Patent Publication Number: US-2016245287-A1

Title: Noise and shock reduction in rotary positive displacement blowers

Description:
CROSS-REFERENCE TO PROVISIONAL APPLICATION(S) 
     This application is a divisional of U.S. patent application Ser. No. 12/931,093, filed Jan. 24, 2011, which claims the benefit of U.S. Provisional Application No. 61/336,495, filed Jan. 22, 2010, the disclosures of which are incorporated herein by this reference thereto. 
    
    
     BACKGROUND AND SUMMARY OF THE INVENTION 
     The invention relates to rotary positive displacement blowers (of the Roots type) and, more particularly, to a back-pass loop for gradually pressurizing the working cell to outlet pressure in order to weaken the strength of the pulsations that would otherwise happen without such a back-pass loop, and thereby reduce noise and shock (and perhaps better efficiency as well). 
     Briefly, the performance of such blowers is typically measured (or specified) in terms of the following factors:—flow, pressure, efficiency, noise, and reliability. 
     It is an object of the invention to provide improvements in particular for at least two or three of those factors, namely, noise and reliability, plus perhaps efficiency. 
     Although the invention perhaps neither betters nor harms flow rate and pressure performance to a significant degree, to be sure, these are important factors to users. 
     So, briefly (and very briefly), the following remarks are offered about flow and pressure. Regarding flow rate, blowers of this type can be built to all kinds of sizes (including very large). Hence design flow rate is an operating point that is scalable over a wide range. 
     As for pressure, the operating pressure differential (Δp) across such blowers might typically vary under the circumstances between very slight (eg., 1 to 2 psi or ˜ 1/15th to 2/15th atm) to something typical (eg., 15 psi or ˜1 atm). It might be just as typical that a blower of this type be rated for up to 18 psi duty (˜1- 3/15ths atm pressure differential). 
     As concerns a separate consideration, some end-use applications may require that the discharge line supply flow at a pressure as high as 100 psig (˜7-2/3rds atm). To do this, the pressure in the inlet line has to be elevated to within 18 psi (˜1- 3/15ths atm pressure differential) or less of the target pressure for the discharge line. 
     Moreover, high reliability is expected of these kinds of blowers. They might be designed and expected to operate more or less continuously (excluding routine maintenance) for years on end. 
     This application is owned by assignment in common with the same owner of U.S. Pat. No. 5,702,240 —O&#39;Neal et al., namely TUTHILL CORPORATION of Burr Ridge, Ill. This blower was referred to by the TUTHILL CORPORATION as the “ACOUSTIC AIR”™ design. 
     The “ACOUSTIC AIR”™ blower introduced some matters in blower design which have been changed, substantially or so, here for better meeting the objects of the invention. These changes fall under two major categories. One major category comprises changes in design for purely or substantially pneumatic reasons. The other major category comprises changes in design for purely or substantially ease of manufacture reasons. 
     In common with one of the objects of the invention here, an object of the invention for the “ACOUSTIC AIR”™ design included reducing pressure pulsations, and thereby reducing resulting noise and vibration. 
     The “ACOUSTIC AIR”™ design sought to do this by the following two ways. One, the “ACOUSTIC AIR”™ design included a backflow loop. Generally speaking, a backflow loop is meant to gradually pre-pressurize a low-pressure closed cell (eg.,  64  or  66 ) so that when the closed cell (eg.,  64  or  66 ) opens across an edge  78  or  80  into the higher-pressure discharge chamber  46 , the backflow loop eliminates or weakens the direct backflow from the discharge chamber  46  into the opening closed cell (eg.,  64  or  66 ). Without a backflow loop, the backflow from the discharge chamber  46  flows directly into the opening closed cell (eg.,  64  or  66 ) and is the source of the sonic pop (eg., the noise) as well as the momentary opposition to the rotation of the rotors  50 ,  52  (eg., the vibration). 
     With reference to  FIGS. 3 and 6  therein, the “ACOUSTIC AIR”™ blower has backflow chambers  106 ,  108 ,  120 ,  122  filled by backflow ports  112 ,  116 ,  126 ,  130  and for pre-pressurizing fluid in the sealed pocket (eg.,  64 ,  66 ) by injector ports  110 ,  114 ,  124 ,  128 . The patent contains this remark on the effectiveness of this design.
         Therefore, after a pocket  64  has been in fluid communication with the injector ports  110  and  114 , the pressure in the now pre-pressurized pocket  64  is greater than the first pressure of the fluid within the intake chamber  44 , but is usually still somewhat lower than the second pressure of the fluid within the discharge chamber  46 . U.S. Pat. No. 5,702,240, col. 6, lines 28-34.       

     In other words, the backflow loop was not as effective as hoped for. The other way the “ACOUSTIC AIR”™ design sought to eliminate or weaken backflow was by curved edges  78  and  80  opening into the discharge chamber  46 . 
     Despite owning the rights to the “ACOUSTIC AIR”™ design, the owner of the patent thereon put together the present team of inventors to do even better. Noise and vibration are serious problems. It is an object of the invention to overcome the shortcomings of the prior art. 
     Now to turn to the improvements herein, various features and objects of the invention will be apparent in connection with the following discussion of preferred embodiments and examples. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       There are shown in the drawings certain exemplary embodiments of the invention as presently preferred. It should be understood that the invention is not limited to the embodiments disclosed as examples, and is capable of variation within the scope of the skills of a person having ordinary skill in the art to which the invention pertains. In the drawings, 
         FIG. 1  is a perspective view of a rotary positive displacement blower with noise and shock reduction improvements in accordance with the invention; 
         FIG. 2  is an exploded view thereof; 
         FIG. 3  is an enlarged scale detail view of the rotor chamber and discharge plenum in  FIG. 2 ; 
         FIG. 4  is a vertical sectional view taken through the rotary positive displacement blower of  FIG. 1 , taken perpendicular to the plane of the rotor axes, and, taken along an offset plane to contain not only the centerline of one inner port(s) in the rotor chamber (as well as portions thereabove), but also, the centerline of one (of the two) outer port(s) in the discharge plenum (as well as portions therebelow); and 
         FIG. 5  is a chart showing the effect of back-pass manifold chamber volume on the fluctuation away from mean discharge flowrate. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIGS. 1 through 4  provide line drawings of a rotary positive displacement blower  210  with noise and shock reduction improvements in accordance with the invention. It is an aspect of the invention to incorporate a pair of back-pass manifolds  212 . 
     This is a Roots style blower.  FIG. 4  shows better that, it has a substantially hollow housing  214  defining an inlet plenum  220 , a rotor chamber  224 , and a discharge plenum  228 . (Preferably the housing  214  is cast, but the flange surfaces would be machined and ground.) 
     A pair of rotors  230  are disposed in the rotor chamber  224 . The rotors  230  would be sealed inside by a pair of opposed end plates (far side end plates shown in  FIGS. 1 and 2 ). The rotors  230  are driven to rotate counter-rotationally to each other. For instance, the left rotor  230  rotates counter-clockwise (CCW). 
     In the drawings, the blower  210  is shown with the inlet port  220 P up and the discharge port  228 P down. However, the blower  210  can be mounted in any orientation, and accordingly, terms like “up” and “down” , “left” and “right” are used merely for convenience in this description and do not limit the installation of the blower  210  to any particular orientation. 
     The rotors  230  are identical. Each rotor  230  comprises three lobes  232 . Each lobe  232  culminates in a tip  232 T. The lobes  232  are spaced by pockets  240 . 
     The inlet plenum  220  transitions into the rotor chamber  224  at a pair of spaced ledges  242 L, and these define an inlet opening  242  for the blower  210 . Likewise, the rotor chamber  224  transitions into the discharge plenum  228  at another pair of spaced ledges  244 L, and these define a discharge opening  244  for the blower  210 .  FIG. 4  shows that the left rotor  230 &#39;s upper lobe tip  232 T is about to sweep (counterclockwise) past the left ledge  242 L of the inlet opening  242 . When it does so, that lobe  232  will trap gas in the pocket  240 X immediately ahead of it, between the surface of the rotor  230  and surface of the housing  214 . The pocket  240 X which temporarily traps gas in it, carrying the trapped gas from the inlet plenum  220  to the discharge plenum  228 , is referred to as the ‘closed cell’ (ie., indicated as  240 X). Each pocket  240  in turn will form the temporarily existing closed cell  240 X, successively, and in an endless succession. 
     The trapped gas is carried around in the closed cell  240 X, from the inlet plenum  220  to the discharge plenum  228 , at the pressure of the inlet plenum  220  while being carried around like that. In contrast, the trapped gas will be ultimately discharged into the discharge plenum  228 , at the pressure of the discharge plenum  228 . 
     When the lobe tip  232 T of the lobe  232  leading the closed cell  240 X sweeps past the ledge  244 L of the discharge opening  244 , suddenly something happens. Two different pressurized spaces at two different pressures have open communication with each other. This allows for the free exchange of gases between the (formerly) closed cell  240 X and the discharge plenum  228 . This ‘opening’ of the (formerly) closed cell  240 X to the discharge plenum  228  also allows for the consequential equalization of pressure between the two. That is, the closed cell  240 X, as it travels from inlet space to discharge space, holds fairly steady at the inlet pressure. But that changes, suddenly, when the lobe tip  232 T of the leading lobe  232  crosses the ledge  244 L of the discharge opening  244 . At that moment, the closed cell  240 X is suddenly no longer closed but ‘open’ to the discharge plenum  228 . Gases in the discharge plenum  228  are free to flow back into the (formerly) closed cell  240 X. 
     There are numerous consequences to this ‘moment’ that the closed cell  240 X opens to discharge space. There is noise (eg., an audible sonic pop or snap, something akin to a popping balloon or snapped cell of bubble wrap), and there is a puff of reverse flow from the discharge plenum  228  into the opening closed cell  240 X. Noise aside (for the moment), the reverse flow is a problem of its own. The reverse flow creates an opposing force in opposition to the turning rotors  230 , and the rotors  230  have to power through the reverse flow. Hence the reverse flow is a readily identifiable source of inefficiency. The reverse flow also has another effect, which is likewise detrimental, which is that of causing mechanical shock through the blower (vibration), and not just to the blower&#39;s castings but also to the joints, couplings, bearings, seals and so on. 
     To come to terms with the problematic effects of reverse flow, it pays to appreciate that the reverse flow comprises a pulsing phenomenon. That is, for each revolution of the rotors  230 , there are six reverse flow events. The rotors  230  are typically driven at 1200, 1800 or 3600 RPM. At the high value given there, that corresponds to 1.3 million reverse flow pulses—each hour. 
     Hence the effects of reverse flow comprise an unceasing hammering on the blower, and over its whole lifetime. Accordingly, it is an object of the invention to not just weaken but eliminate each reverse flow event. It is a further object of the invention to reduce vibration, and not so much the frequency of the vibration but the shock value (amplitude) of each pulse. It is a corresponding object of the invention to enhance reliability. 
     These and other objects and aspects are provided according to the invention in a rotary positive displacement blower  210  (of the Roots type) with a back-pass loop  250 - 52  for gradually pressurizing the closed cell  240 X to the pressure of the discharge plenum  228  in order to weaken the strength of the pulsations that would otherwise happen, and thereby reduce noise and shock. 
       FIGS. 1 and 4  show a rotary positive displacement blower  210  provided with a pair of flanking manifolds  212 . In  FIG. 2 , both manifolds  212  are shown dismounted and apart from the main housing  214 . Conversely in  FIGS. 1 and 4 , both manifolds  212  are shown mounted to the main housing  214 . 
     Just as the main housing  214  is a monolithic casting of (preferably) gray iron, so is each manifold  212  its own separate monolithic casting of gray iron. The flange surfaces for the bolt-on surfaces are preferably very smooth, as are the mating surfaces on the main housing  214 . 
       FIGS. 1 and 2  allow discernment that the manifolds  212  mount to the main housing  214  by a pattern of bolts (bolts not shown). Each manifold  212  defines a back-pass chamber  250 . 
     The main housing  214  is bored through from both sides in order to form a number of channels  251  and  252  for connecting each back-pass chamber  250  into a back-pass loop  250 - 52  with the blower  210 . That is, the main housing  214  is bored through a series of times into each side of the rotor chamber  224  to form a pattern—a line parallel with the axis of the rotor  230 —of inner channels  251  to the rotor chamber  224 . The main housing  214  is furthermore bored through at least two times into each side of the discharge plenum  228  to form a pattern of (eg., two in-line) outer channels  252  (‘outer’ relative to the rotor chamber  224 ).  FIG. 3  shows better the ports  251 P and  252 P of the inner and outer channels  251  and  252 , respectively, in the rotor chamber  224  and discharge plenum  228 , respectively. 
     It is a design preference at present time that the cumulative cross-sectional flow area for the two outer channels  252  feeding one manifold  212  chamber  250  equals or is substantially close in value to the cumulative cross-sectional flow area of all the inner channels  251  serving the same manifold  212  chamber  250 . Hence for each manifold  212  chamber  250 , the ratio of the cumulative cross-sectional area of the outer channels  251  to that of the inner channels  251  is about one to one (1:1). 
       FIG. 4  shows better that the flow axis of gas through the blower  210  is generally perpendicular to the plane containing the rotor axes. This plane (that contains the rotor axes) is referred to herein for convenience sake as the rotor plane. (It might alternatively be referred to as the dowel plane. As  FIG. 1  shows better, it is typical that a housing  214  for a Roots blower would contain a pair of flanking dowels  255  in this same plane. These dowels  255  provide for alignment to the end plates and support to the housing  214  in this plane, and hence promote proper lobe tip  232 T clearance.) 
     Given the foregoing, the manifolds  212  mount to the main housing  214  on the discharge side of the rotor plane. 
       FIG. 4  allows reckoning of the following matters. The lobes  232  of the rotors  230  are angularly spaced apart by 120°. The ledge  242 L of the inlet opening  242  and the ledge  244 L of the discharge opening  244  are angularly spaced apart by about 180° (relative to rotor rotation). 
     Hence, in the absence of the improvements of the invention, the temporarily existing closed cell  240 X is formed for a time period corresponding to a 60° arc of the rotor rotation. In other words, there is a window of opportunity during that 60° arc in which to gradually pressurize the closed cell  240 X from inlet pressure to discharge pressure. 
     It is an object of the invention to gradually pressurize the closed cell  240 X from inlet pressure to discharge pressure over the last 30° to 40° or so of rotation of the closed cell  240 X to its opening to the discharge opening  244 . 
     The design in accordance with the invention was obtained by virtual prototyping with the use of three-dimensional CFD software from SIMERICS, INC., that goes by the brand name PUMPLINX®. 
     The CFD analysis was performed with an existing blower of TUTHILL VACUUM &amp; BLOWER SYSTEMS, model QX-3208, serving as the basis for blower dimensions. The operating point for the analysis was chosen to be 3600 RPM at 15 psi (˜1 atm pressure differential). 
     Following that, a physical prototype was built, and tested, at the following operating points:
         1200 RPM @10 psig (˜⅔rds atm pressure differential).   1800 RPM @10 &amp; 15 psig (˜⅔rds and 1 atm pressure differential).   3600 RPM @10, 15 &amp; 18 psig (˜⅔rds, 1 and 1- 3/15ths atm pressure diff.).       

     At the CFD operating point of 3600 RPM at 15 psig (˜1 atm pressure differential), the prototype blower  210  in accordance with the invention compares to the un-modified original QX-3208 as follows. There was 8.9 db drop and a 12.4 dBA drop in sound pressure levels. There was an average drop across all tested speeds and pressures of 7.4 dB and 10.7 dBA. The maximum sound pressure level drop was 3600 RPM and 18 psi (˜1- 3/15ths atm differential pressure) for both linear and A-weighted scales. These results were 13.2 dB and 17.1 dBA respectively. 
     (Note: Sound pressure levels were recorded by four microphones located on the horizontal plane bisecting the blowers—the rotor plane—located six inches or roughly 15 cm from the corners of the main housings and centered on axis passing through the inlet and discharge ports.) 
     There is a noticeable difference in not just the quieting of the sound of the blower  210  in accordance with the invention, but also the quality of the sound. Indeed, there are still personnel employed by TUTHILL VACUUM &amp; BLOWER SYSTEMS who can personally recall the “ACOUSTIC AIR”™ blower referenced above in connection with U.S. Pat. No. 5,702,240. One such person includes one of the original inventors. The remarks about the change in sound quality with the blower  210  in accordance with the invention is something as follows:—the blower  210  in accordance with the invention is not just merely a quieter jack hammer, it has sort of lost its jack hammer staccato to where it just sounds like the hum of process machinery. 
     The CFD analysis in combination with building and testing a number of prototypes discovered that perhaps the following five (5) factors are chiefly responsible for the blower  210  in accordance with the invention working so well. 
     These five (5) factors include the following:
         1—ease of manufacture,   2—cumulative flow area of inner channels  251 ,   3—angle of attack of (and like matters with) the inner channels  251 ,   4—timing, or separation between plane of the outer channels  252  and plane of the discharge opening ledges  244 L, and   5—ratio of manifold chamber  250  volume to closed cell  240 X volume.       

     (1) To begin with, a nod is given to ease of manufacture as an important factor. The improved blower  210  was prototyped out of a stock QX- 3208  blower of TUTHILL BLOWER &amp; VACUUM SYSTEMS. The casting of the stock blower had to be beefed up in the regions where the inner and outer channels  251  and  252  were to be drilled, as well as where the manifolds  212  bolt on. However, the manifold  212  is its own casting. In the “ACOUSTIC AIR”™ blower, the backflow chambers were cast to size in the main housing casting for the blower. In accordance with the invention, the method of manufacture of the blower  210  with its separate cast manifolds  212  allowed much more flexibility in specifying different sizes and arrangements of inner and outer channels  251  and  252  as well a volume of the manifold  212  chambers  250 . 
     (2) The second important factor is the cumulative flow area of the inner channels  251 . With a given back-pressure in the manifold  212  chamber  250  and under-pressure in the closed cell  240 X, the cumulative flow area is selected to fill the closed cell  240 X with about 100% plus of the make-up mass of air in the angular time that the inner channels  251  are filling the closed cell  240 X (eg., about 30° to 40° angular degrees). It is preferred that the outer channels  252  cumulatively form about the same cross-sectional flow area for each manifold  212  chamber  250  as do the inner channels  251  therefor. The inner channels  251  cannot be undersized or else there will be backflow when the leading lobe tip  232 T of the closed cell  240 X crosses the discharge ledge  244 L. Conversely, the inner channels  251  cannot be grossly oversized or else it just moves the moment of backflow from—when the leading lobe tip  232 T of the closed cell  240 X crosses the discharge ledge  244 L to—when leading lobe tip  232 T of the closed cell  240 X crosses the inner channels  251 . In sum, the inner channels  251  have to fill gradually, and do so all the way until the leading lobe tip  232 T of the closed cell  240 X crosses the discharge ledge  244 L, and then for a little while longer too. 
     (3) The third important factor is a series of factors, and comprises the angle of attack angle of, and like matters concerning the, inner channels  251 . The angle of attack of the inner channels  251  is preferably is as close to a tangent line with the curve of the rotor chamber  224  and blowing onto the backside of the leading lobe tip  232 T of the closed cell  240 X as it crosses the inner channels  251 . Also, a prototype was built and tested where there were a series of inner channels  251  on three lines. It is believed from that experiment that closed cell  240 X wants to open all the inner channels  251  on one line that is parallel to the rotor axes. Hence the inner channels  251  preferably comprise a series of same diameter bore holes equally spaced from one another and generously distributed along the axial length of the closed cell  240 X in order to fill the closed cell  240 X in an axially even fashion. 
     (4) The fourth most important factor is a timing factor. Briefly, by way of background, the pressure in the discharge plenum  228  oscillates. The back-pass loop  250 - 252  goes a long way to dampening the fluctuations. But it does not flatten the fluctuations to zero. The pressure fluctuations are propagated at the plane of the discharge ledges  244 L and move down (or away in) the discharge plenum until eventually the pressure fluctuations have moved so far away from the plane of the discharge ledges  244 L that they have canceled each other out into a mean pressure (with no fluctuations). But near the plane of the discharge ledges  244 L, there are measurable fluctuations. The timing issue relates to where to locate the outer channels  252  relative to the plane of the discharge ledges  244 L.  FIG. 4  illustrates where the outer channels  252  should be located. Given the right side of  FIG. 4 , it is preferred that a maximum of pressure fluctuation in discharge plenum  228  (even though propagated at the plane of the ledges  244 L) should reside at the plane of the outer channels  252  when the closed cell  240  on the right rotor  230  is about to cross the ledge  244 L. That way, the manifold  212  chamber  250  is pulling mass out of the discharge plenum  228  at the moment the closed cell  240  is about to blow out across the discharge ledge  244 L, which will be experiencing a local minimum in the pressure fluctuation. By scaling the outer channels  252  in connection with other proportions, the timing can be managed such that the opening closed cells  240 / 240 X never experience backflow when crossing the ledges  244 L. 
     (5) The fifth factor is left for last perhaps because its range was most elusive. That is, it has been inventively discovered that the effectiveness of the blower  210  in accordance with the invention is sensitive to the ratio of closed cell  240 X volume to manifold  212  chamber  250 . Moreover, it is believed to be highly preferable that there be one dedicated manifold  212  chamber  250  pursuant to each rotor  230 . In contrast to the fourth factor above, the measure of performance here has to do with flow fluctuations. 
     If the mean discharge flow rate is 100 feet per second (˜30 m/s), then local flowrate at the plane of the discharge ledges  244 L fluctuates. How little it fluctuates is a measure of how effective the back-pass loop  250 - 252  is working. Recall that, in prior art blowers without a backflow loop or the like, the fluctuations can even go negative. 
       FIG. 5  is a chart showing the effect of back-pass manifold  212  chamber  250  volume relative to volume of the closed cell  240 X on the fluctuation away from mean discharge flowrate. 
       FIG. 5  shows that the best performance is obtained when manifold  212  chamber  250  volume relative to closed cell  240 X volume is 100% (eg., the volumes are equal, or, there is one-to-one correspondence. The fluctuation as a percentage of flowrate discharge is 13%. That means that, if the mean discharge centerline flowrate is 100 feet per second (˜30 m/s), then the fluctuations in the flowrate are between about 93 feet per second (˜28 m/s) and 107 feet per second (˜32 m/s). 
       FIG. 5  shows that when manifold  212  chamber  250  volume as a percentage of closed cell  240 X volume is any of the following three values:
         83%,   56%, and/or   117%,
 
the fluctuation percentages of the discharge flowrate is still believed to be within acceptable ranges of 14.6%, 14.8% and 17.5% respectively.
       

     However, it is only when manifold  212  chamber  250  volume as a percentage of closed cell  240 X volume is about 134% that the fluctuation percentage of the discharge flowrate is believed to have climbed to an un-preferred value of 25.3% 
     Given the foregoing, it is a preference of the invention that the manifold  212  chamber  250  volume as a percentage of closed cell  240 X volume should fall between about 56% and 117% in order to obtain the preferred performance of the blower  210 . 
     One way to characterize how the back-pass loop  250 - 52  in accordance improves blower performance to the extent it does, might be the following. The back-pass loop  250 - 52  weakens the pulsations by having an out-of-phase flow with chambers  250  comparable in volume to the closed cell  240 X. 
     To turn to manufacture once more, it is an object of the invention to produce the blower  210  in accordance with the invention from conventional stock housings, except modified to accept the inventive manifolds  212 . 
     A preferred method of manufacturing a roots-style positive displacement blower  210  with a back-pass loop  251 - 252  comprises some of the following steps. 
     A housing  214  is provided, and it is highly preferred if the housing is a monolithic casting. The housing  214  has a rotor chamber  224  portion, an inlet plenum  220  portion defining an inlet plenum  220  and a discharge plenum  228  portion. 
     The rotor chamber  224  portion defines a rotor chamber  224  comprising side-by-side left and right cylindrical cavities partially overlapping one another and meeting at tangent lines. The left and right cylindrical cavities receiving the left and right rotors  230  such that the rotor axes define a rotor plane. 
     The discharge plenum  228  portion comprises a bell shape extending along an axis that projects away from the rotor plane. More preferred still is if the axis of the bell shape is perpendicular to the rotor plane. The bell shape defines a discharge plenum  228  extending between a discharge opening  244  in the rotor chamber  224  and a discharge port  228 P. 
     The housing  214  is formed with left and right flange interfaces on the rotor chamber  224  portion of the housing  214 . Preferably this is done by surface machining. The housing  214  is furthermore formed with left and right inner channels  251  in the rotor chamber  224  portion of the housing  214  that extend between interior ports  251 P in the left and right cylindrical cavities respectfully, and exterior ports in the left and right flange interfaces on the rotor chamber  224  portion of the housing  214 . 
     Additionally, the housing  214  is preferably formed with left and right flange interfaces of the discharge plenum  228  portion of the housing  214 , again as by surface machining. Then, the housing is formed with left and right outer channels  252  in the discharge plenum  228  portion of the housing  214 , which extend between interior ports  252 P in the left and right sides respectively of the discharge plenum  228 , and exterior ports in the left and right flange interfaces of the discharge plenum  228  portion of the housing  214 . 
     It is an aspect of the invention to provide left and right ‘covers’  212  that removably attach to the housing  214  and cover portions of the flange interfaces on the rotor chamber  224  portion of the housing  214  as well as portions of the flange interfaces of the discharge plenum  228  portion of the housing  214  on the left and right sides respectively of the housing  214 . 
     Wherein, these covers  212  concurrently seal over the exterior ports of the inner and outer channels  251  and  252 , respectively, and allow a back-pass flow therebetween underneath said covers  212 . 
     It is another aspect of the flange interfaces on the rotor chamber  224  portion of the housing  214  that they are further outboard from the axis of the discharge plenum  228  than the flange interfaces of the discharge plenum  228  portion of the housing  214 . That way, the covers  212  might be L-shaped and still function sufficiently as covers  212 . 
     As the drawings show, it is more preferential still that the ‘covers’  212  are not just simply L-shaped by comprise a monolithic casting in a tubular C-shape. Hence the ‘covers’ given the tubular C-shape might interchangeably be referred to as manifolds  212 . 
     The manifolds  212  extend between a first interface for mating to the flange interfaces of the discharge plenum  228  portion of the housing  214  and a second interface for mating to the flange interfaces of the rotor chamber  224  portion of the housing  214 . 
     Each manifold  212  furthermore defines a back-pass chamber  250  which allows the back-pass flow between the inner and outer channels  251  and  252 . It is an aspect of the invention that the manifolds  212  are removably attached to the housing  214  by mechanical fastening. 
     It is another aspect of the invention that each manifold  212  expands from being relatively narrower at the first interface to being relatively wider at the second interface. In this context, being relatively narrower and wider is taken in context along axes parallel to the rotor axes. 
     Preferably the left inner channels  251  comprise a series of bore holes axially spread apart on an axis parallel to the rotor axes (the right inner channels  251  comprise symmetric opposites of the left inner channels  251 ). Preferably the left outer channels  252  comprise at least two bore holes axially spread apart on an axis parallel to the rotor axes (right outer channels  252  would be symmetric opposites of the left outer channels  252 ). That way, the spread of the manifold  212  could accommodate the spread apartness of the inner channels  251 . 
     It is preferred again if the left inner channels  251  define a cumulative flow area fairly close in size to the cumulative flow area defined by the left outer channels  252 . 
     As previously mentioned, it is preferred if the inventive housing  214  is modified from conventional stock housings. Conventional stock housing are characterized by many design aspects, a including optionally and without limitation that the discharge plenum  228  portion of the housing  214  further comprises a circular ANSI flange encircling discharge port  228 P, and the bell shape comprises a six-sided subtended diamond-shaped bell flare. 
     The invention having been disclosed in connection with the foregoing variations and examples, additional variations will now be apparent to persons skilled in the art. The invention is not intended to be limited to the variations specifically mentioned, and accordingly reference should be made to the appended claims rather than the foregoing discussion of preferred examples, to assess the scope of the invention in which exclusive rights are claimed.