Patent Publication Number: US-8528359-B2

Title: Economized refrigeration cycle with expander

Description:
FIELD OF THE INVENTION 
     This invention relates generally to vapor compression systems and, more particularly, to refrigerant vapor compression systems equipped with an economizer cycle. 
     BACKGROUND OF THE INVENTION 
     Refrigerant vapor compression systems are well known in the art and commonly used for conditioning air (or other secondary media) to be supplied to a climate controlled comfort zone within a residence, office building, hospital, school, restaurant or other facility. Refrigerant vapor compression systems are also commonly used in transport refrigeration systems for refrigerating air supplied to a temperature controlled cargo space of a truck, trailer, container or the like for transporting perishable items and in commercial refrigeration systems for cooling air supplied to a temperature controlled space in a cold room, a beverage cooler, a diary case or a refrigerated merchandiser for displaying perishable foods item in a chilled or frozen state, as appropriate. Typically, these refrigerant vapor compression systems include a compressor, a condenser, an evaporator, and an expansion device. Commonly, the expansion device, typically a fixed orifice, a capillary tube, a thermostatic expansion valve (TXV) or an electronic expansion valve (EXV), is disposed in the refrigerant line upstream, with respect to refrigerant flow, of the evaporator and downstream of the condenser. These basic refrigerant system components are interconnected by refrigerant lines in a closed refrigerant circuit, arranged in accord with known refrigerant vapor compression cycles. 
     Traditionally, most of these refrigerant vapor compression systems operate at subcritical refrigerant pressures. Refrigerant vapor compression systems operating in the subcritical range are commonly charged with fluorocarbon refrigerants such as, but not limited to, hydrochlorofluorocarbons (HCFCs), such as R22, and more commonly hydrofluorocarbons (HFCs), such as R134a, R410A and R407C. Although HFC refrigerants are more environmentally friendly than the chlorine containing HCFC refrigerants that they replaced, “natural” refrigerants, such as carbon dioxide (also referred to as R744), are being turned to for use in air conditioning and transport refrigeration systems instead of HFC refrigerants. 
     Because carbon dioxide has a low critical point, most refrigerant vapor compression systems charged with carbon dioxide as the refrigerant are designed for operation in the transcritical pressure regime. In refrigerant vapor compression systems operating in a transcritical cycle, the refrigerant discharged from a compressor is a vapor having a temperature and pressure in excess of the refrigerant&#39;s critical point. As in conventional refrigerant vapor compression systems operating in a subcritical cycle, refrigerant vapor compression systems operating in a transcritical cycle, include a compression device, a heat rejecting heat exchanger functioning as a gas cooler rather than a condenser, an evaporator, and an expansion device arranged in accord with known refrigerant vapor compression cycles. Typically, the expansion device is a thermostatic expansion valve (TXV) or an electronic expansion valve (EXV) disposed in the refrigerant line upstream, with respect to refrigerant flow, of the evaporator and downstream of the gas cooler. 
     Refrigerant vapor compression systems utilizing a low critical point refrigerant, such as carbon dioxide, often employ a two-stage compression system, either a pair of compressors disposed in series flow arrangement with respect to refrigerant flow or a single compressor having at least two compression stages. To improve the refrigerant system performance and to control the temperature of the refrigerant vapor discharged from the final stage of the compression system over a wide range of operating conditions, commonly referred to as the discharge pressure or the high-side pressure, it is known to equip such systems with an economizer cycle incorporating a refrigerant-to-refrigerant economizer heat exchanger. The economizer heat exchanger is generally disposed in the refrigerant circuit intermediate the gas cooler and the evaporator to further cool the refrigerant in the main circuit exiting the gas cooler, and to return an expanded (to an intermediate pressure) portion of refrigerant having traversed the economizer heat exchanger in heat transfer interaction with the refrigerant in the main circuit as the supplementary cooling fluid to the compressor. Typically, the refrigerant vapor returned to the compressor is injected into an intermediate stage in the compression process, either through an injection port or ports opening into an intermediate pressure stage of the compression chamber (or chambers) of a single compressor or, in the case of a multiple compressor system, into a refrigerant line extending between the discharge outlet of the upstream compressor and the suction inlet of the downstream compressor. Additionally, liquid refrigerant may be taken from a location downstream of the heat rejecting heat exchanger and returned to the compressor, generally through a separate injection port or ports opening to an intermediate stage of the compression process. It is to be understood that the vapor injection in the economizer cycle and the liquid injection can potentially take place at different intermediate pressures in the compression process, especially in the case when vapor and liquid are injected through separate lines. 
     For example, U.S. Pat. No. 6,571,576 discloses a refrigerant vapor compression system operating in a subcritical cycle and equipped with an economizer heat exchanger wherein vapor refrigerant and liquid refrigerant are returned to an intermediate stage of the compression process through one or more economizer injection ports provided in the compressor. To provide the refrigerant vapor for injection into the compressor, liquid refrigerant is taken from the refrigerant circuit at a location downstream of the condenser, expanded to an intermediate pressure and lower temperature by means of an expansion valve to form a refrigerant liquid/vapor mixture which is thereafter passed through the economizer heat exchanger in heat exchange relationship with the main flow of refrigerant liquid. In traversing the economizer heat exchanger, this refrigerant liquid/vapor mixture extracts heat from the main flow of refrigerant liquid, further cooling this liquid, thereby evaporating any remaining liquid component in the two-phase mixture and typically further heating the vapor. The refrigerant vapor leaving the economizer heat exchanger is then injected into the compressor through the economizer injection ports at the intermediate (between suction and discharge) pressure. Additionally, liquid refrigerant is selectively taken from the refrigerant circuit at a location downstream of the condenser and mixed into the refrigerant vapor being passed from the economizer to the compressor and injected into an intermediate pressure stage of the compression process together with the refrigerant vapor through the same economizer injection ports. 
     U.S. Patent Application Publication No. US 2005/0044885 A1 discloses a transcritical cycle for a carbon dioxide refrigerant vapor compression system including a compressor, a gas cooler, a flash tank economizer, an evaporator, a first expansion valve upstream of the flash tank economizer and a second expansion valve downstream of the flash tank economizer. Refrigerant passing from the gas cooler to the evaporator is expanded to a lower pressure by the first expansion valve before entering the flash tank economizer wherein the refrigerant separates into a liquid component and a vapor component. The liquid refrigerant passes from the flash tank economizer through and is further expanded in the second expansion valve before traversing the evaporator. The vapor refrigerant returns to the compressor at some intermediate pressure. 
     U.S. Pat. No. 6,880,357 discloses a refrigerant cycle apparatus, using carbon dioxide as the refrigerant, that is equipped with an expander and optionally a sub-expander disposed in the refrigerant circuit between an outdoor heat exchanger and an indoor heat exchanger. High pressure refrigerant is taken from the refrigerant circuit and injected into an intermediate pressure stage of the expander. Power recovered during the expansion process in the expander and sub-expander may be used to drive the compressor or an electricity generator. 
     SUMMARY OF THE INVENTION 
     It is a general object of the invention to provide a refrigerant vapor compression system, which includes an expander and economizer cycle incorporating the injection of vapor and/or liquid refrigerant into at intermediate pressure stage in the compression process. 
     It is an object of an aspect of the invention to provide a refrigerant vapor compression system equipped with an expander and an economizer cycle and providing for the injection of vapor refrigerant and liquid refrigerant into at intermediate pressure stage in the compression process through a common line. 
     The refrigerant vapor compression system of the invention includes a compression device disposed in a refrigerant circuit for compressing a refrigerant vapor from a suction pressure to a discharge pressure, a heat rejecting heat exchanger disposed in the refrigerant circuit downstream with respect to refrigerant flow of the compression device, a heat accepting heat exchanger disposed in the refrigerant circuit downstream with respect to refrigerant flow of the heat rejecting heat exchanger and upstream with respect to refrigerant flow of the compression device, an economizer heat exchanger disposed in the refrigerant circuit downstream with respect to refrigerant flow of the heat rejecting heat exchanger and upstream with respect to refrigerant flow of the heat accepting heat exchanger, and an expander device disposed in the refrigerant circuit downstream with respect to refrigerant flow of the economizer heat exchanger and upstream with respect to refrigerant flow of the heat accepting heat exchanger. The economizer heat exchanger has a first pass and a second pass operatively associated in heat transfer relationship. 
     An evaporator bypass line is provided for passing a portion of the refrigerant from the main refrigerant circuit after having traversed the first pass of the economizer heat exchanger out of the expander device at an intermediate pressure during the expansion process and thence through the second pass of the economizer heat exchanger and into an intermediate pressure port of said compression device. An economizer bypass line is provided for passing a portion of the refrigerant from the main refrigerant circuit after having traversed the heat rejecting heat exchanger and partially expanded in the expander into the evaporator bypass line at a location upstream with respect to refrigerant flow of the second pass of the economizer heat exchanger. An expansion valve is disposed in the economizer bypass line for expanding the refrigerant passing therethrough to a lower pressure to provide liquid injection when desired. The economizer vapor injection and liquid injection can be engaged on demand. This invention would be the most beneficial for the transcritical cycle, where the benefits of using the expander as an expansion device are most pronounced. 
     In an embodiment of the invention, the expander device comprises a primary expander and a secondary expander. The primary expander is operatively connected in the refrigerant circuit upstream with respect to refrigerant flow of the evaporator to expand a major portion of the refrigerant flow having traversed the first pass of the economizer heat exchanger and circulating throughout the main refrigerant circuit. The secondary expander is operatively connected in the evaporator bypass line upstream with respect to refrigerant flow of the second pass of the economizer heat exchanger to expand a minor portion of the refrigerant flow having traversed the first pass of the economizer heat exchanger and circulating throughout the economizer loop. In this embodiment, the economizer loop refrigerant can be tapped off upstream of the economizer heat exchanger as well. 
     In another embodiment of the invention, the expander device comprises a single expander having a first stage of expansion for expanding the refrigerant vapor having traversed the first pass of the economizer heat exchanger to a pressure intermediate the discharge pressure and the suction pressure and a second stage of expansion for expanding the refrigerant vapor having traversed the first pass of the economizer heat exchanger to a pressure approximating the suction pressure. In this embodiment, the evaporator bypass line communicates with the expansion device to receive a flow of refrigerant at the intermediate pressure. 
     The compression device may consist of a first compressor having a discharge outlet connected by a refrigerant line in refrigerant flow communication to a suction inlet of a second compressor, with the evaporator bypass line opening into the refrigerant line at location between the discharge outlet of the first compressor and the suction inlet of the second compressor. The compression device may be a single compressor having a compression chamber (or chambers) with the evaporator bypass line communicating into the compression chamber (or chambers) at an intermediate stage in the compression process. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a further understanding of these and other objects and the advantageous of the invention, reference will be made to the following detailed description of the invention which is to be read in connection with the accompanying drawing, where: 
         FIG. 1  is a schematic diagram illustrating a first exemplary embodiment of a refrigerant vapor compression system in accord with the invention; 
         FIG. 2  is a schematic diagram illustrating a second exemplary embodiment of a refrigerant vapor compression system in accord with the invention; 
         FIG. 3  is a schematic diagram illustrating an alternative arrangement of the exemplary embodiment of the refrigerant vapor compression system of the invention depicted in  FIG. 1 ; and 
         FIG. 4  is a schematic diagram illustrating an alternative arrangement of the exemplary embodiment of the refrigerant vapor compression system of the invention depicted in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention will be described further herein with respect to the embodiments of the refrigerant vapor compression system  10  depicted in  FIGS. 1-2  preferably operating in a transcritical cycle and charged with carbon dioxide or other relatively low critical point refrigerant. As conventional systems, the refrigerant vapor compression system  10  includes a compression device  20 , a refrigerant heat rejecting heat exchanger  30 , also referred to as a gas cooler, a refrigerant heat absorbing heat exchanger  40 , also referred to herein as an evaporator, and various refrigerant lines  70 A,  70 B,  70 C and  70 D connecting the aforementioned components in a basic refrigerant circuit  70 . Although the refrigerant vapor compression system of the invention is particularly adapted to operate in a transcritical cycle with a low critical point refrigerant such as, for example, carbon dioxide, it is to be understood that the refrigerant vapor compression system described herein may also be operated in a subcritical cycle when charged with conventional refrigerants having a relatively high critical point temperature. 
     The compression device  20  operates to compress and circulate refrigerant through the refrigerant circuit as will be discussed in further detail hereinafter. In the embodiment depicted in  FIG. 1 , the compression device  20  is a single refrigerant compressor having at least a first compression stage and a second compression stage such as, for example, a scroll compressor or a screw compressor having staged compression pockets, or a reciprocating compressor having at least a first bank and a second bank of cylinders. In the embodiment depicted in  FIG. 2 , the compression device  20  is a pair of compressors  20 A and  20 B, such as, for example, a pair of scroll compressors, screw compressors, reciprocating compressors (or separate cylinders of a single reciprocating compressor) or rotary compressors connected in series, having a refrigerant line  22  connecting the discharge outlet port of the first compressor  20 A, which constitutes a first compression stage, in refrigerant flow communication with the suction inlet port of the second compressor  20 B, which constitutes a second compression stage. 
     In a refrigerant vapor compression system operating in a transcritical cycle, the compressor discharge pressure is high enough that the refrigerant vapor does not condense as it traverses the heat rejection heat exchanger  30 . Consequently, with respect to systems operating in a transcritical cycle, the heat rejection heat exchanger  30  functions as, a refrigerant gas cooler, rather than a refrigerant vapor condenser. Supercritical refrigerant vapor discharged into refrigerant line  70 A from the single compressor  20  in the  FIG. 1  embodiment or from the second stage compressor  20 B in the  FIG. 2  embodiment passes in heat exchange relationship with and is cooled by a secondary cooling fluid, typically ambient outdoor air passed over the refrigerant conveying coils  34  by an air mover, such as one or more fans  32 , operatively associated with the gas cooler  30 . In a transcritical system, the refrigerant flow passes from the coils  34  of the gas cooler  30  into the refrigerant line  70 B at a high pressure, lower temperature conditions. 
     A major portion of the refrigerant leaving the gas cooler  30  passes through refrigerant line  70 B to the evaporator  40 . In doing so, the refrigerant traverses the expansion device  80  and expands to a lower, typically subcritical pressure whereby the refrigerant enters the evaporator  40  as a lower temperature, lower pressure liquid refrigerant or more commonly liquid/vapor refrigerant mixture. In the refrigerant vapor compression system of the invention, the expansion device  80  is an expander, rather than a restrictor type expansion device such as: an expansion valve, capillary tube or fixed orifice. The evaporator  40  constitutes a refrigerant heat absorbing heat exchanger through which the liquid refrigerant passes in heat exchange relationship with a secondary fluid to be cooled and delivered to a conditioned environment whereby the refrigerant is heated thereby evaporating the liquid component and typically superheating the resultant vapor. The secondary fluid passed in heat exchange relationship with the refrigerant in the evaporator  40  may be air passed over the evaporator refrigerant coil  44  by an air mover, such as one or more fans  42  to condition the air by cooling the air and condensing moisture from the air. The conditioned air may be supplied to a climate controlled environment such as a comfort zone associated with an air conditioning system or a perishable product storage zone associated with a transport refrigeration unit or commercial refrigeration unit. 
     The refrigerant vapor compression system  10  of the invention further includes an economizer heat exchanger  60  disposed in the refrigerant circuit  70  between the gas cooler  30  and the evaporator  40 . In the exemplary embodiment of the system  10  depicted in  FIGS. 1 and 2 , the economizer heat exchanger  60  is a refrigerant-to-refrigerant heat exchanger wherein a first flow of refrigerant passes through a first pass  62  of the economizer heat exchanger  60  in heat exchange relationship with a second flow of refrigerant passing through a second pass  64  of the economizer heat exchanger  60 . The first flow of refrigerant comprises a major portion of the high pressure refrigerant vapor passing through refrigerant line  70 B, while the second flow of refrigerant comprises a minor, economizer loop portion of the refrigerant passing through refrigerant line  70 B. 
     As mentioned hereinbefore, the refrigerant vapor compression system  10  of the invention includes an expander device  80  for expanding at least a major portion of the refrigerant passing therethrough. The expander device  80  is disposed in refrigerant line  70 C of the refrigerant circuit  70  downstream with respect to refrigerant flow of the economizer heat exchanger  60  and upstream with respect to refrigerant flow of the evaporator  40 . In the embodiment of the refrigerant vapor compression system  10  depicted in  FIG. 1 , all of the refrigerant having traversed the heat exchange coil  62  of the economizer heat exchanger  60  enters a single expander device  80 . A first portion of that refrigerant, which constitutes a major portion of the refrigerant, fully traverses the expander  80  and is thereby expanded to a lower subcritical pressure. This first portion of the refrigerant exits from the expander  80  into refrigerant line  70 C and thereafter passes through the evaporator  40  as hereinbefore discussed. 
     In this embodiment, a second, economized portion of the refrigerant entering the expander device  80 , which constitutes a minor portion of the refrigerant, does not fully traverse the expander  80 , but rather is drawn off through a line  70 E after having been only partially expanded in the expander  80  to pass through the second pass  64  of the economizer heat exchanger  60  in heat exchange relationship with the first portion of the refrigerant passing through the first pass  62  of the economizer heat exchanger  60 . Having been partially expanded within the expander  80  to a lower pressure, intermediate the discharge pressure and the suction pressure, and lower temperature, the second portion of the refrigerant passing through the second pass  64  of the economizer heat exchanger  60  is cooler than the higher temperature, higher pressure refrigerant passing through the first pass  62  of the economizer heat exchanger  60 . Therefore, the refrigerant flowing through the first pass  62  is cooled by rejecting heat to the second portion of the refrigerant flowing through the second pass  64 , which is heated by the heat absorbed in cooling the refrigerant passing through the first pass  62  of the economizer heat exchanger  60 . 
     In the embodiment of the refrigerant vapor compression system of the invention depicted in  FIG. 2 , the expansion device constitutes a primary expander  80  and a secondary expander  82 . In this embodiment, the primary expander  80  may have only one stage of expansion, as the second stage of expansion is performed by the secondary expander  82 . A portion of the refrigerant having traversed the first pass  62  of the economizer heat exchanger  60 , which again constitutes a minor portion of the refrigerant, is diverted from the refrigerant line  70 C into the refrigerant line  70 E at a location upstream with respect to refrigerant flow of the primary expander  80  and downstream of the economizer heat exchanger  60 . The remaining major portion of the refrigerant having traversed the first pass  62  of the economizer heat exchanger  60  continues on through refrigerant line  70 C to pass through the primary expander  80 , thereafter through the heat exchange coil  44  of the evaporator  40 , and thence through refrigerant line  70 D to return to the suction inlet port of the compressor  20 A. The diverted minor portion of the refrigerant flowing through refrigerant line  70 E passes through the secondary expander  82  disposed in refrigerant line  70 E and is expanded therein to a lower intermediate pressure, lower intermediate temperature state prior to flowing through the second pass  64  of the economizer heat exchanger  60 . Having been expanded within the secondary expander  82  to a lower pressure and lower temperature, the diverted minor portion of the refrigerant passing through the second pass  64  of the economizer heat exchanger  60  is cooler than the higher temperature, higher pressure refrigerant flowing through the first pass  62  of the economizer heat exchanger  60 . Therefore, the refrigerant flowing through the first pass  62  of the economizer heat exchanger  60  is cooled by rejecting heat to the minor portion of the refrigerant flowing through the second pass  64  of the economizer heat exchanger  60 , which is heated by the heat absorbed in cooling the refrigerant flowing through the first pass  62  of the economizer heat exchanger  60 . It has to be understood that, in this embodiment, the minor, economizer portion of refrigerant can be tapped upstream of the economizer heat exchanger  60  as well. 
     In either embodiment of the invention, the second portion of the refrigerant having traversed the second pass  64  of the economizer heat exchanger  60  flows through the downstream leg of the refrigerant line  70 E to return to the compression device  20  at an intermediate pressure state in the compression process. If, as depicted in  FIG. 1 , the compression device is a refrigerant compressor  20 , for example a scroll compressor, a screw compressor or a multi-bank reciprocating compressor, the refrigerant from the second pass  64  of the economizer heat exchanger  60  enters the compressor through at least one injection port opening at an intermediate pressure state of compression within the compressor  20 . If, as depicted in  FIG. 2 , the compression device  20  is a pair of compressors  20 A and  20 B connected in series relationship with respect to refrigerant flow, the refrigerant having traversed the second pass  64  of the economizer heat exchanger  60  is injected into the refrigerant line  22  connecting the discharge outlet port of the first stage compressor  20 A in refrigerant flow communication with the suction inlet port of the second stage compressor  20 B. Also, in both  FIGS. 1 and 2  embodiments, a shutoff valve  74  may be provided to disengage the economizer loop from an active refrigerant circuit, if desired. 
     Additionally, in another aspect of the invention, a portion of the refrigerant vapor passing from the gas cooler  30  to the first pass  62  of the economizer heat exchanger  60  through the refrigerant line  70 B is diverted through the refrigerant line  70 F into the downstream leg of the refrigerant line  70 E to provide additional cooling to the compression process. In passing through refrigerant line  70 F, the diverted flow of refrigerant traverses an expansion valve  50  disposed in refrigerant line  70 F and is expanded to a lower pressure and lower temperature to typically form a liquid refrigerant or a liquid/vapor refrigerant mixture. The resultant lower pressure and lower temperature liquid refrigerant or liquid/vapor refrigerant mixture then passes into the downstream leg of the refrigerant line  70 E to return to the compression device  20 . When the economizer loop is operational, the shutoff valve  74  is open and the refrigerant passing into refrigerant  70 E from refrigerant line  70 F will mix with the refrigerant vapor having traversed the second path  64  of the economizer heat exchanger  60  prior to being returned to the compression device  20  as herein before discussed. 
     The refrigerant vapor passing through the expansion valve  50 , which may be an electrostatic expansion valve, EXV, or a thermostatic expansion valve, TXV, is expanded to a pressure lower than the compressor discharge pressure, but higher than the average refrigerant pressure existing at the intermediate compression stage at which the refrigerant passing through  70 E returns to the compression device  20 . Similarly, the portion of the refrigerant that is diverted to pass through the second pass  64  of the economizer heat exchanger  60  is tapped off the expander  80  at or expanded through the expander  82  to a pressure lower than the compressor discharge pressure, but higher than the average refrigerant pressure existing at the intermediate compression stage at which the refrigerant passing through  70 E returns to the compression device  20 . 
     It should be pointed out that the expansion valve  50  may be positioned on the line  70 E upstream of the second pass  64  of the economizer heat exchanger  60  and upstream of the shutoff valve  74  but downstream of the point within the refrigerant cycle where partial expansion of the minor economized portion of refrigerant flow has occurred. For example, in the exemplary embodiment of the refrigerant vapor compression system depicted in  FIG. 3 , the expansion valve  50  may be disposed in a refrigerant line  70 G which provides a refrigerant flow path for a portion of the partially expanded refrigerant drawn off the primary expander  80  to pass from the refrigerant line  70 E at a point upstream of the shutoff valve  74  to re-enter the refrigerant line  70 E at a point downstream of the second pass  64  of the economizer heat exchanger  60 . This portion of the refrigerant diverted from the refrigerant line  70 E bypasses the economizer heat exchanger  60  and is further expanded as it traverses the expansion valve  50  to provide a liquid refrigerant or a liquid/vapor refrigerant mixture for injection into an intermediate pressure stage of the compression device  20  as hereinbefore discussed. Alternatively, as in the exemplary embodiment of the refrigerant vapor compression system depicted in  FIG. 4 , the expansion valve  50  may be disposed in a refrigerant line  70 G which provides a refrigerant flow path for a portion of the unexpanded refrigerant passing from the refrigerant line  70 C at a point upstream of the primary expander  80  into the refrigerant line  70 E to pass from the refrigerant line  70 E at a point upstream of both the shutoff valve  74  and the secondary expander  82  to re-enter the refrigerant line  70 E at a point downstream of the second pass  64  of the economizer heat exchanger  60 . This portion of the refrigerant diverted from refrigerant line  70 E bypasses both the secondary expander  82  and the economizer heat exchanger  60  and is further expanded as it traverses the expansion valve  50  to provide a liquid refrigerant or a liquid/vapor refrigerant mixture for injection into an intermediate pressure between the first and second compression stages  20 A and  20 B as hereinbefore discussed. 
     The amount of liquid refrigerant flow passed through refrigerant line  70 F and into the downstream leg of the refrigerant line  70 E to mix with the refrigerant vapor passing therethrough from the second pass  64  of the economizer heat exchanger  60  when the shutoff valve  74  is open and be injected into an intermediate stage of the compression device  20  as a liquid/vapor refrigerant mixture, may be controlled by means of a controller  90  operatively associated with the expansion valve  50  disposed in refrigerant line  70 F. The controller  90  is programmed in a conventional manner to control the degree of opening of the expansion valve  50  thereby controlling the flow rate of refrigerant passing through refrigerant line  70 F from refrigerant line  70 B. The controller  90  may also be programmed to monitor the compressor discharge temperature, that is the temperature of the refrigerant vapor discharging into refrigerant line  70 A from the discharge outlet port of the second compression stage, and control the operation of the expansion valve  50  to provide sufficient liquid refrigerant flow into refrigerant line  70 E to ensure that the compressor discharge temperature does not exceed a specified upper limit. The discharge temperature can be measured, for instance, by a temperature transducer  92 . Also, the controller  90  can be operatively associated with the shutoff valve  74  to selectively open this valve when extra system capacity is required to satisfy thermal load demands in a conditioned space. The economized refrigerant flow may also assist in controlling the compressor discharge temperature such that it stays below the specified limit. 
     In an embodiment of the invention, the controller  90  constitutes the main system controller and receives operating data regarding various system operating parameters as in conventional practice, such as for purposes of illustration but not limitation, the refrigerant temperature and/or pressure at the compressor discharge, at the compressor suction inlet, at the evaporator outlet, and other locations, as desired, provided by appropriately disposed sensors (not shown). The controller  90  may also be programmed to control the operation of the expander  80  and the secondary expander  82  in response to selected system operating parameters. For example, the controller  90  may be programmed to control the speed of the expander  80  to adjust the refrigerant flow rate passing through refrigerant line  70 C to the evaporator  40  as a means of controlling the evaporator outlet temperature. The controller  90  may also be programmed to control the speed of the secondary expander  82  to adjust the refrigerant flow rate returning through refrigerant line  70 E to an injection port or ports in an intermediate stage of the compression device  20  as hereinbefore discussed. Alternatively, flow control valves (not shown) operatively associated with and controlled by the controller  90  may be provided in refrigerant line  70 C upstream or downstream of the primary expander  80  to control the flow rate of refrigerant passing through the primary expander  80  and in refrigerant line  70 E to control the flow rate of refrigerant passing through the second pass  64  of the economizer heat exchanger  60 . 
     The particular type of expander used is not germane to the invention. The expanders  80  and  82  may be rotary vane expanders, screw expanders, scroll expanders or other conventional expanders. Using an expander as an expansion device in the refrigerant circuit rather than an expansion valve or fixed orifice, is advantageous as power generated by expansion of the refrigerant passing through the expander may be readily recovered rather than wasted. For example, as illustrated in  FIG. 1 , a generator, G, may be operatively associated with the expander  80  whereby the power recovered in the expander  80  is transferred to the generator, G, to generate electricity which could be used to at least partially power the compression device  20 , the secondary fluid moving devices or for other purposes. As illustrated in  FIG. 2 , the expander  80  may be, for instance, operatively connected to assist in driving the first stage compressor  20 A and the secondary expander  82  may be, for instance, operatively connected to the second stage compressor  20 B, whereby the power recovered in the respective expansion process drives or assists in driving the respective compressors. Further, the expansion process in the expanders  80  and  82  is more thermodynamically efficient than in a restrictor-type expansion device (an expansion valve, an orifice or a capillary tube), since it follows isotropic, rather than isenthalpic, expansion line, where the refrigerant passing through an expander would have a higher thermodynamic potential at the evaporator entrance resulting in overall enhancement of the system efficiency and cooling capacity. 
     While the present invention has been particularly shown and described with reference to the preferred mode as illustrated in the drawings, it will be understood by one skilled in the art that various changes in detail may be effected therein without departing from the spirit and scope of the invention as defined by the claims.