A centrifugal impeller includes a body of antistatic plastic and a back plate made of metal, preferably aluminum. In some examples, a surface plot of a measure of wireless-spectrum usage for a geographic region is generated. The present invention also combines OCR results from multiple frames, in a manner that takes the best recognition results from each frame and forms a single result that can be more accurate than the results from any of the individual frames. The outer shell may be double walled with inner and outer walls. Pipe reactor processes of the present invention have a multitude of advantages over prior art processes including improved heat transfer, volume control, agitation and disengagement functions.