WO2010034994A2

WO2010034994A2 - Air conditioning

Info

Publication number: WO2010034994A2
Application number: PCT/GB2009/002276
Authority: WO
Inventors: Xudong Zhao; Saffa Bashir Riffat; Junming Li; Changhong Zhan; Duan Zhiyin
Original assignee: The University Of Nottingham
Priority date: 2008-09-24
Filing date: 2009-09-23
Publication date: 2010-04-01
Also published as: GB0817468D0; WO2010034994A3; CN102224390B; GB0905178D0; CN102224390A

Abstract

Air conditioning in certain geographical locations would be advantageous. Unfortunately traditional air conditioning depends upon refrigeration systems for air cooling. To avoid the fuel costs and the complexity of refrigeration systems a dew point heat exchanger can be utilised. The heat exchanger comprises a number of thermally conductive layers to define dry paths and wet paths adjacent to each other. Inlet air flow into a dry path is cooled by heat exchange with adjacent wet paths through the thermally conductive layer. A proportion of the inlet air flow is diverted to the wet path such that evaporation through the latent heat of evaporation causes cooling of the air flow whilst the remainder of the inlet air flow is presented to a manifold for utilisation with regard to air conditioning. It will be understood that the relative humidity between the inlet air flow and the wet path is important. In such circumstances utilisation of a dehumidifier either independently or as part of an air conditioning arrangement will facilitate operation.

Description

Air Conditioning

The present invention relates to air conditioning and more particularly to air conditioning utilising dew point heat exchange and/or humidity regulation for air and temperature control within built structures.

It will be understood that creation of a pleasant and comfortable environment within which to work or simply inhabit during leisure time is desirable. Traditionally such environments have been created through air conditioning which not only adjusts the temperature of the environmental air within the built structure but also alters the humidity of that air for more acceptability. Traditionally refrigeration cycles have been utilised for air conditioning systems. Such refrigeration systems are not ideal in terms of cost, complexity and environmental effects.

Conventional mechanical compression refrigeration cycle air conditioning systems consume huge amounts of electrical energy which in turn is largely generated from fossil fuels. Such modes of air conditioning are, therefore, neither sustainable nor environment-friendly. Evaporative cooling utilizes the latent heat of water evaporation, a kind of natural energy existing in the atmosphere, to perform air conditioning of buildings, and is therefore a potential replacement of existing refrigeration based systems. However, evaporative cooling has encountered several technical difficulties that impede its wide application. Direct evaporative cooling adds moisture to room air which causes unpleasant thermal comfort. Indirect evaporative cooling lowers the air temperature and avoids adding moisture to the air, but it limits the temperature of supply air to a few degrees Celcius above the wet bulb of the outdoor air, which is too high to perform air conditioning of buildings in the UK and most Chinese climatic conditions.

Dew point (evaporative) heat exchange breaks the wet bulb limit, and allows the supply air to be cooled to a level below the wet bulb level and above the dew point of the outdoor air. Such dew point evaporation processes provide a simply approach to air conditioning but conventional heat exchangers as indicated above for such processes are inappropriate in terms of their efficiency and operational performance. A convenient means for displacing the humidity differential across the dew point heat exchanger would be beneficial in improving efficiency. However, the de-humidification must be achieved with reduced external energy input to be acceptable in the long term and commercially.

Conventional heat exchanger designs of a cross flow type generally do not have a high level of efficiency or at least as high a level as would be desirable. Conventiona! dehumidifiers ultimately utilise gas. electric or hydrocarbon fuels but even when waste or renewable energy is utilised efficiency levels are not sufficiently high or desirable for long term objectives of substantiality.

In accordance with aspects of the present invention there is provided a heat exchanger for dew point heat exchange, the heat exchanger comprising a plurality of thermally conductive layers with alternate layers separated to define side by side dry paths and wet paths, the dry paths extending from an inlet to an outlet with a side aperture to an adjacent wet path, the wet path having an opening for moisture flow and an exhaust for the exchanger whereby moisture flow is driven at least by evaporation and in use there is heat exchange through the thermally conductive layer shared with an adjacent dry path.

Typically, the heat exchanger is associated with means for driving a proportion of an air flow through the side aperture. Generally, the side aperture and the outlet have a damper to vary the relative proportion of air flow between that presented through the outlet and the side aperture to the wet path. Possibly the damper is adjusted through an actuator associated with the damper and a sensor dependent upon temperature and/or air flow rate through the exchanger. Possibly, the outlets are coupled to a collection manifold for utilisation with regard to environmental air conditioning.

Typically the air flow through the inlet to the dry path is driven or stimulated by natural convection through orientation and configuration of the exchanger. Possibly, the means for driving the air flow comprises a fan. Typically, the heat exchanger can be re-configured to vary air flow dependent upon operational requirements. Possibly, the air flow is in the order of 500 cubic metres per hour.

Possibly, the width of the dry path and the width of the wet path are substantially the same. Possibly, the width is 2 to 10 mm and preferably 6 mm.

Possibly, the length of the dry path and the wet path are substantially the same. Typically, the length is 0.5 to 2 m and preferably 1.2 m.

Generally, the thermally conductive layer is aluminium or copper or another suitable metal or material. Possibly, the conductive layer is water proofed at least to one side.

Possibly, at least some of the wet paths include a hydrophilic dispersion such as cellulose fibre or a sintered metal.

Possibly, the thermally conductive layers are supported upon a corrugated reinforcement.

Possibly, the ratio of air flow through the inlet and the exhaust are adjusted dependent upon operational requirements. Typically, the ratio is 1 :1.

In accordance with second aspects of the present invention there is provided a dehumidifier for an air conditioning arrangement, the arrangement comprising a dessicant presented in an air flow path to one side and a regenerative path on the other, the air flow path arranged to dehumidify an air flow in use whilst the regenerative path is arranged to regenerate the dessicant by removing moisture through heating, the regenerative path including a microwave source and means to focus the microwaves upon the dessicant for localised heating of the dessicant.

Normally, the dessicant is in the form of a wheel.

Typically the means for focusing the microwaves comprise a wave guide. Typically the wave guide is formed from a metal.

Generally, the dessicant is formed from a non-polar plastic material to present hydrophilic material.

Generally, the regeneration path includes means for forced air flow. Typically, the means of forced air flow comprises a fan.

Also in accordance with aspects of the present invention is an air conditioning system or cooling system comprising a heat exchanger and/or a dehumidifier arrangement as described above.

Generally, the dehumidifier arrangement is associated with the inlet. Generally a water tank is provided to facilitate moisture flow along the wet path.

An embodiment of aspects of the present invention will now be described by way of example with reference to the accompanying drawings in which:

Figure 1 is a schematic illustration of an air conditioning or cooling arrangement in accordance with aspects of the present invention;

Figure 2 is a schematic illustration of a cross section of a heat exchanger in accordance with aspects of the present invention;

Figure 3 is a schematic illustration from a direction X as depicted in figure 2;

Figure 4 is a schematic cross section taken in the plane A-A depicted in figure 2; Figure 5 is a schematic cross section taken in the plane B-B depicted in figure 2;

Figure 6 provides a schematic illustration of sealing arrangements between wet paths and dry paths in a heat exchanger in accordance with aspects of the present invention;

Figure 7 provides a schematic illustration of operation of a heat exchanger in accordance with aspects of the present invention;

Figure 8 provides a schematic illustration of a dehumidifier in accordance with second aspects of the present invention;

Figure 9 is a schematic illustration of a central heating system incorporating a heat exchanger in accordance with aspects of the present invention with a dehumidifier in accordance with second aspects of the present invention;

Figure 10 is a schematic cross section of a dehumidifier in accordance with aspects of the present invention;

Figure 11 is a schematic cross section of the dehumidifier as depicted in figure 10 in the plane AA-AA;

Figure 12 is a schematic perspective view of one part of a dew point heat exchanger;

Figure 13 is a schematic perspective view of another part of the dew point heat exchanger depicted in figure 12; and,

Figure 14 is a graphic depiction of dry bulb temperature against absolute humidity.

As indicated above the use of a dew point air conditioning arrangement depends on the natural evaporation process with regard to water. In such circumstances the energy of evaporation is taken from the air in order to cool it but such a process may add to the humidity of the exhaust air and therefore render a closed built environment unacceptable to inhabitants of that environment. Nevertheless, in the right geographical locations and weather conditions dew point air conditioning may be practicable if an appropriate heat exchanger and/or dehumidifier arrangement can be provided either separately or in combination for the air conditioning system.

Figure 1 provides a schematic illustration of a dew point air conditioning arrangement 17 in accordance with first aspects of the present invention. Thus, the arrangement 17 comprises a heat exchanger 5 positioned with a water distribution tank 3 which feeds a water collector 9 through a water distribution pipe 4. Water is recirculated through a pipe 6 between the collector 9 and the tank 3. In such circumstances air flow is stimulated through the heat exchanger 5 through a supply cross fan 2 and controlled through an air damper 1. Air passing through the outlet of the heat exchanger 5 is cooled and is depicted by arrowhead 18.

In order to cool the outlet air 18, initially environmental air 19 is presented through the intake grille 10. A proportion of the inlet air passes through the heat exchanger 5 to be cooled and presented as outlet air 18 whilst another proportion within the casing 12 becomes entrained with moisture from the tank 3 to pass in wet paths through the heat exchanger 5 adjacent to dry paths through which air to be cooled passes. The level of water within the collector 9 is maintained by a water pump and level switch 8. In such circumstances as will be described later layers of relatively thin thermo conductive material between the wet paths and the dry paths enhanced heat exchange and cooling of eventual outlet air 18. The highly humid air having passed through the heat exchanger 5 is exhausted through an exhaust fan 7. In order to maintain operational efficiency and avoid potential problems an overflow pipe 11 is provided whilst if necessary a drain valve 14 and drainpipe 15 are provided to remove liquid from the collector 9 as required for operational performance. It will be understood that the level of liquid within the tanks 3, will to a certain extent determine the potential for evaporation and therefore dew point cooling in the heat exchanger 5 in accordance with aspects of the present invention. The supply tank 3 acts as a header for the collector 9 and has a supply valve 13 and supply pipe 16 to an external source of water. This external source will typically be tap water.

In accordance with first aspects of the present invention the heat exchanger 5 is designed to achieve high efficiency with regard to dew point heat exchange. Further details with regard to this construction are provided below with regard to figures 1 to 6. Generally, the heat exchanger comprises thin thermally conductive plates or layers to define wet and dry paths. The layers are typically formed from aluminium or copper foil and are supported to maintain the wet and dry paths. The wet paths incorporate a hydrophilic dispersion typically in the form of a cellulose fibre. The whole heat exchanger is bonded together through a thermal grease to present an operational heat exchanger in which inlet air passes along the dry paths and is cooled through the latent heat of evaporation of moisture in the wet paths. The hydrophilic dispersion acts to well and provide a resistivity to simple moisture flow. In such circumstances latent heat of evaporation is taken from the dry path to evaporate the moisture in the wet path and so through the heat exchange provided by the thermally conductive layers enhanced cooling of the air flow in the dry path achieved. It will be understood that the thermal conductivity of the thermally conductive layers in the form of aluminium foil will be in the order of 220 watts per metre Kelvin whilst the cellulose fibres have a thermal conductivity of in the order of 50 watts per metre Kelvin.

Generally as illustrated in figure 1 cross apertures and side vents 20 are provided whereby inlet air presented along the dry paths is proportionately directed either to the outlet 18 or to the wet paths to entrain and be entrained with evaporated moisture from the cellulose within the wet path for exhaustion through the exhaust 7. In such circumstances the heat exchanger 5 in accordance with aspects of the present invention achieves enhanced cooling efficiency in comparison with prior arrangements dependent upon simple cross flow techniques. It will be understood through use of appropriate dampers, particularly at the outlet 1 and the exhaust 7, regulation of the relative flows through the heat exchanger 5 to determine the level of cooling efficiency can be achieved.

Figure 2 provides a schematic cross section of the heat exchanger 5 as depicted in figure 1. As can be seen the heat exchanger 5 comprises a number of layers 21 of thermally conductive material. These layers are as indicated above generally formed from thin aluminium sheet to ensure there is good thermal conductivity between the dry/wet channels formed between the layers 21. These channels as indicated above are typically alternate dry and wet paths through the heat exchanger 5 between an inlet end 22 and an outlet end 23. In accordance with aspects of the present invention as described above in order to stimulate enhanced cooling and therefore higher efficiency for the heat exchanger 5 inlet air passing along the dry paths is subject to heat exchange with flows along the wet paths where latent heat of evaporation acts to cool the flows in the dry paths. The wet paths incorporate hydrophilic dispersions in the form of cellulose fibres such that a proportion of the air passing along the dry paths is presented through side apertures into the wet paths. Moisture is evaporated from the cellulose fibres and as a result of the latent heat of evaporation utilising heat taken from the air passing along the dry paths cooling is provided. The higher humidity air is entrained by the flow in the wet path for exhaustion as described previously through the exhaust. Clearly, a proportion of the air passing along the dry paths must also act as output air for air conditioning facilities. In such circumstances as illustrated in figure 2 a collection manifold 24 is provided to collect a proportion of the cooled air through the dry paths for output as required for air conditioning performance.

Figure 3 provides an illustration in the direction X of the heat exchanger 5 depicted in figure 2. Thus dry paths 31 are generally open whilst wet paths 32 incorporate a hydrophilic dispersion 33 along with possibly a corrugated or concertina reinforcement to maintain spacing of the path 32. In such circumstances as described previously inlet air passes along the dry paths 31 and a proportion is released through the outlet whilst a remainder passes along the dry paths 33 for entrainment of evaporated moisture within the paths 33 for dew point and latent heat of evaporation cooling effects upon the inlet flow through the dry paths 31.

Figure 4 provides an illustration in the plane A-A depicted in figure 2. As can be seen paths 31 , 32 are still provided with hydrophilic dispersions in the form of cellulose fibres in the wet paths 32 whilst the dry paths 31 do not include such a hydrophilic dispersion but as illustrated in figure 4 have a reinforcement concertina to maintain separation of the thermo conductive layers 41 defining the heat exchanger 5. The hydrophilic dispersion and reinforcing corrugations still allow air flow along the paths 31 , 32 for performance of a heat exchange function in accordance with aspects of the present invention.

Figure 5 provides a schematic cross section in the plane B-B back towards the main body of the heat exchanger 5. In such circumstances as can be seen the wet paths 32 and the dry paths 31 have side apertures 51 between them. The side apertures 51 are located towards the end of the flow direction for the dry path and towards the beginning of the flow direction in the wet path 32. In such circumstances a proportion of the air passing along the dry path is diverted into the wet path whilst the remainder passes through the outlet of the dry path into the collection manifold 23 (figure 2) for utilisation in air conditioning effects. The proportion of air passing along the wet path will be entrained and therefore exhaust moisture evaporated from and within the wet path for cooling effects on the air passing along the dry path is removed. In such circumstances an enhanced cooling efficiency is achieved.

It will be appreciated that the wet paths towards the outlet end of the dry paths must be sealed and an indicated the dry paths themselves configured to present both a proportion of the air passing along their length to the manifold 24 and also to the wet path either side or at least to one side of the dry path. In such circumstances as depicted in figure 6 whereas paths 32 have sealing 61 generally all about the path 32 towards an exhaust outlet 62 each dry path 31 has seals 63 along the sides between an inlet end 64 and an outlet end 65 to allow air flow from the inlet end 64 to the outlet 65 where a proportion of the air is presented to a manifold for air conditioning purposes and a proportion presented to the wet path 32 as described above for facilitating cooling of subsequent air flows along the dry path.

Generally, the thermally conductive layers in accordance with aspects of the present invention are secured together through appropriate thermally conductive glues or greases and if necessary with sealants. The sheets of thermally conductive material forming the layers will generally have a thickness of 1 mm whilst the concertina material utilised for reinforcement as well as air guiders may be in the order of 0.25 mm.

Figure 7 provides a schematic illustration of air flow within a heat exchanger 5 in accordance with aspects of the present invention. Consistent reference nomenclature has been utilised to facilitate understanding with earlier figures. Thus, dry paths 31 have inlet air 71 passing therealong whilst wet paths 32 receive a proportion 72 of the inlet air 71 through side or cross apertures between the paths 31 , 32. The remainder of the inlet air appropriately cooled passes as outlet air 73 into a collection manifold for utilisation in air conditioning. Typically ends of the wet paths 32 will be closed by closures 74 but with a mechanism whereby moisture in the form of water 75 can pass into the wet paths 32. As indicated above the wet paths 32 include a hydrophilic dispersion in the form of cellulose fibres to act as a brake upon moisture and therefore necessitating greater latent heat of evaporation taken from the inlet flow 71 for cooling effects. In such circumstances exhaust air 76 passes out of the wet paths 32 for dispersion externally from a building within which an air conditioning system incorporating a heat exchanger 5 in accordance with aspects of the present invention is presented.

It will be appreciated that the level of cooling in the outlet air 73 will depend upon operational requirements. By adjusting the speed of inlet air flow 71 , the level of moisture within the wet path 32, the proportioning of flows 72, 73 as well as the dimensions and thicknesses of the flow paths 31, 32 and the layers 77 of thermally conductive material it will be understood a heat exchanger suitable for operation in an air conditioning system can be defined. Typically, air flow distribution across the heat exchanger 5 will be set to achieve a desired cooling efficiency. For example with a room of 20 m² and a 30 watts per m² cooling load a flow rate in the order of 500 cubic metres per hour may be acceptable. The exhaust flow rate will be substantially the same. In such circumstances the ratio between the inlet air 71 and the exhaust air 76 will generally be 1:1.

Normally, the inlet air flow 71 will be forced through use of a fan and similarly through a suction or drawing effect the exhaust air 76 will be drawn from the wet paths 32 to stimulate flow through the heat exchanger 5. Again the rate of such forced air flows at the inlet air 71 location and the exhaust air 76 location can be adjusted by altering the speed of the fans providing means for driving air flow at those positions.

Generally, the flow paths in accordance with aspects of the present invention as indicated will be defined for operational purposes. In such circumstances the channels defining the dry paths 31 and the wet paths 32 may have a width of 6 mm but will normally be in the range 2-10 mm and the length of the paths 31 , 32 will also be substantially the same and in the range

0.5 m to 2 metres dependent upon requirements. It will be understood particularly with regard to the wet paths that the length of the paths should be such that recondensation of the moisture entrained there along does not occur for operational efficiency in terms of cooling the inlet air 71 to ensure that adequate cooled outlet air 73 is provided through the heat exchanger 5.

Generally, the wet paths 32 and the dry paths 31 will be substantially of the same dimensions. However, where desirable the paths 31 , 32 may have different shapes and rather than having a consistent width along their length may be tapered as required. The size of the side apertures determines the relative proportioning between the outlet air 73 and the air 78 presented along the wet paths 32 and will be set simply by relative dimensioning between the open outlet to releases the outlet air 73 and the size of the side aperture for leakage of air across into the wet path 32. Typically all side apertures will all be of the same dimensions but could vary through the width of the heat exchanger formed in accordance with aspects of the present invention. In such circumstances outer apertures could have larger sizes than inner apertures and therefore greater returned air flow rate for entrainment of evaporated moisture within the wet paths, or vice versa.

In order to facilitate heat exchange typically the thermally conductive layers utilised in aspects of the present invention will be relatively thin. In such circumstances these layers may have a thickness which is insufficient for self support and therefore require support upon concertina arrangements as described above. It will also be understood that the hydrophilic dispersion in the form of cellulose fibres or sintered structures may themselves facilitate retention of wet path dimensions.

As the process for proportioning air flow between the outlet for utilisation as air conditioning and return to the wet path is generally a passive process, that is to say utilises an aperture as a side feature between the paths it will be understood through a damper associated with the outlet from the heat exchanger or air conditioning system the proportion of air flow returned along the wet path may be adjusted. This damper as illustrated in figure 1 may be associated with an appropriate actuator which in turn may be dependent upon flow rates and temperatures to adjust the open aperture size and therefore the potential flow rate for the outlet from the heat exchanger. Damping outlet flow rates will result generally in more air being returned along the wet path in accordance with aspects of the present invention.

As indicated above heat exchangers in accordance with first aspects of the present invention are generally incorporated within air conditioning systems and arrangements to achieve air conditioning performance within built structures such as offices, houses and public spaces. Traditional approaches with refrigeration systems for providing air conditioning as indicated are generally expensive in terms of fuel costs and therefore may be no longer sustainable. Furthermore, in situations where access to reliable electrical power supplies is difficult, but where the environmental temperatures and humidity conditions allow, provision of dew point heat exchangers as above would be beneficial. Such heat exchangers would be located within an air flow path and as indicated depend upon the dew point/latent heat of evaporation processes. It will be appreciated that dew point air conditioning systems are more acceptable in drier hot environments in comparison with more humid environments. The dew point system depends upon evaporation and therefore evaporation into a more humid environment is more difficult than into a drier environment. However, dew point air conditioning systems can be combined with an appropriate dehumidifier to render heat exchanges and dew point air conditioning systems more suitable for humid environments.

Traditional dehumidification has been performed as illustrated in figure 8 utilising a dessicant normally in the form of a wheel 81 which is arranged to turn across a dehumidification flow path defined by arrowheads 82 through the wheel 81 to present dry air 83. The air flow path as indicated generally takes relatively wet air 82 propels it through a fan 84 and the dessicant wheel in order to create the dry air 83. The dessicant wheel 81 is turned by a motor 85 or other drive mechanism such that the part of the wheel exposed to the flow path 82 to 83 is not consistent and progressively moves around with the wheel 81 rotating on an axis through the drive 85. In such circumstances whilst one part of the wheel 81 provides a dehumidification process between the flow 82 and the dry air 83 another part is a regenerative path between the heater 86 and means for driving or sucking a reaction or regenerative air flow 87 through the wheel 81. In such circumstances the regenerative path 87 through a heater 86 to remove moisture from the wheel 81 results in the wheel 81 when rotated being again able to remove moisture from the air flow 82 to present dry air 83.

Several porous materials including silica gel, activated alumina, molecular sieves or mixture of those, are commonly used as a desiccant to remove moisture from the air. The desiccant is integrated into a wheel which is keeping turning during operation. Part of the wheel face surface is exposed to the incoming air streams acting as a dehumidifier; part of it is exposed to hot air streams which are supposed to extract moisture from the desiccant to achieve generation of the desiccant; and the remainder is subject to a cooling process by passing the cold air across the surface to enable the reuse of the desiccant for moisture attraction. The schematic of the desiccant system is shown in figure 8.

Using the traditional regeneration method, the process air needs to be heated using a heater driven by gas, electricity, oil or a source of waste/renewable energy, and also to be drawn across the wheel using a fan/ducting system. This process involves inefficient energy transfer occurring in associated heat exchanging equipment, as well as significant energy waste due to unexpected heating of the desiccant and its subsequent cooling. This system is therefore bulky, costly and highly energy consuming. Traditional regeneration is also slow responding due to the low thermal mass of the air, which would lead to a long necessary treatment process time for regeneration.

As indicated above traditional regeneration using a dessicant wheel can be inconvenient and may necessitate a relatively large wheel to provide an adequate area for both dehumidification and then regeneration.

Microwaves are a sort of electromagnetic energy, which interacts with the polar molecules and free ions existing in the objects to be targeted. This would cause molecular friction and subsequently generate heat within the interior of the objects exposed, leading to a temperature rise or evaporation from the object of moisture (if it is a liquid with a suitable evaporation temperature, e.g., water). This process is known as volumetric heating, and commonly used in cooking, drying, medical treatment, e.g., cell destruction for cancer patients.

The frequency of the microwaves varies from 915 MHz and 2450 MHz, depending upon the application for which the microwaves are used. Most microwave ovens operate at 2450 MHz with a wavelength of 4.8" in air. However, small scale domestic ovens may operate at 915 MHz with a wavelength of 13". The object targets of the microwaves at this frequency range are usually water, fats and sugars which comprise of abundant polar molecules. Metals tend to reflect microwaves, and so are able to be used as conduits, called waveguides, or containers to hold material to be irradiated in the microwave fields. Some non-polar compounds, e.g., plasties, giass, ceramics and porous desiccants, are transparent to microwaves. This allows the materials to be free of heating when these are under microwave irradiation. For saturated desiccants, microwave energy becomes selective and focuses only on the water moistures within the desiccants, thus saving energy needed for heating and subsequent cooling of the desiccant material. This unique feature makes microwaves systems much more attractive for dehumidifying requirements than the traditional systems currently used.

In microwaves system, the moisture adsorbed is changed to vapour by absorbing heat generated by interaction between the polar molecules and the electromagnetic waves, which is then led away from the desiccant bed by a forced air flow stream. Because of direct contact between the waves and molecules, heat generation will be immediate and equally spread across the whole desiccant bed, leading to a much more prompt thermal response and enhanced energy conversion efficiency. It is estimated that the microwave system would be 50% more efficient than the traditional systems in terms of energy conversion.

In the above circumstances it will be appreciated use of a microwave source which is appropriately focused through a wave guide upon a part of the dessicant wheel will enable more rapid, efficient and with reduced energy cost operation of a dehumidifier for air conditioning purposes. In such circumstances more convenience can be achieved with regard to utilisation of the dehumidifier to provide a greater humidity gradient for utilisation in a heat exchanger in accordance with first aspects of the present invention. However, the dehumidifier according to second aspects of the present invention may also be utilised with more conventional heat exchangers. Essentially, water is formed by polar molecules which are excited by microwaves whilst the material, that is to say the dessicant is traditionally non polar and therefore transparent to microwaves. In such circumstances the microwave energy is selectively focused upon the water molecules and so more efficient regeneration is achieved, in such circumstances less energy will be required although shielding may be necessary to protect individuals who themselves are substantially made of water molecules from the microwave source. Clearly, the microwave source needs to be shielded from the dehumidification flow path of the dehumidifier or the effects of such dehumidification may be diminished. Thus, generally simple shielding between the air flow path and the regenerative path is required.

In the above circumstances, a dehumidifier in accordance with second aspects of the present invention will have a similar configuration to that depicted in figure 8 except that the traditional heater 86 will be replaced by a microwave source and some form of shielding or wave guide will be utilised to focus and protect microwave energy from environmental problems such as diminishing the initial primary dehumidifier effects upon the air flow 86 to 83 and possibly having detrimental effects upon users about the dehumidifier.

It will be appreciated that microwave dehumidifiers/regeneration arrangements as described above may be formed as an independent unit located at a vent 2 in built environments such as an office or closure or alternatively could be constructed as an integral unit with the air conditioning system whether incorporating a heat exchanger in accordance with first aspects of the present invention or not. When produced as an independent unit as indicated it will be understood that the unit will simply act to reduce the humidity within the environment of the air conditioning system and therefore through the closed nature of a built environment acts as a simple dehumidifier of the whole environment. For more focused utilisation generally a dehumidifier incorporating microwave regeneration as described above will be an integrated unit with an air conditioning system utilising a dew point heat exchanger in accordance with aspects of the present invention. In such circumstances the microwave dehumidifier arrangement will act directly upon the inlet air in order to reduce its moisture content and therefore the humidity differential between that inlet air and the air within the heat exchanger in order that liquid will be entrained by that proportion of the air returned through the side apertures to the wet paths in the heat exchanger.

Figure 9 provides a schematic illustration of an integral air conditioning system incorporating a microwave dehumidifier and a dew point heat exchanger for air conditioning in accordance with aspects of the present invention. In such circumstances a fan unit 91 incorporates a respective inlet fan 93 and an outlet fan 94 for drawing and driving air flow into the air conditioning system. The inlet flow 93 draws air into the system where a dew point heat exchanger 92 and a dehumidifier 93 are provided to extract heat from the inlet air to reduce its temperature as well as humidity. It will be appreciated that the dehumidifier unit 93 may act initially to accentuate the humidity differential across the heat exchanger 92 and then with regard to the outlet air further dehumidify that air if required for operational conditioning. In such circumstances additionally a filter unit 95 may be provided to remove any debris including dust as well as ensure the outlet air flow is at a desirable temperature as well as humidity. It will be understood that it would be easy for the dessicant to remove too much humidity and therefore leave the air too dry for acceptability to users of a room. By mixing with environmental air it will be understood that a more appropriate humidity may be achieved.

In terms of power consumption for a dehumidifier utilising microwaves in accordance with aspects of the present invention it will be appreciated at room temperature and normal atmospheric pressure 1 kw of microwave energy will evaporate approximately 1.135 kg of water in an hour. In such circumstances for a 27 m² room the moisture load would be 0.01 kg and the power consumption of a microwave dehumidifier in such circumstances would be 1 watts. Such energy levels, which are less than a conventional light bulb within a room, would be acceptable in terms of costs and for longer term sustainability. Clearly, use of microwave energy causes special design problems. In such circumstances generally metal honeycomb vents will be utilised to shield openings for ventilation against undesirable electromagnetic waves. Such shielding of electromagnetic radiation is to prevent any possible harm to the health and safety of users, occupants and operators of dehumidifier arrangements, air conditioning systems or heat exchangers in accordance with aspects of the present invention. For safety guarantees the systems should be utilised with safety interlocks, temperatures sensors and leakage detectors that switch off power immediately when a system malfunction or misuse is determined and provide an alarm light and/or audible signal.

Air conditioning systems and dehumidifiers in accordance with aspects of the present invention may be located within a building. In such circumstances the dehumidifier may be arranged to present an inlet and therefore an inlet for the air conditioning system/heat exchanger through a window opening. Figure 10 and figure 11 provide schematic illustrations of a dehumidifier unit 200 for location in a window aperture. Thus, the dehumidifier arrangement comprises a microwave source 201 in the form of a magnetron which is focused and directed towards a dessicant wheel 202 arranged to rotate through a drive motor 203 and a drive belt 204. Electromagnetic shielding 205 is provided about three quarters of the wheel 202 to avoid any reduction in the dehumidifying effect of the wheel 202 other than in the regeneration path created by the heat generation source of the magnetron or microwave source 201. The dehumidifier arrangement is located within a casing 206 with an input air flow generated by a fan 207.

Figure 11 provides a schematic cross section in the direction AA-AA of figure 10. In such circumstances the dessicant wheel 202 is rotated by the motor (not shown) whereby two flow paths are created. An air flow path in the direction of arrowheads 210a, 210b passes through the casing 206 and the dessicant wheel 202 to be ejected through the fan 207 as dry air 210b. A regenerative flow path is created by a regenerative or reactive air flow 211a, 211b passing again through the dessicant wheel 202 to remove moisture from that wheel 202 rather than absorb it in a dehumidifying function. In order to remove moisture the wheel 202 is heated by a magnetron as a microwave source 201 and in such circumstances the air flow 211b is wetted for external ejection and exhaustion from the dehumidifier arrangement. It will be appreciated that electromagnetic shielding 205 will be provided at appropriate positions within the dehumidifier arrangement 200 and in particular at inlets and outlets to ensure microwaves do not cause environmental problems to users of the arrangement 200 as well as diminish the dehumidifying effect in other parts of the arrangement 200. It will be noted that the microwave source 201 itself will have a cooling air source 212 and this may be added to the reactive or regenerative air flow 211.

By a combination of a heat exchanger and/or a dehumidifier in accordance with aspects of the present invention a more acceptable air conditioning system for a building can be achieved. In such circumstances where weather conditions prevail it may be possible to utilise dew point action to provide cooling air flows and either separately or as part of the air conditioning system condition the inlet air flows by dehumidification with a more effective and less costly dehumidifier arrangement utilising microwaves in accordance with aspects of the present invention. Generally, the dew point heat exchanger will utilise tap water and typically the consumption rate will be in the order of 1.1 to 2.4 litres per kilowatt hour cooling output. Such usage should be acceptable in normal operational conditions say for example in a situation where air conditioning is provided during working hours, that is to say between 9.00 am and 5.00 pm the daily consumption rate of a dew point heat exchanger in accordance with aspects of the present invention will be in the order of 60 to 70 litres with a two kilowatt cooling input and an air flow rate in the order to 570 to 1 ,800 m² per hour in a 100 m² building with a 30 watts of square meter cooling demand. However, the consumption rates etc will depend upon humidity and other operational requirements.

Aspects of the present invention utilise as indicated dew point

(evaporative) cooling which breaks the limit for wet bulb humidity constraints and allows the supply air to be cooled to a level below the wet bulb and above the dew point of the outdoor air. To achieve this a dew point heat exchanger is created which can achieve dew point effectiveness of up to 85% in terms of cooling. By such advances with regard to dew point cooling wider application of evaporative cooling is possible.

The heat exchanger comprises a number of polygonal sheets stacked to create guides for air flows of the same material. Typically one side of the thermally conductive sheet forming the heat layer will be coated with a water proof material to avoid water penetration. Inlet of intake air is brought into the dπ/ path channels from typically a lower part of the heat exchanger stack. In such circumstances in operation air flows through the dry channels or dry paths initially for cooling and then at an outlet part of the dry channel a proportion of the air flow continues to move in the same direction to provide air conditioning effects while another part of the air flow is diverted into adjacent wet channels or wet paths where surfaces are wetted by water. The wet paths allow heat to be absorbed through the thermally conductive layers by vapourisation of the water on the surfaces. The air in the wet paths flows in a reverse direction and is finally discharged to atmosphere from exhaust portions. Due to heat transfer between the dry paths and the wet paths the product air conditioning air in the dry paths will be cooled and the working air in the wet channels or paths will be humidified and heated. Figure 12 and figure 13 respectively illustrate such operation of a dew point heat exchanger in accordance with aspects of the present invention. In such circumstances inlet or intake air 301 passes along a dry path 302 formed between sheets of thermally conductive material 304. The sheets of thermally conductive material 304 define the wet paths as well as the dry paths. At a distal or far end of the dry path side apertures or holes 303 act to divert a proportion of the inlet air 301 into wet channels 305 whilst the remainder of the inlet air, now cooled acts as product air for air conditioning in accordance with aspects of the present invention as illustrated by arrowheads 306. Within the wet channel or wet path water and moisture is provided and this water or moisture is evaporated by heat exchange through the sheet 304 in order that the latent heat of evaporation cools the inlet flow 301 and therefore the product or output flow 306 utilised for air conditioning. The now humidified air in the wet path is discharged in the direction of arrowhead 307 through an exhaust.

!t will be appreciated that a large number of layers of thermally conductive material 304 are provided and it is by creating a highly thermally conductive association between the dry paths and the wet paths and forcing air flows that adequate cooling is achieved. It will also be understood that the wet paths incorporate cellulose fibre or other material to act as a hydrophilic dispersion to present the moisture and water for evaporation.

The air treatment process in accordance with aspects of the present invention is illustrated in a graph as depicted in figure 12. Outdoor air O is initially pre-treated using a moisture controller which would allow its moisture content to be lowered to the same level as the indoor air, thus reaching a state O'. The air is then mixed with indoor air with state T, creating a new state '1 ' which is the state of the intake air of the exchanger. The intake air is delivered into the dry channels, where it transfers heat to the adjacent wet channels, and is cooled from state '1' to '2', with no moisture added into the air. Part of the air is delivered to a room space for cooling of the space. The remaining air flows into the adjacent wet channel, where it firstly becomes saturated due to absorbing moisture presented on the channel surface, and then continues to absorb sensible heat and moisture due to heat transfer between the dry and wet channels, which contributes to evaporation of water on the wet surface. The air is finally discharged to the atmosphere as the saturated and hot air streams, defined as state '3'.

For comfort air conditioning, the moisture level of indoor air could vary over a wide range, with the associated humidity ranging from 30 to 70%. This allows the smallest possible moisture removal from the fresh air and minimum energy consumption used for air dehumidification.

Mathematical analyses of the cooling performance of the dew point system. The integers used in the following expressions are as follows:

C_p - specific heat of air, kJ/kg.°C; d - moisture content of air, kg/kg dry air; h - enthalpy of air, kJ/kg dry air; M - water consumption rate per kWh cooling output, litre/kWh;

M_daii_y - daily water consumption for the selected building, litre/day;

Q - cooling capacity of the dew point system, W;

Q_p - cooling output of the supply air, W;

Q_p1 - internal sensible load taken by the supply air, W; Q_p2 - fresh air load taken by the supply air, W; t - temperature of air, ⁰C;

V - air volume flow rate, m³/h;

V_target - air volume flow ate for the target building, m³/h; p - air density, 1.2kg/m³; p_w - density of water, 1 kg/litre; η_d - dew point effectiveness;

Subscripts

O - outdoor air;

0 - fresh air after the dehumidifier; i - indoor air;

1 - intake air; 2 - supply air;

3 - discharging air;

dp - dew point.

The cooling capacity of the dew point system can be calculated as follows:

Q = pV₂(h_]-h₂)/3.6 (1)

The relationship between states /I₁ 2 and 3 can be expressed as follows:

A₁-A₂=^(A₃-A₂) (2)

A₂=A₁-I₇^(A₁-A*,) (3)

d₂=d, (5)

Obtaining the value of h₃, d₃ and t₃ can be acquired from the psychrometric correlation equations of air accordingly.

Water consumption per k Wh cooling output will be

M = 3600 ^^{3 "}^¹ (7)

A₁M-A₂)

To keep room air distribution in balance, fresh air flow volume should be same as the exhaust air flow volume, ie V₀ = V₃, and the return air flow volume would be same as the supply air flow volume, ie V₁ = V₂. In that case, the cooling capacity of the system can be calculated using the following equation:

Q_p = C_pPV₂ {t, - t₂ )/3.6 (8)

If room temperature is t,, the cooling energy used for removing internal sensible heat load can be written as,

Q_Pi *= C_pPV₂ (t, -t₂ )/3.6 (9)

This part of cooling energy is defined as the effective cooling output as it is used to remove interna! load.

The cooling energy used for removing fresh air load can be written as,

Q_p2 = C_pPV_o(t_ϋ -t_i)/3.6 = C_pPV₂(t₁ -t_i)/3.6 (10)

For a 2kW effective cooling output, the required volume flow rate can be calculated as follows:

v,-,_r -≡S- (ID

Taking a 100 m² office building space with 30 W/ m² cooling load as an example, if the system operation is limited to daytime, ie 9:00 am to 5:00pm, then the total cooling energy required would be 100 x 30 x 8/1000 = 24kWh. The water consumption for the daytime office operation would be:

M_ιlωιkl = 24M (12)

It will be understood that the dew point heat exchanger utilised in accordance with aspects of the present invention may be utilised in a number of stand alone air conditioning units positioned within individual rooms of a built structure such as a house or workspaces in an office building. Alternatively, the heat exchanger, or several heat exchangers, may be integrated with a central air conditioning system for an entire building. In such circumstances air conditioning within the building may be centrally controlled with water evaporation and then conditioned air presented to individual rooms or spaces through ducting paths. It will be understood that where accurate control of room air temperature and humidity is required processed air may need to be pre-humidified prior to utilisation of the dew point heat exchanger as described above. In such circumstances simple dehumidifiers in the form of silicon gel pads as dessicants may be used which can then be regenerated utilising microwaves as described above. In such circumstances a possibility for a constant dew point air supply is possible with a reciprocal constant cooling capacity achieved.

In order to provide the dew point heat exchanger in operation in accordance with aspects of the present invention it will be understood that moisture and in particular water must be utilised for the evaporation process in the cooling of the processed air through the dry paths. Due to its availability and easily of supply normally tap water will be used. However tap water is also used for other functions and therefore consideration must be made with regard to acceptability of such use particularly where rain water or untreated river water might be available.

Ideally, the tap water in terms of temperature should be lower than the dew point of the atmosphere about the heat exchanger. Such a situation allows effective cooling to be achieved through an air conditioning system utilising a dew point heat exchanger in accordance with aspects of the present invention. As tap water is generally delivered from a water source through pipes and these pipes are embedded at a level below the ground water temperature will generally be stabilised at the soil temperature at that depth.

In such circumstances water temperature will be at least the same or slightly lower than the dew point of the atmosphere above the ground within which the pipes are located. Such a situation allows dew point heat exchange cooling to be carried out in an effective way. Furthermore, in terms of water volume consumption a calculation can be made based upon a number of assumptions namely that the discharge to air ratio is 0.5 and the dew point effectiveness is 0.85. In such circumstances water consumption rates may be in the order of 2.1 to 2.4 litres per kilowatt hour and as described above this leads to a consumption rate in the range of 64 to 72 litres per day. However such evaluations are very dependent upon the level of humidity and dry and hot climate regions usually consume more water than mild and humid regions.

As described above the cooling capacity of a dew point heat exchanger can be calculated based upon equations 1 to 9. Based on a 1 cubic metre per hour of air supply/discharge rate, the calculations yield the air conditioning system's total capacity as well as the ventilation load associated with the system which is the energy used for bringing the temperature of the intake or inlet air from outdoor values down to an acceptable indoor level. In such circumstances the net cooling output, known as the effective cooling capacity, is a figure of total cooling capacity subtracted from the ventilation load. The effective cooling capacity is dependent upon weather conditions, particularly dry bulb state, wet bulb state and the dew point of the ambient air and therefore will vary from location to location. However, for an example, an effective cooling capacity can be found to be in the range of 2.9 to 9.5 watts per cubic metre per hour air flow rate and this gives an average of 2.97 for the effective cooling capacity of the heat exchanger in accordance with aspects of the present invention. It will be understood in such circumstances during most of the summertime the effective cooling capacity can fall into a cooling band of 1 to 5 watts per cubic metre per hour air flow whilst night cooling capacity is slightly higher than that of the daytime as the ventilation load in night conditions is lower than in daytime conditions.

By reviewing different atmospheric conditions it can be found that higher ambient temperature leads to a lower effective cooling capacity as larger proportions of the cooling energy generated by the air conditioning system are utilised for removing the ventilation load. Higher ambient humidity also reduces an air conditioning system's cooling capacity due to the smaller temperature difference between the dry bulb and dew point conditions. For a fixed effective cooling output of 2 kW the required volume flow rate can be calculated using equation 11 above. It is found that typically for a 2 kW cooling capacity the necessary flow rate is in the order of 540 to 1 ,900 metres per hour.

To achieve a comfortable indoor air condition generally the intake air must be adjusted to an acceptable humidity level of 70% and below. In such circumstances a dehumidifier is required and this dehumidifier may act independently or through the air conditioning system incorporating a heat exchanger in accordance with aspects of the present invention.

It will be understood that by utilisation of a heat exchanger and/or a dehumidifier in accordance with respective aspects of the present invention dew point air conditioning can be found suitable for a number of climatic conditions. Where humidity is high then a pre-dehumidification device as described above or otherwise will be necessary in order to allow the dew point heat exchanger to operate appropriately. A lower relative humidity results in a higher temperature difference between the dry bulb and dew point temperatures, and higher cooling capacity of the dew point heat exchanger utilised in an air conditioning system. If air is at a relative humidity of 70% or below, dew point air conditioning utilising a heat exchanger in accordance with aspects of the present invention can be readily used for cooling within buildings and other structures.

Generally tap water will be easily available and can be used to support cooling within the dew point air conditioning system in accordance with aspects of the present invention. Tap water will be at about or slightly below the dew point of ambient air which ensures that its usefulness in cooling is improved. Being at a lower temperature it will be appreciated that this temperature must be raised by the inlet air through heat exchange and therefore cooling of that inlet air will proportionately increase.

By a dew point heat exchanger in accordance with aspects of the present invention it will be appreciated a construction of heat exchanger is achieved in which thin walled highly thermally conductive layers of material are presented to define respectively dry paths and wet paths. The dry paths allow air cooling such that a proportion of the air can be returned and diverted to the wet path while the bulk of the air or a proportion as required can be presented for air conditioning functions. The thermally conductive layers are made from a thin sheet material such as aluminium and therefore are generally presented upon and supported by corrugations as well as hydrophilic dispersions to facilitate the evaporation process for cooling within the dry path. By creating relatively thin in terms of width but long dry and wet paths appropriate cooling of the air within the dry path can be achieved. Furthermore, by having a large number of such side by side dry paths and wet paths it will be understood that the wetted heat exchange surface area is large such that through forced air flow greater heat exchange is achieved.

The side apertures by which a proportion of the inlet air flow is diverted to the wet path will generally be of a size to achieve the necessary proportioning between cooled air flow and air flow for entrainment of the vapourised water within the wet path. Possibly the size of the apertures may be varied across the width of the heat exchanger but such variation will depend upon operation requirements. Generally a damper will be provided at the outlet which will act as a resistance to processed cold air utilised for air conditioning and therefore will vary the level of air flow, through back pressure, diverted to the wet path in use.

Dehumidifiers which utilise a silicon gel generally in a dessicant wheel format when subject to microwave action will exaggerate the effect of the microwave's action due to the non-polar nature of the material of the dessicant, that is to say the silicon gel. In such circumstances greater regenerative effects are created within a dehumidifier in accordance with second aspects of the present invention in comparison with prior arrangements dependent on radiant heat. In accordance with first aspects of the present invention a polygonal exchanger for dew point cooling has been developed. Results indicate that the new exchanger could achieve an enhanced dew point effectiveness of up to 90%, which is 20% to 30% higher than the conventional counter flow exchangers. Advanced dew point cooling technology opens up the opportunity for wide application of evaporative cooling for air conditioning of buildings in the UK and China. To allow the actual application of the technology, a feasibility study has been conducted to investigate the most prominent factors that affect the performance of dew point cooling for air conditioning, including weather conditions and water resource/availability in both the UK and China. This will lead to development of the dew point air conditioners suitable for use under UK and Chinese climatic conditions.

Adaptations and modifications to aspects of the present invention will be appreciated by people skilled in the technology. Thus for example a heat exchanger may be arranged such that the heat exchanger is zoned and various zones of heat exchanger may be brought into operation as required dependent upon current operational conditions. Furthermore as indicated above generally a hydrophilic dispersion in the form of fibrous cellulose is presented within the wet path to enhance the evaporation function of the dew point heat exchanger in terms of cooling effect on the dry path through the thermally conductive layer between. In such circumstances where zones within the dew point heat exchanger are created it will be understood that these zones may be switched to allow recovery or re-wetting of the hydrophilic dispersion in use. Generally air conditioning systems in accordance with aspects of the present invention will incorporate a heat exchanger in the form of a cassette which can be loaded within an air flow path or conduit between an inlet and an outlet. In such circumstances once the cassette in terms of its operational function has diminished due to a reduction in the capacity of the hydrophilic dispersion or otherwise such as with regard to blockage of a proportion of the dry or wet paths then that cassette can be removed and replaced readily.

Claims

1. A heat exchanger for dew point heat exchange, the heat exchanger comprising a plurality of thermally conductive layers with alternate layers separated to define side by side dry paths and wet paths, the dry paths extending from an inlet to an outlet with a side aperture to an adjacent wet path, the wet path having an opening for moisture flow and an exhaust for the exchanger whereby moisture flow is driven at least by evaporation and in use there is heat exchange through the thermally conductive layer shared with an adjacent dry path.

2. An exchanger as claimed in claim 1 wherein the heat exchanger is associated with means for driving a proportion of an air flow through the side aperture.

3. An exchanger as claimed in claim 1 or claim 2 wherein the side aperture and the outlet have a damper to vary the relative proportion of air flow between that presented through the outlet and the side aperture to the wet path.

4. An exchanger as claimed in claim 3 wherein the damper is adjusted through an actuator associated with the damper and a sensor dependent upon temperature and/or air flow rate through the exchanger.

5. An exchanger as claimed in any preceding claim wherein the outlets are coupled to a collection manifold for utilisation with regard to environmental air conditioning.

6. An exchanger as claimed in any preceding claim wherein the air flow through the inlet to the dry path is driven or stimulated by natural convection through orientation and configuration of the exchanger.

7. An exchanger as claimed in claim 6 wherein the means for driving the air flow comprises a fan.

8. An exchanger as claimed in any preceding claim wherein the heat exchanger can be re-configured to vary air flow dependent upon operational requirements.

9. An exchanger as claimed in any preceding claim wherein the air flow is in the order of 500 cubic metres per hour.

10. An exchanger as claimed in any preceding claim wherein the width of the dry path and the width of the wet path are substantially the same.

11. An exchanger as claimed in claim 10 wherein the width is 2 to 10 mm and preferably 6 mm.

12. An exchanger as claimed in any preceding claim wherein the length of the dry path and the wet path are substantially the same.

13. An exchanger as claimed in claim 12 wherein the length is 0.5 to 2 m and preferably 1.2 m.

14. An exchanger as claimed in any preceding claim wherein the thermally conductive layer is aluminium or copper or another suitable metal or material.

15. An exchanger as claimed in any preceding claim wherein the conductive layer is water proofed at least to one side.

16. An exchanger as claimed in any preceding claim wherein at least some of the wet paths include a hydrophilic dispersion such as cellulose fibre or a sintered metal.

17. An exchanger as claimed in any preceding claim wherein the thermally conductive layers are supported upon a corrugated reinforcement.

18. An exchanger as claimed in any preceding claim wherein the ratio of air flow through the inlet and the exhaust are adjusted dependent upon operational requirements.

19. An exchanger as claimed in claim 18 wherein the ratio is 1 : 1.

20. A heat exchanger substantially as hereinbefore described with reference to the accompanying drawings.

21. A dehumidifier for an air conditioning arrangement, the arrangement comprising a dessicant presented in an air flow path to one side and a regenerative path on the other, the air flow path arranged to dehumidify an air flow in use whilst the regenerative path is arranged to regenerate the dessicant by removing moisture through heating, the regenerative path including a microwave source and means to focus the microwaves upon the dessicant for localised heating of the dessicant.

22. A dehumidifier as claimed in claim 21 wherein the dessicant is in the form of a wheel.

23. A dehumidifier as claimed in claim 21 or claim 22 wherein the means for focusing the microwaves comprise a wave guide.

24. A dehumidifier as claimed in claim 23 wherein the wave guide is formed from a metal.

25. A dehumidifier as claimed in any of claims 21 to 24 wherein the dessicant is formed from a non-polar plastic material to present hydrophilic material.

26. A dehumidifier as claimed in any of claims 21 to 25 wherein the regeneration path includes means for forced air flow.

27. A dehumidifier as claimed in claim 25 wherein the means of forced air flow comprises a fan.

28. A dehumidifier substantially as hereinbefore described with reference to the accompanying drawings.

29. An air conditioning system incorporating a heat exchanger as claimed in any of claims 1 to 20 and/or a dehumidifier as claimed in any of claims 21 to 28.

30. An air conditioning system as claimed in claim 29 wherein the dehumidifier arrangement is associated with the inlet.

31. An air conditioning system as claimed in claim 29 or claim 30 wherein a water tank is provided to facilitate moisture flow along the wet path.

32. An air conditioning system substantially as hereinbefore described with reference to the accompanying drawings.