skill in the art. In particular, the form of the fins may be distinct from the illustrated design. These fins may be individual elements rather than corrugated strips. Furthermore, the channels may be formed in different ways by joining multiple membranes or exchangers together rather than by folding of a single membrane on itself.

Many modifications in addition to those described above may be made to the structures and techniques described herein without departing from the spirit and scope of the invention. Accordingly, although specific embodiments have been described, these are examples only and are not limiting upon the scope of the invention.

Claims

1. A heat and moisture exchanger comprising a membrane that is selectively permeable to water vapour, the membrane having first and second opposed surfaces with heat conducting fins attached to both surfaces, the fins on the first surface being arranged in heat conducting relation with the fins on the second surface for enhancing heat transfer through the membrane, such that a first relatively warm, moist air stream flowing over the first surface can transfer heat and moisture through the membrane to a second relatively cooler and drier air stream flowing over the second surface.

2. The device according to claim 1, wherein the membrane comprises a polymer having polar functional groups, preferably a hydrophilic, non-porous layer of polymeric material having a thickness of less than 100 microns.

3. The device according to claim 2, wherein the polar functional groups are selected from hydroxyl, sulfonyl, sulfonamide, nitro, amino, carboxylate, amido, ureido, sulfinyl, sulfhydryl, cyano, silyl and aryl sulfonyl groups and

combinations thereof, most preferably a sulfonated block copolymer.

4. The device according to any preceding claim, wherein the membrane has a moisture transport capacity of at least lKg/m2/hour at a temperature of 20 C and a relative humidity of 50%.

5. The device according to any preceding claim, further comprising a scaffold and wherein the membrane is laminated to the scaffold.

6. The device according to claim 5, wherein the scaffold comprises a mesh and the membrane is connected to one surface of the mesh.

7. The device according to any preceding claim, wherein the fins comprise aluminium.

8. The device according to any preceding claim, wherein the fins are connected to the first and second surfaces by adhesive, preferably a heat-seal adhesive laminated on the fin.

9. The device according to any preceding claim, wherein the fins comprise corrugated strips of conducting material having peaks and troughs and the fins are connected to the respective surfaces of the membrane at the troughs.

10. The device according to any of claims 1 to 8, wherein the fins pass through the membrane to extend from both the first and the second surface.

11. The device according to any preceding claim, wherein the membrane is folded to form one or more flow channels.

12. The device according to any preceding claim, further comprising inlet and outlet manifolds for guiding the first and second air streams to and from the respective first and second surfaces.

13. The device according to any preceding claim, further comprising a housing in which the membrane is located, wherein the first and second air streams flow through the housing and over the respective first and second surfaces.

14. The device according to any preceding claim, wherein the fins on the first and second surfaces are aligned with each other for flow in counter-flow.

15. The device according to any preceding claim, wherein the fins are provided with boundary layer disrupting formations such as louvers or the like.

16. A method of heat and moisture exchange, comprising:

providing a membrane that is selectively permeable to water vapour, the membrane having first and second opposed surfaces with heat conducting fins attached to both surfaces, the fins on the first surface being arranged in heat conducting relation with the fins on the second surface for enhancing heat transfer through the membrane,

passing a first relatively warm, moist air stream over the first surface of the membrane;

passing a second relatively cooler and drier air stream over the second surface of the membrane;

transferring sensible heat from the first stream to the membrane via the fins on the first surface;

transporting moisture and sensible heat through the membrane from the first surface to the second surface; and

transferring sensible heat from the membrane via the fins on the second surface to the second stream.

17. The method of claim 16, using the device according to any of claims 1 to 15.

18. The method according to claim 16 or claim 17, wherein the first and second air streams are passed over the membrane in counter flow.