Abstract:
A screw compressor for use in a chiller assembly includes cooperating screw rotors configured to increase the pressure of a vaporized refrigerant flowing through the compressor, a venturi tube arranged in a flow path of the refrigerant in the compressor downstream of the rotors, and an inlet port in fluid communication with a throat of the venturi tube and configured to deliver liquid refrigerant from a condenser of the chiller assembly to the flow path of the refrigerant in the compressor. The venturi tube is configured to cause a pressure drop in the refrigerant in the compressor. The liquid refrigerant delivered from the condenser reduces pulsations in the pressure of the refrigerant discharged from the compressor.

Description:
BACKGROUND 
       [0001]    The present invention relates to suppressing noise generated in mechanical systems. In particular, the present invention relates to noise suppression in screw compressors used in commercial and industrial air conditioning and refrigeration systems. 
         [0002]    The use of compression type water-cooled chillers is the most common method of cooling air in medium or large commercial, industrial and institutional buildings. Compression type water-cooled chillers are usually electrically driven, but may also be driven by a combustion engine or other power source. There are several types of compressors employed in water-cooled chillers. One common compressor is a screw compressor, which uses a rotary type positive displacement mechanism to compress a working fluid, such as a refrigerant. 
         [0003]    Water cooled chillers used in air conditioning and refrigeration systems are required to meet stringent noise level requirements, such as those prescribed by the Occupational Safety and Health Association (OSHA). However, screw chillers have a tendency to generate significant noise during operation. The primary source of noise generated in these types of chillers is pressure pulsations originating from the compressor, which generates noise, as well as vibration of adjoining components. In addition to the screw compressor, there is a multitude of secondary sources of noise, such as the evaporator, the condenser, and the economizer. 
         [0004]    Prior screw compressor designs have employed various devices and methods to suppress the noise generated by the compressor, such as mufflers and baffle plates arranged in the discharge chamber. Additionally, prior chillers have injected liquid refrigerant from the condenser into the gas refrigerant flow discharged from the compressor to suppress noise generated from pressure pulsations. However, under many operating conditions, these prior chiller designs have required a pressure application device, such as a pump, to compensate for a negative pressure differential between the condenser and the compressor. The addition of a pump, or other device, increases the cost and complexity of the system. 
       SUMMARY 
       [0005]    A screw compressor for use in a chiller assembly includes cooperating screw rotors configured to increase the pressure of a vaporized refrigerant flowing through the compressor, a venturi tube arranged in a flow path of the refrigerant in the compressor downstream of the rotors, and an inlet port in fluid communication with a throat of the venturi tube and configured to deliver liquid refrigerant from a condenser of the chiller assembly to the flow path of the refrigerant in the compressor. The venturi tube is configured to cause a pressure drop in the refrigerant in the compressor. The liquid refrigerant delivered from the condenser reduces pulsations in the pressure of the refrigerant discharged from the compressor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is a perspective view of a screw chiller assembly according to the present invention. 
           [0007]      FIG. 2  is an axial section view of the screw compressor included in the chiller assembly of  FIG. 1 . 
           [0008]      FIG. 3  is a schematic of the screw chiller assembly of  FIG. 1  illustrating refrigerant flow through the system. 
           [0009]      FIGS. 4A and 4B  are schematics of two embodiments of the compressor from the chiller assembly of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0010]      FIG. 1  is a perspective view of screw chiller assembly  10  including screw compressor  12 , variable frequency drive  14 , condenser  16 , and evaporator  18 . In  FIG. 1 , the inlet of compressor  12  is fluidly connected to evaporator  18  and the outlet of compressor  12  is fluidly connected to condenser  16 . Condenser  16  is fluidly connected to evaporator  18 . Variable frequency drive  14  is mounted on condenser  16 . 
         [0011]      FIG. 2  is an axial section view of screw compressor  12  of  FIG. 1 , which compressor  12  includes compressor housing  20 , drive screw  22 , two opposed screws  24 ,  26 , bearing housing  28 , discharge housing  30 , discharge chamber  32 , discharge ports  34 , and motor  48 . Housing  20  receives central drive screw  22  and two opposed screws  24  and  26 . Housing  20  is connected to motor  48 , which is configured to drive screws  22 ,  24 ,  26 . Bearing housing  28  receives screw bearings  28   a  that facilitate low friction rotation of drive screw  22  and opposed screws  24 ,  26 . Bearing housing  28  also receives compressed refrigerant from compression chambers  36  and delivers this compressed refrigerant through discharge ports  34  in the bearing housing  28  to discharge chamber  32  in discharge housing  30 . The size of the discharge chamber  32  necks down with the inner peripheral surface  38  of the discharge housing  30 . 
         [0012]      FIG. 3  is a schematic of chiller assembly  10  illustrating flow of refrigerant through the system. Chiller assembly  10  is a closed loop system through which refrigerant is cycled in various states, such as liquid and vapor. As a somewhat arbitrary starting point in chiller assembly  10  of  FIGS. 1-4 , a low temperature, low pressure superheated gas refrigerant is sucked into screw compressor  12  through fluid conduit  42 , such as a steel pipe, or other conduit from evaporator  18 . Compressor  12  is driven by motor  48  under the control of variable frequency drive  14 . Variable frequency drive  14  controls the frequency of the alternating current (AC) supplied to motor  48 , thereby controlling the speed of motor  48  and the output of compressor  12 . Refrigerant is sucked into compressor  12  through inlet ports  40 , and compressed between screws  22 ,  24 , and  22 ,  26  and carried towards discharge ports  34  in bearing housing  28 . The compressed refrigerant enters discharge chamber  32  through discharge ports  34 . After the refrigerant is compressed, the high temperature, high pressure superheated gas is discharged from compressor  12  through fluid conduit  42  to condenser  16 . Chiller assembly  10  may also include an oil separator (not shown) between compressor  12  and condenser  16 , which separates compressor lubricant from the refrigerant before delivering the refrigerant to condenser  16 . In condenser  16 , the gaseous refrigerant condenses into liquid as it gives up heat. The superheated gas refrigerant enters condenser  16  and is de-superheated, condensed, and sub-cooled through a heat exchange process with, for example, water flowing through condenser  16  to absorb heat. The liquid refrigerant is discharged from condenser  16  to metering device  44 , which may convert the higher temperature, high pressure sub-cooled liquid to a low temperature saturated liquid-vapor mixture. The low temperature saturated liquid-vapor refrigerant mixture enters evaporator  18  from metering device  44  through fluid conduit  42 . The low pressure environment in evaporator  18  causes the refrigerant to change states to a superheated gas and absorbs the required heat of vaporization from the chilled water, thus reducing the temperature of the water. The low pressure superheated gas is then drawn into the inlet of compressor  12  and the cycle is continually repeated. The chilled water is then circulated through a distribution system to cooling coils for providing air conditioning, or for other purposes. 
         [0013]    Chiller assembly  10  may commonly be located in relatively close proximity to people and as such may be designed to suppress noise production and radiation as much as possible. Screw compressor  12  is a significant contributor to noise generation, because of pressure pulsations created when the refrigerant is compressed. Pressure pulsations in compressor  12  result from unsteady mass flux caused by the refrigerant compression process performed within compressor  12 . The pressure pulsations in compressor  12  produce undesirable noise, which noise in turn is radiated from chiller assembly  10 . Additionally, the pressure pulsations may generate mechanical vibrations in components of chiller assembly  10  such as piping, heat exchangers, or compressor housing  20  itself. Mechanical vibrations propagating through chiller assembly  10  may themselves result in further noise generation and radiation. 
         [0014]    In order to suppress noise generated from the pressure pulsations in compressor  12 , chiller assembly  10  includes liquid refrigerant conduit  46  shown in  FIG. 3 . Conduit  46  is configured to deliver liquid refrigerant from condenser  16  to the superheated gas refrigerant flow in compressor  12 . In particular, conduit  46  is configured to deliver liquid refrigerant from condenser  16  to compressor  12  downstream of compression chambers  36  shown in  FIG. 2 . For example, conduit  46  may deliver liquid refrigerant to channels in bearing housing  28 , which channels deliver the superheated gas refrigerant from compression chambers  36  to discharge chamber  32  through discharge ports  34 . Noise in the gas refrigerant flow in compressor  12  is caused by pressure pulsations at frequencies in the audible range, which may range from approximately 20 to 20,000 Hz. Noise levels can be reduced by reducing the magnitude of such pressure pulsations. The objective of introducing liquid refrigerant from condenser  16  into gas refrigerant flow in compressor  12  is to reduce the strength of the pressure pulsations by transferring energy from the gas to liquid phase. Three mechanisms contribute to reduce pressure pulsations when liquid refrigerant droplets are injected into the gas refrigerant flow: a) viscous drag between liquid and gas refrigerant; b) heat transfer between liquid and gas refrigerant; and c) mass transfer from vaporization of liquid refrigerant to gas. Generally speaking, the magnitude of noise attenuation depends on the mass flow rate and droplet size of liquid refrigerant delivered from condenser  16 . Noise suppression due to viscous drag and heat transfer are both functions of droplet size. Noise suppression due to mass transfer is a function of mass flow rate. Viscous drag and heat transfer are particularly effective to reduce noise at frequencies above 10,000 Hz, while vaporization, i.e. mass transfer, is effective at lower frequencies. 
         [0015]    In order to deliver the liquid refrigerant from condenser  16  to the superheated gas refrigerant flow in compressor  12 , the pressure in the condenser  16  must be greater than in the compressor  12 . However, downstream of compression chambers  36  the superheated gas refrigerant often has a higher pressure than the pressure of the liquid refrigerant in condenser  16 . Embodiments of the present invention therefore provide methods of and systems for inducing a pressure drop in the superheated gas refrigerant flow in compressor  12  sufficient to reduce the pressure in compressor  12  below the pressure in condenser  16  without the addition of work to the system. 
         [0016]      FIGS. 4A and 4B  are schematics of two embodiments of compressor  12  configured to induce a pressure drop in the superheated gas refrigerant flow discharged from compressor  12  through bearing housing  28  and discharge chamber  32 . In  FIGS. 4A and 4B , compressor  12  includes compressor housing  20 , bearing housing  28 , discharge housing  30 , motor  48  and venturi tubes  50 . Arranged in compressor housing  20  is compression chamber  36 , which chamber  36  includes drive screw  22  and two opposed screws  24 ,  26  (shown in  FIG. 2 ). Venturi tubes  50 , also referred to as convergent-divergent or De Laval nozzles, include, in the direction of flow, a converging portion and diverging portion connected at a throat. The throat of venturi tubes  50  defines a location of minimum cross-sectional area and is in fluid communication with condenser  16  through conduit  46 , which may be, for example, a steel pipe. In the embodiment of  FIG. 4A , venturi tubes  50  are arranged in bearing housing  28  and are configured to direct refrigerant flow  52  from compressor  12  to discharge chamber  32  in discharge housing  30 . 
         [0017]    As refrigerant flow  52  passes through venturi tubes  50 , the velocity of flow  52  increases while the pressure of flow  52  decreases. The throat of venturi tubes  50  defines not only the location of minimum cross-sectional area, but also the location of minimum pressure of refrigerant flow  52 . Venturi tubes  50  thereby induce a pressure drop in refrigerant flow  52  being discharged from compressor  12  through bearing housing  28  and discharge chamber  32  to condenser  16 . In embodiments of the present invention, venturi tube  50  is configured to induce a pressure drop in refrigerant flow  52  sufficient to reduce the pressure of flow  52  at the throat of venturi tube  50  below the pressure of liquid refrigerant directed through conduit  46  from condenser  16 . Therefore the liquid refrigerant from condenser  16  used to suppress noise in compressor  12  may freely flow from condenser  16  to compressor  12  without adding work to the system, e.g., without the use of a pressure applicator like a pump. 
         [0018]    In some applications, space constraints in compressor  12  may not permit venturi tubes  50  to be disposed in bearing housing  28 . In an alternative embodiment ( FIG. 4B ), venturi tubes  50  are arranged within discharge chamber  32  of discharge housing  30 . In the embodiment of  FIG. 4B , refrigerant flow  52  passes through bearing housing  28  into venturi tubes  50  in discharge chamber  32  through discharge ports  34 . A pressure drop is induced in refrigerant flow  52  as the refrigerant passes through venturi tubes  50 , which pressure drop enables liquid refrigerant from condenser  16  to freely flow from condenser  16  through conduit  46  to compressor  12  without adding work to the system. 
         [0019]    Embodiments of the present invention provide methods of and systems for inducing a pressure drop in the superheated gas refrigerant flow in a screw compressor of a chiller assembly sufficient to reduce the pressure in the compressor below the pressure in a condenser without the addition of work to the system. Inducing a pressure drop in the compressor refrigerant flow enables liquid refrigerant from the condenser to freely flow to the compressor without the use of a pressure application device, such as a pump. Embodiments of the present invention thereby suppress noise generated from pressure pulsations in the screw compressor by injecting liquid from the condenser into the gas refrigerant flow in the compressor without significantly increasing the cost and complexity of the chiller assembly. 
         [0020]    Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.