Abstract:
A falling film evaporator ( 12 ) for a heating ventilation and cooling (HVAC) system includes a housing ( 52 ) and a plurality of evaporator tubes ( 26 ) positioned at least partially in the housing ( 52 ) through which a volume of thermal energy transfer medium is flowed. A distribution system ( 34 ) is located in the housing to distribute a flow of liquid refrigerant ( 20 ) over the plurality of evaporator tubes ( 26 ). The distribution system ( 34 ) includes a distribution vessel having a plurality of drip openings ( 38 ) to flow the liquid refrigerant onto the plurality of evaporator tubes ( 26 ), a feed pipe ( 42 ) to flow refrigerant into the distribution box ( 36 ), and one or more pressure regulators ( 58 ) in the distribution system, thereby regulating the flow of liquid refrigerant.

Description:
BACKGROUND 
       [0001]    The subject matter disclosed herein relates to heating, ventilation and air conditioning (HVAC) systems. More specifically, the subject matter disclosed herein relates to evaporators for HVAC systems. 
         [0002]    HVAC systems, such as chillers, use an evaporator to facilitate a thermal energy exchange between a refrigerant in the evaporator and a medium flowing in a number of evaporator tubes positioned in the evaporator. In a flooded evaporator, the tubes are submerged in a pool of refrigerant. This results in a particularly high volume of refrigerant necessary, depending on a quantity and size of evaporator tubes, for efficient system operation. Another type of evaporator used in chiller systems is a falling film evaporator. In a falling film evaporator, the evaporator tubes are positioned typically below a distribution manifold from which refrigerant is urged, forming a “falling film” on the evaporator tubes. 
         [0003]    In one type of falling film evaporator, the distribution system includes a plurality of sprayers from which a vapor-liquid refrigerant mixture is sprayed directly onto the evaporator tubes, requiring complex and costly distribution systems and sprayer assemblies. In another, a separator is used to separate vapor refrigerant from liquid refrigerant, and the system relies on gravity working through a column of liquid refrigerant to drip the liquid refrigerant onto the evaporator tubes. This system requires the addition of the separator, and a considerable refrigerant charge to effect the gravity feed. 
       BRIEF SUMMARY 
       [0004]    In one embodiment, a falling film evaporator for a heating ventilation and cooling (HVAC) system includes a housing and a plurality of evaporator tubes positioned at least partially in the housing through which a volume of thermal energy transfer medium is flowed. A distribution system is located in the housing to distribute a flow of liquid refrigerant over the plurality of evaporator tubes. The distribution system includes a distribution vessel having a plurality of drip openings to flow the liquid refrigerant onto the plurality of evaporator tubes, a feed pipe to flow refrigerant into the distribution box, and one or more pressure regulators in the distribution system, thereby regulating the flow of liquid refrigerant. 
         [0005]    In another embodiment, a heating, ventilation and air conditioning (HVAC) system includes a condenser flowing a flow of refrigerant therethrough, and a falling film evaporator in flow communication with the condenser. The falling film evaporator includes a housing and a plurality of evaporator tubes positioned at least partially in the housing through which a volume of thermal energy transfer medium is flowed. A distribution system is located in the housing to distribute a flow of liquid refrigerant over the plurality of evaporator tubes. The distribution system includes a distribution vessel having a plurality of drip openings to flow the liquid refrigerant onto the plurality of evaporator tubes, a feed pipe to flow refrigerant into the distribution box, and one or more pressure regulators in the distribution system, thereby regulating the flow of liquid refrigerant. The system further includes a compressor to receive a flow of vapor refrigerant from the falling film evaporator. 
         [0006]    These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0008]      FIG. 1  is a schematic view of an embodiment of a heating, ventilation and air conditioning system; 
           [0009]      FIG. 2  is a schematic view of an embodiment of a falling film evaporator for an HVAC system; and 
           [0010]      FIG. 3  is a schematic view of another embodiment of a falling film evaporator for an HVAC system. 
       
    
    
       [0011]    The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawing. 
       DETAILED DESCRIPTION 
       [0012]    Shown in  FIG. 1  is a schematic view an embodiment of a heating, ventilation and air conditioning (HVAC) unit, for example, a chiller  10  utilizing a falling film evaporator  12 . A flow of vapor refrigerant  14  is directed into a compressor  16  and then to a condenser  18  that outputs a flow of liquid refrigerant  20  to an expansion valve  22 . The expansion valve  22  outputs a vapor and liquid refrigerant mixture  24  to the evaporator  12 . A thermal energy exchange occurs between a flow of heat transfer medium  28  flowing through a plurality of evaporator tubes  26  into and out of the evaporator  12  and the vapor and liquid refrigerant mixture  24 . As the vapor and liquid refrigerant mixture  24  is boiled off in the evaporator  12 , the vapor refrigerant  14  is directed to the compressor  16 . 
         [0013]    Referring now to  FIG. 2 , as stated above, the evaporator  12  is a falling film evaporator. The evaporator  12  of  FIG. 2  utilizes a pressure assist to significantly reduce the amount of refrigerant in the system  10  compared to those utilizing a prior art gravity-fed evaporator. Further, the evaporator  12  makes possible the use of a smaller separator, which may be incorporated into the evaporator  12  structure. 
         [0014]    The evaporator  12  includes housing  52  with the evaporator  12  components disposed at least partially therein, including a separator  30  to separate liquid refrigerant  20  and vapor refrigerant  14  from the vapor and liquid refrigerant mixture  24 . Vapor refrigerant  14  is routed from the separator  30  through a suction port  32  and toward the compressor  16 , while the liquid refrigerant  20  is routed toward a distribution system  34  of the evaporator  12 . The distribution system  34  includes a distribution box  36  having a plurality of drip openings  38  arrayed along a bottom surface  44  of the distribution box  36 . Though in the embodiment of  FIG. 2  the distribution box  36  is substantially rectangular in cross-section, it is to be appreciated that the distribution box  36  may have another cross-sectional shape, for example, T-shaped or oval shaped. The distribution box  36  and drip openings  38  are configured to drip liquid refrigerant  20  onto evaporator tubes  26  and resulting in the falling film terminating in a refrigerant pool  40  at a bottom of the evaporator  12 . A feed pipe  42  extends from the separator  30  into the distribution box  36  and terminates in the distribution box  36 . Flow of the liquid refrigerant  20  into the distribution box  36  results in the collection of a volume of liquid refrigerant  20 , or liquid head  46 , in the distribution box  36  prior to flowing through the drip openings  38 . In some embodiments, a vent  56  may be located at the distribution system  34 , for example, at the distribution box  36  to allow escape of vapor refrigerant  14  that makes its way into the distribution system  34  from the separator  30  thereby preventing an unwanted buildup of vapor refrigerant  14  in the distribution system  34 . In some embodiments, the vent  56  includes a pressure regulator  58 , which may be, for example, a fixed orifice or orifices or a controlled venting device that vents an amount of vapor refrigerant  14  (based on pressure in the separator  20 ) necessary to effect 100% liquid refrigerant  20  feed to the drip openings  38 . 
         [0015]    In prior art gravity fed evaporators, under some system operating conditions, such as high load conditions, a high level of liquid head is necessary to force flow of liquid refrigerant through the distribution system at the required rate to meet high load needs. Thus, a large amount of refrigerant charge is necessary in such prior art evaporators. This necessarily high level of liquid head consequently increases the refrigeration system height. 
         [0016]    To reduce an amount of refrigerant and system height necessary to drive the flow through the evaporator at high load operating conditions, the evaporator  12  includes a liquid head  46  level sensor in the distribution box  36 , for example, a float  48 . While a float  48  is utilized in the embodiment of  FIG. 2 , it is to be appreciated that other types of level sensors may be utilized. The float  48  is operably connected to a damper  50  or valve or other pressure regulator at the suction port  32  at the separator  30 . With the damper  50  in an open position, vapor refrigerant  14  separated out of the refrigerant at the separator  30  flows through the suction port  32  toward the compressor  16 , since pressure in the separator, P s , is greater than a pressure, P e , on the opposite side of the suction port  32 . Under high load conditions, as the liquid head  46  level rises, the float  48  also rises and urges the damper  50  toward a closed position via a connection, either mechanical, electrical, fluid or the like, between the float  48  and the damper  50 . In the embodiment of  FIG. 2 , the connection is, for example, a mechanical linkage  54 . With the damper  50  moved toward a closed position, the pressure of vapor refrigerant  14 , P s  builds in the separator  30  and the distribution system  34 , thus urging increased flow of the liquid refrigerant  20  through the distribution system  34 . Conversely, under low load conditions, the liquid head level  46  drops, allowing the damper  50  to move toward the opened position. This “turns off” the pressure-assist and forces the liquid refrigerant  20  to flow through the distribution system  34  by gravity. 
         [0017]    Another embodiment of a gravity-fed evaporator  12  is shown in  FIG. 3 , illustrating a “direct feed” approach. In a direct feed evaporator  12 , the vapor and liquid refrigerant mixture  24  is routed from the expansion valve  22  directly to the distribution system  34  via the feed pipe  42 . In this embodiment, the separator is eliminated, and the vapor and liquid refrigerant mixture  24  flows directly into the distribution box  36 . The vapor refrigerant  14  separates from the refrigerant  24  in the distribution box  36 , as the liquid refrigerant  20  settles or is otherwise directed toward the bottom surface  44  of the distribution box  36 . The vapor refrigerant  14  is flowed toward the vent  56  where it exits through the pressure regulator  58  into the housing  52  and is flowed toward the compressor  16 . In this embodiment, the pressure regulator  58  is a variable orifice, or a vent valve (not shown). Varying flow through the pressure regulator  58  by these or other devices allows for control of a pressure in the distribution box  36  and thereby the flow of refrigerant into the distribution box  36  from the feed pipe  42  and out of the distribution box  36  via the drip openings  38 . 
         [0018]    While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.