Mixing Operations, by Gordon Young

-- 3: Equipment for Mixing Liquids --

Paddle agitators: (See Brennan Fig 5.1 for diagrams)

• speeds normally in the range of 20 to 50 rpm
• commonly paddles measure half to three quarters the diameter of the vessel with width of the blades one tenth to one sixth of their length
• high radial and rotational components, little vertical flow
• baffling often used to reduce swirling and vortexing
• pitched blades may be used to increase vertical flow
• multivane or gate agitators used for more viscous fluids, and anchor paddles used to just clear vessel walls to promote heat transfer and minimise deposits, counter-rotating multiblade paddles may be used to develop high localised shear
• easy to fabricate
• single blade paddles often used for gentle mixing action with sensitive materials

Turbine agitators: (See Brennan Fig 5.3 for diagrams)

• impeller has more than four blades in same plane of rotation
• generally smaller than paddles, measuring 30 to 50% of the vessel diameter
• commonly rotate at speeds of 30 to 500 rpm
• baffles and pitched blades may be used in a similar way as with paddles
• velocity of the liquid relatively high and currents travel throughout the vessel, with vertical flow often being set up by the deflection of currents from the vessel walls
• vaned disc impellers may be used to disperse gases in liquids
• particularly effective in moderately viscous liquids

Propeller agitators: (See Brennan Fig 5.5 for diagrams)

• short bladed impellers (usually less than 25% of vessel diameter) rotating at high speeds (500 to several thousand rpm)
• produce primarily longitudinal and rotational velocities
• can be effective in quite large vessels with low viscosity liquids, but best not mounted centrally, or inclined at an angle (even horizontally through the wall)
• some typical designs:

Other types of impeller agitator:

• a wide range of specialist impellers may be designed for specific duties
• pumps may also be used for some mixing tasks

Mixing vessels:

• generally circular with rounded bottoms to minimise "dead spots"
• depth to diameter ratio normally 0.5 to 1.5 (1 often recommended). If tall vessels are used, an impeller should be installed for each vessel diameter of height

Mixing of high viscosity pastes and plastic solids

• uniform mixing may not be the only aim; for example, mechanical action is required for dough development
• the general principle is that mixing performance depends on direct contact of the mixing element and the material
• flow in the material is laminar, not turbulent
• the material must be brought to the mixing elements or the mixing elements must travel to all parts of the vessel
• mixing occurs by kneading the material against the vessel wall or against other material, folding unmixed food into the mixed part, and shearing to stretch the material

Pan mixers:

In the rotating pan mixer the mixing vessel is mounted on a rotating turntable. The mixing elements rotate in a fixed position near the pan wall.

Various designs of mixing elements may be used

Horizontal trough mixers (kneaders, dispersers, masticators):

Pairs of heavy horizontal blades rotate in a trough, often following tangential or interlocking paths See http://www.littleford.com/dblarm.html

Blades vary in design, but a common design is the Z-blade or sigma element:

Continuous paste mixers:

Other devices:

In-line mixers:

Dynamic in-line mixers use a combination of pump pressure and a high speed rotating element

Static in-line mixers utilise the movement of materials passing over specially contoured stationary mixing elements located in a tubular housing which serves as part of the pipeline. A variety of forms are used: helices, vanes, or corrugated plates. See Sulzer Chemtech's page for Static Mixers & Heat Exchangers, at http://www.sulzerchemtech.com/n_proserv.htm

Note: Most liquid foods are non-Newtonian - their consistency changes with rate of shear.

• Pseudoplastic materials decrease in consistency with increasing shear rate. For example sauces may form a zone of thinned material around a small agitator so that the bulk of the fluid does not move at all.

• Dilatant materials increase in consistency with increasing shear rate. For example corn-flour mixtures and chocolate need to be mixed with care; if adequate power is not available as the mixture thickens, drive mechanisms, etc., may be damaged.

• Viscoelastic materials exhibit a mixture of viscous and elastic properties including stress relaxation, creep, and recoil. For example bread doughs require a folding and stretching action to shear the material.