Abstract:
A heating, ventilation and air conditioning (HVAC) system includes a condenser ( 18 ) flowing a flow of refrigerant therethrough and to an output pipe ( 56 ) and a falling film evaporator ( 12 ) in flow communication with the condenser and having an evaporator input pipe ( 58 ) located vertically higher than the output pipe. A plurality of riser pipes ( 60 ) connect the output pipe to the evaporator input pipe. The flow of refrigerant flows through selected riser pipes of the plurality of riser pipes as required by a load on the HVAC system.

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 HVAC systems with falling film evaporators utilizing low or medium pressure refrigerants. 
         [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 systems with flooded evaporators, the tubes are submerged in a pool of refrigerant. In flooded evaporator systems, the evaporator and condenser are located substantially side-by-side. In a single stage system, liquid refrigerant leaving the condenser will go through a metering device, such as an expansion valve, and a two phase mixture of liquid and vapor refrigerant enters the evaporator from the bottom of the evaporator. In a two stage system including an economizer, after passing through the metering device the liquid and vapor refrigerant mixture flows through the economizer where the liquid refrigerant is metered again, with a second liquid and vapor refrigerant mixture flowing into the bottom of the evaporator. 
         [0003]    In a falling film evaporator system, the liquid refrigerant is fed in through the top of the evaporator and falls over the tubes, where it is evaporated. In a stacked arrangement of a falling film system, the condenser is installed on top of the economizer, which is installed on top of the evaporator. In this system, the flow through the components is driven by gravity. If the condenser and evaporator are arranged side-by-side, however, with an evaporator inlet physically higher than the exit of the metering device downstream of the condenser or economizer, the two-phase refrigerant mixture will have to be routed through a two-phase riser into the evaporator. 
         [0004]    Traditionally, when using either medium pressure or high pressure refrigerants, the vertical pipe of the riser is sized such that for all flow conditions (lift and flow rate) the mixture&#39;s momentum is great enough to ensure constant flow rate into the evaporator. This sizing results in very large frictional pressure drops at large flow rates. This is not an issue with the high pressure refrigerants, however, since the pressure differential due to lift in these refrigerants can accommodate the frictional pressure drops. When using low pressure refrigerants in falling film applications, however, the pressure differential due to lift is about 25% of that of a typical medium pressure refrigerant, severely limiting the frictional pressure allowed while still maintaining control of flow through the system using the metering device. 
       BRIEF SUMMARY 
       [0005]    In one embodiment, a heating, ventilation and air conditioning (HVAC) system includes a condenser flowing a flow of refrigerant therethrough and to an output pipe and a falling film evaporator in flow communication with the condenser and having an evaporator input pipe located vertically higher than the output pipe. A plurality of riser pipes connects the output pipe to the evaporator input pipe. The flow of refrigerant flows through selected riser pipes of the plurality of riser pipes as required by a load on the HVAC system. 
         [0006]    In another embodiment, a method of operating a heating, ventilation and air conditioning (HVAC) system includes urging a flow of refrigerant from a condenser into an output pipe. The flow or refrigerant is directed through a select number of riser pipes of a plurality of riser pipes vertically upwardly toward a evaporator input pipe disposed vertically higher than the output pipe. The flow of refrigerant is urged through the evaporator input pipe and into an evaporator. 
         [0007]    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 
         [0008]    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: 
           [0009]      FIG. 1  is a schematic view of an embodiment of a heating, ventilation and air conditioning (HVAC) system; 
           [0010]      FIG. 2  is a schematic view of an embodiment of an evaporator for an HVAC system; 
           [0011]      FIG. 3  is a schematic view of an embodiment of a riser pipe configuration for an HVAC system; and 
           [0012]      FIG. 4  is a schematic view of another embodiment of a riser pipe configuration for an HVAC system. 
       
    
    
       [0013]    The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawing. 
       DETAILED DESCRIPTION 
       [0014]    Shown in  FIG. 1  is a schematic view of 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 . 
         [0015]    Referring now to  FIG. 2 , as stated above, the evaporator  12  is a falling film evaporator. The evaporator  12  includes a shell  30  having an outer surface  32  and an inner surface  34  that define a heat exchange zone  36 . As shown, shell  30  includes a rectangular cross-section however, it should be understood that shell  30  can take on a variety of forms including both circular and non-circular. Shell  30  includes a refrigerant inlet  38  that is configured to receive a source of refrigerant (not shown). Shell  30  also includes a vapor outlet  40  that is configured to connect to an external device such as the compressor  16 . Evaporator  12  is also shown to include a refrigerant pool zone  42  arranged in a lower portion of shell  30 . Refrigerant pool zone  14  includes a pool tube bundle  44  that circulates a fluid through a pool of refrigerant  46 . Pool of refrigerant  46  includes an amount of liquid refrigerant  48  having an upper surface  50 . The fluid circulating through the pool tube bundle  44  exchanges heat with pool of refrigerant  46  to convert the amount of refrigerant  48  from a liquid to a vapor state. In some embodiments, the refrigerant may be a “low pressure refrigerant” defined as a refrigerant having a liquid phase saturation pressure below about 45 psi (310.3 kPa) at 104° F. (40° C.). An example of low pressure refrigerant includes R245fa. 
         [0016]    In accordance with the exemplary embodiment shown, evaporator  12  includes a plurality of tube bundles  52  that provide a heat exchange interface between refrigerant and another fluid. Each tube bundle  52  may include a corresponding refrigerant distributor  54 . Refrigerant distributors  54  provide a uniform distribution of refrigerant onto tube bundles  52  respectively. As will become more fully evident below, refrigerant distributors  54  deliver a refrigerant onto the corresponding ones of tube bundles  52 . 
         [0017]    Referring now to  FIG. 3 , the chiller  10  is arranged such that an output pipe  56  downstream from the expansion valve  22 , is physically lower than an evaporator input pipe  58 . It is to be appreciated that while a single-stage system in shown in  FIG. 3 , the subject matter of this disclosure may be readily applied to multi-stage systems including an economizer. In such systems, the output pipe  56  is downstream of a low stage expansion valve at the economizer, or at an intermediate stage expansion device in systems of three or more stages. An array of riser pipes  60  connect the output pipe  56  to the evaporator input pipe  58  so that the liquid and vapor refrigerant mixture  24  is flowed to the evaporator  12  and over the tube bundles  52  via distributor  54  (shown in  FIG. 2 ). Three riser pipes  60  are shown in the embodiment of  FIG. 3 , but it is to be appreciated that any number of two or more riser pipes  60  is contemplated within the present disclosure. There is no analytical maximum limit, but practically, increasing the number of riser pipes  60  increases complexity of the assembly. 
         [0018]    As shown, the riser pipes  60  have different cross-sectional areas, with large riser pipe  60   a  having the largest, small riser pipe  60   c  having the smallest, and medium riser pipe  60   b  having a cross-sectional area between that of large riser pipe  60   a  and small riser pipe  60   c.  In the embodiment shown, large riser pipe  60   a  is closest to the expansion valve  22  and the small riser pipe  60   c  is furthest from the expansion valve  22 , but other arrangements of the riser pipes  60  are contemplated in the present disclosure. 
         [0019]    The riser pipes  60  are connected to the output pipe  56  at a condenser output pipe bottom  62 . This reduces refrigerant charge necessary, especially during part power operation, as the output pipe  56  will still deliver refrigerant to the riser pipes  60  without needing to completely fill the output pipe  56 . It is to be appreciated, however, that alternate arrangements are contemplated within the scope of the present disclosure, such as that shown in  FIG. 4 , where the riser pipes  60  are connected to an output pipe top  64 . Such embodiments require completely filling the output pipe  56 , but the length of piping utilized for the riser pipes  60  can be decreased. Thus, the length of pipe subjected to two-phase frictional pressure drop is reduced. Referring again to  FIG. 3 , the riser pipes  60  are connected to the evaporator input pipe  58  at an evaporator input pipe top  66 , so that in part load conditions, refrigerant does not flow back from the evaporator input pipe  58  through the riser pipes  60  and into the output pipe  56 . 
         [0020]    Under full load, all three riser pipes  60   a - 60   c  are utilized to flow the vapor and liquid refrigerant mixture  24  to the evaporator input pipe  58 . As load decreases, riser pipes  60  are deactivated, beginning with the large riser pipe  60   a.  This deactivation of riser pipes  60  happens automatically, and outside input is not required. The vapor and liquid refrigerant mixture  24  automatically selects which riser pipes  60  to flow through as there is a fixed pressure differential between the evaporator  12  and the condenser  18 . Because of this fixed pressure differential, the required pressure drop is also fixed and the flow rates of the vapor and liquid refrigerant mixture  24  will balance automatically to achieve the pressure differential. 
         [0021]    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.