Agitation System Design

Typical agitation equipment consists of the prime mover (usually an electric motor) coupled to the shaft through a reduction gear. Impellers and baffles are fitted to the shaft and vessel, respectively, to give the desired liquid motion. The shaft may enter from the top, side or bottom, and is usually fitted with a mechanical seal at the vessel wall. The number and location of impeller units depends on the vessel. The general practice is to use one impeller for each diameter of depth. The type of impeller is more critical and different designs produce different fluid motions.

In a smooth walled tank, the liquid swirls round in the same general direction as the agitator. As the impeller speed is increased, a vortex is formed and the liquid level at the wall is raised above the average liquid level. This is normally undesirable for the following reasons:

1. Power is wasted in holding up the liquid at the wall

2. The relative speed of the impeller to the liquid is reduced

3. Slight radial movements of the vortex cause the liquid to swirl unevenly, and undesirable side thrusts are set up

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