Abstract:
The invention relates to a rotary screw machine that is completely oil-free. Known rotary screw machines with cooling, sealing, lubrication of the screw rotors and oil-lubrication and oil-cooling of the bearings have been replaced with water-lubricated and water-cooled bearings. 
     Each screw rotor ( 102 ) is mounted in a water-lubricated radial slide bearing ( 1 ) and one trunnion ( 31 ) of the rotor co-acts with a water-lubricated thrust bearing ( 9, 10 ) which acts both as an hydrodynamic and an hydrostatic bearing. The trunnion bearing housing ( 21 ) is in fluid connection with the rotor housing, both along the trunnion and via a conduit ( 27 ) from the bearing housing interior to the gas inlet ( 108 ) of the screw rotor or via a channel which is cut-off from said inlet.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a screw rotor machine, and in particular to a screw rotor compressor. 
     Screw rotor machines are chiefly used as compressors for compressing gas, normally air, and to a lesser extent for expanding a compressed gas. 
     U.S. Pat. No. 3,975,123 teaches a screw compressor whose screw rotors are lubricated, cooled and sealed with water. The rotor trunnions are mounted on bearings that are lubricated with oil in a conventional manner. 
     The drawback with this construction is that axial seals between the oil-lubricated part and the oil-free part can burst or oil can leak to the oil-free part in some other way. An oil leakage of this nature will contaminate the gas with oil. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a screw rotor machine in which the gas to be compressed or expanded will not be contaminated by oil from the bearing housings. 
     Another object is to provide a screw rotor machine whose bearings are lubricated and cooled without the use of oil and which is totally oil-free. 
     These objects are achieved with a screw rotor machine that has water-lubricated bearings and rotors. The bearing lubricant used is purely water that contains no additives, or water to which there has been added a freezing-point depressing agent, a viscosity-raising agent and/or a corrosion-inhibiting agent. 
     According to one preferred embodiment of the invention, the screw rotor machine includes a slide thrust bearing that acts both hydrodynamically and hydrostatically in one of each of the rotor bearing housings, wherein that part of the slide bearing located nearest the shaft is in fluid connection with a lubricant source via a radial slide bearing or plain bearing, while the opposite part of said thrust bearing is in connection with a working chamber or with the machine gas inlet. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be described hereinbelow with reference to an exemplifying embodiment thereof and also with reference to the accompanying drawings, in which: 
     FIG. 1 is a longitudinal view of a known screw compressor; 
     FIG. 2 is a sectional view taken on the line II—II in FIG. 1; 
     FIG. 3 is a sectional view of an inventive screw rotor machine; 
     FIG. 4 illustrates part of the rotor machine shown in FIG. 3; 
     FIG. 5 illustrates a thrust bearing disc from above; 
     FIG. 6 is a sectional view of the bearing disc shown in FIG. 1, taken on the line VI—VI in said FIGURE; and 
     FIG. 7 is a schematic illustration of a system for returning water to the inventive compressor. 
    
    
     DETAILED DESCRIPTION 
     The constriction and working principle of a screw compressor will now be described briefly with reference to FIGS. 1 and 2. 
     A pair of mutually engaging screw rotors  101 ,  102  are rotatably mounted in a working chamber defined by two side-walls  103 ,  104  and a barrel wall  105  extending between said end-walls. The barrel wall  105  has a form which corresponds generally to the form of two mutually intersecting cylinders, as evident from FIG.  2 . Each rotor  101 ,  102  includes a plurality of respective lobes  106 ,  107  and respective intermediate grooves  111 ,  112  which extend helically along the rotor. One rotor,  101 , is a male type of rotor with which the major part of each lobe  106  is located outside the pitch circle, and the other rotor,  102 , is a female type of rotor with which the major part of each lobe  107  is located inwardly of the pitch circle. The female rotor  102  will normally have more lobes than the male rotor  101 . A typical combination is one in which the male rotor  101  has four lobes and the female rotor  102  has six lobes. 
     The gas to be compressed, normally air, is delivered to the working chamber of the compressor through an inlet port  108  and is then compressed in V-shaped working chambers defined between the rotors and the chamber walls. Each working chamber moves to the right in FIG. 1 as the rotors  101 ,  102  rotate. The volume of a working chamber thus decreases continuously during the latter part of its cycle, subsequent to communication with the inlet port  108  having been cut-off. The gas is thereby compressed and the compressed gas leaves the compressor through an outlet port  109 . The ratio of the outlet pressure to the inlet pressure is determined by the built-in volumetric relationship between the volume of a working chamber immediately after its communication with the inlet port  108  has been cut-off and the volume of said working chamber when it begins to communicate with the outlet port  109 . 
     A theoretic maximum displacement volume V DP , such as V DP =(A M +A F )×Z M ×L is defined in respect of a screw compressor. This volume is expressed as the volume per male rotor rotation. A M  and A F  are the cross-sectional areas of respective male rotor grooves  111  and female rotor grooves  112 , in cross-section to the rotor shafts. These cross-sectional areas A M  and A F  are hatched areas in FIG. 2. L is the rotor length and Z M  the number of male rotor lobes. 
     An embodiment of the invention in the form of a compressor is described below with reference to FIGS. 3-6. 
     The description is concentrated mainly on a rotor bearing and rotor lubrication. 
     The rotor  102  illustrated schematically in FIG. 3 is mounted in the rotor housing and its trunnions  31 ,  32  each project into a respective bearing housing  21 ,  22 . The bearing housing  21  is located in the side-wall  104  while the other bearing housing  22  is located in the opposing side-wall or side-wall  103 . 
     The actual rotor  102  is about 0.5 mm shorter than the distance at which the side-walls  103 ,  104  are spaced apart in the rotor housing. This feature has not been shown in FIG.  3 . The rotor  102  can thus move axially. 
     Each of the trunnions  31 ,  32  of the rotor  102  is mounted in a respective radial slide bearing  1 . The trunnion  31  is longer than the trunnion  32  and is provided with a thrust slide bearing  9 ,  10  that includes a part  9  which is fixedly mounted on the trunnion  31  and movable with said trunnion, and a stationary part  10  mounted in the bearing housing. That side of the trunnion-mounted annular bearing part which lies proximal to the rotor is at most 0.1 mm from the opposite other part  10  of the slide thrust bearing when the end-wall  13  of the rotor  102  connected with the trunnion  31  lies against the inner surface  12  of the side-wall  104 . The rotor  102  can thus move axially through a distance of at most 0.1 mm from the end-wall  12  of the side-wall  104 . 
     Each of the radial slide bearings  1  has the form of a sleeve-like element that includes at least one chamber  2 , preferably two chambers  2 , that is/are open to respective trunnions  31  and  32 . When the radial slide bearing  1  includes two chambers  2 , said chambers will preferably be disposed asymmetrically, although they may alternatively be disposed diametrically. Each chamber  2  is connected to a lubricant source (not shown) via a channel  3 . The channel  3  extends from the chamber  2  radially through the sleeve-like slide bearing  1 , and thereafter through respective side-walls  104  and  103  and from there (not shown) to the lubricant source. The lubricant is water or a water-based liquid. The water-based liquid may be an aqueous liquid which, in addition to water, may include a corrosion-inhibiting agent, a viscosity-raising agent, and/or a freezing-point depressing agent. Such agents are known and are available commercially. The water content will preferably be at least 60%. 
     The chamber  2  has the form of a groove extending in the center of the sleeve and terminating at a distance from the axial ends of the sleeve-like element  1 . The slide bearing  1  thus has a circular cross-section at its axial ends and therewith a cylindrical abutment area against respective trunnions  31 ,  32 . 
     According to one embodiment, the chamber  2  is connected with the space  7  by a channel  61  that extends through the end-wall  5 . 
     The bearing housings  21 ,  22  are provided internally, i.e. in those parts that lie distal from the rotor  102 , with a respective outlet port  27  and  28  for transporting water (lubricant) to either the compressor inlet or to a working chamber in said compressor that is delimited by the rotor housing and the two helical screw rotors  102 ,  101 . 
     The slide thrust bearing  9 ,  10  will best be seen from FIG.  4 . The part  10  includes a slide disc  10 A against which the fixed part  9  on the shaft  31  abuts with a clearance that varies during operation of the compressor. The slide disc  10 A is adjustably fastened in a stationary element  16  mounted in the bearing housing  21  via a support means  10 B that has a spherical part-area  15 , said element  16  having a part-area which is complementary to the part-area  15 . A clearance  62  is provided between the trunnion  31  and the slide disc  10 A, the support means  10 B and the element  16 . 
     The part  9  includes a ring  9 A which is crimped onto the trunnion  31  or secured thereto in some other way. A bearing element  9 B is fastened to the ring  9 A. The bearing element  9 B is located between the ring  9 A and the slide disc  10 A, with which it is in rotational abutment as the compressor works. 
     The bearing element  9 B is shown in more detail in FIGS. 5 and 6. FIG. 5 shows the slide ring  9 A from above, and FIG. 6 is a cross-sectional view taken on the line VI—VI in FIG.  5 . The bearing element  9 B has the form of an annulus or ring, where the inner diameter corresponds to the diameter of the shaft  31 . The bearing element  9 B forms together with the trunnion  31  a ring-shaped channel  17  which is open towards the slide ring. A plurality of channels  18  extend radially from the channel  17  towards the outer periphery of the bearing element, although terminating short of said periphery. These radial channels  18  are also open towards the slide disc  10 A. Located adjacent each radial channel  18  is a recess area  19  which connects with the radial channel  18 . The surface of the recess area  19  lies much closer to the surface  20  of the bearing element  9 B that the bottom of the radial channels  18 , as evident from FIG.  6 . As will be evident from FIG. 5, the bearing element  9 B has nearest the outer periphery a ring-shaped area intended for abutment with the slide disc  10 A. Those parts of the bearing element  9 B that are not comprised of channels  17 ,  18  or recessed areas  19  are also intended for abutment with the slide disc  10 A. 
     The slide thrust bearing  9 ,  10  functions as a combined hydrostatic and hydrodynamic bearing. 
     When the compressor is working, lubricant, which is water, is delivered under pressure from the lubricant source through the channel  3  to the chamber  2  in which a pressure pl prevails. Those parts of the sleeve-like element  1  that do not include the chamber  2  that is open towards the trunnion  31  define a constriction that limits the flow of water from the chamber  2 . These axial constrictions are referenced  4 ,  5  and preferably lie symmetrically outside the inlet opening of the channel  3  to the chamber  2 . The bearing  1  works in accordance with the hydrodynamic principle. 
     A ring-shaped gap  6  is located to the right of the bearing  1 , between the constriction  4  and the end-wall  12  of the rotor housing, as will be seen particularly from FIG. 4. A mean pressure pm prevails in the gap  6 , said pressure lying between the compressor outlet pressure pd and the compressor inlet pressure p. 
     Water flows from the chamber  1  into the ring-shaped gap  6  through the clearance between the trunnion  31  and the constriction  4 , and from said gap  6  to the working chamber of the compressor. 
     Located to the left of the bearing  1 , between the constriction  5  of said bearing  1  and the bearing element  9 B (FIG.  4 ), is a ring-shaped space  7 , in which a pressure pi prevails, said pressure being lower than pl. The pressure pi is determined by the ratio of the gap areas at the constriction of the slide bearing  1  and at  8  between the bearing element  9 B and the slide disc  10 A. As a result of the described, particular embodiment of the bearing element surface that co-acts with the slide disc  10 , the gap width will vary during operation of the compressor. The pressure pi is also influenced by the pressures pm and pk where pm is the pressure in the ring-shaped gap  6  and pk is the pressure in the bearing housing  21  outside the thrust bearing  9 ,  10 . 
     Thus: pl&gt;pi&gt;pk 
     As earlier mentioned, the thrust bearing  9 ,  10  is a combined hydrodynamic and hydrostatic bearing. The hydrodynamic force component F DYN  is generated by the recessed areas  19  in the rotating bearing element  9 B; the hydrostatic component F STAT  is generated by the ring-area of the bearing  9 ,  10  and the pressure difference (pi-pk). 
     The force F AL  acting on the shaft  31 , to the left in FIG. 4, is thus 
     
       
         F AL =F DYN +F STAT   
       
     
     The end surface  12  of the rotor  102  and the end surface  104  of the rotor housing also function as a counter-pressure bearing. When a state of equilibrium prevails, the trunnion  31  is acted upon by a force F AE  which is equal to the force F AL  but which acts in the opposite direction. When F DYN  increases, the force F AL  acting to the left in the drawing also increases. The rotor  102  is therewith drawn towards the end surface  12  of the rotor housing and the gap therebetween decreases and the pressure in the ring-shaped gap  6  rises. This pressure increase results in an increase in the force F AL  that counteracts the force F AE . 
     The pressure pk in the bearing housing  21  may be chosen to be equal to the working-chamber inlet pressure p or higher than this pressure. The pressure pk is selected by appropriate positioning of the means by which the bearing housing  21  is drained. When the inlet channel  108  is chosen as such means, pk will equal p, whereas pk will be greater than p if there is chosen a position in which the gas volume in the working space is cut-off from the inlet and compression has commenced. 
     Water or an aqueous liquid is delivered to the bearing  1  prior to starting-up the compressor. The areas in the thrust bearing  9 ,  10  are dimensioned so that F AL &gt;F AE , and consequently the non-rotating rotors  101 ,  102  will be drawn towards the end surface  12  of the housing and occupy this position when the compressor is started-up. 
     When the compressor is activated and the rotors rotate, the gas forces generated in the working space of the compressor will act on the rotors in a direction opposite to the force F AL  and therewith move the rotors  101 ,  102  away from the end surface  12 . The distribution of the total clearance or play S=S 1  (end surface clearance)+S 2  (thrust bearing clearance) will be controlled by the gas forces or pd and by the applied water pressure pl and by the pressure pk in the bearing housing  21 . 
     The pressure of the lubricant may fall markedly in the region outside the radial slide bearing  1  when the compressor is relieved of load, so as to result in insufficient lubrication of the slide thrust bearing. In order to obtain satisfactory lubrication, it is necessary to increase the pressure in the clearance  62 . This can be achieved, for instance, by delivering lubricant to the clearance  62  via a radial channel passing through the element  16 . Thus, supplied lubricant that penetrates into the gap-like space  7 , past a constriction provided in the element  16  adjacent the gap-like space  7 , is taken-out through a second channel that departs from the gap-like space  7 . 
     The stationary part of the bearings is made of graphite or a polymeric material. The rotating part of said bearings is comprised of a hard, non-corrosive metal or a hard polymeric composite material. 
     At least one of the rotors is produced from a hard polymeric composite material, e.g. epoxy resin with SiO 2 , the shaft or the trunnion is made of stainless steel, bronze or a metal that is coated with a corrosion-protective coating. 
     Water-cooled slide bearings, or plain bearings, enable the construction of a totally oil-free screw rotor machine. This obviates the need of shaft seals between an oil lubricant system and the oil-free part, and eliminates the risk of malfunctioning of the seals, often mechanical contact seals. 
     It has been found that the power consumption of a completely water-lubricated screw rotor machine is 5-10% lower than the power consumption of a corresponding screw rotor machine that includes oil-lubricated bearings and mechanical seals. 
     The cost of a screw rotor machine that includes water-lubricated bearings is also slightly lower than the cost of a corresponding machine that has oil-lubricated rolling bearings and requisite mechanical seals. 
     The water circulation system illustrated in FIG. 7 includes in series a compressor K, a first known air separator  41 , a heat exchanger  40 , and a second air separator  42 . The first air separator is connected to the outlet port  109  of the compressor K by a conduit  43  and has two outlet ports  44 ,  45 , a first outlet port  44  for compressed gas in its upper part and a second outlet port  45  in its lower part for separated water. The second outlet port  45  of the first air separator  41  is connected with the heat exchanger  40 . The heat exchanger  40  is connected with the second air separator  42  by means of a conduit  53 , which may include a filter  46 . 
     The second air separator  42  also has two outlet ports  47 ,  48 , a first outlet port  47  for gas-containing water and a second outlet port  48  for water that has only a low gas content and that can be used as a source of lubricant for the compressor K. A conduit  50  extending from the outlet port  48  branches-off at a branching point  51  into the conduits for lubricating the bearings of the compressor, these conduits being referenced  3  in FIG.  3 . The water phase in the separator  42  is thus the lubricant source. 
     The gas-containing water leaving the separator  42  through its outlet port  47  is delivered to the working chamber of the compressor K through a conduit  52 . A filter and/or an ion-exchanger may be provided between the second air separator  42  and the compressor K. The filter and/or ion-exchanger have not been shown in the Figure. The water passing through the heat exchanger  40  is cooled by water or air, for instance. 
     In order to minimize the size of the second separator  42 , there is preferably provided a conduit  54  which connects the conduit  53  between the heat exchanger  40  and the second air separator  42  and the working chambers of the compressor K at the outlet end of said compressor. When the conduit  53  includes a filter, the conduit  54  departs from the conduit  53  between the heat exchanger  40  and the filter  46 . 
     When the compressor K is working, air is introduced through the inlet port  108  and then compressed. The compressed air is delivered through the outlet port  109  and the conduit  43  to the first air separator  41 , in which water is separated from the gas and collected at the bottom of the separator. The air from which water has been removed is taken out via the outlet port  44  in the upper part of the separator  41 . Prior to delivering the water from the first air separator  41  to the second air separator  42 , the water is cooled by heat-exchange, for instance a heat exchange with the surrounding atmosphere or with a liquid medium, such as water. 
     Most of the water cooled in the heat exchanger  40  passes directly to the working chambers of the compressor K through the conduit  54 . The remainder of said water is delivered to the second air separator  42 . 
     Air-containing water is taken from the second air separator  42  through the upper outlet port  47  and delivered to the closed working chambers of the compressor K (closed thread close to the inlet end of the compressor K. The water from which essentially all air has been removed in the second air separator  41  exits from the separator through the port  48  and the conduit  50 . This water is used to lubricate the bearings of the compressor K in the aforedescribed manner. 
     A means for separating gas and cations  60  may be provided between the second air separator  42  and the conduit  3 , as shown in FIG.  7 .