Abstract:
A thrust control system for use with a turbocompressor having gas bearings. More specifically, the system concerns an arrangement of gas bearings for use in a turbocompressor or other device where a large temperature difference between the turbine and the compressor housing could cause unacceptable performance of the turbocompressor thrust bearings if located in proximity of the turbine and compressor wheels. This danger is obviated in the system by relocating the gas thrust bearings so as to minimize the axial distance between them. This configuration affords the additional advantage that gasses of different composition may be separately used as a seal gas in cases where the process gas in the turbine and compressor are incompatible.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to gas bearing turbocompressors. More particularly, the invention concerns turbocompressors embodying gas bearings wherein large temperature differences develop between the turbine and the compressor ends of the device, such as is the case in turbochargers for internal combustion engines where the turbine is driven by hot exhaust gases and the compressor receives cool intake air. 
     2. Discussion of the Invention 
     Turbocompressors have been in use as turbochargers for internal combustion engines for many years. These types of turbocompressors have generally embodied oil-lubricated bearings. The trend in recent years in turbocompressor design has been towards higher compression ratios requiring higher revolutions per minute (RPM) of the spindle of the turbocompressor and resulting in higher exhaust temperatures. Under such conditions oil lubrication of the bearings becomes inadequate and can possibly result in cavitation in the bearings as a result of the higher rubbing speed, and in thermal decomposition as a consequence of the higher temperatures. A solution to the aforementioned problems is provided by the use of gas bearings such as the bearings disclosed in U.S. Pat. No. 4,808,070 issued to the present inventor. The novel gas bearings disclosed in U.S. Pat. No. 4,808,070 can easily handle the required RPM and rubbing speeds of most types of modern turbo compressors. 
     In a typical turbocharger the compressor is fed through a filter which causes a significant pressure drop at the inlet to the compressor resulting in an inlet pressure lower than the atmosphere. On the other hand, the conventional turbine discharges to atmosphere through a muffler or catalytic converter also causing a pressure drop which results in a pressure higher than atmosphere at the exhaust of the turbine. The resulting pressure difference between the compressor housing and the turbine housing causes a net thrust to develop in the shaft connecting the compressor wheel and the turbine wheel. In an oil-lubricated turbo-charger this thrust is absorbed by a traditional oil-lubricated thrust washer. However, in the case of gas bearings the thrust is typically compensated by the arrangement described in U.S. Pat. No. 5,567,129 issued to the present inventor. Because of the relevance of the U.S. Pat. Nos. 4,808,070 and 5,567,129 to a complete understanding of the present invention, both of these patents are hereby incorporated by reference as though fully set forth herein. 
     As previously mentioned, the thrust of the present invention is directed towards solving the problems caused by large temperature differences between the ends of the turbocompressor. In this regard, the correct operation of the thrust bearings discussed in U.S. Pat. No. 5,567,129 depends in large measure on maintaining the correct clearances in the gaps between the stationary and moving parts of the thrust bearings. These gap dimensions are affected by changes in length of the housing and the length of the shaft due to thermal expansion. Such changes are particularly large if the temperature distribution in the housing differs appreciably from the temperature distribution in the shaft and the thermal expansion coefficient of the shaft differs substantially from that of the housing. These conditions result in serious operating problems. For example, if the shaft elongates more than the housing, the gaps increase thus decreasing the stiffness of the thrust bearings and increasing the parasitic mass flow through the bearings. Similarly, if the housing elongates more than the shaft the gaps may completely close resulting in contact of the facing surfaces of the adjoining components thereby resulting in severe damage to the bearings. 
     Other problems can occur if the gas flowing in the compressor is not compatible with gas flowing in the turbine. This latter problem can be solved by supplying gas to the thrust bearings from an independent source, which gas is compatible with the gas flowing in the turbine and compressor. 
     Although this invention is presented primarily in the context of turbochargers for use with internal combustion engines, the arrangement of the component parts also solves a different problem of great importance in the context of turbo-compressors for supplying air to fuel cells (see for example U.S. Pat. No. 5,523,176 issued to the present inventor). This problem concerns the danger of oil contamination in the supplied air, which contamination can poison the catalyst in the fuel cell. The turbocompressor of the present invention also permits the use of a buffer gas where any mixing of the gasses in the expander and compressor housings must be positively prevented. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improved gas bearing system for use in a turbocompressor or similar device which embodies radial gas bearings and thrust gas bearings which can tolerate large temperature difference between the turbine and the compressor. More particularly, it is an object of the invention to provide an improved system of the aforementioned character in which two opposing gas thrust bearings are brought into close axial proximity so as to minimize the length over which temperature gradients can cause a variation of the built-in gap dimensions in the gas thrust bearings. 
     Another object of the invention is to provide an apparatus as described in the preceding paragraphs in which the gas supplied to either of the thrust bearings can be other than the process gas used in the turbine and expander. 
     A further object of the invention is to provide a turbocompressor of the class described in which the gas supplied to the thrust bearings performs the function of seal gas to prevent any intermixing of the compressor process gas into the turbine process gas or vice-versa. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic, side elevation, cross-sectional view of a gas bearing turbocompressor apparatus illustrating one embodiment of the present invention in which the opposing thrust bearings are disposed in close axial proximity and are provided with independent gas supply sources. 
     FIG. 2 is a reproduction of FIG. 2 of incorporated by reference U.S. Pat. No. 5,567,129. 
     FIG. 3 is diagrammatic, side elevational, cross-sectional view showing an alternate form of the invention which is useful in cases where the central portion of the device is encumbered by auxiliary apparatus, such as an integral electric motor. 
    
    
     DESCRIPTION OF THE INVENTION 
     Referring to the drawings and particularly to FIG. 1, one form of the improved apparatus of the invention is there shown. The apparatus shown in FIG. 1 can be best understood by a comparison of FIG. 1 of the drawings with FIG. 2 of U.S. Pat. No. 5,567,129, which was issued to the present inventor and which is incorporated herein by reference. Referring particularly to FIG. 2, this figure shows a turbocompressor equipped with two thrust bearings and one compensation chamber. One thrust bearing, which is identified by number  68 , is located proximate wheel  60  and comprises the volume between the inboard surface  60   a  of the turbine wheel  60  and face portions  72  and  74  of support housing  50 . As indicated in FIG. 2, face portion  72  is spaced from surface  60   a  by a gap distance identified by the numeral  75 , while face  74  is spaced from surface  60   a  by a gap distance identified by the numeral  77 . Forming a boundary between face portions  72  and  74  is a step  80 . Step  80  is generally circular in shape and is concentric with the axis  81  of shaft  56 . Outer edge  84  of turbine wheel  60  and a relief edge designated as  83 , which functions to define the outer boundary of thrust bearing  68 , are both concentric with axis  81 . It is to be noted that the inner boundary of thrust bearing  68  is defined by an edge  88  which communicates directly with grooves  64  formed on shaft  56 . 
     A second thrust bearing  70 , which is similar to thrust bearing  68 , is located proximate wheel  62 . Thrust bearing  70 , in exact analogy to thrust bearing  68 , comprises a first face portion extending from the surface  104   a  to concentric step  108  and a second face portion extending from concentric step  108  to outer edge  106 , both being parallel to the inboard surface of compressor wheel  62 . The gaps separating the facing surfaces on the compressor side are not specifically numbered in this figure, because they are mirror images of the gaps  75  and  77 , which are disposed proximate turbine wheel  60 , and perform the same function of generating an axial force as a function of the axial displacement of shaft  56  within housing  50 . The forces generated by the thrust bearings are axially opposite to each other thereby causing the shaft to seek an axial position where the two opposing forces are equal in magnitude. Any displacement from this position causes the gaps on one side to widen and causes the corresponding force to decrease while simultaneously causing the gaps on the other side to narrow by an equal amount and causing the corresponding force to increase. The net difference between the axial forces tends to urge the shaft back toward its equilibrium position. 
     The performance of the thrust bearings depend critically on the dimension of the gaps in relation to the dimensions of the step. This aspect is discussed in detail in the disclosure of incorporated by reference U.S. Pat. No, 5,567,129 and any change in the relative dimensions can be detrimental to the correct operation of the bearings. In particular, if the gaps of both bearings increase (as opposed to the condition where one side decreases while the other side increases, as in the case of axial displacement), then the stiffness of the bearing system decreases. On the other hand, if the gaps on both sides decrease to zero, then contact of the moving surfaces will occur and severe damage to the bearings can result. 
     Gap changes of the character described in the preceding paragraph are due to the change in the length of housing  50  and shaft  56  caused by temperature transients and differences in thermal expansion. Obviously, the gaps widen if the shaft  56  elongates more than the housing  50 . Conversely, the gaps narrow in the opposite case. The change in gap width is proportional to the difference in temperature, the differences (if any) in thermal expansion coefficients, and the distance between the bearings, equal to the length of the shaft  56 . 
     The temperature differences depend on the state of the gases being processed and cannot be changed. The differences in thermal expansion coefficients can be eliminated by using similar materials for the shaft and housing respectively. The distance between the bearings can be minimized by design. This novel design change, which is at the heart of the present invention, is uniquely accomplished by moving the thrust bearings from the location shown in FIG. 2 to the new location shown in FIG.  1 . More particularly, as shown in FIG. 2, the flat faces of the thrust bearings  68  and  70 , as defined by the back faces of the turbine wheels  60  and  62 , are moved to the opposite sides of a single element  162  which is located within a central or intermediate chamber  163 . In this regard, support housing  50   a , which generally corresponds to support housing  50  of the &#39;129 patent, is provided with first, second, third and fourth spaced-apart faces F- 1 , F- 2 , F- 3 , and F- 4  respectively with chamber  163  being disposed between faces F- 3  and F- 4 . With this construction, element  162  is strategically positioned between the two radial bearings each being characterized by longitudinal grooves  64  and recesses or cavities “C” which are described in detail in U.S. Pat. No. 4,808,070. As best seen in FIG. 1, element  162  comprises a substantially flat, thin disk with parallel first and second smooth faces,  162   a  and  162   b . These faces define a third chamber comprising new first thrust bearing  165  on one face and a fourth chamber comprising second new thrust bearing  167  on the other face. As indicated in FIG. 1, the first and second recessed areas of the new thrust bearings extend from the shaft  56  radially out to steps identified by numbers  169  and  171  respectively, each being concentric with the axis  81  of shaft  56 . As depicted in FIG. 1, shaft  56  rotates within generally cylindrically shaped bore  52   a  of housing  50   a . The axial width of the recesses are defined in FIG. 1 by the numerals  174  and  175  respectively. With the construction thus described and shown in FIG. 1, the radial passageways discharge into a common volume  173  which is located beyond the edge  163   c  of disk  162 . 
     In operation of the improved apparatus of the invention, a first seal gas having a composition compatible with the process gas flowing through turbine  24  is supplied through a first supply means or inlet  100  to a circumferential groove  102  which is machined in the inner surface of housing  50   a . The seal gas then flows through longitudinal grooves  63  (identical to grooves  64  in the radial bearings of the &#39;129 patent) to circumferential groove  56   b , where it enters gap  174 , hence a gap to  175  and is then discharged to common volume  173 . Labyrinth  111  substantially restricts the outflow of the first gas from groove  102  to the environment of turbine  24  (see also the &#39;129 patent). 
     In similar fashion, a second seal gas of composition compatible with the process gas flowing through compressor  22  is supplied through a second supply means or inlet  101  to circumferential groove  103 . From groove  103  the seal gas flows through longitudinal grooves  64  to circumferential grove  56   a , where it enters gap  177 , and is discharged to common volume  173 . Labyrinth  110  substantially restricts the outflow of said second gas from groove  103  to the environment of compressor  22  (see also the &#39;129 patent). 
     It is to be understood that first and second seal gases can be of the same or different compositions. For example, a single source of gas can be used to supply the first and second seal gases if one gas is compatible with both the turbine process gas and the compressor process gas. This is the case, for example, if the turbocompressor is used in conjunction with a fuel cell where the compressor process gas is ambient air and the turbine discharges to the atmosphere. In this instance, the seal gas for both sides can be taken directly from the compressor diffuser as shown in FIG.  2 . The used seal gas is vented from volume  173  through outlet  181  where it can be recovered and reprocessed if so desired. (As for example, where discharge of the gas to the atmosphere would be environmentally objectionable or otherwise cost ineffective). 
     The configuration described in the preceding paragraphs solves problems resulting from temperature gradients on the gaps in the thrust bearings. Since the distance between the opposed thrust bearings is the thickness of disk  162 , the temperature difference between the two thrust bearings is minimal even if the difference between the temperatures of the turbine and compressor is large. Furthermore, under transient conditions a temperature difference between the housing, which determines the distance between the recessed faces of the thrust bearings through the thermal expansion coefficient of the housing, and the disk, which determines the distance between the flat faces of the thrust bearings through the thermal expansion coefficient of the disk, has minimal effect on the width of the gaps, determined as the difference between the expansion of the housing and the expansion of the disk, since the length over which thermal expansion operates is minimal. 
     Referring next to FIG. 3, this figure shows an alternate form of the improved apparatus. This alternate embodiment is similar in many respects to the earlier described embodiment, but is particularly useful when the central portion of the housing is used to contain auxiliary equipment such as an electric motor for driving the compressor when the power delivered by the turbine is insufficient to develop the desired pressure ratio in the compressor. 
     In the embodiment of the invention shown in FIG. 3, where like numbers are used to identify like components, the assembly of both thrust bearings is moved from between the two radial bearings to a position between one radial bearing and the corresponding labyrinth. Other variations of detail are also shown in FIG. 3, any one of which may be used singly or in combination depending on the end application of the apparatus without departing from the spirit of the invention. 
     Referring particularly to FIG. 3, there is shown a single gas inlet  100  for the seal gas, a useful simplification if the seal gas is compatible with both the turbine process gas and the compressor process gas. In this case the seal gas is admitted through circumferential groove  102  in the housing  50   b  and delivered through longitudinal grooves  63  to circumferential groove  56   b  in the shaft, thence to gaps  174  and  175  of the first thrust bearing and finally out to volume  173 . The second thrust bearing receives seal gas through openings  179  formed in disk  162  in the proximity to shaft  56 , forming a direct channel of communication between circumferential grooves  56   a  and  56   b . From groove  56   a  the gas then proceeds through gaps  181  and  182  to collecting volume  173 . 
     Another important variation is shown in FIG. 3, wherein the smooth and recessed faces are transposed in either one or both bearings. In particular, FIG. 3 shows both bearings being transposed. In this configuration, the recesses having axial widths  183  and  185  respectively are located on the stepped faces of disk  162 , whereas the faces of the housing are smooth. 
     Additionally, a further variation shown in FIG. 3 involves the provision of a third gas supply means or third gas inlet  187  which is provided in or near the axis of support member  67 . This third gas inlet is interconnected with a circumferential groove  189  located in the middle of labyrinth  191 , and is provided with teeth  193  on either side. A seal gas compatible with both expander and compressor process air is admitted to the middle of the labyrinth and positively prevents any admixture of compressor gas present in inlet manifold  94  with expander gas present in the exhaust manifold  65   a  of turbine  24  via hollow shaft  56 . 
     Having now described the invention in detail in accordance with the requirements of the patent statutes, those skilled in this art will have no difficulty in making changes and modifications in the individual parts or their relative assembly in order to meet specific requirements or conditions. Such changes and modifications may be made with out departing from the scope and spirit of the invention, as set forth in the following claims.