Abstract:
An apparatus has a compressor having suction and discharge ports. One or more conduits form a main flowpath from the discharge port through a condenser, a heat exchanger first leg, a first expansion device, and an evaporator to return to the suction port. The conduits also form a bypass flowpath bypassing the heat exchanger first leg, the first expansion device, and the evaporator but passing through a second leg of the heat exchanger in heat exchange relation with the first leg.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The invention relates to cooling systems. More particularly, the invention relates to the control of refrigerant phase in evaporators of air conditioning and refrigeration systems.  
         [0002]     Many engineering considerations attend the design and operation of closed air conditioning and refrigeration systems. Among these are a variety of efficiency and other considerations attendant to evaporator operation. Proper evaporator operation is important for obtaining efficient and reliable system operation. Considerations include the handling of refrigerant and the management of heat transfer. Among problems that must be managed is excessive icing as this may interfere with heat transfer. Accordingly, much effort has gone into evaporator design and engineering.  
         [0003]     A particular area of emphasis has been the engineering of distributor systems. A distributor receives two-phase refrigerant from the expansion device and provides balanced delivery of liquid and gas refrigerant phases among the various coils of an evaporator so as to prevent uneven performance. Various types of distributors have been developed. These include capillary-type distributors and impingement/turbulence distributors. Exemplary distributors are shown in U.S. Pat. Nos. 2,148,414, 2,461,876, 3,795,259, 4,543,802, 5,832,744, and 5,842351, EP 0160542, and JP 5-322378 and 10-185363.  
         [0004]     Nevertheless there remains room for further improvement in the art.  
       SUMMARY OF THE INVENTION  
       [0005]     One aspect of the invention involves an apparatus including a compressor having suction and discharge ports, a condenser, first and second expansion devices, an evaporator, and a heat exchanger having first and second portions in heat exchange relation with each other. One or more conduits form a main flowpath and a bypass flowpath. The main flowpath runs from the discharge port through the condenser, the heat exchanger first portion, the first expansion device, and the evaporator, and returns to the suction port. The bypass flowpath bypasses the heat exchanger first portion, the first expansion device, and the evaporator, but passes through the second expansion device and the heat exchanger second portion.  
         [0006]     In various implementations, the evaporator may lack a distributor. The second expansion device may be a TXV having a bulb essentially in heat exchange relation with a suction port condition. The second expansion device may be an EXV. A controller may be coupled to the EXV and programmed to control the EXV responsive to indicated superheat. The heat exchanger first portion may be downstream of the condenser and upstream of the evaporator along the main flowpath. The heat exchanger second portion may be downstream of the condenser along the bypass flowpath. The heat exchanger first portion may be upstream of the first expansion device along the main flowpath. The evaporator may be a refrigerant-to-air heat exchanger. In at least a bypass mode, a bypass flow along the bypass flowpath may enter the heat exchanger second portion in a two-phase gas/liquid condition and exit the heat exchanger second portion in a single-phase superheated gas condition. In the bypass mode, a main flow along the main flowpath may remain essentially a single-phase liquid in said heat exchanger second portion. The compressor may be selected from the group consisting of screw compressors and scroll compressors.  
         [0007]     Another aspect of the invention involves a method for operating such an apparatus. At least one operational parameter is detected. Responsive to the detection, at least the second expansion device is operated so as to maintain essentially single-phase liquid refrigerant entering the evaporator along the main flowpath. The at least one operational parameter may include at least one of saturated suction temperature and actual suction temperature.  
         [0008]     Another aspect of the invention involves a method for operating a cooling system. A main flow of refrigerant is caused to pass through an evaporator. The main flow is precooled upstream of the evaporator so as to maintain the main flow essentially as a liquid entering the evaporator. The precooling may comprise controlling a bypass flow in heat exchange relation with the main flow. The method may further comprise determining whether, absent the precooling, the main flow would enter the evaporator essentially as a two-phase flow.  
         [0009]     Another aspect of the invention involves a system comprising a compressor, a condenser, an expansion device, and an evaporator, a discharge line couples the compressor to the condenser to carry at least a main flow of refrigerant from the compressor to the condenser. A suction line couples the evaporator to the compressor to carry refrigerant from the condenser to the compressor. The system includes means for precooling refrigerant entering the expansion device so as to maintain the main flow essentially as a liquid while flowing along a flowpath length at least from the expansion device to the evaporator.  
         [0010]     In various implementations, the evaporator may lack a distributor. Within the evaporator, the main flow may transition to a two-phase liquid/gas flow and then to a one-phase superheated gas flow. The bypass flow may represent  10 %- 35 %, by weight, of a total refrigerant flow through the compressor.  
         [0011]     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a schematic representation of a refrigeration or air conditioning system employing the present invention.  
         [0013]      FIG. 2  is a phase diagram for a prior art system.  
         [0014]      FIG. 3  is a phase diagram for the system of  FIG. 1 . 
     
    
       [0015]     Like reference numbers and designations in the various drawings indicate like elements.  
       DETAILED DESCRIPTION  
       [0016]      FIG. 1 , shows an exemplary closed refrigeration or air conditioning system  10 . The system  10  has a hermetic compressor  12 , from which a compressor discharge conduit or line  14  extends downstream to a condenser  16 . An intermediate line  18  extends downstream from the condenser  16  to an expansion device  20  and an evaporator  22 . A suction line  24  extends downstream from the evaporator  22  to the compressor  12  to complete the main circuit/flowpath  26 .  
         [0017]     To form a bypass circuit/flowpath  28 , a bypass line  30  branches off from the intermediate line  18  and contains an auxiliary expansion device  32  and connects with the suction line  24 . A heat exchanger  34  is located such that the bypass line  30 , downstream of the expansion device  32 , and the line  18 , upstream of the main expansion device  20 , are in heat exchange relationship.  
         [0018]     The exemplary evaporator  22  is a cross-flow refrigerant-to-air heat exchanger having a number of parallel refrigerant coils  36  extending from inlet ends at a liquid collector or manifold  38  to outlet ends at a suction collector or manifold  40 . A fan  42  drives an airflow  44  across the coils  36  so that the refrigerant passing through the coils may draw heat from the airflow.  
         [0019]     Exemplary expansion devices  20  and  32  are electronic expansion valves (EEVs) and are illustrated as coupled to a monitoring/control system  44  (e.g., a microprocessor-based controller) for receiving control inputs via control lines  45  and  46 , respectively. The exemplary control system  44  may receive inputs such as zone inputs from one or more sensors  47 , system condition inputs from one or more sensors (e.g., suction temperature sensor  50  and suction pressure sensor  52 ), and external control inputs from one or more input devices (e.g., thermostats  60 ).  
         [0020]     Alternatively to the EEVs, any of a variety of expansion devices may be used (e.g., a thermal expansion valve (TXV)  32  having a remote bulb  70 , a fixed orifice device, or a capillary tube device).  
         [0021]     A basic prior art system would lack the bypass flowpath  28  and heat exchanger  34 .  FIG. 2  shows pressure  100  and enthalpy  102  for the refrigerant flow in such a basic system (or the present system with no bypass flow). A boundary  104  separates a two-phase gas/liquid mixture domain  106  from a single phase sub-cooled liquid domain  108  and a single phase superheated gas domain  110 . Suction conditions are shown as point or condition  120  at enthalpy  122  and pressure  124 . These conditions are essentially present in the flowpath downstream from the suction manifold  40  to the compressor suction port. The refrigerant is compressed (plot compression segment  125 ) in the compressor  12  to a compressed point  126  with increased enthalpy  128  and increased pressure  130 . During the compression  125 , the refrigerant may typically remain in the superheated gas domain  110  or may transition thereto from the two-phase domain  106 . The refrigerant is condensed (condensing segment  131 ) in the condenser  16  to a condensed point  132  with reduced enthalpy  134  but at the same pressure as the compressed/discharge condition. During the condensing  131 , the refrigerant state may transition from the superheated gas domain  110  to the two-phase domain  106  and even into the sub-cooled liquid domain  108 . The refrigerant is expanded (expansion segment  135 ) in the expansion device  34  to an expanded point  136  with decreased pressure  138 . During the expansion  135 , enthalpy may remain essentially constant at  134 . The refrigerant may reenter or remain in the two-phase domain  106  during the expansion  135 . This expanded two-phase refrigerant must enter the evaporator. The refrigerant is evaporated (evaporation segment  139 ) in the evaporator to return to the suction point  120  with substantially increased enthalpy and slightly decreased pressure relative to the expanded point  136 .  
         [0022]     The presence of the expanded point  136  in the two-phase domain  106  presents substantial problems. With two-phase refrigerant entering the evaporator, it becomes difficult to balance the refrigerant across the evaporator coils. Namely, otherwise similar coils may see different total amounts of refrigerant and/or different ratios of the two phases. This can produce substantially different coil conditions amongst the various coils. The coils with higher amounts of refrigerant and higher relative amounts of liquid may overcool so as to produce excessive frost buildup. Overall efficiency may be reduced. Accordingly, it is known to use complicated distributor arrangements in place of the liquid manifold  38  to balance the ratios among the different coils. Distributors tend to be expensive. Advantageously, under ambient conditions that would otherwise cause the point  136  to be in the two-phase domain  106 , it would be desirable to shift the point  136  into the single phase liquid domain  108  eliminating the need for a distributor. In such a situation, the single-phase liquid inlet flow to the evaporator could readily be separated into similar flows for each coil. The coils could be designed/configured for operating with such an input flow.  
         [0023]      FIG. 3  shows how the bypass flow of the present invention may be utilized to achieve advantageous refrigerant conditions entering the evaporator  22 . The suction condition/point  220  may be essentially the same as point  120  of  FIG. 2 . After compression  225 , the compressed/discharge/point  226  may be similar to the point  126  of  FIG. 2 . The condensing  231  brings the combined main flow and bypass flow to a condensed/point  232  which may be similar to point  132 .  
         [0024]     From the condensed condition, the bypass flow splits from the main flow. The bypass flow refrigerant is expanded (segment  233 ) to reach a point  234  which may be essentially at the suction pressure  124  and the enthalpy  134 . Heat exchange ( 235  for the bypass flow and  236  for the main flow) from the main flow to the bypass flow in the heat exchanger  34  then returns the bypass flow conditions to point  220  and cools the main flow to a precooled/point  238  with further reduced main flow enthalpy  240 . The main flow of refrigerant is expanded (segment  241 ) in the expansion device  20  to a point  242  with decreased pressure  244  (which may be essentially the same as  138 ). The main flow of refrigerant is evaporated (segment  245 ) in the evaporator  22  to return the main flow to the initial suction point  220 . The heat exchange from the bypass flow to the main flow tends to shift the point  242  to a lower enthalpy condition. The required amount of heat exchange may depend upon ambient conditions.  
         [0025]     A basic operation of the expansion device  32  may be responsive to sensed superheat of the refrigerant exiting the evaporator  22 . The degree of superheat (actual temperature minus saturated temperature) may be determined based upon the output of the temperature sensor  50  for the actual temperature and the pressure sensor  52  for the saturated temperature (e.g., in view of known refrigerant properties which may be programmed into the control system  44 ). The expansion device  32  may be opened either in a binary fashion or a progressive fashion in response to the presence or degree of superheat or superheat parameter (e.g., superheat above a threshold). With a TXV as the device  32 , control could be achieved by placing its bulb  70  in heat exchange relation with the refrigerant at suction conditions. Much more complex arrangements are also possible.  
         [0026]     The expansion device  32  and/or other components of the bypass flowpath may be dimensioned in view of main flowpath components to permit an appropriate balance between bypass and non-bypass flows. In an exemplary binary configuration (i.e., the binary flow has only off and on conditions) an exemplary balance involves having the bypass flow be approximately  30 % of the total flow through the compressor (i.e., 3/7 of the non-bypass flow) by weight/mass. Broader exemplary figures for binary operation are 25%-33%, and 10%-35%. Progressive or stepwise operation may permit maximums in such ranges and may, optionally, permit flows smaller than the lower ends of such ranges.  
         [0027]     One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, when implemented as a modification or a reengineering of an existing system, details of the existing system may heavily influence details of the implementation. Although illustrated with regard to a basic system and with simplified conditions, the principles may be applied to more complex system configurations, whether existing or yet-developed. Accordingly, other embodiments are within the scope of the following claims.