Abstract:
A side channel pump, preferably a vacuum pump, includes a driven rotor ( 16 ) and a fixed stator ( 14 ). The rotor ( 16 ) and the stator ( 14 ) define a pump channel circulating in a peripheral direction. Blades are fixed onto the rotor, protruding into the cross-section of the pump channel. The pump channel also includes a blade-free side channel ( 44 ). The pump channel ( 22 ) containing the side channel ( 44 ) extends in a helical manner around the rotor ( 16 ). The pump channel is advantageously not limited to the length of a winding but can have the length of substantially any number of uninterrupted windings. As a result, a high suction performance and a high compression ratio in the pump can be obtained.

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to a side channel pump for supplying liquid and gaseous fluids as well as mixtures of liquid and gas. 
   Among other things, side channel pumps are used for generating a vacuum. From EP-A-0 170 175, a side channel vacuum pump is known that includes several annularly extending pump channels limited by the rotor and by the stator each. At the rotor, blades are arranged, protruding into the respective pump channel cross-section. From radially inside, the blades protrude only into a portion of the pump channel cross-section so that the radial outer portion of the pump channel is free of blades. The blade-free portion of the pump channel is the side channel. 
   During rotation of the rotor, the fluid molecules are seized by the blades and accelerated in circumferential direction. Due to the centrifugal force, the fluid molecules are moved outward into the blade-free side channel. In the side channel, the radially outward directed movement is again deflected radially inward in the direction of the blades, the fluid molecules being strongly braked again by the friction at the fixed stator wall. The fluid molecules leave the side channel in a radially inward direction and are seized by the blades again and accelerated in circumferential direction. Through this continuously repeating process, a circumferentially moving helical fluid whirl develops in the pump channel. 
   The fluid inlet and the fluid outlet are formed by a stop wall radially protruding from the stator into the blade-free cross-sectional area of the side channel. In the region of the stop wall, the incoming fluid flow passes out of the blade-free cross-sectional area of the pump channel to a fluid outlet. The portion of the fluid in the region of the blades at that time is not seized by the stop wall and is therefore entrained by the blades to the fluid inlet at the rear side of the stop wall. 
   The compressed fluid entrained to the suction side expands again to the suction pressure on the suction side and is compressed again. This means that, in the region of the blades, the pump channel forms a short circuit between the pressure side and the suction side of the annular-like pump channel. The pressure losses caused in this manner produce heating and noise. In a vacuum pump, several such annular pump channels are connected in series or combined with another molecular pump stage, with a turbomolecular pump stage, for example, for generating high degrees of compression. Because of their simple mechanical structure, ease of maintenance, and reliability, side channel pumps are well suited for industrial use. Due to the plurality of loss-inflicted fluid inlets and outlets, however, the suction capacity and the compression ratio are limited. 
   The present invention contemplates an improved apparatus and method that overcomes the aforementioned limitations and others. 
   SUMMARY OF THE INVENTION 
   One advantage of the invention is improved compression in the side channel pump. 
   In one embodiment of the invention, the pump channel no longer extends like a screw thread about the rotor, rather than in an annular fashion. In this arrangement, the pump channel can comprise more than one winding, that is, the channel can include a plurality of windings. Moreover, the maximum pump channel length is not limited to one a single rotor circumference but, due to the helical arrangement, can be extended to a multiple of the rotor circumference and is just limited by the axial rotor length. The pump channel can extend continuously over a length of a plurality of windings without the pump channel being interrupted by loss-inflicted fluid inlets and outlets. Therefore, an undisturbed helical fluid flow develops in the pump channel over the entire pump channel length. Thus, a high compression of the pump is realized. Because of the omission of a plurality of fluid inlets and outlets, the noise emission is reduced as well. 
   The stator is configured as a surface area of a body of revolution. For example, the stator can be cylindrical, conical or parabolic. Therefore, the stator has a simple structure and can be produced inexpensively. An easily maintained side channel pump is realized that has a high compression and suction capacity, generates a fluid flow of low pulsation level, occupies a small installation space and is adapted to be produced easily and inexpensively. Since no oil seals are required, a fluid is delivered that is free of contaminations. 
   According to a preferred embodiment of the invention, the rotor comprises a channel wall laterally defining the pump channel, extending helically about the rotor. In the region of the pump channel, the stator is configured so as to have a smooth surface. Most walls of the pump channel are provided at the rotor side, i.e., they are moved in the pumping direction. Therefore, the fluid molecules are braked only at a single wall of the pump channel, namely at the wall formed by the stator. By this arrangement, the suction capacity of the pump is increased as well. 
   According to a preferred embodiment, the pump channel extends continuously over approximately the entire rotor length. The fluid inlet and outlet are provided at the end faces of the rotor, respectively. This means that a single self-contained compression stage extends over a plurality of windings over the entire length of the rotor. The front-face fluid inlet and the front-face fluid outlet are spatially separated from each other; this means that between the compression side and the suction side, there is no short circuit causing a pressure loss. With a single compression stage, a high compression and suction capacity can thus be realized. 
   According to a preferred embodiment, the rotor comprises several channel walls defining several pump channels parallel to each other. Hence, it is a multiple side channel pump having a correspondingly high suction capacity. 
   Preferably, the cross-sectional area of the blades amounts to between one fifth and half of the cross-sectional area of the pump channel. 
   According to a preferred embodiment, the stator surrounds the rotor. Alternatively or in combination therewith, the rotor can also surround the stator. Particularly by the combination of both structural shapes in a single rotor or stator, a very compact pump can be realized. 
   According to a preferred embodiment, the channel wall is arranged so as to be inclined to a radial line of the rotor, namely inclined in the direction of discharge. This means that the channel wall does not protrude vertically from a cylindrical rotor, but is inclined towards the pressure side. That channel wall of a pump channel that is the rear one in discharge direction has an obtuse angle of more than 90° with respect to the fixed stator-side channel wall so that the channel wall located at the rear acts like a scraper scraping the fluid off the stator channel wall and supporting the formation of the helical fluid whirl in the pump channel. 
   According to a preferred embodiment, the blades are arranged so as to be inclined to the radial line of the rotor. This means that the blades do not project vertically from a cylindrical rotor but are inclined in the direction of the channel towards the pressure side. Due to the blades being inclined forwards to the pressure side, the flow component of the fluid in discharge direction is increased, whereby the fluid pressure is simultaneously increased. 
   Preferably, the pump channel cross-section is larger at the suction-side end than at the pressure-side end of the rotor. The fluid increasingly compressed towards the pressure side is delivered in correspondence with its compression in a pump channel with a decreasing cross-section. Thus, the pump channel length is capable of being considerably lengthened, with the axial rotor length remaining constant. In this way, the rotor length can be kept relatively short so that a compact structure of the vacuum pump is realized. 
   According to a preferred embodiment, the pump channel comprises a radial step. The height of a radial step of the pump channel may be smaller than half the pump channel height. The stepwise reduction of the pump channel radius causes a reduction of the circumferential rotor speed, with the fluid compression increasing. Thereby, the friction losses between the rotor-side channel walls and the stator-side channel walls are reduced. Due to the limitation of the radial pump channel step to half the pump channel to height, the preservation of the helical whirl is ensured when the fluid transitions from one pump channel section into the next pump channel section. In this way, the pressure losses in the radial step are kept small. In the respective pump channel sections, the pump channel is still arranged helically. 
   According to a preferred embodiment, the rotor-side pump channel wall and the rotor have a conical configuration. Thus, the cross-sectional area of the pump channel can be reduced in correspondence with the pressure increase in the pump channel towards the pressure side. Further, the circumferential rotor speed is reduced towards the pressure side by reducing the outer diameter of the rotor. The geometry of the pump channel is adapted to the curve of the fluid pressure. Thus, a very compact structure and a rotor operation in the stator at a low friction level can be realized. 
   Preferably, a fluid cooling channel is provided that is arranged between two pump channel sections. In this way, an intermediate cooling of the fluid is effected. The fluid is led out of the pump channel by a scraper projecting into the pump channel, for example, and cooled in a cooled cooling channel and subsequently supplied to a following pump channel section again. Due to the intensive cooling of the fluid in an external cooling channel, the heating of the fluid as well as that of the rotor and the stator is limited. In this way, the compression process approximates isothermal compression, and the input power is reduced. 
   According to yet another preferred embodiment, the pump channel is arranged at an end face of the rotor, the pump channel including the side channel extends spirally on the rotor end face. Moreover, the pump channel can also be arranged on a rotor in the form of a spiral instead of in the form of a helix. Thus, it is also possible to realize a pump channel with several windings which are not interrupted by fluid inlets and outlets. The pump channel extends in a logarithmic spiral or evolvent. The suction side of the pump channel may be arranged on the outside or in the center of the rotor or stator. 
   The aforementioned features referring to a pump with a pump channel on the outside of a rotor can also be applied, in a similar or analogous manner, to the pump in which the spiral pump channel is arranged on the rotor end face. 
   Numerous advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading the following detailed description of the preferred embodiments. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may take form in various components and arrangements of components, and in various process operations and arrangements of process operations. The drawings are only for the purpose of illustrating preferred embodiments and are not to be construed as limiting the invention. 
       FIG. 1  shows a longitudinal cross-sectional view of a first embodiment of a side channel pump with a cylindrical rotor and a cylindrical stator. 
       FIG. 2   a  shows a an enlarged cross-sectional view of the pump channels of the pump of  FIG. 17 . 
       FIG. 2   b  shows a cross-sectional top view of one of the pump channels of the pump of  FIG. 1 . 
       FIG. 3  shows a side view of a portion of the rotor of the pump of  FIG. 17 . 
       FIG. 4  shows a second embodiment of a side channel pump with several pump channels arranged behind each other in a step-like manner. 
       FIG. 5  shows a third embodiment of a pump being a side channel pump with a conical rotor and a conical stator. 
       FIG. 6  shows a fourth embodiment of a side channel pump with a pump channel the cross-section of which reduces towards the pressure side. 
       FIG. 7  shows a fifth embodiment of a side channel pump with a meander-like arrangement of several pump channels. 
       FIG. 8  shows a top view of a sixth embodiment of a side channel pump, with a spiral pump channel arranged on the rotor side. 
       FIG. 9  shows a longitudinal cross-sectional view of the vacuum pump of  FIG. 8 . 
       FIG. 10  shows a cross-sectional view of a seventh embodiment of a side channel pump, which has a pump channel arranged on the outer circumference of the rotor and an annexed pump channel arranged on the rotor end face. 
       FIG. 11  shows a cross-sectional view of an eighth embodiment of a a side channel pump, which has a fluid cooling channel. 
       FIG. 12  shows a cross-sectional view taken along the sectional line XII—XII of the pump of  FIG. 11 . 
       FIG. 13  shows a cross-sectional view of a ninth embodiment of a side channel pump, which has a fluid cooling channel. 
       FIG. 14  shows a cross-sectional view taken along the sectional line XIV—XIV of the pump of  FIG. 13 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In  FIG. 1 , a first embodiment of a pump  10  which is a side channel pump, for delivering a fluid, and preferably for delivering a gas, is illustrated. The pump  10  serves to produce a vacuum on a suction side  11  and to compress the fluid into medium vacuum or rough vacuum on a pressure side  13 . 
   The side channel vacuum pump  10  is substantially formed by a stator  14  forming a fixed housing  12  and a driven rotor  16  in the stator housing  12 . The rotor  16  is driven by an electric motor (not shown) by which the rotor  16  can be rotated at up to 80,000 revolutions/minute. The rotor  16  and the stator housing  12  are preferably made of metal, but may also be made of ceramics, be made of plastics or of a material coated with plastics. The operation of the vacuum pump  10  is preferably lubricant-free so that a contamination of the pumped fluid is avoided. 
   From the suction side  11  of the vacuum pump  10 , the fluid flows through a fluid inlet  48  into the stator housing  12  at the one end face of the rotor  16  and flows through a fluid outlet  50  out of the stator housing  12  towards the pressure side  13  at the other end face of the rotor  16  in a compressed manner. 
   The rotor  16  includes an integral rotor body  18  with a shaft  19  and has, disposed at its outer circumference, a single channel wall  20  projecting radially outward, extending over the axial length of the rotor  16  in the form of a helical line with a constant gradient. The helical thread formed in this way is a single-flight thread. Over the entire rotor length, the channel wall  20  defines therebetween a single pump channel  22  extending helically around the rotor circumference. 
   With continuing reference to  FIG. 1  and with further reference to  FIG. 2   a , in cross-section, a channel bottom  25  formed by the rotor body  18  has an approximately circular configuration. On the outside or stator side, the pump channel  22  is defined by a cylindrical housing wall  24  of the housing  12 . An inside  26  of the housing wall  24  is preferably smooth. The pump channel  22  extends in a single winding over the entire length of the rotor  16 . 
   As illustrated in  FIG. 2   a , the channel wall  20  is inclined to a radial line  30  of the rotor  16  at an angle  28  of approximately 15°. The channel wall  20  is inclined such that it is axially bent forward towards the pressure side  13 . A pressure-side side  32  of the channel wall  20  that forms the suction-side wall of the pump channel  22  assumes an obtuse angle with respect to the stator-side inside  26  of the housing wall  24 . A pressure-side front edge  34  of the channel wall acts like a scraper with respect to the inside  26  of the housing wall  24  and thus peels fluid off the housing inside  26 . 
   In the pressure-side and rotor-side quarter of the pump channel cross-section, a plurality of plate-like blades  38  is arranged at an equal mutual distance. The blades  38  shaped like segments of a circle assume about between a fifth and a half of the cross-sectional area of the pump channel, but may also be larger. The blades  38  are arranged in the region of the suction-side and rotor-side quarter of the channel cross-section. As illustrated in  FIG. 2   b , each blade  38  stands at about right angles to the channel wall  20  and at an angle  40  of 10°–20° to a radial line  42  of the rotor body  18 . Due to the forward inclination of the blade  38  in rotational direction or to the pressure side to the fore, the pressure generated in the fluid is increased in comparison with blades without inclination. The blades  38  bent forward in rotational direction effect an increased flow component that is directly proportional to the increase in pressure. 
   The blade-free stator-side half of the pump channel  22  forms a side channel  44  of the pump channel  22 . The side channel  44  of the pump channel  22  is the outside and blade-free portion of the pump channel  22 . 
   A gap  56  between the channel wall  20  and the inside  26  of the housing wall  24  is sufficiently small so that the backflow caused by the pressure difference between neighboring pump channel passages is substantially smaller than the pressure difference built up in a winding. The flow resistance of the gap  56  is large, such that it is an obstacle to a considerable fluid backflow in the direction of the suction side  11 . The flow resistance in the gap  56  can be changed by using a thicker a channel wall  20  and thus a corresponding axial lengthening of the gap  56 . 
   The fluid flows through the fluid inlet  48  into the stator housing  12  and is accelerated by the channel wall  20 , the channel bottom  25 , and the blades  38  and thus, is tangentially compressed in the circumferential direction into the circumferential pump channel  22  and simultaneously delivered axially towards the fluid outlet. In the closed helical pump channel  22 , the fluid or the fluid molecules are moved on a helical line within the pump channel  22 . 
   As illustrated particularly in  FIGS. 2   a  and  3 , the fluid is accelerated in circumferential direction of the rotor by the blade  38 . Because of the acceleration, the centrifugal force acting upon the fluid is increased so that the fluid flows radially outward into the side channel  44 . Finally, the fluid abuts against the fixed inside  26  of the stator housing wall  24  and is braked and reflected radially inward. During the deceleration at the inside of the stator housing wall  24 , fluid flow  54  mixes with fluid particles from other channel sections, which have already been braked at the stator housing wall  24 . In the radial inner portion of the pump channel  22  or in the region of the blade  38 , the pressure is lower than in the radial outer portion of the pump channel  22 , i.e., in the side channel  44 . A force from the side channel  44  acts radially inward upon the fluid. Further, the braked fluid is peeled off the inside  26  of the stator wall by the channel wall front edge  34  and thus moved axially towards the fluid outlet  50  by the channel wall  20 . From the side channel  44 , the fluid flows along the suction-side channel wall side  32  of the channel wall  20  to the channel bottom  25  in which the fluid is again deflected radially outward by approximately 180°. In doing so, it is seized by the blade  38  and accelerated in the circumferential direction again. This process is repeated until the thus compressed fluid reaches the outlet-side axial end of the rotor  16  and flows out of the fluid outlet  50  there. In the fluid pump channel  22 , a helical fluid flow  54  is thus generated in the course of which the fluid is increasingly compressed. By means of the described pump, gaseous fluids can be compressed from ultrahigh vacuum to approximately atmospheric pressure by a single compression stage. 
   The present vacuum pump  10  can be realized with a pump channel  22  of substantially any length so that very high compression capacities are achievable. Owing to the continuous fluid compression, loss-inflicted transitions between different compressor stages are avoided. The system-determined short circuit between the pressure side and the suction side that exists with conventional side channel compressors that have annular pump channels is eliminated in the screw thread-like pump channel arrangement. Apart from the inside  26  of the stator housing wall  24 , all walls of a pump channel  22  are configured so as to be rotating, i.e., to compress the fluid. Thereby, the compression capacity of the present vacuum pump is increased as well. The flow of the delivered fluid has a low pulsation level. Due to the few movable parts and the simple structure, the present vacuum pump can be manufactured inexpensively and requires only a small extent of maintenance. 
   In  FIG. 4 , a second embodiment of a double-lead side channel pump  70  is illustrated, where four steps  72 ,  73 ,  74 ,  75  with pump channels  80 – 83 ,  80 ′– 83 ′ of different diameters are provided. Each step  72 – 75  comprises two parallel pump channels  80 ,  80 ′;  81 ,  81 ′;  82 ,  82 ′;  83 ,  83 ′, by which the suction capacity of the pump  70  is doubled in comparison with single-lead pumps. A rotor  86  as well as the a stator housing wall  88  are configured so as to be stepped such that the radius of the pump channels  80 – 83  respectively decreases to the pressure side  13  from step to step, whereas the cross-sectional area of the pump channels  80  B  83 ,  80 ′– 83 ′ respectively remains the same. The height of each radial step  90 ,  91 ,  92  amounts to about one third of the radial height of a pump channel  80 – 83 ,  80 ′– 83 ′. By limiting the height of the radial step to half of the radial pump channel height at maximum, the screw thread-like course of the pump channel is largely preserved in the region of the radial steps  90 – 92  as well. In this way, it is ensured that the helical fluid flow is substantially undisturbed. Moreover, a considerable pressure loss in the region of the radial steps  90 – 92  is avoided. Owing to the reduction of the pump channel radius towards the pressure side  13 , the friction losses between the rotor  86  and the stator housing wall  88  are reduced. 
   In  FIG. 5 , a third embodiment of a side channel pump  100  is illustrated where a rotor  102  as well as a housing wall inside  104  of a stator  106  are configured so as to conically taper from the suction side  11  to the pressure side  13 . The rotor  102  comprises two pump channels  110  and  111  arranged next to each other on the rotor outside in a helical manner. The radial height of the two parallel pump channels  110 ,  111  is constant over the entire length of the pump channels  110 ,  111 . By the tapering the rotor  102  and the stator  106  towards the pressure side, friction between rotor  102  and stator  106  is reduced. 
   In a fourth embodiment of a side channel pump  120  illustrated in  FIG. 6 , an inside  122  of a stator housing wall  124  has a cylindrical configuration. An envelope formed by a rotor  125 , which envelope is defined by outer ends of the channel walls  126 , is cylindrical as well. The radial height as well as the axial width of the pump channels  128 ,  128 ′ continuously decrease from the suction side  11  towards the pressure side  13  so that the slope of the pump channels  128 ,  128 ′ decreases towards the pressure side. Due to the continuous reduction of the pump channel cross-section towards the pressure side  13 , the pump channel length can be considerably extended, with the axial rotor length remaining constant, to enable a more compact design. The reduction of the pump channel cross-section towards the pressure side  13  is effected approximately analogously to the increase in pressure of the fluid in the two pump channels  128 ,  128 ′. Thus, it is taken into consideration that the fluid needs less and less space due to the continuous compression in the pump channels  128 ,  128 ′ towards the pressure side  13 . 
   In a fifth embodiment of a pump  140  illustrated in  FIG. 7 , three pump channel ducts  142 ,  144 ,  146  are arranged in a meander-like manner and so as to be nested into each other. Thus, the axial length of rotor  148  can be considerably reduced. In the central pump channel duct  144 , wings  150  are arranged in the pressure-side and radially inner quarter of the pump channel cross-section. Thereby, a helical fluid flow is also generated in the pump channel  152  of the central pump channel duct  144 . 
   In  FIGS. 8 and 9 , a sixth embodiment of a pump  170  being side channel pump is illustrated where a pump channel  172  is arranged spirally on an end face of a rotor  174  in a cross-sectional plane of the rotor  174 . The pump channel  172  is radially defined by a channel wall  176  arranged spirally on rotor body  178 , extending over five windings. The channel wall  176  and the pump channel  172  preferably follow a logarithmic spiral. In the illustrated pump  170 , a fluid inlet  180  at the suction side  11  is located at the outer circumference of the rotor  174 , and a fluid outlet  182  at the pressure side  13  is located in the center of the rotor  174 . In the pump channel  172 , blades  184  in the form of a segment of a circle of 90° are arranged at the inner channel wall side. The pump channel  172  defined by the channel wall  176  and the rotor body  178  is axially defined by a substantially disk-shaped stator housing  171 . The compression of the fluid in the pump channel  172  is effected in the same manner as in the afore-described side channel pumps of  FIGS. 1–7 . 
   In a seventh embodiment of a side channel pump  200  illustrated in  FIG. 10 , two helical pump channels  204 ,  204 ′ are combined with a spiral pump channel  206  annexed thereto on a single rotor  202 . 
   In  FIGS. 11–14 , two exemplary arrangements for providing fluid cooling are illustrated. In each exemplary arrangement, fluid is led out of the respective pump channel, cooled in a cooling channel and finally supplied to the pump channel again. 
   A first embodiment incorporating fluid cooling in a side channel pump  220  is illustrated in  FIGS. 11 and 12 . The pump  220  includes two parallel pump channels  222 ,  222 ′. A fixed strip-shaped scraper  224  disposed on a cylindrical stator wall  232  protrudes radially into the two parallel pump channels  222 ,  222 ′. The scraper  224  has an axial length approximately corresponding to an axial width of a channel and approximately protrudes to half the radial height of the pump channels  222 ,  222 ′ to blades  226  into the pump channel  222 . In the region of the scraper  224 , a channel wall  228  is limited to the radial height of the blades  226  so that it does not collide with the scraper  224 . By the scraper  224 , about half of the delivered fluid is led out of the pump channels  222 ,  222 ′ and led into a cooling channel  230  of a cooling device  223 . The cooling channel  230  extends about the cylindrical stator wall  232  and is, in turn, surrounded by a cooling agent channel  234 . In the cooling agent channel  234 , a cooling agent flows by which the cooling channel  230  and the fluid flowing therein are cooled. The cooling channel  230  and the cooling agent channel  234  extend annularly about the stator housing wall  232 . At the rear side of the scraper  224 , the cooled fluid coming from the cooling channel  230  flows into pump channels  225 ,  225 ′ again. By the cooling device  223 , about half of the fluid from the pump channels  222 ,  222 ′ is led into the cooling channel  230 . The other half of the fluid in the region of the blades  226  passes the scraper  224  and thus the cooling device  223  in a non-cooled manner. While only about half of the fluid is cooled, advantageously the helical fluid flow in the pump channels  222 ,  222 ′,  225 ,  225 ′ is only insignificantly disturbed. 
   In a second embodiment of a side channel pump  240  illustrated in  FIGS. 13 and 14  that incorporates fluid cooling, a scraper  242  of a cooling device  244  radially protrudes beyond the complete radial height of pump channels  248 ,  248 ′ into a rotor  246 . The scraper  242  protrudes into a circumferential annular groove  243  of the rotor  246 . Thus, the entire fluid flow from the pump channels  248 ,  248 ′ is branched off into a cooling channel  250  and cooled there. The cooling channel  250 , in turn, is surrounded by a cooling agent channel  252 . In order to reduce pulsations of the fluid flow, a two-part guide ring  254   1 ,  254   2  protrudes into the annular groove  243 . The guide ring  254   1 ,  254   2  consists of two half rings  254   1 ,  254   2  and is configured so as to extend helically in the same direction as channel walls  256 . In this arrangement, the fluid flow can gradually flow out of the pump channels  248 ,  248 ′ before impinging onto the scraper  242 , before it is deflected into the cooling channel  250  by the scraper  242 . After the fluid has passed the cooling channel  250 , it is supplied to pump channels  249 ,  249 ′ again along the guide ring  254   2 . Thus, the entire fluid flow is led out of the pump channels  248 ,  248 ′, cooled and introduced into the following pump channels  249 ,  249 ′ again, without the occurrence of strong pulsations. Thus, a fluid intermediate cooling can be realized that causes only minor pressure losses. 
   In addition or as an alternative to the afore-described fluid cooling, the stator housing can be cooled by a cooling device. To this end, the stator housing can be surrounded, over its entire circumference and its entire length, by one or several cooling channels in which a cooling liquid, a cooling gas or another cooling agent flows around the stator housing. 
   Through the fluid cooling, the fluid compression approaches an isothermal compression, whereby, in turn, the required rotor power is reduced. 
   The invention has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.