Patent Publication Number: US-2010115984-A1

Title: Dual-circuit series counterflow chiller with intermediate waterbox

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to water cooled chillers and, more specifically, to the interconnection of two vapor compression refrigeration systems in a series-counterflow arrangement. 
     Water cooled chillers in a series-counterflow arrangement consist of two independent vapor compression refrigeration systems with chilled water and condenser water circuits that are common to both circuits and are arranged in series. This arrangement allows for an increased coefficient of performance (COP) over a single refrigeration circuit design because the separate circuits with series counterflow have a lower average pressure differential between the evaporator and condenser, thus requiring less energy to compress refrigerant from the evaporator to the condenser. 
     In such a system, water in each of the evaporators and the condensers flows through a plurality of tubes that span both refrigeration circuits, with the refrigeration circuits being separated by a tubesheet which is located at the middle of the tubes, and with each tube being hermetically sealed to the tubesheet, typically by expansion of the tube to the tubesheet. 
     One problem that arises is that of servicing the tubes such as may be required if a tube fails in operation. Such removal of a tube requires cutting the tube at all locations where it has been expanded and then pulling the tube out. It is not possible to completely remove a tube since there is no access to cut the tube at the center tubesheet location, which is inside the refrigerant boundary. If a tube is cut internally, or if a tube fails in operation, a leak path is created between the circuits that does not allow for operation of either circuit, thus adversely impacting both reliability and serviceability. 
     Another problem with a dual circuit system is that of control. A critical parameter for control of a water cooled chiller is the use of the leaving temperature differential, which is the difference in the temperature of the water leaving a heat exchanger and the refrigerant temperature within the heat exchanger. Since the water tubes span both refrigerant circuits in a dual system, it is not possible to obtain the leaving water temperatures of the upstream circuit&#39;s condenser or evaporator. 
     In addition to serviceability and control as discussed hereinabove, prior art heat exchanger tubes that span dual circuits pose problems of reliability, shipping and performance. That is, because the common tubes extend across both circuits, it is impossible to optimize the heat transfer tubes in each circuit independently, and shipping of machines that are longer due to the longer tubes can be difficult. 
     SUMMARY OF THE INVENTION 
     Briefly, in accordance with one aspect of the invention, each circuit has unique tubesheets that separate the refrigeration circuit from the cooling medium. Between each circuit is an intermediate waterbox that passes water from the upstream circuit to the downstream circuit. The waterbox is removable for service and enables the transporting of the units in pieces with shorter length requirements. 
     In accordance with another aspect of the invention, since each circuit has its separate and unique tubes, a tube failure in either circuit no longer creates a refrigerant leak path to the adjacent circuit, such that operation of the nonfailed circuit can be maintained, thereby increasing reliability. 
     By another aspect of the invention, since the intermediate waterbox is accessible from the outside, temperature measurement instrumentation can be installed to obtain the leaving temperature differential of the first circuit, thereby providing better control of the system. 
     In accordance with another aspect of the invention, the intermediate waterbox causes mixing of the water that leaves the upstream circuit before entering the downstream circuit, thereby increasing heat transfer effectiveness and COP. 
     By yet another aspect of the invention, use of the waterbox allows for multiple parameters that can be varied in order to optimize the efficiency of each of the circuits. In addition to varying the length of each circuit, the tube material, the tube heat transfer enhancement, and the number of tubes are configurable, and can be unique to each circuit 
     In the drawings as hereinafter described, a preferred embodiment is depicted; however, various other modifications and alternate constructions can be made thereto without departing from the spirit and scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of the temperatures in a single circuit chiller in accordance with the prior art. 
         FIG. 2  is a schematic illustration of the temperatures in a dual-circuit chiller in accordance with the prior art. 
         FIG. 3  is a schematic illustration of the condensers and evaporators of a dual-circuit chiller in accordance with the prior art. 
         FIG. 4  is a schematic illustration of dual-circuit chiller system in accordance with the present invention. 
         FIG. 5  is a schematic illustration of the condenser and evaporators in a dual-circuit system of the present invention. 
         FIG. 6  is a schematic illustration of the waterbox portion of the dual-circuit system in accordance with the present invention. 
         FIG. 7  is a perspective view of the waterbox portions of a dual-circuit system in accordance with the present invention. 
         FIG. 8  is an end view of the waterbox portion of a dual-circuit system in accordance with the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  shows a condenser  11  and a cooler or evaporator  12  of a single circuit chiller that is typical of the prior art. As shown, the condenser water and evaporator water flows in a counterflow relationship, and the resulting temperatures entering and leaving the condenser and evaporator are as shown. 
     In order to obtain increased COPs, a dual-circuit is connected in series counterflow arrangement as shown in  FIG. 2 . Here, two independent vapor compression refrigeration circuits,  13  and  14 , are connected by an intermediate tubesheet  15  as shown. The first circuit  13  has a condenser  16  and an evaporator  17 , and the second circuit  14  has its own condenser  18  and evaporator  19 . However, the condenser water circuits of the condenser  16  and  18  are common to both circuits and are arranged in series. Also, the chilled water circuits of the evaporators  17  and  19  are common to both circuits and are arranged in series. This can be best seen by reference to  FIG. 3 . 
     It will be seen in  FIG. 3  that the condenser tubes  21  are long and span the length of each of the condensers  16  and  18  of the circuits  13  and  14 . While the intermediate tubesheet  15  isolates and separates the refrigerant in the respective circuits  13  and  14 , the water flow through the condenser tubes  21  is continuous from the entrance of the condenser  16  to the outlet of the condenser  18 . 
     Similarly, the evaporator tubes  22  are unitary members that extend across both circuits  13  and  14 , with the intermediate tubesheets providing isolation only for the refrigerant in the systems  13  and  14 , but allow for the evaporator water to flow continuously from the inlet end of the evaporator  19  to the outlet end of the evaporator  17 . 
     As discussed hereinabove, such dual-circuit systems with heat exchanger tubes that span both circuits present problems with respect to service, reliability, shipping, performance and control. 
     Referring now to  FIG. 4 , a system is shown to overcome the above-discussed problems. A first circuit,  23 , includes a condenser  24 , an expansion device  26 , an evaporator  27  and a compressor  28 , which operate in serial flow relationship in a well-known manner. A second circuit,  29 , includes a condenser  31 , an expansion device  32 , an evaporator  33  and a compressor  34  which also are connected in serial flow relationship and operate in a well known manner. The two circuits  23  and  29  are interconnected in a manner similar to that shown in  FIG. 3  but with a different structure at the interface between the two circuits and different structure with respect to the tubes within both the condensers and the evaporators. 
     As shown in  FIGS. 4 and 5 , at an intermediate position between the two evaporators  27  and  33  is an evaporator waterbox  36 , and at an intermediate position between the two condensers  24  and  31  is a condenser waterbox  37 . Further, unlike the systems described hereinabove wherein the tubes are unitary tubes extending across both circuits, the condenser tubes  38  of circuit I are separate and independent from the condenser tubes  39  of circuit  2 , and the evaporator tubes  41  in circuit  1  are separate and distinct from the evaporator tubes  42  of circuit  2 . That is, the condenser tubes  38  are fluidly connected to one side of the waterbox  36  and the condenser tubes  39  are fluidly connected to the other side thereof. Similarly, the evaporator tubes  41  are fluidly connected to one side of the waterbox  37  and the evaporator tubes  42  are fluidly connected to the other side thereof. The waterboxes  36  and  37  therefore act as intermediate receptacles for the water as it passes between the first circuit  23  and second circuit  29 . 
     The advantages of the above-described design are numerous. First of all, rather than having long unitary tubes, the tubes, and therefore the refrigeration circuits, are generally only about half as long and can be more easily handled and shipped to a site, with the tubes, and therefore the refrigeration circuits, being independent and separatable from the waterboxes. Second, since the tubes are independent, they can be configurable to optimize performance in each circuit. That is, in addition to the variation in length of the tubes in each circuit, the number of tubes within the second circuit can be different from those in the first circuit as shown in  FIG. 5 , and other variations can be made, such as different tube material, or different heat transfer enhancements. This allows the designer to optimize the desired capacity, efficiency, pressure drop, or cost for each circuit. 
     Other advantages of the present system can be seen by reference to  FIG. 6 . Because the water from the upstream tubes is discharged along one side of the waterbox  36  (or waterbox  37  in the case of the evaporator), it tends to cause a turbulence within the waterbox such that the individual flow streams are mixed so as to become a reservoir of water with a relative uniform temperature before it enters the tubes of the downstream circuit. This mixing is beneficial to the heat transfer effectiveness, thereby increasing COP of the total system. 
     By using the waterbox  36  as described, the intermediate waterbox  36  is now accessible from the outside and temperature measurement instrumentation  43  can easily be used to obtain the leaving temperature differential of the upstream heat exchangers, thus providing improved control of the system. 
     Another advantage of the use of waterboxes as described is that of facilitating service and repair. That is, since the waterbox is attached to the tube circuits in a manner that allows removal of the waterbox, as will be described hereinafter, the removal of the waterbox allows service of the tubes at each circuit&#39;s tubesheet, thereby substantially improving serviceability. Further, since a tube failure in either circuit does not create a refrigerant leak path to the adjacent circuit, the reliability of the system is substantially enhanced. 
     Referring now to  FIGS. 7 and 8 , the structural interface of the intermediate waterbox and the adjacent circuits are shown. As shown the intermediate waterbox  44  comprises a relatively short cylinder with a plurality of holes  46  formed longitudinally from one end  47  to the other, for receiving bolts  48  passing through the respective tubesheets  49  and  51 . The waterbox,  44 , is thus sandwiched between the tubesheets  49  and  51  of the respective circuits and can be easily disassembled by removing the bolts,  48 , to get access to the tubes for repair purposes at the tubesheets between the circuits. It will therefore be recognized that each of the circuits is independent, and access can be gained to the intermediate tube to tubesheet joints without disrupting refrigerant boundary of either circuit. 
     Although the waterbox  44  is shown in  FIGS. 7 and 8  as relatively short in length (i.e. about 4 inches), its configuration, size and shape can be substantially varied while remaining within the scope of the present invention. Further, although described in terms of use with a water cooled chiller, the present invention could also be applicable to an air cooled chiller wherein the evaporators of series connected circuits are interconnected by way of an intermediate waterbox structure.