Noise and shock reduction in rotary positive displacement blowers

Abstract

A roots-style blower ( 210 ) has a housing ( 214 ), rotors ( 230 ) and manifolds ( 212 ) for the housing ( 214 ). The housing ( 214 ) forms an inlet plenum ( 220 ), a rotor chamber ( 224 ) and a discharge plenum ( 228 ). The rotors ( 230 ) have straight lobes ( 232 ) spaced by pockets ( 240 ). The pocket ( 240 X) that traps gas between a leading and following lobe ( 232 ) and an inside wall of the rotor chamber ( 224 ) is a temporary closed cell ( 240 X). The manifolds ( 212 ) and the housing ( 214 ) form a pair of back-pass loops ( 250 - 251 ), one for each rotor ( 230 ). Each back-pass loop ( 250 - 252 ) comprises a back-pass chamber ( 250 ), outer channels ( 252 ) from the discharge plenum ( 228 ) to the back-pass chamber ( 250 ), and inner channels ( 251 ) to the rotor chamber ( 224 ). Wherein, the back-pass chamber ( 250 ) volume as a percentage of closed cell ( 240 X) volume ranges between about fifty-six percent (56%) and one-hundred-seventeen percent (117%).

We claim: 
     
         1 . A straight-lobe positive displacement blower ( 210 ) comprising:
 a housing ( 214 ) defining an inlet plenum ( 220 ), a rotor chamber ( 224 ) and a discharge plenum ( 228 );   a pair of rotors ( 230 ), each comprising at least three axially-straight lobes ( 232 ), wherein each lobe ( 232 ) culminates in a tip ( 232 T) and the lobes ( 232 ) are spaced by pockets ( 240 ); and wherein the pocket ( 240 X) that traps gas between a leading and following lobe ( 232 ) and an inside wall of the rotor chamber ( 224 ) is a temporary closed cell ( 240 X);   a pair of back-pass loops ( 250 - 252 ), one for each rotor ( 230 );   each back-pass loop ( 250 - 252 ) comprising a manifold ( 212 ) of the housing ( 214 ) formed with a back-pass chamber ( 250 ), outer channels ( 252 ) formed in one or both of the manifold ( 212 ) and housing ( 214 ) between the discharge plenum ( 228 ) and back-pass chamber ( 250 ), and inner channels ( 251 ) formed in one or both of the manifold ( 212 ) and housing ( 214 ) between the back-pass chamber ( 250 ) and rotor chamber ( 224 );   wherein the manifold ( 212 ) is sized such that the back-pass chamber ( 250 ) volume as a percentage of closed cell ( 240 X) volume falls in a range between about fifty-six percent (56%) and one-hundred-seventeen percent (117%).   
     
     
         2 . The blower ( 210 ) of  claim 1  wherein the main housing ( 214 ) comprises a casting. 
     
     
         3 . The blower ( 210 ) of  claim 2  wherein each manifold ( 212 ) comprises a separately removable part from the main housing ( 214 ). 
     
     
         4 . The blower ( 210 ) of  claim 3  wherein each manifold ( 212 ) comprises an independent casting which is bolted onto the main housing ( 214 ). 
     
     
         5 . The blower ( 210 ) of  claim 3  wherein the inner channels ( 251 ) are formed by drill holes through the main housing ( 214 ) into the rotor chamber ( 224 ). 
     
     
         6 . The blower ( 210 ) of  claim 3  wherein the inner channels ( 251 ) of each back-pass loop ( 250 - 252 ) are formed by a series of drill holes through the main housing ( 214 ) and into the rotor chamber ( 224 ), all aligned linearly on a line parallel with the rotor axes and equi-distantly spaced. 
     
     
         7 . The blower ( 210 ) of  claim 6  wherein the inner channels ( 251 ) are all aligned with an angle of attack that is close to a tangent line with the rotor chamber ( 224 )'s inside wall's curvature, and aimed at the backside of a leading lobe tip ( 232 T) of the closed cell ( 240 X) as the leading lobe tip ( 232 T) crosses the inner channels ( 251 ). 
     
     
         8 . The blower ( 210 ) of  claim 7  wherein from where a leading lobe tip ( 232 T) of the closed cell ( 240 X) crosses the inner channels ( 251 ) to where said leading lobe tip ( 232 T) crosses a ledge ( 244 L) of a discharge opening ( 244 ) into the discharge plenum ( 228 ) comprises between about 30° to 40° angular degrees. 
     
     
         9 . The blower ( 210 ) of  claim 6  wherein the inner channels ( 251 ) for each back-flow loop ( 250 - 252 ) form a cumulative flow area which, when given a specified back-pressure in the back-pass chamber ( 250 ) and a specified under-pressure in the closed cell ( 240 X) pursuant to the pressure of the inlet plenum ( 220 ), said cumulative flow area is selected to fill the closed cell ( 240 X) with about 100% of the make-up mass of gas in the angular time that a leading lobe tip ( 232 T) of the closed cell ( 240 X) crosses the inner channels ( 251 ) to where said leading lobe tip ( 232 T) crosses a ledge ( 244 L) of a discharge opening ( 244 ) into the discharge plenum ( 228 ), comprising between about 30° to 40° angular degrees. 
     
     
         10 . The blower ( 210 ) of  claim 9  wherein the outer channels ( 252 ) define a cumulative flow area and the inner channels ( 251 ) define a cumulative flow area substantially close to the cumulative flow area of the outer channels ( 252 ). 
     
     
         11 . The blower ( 210 ) of  claim 5  wherein the outer channels ( 252 ) are formed by drill holes through the main housing ( 214 ) into the discharge plenum ( 228 ). 
     
     
         12 . The blower ( 210 ) of  claim 1  wherein the back-pass chamber ( 250 ) volume as a percentage of closed cell ( 240 X) volume ranges between about fifty-six percent (56%) and one-hundred-seventeen percent (117%). 
     
     
         13 . The blower ( 210 ) of  claim 1  wherein the back-pass chamber ( 250 ) volume as a percentage of closed cell ( 240 X) volume ranges between about eighty-three percent (83%) and one-hundred-seventeen percent (117%). 
     
     
         14 . The blower ( 210 ) of  claim 13  wherein the back-pass chamber ( 250 ) volume as a percentage of closed cell ( 240 X) volume comprises substantially close to one-hundred percent (100%). 
     
     
         15 . A method of manufacturing a straight-lobe positive displacement blower ( 210 ) with a back-pass loop ( 251 - 252 ), comprising the steps of:
 providing left and right rotors ( 230 ) having lobes ( 232 ) spaced by pockets ( 240 );   providing a housing ( 214 ) having 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 defining a rotor chamber ( 224 ) comprising side-by-side partially-overlapping left and right cylindrical cavities meeting at tangent lines, as well as receiving the rotors ( 230 ) such that the rotor axes define a rotor plane;   the discharge plenum ( 228 ) portion comprising a bell shape extending along an axis that projects away from the rotor plane and defining a discharge plenum ( 228 ) extending between a discharge opening ( 244 ) in the rotor chamber ( 224 ) and a discharge port ( 228 P);   forming left and right flange interfaces on the rotor chamber ( 224 ) portion of the housing ( 214 ) as well as forming left and right inner channels ( 251 ) in the rotor chamber ( 224 ) portion of the housing ( 214 ) and extending 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 );   forming left and right flange interfaces of the discharge plenum ( 228 ) portion of the housing ( 214 ) as well as forming left and right outer channels ( 252 ) in the discharge plenum ( 228 ) portion of the housing ( 214 ) and extending 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 );   providing left and right covers ( 212 ), each removably attaching to the housing ( 214 ) and covering 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 said 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 ).   
     
     
         16 . The method of  claim 15  wherein:
 said main housing ( 214 ) comprises a monolithic casting. 
 
     
     
         17 . The method of  claim 16  wherein:
 the flange interfaces on the rotor chamber ( 224 ) portion of the housing ( 214 ) 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 ); and 
 each cover ( 212 ) comprises an L-shape. 
 
     
     
         18 . The method of  claim 15  wherein:
 each cover ( 212 ) comprises a monolithic casting in a tubular C-shape, extends 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 cover ( 212 ) furthermore defining a back-pass chamber ( 250 ) which allows the back-pass flow between the inner and outer channels ( 251  and  252 ), and is removably attached to the housing ( 214 ) by mechanical fastening. 
 
     
     
         19 . The method of  claim 18  wherein:
 each cover ( 212 ) expands from being relatively narrower at the first interface to being relatively wider at the second interface and, moreover, relative to narrower and wider being taken along axes parallel to the rotor axes; 
 said left inner channels ( 251 ) comprising a series of bore holes axially spread apart on an axis parallel to the rotor axes, the right inner channels ( 251 ) comprising symmetric opposites of the left inner channels ( 251 ); 
 said left outer channels ( 252 ) comprising at least two bore holes axially spread apart on an axis parallel to the rotor axes, the right outer channels ( 252 ) comprising symmetric opposites of the left outer channels ( 252 ); and 
 the left inner channels ( 251 ) define a cumulative flow area fairly close in size to a cumulative flow area defined by the left outer channels ( 252 ). 
 
     
     
         20 . The method of  claim 16  wherein:
 the discharge plenum ( 228 ) portion further comprises a circular ANSI flange encircling discharge port ( 228 P), and the bell shape comprises a six-sided subtended diamond-shaped bell.