Minimum suspension stirring speed

The cells should all attach within 4-6 h after which a further 150 ml prewarmed and pregassed growth medium should be added and the culture stirred continuously at 60 r.p.m.. This speed may need to be increased to 80 r.p.m. for the larger or denser micro-carriers but should be the minimum required to maintain the beads in suspension. 

Convective bulk transport is also an extremely important factor in the suspension of solids in a stirred tank (this is also responsible for the flow pattern at the tank bottom). P/V cannot be used as a scale-up criterion in this process either. Measurements have shown that the minimum rotational speed, ncrit, of the stirrer which is necessary for the suspension (whirling-up) of particles in the turbulent regime is given by the appropriate Froude number ... 

As homogeneous suspension in nonaerated stirred vessels can hardly be achieved, even with very high stirrer speeds, mainly Nc, needed for complete suspension, is of interest for the design purposes. This value, by definition, is characterized by the just-suspended criterion, i.e., the state where only a small fraction of the solids remains at the bottom of the reactor for one second at maximum (Einenkel, 1979). Zwietering (1958) proposed the following correlation to predict Nc, the minimum rotational speed of agitation required for the complete suspension ... 

Place spinner flask on magnetic stirrer and stir at the minimum speed to ensure that all cells and carriers are in suspension (usually 20-30 rpm). It is advantageous if cells and carriers are limited to the lower 60-70% of the culture for this purpose. Alternatively either ... 

Armenante, P.M. Uehara Nagamine, E. Effect of low off-bottom impeller clearance on the minimum agitation speed for complete suspension of solids in stirred tanks. Chem. Eng. Sci. 1998, 53, 1757-1775. 

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