Abstract:
A system for drying and tempering flowing air that has been compressed to a super atmospheric level prior to treatment by the system. The system includes a precooler and reheater core and a chiller core disposed in adjacent relation for receiving flowing air serially therethrough. A moisture removal section is disposed adjacent to the chiller core for removing entrained moisture from the flowing air. The moisture removal section is defined by sidewalls. The moisture removal section has a demister core disposed therein. The demister core comprises a plurality of fins configured to create an undulating flow through the demister core. The demister core is disposed at an angle with respect to at least one of the sidewalls to create an enlarged entrance area and an enlarged exit area.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application is a continuation-in-part of U.S. patent application Ser. No. 13/001,904 filed on Dec. 29, 2010, which is a 371 of International Application No. PCT/US09/66975 filed on Dec. 7, 2009, which claims priority of U.S. Provisional Patent Application No. 61/176,071 filed on May 6, 2009, which are all incorporated by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to the art of heat transfer; more particularly, to heat exchangers for use in refrigerated air drying; and most particularly to a precooler/chiller/reheater (“PCR”) system with a demister core. 
       BACKGROUND OF THE INVENTION 
       [0003]    Refrigerated air dryers are known in the art of compressed air. In a refrigerated air dryer system, warm, moist air such as from the interior of a factory, and which typically is compressed, is cooled and dried and then conveyed to a location where it is used. In such a compressed air system, it is important to reduce the water content of the compressed air before delivering the compressed air to the points of use to avoid condensation of moisture upon adiabatic decompression. This is known in the prior art to be accomplished by using air- or water-cooled aftercoolers, moisture separators, and air dryers. Air dryers are available in many different types, and the present invention is illustrated with a non-cycling direct expansion refrigerated air dryer wherein the refrigerant compressor operates continuously. This type of air dryer effectively reduces water content in compressed air by physically chilling the compressed air directly with a refrigeration circuit and thus reducing the capacity of the compressed air to hold water vapor. Water vapor in the chilled compressed air condenses as liquid droplets as the temperature of the compressed air is lowered to a desired dew point, typically about 40 degrees Fahrenheit (F). The combination of chilled air and water droplets flows through a moisture separator that mechanically removes the droplets from the air stream. 
         [0004]    Reheating or “tempering” the dried air lowers the relative humidity and prevents formation of condensation at the use point, and also prevents or reduces atmospheric condensation on compressed air piping within the factory, as might occur if the chilled dried air were piped directly without insulation. 
         [0005]    U.S. Pat. Nos. 5,845,505; 6,085,529 and 7,121,102 disclose PCRs, and the disclosures of these patents are hereby incorporated by reference. 
         [0006]    Such prior art PCRs, although functionally effective, have various drawbacks including large manifolds or and/or large moisture separator sections. These features add weight and size to the PCR and manufacturing complexity that add to the cost of manufacture. 
         [0007]    Due to its layout, the prior art PCR design of U.S. Pat. Nos. 5,845,505 and 6,085,529 requires a large and complex return manifold to direct the compressed air flow from the moisture separation section to the reheater section. This manifold is typically constructed from an aluminum casting, the size of which cannot be readily altered to accommodate larger or smaller capacity heat exchangers as may be desired for various end-use applications. 
         [0008]    The prior art PCR systems disclose different means for moisture separation. U.S. Pat. Nos. 5,845,505 and 6,085,529, disclose a system including a mesh pad inserted next to the chiller to capture and coalesce much of the condensed water leaving the chiller. Also, the return manifold is used to reduce the vertical upward velocity of the air flow and allow gravity to separate out any remaining droplets leaving the mesh pad. The manifold must be large enough to accommodate the mesh pad and also to reduce the upward air velocity sufficiently to prevent carryover of water into the reheater. Because the air within the PCR is compressed typically to 100 psig or more, the manifold, being of irregular shape not optimized for burst resistance, must be formed with very thick, heavy walls reinforced by internal bars. 
         [0009]    U.S. Pat. No. 7,121,102 discloses a large moisture separator section disposed between the precooler/reheater core and the chiller core. All dehumidified air passing through the chiller core exits the lower end thereof and then must travel upward the entire length of the separator section. The separator section includes a large number of plates having a large surface area for collection of the moisture. The system relies on marginally efficient low velocity and gravity settling of droplets in an oversized separator section of the heat exchanger core. The oversized section results in many extra passages with heavy fins for structural support that add considerable weight to the system. 
       SUMMARY OF THE INVENTION 
       [0010]    In one aspect, the present invention provides an improved precooler/chiller/reheater system having a precooler/reheater core and a chiller core. Warm, moist air enters the precooler/reheater core at a first end and exits through a crossover manifold. The crossover manifold conveys the precooled air to a first end of the chiller core. In another aspect, precooled air and condensate formed in the precooler enters the chiller core and exits at a second end, which preferably is at or near a low point in the system. A refrigerant source provides liquefied refrigerant to the chiller core. Moisture is condensed from the air in the chiller core and flows therefrom by gravity into a drain. Water flow is assisted by the downward passage of the air through the chiller. 
         [0011]    The components include a refrigeration system, a moisture separator, and two air heat exchangers. The first of these heat exchangers is a precooler/reheater. It precools warm saturated compressed air from an air compressor aftercooler by transferring heat to chilled air that is being returned from the moisture separator. One benefit of this heat exchanger is that it reduces some of the cooling load that the refrigeration system would otherwise have to handle in subsequent dehumidification of the air. The refrigeration system becomes smaller, requiring less power for thriftier operation. Another benefit offered by this first heat exchanger is that it reheats the chilled air coming from the moisture separator, as described below. As noted above, reheating the chilled air reduces the chances that low ambient conditions can cause condensation in the air line downstream of the dryer and also reduces the likelihood of pipeline condensation or “sweating” that can occur on chilled surfaces in humid use conditions downstream of the PCR system. 
         [0012]    The second heat exchanger is an air-to-refrigerant chiller that takes the precooled air from the first heat exchanger and chills it to the desired dewpoint temperature by transferring heat from the air into a cold refrigerant on the other side of the heat exchanger, thereby causing condensation of water from the air. After being thus chilled, the air enters a moisture separator to remove any remaining condensed water, and then the air is returned to the cold side of the first heat exchanger for reheating and exit from the PCR. 
         [0013]    In another aspect, the improved precooler/chiller/reheater system includes a manifold to convey air from the chiller core into a moisture removal section wherein the air passes upward. Entrained water droplets are coalesced and stripped from the airflow and flow downward into the drain. The moisture removal section has a demister core extending from the bottom wall to the top wall of the section such that all of the air passes through it before returning to the precooler/reheater core. In one aspect, the demister core comprises a plurality of fins configured to create an undulating flow through the demister core in order to separate water from compressed air. In yet another aspect, the demister core forms a structure that can be oriented at an angle inside the moisture removal section in order to provide an enlarged entrance area and an enlarged exit area in the moisture removal section. 
         [0014]    In another aspect of the improved precooler/chiller/reheater system, the chilled and dried air passes out of the top of the moisture removal section into a third manifold wherein the air is conveyed to an entrance to the second side of the precooler/reheater core. The chilled, dry air passes through the precooler/reheater core, preferably in a downward direction from top to bottom in counterflow to the direction of the warm, moist air entering the system on the first side of the heat exchanger, and exits the system ready for use as tempered, dried air. All fluid flows in the system are parallel to such that fluid flows through the heat exchangers are counter-flow. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
           [0016]      FIG. 1  is a front perspective view of a PCR system of the present invention; 
           [0017]      FIG. 2  is is a cross-sectional view of the PCR system of the present invention taken along lines  2 - 2  of  FIG. 1 ; 
           [0018]      FIG. 3  is a rear perspective view of the PCR system of  FIG. 1 ; 
           [0019]      FIG. 4  is an end perspective view of the PCR taken from the left side of  FIG. 1 ; 
           [0020]      FIG. 5  illustrates a demister core of one embodiment of the present invention; and, 
           [0021]      FIG. 6  is an enlarged sectional view of the demister core taken along the line  6 - 6  of  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    Referring to  FIGS. 1-4 , a PCR system  10  includes a precooler/reheater core  12  disposed adjacent to a chiller core  14 . The cores are configured like conventional heat exchangers of alternately connectable plates  16 ,  18  and may be formed of aluminum as known to those of ordinary skill in the art. Materials passing through the cores on opposite sides thereof travel in opposite directions, i.e., in counter-flow. A moisture separator section  20 , with a drain  21  located at the bottom of the section  20 , is disposed adjacent to the chiller core  14 . Warm moist air, for example, from the discharge of an air compressor aftercooler, enters core  12  of PCR system  10  through an inlet fitting  22  and a manifold  24  in the direction of arrow  23 . Typically the input air is obtained from a compressor  25  connected to fitting  22  via a conduit  26 . Coolant or refrigerant from a source  28  is supplied to chiller core  14  from conduit  29  via a distributor  30  and a manifold  32  in the direction of arrows  33  ( FIG. 2 ). Refrigerant is returned from chiller core  14  via a collector  34  and outlet fitting  36  to the source  28  in the direction of arrows  37  ( FIG. 2 ). The system also includes a precooler-to-chiller air manifold  38  (best shown in  FIG. 3 ), a moisture separator-reheater manifold  40 , and an air outlet manifold  41  connected to air outlet fitting  44 . The chiller core  14  is in communication with the moisture separator section  20  by means of a manifold  47 . 
         [0023]    Referring to  FIG. 2 , in operation warm, moist air enters system  10  and is distributed into precooler/reheater heat exchanger  12  via manifold  24 . Air flows upward through core  12  in the direction of arrows  23  and is turned via a mitered section (not visible) in an upper portion  13  of core  12  such that air flow is directed sideways from core  12  through a core exit  52  into manifold  38 . The partially-cooled air is conveyed by manifold  38  to a side entrance  54  in chiller core  14 . The air is then turned by another mitered section (not visible) to flow downwards in the direction of arrows  53  through core  14  to an exit  56  into manifold  47 . 
         [0024]    A vapor/liquid mixture of refrigerant is supplied from a source  28  ( FIG. 1 ) into distributor  30  at the lower end of core  14 . The flow rate and thermal load are adjusted to provide a dew point of about 40 degrees Fahrenheit (F) in the dried air exiting core  14 . Air flowing downward in the direction of arrows  53  in core  14  is cooled by heat exchange through plates  18 . The cool vapor/liquid mixture flows upwards in the direction of arrows  33  and  37  in core  14  in counterflow to air flowing downward in the direction of arrows  53  through the chiller core  14 . 
         [0025]    Dried air is then directed upwards in the direction of arrow  57  from the bottom of chiller core  14  through moisture-removal section  20  wherein any residual moisture droplets are coalesced and returned by gravity to drain  21 . A demister core  150  is provided in moisture removal section  20  to promote surface turbulence and increased surface/air contact within section  20 . The term “demister core” herein refers to an apparatus that provides a large surface area to volume ratio that is well suited to provide a contact surface for water droplets to contact and coalesce on. An example of the structure of a brazed demister core is disclosed in U.S. Patent Publication No. 2011/0100594 entitled “Water Separator and System,” which is incorporated herein by reference. The demister core  150  shown in the embodiment of  FIG. 5  is made of individual sheets e.g.,  152 A,  152 B,  152 C of stamped aluminum, forming aluminum fins e.g.,  154 A,  154 B,  154 C. Then when properly oriented, the individual sheets are brazed into integral demister core  150  using the same production methods as those used for brazed aluminum bar and plate heat exchangers, such as air cooled oil coolers and compressed air aftercoolers, and other methods known to those of ordinary skill in the art. The demister core  150  in this embodiment may be easily and inexpensively manufactured by aluminum bar and plate brazing technology. In this embodiment, fins are located where no heat transfer is occurring, i.e., there are no heat transfer/alternating coolant passages in the demister core. The brazed demister core  150  is placed downstream of the heat exchanging chiller core where it removes droplets from already cooled compressed air. In this aspect, the demister core operates as a separator only. 
         [0026]    As shown in  FIG. 6 , and with reference to  FIG. 5 , the brazed demister core  150  separates moisture by causing the saturated air laden with entrained water particles e.g.  159 A,  159 B,  159 C to move at a reduced horizontal velocity and pass through the offset fins e.g.  154 D,  154 E,  154 F of the stacked aluminum sheets in an undulating and/or uneven path as shown by example direction arrows  156 A,  156 B,  156 C. The slower moving condensate impinges on the fins and causes coalescence of the suspended droplets into larger water particles e.g.  158 A,  158 B,  158 C. 
         [0027]    Returning to  FIG. 2 , tie bars  90   a - e  hold the demister core  150  in place within the moisture removal section  20 . The demister core  150  is positioned at an angle  152  inside the moisture removal section  20 . The moisture removal section  20  is bordered by sidewalls  200 ,  202  and bottom wall  204 . The demister core  150  is supported by the bottom wall  204  at a first end  206 . A second end  209  of the demister core  150  is disposed in contact with the side wall  202 . With respect to the orientation of  FIG. 2 , the demister core  150  divides the moisture separator section  20  into two generally triangular shaped spaces. On the right side of the demister core  150 , the space  210  is wider at the bottom and narrows toward the top where the demister core  150  contacts sidewall  202 . The triangular shaped spaces create an enlarged entrance area  95  near the bottom wall  204 . On the opposite or left hand side of the demister core  150 , the triangular shaped space  213  creates an enlarged exit area  98  near the outlet  91  of the moisture removal section  20 . The angled positioning of the demister core  150  provides many advantages. The enlarged entrance area  95  reduces the vertical velocity in the moisture separator section  20  as much as possible to prevent re-entrainment of the coalesced water droplets. The droplets that coalesce inside the moisture separator section  20  must be free to fall toward the drain  21  without getting picked up again and carried over the demister core  150 . Carry-over defeats the purpose of the air dryer. Also, the lower velocity created by the enlarged entrance area  95  gives lower pressure drop which is always advantageous in this type of application. Because the demister core  150  is constructed from sheets of rigid material it can easily be arranged at an angle inside the moisture removal section  20  by means of the tie bars  90   a - e.  The tie bars  90   a - e  also serve as tension members to give the assembly the required mechanical strength for the desired pressure rating. The tie bars  90   a - e  span the entire width of the of the separator section  20  and protrude out from each side where they may be welded to the outer wall of the moisture separation section  20 . Chilled, dried air exits section  20  through outlet  91  in the direction of arrows  93  into manifold  40  wherein it is conveyed to core  12 . 
         [0028]    Air passes downward in the direction of arrows  103  in core  12  in counterflow to the moist incoming warm air and is warmed by heat exchange therewith through the walls of plates  16 . Warmed, dried air is collected by manifold  41  and is discharged from system  10  through outlet  44  for use. The system may also be provided with a sensor port  99 . 
         [0029]    While the invention has been described in connection with certain embodiments, it is not intended to limit the scope of the invention to the particular forms set forth, but, on the contrary, it is intended to cover such alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.