Tank Baffle Design Calculations
As the energy input from a mixer increases, the fluid has a tendency to swirl around the tank, and at high intensity may even draw a vortex above the impeller. These effects are indicative of poor mixing as they show the bulk fluid has begun to flow around the tank in a circular manner and as such there is very little mixing between the different areas of the tank.
Baffles reduce swirling in the bulk fluid, and promote circulation between the top and bottom of the tank ensuring the full liquid volume passes through the high turbulence area of the impeller.
This allows for greater energy input into the fluid effectively reducing the mixing times needed to achieve the desired level of homogeneity
Baffles are normally recommended for mixing applications in cylindrical tanks unless the viscosity of the mixture is very high, or mixing intensity very low. If baffles cannot be installed in the mixing tank, mounting the mixer off centre or at an angle may be an option – our application engineers will be pleased to help.
In rectangular tanks the corners act to baffle the fluid and so baffles are not usually required unless vigorous agitation is needed, and so we will focus mainly on cylindrical tanks below.
Standard baffle configuration utilises 3 or 4 equally spaced vertical baffles T/12 where T is the internal tank diameter. Increasing the size or number of baffles beyond this point does little to increase the effectiveness of the mixing. Baffles are also usually mounted slightly off the tank wall. This helps to prevent dead zones occurring behind the baffles by allowing flow between the edge of the baffles and the tank wall. The recommended off wall clearance is calculated as a function of the baffle width (usually W/5). This standard baffle design works well for general mixing applications in water like fluids, but other process factors can change the optimal baffle design.
Effect of Viscosity on Baffle Design
For mixing fluid of higher viscosity, the size of the baffles can be reduced whilst still maintaining effective mixing. The relationship between viscosity and required baffle width is shown below.
It is important to note that this relationship is only a guide, as it can lead to impractically small baffles with tiny off wall distances for fluids of high viscosity and small mixing vessels.
Plot of W/T vs Viscosity
In solid suspension applications solids can accumulate in dead zones around the base of baffles and can be hard to resuspend. Therefore, in flat bottomed tanks a gap between the base of the baffles and the tank bottom is left.
This is usually sized to be the same as the impeller off bottom distance to allow for a small amount of swirl and ensure that the particles remain moving and suspended at all times.
If entrainment of a low density or hard to wet powder is required, then a gap should be left at the top of the baffles, with the baffles ending below the surface. This, along with an impeller positioned half to one impeller diameter below the liquid surface allows for the formation of a small vortex in the top section of the tank which rapidly draws the solid down into the main bulk of the fluid. The gap between the baffle top and the surface of the fluid is typically ≈300mm.
T – Tank internal diameter
W – Baffle width
C – Baffle off wall clearance
B – Baffle off bottom
Minimum baffle thickness can be calculated from the fluid forces acting on the baffle, and the allowable bending stress for the baffle material of construction. One method shown below assumes that the baffles will have to absorb the total force supplied by the mixer torque.
τ – Mixer torque (Nm)
P – Motor power (kW)
n – Mixer speed (rpm)
The torque figure is then converted into a force applied to the baffles and divided by the number of baffles. The adjustment factor (Df) is used to take into account the distribution of forces along the baffle length, but for a conservative design this can be set to 1 assuming that all of the force imparted by the mixer is concentrated in a single point level with the impeller.
FB – Force on each baffle (N)
Af – Adjustment factor (set to a value of 1 for a conservative design)
NB – Number of baffles
T – Tank Diameter (mm)
Bw – Baffle width (mm)
Bc – Baffle off wall clearance (mm)
Using this value, the baffle thickness can be calculated taking into account the allowable bending stress of the baffle material and mounting arrangements, assuming that the force is applied half way between two baffle supports.
Bt – Baffle thickness (mm)
L – Length between baffle supports (mm)
S – Allowable bending stress (N/mm2) (34 N/mm2 for steel)
Non-cylindrical mixing tanks are usually either rectangular, or horizontal tanks, and as previously mentioned these tanks usually do not require baffles unless the required level of agitation is high. These tanks are asymmetrical with respect to the mixer shaft and as a result are self-baffling for applications where the applied mixer power < 165 W/m3.
For rectangular tank, if baffles are required the baffles can be mounted as shown below. The arrangement is similar to that of a cylindrical tank for tanks that are roughly square.
For longer tanks, only the baffles closest to the impeller are required as the effect of the baffles decreases as they are moved away from the impeller.
For even longer tanks, multiple mixers are usually recommended, and baffles should be installed adjacent to each mixer.
For horizontal tanks, baffles can be mounted as shown below, on the tank centreline. Baffles are not usually installed on the curved sides of the tank due to difficulties with mounting, and ensuring sufficient impeller clearance.
For longer tanks the baffles are positioned away from the tank ends to give ~600mm clearance from the impeller.
As for rectangular tanks, when multiple mixers are used we baffles need to be installed for each mixer. Baffles are usually positioned as below with the same 600mm clearance and baffles installed down the length of the tank.
John Whittle MEng (Hons)