Abstract:
A system is provided for attenuating noise in at least two positive displacement compressors proximately located from each other for use with at least one heating or cooling system. A lead compressor and a lag compressor have a selectably controllable rotational speed and a selectably controllable phase of operation. A controller selectably controls the rotational speed and the phase of operation of each of the compressors. The controller controls the rotational speed of the compressors at a predetermined rotational speed that is substantially the same for each of the compressors. The controller controls the phase of operation of the compressors by shifting the phase of operation of the lag compressor so that an outlet pressure pulse operatively produced by the lag compressor is substantially evenly spaced between successive outlet pressure pulses operatively produced by the reference compressor.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates generally to a method of operation and apparatus for noise attenuation of positive displacement compressors, and more particularly, to a method of operation and apparatus for noise attenuation of screw compressors that decreases the composite pressure pulse of the screw compressors by varying the speed of one or more of the screw compressors.  
         [0002]     Heating and cooling systems typically maintain temperature control in a structure by circulating a fluid within coiled tubes such that passing another fluid over the tubes effects a transfer of thermal energy between the two fluids. A primary component in such a system is a positive displacement compressor which receives a cool, low pressure gas and by virtue of a compression device, exhausts a hot, high gas. One type of positive displacement compressor is a screw compressor, which generally includes two cylindrical rotors mounted on separate shafts inside a hollow, double-barreled casing. The side-walls of the compressor casing typically form two parallel, overlapping cylinders which house the rotors side-by-side, with their shafts parallel to the ground. Screw compressor rotors typically have helically extending lobes and grooves on their outer surfaces forming a large thread on the circumference of the rotor. During operation, the threads of the rotors mesh together, with the lobes on one rotor meshing with the corresponding grooves on the other rotor to form a series of gaps between the rotors. These gaps form a continuous compression chamber that communicates with the compressor inlet opening, or “port,” at one end of the casing and continuously reduces in volume as the rotors turn and compress the gas toward a discharge port at the opposite end of the casing for use in the system.  
         [0003]     These rotors rotate at high rates of speed, and multiple sets of rotors (compressors) may be configured to work together to further increase the amount of gas that can be circulated in the system, thereby increasing the operating capacity of a system. While the rotors provide a continuous pumping action, each set of rotors (compressor) produces pressure pulses as the pressurized fluid is discharged at the discharge port. These discharge pressure pulsations act as significant sources of audible sound within the system. In addition, when multiple rotors (compressors) are proximately located, whether being utilized within the same or independent heating or cooling systems, if the rotors are not operating at substantially the same rotational speed, a phenomenon known as beating may occur. Beating, also referred to as beats, result from a difference between the frequencies of the discharge pressure pulsations. In addition to providing further undesirable sound, beats can potentially damage the compressors.  
         [0004]     To eliminate or minimize beats and the undesirable sound, noise attenuation devices or systems can be used. One example of a noise attenuation system is a dissipative or absorptive muffler system typically located at the discharge of the compressors. The use of muffler systems to attenuate sound can be expensive, depending upon the frequencies that must be attenuated by the muffler system. Typically, the lower the frequency of the sound to be attenuated, the greater the cost and size of the muffler system.  
         [0005]     What is needed is a cost-effective, efficient and easily implemented method or apparatus for compressor noise attenuation that may be used with multiple variable speed compressors.  
       SUMMARY OF THE INVENTION  
       [0006]     The present invention relates to a method for attenuating noise in at least two positive displacement compressors proximately located from each other having a reference compressor for providing reference operational settings for comparison with the remaining compressors. The steps include providing at least two compressors including a reference compressor, the compressors having a selectably controllable rotational speed and a selectably controllable phase of operation; providing a controller for selectably controlling the rotational speed and the phase of operation of each of the compressors; providing a sensor for sensing the rotational speed and the phase of operation of each of the compressors; controlling the rotational speed of the compressors at a predetermined rotational speed that is substantially the same for each of the compressors; and controlling the phase of operation of the compressors wherein the phase of operation of the remaining of the compressors, not including the reference compressor, is shifted so that an outlet pressure pulse operatively produced by each of the remaining compressors is substantially evenly spaced between successive outlet pressure pulses operatively produced by the reference compressor. Note: A three-compressor system would interleave the two remaining compressors&#39; discharge pulsations evenly between the reference compressor&#39;s discharge pressure pulsations, effectively tripling the pressure pulsation fundamental frequency. A four-compressor system would quadruple the pressure pulsations etc. Alternatively, a pair of two-compressor systems could operate independently from one another in regards to speed, if so desired.  
         [0007]     The present invention further relates to a system for attenuating noise in at least two positive displacement compressors proximately located from each other, which includes a reference compressor. The compressors have a selectably controllable rotational speed and a selectably controllable phase of operation. A means of control selectably controls the rotational speed and the phase of operation of each of the compressors. A sensing means senses the rotational speed and the phase of operation of each of the compressors. The means of control controls the rotational speed of the compressors at a predetermined rotational speed that is substantially the same for each of the compressors. The means of control controls the phase of operation of the compressors by shifting the phase of operation of all the compressors with the exception of the reference compressor. The phase of operation of the remaining compressors other than the reference compressor is shifted so that an outlet pressure pulse operatively produced by each of the remaining compressors is substantially evenly spaced between successive outlet pressure pulses operatively produced by the reference compressor.  
         [0008]     An advantage of the present invention is the reduction in the size and cost of dissipative or attenuating muffler systems.  
         [0009]     Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a schematic of a continuously variable speed compressor system of the present invention.  
         [0011]      FIG. 2  is a diagram of compressor pressure pulses shifted by the method of the present invention. 
     
    
       [0012]     Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0013]     One embodiment of the heating, ventilation, air conditioning or refrigeration (HVAC&amp;R) system  10  of the present invention is depicted in  FIG. 1 . A positive displacement lead compressor  12  is connected to a motor  21  and inverter  42 , for selectively controlling operational parameters, such as rotatational speed, of the compressor  12 . Compressor  12  discharges compressed refrigerant gas through discharge line  24 . Similarly, compressor  14 , which operates in parallel with compressor  12 , discharges compressed refrigerant gas through discharge line  22 . These compressors are typically positive displacement compressors, such as screw, reciprocating or scroll, having a wide range of cooling capacity. Sensors  48 ,  50  monitor refrigerant gas parameters, such as pressure pulses, passing through respective discharge lines  22 ,  24  providing parameter inputs to a controller  56  via respective lines  58 ,  60 . The controller  56  includes logic devices, such as a microprocessor or other electronic means, for the generation of speed control signals  46  and  48  for controlling the operating parameters of compressors  12 ,  14  by controlling their respective inverters  42 ,  44  and motors  21 ,  23 . AC electrical power received from an electrical power source  40  is rectified from AC to DC, and then inverted from DC back to variable frequency AC by inverters  42 ,  44  for driving respective compressor motors  21 ,  23 . The compressor motors are typically AC induction, but might also be Brushless Permanent Magnet or Switched Reluctance motors. After refrigerant gas that is compressed by compressors  12 ,  14  is directed downstream of sensors  48 ,  50 , discharge lines  22 ,  24  join and become a common line  26 , although lines  22 ,  24  may remain separate if desired. Optionally, muffler  15  is positioned along the common line  26  to dissipate or absorb the pressure pulses generated by operation of the compressors  12 ,  14 .  
         [0014]     Common line  26  delivers refrigerant gas to the condenser  16 , which enters into a heat exchange relationship with a fluid, preferably water, flowing through a heat-exchanger coil  25  connected to a cooling tower  17 . The refrigerant vapor in the condenser  16  undergoes a phase change to a refrigerant liquid as a result of the heat exchange relationship with the liquid in the heat-exchanger coil  25 . The condensed liquid refrigerant from condenser  16  flows along a conduit  28  to an expansion device  18 , which greatly lowers the temperature and pressure of the refrigerant before entering the evaporator  20  via conduit  30 . Alternately, the condenser can reject the heat directly into the atmosphere through the use of air movement across a series of finned surfaces (direct expansion condenser).  
         [0015]     The evaporator  20  can include a heat-exchanger coil  21  having a supply line  21 S and a return line  21 R connected to a cooling load  19 . The heat-exchanger coil  21  can include a plurality of tube bundles within the evaporator  20 . Water or any other suitable secondary refrigerant, e.g., ethylene, calcium chloride brine or sodium chloride brine, travels into the evaporator  20  via return line  21 R and exits the evaporator  20  via supply line  21 S. The liquid refrigerant in the evaporator  20  enters into a heat exchange relationship with the water in the heat-exchanger coil  21  to chill the temperature of the water in the heat-exchanger coil  21 . The refrigerant liquid in the evaporator  20  undergoes a phase change to a refrigerant gas as a result of the heat exchange relationship with the liquid in the heat-exchanger coil  21 . The gas refrigerant in the evaporator  20  then returns to the compressors  12 ,  14  by suction line  32  which bifurcates at suction plenum  34  to separate suction lines  36 ,  38  which join respective compressors  12 ,  14  to complete the cycle. In another embodiment of the present invention, the suction line  32  from the evaporator  20  to the compressors  12 ,  14  can be continuously separate lines that deliver refrigerant gas to the compressors  12 ,  14 .  
         [0016]     Inverters  42 ,  43  collectively provide variable speed control to the operating parameters of respective compressors  12 ,  14  by independently controlling both the frequency and voltage magnitude of electrical power to the motors  21 ,  23  by power source  40 . Collectively, inverters  42 ,  43  can simultaneously vary both the frequency and voltage, as dictated by the controller  56  via respective speed control signals  46 ,  47  to provide control of the overall system refrigeration capacity through the use of variable speed modulation of compressors  12 ,  14 . Inverters  42 ,  44  are also referred to in the industry as variable speed or variable frequency drives. Alternately, variable speed drives  42 ,  43  may contain a single AC to DC converter and two or more DC to AC inverts to provide a lower cost solution. While the system of the present invention illustrates two variable speed drives for selectively controlling two compressors, so long as each compressor is controlled by a separately designated variable speed drive, it is envisioned that any number of compressors may be employed.  
         [0017]     Inverter  42  controls the operating parameters applied to the motor of lead compressor  12  via speed control signal  46 . The remaining compressors in the system are referred to as lag compressors. Selection of lead compressor  12  is not critical as it is not dependent on size, but is for identifying an operating point of reference for the controller  56 . Thus, the compressors used in system  10  are not required to be of the same capacity.  
         [0018]     Controller  56 , which controls the operations of system  10 , employs continuous feedback from sensors  48 ,  50  to continuously monitor and change the frequency and voltage applied to compressors  12 ,  14  in response to changes in system cooling loads. That is, as the system  10  requires either additional or reduced cooling capacity, which is constantly monitored by the controller  56 , the operating parameters of any of the compressors  12 ,  14  in the system  10  may likewise be revised. To maintain maximum operating efficiency, the operating frequencies of the compressors  12 ,  14  are changing constantly, such as proportionally changing the operating frequencies of all the compressors, or any compressors, as controlled by a capacity control algorithm within the controller  56 . However, separate from system load requirements, the controller  56  also continuously monitors the gas parameter readings provided by sensors  48 ,  50  to minimize the resultant compressor sound level in the system.  
         [0019]     One way for the controller  56  to effect noise attenuation in system  10  is to control the phase of operation of the compressor  14  with respect to compressor  12 . The controller  56  monitors the occurrence of pressure pulses from the lead or reference compressor  12  by use of sensor  50 . From this information, the controller  56  varies the magnitude of speed control signal  47  which is applied to inverter  44  to synchronize the feedback pressure pulses emanating from the lag compressor  14  via sensor  50  with respect to frequency and simultaneously interleave the pulsations with respect to the phase of the pressure pulsations sensed by sensor  48 . Referring to  FIG. 2 , which depicts the pressure pulses as square waves, wave  52  corresponding to lead compressor  12  pressure pulses and wave  54  corresponding to lag compressor  14  pressure pulses. Preferably, the phase of wave  54  is shifted such that the pulse of wave  54  is positioned substantially equidistant between successive pulses of wave  52 . This shifting preferably produces a resultant or effective output wave that is twice the frequency of wave  52  having a wavelength half that of wave  52 . Higher frequency waves are easier to attenuate, requiring smaller, less expensive dissipating or absorption mufflers.  
         [0020]     In an alternate embodiment, additional lag compressors may be employed. By placing additional lag compressor waves in the system which are substantially equally spaced between successive pulses of the lead compressor, the resultant wave frequency is multiplied by the total number of compressors. Preferably, two to four compressors are employed in this arrangement. Therefore, if there are four compressors, whose pulse pattern is shifted in accordance with the present invention, the resultant pulse wave frequency is multiplied by four, although any number of compressors may be used in a system.  
         [0021]     While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.