A centrifugal fan includes a scroll casing having first and second flat base walls, and a circumferential side wall. The present invention may be incorporated into a wide range of microelectronic device manufacturing processes and equipment types for which an array of more forceful, more focused process streams are desired for treating microelectronic workpieces. The sections may be made in different shapes but are capable of being connected end-to-end to define a variety of patterns, either as an endless or open track.