Solids Suspension Measurement of NjS

The measurement (indeed, the existence of) of the just suspension speed is subject to the assumption that there exists a speed where there is equilibrium between the particles lifted from the vessel base and those settling back onto the base. This assumption is reasonable with fast settling particles but becomes more and more dubious as the settling velocity of the particles decreases. 

Different workers, even when working with the same system and using the same suspension criterion, often obtain different values of Njs. Where possible, the entire set of measurements should be made by one person. When this is not possible, care should be taken to ensure that each operator tt-ained in the technique produces consistent results. 

The operator should have access to the impeller speed controller and a digital readout from the position in which he or she makes the measurements. Several repeat measurements should be made by slowly stepping up the impeller speed and observing the vessel base for stagnant particles. Sufficient time should be allowed for steady state to be reached before checking to see if the chosen just suspension criterion has been reached. If not, the impeller speed should be stepped up and the observation repeated. With fast-settling particles in a reasonably small vessel (say T 1.0 m), the time required to reach steady state is 

When making visual Njs measurements, it is important to note not only the speed required to meet the chosen just suspended criterion but also the location of the point of last suspension. Some impeller and vessel geometry combinations tend to produce a point of last suspension in the middle of the base of the vessel other combinations tend to leave a ring of settled solids around the base next to the vessel walls. 

Similar visual techniques can be used to measure the equivalent speed for systems with floating particles, the just drawn-down criterion. The visual observations are subject to exactly the same issues and problems as the just suspended measurements however, access to see the particles on the free surface of the fluid is generally easier to obtain than at the base of the vessel, as no transparent vessel parts are required. 

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