Abstract:
A micro-evaporator is disclosed which is easy to manipulate and can be operated problem-free over a large temperature range. The micro-evaporator has micro-evaporator channels in a trapezoidal region, comprising an inlet region opening in the liquid feed chamber with a small diameter, and an outlet region opening into the vapor collecting chamber with a large diameter.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a micro-evaporator according to the preamble of claim  1   
     2. Description of Related Art 
     Micro-evaporators are used to evaporate liquid media such as water, alcohols or alcohol/water mixtures, liquid gases or liquid alkanes for further processing. Such micro-evaporators are for example used in the fields of fuel cell engineering. 
     Different designs of evaporators are known. So-called plate heat exchangers consist of corrugated steel plates provided with channels for transferring the liquid like filter presses. Resistant rubber seals hold the plates at a distance from each other and seal the channels of the two fluids from each other. Due to the rubber seals, the plate heat exchangers are restricted to a maximum temperature of 250° C. The two fluids flow in a concurrent or a countercurrent flow as thin layers alternating up or down through a series of chambers and contact each other on both chamber walls. Corrugated plate profiles increase turbulence and improve the transfer of heat. Such evaporators are not compact and, when they are operated with heating gas, they have low power densities. Such plate heat transfer systems are for example known from Vauck/Muller; Grundoperationen der Verfahrenstechnik; Leipzig 1994. 
     An evaporator with micro-structured components for the partial or complete evaporation of liquids is for example known from U.S. Pat. No. 6,126,723. The reactor consists of two fluid chambers for a first and second fluid, and the fluid chambers are connected by means of plates that are porous or provided with micro-structured holes. The reactor is intended for transferring a working fluid contained in a first liquid into a second fluid, and the plates provided with micro-structured holes are only permeable to the working fluid. This reactor can also be used as an evaporator. 
     Micro-structured devices for conducting media are known from DE 199 63 594 A1 and DE 100 10 400 C2. These devices are especially suitable for evaporating liquid media. 
     The device in DE 199 63 594 A1 has a layered structure, and at least one layer possesses a number of micro-channels through which the medium to be evaporated flows. A second layer also has a number of micro-channels through which a heat-transferring medium flows. The micro-channels each have an inlet and outlet. To satisfactorily evaporate the liquid medium, the outlets of the micro-channels have a smaller area for the medium to be evaporated and/or have a different geometrical structure than the respective inlets. The pressure of the liquid medium is thereby increased in the micro-channels so that overheated, still-liquid medium suddenly transitions from a liquid to a vapor after leaving the smaller area outlets. 
     According to DE 100 10 400 C2, the heat output of the heating device is adjustable at least in areas independent of other areas to adjust a desired temperature profile on the surface of the flow channels, at least in individual areas of the flow channels in the direction of flow. 
     According to this state-of-the-art, the micro-channels for guiding the liquid to be evaporated are parallel. The disadvantage of this micro-evaporator is that the vapor bursts or vapor bubbles can arise if the optimum temperature range is not adjusted. The optimum operating point must be maintained which requires several parameters to be harmonized such as the flow speed and pressure of the medium to be evaporated, the heat output, etc. Such micro-evaporators therefore malfunction easily and cannot be flexibly used. 
     The problem of the invention is to provide a micro-evaporator that is easier to handle and can be operated problem-free over a wider temperature range 
     BRIEF SUMMARY OF THE INVENTION 
     This problem is solved with a micro-evaporator where the micro-evaporator channels are in a trapezoidal area having an inlet area with a smaller cross-section ending in the feed chamber, and an outlet area with a larger cross-section ending in the vapor collection chamber. 
     By arranging the micro-evaporator channels in a trapezoidal area, the inlets of the micro-channels lie directly next to each other. The width of segments separating the micro-evaporator channels can become 0 at the inlet area of the trapezoidal area. The liquid in the feed chamber is therefore discharged very quickly which effectively prevents premature evaporation in the feed chamber. 
     The outlet area of the trapezoidal area is larger than the inlet area with the advantage that the outlets of the micro-channels are clearly spaced which allows the (overheated) vapor whose volume is very much more than that of the initial liquid to exit unrestricted into the corresponding large-volume vapor collection chamber. This prevents overpressure at the outlet area and keeps the liquid columns from being pressed back in the micro-channels opposite the direction of flow which enhances problem-free operation. 
     It has been demonstrated that the micro-evaporator can operate smoothly over a wide temperature range of 100 to 500° C. for water. The micro-evaporator is insensitive to parameter changes, and a change in the mass flow of the medium to be evaporated is not problematic at a given heating temperature. 
     Another advantage of the micro-evaporator is that it can be positioned independently in use. 
     The substantial compactness and the high energy density are additional advantages. In addition, the system can be operated free of carrier gas. The system can be heated with heating gas whose temperature can lie several hundred degrees centigrade above the boiling temperature of the liquid to be evaporated. Furthermore, a chemical reaction can be used for heating whose reaction temperature can lie several hundred degrees centigrade above the boiling temperature of the liquid to be evaporated. The liquid can also be evaporated at heating temperatures that are slightly above the evaporation temperature of the medium to be evaporated, which is advantageous in regard to energy consumption. 
     The micro-evaporator can be operated within a variable pressure range of the liquid to be evaporated. 
     The micro-evaporator channels are preferably located in the trapezoidal area in a fan-like arrangement. This means that the micro-evaporation channels lead radially from the liquid feed chamber and enter the vapor collection chamber at a marked distance from each other. 
     The micro-evaporator channels preferably have a cross-section of 100 μm 2  to 0.01 mm 2 . The cross-sectional areas are more preferably 100 μm 2  to 0.005 mm 2  and 100 μm 2  to 0.0025 μm 2 . Rectangular micro-evaporator channels have corresponding edge lengths of 10 μm to 100 μm, or a especially range from 10 μm to 50 μm. Due to the small dimensions of the micro-evaporator channels, the contact surface/volume ratio is very high, and a high transfer of heat is achieved that causes the liquid to quickly evaporate. These small dimensions of the micro-evaporator channels, especially in the area where the liquid/gas transition occurs, prevent boiling delays which enables the liquid to evenly evaporate. 
     The micro-evaporator channels preferably have a constant cross-section over their entire length. The micro-evaporator channels can for example be produced by electrical discharge machining, molding, laser ablation or other processing methods. 
     The micro-evaporator channels can run in a straight line or meander. The advantage of meandering or wave-shaped micro-evaporator channels is the greater contact between the liquid to be evaporated and the channel wall while the liquid is flowing due to the curves in the channel. This substantially improves the transfer of heat. Another advantage of the meandering arrangement and design of the micro-evaporator channels is that, despite the greater length of the micro-evaporator channels, the design can be more compact. 
     In another embodiment, the micro-evaporator channels have sections with larger and smaller cross-sections that alternate sequentially. This embodiment of the micro-evaporator channels also has the advantage of greater heat transfer from the increased contact of the liquid due to the necessary diversion in the micro-evaporator channels. 
     In these embodiments of the micro-evaporator channels, the distance between the micro-evaporator channels increases toward the vapor collection chamber. 
     An alternate embodiment provides that each micro-evaporator channel consists of offset, sequential channel sections. In a development of this embodiment, the adjacent channel sections can be connected with each other. Consequently, the volume available for the vapor in the area of the micro-evaporator channels increases toward the vapor collection chamber. This takes into account the increased volume of the vapor and helps the (overheated) vapor to be quickly discharged into the vapor collection chamber. 
     The vapor collection chamber preferably has a larger volume than the liquid feed chamber. 
     The means to heat the liquid to be evaporated are preferably located at least in the area above and/or below the micro-evaporator channels. By means of this arrangement, the area to be heated can be preferably concentrated on the area where the micro-evaporator channels are located. Given the relatively fast discharge of the liquid to be evaporated out of the feed chamber into the micro-evaporator channels, the entire micro-evaporator does not necessarily have to be correspondingly heated. 
     The means for heating preferably comprise micro-structured heating channels, electrical heating cartridges or lamps such as quartz lamps. In this case, the heat is provided by radiation energy. 
     A preferred embodiment of the micro-evaporator is characterized by at least one evaporator plate, on the front side comprising the structure of the liquid supply chamber, the micro-evaporator channels and vapor collection chamber. The essential components can therefore be located in a plate which simplifies the production of the micro-evaporator and also allows for a modular design in which numerous such plates can be stacked, and the evaporation plates are only separated from each other by simple, preferably non-structured intermediate plates. 
     When the evaporator plate preferably has micro-structured heating channels in its back, it additionally contributes to the compact design and simplified production of the micro-evaporator. 
     The cross-sections of the heating channels preferably range from 0.1 mm to 10 mm. 
     The heating channels can be coated with a catalyst material. The heating in this instance can be provided by catalytic combustion. 
     The heating channels are preferably parallel, and preferably extend in the same direction as the micro-evaporator channels. This same alignment of the heating channels means that both the heating channels as well as the micro-evaporator channels extend essentially in the lengthwise direction of the plate. The heating channels can be operated in a concurrent flow or in a counter-current flow with reference to the direction of flow of the liquid to be evaporated. This depends on the heating liquid, the liquid to be evaporated and the required transfer of heat. 
     The residual wall thickness of the evaporator plate is preferably &lt;1 mm. The advantage is that there is a good transfer of heat between the heating medium and the liquid to be evaporated. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
       The foregoing summary, as well as the following detailed description of preferred embodiments of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there is shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. Exemplary embodiments of the invention are further explained below with reference to the drawings. The following are shown: 
         FIG. 1  an exploded view of a micro-evaporator in a first embodiment, 
         FIG. 2  a plan view of a micro-evaporator plate of the micro-evaporator shown in  FIG. 1 , 
         FIG. 3   a - c  enlarged sections from the plan view of the micro-evaporator shown in  FIG. 2 , 
         FIG. 4-7  enlarged sections of a schematic representation of the micro-evaporator plate to illustrate different embodiments of the micro-evaporator channels, 
         FIG. 8  a perspective view of the underside of an evaporator plate 
         FIGS. 9   a  and  b  perspective plan and bottom views of a micro-evaporator in another embodiment, and 
         FIG. 10  a schematic representation of an evaporator system using a micro-evaporator according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a micro-evaporator  1  having (from top to bottom) a heating plate  2  with holes  3  to receive heating elements  4 , a cover plate  5 , a micro-evaporator plate  10  and a baseplate  6 . The cover plate  5  lies on the micro-evaporator plate  10  that has the micro-structured micro-evaporator channels in a trapezoidal area  15  as explained in detail with reference to the subsequent Figures. 
     The micro-evaporator plate  10  where the liquid is evaporated is connected to a feed line  30  for the liquid to be evaporated and a discharge line  31  to remove the vapor. The micro-evaporator plate  10  is in a groove-like recess  8  of the base plate  6 . Overall, this yields a cuboid micro-evaporator with outer dimensions less than 50 mm that is expandable with additional micro-evaporator plates  10  and inserted intermediate plates  7  as explained with reference to  FIGS. 9   a  and  9   b.    
       FIG. 2  shows a plan view of the front side  11  of the micro-evaporator plate  10  shown in  FIG. 1 , said plate consisting of a highly-conductive material such as steel. An inlet  12  runs upward from the bottom and ends in a liquid feed chamber  13 . The liquid feed chamber  13  expands in the direction of flow and has a curved end area which connects with the likewise curved inlet area  14  of the trapezoidal area  15  in which the micro-evaporator channels  20  are located that are best viewed in  FIG. 3   a  to  3   c.    
     The opposite end of the trapezoidal area  15  has an outlet area  16  that connects with the vapor collection chamber  17  which is provided with an outlet  18 . The vapor collection chamber  17  also has a trapezoidal shape, and the vapor collection chamber  17  narrows from the outlet area  16  of the trapezoidal area  15  toward the outlet  18 . The depth of all the microstructures is 30 μm. 
       FIG. 3   a  shows an enlarged section of the inlet area  14  of the trapezoidal area  15 . Numerous straight micro-evaporator channels  20  are separated from each other by a corresponding number of segments  21 . The width of the segments  21  is minimal at the inlet area  14  so that the inlet openings of the micro-evaporator channels  20  lie directly next to each other to quickly intake the liquid in the feed chamber  13  and conduct it into the micro-evaporator channels  20 . 
     The micro-evaporator channels  20  in this embodiment are fan-shaped so that the distance between the micro-evaporator channels  20  increases toward the vapor collection chamber  17  as a result of the corresponding widening of the segments  21 . This can be seen by comparing  FIGS. 3   a  and  3   c  that show a section of the middle area and outlet area  16  of the trapezoidal area  15 . The cross-section of the micro-evaporator channels  20  remains the same over the entire length. The cross-section dimensions of the micro-evaporator channels  20  are 30 μm×30 μm. In this manner, several hundred evaporator channels (such as  200 ) can be arranged next to each other. 
       FIG. 4  again schematically illustrates the embodiment shown in  FIG. 3   a  to  3   c . We can see that the width B of the radiating or fan-shaped micro-evaporator channels  20  remains constant over their entire length in the trapezoidal area  15 , whereas the width of the segments  21  separating the micro-evaporator channels  20  increases from width A 1  to width A 2  that can be a multiple of width B. 
       FIG. 5  schematically portrays another embodiment where the micro-evaporator channels  20 ′ have a wave-shaped or meandering design. The liquid to be evaporated necessarily follows the curves of the meandering micro-evaporator channels  20 ′ and is alternately pressed against the left and right side of the micro-evaporator channel walls due to the flow to improve the transfer of heat. At the same time, the construction is compact with a greater length of the micro-evaporator channels  20 ′. In this embodiment as well, the meandering micro-evaporator channels  20 ′ are fan-shaped arising from the increased width of the likewise wave-shaped segments  21 ′. 
     According to the embodiment shown in  FIG. 6 , the micro-evaporator channels  20 ″ have channel sections  22  with a smaller diameter, and channel sections  23  with a larger diameter. This embodiment also increases the contact of the liquid to be evaporated with the channel walls due to the flow and thereby improves the transfer of heat. The individual channel sections  22 ,  23  of the neighboring micro-evaporative channels  20 ″ are offset in relation to each other to save space. The design of the segments  21 ″ follows this arrangement. 
       FIG. 7  shows another embodiment in which individual adjacent, offset, column-shaped segments  25  are sequentially arranged. The individual micro-channels are formed by the channel sections  24  where two channel sections are given dashed lines between two column-shaped segments  25  for clarity. We can clearly see that these channel sections  24  are also connected to the neighboring channel sections  24 . The volume within the micro-evaporator channels thereby increases from the inlet area  14  to the outlet area  16  taking into account the expansion of the vapor in the trapezoidal area  15 . 
       FIG. 8  shows an enlargement of the underside  19  of the micro-evaporator plate  10 . The heating micro-channels  27  are also separated from each other with segments  28 . An antechamber  26  and a post-chamber  29  are provided that are connected with each other by means of heating channels  27 . These chambers  26 ,  29  serve to distribute and collect the heating gas that flows through the heating channels  27 . In addition,  FIG. 8  also shows the feed line  32  and the discharge line  33  for the heating gas as well as an intermediate plate  7  that is only structured in the area of the feed and discharge of the steam or liquid to be evaporated. 
       FIGS. 9   a  and  b  show another embodiment of the micro-evaporator with numerous evaporator plates  10  that are separated from each other by non-structured intermediate plates  7 . This modular design allows the micro-evaporator to be constructed to have any desired throughput. 
     The micro-evaporator  1  is sealed at the top by a cover plate  5 . The base plate  6  is not shown in this figure. The liquid to be evaporated is supplied and discharged by feed and discharge lines  30  and  31  that are perpendicular to the evaporator plates  10 . The heating gas is supplied by the feed line  32 , and the heating gas is discharged by the discharge line  33 . The cross-sections of individual lines  30  to  33  are adapted to the requirements of the utilized media. 
     In  FIG. 9   b , one can see the structuring of the heating channels  27  on the bottom of the evaporator plate  10 . 
     An evaporation system is portrayed in  FIG. 10  that uses a micro-evaporator  1 . The liquid (water in this instance) was held by a pressurized reservoir  40 . The flow of the unevaporated liquid was determined by a thermal mass flow regulator is  41 . After evaporation, secondary heaters  42  (indicated by the bold arrows) prevent the liquid from condensing. The quality of the generated vapor was determined with a Coriolis flowmeter  43 . The amount of evaporated water can be regulated by the preliminary pressure regulator  44 . 
     REFERENCE NUMBER LIST 
     
         
         
           
               1  micro-evaporator 
               2  heating plate 
               3  hole 
               4  heating element 
               5  cover plate 
               6  baseplate 
               7  intermediate plate 
               8  groove-like recess 
               10  micro-evaporator plate 
               11  front 
               12  inlet 
               13  liquid feed chamber 
               14  inlet area 
               15  trapezoidal area 
               16  outlet area 
               17  vapor collection chamber 
               18  outlet 
               19  underside 
               20 , and  20 ′,  20 ″ micro-evaporator channel 
               21 ,  21 ′,  21 ″ segment 
               22  channel section with a smaller cross-section 
               23  channel section with a larger cross-section 
               24  channel section 
               25  column-shaped segment 
               26  antechamber 
               27  heating channel 
               28  segment 
               29  post-chamber 
               30  feed line for liquid 
               31  discharge line for vapor 
               32  feed line for heating gas 
               33  discharge line for heating gas 
               40  reservoir 
               41  mass flow controller 
               42  secondary heater 
               43  Coriolis flowcontroller 
               44  upstream pressure regulator 
           
         
       
    
     It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.