Abstract:
A scroll compressor which comprises a scroll assembly comprising two scroll walls, and a drive for causing a relative orbiting motion between the scroll walls for compressing fluid on two fluid flow paths between an inlet and an exhaust of the scroll assembly. A first fluid flow path is formed between a wall surfaces of the scroll walls and a second fluid flow path is formed between another facing wall surfaces of the scroll walls. A first ambient clearance A 1  is selected between the first two facing surfaces, and a second ambient clearance A 2  is selected between the second two facing surfaces, and where the first and the second ambient clearances are selected independently from each other. Independent selection permits each ambient clearance to be designed according to its own performance requirements, to take into account thermal expansion and manufacturing tolerances.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a scroll compressor and particularly to a scroll wall of the compressor.  
         BACKGROUND OF THE INVENTION  
         [0002]    [0002]FIG. 3 shows in accordance with the prior art a scroll compressor  10  know which comprises a fixed scroll  12  supported by a compressor casing  14  and an orbiting scroll  16  disposed opposite the fixed scroll supported by a crank shaft  18 . The orbiting scroll and the fixed scroll wall form a scroll assembly  19 . A motor or drive unit  20  is supplied for causing an orbiting movement of the orbiting scroll  16  in relation to the fixed scroll  12 . The fixed scroll comprises a base plate  22  from which a scroll wall  24  extends generally orthogonally. The orbiting scroll  16  comprises a base plate  26  from which an orbiting scroll wall  28  extends generally orthogonally so as to co-operate with the fixed scroll wall  28  for compressing fluid between inlet  30  and an outlet  32  of the compressor  10  when the orbiting scroll  16  orbits about the fixed scroll  12 .  
           [0003]    In the arrangement shown, the orbiting scroll  16  is disposed towards the centre of the compressor  10  (i.e. towards the left of the fixed scroll in FIG. 3) and towards the moving parts of the compressor. This causes the orbiting scroll  16  to increase in temperature during use and causes thermal expansion of the orbiting scroll. The orbiting scroll  16  does not easily dissipate heat because generally it is positioned at a low pressure side of the compressor where conductive heat transfer to the fluid being pumped is limited and there is no access for ambient air. The fixed scroll  12  on the other hand is positioned with its rear face in the ambient air which is used to provide cooling. It will be understood therefore that the orbiting scroll  16  undergoes thermal expansion relative to the fixed scroll  12  between ambient temperature when the compressor is not in use (or when the components of the compressor are at the same temperature) and working temperatures when the pump is in use. A converse arrangement is possible, but not shown, in which the relative orientation of the fixed scroll and the orbiting scroll leads to the fixed scroll being heated and expanding relative to the orbiting scroll, although this arrangement is not currently preferred and will not be discussed further.  
           [0004]    Thermal expansion of the orbiting scroll  16  translates into radial expansion of base plate  26 . The radial expansion is generally dependent on the distance from the centre of the plate so that a radially outer portion of the plate expands more than a radially inner portion of the plate and accordingly, a radially outer portion of the orbiting scroll wall  28  expands more than a radially inner portion thereof. The inner section of the scroll wall expands about 10 to 50 (typically 30) microns whilst radially outer portions may expand by many times the inner clearance, about 100 to 500 microns (typically 200 to 300 microns for a 50° C. rise depending on the diameter of the scroll).  
           [0005]    FIGS.  4  to  8  show cross-sections of the prior art scroll assembly, taken along line II in FIG. 3. Fluid enters the scroll arrangement at inlet  30  where orbiting movement of the orbiting scroll causes it to be compressed along two fluid flow paths and to be exhausted from outlet  32 . The first path passes between a first two facing wall surfaces, that is, a radially outer wall surface  36  of the fixed scroll wall  28  and a radially inner wall surface  38  of the orbiting scroll wall  24 . The second path extends between a second two facing surfaces, that is, a radially inner wall surface  40  of the fixed scroll wall  28  and a radially outer wall surface  42  of the orbiting scroll wall  24 . Fluid on the first flow path is trapped in crescent-shaped fluid pockets  44  which are forced to shrink in size as they are caused to move inwardly by the motion of the orbiting scroll wall as can be seen from a comparison of the position of the single highlighted pocket  44  shown in FIGS.  4  to  8 . Although a single pocket  44  is highlighted in FIGS.  4  to  8 , it will be seen that the first fluid flow path contains as many pockets of trapped fluid being compressed as there are wraps of the scroll walls. In the same way, fluid on the second fluid flow path is trapped in crescent-shaped fluid pockets  46  and is forced inwardly by motion of the orbiting scroll  16 .  
           [0006]    During compression, each fluid pocket  44 , 46  extends for less than 3608 about the circumference of the scroll assembly. The first two wall surfaces  36 , 38  are separated by just enough space, or clearance, at the circumferential ends of the pockets  44  to resist the seepage of fluid. The second two facing wall surfaces  40 , 42  are also separated by a clearance at each circumferential end of pockets  46 . These clearances are hereinafter referred to as running, or working, clearances. No sealant or lubricant is, therefore, required in the swept volume of the pump.  
           [0007]    As will be seen from FIG. 4, fluid pockets on the first fluid path extend between clearances C 1  and fluid pockets on the second fluid path extend between clearances C 2 . Clearances C 1  are substantially radially aligned and clearances C 2  are substantially radially aligned, however, clearances C 1  and C 2  are substantially diametrically opposed in the scroll assembly.  
           [0008]    It is important to accurately maintain running clearances between the scroll walls since if the running clearance is too large seepage out of the pockets occurs leading to loss in efficiency. If the running clearance is too small, there is a possibility that the scroll walls collide. It is apparent that thermal expansion of one of the scroll walls affects the running clearances between the scroll walls between ambient and running conditions. This thermal expansion causes a problem which will be explained with reference to expansion of the orbiting scroll wall  28  relative to the fixed scroll wall  24 . First, the radially outer wall surface  36  of the orbiting scroll wall  28  expands towards the radially inner wall surface  38  of the fixed scroll wall  24  thereby reducing clearance C 1  with the risk of collision between the scroll walls. Secondly, the radially inner wall surface  40  of the orbiting scroll wall  28  expands away from the radially outer wall surface  42  of the fixed scroll  24  thereby increasing the clearance C 2  therebetween and causing seepage. It is desirable therefore that when the pump is at ambient temperature (i.e. all components are at the same temperature), the scrolls do not collide with each other, but when the pump is at running temperature the clearances are neither too small that the scrolls collide nor too large that the pump does not achieve its vacuum performance.  
           [0009]    [0009]FIG. 9 shows a representation of the relationship between running clearances C 1  and C 2  and clearances A 1  and A 2  (where ‘A 1 ’ represents the clearance at ambient temperatures between the first two facing wall surfaces  36  and  38 , and ‘A 2 ’ represents the clearance at ambient temperatures between the second two facing wall surfaces  40  and  42 ). The relationship is plotted between the exhaust (radial centre) and the inlet (outer radial portion) of the scroll assembly. It will be seen that FIG. 9 does not show the actual spacing between the orbiting scroll wall  28  and the fixed scroll wall  24 , which would be represented by cyclic curves forming pockets  44 , 46 .  
           [0010]    According to the prior art, sufficient ambient clearance A 1  is provided between the first two wall surfaces  36  and  38  to allow the orbiting scroll wall to expand without colliding with the fixed scroll wall and so that at working conditions a desired running clearance C 1  is achieved. According to the prior art, the ambient clearance A 1  is increased by angularly displacing the orbiting scroll wall relative to the fixed scroll wall. This angular displacement causes the radius of the orbiting scroll wall to be reduced relative to the fixed scroll wall at any given angle about the centre of the scroll assembly, even though the actual shape and pitch of both scroll walls remains the same. If ambient clearances A 1  are increased by this angular displacement, ambient clearance A 2  will be decreased. As shown in FIG. 9, clearance A 1  is the same as clearance A 2 . At running temperatures, running clearance C 1  gradually reduces towards the inlet of the scroll assembly since thermal expansion increases depending on the radial distance from the centre of the scroll assembly. Conversely, running clearance C 2  gradually increases towards the inlet of the scroll assembly. As shown, the orbiting scroll wall  28  collides with the fixed scroll wall  24  towards the inlet of the scroll assembly. Further, compression of fluid on the first and the second fluid flow paths are different because C 1  is less than C 2  and therefore more seepage occurs in the second flow path thereby reducing efficiency.  
           [0011]    A second prior art scroll compressor is described with reference to FIG. 10 which shows the same relationship between ambient clearances A 1  and A 2 , and running clearances C 1  and C 2  as shown in FIG. 9. The second prior art scroll compressor to some extent reduces the extent of the problem highlighted above.  
           [0012]    In the second depicted prior art scroll compressor, ambient clearance A 1  between a first two facing wall surfaces  50 , 52  gradually increases as the radial distance from the centre of the scroll assembly increases and ambient clearance A 2  between a second two facing wall surfaces  54 , 56  gradually decreases as the radial distance from the centre of the scroll assembly increases such that the rate of change of A 1  and A 2  are equal and respectively constant. The first two facing wall surfaces  50 ,  52  are, respectively, a radially inner surface  50  of a fixed scroll wall  58  and a radially outer surface of an orbiting scroll wall  60 . The second two facing wall surfaces  54 ,  56  are, respectively, a radially inner surface  50  of the orbiting scroll wall  60  and a radially outer surface of the fixed scroll wall  58 .  
           [0013]    The above relationship between A 1  and A 2  is enabled by providing the orbiting scroll wall  60  with a spiral with a different pitch to that of the fixed scroll wall  58 . In more detail, the orbiting scroll wall  60  has a spiral with reduced pitch in that its radius increases more slowly as it extends away from its centre than the increase in radius of the fixed scroll wall  58 . Therefore, as the orbiting scroll wall  60  extends radially outwardly, A 1  gradually increases to compensate for the affect of thermal expansion which increases as distance from the centre (exhaust) increases. As will be seen in FIG. 10, A 1  is increased as compared to the prior art in FIG. 9. The second prior art scroll compressor allows for greater thermal expansion of the orbiting scroll wall without colliding with the fixed scroll wall at running temperatures, and without allowing C 2  to increase to allow significant seepage of gas between the second two facing wall surfaces  54 , 56 . However, clearance C 1  and C 2  are not equal and therefore there will be some difference between fluid compression on the first fluid path and on the second fluid path. However, ambient clearance A 2 , particularly towards the inlet, cannot be further increased without the risk of collision between the scroll walls.  
           [0014]    It is desirable to provide an improved solution to the above problem.  
         SUMMARY OF THE INVENTION  
         [0015]    The present invention provides a scroll compressor comprising: a scroll assembly comprising two scroll walls; and a drive for causing a relative orbiting motion between the scroll walls for compressing fluid on two fluid flow paths between an inlet and an exhaust of the scroll assembly, a first fluid flow path being formed between a first two facing wall surfaces of the scroll walls and a second fluid flow path being formed between a second two facing to wall surfaces of the scroll walls; wherein a first ambient clearance is selected between the first two facing surfaces, and a second ambient clearance is selected between the second two facing surfaces, and wherein the first and the second ambient clearances are selected independently from each other.  
           [0016]    The present invention also provides a scroll compressor comprising: a scroll assembly comprising two scroll walls; and a drive for causing a relative orbiting motion between the scroll walls for compressing fluid on two fluid flow paths between an inlet and an exhaust of the scroll assembly, a first fluid flow path being formed between a first two facing wall surfaces of the scroll walls and a second fluid flow path being formed between a second two facing wall surfaces of the scroll walls; wherein a first ambient clearance between the first two facing wall surfaces of the scroll walls increases as the radial distance from the exhaust increases and a second ambient clearance between the second two facing wall surfaces of the scroll walls decreases as the radial distance from the exhaust increases and the rate of change of the first ambient clearance is different to the rate of change of the second ambient clearance.  
           [0017]    The present invention also provides a scroll compressor comprising: an orbiting scroll wall having a radially outer wall surface and a radially inner wall surface; and a fixed scroll wall having a radially inner wall surface and a radially outer wall surface, and the orbiting scroll wall is adapted to be driven by a drive to orbit relative to the fixed scroll wall; and wherein one or both of the orbiting scroll wall and the fixed scroll wall has at least one portion in which the wall thickness is tapered and at least one portion in which the wall surfaces thereof are parallel.  
           [0018]    The present invention also provides a scroll compressor comprising: a scroll assembly comprising two scroll walls; and a drive for causing an orbiting motion of one of the scroll walls relative to the other of the scroll walls for compressing fluid between an inlet and an exhaust of the scroll assembly, the exhaust being at an axial centre of the scroll assembly and the inlet being at a radial outer portion of the scroll assembly; wherein one or both of the scroll walls are tapered so that the walls or walls have a greater radial thickness towards the exhaust and a smaller radial thickness towards the inlet.  
           [0019]    Other aspects of the invention are defined in the accompanying claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    In order that the present invention may be well understood, two embodiments thereof, which are given by way of example only, will now be described with reference to the accompanying drawings, in which:  
         [0021]    [0021]FIG. 1 is a representation of the scroll walls in accordance with a scroll compressor of a first embodiment of the present invention;  
         [0022]    [0022]FIG. 2 is a representation of the scroll walls in accordance with a scroll compressor of a second embodiment of the present invention;  
         [0023]    [0023]FIG. 3 is a side-section of a scroll compressor in accordance with the prior art;  
         [0024]    FIGS.  4  to  8  are sections showing a scroll assembly of the prior art scroll compressor shown in FIG. 3;  
         [0025]    [0025]FIG. 9 is a representation of the prior art scroll walls shown in FIGS.  4  to  8 ; and  
         [0026]    [0026]FIG. 10 is a representation of a scroll assembly in accordance with a second prior art scroll compressor. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]    The scroll compressors described hereinafter differ from the prior art described above in the shape of the scroll walls. Other aspects of the following scroll compressors are the same as the scroll compressor shown in FIG. 3 and the scroll assembly shown in FIGS.  4  to  8 , and therefore will not be described in further detail.  
         [0028]    [0028]FIG. 1 shows the relationship between ambient clearances A 1  and A 2 , and running clearances C 1  and C 2  for a scroll compressor of the first embodiment. The scroll compressor described with reference to FIG. 1 differs from the second prior art apparatus described above in that in the prior art the rate of change of ambient clearance A 1  is the same as the rate of change of the ambient clearance A 2  whereas in the first embodiment, the rate of change of A 1  is different to the rate of change of A 2  because the radially inner wall surface  70  and the radially outer wall surface  78  of the orbiting scroll  72  are not parallel as each wall surface is designed independently based on its performance requirements. In other words, ambient clearance A 1  and ambient clearance A 2  are selected independently from each other. As shown, the first ambient clearance increases uniformly (i.e. at a constant rate) as a function of the radial distance from the exhaust and the second ambient clearance decreases uniformly (i.e. at a constant rate) as a function of the radial distance from the exhaust. It should also be noted that the rate of change of the first ambient clearance is greater than the rate of change of the second ambient clearance.  
         [0029]    In the scroll compressor described with reference to FIG. 1, as compared with the second prior art apparatus, the radial outer wall surface  70  of the orbiting scroll wall  72  is a spiral with a reduced pitch at ambient temperatures to allow for thermal expansion during use and to avoid collision with a radially inner wall surface  74  of a fixed scroll wall  76  at running temperatures. On the other hand, a radially inner wall surface  78  of the orbiting scroll wall  72  is a spiral with greater pitch than the pitch of the radially inner wall surface  70  to avoid collision with a radially outer wall surface  80  of the fixed scroll wall  76  at ambient temperatures during start-up of the compressor and to allow C 1  to be approximately equal to C 2  thus optimising compressor performance. The affect of designing the wall surfaces  70  and  78  independently is to provide an orbiting scroll wall  72  which is tapered towards the inlet so that the orbiting scroll wall has a greater radial thickness towards the exhaust and a lesser radial thickness towards the inlet.  
         [0030]    The manufacture of a tapered orbiting scroll wall can be achieved by either gradually reducing the pitch of the spiral of one its wall surfaces, or gradually increasing the pitch of the spiral of the other one of its wall surfaces, or both of the above as shown in FIG. 1.  
         [0031]    It is to be understood that the important aspect of the scroll compressor described with reference to FIG. 1 is the relationship between the first two facing wall surfaces  70 , 74  (i.e. ambient clearance A 1 ) and the relationship between the second two facing wall surfaces  78 , 80  (i.e. ambient clearance A 2 ). These relationships are affected as shown in FIG. 1 by altering the pitch of the spiral of wall surfaces  70  and  78 . In a modification, the pitch of the spiral of wall surfaces  74  and/or  80  is altered to achieve a similar affect (i.e. the fixed scroll wall has a radial wall thickness which varies between the inlet and the exhaust). In a further modification, the spiral of one of the surfaces  70  and  74  and one of the surfaces  78  and  80  are altered to achieve a gain in performance (i.e. both the orbiting and the fixed scroll walls have respective radial wall thickness which vary between the exhaust and inlet).  
         [0032]    A further embodiment of the present invention is designed taking into account local variations in temperature and differing manufacturing tolerances within the scroll wall assembly. To take account of these, the rate of change of the first and/or second ambient clearances are not constant from the exhaust to the inlet i.e. ambient clearances A 1  and/or A 2  change non-uniformly. Accordingly, the embodiment benefits from improved performance in comparison with the scroll compressor described with reference to FIG. 1.  
         [0033]    As discussed above in relation to the prior art, thermal expansion of the orbiting scroll wall is greater at the outer radial portions thereof closer to the inlet  30  because expansion at outer radial portions is compound to the expansion at inner radial portions. Therefore, in the embodiment, the orbiting scroll wall has a tapered portion  86  closer to the inlet  30  and a parallel portion  88  closer to the exhaust. This means that the first ambient clearance A 1  changes at a different rate over the extent of the tapered portion to the rate of change of the first ambient clearance over the extent of the parallel portion. Likewise, the second ambient clearance A 2  changes at a different rate over the extent of the tapered portion to the rate of change of the second ambient clearance over the extent of the parallel portion. In the tapered portion, the radial wall thickness is gradually reduced towards the inlet whereas with the parallel portion the thickness is constant. The tapered portion  86  reduces the possibility of collisions at radially outer portions of the scroll assembly, whilst the parallel portion  88  increases performance where little thermal expansion takes place. The pressure during the inlet stages of the scroll assembly is less than that during the compression and exhaust stages, and therefore clearances between the scroll walls at the inlet stages can be larger than those during the compression and exhaust stages, because less seepage takes place at low pressures. Accordingly, a further advantage of the arrangement shown with reference to FIG. 2 is that less manufacturing accuracy is required at the outer radial portion of the scroll assembly thereby reducing costs.  
         [0034]    The manufacture of an orbiting scroll wall with a tapered portion can be achieved by either gradually reducing the pitch of the spiral of one its wall surfaces, or gradually increasing the pitch of the spiral of the other one of its wall surfaces, or both of the above as shown in FIG. 2. In addition to, or instead of, providing the orbiting scroll with a tapered portion it would be possible to provide the fixed scroll wall with a tapered portion in which one or both of the wall surfaces  94 , 96  of the fixed scroll wall  98  have a spiral with increasing/decreasing pitch.  
         [0035]    Depending on the characteristics of the compressor, the fluid being compressed, acceptable manufacturing tolerances, it can be desirable to provide either or both of the fixed or orbiting scroll walls with more than one tapered portion and/or more than one parallel portion. In this regard, one or both of the scroll walls can have tapered and parallel portions in a similar way to that shown in FIG. 2, and a further tapered portion towards the exhaust of the scroll assembly (i.e. the wall thickness gradually increases between the parallel portion and the exhaust) in view of the reduced level of expansion which occurs at the centre of the scroll assembly. In this way, there is a tapered portion at a radially inner part of the scroll assembly with increasing radial wall thickness towards the exhaust where the scroll assembly expands least, a parallel portion at a radial intermediate part of the scroll assembly for increased efficiency, and a tapered portion at a radially outer part of the scroll assembly with decreasing thickness towards the inlet where more thermal expansion takes place, and where greater manufacturing tolerances are allowable. The rate of change of the wall thickness of the tapered portions can be the same or different.  
         [0036]    A further modification of the scroll compressor described with reference to FIG. 2 comprises one or both of the scroll walls with a first tapered portion and a second tapered portion and the rate of change of the first and the second ambient clearances are different over the extent of the first tapered portion and over the extent of the second tapered portion. In other words, the radial thickness of the walls of at least one of the scroll walls varies at a different rate in the first tapered portion and in the second tapered portion. One example of this arrangement comprises a first tapered portion provided at a radially inner part of the scroll assembly with a wall thickness that decreases from the exhaust towards the inlet at a first rate. A second tapered portion is provided at an outer radial part of the scroll assembly with a wall thickness that decreases towards the inlet at a second rate different to the first rate. This arrangement seeks to increase efficiency towards the radial centre of the scroll assembly and allow for greater expansion towards the radially outer part of the assembly and/or where greater manufacturing tolerances are allowable. The same effect could be achieved by providing one scroll wall with a tapered portion towards the radial centre of the scroll assembly and a parallel portion towards the radially outer part of the assembly and the second scroll wall with a parallel portion towards the radial centre of the scroll assembly and a tapered portion towards the radially outer part of the assembly.  
         [0037]    In summary, the embodiment provides a scroll compressor comprising a scroll assembly including two scroll walls at least one of which has a portion in which the radial wall thickness varies between an inlet and an outlet of the scroll assembly and a second portion in which the radial wall thickness is constant or varies at a different rate between the inlet and the outlet.  
         [0038]    Reference has been made to ambient clearances A 1  and A 2 , and running clearances C 1  and C 2  in the description of the embodiment and prior art. These clearances have been greatly exaggerated in the Figures since they are usually only of the order of 10 to 500 microns.  
         [0039]    The invention has been described with reference to a scroll compressor comprising a scroll assembly as shown in FIG. 3. However, the present invention covers a scroll compressor comprising a scroll assembly in which a fixed scroll comprises a base plate having two scroll walls extending orthogonally from respective sides of the base plate and intermeshing with respective orbiting scroll walls of two orbiting scrolls.  
         [0040]    While the present invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be apparent to by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.