A common (universal?) feature of industrial metal chimneys/smoke stacks, affixed to the outer surface of their top few metres. Presumably they function in the following manner:
- industrial process generates hot gaseous emissions
- emissions are directed toward the chimney intake
- emissions warm the chimney as they rise within it
- chimney warms the air immediately surrounding it
- this air rises and encounters the spirals
- spirals set up a vortex
- vortex extends ?high above the upper limit of the chimney
- emissions emerging from the top of the chimney encounter this vortex
- chimney becomes functionally equivalent to a much taller spiral-less chimney, delivering the emissions much higher into the atmosphere.