Patent Publication Number: US-7587911-B2

Title: Compact evaporator for chiller application

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This patent application is a continuation of U.S. Utility application Ser. No. 11/271,764, filed on Nov. 10, 2005, allowed, entitled “Compact Evaporator for Chiller Application”, the disclosure of which is incorporated as if fully rewritten herein. 

   FIELD OF THE INVENTION 
   The present invention relates generally to heating, ventilation, air conditioning and refrigeration (HVAC&amp;R) systems. In particular, the present invention relates to HVAC&amp;R systems that utilize a chilled water system. 
   BACKGROUND OF THE INVENTION 
   One type of heating ventilation and air conditioning (HVAC&amp;R) system uses a chilled fluid to remove heat from a building and is typically referred to as a chilled water system. The fluid utilized in the chilled water system is not limited to water and may include liquids, such as glycol or brine. In this type of system, a chilled fluid is provided to a building having a heating load. The chilled fluid is placed in a heat exchange relationship with the heating load from the building, usually warm air. During the heat exchange with the heating load, the chilled fluid receives heat from the heating load and generally increases in temperature. In order to remove the heat from the fluid and to lower the temperature of the fluid, a closed loop refrigeration system is utilized. The fluid circulated through the building is chilled by placing the fluid in a heat exchange relationship with another cooler fluid, usually a refrigerant, in a heat exchanger, commonly referred to as an evaporator or chiller. The refrigerant in the evaporator removes heat from the fluid during the evaporation process, thereby cooling the fluid. The chilled fluid is then circulated back to the building for subsequent heat exchanging with the heating load, and the cycle repeats. 
   Chillers may include a shell and tube heat exchanger design. The shell and tube heat exchanger may include a bundle of heat exchange tubes located in a shell. The tubes are typically fabricated from a metal, such as copper, and may be horizontally mounted. At either end of the tubes are tube sheets that support the individual tubes. Refrigerant may flow through the tubes in order to cool a fluid, usually water or an aqueous solution, flowing through the shell. The use of this type of shell and tube heat exchanger design in an evaporator is commonly referred to as a direct expansion (DX) evaporator. A typical design for DX evaporators includes a single inlet connection and a single outlet connection for the fluid flowing through the shell. The single inlet and single outlet provide a single flow stream of fluid that exchanges heat with the refrigerant flowing inside the tubes. The shell side flow of the fluid follows a serpentine path due to the use of a plurality of baffles inside the shell on the shell side. The shell side fluid flow is generally in one direction providing uneven heat exchange over the length of the shell. Furthermore, DX evaporators incorporating tubes for multiple refrigerant circuits must flow the refrigerant in a single, concurrent direction. For a given shell side fluid flow, the DX evaporators effectiveness depends upon the direction of the refrigerant flow. Known evaporators provide efficient operation and superheated refrigerant by exchanging heat between the outlet flow of refrigerant and the inlet flow of fluid, i.e., by having the shell side fluid flow be opposite the refrigerant flow. The inlet flow of fluid contains an amount of heat greater than the outlet flow of fluid. Therefore, in order to operate efficiently, known DX evaporators must flow the shell side fluid in a single direction in order to efficiently provide heat to the refrigerant outlet. 
   The refrigerant in the tubes may make multiple passes across the shell through the use of baffling in the headers of the evaporator. However, known DX evaporators utilized in chilled water systems suffer from the drawback that the diameter of the shell of the evaporator becomes relatively large as the total heat exchange capacity increases and the shell requires a larger vertical clearance (i.e., heat exchanger height) in which to install. In particular, known DX evaporators having multiple passes require a large vertical clearance in the chiller platform providing for increased difficulty in installation. In addition, water flowing into the shell through the single inlet could cause excessive tube vibration, which could eventually cause failure of the tubes due to fatigue. 
   What is needed is an evaporator that permits refrigerant flow in either direction through the tubes, has a relatively small shell diameter, and a reduced tube vibration. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to an evaporator including a shell having a first end and a second end. A plurality of tubes are disposed within the shell to circulate refrigerant through the shell. A plurality of shell inlets are in fluid communication with the shell to deliver a fluid to exchange heat in the plurality of tubes, preferably through a baffle arrangement. At least one of the shell inlets may be arranged to deliver fluid to the shell adjacent to the first end. In addition, at least one of the shell inlets may be arranged to deliver fluid adjacent to the second end. A shell outlet is in fluid communication with the shell to discharge fluid from the shell. The shell outlet is arranged to receive the combined liquid delivered to the shell by the plurality of shell inlets. 
   In another embodiment, the present invention includes a chilled water system having a refrigerant loop and a cooling loop. The refrigerant loop includes a compressor, a condenser, an expansion device and an evaporator connected in a closed loop. At least three openings are present in the shell and are arranged and disposed to deliver fluid to and from the shell. The cooling loop includes at least one second heat exchanger in fluid communication with the evaporator. A fluid is circulated between the evaporator and at least one second heat exchanger. The evaporator is configured to place the fluid and the refrigerant in a heat exchange relationship. 
   An advantage of the present invention is that the split fluid flow on the shell side results in relatively lower shell side pressure drop across the evaporator. 
   Another advantage of the present invention is that the split fluid flow on the shell side results in a reduced quantity of cross-flow over the tubes, thereby reducing flow induced tube vibration. Reduced tube vibrations, reduces noise and material fatigue in the tubes. 
   Still another advantage of the present invention is that the shell diameter may be smaller than conventional evaporator shell designs while providing an almost identical capacity to that of larger diameter, conventional evaporators. This permits easier installation of the chiller system due to the smaller profile of the evaporator. 
   A further advantage of the present invention is that the performance of the evaporator is substantially unaffected by direction of refrigerant flow due to the split flow of fluid on the shell side. In addition, the performance, including capacity, efficiency and evaporating temperature, of the evaporator is substantially unaffected in embodiments where refrigerant in some circuits flow in one direction, while refrigerant in other circuits flow in the opposite direction. Because the evaporator performance is independent of refrigerant flow direction, a chiller system utilizing the present invention could have refrigerant circuits with refrigerant flowing in the same or different directions relative to each other. 
   Another advantage of the present invention is that the evaporator of the present invention includes a smaller number of tubes than an evaporator having a single inlet and a single outlet, simplifying the manufacture and assembly of the evaporator. 
   Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a schematic view of a chilled water system. 
       FIG. 2A  shows a top view of a prior art evaporator. 
       FIG. 2B  shows a front view of a prior art evaporator. 
       FIG. 3A  shows a top view of an alternate prior art evaporator. 
       FIG. 3B  shows a front view of an alternate prior art evaporator. 
       FIG. 4A  shows a top view of an evaporator according to an embodiment of the present invention. 
       FIG. 4B  shows a front view of an evaporator according to an embodiment of the present invention. 
       FIG. 5  shows a perspective view of an evaporator according to an embodiment of the present invention. 
       FIG. 6  shows a cutaway view of an evaporator according to an embodiment of the present invention 
       FIG. 7  shows a cutaway view of an evaporator according to another embodiment of the present invention. 
       FIG. 8  shows a cutaway view of an evaporator according to still another embodiment of the present invention 
       FIG. 9  shows a temperature profile graph over a prior art heat exchanger. 
       FIG. 10  shows a temperature profile graph over an embodiment of the present invention. 
   

   Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention may be utilized with an HVAC system such as a chilled water system. A suitable system for use with the present invention is illustrated, by means of example, in  FIG. 1 . As shown, the chilled water system  100  utilizes a refrigeration cycle or circuit, including a compressor  101 , a condenser  103  and evaporator  107 . In addition, the chilled water system  100  further includes a cooling loop or circuit, or secondary coolant loop or circuit, including evaporator  107  and one or more heat exchangers  115 . The HVAC or refrigeration system according to the present invention may include many other features that are not shown in  FIG. 1 . The features not shown have been purposely omitted to simplify the drawing for ease of illustration. 
   During operation of the chilled water system  100 , the compressor  101  compresses a refrigerant vapor and delivers it to the condenser  103 . The compressor  101  may be any suitable type of compressor including, but not limited to, reciprocating compressors, scroll compressors, screw compressors, centrifugal compressors and rotary compressors. The refrigerant vapor delivered by the compressor  101  to the condenser  103 , which transfers heat from the refrigerant to a medium, such as air or water, undergoes a phase change to a refrigerant liquid as a result of the heat exchange with the medium. The condensed liquid refrigerant from condenser  103  flows though an expansion device  105 , which reduces the pressure of the refrigerant. The lower pressure refrigerant is then delivered to the evaporator  107  that evaporates the lower pressure refrigerant to a vapor. The evaporating refrigerant in the evaporator  107  enters into a heat exchange relationship with a fluid to remove heat from the fluid. The vaporous refrigerant exits the evaporator  107  and returns to the compressor  101  by a suction line of the compressor  101  to complete the cycle. It is to be understood that any suitable configuration of condenser  103  may be used in the system  100 , provided that the appropriate phase change of the refrigerant in the condenser  103  is obtained. Chilled water systems utilize the heat exchange in the evaporator  107  in order to cool a fluid, which is utilized to provide cooling to a heat load  113  (e.g., building, structure or other heat source). Chilled water systems are not limited to water, but may include any suitable fluid capable of transferring an amount of heat from a heat load to an evaporator  107 . Suitable fluids for use in the chilled water system include, but are not limited to, liquids, such as water, glycol or brine. In the cooling loop, warm fluid  109  returns from heat exchanger(s)  115  within heat load  113  and enters the evaporator  107 . The warm fluid then exchanges heat with the evaporating refrigerant. The evaporator  107  cools the fluid and the cool fluid  111  returns to the heat load  113 . The heat load  113  may be any application requiring cooling, including a building or a structure. In addition, heat exchanger  115  may be any suitable heat exchange device that is capable of exchanging heat from the heat load to the fluid circulating in the cooling loop. The cool fluid  111  exchanges heat with the heat load  113  via heat exchanger(s)  115  and returns as warm fluid  109  to repeat the cycle. 
     FIGS. 2A and 2B  show schematic views of a known evaporator  200 .  FIG. 2A  shows a schematic top view of the known evaporator  200  and  FIG. 2B  shows a schematic front view of the known evaporator  200 . The evaporator  200  includes a shell extending between a first header  203  and a second header  205 . A refrigerant inlet  207  is in fluid communication with the first header  203  and is arranged to provide refrigerant to the first header  203 . The first header  203  is in fluid communication with tubes (not shown) arranged within the shell  201 . Refrigerant flowing into the tubes from the first header  203  flows through the tubes along the length of the shell  201  and is delivered to the second header  205 . The second header  205  is in fluid communication with an outlet  211 . The shell  201  includes fluid inlet  215 . The fluid enters the shell  201  at fluid inlet  215  and exchanges heat with the refrigerant in the tubes located in the shell  201 . The fluid then exits through fluid outlet  219 . In a chilled water system building cooling application, the fluid is then transported to a heating load in order to cool, for example, a building or other structure prior to the refrigerant leaving the evaporator  200  via refrigerant outlet  211 , the refrigerant exchanges heat with the fluid entering the shell  201  via fluid inlet  215 . The fluid entering the shell  201  has an amount of heat greater than the fluid exiting the shell  201 . The heat exchange with the fluid entering the shell  201  allows the refrigerant to receive a maximum amount of heat from the fluid stream, thereby allowing the refrigerant to be superheated by the fluid entering the shell  201 . 
     FIGS. 3A and 3B  show schematic views of an alternate known evaporator  200 .  FIG. 3A  shows a schematic top view of the known evaporator  200  and  FIG. 3B  shows a schematic front view of the known evaporator  200 . The fluid inlet  215  is arranged substantially adjacent to the first header  203 . The fluid outlet is arranged substantially adjacent to the second header  205 . Refrigerant inlet  207  and refrigerant outlet  211  are in fluid communication with first header  203 . Baffles (not shown in  FIGS. 3A and 3B ) are utilized in first header  203  and second header  205  in order to direct the refrigerant in two or more passes. Prior to the refrigerant leaving the evaporator  200  via refrigerant outlet  211 , the refrigerant exchanges heat with the fluid entering the shell  201  via fluid inlet  215 . As discussed above, the fluid entering the shell contains an amount of heat greater than the fluid exiting the shell  201  at the fluid outlet  219 . The heat exchange with the warm fluid entering the shell  201  allows the refrigerant to receive a maximum amount of heat from the fluid stream, thereby allowing the refrigerant to be superheated by the fluid entering the shell. Evaporator  200  has a relatively large diameter, making installation in certain applications difficult and subjecting the tubes within shell  201  to substantial cross-flow, thereby causing excessive vibration of the tubes. 
     FIG. 4A  and  FIG. 4B  show schematic views of an evaporator  300  according to an embodiment of the present invention.  FIG. 4A  shows a schematic top view of the evaporator  300  and  FIG. 4B  shows a schematic side view of the evaporator  300 . Although  FIGS. 4A and 4B  are designated as a top and front view, the installation of the evaporator  300  may be in any suitable configuration that provides an appropriate arrangement of refrigerant and fluid flow through the evaporator  300  to transfer heat and provide the advantages of the present invention. The evaporator  300  according to the present invention includes a shell  201  extending for a length between a first header  203  and a second header  205 . Refrigerant inlet  207  is in fluid communication with the first header  203  to deliver refrigerant to the first header  203 . Refrigerant may travel through the shell  201  and exit the shell through second header  205  and the refrigerant outlet  211 . Refrigerant may include any type of refrigerant suitable for use with evaporators having a shell and tube arrangement. Suitable refrigerants include, but are not limited to R-134a, R-22, R-410A, R-407C, Ammonia, carbon dioxide, etc. 
   The shell  201  of the evaporator  300  includes a first fluid inlet  301  adjacent to the first header  203 . The shell  201  also includes a second fluid inlet  303  adjacent to the second header  205 . The fluid travels through the shell  201  and exchanges heat with tubes (not shown) in the shell  201 . The fluid in the shell  201  then exits through fluid outlet  305 . As shown in  FIG. 1 , the fluid preferably is circulated in a cooling loop between the evaporator  300  and heat exchanger(s)  115 . The fluid outlet flow includes the combined fluid inlet flows from the first fluid inlet  301  and the second fluid inlet  303 . 
     FIG. 5  shows a perspective side view of an evaporator  300  according to an embodiment of the present invention. As shown in  FIGS. 4A and 4B , the evaporator  300  includes a substantially cylindrical shell  201 . Refrigerant inlet  207  and refrigerant outlet  213  include substantially cylindrical piping in fluid communication with the first header  203  and second header  205 , respectively. The shell  201  also has substantially cylindrical first fluid inlet  301 , second fluid inlet  303  and fluid outlet  305 . The evaporator  300  is not limited to the geometry shown in  FIG. 5 . The evaporator  300  may be provided in any suitable geometry for the shell  201 , the fluid inlets  301  and  303 , the fluid outlet  305  and the refrigerant inlets  207  and  213  that provide the refrigerant flow and fluid flow such that heat transfer may take place. 
     FIG. 6  shows a cutaway view of an evaporator  300  according to an embodiment of the present invention.  FIG. 6  shows an inlet refrigerant flow  209  entering the first header  203  via the refrigerant inlet  207 . The refrigerant entering the first header  203  is distributed to the tubes  601  and is transported through the tubes  601  to the second header  205 . From the second header  205 , outlet refrigerant flow  213  is discharged from the refrigerant outlet  211 . The first header  203  and second header  205  are not limited to the configuration shown, the first and second headers  203  and  205  may be any refrigerant distribution device that is capable of delivering refrigerant to the tubes  601  within the shell  201 . As discussed above, the refrigerant may also flow in the opposite direction, wherein the refrigerant is delivered to the second header  205  via refrigerant outlet  211  and travels from the second header  205 , through the tubes  601 , and into the first header  203 , wherein the refrigerant inlet  207  discharges the refrigerant from the evaporator  300 . The rate of heat transfer between the refrigerant and the fluid, the efficiency of heat exchange, and the capacity is approximately the same whether the refrigerant flows from the first header  203  to second header  205  or the refrigerant flows from second header  205  to first header  203 , thereby permitting the refrigerant flow to be reversed without any substantial reduction in evaporator  300  performance. As shown in  FIGS. 4A and 4B , shell  201  of the evaporator  300  includes a first fluid inlet  301  adjacent to the first header  203 . The shell  201  also includes a second fluid inlet  303  adjacent to the second header  205 . The fluid travels through the shell  201  and exchanges heat with tubes  601  disposed in the shell  201 . Baffles  603  may be included in shell  201  to direct fluid over tubes  601  and to fluid outlet  305 . Baffles  603  are not limited to the configuration shown in  FIGS. 6-8  and may be arranged in any suitable configuration that directs the flow of fluid through the shell  201  and supports tubes  601 . The combined fluid entering the shell  201  from the first fluid inlet  301  and the second fluid inlet  303  then exits through fluid outlet  305 . Fluid outlet flow  306  includes the combined fluid inlet flows  302  from the first fluid inlet  301  and the second fluid inlet  303 . 
     FIG. 7  shows a cutaway view of an evaporator  300  according to another embodiment of the present invention.  FIG. 7  shows an evaporator  300  with multiple independent refrigerant circuits. The multiple independent refrigerant circuits are preferably refrigerant circuits that each include, in addition to the evaporator  300 , a compressor, a condenser arrangement and an expansion device. The evaporator  300  includes a first circuit that is configured substantially as shown and described with respect to  FIG. 6 , including the refrigerant inlet flow  209 , the refrigerant inlet  207 , the first header  203 , the second header  205 , the refrigerant outlet  211  and the outlet refrigerant flow  213 . In addition, the evaporator  300  shown in  FIG. 7  includes a first fluid inlet  301 , a second fluid inlet  303 , a fluid outlet  305 , an inlet fluid flow  302  and an outlet fluid flow  306  that are arranged substantially as shown an described with respect to  FIG. 6 . However,  FIG. 7  includes a second refrigerant circuit including a second circuit inlet  707 , which receives a second refrigerant inlet flow  711  that is delivered to the second circuit second header  703 . The second circuit second header  703  is in fluid communication with tubes  601 . The refrigerant travels through tubes  601  to the second circuit first header  701 . The refrigerant is discharged from the second circuit first header  701  via second circuit outlet  705  as second refrigerant outlet flow  709 . In the arrangement shown in  FIG. 7 , the flow of refrigerant in the first circuit and the flow of refrigerant in the second circuit is countercurrent; however, the flow arrangement is not limited to being countercurrent. The flow in the first and second circuit may be concurrent, countercurrent or any combination of concurrent and countercurrent. In addition, the present invention is not limited to two circuits. A plurality of independent circuits may be utilized and may include circuits having varying heat transfer requirements. 
     FIG. 8  shows a cutaway view of an evaporator  300  according to another embodiment of the present invention.  FIG. 8  shows an evaporator  300  according to the present invention wherein the refrigerant travels in multiple passes through the shell  201 . The evaporator  300  includes a dual refrigerant pass arrangement, including the refrigerant inlet flow  209 , the refrigerant inlet  207 , the first header  203 , the second header  205 , the refrigerant outlet  211  and the outlet refrigerant flow  213 . In addition, the evaporator  300  shown in  FIG. 8  includes a first fluid inlet  301 , a second fluid inlet  303 , a fluid outlet  305 , an inlet fluid flow  302  and an outlet fluid flow  306  that are arranged substantially as shown and described with respect to  FIG. 6 . However, in  FIG. 8 , the refrigerant inlet  207  is in fluid communication with an upper portion  802  of the first header  203 . The first header is divided by baffle  801  into the upper portion  802  and a lower portion  807 . The baffle  801  divides the first header in order to provide inlet refrigerant flow  209  to only a portion of the tubes  601 . The portion of the tubes  601  that receive refrigerant from the upper portion  802  travels through the tubes  601  along the length of the shell  201  in a first pass to the second header  205 . The second header  205  preferably includes baffle  805  that directs return refrigerant flow  803  into the tubes  601  to travel countercurrent to the first pass to the lower portion  807  of the first header  203 . The lower portion  807  of the first header  203  delivers the refrigerant from the tubes  601  to the refrigerant outlet  211  and is discharged as outlet refrigerant flow  213 . Although  FIG. 8  shows a dual pass evaporator  300 , any number of passes may be utilized. Further, the refrigerant inlets and outlets are not limited to being in fluid communication with a single header. Any combination of refrigerant inlets and outlets may be utilized. In addition, multiple circuits, as shown and described with respect to  FIG. 7 , may be used in combination with multiple passes. 
   An advantage of the split flow of fluid in the shell  201  is that the evaporator  300  may include a high ratio of shell length to shell diameter. The shell length is defined as the length of the shell  201  between the first header  203  and the second header  205 . The shell diameter is defined as the inner diameter of the shell  201  available for receiving fluid from the fluid inlets  301 ,  303  and providing fluid to fluid outlet  305 . The utilization of the multiple inlets to the shell  201  to divide fluid flow decreases the volume of flow entering the shell at a given shell diameter. Therefore, a reduced diameter may be utilized in the evaporator  300  to maintain substantially identical capacity, efficiency and heat exchange rate as the known evaporator  200  having a single inlet and single outlet shell with a given shell diameter. The reduced diameter provides additional advantages including reduced cross-flow of fluid over the tubes, thereby reducing flow induced tube vibration, and permitting easy installation in areas having reduced clearance. Suitable ratios of the shell length to the shell diameter include from greater than about 5:1, preferably about 5:1 to about 20:1. In one embodiment of the present invention, the ratio of the shell length to the shell diameter includes greater than about 7:1. The high ratio of shell length to shell diameter permits the evaporator  300  to have a reduced height, which permits the installation of the evaporator  300  into chiller platforms having a smaller clearance than can be obtained with conventional heat exchanger systems. The reduction in aspect ratio compared to known heat exchangers may be provided in any arrangement of refrigerant flow, including multiple refrigerant circuits having independent flow directions (e.g.,  FIG. 7 ) and multiple pass evaporators (e.g.,  FIG. 8 ). 
   EXAMPLE 1 
   Example 1 includes a DX evaporator having a 375-ton cooling capacity. Table 1 includes the aspect ratio of a prior art evaporator having a single inlet and a single outlet on the shell-side of the evaporator (see e.g.,  FIGS. 3A and 3B ) in comparison to an evaporator according to an embodiment of the present invention having the same capacity. Aspect ratio is defined as a ratio of the length to the height (i.e., length/height). 
   
     
       
         
             
           
             
               TABLE 1 
             
           
          
             
                 
             
             
               375 Ton Evaporator 
             
          
         
         
             
             
             
             
          
             
                 
               Diameter 
               Length 
               Aspect Ratio 
             
             
                 
               [inch] 
               [inch] 
               [/] 
             
             
                 
                 
             
          
         
         
             
             
             
             
          
             
               Comparative Example 1* 
               30 
               103.2 
               3.44 
             
             
               Example 1** 
               22 
               186 
               8.45 
             
             
                 
             
             
               *375 Ton Evaporator arranged as shown in FIGS. 3A and 3B 
             
             
               **375 Ton Evaporator arranged as shown in FIGS. 4A, 4B and 6 
             
          
         
       
     
   
   EXAMPLE 2 
   Example 2 includes a DX evaporator having a 500-ton cooling capacity. Table 2 includes the aspect ratio of a prior art evaporator having a single inlet and a single outlet on the shell-side of the evaporator (see e.g.,  FIGS. 3A and 3B ) in comparison to an evaporator according to an embodiment of the present invention having the same capacity. 
   
     
       
         
             
           
             
               TABLE 2 
             
           
          
             
                 
             
             
               500 Ton Evaporator 
             
          
         
         
             
             
             
             
          
             
                 
               Diameter 
               Length 
               Aspect Ratio 
             
             
                 
               [inch] 
               [inch] 
               [/] 
             
             
                 
                 
             
          
         
         
             
             
             
             
          
             
               Comparative Example 2* 
               34 
               97.2 
               2.86 
             
             
               Example 1** 
               25.5 
               192 
               7.53 
             
             
                 
             
             
               *500 Ton Evaporator arranged as shown in FIGS. 3A and 3B 
             
             
               **500 Ton Evaporator arranged as shown in FIGS. 4A, 4B and 6 
             
          
         
       
     
   
   In addition to having the reduced aspect ratio shown in Tables 1 and 2, Examples 1 and 2 also may flow refrigerant from the first header to the second header or from the second header to the first header with little or no reduction in evaporator performance. In addition, the evaporating temperature of the evaporator  300  is maintained regardless of direction of refrigerant flow. 
     FIG. 9  shows a temperature profile across the length of a prior art evaporator  200  having the arrangement shown in  FIGS. 2A and 2B . The axial location is shown as a function of distance across the evaporator, shown schematically below the graph.  FIG. 9  shows a substantially linear reduction in fluid temperature on the shell side of the evaporator. When refrigerant is flowing in Direction  1  (i.e., from right to left, as shown in  FIG. 9 ), evaporating temperature (T evap ) of the evaporator is higher than the T evap  of the evaporator when the refrigerant is flowing in Direction  2  (i.e., from left to right, as shown in  FIG. 9 ), wherein the efficiency and capacity of the evaporator are likewise reduced. 
     FIG. 10  shows a temperature profile across the length of an evaporator  300  according to an embodiment of the present invention (e.g.,  FIG. 6 ). The axial location is shown as a function of location across the evaporator, shown schematically below the graph. The fluid temperature linearly decreases to a center point from each of the fluid inlets  301  and  303 . When refrigerant is flowing in Direction  1  (i.e., from right to left, as shown in  FIG. 10 ), the evaporating temperature (T evap ) of the evaporator is substantially identical to the T evap  of the evaporator when the refrigerant is flowing in Direction  2  (i.e., from left to right, as shown in  FIG. 10 ), wherein the efficiency and capacity of the evaporator are likewise substantially identical. As shown in  FIG. 10 , the T evap  of the evaporator  300  according to the present invention is independent of the direction of flow of refrigerant, permitting a variety of possible configurations including multiple refrigerant circuits, multiple passes, reversible refrigerant flows. 
   While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.