Patent Publication Number: US-2009238701-A1

Title: Compressor having a piston received on a gas bearing

Description:
The present invention relates to a compressor having a gas-bearing piston, in particular for use as a refrigerant compressor in a domestic refrigerator. 
     Compressors with rotary-driven lifting pistons and oil lubrication are conventionally used in domestic appliances. Owing to the rotary drive, during the course of its lifting movement in a working chamber the piston in such a compressor is also exposed to forces transverse to the lifting direction which push the piston against the wall of the working chamber. To protect the compressor against frictional wear there must be a continuous film of oil between piston and chamber wall. The viscosity of the oil must be high enough to prevent the oil from being displaced between wall and piston under the influence of a transverse force. The greater the transverse forces are, the more viscous the oil must be and the friction losses are all the greater owing to the viscosity of the oil. 
     To improve the efficiency of the compressors increased efforts have recently been made to develop compressors with a linear motor drive. A linear motor of this kind exerts smaller transverse forces on the piston so lower viscosity lubricants can be used, through to bearing of the piston by the gas that is itself to be compressed. With a compressor of this kind with gas bearing, openings are formed in a wall of the working chamber covered by the piston and communicate with an outlet connection of the compressor, so a small portion of the compressed medium can flow from the outlet connection, via the openings and back into the working chamber and in the process generates a gas cushion between piston and wall which ideally allows movement of the piston without any contact with the wall at all and therefore without any frictional wear either. 
     These ideal conditions prove to be difficult to implement, however. In practice it has been found that the linear compressor frequently fails after a relatively short service life owing to the frictional wear between piston and chamber wall. 
     It is therefore the object of the invention to create a compressor with gas bearing which allows such frictional wear to be reliably prevented. 
     The object is achieved by a compressor comprising an inlet connection and an outlet connection for a gas to be compressed, and a working chamber in which a piston for compressing the gas can be displaced, openings communicating with the outlet connection being configured in a wall of the working chamber covered by the piston, said compressor being characterized by a particle filter arranged in a flow path of the gas through the compressor between the inlet connection and the outlet connection. 
     More detailed examinations of the failed compressors have shown that contaminants found therein could be attributed not only to abrasion of the piston and chamber wall, but were in some cases introduced from other areas of the refrigerant circuit as well, and it is assumed that even if the quantity and particle size of these contaminants, which are alien to the compressor, is not sufficient per se to affect the mobility of the piston in the compressor, they initially block several of the wall openings if they circulate with the flow of refrigerant, so the gas cushion protecting the piston becomes incomplete. Contact between piston and wall can no longer be reliably prevented in the region of these holes, so the abrasion resulting from contact quickly destroys the compressor. 
     The inventive particle filter remedies this by preventing these foreign bodies from reaching the wall openings. 
     A particle filter of this kind may be provided at various sites in the compressor. According to a first configuration the particle filter is arranged between the inlet connection and the working chamber. A particle filter of this kind has the advantage that foreign bodies can be prevented from reaching the working chamber from the outset. This is particularly desirable in the case of coarse foreign bodies which can cause damage solely by virtue of their presence in the working chamber. 
     One drawback of such placement of the particle filter however is that the filtering of the entire gas flow passing through the compressor that it brings about requires considerable driving power and as a result affects the efficiency of the compressor. Additionally, the drop in pressure at a particle filter of this kind can never be greater than the pressure at the inlet connection itself, so to achieve a sufficient throughput a high conductance and therefore a large filter cross-section may be necessary. In particular when using the compressor in a refrigerating machine it is important that the particle filter does not build up excessive dynamic pressure as this would impair evaporation of the refrigerant in the evaporator located upstream of the filter. 
     According to a second embodiment the particle filter is arranged in a pipe, connecting an egress of the working chamber to the outlet connection, between the egress and a branch pipe leading to the openings. This arrangement of the filter does not lead to an undesirable increase in pressure at the inlet connection of the compressor. In addition a greater drop in pressure can be built up at such a filter arranged on the pressure side of the compressor than at the inlet-side filter described above, so sufficient throughput can also be achieved in a compact filter although such a high drop in pressure does in turn affect the efficiency of the compressor. 
     According to a third, particularly preferred embodiment a branch pipe leading to the openings branches off from a pressure pipe connecting an egress of the working chamber to the outlet connection, and the particle filter is arranged in the branch pipe. As a particle filter of this kind only acts on the fraction of the gas throughput which is required for generating the gas cushion, the driving power that has to be applied for filtering is low, even if the drop in pressure at the particle filter is the same as that according to the second embodiment. 
     While the filter arranged in the branch pipe does not prevent foreign bodies from passing through the compressor from the start, it does collect them during the course of compressor operation as soon as they pass into the branch pipe. 
     According to a preferred development the branch pipe comprises a distribution chamber and a hole connecting the distribution chamber to the pressure pipe, the wall covered by the piston forming a partition between the working chamber and the distribution chamber. With a branch pipe of this kind the particle filter can be small and compactly arranged in the hole, or it can be arranged in the distribution chamber so as to cover the partition. 
     In the case of a working chamber with lifting piston the distribution chamber is preferably a hollow cylinder that extends around the cylindrical working chamber. 
    
    
     
       Further features and advantages of the invention emerge from the following description of exemplary embodiments with reference to the accompanying figures, in which: 
         FIG. 1  shows a schematic section through a compressor according to the invention with suction-side particle filter; 
         FIG. 2  shows a section analogous to  FIG. 1  through an inventive compressor with pressure-side particle filter; 
         FIG. 3  shows a section analogous to  FIG. 1  through an inventive compressor with particle filter arranged in a branch pipe, and 
         FIG. 4  shows a section through a modification of the compressor with particle filter arranged in a branch pipe. 
     
    
    
       FIG. 1  shows a schematic section through a compressor according to a first embodiment of the invention. The compressor is part of a refrigerating machine of a domestic refrigerator which is not shown as its remaining components are known. A piston  2  can be reciprocated by a driving unit, likewise known per se and not shown, in a cylindrical working chamber  1  of the compressor. The driving unit can for example be a linear motor as described for instance in DE 10 2004 010 849 AG, but in general any desired driving unit which is capable of generating a driving force, and whose transverse components are small enough that frictional contact between the walls of the working chamber  1  and the piston  2  can be reliably avoided, may be considered. 
     A suction valve  3  and a pressure valve  4  which connect the working chamber  1  to a suction pipe  5  and a pressure pipe  6  respectively and via these to an inlet connection  12  and an outlet connection  13  respectively are located at one end face of the working chamber. A particle filter  7  is provided in the suction pipe  5 . A hole  8  leads from the pressure pipe  6  to a distribution chamber  9  with a hollow cylindrical shape and which surrounds the working chamber  1 , so the wall  10  of the working chamber  1  covered by the piston  2  as it moves is a partition between the working chamber  1  and the distribution chamber  9 . A large number of narrow openings  11  is distributed in this wall  10 . 
     During operation the piston  2 , when moved to the right, sucks in refrigerant from the inlet connection  12  via the suction pipe  5 , the particle filter  7  and the suction valve  3  into the working chamber  1 . When the piston  2  is moved to the left the contents of the working chamber  1  are firstly compressed at the pressure prevailing in the pressure pipe  6  before the pressure valve  4  opens and the compressed gas runs off into the pressure pipe  6 . A small portion of the gas flowing in the pressure pipe  6  passes through the hole  8  and into the distribution chamber  9  and from there via the openings  11  in the wall  10  back into the working chamber  1  where it forms a gas cushion between the piston  2  and the wall  10 . 
     Foreign particles which were present in the various other components of the refrigerating machine even before it was assembled or which came about as a result of assembly, are captured by the particle filter  7  before they can pass with the flow of refrigerant into the working chamber  1 . The openings  11  are therefore protected from being blocked by the foreign particles. As the gas cushion between piston  2  and wall  10  is therefore complete, no abraded particles are produced between piston and wall which could, in turn, pass via the hole  8  and the distribution chamber  9  to the openings  11  and block them. 
     The embodiment shown in  FIG. 2  differs from that in  FIG. 1  in that the particle filter  7  is arranged in the pressure pipe  6 , between the pressure valve  4  and the hole  8 , and not in the suction pipe  5 . As in the embodiment of  FIG. 1  all of the refrigerant passing through the compressor passes through the particle filter  7 . Owing to the arrangement of the particle filter  7  in the pressure pipe  6  a greater drop in pressure can be built up at the particle filter  7  than in the case of the embodiment of  FIG. 1 , so sufficient throughput can be achieved even with a compact filter. 
     The embodiment of  FIG. 3  differs from the two described above in the placement of the particle filter  7  in the distribution chamber  9 . Here it rests on the wall  10 , so it is pressed against the wall  10  as a result of the pressure difference between its outer and inner sides. As only a fraction of the refrigerant throughput of the compressor passes through the filter and the driving power consumed during filtering matches the product of filter throughput and drop in pressure, in this embodiment the increased demand for power owing to filtering is only a fraction of the increased demand of the first two embodiments. 
     The provision of the particle filter  7  in the distribution chamber  9  has the advantage that refrigerant can flow through the particle filter  7  over a very large cross-sectional area, so even when using a very porous filter a throughput sufficient to maintain the gas cushion can be attained. 
     To ensure uniform distribution of the gas among the openings  11  the particle filter  7  preferably has a layered structure here with a fine-pored outer layer  7   a  and a coarse-pored, easily permeable inner layer whose function is substantially to maintain a spacing between the fine-pored layer  7   a  and the wall  10  in which the gas can uniformly distribute itself among the openings  11 . 
       FIG. 4  shows the currently preferred embodiment of the invention. This embodiment differs from those described above in that the hole  8 , which connects the distribution chamber  9  to the pressure pipe  6 , is long enough and wide enough to receive the particle filter  7  in the form of a filter cartridge  14 . The filter cartridge is introduced into the hole  8  from its end that faces the pressure pipe  6 . A pressure-side flange  15  of the filter cartridge rests on a shoulder  16  formed at the entrance to the hole  8 , so the filter cartridge  14  is pressed against the shoulder  16  by the drop in pressure which it generates on the refrigerant flowing through. 
     According to one development (not shown in a figure) the suction-side particle filter  7  from the embodiment of  FIG. 1  can also be combined with the particle filter according to  FIG. 3  or  4  that is arranged on the route of the refrigerant from the pressure pipe  6  to the openings  11 . In this case the suction-side particle filter can be a coarse-pored filter with low flow resistance which is used solely to keep coarse contaminants away from the working chamber  1  while finer contaminants are caught by the pressure-side particle filter.