Understanding the Driving Force Behind Efficient Mixing
Thrust & Submersible Mixers
What is Thrust?
Thrust is a reaction force explained by Newton’s third law:
When a system expels or accelerates mass in one direction, an equal force pushes back in the opposite direction. This pushback is what we call thrust.
For example, a rocket generates thrust by expelling hot gases out of its engines at high speed. The gases push downward, and in reaction, the rocket is pushed upward with an equal force. This upward force is called thrust.
Thrust can also be seen in how a boat’s propeller moves the vessel. A boat moves forward because its propeller pushes water backward. The water pushes back on the propeller with an equal force in the opposite direction, which moves the boat forward. This pushing force is called thrust.
Thrust and Submersible Mixers
Thrust, in the context of a submersible mixer, refers to the force generated by the mixer’s propeller to move liquid in a specific direction. The mixer’s motor spins the propeller, which pushes liquid to generate directional force, or thrust. This action creates a flow pattern that keeps solids in suspension and liquids evenly mixed throughout the tank.
Unlike pumps, which have casings or volutes to build pressure and move liquid from one location to another, mixers are designed solely to create flow within the same tank or basin. Mixers do not generate pressure; instead, they produce a continuous flow that keeps solids suspended and liquids evenly mixed.
Because mixers work by creating flow rather than pressure, their performance cannot be compared using head or pressure metrics like pumps. Instead, thrust is the standard used to compare mixers, as it measures the force generated to move liquid and maintain proper circulation.
The measurement of thrust for submersible mixers follows ISO 21630, which defines standardized test methods specific to wastewater applications. This standard recognizes similarities to pumps while accounting for the critical design and functional differences.
Thrust is measured in either Newtons (N) in metric or pounds-force (lbf) in imperial.
Why is Thrust Important?
Thrust is important in wastewater treatment plants because stronger thrust equals better circulation. This circulation prevents sediment buildup at the bottom of the tanks, maintains consistent conditions for processes like wastewater treatment, and helps avoid dead zones within the tank where solids settle.
Since thrust is a standard used in the pump industry, it is critical for the sizing of mixers, based on tank volume, shape, and fluid characteristics.
Factors affecting Thrust
Based on ISO 21630, thrust is used to guide mixer selection, ensuring a consistent basis for comparison.
Testing standards under ISO 21630 are the most accurate way to size a mixer as it incorporates the most influential factors in the system, the basin's geometrical shape, as well as the propellers’ design.
The mixer theory equation indicates thrust is primarily influenced by the propeller’s diameter and its rotational speed. As you see below in the mixer theory equation, the biggest influence on mixing power is the propeller diameter, with the second most influential factor being the propeller speed. The application and basin size will dictate the type of mixer: high speed, medium speed, or low speed.
There are several factors affecting thrust:
- Propeller diameter and design: The amount of thrust is dependent on the size of the propeller. The larger the diameter, the higher the thrust.
- Motor power and speed: More power and/or higher speed generally means more thrust.
- Rotation speed (RPM): Lower RPM with larger propellers equals efficient thrust.
- Fluid properties: Higher viscosity liquids require more thrust for proper mixing.
- Small/High speed mixers: 200-500 N (45-110 lbf)
- Medium speed mixers: 500-2,000 N (110-450 lbf)
- Large/low speed mixers: 2,000+ N (450+ lbf)
*Actual size depends on tank volume, shape, and process needs.
Wilo is Your Solutions Provider
Wilo USA headquartered in Cedarburg, WI, is a multi-national pump manufacturer and one of the world’s leading premium suppliers of pumps and pumps systems for building services, water management, and the industrial sector. With innovative solutions, smart products, and individual services, Wilo is your solution provider in making water move using intelligent, efficient, and eco-friendly techniques.
Wilo manufactures a wide range of centrifugal pumps and submersible mixers, including the Wilo-Flumen OPTI-TR and -Flumen EXCEL-TRE product lines and the Wilo-EMU TR(E) 216-326 line. For more information regarding Wilo’s submersible mixers, how to size them, and other sustainable solutions, visit the Wilo USA website or take a quick look through the Wilo Product Guide for more options.
August 2025 | tlk
