Abstract:
A system is provided for attenuating tonal acoustic noise associated with a single positive displacement compressor, or multiple positive displacement compressors proximately located from each other. A controller selectably controls the rotational speed and the frequency of operation of each of the compressors. The controller controls the rotational speed of the compressors about a predetermined rotational speed, in a random manner, within the pre-selected speed band to reduce the magnitude of the central tonal acoustic frequency of the pressure pulsations and disperse the sound power over a wider acoustic bandwidth.

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 audible noise generated by one or more of the screw compressors by varying the operating speed of each screw compressor about its respective central frequency.  
         [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 pressure 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 one another. 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.  
         [0003]     These rotors rotate at high rates of speed, and multiple sets of rotors, or multiple 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 produces pressure pulses as the pressurized fluid is discharged at the discharge port. These pressure pulses are generated by the compressor at increments of the operating speed of the driven screw, which is typically about 5 or 6 times the driven or operating RPM. These discharge pressure pulsations act as significant sources of audible sound within the system.  
         [0004]     To eliminate or minimize 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 an effective, low cost, efficient and easily implemented method or apparatus for compressor rotor noise attenuation.  
       SUMMARY OF THE INVENTION  
       [0006]     One embodiment of the present invention relates to a method for attenuating noise in at least one positive displacement compressor. The steps include providing at least one compressor, the at least one compressor having a selectably controllable rotational speed; and varying the rotational speed of the at least one compressor in a random manner about a predetermined rotational speed to reduce the magnitude of the tonal acoustic noise associated with the at least one compressor.  
         [0007]     The present invention further relates to a system for attenuating noise in at least one positive displacement compressor, the system includes at least one compressor, the at least one compressor having a selectably controllable rotational speed. A control panel comprises a microprocessor and a memory device, the control panel being configured to vary a rotational speed of the at least one compressor in a random manner about a predetermined rotational speed to reduce the magnitude of the tonal acoustic noise associated with the at least one compressor.  
         [0008]     The present invention yet further relates to a chiller system including at least one refrigerant circuit. The at least one refrigerant circuit comprises at least one compressor each driven by a motor, a condenser arrangement and an evaporator arrangement connected in a closed refrigerant loop, the at least one compressor having a selectably controllable rotational speed. A control panel comprises a microprocessor and a memory device, the control panel being configured to vary a rotational speed of each motor of the at least one compressor in a random manner about a predetermined rotational speed to reduce the magnitude of the tonal acoustic noise associated with the at least one compressor.  
         [0009]     An advantage of the present invention is the reduction in tonal acoustic noise associated with compressors driven by variable speed drives.  
         [0010]     A further advantage of the present invention is that it can be used in single compressor systems as well as multiple compressor systems.  
         [0011]     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  
       [0012]      FIG. 1  illustrates schematically an embodiment of a refrigeration or chiller system used with the present invention.  
         [0013]      FIG. 2  illustrates schematically a variable speed drive used with the present invention. 
     
    
       [0014]     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  
       [0015]      FIG. 1  illustrates generally one embodiment of a refrigeration system that Q_ ) can incorporate the present invention. As shown in  FIG. 1 , the HVAC, refrigeration or liquid chiller system  100  has two compressors incorporated in corresponding refrigerant circuits, but it is to be understood that the system  100  can have one refrigerant circuit or more than two refrigerant circuits for providing the desired system load and can have more than one compressor for a corresponding refrigerant circuit. The system  100  includes a first compressor  102 , a second compressor  104 , a condenser arrangement  106 , expansion devices, a water chiller or evaporator arrangement  108  and a control panel  110 . The control panel  110  can include an analog to digital (A/D) converter, a microprocessor, a non-volatile memory, and an interface board to control operation of the refrigeration system  100 . The control panel  110  can be used to control the operation of a VSD  112 , which receives its electrical power from an AC power source  116 , the motors  114  and the compressors  102  and  104 . A conventional HVAC, refrigeration or liquid chiller system  100  includes many other features that are not shown in  FIG. 1 . These features have been purposely omitted to simplify the drawing for ease of illustration.  
         [0016]     The compressors  102  and  104  compress a refrigerant vapor and deliver it to the condenser  106 . The compressors  102  and  104  are preferably connected in separate refrigeration circuits, i.e., the refrigerant output by the compressors  102  and  104  are not mixed and travel in separate circuits through the system  100  before reentering the compressors  102  and  104  to begin another cycle. The separate refrigeration circuits preferably use a single condenser housing  106  and a single evaporator housing  108  for the corresponding heat exchanges. The condenser housing  106  and evaporator housing  108  maintain the separate refrigerant circuits either through a partition or other dividing means with the corresponding housing or with separate coil arrangements. In another embodiment of the present invention, the refrigerant output by the compressors  102  and  104  can be combined into a single refrigerant circuit to travel through the system  100  before being separated to reenter the compressors  102  and  104 .  
         [0017]     The compressors  102  and  104  are preferably screw compressors, although other positive displacement compressors such as reciprocating compressors, scroll compressors, rotary compressors or other type of compressor may also benefit from the motor control apparatus of the present invention. The output capacity of the compressors  102  and  104  can be based on the operating speed of the compressors  102  and  104 , which operating speed is dependent on the output speed of the motors  114  driven by the VSD  112 . The refrigerant vapor delivered to the condenser  106  enters into a heat exchange relationship with a fluid, e.g., air or water, and undergoes a phase change to a refrigerant liquid as a result of the heat exchange relationship with the fluid. The condensed liquid refrigerant from condenser  106  flows through corresponding expansion devices to an evaporator  108 .  
         [0018]     The evaporator  108  can include connections for a supply line and a return line of a cooling load. A secondary liquid, which is preferably water, but can be any other suitable secondary liquid, e.g., ethylene, calcium chloride brine or sodium chloride brine, travels into the evaporator  108  via return line and exits the evaporator  108  via supply line. The liquid refrigerant in the evaporator  108  enters into a heat exchange relationship with the secondary liquid to chill the temperature of the secondary liquid. The refrigerant liquid in the evaporator  108  undergoes a phase change to a refrigerant vapor as a result of the heat exchange relationship with the secondary liquid. The vapor refrigerant in the evaporator  108  then returns to the compressors  102  and  104  to complete the cycle. It is to be understood that any suitable configuration of condenser  106  and evaporator  108  can be used in the system  100 , provided that the appropriate phase change of the refrigerant in the condenser  106  and evaporator  108  is obtained.  
         [0019]     The AC power source  116  provides single phase or multi-phase (e.g., three phase), fixed voltage, and fixed frequency AC power to the VSD  112  from an AC power grid or distribution system that is present at a site. The AC power source  116  preferably can supply an AC voltage or line voltage of 200 V, 230 V, 380 V, 460 V, or 600 V at a line frequency of 50 Hz or 60 Hz, to the VSD  112  depending on the corresponding AC power grid.  
         [0020]     The VSD  112  receives AC power having a particular fixed line voltage and fixed line frequency from the AC power source  116  and provides AC power to each of the motors  114  at desired voltages and desired frequencies, both of which can be varied to satisfy particular requirements. Preferably, the VSD  112  can provide AC power to each of the motors  114  that may have higher voltages and frequencies and lower voltages and frequencies than the rated voltage and frequency of each motor  114 . In another embodiment, the VSD  112  may again provide higher and lower frequencies but only the same or lower voltages than the rated voltage and frequency of each motor  114 .  
         [0021]     The motors  114  are preferably induction motors that are capable of being operated at variable speeds. The induction motors can have any suitable pole arrangement including two poles, four poles or six poles. However, any suitable motor that can be operated at variable speeds can be used with the present invention.  
         [0022]      FIG. 2  illustrates schematically some of the components in one embodiment of the VSD  112 . The VSD  112  can have three stages: a converter or rectifier stage  202 , a DC link stage  204  and an output stage having a plurality of inverters  206 . The converter  202  converts the fixed line frequency, fixed line voltage AC power from the AC power source  116  into DC power. The converter  202  can be in a rectifier arrangement composed of electronic switches that can only be turned on either by gating, when using silicon controlled rectifiers, or by being forward biased, when using diodes. Alternatively, the converter  202  can be in an active converter arrangement composed of electronic switches that can be gated or switched both on and off, to generate a controlled DC voltage and to shape the input current signal to appear sinusoidal, if so desired. The active converter arrangement of converter  202  has an additional level of flexibility over the rectifier arrangement, in that the AC power not only can be rectified to DC power, but that the DC voltage level can also be controlled to a specific value. In one embodiment of the present invention, the diodes and silicon controlled rectifiers (SCRs) can provide the converter  202  with a large current surge capability and a low failure rate. In another embodiment, the converter  202  can utilize a diode or thyristor rectifier coupled to a boost DC/DC converter or a pulse width modulated boost rectifier to provide a boosted DC voltage to the DC link  204  in order to obtain an output voltage from the VSD  112  greater than the input voltage of the VSD  112 .  
         [0023]     The DC link  204  filters the DC power from the converter  202  and provides energy storage components. The DC link  204  can be composed of capacitors and inductors, which are passive devices that exhibit high reliability rates, i.e., very low failure rates. Finally, the inverters  206  are connected in parallel on the DC link  204  and each inverter  206  converts the DC power from the DC link  204  into a variable frequency, variable voltage AC power for a corresponding motor  114 . The inverters  206  are power modules that can include power transistors, e.g., insulated gate bipolar transistors (IGBTs), with diodes connected in anti-parallel. Furthermore, it is to be understood that the VSD  112  can incorporate different components from those discussed above and shown in  FIG. 2  so long as the inverters  206  of the VSD  112  can provide the motors  114  with appropriate output voltages and frequencies.  
         [0024]     In a preferred embodiment, each motor  114  to be powered by the VSD  112  has a corresponding inverter  206  in the output stage of the VSD  112 . Preferably, the number of motors  114  that can be powered by the VSD  112  is dependent upon the number of inverters  206  that are incorporated into the VSD  112 . In a preferred embodiment, there can be either 2 or 3 inverters  206  incorporated in the VSD  112  that are connected in parallel to the DC link  204  and used for powering corresponding motors  114 . While it is preferred for the VSD  112  to have between 2 and 3 inverters  206 , it is to be understood that in a preferred embodiment more than 3 inverters  206  can be used so long as the DC link  204  can provide and maintain the appropriate DC voltage to each of the inverters  206 . In certain embodiments, it may be preferable to utilize a single suitably sized inverter stage to drive multiple motors. Alternately, for a single compressor refrigerant system, only a single inverter  206  is required.  
         [0025]     The VSD  112  can prevent large inrush currents from reaching the motors  114  during the startup of the motors  114 . In addition, the inverters  206  of the VSD  112  can provide the AC power source  116  with power having about a unity power factor. Finally, the ability of the VSD  112  to adjust both the input voltage and input frequency received by the motor  114  permits a system equipped with VSD  112  to be operated on a variety of foreign and domestic power grids without having to alter the motors  114  for different power sources.  
         [0026]     In a preferred embodiment of the present invention, the control panel  110  generates the switching signals for the IGBT power switches in the inverter modules  206  using a random frequency modulation (RFM) technique for the modulating frequency, which is the frequency that drives the motors at the desired rotational speeds. The control panel  110  preferably has a single RFM modulator for each inverter module  206  to generate the corresponding switching signals for the IGBT power switches in the inverter module  206  when the motor  114  is rotating within a predetermined range or ranges. The RFM modulator applies a random modulating frequency dithering, i.e., a random variation or fluctuation, to the IGBT power switches to randomly vary the modulating waveform to the motor  114  to vary the rotational speed of the motor  114 . By randomly dithering the rotational speed of the motor  114 , the frequency of the pressure pulses generated by the meshing rotors of the screw compressors are randomly varied about the central frequency of the pressure pulsations, resulting in a reduction in the tonal peaks, an increase in the bandwidth of the acoustic noise and a significant reduction in the annoyance associated with the tonal acoustic noise of the compressor operation.  
         [0027]     RFM modulators can include, but are not limited to, a white noise generator that is sufficiently amplified to generate the desired random excitation, a random number generator within software running in a microprocessor, or an oscillator, such as a crystal clock oscillator model Maxim DS 1086 manufactured by Dallas Semiconductor of Dallas, Tex. Preferably, the RFM source can be selectably adjusted to the central frequency of the motor  114 . The central frequency refers to the desired steady state operating rotational speed of the motor  114 , if the RFM modulator is not used. For example, for a fixed speed screw compressor, the central frequency can be 3,000 RPM, or 3,600 RPM, depending whether the electrical power source is 50 or 60 Hz. For variable speed screw compressor operation, the central frequency is dependant upon the operational conditions of the system, with a preferred operating range of about 1,200 to about 10,000 RPM. However, it is to be understood that the present invention is not limited to this frequency range, and may be implemented on screw compressors having operating ranges significantly outside the preferred operating range. If the desired central frequency is 3,000 RPM, and the RFM modulator is set at two percent, the RFM modulator will operate to randomly vary the rotational speed of the motor  114  from 2,940 to 3,060 RPM, or a range of two percent above and two percent below the 3,000 RPM central frequency. Thus, while the rotational speed of the motor  114  operates in a band of speeds between 2,940 RPM and 3,060 RPM, the RFM modulator, on the average, provides a motor rotational speed of 3,000 RPM. It is to be understood that the RFM modulator&#39;s operational band can be set at a significantly higher percentages of the central frequency, such as at least ten percent of the maximum rated RPM, to provide a further widening of the bandwidth of the acoustic noise and an associated further reduction in the magnitude tonal acoustic noise. For screw compressors, RFM can be employed both with standard speed screws (3,000/3,600 RPM rated maximum) and high-speed screws (6,000 to 10,000 RPM rated maximum). Moreover, it is easier to implement RFM on high-speed screws, since the motors required to drive the compressors are typically smaller and have less inertia to accelerate/decelerate. Although inductions motors are preferred, any type of motor can be used, such as permanent magnet and switched reluctance motors.  
         [0028]     In one embodiment, the control panel  110  executes a control algorithm(s) or software to control operation of the RFM modulators. The control algorithm(s) can be computer programs or software stored in the non-volatile memory of the control panel  110  and can include a series of instructions executable by the microprocessor of the control panel  110 . While it is preferred that the control algorithm be embodied in a computer program(s) and executed by the microprocessor, it is to be understood that the control algorithm may be implemented and executed using digital and/or analog hardware by those skilled in the art. If hardware is used to execute the control algorithm, the corresponding configuration of the control panel  110  can be changed to incorporate the necessary components and to remove any components that may no longer be required.  
         [0029]     Use of RFM modulators to reduce the magnitude of the tonal noise associated with compressor operation offers advantages over other types of noise reduction in that many other types of noise reduction require the use of two compressors operating in tandem. RFM allows for a decrease in audible noise for single screw compressor chillers as well as chillers that might utilize multiple compressors. Typically these multiple compressor systems require that the system operate only one compressor, for the purpose of capacity reduction, or for the retention of cooling capability when one compressor is unable to operate, such as by a failure in a refrigerant system. In these situations the use of two compressors to reduce/eliminate audible noise is no longer an option, as failure of one of the two compressors eliminates the means for a reduction in the system&#39;s audible noise level. Furthermore, in multiple compressor systems, RFM modulators may be selectably configured for each compressor so that compressor synchronization is not necessary. Stated another way, each compressor can be operated independently of the others, including having a different RFM modulator operating band frequency percentage for each compressor central frequency, if desired. Additionally, any of the RFM modulators can be deactivated if desired.  
         [0030]     It is appreciated by those having ordinary skill in the art that the present invention is not restricted to HVAC&amp;R application, and can be applied to any application that might require the use of a positive displacement compressor, such as an air compressor.  
         [0031]     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.