Abstract:
Refrigerant gas at a few psi higher than suction is supplied via labyrinth or annular groove type seals located between the suction side bearings and suction and between the suction side motor bearing and the motor. The buffer gas flow divides with part going to suction and part to a drain along the oil flowing from the bearings.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    There is an affinity between refrigerants and lubricants such that oil is normally present in the refrigerant circulating in a refrigeration system. In screw compressors, the oil entrained in the refrigerant helps to seal the rotors. In other parts of a refrigeration system, the oil interferes with heat transfer by coating the heat transfer surfaces. In addition to the oil present in the suction gas, oil supplied to lubricate the bearings may leak into the suction gas since oil supplied to the inlet side bearings must be at a pressure greater than suction pressure. Excess oil reduces compressor efficiency. If, for example an initial volume of 1000 cc is to be reduced to 200 cc, a 5:1 compression ratio, the presence of 10 cc of oil will result in 990 cc of refrigerant being reduced to 190 cc, a 5.2:1 compression ratio. Thus a reduced volume of refrigerant will be compressed to an excess pressure. Additionally, an oil separator will be required immediately downstream of the compressor to prevent excess oil circulation or a still will be required to remove and recover excess oil. Where an oil separator is used, a larger unit will be required due to the inlet bearing lubricant flowing into the suction flow.  
         SUMMARY OF THE INVENTION  
         [0002]    The present invention has an inlet bearing lubrication system which is isolated from the refrigerant flow through the compressor. During the compression process, the pressure of the trapped volume of refrigerant gas goes from suction to discharge. Accordingly, controlled amounts of gas can be diverted from the trapped volumes over a range of pressures. According to the teachings of the present invention, buffer gas is drawn off from the trapped volumes at a pressure just a few psi higher than the suction pressure. The buffer gas is directed to labyrinth seals at locations between the suction side rotor bearings and suction and between the suction side motor bearings and the motor. The buffer gas flows divide with part going to suction and part to a drain to the oil sump along with the oil flowing from the bearings. The buffer gas flow at the motor side labyrinth seal divides with part going to a drain to the oil sump and part going to the motor chamber which is at suction. Since the buffer gas is just a few psi higher than the compressor suction pressure, there is a minimal impact on the compressor capacity and efficiency.  
           [0003]    It is an object of this invention to eliminate, or at least reduce the size of, oil separators or stills in screw compressor refrigeration systems.  
           [0004]    It is another object of this invention to provide a separate inlet bearing lubrication system. These objects, and others as will become apparent hereinafter, are accomplished by the present invention.  
           [0005]    Basically, refrigerant gas at a few psi higher than suction is supplied via labyrinth or annular groove type seals located between the suction side bearings and suction and between the suction side motor bearing and the motor. The buffer gas flow divides with part going to suction and part to a drain along the oil flowing from the bearings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    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:  
         [0007]    [0007]FIG. 1 is a schematic representation of a lubrication system of a refrigeration system where a still is employed;  
         [0008]    [0008]FIG. 2 is a partial sectioned view of the inlet portion of a tri-rotor screw compressor employing the present invention;  
         [0009]    [0009]FIG. 3 is a sectional view of the inlet seal structure of FIG. 2 together with their fluid connections; and  
         [0010]    [0010]FIG. 4 shows a sectional view of a portion of the FIG. 3 structure with the addition of drain structure. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0011]    In FIG. 1, the numeral  10  generally designates a refrigeration system. Refrigeration system  10  includes a screw compressor  12  having a plurality of rotors  12 - 1 ,  12 - 2  and  12 - 3  which are supported at their ends by a plurality of roller bearings, as best shown in FIG. 2. Refrigeration system  10  includes a refrigerant circuit serially including screw compressor  12 , discharge line  14 , condenser  16 , expansion device  20 , cooler or evaporator  24  and suction line  28 . Branch line  30  extends from condenser  16  to cooler  24  via still  32  where heat from the refrigerant flowing in line  30  separates an oil-refrigerant mixture flowing into still  32  via line  26  from cooler  24 . The heating separates the mixture with refrigerant flowing via line  34  from still  32  to suction line  28  and oil draining from still  32  via line  36  into oil sump  40 . System  10  uses still  32  to achieve oil separation rather than an oil separator located immediately downstream of compressor  12 . Oil sump  40  is at suction pressure and oil pump  42  delivers oil from sump  40  to line  44  at a pressure on the order of twenty to twenty five psi above suction pressure. Line  44  branches and/or supplies a plurality of passages in compressor  12  such that the inlet and discharge side bearings are lubricated. Excess oil drains from compressor  12  via line  46  back to sump  40 . Since the oil supplied to the inlet bearings is at a pressure greater than suction, there is a tendency for the oil supplied to the inlet bearings to leak into the suction chamber and to be entrained with the refrigerant being compressed by the compressor. A portion of the refrigerant with the entrained oil serially passes through discharge line  14 , condenser  16  and line  30  into still  32 . In still  32 , by heating and/or flashing, the oil is separated from the refrigerant and drains via line  36  into sump  40 . The present invention permits the elimination, or at least a size reduction, of still  32  and its associated lines and connections without requiring an oil separator by isolating the bearing lubrication system from the refrigerant circuit.  
         [0012]    Referring specifically to FIG. 2, rotors  12 - 1  and  12 - 3  are female rotors and rotor  12 - 2  is a male rotor which is driven by motor  13  and, in turn, drives rotors  12 - 1  and  12 - 3 . The suction or inlet ends of rotors  12 - 1 ,  12 - 2  and  12 - 3  are supported by bearings  50 ,  51  and  52 , respectively. Bearings  50 ,  51  and  52  require lubrication which is supplied from oil sump  40  via line  44  and its branches  44 - 1 ,  44 - 2  and  44 - 3 , respectively, at a pressure of twenty to twenty five psi above suction. All of the spaces indicated by the numeral  54  are at suction pressure and, absent the structure of the present invention, the oil supplied to lubricate bearings  50 ,  51  and  52  would drain into the regions at suction pressure. According to the teachings of the present invention, seals  60  and  62  which are represented as labyrinth seals are respectively located between bearings  50  and  52  and corresponding regions at suction pressure. Bearing  51  is located between two regions at suction pressure. Seal  64 , which is t 0  represented as a labyrinth seal, is located between bearing  51  and one region at suction pressure. Seal  66 , which is represented as a labyrinth seal, is located between bearing  51  and a second region at suction pressure which is the chamber in which motor  13  is located. As shown in FIG. 2, refrigerant at a pressure twenty to thirty pounds above suction pressure is diverted from trapped volumes via passages  12 - 4  and  12 - 5  to seals  60  and  62 , respectively, to act as a buffer gas. Stippling has been added to unhatched structure to indicate areas at suction and buffer gas pressures with the greater density of stippling corresponding to buffer gas pressure.  
         [0013]    Referring specifically to FIG. 3, the flow paths for the buffer gas are shown relative to seals  60 ,  62 ,  64  and  66 . Seal  60  is supplied with buffer gas via passage  12 - 4  which is fluidly connected to radial passage  60 - 1  in seal  60 . Passage  60 - 1  extends radially inward to annular chamber  60 - 2 . Flow from annular chamber  60 - 2  is (1) radially inward via ports/annular slots  60 - 3 , (2) radially outward via passage  60 - 4 , or (3) axially via passage  60 - 5 . The flow via ports/annular slots  60 - 3  is between labyrinth seals  60 - 6  and  60 - 7  such that a portion of the flow passes over labyrinth seal  60 - 6  and flows into suction. A second portion of the flow passes over labyrinth  60 - 7  and flows into drain  46 - 1  where it combines with oil flowing from bearing  50 . The flow through passage  60 - 4  together with flow through passage  62 - 4  are supplied via radial passages  64 - 1  and  64 - 2 , respectively, into annular chamber  64 - 3 . The flow from annular chamber  64 - 3  is radially inward via ports/annular slots  64 - 4 . The flow via ports/annular slots  64 - 4  is between labyrinth seals  64 - 5  and  64 - 6  such that a portion of the flow passes over labyrinth seal  64 - 5  and flows into suction. A second portion of the flow passes over labyrinth  64 - 6  and flows into drain  46 - 2  where it combines with oil flowing from bearing  51 .  
         [0014]    Seal  62  is identical to seal  60  and is supplied with buffer gas via passage  12 - 5  which is fluidly connected to radial passage  62 - 1  in seal  62 . Passage  62 - 1  extends radially inward to annular chamber  62 - 2 . The flow from annular chamber  62 - 2  is (1) radially inward via ports/annular slots  62 - 3 , (2) radially outward via passage  62 - 4  to seal  64 , as noted above, or (3) axially via passage  62 - 5 . The flow via port/annular slots  62 - 3  is between labyrinth seals  62 - 6  and  62 - 7  such that a portion of the flow passes over labyrinth seal  62 - 6  and flows into suction. A second portion of the flow passes over labyrinth seal  62 - 7  and flows into drain  463  where it combines with oil flowing from bearing  52 .  
         [0015]    The flows through axially extending passages  60 - 5  and  62 - 5  flow via passages  66 - 1  and  66 - 2 , respectively, into annular chamber  66 - 3  of seal  66 . The flow from annular chamber  66 - 3  is radially inward via ports/annular slots  66 - 4 . The flow via port/annular slots  66 - 4  is between labyrinth seals  66 - 5  and  66 - 6  such that a portion of the flow passes over labyrinth seal  66 - 5  and flows into drain  46 - 4  where it combines with oil flowing from bearing  51 . A second portion of the flow passes over labyrinth seal  66 - 6  and flows into the motor chamber which is at suction. Labyrinth seal  66 - 7  is located on the opposite side of drain  46 - 4  from labyrinth seal  66 - 5  and serves to prevent the flow of oil supplied to bearing  51  from flowing unrestricted into drain  46 - 4  rather than passing through bearing  51 .  
         [0016]    [0016]FIG. 4 adds rotors  12 - 1 ,  12 - 2  and  12 - 3  as well as the bearings  50 ,  51  and  52  and their oil supply structure to the structure illustrated in FIG. 3. Only the oil supply and drain flows are indicated by arrows. As is clear from FIG. 4, lubricant supplied via line  44 - 1  flows through bearing  50  and combines with the buffer gas flowing over labyrinth seal  60 - 7  in drain line  46 - 1  which is at suction pressure. Lubricant supplied via line  44 - 2  flows through bearing  52  and combines with the buffer gas flowing over labyrinth seal  62 - 7  in drain line  46 - 3  which is at suction pressure. Lubricant supplied via line  44 - 3  to lubricate bearing  51  has two possible paths to drain. The first path is through bearing  51  to drain  46 - 2  where the lubricant combines with buffer gas flowing over labyrinth  64 - 6 . The second path is over labyrinth seal  66 - 7  to drain  46 - 4  where the oil combines with buffer gas flowing over labyrinth  66 - 5 . Drains  46 - 1 ,  46 - 2  and  46 - 3  and  46 - 4  combine to form line  46 . Although both oil and gaseous refrigerant are flowing to oil sump  40  via line  46 , the refrigerant gas remains separated beyond the natural affinity and is drawn off from oil sump  40  via line  48  which feeds into suction line  28 .  
         [0017]    In the drawings, the fluid paths for the lubricant and buffer gas were chosen to provide the clearest understanding of the present invention. Accordingly, as necessary, or desired, internal or external paths, or a combination thereof may be employed. For example, passages  12 - 4  and  12 - 5  may be from an external source of pressure as where a constant buffer pressure is desired.  
         [0018]    Although a preferred embodiment of the present invention has been illustrated an 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.