Abstract:
The economizer flow to the intermediate compression chamber inside a compressor is controlled via a variable restriction. The size of the restriction is selected to optimize unit performance in relation to operating conditions.

Description:
BACKGROUND OF THE INVENTION 
     An economizer consists of a flash tank or heat exchanger with an associated dedicated expansion device and piping. It is located in a refrigeration circuit downstream of the condenser. In the case of the heat exchanger, as is specifically disclosed, the flow upstream of the economizer circuit is divided with a minor portion of the condensed refrigerant flow passing through an expansion device thereby undergoing a pressure drop and partially flashing as it passes into the economizer heat exchanger. In the economizer heat exchanger, the remaining liquid refrigerant evaporates due to heat transfer with the major portion of the condensed refrigerant which is further cooled, thereby increasing the cooling capacity of the unit. The gaseous minor flow is at an intermediate pressure and can pass to the compressor, to cool the motor, or it may be supplied directly to intermediate compression volumes in the compressor to increase the mass of refrigerant being compressed. 
     SUMMARY OF THE INVENTION 
     Because the economizer flow line leading to the compressor is connected to a variable pressure inside the intermediate compression volumes, the flow may go back and forth as the intermediate compression volume pressure changes. The present invention places a variable restriction in the economizer line supplying the intermediate compression volumes which may be trapped volumes, as in a positive displacement compressor, or interstage for a multiple stage compressor. The size of the restriction affects the efficiency and capacity of the economized cycle. However, the optimum restriction size varies with operating conditions. For example, for higher pressure ratio applications the optimal size of the restriction or injection port is larger than for lower pressure ratio applications. The present invention varies the size of the restriction as a function of compressor operating conditions to maximize the unit operating efficiency or capacity. Additionally, there is also an optimum size of the restriction or injection port for maximum unit capacity and the opening would be larger if the unit is optimized for maximum capacity rather than for maximum efficiency operation. 
     In some refrigeration systems, such as transport refrigeration, the temperature is very precisely controlled and may be held to 0.1° C. Accordingly, in such systems, the suction and discharge temperatures and/or pressures are monitored in addition to the zone temperatures etc. and provide the necessary information for controlling the size of the restriction or injection port. 
     It is an object of this invention to provide a method and apparatus to increase the efficiency and/or capacity of a refrigeration system cycle. 
     It is another object of this invention to precisely control the economizer flow into a compressor to variably control refrigeration system capacity. These objects, and others as will become apparent hereinafter, are accomplished by the present invention. 
     Basically, the economizer flow to the intermediate compression volumes is controlled via a variable restriction or injection port which is optimized to maximize efficiency and/or capacity and/or to vary capacity. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a fuller understanding of the present invention, reference should now be made to the following detailed description thereof taken in conjunction with the accompanying drawings wherein: 
     FIG. 1 is a schematic representation of a refrigeration or air conditioning system employing the present invention; 
     FIG. 2 is a partial sectional view of a scroll compressor employing the present invention; 
     FIG. 3 is a partial sectional view of a scroll compressor employing a modification of the present invention; 
     FIG. 4 is a plot of efficiency vs. orifice size for various pressure ratios; and 
     FIG. 5 is a plot of capacity vs. orifice size for various pressure ratios. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1, the numeral  10  generally designates a refrigeration or air conditioning system. Refrigeration or air conditioning system  10  has a compressor  12  such as a screw compressor, scroll compressor, multi-stage reciprocating compressor, a multi-stage centrifugal compressor, or an axial compressor. Refrigeration or air conditioning system  10  includes a fluid circuit serially including compressor  12 , discharge line  14 , condenser  16 , line  18 , economizer heat exchanger  20 , line  22  containing expansion device  24  which is illustrated as an electronic expansion valve (EEV), evaporator  26 , and suction line  28 . Line  30  branches from line  18  and contains expansion device  32  which is illustrated as an EEV, passing through economizer heat exchanger  20 , into line  33  containing variable restriction  34  and terminating at an intermediate compression volume (not illustrated) in compressor  12  at an intermediate pressure. 
     Compressor suction temperature and/or pressure data is supplied to microprocessor  100  by sensor  40  and condenser subcooling data is supplied to microprocessor  100  by sensor  60 . Compressor discharge temperature and/or pressure data is supplied to microprocessor  100  by sensor  50 . If necessary, or desired, other sensors can be installed to provide equivalent or alternative information for controlling system  10 . Microprocessor  100  also receives data identified on FIG. 1 as “zone inputs” and would include data such as zone temperature, zone set point etc. Microprocessor  100  controls compressor  12  through motor  13  and controls EEVs  24 ,  32  and variable restriction  34 . Except for the presence of variable restriction  34  all of the structure described is generally conventional. 
     In a conventional system without an economizer circuit or with variable restriction or injection port  34  being completely closed, gaseous refrigerant is drawn into compressor  12  via suction line  28  and compressed with the resultant hot, high pressure refrigerant gas being supplied via discharge line  14  to condenser  16 . In condenser  16 , the gaseous refrigerant condenses as it gives up heat due to heat transfer via air, water or brine-cooled heat exchangers (not illustrated). The condensed refrigerant passes from condenser  16  into line  18 . 
     If economizer heat exchanger  20  is in operation and restriction  34  is not completely closed, a portion of the condensed refrigerant flowing in line  18  is diverted into line  30  and passes through expansion device  32  thereby undergoing a pressure drop and partially flashing as it passes into economizer heat exchanger  20 . The remainder of the condensed refrigerant from condenser  16  flows via line  18  into economizer heat exchanger  20 . The remaining liquid refrigerant in line  30  supplied to economizer heat exchanger  20  evaporates due to heat transfer with the liquid refrigerant in line  18  which is thereby additionally subcooled. The subcooled condensed refrigerant passes via line  22  through expansion device  24  thereby undergoing a pressure drop and partially flashing as it passes into evaporator  26 . In evaporator  26 , the remaining liquid refrigerant evaporates due to heat transfer via air, water or brine-cooled heat exchangers (not illustrated). The gaseous refrigerant is then supplied via suction line  28  to compressor  12  to complete the cycle. The gaseous refrigerant from economizer  20  is at an intermediate pressure and passes via line  33  to an intermediate compression volume in compressor  12 . Microprocessor  100  controls compressor  12  through motor  13  and controls expansion devices  24  and  32  responsive to the data supplied by sensors  40 ,  50  and  60  and the zone inputs. 
     The foregoing is generally conventional. In a compressor, refrigerant pressure is continuously increasing inside the compression volume. Thus, during communication of refrigerant in line  33  with refrigerant in the intermediate compression volume, the communication will take place over a range of pressures/volumes in the compression process. Stated otherwise, gaseous refrigerant in line  33 , at an intermediate pressure, is in fluid communication with an intermediate compression volume at a varying intermediate pressure. Since flow always is from a higher pressure to a lower pressure, flow can initially be from line  33  to the intermediate compression volume and then as pressure in the intermediate compression volume increases above that in line  33  a flow reversal can take place with flow from intermediate compression volume into line  33 . 
     The present invention adds a variable restriction or injection port  34 . The variable restriction or injection port  34  may either be in the economizer injection line  33  outside of the compressor or in the economizer passage internal to the compressor. If compressor  12  is a scroll compressor or a multi-rotor screw compressor there may be more than one economizer injection port in order to maintain a balance between different compression pockets inside the compressor, and thus more than one variable restriction may be necessary, or desired. 
     Referring specifically to FIG. 2, compressor  12  is illustrated as a scroll compressor. Flow through line  33  into compressor  12  is controlled by microprocessor  100  through variable restriction  34 . Flow from line  33  into compressor  12  is supplied to annular cavity  12 - 1  which feeds trapped volumes via passages  12 - 2  and  12 - 3 , respectively. Compressor  112  of FIG. 3 differs from compressor  12  of FIG. 2 in that two variable restrictions or injection ports  134 - 1  and  134 - 2  are provided and they are located within compressor  112 . Economizer flow supplied via line  133  into compressor  112  flows into annular cavity  112 - 1  which feeds intermediate compression volumes via passages  112 - 2  and  112 - 3  containing variable restrictions or injection ports  134 - 1  and  134 - 2 , respectively. It should be understood that passages  112 - 2  and  112 - 3  are not shown to scale and their length and/or diameter may need to be increased, relative to conventional non-variable restrictions, in order to accommodate suitable commercially available variable restriction devices indicated schematically by  134 - 1  and  134 - 2 . Also, variable restriction devices  134 - 1  and  134 - 2  will be connected to and controlled by microprocessor  100 . 
     The size of the variable restriction  34  in economizer line  33  affects the efficiency of the economized cycle but the optimum restriction size of restriction  34  varies with operating conditions. For example, for higher pressure ratio applications the optimal size of restriction  34  is larger than for lower pressure ratio applications. Therefore, according to the teachings of the present invention the size of restriction  34  can be varied as a function of the compressor operating conditions to maximize the operating efficiency. The optimum size of restriction  34  for maximum capacity is larger than for maximum efficiency operation. If the restriction  34  or restrictions  134 - 1  and  1342  are too small for a given operating condition then the efficiency of the economized cycle is reduced. For example, in one extreme case when the restriction size is zero, or the restriction is completely closed, then there is no economized flow at all and the compressor  12 , or  112 , operates in a non-economized mode that is normally less efficient then the economized mode. In another extreme case where the restriction size is too large for the operating condition, the efficiency of the economized cycle is compromised because of additional flow losses associated with increased sloshing of fluid in and out of the economizer line relative to the compressor. Therefore, there is an optimum size restriction that will result in the most efficient unit operation for each set of operating conditions. Furthermore, if the goal of a designer is to maximize the unit refrigeration capacity rather than the unit efficiency, then the optimum restriction size for unit capacity would be, typically, larger than the restriction size where the unit is optimized for best efficiency. Reduction in sloshing losses has been addressed in U.S. Pat. No. 6,202,438, entitled “Compressor Economizer Circuit With Check Valve”. The invention disclosed in that patent does not allow for variable capacity control. Delays associated with the opening and closing of a check valve present difficulties in operating at optimum efficiency or maximum capacity. Additionally, check valves can be noisy and leak. 
     FIGS. 4 and 5 show how the efficiency and capacity of the refrigeration system is affected by the size of restriction  34  or the total size of restrictions  134 - 1  and  134 - 2 . 
     The line that connects the efficiency maximas in FIG. 4 corresponds to optimum orifice size for the best efficiency for a given pressure ratio operation. FIG. 5 shows the line for peak capacity for given pressure ratios. The orifice size corresponding to the best efficiency and/or best capacity line can be programmed into the system control logic of microprocessor  100  for best performance. The optimum restriction orifice size varies with compressor size, the type of compressor, the operating speed, the position of the injection ports in the compression cycle, etc. Therefore, the exact shape of the curves in FIGS. 4 and 5 can only be shown in a generalized qualitative form. As an example, for a scroll compressor with 15 CFM displacement, operating at 60 Hz and with the injection ports located at a location in the compression cycle immediately after seal off from suction, the optimum size of the injection port into each compression pocket was, roughly, six square millimeters for an operating pressure ratio of five. In general, the optimum total restriction size would vary from one square millimeter to four thousand square millimeters for maximum capacity in compressors in the range of 1 CFM to 300 CFM. 
     Microprocessor  100  can control variable restriction  34  or variable restrictions  134 - 1  and  134 - 2  based upon sensed operating conditions. Suction pressure and/or temperature sensed by sensor  40 , discharge pressure and/or temperature sensed by sensor  50  and condenser subcooling sensed by sensor  60  are suitable data inputs. In general, a higher pressure ratio and a lower condenser subcooling will require a larger orifice size for variable restriction  34  or variable restrictions  134 - 1  and  134 - 2  for optimum operation. While suction and discharge pressure can be measured directly by pressure sensors, they can be measured indirectly based on the measurements of the saturated suction and discharge temperature, respectively. For example, microprocessor  100  will be programmed for maximum efficiency or maximum unit capacity and based upon the programming and data supplied by sensors  40 ,  50  and  60  and zone inputs will operate as described above with the additional control of the size of restriction  34  or restrictions  134 - 1  and  134 - 2 . 
     Although preferred embodiments of the present invention have been illustrated and described, other changes will occur to those skilled in the art. For example, the economizer can be a flush tank or economized heat exchanger, as illustrated. A pulsed valve may be used in place of the variable orifice if it can be pulsed at a sufficient rate. It is therefore intended that the scope of the present invention is to be limited only by the scope of the appended claims.