FIELD OF THE INVENTION The present invention relates to turbomachinery, and more specifically, to the design of compressors for the cooling systems for combustion turbines. BACKGROUND OF THE INVENTION

It is known to bleed compressed air from a compressor, cool the air and use it to cool the turbine section of a combustion turbine. Several embodiments of such systems are set forth in U.S. Patent No. 5, 255, 505--Cloyd et al. , which is assigned to the assignee of the present invention and is incorporated herein by reference as if set forth in its entirety.

Increases in combustion turbine performance can be created using closed-loop air cooling systems. Additionally, combustion turbines with ultra-low NO _x emission combustion systems require nearly all of the compressor air to be pre- mixed with fuel. In either of these systems, it is necessary to extract a portion of the compressor delivery air, cool it, compress it in an outside compressor. The compressed air is then re-introduced into the combustion turbine at a point where it is used for cooling. However, the addition of an outside compressor results in increased system complexity and cost, and also creates the possibility of shutdowns due to a malfunction of either the auxiliary compressor or its controls.

Providing a supplemental centrifugal compressor at the inlet to the turbine blades and adjacent the exit of the combustion chamber is disclosed in U.S. Patent No. 3,936,215-- Hoff. This patent discloses that the compressor exit air loses sufficient pressure such that it cannot be effectively introduced into the turbine section at the point where the combustion chamber exhausts hot gases. It is at this point that the temperature is highest and cooling is needed; therefore, a small centrifugal compressor is interposed at the point between the exhaust of the combustion chamber and the inlet to the turbine section. The centrifugal compressor is provided with air from the compressor, that, due to losses, is at a lower pressure than would be effective for cooling.

Thus, although providing an external centrifuged compressor to increase the pressure of the compressor exhaust is known, such a system suffers from the aforementioned complexity problems. Prior art compressors of the centrifugal type that are internal to the turbine do not provide the same flow parameters and heat exchange that would be required for closed-loop air cooling systems or for combustion turbines with ultra-low NO _x emission combustion systems that require nearly all of the compressor air to be pre-mixed with fuel . Therefore, there exists a long-felt and as of yet unsolved need for methods and apparatus whereby additional pressure can be created in the cooling air exiting form the compressor.

SUMMARY OF THE INVENTION It has now been found, however, that the deficiencies of the prior art can be overcome by an auxiliary compressor that supplies the required amount of increased pressure air for cooling hot end components, either directly or by extracting this air from the combustor cylinder, cooling it, and then reintroducing the air to the components to be cooled. In a preferred embodiment, a portion of compressor delivery air is diverted at the compressor exit inner diameter and then compressed in one or more reduced height axial compressor stages located below the inner wall of the main compressor exit diffuser that is itself located adjacent and

downstream from the auxiliary compressor. The compressed air is then ducted to cool combustor and turbine components, or taken out of the cylinder through conduits. Those of skill in the art will be familiar with the use of such conduits and their use to cool combustion turbines.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a partially schematic, partially diagrammatic view of a portion of a combustion turbine and associated components incorporating the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG 1, the general layout and operation of the components of preferred embodiments of the present invention are illustrated. Those of skill in the art will be familiar with the construction and operation of the equipment used in combustion turbines. In order to more clearly illustrate the present invention, miscellaneous equipment is not shown, while certain components of the combustion turbine are shown diagrammatically, in a broken away cross-section. As seen in FIG. 1, a typical system includes a combustion turbine and compressor 10 that has rows of compressor blades. The last row of compressor blades 12 is illustrated in FIG. 1. Immediately downstream of the last row compressor blade 12 is a set of last row stators and outlet guide vanes 20. After passing through the stators and guide vanes 20, the air flow passes through a main compressor exit diffuser 30. However, in accordance with the present invention, the main airflow is partitioned or otherwise split into a main section 32 and an auxiliary air flow 34 by an inner wall or partition 36. As shown by the arrows in FIG. 1, the auxiliary air flow 34 is directed through an auxiliary air compressor 50. The auxiliary air compressor 50 is connected to and driven by the main shaft 11 of the combustion turbine 10, and is spaced from the last row compressor blades 12 by a spacer disc 21. The auxiliary compressor 50 is comprised of one or more auxiliary compressor blades 52, that are most preferably of reduced height as compared to the

compressor blades and located adjacent and below the inner wall 36 of the main compressor exit diffuser 30. As the auxiliary air flow 34 enters the blades, is compressed, and then exits via the auxiliary exit diffuser 34. The auxiliary compressor 50 supplies air at increased pressure that is used for cooling hot components, either directly or by extracting this air from the combustor cylinder, cooling it, and then reintroducing the air to the components to be cooled.

The compressed air is thus either then ducted directly to cool combustor and other combustion turbine components, or is taken out of the cylinder through conduits. Those of skill in the art will be familiar with the use of such conduits and the cooling of combustion turbines. The on¬ board auxiliary compressor efficiency provided by the present invention is similar to that of an outside compressor used for the same application.

In addition to the foregoing, it will be realized that the present invention also provides improvements in methods of cooling a combustion turbines. As explained above, in prior art systems the air exiting the compressor section was bled off from the compressor shell, or the combustion chamber, and was cooled as shown in U.S. Patent No. 5,255,505, which is incorporated herein by reference. However, in accordance with the present invention, improvements in efficiency and reliability are created by first dividing the compressor outlet stream into a major portion and a minor portion and then routing the minor portion of the compressor outlet stream into an auxiliary compressor. In accordance with preferred embodiments of the present invention, the compressor outlet stream is divided by providing a partition across a main compressor exit diffuser.

The auxiliary compressor of the present invention is most preferably disposed on-board and the minor portion of the compressor outlet stream is compressed into an auxiliary compressor outlet stream, preferably by flowing the minor portion of the compressor outlet stream over one or more auxiliary axial compressor blades. In preferred embodiments,

the compressor is driven by a main shaft and the auxiliary axial compressor blades are also connected to and rotated by the central shaft 11. The auxiliary compressor outlet stream may be ducted directly to one or more of a combustor and other combustion turbine components, or it may be transferred to one or more conduits and carried to a heat exchanger. In either case, the auxiliary compressor outlet stream is then cooled and reintroduced to the turbine.

Finally, the function of the axial on-board auxiliary compressor described above can also be accomplished by either a centrifugal or mixed flow compressor. In accordance with these embodiments of the present invention, the compressor is again most preferably located under and adjacent the main compressor exit diffuser. Although certain embodiments of the present invention have been set forth herein and described with particularity, these descriptions are meant to illustrate the nature of the present invention and are not limitative. Upon review of the foregoing specification, various modifications, adaptations and improvements to the methods and apparatus disclosed will immediately present themselves to those of skill in the art. Such modifications, adaptations and improvements will not depart from the spirit of the present invention and thus, in order to apprehend the full scope of the present invention, reference should be made to the appended claims.