Patent Publication Number: US-6663341-B2

Title: Process fluid recycle system for a compressor assembly

Description:
FIELD OF THE INVENTION 
     The present invention relates to a process fluid recycle system for a compressor assembly having at least a compressor driven from a gear case to recycle process fluid flowing through a shaft seal, from the compressor to the gear case. More particularly, the present invention relates to such a process fluid recycle system in which an anti-back flow compressor is employed to prevent gear oil contained in the gear case from entering the compressor through the shaft seal. 
     BACKGROUND OF THE INVENTION 
     Prior art compressor assemblies employ one or more stages of compression, formed for instance, by a centrifugal compressor driven from an adjacent gear case by a shaft extending through a shaft seal between the stage of compression and the gear case. The shaft seal can be a labyrinth seal that is designed to allow rotation of the shaft while at least inhibiting loss of a process fluid being compressed by the compressor. 
     The shaft seal itself can be designed to accept a certain flow of the process fluid and therefore, a loss from the compressor. This is done to have a non-contact zero wear gas seal. As a result the gear oil within the gear case will not back flow through the shaft seal, in a direction from the gear case to the stage of compression, and thereby contaminate the process fluid. 
     As may be appreciated, the flow of process fluid into the gear case must either be vented or recycled back to the stage or stages of compression. When a compressor assembly is used in a refrigeration system, there is no option but to recycle the process fluid in that certain refrigerants are either toxic or potentially destructive to the environment. The potential loss of refrigerant can also degrade the performance of the refrigeration system. For instance, the composition of refrigerants used in mixed gas refrigerant systems, will change due to loss through shaft seals and the like. A typical mixed gas refrigerant is made up of nitrogen, argon, carbon tetrafluoride, pentabromoethane and perfluoropropl methyl ether and such constituents will be lost in unequal amounts due to their different properties. Furthermore, such refrigerants are expensive and any loss of refrigerant is a significant cost penalty to the process. 
     The prior art provides many examples of compressor assemblies that recycle process fluid flow through the shaft seal back to a low pressure inlet side of the compressor. An example of this can be found in U.S. Pat. No. 6,018,962. In this patent, the compressor assembly is housed in an air tight enclosure. Refrigerant flowing into the gear case mixes with gear oil and a mixture of refrigerant and gear oil collects in the oil sump of the gear case. The mixture is drawn through a demister element to separate the gear oil from the mixture under suction provided by the low pressure side of the compressor. The suction further draws the refrigerant from the demister element back into the low pressure side of the compressor. 
     In U.S. Pat. No. 4,213,307 the gear case of the compressor assembly is vented to a coalescing filter. The coalescing filter separates the refrigerant from the gear oil. The gear oil, after separation from the refrigerant, is pumped back into a oil sump of the gear case by a jet pump. The refrigerant is drawn from the coalescing filter back to the low pressure side of the compressor. A separate oil pump is used to pump oil to bearings contained within the gear case and also to supply pressurized oil as a motive fluid to the jet pump. 
     A problem inherent in all of the prior art devices is that when low pressure transients are encountered or during time periods in which the compressor assembly is started or shut down, the pressure within the gear case can be higher than that of the compressor being driven from the gear case. Normally, during operation, the pressure within the compressor is higher than the pressure within the gear case. When such pressure reversal occurs, the gear oil can back flow, that is, be driven through the shaft seal, from the gear case to the compressor to contaminate the refrigerant or other process fluid being compressed. 
     Furthermore, the illustrative, prior art compressor assemblies, described above, are integrated systems that are not very applicable to large scale installations of compressors or assemblies in which the separate components of the compressor assembly, namely, the motor, gear case, and compressor, are provided with separate enclosures and the components are separately installed on site. 
     As will be discussed, the present invention provides a system for recycling process fluid flow through a shaft seal of a compressor assembly that is specifically designed to prevent back flow of the gear oil into a compressor. Moreover, any application of the system of the present invention inherently requires very little modification of the components making up the compressor assembly. 
     SUMMARY OF THE INVENTION 
     The present invention provides a process fluid recycle system for a compressor assembly. The compressor assembly has at least a compressor driven from a gear case. The system acts to recycle process fluid vapor flowing through a shaft seal, from the compressor to the gear case. 
     In accordance with the present invention, the process fluid recycle system includes at least one coalescing filter to separate oil mist made up of the gear oil from the process fluid vapor. A recycle conduit is connected, at one end, to a low pressure inlet of the compressor assembly. The other end of the recycle conduit is in flow communication with the at least one coalescing filter to return the process fluid vapor to the compressor assembly. Two alternate flow paths are provided to conduct the oil mist and the process fluid vapor from the compressor assembly to the at least one coalescing filter. One of the two alternate flow paths is formed by an anti-back flow compressor in flow communication with the gear case such that operation of the anti-back flow compressor reduces pressure within the gear case below the compressor. The other of the two alternate flow paths is formed by a conduit also in flow communication with the gear case. A valve is located in the conduit to prevent the flow of the oil mist and the process fluid to the gear case during operation of the anti-back flow compressor. A controller activates the anti-back flow compressor to ensure pressure within the gear case is less than that of the compressor, thereby to prevent gear oil from being driven from the gear case into the compressor through the shaft seal. 
     The compressor assembly can also be provided with an oil sump connected to the gear case such that the oil mist and process fluid vapor collects in a headspace region thereof. The two alternate flow paths are connected to the oil sump so as to receive the oil mist and the process fluid vapor from the headspace region thereof. An oil return pump is connected between the at least one coalescing filter and the oil sump to return the gear oil to the oil sump. 
     As can be appreciated from the above description, the use of the anti-back flow compressor prevents the back flow of gear oil through the shaft seal that might otherwise occur during start-up and shutdown and other low pressure transients. Moreover, since the recycle system of the present invention is applied to existing compressor assemblies the compressor assemblies do not have to be modified to take advantage of the present invention. In this regard, one or more coalescing filters can be applied to prevent any oil from being recycled back to the compressor because, unlike some prior art designs, the filter does not have to be incorporated into the compressor assembly itself. 
     The present invention is applicable to multistage compression assemblies and in one aspect recycles process fluids through replication of the process fluid recycle system, described above, for each compressor thereof. In such an assembly, a second compressor is connected in series with a first compressor such that process fluid is initially compressed in the first compressor and is further compressed in the second compressor. The compressor assembly has first and second low pressure inlets to the first and second recycle compressors and first and second gear cases associated therewith. 
     A recycle system in accordance with this aspect of the present invention has a recycle conduit that is a first recycle conduit connected to the low pressure inlet of the first compressor. A recycle conduit, constituting a second recycle conduit, is connected to a low pressure inlet of the second compressor. The at least one coalescing filter is at least one first coalescing filter in flow communication with the other end of the first recycle conduit. At least one second coalescing filter is in flow communication with the other end of the second recycle conduit. A first of two alternate flow paths in flow communication with the first gear case to conduct the oil mist and the process fluid vapor to the at least one first coalescing filter. A second of the two alternate flow paths is in flow communication with the second gear case to conduct the oil mist and the process fluid vapor to the at least one second coalescing filter. The controller activates each anti-back flow compressor of the first and second of the two alternative flow paths to ensure pressure within the first and second gear case is less than that of the first and second compressor, respectively. 
     The compressor assembly of a multi-stage unit can also have first and second oil sumps connected to the first and second gear cases such that the oil mist and process fluid vapor collects in first and second headspace regions thereof. The first and second of the two alternate flow paths are connected to the first and second oil sumps so as to receive the oil mist and the process fluid vapor from the first and second headspace regions, respectively. A first oil return pump is connected between the at least one first coalescing filter and the first oil sump to return the gear oil to the first oil sump. A second oil return pump is connected between the at least one second coalescing filter and the first oil sump to return the gear oil to the first oil sump. 
     The present invention in another aspect is applied to multi-stage compressor assemblies in a more simplified fashion by combining elements. For instance in a multi-stage compressor assembly, the process fluid enters the first compressor through the low pressure inlet thereof, which thus constitutes a system inlet for the compressor assembly. The one end of the recycle conduit is connected to the system inlet. A first of two alternate flow paths is in flow communication with the first gear case to conduct the oil mist and the process fluid vapor to the at least one coalescing filter. A second of the two alternate flow paths is in flow communication with the second gear case to also conduct the oil mist and the process fluid vapor to the at least one coalescing filter. The controller activates each anti-back flow compressor of the first and second two alternate flow paths to ensure pressure within the gear case is less than that of the first and second compressors. 
     The compressor assembly can be provided with first and second oil sumps connected to the first and second gear cases such that the oil mist and process fluid vapor collects in first and second headspace regions thereof. The first and second of the two alternate flow paths are connected to the first and second oil sumps so as to receive the oil mist and the process fluid vapor from the first and second headspace regions, respectively. An oil return pump is connected between the at least one coalescing filter and the first and second oil sump to return the gear oil to the first and second oil sump. 
     Alternatively, first and second phase separators are connected to the first and second gear cases to separate the oil mist and the process fluid vapor from the gear oil. The compressor assembly also has a common oil sump connected to the first and second phase separators to receive the gear oil therefrom. The first and second of the two alternate flow paths are connected to the first and second phase separators to receive the oil mist and the process fluid vapor therefrom. An oil return pump is connected between the at least one coalescing filter and the common oil sump to return the gear oil to the common oil sump. 
     In a yet further aspect of the present invention involving its application to multi-stage compressors, still further simplification is possible. In such aspect of the present invention, the one end of the recycle conduit is connected to the system inlet. The compressor assembly is provided with a common oil sump connected to the first and second gear cases such that the process fluid vapor and the oil mist collecting in a headspace region thereof. This allows the two alternate flow paths to be connected to the common oil sump so as to receive the process fluid vapor and the oil mist from the headspace region. An oil return pump is connected between the at least one coalescing filter and the common oil sump to return the gear oil to the first and second oil sumps. 
     In all aspects of the present invention, an oil vapor adsorption trap and a water vapor adsorption trap can be interposed between the at least one coalescing filter and the conduit or each of the at least one first and second coalescing filters and each of the first and second recycle conduits. Furthermore, the controller can be a pressure differential switch connected to the anti-back flow compressor. In aspects of the invention involving multi-stage compression, the controller can comprise two pressure differential switches each respectively connected to the anti-back flow compressor of the first and second two alternate flow paths. Two pressure differential switches are positioned to react to pressure differentials between the first gear case and the first compressor and the second gear case and the second compressor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     While the specification concludes with claims distinctly pointing out the subject matter that Applicants regard as their invention, it is believed that the invention will be better understood when taken in connection with the accompanying drawings in which: 
     FIG. 1 is a schematic illustration of a process fluid recycle system in accordance with the present invention; 
     FIG. 2 is a schematic illustration of a process fluid recycle system in accordance with the present invention of the type shown in FIG. 1 that is applied to successive compressors of a compressor assembly; 
     FIG. 3 is a schematic illustration of a process fluid recycle system in accordance with the present invention employed in connection with a compressor assembly having two compressors; 
     FIG. 4 is a schematic illustration of an alternative embodiment of the process fluid recycle system illustrated in FIG. 3; and 
     FIG. 5 is a schematic illustration of an alternative embodiment of the process fluid recycle system illustrated in FIG.  3 . 
    
    
     In order to avoid repetitious explanations of the elements in the Figures, the same reference numbers are used for the same elements that appear in the various figures without modification. Furthermore, where two identical elements are present, a suffix “a” to the reference number is used to indicate the first of the elements and a suffix “b” is used to indicate the second of the elements. 
     DETAILED DESCRIPTION 
     With reference to FIG. 1, a process fluid recycle system  1  is illustrated in connection with the recycle of process fluid being compressed by a compressor assembly  2 . Compressor assembly  2  is provided with a single compressor  10  which can be a centrifugal compressor of the type having an impeller to compress a process fluid. In such a compressor, the impeller is driven by a shaft connected to gears located within a gear case  12 . The gears are lubricated by gear oil. In case of a mixed gas refrigerant, a suitable gear oil is polyalphaolefin. The gears within gear case  12  and therefore, the impeller of compressor  10  are driven by an electric motor. 
     The gear oil drains via a conduit  14  into an oil sump  16 . Oil sump  16  has a headspace region  18  situated above liquid gear oil  20 . A submersible oil pump  21  pumps oil through a return path  22  back to gear case  12 . Known features of return path  22  are not illustrated for purposes of simplicity. However, as would be known to those skilled in the art, return path  22  could include an oil cooler and filter. Gear oil could be returned to gear case  12  through an emergency oil reservoir. Additionally, suitable known controls are also not illustrated. 
     In compressor assembly  2 , a shaft seal  26  is provided to seal the shaft that is used to drive compressor  10  from gear case  12 . Shaft seal  26  acts to prevent gear oil from entering compressor  10  to contaminate the process fluid being compressed. Shaft seal  26 , which can be a labyrinth seal, is continually self-purged with process fluid during normal operation. During such times, the compressor discharge pressure, which is greater than the gear case pressure, forces process fluid vapor to flow as a purge gas flow through the shaft seal  26 . Thus, the process fluid, which can be a mixed gas refrigerant, is constantly being forced through shaft seal  26  into gear case  12 . 
     During flow of gear oil through gear oil passage  14  to oil sump  16 , the process fluid vapor will collect within headspace region  18  of oil sump  16  along with oil mist made up of the gear oil. 
     In accordance with the present invention, the process fluid vapor is recycled from headspace region  18  of oil sump  16  and recirculated back to compressor  10 . Since, oil mist and process fluid vapor collects within headspace region  18 , the oil mist must be separated from the process fluid vapor before the process fluid vapor is returned to compressor  10 . This is accomplished by provision of one or more coalescing filters  28 . Coalescing filters  28  can be obtained from Parker Company of 500 Glaspie St., Oxford, Mich. 48371 and Hankinson, Inc. of 1000 Philadelphia St., Canonsburg, Pa. 15317. It has been found that the concentration of oil within the process fluid leaving the last coalescing filter  28  will be in the part per billion range. The oil collects within the bottom of coalescing filters  28 . It is preferable that the collected oil be directly reused. As such, an oil return circuit  30  is provided having an oil pump  32  to pump the collected oil back into oil sump  16 . A check valve  34  is provided to prevent the back flow of oil  20  from oil sump  16 . Coalescing filters  28  are in flow communication with gear case  12  through headspace region  18  of oil sump  16 . There are two alternate flow paths for such flow communication. One of the two alternate flow paths is provided by a conduit  36  having preferably a check valve  38  to prevent the back flow of oil during operation of an anti-back flow compressor  50  (discussed hereinafter) that serves as the other of the two alternate flow paths. An oil vapor adsorption trap  40  can be connected to coalescing filters  28  to adsorb oil vapor. The adsorbent used may be carbon, molecular sieve or the like. A suitable oil vapor trap may be obtained from the Parker Company and Hankinson, Inc. It is to be noted that in come cases it may not be necessary to include oil vapor trap  40 , dependent upon the vapor pressure of the gear oil. In systems in which water vapor may be present, a water vapor trap  42  can be provided to remove any water. Water vapor trap  42  can contain an adsorbent such as silica gel, molecular sieve, alumina or the like. Suitable water vapor traps can be obtained from Sporlan Valve Company of 206 Lange Dr., Washington, Mo. 63090 and Watsco Inc. of 2665 South Bayshore Dr., Coconut Grove, Fla. 33133. 
     As may be appreciated by those skilled in the art, in suitable applications of the present invention, oil vapor adsorption trap  40  and water vapor trap  42  could be deleted. Additionally, embodiments of the present invention are possible in which only a single coalescing filter  28  is employed or multiple coalescing filters  28  are used. 
     The process fluid after filtering is returned by a recycle conduit  44  connected at opposite ends to water vapor trap  42  and the low pressure inlet  46  of compressor  2 . A surge check valve  48  is provided to prevent back flow of process fluid. It is to be noted that the process fluid after filtering is returned back to the low pressure inlet  46  of compressor  10  at a point that would be upstream of inlet vanes of the compressor. The suction pressure produced by compressor  10  is sufficiently low, compared to the pressure within gear case  12 , to cause process fluid to flow from gear case  12 , headspace region  18 , coalescing filters  28 , oil vapor adsorption trap  40  and water vapor adsorption trap  42 . 
     During startup or shutdown of compressor assembly  2 , the discharge pressure of compressor  10  is approximately the same as the suction pressure. Under these conditions, oil can back flow through shaft seal  26  into compressor  10 . In order to assure that process fluid vapor always flows through shaft seal  26  and into gear case  12  without oil seeping in the opposite direction, the anti-back flow compressor  50  operates to lower the pressure within gear case  12  with respect to that of compressor  10 . In such manner, the back flow of gear oil into the compressor  10  is prevented, thereby to prevent contamination of the process fluid being compressed by compressor  10 . 
     Anti-back flow compressor  50  can be controlled by a known differential pressure switch  52 . Differential pressure switch  52  is connected to anti-back flow compressor  50  by way of a conductor  54 . The differential pressure switch is preferably set to trigger anti-back flow compressor  50  when the pressure within gear case  12  approaches that within compressor  10 . The differential pressure switch can preferably be set to maintain the pressure within gear case  12  is 5-15 psig below compressor  10 . Suction is thereby applied to gear case  12  to draw the process fluid vapor and oil mist from headspace region  18  and gear case  12  to the coalescing filter of filters  28 . This will normally happen during startup and shutdown. Additionally, other low pressure transient conditions are possible in which anti-back flow compressor will be triggered. 
     It is understood that other known pressure controllers could be employed that have greater control capability than differential pressure switch  52 . They are less preferred, however, due to the costs involved in obtaining such controllers. 
     In the illustrated embodiment, oil sump  16  provides a phase separation space to allow the process fluid vapor and the oil mist to separate from the liquid gear oil. As will be discussed below, other embodiments are possible in which separate phase separators are employed for such purposes. Although not illustrated, appropriately sized gear cases  12  could also be employed to provide a phase separation space to additionally perform phase separation between the gear oil liquid and the process vapor and gear oil mist. This is not preferred, however, in that such a possible embodiment might involve modification of a gear case  12  provided by a compressor manufacturer. 
     With reference to FIG. 2, a compressor assembly  3  is illustrated having first and second compressors  10   a  and  10   b  connected in series by a conduit  56  such that the process fluid is initially compressed in first compressor  10   a  and then is further compressed in the second compressor  10   b.  Each of the compressors,  10   a  and  10   b,  is provided with a low pressure inlet, numbered  46   a  and  46   b , respectively. Compressors  10   a  and  10   b  are driven by shafts connected to gears in a gear cases  12   a  and  12   b . In this regard, the design of the compressors of compressors  10   a  and  10   b  might differ from one another, in a known manner, due to the respective pressure ranges of the compression required in each of the compressors  10   a  and  10   b.  Compressors  10   a  and  10   b  could, however, be identical. 
     During operation, process fluid vapor flows from each compressor  10   a  and  10   b  to its associated gear case  12   a  and  12   b , respectively, through shaft seals  26   a  and  26   b  thereof and collects as a vapor. In order to recycle the process fluid, two process fluid recycle systems  1 A and  1 B, each having the same design and function as process fluid recycle system  1 , are applied to first and second compressors  10   a  and  10   b , respectively, as first and second sets of components. In this regard, a first of two alternate flow paths is formed by a conduit  36   a  having a check valve  38   a  and an anti-back flow compressor  50   a  and a second of two alternate flow paths is formed by a conduit  36   b  having a check valve  38   b  and an anti-back flow compressor  50   b . The two alternate flow paths conduct the oil mist and process fluid vapor that collects in headspace regions  18   a  and  18   b  of oil sumps  16   a  and  16   b  to coalescing filters  28   a  and  28   b , respectively. First and second oil return circuits  30   a  and  30   b  conduct the separated gear oil back to oil sumps  16   a  and  16   b.    
     With reference to FIG. 3, a process fluid recycle system  4  is illustrated that is designed to be used in connection with a compressor assembly  3 , described above in connection with the embodiment shown in FIG.  2 . Process fluid recycle system  4  uses common components to avoid the entire duplication of a process fluid recycle system for each compressor in the manner shown in FIG.  2 . 
     As illustrated, process fluid recycle system  4  utilizes a single recycle conduit  44  connected, at one end, to the low pressure inlet  46   a  associated with compressor  10   a  which constitutes the first compression stage. Low pressure inlet  46   a  functions as the system inlet to compressor assembly  3 . A single set of one or more coalescing filters  28  connected in series and oil and water vapor adsorption traps  40  and  42  is connected to the other end of recycle conduit  44 . Process fluid vapor and oil mist is separated from gear oil within phase separation spaces provided by oil sumps  16   a  and  16   b  and collects within headspace regions  18   a  and  18   b  thereof. 
     The separated oil mist and process fluid vapor is conducted to the a coalescing filter  28  that constitutes part of a single set of coalescing filters  28  and oil and water vapor adsorption traps  40  and  42  by a first and a second of two alternate flow paths (described above) by way of two conduits  62  and  64  that meet at a junction  65 . 
     First and second anti-back flow compressors  50   a  and  50   b  are controlled by pressure differential switches  52   a  and  52   b  to prevent overpressures within gear cases  12   a  and  12   b  from building up and driving gear oil into compressors  10   a  and  10   b  in a manner described above. Since second compressor  10   b  operates at a higher pressure than first compressor  10   a , the pressure within the gear case  12   b  associated with second compressor  10   b  will be higher than that of the gear case  12   a  associated with first compressor  10   a . In order to equalize the pressure, pressure reduction is provided by such means as a throttle valve  66  located in conduit  36   b  to equalize pressure within conduit  36   b  to that of conduit  36   a . It is to be noted that other means of throttling are possible, for instance, sizing various runs of piping differently to control the flow. 
     Additional efficiencies are realized by the use of a single oil return circuit  30  having a single pump  32  to pump gear oil recycled from coalescing filters  28  back to first and second oil sumps  16   a  and  16   b . Thus two oil return conduits  67  and  68  are provided. In order to pump the oil to both first and second oil sumps  16   a  and  16   b , an over pressure must be developed. Thus, throttle valve valves  70  and  72  are provided to reduce the pressure sufficiently to allow a gear oil to be returned to both first and second oil sumps  16   a  and  16   b  simultaneously. 
     With reference to FIG. 4, a process fluid recycle system  5  is provided that is designed to be used in connection with a two stage compressor assembly  6  having first and second compressors  10   a  and  10   b.  A further efficiency is realized in compressor assembly  6  by the use of a common oil sump  74  connected to gear cases  12   a  and  12   b  of first and second compressors  10   a  and  10   b . 
     Oil mist and process fluid vapor to be recycled is separated from liquid gear oil by means of first and second phase separation spaces provided by first and second phase separators  76  and  78  interposed between gear cases  12   a  and  12   b  and the common oil sump  74 . The separated liquid gear oil is introduced into common oil sump  74  by oil lines  80  and  82  leading from phase separators  76  and  78 . A pressure control valve  84  is provided in oil line  80  to prevent higher pressures produced in second compressor  10   b  from driving oil from the common oil sump  74  back into first phase separator  76 . 
     First and second of two alternate flow paths provide flow communication with gear cases  12   a  and  12   b  via connection of conduit  36   a  and anti-back flow compressor  50   a  to first phase separator  76  and connection of conduit  36   b  and anti-back flow compressor  50   b  to second phase separator  78 . A single set of coalescing filters  28  and etc. is used as in the previous embodiment in which one or more coalescing filters  28  and oil and water vapor traps  40  and  42 , if necessary, are connected in series and to both the first and second of the two alternate flow paths to separate oil mist, oil, and water from the process fluid vapor. The connection of such single set can be accomplished by way of a conduit  86  connected to first anti-back flow compressor  50   a  and first conduit  36   a  which meets second conduit  36   b  and second anti-back flow compressor  50   b  at a junction  88 . A conduit  90  in turn communicates between junction  88  to the first in series of the coalescing  28 . A throttle valve  92  is provided in second conduit  36   b  to prevent the high pressure produced within compressor  10   b  from driving oil mist and process fluid into conduit  88  by reducing the pressure within conduit  36   b.    
     With further reference to FIG. 5, a further simplified process fluid recycle system  7  is used in connection with compressor assembly  6  utilizing a common oil sump  74 . Gear oil, oil mist, and process fluid vapor drain to common oil sump  74  through first and second conduits  14   a  and  14   b  which meet at a junction  94 . A conduit  96  connects junction  94  to headspace region  18  of common oil sump  74 . In order to prevent gear oil and etc. from being driven by the higher pressure of compressor  10   a  into second conduit  14   a  to gear case  12   a , a throttle valve  98  is provided within conduit  14   a . An alternative is to eliminate throttle valve  98  such that gear case  12   b  is maintained at the same pressure of gear case  12   a.    
     The feature distinguishing this embodiment from the other embodiments having compressor assemblies employing multiple stages is the use of a single set of two alternate flow paths formed by an anti-back flow compressor  50  and a conduit  36 . 
     Common oil sump  74  provides a phase separation space to separate oil mist and process fluid vapor from gear oil liquid. The use of common oil sump  74  also allows anti-back flow compressor  50  to lower pressure within both gear cases  12   a  and  12   b  and thereby ensure the proper pressure differential is maintained between gear cases  12   a  and  12   b  and the respective compressors  10   a  and  10   b.    
     Anti-back flow compressor  50  is controlled by pressure differential switches  52   a  and  52   b  triggered by a pressure differential existing either between compressor  10   a  and gear case  12   a  or compressor  10   b  and gear case  12   b  that would drive gear oil through shaft seals  26   a  and  26   b  into compressors  10   a  and  10   b . 
     While the present invention has been described with reference to preferred embodiment, as will occur to those skilled in the art, numerous changes, additions and omissions may be made without departing from the spirit and the scope of the present invention.