the constriction and working principle of a screw compressor will now be described briefly with reference to fig1 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 fig2 . 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 fig1 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 fig2 . 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 fig3 - 6 . the description is concentrated mainly on a rotor bearing and rotor lubrication . the rotor 102 illustrated schematically in fig3 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 fig3 . 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 fig4 . 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 fig5 and 6 . fig5 shows the slide ring 9 a from above , and fig6 is a cross - sectional view taken on the line vi — vi in fig5 . 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 fig6 . as will be evident from fig5 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 fig4 . 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 ( fig4 ), 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 . 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 fig4 is thus 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 fig7 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 fig3 . 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 fig7 .