Abstract:
Lubrication supply to the rotors of a variable speed screw compressor is optimized for full load and a supplemental supply path is provided which is opened during low speed operation.

Description:
BACKGROUND OF THE INVENTION 
     Screw compressors used in water cooled chillers are of the oil flooded type. Oil provides a seal between adjacent trapped volumes during the compression process and an oil separator removes oil from the hot, compressed gas downstream of the compression process. At high speeds, the percentage of gas leakage between the rotor and housing and interlobes is small as compared to the inlet flow. This is partly due to the oil film formed between the rotor and housing. Additionally, to reduce drag loses, the amount of oil injected should be kept at a minimum. For screw compressors the loading or flow is directly proportional to speed. At low speeds the sealing at the tips and interlobes is poor which reduces the volumetric efficiency of the compressor. 
     SUMMARY OF THE INVENTION 
     Volumetric efficiency is increased at low speeds by increasing the oil flow at low loads to offset the poorer sealing at low speeds. Oil from the cooler or evaporator is injected at the rotor inlet for sealing. An oil-rich layer tends to form at the top of the cooler and an ejector is used to draw oil from the top surface of the cooler. The drive or motive fluid for the ejector is, preferably, taken from the last closed lobe, although the discharge pressure may be used. The last closed lobe pressure is preferred because it is determined by compressor operation whereas discharge pressure is determined by system conditions. The amount of oil injected is optimized for full load operation by proper selection of ejector size. Because, at low loads the oil supply should be increased for better volumetric efficiency, a supplemental oil supply is provided. One approach is to provide a parallel supply path from the ejector which is controlled by a solenoid valve responsive to motor speed. Another approach is to provide a second ejector and supply path to the rotors with a solenoid valve in the second path responsive to motor speed. 
     It is an object of this invention to improve part load performance of variable speed screw compressors. 
     It is another object of this invention to increase volumetric efficiency at low speeds by increasing oil flow to the rotors at low speeds. These objects, and others as will become apparent hereinafter, are accomplished by the present invention. 
     Basically, lubrication supply to the rotors of a variable speed screw compressor is optimized for full load and a supplemental supply path is provided which is opened during low speed operation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a fuller 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 diagram of a closed refrigeration or air conditioning system employing the present invention; and 
     FIG. 2 is a schematic diagram of a closed refrigeration or air conditioning system employing a modified embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1, the numeral  10  generally designates a closed refrigeration or air conditioning system. As is conventional, there is a closed circuit serially including compressor  12 , discharge line  14  connected to the discharge port, condenser  16 , line  18  which contains expansion device  20 , cooler or evaporator  22  and suction line  24  leading to the suction port. Compressor  12  is a multi-rotor, hermetic, screw compressor. Compressor  12  is driven by electric motor  26  which is under the control of inverter  28  through microprocessor or controller  30 . 
     An oil-rich layer forms an upper layer in cooler  22  and is used for lubricating and sealing the rotors  12 - 2  and  12 - 2 . The lubricant is drawn from cooler  22  and supplied via line  40  to ejector  50  which is sized to supply the maximum required amount of lubricant. Line  52  supplies refrigerant gas at last closed lobe pressure to ejector  50  causing the oil in a refrigerant oil mixture to be drawn from cooler  22  via line  40  and to be supplied to compressor  12  for lubricating and sealing the rotors  12 - 1  and  12 - 2 . Line  52  branches downstream of ejector  50  into lines  52 - 1  and  52 - 2 . Line  52 - 1  contains a restriction  60  sized to provide the proper lubricant flow to the rotors  12 - 1  and  12 - 2  during full load conditions. Line  52 - 2  contains a normally closed solenoid valve  62  and is under the control of controller or microprocessor  34  responsive to the speed of motor  26  via the output of inverter  28 . Microprocessor  30  receives temperature inputs from thermal sensor  32  which senses the outlet chilled water temperature and responsive thereto controls inverter  28 , and thereby compressor  12 . When the motor speed has been reduced sufficiently, solenoid valve  62  is opened by controller  34  to permit more oil to be supplied to the rotors  12 - 1  and  12 - 2  for sealing and lubrication. Typically, the motor sped would be 50-60% of the full load speed when solenoid valve  62  is opened. 
     Referring now to FIG. 2, system  10 ′ differs from system  10  in dividing line  40  and replacing ejector  50  with two smaller ejectors,  50 - 1  and  50 - 2 , in parallel. Specifically, line  40  divides into lines  40 - 1  and  40 - 2  which are connected to ejectors  50 - 1  and  50 - 2 , respectively. Line  52  divides into lines  52 - 1  and  52 - 2  which recombine into line  52  after supplying refrigerant gas at last closed lobe pressure to ejectors  50 - 1  and  50 - 2 , respectively. Ejector  50 - 1  is sized such that the oil refrigerant mixture drawn from cooler  22  via line  40 - 1  is the proper amount to provide sealing and lubrication of rotors  12 - 1  and  12 - 2  at full load Line  52 - 2  contains normally closed solenoid valve  62  in addition to ejector  50 - 2 . When the speed of motor  26  has been sufficiently reduced by inverter  28  under control of controller  30 , solenoid valve  62  is opened by controller  34  to permit more oil to be supplied to the rotors  12 - 1  and  12 - 2  for sealing and lubrication. Typically, the motor speed would be 50-60% of the full load speed when solenoid valve  62  is opened. 
     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.