VAPOUR EXTRACTION DEVICE

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The present invention relates to vapour extraction devices, in particular, to devices for the removal of water vapour from air and an evaporative cooler incorporating such a device. The invention further relates to a method of extracting vapour from an air stream.

2. Description of the Related Art

[0002] Vapour extraction devices are conventionally used in many situations where it is desired to reduce the vapour content of or otherwise dry an air stream. In particular, in heating ventilating and air conditioning systems removal of excess moisture from an air stream is often desirable. Other situations where water vapour may be extracted include clothes dryers, industrial desiccation and dehumidtfters,

[0003] One form of vapour extracting device used in clothes dryers of the tumble dryer type is the condenser. Warm air saturated with water vapour is passed through a heat exchanger where it is cooled. Some of the water vapour in the air condenses on the surface of the heat exchanger and is collected. The cooled air exiting the condenser carries less moisture and has a lower absolute humidity than on entry. It nevertheless remains saturated with moisture. A characteristic of such condensers is thus that they can only operate close to the saturation line of 100% humidity. An additional heating step is then required to lower the relative humidity and move away from the saturation line.

[0004] For drying of unsaturated air having a humidity of below 100%, hygroscopic materials have conventionally been used. One conventional device is known as a desiccant wheel and uses a desiccant such as silica gel to absorb moisture. The desiccant is provided on a carrier layer, convoluted or corrugated to form a multitude of passages having a large surface area. The carrier layer is rolled up or otherwise arranged to form a wheel -shaped structure with the passages aligned with an axis of the wheel. In use, the air to be dried is passed through a first sector of the wheel as a first air stream. The desiccant has a greater affinity to water than does the air and moisture in the air is taken up by the desiccant. Silica gel in particular is extremely effective in that it can absorb many times its own weight in

water until it finally becomes saturated. During operation, the wheel turns and the parts of the wheel that have become saturated rotate out of the first air stream. They are then exposed to a second stream of high temperature air. The second air stream operates to dry the desiccant by effectively boiling off the absorbed water. Considerable energy, equal to the latent heat of evaporation, is required in order to evaporate this water. Such desiccant devices are also generally relatively large and cumbersome. More recently, alternative (smart) materials have been discovered that are capable of selectively absorbing particular substances and releasing them in response to a stimulus. One class of such materials are referred to as LCST polymers. These materials are known for their ability to change state at the so-called Lower Critical Solution Temperature (LCST) from a relatively hydrophilic to a relatively hydrophobic form. At present these polymers have been used primarily for bio-medical purposes. An example of such materials is given in EP 501 301, the contents of which are herein incorporated by reference in their entirety.

[0005] One area where vapour extraction is useful is in the field of evaporative cooling systems. Evaporative cooling systems make use of the latent heat of evaporation of water into an air stream to extract heat. Indirect evaporative coolers and "dew-point coolers" cool a product air stream by evaporation into a working air stream. If the working air stream already has high relative humidity, then the amount of water vapour that it can absorb is limited. One such dew-point cooler is known from WO03/091633, the contents of which are herein incorporated by reference in their entirety. A further device is known from document US 6,050,100, the contents of which are also incorporated by reference in their entirety. This document describes how a desiccant wheel could be incorporated in a system comprising an indirect evaporative cooler. A burner is required to regenerate the desiccant wheel, requiring significant energy input. Furthermore, the heat of absorption of the moisture and the regeneration of the wheel can cause the air flow to be heated to as much as 80 ⁰ C, It would therefore be desirable to provide an efficient manner of reducing the humidity of the incoming air to such evaporative coolers in order to increase the effective cooling capacity. The energy required to perform vapour extraction should nevertheless be minimal.

[0006] There is thus a need for alternative vapour extracting devices that can operate away from the saturation line and that do not require elevated energy input. Such devices should be 05589.0024,00PCOO

cheap and simple to produce and also be relatively small for better integration into existing systems.

BRIEF SUMMARY OF THE INVENTION

[0007] According to the invention there is provided a device for extracting water vapour from a fluid stream that attempts to alleviate some of the above-mentioned drawbacks. The device comprises a flow channel for the fluid stream; a quantity of selectively absorbing material located in the flow channel, having a first state in which the material is capable of holding a first higher quantity of water and having a second state in which the material is capable of holding a second lower quantity of water; and an actuator for selectively switching the material from the first state to the second state whereby product in excess of the second quantity is released. By actuating the material to change from the first state to the second state after absorbing a quantity of water, the material may be effectively regenerated. It may then be used again to absorb a further quantity of vapour. Clearly, the second lower quantity may be substantially zero, whereby in the second state no significant water is retained. Because of the nature of such materials however it is understood that the material will always have a minimal affinity to the water and a certain amount will be retained in the second state. Suitable materials may also be referred to as stimulus responsive materials as they may switch from the first state to the second state in response to a stimulus. In the present context, reference will be made to absorption of moisture, nevertheless, materials that adsorb moisture are also considered to be included within the scope of this term.

[0008] According to a preferred form of the invention, the device comprises a carrier structure and the material is provided on the carrier structure, hi this context, the term "provided on" is understood to include cases where the material comprises an integral part of the carrier structure. The carrier structure may be a foil or gauze and may be formed of e.g. a metal or a plastics material. Paper or carton may also be used. Preferably the material is arranged in the flow channel to offer a relatively large surface area and relatively low flow resistance to the fluid stream. In particular, the material or the carrier structure may be in the form of a plurality of passageways aligned with the flow direction in the manner of prior art desiccant wheels. Alternatively, open mesh structures offer advantageous flow characteristics

as they stimulate turbulent flow and can help to reduce overall flow resistance. Most preferably the material should remain in solid or gel form in order to maintain its structural form and ensure its attachment to the carrier structure.

[0009] According to a most favoured embodiment of the invention, the fluid stream comprises air. In this manner, the device can operate as a dehumidifier whereby the material absorbs a quantity of the water vapour from the air when it is in the first state. Preferably, the air is atmospheric air and the device is used for conditioning the atmospheric air as part of a climate control system.

[0010] For use in extracting water vapour, a material may be chosen that is relatively hydrophilic in the first state and relatively hydrophobic in the second state. In one form of operation, after absorbing water vapour in the first state the material may then release liquid water on switching to the second state. Of particular significance in this case, the phase change from vapour to liquid takes place on absorption of the vapour by the material and is not reversed on releasing the water on switching to the second state. The energy required to release the water from the material on switching may thus be considerably less than the energy required to evaporate a similar quantity of water from a silica gel or similar desiccant.

[0011] Exemplary materials that can be used for extracting water vapour are stimulus responsive polymers, more particularly LCST polymers. These materials are known for their ability to change state at the so-called Lower Critical Solution Temperature (LCST) from a relatively hydrophilic to a relatively hydrophobic form. LCST polymers exhibit thermally reversible soluble-insoluble changes in aqueous solutions in response to temperature changes. One form of LCST polymer is poly(N-isopropyl acrylamide) (PNIPAM). Other LCST polymers include polysilanes and polysilynes such as ρoly(4,7,10-trioxaundecylsilyne) and poly(4,7,10,13-tetraoxatetradecylsilyne), poly(dimethylamino ethyl methacrylate) (PDMAEMA) and polyoxazolines using ethyl and isopropyl groups, in particular poly(2- ethyl-2-oxazoIine) (PETOX) and poly(2-isoproρyl-2-oxazoline). Upon raising the temperature of an aqueous solution of such polymers, reversible phase separation occurs at the lower critical solution temperature (LCST). In aqueous solution at ambient temperatures below the LCST, the polymer is present as a highly folded random coil. Above the LCST its polymer backbone adopts a more extended conformation and water is released. The position

of the LCST can be tuned over a large temperature range (27 to 75 ⁰ C) by addition of inorganic salts or co-monomers. Furthermore, by the addition of cross-linking agents, the stability of the structure can be improved in order to ensure that the material remains in solid or gel form., m the case of ρoly(ethyloxazolines) it has been found that the presence of more than 30% of a cross-linking agent (2-isopropenyl-2-oxazoline) can prevent the polymer becoming liquid on absorption of water.

[0012] Preferably the material switches from the first state to the second state in response to heat and the actuator comprises a heater. This is the preferred form of operation for polysilane materials whereby heating to the LCST causes switching to occur. For use in a climate control system, the LCST may be set to a switching temperature slightly above the highest temperatures usually encountered. Heating the material to this switching temperature causes regeneration of the material to take place. The switching temperature will nevertheless be below about 100 ⁰ C as elevation to such a temperature would effectively require boiling of the water and significant energy loss.

[0013] Other activation forms may be used to cause the material to switch from the first state to the second state e.g. in response to an electric potential, an electric current, a magnetic field, electromagnetic radiation, pH, vibration or mechanical stress.

[0014] Preferably, the device may further comprise a drain for collecting the released water, which may be collected for subsequent use. In this way, the device may be used to provide clean, fresh water by extraction from humid air passing over e.g. the sea. In order to facilitate collection a gravity flow structure may be provided leading to the drain. Alternatively or additionally, centrifugal or vibratory devices may be used to improve the collection of the liquid.

[0015] Associated with the absorption of water vapour, there will be an increase in temperature caused by the latent heat of evaporation/absorption. This can lead to warming of both the fluid stream and the material. In order maintain the material at a desired temperature and/or to prevent heating of the fluid stream, heat exchange e.g. with the ambient air or another fluid may be provided. This may be achieved by providing suitable cooling fins on or

around the flow channel or by subsequently directing the fluid stream through a heat exchanger.

[0016] The device is particularly useful in combination with an evaporative cooling device, preferably a dew-point cooler, wherein in use the flow stream is directed from the flow channel to an inlet e.g of working fluid to the evaporative cooling device. In this manner, the fluid entering the cooling device may be dehumidified in order to allow a greater uptake of moisture during cooling.

[0017] The invention further provides for a method of extracting an entrained product such as water from a fluid stream comprising: providing a quantity of a selectively absorbing material, having a first state in which the material is capable of holding a first higher quantity of the product and having a second state in which the material is capable of holding a second lower quantity of the product; exposing the material to the fluid stream during a first period of time such that the material absorbs a first quantity of the product; and selectively switching the material from the first state to the second state during a second period of time whereby product in excess of the second quantity is released.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The features and advantages of the invention will be appreciated upon reference to the following drawings of a number of exemplary embodiments, in which:

[0019] Figure 1 shows a perspective view of a conventional desiccant wheel;

[0020] Figure 2 shows a detail of the wheel of Figure 1 illustrating the construction of the disposition of the layers;

[0021] Figure 3 shows a vapour extraction device according to the present invention; [0022] Figure 4 shows a detail of the vapour extraction device of Figure 3; and

[0023] Figure 5 shows an evaporative cooling system in which a vapour extraction device according to the present invention is used.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0024] The following is a description of certain embodiments of the invention, given by way of example only and with reference to the drawings.

[0025] Figure 1 shows a conventional desiccant wheel 1, mounted for rotation about an axis X-X. A first flow of product air 4 passes through a first sector 6 of the wheel 1. A second flow of regenerative air 8 passes through a second sector 10. A third recovery sector 12 is also provided. Appropriate flow structures are provided to channel the flows into their respective sectors and to keep them separated. As these flow structures are generally conventional, they will not be further disclosed.

[0026] Figure 2 shows a portion A of the desiccant wheel 1 of Figure 1 in greater detail. The wheel 1 is formed of layers of carrier material 14. A separator layer 16 and a corrugated spacer layer 18 are arranged in a repeating structure to form a multitude of passageways 20 aligned with the axis X-X. The carrier material 14 is formed of carton and is coated on both of its surfaces with a layer of silica gel 22.

[0027] In use, the wheel 1 is set to rotate at a low speed of less than 1 rpm. The portion A is initially in the first sector 6 of the wheel 1. Product air 4 carrying moisture vapour flows through the passageways 20 and is in intimate contact with the silica gel 22. The hygroscopic property of the silica gel 22 causes it to absorb moisture from the product air 4. Absorption continues until the silica gel 22 is saturated with water and can absorb no more. This is effectively the point at which an equilibrium is reached between the vapour content of the product air 4 and the water content of the silica gel 22. The product air 4' exiting from the first sector 6 is substantially dryer than the on entry.

[0028] As the wheel 1 rotates, the portion A moves into the second sector 10. A flow of regenerative air 8 having a temperature of around 100 ⁰ C is passed through the passageways 20 in the opposite direction to the first flow 4. As a result of the elevated temperature, the silica gel 22 gives up its moisture by evaporation and the regenerative air 8' exiting from the second sector 10 has an elevated moisture content. Clearly, considerable energy is required to evaporate the water out of the silica gel structure. This energy corresponds to the latent heat

of evaporation of water in addition to the energy required to heat the regenerative air to 100 ⁰ C.

[0029] It may be noted that the silica gel absorbs and releases water in a reversible way. The amount of water released is proportional to the energy input

[0030] Further rotation of the wheel 1 brings the portion A into the third recovery sector 12. The silica gel 22 is now completely dried and ready to absorb further water, hi the recovery sector 12 the temperature of the carrier 14 may be allowed to return to its initial or ambient value. Although not shown, this may be achieved by a further cooling or scavenging air flow.

[0031] Figure 3 shows a device according to the present invention. The device of Figure 3 is generally similar in structure to that of Figure 1 comprising a wheel 40, mounted for rotation about an axis Y-Y. A first flow of product air 44 passes through a first sector 46 of the wheel 40. A second flow of regenerative air 48 passes through a second sector 50. Again, appropriate flow structures are provided to channel the flows into their respective sectors and to keep them separated. It may be noted that in this embodiment the axis Y-Y is directed vertically, the second sector 50 is also considerably smaller than that of Figure 1 and the third sector has been omitted. A drain 66 is located below the second sector 50.

[0032] Figure 4 shows a portion B of the wheel 40 of Figure 3 in greater detail. As in the device of Figures 1 and 2, the wheel 40 is formed of layers of carrier material 54 forming a separator layer 56 and a corrugated spacer layer 58 arranged in a repeating structure to form a multitude of passageways 60 aligned with the axis Y-Y. The carrier material 54 is formed of a suitable material such as aluminium or plastics and is coated on both of its surfaces with a layer of LCST polymer 62.

[0033] In use, the wheel 40 is set to rotate. The portion B is initially in the first sector 46 of the wheel 40. Product air 4 carrying moisture vapour flows through the passageways 60 and is in intimate contact with the LCST coating 62. The LCST polymer is initially below its LCS temperature and its hydrophilic property causes it to absorb moisture from the product air 4. Absorption continues until the coating 62 is saturated with water and can absorb no more. The product air 4' exiting from the first sector 6 is substantially dryer than the on entry.

[0034] As the wheel 40 rotates, the portion B moves into the second sector 50. A flow of regenerative air 8 having a temperature of around 6O ⁰ C is passed through the passageways 60 in the opposite direction to the first flow 4. As a result of the elevated temperature, the LCST coating 62 is heated to above the LCS temperature and enters a hydrophobic phase. It is thus caused to give up its moisture in the form of liquid droplets 68 which are collected by drain 66.

[0035] According to Figure 5, there is shown a further development of the invention in which an evaporative cooler 100 of the dew point type is supplied with air flow 110 having a reduced level of humidity. Evaporative cooler 100 comprises a primary channel 102, in heat exchanging contact with a secondary channel 104 via a heat exchanger 106. Air in the secondary channel 104 is humidified by the evaporation of water, the heat of evaporation thus driving the cooling of the air flow 110 in the primary channel 102. Water may be supplied to the secondary channel 104 by a wetting device 108. The air flow 1 10 passing through the primary channel 102 may be separated (indicated by the dotted line) and returned through the secondary channel 104 in the manner described in WO03/091633. Fans or other air circulation devices (not shown) are provided at appropriate locations within the system in order to ensure the desired air circulation. Other configurations for the air flow through an evaporative cooler such as that of US 6,050,100 are also known and could also be used in the present context.

[0036] Prior to being supplied to the primary channel 102, the air flow 110 is dehumidified by passing through dehumidifier 112 according to the present invention. Dehumidifier 112 comprises a quantity of an LCST polymer supported on a carrier 118. As described above, the LCST polymer absorbs water vapour from the air in its first state when below its LCST. Periodically, the dehumidifier 112 is regenerated by heating to a temperature above the LCST using e.g. heating element 114. Liquid water is collected in drain 116 and may if required be supplied to the wetting device 108. It is noted that dehumidifier 112 could also be of the type as described in Figure 3.

[0037] On passing through the dehumidifier 112, air flow 1 10 becomes heated by the energy of absorption released due to the phase change of water from a vapour to its liquid form in the LCST polymer. In order to return the airflow 110 to its original ambient

temperature, a second heat exchanger 120 is provided. Second heat exchanger 120 may be a conventional cross-flow or counter-flow exchanger through which ambient air 122 is circulated in heat exchanging contact with air flow 110. In this manner, it is achieved that on entry to evaporative cooler 100, air flow 110 has a similar temperature to the ambient air 122 but lower humidity. As a consequence, of its lower humidity, it will have a reduced dew point and evaporative cooler 100 will be able to operate to deliver primary air at a lower temperature.

[0038] 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. 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.