Patent Publication Number: US-9845797-B2

Title: Reciprocating compressor and method for driving same

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
     This application is a U.S National Stage Application under 35 U.S.C. §371 of PCT Application No. PCT/KR2013/007814, filed Aug. 30, 2013, which claims priority to Korean Patent Application Nos. 10-2012-0097276 and 10-2012-0097278, both filed Sep. 3, 2012, whose entire disclosures are hereby incorporated by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to a reciprocating compressor, and more particularly, a reciprocating compressor having a fluid bearing, and a method for driving the same. 
     BACKGROUND ART 
     Generally, a reciprocating compressor serves to intake, compress, and discharge a refrigerant as a piston linearly reciprocates within a cylinder. The reciprocating compressor may be classified into a connection type reciprocating compressor or a vibration type reciprocating compressor according to the method employed to drive the piston. 
     In the connection type reciprocating compressor, the piston is connected to a rotating shaft associated with a rotation motor by a connection rod, which causes the piston to reciprocate within the cylinder, thereby compressing the refrigerant. On the other hand, in the vibration type reciprocating compressor, the piston is connected to a mover associated with a reciprocating motor, which vibrates the piston while the piston reciprocates within the cylinder, thereby compressing the refrigerant. The present invention relates to the vibration type reciprocating compressor, and the term “reciprocating compressor” will hereinafter refer to the vibration type reciprocating compressor. 
     To enhance the performance of a reciprocating compressor, a portion between the cylinder and the piston, being hermetically sealed, has to be properly lubricated. To this end, there has been conventionally known a reciprocating compressor which seals and lubricates the portion between the cylinder and the piston by supplying a lubricant such as oil between the cylinder and the piston and forming an oil film. However, the supplying of the lubricant requires an oil supply apparatus, and an oil shortage may occur depending on operation conditions, thereby degrading compressor performance. Also, the compressor size needs to be increased because a space for receiving a certain amount of oil is required, and the installation direction of the compressor is limited because the entrance of the oil supply apparatus should always be kept immersed in oil. 
     Taking into consideration the disadvantages of the oil-lubricated type reciprocating compressor, as illustrated in  FIGS. 1 and 2 , there has been conventionally known a technique of forming a fluid bearing between a piston  1  and a cylinder  2  by bypassing a part of compressed gas between the piston  1  and the cylinder  2 . A plurality of gas holes  2   a  each having a small diameter are formed through the cylinder  2  to inject the compression gas into an inner circumferential surface of the cylinder  2 . 
     This technique can simplify a lubrication structure of the compressor because it requires no oil supply apparatus, unlike the oil-lubricated type for supplying oil between the piston  1  and the cylinder  2 , and can maintain constant compressor performance by preventing an oil shortage depending on operating conditions. Also, this technique has the advantage that the compressor can be smaller in size and the installation direction of the compressor can be freely designed because no space for receiving oil is required in the casing of the compressor. Unexplained reference number  3  denotes a plate spring (a leaf spring),  5   a  to  5   c  denote connecting bars, and  6   a  and  6   b  denote links. 
     However, in the related art reciprocating compressor, foreign substances mixed with refrigerant gas are introduced into the a fluid bearing to block the fluid bearing, thereby preventing the refrigerant gas from being supplied between the cylinder  2  and the piston  1 . Accordingly, concentricity between the piston  1  and the cylinder  2  is destroyed, thereby causing a friction loss and abrasion while the piston  1  reciprocates with being closely adhered to the cylinder  2 . 
     Also, as high-temperature refrigerant gas discharged from a compression space is introduced into the fluid bearing to heat the cylinder  2 , a specific volume of a compression space increases and thereby a suction loss is caused. 
     Furthermore, discharge noise and vibration which are generated while a refrigerant compressed in the compression space is discharged cannot effectively be offset, thereby increasing vibration noise of the compressor. 
     DISCLOSURE OF THE INVENTION 
     Therefore, to obviate those problems, an aspect of the detailed description is to provide a reciprocating compressor, capable of preventing a friction loss and abrasion between a cylinder and a piston, which are caused when a fluid bearing is blocked by foreign materials (or substances) mixed with refrigerant gas, in a manner of blocking the foreign materials from being introduced into the fluid bearing, and a method for driving the same. 
     Another aspect of the present invention is to provide a reciprocating compressor, capable of preventing in advance suction loss, caused due to an increased specific volume of a compression space, in a manner of preventing a cylinder from being heated by high-temperature refrigerant gas discharged from a compression space, and a method for driving the same. 
     Another aspect of the present invention is to provide a reciprocating compressor, capable of reducing vibration noise of the compressor by effectively offsetting vibration and noise which are generated as a refrigerant is discharged from a compression space, and a method for driving the same. 
     To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described herein, there is provided a reciprocating compressor including a casing having an inner space communicating with a suction pipe, a frame provided in the inner space of the casing, a reciprocating motor coupled to the frame, and having a mover, the mover performing a linear reciprocating motion, a cylinder coupled to the frame and having a compression space, a piston inserted into the cylinder to perform a reciprocating motion, the piston having a suction passage formed therethrough in a lengthwise direction to guide a refrigerant into the compression space, a discharge cover installed at an end side of the cylinder and having a discharge space communicating with a discharge pipe, a fluid bearing having gas holes formed through the cylinder and configured to inject fluid therethrough into a portion between the cylinder and the piston so as to support the piston with respect to the cylinder, and a block-preventing unit configured to prevent the gas holes of the fluid bearing from being blocked due to foreign materials. 
     The reciprocating compressor may further include a discharge cover provided at an end side of the cylinder and having the discharge space to communicate with the discharge pipe. The discharge space and inlets of the gas holes may communicate with each other through a gas guiding pipe. The gas guiding pipe may be partially exposed to the outside of the discharge cover, and a filtering unit may be installed at the exposed gas guiding pipe to filter off the foreign materials. 
     Also, the reciprocating compressor may further include a vibration unit configured to vibrate the cylinder. 
     To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described herein, there is provided a method for driving a reciprocating compressor, the method including determining whether or not a foreign material-removing operation is required, shaking out foreign materials from gas holes of a cylinder by increasing the number of vibrations of a piston when the foreign material-removing operation is required, and executing a normal operation by decreasing the number of vibrations of the piston. 
     Advantageous Effect 
     In a reciprocating compressor and a method for driving the same according to the present invention, a friction loss and abrasion, which are caused between a cylinder and a piston because the piston is closely adhered on the cylinder due to gas holes of a fluid bearing being blocked by foreign materials mixed with refrigerant gas, can be prevented by preventing the foreign materials from being introduced into the fluid bearing. 
     Also, as a gas guiding pipe is provided in an inner space of a casing, separate from a discharge cover, high-temperature refrigerant gas discharged from a compression space can be cooled by performing heat exchange with a sucked refrigerant filled in the inner space of the casing, and accordingly a cylinder forming a gas pocket can be cooled. This may result in reducing a specific volume of the compression space and thus improving compressor performance. 
     Also, vibration and noise which are generated as a refrigerant is discharged from a compression chamber can be offset by a guide guiding unit, thereby reducing vibration noise of the compressor. 
     In addition, even though gas holes are blocked due to foreign materials being introduced into a fluid bearing along with a refrigerant, a cylinder may vibrate by temporarily increasing the number of vibrations of a mover, so as to remove the foreign materials stuck in the gas holes. This may result in preventing a friction loss and abrasion, which are caused between the cylinder and a piston because the piston is closely adhered on the cylinder due to the gas holes of the fluid bearing being blocked by the foreign materials. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a longitudinal sectional view illustrating an example that a gas bearing is applied to a reciprocating compressor according to the related art. 
         FIG. 2  is a perspective view illustrating an example that a leaf spring is applied to a reciprocating compressor according to the related art. 
         FIG. 3  is a longitudinal sectional view of a reciprocating compressor in accordance with the present invention. 
         FIG. 4  is an enlarged view of a part “A” of  FIG. 3 , namely, a sectional view illustrating one embodiment of a fluid bearing. 
         FIG. 5  is a perspective view illustrating a gas guiding unit of the fluid bearing according to  FIG. 3 ; 
         FIG. 6  is a sectional view illustrating one example of a filtering unit of  FIG. 5 . 
         FIGS. 7 to 10  are sectional views illustrating other embodiments of a gas guiding unit of the fluid bearing according to  FIG. 3 . 
         FIG. 11  is a sectional view illustrating another embodiment of a filtering unit of the fluid bearing according to  FIG. 3 . 
         FIG. 12  is a longitudinal sectional view illustrating a main portion for another embodiment of a fluid bearing of a reciprocating compressor in accordance with the present invention. 
         FIG. 13  is a schematic view illustrating a structure of a controller of the compressor to remove foreign materials according to  FIG. 12 . 
         FIG. 14  is a block diagram illustrating a foreign material removing process according to  FIG. 13 . 
     
    
    
     MODES FOR CARRYING OUT THE PREFERRED EMBODIMENTS 
     Hereinafter, description will be given in detail of a reciprocating compressor according to one embodiment illustrated in the accompanying drawings. 
       FIG. 3  is a longitudinal sectional view of a reciprocating compressor in accordance with the present invention. 
     As illustrated in  FIG. 3 , in a reciprocating compressor according to an embodiment disclosed herein, a suction pipe  12  may be connected to an inner space  11  of a casing  10 , and a discharge pipe  13  may be connected to a discharge space S 2  of a discharge cover  46  to be explained later. A frame  20  may be disposed in the inner space  11  of the casing  10 . A stator  31  of a reciprocating motor  30  and a cylinder  41  may be fixed to the frame  20 . A piston  42  which is coupled to a mover  32  of the reciprocating motor  30  may be inserted into the cylinder  41  so as to reciprocate therein. Resonant springs  51  and  52  for inducing a resonating motion of the piston  42  may be provided at both sides of the piston  42  in a motion direction of the piston  42 . 
     A compression space S 1  may be defined in the cylinder  41 , and a suction passage F may be formed in the piston  42 . A suction valve  43  for opening and closing the suction passage F may be provided at an end of the suction passage F. A discharge valve  44  for opening and closing the compression space S 1  of the cylinder  41  may be provided at an end surface of the cylinder  41 . 
     In the reciprocating compressor having such configuration, when power is applied to the reciprocating motor  30 , the mover  32  of the reciprocating motor  30  reciprocates with respect to a stator  31 . The piston  42  coupled to the mover  32  then linearly reciprocates within the cylinder  41 . Accordingly, a refrigerant can be sucked, compressed and discharged. 
     Explaining the process in detail, when the piston  42  is moved backwards, a refrigerant of the casing  10  is introduced into the compression space S 1  through the suction passage F of the piston  42 . When the piston  42  is moved forwards, the suction passage F is closed such that the refrigerant can be compressed in the compression space S 1 . When the piston  42  is further moved forwards, the refrigerant compressed in the compression chamber S 1  is discharged as the discharge valve  44  is open, so as to flow toward a refrigerating cycle. 
     Here, a coil  35  may be inserted into the stator  31  of the reciprocating motor  30  to be coupled thereto, and an air gap may be formed only at one side of the coil  35 . The mover  32  may be provided with magnets  36  each of which is inserted into the air gap of the stator  31  so as to reciprocate in a motion direction of the piston  42 . 
     The stator  31  may include a plurality of stator blocks  31   a , and a plurality of pole blocks  31   b  coupled to sides of the stator blocks  31   a , respectively, to form air gap portions  31   c  along with the stator blocks  31   a.    
     The stator blocks  31   a  and the pole blocks  31   b  may be configured in a manner of laminating a plurality of thin stator cores sheet by sheet into an arcuate shape when axially projected. The stator blocks  31   a  may be formed in the shape of recesses ( ) when axially projected, and the pole blocks  31   b  may be formed in a rectangular shape ( ) when axially projected. 
     The mover  32  may include a magnet holder  32   a  formed in a cylindrical shape, and a plurality of magnets  36  coupled to an outer circumferential surface of the magnet holder  32   a  in a circumferential direction so as to form a magnetic flux along with the coil  35 . 
     The magnet holder  32   a  may preferably be formed of a non-magnetic substance to prevent a leakage of magnetic flux, but may not be limited thereto. The outer circumferential surface of the magnetic holder  32   a  may be formed in a circular shape so that the magnets  36  are in line contact therewith and adhered thereto. A magnet mounting groove (not illustrated) may be formed in a strip shape on the outer circumferential surface of the magnet holder  32   a  so as to insert the magnets  36  therein and support them in the motion direction. 
     The magnets  36  may be formed in a hexahedral shape and adhered one by one to the outer circumferential surface of the magnet holder  32   a . In the case of attaching the magnets  36  one by one, supporting members (not shown), such as fixing rings or a tape made up of a composite material, may be fixed to outer circumferential surfaces of the magnets  36  in a covering manner. 
     Although the magnets  36  may be serially adhered in a circumferential direction to the outer circumferential surface of the magnet holder  32   a , it is preferable that the magnets  36  are adhered at predetermined intervals, i.e., between the stator blocks in a circumferential direction to the outer circumferential surface of the magnet holder  32   a  to minimize the use of the magnets, because the stator  31  comprises the plurality of stator blocks  31   a  and the plurality of stator blocks  31   b  are arranged at predetermined intervals in the circumferential direction. 
     Preferably, the magnet  36  may be configured such that its length in a motion direction is not shorter than a length of the air gap portion  31   c  in the motion direction, more particularly, longer than the length of the air gap portion  31   c  in the motion direction. At its initial position or during its operation, the magnet  36  may be disposed such that at least one end thereof is located inside the air gap portion  31   c , in order to ensure a stable reciprocating motion. 
     Moreover, though only one magnet  36  may be disposed in the motion direction, a plurality of magnets  36  may be disposed in the motion direction in some cases. In addition, the magnets may be disposed in the motion direction so that an N pole and an S pole correspond to each other. 
     Although the above-described reciprocating motor may be configured such that the stator has one air gap portion  31   c , it may be configured such that in some cases the stator has air gap portions  31   c  on both sides of the coil in the lengthwise direction. In this case, the mover may be formed in the same manner as the foregoing embodiment. 
     In the above-stated reciprocating compressor, it is required to reduce a frictional loss between the cylinder  41  and the piston  42  to improve the performance of the compressor. To this end, there has been conventionally known a fluid bearing which lubricates between the cylinder  41  and the piston  42  by gas force by bypassing a part of compressed gas between an inner circumferential surface of the cylinder  41  and an outer circumferential surface of the piston  42 . 
       FIG. 4  is an enlarged view of a part “A” of  FIG. 3 , namely, a sectional view illustrating one embodiment of a fluid bearing. 
     As illustrated in  FIGS. 3 and 4 , a fluid bearing (or a hydraulic bearing)  100  may include a gas pocket  110  formed on an inner circumferential surface of the frame  20  by a predetermined depth, and a plurality of columns of gas holes  120  communicating with the gas pocket  110  and penetrating through the inner circumferential surface of the cylinder  41 . Here, the column of gas holes refers to gas holes which are formed on the same circumference at positions corresponding to the same length along a lengthwise direction of the cylinder. 
     The gas pocket  110  may be formed in an annular shape along the entire inner circumferential surface of the frame  20 , but in some cases, may be provided in plural arranged with predetermined intervals along the circumferential direction of the frame  20 . 
     A gas guiding unit  200  may be coupled to an inlet of the gas pocket  110  to guide some of the compression gas, which has been discharged from the compression space into the discharge space S 2 , from the discharge space S 2  to the fluid bearing  100 . 
     Here, the gas pocket  110  may be located between the frame  20  and the cylinder  41 . Alternatively, the gas pocket  110  may be provided at an end surface of the cylinder  41  along the lengthwise direction of the cylinder  41 . In this instance, since the gas pocket  110  is formed to communicate directly with the discharge space S 2  of the discharge cover  46 , a separate gas guiding unit may not be needed. This may simplify an assembly process and reduce fabricating costs. 
     Referring to  FIG. 3 , the resonant springs may include a first resonant spring  51  and a second resonant spring  52 , both of which are provided at both sides in a back-and-forth direction of a spring supporter  53 , which is coupled to the mover  32  and the piston  42 . 
     The first resonant spring  51  and the second resonant spring  52  each are provided in plural and arranged along a circumferential direction. However, either the first resonant spring  51  or the second resonant spring  52  may be provided in plural and the other may be provided in singular. 
     The first resonant spring  51  and the second resonant spring  52 , as aforementioned, may be implemented as a compression coil spring. Accordingly, when the resonant springs  51  and  52  are expanded, side force may be produced. Therefore, the resonant springs  51  and  52  may be arranged to offset the side force or torsion moment of the resonant springs  51  and  52 . 
     For example, in the case that the first resonant spring  51  and the second resonant spring  52  are arranged alternately by twos in a circumferential direction, distal ends of the first and second resonant springs  51  and  52  may be wound at the same position in opposite directions (counterclockwise) relative to the center of the piston  42 , and the resonant springs on the same side positioned in their respective diagonal directions may be arranged to symmetrically engage each other so that a side force and a torsion moment are produced in opposite directions. 
     Also, the first resonant spring  51  and the second resonant spring  52  may be arranged to symmetrically engage the distal ends of the resonant springs with each other so that side force and torsion moment are produced in opposite directions along the circumferential direction. 
     Preferably, spring fixing protrusions  531  and  532  are respectively formed on a frame or spring supporter  53 , to which the ends of the first and second resonant springs  51  and  52  are fixed, in order for the resonant springs  51  and  52  to be press-fitted into the spring fixing protrusions  531  and  532 , because the engaged resonant springs are prevented from turning. 
     The number of first resonant springs  51  may be equal to or different from the number of second resonant springs  52  as long as the first resonant spring  51  and the second resonant spring  52  have the same elasticity. 
     When the resonant springs  51  and  52  configured as the compression coil spring are applied, side force may be produced while the compression coil spring is expanded and accordingly linearity of the piston  42  may be lost. However, as illustrated in this embodiment, when the first resonant spring  51  and the second resonant spring  52  each are provided in plural and arranged to be wound in opposite directions to each other, the side force and the torsion moment produced by each of the resonant springs  51  and  52  may be offset by the resonant springs, symmetrical in the diagonal direction, thereby maintaining the linearity of the piston  52  and preventing in advance abrasion of a surface of the piston  52  in contact with the resonant springs  51  and  52 . 
     Also, since the resonant springs  51  and  52  are implemented as the compression coil spring which is not locked in a horizontal direction and exhibits less transformation in a vertical direction, the compressor can also be installed in a vertical manner as well as a horizontal manner. Also, with no need of separate connecting bars or links to connect the mover  32  and the piston  42  to each other, material costs and the number of assembling stages can be reduced. 
     Meanwhile, in this embodiment, although the weight of the piston increases because the piston is formed longer than the cylinder, since the resonant springs are configured as the compression coil spring, the piston is likely to be hung down in view of the characteristic of the compression coil spring. This may bring about a friction loss and abrasion between the piston and the cylinder. Specifically, when the piston is supported by supplying gas, not supplying oil, between the cylinder and the piston, gas holes should be appropriately arranged, in order to prevent the piston from being hung down and thus prevent the friction loss or the abrasion between the cylinder and the piston. 
     For example, gas holes  120  which penetrate through the inner circumferential surface of the cylinder  41  may be formed with predetermined intervals over an entire region of the piston  42  in a lengthwise direction of the piston  42 . That is, when the length of the piston  42  is longer than that of the cylinder  41  and the piston  42  performs a reciprocating motion in a horizontal direction, the positions of the gas holes  120  for injecting gas therethrough into a portion between the cylinder  41  and the piston  42  may be uniformly formed even on a rear region of the piston  42  as well as front and central regions of the piston  42 , adjacent to a compression space S 1 . In such a manner, the fluid bearing  100  can stably support the piston  42  and thus the friction loss and the abrasion between the cylinder  41  and the piston  42  can be prevented in advance. 
     Specifically, when the compression coil spring is employed as the resonant springs  51  and  52  for inducing the resonating motion of the piston  42 , the piston  42  may be more hung down due to the great vertical transformation of the compression coil spring. However, since the gas holes  120  are evenly provided all over the regions (A), (B) and (C) along the lengthwise direction of the piston  42 , the piston  42  may not be hung down and can smoothly perform the reciprocating motion, thereby effectively preventing the friction loss and the abrasion between the cylinder  41  and the piston  42 . 
     In the meantime, in order to prevent drooping of the piston to avoid the friction loss and the abrasion between the cylinder and the piston, the reciprocating compressor according to this embodiment should be configured such that a total cross-section of the gas holes arranged at a lower portion of the cylinder is greater than a total cross-section of the gas holes arranged at an upper portion of the cylinder. 
     To this end, the gas holes  120  may be provided in a manner that the number of gas holes located at the lower portion is greater than the number of gas holes located at the upper portion of the cylinder  41  or the cross-section of the gas holes located at the lower portion is greater than the cross-section of the gas holes located at the upper portion. And, the gas holes may be configured such that the number or cross-section thereof increases from a top to a bottom of the cylinder  41 , thereby increasing a supporting force for supporting a lower side of the fluid bearing. 
     A gas guiding groove  125  which guides compressed gas introduced into the gas pocket  110  into the gas holes  120  and simultaneously serves as a type of buffer may be formed at entrances of the gas holes  120 , respectively. The gas guiding grove  125  may be formed in an annular shape such that the gas holes arranged in each column can communicate with one another, or be provided in plural and arranged with predetermined intervals along a circumferential direction such that the gas holes in each column can be independent of one another. However, it may be preferable that the plurality of gas guiding grooves  125  are provided to the gas holes  120 , respectively, with predetermined intervals along the circumferential direction, so as to equalize compressed gas and compensate for strength of the cylinder. 
     Meanwhile, upon employing the fluid bearing as illustrated in this embodiment, when foreign substances mixed with a refrigerant are introduced into the fluid bearing, the foreign substances may block the fine gas holes so as to interfere with a smooth introduction of refrigerant gas between the cylinder and the piston. When the refrigerant gas is not supplied between the cylinder and the piston, the piston comes in contact with the cylinder, thereby causing a friction loss and abrasion between them. Hence, it is important to block the introduction of the foreign materials into the fluid bearing, in terms of enhancing reliability of the compressor. 
       FIG. 5  is a perspective view illustrating a gas guiding unit of a fluid bearing according to  FIG. 3 ,  FIG. 6  is a sectional view illustrating one example of a filtering unit of  FIG. 5 , and  FIGS. 7 to 10  are sectional views illustrating other embodiments of a gas guiding unit of the fluid bearing according to  FIG. 3 . 
     As illustrated in  FIG. 5 , a filtering unit may be provided at a middle portion of a gas guiding pipe. That is, a gas guiding pipe  210  may branch out at a middle portion of the discharge pipe  13  and be connected to an inlet of the gas pocket  110 . A filtering unit  220  configuring a block-preventing unit may be connected to a middle portion of the gas guiding pipe  210  so as to filter off foreign substances from a refrigerant which flows into the gas pocket  110 . 
     The gas guiding pipe  210  may preferably be formed as long as possible, such that refrigerant gas introduced into the gas pocket  110  through the gas guiding pipe  210  can be cooled and decompressed by performing heat exchange with a low-temperature sucked refrigerant, which is filled in the inner space  11  of the casing  10 . To this end, the gas guiding plate  210  may preferably be wound several times to cover surroundings of the discharge cover  46  with being spaced apart from an outer circumferential surface of the discharge cover  46 . Alternatively, the gas guiding pipe  210  may also be connected directly to the discharge space S 2  of the discharge cover  46 , which is coupled to the end surface of the cylinder  41 . 
     The filtering unit  220 , as illustrated in  FIG. 5 , may include a filter housing  221  connected to a middle portion of the gas guiding pipe  210 , and a filter  222  located in the filter housing  221  to filter off foreign materials. 
     The filter housing  221  is a filtering space in which the foreign materials are filtered off. An inlet of the filtering space may communicate with the discharge space S 2  through the gas guiding pipe  210  while an outlet of the filtering space may be connected to the gas pocket  110  through the gas guiding pipe  210 . A cross-section of the filtering space may be greater than a cross-section of the gas guiding pipe  210 . 
     The filter  222 , as illustrated in  FIG. 6 , may be configured as a cyclone filter for filtering and collecting foreign materials, such as metal pieces, using a cyclone effect, or as a mesh filter using a filtering effect. When a filtering space is not separately required, the filter  222  such as the mesh filter may be located at the outside (for example, at the inlet of the gas pocket  110 ) of the filter housing  221 . 
     The filter housing  221  may be provided in singular, but, as illustrated in  FIG. 7 , a plurality of filter housings  221   a  to  221   e  may be serially connected by a single gas guide pipe  210 . When the filter housing is provided in plural, it may be preferable to install a filter (not illustrated) in only one of the plurality of filter housings, in terms of reducing installation costs and preventing pressure of compressed gas from being excessively lowered due to flow resistance. 
     The filter housing  221  may also be installed in the discharge cover  46 , as illustrated in  FIG. 8 . In this instance, the discharge cover  46  may be divided into a first discharge space S 21  having the discharge valve  44  installed therein, and a second discharge space S 22  having the filter  222  installed therein. The first discharge space S 21  and the second discharge space S 22  may communicate with each other. The discharge pipe  13  and the gas guiding pipe  210  may branch out at an outlet of the filter housing  221 . 
     Also, the filter housing  221  may be installed to cover an outside of the discharge cover  46 , as illustrated in  FIG. 9 . In this instance, the discharge space S 2  of the discharge cover  46  may communicate with the filtering space  225  of the filter housing  221 , and the discharge pipe  13  may be connected to the filter housing  221 . 
     Here, a truncated conical filter  222  may be provided on an inner circumferential surface of the filter housing  221  so as to configure a cyclone filter. A gas through hole  222   a  may be formed at one side of the filter  222  to communicate with the gas guiding pipe  210 . 
     In this instance, the filtering space  225  of the filter housing  221  may be coupled to accommodate therein the inlet of the gas pocket  110 . 
     Meanwhile, as illustrated in  FIG. 10 , the inlet of the gas pocket  110  may be located at the outside of the filter housing  221 , the filter housing  221  and the gas pocket  110  may be connected to each other through the gas guiding pipe  210 , and a muffler  230  may be provided at a middle portion of the gas guiding pipe  210 . Here, pulsation noise and vibration which are generated when compressed gas is discharged can be more offset because they are offset through the muffler  230  once more. In this instance, a mesh filter may further be provided at an outlet side of the muffler  230 . 
     In the reciprocating compressor according to the embodiment disclosed herein, when the filtering unit  220  is installed at a discharge side of the compression space S 1 , a part of compressed refrigerant gas may be introduced into the filter housing  221  through the gas guiding pipe  210  or directly introduced into the filter housing  221  through the discharge space S 2 , thereby passing through the filter  222  located in the filtering housing  221 . Accordingly, foreign materials mixed with the refrigerant gas may be filtered off by the filter  222 , thereby preventing in advance the introduction of the foreign materials into the fluid bearing  100 . 
     In such a manner, the gas holes which are configured as fine holes can be prevented from being blocked due to the foreign materials, such that the fluid bearing can stably support a portion between the cylinder and the piston while the compressor smoothly operates. 
     In addition, the filter housing can serve as a type of a muffler and simultaneously reduce pressure pulsation of a discharged refrigerant, thereby reducing discharge noise of the compressor. 
     Also, as the gas guiding pipe is installed at the outside of the discharge cover and simultaneously formed long in length, the compressed gas introduced into the gas pocket of the fluid bearing can be cooled by a low-temperature sucked refrigerant filled in the inner space of the casing, which may allow for cooling the cylinder defining the gas pocket and thus reducing a specific volume of the compression space, thereby enhancing compressor efficiency. 
     In the meantime, description will be given of another embodiment of a filtering unit of the reciprocating compressor according to the present invention. 
     That is, the foregoing embodiments illustrate that the filtering unit is located at the discharge side of the compression space, but these embodiments illustrate that the filtering unit is provided at an inlet side of the compression space. 
     To this end, as illustrated in  FIG. 11 , filters  222   a  to  222   d  may be provided in a suction muffler  47  which is coupled to an inlet of the suction passage F of the piston  42 , in an intermediate pipe  22  coupled to a back cover  21 , in a suction pipe  12  coupled to the casing  10 , or in a suction muffler  15  coupled to the casing  10 . 
     Even in this instance, as aforementioned, those filters may be implemented as a mesh filter or a cyclone filter. Even when the filtering unit is provided at the suction side of the compression space as illustrated in these embodiments, the operation effect may be the same as or similar to those of the foregoing embodiments. However, in these embodiments, as the filtering unit is provided at the suction side of the compression space, foreign materials may be filtered off from a refrigerant before the refrigerant is sucked into the compression space and accordingly the cylinder and the piston may be prevented in advance from being abraded due to foreign materials within the compression space. 
     The foregoing embodiments illustrate that the cylinder is inserted into the stator of the reciprocating motor. However, those positions of the gas holes may be equally applied even when the reciprocating motor is mechanically coupled with a predetermined interval to a compression unit including the cylinder. Detailed description thereof will be omitted. 
     Also, the foregoing embodiments illustrate that the piston is configured to perform a reciprocating motion and thus the resonant springs are provided at both sides of the piston in the motion direction of the piston. However, in some cases, the cylinder may also be configured to perform a reciprocating motion and thus the resonant springs may be installed at both sides of the cylinder. Even in this instance, the positions of the gas holes may be equal to those in the foregoing embodiments, of which detailed description will be omitted. 
     In the meantime, the foregoing embodiments illustrate that the filtering unit is provided on a passage of refrigerant gas to filter off foreign materials before the refrigerant gas is introduced into the gas holes. However, these embodiments illustrate that the cylinder is periodically shaken to remove foreign materials stuck in the gas holes of the cylinder when the compressor continuously operates for a predetermined period of time, thereby preventing the blocking of the gas holes in advance. 
       FIG. 12  is a longitudinal sectional view illustrating a main portion for another embodiment of a fluid bearing of a reciprocating compressor in accordance with the present invention,  FIG. 13  is a schematic view illustrating a structure of a controller of the compressor to remove foreign materials according to  FIG. 12 , and  FIG. 14  is a block diagram illustrating a foreign material-removing process according to  FIG. 13 . 
     For example, as illustrated in  FIGS. 12 to 14 , an operation duration time t 1  of the compressor is detected using a timer  310  which is provided at a controller  300  of the compressor (S 1 ). 
     When the detected operation duration time t 1  reaches a preset foreign material-removing operation time (t 2 ), the controller  300  increases the number of vibrations of the mover  32  (namely, the number of vibrations of the piston), which typically vibrates at 30 to 120 Hz, for example, up to 1 kHz or more (S 2 ). Accordingly, the piston  42  coupled to the mover  32  performs a fast reciprocating motion. While the piston  42  fast reciprocates, a resonant frequency of the resonant springs  51  and  52  increases as high as the change in the number of vibrations of the piston  42 , thereby exciting the stator  31 . In response to the excitation of the stator  31 , the cylinder  41  is excited by the frame  20  coupled to the stator  31  so as to generate a type of “shaking effect (or vibration effect),” thereby removing foreign materials stuck in the gas holes  120 . 
     Here, when a supporting spring  15  is provided on a bottom surface of the casing  10  to elastically support an installation surface of the compressor, the casing  10  may greatly be excited in response to the change in the vibration of the piston  42 , and accordingly the effect of shaking the cylinder  41  may be more increased. 
     Afterwards, when a predetermined time elapses after the foreign material-removing operation t 2  has started, the controller  300  controls the number of vibrations of the mover  32  (namely, the number of vibrations of the piston  41 ) to be decreased down to the number of vibrations at a typical operation, such that the compressor executes a normal operation (S 3  and S 4 ). 
     Here, the compressor may also be controlled to return to its normal operation state immediately after executing the operation of shaking the foreign materials out, but a process of pausing (or stopping) the mover  32  (namely, the piston  41 ) for a predetermined time is further executed (S 31 ) in some cases. Through the process, the foreign materials may be removed from the gas holes  120  while the compressor is paused, thereby increasing the effect of removing the foreign materials. 
     In such a manner, even though some of the gas holes are blocked due to foreign materials introduced into the fluid bearing along with the compressed refrigerant gas, the cylinder may periodically be vibrated to remove the foreign materials stuck in the gas holes. This may result in preventing the gas holes as the fine holes from being blocked due to the foreign materials so as to allow for a smooth operation of the fluid bearing and a stable support of a portion between the cylinder and the piston. 
     In the meantime, the foregoing embodiments illustrate that the cylinder is inserted into the stator of the reciprocating motor, but those positions of the gas holes may equally be applied even when the reciprocating motor is mechanically coupled with a predetermined interval to a compression unit including the cylinder. Detailed description thereof will be omitted. 
     Also, the foregoing embodiments illustrate that the piston is configured to perform a reciprocating motion and thus the resonant springs are provided at both sides of the piston in the motion direction. However, in some cases, the cylinder may be configured to perform a reciprocating motion and thus the resonant springs may be installed at both sides of the cylinder. Even in this instance, the positions of the gas holes may be equal to those in the foregoing embodiments. Detailed description thereof will be omitted.