13 Other Components and Variations Available to Purchase

The preceding chapters have considered the basic aerodynamic design and analysis functions relative to typical axial-flow compressor configurations. Blade row, stage and compressor design and analysis have been considered in the context of the compressor flow path from the first blade row to the last, including operation at off-design speeds and mass flow rates. This chapter addresses other design and analysis functions that are often required by the specific application. One obvious consideration for industrial axial-flow compressors is dictated by the need to supply the compressor discharge flow to the process through a discharge pipe, typically exiting in the radial direction. This requires the design and analysis of an exhaust diffuser and flow collection system, which can have significant influence on the overall performance of the compressor. Another important consideration is the use of adjustable inlet guide vanes and stators as a means of improving the surge margin. Normally this provides a substantially better surge margin at mass flows that are significantly less than the design flow than can be achieved with simple variable speed operation. Indeed, variable geometry is an essential requirement for many industrial compressor applications. The influence of surface roughness on performance is discussed to provide a basis for its evaluation when required. Finally, the axial-centrifugal compressor is briefly discussed. This configuration includes a centrifugal compressor stage following a series of typical axial-flow compressor stages. Under appropriate operating conditions, this can offer significant cost and performance benefits.