Baffles are internals, generally flat plates, used in agitated vessels to optimize and stabilize the mixing flow pattern and minimize variation in agitator power draw. Baffle recommendations are part of the agitator vendor’s scope although the baffles are designed and fabricated by others. Proper baffle implementation dramatically impacts process results.
Low viscosity fluids in agitated vessels without baffles swirl and have surface vortices with little top-to-bottom vessel turnover. Velocity gradients are minimal. Particle tracing within a horizontal plane shows circular motion almost like the horses on a carousel — rotation but not interaction. Particle traces in a vertical plane show minimal motion, a poor configuration for blending or solid suspension.
As Figure 1 illustrates, baffles provide advantages with such fluids. Baffles establish an axial flow pattern, minimizing the tangential or swirl component imparted by the rotation of mixing impellers. The baffled flow pattern facilitates top-to-bottom bulk motion, increasing the velocity across heat transfer surfaces and facilitating blending and solid suspension. However, top entry on-center-mounted agitators on a properly baffled vessel draw more power than on an unbaffled vessel because the impeller pumps more fluid in a given amount of time.
In mass transfer reactions, where power draw is a critical parameter, proper baffling increases impeller power draw and improves blending, which increases the mass transfer capabilities of the mixer.
For vertical cylindrical vessels, “standard” baffles — defined as four flat plates of 1/12 vessel diameter, installed radially along the vessel straight side and spaced at 90° — are common for top entry agitators mounted on center. They are recommended based upon “standard” assumptions about the agitator, vessel, fluids and mixing requirements. Many processes frequently deviate from these assumptions! We will discuss frequently encountered deviations.
A common misconception is that the number of baffles must equal the number of impeller blades. This arose from the change in baffle recommendations that occurred when high efficiency impellers came into the market, replacing many pitched blade turbines. More accurately, the optimum number of baffles is a function of the ability of an impeller to generate axial flow in the process fluids.
When processing low viscosity fluids with high efficiency axial flow impellers, performance differences between three and four equally spaced baffles, are barely discernable. In these fluids, four bladed pitched blade turbines and six bladed radial flow impellers require four baffles.
Practical considerations or vessel internals often rule out spacing baffles equidistant around the vessel circumference. Redistributing baffles within a few degrees of equal spacing is fine. However, removing a baffle without respacing the remaining baffles is problematic. In other words, it’s much better to have three baffles at 0°, 120° and 240° than three at 0°, 90° and 180°.
When you must reduce the number of baffles, adjustment of baffle width can maintain power draw and facilitate an axial flow pattern. For a low viscosity fluid, an installation with two baffles at widths of 0.1 × vessel diameter will draw approximately the same power as three baffles at 0.062 × vessel diameter.
As seen in Figure 1, off-center mounting in an unbaffled vertical cylindrical vessel will create an axial flow pattern and less swirl than an on-center mounting in the unbaffled vessel. For small vessels, an angle mounted agitator, often called a portable agitator, may make sense. These asymmetric options usually are impractical in large mixers from a mechanical design point of view.