Abstract:
The gas flow direction and the noise generation direction are in opposite directions in the suction muffler. The flow path in the muffler has a number of changes in direction and the flow path cross section decreases at each change of direction in the direction of flow.

Description:
BACKGROUND OF THE INVENTION 
     In positive displacement compressors, discrete volumes of gas are trapped and compressed with the trapped, compressed volumes being discharged from the compressor. The trapping of the volumes at suction pressure and their discharge at discharge pressure each produce pressure pulsations and the related noise generation. In the case of chillers, the suction pipe extends into the cooler and the suction gas pulsation in the cooler has been found to be one of the major noise sources in a chiller. This noise source becomes significant after vibration and acoustic treatments have been performed to control compressor vibration and discharge gas pulsation utilizing compressor isolators and a discharge muffler. 
     The flow of gas is along a flow path defined by a pressure differential and, for the suction flow, is through the suction pipe into the compressor. The direction of noise generation is not dictated by the flow direction. 
     The gas pulsation resulting from the intermittent nature of gas intake is exacerbated by variable speed operation which greatly extends the frequency range over which noise can be generated during operation. A suction inlet muffler, an acoustic treatment or lagging the cooler, partially or completely, can be employed for noise attenuation. While an absorptive suction muffler is an obvious choice, they are made to attenuate noise in a particular frequency range, or ranges, much less extensive than the frequency range associated with variable speed operation. 
     SUMMARY OF THE INVENTION 
     In the suction pipe of a chiller compressor the gas flow and the gas intake noise pulsations are traveling in primarily opposite directions, although some acoustic energy is reflected back towards the compressor where the suction pipe terminates in the cooler. The present invention locates a dissipative-type muffler at the inlet end of the suction pipe which is within the cooler. The inlet cross section of the muffler is oversized so as to permit a series of reductions in cross section down to the cross section of the suction pipe which is suitable for feeding the suction inlet of the compressor. The changes in the areas of the cross sections are primarily for reducing flow losses but could have slight acoustic benefits as where the wave propagation in the suction pipe is highly modal in nature, i.e. only is beneficial where the pipe cross section is small compared to an acoustic wavelength of 300 Hz, or less. The flow path through the muffler into the suction pipe involves a number of changes in flow direction. At each directional change in the muffler, the cross section of the flow path is decreased in the direction of flow. A preferred area reduction at each directional change is on the order of one third which keeps the flow and turning losses small. 
     It is an object of this invention to provide enhanced muffler performance. 
     It is another object of this invention to attenuate suction gas pulsation in a variable speed chiller compressor. These objects, and others as will become apparent hereinafter, are accomplished by the present invention. 
     Basically, the gas flow direction and the noise generation direction are in opposite directions in the suction muffler. The flow path in the muffler has a number of changes in direction and the flow path cross section decreases at each change of direction in the direction of flow. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a fuller and understanding of the present invention, reference should now be made to the following detailed description thereof taken in conjunction with the accompanying drawings wherein: 
     FIG. 1 is a schematic representation of a chiller employing the present invention; 
     FIG. 2 is a cross section of a suction muffler made according to the teachings of the present invention; 
     FIG. 3 is a cross section of a modified suction muffler; and 
     FIG. 4 is a cross section of a second modified suction muffler. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1, the numeral  10  generally designates a chiller. Chiller  10  includes a positive displacement compressor  12 , such as a screw compressor, which discharges hot, compressed refrigerant gas into condenser  14 . The gaseous refrigerant condenses in condenser  14  and high pressure liquid refrigerant passes from condenser  14  to cooler  20  via expansion device  16  whereby the liquid refrigerant partially flashes. The refrigerant is in a heat exchange relationship with water, or the like) in cooler  20  such that the liquid refrigerant is evaporated and the water is cooled so as to be available for air conditioning. The gaseous refrigerant is drawn from cooler  20  by compressor  12  via suction muffler  30  and suction pipe  22 . Suction pipe  22  is sized to deliver suction gas to compressor in the absence of a muffler and all of the flow path segments in the muffler are successively larger in cross section at each change in flow direction in an upstream direction. 
     As best shown in FIG. 2, suction pipe  22  extends downwardly into cooler  20  and is received in a spaced relationship within muffler  30 . Muffler  30  is in the form of an annular cylinder with one closed end  30 - 1 . End  30 - 1  is serially overlain by an acoustical lining  31 , such as fiberglass, and a perforate member  32  which may be metal or plastic and preferably having a porosity of 40-70%. Member  32  faces and is spaced from the inlet end  22 - 1  of suction pipe  22 . For the purposes of the present invention, a change in flow direction will normally be a nominal 90° . Other angles are possible but make the device less compact. 
     Assuming that the cross sectional area of suction pipe  22  is S, the area of the surface S′ defined by the extension of pipe  22  to perforate member  32  is greater than S. Preferably S′ is 125% to 175% of S with 150% being preferred. The area of the annular area S″ defined between annular wall  30 - 2  of muffler  30  and suction pipe  22  will be 125% to 175% of S′ with 150% being preferred. Wall  30 - 2  is spaced from inner surface  20 - 1  of cooler  20  and the area of the surface S′″ defined by the extension of wall  30 - 2  to the inner surface  20 - 1  is greater than S″. Preferably S′″ is 125% to 175% of S″ with 150% being preferred. 
     Operation of compressor  12  serially draws gaseous refrigerant from cooler  20  through muffler  30  in a flow path serially having the reduced cross sections of S′″, S″ and S′ before entering suction pipe  22  which has a cross section of S. The reductions in cross section at the locations of change in the flow direction keeps flow/turning losses small. The noise generated by the suction process in compressor  12  reflects along the interior of suction pipe  22  before impinging upon the surface defined by perforate member  32  and the underlying acoustical lining  31 . Sound passing through perforations  32 - 1  of perforate member  32  are attenuated by acoustical lining  31 . 
     Muffler  130  of FIG. 3 is the same as muffler  30  except that acoustical lining  131  covers both end  130 - 1  and annular wall  130 - 2  and is overlain by perforate member  132 . The additional flow path length over the portion of perforate member  132  covering the wall  130 - 2  and the underlying acoustical lining  131  would tend to provide increased flow resistance over muffler  30  but the oversized flow path cross section area, S″, in that region mitigates flow losses. Additionally, flow is over perforate member  132  and its perforations  132 - 1  in passing through the area having cross section S′″. The increased area provided by perforate member  132 , perforations  132 - 1  and the acoustical lining  131  for noise impingement provides further attenuation. 
     Muffler  230  is the same as muffler  130  except that the outer end portion of suction pipe  222  has been covered with acoustical segments lining  231 - 1  which is spaced by spacer(s)  232 - 2  overlain by perforate member  232  having perforations  232 - 1  and acoustical lining  131  has been replaced by a series of segments  131 - 1  spaced by spacers  132 - 2 . Acoustic liners  31  and  131  are illustrated as bulk type liners but could be of the locally reacting type such as acoustic liner segments  131 - 1 . Because acoustic liner segments  131 - 1  and  231 - 1  are separated by spacers  132 - 2  and  232 - 2 , respectively, acoustic wave propagation in the liner segments  131 - 1  and  231 - 1  is prevented so that there is primarily propagation normal to the liner only. This results in enhanced low frequency performance where the distance between spacers  132 - 2  is small compared to the acoustic wavelength, i.e. less than about one eighth of the wavelength. The flow path between perforate members  132  and  232  would have the cross sectional areas S″, as defined above. Perforate member  132  is spaced from inner surface  20 - 1  of cooler  20  and the area of the surface S′″ defined between perforate member  132  to the inner surface  20 - 1  is greater than S″. Preferably S′″ is 125% to 175% of S″ with 150% being preferred. 
     Muffler  230  has an additional flow path length over the perforate member  232  when compared to muffler  130  but the oversized flow path cross sectional area S″ in that region mitigates flow losses. The provision of a flow path portion where noise reflects and impinges between two perforate members underlain by acoustical lining provides further attenuation. The shape of spacers  132 - 2  and  232 - 2  is arbitrary in that they only need to block wave travel longitudinally between liner segments  131 - 1  and  231 - 1 , respectively, and can be a series of annular discs for the annular walls and spaced circles or a grid for end  230 - 1 . 
     Although preferred embodiments of the present invention have been illustrated and described other changes will occur to those skilled in the art. It is therefore intended that the scope of the present invention is to be limited only by the scope of the appended claims.