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Abstract

Centrifugal compressors with oil lubricated bearings are widely used to pressurise the refrigerant in large air-conditioning chiller systems. If the bearings, were lubricated with liquid refrigerant, the friction losses would be reduced and the oil system could be eliminated. This would increase the COP. Also, the absence of oil would increase the heat exchanger effectiveness and reduce manufacturing costs.

This thesis describes work carried out to develop a centrifugal compressor with refrigerant lubricated fluid film bearings of either:

i) The externally pressurised hydrostatic type or

ii) The simple hydrodynamic type, which do not need a high-pressure external pump.

The low refrigerant viscosity makes flow through both types of bearing turbulent.

The hydrostatic bearing was analysed using a computer program based on a 2-D Navier-Stokes turbulent model. Analysis showed that bearing load, dynamic properties and compressor vibrations are a strong function of supply pressure, radial clearance and orifice diameter. Vibration analysis carried out on the rotor-bearing system, predicted the onset of instability at supply pressures of less than 2.1 MPa.

An experimental programme was carried out on a centrifugal compressor to confirm this. Bearing instability was detected even at pressures as high as 2.1 MPa but with acceptable levels of vibration amplitude. At lower supply pressures, the vibration amplitudes were unacceptable.

A computer code was used to analyse hydrodynamic bearings, assuming two- dimensional flow and a simple zero-equation turbulence model. This showed that turbulence has a significant effect on the load carrying capacity and dynamic properties. A CFD code was used for a more detailed 3-D analysis to determine the effect of three different turbulence models on the pressure profile within the clearance. Nearly equal pressure profiles were predicted in all cases, but the peak pressures were 13% higher than the values obtained from the 2-D model. The predicted bearing size could thus be reduced.

Publication Type:	Thesis (Doctoral)
Subjects:	T Technology > TJ Mechanical engineering and machinery
Departments:	School of Science & Technology > Engineering > Mechanical Engineering & Aeronautics School of Science & Technology > School of Science & Technology Doctoral Theses Doctoral Theses

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