Patent Publication Number: US-6910349-B2

Title: Suction connection for dual centrifugal compressor refrigeration systems

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Application No. 60/401,354 filed Aug. 6, 2002. 

   BACKGROUND OF THE INVENTION 
   The present invention relates generally to a suction connection for a compressor. Specifically, the present invention relates to a suction connection in the evaporator that increases the aerodynamic stability of multiple centrifugal compressors operating in parallel in a refrigeration system. 
   To obtain increased capacity in a refrigeration system, two centrifugal compressors can be connected in parallel to a common refrigerant circuit. Frequently, for capacity control, one of the compressors is designated as a “lead” compressor and the other compressor is designated as a “lag” compressor. The capacity of the refrigeration system, and of each compressor, can be controlled by the use of adjustable pre-rotation vanes or inlet guide vanes incorporated in or adjacent to the suction inlet of each compressor. Depending on the particular capacity requirements of the system, the pre-rotation vanes of each centrifugal compressor can be positioned to control the flow of refrigerant through the compressors and thereby control the capacity of the system. The positions of the pre-rotation vanes can range from a completely open position to a completely closed position. The pre-rotation vanes for a centrifugal compressor can be positioned in a more open position to increase the flow of refrigerant through the compressor and thereby increase the capacity of the system or the pre-rotation vanes of a centrifugal compressor can be positioned in a more closed position to decrease the flow of refrigerant through the compressor and thereby decrease the capacity of the system. 
   During operation, a compressor instability or surge condition can occur in a centrifugal compressor, wherein the compressor cannot pump the flow against its discharge pressure. Surge or surging is an unstable condition that may occur when compressors, such as centrifugal compressors, are operated at light loads and high pressure ratios. A high compressor pressure ratio, sometimes called lift or head, may be expressed in a number of fashions. A simplified representation of this compressor pressure ratio is (discharge pressure minus suction pressure (differential pressure or “ΔP”)) divided by suction pressure (“P”), or expressed symbolically, (ΔP)/P. A lower suction pressure will increase the compressor ratio and decrease the stability of a centrifugal compressor. Surge is a transient phenomenon characterized by high frequency oscillations in pressures and flow, and, in some cases, the occurrence of a complete flow reversal through the compressor. Surging, if uncontrolled, can cause excessive vibrations in both the rotating and stationary components of the compressor, and may result in permanent compressor damage. During a surge condition there can exist a momentary reduction in flow and pressure developed across the compressor. Furthermore, there can be a reduction in the net torque and mechanical power at the compressor driving shaft. In the case where the drive device of the compressor is an electric motor, the oscillations in torque and power caused by a surge condition can result in oscillations in motor current and excessive electrical power consumption. 
   In dual compressor applications, the occurrence of a surge or lack of pumping condition on one compressor results in the other compressor having an increase in refrigerant flow. This increase in refrigerant flow to the non-surging compressor makes it more difficult for the surging compressor to recover from its instability. Axial gas flow within the evaporator to the stable compressor will pass over a suction opening of the unstable compressor, thereby lowering the pressure at the unstable compressor suction connection which further contributes to instability. Several different techniques have been used to limit the potential aerodynamic impact one compressor may have upon the other compressor in a dual compressor system. Some chiller systems with two compressors utilize two completely separate refrigerant circuits to avoid the problem of one compressor aerodynamically impacting the other compressor. Other dual compressor chiller systems which use a common refrigerant circuit have a baffle in the gas plenum space of the evaporator between the suction connections of the compressors to reduce the aerodynamic impact of one compressor upon the other compressor. In this type of system each of the two suction connections are typically located approximately one quarter of the evaporator shell&#39;s length from the ends of the evaporator shell, because of the baffle or partition bisecting the evaporator shell into substantially equal halves. Both of these solutions have several drawbacks including a more complicated and expensive implementation of the evaporator. A completely separated evaporator shell would result in less heat exchanger surface being available during single compressor operation, and therefore would provide less effective heat transfer and reduced performance. Flooded shell and tube evaporators boil refrigerant liquid on the shell side to cool water flowing through the tubes. The refrigerant gas flow evaporating off the liquid surrounding the tubes will carry some of the liquid along with the gas. Evaporator heat exchangers typically use baffle passages or mesh pad eliminators to remove the liquid droplets from the gas before entering the compressor suction. If the vapor space above the baffle or mesh pad is separated into halves, as in some systems, the boiling activity in single compressor operation is concentrated in one half of the evaporator using one half of the mesh pads. This provides less effective vapor separation than if the entire baffle or mesh pad section were utilized. 
   Therefore, what is needed is a simple and economical suction connection for use in a dual compressor refrigeration system that can increase pressure at the suction connection to encourage the flow of refrigerant vapor into a surging compressor to thereby enhance the ability of the surging compressor to recover from its instability. 
   SUMMARY OF THE INVENTION 
   One embodiment of the present invention is directed to a suction connection for a compressor of a refrigeration system. The suction connection is in fluid communication with an evaporator of the refrigeration system. The suction connection includes a protrusion extending into the evaporator upon installation of the suction connection. The protrusion is configured and disposed to disturb axial flow of refrigerant vapor in the evaporator. This disturbance or disruption of the axial flow of refrigerant vapor in the evaporator can provide a flow of refrigerant to a surging compressor in a dual compressor system to permit the surging compressor to recover from its instability. 
   An alternate embodiment of the present invention is directed to a suction connection for a plurality of compressors of a refrigeration system in fluid communication with an evaporator of the refrigeration system. The suction connection includes at least one protrusion extending into the evaporator upon installation of the suction connection. The at least one protrusion is configured and disposed to disturb axial flow of refrigerant vapor in the evaporator. 
   A further alternate embodiment of the present invention is directed to a multiple compressor refrigeration system including two or more compressors, a condenser in fluid communication with the two or more compressors; an evaporator in fluid communication with the condenser, and a suction connection connecting the evaporator and the two or more compressors. The suction connection has at least one protrusion extending into the evaporator. The evaporator is configured to develop axial flow of refrigerant vapor adjacent to the suction connection and the at least one protrusion is configured and disposed to disturb the axial flow of refrigerant vapor in the evaporator. 
   One advantage of the present invention is that it encourages refrigerant vapor to flow into the suction connection of a surging compressor in a dual compressor system. 
   Another advantage of the present invention is that it can provide a more equal distribution and improved liquid/vapor separation with the evaporator heat exchanger. 
   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  illustrates schematically a refrigeration system of the present invention. 
       FIG. 2  illustrates an evaporator of the refrigeration system of the present invention. 
       FIG. 3  illustrates an end view of the evaporator of the refrigeration system of the present invention taken along line  3 — 3  of FIG.  2 . 
       FIG. 4  illustrates a cross-sectional side view of the evaporator of the refrigeration system of the present invention taken along line  4 — 4  of  FIG. 3 , and additionally illustrates internally protruding features of the suction connections. 
       FIG. 5  illustrates a cross-sectional side view of the evaporator of the refrigeration system of the present invention taken along line  4 — 4  of  FIG. 3 , and additionally illustrates an alternate embodiment of the suction connections. 
       FIGS. 6-12  illustrate different views of the suction connection 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 
   A general dual compressor system to which the invention can be applied is illustrated, by means of example, in FIG.  1 . As shown, the HVAC, refrigeration or liquid chiller system  100  includes a first compressor  108 , a second compressor  110 , a condenser  112 , a water chiller or evaporator  126 , and a control panel (not shown). In another embodiment of the present invention, the liquid chiller system  100  could use one compressor or three or more compressors connected in parallel similar to the connection of the first and second compressors  108 ,  110 . The control panel receives input signals from the system  100  that indicate the performance of the system  100  and transmits signals to components of the system  100  to control the operation of the system  100 . The conventional liquid chiller system  100  includes many other features which are not shown in FIG.  1 . These features have been purposely omitted to simplify the drawing for ease of illustration. 
   The compressors  108  and  110  compress a refrigerant vapor and deliver it to the condenser  112  by separate discharge lines. In another embodiment of the present invention, the discharge lines from the compressors  108  and  110  can be combined into a single line that delivers refrigerant vapor to the condenser  112 . The compressors  108  and  110  are preferably centrifugal compressors, however the present invention can be used with any type of compressor suitable for use in a chiller system  100 . The refrigerant vapor delivered to the condenser  112  enters into a heat exchange relationship with a fluid, preferably water, flowing through a heat-exchanger coil  116  connected to a cooling tower  122 . The refrigerant vapor in the condenser  112  undergoes a phase change to a refrigerant liquid as a result of the heat exchange relationship with the liquid in the heat-exchanger coil  116 . The condensed liquid refrigerant from condenser  112  flows to an evaporator  126 . 
   The evaporator  126  can include a heat-exchanger coil  128  having a supply line  128 S and a return line  128 R connected to a cooling load  130 . The heat-exchanger coil  128  can include a plurality of tube bundles within the evaporator  126 . Water or any other suitable secondary refrigerant, e.g., ethylene, calcium chloride brine or sodium chloride brine, travels into the evaporator  126  via return line  128 R and exits the evaporator  126  via supply line  128 S. The liquid refrigerant in the evaporator  126  enters into a heat exchange relationship with the water in the heat-exchanger coil  128  to chill the temperature of the water in the heat-exchanger coil  128 . The refrigerant liquid in the evaporator  126  undergoes a phase change to a refrigerant vapor as a result of the heat exchange relationship with the liquid in the heat-exchanger coil  128 . The vapor refrigerant in the evaporator  126  exits the evaporator  126  through suction connections  132  and  134  as shown in FIG.  2  and returns to the compressors  108  and  110  by separate suction lines to complete the cycle. 
   At the input or inlets to the compressors  108  and  110  from the evaporator  126 , there are one or more pre-rotation vanes or inlet guide vanes  120  and  121  that control the flow of refrigerant to the compressors  108  and  110 . Actuators are used to open the pre-rotation vanes  120  and  121  to increase the amount of refrigerant to the compressors  108  and  110  and thereby increase the cooling capacity of the system  100 . Similarly, the actuators are used to close the pre-rotation vanes  120  and  121  to decrease the amount of refrigerant to the compressors  108  and  110  and thereby decrease the cooling capacity of the system  100 . 
   To drive the compressors  108  and  110 , the system  100  includes a motor or drive mechanism  152  for the first compressor and a motor or drive mechanism  154  for the second compressor  110 . While the term “motor” is used with respect to the drive mechanism for the compressors  108  and  110 , it is to be understood that the term “motor” is not limited to a motor but is intended to encompass any component(s) that can be used in conjunction with the driving of the compressors  108  and  110 , such as a variable speed drive and/or a motor starter in addition to the motor. In a preferred embodiment of the present invention, the motors or drive mechanisms  152  and  154  are electric motors and associated components. However, other drive mechanisms such as steam or gas turbines or engines and associated components can be used to drive the compressors  108  and  110 . 
   In previous evaporators, the gas flowing from a refrigeration evaporator into a compressor suction connection typically leaves through a pipe opening contoured closely to the outside cylindrical shell wrapper of the evaporator vessel. When operating two or more compressors in parallel that draw refrigerant gas or vapor from one evaporator with the previous suction connection, a lack of pumping or “surge” condition can be observed in response to certain suction flow conditions. As one compressor enters a surge condition or state, the other compressor(s) have a stronger axial pull or draw of the gas through the evaporator gas passage. The evaporator gas passage is a section located above a liquid separation means, typically a mesh eliminator or a suction baffle passage. As this axial flow of the gas passes over the suction opening of the surging compressor it can create a lower relative dynamic suction pressure at the opening, making it more difficult for the surging compressor to recover and begin pumping gas again. 
   In contrast, the present invention has modified suction connections  132 ,  134 , as shown in  FIGS. 3-5 , to achieve a more stabilized flow of refrigerant vapor to the compressors  108 ,  110 . In a preferred embodiment of the present invention, suction connections  132 ,  134  can include an insert portion or member  156 . Providing a convenient connection of suction connections  132 ,  134  with the compressors  108 ,  110 , an end  165  of the insert member  156  may be connected to an annular flange  163 , although other fastening arrangements as known in the art, such as clamping or bonding, may be employed. The insert member  156  is preferably formed from a single, straight continuous piece of material (FIG.  5 ). However, the insert member  156  can also be formed from one or more separate pieces securely connected, fastened or joined together, or a single, curved continuous piece of material (FIG.  3 ), if required, to connect with the compressors  108 ,  110 . 
   Insert member  156  includes a tongue or protruding portion  160  that extends into and is positioned inside the evaporator shell  126  as shown in  FIGS. 3-5 . The protruding portion  160  preferably has the same profile, preferably cylindrical, as insert member  156 , i.e., the protruding portion  160  is a direct extension of the insert member  156 . However, in other embodiments of the present invention, the protruding portion  160  can have a profile different from the profile of insert member  156 . In other words, one or more portions or segments of the protruding portion  160  can be disposed outside of the extended profile of insert member  156 . For example, the protruding portion  160  can be disposed at an angle with respect to a portion of the insert member  156  ( FIG. 3 ) or the protruding portion  160  and the insert member  156  can extend substantially axially within the evaporator  126 . As shown in  FIGS. 3 and 5 , which are embodiments of the present invention, the center axis  175  of the protruding portion  160  can, but does not necessarily, extend toward the center of the evaporator  126 , and may, in fact, extend away from the center of the evaporator  126 . In addition, and in another embodiment of the present invention, the protruding portion  160  can include one or more apertures disposed within the protruding portion  160  and/or one or more slots disposed along the edge of the protruding portion  160  to permit partial flow of refrigerant vapor or gas through the protruding portion. 
   Referring to  FIGS. 6-9 , namely  FIG. 6  which is a flat pattern of an embodiment of insert member  156 , protruding portion  160  has a peripheral edge  162  that does not span the entire peripheral edge of insert member  156 , terminating at bisecting line  166 . The peripheral edge  162  extends from reference point  169 , which defines the lower bound of end  194  of insert member  156 , to reference point  167 , that similarly defines the lower bound of bisecting line  166 . While the peripheral edge  162  shown in  FIG. 6  is substantially in the shape of an arc, it is to be understood that peripheral edge  162  can have any suitable shape including a shape having one or more linear segments or a shape having a wavy pattern. The bisecting line  166  is substantially equidistant between opposed ends  192  to  194  of insert member  156 . In other embodiments of the present invention, such as shown in  FIGS. 10-12 , bisecting line  166  and reference point  167  can be positioned closer to either end  192  or end  194  to form a respectively larger or smaller protruding portion  160 . 
   To form insert member  156  as used in the embodiment of the present invention shown in  FIG. 6 , ends  192 ,  194  are brought into physical contact with each other and bonded together, forming a cylindrical profile having the center axis  175 . In the assembled embodiment of  FIG. 6 , any line passing through bisecting line  166  and end  194  that is also perpendicular to both bisecting line  166  and end  194  defines a diameter of insert member  156 . Likewise, the line connecting reference points  167  and  169  defines a diameter of insert member  156  which is a reference axis  173 . In another embodiment of the present invention, the inset member  156  can be formed of a single, continuous piece that has a profile or shape similar to the assembled shape of the insert member  156  shown in FIG.  6 . 
   Referring to  FIG. 8 , protruding portion  160  is bound along its lower end, i.e., the end that is disposed or extended into the evaporator  126 , by peripheral edge  162 . The peripheral edge  162  preferably has one or more or points that correspond to the furthest extension, preferably along center axis  175 , of the peripheral edge  162  into the evaporator  126 . In a preferred embodiment of the present invention, the furthest extension points of the peripheral edge  162  preferably extend between about 6-11 inches into the evaporator  126 . This extension of the peripheral edge  162  into the evaporator  126  proportionally corresponds from about 15 percent to about 25 percent of the outer perimeter of the protruding portion  160 . A proportion of the peripheral surface of the protruding portion  160  extending into the evaporator  126  is between about one-fifteenth to about two-thirds of the outer perimeter of the insert member  156 . Alternately, an extension of about one-half the outer perimeter of insert member  156  ( FIG. 11 ) may be preferable. However, it is to be understood that any suitable extension depth for the peripheral edge  162  and proportion of the peripheral surface of protruding portion  160  can be used depending on the size and configuration of the evaporator  126 , provided that the protruding portion  160  can disturb, but not block, the axial flow of refrigerant gas or vapor in the evaporator  126  and that if more than one insert member  156  is employed, the peripheral edge  162  and the protruding portion  160  may be, but are not necessarily, substantially identical. 
   Alternatively, an insert angle  170  as shown in  FIG. 8  can be defined as the angle between the center axis  175  and a plane that passes through reference axis  173  and the furthest extension point(s) of peripheral edge  162 . Points  167  and  169  of reference axis  173  are coincident with the periphery of evaporator shell  126 . In a preferred embodiment, the insert angle  170  measures about 35°, but may vary substantially either above or below this measured value, due to variations in operating parameters including, but not limited to, the type of refrigerant employed, evaporator shell dimensions, spacing between components within the evaporator, and vapor refrigerant flow rate. It is to be understood that different configurations of the protruding portion  160  and peripheral edge  162  may require slightly different techniques for measuring the insert angle  170 . The protruding portion  160  also has a peripheral edge  164 , which is opposite the peripheral edge  162 . Peripheral edge  164  is formed to be substantially coincident to the periphery of the evaporator shell  126  upon assembly. As shown in  FIGS. 7-9 , protruding portion  160  as bound by peripheral edge  162  resembles a tongue, the profile, namely the “tip” of the tongue, becoming increasingly pronounced as the insert angle  170  is increased. 
   Referring to  FIGS. 3-4 , suction connections  132 ,  134  have a substantially similar radial position with respect to the center axis of the evaporator shell  126 . Suction connection  132  is positioned at approximately the mid span of the axial length of the evaporator shell  126 , while suction connection  134  is positioned adjacent one end of the evaporator shell  126 . This spacing arrangement permits effective use of the entire length of the evaporator shell  126  for drawing vapor refrigerant into suction connections  132 ,  134 . For purposes of orientation, suction connection or connector  134  is preferably positioned opposite the direction of axial refrigerant vapor flow  188  created by the phase change of refrigerant liquid resulting from the heat exchange with the liquid in the heat-exchanger coil  128  ( FIG. 1 ) as previously discussed. That is, at least a portion of the refrigerant vapor axial flow stream  188  will travel almost the entire length of the evaporator shell  126  prior to reaching suction connector  134 . The protruding portions  160  of respective suction connections or connectors  132 ,  134  are oriented to open into and substantially fully face the direction of refrigerant vapor axial flow  188  that is discussed in greater detail below. In other words, the refrigerant vapor axial flow stream  188  emanating adjacent the end opposite suction connector  134  is first directed past peripheral edge  164  of insert member  156  of suction connector  132  prior to encountering the protruding portion  160 . This encounter with protruding portion  160  disturbs the flow stream  188  of refrigerant vapor passing along suction connector  132  and generates turbulence  190  in the flow stream  188 . The turbulence  190  joined by additional refrigerant vapor axial flow  188  likewise encounters the protruding portion  160  of suction connector  134 , producing similar turbulence in the flow. These combined disturbances in vapor refrigerant suction flow enhances the stability of the two compressors  108 ,  110  by disturbing the laminar flow of refrigerant vapor and generating turbulence, which, in turn, enables flow into the suction connection of the weaker or surging compressor, which typically would be compressor  110  that receives refrigerant from suction connector  132 . In a preferred embodiment as shown in  FIG. 4 , both suction connections  132 ,  134  have a protruding portion  160 . However, both suction connections  132 ,  134  do not require a protruding portion  160 . If only one suction connection  132 ,  134  has a protruding portion  160 , it is preferably suction connection  132 , although it could be suction connection  134 . In addition, protruding portion  160  can be incorporated into the suction connection for a compressor in a single compressor refrigeration system. 
   To provide effective vapor refrigerant flow over substantially the entire length of the evaporator shell  126 , a cap plate  176  is provided that spans substantially the entire length of the evaporator shell  126 . The cap plate  176  includes opposed sloped portions  183  that are each secured to the inside wall of the evaporator shell  126 . Each sloped portion  183  extends to opposed vertical portions  185  that are spanned by a cap portion  187 . The cap portion  187  has a plurality of apertures  177  formed therethrough along substantially the entire length of the cap portion  187  to permit the flow of vapor refrigerant  188  through the apertures  177  of the cap plate  176  and the suction connectors  132 ,  134  of the evaporator shell  126  in response to the suction from suction connectors  132 ,  134 . By forming the apertures  177  in a substantially uniform pattern over the entire length of the cap portion  187 , a small pressure drop is generated, which is nonetheless more than the axial pressure drop in the evaporator. This ensures uniform loading of the evaporator tube bundle along its length and minimizes liquid droplets mixing with the vapor. Further, an optional filtering means, such as a mesh pad  178  or baffle is secured within the recess formed by the collective vertical portions  185  and cap plate  176 . Securing the mesh pad  178  in this position is a plurality of support members  186  which span along the lower portion of vertical portions  185 . Mesh pad  178  is composed of a material that permits vapor refrigerant to flow therethrough while obstructing droplets striking the mesh pad  178  to prevent their entry into the suction connections  132 ,  134 . 
   One having ordinary skill in the art will appreciate that both the shape of protruding portions  160  and the location of suction connectors  132 ,  134  may vary significantly from the positions described in the preferred embodiment. That is, protruding portions  160  employed in suction connectors  132 ,  134  may differ in both profile and size, not being constrained to the cylindrical walls of insert member  156 , such as forming a flat or even a curved plate as long as the protruding portion  160  is secured substantially full face in the stream of suction vapor refrigerant to disrupt the axial flow of vapor refrigerant over the suction connections  132 ,  134  and provide improved stability of the compressors  108 ,  110  against surging. Protruding portion  160  may be an insert, may be a contoured or cut shape in the end of the suction pipe connections  132 ,  134  themselves, or may be an elbow. Finally, the protruding portions  160  can be used in conjunction with other known surge control techniques and procedures. 
   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.