Abstract:
A multistage turbocompressor is equipped with a tapping point between two compressor stages. The guide-blade cascade of the stage arranged directly upstream or downstream of the tapping point is designed with adjustable geometry. The pressure at the tapping point can be set within wide limits, irrespective of the compressor operating point, by an adjustment of this guide-blade cascade.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a multistage turbocompressor which, when in operation, conveys a medium from an inlet pressure to an outlet pressure, including, in each case, a plurality of guide-blade cascades and moving-blade cascades which are arranged alternately in the conveying direction, in such a way that, in each case, a moving-blade cascade forms, with a guide-blade cascade following it in the conveying direction, a compressor stage, the moving-blade cascades being arranged on a rotor and the guide-blade cascades on a stator, said turbocompressor having at least one tapping point for at least one further pressure which is between the inlet pressure and the outlet pressure, at which tapping point a part stream of a partly compressed medium is extracted, to a extracted. The invention also relates method for operating a turbocompressor. 
     A highly efficient method for regulating turbocompressors involves adjusting the guide-blade cascade of one or more stages (Dubbel, Taschenbuch fur den Maschinenbau [Mechanical Engineering Manual], 14 th  edition 1981 pp 877; Traupel, Thermische Turbomaschinen [Thermal Turbomachines], 3 rd  Ed. 1982, Vol. 2, pp 111). This regulating method has been employed for a long time, particularly in the case of machines which have to cover a large rotational speed range: in aircraft engines, it is perfectly possible for half the compressor stages to be equipped with adjustable guide-blade cascades. Even with regard to stationary gas turbines of the latest generation, development tends toward designing a plurality of compressor stages with adjustable geometry, as described, for example, in ABB Technology 2/1994, pp 4. 
     Multistage turbocompressors, precisely those used in gas turbines, have tapping points for extracting compressor air at one or more pressures, the tapping pressure being between the inlet pressure and the outlet pressure. The auxiliary air required at the tapping points is usually needed at a very high pressure level, for example in order to cool the turbine components. The tapping points are therefore mostly located in the rear stages of a compressor. The tapping pressure is thus predetermined essentially by the compressor operating pressure. On the one hand, it is invariable in the case of a specific compressor outlet pressure, and, on the other hand, it varies directly with the operating point of the compressor. 
     With the turbocompressors customary at the present time and the methods for operating them, it is therefore not possible to provide a defined tapping pressure, irrespective of the operating point of the turbocompressor. 
     SUMMARY OF THE INVENTION 
     The invention, then, intends to remedy this. The object of the invention, therefore, in a multistage turbocompressor which, when in operation, conveys a medium from an inlet pressure to an outlet pressure, including, in each case, a plurality of guide-blade cascades and moving-blade cascades which are arranged alternately in the conveying direction, in such a way that, in each case, a moving-blade cascade forms, with a guide-blade cascade following it in the conveying direction, a compressor stage, the moving-blade cascades being arranged on a rotor and the guide-blade cascades in a stator, said turbocompressor having at least one tapping point for at least one further pressure which is between the inlet pressure and the outlet pressure, at which tapping point a part stream of a partly compressed medium is extracted, is to provide a possibility for setting the tapping pressure within a wide range, irrespective of the operating point of the compressor. 
     This is achieved, according to the invention, in that the guide-blade cascade of the stage located immediately upstream of the tapping point and/or of at least one stage arranged downstream of the tapping point is designed with adjustable geometry. 
     As mentioned initially, in turbocompressors operating over a relatively wide rotational speed range, for adapting the stage kinematics it is perfectly normal to design a relatively large number of moving-blade cascades so as to be adjustable. Here, however, the adjustment of the row of guide blades in connection with the invention serves for controlling the pressure at a specific point in the compressor, irrespective of the compressor operating point. It is therefore expedient and desirable to use the subject of the invention even in compressors which are provided for operating at a constant rotational speed. 
     By virtue of this design, it is possible for the compressor to have an operating mode in which the work output of the compressor is shifted into the front or the rear compressor stages as a function of the requirements of the tapping point and of the operating point of the compressor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows by way of example a five-stage axial compressor with an intermediate tapping point after the third compressor stage. 
     FIG. 2 illustrates the pressure profile in this compressor at two operating points in the design and operating mode according to the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 illustrates by way of example an axial compressor which conveys a medium  25 ,  26  in the direction of the arrow from a pressure p=p 0  to a pressure p=p 5 . The compressor moving-blade cascades LA 1 , LA 2 , LA 3 , LA 4 , LA 5  are arranged on a shaft  11  of the compressor and rotate together with the shaft about a machine axis  10 . Each moving-blade cascade is followed in the direction of flow by a guide-blade cascade LE 1 , LE 2 , LE 3 , LE 4 , LE 5  which is fastened in the housing  12 . Each of the compressor moving-blade cascades LA 1 , LA 2 , LA 3 , LA 4 , and LA 5 , and each immediately adjacent (in the direction of flow) corresponding guide-blade cascades LE 1 , LE 2 , LE 3 , LE 4 , and LE 5 , respectively, constitute a compressor stage  16 . The compressor illustrated is equipped, furthermore, with an adjustable preguide-blade row VLE which determines essentially the inlet volume flow of the compressor. 
     The five-stage axial compressor illustrated should in no way be interpreted here in a restrictive sense, specifically, in particular, not in terms of the number of stages and the design. An axial compressor as an integral part of a gas turbine plant has, in general, many more stages. On the other hand, the idea of the invention can also be implemented, for example, in radial compressors or in compressors which have both axial and radial stages or stages through which the flow passes diagonally. Nor is the preguide-blade row any more mandatory. 
     The operating point of the compressor, in particular the pressure p 5 , is predetermined by a consumer arranged downstream of the last compressor stage  16  and of the level  5 , whilst the pressure p 0  corresponds essentially to ambient pressure. If the compressor stages have a given geometry, the pressure at the levels  1 ,  2 ,  3 ,  4  within the compressor is determined by the inlet and outlet pressure and the volume flow. 
     Furthermore, in the compressor illustrated, a tapping point  14  is provided in the housing  12 , a quantity of partly compressed medium  14  being extracted through said tapping point downstream of the third compressor stage LA 3 , LE 3 . This may be, for example, cooling air which, in a gas turbine plant, is supplied at a point of suitable pressure to the flow in the thermally highly loaded turbine. 
     The unbroken line in FIG. 2 shows diagrammatically the pressure profile in the direction of flow of the compressor at the design operating point, on the assumption that all the compressor stages have an identical pressure ratio. 
     If the compressor outlet pressure falls, then, the pressure at any of the intermediate levels of the compressor likewise falls, of course, that is to say also the pressure at the tapping point  14  at level  3 . If this pressure is to be kept constant, the compressor characteristic map must be changed for this purpose, and this may be carried out, in particular, by changing the compressor stage geometry. As mentioned initially, adjustable compressor guide-blade cascades are known. As likewise mentioned, their use is restricted to the front compressor stages, where they serve for adapting the characteristic map of the compressor to that of the consumer, or as a preguide-blade row for controlling the inlet volume flow. 
     In a turbocompressor according to the invention, then, guide-blade cascades immediately adjacent to the tapping point are designed adjustably, in order to act as directly as possible on the available tapping pressure. In the exemplary embodiment, for example, the guide blades LE 3  will be assumed to be adjustable, and, with a falling compressor outlet pressure, the geometry of the guide-blade cascade can then be changed in such a way that the enthalpy rate, and therefore the pressure build-up, in the third compressor stage rises, as compared with the other stages. This results in the pressure profile depicted by a broken line: when the compressor outlet pressure falls, the tapping pressure at level  3  can be kept constant. 
     The adjustment of the guide-blade row LE 3  may, of course, also be used to reduce the pressure build-up in the third compressor stage in the event of an increase in the compressor outlet pressure and to keep the tapping pressure constant. The adjustment of the guide-blade cascade may likewise also be utilized in order, when the compressor operates under conditions which are constant overall, to adapt the pressure at the tapping point  14  to the variable requirements of a consumer connected there. 
     It is, of course, also possible to carry out pressure adaption by adjusting the guide-blade cascade of the compressor stage located immediately downstream of the tapping point, LE 4  in the exemplary embodiment.