Patent Publication Number: US-6669436-B2

Title: Gas compression apparatus and method with noise attenuation

Description:
BACKGROUND 
     This invention is directed to a gas compression apparatus and method in which the acoustic energy caused by a rotating impeller is attenuated. 
     Gas compression apparatus, such as centrifugal compressors, are widely used in different industries for a variety of applications involving the compression, or pressurization, of a gas. These type of compressors utilize an impeller adapted to rotate in a casing at a relatively high rate of speed to compress the gas. However, a typical compressor of this type produces a relatively high noise level, caused at least in part, by the rotating impeller, which is an obvious nuisance and which can cause vibrations and structural failures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of a portion of a gas compression apparatus incorporating acoustic attenuation according to an embodiment of the present invention. 
     FIG. 2 is an isometric view of a base plate with a plurality of diffuser vanes used in the apparatus of FIG.  1 . 
     FIG. 3 is an enlarged view of a portion of the apparatus of FIG.  1 . 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 depicts a portion of a high pressure, gas compression apparatus, such as a centrifugal compressor, including a casing  10  having an inlet  10   a  fluid to be compressed, and an impeller cavity  10   b  for receiving an impeller  12  which is mounted for rotation in the cavity. It is understood that a power-driven shaft (not shown) rotates the impeller  12  at a high speed, sufficient to impart a velocity pressure to the gas drawn into the casing  10  via an inlet  10   a . The casing  10  extends completely around the shaft and only the upper portion of the casing is depicted in FIG.  1 . 
     The impeller  12  includes a plurality of impeller blades  12   a  arranged axisymmetrically around the latter shaft and defining a plurality of passages  12   b . The impeller  12  discharges the pressurized gas into a diffuser passage, or channel,  14  defined between two annular facing interior walls  10   c  and  10   d  in the casing  10 . The channel  14  extends radially outwardly from the impeller  12  and receives the high pressure gas from the impeller  12  before the gas is passed to a volute, or collector,  16  also formed in the casing  10  and in communication with the channel. The channel  14  functions to convert the velocity pressure of the gas into static pressure, and the volute  16  couples the compressed gas to an outlet (not shown) of the casing. 
     Due to centrifugal action of the impeller blades  12   a  and the design of the casing  10 , gas entering the impeller passages  12   b  from the inlet  10   a  is compressed to a relatively high pressure. It is understood that conventional labyrinth seals, thrust bearings, tilt pad bearings and other similar hardware can also be provided in the casing  10  which are conventional and therefore will not be shown or described. 
     An annular plate  20  is mounted in a recess, or groove, formed in the interior wall  10   a , with only the upper portion of the plate being shown, as viewed in FIG.  1 . As better shown in FIG. 2, a plurality of discharge vanes  24  are angularly spaced around the plate  20 , with each vane extending from the plate and at an angle to the corresponding radius of the plate. The plate  20  and the vanes  24  can be milled from the same stock or can be formed separately. The vanes  24  increase the efficiency of the apparatus by improving static pressure recovery in the diffuser channel  14 , and since their specific configuration and function are conventional, they will not be described in further detail. 
     As better shown in FIGS. 2 and 3, a series of relatively large cells, or openings,  34  are formed through one surface of the plate  20  between each pair of adjacent vanes  24 . The cells  34  extend through a majority of the thickness of the plate  20  but not through its entire thickness. As shown in FIG. 3, a series of relatively small cells, or openings,  36  extend from the bottom of each cell  34  to the opposite surface of the plate  20 . Each cell  34  is in the form of a bore having a relatively large-diameter cross section, and each cell  36  is in the form of a bore having a relatively small-diameter cross section, it being understood that the shapes of the cells  34  and  36  can vary within the scope of the invention. The cells  34  and  36  can be formed in any conventional manner such as by drilling counterbores through the corresponding surface of the plate  20 . The cells  34  are capped by the underlying wall of the plate  20 , and the open ends of the cells  36  communicate with the diffuser channel  14 . 
     Preferably, the cells  34  are formed in a plurality of annular extending rows between each adjacent pair of diffuser vanes, with the cells  34  of a particular row being staggered, or offset, from the cells of its adjacent row(s). The cells  36  can be randomly disposed relative to their corresponding cell  34 , or, alternately, can be formed in any pattern of uniform distribution. 
     In operation, a gas is introduced into the inlet  10   a  of the casing  10 , and the impeller  12  is driven at a relatively high rotational speed to force the gas through the inlet  10   a , the impeller passage, and the channel  14 , as shown by the arrows in FIG.  1 . Due to the centrifugal action of the impeller blades  12   a , the gas can be compressed to a relatively high pressure. The channel  14  functions to convert the velocity pressure of the gas into static pressure, while the vanes  24  increase the efficiency of the operation by boosting static pressure recovery in the diffuser. The compressed gas passes through the channel  14  and the volute  16  and to the casing outlet for discharge. 
     Due to the fact that the cells  36  connect the cells  34  to the diffuser channel  14 , the cells work collectively as an array of acoustic resonators which are either Helmholtz resonators or quarter-wave resonators in accordance with conventional resonator theory. This significantly attenuates the sound waves generated in the casing  10  in the area of the diffuser vanes  24  caused by the fast rotation of the impeller  12 , and by its interaction with the diffuser vanes, and eliminates, or at least minimizes, the possibility that the noise bypass the plate  20  and pass through a different path. 
     Moreover, the dominant noise component commonly occurring at the passing frequency of the impeller blades  12   a , or at other high frequencies, can be effectively lowered by tuning the cells  34  and  36  so that the maximum sound attenuation occurs around the latter frequency. This can be achieved by varying the volume of the cells  34 , and/or the cross-sectional area, the number, and the depth of the cells  36 . Also, given the fact that the frequency of the dominant noise component varies with the speed of the impeller  12 , the number of the smaller cells  36  per each larger cell  34  can be varied spatially across the plate  20  so that noise is attenuated in a broader frequency band. Consequently, noise can be efficiently and effectively attenuated, not just in constant speed devices, but also in variable speed devices. 
     In addition, the employment of the acoustic resonators in the plate, as a unitary design, preserves or maintains a relatively strong structure which has less or no deformation when subject to mechanical and thermal loading. As a result, the acoustic resonators formed by the cells  34  and  36  have no adverse effect on the aerodynamic performance of the gas compression apparatus. 
     Variations and Equivalents 
     The specific technique of forming the cells  34  and  36  can vary from that discussed above. For example, a one-piece liner can be formed in which the cells are molded in their respective plates. 
     The vanes  24  can be integral with, or attached to, the plate  20 . 
     The relative dimensions, shapes, numbers and the pattern of the cells  34  and  36  can vary. 
     The above design is not limited to use with a centrifugal compressor, but is equally applicable to other gas compression apparatus in which aerodynamic effects are achieved with movable blades. 
     The plate  20  can extend for 360 degrees around the axis of the impeller as disclosed above; or it can be formed into segments each of which extends an angular distance less than 360 degrees. 
     The spatial references used above, such as “bottom”, “inner”, “outer”, “side” etc, are for the purpose of illustration only and do not limit the specific orientation or location of the structure. 
     Since other modifications, changes, and substitutions are intended in the foregoing disclosure, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention.