Abstract:
A refrigerant system utilizes an expander to expand refrigerant and to drive or assist in driving an associated compressor. By varying the compressor load, the speed of the expander can be adjusted to achieve the desired thermodynamic characteristics of the expanding refrigerant and enhance expander operation.

Description:
BACKGROUND OF THE INVENTION 
     Refrigerant systems are known to utilize refrigerant circulating throughout a closed-loop circuit to condition a secondary fluid. Typically, a refrigerant system includes a compressor for compressing the refrigerant, and delivering the refrigerant to a downstream heat exchanger. Refrigerant from that downstream heat exchanger passes through an expansion device, and then to an evaporator. In traditional refrigerant systems, the expansion device is a fixed area restriction or a valve that may be controlled such that the amount of expansion is tailored to achieve desired characteristics in operation of the refrigerant system. 
     In some advanced refrigerant systems, the work which is available from the expansion process of the refrigerant is utilized to drive or assist in driving at least one component within the refrigerant system. 
     In one known refrigerant system configuration, a secondary compressor operates in parallel with a main compressor. This secondary compressor compresses a portion of the refrigerant circulated throughout the refrigerant system. The secondary compressor is driven by the expander, with the expander operating much like a turbine, to receive the compressed refrigerant, and expand that refrigerant to a lower pressure and temperature. The work from this expansion process is utilized to drive the secondary compressor. This known combination of a compressor and an expander, located on the same shaft, is called an expresser. The use of the expresser is known in the industry, where the expander drives or assists in driving the corresponding compressor. The refrigerant exiting a heat rejection heat exchanger enters the expander, and then is expanded to a lower pressure and temperature. A two-phase flow exiting the expander enters the evaporator. The work extracted from the expansion process in the expander is used to drive the secondary compressor that is quite often located on the same shaft as the expander. In addition to extracting useful work from the expansion process, the refrigerant passing through the expander acquires a higher cooling thermodynamic potential, as it expands through the expander, since it follows a more efficient isentropic process. The use of the expresser technology is especially expected to grow in CO 2  applications, where the potential for the expansion energy recovery is higher than for the conventional refrigerants. 
     One of the disadvantages of positioning the expander and the associated compressor into a closely coupled mechanical engagement, such as locating them on the same shaft, is that the expander speed is not actively controlled. In other words, the expander will settle at a speed at which the power extracted by the expander from the refrigerant expansion process is roughly equal to and is balanced by the power delivered to the compressor. Since the expander speed cannot be actively controlled, the expansion process through the expander is typically not optimal. If the expansion process is not optimal, then the amount of refrigerant delivered to the evaporator, and its thermodynamic state, cannot be precisely controlled. If a delivered amount of refrigerant cannot be adjusted, it may result, for instance, in less than optimal gas cooler pressure, in transcritical applications, and/or undesirable conditions at the compressor entrance. 
     In other words, to optimize the expansion process for given operating and environmental conditions, such as gas cooler pressure, suction superheat, etc., flexibility in varying the expander speed must be provided. One way to enhance the control of the expander is to install an expansion valve that is located in series with the expander. However, the expansion valve would reduce/limit the amount of the work extracted from the expansion process by the expander. This reduction would occur, as part of the expansion process would take place in the expansion valve, and not in the expander. Therefore, a need exists to optimize the expresser operation. 
     SUMMARY OF THE INVENTION 
     In this invention, the expansion process in the expander is controlled by adjusting the speed of the expander. The higher the expander speed, the more refrigerant can be passed through the expander. Similarly, the lower the expander speed, the less refrigerant passes through the expander. The expander speed of the expresser (a mechanically coupled compressor-expender configuration) is adjusted by changing the load on the compressor component of the expresser. Compressor unloading can be accomplished by using various unloading techniques such as, for example, moving a slide valve of a screw compressor, opening a bypass port of the scroll compressor, using suction cutoff of a reciprocating compressor, installing a suction modulation valve, or utilizing any other known techniques to reduce the compressor load. This compressor load reduction causes the expander speed to increase. 
     Similarly, loading the associated compressor component of the expresser results in a speed decrease of the expander component of the expresser. Therefore, by utilizing the proper amount of compressor unloading we can very the expresser speed and thus optimize the expansion process. This is true since the expander speed varies along with the expresser speed, as both the compressor and expander are closely mechanically coupled, such as located on the same shaft. An ability to change the expander speed is similar to adjusting the amount of flow by using a variable restriction expansion device, such as an electronic expansion valve, in comparison to inefficient fixed cross-sectional area expansion device, such as a capillary tube or orifice. 
     These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a refrigerant system incorporating the present invention. 
         FIG. 2  is a view of another schematic. 
         FIG. 3  is a view of another schematic. 
         FIG. 4  is a view of another schematic. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A refrigerant system  20  is illustrated in  FIG. 1 . A main compressor  22  compresses a refrigerant received from a main suction line  24 . As shown, a secondary suction line  26  delivers a portion of the refrigerant flow through a secondary compressor  28 . Refrigerant compressed by the secondary compressor  28  is delivered through a secondary discharge line  30  to a main discharge line  46 , positioned on a high side of the refrigerant system  20 , to be combined with the refrigerant delivered from the main compressor  22 . The combined refrigerant flow passes through a heat rejection heat exchanger  32 , where the heat is removed from the refrigerant by a secondary fluid typically delivered to an ambient environment. The heat rejection heat exchanger  32  is called a condenser, if the refrigerant passes through the thermodynamic states within the heat exchanger  32  that are below the critical point, or a gas cooler, if the refrigerant passes through the thermodynamic states within the heat exchanger  32  that are above the critical point. 
     Downstream of the condenser  32 , an expansion process, to a lower pressure and temperature, occurs in an expander  34 . As known, the expander  34  takes the compressed refrigerant from the heat rejection heat exchanger (a subcritical condenser or a supercritical gas cooler)  32 , and utilizes energy from that compressed refrigerant to drive the expander, while the compressed refrigerant is “isentropically” expanded to a lower pressure and temperature. A shaft  36  (alternatively a generator) is driven by the expander  34 , and this shaft (or power from the generator) in turn drives the secondary compressor  28 . Such systems are known as “expressers.” 
     A heat exchanger, or an evaporator,  38  is positioned downstream of the expander  34 . The evaporator  38  is located on a lower pressure side of the refrigerant system  20 , and heat is transferred to the refrigerant in the evaporator  38  from a secondary fluid to be delivered to a climate-controlled space. Refrigerant passes from the expander  34 , through the evaporator  38 , and back into the suction line  24  to return to the compressors  22  and  28 . The refrigerant system  20 , as described to this point, is as known in the art. Obviously, the basic refrigerant system  20  may have additional features or enhancement options. All these variations in refrigerant system configurations are within the scope and can equally benefit from the invention. 
     A control  50  for the refrigerant system  20  operates components such as a bypass valve  40 , and/or a suction modulation valve  44 , both associated with the secondary compressor  28 , to limit the amount of refrigerant compressed by the secondary compressor  28 , and thus to unload the compressor  28 . By reducing the amount of refrigerant compressed by the secondary compressor  28 , the speed of the expander  34  mechanically coupled with the compressor  28  can be increased. The expander speed adjustment achieves desired thermodynamic characteristics of the expanding refrigerant that can be optimized for specific operating conditions. The desired thermodynamic characteristics of the expanding refrigerant tailored to a specific set of operating conditions are as known in the art, and have been utilized for operation and control of electronic expansion valves. However, achieving desired thermodynamic characteristics of the expanding refrigerant have been limited with systems utilizing expanders, since the expander speed is not usually actively controlled. 
     However, by utilizing the control  50 , and selectively operating, for example, either the bypass valve  40  to control the amount of refrigerant bypassed through a bypass line  42 , or by limiting the amount of refrigerant passing through a suction modulation valve  44  and reaching the secondary compressor  28 , the amount of refrigerant compressed by the secondary compressor  28 , and thus the speed of the expander  34 , can be controlled. The control  50  may also be operated in a pulse width modulation mode to rapidly cycle either valve  40  or  44  between open and closed positions to achieve precise control over the amount of refrigerant compressed by the secondary compressor  28 . Obviously, the valves  40  and  44  may operate in conjunction with each other to achieve the desired level of unloading of the secondary compressor  28 . 
     Compressor unloading can be accomplished by using various unloading techniques such as, for example, moving a slide valve of a screw compressor, opening a bypass port of the scroll compressor, using suction cutoff of a reciprocating compressor, installing a suction modulation valve, or utilizing any other known techniques to reduce the compressor load. 
     To be operational and to take advantage of the invention, the expander  34  does not have to be connected to the high source of pressure associated with the heat rejection heat exchanger  32  and to the source of low pressure associated with the evaporator  38 . To perform the expansion function, the expander can be connected to an intermediate pressure point in the refrigerant system  120  as shown in  FIG. 2 . In refrigerant system  120 , the main compressor may consist of two compressor stages  22  and  222  connected in series. In the embodiment shown in  FIG. 2 , the expander  34  is incorporated into a loop associated with a vapor injection or economizer cycle, where the expander  34  is expanding the refrigerant from the pressure associated with the heat rejection heat exchanger  32  to the intermediate cycle pressure approximated by the pressure between the first compression stage  22  and the second compression stage  222 . Economizer cycles are known in the art, and the benefits provided by economizer cycles are associated with additional subcooling obtained in the economizer heat exchanger  224  and a more efficient compression process, due to refrigerant vapor injection between sequential compression stages  22  and  222 . The refrigerant undergoing expansion in the expander  34 , from a high-side to intermediate pressure, provides even greater subcooling to the main flow in the economizer heat exchanger  224 , where the main flow undergoes expansion in a main expansion device  226 . This greater subcooling, and higher cooling thermodynamic potential for refrigerant entering the evaporator  38 , is achieved due to more efficient isentropic expansion process, in comparison to isenthalpic expansion process provided by traditional expansion devices. The expansion device  226  can be, for example, a fixed area orifice, a capillary tube, a thermostatic expansion valve, an electronic expansion valve, another expander or a combination of different expansion devices. As in the embodiment shown in  FIG. 1 , the expander  34  of the  FIG. 2  embodiment is associated with secondary compressor  28  and takes advantages of the selective unloading of this compressor, as discussed above. In this case, the secondary compressor  28  operates in a parallel arrangement (or in tandem) with the primary compressor  22 , which in combination with the compressor  28 , provide the first stage of compression, from a suction pressure to an intermediate pressure. Of course, as known in the art, the two compression stages  22  and  222  may be provided within a single compressor housing. 
     Similarly, in the embodiment  220  shown in  FIG. 3 , the secondary compressor  28  may be positioned to operate in parallel (or in tandem) with the second compression stage  222  and to compress refrigerant from an intermediate pressure to a discharge pressure. Other arrangements are also possible, where for instance, the main and secondary compressor operating in tandem may compress refrigerant to a pressure lower then the pressure associated with the heat rejection heat exchanger  32 . Further, if multiple intermediate pressure levels are available within the refrigerant cycle, the secondary compressor  28  may operate between its own pressure levels, and not exactly in tandem with any of the primary compressors. These arrangements would also be typical of compressors installed in series. 
     Even further arrangements are possible, where, for example, the secondary compressor  28  is not compressing the refrigerant, but instead is compressing some other process fluid. In this case, in the embodiment  320  shown in  FIG. 4 , the secondary compressor may be used, for example, to compress air and deliver it from an inlet line  321  to an outlet line  322 . As described above, a similar bypass arrangement may be used to control the amount of the bypassed air to shed off the compressor load to control the speed of the expander. Of course, in this case, since both the compressor  28  and the expander  34  are located on the same shaft, a special seal needs to be added onto the rotating shaft, as known, that would prevent the leakage of the refrigerant to the ambient environment. 
     Further, in all the embodiments above, a clutch can be installed on the rotating shaft  36  connecting the secondary compressor  28  and the expander  34  to selectively engage and disengage a mechanical coupling of these two expresser components. 
     It should be pointed out that many different compressor and expander types could be used in this invention. For example, scroll, screw, rotary, centrifugal or reciprocating compressors and expanders can be employed. 
     The refrigerant systems that utilize this invention can be used in many different applications, including, but not limited to, air conditioning systems, heat pump systems, marine container units, refrigeration truck-trailer units, and supermarket refrigeration systems. 
     Furthermore, it has to be understood that although this invention can be applied to any economized refrigerant systems, the refrigerant systems employing CO 2  as a refrigerant would particularly benefit from this invention, since these systems have inherit deficiencies and require additional means for the performance enhancement. 
     Although a preferred embodiment of this invention has been disclosed, a worker of ordinary skill in the art would recommend that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.