Abstract:
The present invention relates to a screw compressor and a method for operating the compressor primarily for gaseous refrigerants, comprising a compressor housing ( 4 ) and a male ( 8 ) and a female ( 10 ) screw rotor arranged in screw rotor bores in the compressor housing ( 4 ), which male and female rotor are co-rotatingly drivable and interacting for compressing the refrigerant, which screw compressor comprises at least one slider ( 22 ) movable in relation to the male and the female rotor, where movement of the slider controls the internal volume ratio of the screw compressor, which slider is moveable in a slider housing ( 20 ) in a direction sideways to the plane formed by the rotational axes of the male and the female rotor, which slider, interacting with the male and the female rotor, comprises at least two surfaces ( 152, 154 ) cooperating with subsequent surfaces ( 156, 158 ) placed in conjunction with the slider housing for controlling the angular and linear placement of the slider in its forward position.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a screw compressor primarily for gaseous refrigerants, comprising a compressor housing, which comprises a male and a female screw rotor arranged in screw rotor bores in the compressor housing, which male and female rotor are co-rotatingly drivable and interacting for compressing the refrigerant, which screw compressor comprises at least one slider, which slider is movable in relation to the male and the female rotor, where movement of the slider controls the internal volume ratio of the screw compressor, which slider is moveable in a slider housing, which slider is in a direction sidewards to the plane formed by the rotational axes of the male and the female rotor, which slider is interacting with the male and the female rotor. 
     The present invention further relates to a method for controlling the discharge of a screw compressor, which compressor comprises a male and a female screw rotor arranged in screw rotor bores in the compressor housing, which male and female rotor are co-rotatingly drivable and interacting for compressing the refrigerant, where a slider is moved in relation to the male and the female rotor from a sidewards direction, which direction deviates several degrees from the longitudinal axes of rotation of the male and the female rotor, which slider is operable for controlling the discharge of the compressor. 
     BACKGROUND OF THE INVENTION 
     US2005/001302 describes a screw compressor for gaseous media, in particular refrigerants, comprising a compressor housing, two screw rotors which are arranged in screw rotor bores in the compressor housing, which are rotatingly drivable and interact to compress the medium, and a control slide arranged adjacent the screw rotors and movable in a direction of displacement for controlling the compression of the screw compressor. In order to solve the problem that the compression of the screw compressor can be controlled, but not precisely regulated, it is proposed that a scannable element, which is scannable with a measuring sensor so as to recognize a position of the control slide in the direction of displacement, is coupled with the control slide. 
     U.S. Pat. No. 4,913,634 describes a screw compressor in which a pair of screw rotors supported by bearings and accommodated in a casing acts to compress a gas and in which a slide valve disposed between an inner wall of the casing and the pair of screw rotors are capable of moving axially while maintaining a small gap between itself and the outer peripheries of the screw rotors, the rate of gas flow bypassed to an inlet port during compression being regulated by axially moving the slide valve. The screw compressor has a side cover in which a bearing for supporting the pair of screw rotors on the discharge side and a slide valve driving hydraulic means are incorporated and which are disposed on the discharge side of the screw rotors; a discharge axial port formed in the side cover; and at least one projection continuous with an opening edge of the discharge axial port and in contact with a semi-circular surface of the slide valve facing in the radial direction thereof, the projection acting to limit the radial movement of the slide valve while the slide valve is moving in contact with the top end of the projection 
     U.S. Pat. No. 4,281,975 describes a screw compressor which includes a contacting a male and female rotor with respect to which a slide valve is operable to control the discharge of the compressor and also regulate the pressure of the fluid pumped or compressed thereby, where the improvement comprises a limited number of different embodiments of anti-friction constructions for supporting the slide valve within the cylindrical bore provided therefore in the housing within which the male and female rotors operate, the wear upon the slide valve within the bore therefore is thereby minimized and the force required to move the slide valve is minimized. 
     U.S. Pat. No. 5,044,909 concerns a rotary compressor of a cooling or heat pump system, the inner volume relation should be related in a predetermined way to the pressure relation of the compressor for an optimal efficiency to be achieved. The built-in volume relation must therefore be variable to be adapted, for example, to full load and partial load. In order to achieve the highest efficiency with respect to loading requirements, a valve device has been developed, in which the discharge port is formed in such a way as to substantially correspond to the theoretically correct radial discharge port and in which a valve body adapted for the purpose has its line of action oriented towards the outlet plane. The valve body is arranged in such a way that in its fully inserted position in the outlet port the mantle wall will correspond to the mantle wall of the working space of the compressor, and will be adjacent to the rotors at a minimum amount of play by means of the end face having been provided with a pointed line surrounded by two concave surface. The outlet port in the mantle wall of the working space of the rotary compressor is delimited by an outlet plane of the compressor and by the screw lines in the mantle wall, which correspond to the cam surfaces of the rotors, which cam surfaces interact in the direction of the outlet plane of the compressor. 
     It is of the utmost importance for the valve body to move on a certain plane in order not to get into a wrong position relative to its correct position in relation to the outlet port and the outlet plane; for example, the valve body can be given, for example, a substantially circular-cylindrical cross-section, whereby a good guiding surface for the valve body is achieved. 
     In order for the valve body not to rotate during its movement up to the outlet port and back again, the valve body can be provided with guiding means, such as grooves. With the valve body in an inclined position relatively to the running rotors, the outlet port can be formed more or less in correspondence with the theoretical outlet port, and at the same time a fully closed sealing line between the rotor housing and the outlet plane is achieved, which also contributes towards making the rotary compressor easier to handle; for example, when the rotors are to be inspected, only the valve body has to be removed. Further, the inclined position implies forming the valve body with an oblique surface, which will move towards the outlet plane, whereby the valve body will be guided in its movement towards the outlet port and will finally be fixed in its fully inserted position. 
     OBJECT OF THE INVENTION 
     It is the object of the invention to develop a highly effective screw compressor, which screw compressor can regulate the discharge pressure. 
     DESCRIPTION OF THE INVENTION 
     This can be achieved by a screw compressor as described in the opening paragraph if the compressor comprises discharge end plates, which discharge endplates can be place between the discharge end of the male and female screw rotors and the end plate of the compressor housing, which discharge end plates comprises a discharge opening, which discharge end plates comprise discharge end walls, where the slider comprises at least two areas, which areas are cooperating with subsequent areas placed in conjunction with the discharge end walls for controlling the angular and linear placement of the slider for avoiding rotor contacts. 
     It can hereby be achieved that independent pressure volumes formed between the male and the female rotor during the interaction of the rotors are connected to the discharge before the last compression takes place, when this slider is moved away from its forward position. This will lead to a reduction of the discharge pressure of the compressor. This shunting of the discharge from the rotors will automatically also increase the volume of compressed gas that leaves the compressor. The sideways placement of the slider and the slider housing lead to the rotor bores in the compressor housing being formed in the housing without any cut-outs along most of the length of the rotor bores before the sideways opening for the slider housing. This can lead to a very tight screw compressor without any leaks along for example a slider operating parallel to the male and the female rotor. The screw compressor with the sideways acting slider is much cheaper to produce than the traditional screw compressors with sliding elements. It is easy to produce the two bores for the male and the female rotor and afterwards to form the sideways slider housing. 
     The angle of the slider and the slider housing is preferably more than 45 degrees in relation to the longitudinal rotational axes of the male and the female rotor. This can lead to an effective placement of the slider if the angle of operation is higher than 45 degrees. 
     The angle of the slider and the slider housing can be more than 60 degrees in relation to the longitudinal rotational axes of the male and the female rotor. It can hereby be achieved that the angle for the slider and the slider housing could be up to 90 degrees so that the slider is acting towards the male and the female rotor directly perpendicular. In certain embodiments of the invention, the angle could be the supplementary angle to the above-mentioned one. 
     The slider can be movable in the housing, which housing comprises at least one pressure chamber, which pressure chamber is connected to the discharge volume of the compressor, where the slider has a front surface, at which surface the pressure is forcing the slider to move backwards from the male and the female rotor. It is hereby achieved that the actual discharge pressure of the screw compressor will force the slider backwards. Means for forcing the slider in the opposite direction are can be achieved by different force acting methods and components. A primitive solution could be that the slider is operating against the pressure of a spring. This could lead to an automatic pressure adjustment of the screw compressor. Other actuating means would also be possible. An example is that a hydraulic pressure is actuating the slider into a direction towards the male and the female screw. A combination of a spring and hydraulic or gas activation would probably also be possible for pressing the slider towards the rotors. 
     The housing can comprise at least a second pressure chamber, which second pressure chamber contains a back surface of the slider for forcing the slider to move forwards towards the male and the female rotor. It is hereby achieved that a pressure chamber is placed behind the slider, and this pressure chamber can be activated for example by refrigerant. In this way, the compressor can operate without use of an extra media for activation. 
     The front of the slider can be placed in the first pressure chamber, which front of the slider has an active pressure surface, which pressure surface is smaller than the active pressure surface of the back surface of the slider placed in the second pressure chamber. It is hereby achieved that the back-side pressure can be controlled so that this pressure can achieve nearly all pressure values between the suction pressure of the compressor and the discharge pressure of the compressor. In this way, the slider can be activated with a pressure that can be regulated to every value between these two outer limits. This can lead to a possible solution where the slider is placed in different positions between two outer positions. 
     The second pressure chamber can be connected through an orifice towards the suction side of the compressor, which second chamber further is connected through a first electromagnetic valve to the discharge pressure of the compressor. This will lead to a constant pressure reduction in the second chamber to achieve an inlet pressure in that chamber if the electromagnetic valve is closed. 
     The first electromagnetic valve can be connected to an electronic circuit comprising computer means. It can hereby be achieved that an increase of the pressure in the second chamber can be achieved by periodical opening of the electromagnetic valve. Depending on the size of the previously described orifice and the opening of the electromagnetic valve, the pressure in the second chamber can achieve a value that maximally is very close to the discharge pressure of the compressor. This pressure will then always move the slider forwards. 
     It is hereby possible that the computer takes over the control of the discharge pressure for the compressor. These computer means can also comprise further means for controlling a motor, which drives the compressor. If the motor is an electromotor, a kind of semiconductor switches will probably be used. 
     The slider can cooperate with at least one mechanical stop when the slider is placed in a forward position for increasing the discharge pressure of the compressor. This assures that the slider will not come in touch with the rotors in the compressor. By using this stop, a very effective and very close position towards the rotors can be achieved. 
     The invention further comprises a method for controlling the slider, where a discharge opening is formed in discharge endplates, where the discharge endplates comprises discharge end walls, where the slider comprises guiding areas which areas interacts with corresponding areas placed in conjunction with the discharge end wall for controlling the angular and linear placement of the slider when the slider is operating adjacent to the rotors at a minimum amount of play towards the male and the female screw rotor. 
     In this way, the interaction between the slider and the male and the female rotor can be made only in a limited area near the pressure end of the male and the female rotor. In fact, the slider can when it is away from interaction make a shortcut between the two last compression volumes formed between the male and the female rotor. This shortcut will lead to an increasing volume and a decreasing pressure at the pressure discharge. The sideways interaction reduces the number of possible leaks that typically occur if a slider operates along the male and the female screw rotor. This parallel slider will end up in a very difficult construction of the compressor. Furthermore, there will be very small openings towards the slider, and these openings can be a kind of shortcuts that is decreasing the effectiveness of the compressor. 
     The slider can move from a first position to a second position in a slider housing, which slider housing can be placed at an angle of more than 45 degrees in relation to the longitudinal rotational axes of the male and the female rotor. This relatively high angle is effective because the slider housing has limited influence on the construction of the rest of the compressor housing. 
     The angle of the slider and the slider housing can be more than 60 degrees in relation to the longitudinal rotational axes of the male and the female rotor. A still steeper angler for the slider seems more and more effective. The limitation for this patent application is as such not 90 degrees, even slider angles over 90 degrees would be possible in a certain embodiment, depending on which end of the compressor that is connected to a driving motor. 
     The slider can be movable by the discharge pressure, which discharge pressure acts on the effective area of the front surface of the slider to force the slider to move backwards from the male and the female rotor. By letting the slider be movable backwards of the discharge pressure of the compressor, an automatic pressure control can easily be achieved. As soon as the slider is moved backwards, the output pressure is reduced, and then the pressure of the front end of the slider is reduced. Depending on which activation means that are placed in conjunction with the slider, it could be moved forward, for example by a spring, or it could be moved forward by other activation means which could be hydraulic or also mechanical or electromagnetic means would be possible. Even a kind of analogue pressure regulation could be formed in this way because it would be possible to let the slider take a position where a certain pressure occurs, where there is a limited effect on the slider and where a backwards movement of the slider will reduce the effect of the slider. 
     The backside of the slider can be under influence of a pressure, which pressure can be regulated to force the slider to move forwards towards the male and the female rotor. It is hereby achieved that a pressure on the backside will move the slider forwards. This pressure can be hydraulic where for example high pressure oil from the compressor could be used, or it is possible to use the high pressure refrigerant delivered from the discharge of the compressor as the medium. 
     The front side of the slider has an active pressure surface, which pressure surface is smaller than the active pressure surface of the back surface of the slider. It is hereby achieved that a smaller pressure of the backside in relation to the pressure of the front side can keep the slider in a kind of balance. Increasing pressure on the backside will move the slider forwards, and decreasing pressure will move the slider backwards. 
     The pressure in the second pressure camber can be decreased by an orifice connected towards the suction side of the compressor, which pressure in the second chamber further is controlled by a first electromagnetic valve connected to the discharge pressure of the compressor. By using an orifice to decrease the pressure and a magnetic valve to increase the pressure, it will be possible to adjust the pressure in the chamber between the suction pressure of the compressor and the discharge pressure of the compressor. The orifice can have an opening that is relatively small in relation to the opening degree of the magnetic valve. This will lead to an increasing pressure as soon as the magnetic valve is opened, but in the other way, as soon as the magnetic valve is closed, the pressure will start decreasing in the chamber. Depending again on the size of the orifice, this pressure decrease can be relatively slow which will let the slider move relatively slowly. This can automatically lead to a dampening of oscillations of the slider. 
     An electronic circuit comprising computer means can control the first electromagnetic valve. Computer means can hereby control the opening degree of the magnetic valve. It would be possible that the computer means controls the valve in a way of modulation so that different positions of the slider could be obtained. 
     The slider can be controlled by one mechanical stop, when the slider is placed in a position for increasing the discharge pressure of the compressor. The mechanical stop is necessary if the slider shall be operating very closely to the rotating male and female rotor. Only by using a stop, it will be possible to come that close to the rotating elements, which are necessary for making an effective compression. The distance between the slider and the rotating rotors should be limited to the size of an oil film. 
    
    
     
       DESCRIPTION OF THE DRAWING 
         FIG. 1  shows a sectional view of a part of a screw compressor 
         FIG. 2  shows a enlarged sectional view of the part of a screw compressor shown at  FIG. 1 , 
         FIG. 3  shows a sectional view of the same elements as seen in  FIG. 2   
         FIG. 4  shows an enlarged view of  FIG. 3 . 
         FIGS. 5 ,  6  and  7  show the same components in different situations, 
         FIGS. 8 ,  9  and  10  show the slider in its forward position 
         FIGS. 11 ,  12  and  13  show an alternative embodiment 
         FIG. 12  shows a compressor housing and a slider housing. 
         FIGS. 14 ,  15 , and  16  show the same embodiment as the one shown in  FIGS. 11 ,  12 , and  13 . 
         FIG. 17  shows an inner slider 
         FIG. 18  shows an outer slider 
         FIG. 19  shows the same embodiment as the one shown in  FIG. 11 , were the rotors are shown above the sliders. 
         FIG. 20  shows the same embodiment as the one shown in  FIG. 12 , where the rotors  0  are shown 
         FIG. 21  is an enlarged view of  FIG. 20   
         FIG. 22  shows a compressor which has a suction pipe connected to the suction side of the compressor. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a sectional view of a screw compressor  2  comprising a housing  4 , which comprises a bore  6 , which contains the male  8  and the female  10  rotors. A slider housing  20  comprises a slider  22  which slider  22  has a front area  24  placed in a pressure chamber  26 . Furthermore, the slider  22  comprises a backside  28  placed in a second pressure chamber  30 .  FIG. 1  shows moreover a channel  32  connected to the suction side of the compressor. 
     In operation, the slider  22  can be moved away from interaction with the male  8  and the female  10  rotors in the rotor housing  6 . The slider  22  is shown in its forward position where the slider front has a form that is equal to the form of the housing  6 , but on the front surface of the slider  22 , the pressure volume  26  is acting. The discharge pressure of the compressor will here try to force the slider  22  to perform a backwards movement in the slider bore  20 . This backwards movement will reduce the second volume  30 , which can be connected through a not shown orifice. This will automatically lead to a dampening of the movement of the slider  22 . By changing the pressure in the second chamber  30 , the back surface  28  of the slider can come under pressure, which will press the slider  22  back to its position where it is in touch with a stop. 
       FIG. 2  shows a sectional view of the part of a screw compressor, which comprises the previously described slider. In  FIG. 2 , a slider  122  is shown which slider  122  has a front surface  124  placed in conjunction with a pressure chamber  126 . The backside  128  of the slider  122  is placed in a second pressure chamber  130 . The discharge pressure port  140  is placed in a discharge endplate  141 , where the discharge port  140  cooperates with a cut-out  146  in the front of the slider  122 . The slider  122  has a surface  152 , which cooperates with a surface  156  placed in conjunction with the discharge port  140  of the screw compressor. A guiding-in  160  cooperates with a groove  162  which groove  162  is formed in the backside  128  of the slider  122 . 
     In  FIG. 2 , the slider  122  is pressed backwards away from the discharge port  140  by the discharge pressure. It is hereby achieved that the discharge port  140  is increased in size along the discharge end of the male and female rotors so that the pressure chambers in the end of the volumes between the two rotors are connected to the outlet. This leads to a reduced pressure and a higher flow of gas. 
       FIG. 3  shows a sectional view of the same elements as seen in  FIG. 2  but seen from the upper side of  FIG. 2 .  FIG. 3  shows a compressor housing  104  and a slider housing  120 . Inside, the discharge port  140  is placed in the discharge endplate  141 . Furthermore, the surfaces  142 ,  144  of front end of the slider  122 . Between the two surfaces  142 , 144 , the cut-out  146  is shown. 
       FIG. 4  shows an enlarged view of  FIG. 3 . The discharge port  140  in the discharge endplate  141  is shown, and the front end of the slider  122  is shown with its surface  142 , 144 . The number  148  indicates the centre line of the compressor housing, and the number  150  indicates the centre line of the slider  122 . The slider  122  has a surface  152  and a further surface  154 . The end surface  152  cooperates with a surface  156  placed in conjunction with the discharge opening in the compressor. Further, the end surface  154  will cooperate with a surface  158  also in conjunction with the pressure outlet. 
     As already mentioned for  FIG. 2 , it is also indicated in  FIGS. 3 and 4  that the slider  122  is placed in its backwards position. In this position, in  FIG. 4  a difference between the centre line  150  of the slider and the centre line  148  of the housing is shown. This indicates that the slider is rotated out of its optimal position when it is placed in its backwards position. This leads to a large opening between the surface  154  and the surface  158 . 
       FIGS. 5 ,  6  and  7  shows exactly the same components as already mentioned for  FIGS. 2 ,  3  and  4 . The difference between the figures is that the slider  122  is now placed in a position between its two end situations. This can be seen in  FIG. 5  where the front of the slider  124  is closer to the housing of the rotors. But especially in  FIG. 7 , the function taking place when the slider is moved downwards is indicated. The slider  122  has been moved forwards so its surface  152  and its surface  154  are rather close to get in touch with the corresponding surface  156  and the surface  158  at the corresponding surfaces at the compressor outlet components. Also, the line  150  in relation to the line  148  is now partly aligned. The open area formed by the cut-out  146  in the slider is now reduced because the opening along the front  124  of the slider is smaller. 
       FIGS. 8 ,  9  and  10  show the slider  122  in its forward position. The front end  124  of the slider  122  is now parallel with the inner wall of the rotor housing. The surface  152  of the slider  122  is now in touch with the surface  156 . This is further indicated in  FIG. 10  where the surface  152  and the surface  156  are now in touch, and the surface  154  and the surface  156  are also touching each other. It is indicated that the centre line  148  of the compressor housing and the centre line  150  of the slider are now totally aligned. It is also indicated that the opening  146  is now reduced. 
     By using the direct touch of the slider with the discharge pressure components having the surfaces  156  and  158 , this will lead to a nearly perfect alignment of the slider  122  when it is in its front position. This is absolutely necessary; otherwise the slider  122  could get in mechanical touch with the rotating screws. 
       FIGS. 11 ,  12  and  13  show an alternative embodiment where a slider housing  220  contains an inner slider  222  and an outer slider  223 . The inner slider has a back end  228 , which cooperates with a pressure chamber  230 . The outer slider  223  has an end  229 , which cooperates with a pressure chamber  231 . The slider  222  has a surface  252  which cooperates with a surface  256  formed at the pressure outlet. The outer slider  223  has a surface  253 , which cooperates with a surface  257  at the pressure outlet of the compressor. 
     In  FIG. 12 , a compressor housing  204  and a slider housing  220  are seen. The slider  222  has a front surface  242 , 244 , and the slider  223  has surfaces  243 , 245 . In  FIG. 13 , an enlarged view of the central part of  FIG. 12  is shown.  FIG. 13  shows a pressure outlet opening  240  placed in the discharge endplate  241 . The slider  222  has end surfaces  242 , 244 . The slider  223  has surfaces  243 , 245 . The surface  252  cooperates with a surface  256  at the discharge endplate  241 , where the surface  253  cooperates with a surface  257 . Further, the surface  254  cooperates with a surface  258 . Also the surface  255  cooperates with a surface  259 . A centre line of the housing  248 , a centre line  250  of the slider  222  and the centre line  251  of the slider  223  are shown. 
     In operation, it is possible to force the slider  222  backwards by reducing pressure in the chamber  230 . This will reduce the discharge pressure of the compressor. Further reduction of the discharge pressure can be achieved by also forcing the slider  223  backwards. This can be achieved if the slider  222  is moved, and the pressure in the chamber  231  is reduced. The two sliders  222 ,  223  can be moved back in their front position by first increasing the pressure in the chamber  231  and afterwards increasing the pressure in the chamber  230 . Hereby, it is achieved that a compressor can be adjusted in at least three different steps. It will probably be possible to also achieve analogue discharge control of the compressor by placing one of the sliders or maybe both sliders in a position between the minimum or maximum positions. 
       FIGS. 14 ,  15 , and  16  show the same embodiment as the one shown in  FIGS. 11 ,  12 , and  13 . Components already mentioned above will not be mentioned again in the following. 
     The difference between the embodiment shown in  FIGS. 11 ,  12 ,  13  and the embodiment shown in  FIGS. 14 ,  15 ,  16  is that the sliders  222  and  223  in the latter are shown in their backward position. A working chamber  233  is now visible under the backside  229  of the slider  233 . Further, the chamber  231  is visible between the backside  229  of the outer slider and the backside  228  of the inner slider. From  FIG. 16  it appears that the outer slider has a front edge  255  which can engage with the edge  259  at the discharge end plate  241 . The inner slider also has a front edge  254  which can engage with the edge  258  positioned on the edge of the discharge end plate  241 . Furthermore the edge  252  at the inner slider is shown which will cooperate with the edge  256  at the edge of the discharge end plate  241 . Further, the outer slider has a front edge  253  which will cooperate with the edge  257  at the discharge end plate  241 . Further in  FIG. 16 , the centre line  248  for the compressor house  204  and the centre line  250  of the inner slider can be seen. It is clearly indicated that the inner slider is misaligned in the angular and sideward position in relation to the centre line  248 . 
     In operation this misalignment is harmless in the backward position shown. However, when the sliders  222 , 223  is moved into contact with the discharge end plate  241 , both sliders  222 , 223  will be aligned. 
       FIG. 17  shows an inner slider  222  which has contact areas  242  and  244  for being aligned when they engages with the discharge end plate  241 . Furthermore edges  252 ,  254  are shown which are positioned in such a way that they will engage the edge of the discharge end plate  241 . 
       FIG. 18  shows an outer slider  223  which comprises contact areas  243  and  245  for engaging with the discharge end plate  241  when it is in its forward position of the slider. Further contact areas  253  and  255  are shown. These contact areas will engage the edge of the discharge end plate  241 . 
       FIG. 19  shows the same embodiment as the one shown in  FIG. 11 , the difference being that the rotors are shown above the sliders. 
       FIG. 20  shows the same embodiment as the one shown in  FIG. 12 , the difference being that the rotors  208  and  210  are shown. 
       FIG. 21  is an enlarged view of  FIG. 20  and also shows the two rotors  208  and  210 . 
       FIG. 22  shows a compressor  302  which has a suction pipe  382  connected to the suction side of the compressor. The compressor  302  has a discharge pipe  378  which is connected to an oil management system  376  from which refrigerant is flowing to a pipe  380 . In the compressor  302  one of the rotors  308  and  310  can be seen. A slider  322  can also be seen. This slider has a front area  324  which engage the rotors  308  and  310 . The slider has a backside  328 , which is placed in a working chamber  380 . The working chamber  330  is connected to the chamber  333  through a flow-restriction  370 . From chamber  333  there is a connection towards the compressor to a point in the compressor having a relatively low pressure P 4 . The chamber  330  is further connected to a line  371  to a magnetic valve  372  and further to a line  374  to the oil management system  376 . 
     In operation, the discharge pressure P 2  will be present at the area A 1 , which is the front of the slider  322 . This pressure P 2  working on A 1  will press the slider backwards. In the chamber  333  a pressure P 4  is operating which is slightly above P 1 , which is the suction pressure of the compressor. The flow-restriction unit  370  also connects the pressure P 4  to the chamber  330  so that oil can flow from this chamber through the flow-restriction unit  370  towards the chamber  333  and from here towards the compressor at the pressure P 4 . 
     If the valve  372  is open, oil will flow through the line  374  through the valve  372  and further through the line  371  towards the chamber  330 . The high-pressure oil also flows through the flow-restriction  370 , but in an amount so that the pressure in the chamber  330  will increase. This increasing pressure P 5  will now, due to its increase, move the slider  332  forwards. This will bring the slider into its operational forward position where the slider is engaging the rotors and the discharge area at the end of the rotors is reduced. By using oil for hydraulic activation and de-activation of the slider it is achieved that a medium usually applied in the compressor can also be used for this hydraulic activation. By using the oil there will be no leakages for refrigerant and as such no reduction in the amounts of refrigerant, which can be supplied over line  380  to a refrigeration system.