Abstract:
In compression systems having multiple sources of pulsation induced noise, provisions are made to reduce the noise by selectively varying the length of adjacent refrigerant lines leading to a common refrigerant manifold. Each of the refrigerant lines is associated with a particular noise source. The feature can be incorporated into compressors operating in parallel or in a single multi-rotor compressor having multiple suction or/and discharge ports. It is equally applicable to both discharge and suction ports.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates generally to compressors and, more particularly, to a method and apparatus for reducing pulsation induced noise generated by compression elements operating in parallel. 
       BACKGROUND OF THE INVENTION 
       [0002]    Typical air conditioning, heat pump and refrigeration systems include at least one compressor for circulating a refrigerant throughout the system which includes at least a condenser (or a gas cooler), an expansion device and an evaporator. In such vapor compression systems, the compressor is a major source of pulsation induced noise. During operation, the compressor generates refrigerant pulsations propagating throughout the refrigerant system and contributes about fifty percent to the overall system sound level. These pulsations transmitted through the refrigerant system can result in damage and distraction of the system components and can be disruptive to occupants of a conditioned space. 
         [0003]    The types of compressors that are used in heating, ventilation, air conditioning and refrigeration (HVAC&amp;R) applications include, for example, reciprocating compressors, rotary compressors, scroll compressors and screw compressors. Screw compressor installations are particularly recognized to generate pulsation induced noise. 
         [0004]    It is common in HVAC&amp;R applications to have multiple compressor installations with compressors operating in parallel (or in tandem) or have a plurality of compression elements that may be driven, for example, by a common shaft. In these applications, pulsation induced noise can be particularly disruptive and bothersome, since the amplitude of pulsations may be additive from these multiple sources. 
         [0005]    The HVAC&amp;R equipment manufacturers have made efforts to reduce pulsation induced noise levels generated by refrigerant systems implementing various techniques such as sound attenuating materials, active noise control and the like. Many of these methods involve additional cost and complexity or have unproven effectiveness and reliability record. Also, for the occupant comfort and safety and protection of the environment, various governmental regulations and legislative measures imposing limits on the sound levels emanating from HVAC&amp;R systems have been implemented. Therefore, there is a need for efficient and inexpensive methods for sound attenuation and pulsation reduction for multiple compressors or compression elements operating in parallel. 
       DISCLOSURE OF THE INVENTION 
       [0006]    In an HVAC&amp;R system having a plurality of pulsation induced noise sources associated with compressor discharge/suction ports, connected to a common manifold, such that the lengths of the connecting ducts are selectively varied in such a way that the pressure pulsations in the respective ducts cancel each other at the point where they reach the common manifold. 
         [0007]    By one aspect of the invention, a plurality of compressors operating in parallel have discharge ducts of varying lengths which are connected to a common outlet manifold. 
         [0008]    By another aspect of the invention, a compressor, such as a screw compressor or scroll compressor, including at least a pair of discharge ports has discharge ducts of differing lengths that interconnect to a common discharge manifold. 
         [0009]    By yet another aspect of the invention, a duality of discharge/inlet ducts vary in length in accordance with the equation L Dif =n*(c/f), where c is the speed of sound in the refrigerant, f is the dominant pulsation frequency and n is an integer number. 
         [0010]    In the drawings as hereinafter described, a preferred embodiment is depicted; however, various other modifications and alternate constructions can be made thereto without departing from the spirit and scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a schematic illustration of a vapor compression system having multiple compressors with the present invention incorporated therein. 
           [0012]      FIG. 2  is a graphic illustration of the two sound waves emanating from two sound sources and cancelling each other. 
           [0013]      FIG. 3  is a graphic illustration of three sound waves emanating from three compressors or compression elements in accordance with the present invention. 
           [0014]      FIG. 4  is a graphic illustration of a tri-rotor screw compressor with dual discharge ports to which the present invention is applicable. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    Shown in  FIG. 1  is a schematic illustration of a basic vapor compression system  11  which includes, in a serial flow relationship, a condenser  12 , an expansion device  13 , an evaporator  14  and a compression system  16 . In the cooling or refrigeration mode of operation, refrigerant flows serially from the compression system  16  to the condenser  12 , the expansion device  13 , the evaporator  14  and back to the compression system  16  which includes dual compressors  17  and  18  operating in tandem. In a heat pump mode of operation, the functions of the evaporator  14  and the condenser  12  are reversed. Obviously, by incorporation of appropriate flow control devices such as a four-way valve, the vapor compression system  11  can be made capable to run in both cooling and heating modes of operation. 
         [0016]    As will be seen, the compression system  16  includes two compressors  17  and  18  arranged in parallel with a common suction manifold  19  and a common discharge manifold  21 . The compressor  17  is connected to the common suction manifold  19  by a refrigerant suction line  22  and to the common discharge manifold  21  by a refrigerant discharge line  23 . Similarly, the compressor  18  is fluidly connected to the suction manifold  19  by a refrigerant suction line  24  and to the common discharge manifold  21  by the refrigerant discharge line  26 . Although only two compressors  17  and  18  operating in tandem are shown in  FIG. 1 , a larger number of compressors can be included into the compression system  16 . 
         [0017]    Sound pressure waves tend to emanate from the compressors  17  and  18 , most significantly from their respective discharge ports  27  and  28 , but also from their suction ports  29  and  31 . The present invention is implemented to minimize the generated noise effect of these pressure pulsations. 
         [0018]    Recognizing that the pressure pulsations are typically transmitted by plane wave propagation, the respective lengths of the refrigerant discharge line  23  and the refrigerant discharge line  26  are varied in such a manner as to cause the respective pressure pulsations to cancel each other out at the point where they enter the common discharge manifold  21 . The same is true for the lengths of the refrigerant suction line  22  and the refrigerant suction line  24  where the pressure pulsations travel toward the common suction manifold  19 . In order for this wave cancellation to occur, the lengths of the refrigerant discharge lines  23  and  26  and/or the lengths of the refrigerant suction lines  22  and  24  are selected such that the difference in length is L Dif =n*(c/f), where c is the speed of sound in the particular refrigerant, f is the dominant pulsation frequency and n is an integer number. 
         [0019]    As an example, a typical speed of sound in a refrigerant would vary from 150 m/s to 250 m/s and a typical dominant pulsation frequency would be in the range of 30 Hz to 600 Hz Thus, L Dif  would vary from 0.25 m to 8.3 m (with n=1). 
         [0020]    Shown in  FIG. 2  is a graphic illustration of the sound waves amplitude as a function of time at the point in each refrigerant line communicating with each compressor or compression element  17  and  18  within the compression system  16 , where the refrigerant lines located either on the suction or discharge side, merge together into the common suction or discharge manifold respectively. Each of these sound waves represents the pulsation at this location as emanating from each of the two compressors. The sound wave shown as “x” is the wave emanating from the first compressor  17 , and the sound wave shown as “y” is the wave emanating from the compressor  18 . In this case, each of those two refrigerant lines are chosen to be of a different length, such that each sound wave arrives at the junction point to be 180 degrees out of phase with the other wave. Due to of the difference in the lengths of the refrigerant lines, as described hereinabove, the two sound waves cancel each other out. 
         [0021]    As mentioned above, it should be recognized that the present invention is applicable not only to dual compressors but to the compression systems including multiple compressor in general. For example, in a three compressor configuration, the discharge refrigerant lines (and/or the suction refrigerant lines) are of three different lengths for the purpose discussed hereinabove in accordance to the following formula L Dif =(2/3)*n*(c/f). 
         [0022]    In a three compressor configurations, the respective sound waves that result from the differing duct lengths thus correspond by the wave forms A, B and C as shown in  FIG. 3 . Again, at the point where the sound waves arrive to the common discharge manifold  21 , or common suction manifold  19 , they cancel each other out. 
         [0023]    These embodiments would be especially applicable to the compression systems with multiple compressors whose rotating speed of operation is synchronized. In a more general sense, for multiple synchronous pulsation sources connected to a common manifold to cancel each other, the length of each corresponding refrigerant line can be offset by L Dif =(2/k)*n*(c/f), where k is the number of pulsation sources or compressor ports. 
         [0024]    Referring now to  FIG. 4 , a basic vapor compression system  60  includes in a serial flow relationship a tri-rotor compressor  32 , a condenser  33 , an expansion device  34 , and an evaporator  36 . An economizer circuit, with additional vapor injection and unloading capability, may be included but is not shown. 
         [0025]    The tri-rotor compressor  32  includes a centrally located drive rotor  37  and a pair of driven rotors  38  and  40  on either side thereof. The drive rotor  37  may be driven by an electric motor (not shown), and in turn drives the driven rotors  38  and  40 . Compression chambers are defined between the screw flutes on the drive rotor  37  and the respective drive rotors  38  and  40 . Refrigerant, which is compressed in the compression chambers between the rotors, is discharged through discharge ports  39  and  41 , to respective refrigerant discharge line  42  and  43  and finally to a discharge manifold  44  prior to passing to the condenser  33 . 
         [0026]    The present invention is incorporated in such a tri-rotor system by selectively varying the lengths of the two discharge ducts  42  and  43  in a manner described herein above. 
         [0027]    On the suction side of the tri-rotor compressor, refrigerant flows from the evaporator  36  to an inlet manifold  46 , to the refrigerant inlet lines  47  and  48 , and then to the respective inlet ports  49  and  51 . The respective lengths of the refrigerant inlet lines  47  and  48  can be selectively chosen so as to cancel out their respective pressure pulsations emanating from the inlet ports  49  and  51  in a manner as described hereinabove. 
         [0028]    Once again, this invention is applicable to the compressors including multiple compression elements, where the lengths of the refrigerant lines leading to suction or/and discharge port for each compression element is established in accordance to the formula disclosed hereinabove for multiple compressors. Also, although the invention is described with respect to a tri-rotor screw compressor, other compressor types, such as root compressors or scroll compressors connected to the same shaft, are within the scope and can equally benefit from the invention. 
         [0029]    Furthermore, although the invention is disclosed with respect to the sound level attenuation, other benefits, such as for instance vibration level reduction or sound quality improvement, can be also obtained. These and other benefits of the invention could be recognized by a person ordinarily skilled in the art. 
         [0030]    It should be understood that this invention applies to a broad range of refrigerant systems, including container refrigeration units, truck-trailer systems, residential cooling units and heat pumps, roof top installations and the like. 
         [0031]    While the present invention has been particularly shown and described with reference to a preferred mode as illustrated in the drawings, it will be understood by one skilled in the art that various changes in detail may be effected therein without departing from the spirit and scope of the invention as defined by the claims.