WO2013094206A1

WO2013094206A1 - Heat and mass exchanger for liquid desiccant air conditioners

Info

Publication number: WO2013094206A1
Application number: PCT/JP2012/008152
Authority: WO
Inventors: Michael Charles TOMLIN
Original assignee: Sharp Kabushiki Kaisha
Priority date: 2011-12-21
Filing date: 2012-12-20
Publication date: 2013-06-27
Also published as: CN103958995A; MY168602A; CN103958995B; GB2497789A; GB201122045D0

Abstract

A heat and mass exchanger for exchanging heat and mass between an airsteam (16), a liquid desiccant (26) and a refrigerant, comprises one or more roll-bond plates (10). Each roll-bond plate has at least one internal passage (18), the passage(s) being generally in the plane of the respective roll-bond plate. In use, a refrigerant is caused to flow through the internal passage(s) of the roll-bond plate(s). One or more flow areas (14) for a liquid desiccant are defined over at least part of an external surface of the roll-bond plate(s). In operation the air stream (16) makes contact with a large area wetted by flow of liquid desiccant (26), providing effective transfer to the desiccant of heat from the air and of latent heat from water vapour carried in the air. At the same time, the refrigerant flowing in the internal passage(s) (18) of the roll-bond plates (10) continuously cools the desiccant.

Description

HEAT AND MASS EXCHANGER FOR LIQUID DESICCANT AIR CONDITIONERS

The present invention relates to a heat and mass exchanger and, more particularly, a heat and mass exchanger for the extraction of sensible and latent heat from a conditioned space.

Conventional vapour compression air conditioners extract latent heat (water vapour) from air by condensation on the surfaces of an evaporator that is maintained below the dewpoint temperature. The major problem with this conventional method is that latent heat removal, dehumidification, is controlled by the evaporator temperature and thus cannot be controlled independently of the thermostat-regulated air temperature. In high humidity environments, where the latent load is high, the dependency of humidity on thermostat temperature leads to uncomfortably high relative humidity in the conditioned air with consequent detrimental effects of immediate comfort and the long-term health of users due to the build-up of condensation and mould on interior walls. Furthermore, the requirement to cool air to the dewpoint temperature corresponding with the desired outlet moisture content requires either "overcooling" and reheating the air, or supplying air at low flow rates with highly efficient air mixing in order to achieve comfortable air conditions across the room; both approaches are thermodynamically inefficient.

Desiccants are capable of absorbing water vapour from an air stream at temperatures above the dewpoint. The rate of absorption can be controlled independently of temperature by regulating the desiccant drying conditions, termed "regeneration" or "desorption". Liquid desiccants have the advantage of being able to transport the absorbed moisture in a liquid state to a "sink" outside the conditioned space where regeneration can occur. This function is highly beneficial for split-type air conditioners which dominate the market today. It is the aim of this invention to provide a compact, lightweight and cost-effective air-desiccant-refrigerant contactor which is capable of continuously cooling a low flow of corrosive liquid desiccant such as aqueous lithium chloride, resulting in a higher evaporating pressure than existing liquid desiccant vapour compression air conditioning (LDVCAC) systems and thus a higher coefficient of performance (COP) in the operating cycle.

US 2122012 (Smith) describes an air conditioner in which a hygroscopic liquid (a desiccant) is precooled by an evaporating refrigerant such that it is then capable of absorbing water vapour from an indoor air stream. The heat released from condensing the refrigerant is used to heat the liquid desiccant which is then subsequently exposed to an outdoor airstream which dries or "regenerates" the desiccant, returning its concentration to the first state. In the absorbing device the desiccant is distributed to the top of a plurality of upright and substantially parallel plates and the desiccant falls in a film, thus exposing a large surface area to air.

Various LDVCAC patents including but not limited to US 4259849 (Griffiths), US 6976365 (Drykor), US7360375 (CRF) and commercial systems from Advantix, Kathabar and American Genius Corporation have followed the basic approach set out in US 2122012 whereby the desiccant is precooled and allowed to gain sensible heat during absorption. In these systems the desiccant flow rate must be high in order to minimise desiccant temperature gain across the absorber. At such high flow rates desiccant droplet creation and consequent carryover in the air stream require substantial filtration of the exiting air resulting in high pressure losses. Additionally the high flow rate requires a large internal heat exchanger to recover heat between the diluted and concentrated desiccant flows.

Whilst the formerly noted patents absorbed moisture through adiabatic mass exchangers by utilizing precooled desiccant, various examples can be found in the prior art where the mass exchanger is cooled to facilitate improved mass transfer; such exchangers are herein termed heat and mass exchangers (HMX).

US 5022241 (Wilkinson) describes a liquid desiccant air conditioner with an air-cooled HMX. Precooled process air travels horizontally through alternate channels whilst the conditioned air travels vertically across a falling film of liquid desiccant. In this invention the process air is sensibly cooled through humidification in an evaporative cooler. The process air must be sufficiently dry to enable all the heat generated by desiccant absorption to be carried away as latent heat in the exhausting process air stream. Therefore such a system is unsuitable for humid climates.

Water-cooled heat and mass exchangers for liquid desiccant systems have been proposed in US 6745826 B2 (Lowenstein), WO 03/019081 A1 (Lavemann), and WO 10/89315 A2 (Krause). In all cases the exchanger is composed wholly of a polymer with a plurality of extruded coolant channels. These inventions have the advantage of simple low-cost production and resistance to corrosive desiccants but require a structured or coated surface to enable good wetting due to the hydrophobic nature of the material. Water is also less practical as a coolant compared to a refrigerant such as R410a due to its high boiling point at atmospheric pressure. As such water-based cooling systems are limited to single-phase use which restricts the lowest entry coolant temperature to that which can be achieved with a cooling tower or auxiliary chiller. This reliance on auxiliary cooling plant makes water an unattractive coolant for small air conditioning (AC) systems.

US 4819444 (Meckler) is the first patent to describe a HMX cooled by an evaporating refrigerant, thus enabling a LDVCAC system similar to US 2122012 where heat is extracted from the desiccant to the refrigerant during absorption to rejected from the condensing refrigerant during "regeneration" of the desiccant. Whilst US 4819444 does not disclose how such an air-desiccant-refrigerant contactor may be designed, US 4941324 (Howell & Peterson) describes a very similar LDVCAC system and goes further to propose the use of finned tube evaporators to serve as the HMX. In this system the tubes run horizontally, the fins are mounted perpendicular to the tubes and all components are preferably metallic. The use of aqueous lithium chloride is recognised many times in the literature as the most suitable liquid desiccant for most applications, however its high corrosiveness present a problem for use with metallic heat and mass exchangers. In US 4941324 the use of non-metallic fins is proposed for compatibility with aqueous lithium chloride but the detailed design or likely performance is not disclosed.

WO 94/00724 (Lavemann) describes a similar finned-tube HMX but attempts to resolve the corrosion problem by limiting the desiccant to contact with the refrigerant tubes only. To achieve even wetting over the tube surfaces a means of wicking the surface is proposed. This solution suffers from creation of droplets as the desiccant travels from upper to lower pipes and the relatively small and expensive surface area available for mass transfer from desiccant to air. Both problems make this solution unsuitable for small AC devices.

US 7269966 B2 (Lowenstein) describes a further development of the finned-tube HMX for LDVCAC systems. In this invention the desiccant flows principally over a stack of vertical closely-packed polymer film fins which are interrupted by horizontal refrigerant tubes, as shown schematically in figure 10. The design allows for very large mass transfer area but, as with WO 94/00724, it relies on a very small area to transfer heat to the refrigerant and therefore imposes a high thermal gradient both through the tube wall and between tubes as the desiccant gains sensible heat while falling. The material cost of this invention is also relatively high due to the proposed use of copper-nickel or pure copper tubes together with liquid desiccant LIMIT 301 which consists of 37% lithium chloride solution with a corrosion-inhibitor. Fundamentally US 7269966 B2 resolves the difficulty of using aqueous lithium chloride with a HMX but at the expense of reducing the evaporating temperature compared to other LDVCAC systems, thus offering only negligible improvement to COP over conventional AC.

The prior art lacks a solution for high efficiency air conditioning under conditions of high latent heat which is similar in both cost and size with conventional vapour compression air conditioners for domestic use; i.e. below 5kW cooling capacity. Existing inventions and commercial LDVCAC products rely on i) volatile desiccants, which are not cost effective for intermittent use; ii) the availability of auxiliary water coolers, which is not practical in most residential properties; or iii) evaporating temperatures close to or below the dewpoint, which produce negligible efficiency benefits over conventional AC systems.

The present invention provides a heat and mass exchanger for exchanging heat and mass between an airsteam, a liquid desiccant and a refrigerant, the exchanger comprising:

one or more roll-bond plates, the or each roll-bond plate having at least one internal passage, the passage being generally in the plane of the respective plate;

one or more flow areas for a liquid desiccant defined over at least part of an external surface of the roll-bond plate(s); and

means for causing a flow of refrigerant through the internal passage(s) of the roll-bond plate(s).

The present invention improves upon the prior art by facilitating the use of highly effective liquid desiccants such as aqueous lithium chloride at evaporating temperatures higher than those possible in the prior art. This is made possible with the invention of a heat and mass exchanger (HMX) which incorporates specially coated roll-bond heat exchangers having both high corrosion resistance yet very low thermal resistance between the refrigerant and the desiccant. The higher evaporating temperature reduces specific compression work thus improving thermodynamic efficiency.

The following drawings in which like reference characters indicate like parts are illustrative of embodiments of the invention and are not intended to limit the invention as encompassed by the claims forming part of the application.
Fig. 1a is a perspective view of a heat and mass exchanger (HMX) plate in the form of a conditioner in accordance with a first embodiment of the invention. Fig. 1b is a sectioned perspective view of a plurality of HMX plates forming a conditioner in accordance with a first embodiment of the invention. Fig. 1c is a sectioned perspective view the heat and mass exchanger of Fig 1b in the direction of the desiccant flow illustrating horizontally offset refrigerant channels between adjacent heat and mass exchanger plates to yield a meandering process air stream. Fig. 2 is an illustration of a counter-flow HMX in accordance with a second embodiment of the invention wherein the process air stream travels upwards counter to the flow of desiccant. Fig. 3 is an illustration of a HMX wherein the plates are curved in accordance with a third embodiment of the invention. Fig. 4 is a perspective view of a counterflow heat and mass exchanger and axial exhaust fan in the form of a conditioner in accordance with a fourth embodiment of the invention. Fig. 5a is a sectioned perspective view of a heat and mass exchanger with gaps through the plates between refrigerant channels in accordance with a fifth embodiment of the invention. Fig. 5b is a sectioned perspective view of an uncoated heat and mass exchanger in accordance with a fifth embodiment of the invention but with deformed edges around the central gaps. Fig. 6 is a perspective view of a heat and mass exchanger in the form of a conditioner with variable air entry aperture in accordance with a sixth embodiment of the invention. Fig. 7a is a perspective view of a heat and mass exchanger in the form of a conditioner in accordance with a seventh embodiment of the invention. Fig. 7b is a sectioned perspective view of the heat and mass exchanger of Fig 7a. Fig. 8 is a perspective view of a heat and mass exchanger assembly in the form of a conditioner in accordance with an eighth embodiment of the invention showing only the conditioning plates and liquid distributors. Fig. 9 is a side elevation view of a single heat and mass exchanger plate for a conditioner in accordance with a ninth embodiment of the invention. Fig. 10 is a perspective view of a conventional heat and mass exchanger in the form of an LDVCAC absorber as described in US 7269966 B2.

The present invention is directed to a heat and mass exchanger for application in liquid desiccant air conditioning. The HMX of the present invention is resistant to corrosive liquid desiccants, has a very low thermal resistance between the refrigerant and desiccant, and is designed to minimise the formation of droplets in the desiccant flow.

The present invention will be described as it is applied to a conditioner for a liquid desiccant vapour compression air conditioner (LDVCAC), although the same invention may be used for the desiccant regenerator of said system. The HMX of the present invention may be incorporated into many types of LDVCAC devices including, but not limited to, single-split units, multi-split units, air handling units, dehumidifiers, and air source heat pumps. In the latter application the LDVCAC device may be used to slow the accumulation of frost and thus reduce or avoid the need for defrosting cycles.

The heat and mass exchanger of the present invention is well suited to mass production processes and requires minimal maintenance.

Figure 1a shows a heat and mass exchanger consisting of a roll-bond plate 10 possessing a surface 14 capable of supporting a flow of liquid desiccant 26 thereon while in contact with an air stream 16 wherein the liquid desiccant is capable of modifying the water vapour content of the air. Optionally, the liquid desiccant flows over both major surfaces of the roll-bond plate. Each HMX plate consists of an enclosed channel 18 which travels across a substantial fraction of the plate surface, transporting a refrigerant fluid from an entry pipe 20 to an exit pipe 22. The entry pipe 20 may comprise a capillary tube or expansion valve or be of identical internal diameter as the exit pipe 22. The channel path 18 is such that the refrigerant is capable of extracting heat from the desiccant-wetted surface 14 of each plate with a mean thermal resistance from desiccant to refrigerant of less than 1.7 x 10^-3 Km²/W. The roll-bond plates are preferably fabricated from an alloy of aluminium but could also be fabricated from any other material of sufficiently high thermal conductivity and tensile strength. All the exterior surfaces of each HMX plate 10, excluding the refrigerant entry and

exit pipes

20 and 22, are treated with a pin-hole free coating 12 of no greater than 250 microns thickness and of a epoxy material which has resistance to an aqueous solution of lithium chloride of 40% concentration by mass. Alternatively, the coating 12 may comprise an enamel, phenolic, polyurethane or anodising material instead of an epoxy material. Also, the coating is not limited to a single layer of material, but may alternatively comprise two or more layers of different materials (optionally with each material being selected from the above materials). One example of a suitable coating comprises a layer of chromated epoxy primer overlaid by a polyurethane topcoat layer.

At least one portion of each face of each plate 14 is then further treated to make it more hydrophilic such that an operating desiccant flow rate of 0.5 - 1.0 litres/m²/hour will totally wet the defined region. This may be achieved either by coating with a very hydrophilic material, by fabrication of a micro-structured surface in a moderately hydrophilic material or by constraining the desiccant behind a vapour-permeable membrane. Such a microstructure may be produced at a macro-scale by flocking with polymer or metallic fibres, by the use of microchannels or by application of a porous textile to the surface. Alternatively a nano-scale hydrophilic structure may be produced by proprietary surface treatment. Any combination of the above methods may also be employed to achieve even wetting at low desiccant flow rate. The hydrophilic coating must be corrosion resistant and prevent the accumulation of droplets. Applicants have observed that a flocked surface of 0.5mm Nylon fibres, when saturated with liquid desiccant, provides adequate surface wetting at the stated operating desiccant flow rates.

The absorber, as illustrated in Figure 1b comprises a plurality of HMX plates 10 and operates to allow a process air stream 16 to pass in contact with a large area wetted by a flow of liquid desiccant 26. The process air is forced substantially horizontally across and between the plates of the absorber by a fan. The air to desiccant contact facilitates the transfer of: i) sensible heat from the air; and ii) latent heat from water vapour carried in the air; to the desiccant. The desiccant gains additional heat as it becomes diluted from the exothermic absorption of water vapour. As the desiccant gains heat its temperature increases. However, unlike prior HMX inventions, the present invention allows for the desiccant to be continuously cooled by a cold refrigerant fluid. In so doing, the refrigerant gains heat from the desiccant, causing liquid refrigerant entering via the entry pipe 20 to at least partially vaporize as it passes along the channel 18, and causing vapour refrigerant to increase in temperature. The refrigerant extracted at the exit 22 carries this heat away from the HMX plates by the presence of suction pressure imposed by a subsequent compressor (not shown). The refrigerant is then condensed, sub-cooled and expanded before it is re-introduced into the entry pipe 20. The refrigerant delivered to the absorber is metered through an expansion valve or capillary tube in order to regulate the evaporating pressure and amount of superheat, (the specific gain in sensible heat), required. (This description assumes that the HMX is configured to act as an absorber but, as noted, the HMX may alternatively be configured to act as a desorber or regenerator. In this case, refrigerant entering via the entry pipe 20 will at least partially condense as it passes along the channel 18, and liquid refrigerant will decrease in temperature as it passes along the channel 18.

The process air stream in contact with the desiccant becomes partially dehumidified and cooled to a vapour pressure and temperature determined by the equivalent vapour pressure of the desiccant, the desiccant temperature, and the heat and mass transfer coefficients. The heat and mass transfer coefficients can be altered through the dimensional parameters of the absorber, the air and desiccant flow rates, and the various geometric variations described in subsequent embodiments.

Desiccant is delivered to the top of each absorber plate via one or more pipes or preferably a parallel flow manifold 28 feeding a micro-channel 32 or open-cell foam 30 distributor to ensure even distribution across the width of the plate. Desiccant evenly wets the hydrophilic surfaces and falls under gravity in a very thin film to the bottom edge of each plate where it runs into a sump 34 below the absorber.

The smooth profile of the roll-bond heat exchangers minimizes air stream turbulence, thus minimising both the potential for desiccant carryover and the fan power demand.

A strainer (not shown) sits immediately above the sump for the purpose of filtering macro-particles from the desiccant to prevent blockage or damage to the desiccant pump.

The desiccant is an aqueous solution of either lithium chloride, calcium chloride, lithium bromide or other halide salt or potassium formate.

The plates may be connected via through-bolts, with the mounting holes located in marginal regions away from the hydrophilic surface both to avoid creating a corrosion path to the metal plates and to avoid the formation of droplets in the desiccant flow.

Alternatively the plates may be bonded together, either by application of adhesive to one or both facing surfaces, or by thermal methods which would cause a region of one or both plate surfaces to melt and fuse to a facing plate. In all cases the air gap may be defined and maintained by the use of spacers or by

desiccant distributors

30 or 32 with integral spacers such as the distributor 30 shown in Figure 1b.

In the preferred embodiment the HMX plates are tall to maximise the intake air aperture with the minimal number of plates.

The HMX plates may also be fabricated from a non-metallic material of high thermal conductivity and tensile strength, such as a bespoke ceramic composite, which has one or more of the following advantages i) lower specific gravity; ii) lower cost per unit exposed surface area; iii) inherent chemical resistance to concentrated halide salt solutions; iv) inherently hydrophilic surface.

The plates in each absorber stack may be connected in series or parallel with respect to the refrigerant flow.

The pattern of refrigerant channels is offset from the centre of each HMX plate such that when a plurality of plates are stacked with alternate plates being oriented in reverse the refrigerant channels of adjacent plates are not facing one another but instead each channel faces a flat section of plate as can be seen in Fig 1c. Applicants have observed in computational fluid dynamic modelling that as the air stream meanders through the passage created by the aforementioned offset channels the viscous boundary layer between the air and desiccant surface is disturbed at each internal bend. This disturbance, whose magnitude can be controlled by the air channel width, air face velocity and refrigerant channel profile and channel spacing, can be observed to increase the convective heat and mass transfer within the HMX relative to flow through flat plates in conventional tube and fin HMX. This gain in specific performance afforded by the roll-bond plates with alternate offset positioning is associated with minimal increases in stack volume, air side pressure loss and the potential for droplet creation.

Refrigerant perpendicular to desiccant

Referring to Figure 2, a stack of absorber plates 10 are shown for a second embodiment of the present invention. The absorber is similar to that of the first embodiment but oriented and with particular features to permit the process air stream 16 to be drawn upwards against the flow of liquid desiccant 26 resulting in a contraflow heat and mass exchange. In this embodiment cut-outs 38 are made along the bottom edge of each absorber plate to permit the entry of process air at an angle perpendicular to the flow of air in the absorber, thus allowing the desiccant to be collected in a sump, which is not shown, immediately beneath the absorber and extending the full width and length of the absorber. The base cut-outs 38 are shaped to minimise process air stream pressure loss while permitting the desiccant to be conveyed to the sump with minimum risk of droplet creation. A hydrophilic coating, which is not shown, is applied to the plate surfaces, extending to the full height of each plate and to the full width less a small margin. A parallel flow manifold 28 distributes desiccant at equal pressure to a discrete number of open-cell foam distributors 30 which lie across the plurality of absorber plates. The provision of cut-outs 40 along the top edge of each absorber plate permits the process air to be drawn out of the absorber in a range of directions, from vertically to laterally. The top cut-outs are shaped to minimise process air stream pressure loss while permitting the desiccant to spread out smoothly to wet the full width of each absorber plate. The refrigerant may enter and leave the plate at the same side of the HMX, or on opposite sides, enabling more flexibility for refrigerant channel design.

Curved plates

As shown in Fig 3 the evaporator plates 10 may be fabricated in a curved fashion with respect to the direction of process air flow 16. The benefit of curved passages is to avoid the space and pressure losses that result from plenum chambers where the inlet and outlet of the process air flow are required to be in different planes. The avoidance of plenum chambers suits compact and high air flow air conditioner designs.

Radial configuration

Axial fans are the most efficient devices for accelerating air at high flow rates and with low suction and discharge pressure heads. Referring to Fig 4, a fourth embodiment of the invention provides for plurality of roll-bond HMX plates 10 to be arranged in a generally radial configuration which is tailored to suit axial exhaust fans. The process air stream 16 is drawn through gaps 50 between the plates 10 and substantially upwards by an axial fan 42. By surrounding the upper part of the HMX stack in a circular duct 44, air is forced to enter the stack near the base, causing the process air stream to travel in contraflow to the falling liquid desiccant 26. This counterflow arrangement can be shown to offer improved mass transfer performance under isothermal conditions compared to cross-flow mass exchangers. Desiccant is delivered radially to one or more open-cell foam distributors 30 on each plate from a central supply pipe (which is not visible in the drawing). The distributors are preferably narrow, to limit their effect on the air flow exiting the HMX.

Figure 4 shows the plates as curved, but in principle the embodiment of figure 4 could have non-curved plates.

Turbulence-generating gaps in plates

As shown in Fig 5a the absorber plates 10 may contain gaps or holes 46 located between refrigerant channels to facilitate the smoother passage of process air 16 around opposing refrigerant channels 18 and the interchange of air between plates. This embodiment has the advantage of disrupting the viscous boundary layer without reducing the air gap, thus enabling the HMX plates to be stacked at closer spacing than the continuous plate embodiment for the same air side pressure loss. The disadvantage of this approach is that the plates must become longer, in the direction of air flow, to supply the same contact surface area as plates without gaps.

Figure 5b shows a further embodiment of Figure 5a whereby parts of the plate edges surrounding the gaps are deformed in directions normal to the plane of the plate. These deformations 48, in conjunction with the gaps 46, encourage mixing in the process air 16, thus disrupting the viscous boundary layer and improving convective heat and mass transfer. The use of inversely deformed edges on opposing sides of the gaps, as shown in Fig 5b, further encourages mixing of air flow and reduces obstruction and consequent pressure losses. The use of alternating positive and negative deformations along the length of each exposed edge serves to equalize the mass flow of air between passages. The amplitude of such deformations must be small and to avoid damaging the roll-bond integrity and to ensure pin-hole free coating.

It should be noted that in the embodiment of Figures 5a and 5b the gaps 46 cannot extend over the entire height of the plate. The gaps preferably extend over as much of the height of the plate as possible, while still leaving the plate with sufficient structural strength, and preferably with the minimum number of divisions - for example, a single continuous gap, starting perhaps 25mm from the top edge of the plate and finishing 25mm from the bottom edge. However, other configurations for the gaps are possible.

Variable air aperture

As shown in Fig 6, a further embodiment of the present invention allows for the HMX plates to be arranged such that the gap between plates increases or decreases from the air inlet side to the air outlet side. This arrangement allows the turbulence of the process air stream to be maximised towards the inlet or outlet, which is not easy to produce with conventional tube and fin heat exchangers. By increasing turbulence in the air flow both the heat and mass transfer coefficients can be increased, thereby reducing the total contact area required. The variable inlet and outlet widths may also be advantageous for compact designs.

Central gaps in plates to reduce plate number

A further embodiment of the invention is exemplified in Fig 7a, wherein a conditioner composed of a stack of several HMX plates 10 is fabricated with a gap in each of those plates and the gaps are aligned to form a void 54 within the stack. This void may contain a fan or a duct leading to a fan for the purpose of extracting air from two sides of the conditioner simultaneously, as exemplified in Fig 7b,. This design has the advantage of increasing the intake air aperture for a given quantity of HMX plates thus reducing component and assembly costs.

Dual mass exchange

A further embodiment of the invention is exemplified in Fig 8, wherein one or more plates 10 of a conditioner stack is fabricated with one or more fins 56 adhered to or located in the proximity of the air exit edge. The fins are preferably made of a cheap thermoplastic such as polypropylene and may be adhered to the coated HMX plates with an epoxy adhesive. The fins themselves are coated with a secondary region of hydrophilic material 60 for the purpose of holding and controlling a low flow of water or a dilute aqueous solution 62 from top to bottom. The hydrophilic material on the fin must be separated by a gap from the aforementioned region of hydrophilic material 14 which serves to hold and control the desiccant flow 26 so as to avoid mixing of the two liquids. The fin may also be made of a thin metal sheet, a non-metallic hydrophilic material, or a polymer coated with a hydrophilic material. The material is not necessarily required to have chemical resistance to the desiccant but this property may be advantageous to applications of the invention where the carryover of desiccant is not critical. The water or dilute aqueous solution may be delivered to the secondary hydrophilic surface 60 by a secondary distributor 58, which may of similar design to the aforementioned desiccant distributors.

As the process air stream passes between the fins, water is evaporated from the fin surface, humidifying and cooling the air and thereby converting a proportion of the sensible heat in the air stream to latent heat. By controlling the water flow rate, the addition of these fins and water delivery system would enable the user to control the ratio of sensible to latent cooling done by the LDVCAC device. Such control is not afforded to conventional vapour compression air conditioners because the exiting process air stream is already saturated. Humidification of chilled air from LDVCACs is described in the prior art, specifically in US 4941324, but has not been proposed to be manufactured in this compact and inexpensive way.

Optimised temperature distribution

A further embodiment of the invention is exemplified in Fig 9, wherein the refrigerant connections to the plates 10 are made in parallel, that is the internal channels of the plates are connected in parallel to one another, such that the refrigerant flowing through the exchanger is divided between the plates (and would be divided equally between the plates if all plates are identical to one another). The refrigerant channel 18 of each plate preferably is, in this embodiment, a single continuous path from entry 20 to exit 22, preferably a serpentine path from entry 20 to exit 22. In this way, the portion of refrigerant in each plate enters at the lowest temperature and exits at the highest temperature. The inset graph at the bottom of figure 9 shows the temperature (T) of the air (16) and refrigerant (18) as a function of the horizontal distances across each HMX plate.

Connecting the internal channels of the plates in parallel to one another has the further advantage that dividing the refrigerant flow through the exchanger between the plates results in a reduced flow per plate, and so allows the cross-sectional area of the internal channel to be reduced. Since the maximum width of the roll bond plate is determined by the width of the internal passage, reducing the cross-sectional area of the internal channel allows the width of the roll-bond plate to be reduced, thus reducing the spacing required between adjacent HMX plates.

A further optional aspect of this invention which represents an advantage over prior art for vapour compression (refrigerant) based liquid desiccant systems is the use of a zeotropic refrigerant (such as R407C) having a temperature glide (an evaporating temperature range at a constant pressure) and flowing through a passage in the HMX plate in contraflow to the air, desiccant, or both. This has the advantage of improving heat and/or mass transfer by increasing the log mean temperature difference and/or the log mean vapour pressure difference. To achieve this, all the plates must be connected in parallel as described above such that the refrigerant is divided between all plates. In this way the portion of refrigerant within each plate experiences the complete temperature glide, entering at the lowest temperature, and leaving at the highest temperature. This arrangement also enables the HMX to benefit from refrigerant superheat and subcooling. The serpentine passage design is the simplest way to maximise the difference between the entry and exit edges (with respect to the air stream), but other passage configurations/designs may be beneficial in certain circumstances. This arrangement of parallel refrigerant channels to produce counterflow heat and/or mass exchange can benefit both the conditioner and regenerator.

Use of a lower pressure refrigerant (e.g. R407C in place of R410A) has the additional benefits of reducing material cost - since the roll bond plates are required to support a lower internal pressure they may be made using thinner sheets.
R407C and R410 are both well-known refrigerants. R-407c is a zeotropic refrigerant, being a blend of difluoromethane (R-32), pentafluoroethane (R-125), and 1,1,1,2-tetrafluoroethane (R-134a). R-410A, often sold under the trade marks PURON, ECOFLUOR R410 (or EcoFluor R410), GENETRON R410A, or AZ-20, is a zeotropic but near-azeotropic mixture of difluoromethane (CH2F2, called R-32) and pentafluoroethane (CHF2CF3, called R-125), which is used as a refrigerant in air conditioning applications.

Although the invention has been shown and described with respect to a certain embodiment or embodiments, equivalent alterations and modifications may occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described elements (components, assemblies, devices, compositions, etc.), the terms (including a reference to a "means") used to describe such elements are intended to correspond, unless otherwise indicated, to any element which performs the specified function of the described element (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein exemplary embodiment or embodiments of the invention. In addition, while a particular feature of the invention may have been described above with respect to only one or more of several embodiments, such feature may be combined with one or more other features of the other embodiments, as may be desired and advantageous for any given or particular application.

For example, it is possible for certain embodiments of the invention to be combined with one another. As an example, an exchanger could have both the gaps of Figure 5a or 5b and the central void of Figure 7a. As another example, the radial embodiment of Figure 4 could additionally or alternatively be provided with cut-outs in the upper edge and/or lower edge of the plates (as in Figure 2). As a further example, gaps similar to the gaps of figure 5a or 5b could be provided in the free, vertical edges of the plates of the radial embodiment of figure 4, with the gaps extending horizontally as the embodiment is depicted in figure 4. As a yet further example, adjacent plates of the radial embodiment of figure 4 could be offset relative to one another as described with reference to figure 1c so that the refrigerant channels of adjacent plates are not facing one another but instead refrigerant channels of one plate face a flat section of an adjacent plate

As noted above there are various aspects of this invention that are capable of mitigating the increased material requirements caused by the material thickness likely to be required for the roll-bond plates used in the present invention. These aspects include one or more of the following:
I Reducing the thermal resistance from refrigerant to desiccant (less distance for heat to travel, and thicker thermal conduit {aluminium plate})
II Creating contraflow rather than crossflow mass exchange
III Creating contraflow rather than crossflow heat exchange
IV Increasing turbulence (i.e. higher localised velocities)
V Enabling higher flow bulk air velocities without risking droplet carryover

The use of a roll-bond plate, with its relatively smooth outer surfaces, achieve aspects I and V relative to conventional (prior art) methods of achieving the same.

The recesses of Figure 2 or 4a enable contraflow mass exchange by permitting the air stream to flow in counterflow to the desiccant, (i.e. they affect aspect II).

The use of the zeotropic refrigerant R407C in a parallel flow absorber design can create contraflow heat exchange conditions (aspect III).

The gaps of Figure 5a increase turbulence and disrupt the thermal boundary layer (i.e. affecting aspect IV), and the shaped edges to the gaps in figure 5b further increase turbulence.

In the description of the embodiments the internal passage(s) in the roll bond plate are shown as having an oval or elliptical cross-section. The invention is not limited to this, and the internal passage(s) in the roll bond plate may have other sections, for example an hexagonal cross-section.

In any embodiment of the invention the hydrophilic coating may be absorbent, enabling faster wetting by desiccant during intermittent operation.

The present invention provides a heat and mass exchanger for exchanging heat and mass between an airsteam, a liquid desiccant and a refrigerant, the exchanger comprising: one or more plates, the or each plate having at least one internal passage, the passage being generally in the plane of the respective plate; one or more flow areas for a liquid desiccant defined over at least part of an external surface of the plate(s); and means for causing a flow of refrigerant through the internal passage(s) of the plate(s).

The or each plate may be a roll-bond plate. As is know, In a roll-bond process (or roll bond process), two sheets of metal are bonded together with a true metallurgical bond, except in one or more desired flow channels, in which the two sheets are prevented from bonding. In a roll-bond process, "stopweld" material is disposed on one of the sheets in a desired pattern (corresponding to the desired flow channel(s)), and the other sheet is then stacked on top. The sheets are then heated and rolled under pressure to provide a metallurgical bond between the sheets, except for where stopweld material was provided. One end of the channel is then opened, and high-pressure air is used to inflate the flow channel. The term "roll-bond" plate as used herein denotes a plate obtained by such a roll-bond process.

The term "refrigerant" as used herein preferably denotes a material classified in the R-# classification system for refrigerants as developed by DuPont. The refrigerant may be a pure fluid (for example such as butane), or it may be a mixture of two or more fluids (for example such as R407C or R410, described below). It should be noted that while air and water are, according to some sources, classified as refrigerants, they are not preferred refrigerants for the present invention.

The exchanger may further comprise a non-metallic coating over at least the part of an external surface of the plate(s) over which, in use, the liquid desiccant flows, the non-metallic coating being corrosion resistant to the liquid desiccant.

The non-metallic coating may comprise at least one of an enamel, epoxy, phenolic, polyurethane or anodising material.

The plate(s) may be fabricated from a non-metallic material that provides corrosion resistance against the liquid desiccant.

The internal passage(s) in the plate(s) may be generally substantially parallel to the flow, in use, of the liquid desiccant, or they may be generally substantially perpendicular to the flow, in use, of the liquid desiccant.

The exchanger may be adapted to exchange heat and mass with an airstream directed generally perpendicular to the desiccant flow and generally parallel to the plane of the plate(s). Alternatively, it may be adapted to exchange heat and mass with an airstream directed generally opposite to the desiccant flow.

The exchanger may comprise a plurality of plates extending generally parallel to one another in at least one direction.

The plates may be generally parallel to one another. Alternatively, the separation between adjacent plates may vary along the direction of the airstream.
An edge of the plates may be shaped to promote introduction or extraction of air from spaces between adjacent plates.

An internal passage in one plate may be offset, in the direction of the airstream, with respect to an internal passage in an adjacent plate.

One or more first apertures may be formed in the plates, a first aperture in one plate being disposed generally opposite the internal passage in an adjacent plate and extending generally in the same direction as the internal passage in the adjacent plate passage.

At least part of one of the edges of the first apertures in a plate may be deformed in a direction normal to the plane of the plate.

The median plane of the plates may be non-flat. For example, the median plane of the plates may be curved, or include a curved section, so that a section of the plates is crossed with the direction of the airstream, as shown in figure 3. (By "median plane" of a plate is meant the plane that is midway between the front surface of the plate and the back surface of the plate.)

A second aperture may be formed in each plate, a second aperture in one plate being disposed adjacent, and being substantially co-extensive with, the second aperture of an adjacent plate thereby to form a void extending through the plurality of plates.

The exchanger may comprise a plurality of plates extending, when viewed along the direction of desiccant flow, radially from a central support, the support extending along the direction of desiccant flow.

The plates may be enclosed, over at least part of their length along the direction of desiccant flow, thereby to promote airflow over a predetermined part of the length, along the direction of desiccant flow, of the plates. The plates may be enclosed so as to promote airflow over substantially the entire length, along the direction of desiccant flow, of the plates.

The plate(s) may be arranged substantially vertical in use, and the exchanger may comprise a distributor for releasing a flow of said liquid desiccant at or near an upper edge of the plate(s).

The distributor may comprise a micro-channel distributor or open-cell porous material.

The exchanger as claimed in any preceding claim and comprising a receiver for collecting said liquid desiccant after passing over the plate(s).

The exchanger may comprise a first hydrophilic coating disposed over at least part of an external surface of the plate(s) thereby to define the flow area(s) for the liquid desiccant .

The first hydrophilic coating may be a microstructured surface.

Alternatively, the exchanger may comprise a vapour-permeable membrane disposed over at least part of an external surface of the plate(s) thereby to define the flow area(s) for the liquid desiccant.

The or each plate may have a single continuous internal passage.

The or each plate may have an internal passage having a serpentine shape.

At least one plate may be provided with one or more fins attached to the plate, the or each fin being coated with a region of hydrophilic material to define a flow area for a coolant. The fins extend the effective width, in the direction of the airflow, of the plates.

The coolant may be water or an aqueous solution.

The exchanger may further comprise a second distributor for distributing the coolant to the flow area of the fin(s).

The second distributor may comprise a micro-channel distributor or an open-cell porous material.

The liquid desiccant may be an aqueous solution of lithium chloride, calcium chloride or lithium bromide. Alternatively, another halide salt may be used.

The refrigerant passages in each plate may be arranged to optimise the location of refrigerant superheating and/or cooling relative to the evaporating temperature, and/or the refrigerant channel may be designed to optimise the temperature distribution across each plate. In particular, where the exchanger comprises a plurality of plates the internal channels of the plates may be connected in parallel to one another, such that the refrigerant flowing through the exchanger is divided between the plates (and would be divided equally between the plates if all plates are identical to one another). In this way, the portion of refrigerant in each plate enters at the lowest temperature and exits at the highest temperature.

In an exchanger having a plurality of plates, the internal passage(s) of some, and optionally all, of the plates are connected in parallel with one another.

The exchanger may be configured such that refrigerant at least partially changes phase in an internal passage, as it passes from one end of the passage to another - for example the refrigerant may partially or totally evaporate as it passes along the internal passage (if it is desired to cool the airstream) or may partially or totally condense as it passes along the internal passage (if it is desired to heat the airstream).

The refrigerant may be a zeotropic refrigerant.

The invention further provides an air-conditioning unit comprising a heat and mass exchanger as defined above.

The air-conditioning unit may comprise means for generating the airstream over the plate(s).

The invention further provides a refrigeration unit comprising a heat and mass exchanger as defined above.

All embodiments of the present invention are designed to operate within an LDVCAC cycle similar to that described in US 4941324 wherein a heat and mass exchanger absorbs water vapour and sensible heat from a process air stream to a liquid desiccant, being both actively and continuously cooled by a refrigerant in a vapour compression circuit. While the following description applies to the absorber of a LDVCAC system, a person who is skilled in the art will notice that the HMX of the present invention may be readily configured to operate as the desorber or regenerator in the same system, functioning to humidify and heat an external process air stream.
The present invention may be used to replace the heat and mass exchangers in the US 4941324 cycle thus enabling non-volatile desiccants such as aqueous lithium chloride to be used in a compact and low-maintenance device which is well-suited to mass production.

A first aspect of the present invention comprises: a plurality of aluminium alloy roll-bond heat exchanger plates arranged in parallel so as to form a stack with process air drawn through gaps between the plates by a suction force such as that supplied by an exhaust fan; a sealed refrigerant channel within each HMX plate which is substantially perpendicular to the process air stream and substantially parallel to the flow of liquid desiccant; a means of distributing liquid desiccant to the top of both sides of each roll-bond plate; at least one region of each plate coated in a hydrophilic material to create at least one region where the distributed desiccant will easily wet each plate; at least one region of each plate not coated in a hydrophilic material; and a desiccant sump.

The means of desiccant distribution may be a pressurized reservoir feeding a plurality of small channels or a porous material saturated with the desiccant, either of which may be positioned at the top of and in contact with each plate.

The desiccant may be a halide salt solution such as aqueous lithium chloride or it may be potassium formate.

The process air stream may be provided by a motorized suction device such as an axial or tangential fan or by a passive source of suction such as a stack effect or diffusion gradient.

The HMX is designed to be positioned so as to allow the smooth passage of air into and out of the device. The sump is to be positioned beneath the plates so as to recover and filter the desiccant and to provide pressure head to feed a desiccant pump. The HMX plates are designed to be positioned such that the refrigerant channels of adjacent plates are offset in the direction of the process air stream.

Roll-bond heat exchangers are fabricated from just two metal sheets which together combine the functions of refrigerant containment and large area thermal contact to air both in the same plane and the same material, thus providing very smooth geometry and low thermal resistance. The smooth profile facilitates pin-hole free coating over the entire exposed surface with a corrosion-resistant layer whilst the relatively thick fin section between refrigerant channels provides high structural rigidity. This invention employs roll-bond technology in a novel and unintuitive way, taking a technology which is designed primarily to enable the interior surfaces of domestic refrigerators to be easily cleaned and then exploiting the smooth roll-bond geometry in an entirely new way to achieve heat and mass exchange plates which can be easily coated with corrosion resistant and hydrophilic materials.

It should be noted that, compared to the fins of conventional brazed finned-tube heat exchangers, roll-bond plates used in the present invention are likely to between five and ten times thicker due to the need for the thickness of the fin section to be equal to twice the minimum refrigerant containment thickness, resulting in a greater material cost. However, as described below, there are various aspects of this invention that are capable of mitigating the increased material requirements.

Against the prior art for LDVCAC devices, the roll-bond HMX stack offers reduced obstruction to process air flow due to the elimination of elements perpendicular to the air stream and due to the smooth profile of the surfaces. The smoother flow path results in two benefits: i) lower pressure loss in the air stream which reduces the electrical load on fans; and ii) reduced potential for desiccant droplet formation, thus enabling higher air channel velocities without risking the entrainment of desiccant droplets in the air stream.

Applicants have observed in computational fluid dynamic modelling that the offset positioning of refrigerant channels in the direction of the air stream causes the viscous laminar boundary layer between the air and desiccant surfaces to be disturbed as the air passes between adjacent HMX plates. This disturbance allows the desiccant to come periodically into close contact with the bulk air stream in the centre of the air channel, thus improving heat and mass transfer compared to flat HMX plates.

#2 According to a second aspect of the invention, the refrigerant channels are designed to be substantially perpendicular to the flow of desiccant and the process air stream is directed in contraflow to the desiccant. The top and bottom edges of each plate are profiled to facilitate the entry and exit of process air normal to the direction of the air stream within the HMX.

#3 According to another aspect of the invention, the HMX plates are curved to provide the air to be turned within the HMX assembly.

#4 According to another aspect of the invention, the refrigerant channels are substantially perpendicular to the desiccant flow and one or more HMX plates are curved in a plane normal to the flow of desiccant and arranged in a spiral form.

#5 According to another aspect of the invention, gaps are present in the HMX plates between refrigeration channels. The edges of these gaps may be deformed to improve air flow conditions within the HMX.

#6 According to another aspect of the invention, the HMX plates may be configured in a non-parallel stack such that the air entry aperture between plates may be wider or narrower than the air exit aperture.

#7 According to another aspect of the invention, the HMX plates are fabricated from a composite material containing a fraction of ceramic particles or fibres embedded in a low-cost polymer matrix.

#8 According to another aspect of the invention, the HMX plates are coated with a wick composed of polymer fibres.

#9 According to another aspect of the invention, at least one of the HMX plates in the device possesses a gap within a flat section of the plate for the extraction of air.

#10 According to another aspect of the invention, two independent distributors deliver two different liquids to two or more separate hydrophilic regions on each side of each HMX plate. For example in the embodiment of figure 8 a coolant (eg. water) is delivered to the fin of the HMX plate while a liquid desiccant is delivered to the plate. It is known to adiabatically humidify air that has been isothermally dehumidified by a liquid desiccant. In this way air may be chilled to a temperature below the coolant (refrigerant) temperature in the HMX. This aspect of the invention combine the two functions (HMX and adiabatic humidifier) in a single assembly and with common geometry, and provides the advantage of a reduced pressure loss in the air flow relative to a conventional humidifier.

To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.

The invention described herein can be utilised for any of the following applications:
(1) Single split air conditioning systems - typically for residential or small commercial or office premises.
(2) Multi split air conditioning systems.
(3) Air handling units (AHUs).
(4) Refrigerated vehicles where dehumidification is necessary or desirable.

10 heat and mass exchanger (HMX) plate
12 corrosion-resistant coating
14 hydrophilic surface
16 process air stream flow
18 refrigerant channel
20 refrigerant entry pipe
22 refrigerant exit pipe
24 desiccant delivery pipe
26 liquid desiccant solution flow
28 parallel flow desiccant manifold
30 open-cell foam desiccant distributor
32 micro-channel desiccant distributor
34 desiccant sump
36 fixing hole
38 base cut-out in HMX plate
40 top cut-out in HMX plate
42 axial exhaust fan
44 air duct
46 gap in HMX plate
48 deformed edge
50 process air inlets
52 process air outlets
54 air evacuation passage
56 adiabatic fin
58 secondary distributor
60 secondary hydrophilic region
62 water or dilute aqueous solution flow

Claims

A heat and mass exchanger for exchanging heat and mass between an airsteam, a liquid desiccant and a refrigerant, the exchanger comprising:
one or more roll-bond plates, the or each roll-bond plate having at least one internal passage, the passage being generally in the plane of the respective plate;
one or more flow areas for a liquid desiccant defined over at least part of an external surface of the roll-bond plate(s); and
means for causing a flow of refrigerant through the internal passage(s) of the roll-bond plate(s).
An exchanger as claimed in claim 1 and further comprising a non-metallic coating over at least the part of an external surface of the plate(s) over which, in use, the liquid desiccant flows, the non-metallic coating being corrosion resistant to the liquid desiccant and comprising at least one of an epoxy, enamel,
An exchanger as claimed in claim 2 wherein the non-metallic coating comprises a phenolic, polyurethane or anodising material.
An exchanger as claimed in claim 1 or 2 and adapted to exchange heat and mass with an airstream directed generally opposite to the desiccant flow.
An exchanger as claimed in any one of claims 1 to 4 and comprising a plurality of plates extending generally parallel to one another in at least one direction.
An exchanger as claimed in claim 5, wherein the separation between adjacent plates varies along the direction of the airstream.
An exchanger as claimed in any one of claims 4 and 5, wherein an internal passage in one plate is offset, in the direction of the airstream, with respect to an internal passage in an adjacent plate.
An exchanger as claimed in any one of claims 5, 6 and 7, wherein one or more first apertures are formed in the plates, a first aperture in one plate being disposed generally opposite the internal passage in an adjacent plate and extending generally in the same direction as the internal passage in the adjacent plate passage.
An exchanger as claimed in claim 8 wherein at least part of one of the edges of the first apertures in a plate is deformed in a direction normal to the plane of the plate.
An exchanger as claimed in any one claims 5 to 9, wherein the median plane of the plates is non-flat.
An exchanger as claimed in any of claims 1 to 5, and 6 to 10 when dependent directly or indirectly from claim 5, and comprising a plurality of plates extending, when viewed along the direction of desiccant flow, radially from a central support, the support extending along the direction of desiccant flow.
An exchanger as claimed in claim 11 wherein the plates are enclosed, over at least part of their length along the direction of desiccant flow, thereby to promote airflow over a predetermined part of the length, along the direction of desiccant flow, of the plates.
An exchanger as claimed in any preceding claim, wherein the or each plate has an internal passage having a serpentine shape.
An exchanger as claimed in any preceding claim and wherein at least one plate is provided with one or more fins attached to the plate, the or each fin being coated with a region of hydrophilic material to define a flow area for a coolant.
An exchanger as claimed in claim 14, wherein the coolant is water or an aqueous solution.
An exchanger as claimed in claim 14 or 15 and further comprising a second distributor for distributing the coolant to the flow area of the fin(s).
An exchanger as claimed in claim 16, wherein the second distributor comprises a micro-channel distributor or an open-cell porous material.
An exchanger as claimed in claim 5 or in any one of claims 6 to 17 when directly or indirectly dependent from claim 5, wherein the internal passage(s) of the plates are connected in parallel with one another.
An exchanger as claimed in any preceding claim and configured such that refrigerant at least partially changes phase in an internal passage.
An exchanger as claimed in any preceding claim, wherein the refrigerant is a zeotropic refrigerant.