Sealing of stirrer shafts

In closed stirred tanks the rotating shaft must be sealed against the tank lid (or against the tank bottom). The type of seal depends upon whether the gas or liquid space has to be sealed, on the stirrer speed and on the pressure difference Ap between the operating pressure (system pressure) in the tank and the outside pressure (generally atmospheric pressure). 

At high shaft speeds and negligible Ap values labyrinth seals can be utilized, whose narrow openings build up a high dynamic pressure [109]. Lip seals are practicable at Ap values 1 bar, particularly for laboratory devices. They are made of rubber or silicone plastics and often have to be cooled. Generally slip properties have to be maintained with a liquid (usually silicone oil). 

For high pressure differences the usual type of sealing device is the mechanical seal, of which there are a range of designs (internal and external single and double mechanical seals with and without throttle bushing, with or without pressure relief). Some of them can be dismantled and replaced in filled tanks under pres- 

14 Classification of shaft sealing devices with regard to system pressure and the shaft speed 

Typical operating limits for single sliding ring seals are velocities of v 5 m/s and txp 1 bar and for double sliding ring seals Ap 26 bar (in special designs up to 40 bar) [0.14, 526]. 

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The entrainment of gas in the liquid is undesirable in most mixing operations. In addition, the bearings and seals of stirrer shaft are subjected to exceptionally strong mechanical stress, due to the inhomogeneous gas/liquid mixture and the fact that the stirrer partly rotates in air and therefore the liquid bearing is absent. It is therefore of practical interest to know how a stirrer can be operated in an unbaffied vessel in such a way that vortex formation can be tolerated. 

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