Impellers
Two types of impellers are commonly used in industrial fermentation processes. Propeller agitators resemble marine propellers, with the main difference being that in industrial use, the vessel remains stationary while the fluid moves. Axial impellers are similar to radial ones, except that the blades are pitched, generally at a 45 degree angle. This causes flow to move downward parallel to the shaft, and then up along the wall of the tank. They are generally installed vertically in a tank, so as to allow for the fluid to circulate in one direction along the axis, while going in the opposite direction along the walls. Propellers are generally only employed in small-scale applications where their flexibility is desirable. They are often used to disperse gases or non-wetting solids in liquids due to the deep vortex they are able to create. They generally have diameters of not greater than 1.5 meters, and are characterized by their high rotational speed. They may range in power from those of laboratory size up to 50 kW. This type of impeller also serves as a means to achieve bulk mixing and produce a homologous sample. The accompanying figure shows both a picture and the circulation pattern associated with this type of impeller.
The second type is a turbine impeller. Turbine impellers are mounted on shafts like propellers, but are usually much larger, and rotate at slower speeds. Turbines, because they are available in many impeller designs, are more flexible and efficient than propellers in several ways. Radial-flow impellers are similar to centrifugal pumps, in that they discharge liquids at high velocities in the radial direction. This acts like a jet mixer, causing entrainment of the surrounding fluid, while setting up two circulation systems. One of these is above the impeller, the other below. Liquid flowing outward separates at the wall, with some flowing up to the surface and returning to the eye of the impeller along the shaft. The remainder flows down along the wall, across the bottom of the tank, and returns back to the center of the impeller. Practical turbine impellers are limited to approximately 5 meters in diameter, translating to multiple tanks or multiple agitators necessary for larger plant capacities. In Fermentation plants, these impellers serve the purpose of creating mass transfer in order to break up oxygen bubbles. The high shear associated with the impeller necessitates a power demand 10-15 times that of the propeller-type impeller. The accompanying figure again shows a picture and the circulation pattern associated with this type of impeller.
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last updated April 22, 1997