|Retreat curve agitator (Credit: Southern Glass)|
For a little tutorial on the retreat curve impeller, I refer us to "Understand Flow Patterns in Glass-Lined Reactors", an article by Dickey et al. in the November 2004 issue of Chemical Engineering Professional:
Glass-lined reactors are essential process equipment in the pharmaceutical and speciality chemicals industries. A typical glass-lined reactor included a retreat curve impeller (RCI) near the bottom of the vessel and usually a single baffle mounted through a nozzle in the vessel head. The RCI with rounded blade corners may limit harmful turbulence effects while maintaining circulation throughout the vessel. Glass lining (the term "lining" is used to refer to the glass coating on the agitator and the inside of the tank) provides corrosion resistance, is easy to clean, and eliminates product contamination.
The retreat curve of the RCI blades provids better radial flow than radial flow impellers with similar power characteristics. The impeller is placed near the bottom of the vessel to maximize the allowable range of liquid levels and to produce circulation from the bottom to the top of the vessel. The baffle (occasionally two baffles) is mounted from a nozzle in the top head because mounting to the side of a glass-lined vessel is difficult. The impeller and baffle always have a rounded cross-section without sharp corners because high stresses in the glass can cause the brittle coating to fail.There's a lot in those paragraphs that I don't really understand (radial flow? turbulence effects are harmful?), but it's interesting to me nonetheless.