Patent Publication Number: US-9890779-B2

Title: Linear compressor

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority under 35 U.S.C. 119 and 35 U.S.C. 365 to Korean Patent Application No. 10-2014-0077507, filed in Korea on Jun. 24, 2014, which is hereby incorporated by reference in its entirety. 
     BACKGROUND 
     1. Field 
     A linear compressor is disclosed herein. 
     2. Background 
     Cooling systems are systems in which a refrigerant is circulated to generate cool air. In such a cooling system, processes of compressing, condensing, expanding, and evaporating the refrigerant may be repeatedly performed. For this, the cooling system may include a compressor, a condenser, an expansion device, and an evaporator. The cooling system may be installed in a refrigerator or air conditioner, which is a home appliance. 
     In general, compressors are machines that receive power from a power generation device, such as an electric motor or turbine, to compress air, a refrigerant, or various working gases, thereby increasing in pressure. Compressors are being widely used in home appliances or industrial fields. 
     Compressors may be largely classified into reciprocating compressors, in which a compression space into and from which a working gas, such as a refrigerant, is suctioned and discharged, is defined between a piston and a cylinder to allow the piston to be linearly reciprocated in the cylinder, thereby compressing the working gas; rotary compressors, in which a compression space into and from which a working gas is suctioned or discharged, is defined between a roller that eccentrically rotates and a cylinder to allow the roller to eccentrically rotate along an inner wall of the cylinder, thereby compressing the working gas; and scroll compressors, in which a compression space into and from which a working gas is suctioned or discharged, is defined between an orbiting scroll and a fixed scroll to compress the working gas while the orbiting scroll rotates along the fixed scroll. In recent years, a linear compressor, which is directly connected to a drive motor and in which a piston is linearly reciprocated, to improve compression efficiency without mechanical losses due to movement conversion and having simple structure, is being widely developed. The linear compressor may suction and compress a working gas, such as a refrigerant, while the piston is linearly reciprocated in a sealed shell by a linear motor and then discharge the refrigerant. 
     The linear motor is configured to allow a permanent magnet to be disposed between an inner stator and an outer stator. The permanent magnet may be linearly reciprocated by an electromagnetic force between the permanent magnet and the inner (or outer) stator. As the permanent magnet operates in a state in which the permanent magnet is connected to the piston, the refrigerant may be suctioned and compressed while the piston is linearly reciprocated within the cylinder, and then the refrigerant may be discharged. 
     The present Applicant filed for a patent (hereinafter, referred to as “prior art document” and then registered the patent with respect to the linear compressor, as Korean Patent No. 10-1307688, filed in Korea on Sep. 5, 2013 and entitled “linear compressor”, which is hereby incorporated by reference. 
     The linear compressor according to the prior art document includes a shell to accommodate a plurality of components. A vertical height of the shell may be somewhat large, as illustrated in  FIG. 2  of the prior art document. Also, an oil supply assembly to supply oil between a cylinder and a piston may be disposed within the shell. When the linear compressor is provided in a refrigerator, the linear compressor may be disposed in a machine chamber provided at a rear side of the refrigerator. 
     In recent years, a major concern of customers is increasing an inner storage space of the refrigerator. To increase the inner storage space of the refrigerator, it may be necessary to reduce a volume of the machine room. To reduce the volume of the machine room, it may be important to reduce a size of the linear compressor. 
     However, as the linear compressor disclosed in the prior art document has a relatively large volume, the linear compressor in the prior art document is not applicable to a refrigerator, for which increased inner storage space is sought. 
     To reduce the size of the linear compressor, it may be necessary to reduce a size of a main component of the compressor. In this case, the compressor may deteriorate in performance. 
     To compensate for the deteriorated performance of the compressor, it may be necessary to increase a drive frequency of the compressor. However, the more the drive frequency of the compressor is increased, the more a friction force due to oil circulating into the compressor increases, deteriorating performance of the compressor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments will be described in detail with reference to the following drawings in which like reference numerals refer to like elements, and wherein: 
         FIG. 1  is a cross-sectional view of a linear compressor according to an embodiment; 
         FIG. 2  is a cross-sectional view of a suction muffler of the linear compressor of  FIG. 1 ; 
         FIG. 3  is a partial cross-sectional view of the linear compressor of  FIG. 1 , illustrating a position of a second filter; 
         FIG. 4  is an exploded perspective view of a cylinder and a frame of the linear compressor of  FIG. 1 ; 
         FIG. 5  is a cross-sectional view illustrating a state in which the cylinder of  FIG. 4  and a piston are coupled to each other; 
         FIG. 6  is an exploded perspective view of the cylinder according to embodiments; 
         FIG. 7  is an enlarged cross-sectional view of portion A of  FIG. 5 ; 
         FIG. 8  is a view of a nozzle according to embodiments; 
         FIG. 9  is a graph illustrating variation in pressure loss depending on an inlet/outlet diameter ratio and length of the nozzle according to embodiments; and 
         FIG. 10  is a cross-sectional view illustrating refrigerant flow in the linear compressor according of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, embodiments will be described with reference to the accompanying drawings. The embodiments may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein; rather, alternate embodiments falling within the spirit and scope will fully convey the concept to those skilled in the art. 
       FIG. 1  is a cross-sectional view of a linear compressor according to an embodiment. Referring to  FIG. 1 , the linear compressor  100  according to an embodiment may include a shell  101  having an approximately cylindrical shape, a first cover  102  coupled to a first side of the shell  101 , and a second cover  103  coupled to a second side of the shell  101 . For example, the linear compressor  100  may be laid out in a horizontal direction. The first cover  102  may be coupled to a right or first lateral side of the shell  101 , and the second cover  103  may be coupled to a left or second lateral side of the shell  101 , with reference to  FIG. 1 . Each of the first and second covers  102  and  103  may be understood as one component of the shell  101 . 
     The linear compressor  100  may include further a cylinder  120  provided in the shell  101 , a piston  130  linearly reciprocated within the cylinder  120 , and a motor assembly  140  that serves as a linear motor to apply a drive force to the piston  130 . When the motor assembly  140  operates, the piston  130  may be linearly reciprocated at a high rate. The linear compressor  100  according to this embodiment may have a drive frequency of about 100 Hz, for example. 
     The linear compressor  100  may include a suction inlet  104 , through which refrigerant may be introduced, and a discharge outlet  105 , through which the refrigerant compressed in the cylinder  120  may be discharged. The suction inlet  104  may be coupled to the first cover  102 , and the discharge outlet  105  may be coupled to the second cover  103 . 
     The refrigerant suctioned in through the suction inlet  104  may flow into the piston  130  via a suction muffler  150 . Thus, while the refrigerant passes through the suction muffler  150 , noise may be reduced. The suction muffler  150  may include a first muffler  151  coupled to a second muffler  153 . At least a portion of the suction muffler  150  may be disposed within the piston  130 . 
     The piston  130  may include a piston body  131  having an approximately cylindrical shape, and a piston flange  132  that extends from the piston body  131  in a radial direction. The piston body  131  may be reciprocated within the cylinder  120 , and the piston flange  132  may be reciprocated outside of the cylinder  120 . 
     The piston  130  may be formed of a non-magnetic material, such as an aluminum material, such as aluminum or an aluminum alloy. As the piston  130  may be formed of the aluminum material, a magnetic flux generated in the motor assembly  140  may not be transmitted into the piston  130 , and thus, may be prevented from leaking outside of the piston  130 . The piston  130  may be manufactured by a forging process, for example. 
     The cylinder  120  may be formed of a non-magnetic material, such as an aluminum material, such as aluminum or an aluminum alloy. The cylinder  120  and the piston  130  may have a same material composition, that is, a same kind of material and composition. 
     As the cylinder  120  may be formed of the aluminum material, a magnetic flux generated in the motor assembly  200  may not be transmitted into the cylinder  120 , and thus, may be prevented from leaking outside of the cylinder  120 . The cylinder  120  may be manufactured by an extruding rod processing process, for example. 
     As the piston  130  may be formed of the same material as the cylinder  120 , the piston  130  may have a same thermal expansion coefficient as the cylinder  120 . When the linear compressor  100  operates, a high-temperature (a temperature of about 100° C.) environment may be created within the shell  100 . Thus, as the piston  130  and the cylinder  120  may have the same thermal expansion coefficient, the piston  130  and the cylinder  120  may be thermally deformed by a same degree. As a result, the piston  130  and the cylinder  120  may be thermally deformed with sizes and in directions different from each other to prevent the piston  130  from interfering with the cylinder  120  while the piston  130  moves. 
     The cylinder  120  may accommodate at least a portion of the suction muffler  150  and at least a portion of the piston  130 . The cylinder  120  may have a compression space P, in which the refrigerant may be compressed by the piston  130 . A suction hole  133 , through which the refrigerant may be introduced into the compression space P, may be defined in a front portion of the piston  130 , and a suction valve  135  to selectively open the suction hole  133  may be disposed on or at a front side of the suction hole  133 . A coupling hole, to which a predetermined coupling member may be coupled, may be defined in an approximately central portion of the suction valve  135 . 
     A discharge cover  160  that defines a discharge space or discharge passage for the refrigerant discharged from the compression space P, and a discharge valve assembly  161 ,  162 , and  163  coupled to the discharge cover  160  to selectively discharge the refrigerant compressed in the compression space P, may be provided at a front side of the compression space P. The discharge valve assembly  161 ,  162 , and  163  may include a discharge valve  161  to introduce the refrigerant into the discharge space of the discharge cover  160  when a pressure within the compression space P is above a predetermined discharge pressure, a valve spring  162  disposed between the discharge valve  161  and the discharge cover  160  to apply an elastic force in an axial direction, and a stopper  163  to restrict deformation of the valve spring  162 . 
     The compression space P may be understood as a space defined between the suction valve  135  and the discharge valve  161 . The suction valve  135  may be disposed at a first side of the compression space P, and the discharge valve  161  maybe disposed at a second side of the compression space P, that is, a side opposite of the suction valve  135 . 
     The term “axial direction” may refer to a direction in which the piston  130  is reciprocated, that is, a transverse direction in  FIG. 3 . Also, in the axial direction, a direction from the suction inlet  104  toward the discharge outlet  105 , that is, a direction in which the refrigerant flows may be referred as a “frontward direction”, and a direction opposite to the frontward direction may be referred as a “rearward direction”. On the other hand, the term “radial direction” may refer to a direction perpendicular to the direction in which the piston  130  is reciprocated, that is, a vertical direction in  FIG. 1 . 
     The stopper  163  may be seated on the discharge cover  160 , and the valve spring  162  may be seated at a rear side of the stopper  163 . The discharge valve  161  may be coupled to the valve spring  162 , and a rear portion or rear surface of the discharge valve  161  may be supported by a front surface of the cylinder  120 . The valve spring  162  may include a plate spring, for example. 
     While the piston  130  is linearly reciprocated within the cylinder  120 , when the pressure of the compression space P is below the predetermined discharge pressure and a predetermined suction pressure, the suction valve  135  may be opened to suction the refrigerant into the compression space P. On the other hand, when the pressure of the compression space P is above the predetermined suction pressure, the refrigerant may be compressed in the compression space P in a state in which the suction valve  135  is closed. 
     When the pressure of the compression space P is above the predetermined discharge pressure, the valve spring  162  may be deformed to open the discharge valve  161 . The refrigerant may be discharged from the compression space P into the discharge space of the discharge cover  160 . 
     The refrigerant flowing into the discharge space of the discharge cover  160  may be introduced into a loop pipe  165 . The loop pipe  165  may be coupled to the discharge cover  160  to extend to the discharge outlet  105 , thereby guiding the compressed refrigerant in the discharge space into the discharge outlet  105 . For example, the loop pipe  165  may have a shape which is wound in a predetermined direction and extends in a rounded shape. The loop pipe  165  may be coupled to the discharge outlet  105 . 
     The linear compressor  100  may further includes a frame  110 . The frame  110  may fix the cylinder  120  and be coupled to the cylinder  120  by a separate coupling member, for example. The frame  110  may be disposed to surround the cylinder  120 . That is, the cylinder  120  may be accommodated within the frame  110 . The discharge cover  160  may be coupled to a front surface of the frame  110 . 
     At least a portion of the high-pressure gas refrigerant discharged through the opened discharge valve  161  may flow toward an outer circumferential surface of the cylinder  120  through a space formed at a portion at which the cylinder  120  and the frame  110  are coupled to each other. The refrigerant may be introduced into the cylinder  120  through a gas inflow (see reference numeral  122  of  FIG. 7 ) and a nozzle (see reference numeral  123  of  FIG. 7 ), which may be defined in the cylinder  120 . The introduced refrigerant may flow into a space defined between the piston  130  and the cylinder  120  to allow an outer circumferential surface of the piston  130  to be spaced apart from an inner circumferential surface of the cylinder  120 . Thus, the introduced refrigerant may serve as a “gas bearing” that reduces friction between the piston  130  and the cylinder  120  while the piston  130  is reciprocated. 
     The motor assembly  140  may include outer stators  141 ,  143 , and  145  fixed to the frame  110  and disposed to surround the cylinder  120 , an inner stator  148  disposed to be spaced inward from the outer stators  141 ,  143 , and  145 , and a permanent magnet  146  disposed in a space between the outer stators  141 ,  143 , and  145  and the inner stator  148 . The permanent magnet  146  may be linearly reciprocated by a mutual electromagnetic force between the outer stators  141 ,  143 , and  145  and the inner stator  148 . The permanent magnet  146  may be provided as a single magnet having one polarity, or a plurality of magnets having three polarities. 
     The permanent magnet  146  may be coupled to the piston  130  by a connection member  138 . The connection member  138  may be coupled to the piston flange  132  and be bent to extend toward the permanent  146 . As the permanent magnet  146  is reciprocated, the piston  130  may be reciprocated together with the permanent magnet  146  in the axial direction. 
     The motor assembly  140  may further include a fixing member  147  to fix the permanent magnet  146  to the connection member  138 . The fixing member  147  may be formed of a composition in which a glass fiber or carbon fiber is mixed with a resin. The fixing member  147  may surround an outside of the permanent magnet  146  to firmly maintain a coupled state between the permanent magnet  146  and the connection member  138 . 
     The outer stators  141 ,  143 , and  145  may include coil winding bodies  143  and  145 , and a stator core  141 . The coil winding bodies  143  and  145  may include a bobbin  143 , and a coil  145  wound in a circumferential direction of the bobbin  145 . The coil  145  may have a polygonal cross-section, for example, a hexagonal cross-section. The stator core  141  may be manufactured by stacking a plurality of laminations in the circumferential direction and be disposed to surround the coil winding bodies  143  and  145 . 
     A stator cover  149  may be disposed at one side of the outer stators  141 ,  143 , and  145 . A first side of the outer stators  141 ,  143 , and  145  may be supported by the frame  110 , and a second side of the outer stators  141 ,  143 , and  145  may be supported by the stator cover  149 . The inner stator  148  may be fixed to a circumference of the frame  110 . Also, in the inner stator  148 , the plurality of laminations may be stacked in a circumferential direction outside of the frame  110 . 
     The linear compressor  100  may further include a support  137  to support the piston  130 , and a back cover  170  spring-coupled to the support  137 . The support  137  may be coupled to the piston flange  132  and the connection member  138  by a predetermined coupling member, for example. 
     A suction guide  155  may be coupled to a front portion of the back cover  170 . The suction guide  155  may guide the refrigerant suctioned through the suction inlet  104  to introduce the refrigerant into the suction muffler  150 . 
     The linear compressor  100  may further include a plurality of springs  176 , which are adjustable in natural frequency, to allow the piston  130  to perform a resonant motion. The plurality of springs  176  may include a first spring supported between the support  137  and the stator cover  149 , and a second spring supported between the support  137  and the back cover  170 . 
     The linear compressor  100  may additionally include plate springs  172  and  174  disposed, respectively, on both sides of the shell  101  to allow inner components of the compressor  100  to be supported by the shell  101 . The plate springs  172  and  174  may include a first plate spring  172  coupled to the first cover  102 , and a second plate spring  174  coupled to the second cover  103 . For example, the first plate spring  172  may be fitted into a portion at which the shell  101  and the first cover  102  are coupled to each other, and the second plate spring  174  may be fitted into a portion at which the shell  101  and the second cover  103  are coupled to each other. 
       FIG. 2  is a cross-sectional view of a suction muffler of the linear compressor of  FIG. 1 . Referring to  FIG. 2 , the suction muffler  150  according to this embodiment may include a first muffler  151 , a second muffler  153  coupled to the first muffler  151 , and a first filter  310  supported by the first and second mufflers  151  and  153 . 
     A flow space, in which the refrigerant may flow, may be defined in each of the first and second mufflers  151  and  153 . The first muffler  151  may extend from an inside of the suction inlet  104  in a direction of the discharge outlet  105 , and at least a portion of the first muffler  151  may extend inside of the suction guide  155 . The second muffler  153  may extend from the first muffler  151  inside of the piston body  131 . 
     The first filter  310  may be a component disposed in the flow space to filter foreign substances. The first filter  310  may be formed of a material having a magnetic property. Thus, the foreign substances contained in the refrigerant, in particular, metallic substances, may be easily filtered. 
     For example, the first filter  310  may be formed of stainless steel, and thus, have a magnetic property to prevent the first filter  310  from rusting. Alternatively, the first filter  310  may be coated with a magnetic material, or a magnet may be attached to a surface of the first filter  310 . 
     The first filter  310  may be a mesh-type structure and have an approximately circular plate shape. Each of filter holes may have a diameter or width less than a predetermined diameter or width. For example, the predetermined size may be about 25 μm. 
     The first muffler  151  and the second muffler  153  may be assembled with each other using a press-fit manner, for example. The first filter  310  may be fitted into a portion into which the first and second mufflers  151  and  153  are press-fitted, and then, may be assembled. For example, a groove  151   a  may be defined in one of the first and second mufflers  151  and  153 , and a protrusion  153   a  inserted into the groove may be disposed on the other one of the first and second mufflers  151  and  153 . 
     The first filter  310  may be supported by the first and second mufflers  151  and  153  in a state in which both sides of the first filter  310  are disposed between the groove  151   a  and the protrusion  153   a . In the state in which the first filter  310  is disposed between the first and second mufflers  151  and  153 , when the first and second mufflers  151  and  153  move in a direction that approach each other and then are press-fitted, both sides of the first filter  310  may be inserted and fixed between the groove  151   a  and the protrusion  153   a.    
     As described above, as the first filter  310  is provided on the suction muffler  150 , a foreign substance having a size greater than a predetermined size of the refrigerant suctioned through the suction inlet  104  may be filtered by the first filter  310 . Thus, the first filter  310  may filter the foreign substance from the refrigerant acting as the gas bearing between the piston  130  and the cylinder  120  to prevent the foreign substance from being introduced into the cylinder  120 . Also, as the first filter  310  is firmly fixed to the portion at which the first and second mufflers  151  and  153  are press-fitted, separation of the first filter  310  from the suction muffler  150  may be prevented. 
       FIG. 3  is a partial cross-sectional view of the linear compressor of  FIG. 1 , illustrating a position of a second filter.  FIG. 4  is an exploded perspective view of a cylinder and a frame of the linear compressor of  FIG. 1 . 
     Referring to  FIGS. 3 and 4 , the linear compressor  100  according to embodiments may include a second filter  320  disposed between the frame  110  and the cylinder  120  to filter a high-pressure gas refrigerant discharged through the discharge valve  161 . The second filter  320  may be disposed on or at a portion of a coupled surface at which the frame  110  and the cylinder  120  are coupled to each other. 
     In detail, the cylinder  120  may include a cylinder body  121  having an approximately cylindrical shape, and a cylinder flange  125  that extends from the cylinder body  121  in a radial direction. The cylinder body  121  may include at least one gas inflow  122 , through which the discharged gas refrigerant may be introduced. The gas inflow  122  may be recessed in an approximately circular shape along a circumferential surface of the cylinder body  121 . 
     The at least one gas inflow  122  may comprise a plurality of gas inflows  122 . The plurality of gas inflows  122  may include gas inflows (see reference numerals  122   a  and  122   b  of  FIG. 6 ) disposed on a first side with respect to a center or central portion  121   c  of the cylinder body  121  in an axial direction, and a gas inflow (see reference numeral  122   c  of  FIG. 6 ) disposed on a second side with respect to the center or central portion  121   c  of the cylinder body  121  in the axial direction. 
     One or more coupling portion  126  coupled to the frame  110  may be disposed on the cylinder flange  125 . The one or more coupling portion  126  may protrude outward from an outer circumferential surface of the cylinder flange  125 . Each coupling portion  126  may be coupled to a cylinder coupling hole  118  of the frame  110  by a predetermined coupling member. 
     The cylinder flange  125  may have a seat surface  127  seated on the frame  110 . The seat surface  127  may be a rear surface of the cylinder flange  125  that extends from the cylinder body  121  in a radial direction. 
     The frame  110  may include a frame body  111  that surrounds the cylinder body  121 , and a cover coupling portion  115  that extends in a radial direction of the frame body  111  and coupled to the discharge cover  160 . The cover coupling portion  115  may have a plurality of cover coupling holes  116  in which the coupling member coupled to the discharge cover  160  may be inserted, and a plurality of the cylinder coupling holes  118 , in which the coupling member coupled to the cylinder flange  125  may be inserted. The plurality of cylinder coupling holes  118  may be defined at positions that are recessed somewhat from the cover coupling portion  115 . 
     The frame  110  may have a recess  117  recessed backward from the cover coupling portion  115  to allow the cylinder flange  125  to be inserted therein. That is, the recess  117  may be disposed to surround the outer circumferential surface of the cylinder flange  125 . The recess  117  may have a recessed depth corresponding to a front to rear width of the cylinder flange  125 . 
     A predetermined refrigerant flow space may be defined between an inner circumferential surface of the recess  117  and the outer circumferential surface of the cylinder flange  125 . The high-pressure gas refrigerant discharged from the discharge valve  161  may flow toward the outer circumferential surface of the cylinder body  121  via the refrigerant flow space. The second filter  320  may be disposed in the refrigerant flow space to filter the refrigerant. 
     In detail, a seat  113  having a stepped portion may be disposed on or at a rear end of the recess  117 . The second filter  320  having a ring shape may be seated on the seat  113 . 
     In a state in which the second filter  320  is seated on the seat, when the cylinder  120  is coupled to the frame  110 , the cylinder flange  125  may push the second filter  320  from a front side of the second filter  320 . That is, the second filter  320  may be disposed and fixed between the seat of the frame  110  and the seat surface  127  of the cylinder flange  125 . 
     The second filter  320  may prevent foreign substances in the high-pressure gas refrigerant discharged through the opened discharge valve  161  from being introduced into the gas inflow  122  of the cylinder  120  and be configured to absorb oil contained in the refrigerant thereon. For example, the second filter  320  may include a felt formed of polyethylene terephthalate (PET) fiber or an adsorbent paper. The PET fiber may have superior heat-resistance and mechanical strength. Also, a foreign substance having a size of about 2 μm or more, which is contained in the refrigerant, may be blocked. 
     The high-pressure gas refrigerant passing through the flow space defined between the inner circumferential surface of the recess  117  and the outer circumferential surface of the cylinder flange  125  may pass through the second filter  320 . In this process, the refrigerant may be filtered by the second filter  320 . 
       FIG. 5  is a cross-sectional view illustrating a state in which the cylinder of  FIG. 4  and a piston are coupled to each other.  FIG. 6  is an exploded perspective view of the cylinder according to embodiments.  FIG. 7  is an enlarged cross-sectional view of portion A of  FIG. 5 .  FIG. 8  is a view of a nozzle according to embodiments. 
     Referring to  FIGS. 5 to 8 , the cylinder  120  according to embodiments may include the cylinder body  121  having an approximately cylindrical shape to form a first body end  121   a  and a second body end  121   b , and the cylinder flange part  125  that extends from the second body end  121   b  of the cylinder body  121  in a radial direction. The first body end  121   a  and the second body end  121   b  form both ends of the cylinder body  121  with respect to the center or central portion  121   c  of the cylinder body  121  in an axial direction. 
     The cylinder body  121  may includes a plurality of the gas inflows  122 , through which at least a portion of the high-pressure gas refrigerant discharged through the discharge valve  161  may flow. A third filter  330  may be disposed in the plurality of gas inflows  122 . 
     Each of the plurality of gas inflows  122  may be recessed from the outer circumferential surface of the cylinder body  121  by a predetermined depth and width. The refrigerant may be introduced into the cylinder body  121  through the plurality of gas inflows  122  and the nozzle  123 . 
     The introduced refrigerant may be disposed between the outer circumferential surface of the piston  130  and the inner circumferential surface of the cylinder  120  to serve as the gas bearing with respect to movement of the piston  130 . That is, the outer circumferential surface of the piston  130  may be maintained in a state in which the outer circumferential surface of the piston  130  is spaced apart from the inner circumferential surface of the cylinder  120  by the pressure of the introduced refrigerant. 
     The plurality of gas inflows  122  may include the first and second gas inflows  122   a  disposed on the first side with respect to the central portion  121   c  in the axial direction of the cylinder body  121 , and a third gas inflow  122   c  disposed on the second side with respect to the central portion  121   c  in the axial direction. The first and second gas inflows  122   a  and  122   b  may be disposed at positions closer to the second body end  121   b  with respect to the central portion  121   c  in the axial direction of the cylinder body  121 , and the third gas inflow  122   c  may be disposed at a position closer to the first body end  121   a  with respect to the central portion  121   c  in the axial direction of the cylinder body  121 . That is, the plurality of gas inflows  122  may be provided in numbers which are not symmetrical to each other with respect to the central portion  121   c  in the axial direction of the cylinder body  121 . 
     Referring to  FIG. 6 , the cylinder  120  may have a relatively high inner pressure at a side of the second body end  121   b , which is closer to a discharge-side of the compressed refrigerant when compared to that of the first body end  121   a , which is closer to a suction-side of the refrigerant. Thus, more gas inflows  122  may be provided at the side of the second body end  121   b  to enhance a function of the gas bearing, and relatively less gas inflows  122  may be provided at the side of the first body end  121   a.    
     The cylinder body  121  may further include the nozzle  123  that extends from the plurality of gas inflows  122  toward the inner circumferential surface of the cylinder body  121 . The nozzle  123  may have a width or size less than a width or size of the gas inflow  122 . 
     A plurality of nozzles  123  may be provided along the gas inflow  122  which may extend in a circular shape. The plurality of nozzles  123  may be spaced apart from each other. 
     The plurality of nozzles  123  may each include an inlet  123   a  connected to the gas inflow  122 , and an outlet  123   b  connected to the inner circumferential surface of the cylinder body  121 . The nozzle  123  may have a predetermined length from the inlet  123   a  toward the outlet  123   b.    
     The refrigerant introduced into the gas inflow  122  may be filtered by the third filter  330  to flow into the inlet  123   a  of the nozzle  123 , and then, may flow toward the inner circumferential surface of the cylinder  120  along the nozzle  123 . The refrigerant may be introduced into the inner space of the cylinder  120  through the outlet  123   b.    
     The piston  130  may operate to be spaced apart from the inner circumferential surface of the cylinder  120 , that is, be lifted or spaced from the inner circumferential surface of the cylinder  120  by the pressure of the refrigerant discharged from the outlet  123   b . That is, the pressure of the refrigerant supplied into the cylinder  120  may provide a lifting force or pressure to the piston  130 . 
     Referring to  FIG. 8 , the nozzle  123  may have a length L (mm), the inlet  123   a  may have a diameter D 1  (μm), and the outlet  123   b  may have a diameter D 2  (μm). A recessed depth and width of each of the plurality of gas inflows  122  and a length L of the nozzle  123  may be determined to have adequate dimensions in consideration of a rigidity of the cylinder  120 , an amount of third filter  330 , or an intensity in pressure drop of the refrigerant passing through the nozzle  123 . 
     For example, if the recessed depth and width of each of the plurality of gas inflows  122  are too large, or the length L of the nozzle  123  is too short, the rigidity of the cylinder  120  may be weak. On the other hand, if the recessed depth and width of each of the plurality of gas inflows  122  are too small, an amount of third filter  330  provided in the gas inflow  122  may be too small. Also, if the length L of the nozzle  123  is too long, a pressure drop of the refrigerant passing through the nozzle  123  may be too large, it may be difficult to perform the function of the gas bearing. 
     The inlet  123   a  of the nozzle  123  may have the diameter D 1  greater than the diameter D 2  of the outlet  123   b . In a flow direction of the refrigerant, a flow section area of the nozzle  123  may gradually decrease from the inlet  123   a  to the outlet  123   b.    
     In detail, if the diameter of the nozzle  123  is too small, an amount of refrigerant, which is introduced from the nozzle  123 , of the high-pressure gas refrigerant discharged through the discharge valve  161 , may be too small, increasing flow loss in the compressor. On the other hand, if the diameter of the nozzle  123  is too small, the pressure drop in the nozzle  123  may increase, reducing performance of the gas bearing. 
     Thus, in this embodiment, the inlet  123   a  of the nozzle  123  may have the relatively large diameter D 1  to reduce the pressure drop of the refrigerant introduced into the nozzle  123 . In addition, the outlet  123   b  may have the relatively small diameter D 2  to control an inflow amount of gas bearing through the nozzle  123  to a predetermined value or less. 
     The third filter  330  may prevent a foreign substance having a predetermined size or more from being introduced into the cylinder  120  and perform a function of absorbing oil contained in the refrigerant. The predetermined size may be about 1 μm. The third filter  330  may include a thread that is wound around the gas inflow  122 . In detail, the thread may be formed of a polyethylene terephthalate (PET) material and have a predetermined thickness or diameter. 
     The thickness or diameter of the thread may be determined to have adequate dimensions in consideration of a rigidity of the thread. If the thickness or diameter of the thread is too small, the thread may be easily broken due to very weak strength thereof. On the other hand, if the thickness or diameter of the thread is too large, a filtering effect with respect to the foreign substances may be deteriorated due to a very large pore in the gas inflow  122  when the thread is wound. 
     For example, the thickness or diameter of the thread may be several hundreds μm. The thread may be manufactured by coupling a plurality of strands of a spun thread having several tens μm to each other, for example. 
     The thread may be wound several times, and an end of the thread may be fixed with a knot. A number of windings of the thread may be adequately selected in consideration of pressure drop of the gas refrigerant and a filtering effect with respect to foreign substances. If the number of windings of the thread is too large, the pressure drop of the gas refrigerant may increase. On the other hand, if the number of windings of the thread is too small, the filtering effect with respect to the foreign substances may be reduced. 
     Also, a tension force of the wound thread may be adequately controlled in consideration of a strain of the cylinder and fixation of the thread. If the tension force is too large, deformation of the cylinder  120  may occur. On the other hand, if the tension force is too small, the thread may not be adequately fixed to the gas inflow  122 . 
       FIG. 9  is a graph illustrating variation in pressure loss depending on an inlet/outlet diameter ratio and length of the nozzle according to embodiments. The graph of  FIG. 9  illustrates a degree or variation in occurrence of a pressure loss ΔP of refrigerant depending on length L of the nozzle  123  and a ratio of diameter D 1  of the inlet  123   a  of the nozzle  123  to diameter D 2  of the outlet  123   b  of the nozzle  123  according to embodiments. 
     The pressure loss ΔP may refer to a value obtained by subtracting a pressure P 2  at the outlet  123   b  from a pressure P 1  at the inlet  123   a . That is, the refrigerant may be gradually reduced in pressure in a flow direction from the inlet  123   a  to the outlet  123   b  of the nozzle  123 . 
     It may be necessary to set a pressure of the refrigerant supplied toward the inner circumferential surface of the cylinder  120  to a predetermined pressure or more. When the pressure of the refrigerant supplied toward the inner circumferential surface of the cylinder  120  is less than the preset or predetermined pressure, a sufficient pressure to lift the piston  130  may not be provided, and thus, the refrigerant may not perform the function as the gas bearing. 
     If a pressure (a discharge pressure) of the refrigerant discharged through the discharge valve  161  is substantially uniform under preset or predetermined conditions of external air, the pressure of the refrigerant supplied toward the inner circumferential surface of the cylinder  120  may vary according to the pressure loss that occurs in the nozzle  123 . If the pressure loss occurring in the nozzle  123  is too large, the pressure of the refrigerant supplied toward the inner circumferential surface of the cylinder  120  may be less than an inner pressure of the piston  130  or may not be sufficiently larger than the inner pressure of the piston  130 . Thus, the piston  130  may not be lifted within the cylinder  120 , and thus, performance of the gas bearing may deteriorate. 
     More particularly, if external air conditions, in particular, an external air temperature is low, a difference between the suction pressure and the discharge pressure of the compressor is not large. For example, a difference between the suction pressure Ps and the discharge pressure Pd may be about 1 bar (about 100 kpa). In this case, the inner pressure of the piston  130  may be above at least the suction pressure Ps. 
     Also, in a state in which the discharge pressure Pd of the refrigerant discharged through the discharge valve  161  is greater by about 1 bar than the suction pressure Ps, when the pressure loss at the nozzle  123  is too large, the pressure of the refrigerant supplied toward the inner circumferential surface of the cylinder  120  is less than the inner pressure of the piston  130  or is not sufficiently larger than the inner pressure of the piston  130 . As a result, performance of the refrigerant as the gas bearing may deteriorate. 
     Thus, in this embodiment, to maintain the pressure loss to a preset or predetermined loss value ΔPa or less, a test may be performed by changing a length of the nozzle  123  and a ratio of the inlet/outlet diameters. For example, the preset or predetermined loss value ΔPa may be set to about 0.20 bar (about 20 kpa).  FIG. 9  illustrates a test result obtained under the above-described conditions. 
     Referring to  FIG. 9 , a horizontal axis in the graph may represent a ratio of the diameter D 1  of the inlet  123   a  to the diameter D 2  of the outlet  123   b  of the nozzle  123 . Also, a vertical axis in the graph may represent a pressure loss ΔP at the nozzle  123 , that is, a value obtained by subtracting the pressure at the outlet  123   b  from the pressure at the inlet  123   a . As described above, when the pressure loss ΔP is less, performance of the refrigerant as the gas bearing may improve. 
     In the test, the ratio may be adjusted by changing the diameter D 1  of the inlet  123   a  in a state in which the diameter D 2  of the outlet  123   b  of the nozzle  123  is fixed. For example, in a state in which the diameter D 2  of the outlet  123   b  is fixed to about 25 μm, the diameter D 1  of the inlet  123   a  may vary to perform the test. 
     Also, a variation in pressure loss ΔP with respect to the ratio may be measured when the length L of the nozzle  123  is changed to lengths L 1  L 2 , or L 3 . For example, L 1  may be about 0.5 mm, L 2  may be about 0.8 mm, and L 2  may be about 1.2 mm. 
     The length of the nozzle  123  according to this embodiment may be selected from the lengths L 1  to L 3 . If the length of the nozzle  123  is less than L 1 , rigidity of the cylinder  120  may deteriorate. On the other hand, when the length of the nozzle  123  is greater than L 3 , the pressure loss may increase with respect to the predetermined ratio, and material costs of the cylinder  120  may increase. 
     When the ratio is 1, the diameter D 1  at the inlet  123   a  may be equal to the diameter D 2  at the outlet  123   b . When the ratio is less than 1, the diameter D 2  at the outlet  123   b  may be greater than the diameter D 1  at the inlet  123   a . When the ratio is 1 or less than 1, the pressure loss ΔP may be significantly greater than the preset or predetermined loss valve ΔPa. 
     In detail, when the ratio is less than 1, for example, when the ratio is about 0.5, in a case in which the length of the nozzle  123  is L 1 , the pressure loss ΔP may be about 0.40 bar. Also, in a case in which the length of the nozzle  123  is L 2 , the pressure loss ΔP may be about 0.37 bar, and in a case in which the length of the nozzle  123  is L 3 , the pressure loss ΔP may be about 0.29 bar. 
     When the ratio is 1, that is, the inlet/outlet diameters of the nozzle  123  are the same, in a case in which the nozzle length is L 1 , L 2 , and L 3 , the pressure loss ΔP may be about 0.38 bar, about 0.35 bar, and about 0.24 bar. When the ratio is greater than 1, as the ratio increases, the pressure loss ΔP may gradually decrease. For example, in a case in which the length of the nozzle  123  is L 1 , when the ratio is 2, the pressure loss may slightly increase above the preset or predetermined loss value ΔPa. Also, when the pressure loss corresponds to the preset or predetermined loss value ΔPa, the ratio may be a value A. The value A may correspond to about 2.0. That is, when the length of the nozzle  123  is about 0.5 mm, and the diameter D 2  of the outlet  123   b  is about 25 μm, the diameter D 1  of the inlet  123   a  may be about 50 μm or more. 
     As another example, in a case in which the length of the nozzle  123  is L 2 , when the pressure loss corresponds to the preset or predetermined loss value ΔPa, the ratio may be a value B. The value B may correspond to about 2.8. That is, when the length of the nozzle  123  is about 0.8 mm, and the diameter D 1  of the outlet  123   b  is about 25 μm, the diameter D 2  of the inlet  123   a  may be about 70 μm or more. 
     As another example, in a case in which the length of the nozzle  123  is L 2 , when the pressure loss corresponds to the preset or predetermined loss value ΔPa, the ratio may be a value C. The value C may correspond to about 3.8. That is, when the length of the nozzle  123  is about 1.2 mm, and the diameter D 2  of the outlet  123   b  is about 25 μm, the diameter D 1  of the inlet  123   a  may be about 95 μm or more. 
     In summary, in this embodiment, when the length of the nozzle  123  is selected as one value of L 1  to L 3 , the ratio may be 2 or more so as to maintain the pressure loss at the nozzle  123  to the preset or predetermined loss value ΔPa or less. Also, as the length of the nozzle  123  increases, the ratio may increase (A&lt;B&lt;C) to maintain the pressure loss to the preset or predetermined loss value ΔPa or less. 
       FIG. 10  is a cross-sectional view illustrating refrigerant flow in the linear compressor of  FIG. 1 . Referring to  FIG. 10 , refrigerant flow in the linear compressor according to embodiments will be described hereinbelow. 
     Referring to  FIG. 10 , the refrigerant may be introduced into the shell  101  through the suction inlet  104  and flow into the suction muffler  150  through the suction guide  155 . The refrigerant may be introduced into the second muffler  153  via the first muffler  151  of the suction muffler  150  to flow into the piston  130 . In this process, suction noise of the refrigerant may be reduced. 
     A foreign substance having a predetermined size (about 25 μm) or more, which is contained in the refrigerant, may be filtered while passing through the first filter  310  provided on the suction muffler  150 . The refrigerant within the piston  130  after passing though the suction muffler  150  may be suctioned into the compression space P through the suction hole  133  when the suction valve  135  is opened. 
     When the refrigerant pressure in the compression space P is above the discharge pressure, the discharge valve  161  may be opened. Thus, the refrigerant may be discharged into the discharge space of the discharge cover  160  through the opened discharge valve  161 , flow into the discharge outlet  105  through the loop pipe  165  coupled to the discharge cover  160 , and be discharged outside of the compressor  100 . 
     At least a portion of the refrigerant within the discharge space of the discharge cover  160  may flow toward the outer circumferential surface of the cylinder body  121  via the space defined between the cylinder  120  and the frame  110 , that is, the inner circumferential surface of the recess  117  of the frame  110  and the outer circumferential surface of the cylinder flange  125  of the cylinder  120 . The refrigerant may pass through the second filter  320  disposed between the seat surface  127  of the cylinder flange  125  and the seat  113  of the frame  110 . In this process, a foreign substance having a predetermined size (about 2 μm) or more may be filtered. Also, oil in the refrigerant may be adsorbed onto or into the second filter  320 . 
     The refrigerant passing through the second filter  320  may be introduced into the plurality of gas inflows  122  defined in the outer circumferential surface of the cylinder body  121 . Also, while the refrigerant passes through the third filter  330  provided on or in the gas inflows  122 , a foreign substance having a predetermined size (about 1 μm) or more, which is contained in the refrigerant, may be filtered, and the oil contained in the refrigerant may be adsorbed. 
     The refrigerant passing through the third filter  330  may be introduced into the cylinder  120  through the nozzle(s)  123  and be disposed between the inner circumferential surface of the cylinder  120  and the outer circumferential surface of the piston  130  to space the piston  130  from the inner circumferential surface of the cylinder  120  (gas bearing). The inlet  123   a  of each nozzle  123  may have a diameter greater than a diameter of the outlet  123   b . Thus, a refrigerant flow section area of the nozzle  123  may gradually decrease with respect to the flow direction of the refrigerant. For example, the inlet  123   a  may have a diameter greater than two times a diameter of the outlet  123   b.    
     As described above, the high-pressure gas refrigerant may be bypassed within the cylinder  120  to serve as the gas bearing with respect to the piston  130  which is reciprocated, thereby reducing abrasion between the piston  130  and the cylinder  120 . Also, as oil is not used for the bearing, friction loss due to oil may not occur even though the compressor  100  operates at a high rate. 
     Also, as the plurality of filters may be provided on a path of the refrigerant flowing into the compressor  100 , foreign substances contained in the refrigerant may be removed. Thus, the refrigerant acting as the gas bearing may be improved in reliability. Thus, it may prevent the piston  130  or the cylinder  120  from being worn by the foreign substances contained in the refrigerant. Also, as oil contained in the refrigerant may be removed by the plurality of filters, it may prevent friction loss due to the oil from occurring. 
     The first, second, and third filters  310 ,  320 , and  330  may be referred to as a “refrigerant filter device” in that the filters  310 ,  320 , and  330  filter the refrigerant that serves as the gas bearing. 
     According to embodiments, the compressor including inner parts or components may be decreased in size to reduce a volume of a machine room of a refrigerator and increase an inner storage space of the refrigerant in which the compressor is employed. Also, a drive frequency of the compressor may be increased to prevent performance of the inner parts from being deteriorated due to decreased size thereof. In addition, as the gas bearing is applied between the cylinder and the piston, the friction force occurring due to oil may be reduced. 
     Further, as the nozzle to guide introduction of the refrigerant is provided on the outer circumferential surface of the cylinder, and an optimum value or ratio with respect to inlet/outlet diameters of the nozzle and a length of the nozzle is applied, pressure loss of the refrigerant passing through the nozzle may be minimized, and the cylinder may be maintained at a preset or predetermined rigidity or more. Furthermore, as the plurality of filtering device are provided in the compressor, it may prevent foreign substances or oil contained in the compression gas (or discharge gas) introduced to the outside of the piston from the nozzle of the cylinder from being introduced. More particularly, the first filter may be provided on the suction muffler to prevent the foreign substances contained in the refrigerant from being introduced into the compression chamber. Also, the second filter may be provided on the coupling part or portion between the cylinder and the frame to prevent the foreign substances and oil contained in the compressed refrigeration gas from flowing into the gas inflow of the cylinder. Also, the third filter may be provided on the gas inflow of the cylinder to prevent the foreign substances and oil from being introduced into the nozzle of the cylinder from the gas inflow. 
     As described above, as foreign substances or oil contained in the compression gas that acts as the bearing may be filtered through the plurality of filtering device provided in the compressor, it may prevent the nozzle of the cylinder from being blocked by the foreign substances or oil. As the blocking of the nozzle of the cylinder is prevented, the gas bearing effect may be effectively performed between the cylinder and the piston, and thus, abrasion of the cylinder and the piston may be prevented. 
     Embodiments disclosed herein provide a linear compressor in which a gas bearing may easily operate between a cylinder and a piston. 
     Embodiments disclosed herein provide a linear compressor that may include a shell including a suction part or inlet; a cylinder provided in the shell to define a compression space for a refrigerant; a piston reciprocated in an axial direction within the cylinder; a discharge valve provided on or at one or a first side of the cylinder to selectively discharge the refrigerant compressed in the compression space; and a nozzle part or nozzle, through which at least a portion of the refrigerant discharged through the discharge valve may flow, the nozzle part being disposed in the cylinder. The nozzle part may include an inlet part or inlet, through which the refrigerant may be introduced, and an outlet part or outlet having a diameter less than a diameter of the inlet part. 
     The nozzle part may be recessed inward from the cylinder in a radial direction from the inlet part toward the outlet part. The nozzle part may extend to have a preset or predetermined length (L), and the inlet part may have a diameter (D 1 ) greater than two times a diameter D 2  of the outlet part. As the nozzle increases in preset length (L), a ratio of the diameter (D 1 ) of the inlet part to the diameter (D 2 ) of the outlet part may gradually increase. 
     When the preset length (L) of the nozzle part is about 0.5 mm, the ratio may be 2 or more. When the preset length (L) of the nozzle part is about 0.8 mm, the ratio may be 2.8 or more. When the preset length (L) of the nozzle part is about 1.2 mm, the ratio may be 3.8 or more. 
     The linear compressor may further include a gas inflow part or inflow recessed from an outer circumferential surface of the cylinder to communicate with the nozzle part, and a filter member disposed in the gas inflow part. The filter member may include a thread having a preset or predetermined thickness or diameter. 
     Embodiments disclosed herein further provide a linear compressor that may include a shell including a suction part or inlet; a cylinder provided in the shell to define a compression space for a refrigerant; a piston reciprocated in an axial direction within the cylinder; a discharge valve provided on or at one or a first side of the cylinder to selectively discharge the refrigerant compressed in the compression space; a gas inflow part or inflow, in which a filter member may be disposed, the gas inflow part being recessed from an outer circumferential surface of the cylinder; and a nozzle part or nozzle that extends from the gas inflow part toward an inner circumferential surface of the cylinder. The nozzle part may have a flow cross-section area that gradually decreases with respect to a flow direction of the refrigerant. 
     The nozzle part may include an inlet part or inlet connected to the gas inflow part, and an outlet part or outlet connected to the inner circumferential surface of the cylinder. The nozzle part may have a preset or predetermined length from the inlet part toward the outlet part. 
     The outlet part may have a diameter (D 2 ) less than a diameter (D 1 ) of the inlet part. The inlet part may have a diameter (D 1 ) greater than two times the diameter D 2  of the outlet part. The filter member may include a thread formed of a polyethylene terephthalate (PET) material. 
     The details of one or more embodiments are set forth in the accompanying drawings and the description. Other features will be apparent from the description and drawings, and from the claims. 
     Any reference in this specification to “one embodiment,” “an embodiment,” “example embodiment,” etc., means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. The appearances of such phrases in various places in the specification are not necessarily all referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with any embodiment, it is submitted that it is within the purview of one skilled in the art to effect such feature, structure, or characteristic in connection with other ones of the embodiments. 
     Although embodiments have been described with reference to a number of illustrative embodiments thereof, it should be understood that numerous other modifications and embodiments can be devised by those skilled in the art that will fall within the spirit and scope of the principles of this disclosure. More particularly, various variations and modifications are possible in the component parts and/or arrangements of the subject combination arrangement within the scope of the disclosure, the drawings and the appended claims. In addition to variations and modifications in the component parts and/or arrangements, alternative uses will also be apparent to those skilled in the art.