Print page

Print page

Designing a Support Bracket for a Baffle

Q: I have a vertical 3100mm dia nad 7000 overall length agitator. The baffle supports were failing due to improper loading. I do not have any book so that I can use the formulae for redesign the support. Please provide me a sample calculation for designing support bracket for the baffle.

A:

The baffle supports must be designed to handle the torque input by the mixer.  Mixer torque should be calculated using the motor power and dividing by pi times the mixer shaft speed, with the proper conversion factor for units.  The torque formula for power in horsepower, shaft speed in revolutions per minute, and torque in inch-pounds is torque [in-lbs] = 63025 * motor horsepower [hp] / output shaft speed [rpm].  Note that the correct speed to use is the mixer shaft speed, not the motor speed.  A speed reducer is a constant power device and a torque increaser.

A variety of methods can be used to get from the input torque to the baffle loads.  The simplest method is to simply distribute the torque input to a force load on each baffle, dividing by the number of baffles and the distance from the tank center to the tips of the baffles.  Then the baffle thickness, supports shape, number of supports, etc. must be designed with adequate strength.   Using maximum design limits of 6,000 psi for shear stresses and 10,000  psi for tensile stress levels usually works well for steels, carbon or  stainless.  These strength levels take into account that the failures will occur because of fatigue, not yield strength.

The dimensions given are not sufficient to provide more detailed calculation.

Here are more of the latest questions on: Mixing

How do I select the optimal location for an impeller inside a tank?
I am new to tank mixing and I have a question about selecting the optimal location (and orientation) for an impeller inside a tank. I am dealing with a tank that contains very light materials (0.7 and 0.8 SG). The contents have been sitting in the tank and are stratified. Please let me know if there is some sort of general process for selecting the proper impeller position.

The tank is 120 ft. in diameter, 45 ft. high. It is allowed to only have one mixer. The first layer of material has a specific gravity of 0.75 is 32 ft. high. The second layer of the tank has a specific gravity of 0.67 and is 10 ft. high; making the total contents of the tank 42 ft. high. The purpose of the mixer is to have complete mixing within 6 hours.

Is a UDIF impeller suitable?
We have been using a UDIF agitator for emulsion polymerization mixing for 1cp to 8,000 cp. Is this type of agitator suitable for mixing?

What kind of agitator is suitable for a range of viscosities?
Our process dissolves several types of polymers in appropriate solvents. During the solving process, the viscosity might reach to the value of 8,000 cp. Our vessel dimension is 30 cm in diameter and 70 cm in height. What kind of agitator is suitable for this purpose?

What is the best way to approach gas-gas mixing?
I want to mix two low viscous gas streams in laminar regime. I am planning to mix them by passing through a pipe filled with spherical pebbles or some structured packing. I want to calculate the length required for the mixing expected. My required process stream details are as follows:

Q = 1Nm³/hr
He= 0.99
H₂= 0.01
P= 1.2 bar
Dia of pipe = 1/2 inch
Temp= 25 degrees C

I have gone through the text book "Handbook of Industrial Mixing." But that textbook has procedure for only commercially available static mixers. Please guide me through the design. Is dispersion model helpful in finding the length? If so, how?

We would like to have a general idea of pumping number for our reactors. Can you help us with blend-time calculations?
To characterize one of the reactors at plant scale, we use the discoloration method. The parameters for the mixing time are calculated based according to the following formula:

t_mix= K/(aN(D/T)^b (T/Z)^0.5 )
N = impeller speed
D = diameter stirrer
T = diameter tank
Z = liquid height
The divisor is known as Kmix

Because we don't really know the uniformity (U) reached with this method we don't replace K with

K = -ln (1-U)

But get the best fit for K, a and b by means of the least square method. It is known that the pumping number can be determined by

N_Q= (Vk_mix)/(ND^3 )

We would like to have a general idea of pumping number for our reactors. So if we would like to deviate the pumping number from the above method would it be correct that for T=Z (so at a fixed volume based on the reactor geometry) we use the following formula.

N_Q= (VaN(D/T)^b)/(ND^3 )

Working in a turbulent regime, this result in a constant pumping number related to the tank geometry. Would the above approach be correct to compare pumping and mixing time capabilities of different reactor set ups? I think that not working at a fixed uniformity results in a gap in the above approach.

Back to Ask the Experts