Patent Publication Number: US-2018051685-A1

Title: Compressor with a discharge valve

Description:
FIELD OF THE INVENTION 
     The present subject matter relates generally to compressors and discharge valves for compressors. 
     BACKGROUND OF THE INVENTION 
     Certain refrigerator appliances include sealed systems for cooling chilled chambers of the refrigerator appliance. The sealed systems generally include a compressor that generates compressed refrigerant during operation of the sealed system. The compressed refrigerant flows to an evaporator where heat exchange between the chilled chambers and the refrigerant cools the chilled chambers and food items located therein. 
     Recently, certain refrigerator appliances have included linear compressors for compressing refrigerant. Linear compressors generally include a piston and a driving coil. The driving coil receives a current that generates a force for sliding the piston forward and backward within a chamber. During motion of the piston within the chamber, the piston compresses refrigerant. A discharge valve regulates a flow of pressured refrigerant from the chamber. 
     Over-pressurization of the chamber can negatively affect performance of the linear compressor, and the discharge valve design frequently exacerbates the over-pressurization. In particular, a mass of the discharge valve can require a cylinder pressure to exceed a discharge muffler pressure by a certain margin before the discharge valve opens. A high mass discharge valve responds slowly and increases the amount of work required to open the discharge valve. However, high mass is generally required to provide a discharge valve that covers the chamber at an end of the cylinder and thereby allow the piston to run and “crash” into the discharge valve without damaging the linear compressor. Thus, a full cylinder diameter discharge valve is disadvantageously massive but has other benefits. 
     Accordingly, a linear compressor with a discharge valve having features limits over-pressurization of a chamber while also permitting crashing of a piston would be useful. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The present subject matter provides a compressor. The compressor includes a discharge valve with a first valve head and a second valve head. The first valve head defines a passage that extends through the first valve head along an axial direction. A first spring urges the first valve head towards a casing. A second valve head is positioned at the passage of the first valve head. A second spring urges the second valve head towards the first valve head. Additional aspects and advantages of the invention will be set forth in part in the following description, or may be apparent from the description, or may be learned through practice of the invention. 
     In a first exemplary embodiment, a compressor is provided. The compressor includes a casing that defines a chamber. A piston is disposed within the chamber of the casing. The piston is reciprocable within the chamber of the casing along an axial direction. A discharge valve includes a housing. A first valve head is positioned adjacent the chamber of the casing. The first valve head has a width along a radial direction that is perpendicular to the axial direction. The first valve head also defines a passage that extends through the first valve head along the axial direction. A first spring is coupled to the housing and the first valve head such that the first spring urges the first valve head towards the casing. A second valve head is positioned at the passage of the first valve head. The second valve head has a width along the radial direction. The width of the second valve head is less than the width of the first valve head. A second spring is coupled to the second valve head such that the second spring urges the second valve head towards the first valve head. 
     In a second exemplary embodiment, a compressor is provided. The compressor includes a casing that defines a chamber. A piston is disposed within the chamber of the casing. The piston is reciprocable within the chamber of the casing along an axial direction. A discharge valve includes a housing. A first valve head is positioned adjacent the chamber of the casing. The first valve head has a mass. The first valve head also defines a passage that extends through the first valve head along the axial direction. A first spring is coupled to the housing and the first valve head such that the first spring urges the first valve head towards the casing. The first spring has a stiffness. A second valve head is positioned at the passage of the first valve head. The second valve head has a mass. The mass of the second valve head is less than the mass of the first valve head. A second spring is coupled to the second valve head such that the second spring urges the second valve head towards the first valve head. The second spring has a stiffness. The stiffness of the second spring is less than the stiffness of the first spring. 
     In a third exemplary embodiment, a compressor is provided. The compressor includes a casing that defines a chamber. A piston is disposed within the chamber of the casing. The piston is reciprocable within the chamber of the casing along an axial direction. A discharge valve includes a housing. A valve head is positioned adjacent the chamber of the casing. The valve head defines a passage that extends through the valve head along the axial direction. A spring is coupled to the housing and the valve head such that the spring urges the valve head towards the casing. A reed is positioned at the passage of the valve head. The reed is mounted to the valve head such that the reed is cantilevered over the passage of the valve head. 
     These and other features, aspects and advantages of the present invention will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended figures. 
         FIG. 1  is a front elevation view of a refrigerator appliance according to an exemplary embodiment of the present subject matter. 
         FIG. 2  is schematic view of certain components of the exemplary refrigerator appliance of  FIG. 1 . 
         FIG. 3  provides a section view of a linear compressor according to an exemplary embodiment of the present subject matter. 
         FIG. 4  provides a partial, section view of a discharge valve on a linear compressor according to an exemplary embodiment of the present subject matter. 
         FIG. 5  provides a section view of a discharge valve according to an exemplary embodiment of the present subject matter. 
         FIG. 6  provides a section view of a discharge valve according to another exemplary embodiment of the present subject matter. 
         FIG. 7  provides a perspective view of components of a discharge valve according to an additional exemplary embodiment of the present subject matter. 
         FIG. 8  provides a section view of the components of the discharge valve of  FIG. 7 . 
         FIG. 8  provides an exploded view of the components of the discharge valve of  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION 
     Reference now will be made in detail to embodiments of the invention, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents. 
       FIG. 1  depicts a refrigerator appliance  10  that incorporates a sealed refrigeration system  60  ( FIG. 2 ). It should be appreciated that the term “refrigerator appliance” is used in a generic sense herein to encompass any manner of refrigeration appliance, such as a freezer, refrigerator/freezer combination, and any style or model of conventional refrigerator. In addition, it should be understood that the present subject matter is not limited to use in appliances. Thus, the present subject matter may be used for any other suitable purpose, such as vapor compression within air conditioning units or air compression within air compressors. 
     In the illustrated exemplary embodiment shown in  FIG. 1 , the refrigerator appliance  10  is depicted as an upright refrigerator having a cabinet or casing  12  that defines a number of internal chilled storage compartments. In particular, refrigerator appliance  10  includes upper fresh-food compartments  14  having doors  16  and lower freezer compartment  18  having upper drawer  20  and lower drawer  22 . The drawers  20  and  22  are “pull-out” drawers in that they can be manually moved into and out of the freezer compartment  18  on suitable slide mechanisms. 
       FIG. 2  is a schematic view of certain components of refrigerator appliance  10 , including a sealed refrigeration system  60  of refrigerator appliance  10 . A machinery compartment  62  contains components for executing a known vapor compression cycle for cooling air. The components include a compressor  64 , a condenser  66 , an expansion device  68 , and an evaporator  70  connected in series and charged with a refrigerant. As will be understood by those skilled in the art, refrigeration system  60  may include additional components, e.g., at least one additional evaporator, compressor, expansion device, and/or condenser. As an example, refrigeration system  60  may include two evaporators. 
     Within refrigeration system  60 , refrigerant flows into compressor  64 , which operates to increase the pressure of the refrigerant. This compression of the refrigerant raises its temperature, which is lowered by passing the refrigerant through condenser  66 . Within condenser  66 , heat exchange with ambient air takes place so as to cool the refrigerant. A fan  72  is used to pull air across condenser  66 , as illustrated by arrows A C , so as to provide forced convection for a more rapid and efficient heat exchange between the refrigerant within condenser  66  and the ambient air. Thus, as will be understood by those skilled in the art, increasing air flow across condenser  66  can, e.g., increase the efficiency of condenser  66  by improving cooling of the refrigerant contained therein. 
     An expansion device (e.g., a valve, capillary tube, or other restriction device)  68  receives refrigerant from condenser  66 . From expansion device  68 , the refrigerant enters evaporator  70 . Upon exiting expansion device  68  and entering evaporator  70 , the refrigerant drops in pressure. Due to the pressure drop and/or phase change of the refrigerant, evaporator  70  is cool relative to compartments  14  and  18  of refrigerator appliance  10 . As such, cooled air is produced and refrigerates compartments  14  and  18  of refrigerator appliance  10 . Thus, evaporator  70  is a type of heat exchanger which transfers heat from air passing over evaporator  70  to refrigerant flowing through evaporator  70 . 
     Collectively, the vapor compression cycle components in a refrigeration circuit, associated fans, and associated compartments are sometimes referred to as a sealed refrigeration system operable to force cold air through compartments  14 ,  18  ( FIG. 1 ). The refrigeration system  60  depicted in  FIG. 2  is provided by way of example only. Thus, it is within the scope of the present subject matter for other configurations of the refrigeration system to be used as well. 
       FIG. 3  provides a section view of a linear compressor  100  according to an exemplary embodiment of the present subject matter. As discussed in greater detail below, linear compressor  100  is operable to increase a pressure of fluid within a chamber  112  of linear compressor  100 . Linear compressor  100  may be used to compress any suitable fluid, such as refrigerant or air. In particular, linear compressor  100  may be used in a refrigerator appliance, such as refrigerator appliance  10  ( FIG. 1 ) in which linear compressor  100  may be used as compressor  64  ( FIG. 2 ). As may be seen in  FIG. 3 , linear compressor  100  defines an axial direction A, a radial direction R and a circumferential direction C. Linear compressor  100  may be enclosed within a hermetic or air-tight shell (not shown). The hermetic shell can, e.g., hinder or prevent refrigerant from leaking or escaping from refrigeration system  60 . 
     Turning now to  FIG. 3 , linear compressor  100  includes a casing  110  that extends between a first end portion  102  and a second end portion  104 , e.g., along the axial direction A. Casing  110  includes various static or non-moving structural components of linear compressor  100 . In particular, casing  110  includes a cylinder assembly  111  that defines a chamber  112 . Cylinder assembly  111  is positioned at or adjacent second end portion  104  of casing  110 . Chamber  112  extends longitudinally along the axial direction A. Casing  110  also includes a motor mount mid-section  113  and an end cap  115  positioned opposite each other about a motor. A stator, e.g., including an outer back iron  150  and a driving coil  152 , of the motor is mounted or secured to casing  110 , e.g., such that the stator is sandwiched between motor mount mid-section  113  and end cap  115  of casing  110 . Linear compressor  100  also includes valves (such as a discharge valve assembly  117  at an end of chamber  112 ) that permit refrigerant to enter and exit chamber  112  during operation of linear compressor  100 . 
     A piston assembly  114  with a piston head  116  is slidably received within chamber  112  of cylinder assembly  111 . In particular, piston assembly  114  is slidable along the axial direction A. During sliding of piston head  116  within chamber  112 , piston head  116  compresses refrigerant within chamber  112 . As an example, from a top dead center position, piston head  116  can slide within chamber  112  towards a bottom dead center position along the axial direction A, i.e., an expansion stroke of piston head  116 . When piston head  116  reaches the bottom dead center position, piston head  116  changes directions and slides in chamber  112  back towards the top dead center position, i.e., a compression stroke of piston head  116 . It should be understood that linear compressor  100  may include an additional piston head and/or additional chamber at an opposite end of linear compressor  100 . Thus, linear compressor  100  may have multiple piston heads in alternative exemplary embodiments. 
     As may be seen in  FIG. 3 , linear compressor  100  also includes an inner back iron assembly  130 . Inner back iron assembly  130  is positioned in the stator of the motor. In particular, outer back iron  150  and/or driving coil  152  may extend about inner back iron assembly  130 , e.g., along the circumferential direction C. Inner back iron assembly  130  also has an outer surface  137 . At least one driving magnet  140  is mounted to inner back iron assembly  130 , e.g., at outer surface  137  of inner back iron assembly  130 . Driving magnet  140  may face and/or be exposed to driving coil  152 . In particular, driving magnet  140  may be spaced apart from driving coil  152 , e.g., along the radial direction R by an air gap. Thus, the air gap may be defined between opposing surfaces of driving magnet  140  and driving coil  152 . Driving magnet  140  may also be mounted or fixed to inner back iron assembly  130  such that an outer surface of driving magnet  140  is substantially flush with outer surface  137  of inner back iron assembly  130 . Thus, driving magnet  140  may be inset within inner back iron assembly  130 . In such a manner, the magnetic field from driving coil  152  may have to pass through only a single air gap between outer back iron  150  and inner back iron assembly  130  during operation of linear compressor  100 , and linear compressor  100  may be more efficient relative to linear compressors with air gaps on both sides of a driving magnet. 
     As may be seen in  FIG. 3 , driving coil  152  extends about inner back iron assembly  130 , e.g., along the circumferential direction C. Driving coil  152  is operable to move the inner back iron assembly  130  along the axial direction A during operation of driving coil  152 . As an example, a current may be induced within driving coil  152  by a current source (not shown) to generate a magnetic field that engages driving magnet  140  and urges piston assembly  114  to move along the axial direction A in order to compress refrigerant within chamber  112  as described above and will be understood by those skilled in the art. In particular, the magnetic field of driving coil  152  may engage driving magnet  140  in order to move inner back iron assembly  130  and piston head  116  along the axial direction A during operation of driving coil  152 . Thus, driving coil  152  may slide piston assembly  114  between the top dead center position and the bottom dead center position, e.g., by moving inner back iron assembly  130  along the axial direction A, during operation of driving coil  152 . 
     Linear compressor  100  may include various components for permitting and/or regulating operation of linear compressor  100 . In particular, linear compressor  100  includes a controller (not shown) that is configured for regulating operation of linear compressor  100 . The controller is in, e.g., operative, communication with the motor, e.g., driving coil  152  of the motor. Thus, the controller may selectively activate driving coil  152 , e.g., by inducing current in driving coil  152 , in order to compress refrigerant with piston assembly  114  as described above. 
     The controller includes memory and one or more processing devices such as microprocessors, CPUs or the like, such as general or special purpose microprocessors operable to execute programming instructions or micro-control code associated with operation of linear compressor  100 . The memory can represent random access memory such as DRAM, or read only memory such as ROM or FLASH. The processor executes programming instructions stored in the memory. The memory can be a separate component from the processor or can be included onboard within the processor. Alternatively, the controller may be constructed without using a microprocessor, e.g., using a combination of discrete analog and/or digital logic circuitry (such as switches, amplifiers, integrators, comparators, flip-flops, AND gates, and the like) to perform control functionality instead of relying upon software. 
     Linear compressor  100  also includes a spring  120 . Spring  120  is positioned in inner back iron assembly  130 . In particular, inner back iron assembly  130  may extend about spring  120 , e.g., along the circumferential direction C. Spring  120  also extends between first and second end portions  102  and  104  of casing  110 , e.g., along the axial direction A. Spring  120  assists with coupling inner back iron assembly  130  to casing  110 , e.g., cylinder assembly  111  of casing  110 . In particular, inner back iron assembly  130  is fixed to spring  120  at a middle portion of spring  120  as discussed in greater detail below. 
     During operation of driving coil  152 , spring  120  supports inner back iron assembly  130 . In particular, inner back iron assembly  130  is suspended by spring  120  within the stator or the motor of linear compressor  100  such that motion of inner back iron assembly  130  along the radial direction R is hindered or limited while motion along the axial direction A is relatively unimpeded. Thus, spring  120  may be substantially stiffer along the radial direction R than along the axial direction A. In such a manner, spring  120  can assist with maintaining a uniformity of the air gap between driving magnet  140  and driving coil  152 , e.g., along the radial direction R, during operation of the motor and movement of inner back iron assembly  130  on the axial direction A. Spring  120  can also assist with hindering side pull forces of the motor from transmitting to piston assembly  114  and being reacted in cylinder assembly  111  as a friction loss. 
     Inner back iron assembly  130  includes an outer cylinder  136  and a sleeve  139 . Outer cylinder  136  defines outer surface  137  of inner back iron assembly  130  and also has an inner surface  138  positioned opposite outer surface  137  of outer cylinder  136 . Sleeve  139  is positioned on or at inner surface  138  of outer cylinder  136 . A first interference fit between outer cylinder  136  and sleeve  139  may couple or secure outer cylinder  136  and sleeve  139  together. In alternative exemplary embodiments, sleeve  139  may be welded, glued, fastened, or connected via any other suitable mechanism or method to outer cylinder  136 . 
     Sleeve  139  extends about spring  120 , e.g., along the circumferential direction C. In addition, a middle portion of spring  120  is mounted or fixed to inner back iron assembly  130  with sleeve  139 . Sleeve  139  extends between inner surface  138  of outer cylinder  136  and the middle portion of spring  120 , e.g., along the radial direction R. A second interference fit between sleeve  139  and the middle portion of spring  120  may couple or secure sleeve  139  and the middle portion of spring  120  together. In alternative exemplary embodiments, sleeve  139  may be welded, glued, fastened, or connected via any other suitable mechanism or method to the middle portion of spring  120 . 
     Outer cylinder  136  may be constructed of or with any suitable material. For example, outer cylinder  136  may be constructed of or with a plurality of (e.g., ferromagnetic) laminations. The laminations are distributed along the circumferential direction C in order to form outer cylinder  136  and are mounted to one another or secured together, e.g., with rings pressed onto ends of the laminations. Outer cylinder  136  defines a recess that extends inwardly from outer surface  137  of outer cylinder  136 , e.g., along the radial direction R. Driving magnet  140  is positioned in the recess on outer cylinder  136 , e.g., such that driving magnet  140  is inset within outer cylinder  136 . 
     A piston flex mount  160  is mounted to and extends through inner back iron assembly  130 . In particular, piston flex mount  160  is mounted to inner back iron assembly  130  via sleeve  139  and spring  120 . Thus, piston flex mount  160  may be coupled (e.g., threaded) to spring  120  at the middle portion of spring  120  in order to mount or fix piston flex mount  160  to inner back iron assembly  130 . A coupling  170  extends between piston flex mount  160  and piston assembly  114 , e.g., along the axial direction A. Thus, coupling  170  connects inner back iron assembly  130  and piston assembly  114  such that motion of inner back iron assembly  130 , e.g., along the axial direction A, is transferred to piston assembly  114 . Coupling  170  may extend through driving coil  152 , e.g., along the axial direction A. 
     Coupling  170  may be a compliant coupling that is compliant or flexible along the radial direction R. In particular, coupling  170  may be sufficiently compliant along the radial direction R such that little or no motion of inner back iron assembly  130  along the radial direction R is transferred to piston assembly  114  by coupling  170 . In such a manner, side pull forces of the motor are decoupled from piston assembly  114  and/or cylinder assembly  111  and friction between piston assembly  114  and cylinder assembly  111  may be reduced. 
     Piston flex mount  160  defines at least one suction gas inlet  162 . Suction gas inlet  162  of piston flex mount  160  extends, e.g., along the axial direction A, through piston flex mount  160 . Thus, a flow of fluid, such as air or refrigerant, may pass through piston flex mount  160  via suction gas inlet  162  of piston flex mount  160  during operation of linear compressor  100 . 
     Piston head  116  also defines at least one opening  118 . Opening  118  of piston head  116  extends, e.g., along the axial direction A, through piston head  116 . Thus, the flow of fluid may pass through piston head  116  via opening  118  of piston head  116  into chamber  112  during operation of linear compressor  100 . In such a manner, the flow of fluid (that is compressed by piston head  116  within chamber  112 ) may flow through piston flex mount  160  and inner back iron assembly  130  to piston assembly  114  during operation of linear compressor  100 . 
       FIG. 4  provides a partial, section view of a discharge valve  200  according to an exemplary embodiment of the present subject matter. Discharge valve  200  is described in greater detail below in the context of linear compressor  100 . Thus, discharge valve  200  may be used as discharge valve assembly  117 . However, it should be understood that discharge valve  200  may be used in or with any suitable compressor in alternative exemplary embodiments, e.g., to regulate pressurized fluid flow from a chamber. As discussed in greater detail below, discharge valve  200  includes features for limiting over-pressurization of chamber  112  and thereby increasing an efficiency of linear compressor  100 , e.g., by requiring less work to open discharge valve  200  relative to other discharge valves. As may be seen in  FIG. 4 , discharge valve  200  includes a housing  210 , a first valve head  220 , a first spring  230 , a second valve head  240  and a second spring  250 . 
     Housing  210  may include an end wall  212  and a cylindrical side wall  214 . Cylindrical side wall  214  is mounted to end wall  212 , and cylindrical side wall  214  extends from end wall  212 , e.g., along the axial direction A, to cylinder assembly  111  of casing  110 . Housing  210  may be mounted or fixed to casing  110 , and other components of discharge valve  200  may be disposed within housing  210 . For example, a plate  218  of housing  210  at a distal end of cylindrical side wall  214  may be positioned at or on cylinder assembly  111 , and a seal  219  may extend between cylinder assembly  111  and plate  218  of housing  210 , e.g., along the axial direction A, in order to limit fluid leakage at an axial gap between casing  110  and housing  210 . Fasteners (not shown) may extend through plate  218  into casing  110  to mount housing  210  to casing  110 . First valve head  220 , first spring  230 , second valve head  240  and/or second spring  250  may be disposed within housing  210  when housing  210  is mounted to casing  110 . 
     First valve head  220  is positioned at or adjacent chamber  112  of cylinder assembly  111 . First valve head  220  defines a passage  222  that extends through first valve head  220 , e.g., along the axial direction A. Passage  222  may be contiguous with chamber  112 . First spring  230  is coupled to housing  210  and first valve head  220 , and first spring  230  is configured to urge first valve head  220  towards or against cylinder assembly  111 , e.g., along the axial direction A. As shown in  FIG. 4 , one end of first spring  230  may be mounted to end wall  212  of housing  210  at a bracket  216  of end wall  212 , and another end of first spring  230  may be mounted to an outer diameter of a support  224  of first valve head  220 . Thus, first spring  230  may be compressed between end wall  212  (e.g., bracket  216  of end wall  212 ) and first valve head  220  within housing  210 . First spring  230  may be a coil or helical spring in certain exemplary embodiments. 
     Second valve head  240  is positioned at passage  222  of first valve head  220 , e.g., on first valve head  220 . Second spring  250  is coupled to second valve head  240 , and second spring  250  is configured for urging second valve head  240  towards or against first valve head  220 . As shown in  FIG. 4 , discharge valve  200  may include a retainer  260  with a post  262 . Retainer  260  is mounted to first valve head  220 . For example, retainer  260  may be snap-fit, press-fit, ultra-sonically welded, fastened, adhered or otherwise suitable mounted to support  224  of first valve head  220  at an inner diameter of support  224 . Post  262  is positioned at a central portion of retainer  260  and extends, e.g., along the axial direction A, towards second valve head  240 . Second spring  250  may be compressed between post  262  and second valve head  240 . Second spring  250  may be a coil or helical spring in certain exemplary embodiments. 
     First valve head  220  and second valve head  240  are each adjustable between an open position (not shown) and a closed position ( FIG. 4 ). Thus, first valve head  220  and second valve head  240  may be moveable, e.g., along the axial direction A, relative to casing  110 . In particular, during operation of linear compressor, piston assembly  114  reciprocates within chamber  112  and pressurizes fluid, and first and second valve heads  220 ,  240  shift between the open and closed positions. For example, first and second springs  230 ,  250  bias first and second valve heads  220 ,  240  towards the closed position, respectively. Thus, first and second valve heads  220 ,  240  are normally closed. When first valve head  220  is in the closed position, first valve head  220  may be seated against cylinder assembly  111  and thus assist with sealing chamber  112 . Similarly, second valve head  240  may be seated against first valve head  220 , e.g., opposite chamber  112 , when second valve head  240  is in the closed position. Thus, when first and second valve heads  220 ,  240  are both closed, discharge valve  200  may seal chamber  112  and thereby assist with pressurization of fluid due to motion of piston assembly  114  within chamber  112 . 
     When the fluid in chamber  112  reaches a first threshold pressure, second valve head  240  may open. For example, fluid within chamber  112  may apply a force onto second valve head  240  that overcomes the force applied to second valve head  240  by second spring  250  such that second valve head  240  moves, e.g., along the axial direction A, away from first valve head  220  to the open position. When second valve head  240  is in the open position, fluid from chamber  112  may flow through passage  222  out of chamber  112  and into housing  210 . 
     First valve head  220  may open when the fluid in chamber  112  reaches a second threshold pressure, e.g., that is greater than the first threshold pressure. For example, fluid within chamber  112  may apply a force onto first valve head  220  that overcomes the force applied to first valve head  220  by first spring  230  such that first valve head  220  moves, e.g., along the axial direction A, away from cylinder assembly  111  to the open position. When first valve head  220  is in the open position, fluid from chamber  112  may flow through out of chamber  112  through an axial gap between first valve head  220  and cylinder assembly  111  into housing  210 . 
     First valve head  220  may also move from the closed position to the open position when piston assembly  114  strikes or impacts first valve head  220 . Thus, second valve head  240  may correspond to a flow path for pressurized fluid from chamber  112  during normal operation of linear compressor  100 , and first valve head  220  may correspond to a movable end wall of cylinder assembly  111  that seals chamber  112  during normal operation of linear compressor  100  but is movable, e.g., along the axial direction A, to limit damage to piston assembly  114  when piston assembly  114  strikes or impacts first valve head  220 . 
     As may be seen from the above, discharge valve  200  may have a valve-on-valve design that includes at least two mechanisms for releasing pressurized fluid from chamber  112 . In particular, second valve head  240  may be piggybacked onto first valve head  220 . First valve head  220  may be larger and less responsive while second valve head  240  is smaller and more responsive. For example, second valve head  240  may react and open under normal operating conditions and thereby improve compressor efficiency relative to utilizing only first valve head  220 . 
     Various parameters of first valve head  220  and second valve head  240  may be varied to allow second valve head  240  to be smaller and more responsive relative to first valve head  220 . For example, a mass of first valve head  220  may be greater than a mass of second valve head  240 . As a particular example, the mass of first valve head  220  may be no less than twice the mass of second valve head  240 . First valve head  220  may also have a width W 1 , e.g., along the radial direction R, and second valve head  240  may have a width W 2 , e.g., along the radial direction R. The width W 2  of second valve head  240  may be less than the width W 1  of first valve head  220 . As a particular example, the width W 1  of first valve head  220  may be no less than twice the width W 2  of second valve head  240 . First and second valve heads  220 ,  240  may have circular outer perimeters, and the widths W 1 , W 2  of first and second valve heads  220 ,  240  may be diameters. Parameters of first spring  230  and second spring  250  may also be varied to allow second valve head  240  to be more responsive relative to first valve head  220 . For example, a stiffness of first spring  230  may be greater than a stiffness of second spring  250 . As a particular example, the stiffness of first spring  230  may be no less than twice the stiffness of second spring  250 . Such relative sizing of first and second valve heads  220 ,  240  and/or first and second springs  230 ,  250  assists with providing the efficiency increase in linear compressor  100  noted above. 
     As may be seen in  FIG. 4 , first spring  230  may assist with seating first valve head  220  on casing  110  at chamber  112 . In particular, first valve head  220  may extend radially over chamber  112 . Thus, the width W 1  of first valve head  220  may be greater than a width of chamber  112 , e.g., along the radial direction R. Such sizing of first valve head  220  relative to chamber  112  provides that first valve head  220  may be wider than piston assembly  114 , e.g., along the radial direction R. Thus, first valve head  220  may be sized to seal chamber  112  while also allowing crashing of piston assembly  114  against first valve head  220  rather than other fixed components of cylinder assembly  111 . 
     Second spring  250  may assist with seating second valve head  240  on first valve head  220  at passage  222 . In particular, second valve head  240  may extend radially over passage  222 . Thus, the width W 2  of second valve head  240  may be greater than a width of passage  222 , e.g., along the radial direction R. 
       FIG. 5  provides a section view of a discharge valve  300  according to an exemplary embodiment of the present subject matter. Discharge valve  300  is constructed in a similar manner to discharge valve  200  ( FIG. 4 ) and includes numerous common components, as shown with common reference numerals. However, first and second springs  230 ,  250  are mounted in a different manner in discharge valve  300 . 
     As shown in  FIG. 5 , first spring  230  is coupled to housing  210  and first valve head  220 . In particular, one end of first spring  230  may be mounted to end wall  212  of housing  210  at a bracket  216  of end wall  212 , and another end of first spring  230  may be received within a retainer  310  mounted on first valve head  220 . Thus, first spring  230  may extend between end wall  212  (e.g., bracket  216  of end wall  212 ) and retainer  310  within housing  210 . Retainer  310  may be snap-fit, press-fit, ultra-sonically welded, fastened, adhered or otherwise suitable mounted to support  224  of first valve head  220  at an outer diameter of support  224 . Retainer  310  includes a post  312 . Post  312  is positioned at a central portion of retainer  310  and extends, e.g., along the axial direction A, towards second valve head  240 . Second spring  250  may extend between post  312  and second valve head  240 . 
       FIG. 6  provides a section view of a discharge valve  400  according to another exemplary embodiment of the present subject matter. Discharge valve  400  is constructed in a similar manner to discharge valve  200  ( FIG. 4 ) and includes numerous common components, as shown with common reference numerals. However, second spring  250  is mounted in a different manner in discharge valve  400 . 
     As shown in  FIG. 6 , discharge valve  400  includes a post  410 . Post  410  is mounted to end wall  212  of housing  210 . For example, post  410  may be mounted to or formed with bracket  216  of end wall  212 . Post  410  extends, e.g., along the axial direction A, from end wall  212  towards second valve head  240 . Second spring  250  extends between post  410  and second valve head  240  within housing  210 . 
       FIG. 7  provides a perspective view of components of a discharge valve  500  according to an additional exemplary embodiment of the present subject matter.  FIG. 8  provides a second view of the components of discharge valve  500 .  FIG. 9  provides an exploded view of the components of discharge valve  500 . Discharge valve  500  may be used in or with any suitable compressor, such as linear compressor  100 . As an example, the components of discharge valve  500  may be used with linear compressor  100  in a similar manner to that shown in  FIG. 4  for discharge valve  200 . 
     As may be seen in  FIG. 7 , discharge valve  500  includes a valve head  510 . Valve head  510  may be used and positioned in the same or similar manner to that shown in  FIG. 4  for first valve head  220 . Thus, valve head  510  may be coupled to first spring  230  and positioned at or adjacent chamber  112 . Valve head  510  defines a passage  512  that extends through valve head  510 , e.g., along the axial direction A. 
     A reed  520  is positioned at passage  512  of valve head  510 . Reed  520  may be mounted to valve head  510  such that reed  520  is cantilevered over passage  512  of valve head  510 . Reed  520  may adjust between an open position (not shown) and a closed position ( FIG. 8 ). A distal end of reed  520  is seated on valve head  510  when reed  520  is closed. Conversely, the distal end of reed  520  is spaced apart from valve head  510  when reed  520  is open. Thus, reed  520  limits or blocks fluid flow through passage  512  when reed  520  is in the closed position while reed  520  permits fluid flow through passage  512  when reed  520  is in the open position. 
     A reed damper or additional reed  525  may be disposed on or over reed valve  520 . Thus, reed valve  520  may be positioned between passage  512  and additional reed  525 . Additional reed  525  may be stiffer than reed valve  520 , e.g., along the axial direction A, in certain exemplary embodiments. Additional reed  525  may dampen opening or deformation of reed  520 . For example, additional reed  525  may limit or prevent reed  520  from impacting a valve stop  528  as reed  520  shifts open. Valve stop  528  restricts movement of reed  520  and additional reed  525  during the discharge process, e.g., thereby preventing excess stress in reed  520  and additional reed  525 . Thus, valve stop  528  may limit displacement or deformation of reed  520  and/or additional reed  525 , e.g., along the axial direction A, away from passage  512 . As an example, the distal end of reed  520  and/or additional reed  525  may be positioned against valve stop  528  when reed  520  is open, and the distal end of reed  520  and/or additional reed  525  may be spaced apart from valve stop  528  when reed  520  is closed. Discharge valve  500  may also include a shaft  530  mounted to valve head  510 , e.g., above reed  520 . Shaft  530  may assist with mounting reed  520 , additional reed  525  and/or valve stop  528  to valve head  510 . 
     In a similar manner to that described above for discharge valve  200 , discharge valve  500  may have a valve-on-valve design that includes at least two mechanisms for releasing pressurized fluid from chamber  112 . In particular, reed  520  may be piggybacked onto valve head  510 . Valve head  510  may be larger and less responsive while reed  520  is smaller and more responsive. For example, reed  520  may react and open under normal operating conditions and thereby improve compressor efficiency relative to utilizing only valve head  510 . Thus, e.g., a mass of reed  520  may be less than a mass of valve head  510 . 
     Discharge valve  200  ( FIG. 4 ), discharge valve  300  ( FIG. 5 ), discharge valve  400  ( FIG. 6 ) and discharge valve  500  ( FIG. 7 ) may be used in or with any suitable compressor. For example, discharge valve  200 , discharge valve  300 , discharge valve  400  and discharge valve  500  may be used in or with linear compressor  100 , as discharge valve assembly  117 . As another example, discharge valve  200 , discharge valve  300 , discharge valve  400  and discharge valve  500  may be used in or with the linear compressor described in U.S. Patent Publication No. 2015/0226197 of Gregory William Hahn et al., which is hereby incorporated by reference in its entirety for all purposes. Thus, e.g., discharge valve  200 , discharge valve  300 , discharge valve  400  and discharge valve  500  may be used in or with linear compressors with planar springs rather than a machined spring. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they include structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.