an Engineering & Service Bulletin

Technical Topics

J.H. WRIGHT & ASSOCIATES

MIXING OR AGITATION OF SOLIDS IN LIQUIDS

The most common mixing or agitation applications involve the suspension of solids in water or water like liquids. Correctly defining the type or degree of mixing required is critical to mixer selection

Since the horsepower needed for a particular mixing job can vary dramatically, defining the degree or the amount of suspension required is a subject worthy of attention.

Listed below are five criteria which can be used to define the amount of mixing really needed. Use these when writing mixing specifications. Remember that if it takes a 10 HP mixer to achieve complete motion on the tank bottom - it will probably take over 250 HP to achieve complete uniformity - "something you may not really require"

(1) COMPLETE UNIFORMITY

This implies that the percent suspension at any point is 100%. The upper layer of liquid in the tank is the most difficult to bring to 100% suspension. It is difficult to get particles with settling velocities above 6 ft/min suspended uniformly in the upper 2% of the tank volume. Since the primarily horizontal flow pattern at this point cannot keep high-settling velocity solids in suspension.

(2) COMPLETE OFF-BOTTOM SUSPENSION

This is defined as all particles moving up off the tank bottom. It does not have any further requirement for a particular percent suspension at any other point in the tank.

(3) COMPLETE MOTION ON TANK BOTTOM

This means that all particles are either suspended off the tank bottom or are rolling around on the tank bottom. No statement is made concerning the percent uniformity in the vessel.

(4) FILLETING PERMITTED BUT NO PROGRESSIVE FILLET BUILD-UP

A fillet is a stationary or stagnant deposit of solids most commonly at the outside periphery of the bottom where it joins the tank wall, but it could exist at any other part of the tank bottom depending on the fluid flow pattern.

(5) HEIGHT OF SUSPENSION

The liquid height in the tank to which solids are suspended may be used to describe the operation. It is most commonly expressed as the percent solids of each of the various particle size fractions at various liquid heights off bottom. This can also be expressed as the particle size distribution in samples taken at various points.

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