Patent Application
20060065478
The invention relates to compressors. More particularly, the invention relates to sound and vibration suppression in screw-type compressors.
In positive displacement compressors, discrete volumes of gas are: trapped at a suction pressure; compressed; and discharged at a discharge pressure. The trapping and discharge each may produce pressure pulsations and related noise generation. Accordingly, a well developed field exists in compressor sound suppression.
One class of absorptive mufflers involves passing the refrigerant flow discharged from the compressor working elements through an annular space between inner and outer annular layers of sound absorptive material (e.g., fiber batting or foam). U.S. Patent Application Pub. No. 2004/0065504 A1 discloses a basic such muffler and then improved versions having integral helmholtz resonators formed within the inner layer. The disclosure of this '504 publication is incorporated by reference herein as if set forth at length.
Commonly owned and concurrently filed U.S. patent application docket 04-331, entitled Compressor Sound Suppression discloses a discharge plenum centerbody and methods for configuring such a centerbody. The disclosure of that application is incorporated by reference herein as if set forth at length.
One aspect of the invention involves a compressor including a housing and one or more working elements. A muffler is located downstream of the discharge plenum and a helmholtz resonator is located in the discharge plenum upstream of the muffler.
The helmholtz resonator may be formed in a centerbody between a bearing case and an inner element of the muffler. The helmholtz resonator may be added in a redesign or reengineering of an existing compressor configuration and/or a remanufacturing of an existing compressor previously lacking such a resonator. During the redesign/reengineering, parameters of the resonator may be optimized to provide a desired degree of suppression of a desired target type of vibration. The resonator may limit external sound radiated by the discharge housing and downstream piping due to resonating of discharge pulsation from the one or more working elements
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.
Like reference numbers and designations in the various drawings indicate like elements.
The exemplary muffler 52 includes annular inner and outer elements 70 and 72 separated by a generally annular space 74 (e.g., interrupted by support webs for retaining/positioning the inner element 70). These elements may be formed of sound absorption material (e.g., fiberglass batting encased in a nylon and steel mesh) In the exemplary embodiment, the inner element 70 is retained and separated from the space 74 by an inner foraminate sleeve 76 (e.g., nylon or wire mesh or perforated/expanded metal sheeting) and the outer element 72 is similarly separated and retained by an outer foraminate sleeve 78. In the exemplary embodiment, the outer element 72 is encased within an outer sleeve 80 (e.g., similarly formed to the sleeves 76 and 78) telescopically received within the housing 54. The sleeves 80 and 78 are joined at upstream and downstream ends by annular plates 82 and 84. In the exemplary embodiment, the upstream end of the sleeve 76 is closed by a circular plate 86 and the downstream end closed by an annular plate 90. In the exemplary embodiment, a non-foraminate central core 94 (e.g., steel pipe) extends through the inner element 70 and protrudes beyond a downstream end thereof.
In operation, compressed gas flow exits the compression pockets of the screw rotors 26, 28, 30 and flows into the discharge plenum 42. Upon exiting the compressor discharge plenum, the gas enters the muffler case 54 and flows down the annular space 74. Upon exiting the muffler the gas flow, which typically has entrained oil droplets, flows through the oil separating mesh 64. The mesh 64 captures any oil entrained in the gas and returns it to the oil management system by means of the conduit 66. The gas leaves the oil separating mesh and enters the plenum 68 and exits the outlet 69 toward the condenser (not shown).
As so far described, the compressor may be of an existing configuration although the principles of the invention may be applied to different configurations.
According to the present invention, a centerbody 120 is positioned in the flowpath between the rotors and the muffler.
Various materials and techniques may be used to manufacture the centerbody. The centerbody may consist essentially of at least one of molded plastic (e.g., non-foam polypropylene or glass-filled nylon) or of polymeric foam or expanded bead material (e.g., molded in one or more pieces or cut from one or more pieces).
In the exemplary embodiment, the overall size and shape of the centerbody are chosen to provide a smooth transition from the discharge ports to the muffler. For example, a streamwise variation in centerbody exterior cross-section (e.g., a frustoconical taper) may be selected to limit a pressure drop across the discharge plenum between the rotors and the muffler. Accordingly, the upstream/front face 124 may be sized to correspond to the inboard contours of the ports 200 and 202 defined by the plate 50. This may be at a radius essentially equal to the root radius of the working portion of the rotor 26.
Similarly, the downstream/aft face 126 may be dimensioned correspondingly to the inner element of the muffler (e.g., having a similar outer radius). The openings in the centerbody leading to the cavities are advantageously oriented normal to the local compressor discharge flow. The cavity volume and the number of cavities may be selected to address one or more particular sound frequencies. For example the first cavity 142 and passage 150 could be tuned for one frequency. The second cavity 144 and passage 152 could be tuned for a different frequency. Also the cavities within the centerbody are not limited to two. One or multiple could be provided. Further, the cavities could be filled fully or partially with sound absorption material, depending on the frequency of sound to be controlled.
The volumes of the chambers 142 and 144 and the shape, cross-sectional areas, and lengths of the passageways 150 and 152 may be selected to provide advantageous sound suppression. The engineering and/or optimization of the resonator may be undertaken at a variety of levels from basic to detailed and may involve a variety of theoretical/simulation and/or practical/experimentation steps. Well-developed helmholtz resonator optimization techniques may be applied. For example, the parameters may be optimized to provide maximum suppression at a particular target operating speed. Alternatively, the parameters may be optimized to provide a desired level of suppression over a desired range of speed or series of discrete speeds. The parameters may be optimized to provide desired suppression at non-target operating speeds which otherwise experience particularly significant resonance-associated noise.
For example, a target frequency may be calculated as a function of a target rotational speed and the rotor geometry—numbers of lobes/pockets). A first approximation of port size and volume size may be calculated based upon that target frequency and a prototype built. With the prototype, sound intensity at the target frequency may be measured. At least one parameter of the at least one internal volume or the at least one port may be selected/varied and the intensity remeasured in an iterative process to achieve a desired level of said intensity.
The centerbody may be incorporated in the remanufacturing of a compressor or reengineering of a compressor configuration. In the reengineering or remanufacturing, various existing elements may be essentially preserved.
One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, in a reengineering or remanufacturing situation, details of the existing compressor may particularly influence or dictate details of the implementation. Accordingly, other embodiments are within the scope of the following claims.
