In thrust we trust: How to select a submersible mixer with optimal efficiency

When comparing submersible mixers, how can you know which mixer will achieve the best results with the least amount of energy used? There are a number of parameters to consider, but the most relevant is the mixer’s thrust-to-power ratio. This ratio will tell you how well the submersible mixer moves liquid compared to how much energy the mixer uses. This article explains the basics of thrust, how to compare power ratings, and why Flygt mixers are designed based on thrust performance.

For many mixing operations, the objective is to create a flow that ensures the blending of fluid, solids suspension and solids distribution. More specifically, the mixing should establish a bulk flow, which means that the fluid in the entire tank volume is in motion. The overall mixing result is determined by the strength of the bulk flow.

How does bulk flow work? Bulk flow is created by the jet issuing from the of a submersible mixer. As the propeller rotates, it imparts momentum into the fluid. This thrust, which is in the form of a jet of water, moves the fluid and does the most intense mixing. As the jet expands, it affects more of the surrounding fluid. The jet gets wider and slower, until it is moving the entire contents of the tank along with it, achieving an efficient bulk flow for effective mixing.

There are two important parameters to consider for effective mixing: mixer thrust and mixer power consumption, which is the price you pay for the final mixing result. Ideally you want to create optimal mixer thrust with minimal power consumption.

Understanding mixer power when making comparisons

There are several ways to define the power of a submersible mixer, including rated power and input power, mechanical (shaft) and electrical. Rated electrical power, measured in watts (W) or kilowatts (kW), refers to the maximum power allowed to flow into a mixer, while input power refers to the power that the mixer actually uses during operation.

When comparing the efficiency of mixers, input power is the more relevant metric. Power consumption alone, however, doesn’t tell you much about the effectiveness of the mixer, as two mixers with equal power inputs can have dramatically different effects based on the design of the mixers.

In the context of mixing, the terms “specific power” or “power density/intensity” can also be used by manufacturers to refer to the mixer power used per unit volume of liquid in a tank, such as the number of watts (W) used to mix a cubic meter of liquid. The power intensity of a mixer, however, can be determined in completely different ways and can be difficult to measure, making it hard to use for comparisons. Power intensity may give you an idea of how efficient a mixer is (in terms of energy use), but it is less helpful in telling you how effective a mixer is (how well it gets the job done).

When comparing submersible mixers, therefore, the thrust value of a mixer – which can be reliably measured – should be your starting point.

Using the thrust-to-power ratio to compare submersible mixers

The performance of a submersible mixer is measured by the total amount of thrust produced, measured in Newtons (N). This metric describes the strength of the bulk flow. The thrust-to-power ratio (R) is calculated by dividing thrust by input power (N/kW), which measures performance against energy consumption.

Xylem’s Flygt brand pioneered the use of thrust force (N) as the main parameter to determine the bulk flow required to achieve a good mixing result. In 1980, Flygt began using the thrust parameter in designing submersible mixers, and we have been publishing mixer thrust ranges in our documentation since 2000. Flygt also contributed to the international standard ISO 21630:2007, in which output thrust compared to power consumption (N/kW) is defined as the most relevant and single parameter to state efficiency for a single mixer.

Based on decades of research and the installation of over 300,000 Flygt mixers worldwide, we carefully design and engineer our mixers to ensure maximum thrust performance with minimal energy usage. 40 years ago, Flygt developed our signature “banana blade” mixers, with self-cleaning propellers designed to provide the best thrust-to-power ratio on the market.

We continue to evolve our mixers, focusing on optimizing thrust and energy savings. Our Flygt Adaptive mixers, for example, offer variable thrust to match mixer performance to actual demand, which avoids excessive mixing and wasted energy. Thanks to their high-efficient IE4 motor and integrated control electronics, Flygt Adaptive mixers can save up to 50% in energy consumption, with easy access to state-of-the-art control and monitoring functions.

Need help determining the required thrust for your application?

The thrust required for each mixing application depends on tank shape and size, the liquid’s properties and the mixing duty. Xylem has a complete line of mixers with superior thrust-to-power ratios, for every tank configuration and every substrate blend. Our highly skilled engineers and technicians can help you determine the thrust required to generate the bulk flow that best meets your mixing needs.