Axial compressors bearings

The bearings used in axial compressors are the same journal and thrust type used in the centrifugal compressor. Refer to Chapter 5 for a complete description of these bearings. 

F(ir axial compressors, the journal bearings are of the plain sleeve type for the larger, slower speed compressors. They are of the tilting pad type for the smaller, higher speed machines. The sleeve bearing is normally housed io a spherically seated carrier. The bearings require pressure lubrication as do most of the other compressors. 

This definition of efficiency does not consider the work done on the shaft driving the compressor. To bring this in we should also consider the mechanical efficiency which takes into account the friction in the bearings, or between the rotors and the outer casing (if any). In the case of centrifugal and axial compressors, this is very high, almost certainly over 98%. We can then say that... 

Like centrifugal compressor, axial compressor needs shafr seal to reduce the gas leakage. It uses balance drum to reduce the axial hydraulic imbalance and thrust bearing for the remaining axial imbalance. Journal bearings are used for the radial hydraulic imbalance. 

The axial flow compressors in aero gas turbines are heavily loaded. The aspecl ratio of the blades, especially the first few stages, can be as high as 4.0, and the effecl of streamhne curvature is substantial. The streamline configuration is a function of the annular passage area, the camber and thickness distribution of the blade, and the flow angles at the inlet and outlet of the blades. The shafts on these units are supported on antifriction bearings (roller or ball bearings). 

Monitoring should include the usual parameters worthy of surveillance in high-speed turbo machinery the temperature of journal bearings, vibration and axial position of the pinions, inlet and discharge temperatures combined with discharge pressure from the individual compressor stages, and various lube oil system devices. Competent manufacturers make sure all measurement locations are completely prewired on the machine and made available at predefined interfaces or in terminal boxes for connection at the plant site. 

Centrifugal compressor impellers, with the exception of the open impeller, are thrust unbalanced. The machine also has a requirement for a location device to maintain axial clearances. For these reasons, all centrifugal compressors use some form of thrust bearing. 

Analysis parameters should be established to monitor the key indices of the compressor s dynamics and failure modes. These indices should include bearings, gear mesh, rotor passing frequencies, and mnning speed. However, because of its sensitivity to process instability and the normal tendency to thmst, the most critical monitoring parameter is axial movement of the male and female rotors. 

First, one must know the locations of the shaft currents. Brushes are then positioned so that the current can travel the path of least resistance to leave the unit without any sparking. They can be placed either axially or radially, as shown in Fig. 2. Axial brushes are effective when shaft currents are generated at the extreme end of a turbomachinery set, such as at the end of a compressor-turbine train. Radial brushes can be used in a turbine where the shaft is projected between the casing and bearing housing, and, if possible, should be mounted on the shaft inside the bearing housing. 

---