The concept of baffle and its application in agitators
Good mixing by mixers in the tank occurs when vortexes and flow rotations are minimized and the flow pattern is parallel to the axis of rotation. Baffles are vertical plates with a very small thickness, which, when installed on the tank wall, minimize vortexes and make the flow pattern more uniform.
Tank picture with baffles
In the absence of baffles, the flow created by impellers is two-dimensional and with by rotation and vortex. Baffles convert tangential and swirling flow into vertical flows. This creates a thorough mixing in the tank without swirling and with minimization of bubbles. Baffles also increase the efficiency of impellers by creating uniform mixing.
Flow pattern in tank without baffle
Flow pattern in tank with baffle
The existence of baffles is more necessary in low-viscosity fluids, because in viscous fluids, the resistance of the fluid automatically makes the flow pattern uniform and reduces vortexes. The number of baffles is usually 4 in standard mode and at an angle of 90 degrees, and more than that does not make a noticeable change in the flow pattern. Baffles in low viscosity fluids have a width of about 1/12 of the tank diameter. In fluids with higher viscosity, this ratio gradually decreases.
Ratio of the baffle width to the tank diameter against the increase in viscosity
Baffles are usually installed at a distance from the wall, which is approximately 1/6 of the width of the baffle. If the baffles are installed too close to the wall, they cause the accumulation and deposition of particles and create a so-called stagnant zone.
In fluids that have low viscosity (range between 1 and 500 centipoise), baffles can be installed too close to the wall. In fluids with high viscosity (10000 centipoise and above), the minimum distance between the baffle and the wall should be 25 mm.
In square or rectangular tanks, the corners of the tank break the tangential flow pattern and thus create the effect of a baffle, and in this case, a baffle may not be needed.
Baffles are also not used for side mixers in large tanks and angle mixers in small tanks.
In tanks where helical coils are installed for heat exchange, these coils have a small baffle effect, and therefore baffles are needed in these tanks. In these tanks, the coils are 1.5 to 2 times the diameter of the coil apart, and the baffles are mounted between the tank wall and the coils. In this case, the baffles are also used to support the coils.
