Patent Abstract:
A mass and heat exchanger includes at least one first substrate with a surface for supporting a continuous flow of a liquid thereon that either absorbs, desorbs, evaporates or condenses one or more gaseous species from or to a surrounding gas; and at least one second substrate operatively associated with the first substrate. The second substrate includes a surface for supporting the continuous flow of the liquid thereon and is adapted to carry a heat exchange fluid therethrough, wherein heat transfer occurs between the liquid and the heat exchange fluid.

Full Description:
RELATED APPLICATIONS  
       [0001]     This is a continuation application of U.S. patent application Ser. No. 11/103,136 filed Apr. 11, 2005 which claims priority to U.S. Provisional Patent Application Ser. No. 60/561,182 filed April 9, 2004. 
     
    
     GOVERNMENT INTEREST  
       [0002]     The invention described and claimed herein may be manufactured, used and licensed by or for the United States Government.  
         [0003]     This invention is made with Government support under SBIR Grant No. DE-FG02-03ER83600 awarded by the Department of Energy. The Government has certain rights in this invention. 
     
    
     FIELD OF THE INVENTION  
       [0004]     The present invention relates to thermodynamic devices, and more particularly to a heat and mass exchanger.  
       BACKGROUND OF THE INVENTION  
       [0005]     Proper ventilation and regulation of humidity are essential for maintaining healthy and comfortable air quality indoors. However, these two factors can be in conflict in certain situations. For example, when ventilation rates are increased to improve indoor air quality, humidity can soar to levels that are uncomfortable or even unhealthy. Nearly all residential heating, ventilation and air conditioning (HVAC) systems are capable of regulating air temperature within acceptable ranges. However, few systems are able to effectively regulate air humidity.  
         [0006]     People living in the eastern portion of the United States are familiar with the problem of less than adequate humidity control. A rainy summer night with temperatures in the range of upper 60s to low 70s can have a humidity ratio above 0.015 lb/lb (dewpoint above 68° F.). Since the sun is down and the air temperature is moderate, the cooling load on the house is almost zero. If the air conditioner does not run, the absolute humidity within the house will equal or exceed that of the outdoors. For a 75° F. indoor temperature, the relative humidity will be at least 80%—a level that is not only uncomfortable, but exceeds the 70% threshold at which mold and mildew proliferate.  
         [0007]     Conventional HVAC equipment under such conditions is limited in its ability to restore comfortable air quality. All conventional systems dehumidify by cooling air below its dewpoint. A conventional vapor compression dehumidifier operates by cooling the air to condense the water vapor, and thereafter re-heating the air. However, this process is generally inefficient.  
         [0008]     Desiccants provide a very efficient means to control indoor humidity independent of temperature. The concepts described herein integrate desiccant technology with a vapor-compression air conditioner to produce a system that yields an enhanced dehumidifier exhibiting higher efficiency.  
         [0009]     Attempts have been made to develop vapor-compression air conditioners that directly coupled a liquid desiccant to both the evaporator and condenser of the air conditioner. The earliest work was done by John Howell and John Peterson at the University of Texas. The concept involved spraying desiccant directly onto the air conditioner&#39;s evaporator and condenser. The process air stream that flows through the evaporator is simultaneously cooled and dehumidified as the desiccant absorbs water vapor from the air. The cooling air that flows through the condenser, in addition to carrying away the heat rejected by the air conditioner, regenerates the desiccant by carrying away water desorbed by the warm desiccant.  
         [0010]     Although Howell and Peterson modeled the performance of a liquid-desiccant vapor-compression air conditioner (LDVCAC) that used lithium chloride, the prototype that they built and tested used ethylene glycol. Unfortunately, the use of glycol as a desiccant was impractical. All glycols have a finite vapor pressure. In both the evaporator and the condenser, glycol will evaporate into the air streams, thus undesirably requiring periodic recharging of the system.  
         [0011]     More recently, the Drykor Corporation of Israel introduced several models of liquid-desiccant vapor-compression air conditioners (LDVCAC) based on the teachings of U.S. Published Patent Application No. 2002/0116935. The Drykor technology uses lithium chloride as the liquid desiccant. This is an improvement over the Howell and Peterson work since solutions of all ionic salts including lithium chloride do not “evaporate” the salt, i.e., the vapor pressure of an ionic salt is essentially zero.  
         [0012]     In the Drykor system, the liquid desiccant is first cooled in the evaporator in the form of a refrigerant-to-desiccant heat exchanger, and then the cool desiccant is delivered to a porous bed of contact media where the process air is dried and cooled. Similarly, the desiccant is regenerated by first heating it in the condenser in the form of a second refrigerant-to-desiccant heat exchanger and then flowing the warm desiccant over a porous bed of contact media where a stream of ambient air is flowing therethrough.  
         [0013]     The American Genius Corporation (AGC) is marketing a liquid desiccant air conditioner that functions similarly to the Drykor unit. The AGC system uses a mixture of lithium chloride and lithium bromide as the liquid desiccant.  
         [0014]     In one important way, the LDVCAC of Howell and Peterson is superior to those of both Drykor and AGC in that the Howell and Peterson system uses the evaporator and condenser of the vapor-compression air conditioner as the contact surface for mass and heat exchange between the desiccant and the air streams, whereas the other two systems either heat or cool the desiccant and then, in separate sections bring the desiccant in contact with the air streams. The LDVCACs of Drykor and AGC therefore introduce additional temperature drops that degrade the efficiency of the air conditioners.  
         [0015]     The LDVCAC of Howell and Peterson, however, cannot be easily used with aqueous solutions of either lithium chloride or lithium bromide because these solutions are very corrosive to the metals that are commonly used to make evaporators and condensers. While the evaporator and condenser can be made from an expensive alloy that resists corrosion, the resulting air conditioner would be too expensive to sell in the broad HVAC market. Howell and Peterson suggested that corrosion-resistant metallic tubes with plastic or ceramic-coated fins may be a compromise surface for combined heat and mass transfer. However, these approaches of protecting the evaporator and condenser from corrosion have important limitations: plastics have a low surface energy and so are not easily wetted by liquids; and ceramics are very difficult to apply in the thin pin-hole-free coatings needed in this application.  
         [0016]     All LDVCACs must also prevent droplets of desiccant from being entrained by the air that flows through the dehumidifying and the regenerating sections of the air conditioner. While it is possible to add a droplet filter or demister at the air exits from both the dehumidifying and regenerating sections of the LDVCAC so that droplets do not escape from the system, this approach will create large maintenance requirements associated with keeping the filters unblocked by liquid, and increase the pressure drop that must be overcome by the system&#39;s fans.  
         [0017]     U.S. Pat. Nos. 5,351,497 and 6,745,826 teach that desiccant droplets can be suppressed in a mass and heat exchanger by flowing very low rates of desiccant onto the surfaces of the mass and heat exchanger, and preparing the surfaces so that the low flow of desiccant still provides uniform coverage. This approach to suppressing droplets cannot be used in the LDVCACs proposed by Howell-Peterson, Drykor or AGC. As previously described, in the Drykor and AGC systems the desiccant is first heated or cooled in a refrigerant-to-desiccant heat exchanger and then the desiccant is brought in contact with air in a bed of porous contact media. The bed is adiabatic (i.e. the bed does not exchange thermal energy with the desiccant). The flow rate of desiccant, therefore, must be high enough to prevent the temperature of the desiccant from either decreasing too much (in the regenerating section where the desorption of water is endothermic) or increasing too much (in the dehumidifying section where the absorption of water is exothermic). This prevents the use of Lowenstein&#39;s low-flow approach to suppressing droplets.  
         [0018]     In the Howell-Peterson LDVCAC, the contact surface on which the desiccant and air exchange heat and mass is either the surface of the evaporator or the condenser. Thus, if these heat exchangers have metallic fins, the desiccant will be continually cooled or heated as it interacts with the air. However, the Howell-Peterson LDVCAC does not readily achieve uniform distribution of the desiccant on the surfaces of the evaporator and condenser. As noted earlier, Howell and Peterson propose that the evaporator and condenser can be coated with plastic or ceramic to protect them from a corrosive desiccant. However, these coatings do not enhance and may deter the spreading of the desiccant over the external surfaces of the heat exchangers. Furthermore, Lowenstein&#39;s low-flow approach to suppressing droplets would be difficult to implement with plain plastic surfaces.  
         [0019]     Howell and Peterson&#39;s suggestion that corrosion-resistant metallic tubes be used with plastic fins is also disadvantageous because of the poor thermal conductivity of plastics. Although a plastic fin can be used to provide contact between the liquid desiccant and the air that flows over the fin, the fin will not effectively heat or cool the desiccant. It is essential in a heat and mass exchanger that the liquid that flows on the fins periodically comes into close thermal contact with the metallic tubes. We have observed that the most common configuration for finned-tube HVAC heat exchangers (e.g.  FIG. 3  of U.S. Pat. No. 4,984,434), in which the tubes pass through holes in the fins, will not effectively heat or cool the desiccant if the fins are plastic, even if the surface of the fins are treated so that uniform films of desiccant are created. This is because the plastic fins are poor thermal conductors and they provide a path for the desiccant to bypass the tube i.e., the liquid desiccant can flow on a fin from the top of the evaporator/condenser to the bottom without ever coming in thermal contact with a metallic tube.  
         [0020]     The evaporator and the condenser of a LDVCAC are heat and mass exchangers whereby in the form of an evaporator both thermal energy (heat) and water vapor (mass) are absorbed from an air stream, and whereby in the form of a condenser both heat and mass are added to an air stream. Many processes in industry rely on mass and heat exchangers, and the invention can be used to both lower the cost and improve the efficiency of some of these processes. Examples of processes that may benefit from the invention are: ( 1 ) evaporative condensers for air conditioners and refrigeration systems, ( 2 ) gas scrubbers used in emission control systems and gas purification systems, ( 3 ) desalination plants, ( 4 ) driers, distillers and concentrators where water or other volatile species are removed from a less-volatile liquid, and ( 5 ) absorption chillers.  
         [0021]     The heat and mass exchangers for the preceding processes are commonly configured as an array of tubes that can be oriented vertically or horizontally. If the process is endothermic, as would be the case for most evaporation, distillation or desorption processes, the tubes are heated internally through a fluid or condensing vapor such as steam. The second fluid that is to be evaporated or that contains the volatile specie that is to be desorbed flows as a film over the outside of the tubes.  
         [0022]     In at least one configuration of a heat and mass exchanger, which is described by Goel and Goswami in the Fall 2004 Newsletter of the ASME Solar Energy Division, the external surface of the tubes is enhanced with a screen, mesh or fabric. For a vertical column of spaced-apart horizontal tubes, the screen, mesh or fabric is interlaced with the tubes so that it alternately contacts the left and right sides of the tubes at a limited region of contact. As an absorbing fluid flows downward in the screen, mesh or fabric, it contacts each tube in the column in this limited region of contact, but the liquid is not forced to flow around the tube.  
         [0023]     Accordingly, there is a need for a heat and mass exchanger for use in a thermodynamic device that is designed to overcome the limitations described above. There is a need for a heat and mass exchanger that can carry a liquid on the surface of the exchanger that either absorbs, desorbs, evaporates or condenses one or more gaseous species from or to a surrounding gas such as a process air stream, while maintaining the temperature of the liquid at a desired level to improve the efficiency of the heat and mass exchange. There is a further need for a heat and mass exchanger compatible with corrosive liquids such as liquid desiccants, and which is capable of suppressing droplet formation of the liquid, while maintaining both elevated levels of efficiency and ease of maintenance.  
       SUMMARY OF THE INVENTION  
       [0024]     The present invention is directed to a heat and mass exchanger designed to exchange a gas with a liquid, while independently maintaining the temperature of the liquid so as to maintain an efficient exchange. By way of example, the heat and mass exchanger of the present invention utilizes a liquid desiccant that is capable of altering the water vapor content of a process air stream in an efficient manner. The heat and mass exchanger includes a substrate having a surface capable of supporting the flow of the liquid thereon in contact with a gas, the surface further functioning to enhance the exchange of thermal energy between the liquid and a heat exchange fluid (gas or liquid or the same undergoing a phase change) that flows within the heat and mass exchanger.  
         [0025]     In one aspect of the invention, there is provided a heat and mass exchanger for exchanging heat and mass between a gas and a liquid comprising: 
        a plurality of substantially parallel tubes in spaced apart relationship including at least one upper tube which is above and spaced apart from at least one lower tube, said tubes having an outer surface;     a substrate positioned in the space between the upper and lower tubes, said substrate comprising at least one surface in contact with the gas and providing at least one pathway for the liquid to flow by gravity from the upper to the lower tubes without forming droplets; and that cause a substantial portion of the liquid to flow onto the outer surface of at least one lower tube;     a liquid supply assembly for delivering the liquid to the at least one upper tube; and     means for internally heating or cooling at least some of the tubes.        
 
         [0030]     In another aspect of the present invention, there is provided an extruded plate having a longitudinal axis and opposed end portions for use in a heat and mass exchanger comprising: 
        a front wall and a rear wall spaced apart from each other;     a plurality of parallel channels in the space between the front and rear walls running between the opposed end portions of the plate, wherein adjacent channels are separated from each other by webs;     fluid entry means for enabling a fluid to enter at least some of the channels through at least one of the front and rear walls;     fluid exit means for enabling the fluid to exit at least some of the channels through at least one of the front and rear walls;     means for preventing the fluid from entering or leaving the channels at the opposed end portions; and     fluid communication means through at least some of the webs creating a path for fluid to flow within the plate from the fluid entry means to the fluid exit means of the plate.        
 
         [0037]     In a further aspect of the invention there is provided a heat and mass exchange assembly comprising: 
        a plate assembly comprising a plurality of spaced apart plates, each plate having an upper region and a lower region;     means for internally heating or cooling each plate;     a wettable substrate positioned in the spaces between adjacent plates and in contact with the adjacent plates at a plurality of locations, said wettable substrate allowing a gas to move through the spaces between the plates; and     a liquid supply assembly comprising a source of a liquid and means for delivering the liquid from the source to the upper regions of the plates.       
 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0042]     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.  
         [0043]      FIG. 1  is a perspective view of a heat and mass exchanger in the form of an evaporator for one embodiment of the present invention;  
         [0044]      FIG. 2  is a perspective view of a heat and mass exchanger in the form of an evaporator for a second embodiment of the present invention;  
         [0045]      FIG. 3  is a perspective view of a heat and mass exchanger in the form of an evaporator for a third embodiment of the present invention;  
         [0046]      FIG. 4  is a perspective view of a heat and mass exchanger in the form of an evaporator for a fourth embodiment of the present invention;  
         [0047]      FIGS. 5A through 5D  are perspective views of a pair of adjacent fins illustrating various spacer configurations in accordance with the present invention;  
         [0048]      FIG. 6  is a perspective view of a portion of the evaporator of  FIG. 1  in combination with a spacer configuration in accordance with the present invention;  
         [0049]      FIG. 7  is a partial cutaway perspective view of a heat exchange tube illustrating one surface design in accordance with the present invention;  
         [0050]      FIG. 8  is a perspective view of a portion of an evaporator with multiple heat exchange tubes having elongated cross sections shown in combination with a spacer configuration in accordance with the present invention;  
         [0051]      FIG. 9  is a perspective view of an evaporator with multiple heat exchange tubes in combination with a plurality of fins each disposed between the corresponding tubes in accordance with the present invention;  
         [0052]      FIG. 10A  is a perspective view of an evaporator comprising an array of vertical plates and a corrugated fin disposed between adjacent plates for another embodiment of the present invention;  
         [0053]      FIG. 10B  is an enlarged view of the portion marked  FIG. 10B  of  FIG. 10A  in accordance with the present invention;  
         [0054]      FIG. 11  is a transverse cross sectional view of a heat exchange plate showing internal channels separated by internal webs for use in the present invention;  
         [0055]      FIG. 12  is a perspective view of a triangular insert coupled to a heat exchange plate to yield a two-pass flow circuit within the plate for use with the present invention;  
         [0056]      FIG. 13A  is a partial cutaway perspective view of a heat exchange plate having a series of holes bored through a sidewall portion intersecting the internal channels to yield a two-pass flow circuit within the plate in accordance with the present invention;  
         [0057]      FIG. 13B  is an enlarged view of the portion marked  FIG. 13B  of  FIG. 13A  in accordance with the present invention;  
         [0058]      FIG. 14  is a partial cutaway perspective view of a heat exchange plate having a series of holes bored at an angle intersecting the internal channels to yield a two-pass flow circuit within the plate in accordance with the present invention; and  
         [0059]      FIG. 15  is a perspective view of a distribution insert for delivering a liquid desiccant to a corresponding pair of heat exchange plates in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0060]     The present invention is directed to a heat and mass exchanger that can readily be implemented in air conditioning, dehumidification, and other applications that require the transfer of heat and mass between corresponding fluids. In one embodiment, the heat and mass exchanger of the present invention is adapted to facilitate the transfer of a mass in the form of a water vapor between a process air stream and a liquid desiccant, while at the same time, regulating the exchange of heat. The heat and mass exchanger of the present invention is resistant to corrosive substances including liquid desiccants, and is designed to suppress undesirable droplet formation of the liquid, control the temperature of the liquid, and exhibit good thermodynamic efficiency. The heat and mass exchanger of the present invention is cost efficient to fabricate and implement, and requires low maintenance.  
         [0061]     The heat and mass exchanger of the present invention can be incorporated into a variety of thermodynamic devices including, but not limited to, evaporative condensers for air conditioners and refrigeration systems, gas scrubbers used in emission control systems and gas purification systems, desalination plants, driers, distillers and concentrators where water or other volatile species is removed from a less-volatile liquid, and absorption chillers.  
         [0062]     In one embodiment of the present invention, there is provided a heat and mass exchanger that includes a substrate having a surface capable of supporting a flow of a liquid such as a liquid desiccant thereon while in contact with a gas such as a process air stream wherein the liquid desiccant is capable of modifying the content of a component of the gas such as a water vapor, and a heat exchange element having a surface capable of supporting the flow of the liquid desiccant thereon and a heat exchange fluid flowing therein wherein heat energy is transferred between the liquid desiccant and the heat exchange fluid. The substrate is preferably made from a material having a thermal conductivity of less than 10 w/m-C.  
         [0063]     Although not limited to this application, the detailed design and operation of the present invention, namely a heat and mass exchanger, will be described as it is applied to an evaporator of a liquid desiccant vapor compression air conditioner (LDVCAC). An evaporator operates to allow a gas such as a process air stream to pass therethrough in contact with a liquid desiccant, and absorb water vapor and heat from the passing process air stream. The heat is absorbed in the evaporator by a heat exchange fluid delivered from a condenser in the form of a refrigerant liquid. The heat exchange fluid is metered through a control valve or capillary tube to the evaporator. The pressure within the evaporator is maintained at a low level by a compressor. At low pressure, the heat exchange fluid in the form of a liquid begins to boil, and absorbs heat from the liquid desiccant and from the process air stream. The reverse process occurs in the heat and mass exchanger operating as a condenser.  
         [0064]     Referring to  FIG. 1 , an evaporator  10  is shown for one embodiment of the present invention. The evaporator  10  comprises heat exchange tubes  12  for carrying therethrough a heat exchange fluid  14  in the form of a coolant or evaporating refrigerant, for example. The heat exchange tubes  12  are shown circular in cross section but may have other shapes including non-circular cross section shapes as desired including an elongated cross-section with a major axis of the cross-section in a vertical orientation as shown specifically in  FIG. 8 .  
         [0065]     The tubes  12  are arranged horizontally in rows of three stacked upon each other in spaced apart relationship thus forming corresponding columns of tubes. A plurality of substrates into the form of spaced-apart fins  16  are disposed between adjacent rows of tubes  12  which separates upper tubes from lower tubes. The number of tubes  12  in each row, the number of rows of tubes  12 , and the number of fins  16  are not limited to those shown herein, and may be modified or adjusted to meet the requirements of the application. The fins  16  are arranged to be at least substantially parallel to one another, and preferably equally spaced apart with the space between adjacent fins  16  larger than the thickness of the fin  16 . The fins may be planar, bowed, corrugated or other suitable shapes.  
         [0066]     The fins  16  shown in the embodiment of  FIG. 1  are arranged at least substantially perpendicular to the longitudinal axis of the tubes  12 . The fins include top and bottom edge portions  18  and  20  positioned proximate to the tubes  12 . The tubes  12  may be in contact or separated by a small gap from the corresponding edges  18  and  20 , respectively, of the fins  16 .  
         [0067]     A liquid desiccant  22  delivered from a regenerator (not shown) by a distribution manifold  24  is carried to distribution tubes  26 . Suitable liquid desiccants may be selected from lithium chloride, lithium bromide, calcium chloride, potassium acetate and the like. The regenerator (not shown) functions to drive off excess water from the liquid desiccant that may be present prior to delivery to the evaporator  10 . The liquid desiccant  22  is released from the distribution tubes  26  through outlets  27  onto corresponding porous distribution pads  28 . The distribution pads  28  are preferably composed of a porous material such as open cell foams, non-woven fabrics and the like. The purpose of the pad is to spread the liquid over a relatively large area from a liquid source of smaller area to facilitate distribution of the liquid about the tubes. Each distribution pad  28  is positioned in contact with the corresponding tube  12 . The liquid desiccant  22  disperses throughout the pad  28  and eventually flows onto the outer surface of the top row of the tubes  12 . Through selection of thickness and porosity, the distribution pads  28  can be adapted to uniformly distribute the liquid desiccant  22  over at least a substantial portion of the outer surface of the tubes  12 .  
         [0068]     In another embodiment of the present invention, where the spacing between the tubes  12  is sufficiently close to avoid dripping, it may be preferable to utilize a single distribution pad (not shown) extending across the span of the tubes  12 . The liquid desiccant  22  is delivered to the single distribution pad via spray nozzles (not shown) or drip pans (not shown). The use of spray nozzles or drip pans may require the use of baffles or partitions constructed around the distribution pad and the spray nozzles or drip pans to prevent the process air stream  30  from picking up the sprayed droplets of liquid desiccant  22 .  
         [0069]     Referring back to  FIG. 1 , the liquid desiccant  22  flows around the outer surface of the top row of tubes  12 , and is cooled by contact with the tubes  12 . Drawn downward by gravity, the liquid desiccant  22  flows to the top of the adjacent fins  16 . The liquid desiccant  22  spreads across the outer surface of the fins  16  as a continuous flow without undesirably forming drips or droplets. A process air stream  30  that is to be cooled and dried is passed through the spaces between the fins  16  and around the tubes  12 . The process air stream  30  may be introduced horizontally, vertically or at an angle to the evaporator  10 . The process air stream  30  comes into contact with the liquid desiccant  22 . The liquid desiccant  22  absorbs the heat and water vapor from the process air stream  30 . The process air stream  30  leaving the evaporator  10  possesses a lower water content, while maintaining at least the same or lower temperature than entering the evaporator  10 .  
         [0070]     Since the water absorbing process is exothermic, the temperature of the liquid desiccant  22  increases as it flows down the outer surface of the fin  16  in contact with the process air stream  30 . As a result of the temperature increase, the residence time of the liquid desiccant on the fins  16  must be controlled because the ability of the liquid desiccant  22  to absorb water vapor is diminished, and if the temperature exceeds a certain threshold level, the liquid desiccant  22  stops absorbing water vapor. Therefore, the distance between the top edge  18  and the bottom edge  20  of the fins  16  is selected to prevent the liquid desiccant  22  from exceeding the temperature threshold prior to coming into contact with and being cooled by the next row of tubes  12 .  
         [0071]     At this point, the liquid desiccant  22  reaches the next row of tubes  12  and is cooled by the heat exchange fluid  14  flowing through the tubes  12 . The temperature of the liquid desiccant  22  is lowered, which enhances the ability of the liquid desiccant  22  to absorb more water vapor. This process of the liquid desiccant  22  being cooled while on the tubes  12 , followed by the absorption of heat and water vapor while on the fins  16  is repeated several times as the liquid desiccant  22  flows from the top of the evaporator  10  to the bottom. When the liquid desiccant  22  reaches the bottom, the water-containing liquid desiccant  22  is collected in a reservoir (not shown) for delivery back to the regenerator (not shown) for re-charge and re-use.  
         [0072]     As shown in  FIG. 1 , the top and bottom edges  18  and  20  of the fins  16  include contoured edge portions  32  that match the curvature of the tubes  12 . This enables the fins  16  to be securely seated therebetween, while facilitating the flow of the liquid desiccant  22  between the tube  12  and the corresponding edge  18  or  20  of the fin  16 .  
         [0073]     Applicants have observed that a fillet of liquid desiccant forms where the edge  18  or  20  of the fin  16  is positioned in proximity to the tube  12 . The fillet of relatively thick liquid desiccant  22  forms a region where the liquid desiccant  22  flows freely, but due to the thickness, poor thermal contact is made with the tube  12  and therefore only small amounts of heat are exchanged between the liquid desiccant  22  and the tube  12 . As a result, the liquid desiccant  22  passing through the fillet is not effectively cooled upon contact with the tube  12 . Thus, if the contoured edge portions  32  extend too far around the circumference of the tube  12  and no provision is made to prevent a fillet from forming, the contoured edge portions  32  form a path for the liquid desiccant  22  to flow around the tube  12  without being cooled.  
         [0074]     The fins  16  further include drip preventing means to prevent the liquid desiccant from dropping off of the substrate. As shown in  FIG. 1 , the fins  16  include notches  34  located at the bottom edges  20  of the fins  16  between adjacent tubes  12 . The notches  34  may include inclined edge portions that greatly reduce the tendency of the liquid desiccant  22  to drip off the bottom edge  20 , and function to channel the downward-flowing liquid desiccant  22  towards the adjacent tube  12 . In this manner, the liquid desiccant  22  is prevented from accumulating along the edge  20  of the fin  16  away from the tube  12  and dripping between the tubes  12 .  
         [0075]     The fins  16  are composed of a suitable material that facilitates wetting of the liquid desiccant  22  on substantially the entire surface or selected portions thereof, and which provides a suitable wicking surface for allowing the liquid desiccant  22  to flow uniformly over the fin  16 . Such suitable materials are in the form of screens, meshes, non-woven sheets and the like typically made from fibers of plastics, metal, carbon, glass, ceramic, and cellulose. The fins  16  may be made in the form of thin films in which grit or fibers are adhered thereto which may be selected from plastic, metal, carbon, glass, ceramic, minerals, cellulose, and the like. In one embodiment the fins comprise a thin film of plastic material of less than  15  mils, and a layer of wicking material on each side of the thin film.  
         [0076]     In the present embodiment, the evaporator  10  is constructed to facilitate the removal of the fins  16  for simple replacement, while keeping the evaporator  10  at least substantially intact. The fins  16  can be easily slipped out from between the tubes  12  and thereafter replaced.  
         [0077]     Referring to  FIG. 2 , an evaporator  40  is shown for a second embodiment of the present invention. The evaporator  40  is similar to the evaporator  10  except for the liquid desiccant distribution system. The evaporator  40  comprises a single distribution pad  34  in direct contact with the top edge  18  of the corresponding fins  16 , and a plurality of distribution tubes  36  in fluid communication with the distribution manifold  24 . The distribution tubes  36  each include a series of spray nozzles  38  disposed along the length thereof. The spray nozzles  38  are adapted to spray streams of the liquid desiccant  22  onto the top surface of the single distribution pad  34 . The sprayed liquid desiccant  22  permeates throughout the pad  34  eventually flowing onto the surface of the fins  16 . Since the fins  16  are closely spaced to one another, the formation of droplets under the pad  34  is eliminated.  
         [0078]     When using the single distribution pad  34  and spray system for supplying the liquid desiccant  22 , a partition  42  is mounted on top of the distribution pad  34  and enclosing the distribution tubes  36  and spray nozzles  38 . The partition  42  isolates and prevents the liquid desiccant  22  sprayed from the nozzles  38  from becoming entrained in the process air stream  30 .  
         [0079]     Referring to  FIG. 3 , an evaporator  50  absent a liquid desiccant distribution assembly is shown for a third embodiment of the present invention. The evaporator  50  is similar to the evaporator  10  except for the fin configuration. The evaporator  50  includes the heat exchange tubes  12  through which the heat exchange fluid  14  flows, and a plurality of fins  44  extending contiguous from the upper rows to the lower rows of tubes  12 . The fins  44  are arranged in a spaced apart configuration. Each fin  44  includes a plurality of holes  46  for receiving the tubes  12 . The surface of the fins  44  is treated as described above to yield a wettable, wicking region  48  disposed between each row of tubes  12 . The wicking region  48  is created to induce the liquid desiccant  22  to flow towards one of the tubes in the next row of tubes  12  during the downward flow. The surface portion of the fins  44  on either side of a tube  12  remains untreated (i.e. non-wettable, non-wicking) to deter any fluid from flowing on the fin around the tube  12 . In this manner, the flow of the liquid desiccant  22  is directed onto the surface of the tube  12  during the course of the downward flow.  
         [0080]     Referring to  FIG. 4 , an evaporator  60  absent a liquid desiccant distribution assembly is shown for a fourth embodiment of the present invention. The evaporator  60  is similar to the evaporator  50  except for the heat exchange tube configuration. The evaporator  60  comprises a plurality of heat exchange tubes  12  in rows of five and spaced closely to one another in the same row, and a plurality of fins  52  spaced uniformly apart from one another. The entire surface of the fin  52  is treated in the manner described above to yield a wettable wicking region  54 . Each tube  12  includes a wicking pad  56  disposed on the top surface thereof in contact with the wicking region  54  of the fin  52 . The liquid desiccant  22  flows downward along the wicking region  54  and is drawn by the wicking pads  56  onto the tubes  12 . Once drawn on top of the tubes  12 , the liquid desiccant  12  flows around the tube  12  as a thin film to form a suitable thermal contact. This process is repeated at each row of tubes  12 .  
         [0081]     It is essential that the space between the fins be uniform along the length thereof. Non-uniformity of the space can induce bridging of the liquid desiccant between the adjacent fins particularly at points when the space is narrow. Bridging of the liquid desiccant creates a low resistance path for the liquid desiccant to flow from one tube to the next lower one. This creates a non-uniform flow that adversely reduces the surface area of the fin on which heat and mass exchange can occur. Bridging further creates a non-stable flow feature, where the bridges tend to break and reform. When a bridge breaks, droplets of liquid desiccant can form and be undesirably entrained into the process air stream.  
         [0082]     Referring to  FIGS. 5A  to  5 D, there is shown four methods of maintaining a uniform space between adjacent fins  16 . As shown in  FIG. 5A , the fins  12  comprise small dimples  58  stamped or thermoformed onto the surface thereof. When the fins  16  are stacked, each dimple  58  comes into contact with either another dimple  58  on an adjacent fin  16  or the surface of the adjacent fin  16 . Since the dimples  58  can be formed to have consistent heights, the dimples  58  provide a reliable means for maintaining uniform spaces between the fins  16 .  
         [0083]     As shown in  FIG. 5B , a plurality of spacers  62  are applied to the surface of the fins  16  through a suitable fastening means including, but not limited to, adhesives, welding, and bonding. The spacers  62  maintain a uniform space between adjacent fins  16 . In the alternative, the spacers  62  can be formed from a bead of adhesive that spans the space between adjacent fins  16 . The adhesive is initially flowable after application. The adhesive eventually cures into a hard spacer.  
         [0084]     As shown in  FIG. 5C , a series of spacer rods  64  are inserted through a stack of fins  16  to maintain the spaced apart arrangement. The fins  16  are either bonded to the rods  64  at the desired positions or the fins  16  are held in place by friction between the fins  16  and the rods  64 . A separating means is preferable to maintain the fins  16  in a spaced apart arrangement during insertion of the spacer rods  64 .  
         [0085]     As shown in  FIG. 5D , a pair of fins  66  include corrugations  68  formed thereon. The fins  66  are placed adjacent to one another and are maintained in a spaced apart arrangement by the corrugations  68 . As previously indicated the fins as shown in  FIGS. 5A-5D  may be planar, bowed, corrugated or the like.  
         [0086]     Referring to  FIG. 6 , a portion of the evaporator  10  of  FIG. 1  is shown. The evaporator  10  includes a plurality of spacers  68 A,  68 B. Typically, the liquid desiccant  22  tends to thicken under a spacer. This can cause bridging between adjacent fins  16 . The spacers  68 A are positioned on the fin  16  in close proximity to a corresponding tube  12  where bridging does not cause problems. The spacers  68 B are positioned in an area where the liquid desiccant flow will be low and so there is less tendency for the liquid desiccant  22  to bridge between adjacent fins  16 .  
         [0087]     It is essential that the surface of the heat exchange tube is readily wettable by the liquid desiccant. If the tube is not readily wettable, there is a tendency for discrete rivulets to form on the surface of the tube. The presence of rivulets indicates that only a portion of the surface of the tube is exchanging heat with the liquid desiccant  22 .  
         [0088]     However, even if the entire surface of the tube is wetted with the liquid desiccant  22 , it has been observed that the film thickness of the liquid desiccant that flows around the tube may result in a non-uniform film thickness. This non-uniformity can also reduce the heat exchange between the liquid desiccant and the tube. It may also be desirable for the surface of the tube to be wicking to insure that the flow of the liquid desiccant  22  on the surface of the tube has a relatively uniform thickness. However, the use of a wick on the surface of the tube must be used with discretion since the wick itself can interfere with the flow of heat between the liquid desiccant  22  and the tube if it is too thick.  
         [0089]     Wicks that can be used on the tubes of the evaporator are similar to those that have been described for the fins. Applicants have successfully used fibers of glass, carbon, acrylic, polyester and nylon as wicking material that can be adhered to the surface of the tube. In all instances the thickness of the wicking material in the form of a fiber layer ranges from about  10  mils to  25  mils.  
         [0090]     Referring to  FIG. 7 , a portion of a heat exchange tube  70  is shown for one embodiment of the present invention. It is important to provide a sufficient thermal contact between the liquid desiccant  22  and the heat exchange tube  70 . The tube  70  includes a plurality of circumferential grooves  72  extending along the length thereof. The grooves  72  may also form a helix. The grooves  72  substantially increase the area for heat transfer between the tube  70  and the liquid desiccant  22 . The grooves  72  also reduce the formation of discrete rivulets from the liquid desiccant  22  that would otherwise form. The formation of rivulets adversely reduces the surface area on which heat is exchanged with the liquid desiccant.  
         [0091]     In one embodiment that was tested, the grooves  72  have a pitch of 40 per inch and a peak-to-trough height of 0.020 inch. Applicants have observed a 300% increase in the heat transfer coefficient between the tube  70  and the liquid desiccant  22  when the tubes have grooves as described above.  
         [0092]     Referring to  FIG. 8 , there is shown a portion of an evaporator  80  with multiple heat exchange tubes  74  having oblong cross sections shown in combination with a plurality of spacers  76 . The spacers  76  are each disposed on the surface of the fins  16  proximate the heat exchange tubes  74 . The tubes  74  exhibit a flattened cross section which increases the surface area on which the liquid desiccant  22  exchanges heat. Furthermore, the substantially vertically oriented surface of the tube  74  increases the velocity of the flow of the liquid desiccant, thus reducing the thickness of the liquid desiccant  22  flowing over the tube surface, and enhancing the transfer of heat. Alternatively, the tubes  74  may be modified with an oval cross section to yield similar enhanced heat transfer efficiency.  
         [0093]     Referring to  FIG. 9 , an evaporator  90  is shown without a liquid desiccant distribution system for an alternate embodiment of the present invention. The evaporator  90  includes a plurality of fins  78  each disposed between adjacent heat exchange tubes  82 . The fins  78  each extend from one tube (e.g.  82 A) to the lower adjacent tube (e.g.  82 B), and they lie in a plane defined by the axes of the tubes. The liquid desiccant that flows down the surface of a fin  78  must flow around and exchange heat with a tube  82  before it can continue flowing down on the next lower fin  78 . This arrangement ensures that the entire surface of the tube  82  exchanges heat with the liquid desiccant flowing down the fin  78 . This embodiment may benefit from the use of tubes  82  with a flattened or elongated cross section and a tube surface that is grooved or lined with a wicking material.  
         [0094]     Referring to  FIGS. 10A and 10B , an evaporator  140  is shown for another embodiment of the present invention. The evaporator  140  includes a plurality of vertical heat exchange plates  104  arranged in a spaced apart configuration, and a plurality of corrugated fins  106  each disposed between corresponding adjacent plates  104 . The evaporator further includes a distribution manifold  24  for delivering a liquid desiccant from a regenerator (not shown), and a plurality of distribution tubes  26  for distributing the liquid desiccant from the distribution manifold  24  to a plurality of distribution pads  28  each positioned between adjacent plates  104 . The liquid desiccant  22  disperses throughout the pad  28  and uniformly flows down the surface of the plates  104 . The liquid desiccant  22  is eventually collected in a reservoir (not shown) and returned to the regenerator (not shown) for reprocessing.  
         [0095]     The exterior portion of the plates  104  and the corrugated fins  106  are treated to yield a wettable, wicking surface in the manner described above. The wicking surface of the plates  104  facilitates a uniform flow of liquid desiccant  22 . The corrugated fins  106  are disposed in close proximity or in contact with the corresponding adjacent plates  104  at discrete contact locations  108 . The contact locations  108  allows the liquid desiccant  22  flowing down the plate  104  to continue the flow on the surface of the plate  104  or move onto the surface of the corrugated fin  106 .  
         [0096]     The corrugated fins  106  are preferably composed of a wettable, wicking material which provide a wicking surface on the fin  106  so that the liquid desiccant  22  is able to flow uniformly. Suitable forms of the fins include screens, meshes, or non-woven sheets made from plastic, metal, carbon, glass, ceramic or cellulose fibers, and thin films that have a grit or fiber composed materials such as plastic, metal, carbon, glass, ceramic, mineral or cellulose adhering to the surface of the fin  106 .  
         [0097]     The heat exchange plate  104  includes a heat exchange fluid flowing internally to facilitate heat transfer with the liquid desiccant  22 . It may be desirable for the heat exchange fluid flowing internally within the plate  104  to make multiple passes therein as will be described hereinafter. Details of such heat exchange plates are further disclosed in U.S. Pat. No. 6,079,481, the content of which is incorporated herein by reference. A process air stream is passed through the space between the fins  106  and the plates  104  where the stream is cooled and dried by contact with the liquid desiccant  22  flowing down the fins  106  and the plates  104 .  
         [0098]     Referring to  FIG. 11 , a cross section of a heat exchange plate  104  is shown. The plate  104  comprises a pair of plate walls  112  maintained uniformly spaced apart by a plurality of spaced apart webs  114 . The webs  114  define a plurality of fluid carrying channels  116  for conveying the heat exchange fluid therethrough.  
         [0099]     Referring to  FIG. 12 , the heat exchange plate  104  includes a triangular insert  118  comprising a plurality of channels  122  extending transversely therethrough. The channels  122  of the insert  118  are oriented in a manner that when the insert  118  is coupled to the plate  104 , the channels  122  fluidly connect channels  116  of one side of the plate  104  to channels  116  of the other side of the plate  104  to yield a two-pass fluid circuit. The heat exchange fluid enters the plate  104  through channels  116  in one side and enters the channels  122  of the insert  118  and undergoes a 180-degree turn into the channels  116  in the other side of the plate  104 . The turning of the heat exchange fluid is executed within the plane of the plate  104  without using an external manifold or additional fittings attached to the plate  104 .  
         [0100]     Referring to  FIGS. 13A and 13B , a heat exchange plate  150  is shown for another embodiment of the present invention. The heat exchange plate  150  is similar to the heat exchange plate  104 . The heat exchange plate  150  comprises a plurality of fluid conveying channels  124  extending longitudinally therein, and a plurality of bores  126  extending perpendicularly to and intersecting the channels  124  at one end of the plate  150 . The intersecting channels  124  and bores  126  form an fluid turning area  134  that permits fluid passing through the channels  124  to turn 180-degrees, thus yielding a two-pass or multiple-pass fluid circuit. A side cover member  128  is attached to the plate  150  to maintain the bores  126  fluidly sealed from the outside. An end cover member  132  is attached to the plate  150  to maintain the channels  124  fluidly seal from the outside.  
         [0101]     Referring to  FIG. 14 , a heat exchange plate  160  is shown for another embodiment of the present invention. The heat exchange plate  160  is similar to the heat exchange plate  150  except for the absence of the side cover member. The heat exchange plate  160  comprises a plurality of channels  136  extending longitudinally therein and in communication with a plurality of bores  138  extending within the plate  160  and intersecting the channels  136  at an angle. The intersecting channels  136  and bores  138  form an fluid turning area  144  that permits fluid passing through the channels  136  to turn 180-degrees, thus yielding a two-pass or multiple-pass fluid circuit. The bores  138  do not intersect the sidewall of the heat exchange plate  180 . An end cover member  146  is attached to the plate  150  to maintain the channels  136  and bores  138  fluidly seal from the outside. Alternatively, the open end of the plate  160  may be sealed by suitable means including welding or plugging with an adhesive.  
         [0102]     Referring to  FIG. 16 , a distribution insert  170  is shown for one embodiment of the present invention. The distribution insert  170  can be utilized to replace the distribution pads  28  of  FIG. 11  to deliver the liquid desiccant  22  to the top end of the plate  104  of the evaporator  140 . Each distribution insert  170  is adapted to receive and accommodate the top end portions of adjacently positioned heat exchange plates  104 .  
         [0103]     Liquid desiccant is delivered to the distribution insert  170  from the distribution manifold  24  and the distribution tube  26  to a small diameter inlet  148 . The structural elements of one side of the distribution insert  170  are the same on the other side. The small diameter inlet  148  is in fluid communication with a throughhole  152  extending perpendicularly with the face portions of the distribution insert  170 . The distribution insert  170  further includes a delivery groove  154  disposed on each side thereof to deliver the liquid desiccant from the throughhole  152  to the top portions of the adjacent pair of the heat exchange plates  104  that are positioned on each side thereof.  
         [0104]     In order in ensure that substantially equal amount of liquid desiccant is delivered to each plate  104 , the resistance to the flow in the distribution manifold  24  is small compared to the resistance in the flow path in the distribution insert  170  to the surface of each plate  104 . The flow resistance may be increased through reducing the width and depth of the grooves  154 . However, the width and depth should be sufficiently large to avoid blockages by either scale or solid particles that may be deposited on the inner surfaces of the flow path. Alternatively, the flow length of the grooves  154  may be lengthened to increase flow resistance, while preventing flow blockages.  
         [0105]     Applicants have observed that streams of liquid desiccant that flow from the distribution insert  170  onto the opposed sides of the plates  104  can combine to bridge the gap across adjacent plates  104 . This can cause the process air stream to interact with the bridge of liquid desiccant and strip away droplets.  
         [0106]     To minimize such occurrences, the distribution insert  170  further includes a thinner skirt  156  extending along the lower edge thereof. The skirt  156  effectively prevents bridging between the liquid desiccant flows on the opposed surfaces of the plates  104 .  
         [0107]     The distribution insert  170  further includes a raised sealing barrier  158  and a secondary drain groove  162  that directs liquid desiccant onto the surface of the plates  104  that may leak from the sides of the deliver groove  154 .  
       EXAMPLE  
       [0108]     In this example, a mass and heat exchanger that is designed according to the principles taught herein is installed in a vapor-compression air conditioner to replace a conventional evaporator. The replaced conventional evaporator is an industry-standard finned-tube heat exchanger with copper tubes and aluminum fins. The conventional evaporator possesses the following characteristics:  
                                       Total number of tubes   92       Number of tubes in vertical column   23       Number of tube columns    4       Tube outer diameter   0.3325 in       Fin orientation   vertical and perpendicular to tubes       Fin height   24.0 in       Fin width   2.5 in       Fin thickness   0.010 in       Fin spacing   13 fins per inch       Volume of air processed   1000 cfm       Face velocity for incoming air   263 fpm                  
 
         [0109]     With R-22 refrigerant evaporating at a saturation temperature of 49° F. within the tubes of this heat exchanger and 1000 CFM of air entering at 80° F dry-bulb temperature and 67° F. wet-bulb temperature flowing over the outside of the fins and tubes, the conventional heat exchanger absorbs 30,100 Btu per hour from the air and remove 8.6 lbs per hour of water.  
         [0110]     The conventional evaporator is replaced with a mass and heat exchanger in the form of an evaporator that is designed according to the principles taught herein. A 37% (by weight) solution of lithium chloride, a strong liquid desiccant, is applied as a flow on the outside of the mass and heat exchanger. To facilitate a useful comparison of the conventional evaporator and the present invention, the mass and heat exchanger is designed to match the above listed characteristics of the conventional evaporator particularly with regard to (1) total number of tubes (approximately), (2) tube outer diameter, (3) volume of air processed, (4) face velocity for incoming air, and (5) the temperature of the evaporating refrigerant within the tubes.  
         [0111]     The tubes, oriented horizontally, are arranged in a square array of five per row and eighteen per column. (The process air stream is generated to flow in the direction of the rows and the liquid desiccant is delivered to flow in the direction of the columns.) The five tubes in each row are aligned with a ¼ inch gap between adjacent tubes. The 18 tubes in each column are also aligned with a one inch gap between them. The tubes include helical saw-tooth grooves on the outer surface. There are 40 grooves per inch, and each groove has a 20 mil trough-to-peak dimension.  
         [0112]     The tubes are fabricated from either copper or a 90/10 copper-nickel alloy. If copper tubes are used, a corrosion inhibitor such as LIMIT 301, which is manufactured by FMC Lithium of Gastonia, N.C., is added to the lithium-chloride solution. (FMC reports that the corrosion rate of copper in lithium chloride with LIMIT 301 at 100° F. is 2.0 mils per year. This corrosion rate is significantly lower at the 50° F. operating temperature of this example.) Thin, wicking fins are inserted in the one inch gap between tube rows and perpendicular to the tubes. The fins are made from a PVC film with a thickness of 10 mils. Each fin is prepared with acrylic fibers adhesively applied on both sides thereof. The fibers are 20 mils long and 3 denier. (The “denier” is the standard measure of fiber diameter.) The fins are 3 inches by 1 inch, and stacked to yield seven fins per inch.  
         [0113]     A total of 630 ml per minute of desiccant is pumped to open-cell melamine foam pads that sit on top of the tubes in the uppermost row. The liquid desiccant is first filtered before delivery to the pads. From the pads, the desiccant flows by gravity onto all 18 rows of tubes and fins, flowing off of the lowermost row of fins into a collection sump. In traveling from the foam pad to the collection sump, the desiccant does not traverse any air gaps that may cause it to breakup into droplets.  
         [0114]     The performance of the liquid-desiccant mass and heat exchanger is modeled by separately calculating the heat transfer between the tubes and the desiccant films that flow around the tubes, and the heat and mass transfer between the process air stream and the liquid desiccant films that flow on the fins. The heat transfer between the tubes and the desiccant films is calculated assuming that U, the heat transfer coefficient is 500 Btu/h-ft2-F. Values of U between 520 and 680 Btu/h-ft2-F have been measured in bench-top experiments. Since a higher value of U will lead to a more compact and efficient mass and heat exchanger, the assumption that U is 500 Btu/h-ft2-F is conservative. Knowing the temperature of the liquid desiccant that flows onto the tube, the surface area available for heat transfer, the heat transfer coefficient U, the temperature within the tubes (i.e., the temperature of the evaporating refrigerant), the flow rate of desiccant, and the heat capacity of the desiccant, one can calculate from the conservation of energy the temperature of the desiccant as it flows off of the tube onto the fins.  
         [0115]     The fins form parallel-wall channels for the flow of the process air stream. For the design studied here the velocity of the air in these channels is 525 fpm. The Reynolds number for this air flow is about 900, which means that the air flow will be laminar. Heat and mass transfer coefficients for laminar flows between parallel walls are well known as functions of Reynolds number and Prandtl number (which will be 0.7 for air). Using these heat and mass transfer coefficients and the properties for the liquid desiccant, the exchange of heat and mass between the air and the desiccant films is calculated. With these exchanges known, the temperature and humidity of the air that leaves the channels between the fins are calculated and the temperature and concentration of the liquid desiccant leaving the fins and flowing onto the next row of tubes are calculated.  
         [0116]     The preceding calculational procedure is repeated for each row of tubes and fins.  
         [0117]     The completed performance calculation shows that for the desiccant flow rate and the fin height that has been selected, the temperature of the desiccant increases 10° F. while it is absorbing water vapor on the fin. This change in temperature produces an acceptable 10% decrease in the driving potential for water absorption. Also, after passing over all the fins and tubes the desiccant&#39;s concentration decreases to 34.7% from its initial value of 37.0%. This 2.3 point change in concentration produces an acceptable 4.0% decrease in the driving potential for water absorption.  
         [0118]     The complete performance calculation shows that the liquid-desiccant mass and heat exchanger absorbed 31,100 Btu per hour of heat and 17.4 lbs per hour of water from the air. This heat absorption is almost 4% higher than the conventional evaporator and the water removal is more than 2 times higher. The increased water removal is very important in HVAC applications where humidity control is critical, and provides a strong incentive for air conditioners to replace their conventional evaporator with a liquid-desiccant mass and heat exchanger of the present invention.  
         [0119]     The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims, that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.

Technology Classification (CPC): 5