Patent Publication Number: US-9903659-B2

Title: Low pressure chiller

Description:
BACKGROUND 
     The subject matter disclosed herein relates to heating, ventilation and air conditioning (HVAC) systems. More specifically, the subject matter disclosed herein relates to chillers. 
     As regulatory &amp; industry trends continue to drive towards replacement of conventional HFC&#39;s like R134a of particular interest are the class of “low pressure refrigerants”, i.e. refrigerants that are near, or below atmospheric pressure at the boiling temperatures in a chiller. These have long been known to provide better thermodynamic cycle performance over medium (R134a) or higher (R410A) pressure refrigerants, due to their higher latent heats of vaporization and other thermodynamic properties. However, yet other thermodynamic properties such as vapor density or transport properties such as surface tension can reduce heat transfer performance and offset a significant portion of the thermodynamic cycle performance gains. Further, low pressure refrigerants have significantly greater specific volumes, resulting in the need for larger vapor spaces and pipes to connect the components of the chiller system. The larger vapor spaces and pipes are more costly and increase the volumetric footprint required to accommodate the chiller system. 
     BRIEF SUMMARY 
     In one embodiment, a heating, ventilation and air conditioning (HVAC) system includes a condenser to condense a flow of refrigerant into a liquid state. The system further includes an economizer assembly having at least one separator chamber to separate liquid refrigerant from vapor refrigerant. The economizer assembly shares an upper common wall with at least a portion of the condenser and the flow of refrigerant from the condenser into the economizer assembly proceeds through a flow opening in the upper common wall. A falling film evaporator exchanges thermal energy between the liquid refrigerant and a medium flowed through a plurality of evaporator tubes in the evaporator. 
     In another embodiment, a method of operating a heating, ventilation and air conditioning (HVAC) system includes condensing a flow of refrigerant into a liquid state in a condenser and flowing the flow of refrigerant from the condenser to an economizer assembly via a flow opening in an upper common wall shared by the condenser and at least a portion of the economizer assembly. Liquid refrigerant is separated from vapor refrigerant in the flow of refrigerant at at least one separator chamber of the economizer assembly. The liquid refrigerant is flowed into a falling film evaporator to exchange thermal energy between the liquid refrigerant and a medium flowed through a plurality of evaporator tubes in the evaporator. 
     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 
       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: 
         FIG. 1  is an elevation view of an embodiment of a chiller; 
         FIG. 2  is an end view of an embodiment of a chiller; and 
         FIG. 3  is a schematic view of an embodiment of an evaporator for a chiller. 
     
    
    
     The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawing. 
     DETAILED DESCRIPTION 
     Embodiments of low pressure refrigerant chiller systems are disclosed herein. Initially, it should be understood that the term “low pressure refrigerant” defines 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. It should also be understood that while described as employing a low pressure refrigerant, the exemplary embodiments could also employ a medium pressure refrigerant. The term “medium pressure refrigerant” defines a refrigerant having a liquid phase saturation pressure between 45 psia (310.3 kPa) and 170 psia (1172 kPa) at 104° F. (40° C.). 
     Specifically, embodiments of low pressure refrigerant chiller systems are disclosed which are configured to better take advantage of thermodynamic cycle performance advantages over medium or high pressure refrigerant chiller systems, by reducing the impact of heat transfer disadvantages noted regarding low pressure refrigerants. These improvements include the use of a falling film evaporator in low pressure systems, which ensures that the boiling temperature is substantially uniform in the falling film tube bundle, since the tube bundle is not submerged in a refrigerant pool. Such submersion results in higher boiling temperatures in the submerged portions and the reduction in heat transfer performance. Additionally, the use of a falling film evaporator facilitates the efficient removal of the large refrigerant vapor flow from the tubes, ensuring continuous liquid feed, and thus increasing heat transfer performance. Low pressure systems allow use of cost effective rectangular components. As such, the aspect ratio of the condenser can be optimized to correct the heat transfer deficiency in the condenser by virtue of the poorer thermodynamic and transport properties of lower pressure refrigerants. In addition, the aspect ratio of the evaporator can be optimized to maximize heat transfer performance in the falling film tube bundle by ensuring more complete wetting of the tubes. 
     Further, the disclosed embodiments reduce the footprint necessary to accommodate the chiller system by nesting rectangular components, eliminating piping connections between components and providing flow therethrough via openings in shared walls of the components. 
     Shown in  FIG. 1  is an embodiment of a heating, ventilation and air conditioning (HVAC) unit, for example, a chiller  10  utilizing a low pressure refrigerant and a falling film evaporator  12 . The chiller  10  is gravity fed, with the evaporator  12  beneath an economizer assembly  14  and a condenser  16 . The chiller  10  also includes a compressor  18 , as shown in  FIG. 2 . The compressor  18  of the embodiment shown is a two-stage compressor  18  that discharges upwardly into a corner  20  of the condenser  16 . A discharge area  22  of the compressor  18  is separated from the condenser  16  by a baffle plate  24  to prevent impingement of high velocity vapor condenser output  26  on condenser tubes  28  and prevent tube  28  vibration issues. 
     The condenser  16  is separated into a main condenser  30  and a flasc subcooler  32 . Use of the subcooler  32  in the condenser  16  ensures that all of the refrigerant  34  flowing through the chiller  10  reaches the evaporator  12  in a liquid state. Referring again to  FIG. 1 , the condenser  16  is a vertically short and horizontally long vessel, and is substantially cuboid, having six rectangular faces. It is to be appreciated that, throughout this application, the term “rectangular” is used to denote rectangular shapes having either sharp corners or rounded corners. As shown, condenser length  110  is defined along a length of condenser tubes  28 , while condenser height  112  is vertically up-down in  FIG. 1  and  FIG. 2 , and condenser width  114  is horizontal in the side view of  FIG. 2 . In some embodiments, the aspect ratio of condenser width  114  to condenser height  112  is greater than 1 and less than about 3. The condenser tubes  28  have a flow of liquid  36 , for example, water, flowed therethrough between a condenser water inlet nozzle  38  and a condenser water outlet nozzle  40 . The refrigerant  34  output from the compressor  18  as vapor is condensed to liquid by the liquid  36  flowing through the condenser tubes  28 . 
     From the condenser  16 , the refrigerant  34  is fed into the economizer assembly  14 . The economizer assembly  14  of the embodiment of  FIG. 1  includes three chambers, but it is to be appreciated that other quantities of chambers may be utilized. In some embodiments, the economizer assembly is substantially cuboid, with six rectangular faces. Further, in some embodiments, the condenser  16  and economizer assembly  14  are arranged to share at least a portion of an upper common wall  116  between the condenser  16  and the economizer assembly  14 , with the two components substantially abutting one another. This allows for flow between the condenser  16  and the economizer assembly  14  via a flow opening  118  in the upper common wall  116  without additional external tubing or piping. 
     The refrigerant  34  initially flows into a high-side chamber  42  of the economizer assembly  14  in which a high side refrigerant level  44  is controlled via a high-side float  46  or other metering device that allows refrigerant  34  flow through from the high side chamber  42  to an economizer chamber  48 . From the high side chamber  42 , the refrigerant  34  flows into the economizer chamber  48  and is flashed therein resulting in a volume of refrigerant vapor  52  and a volume of chilled refrigerant  34 . The flow of refrigerant  34  between the high side chamber  42  and the economizer chamber  48  is driven by a pressure differential between the two chambers  42  and  48 . The resulting refrigerant vapor  52  is introduced into the compressor  18  in, for example, a second stage of the compressor  18  (shown in  FIG. 2 ) through an economizer nozzle  54  located in a common economizer wall  120  between the economizer assembly  14  and the compressor  18 . The liquid refrigerant  34  settles in the economizer chamber  48  and proceeds into a separator chamber  50  by operation of a low side float  56 , or other metering device that controls flow between the economizer chamber  48  and the separator chamber  50 . Vapor refrigerant  52  in the separator chamber  50  is routed to a suction plenum  58  (shown in  FIG. 2 ) located adjacent to and sharing a suction plenum wall  122  with the economizer assembly  14 , via a separator chamber port  60 . Locating the suction plenum  58  and the economizer assembly  14  with a shared wall allows the separator chamber port  60  to be merely a hole in the wall, thus eliminating piping and fittings that are typically used in such a connection between a suction plenum and separator. The liquid refrigerant  34  in the separator chamber  50  reaches a separator chamber level  62  and is allowed to flow into the evaporator  12  via gravity. The evaporator  12  is configured as a cuboid structure with six substantially rectangular faces, and is located below the economizer assembly  14 . In some embodiments, the evaporator  12  abuts the economizer assembly  14  at a lower common wall  124  separating the two components, with an evaporator opening  126  in the lower common wall  124  allowing for flow from the economizer assembly  14  into the evaporator  12 . The evaporator  12  has an evaporator length  128  extending substantially parallel to the condenser length  110  as shown in  FIG. 1 , and an evaporator height  130  extending up-down as shown in  FIG. 1 . Further, the evaporator  12  has an evaporator width  132  extending left-right in the cross-sectional view of  FIG. 2 . In some embodiments, the evaporator  12  has an aspect ratio of evaporator height  130  to evaporator width  132  of greater than 1 and less than about 3. 
     In some embodiments, separator chamber port  60  is adjustable to increase or decrease pressure in third chamber  50 . For example, when separator chamber port  60  is opened, pressure in separator chamber  50  decreases, thereby increasing the refrigerant  34  urged from the second chamber  48  to the separator chamber  50 , raising the separator chamber level  62 . As the separator chamber level  62  rises, separator chamber port  60  may be constricted to increase the pressure in separator chamber  50  to drive an increased amount of liquid refrigerant  34  from the separator chamber  50  into an evaporator manifold  64 . Such increased flow of liquid refrigerant  34  is desired under certain operating conditions, for example, high load conditions. 
     Referring now to  FIG. 3 , the evaporator  12  includes a shell  66  having an outer surface  68  and an inner surface  70  that define a heat exchange zone  72 . In the exemplary embodiment shown, shell  66  includes a non-circular cross-section. For example, shell  66  may have a rectangular cross-section with a horizontal width (as shown in  FIG. 2 ) less than a vertical height. Shell  66  includes a refrigerant inlet  74  from the evaporator manifold  64  to receive the liquid refrigerant  34 . Shell  66  also includes a vapor outlet  76  that is connected to the compressor  18 . Evaporator  12  is also shown to include a low pressure refrigerant pool zone  78  arranged in a lower portion of shell  66 . Low pressure refrigerant pool zone  78  includes a pool tube bundle  80  that circulates a fluid through a pool of low pressure refrigerant  82 . Pool of low pressure refrigerant  82  includes an amount of liquid low pressure refrigerant  34  having an upper surface  84 . The fluid circulating through the pool tube bundle  80  exchanges heat with pool of low pressure refrigerant  82  to convert the amount of low pressure refrigerant  82  from a liquid to a vapor state. 
     In accordance with the exemplary embodiment shown, evaporator  12  includes a plurality of tube bundles  86 - 88  that provide a heat exchange interface between low pressure refrigerant and another fluid. At this point it should be understood that while shown with a plurality of tube bundles  86 - 88 , a single tube bundle could also be employed in connection with economizer assembly  14 . Each tube bundle  86 - 88  is connected to evaporator manifold  64 . Evaporator manifold  64  provides a uniform distribution of refrigerant onto tube bundles  86 - 88 . As will become more fully evident below, evaporator manifold  64  delivers low pressure refrigerant  34  onto tube bundles  86 - 88 . Tube bundles  86 - 88  are spaced one from another to form first and second vapor passages  89  and  90 . In addition, tube bundles  86  and  88  are spaced from inner surface  70  to establish first and second outer vapor passages  91  and  92 . As each tube bundle  86 - 88  is substantially similarly formed, a detailed description will follow with reference to tube bundle  88  and evaporator manifold  64  with an understanding the tube bundles  86  and  87  are similarly constructed. 
     In further accordance with the exemplary embodiment shown, tube bundle  88  includes first and second wall members  93  and  94 . First and second wall members  93  and  94  are spaced one from another to define a tube channel  95  through which pass a plurality of tubes  96  that are configured to carry a liquid. As will become more fully evident below, liquid passing through the plurality of tubes  96  is in a heat exchange relationship with the low pressure refrigerant flowing into tube channel  95 . First wall member  93  includes a first end  97  and extends to a second end  98 . Similarly, second wall member  94  includes a first end  99  and extends to a second end  100 . Each first end  97  and  99  is spaced below evaporator manifold  64  while each second end  98  and  100  is spaced above low pressure refrigerant pool  34 . With this arrangement, liquid low pressure refrigerant flowing from evaporator manifold  64  flows, under force of gravity, through tube channel  95 , over tubes  96  and passes into low pressure refrigerant pool  34 . In this manner, the refrigerant reduces a temperature of liquid, for example, water, flowing through tubes  96  before transitioning to a vapor for return to, the compressor  16  via the vapor outlet  76 . Liquid flows through tubes  96  via evaporator liquid inlet  102  and evaporator liquid outlet  104 . 
     At this point it should be understood that the example embodiments describe a shell and tube evaporator that employs a low pressure refrigerant to facilitate heat exchange with a secondary medium. The use of falling film systems and low pressure refrigerant provides various advantages over prior art systems. For example, the use of falling film systems employing low pressure refrigerant reduces pressure losses associated with flow through the tube bundles as compared to conventional flooded evaporator bundles of similar size. In addition, falling film systems employ a lower refrigerant charge, thereby leading to an overall cost reduction. Additional benefits are realized by higher heat transfer coefficients associated with using falling film evaporation in a low pressure refrigerant. It should be also understood, that while shown as having a circular cross-section, the tubes in the tube bundles can be formed from tubes having non-circular cross-sections and/or tubes formed of assemblies of brazed channels. 
     Further, the arrangement described herein utilizes gravity to drive flow from the economizer assembly  14  into the evaporator manifold  64 . Configuring the condenser as vertically short increases condenser efficiency, in some embodiments by about 30% over traditionally configured condensers as well as allows for a compact arrangement of system components. Further, the compressor and evaporator/separator structures are load bearing thus reducing structural support requirements for the system. 
     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.