Patent Publication Number: US-7213405-B2

Title: Two-stage linear compressor

Description:
BACKGROUND 
   The present invention relates to a refrigeration system including a dual-opposed piston linear compressor, and more particularly to an application of an economizer cycle to the linear compressor. 
   In refrigeration systems, such as those used in cooling display cases of refrigeration merchandisers, it is necessary to maintain a constant temperature in the display cases to ensure the quality and condition of the stored commodity. Many factors cause varying cooling loads on evaporators cooling display cases. Therefore, selective operation of the compressor of the refrigeration system at different cooling capacities corresponds to the cooling demand of the evaporators. Further, as ambient outdoor temperature decreases, compressor loading typically decreases due to lower system lift. In refrigeration systems utilizing existing scroll and screw compressors, an economizer cycle is used to increase the refrigeration capacity and improve efficiency of the refrigeration system. In the economizer cycle of existing scroll and screw compressors, gas pockets in the compressor create a second “piston” as the mechanical elements of the compressor proceed through the compression process. 
   Further, scroll compressors use oil for operation, which results in inefficient performance due to oil film on evaporator and condenser surfaces, requires the use of expensive oil management components, and increases the installation cost of the refrigeration system. Some refrigeration systems utilize a linear compressor, which provides variable capacity control of the refrigeration system. 
   SUMMARY 
   In one embodiment, the invention provides a refrigeration system including a dual-piston linear compressor having a first piston disposed in a first cylinder and a second piston opposed to the first piston and disposed in a second cylinder. The first piston divides the first cylinder into a first suction chamber and a first discharge chamber, and the second piston divides the second cylinder into a second suction chamber and a second discharge chamber. The refrigeration system also includes a first gas flow path through the linear compressor, a second gas flow path through the linear compressor, and a controller operable to switch the linear compressor between an economizer cycle with two stage compression and a single stage cycle. In the economizer cycle, flow of gas is along the first gas flow path, and in the single stage cycle flow of gas is along the second gas flow path. At least one discharge control valve is coupled to the controller and responsive to control signals from the controller. The discharge control valve is operable to direct gas from the first and second discharge chambers to the first gas flow path or the second gas flow path. At least one suction control valve is coupled to the controller and responsive to control signals from the controller. The suction control valve is operable to direct gas to the first and second suction chambers along the first gas flow path or the second gas flow path. 
   In another embodiment, the invention provides a dual-piston linear compressor switchable between an economizer cycle and a single stage cycle. The linear compressor includes a housing divided into a first chamber and a second chamber, a first piston disposed in the first chamber, and a second piston disposed in the second chamber wherein the first and second pistons are opposed and each piston moves back and forth within the respective chamber in opposite directions of movement. The first chamber includes a first input to receive refrigerant into the first chamber and a first output to discharge refrigerant from the first chamber. The second chamber includes a second input to receive refrigerant into the second chamber and a second output to discharge refrigerant from the second chamber. In the economizer cycle, the first input receives refrigerant from an evaporator line, the first output discharges refrigerant to an economizer line, the second input receives refrigerant from the economizer line, and the second output discharges refrigerant to a condenser line. In the single stage cycle, the first and second inputs receive refrigerant from the evaporator line and the first and second outputs discharge refrigerant to the condenser line. The linear compressor further includes a controller operable to switch between the economizer cycle and the single stage cycle. 
   In another embodiment, the invention provides a refrigeration system including a dual-piston linear compressor including a first piston disposed in a first cylinder and a second piston opposed to the first piston and disposed in a second cylinder. The first cylinder defines in part a first suction chamber and a first discharge chamber, and the second cylinder defines in part a second suction chamber and a second discharge chamber. The refrigeration system includes at least two refrigerant flow paths through the linear compressor wherein the at least two refrigerant flow paths deliver refrigerant from the linear compressor to a condenser and deliver refrigerant to the linear compressor from at least one evaporator. The refrigeration system also includes a controller operable to select one of the at least two refrigerant flow paths through the linear compressor. At least one discharge control valve is coupled to the controller and responsive to control signals from the controller. The discharge control valve is operable to direct refrigerant from the first and second discharge chambers to either of the at least two refrigerant flow paths. At least one suction control valve is coupled to the controller and responsive to control signals from the controller. The suction control valve is operable to direct refrigerant from either of the at least two refrigerant flow paths to the first and second suction chambers. 
   In yet another embodiment, the invention provides a dual-piston linear compressor operable in an economizer cycle. The linear compressor includes a housing divided into a first chamber and a second chamber, a first piston disposed in the first chamber, and a second piston disposed in the second chamber wherein the first and second pistons are opposed and each piston moves back and forth within the respective chamber in opposite directions of movement. The first chamber includes a first input to receive refrigerant into the first chamber and a first output to discharge refrigerant from the first chamber. The second chamber includes a second input to receive refrigerant into the second chamber and a second output to discharge refrigerant from the second chamber. The first input receives refrigerant from an evaporator line, the first output discharges refrigerant to the second input, the second input receives refrigerant from the first output and an economizer line, and the second output discharges refrigerant to a condenser line. 
   Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of a refrigeration system including a two-stage linear compressor with dual-opposed pistons embodying the present invention. 
       FIG. 2  is a schematic diagram of the two-stage linear compressor shown in  FIG. 1  operating in an economizer cycle. 
       FIG. 3  is a schematic diagram of the two-stage linear compressor shown in  FIG. 1  operating in a single stage cycle. 
       FIG. 4  is a sectional view of a dual opposing, free-piston linear compressor used in the refrigeration system of  FIG. 1 . 
       FIG. 5  is a schematic diagram of a two-stage linear compressor operable in an economizer cycle. 
   

   Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless specified or limited otherwise, the terms “mounted,” “connected,” “supported,” and “coupled” and variations thereof are used broadly and encompass both direct and indirect mountings, connections, supports, and couplings. Further, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings. 
   DETAILED DESCRIPTION 
     FIG. 1  is a schematic diagram of a refrigeration system  10  including a two-stage linear compressor  14  with dual-opposed pistons. In  FIG. 2 , the linear compressor  14  is shown in an economizer cycle in which refrigerant flows through the refrigeration system along an economizer gas flow path  16  (shown as a bold line in  FIG. 2 ). In the illustrated embodiment, components of the refrigeration system  10  include the linear compressor  14 , a condenser  18 , an economizer  22  (also referred to as a liquid subcooler), an expansion device  26  (typically referred to as the expansion valve), and an evaporator  30  (or a group of evaporators), all of which are in fluid communication. In a further embodiment, the refrigeration system  10  includes other components, such as a receiver, a filter dryer, etc. The refrigeration system  10  includes a controller  34  for controlling operation of the linear compressor  14  and operable to switch the linear compressor  14  between the economizer cycle (shown in  FIG. 2 ) and a single stage cycle (shown in  FIG. 3 ). In an alternative embodiment, one controller operates the linear compressor and another controller operates to switch the linear compressor  14  between the economizer cycle and the single stage cycle. 
   In general, compressed refrigerant discharged from the linear compressor  14  travels to the condenser  18  through a condenser line  38 . After leaving the condenser  18 , the refrigerant next travels to the economizer  22  located upstream of the evaporator  30  through a refrigerant line  42  that divides into a first line  46  and a second line  50 . Refrigerant directed to the first line  46  passes through a first side  54  of the economizer  22  by way of a heat exchanger element (not shown) to the evaporator  30 . After the refrigerant passes through the evaporator  30 , the refrigerant is delivered to the linear compressor  14  through an evaporator line  56 . 
   When the linear compressor  14  is in the economizer cycle, a portion of the refrigerant is diverted to travel through the second line  50 . The second line  50  is fluidly connected to the expansion valve  26 . Refrigerant directed to the second line  50  passes through the expansion valve  26 , through a second side  58  of the economizer  22 , and out to an economizer line  62 . Refrigerant that passes through the second side  58  of the economizer  22  is used to cool refrigerant that passes through the first side  54  of the economizer  22 . The economizer line  62  delivers refrigerant to the linear compressor  14 . In another embodiment, the refrigerant line  42  divides into a first line and a second line after the refrigerant exits the first side  54  of the economizer  22 . The first line directs refrigerant to the evaporator  30  and the second line directs refrigerant through the expansion valve  26  and to the second side  58  of the economizer  22 . 
   A schematic of the dual-opposed piston linear compressor  14  is shown in  FIGS. 1–3 . The linear compressor  14  includes a first cylinder  66  and a second cylinder  70  separated by a dividing wall  74 . A primary piston  78  is disposed in the first cylinder  66  and divides the first cylinder  66  into a suction chamber  82  and a discharge chamber  86 . The primary piston  78  is secured to a spring  90 . Refrigerant enters the suction chamber  82  of the first cylinder  66  from a refrigerant flow path and is discharged from the discharge chamber  86  of the first cylinder  66  to a refrigerant flow path (e.g, the economizer gas flow path  16  shown in  FIG. 2  or a single stage gas flow path  98  shown in  FIG. 3 ). 
   A secondary, or economizer, piston  102  is disposed in the second cylinder  70  and divides the second cylinder  70  into a suction chamber  106  and a discharge chamber  110 . The secondary piston  102  is secured to a spring  114 . The primary and secondary pistons  78 ,  102  are opposed and each piston moves back and forth in its respective cylinder in opposite directions of movement. Refrigerant enters the suction chamber  106  of the second cylinder  70  from a refrigerant flow path and is discharged from the discharge chamber  110  of the second cylinder  70  to a refrigerant flow path (e.g, the economizer gas flow path  16  shown in  FIG. 2  or the single stage gas flow path  98  shown in  FIG. 3 ). In the illustrated embodiment, the controller  34  controls piston stroke of the primary and secondary pistons  78 ,  102  within the first and second cylinders  66 ,  70 . A linear motor (shown in  FIG. 4 ) for each piston is coupled to the controller  34  and responsive to control signals from the controller  34 . 
   The controller  34  switches the linear compressor  14  between economizer operation ( FIG. 2 ) and single stage operation ( FIG. 3 ) by actuating appropriate control valves  118 A and  118 B. The control valve  118 A is positioned in the refrigerant line between the condenser line  38  and a discharge line  122  proximate the linear compressor  14 . The control valve  118 A includes three ports, one port communicating with the condenser line  38  and two ports communicating with the discharge line  122 . The control valve  118 B is positioned in the refrigerant line between the evaporator line  56  and the economizer line  62 . The control valve  118 B includes three ports, one port communicating with the refrigerant line to the secondary piston suction chamber  106 , one port communicating with the evaporator line  56 , and one port communicating with the economizer line  62 . In the illustrated embodiment, two, three-way valves are shown, however, in further embodiments fewer or more valves and valves of different configurations may be used to direct refrigerant along one of the at least two refrigerant flow paths For example, four two-way valves or a dual switching valve may be used. 
   In the single stage cycle, refrigerant flows along the single stage gas flow path  98 , shown by the bold line in  FIG. 3 . The linear compressor compresses refrigerant in a single step, whereby the refrigerant is compressed by the primary and secondary pistons  78 ,  102 , with gas flow in parallel. The control valves  118 A and  118 B are actuated to direct refrigerant along the single stage gas flow path  98 . The control valve  118 A is actuated to a first position (shown in  FIG. 3 ) to permit refrigerant to flow from the primary piston discharge chamber  86  to the condenser line  38  and the control valve  118 B is actuated to a first position (shown in  FIG. 3 ) to permit refrigerant to flow from the evaporator line  56  to the secondary piston suction chamber  106 . 
   In the single stage cycle, the suction chambers  82 ,  106  for the primary and secondary pistons  78 ,  102  receive refrigerant through the evaporator line  56  and the pistons  78 ,  102  compress the refrigerant, which increases the temperature and pressure of the refrigerant. The compressed refrigerant is discharged from the discharge chambers  86 ,  110  for the primary and secondary pistons  78 ,  102  as a high-temperature, high-pressure heated gas to the condenser line  38 . The refrigerant travels to the condenser  18  and the condenser  18  changes the refrigerant from a high-temperature gas to a warm-temperature liquid. Air and/or liquid, such as water, are generally used to cause this transformation in the condenser  18 . 
   The high-pressure liquid refrigerant then travels to the economizer  22  through the first line  46 . In the single stage cycle, the control valve  118 B is actuated to the first position to prevent refrigerant from traveling through the second line  50 , and thereby the economizer line  62 . Therefore, the entire refrigerant from the condenser  18  is directed to the first line  46 , through the economizer  22  and to the evaporator  30 . In other arrangements the refrigeration system  10  can also include a receiver positioned prior to the economizer  22  for storing refrigerant before the refrigerant is provided to the economizer  22 . 
   When the linear compressor  14  is operating as a single-stage compressor (shown in  FIG. 3 ), the warm-temperature, high-pressure liquid refrigerant passes through the heat exchanger (not shown) on the first side  54  of the economizer  22 , which generally does not change the state of the refrigerant. The warm refrigerant then enters the evaporator  30 , which cools environmental spaces storing a commodity (not shown). In some embodiments, a second expansion device can be positioned between the economizer  22  and the evaporator  30  for controlling or metering the proper amount of refrigerant into the evaporator  30 . In some constructions, air (e.g., a fan) and/or a liquid can be used with the evaporator  30  to promote the cooling action of the environmental spaces. After leaving the evaporator  30 , the cool refrigerant re-enters the suction chambers  82 ,  106  of the linear compressor  14  to be pressurized again and the cycle repeats. 
   In the economizer cycle, refrigerant flows along the economizer gas flow path  16 , shown by the bold line in  FIG. 2 . The linear compressor  14  compresses refrigerant in a two step process, whereby the refrigerant is compressed first by the primary piston  78  and subsequently by the secondary piston  102 . The control valves  118 A and  118 B are actuated to direct refrigerant along the economizer gas flow path  16 . The control valve  118 A is actuated to a second position (shown in  FIG. 2 ) to permit refrigerant to flow from the primary piston discharge chamber  86  to the discharge line  122  and the control valve  118 B is actuated to a second position (shown in  FIG. 2 ) to permit refrigerant to flow from the economizer line  62  to the secondary piston suction chamber  106 . 
   The suction chamber  82  for the primary piston  78  receives refrigerant from the evaporator line  56 , and the discharge chamber  86  for the primary piston  78  discharges refrigerant to the discharge line  122  that feeds the economizer line  62 . The suction chamber  106  for the secondary piston  102  receives refrigerant from the economizer line  62 , which includes refrigerant from both the primary piston chamber  86  and the economizer  22 , and the discharge chamber  110  for the secondary piston  102  discharges refrigerant to the condenser line  38 . 
   In the illustrated embodiment, after being discharged from the primary piston discharge chamber  86 , the refrigerant passes through an air-cooled de-superheater  126 . The de-superheater  126  cools the refrigerant before it is mixed with refrigerant from the economizer line  62 . Therefore, the mixed refrigerant entering the secondary piston suction chamber  106  will be cooler, which reduces the work required for the second stage compression by the secondary piston  102 . In further embodiments, the de-superheater uses natural convection or water to cool the refrigerant, or no de-superheater is used. 
   In the economizer cycle, the suction chamber  82  for the primary piston  78  receives cool refrigerant through the evaporator line  56  and the primary piston  78  compresses the refrigerant, which increases the temperature and pressure of the refrigerant. The compressed refrigerant is discharged from the discharge chamber  86  for the primary piston  78  as a warm-temperature, medium-pressure heated gas to the discharge line  122 . Low-temperature, medium-pressure refrigerant gas from the economizer  22  is mixed with the discharged gas from the primary piston chamber  86  in the economizer line  62 . The mixed refrigerant enters the suction chamber  106  of the secondary piston  102  from the economizer line  62 . Mixing the refrigerant from the primary piston chamber  86  with the refrigerant from the economizer  22  lowers the temperature of the refrigerant entering the secondary piston suction chamber  106 , which prevents overheating of the linear compressor. The secondary piston  102  compresses the mixed refrigerant, which increases the temperature and pressure of the refrigerant. The compressed refrigerant is discharged from the discharge chamber  110  of the secondary piston  102  as a high-temperature, high-pressure heated gas to the condenser line  38 . 
   The refrigerant travels to the condenser  18  and the condenser  18  changes the refrigerant from a high-temperature gas to a warm-temperature liquid. The high-pressure liquid refrigerant then travels to the economizer  22  through the refrigerant line  42 . In the economizer cycle, the control valve  118 B is actuated to the second position to divert refrigerant from the refrigerant line  42  to the second line  50 . A portion of the refrigerant is directed to the first line  46  through the first side  54  of the economizer  22  and the remaining refrigerant is directed to the second line  50  through the second side  58  of the economizer  22 . 
   The warm-temperature, high-pressure liquid refrigerant that passes through the heat exchanger (not shown) on the first side  54  of the economizer  22  and is cooled further to a cool-temperature liquid refrigerant. Warm-temperature, high-pressure gas/liquid refrigerant from the second line  50  passes through the expansion valve  26 , which creates a pressure drop between the two refrigerant lines  46 ,  50 . Low-temperature, medium-pressure refrigerant exits the expansion valve  26  and passes through the second side  58  of the economizer  22 , which cools the refrigerant passing through the first side  54  of the economizer  22 . 
   In the illustrated embodiment, the expansion valve  26  is a thermal expansion valve controlled by temperature and pressure at the outlet of the second side  58  of the economizer  22 , i.e., the refrigerant temperature and pressure in the economizer line  62 . In a further embodiment, the expansion valve  26  is an electronic valve controlled by the controller  34  based upon measured interstage and/or discharge temperature. 
   The refrigerant from the first side  54  of the economizer  22  enters the evaporator  30  and cools commodities stored in the environmental spaces (not shown). After leaving the evaporator  30 , the cool refrigerant re-enters the suction chamber  82  of the primary piston  78  to be pressurized again and the cycle repeats. The refrigerant from the second side  58  of the economizer  22  enters the economizer line  62  to be mixed with the gas discharged from the discharge chamber  86  of the primary piston  78 . The mixed refrigerant enters the suction chamber  106  for the secondary piston  102  from the economizer line  62  to be pressurized again. 
   To determine whether to operate the linear compressor  14  in the economizer cycle, the controller  34  calculates an overall compression ratio of the linear compressor  14 , i.e., the pressure ratio between the condensing pressure and the main cooling load&#39;s evaporating pressure. When an overall compression ratio is greater than a pre-determined value, the linear compressor  14  operates in the economizer cycle. For example, in one embodiment the pre-determined value for the overall compression ratio is between about 2:1 and about 10:1, and is preferably about 5:1. 
   If the linear compressor  14  is operating in the single stage cycle, the controller  34  switches operation of the linear compressor  14  to the economizer cycle by actuating the control valves  118 A and  118 B to the first position to direct refrigerant along the single stage gas flow path  98 . When the overall compression ratio falls below the pre-determined value, the controller  34  switches operation of the linear compressor  14  to the single stage cycle by actuating the control valves  118 A and  118 B to the second position to direct refrigerant along the economizer gas flow path  16 . In one embodiment, the pre-determined value is within a “dead band” where the linear compressor  14  operates in either the economizer cycle or the single stage cycle. Within the “dead band” the control point for switching cycles depends on whether the overall compression ratio is increasing (i.e., switch to the economizer cycle) or decreasing (i.e, switch to the single stage cycle). In another embodiment, the overall compression ratio is calculated based upon secondary discharge pressure and primary suction pressure, however, in further embodiments, other measurements from the refrigeration system  10  are used to determine whether operation in the economizer cycle is necessary. 
   An economizer cycle is typically more effective at relatively high compression ratios, such as the compression ratios found in low temperature refrigeration, i.e., below 0° F. evaporating. Generally, at higher evaporating temperatures, single stage compression without the economizer cycle is used. An economizer cycle provides more efficient operation of the refrigeration system and cooling of the evaporator. In the economizer cycle, the compression process is split into two stages. The combined compression ratio of the primary and secondary pistons is substantially equal to the compression ratio in the single stage cycle. However, in the economizer cycle compression is a two step process. Because individual compression of the pistons remains relatively low, there is less wear on the pistons and less leakage occurs. 
   In a further embodiment of the linear compressor, the primary piston  78  has a larger displacement than the secondary piston  102  to increase the compression ratio of the first stage of the linear compressor  14  (i.e., by the primary piston  78 ) and increase the density of the refrigerant discharged from the first stage of the linear compressor  14  (i.e., from the discharge chamber  86 ). For example, the primary piston  78  has a larger diameter than the secondary piston  102  or the primary piston  78  has a longer piston stroke than the secondary piston  102 . In one embodiment, piston stroke of the primary and secondary pistons  78 ,  102  is controlled by the controller  34 . 
   One embodiment of a dual-opposed piston linear compressor  140  is shown in  FIG. 4  at an intake stroke. The dual-opposed piston linear compressor  140  includes a housing  144  supporting a main body block  148 . Inner and outer laminations  152  and  156  are secured to the main body block  148  and coils  160  are wound on the outer laminations  156 , thereby resulting in stators. The stators, when energized, interact with magnet rings  164  mounted on outer cylinders  168 . The outer cylinders  168  are fastened to a first piston  172  and a second piston  176 , which are secured to springs  180 . The interaction between the magnet rings  164  and the energized stators results in the outer cylinders  168  moving the pistons  172 ,  176  linearly along an axis of reciprocation  184 . 
   A dividing wall  188  separates the first piston  172  and the second piston  176  into a first chamber  192  and a second chamber  196 , respectively. Each chamber includes a suction portion  192 A and  196 A and a compression portion  192 B and  196 B, or discharge portion. When the first and second pistons  172 ,  176  are at the intake stroke, refrigerant is allowed to flow from a suction port  200  at the suction portion  192 A,  196 A of each chamber  192 ,  196  through channels  204  to the compression chambers  192 B,  196 B. When moving from the intake stroke to a compression stroke, the channels  204  are closed by suction valves  208  and refrigerant is compressed out of the compression chambers  192 B,  196 B through discharge valves  212  and discharge ports  216 . 
   The linear motor allows for variable stroke by the pistons, and therefore, the linear compressor provides variable capacity control. In other words, the linear motors can cause the pistons to move a small stroke for a first volume, or to move a larger stroke for a second, larger volume. 
     FIG. 5  illustrates a two-stage linear compressor  220  that operates in an economizer cycle, but is not switchable to a single stage cycle. The linear compressor  220  may be used in the refrigeration system  10  discussed above with respect to  FIG. 1 . The linear compressor  220  includes a first cylinder  224  and a second cylinder  228  separated by a dividing wall  232 . A primary piston  236  is disposed in the first cylinder  224  and divides the first cylinder  224  into a suction chamber  240  and a discharge chamber  244 . The primary piston  236  is secured to a spring  248 . Refrigerant enters the suction chamber  240  of the first cylinder  224  from an evaporator line  252  and is discharged from the discharge chamber  244  of the first cylinder  224  to a discharge line  256 . The evaporator line  252  delivers refrigerant from an evaporator (not shown) to the suction chamber  240  of the first cylinder  224 . 
   A secondary, or economizer, piston  260  is disposed in the second cylinder  228  and divides the second cylinder  228  into a suction chamber  264  and a discharge chamber  268 . The secondary piston  260  is secured to a spring  272 . The primary and secondary pistons  236 ,  260  are opposed and each piston moves back and forth in its respective cylinder in opposite directions of movement. Refrigerant enters the suction chamber  264  of the second cylinder  228  from the discharge line  256  and is discharged from the discharge chamber  268  of the second cylinder  228  to a condenser line  276  that delivers the refrigerant to a condenser (not shown). In the illustrated embodiment, a controller  280  controls piston stroke of the primary and secondary pistons  236 ,  260  within the first and second cylinders  224 ,  228 . A linear motor (shown in  FIG. 4 ) for each piston is coupled to the controller  280  and responsive to control signals from the controller  280 . 
   The linear compressor  220  illustrated in  FIG. 5  operates in the economizer cycle and compresses refrigerant in a two step process, whereby the refrigerant is compressed first by the primary piston  236  and subsequently by the secondary piston  260 . The suction chamber  240  for the primary piston  236  receives refrigerant from the evaporator line  252 , and the discharge chamber  244  for the primary piston  236  discharges refrigerant to the discharge line  256  that feeds an economizer line  284 . The refrigerant passes through a de-superheater  288  to cool the refrigerant before it is mixed with refrigerant from the economizer line  284 . The suction chamber  264  for the secondary piston  260  receives refrigerant from the economizer line  284 , which includes refrigerant from both the primary piston chamber  244  and an economizer (not shown). The discharge chamber  268  for the secondary piston  260  discharges refrigerant to the condenser line  276 . 
   Various features and advantages of the invention are set forth in the following claims.