Abstract:
Hybrid dry cooler heat exchangers with one or more tube-fin heat exchange assemblies including at lease one such assembly wetted by water dropped on it with the first of such assembly stamped to provide arrays of slit/louver combinations breaking up water droplets and averting flight of droplets into a gas flowing between adjacent fins.

Description:
FIELD AND BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to heat exchangers, often used for large air-conditioning and large refrigeration systems and more particularly to the finned tube heat exchangers used therein for the hybrid cooling of a liquid medium or for the liquefaction of refrigerants. Such types of apparatus have an air-side heat transfer surface wettable with water or other liquid and exposed to ambient air sucked in by at least one fan as coolant, for evaporation of water carried in the circulation system. 
         [0002]    Hybrid dry coolers are known (see e.g., European Patent EP 428 647 B1) in which the liquid medium of the primary cooling circuit flows through a finned heat exchanger and the heat to be dissipated via the cooling fins is emitted to the air stream partially as sensible and partially as latent heat. 
         [0003]    One or several fans suck the air stream through the heat exchanger and always run at the same rotating speed. The wetting water is pumped from a collection dish integrated in the cooler into open ducts via two heat exchanger elements, usually arranged in a V-shape. There it is given off laterally to the direction of air flow as a droplet-forming liquid film on the air-side heat exchanger surface. Directly below the heat exchanger elements, the excess water drips back into the collection dish. The term “hybrid” stands for selective operation of the drying tower, the heat exchanger with water and with wetting. 
         [0004]    These types of hybrid dry coolers are normally designed for a maximum permissible air load, which results from the fact that no droplet flight results during the wetting of the heat exchanger. This means the specific volume flow of the cooling air is extremely limited. The switchover point from wet operation (hybrid) with wetting to the purely dry operation calculated for a 100% cooling load usually lies somewhat below the annual average air temperature. As a result, the hybrid dry cooling tower cannot be operated dry, i.e. without wetting, for even half its annual operating time, and therefore requires a great deal of expensive additional water. 
         [0005]    Depending on the chosen heat exchanger geometry, droplet flight results at a specific air load of approximately 5 t/h on a face area of 10.8 ft 2  (1 m 2 ). The vectorial speed profile of a droplet in the more or less laminar air stream between 2 smooth finned plates clearly shows that once a droplet has torn off, it can no longer actively participate in the cooling process via the lamellar passage. At best, the water loss problem can be reduced with so-called droplet separators. For this reason, the greatest disadvantage, i.e. the performance loss due to the missing wetting area with an air flow rate which is insufficient anyway, is retained. This greatly endangers the economical operation of a hybrid dry cooler. 
         [0006]    It is already known that with purely sensible heat transfer during drying operation of a heat exchanger, a more effective utilization of the cooling stream results when it is swirled by so-called turbulators between the fins or cooling ribs. In other designs, openings are provided in the fins to enable movement of the air into another fin gap during its passage. In a further elaboration of this design, louvers adjacent to the openings are already provided for the forced deflection of the air at these openings. 
         [0007]    However, these fins equipped with louvers demonstrate certain undesirable properties, especially during wetting. Due to improper positioning, alignment and shaping, a very high flow resistance or even a misdirection of the air stream with back flow, or even increased droplet flight results leading to excessive turbulence, which reduces the efficiency of the entire cooling system. Certain design weaknesses exist in the shape and alignment, which leads to an undesirable distortion of the cooling fin. 
         [0008]    Experience has shown that with large heat exchangers, like those used for hybrid cooling, the unsuitable formation of the fin surface at certain air stream speeds can lead to the formation of eddies. This is expressed by high-frequency whistling noises. Especially with louvers arranged regularly in a row with uniform spacing and a multiple repetition of a certain elementary geometric pattern, the increasing intensity of the sound produces damaging vibrations in the entire system. 
         [0009]    If hybrid cooling is dimensioned for high cooling-medium entry temperatures of, for example, 113° to 140° F. (45° to 60° C.) and a low coolant exit temperature of, for example 79° to 90° F. (26° to 32° C.) at an air temperature on the moist thermometer of 72° F. (22° C.), then numerous disadvantages result for hybrid cooling. The high initial temperature difference, formed from the difference of the cooling-medium entry temperature and the resulting cooling air temperature, reduces the necessary dimensions of the heat-exchanging, wettable surface so greatly that cooling operation with a purely sensible heat transfer in the positive range of the air temperature becomes impossible. The cooler surface would therefore already have to be wetted in the negative air temperature range, which leads both to an undesirable formation of visible cooling air vapors and to an increased danger of freezing of the excess wetting water. In addition, the very high annual consumption of processed additional water for the latent heat transfer leads to uneconomically high operation costs for the hybrid cooling system. An especially weighty disadvantage is the depositing of ingredients contained in the water (e.g. lime) on the cooling fins as a result of the high evaporation and drying effect of the wetting water at high fin temperatures of, for example, over 122° F. (50° C.). Reduced performance due to deposits on the fin surfaces requires frequent cleaning with high maintenance and operating costs, which leads to an uneconomical operation of the hybrid cooling system. 
         [0010]    It is therefore an object of the invention to provide a hybrid heat exchanger that permits a higher air load at the design point during wetted operation without droplet flight. 
         [0011]    Further objects are that the percentage of the wetted surface relative to the entire heat exchange surface be increased, the efficiency in the mass transfer during wetting be improved, the cooling capacity be considerably enhanced, eddies causing noises over the entire air loading range cannot occur, that drying operating be extended due to higher air loading and higher cooling ranges without impermissible thermal tensions and without visible post-condensation (most formation) become feasible. 
       SUMMARY OF THE INVENTION 
       [0012]    The foregoing problems are solved and the objects achieved through several means as follows. 
         [0013]    A series of water-droplet slit and water-droplet splitting louvers are cut and stamped in the smooth fins. A tube carrying the liquid coolant is assigned two groups of water-droplet slit and water-droplet splitting louvers, which are different distances from each other. The water-droplet slits and water-droplet splitting louvers are positioned perpendicular to the main direction of flow of the gaseous coolant with lower tips extending 45-60% into the clear fin gap. 
         [0014]    With smooth fins, the droplets unavoidably produced when wetting the fins with excess water are held suspended between the fins by the tube effect of the air stream, and are carried out as a result of excessively high surface tension. Now it is important that the gaseous medium laden with water droplets flows toward the opened gap and not, as is otherwise the case with ribbed heat exchanger fins, via the heat-conducting bar connected to the finned plate. If a droplet of the wetting water now reaches a water-droplet slit and water-droplet splitting louver, then the droplet is caught on the front edge of the rib and deflected so that it wets the surface touched as a film. This filmy wetting inevitably leads to a major increase in the surface wet with water, and therefore to a noticeable increase in the latent heat transfer through evaporation. 
         [0015]    Even at low flow velocities, the uneven geometric arrangement of an elementary formation of the water-droplet slit and water-droplet splitting louvers produces sufficient turbulence to increase the heat transfer coefficient in pure dry operation and the evaporation percentage during wetting. 
         [0016]    The louvers are arranged at different distances from each other and produce no noises over a very broad range of different flow velocities. Both the exciter frequencies and the natural frequencies differ as a result of the geometric arrangement. This permits a multiple repetition of the elementary arrangement over the entire fin surface, or an improvement to this heat exchanger by increasing the heat transfer coefficient in pure dry operation and an increase in the evaporation percentage during wetting at a very broad range of different flow velocities with simultaneous elimination of troublesome noise emissions. The system also does not tend to vibrate. 
         [0017]    As a direct consequence of an arrangement of a water-droplet slit and water-droplet splitting louver, hybrid (water-wettable) heat exchangers can be designed for a higher specific air load. Measurements have shown that the permissible air load of a heat exchanger with wetting up to the beginning of droplet flight can be increased by up to 18% with the water-droplet slit and water-droplet splitting louvers in accordance with the invention. Even from the standpoint of an enlargement of the wetted surface, this increase in the air throughput causes a performance increase between 22 and 25%. 
         [0018]    This results in a noticeable decrease in the heat transfer area required for a certain performance, and with it a reduction in the investment costs. Thanks to higher air loading, the switchover point from dry operation to wetting does not shift noticeably, despite the reduced surface, enabling the economical operation of the hybrid cooler to be retained. 
         [0019]    Other advantages that result from higher air loading during wetting include a more economical operation of a connection in series of a drying cooler for purely sensible heat transfer with a hybrid (water-wettable) cooler for mainly latent heat emission, especially at high cooling-medium entry temperatures, a very large cooling range and use of the same fans for forced-air feeding through both heat exchangers. The same advantages also result during completely wetted operation of heat exchangers connected in series or in parallel. 
         [0020]    If the specific cooling air quantity can be increased by a wettable heat exchanger, then the air quantity in the dry heat exchanger even increases by the factor 1.18, because the pressure loss with a lack of water is lower despite the same fin geometry. 
         [0021]    Other objects, features and advantages will be apparent from the following detailed description of preferred embodiments taken in conjunction with the accompanying drawings in which 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]      FIG. 1  shows a view of a cooling fin in the preferable angled or vertical installation position with horizontal tubes arranged offset according to a preferred embodiment of the invention; 
           [0023]      FIG. 2 . shows the arrangement of a water-droplet slit and water-droplet splitting louver assigned to a tube; 
           [0024]      FIG. 3 . shows the enlarged depiction of a detail (II) from  FIG. 2 ; 
           [0025]      FIG. 4 . shows a cross section through a water-droplet slit and water-droplet splitting louver of two fins arranged parallel to each other; 
           [0026]      FIGS. 5-7  show alternate designs of a water-droplet slit and water-droplet splitting louver; and 
           [0027]      FIG. 8  shows an application example of a water-wettable heat exchanger designe with a water-droplet slit and water-droplet splitting louver in combination with a heat exchanger for purely sensible heat emission, for the application case with high coolant entry temperatures and a large coolant cooling range. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0028]      FIG. 1  shows a hybrid heat exchanger  1  with several tubes  2  which run parallel to each other and are arranged at a selected distance from each other. The tubes  2  are positioned horizontally. The tubes  2  begin and end in so-called collectors, not shown in  FIG. 1 . A liquid flows inside the tubes  2 . The temperature of the liquid which must lie above that of the wet-bulb temperature of the surrounding air. 
         [0029]    Along the reach of the fin depth in the air flow direction  3 , 8 , the tubes  2  may be arranged behind each other or preferably offset. In accordance with the number of tube rows, the liquid can flow through the heat exchanger one to several times. 
         [0030]    The feed  6  of the wetting water  7  is located above the upper face  4  of the fins  5 . A larger quantity of this water is output than can evaporate in the air stream  8 . The excess wetting water  9  is collected in a catch bowl  10  and recirculated via a pump  11 . 
         [0031]    In accordance with the preferred embodiment, the heat exchanger  1  is a so-called “finned tube heat exchanger” in which the tubes  2  are designed as round tubes, as droplet-shaped tubes, or as oval tubes and the fins  4  produced from one piece can hold a large number of tubes  2 . In the longitudinal direction of the tubes  2 , several fins  5  are arranged behind each other. Their distances from each other are optimized for pressure loss and heat transfer in accordance with the respective needs. 
         [0032]    The plane of the fins  5  is vertical and is shown on the drawing level in  FIG. 1 . On the other hand, the heat exchanger  1  encloses an angle α relative to the axis  12  of the fan  13 , which can be between 0 and 30°. 
         [0033]    Water-droplet slit and water-droplet splitting louvers  14 , 15  are arranged between the adjacent tubes  2  in each case in the air flow direction. The design of the water-droplet slit and water-droplet splitting louver  14 ,  15  in accordance with the invention is described in the following with reference to  FIGS. 2-7 . 
         [0034]    In  FIGS. 2 and 5 , Detail II from  FIG. 1  is shown enlarged.  FIG. 2  shows the arrangement of the group of a water-droplet slit and water-droplet splitting louver assigned to a tube. It consists of baffles located parallel to each other at differing distances  16 - 23  on both sides of the tube. With regard to the distances of the baffles  24  from each other, it should be noted that a detuning of the exciter vibrations of the cooling air is achieved with preferably different distances. However, the desired effect of water droplet slitting and water droplet splitting can also be achieved with similar baffles at equal distances. 
         [0035]    As a result of a backup effect of the air stream  8  before the tube  2 , the two baffles  24  flowed against first by the air, which are located next to each other in the longitudinal fin direction Y, can be positioned relatively far apart. The baffles  24  which follow in the air flow direction  8  then preferably lie in the area of the highest air flow velocity and generally in areas in which the free water droplets are transported in the cooling air. As the baffles only serve the purpose of droplet catching and droplet splitting, it is therefore not necessary for the entire air area between to adjacent tubes  2  to be covered. An air deflection is also of secondary importance, which is why the focus is on a reduction of the air-side pressure loss. Even with flows with a laminar character, new water droplets always come into immediate contact with the baffles  24 . For production-related reasons, a multiple repetition of geometrically identical elementary formations assigned to the tubes  2  is advantageous. 
         [0036]    The design of a baffle  24  is shown spatially in  FIG. 3 . The finned plate is cut open over the length of the baffle  24  and then the baffle  24  stamped with side seals  25  from its own material. The side seals  25  prevent the caught water from flowing out again as a result of turbulence and counter-pressure from the baffle  24  located below. Depending on the design of the baffle  24  the cooling air laden with water droplets strikes the cut edge  25  vertically or at a slight angle. 
         [0037]      FIG. 4  shows a cross section through two baffles  24  of two fins  5  spaced parallel to each other. To catch and cut the greatest possible number of droplets  28  suspended in the cooling air stream, the openings  26  of the baffles  24  must face opposite the flight direction  27  of the droplets  28 . The angle β must be selected so that the cross section  29  of the baffles  24  is located approximately in the center of the fin gap  30 . The flattest possible angle of incidence minimizes turbulence, and therefore minimizes the air-side pressure loss, without hindering droplet catching. The radii R 1  and R 2  ( FIG. 4 ) must be designed as large as possible to largely prevent pressure losses through turbulences. 
         [0038]    The impact of a droplet  28  on the cut edge  29  presses part of the water contained in a droplet  28  onto the upper fin and part below the actual baffle. During this impact the surface tension of the water droplet  28  is affected and filmy wetting forms on the surface concerned with an enlarged working surface for the evaporation of the wetting water. 
         [0039]      FIG. 5  shows another design of the water-droplet slit and water-droplet splitting louver. These baffle groups  31  and  32  are in turn assigned to a single tube  2  and for production-related reasons can be assigned to other tubes  2  through multiple repetition of geometrically identical elementary formations assigned to the tubes  2 . The baffles  33 ,  34 ,  35  and  36  are shorter in the direction lateral to the air flow  8 , however are more numerous than the baffles  24  shown in  FIG. 2 . 
         [0040]    Due to a lack of mechanical resistance, the air flow between the individual baffles  33  is somewhat accelerated and directed directly at the second row of baffles  34  located in the air flow direction. Air droplets carried along which have avoided the first row of baffles  33  are caught and cut open in the course of the baffle row  34 . The same process is repeated in a similar manner in the subsequent rows  35  and  36 . 
         [0041]      FIG. 6  shows the design of a larger number of baffles  37  and  38 . To minimize the backpressure of the air stream  8 , the baffles  37  and  38  are stamped slightly rounded. The baffles  38  located at the rear in the air flow  8  in each case cover the spaces  39  of the baffles  37 . 
         [0042]      FIG. 7  shows a cross section through two baffles  40  of two fins  5  spaced parallel to each other. Once again, for the purpose of reducing the backpressure, the transition of the water-droplet slit and water-droplet splitting louver must be designed with the largest possible radii R 1  and R 2 . On the other hand, the radii R 3  and R 4  must be selected so that the splitting edge  41  of the baffles  40  lies approximately in the center of the fin gap  42   
         [0043]      FIG. 8  shows an especially advantageous design of a hybrid heat exchanger  43  equipped with a water-droplet slit and water-droplet splitting louver with the hydraulic upstream connection of a heat exchanger  44  which emits purely sensible heat to the cooling air. This design of the liquid, coolant-side series connection of the heat exchangers  44  and  43  only achieves the economy of the cooling method, because the hybrid heat exchanger  43  can be designed for as high a specific cooling air load as the sensible heat exchanger  44  requires for economical operation, and the sensible heat exchanger  44  is first charged with the hottest coolant temperature. 
         [0044]    In accordance with the invention, the liquid coolant  45  of the primary cooling circuit P, coming from the heat source  46  and circulated by the circulation pump  47 , is fed to the heat exchanger  44  with the highest cooling circuit temperature, which must always lie above the ambient air temperature to be cooled, for a purely sensible heat emission. This heat exchanger  44  has the same or a different fin geometry than the hybrid heat exchanger  43 . 
         [0045]    The high initial temperature difference (t air inlet −t water inlet ) results in a very large heat flow density for the transfer to the heat from the liquid coolant via the walls of the tubes  2  to the fins  5 , and ultimately to the cooling air, the flow rate of which is symbolized by the arrows  48 . The liquid coolant  49  pre-cooled by means of the ambient air and without evaporation is now transferred from the heat exchanger  44  to the hybrid (water-wettable) heat exchanger. The reduced temperature of the coolant  49  at the inlet into the hybrid heat exchanger  43  now enables a primarily latent heat emission from the fins  5  to the cooling-air flow-through quantity, symbolized by the arrow  48 . 
         [0046]    The secondary cooling circuit S for wetting the fin surface of the heat exchanger  43  mainly consists of a water reservoir basin  50  with the feed pump  51  and the supply line  52  to the water feed  53 . The excess wetting water  54  which has not evaporated drips over a drain plate  55  and is routed back through the return line  56  into the reservoir basin  50 . A level sensor  57  regulates the water supply in the reservoir basin  50  by keeping the water level within certain limits by opening and closing the feed valve  58 . By means of a drain valve  59 , water with an excessively high concentration of components can be drained out of the reservoir basin  50  into a drain line  60  either conductivity-controlled or in proportion to the quantity. The overflow  61  of the reservoir basin  50  also empties into the overflow line by bypassing the drain valve  59 . 
         [0047]    The cooling air entering the heat exchanger, symbolized by the arrows  63 , has the same state for both heat exchangers  44 ,  43 . However, for the sensible heat transfer in the heat exchanger  44 , primarily the air temperature, and for the primarily latent heat transfer in the hybrid heat exchanger  43 , the air temperature with the related moisture content, must be included in the calculation of the thermal output. 
         [0048]    The content of the mixed-air water vapor, formed from the exhaust-air streams (arrows  65  and  66 ), at the transition from the cooler to the ambient air is decisive for the prevention of a visible, disturbing plume of steam as the result of condensation of the water vapor of the exiting cooling air, symbolized by the arrow  64 , in the cooler ambient air. The air exiting from the heat exchanger  44 , symbolized by the arrow  65 , which is heated without adding water, is greatly reduced in its relative moisture content at the outlet. On the other hand, the relative moisture content of the air exiting the heat exchanger  43 , symbolized by the arrow  66 , is increased greatly, or up to almost 100%, by evaporation of water. 
         [0049]    These two air streams (arrow  65 ,  66 ) are sucked in by the fan  62  and thoroughly mixed by its rotation, symbolized by arrow  67 . This results in a relative water vapor content that lies between the sensible and latent heat transfer of the heat exchangers  44 , 43  and far from the mist area with 100% saturation. A re-condensation of the cooling air in the area around the cooler can therefore certainly be excluded. 
         [0050]    Another advantageous property of the liquid heat exchangers  44  and  43  connected in series on the coolant side is the division of a high total cooling range between the coolant entry and exit temperature to a sensible heat sink in the high temperature range and a latent heat sink in the lower temperature range with a smaller cooling range in each case. This may exclude damaging influences on the tube-fin connection or even on the entire heat exchanger geometry as a result of excessively high thermal stress. 
         [0051]    The high specific air throughput for sensible and latent heat transfer in the design point also causes an increase in the air throughput for purely sensible heat transfer in the lower air temperature range. This in turn causes a shifting of the switchover point from purely sensible to sensible/latent heat transfer in the upper air temperature range. The extended drying operation with purely sensible heat transfer saves a great deal of expensive, processed additional water over the year, and therefore leads to economical cooling operation of the system. 
       CONCEPTUAL EXAMPLE  
       [0052]    In two sensible and latent heat exchangers connected in series, the coolant of a liquid coolant mass flow from a cooling circuit of a process of 17.05 kg/s with an ambient temperature state of 91° F. (33° C.)/38% relative humidity and by means of the fan with a sucking action for the two heat exchangers is to be cooled down from 149° F. (65° C.) to 86° F. (30° C). This is equivalent to a thermal output of 2,250 kW. 
         [0053]    In the sensible heat exchanger first charged with the liquid coolant, into which the coolant enters at 149° F. (65° C.), it is cooled down to 123.94° F. (51.08° C.) with the air mass flow of 45.36 kg/s sucked in by the fan with an air-side pressure loss of 118 Pa. 
         [0054]    The water pre-cooled to 123.94° F. (51.08° C.) then flows to the latent heat exchanger of the same geometry and the same dimensions. There it is cooled down to 86° F. (30° C.) with an air mass flow of 29.53 kg/s sucked in by the same fan with an air-side pressure loss of 118 Pa by means of evaporation of water from a secondary circuit. The smaller air mass flow with latent heat transfer is due to the somewhat increased pressure loss of the water-droplet slit and water-droplet splitting louver required for effective wetting. 
         [0055]    The initial air state at the inlet into the two heat exchangers  44 ,  43  of 91° F. (33° C.)/38% relative humidity changes in the process at the outlet of the sensible heat exchanger  44  to 133.52° F. (56.4° C.)/11% relative humidity, and at the outlet of the latent heat exchanger  43  to 90.5° F. (32.5° C.)/94% relative humidity. The two air streams with the different air quantities as a result of different pressure losses in the fin gaps of the two heat exchangers, and with different air states as a result of sensible cooling in the first and latent cooling in the second heat exchanger, are mixed in the cooling air fan and reach a mixed air state of 114.62° F. (45.9° C.)/31% relative humidity at the outlet of the cooler and at the transition to the ambient air respectively. With the very large difference of 69% relative humidity to the saturation limit of 100% relative humidity, the formation of visible plumes is completely excluded. 
         [0056]    At a constant cooling capacity of 2,250 kW but dropping temperatures of the ambient air, the switchover point to purely sensible heat transfer in both heat exchangers and at the same fan performance as in the design point is reached at an ambient air temperature of 55.94° F. (13.3° C.). In moderate climatic regions, this lies far above the annual average temperature. 
         [0057]    It will now be apparent to those skilled in the art that other embodiments, improvements, details, and uses can be made consistent with the letter and spirit of the foregoing disclosure and within the scope of this patent, which is limited only by the following claims, construed in accordance with the patent law, including the doctrine of equivalents.