How To Choose The Right Agitator

Mixing technology is deceptively complex and can be difficult to understand for someone who is not well-versed in science. Although industrial agitators are often grouped with pumps and other fluid transport methods, more knowledge is needed when choosing an agitator. For example, pumps can often be selected by establishing the required head and flow required for the process and reading off the manufacturer’s specifications. 

Mixer selection is much more complex because there are often process goals that need to be achieved or material considerations that need to be taken into account and these requirements can be much more difficult to ascertain

To help you understand what to look for when purchasing an agitator, we’ve put together this guide. If you have any further questions, don’t hesitate to contact us!

Here’s How You Can Choose The Right Industrial Agitator

There are three key elements of mixer design. Understanding each of these three elements will help you choose the right agitator while enabling your new equipment to be a key element for efficient process operation. 

1. Process Considerations

This requires matching your process requirements with the correct impeller design. Based on your requirements in terms of product, blend/residence time, and rheology, we match the impeller type, vessel design, power, and speed, to arrive at an optimum impeller selection. 

We also need to match the impeller output in terms of flow and shear with the process requirement. A flocculation process, for example, needs minimum shear to keep the flocculant intact.

2. Power Considerations

Once the process aspects of the mixer design are finalized, we’ll need to optimize a final design that uses power efficiently. This could mean altering the D/T (impeller to Tank Ø) ratio to produce the same process result at reduced power. The price to pay will be torque, which could require larger gearboxes, using lower power. This is a trade-off between operating cost and capital cost, often requiring input from the customer.

3. Mechanical Design

Once the process and power considerations have been made, and the agitator design has been optimized for efficient operation, we can advance to its mechanical design. Understanding the mechanical design of your mixer will prolong its operating life and minimize downtime. The goal here is to choose a design that is powerful enough to prevent premature failure without being excessively strong and overly heavy, resulting in unnecessary cost. Industrial agitators should be designed to have a mechanical life of 20 years. The key to efficient mechanical design is understanding the forces acting on the mixer. Mixers are dynamic machines with fluctuating loads and forces. Shafts are usually overhung with no lower support. 

The forces acting on the shaft, concentrated around the mounting base, are:

• Axial load and thrust
• Bending moment 
• Torque

Special consideration must be paid to critical speed, which is the first natural harmonic of the shaft and impeller systems.

Material, shaft length, bearing centres, impeller weight and quantity are critical factors in determining the critical speed relative to the operating speed. For safe mechanical design, mixers should not operate within 75% of the first critical speed. For critical speed ratios in excess of 40%, the lower impellers should be fitted with stabilizing fins. 

Contact The Mixing Experts

It is not sufficient to design an agitator only for efficient process operation; it must also be mechanically reliable and efficient in terms of power usage. Choosing the right industrial agitator will provide you with decades of efficient operation. Contact Sepro Mixing for more information and a free consultation.