The challenge in mixer system design is to maximize the produced thrust while minimizing power consumption.
Most wastewater treatment applications using submersible jet mixers are typically termed “flow-controlled.” Mixing in flow-controlled systems is accomplished through large-scale bulk flow circulation encompassing all parts of the tank.
Bulk Flow Mixing
In flow-controlled systems, the bulk flow provides mixing through:
A) convection or transportation of fluid elements (macro mixing)
B) Distribution of turbulence throughout the tank (micro mixing)
The degree of bulk flow velocity, and its associated turbulence intensity, depend on the mixing requirement of the specific application. The tougher the mixing requirement, the larger the bulk flow velocity must be.
For instance, if the mixing requirement is to keep a liquid/solid system homogenous, the bulk flow velocity and turbulence intensity must be sufficient to allow horizontal and vertical transportation of the solid matter. It must also provide sufficient scour to prevent sediment buildup on the tank bottom.
Mixer Thrust Provides Velocity
In order to give adequate speed to the bulk flow circulation, the liquid must be “pushed” with a thrust force produced by the mixer’s propeller. This thrust force is both well-defined and measurable. Therefore, the degree of thrust force produced by a mixer’s propeller is the most relevant performance parameter for any axial flow mixer.
Thrust may be best understood by an analogy with a ship, where the ship’s speed is a result of the thrust exerted by its propeller. Similarly, the liquid velocity in a mixed tank is a result of the thrust of the mixer. The higher the thrust, the higher the velocity. And, for any given thrust, the magnitude of the attained velocity depends on the design, whether it be a ship or a tank.
The Cost Of Mixer Thrust
Thrust, unfortunately, comes with a cost. In mixing, the cost is the mixer’s input power consumption.
The challenge in mixer system design is to maximize the produced thrust while minimizing power consumption. Thus, the ratio of produced thrust to consumed (input) power is a highly relevant measure of mixer performance. These parameters (produced thrust, the input power required to produce the thrust, and the ratio of the two) are defined in the international standard ISO 21630, which sets the conditions and procedures for testing submersible mixer performance.
With these well-defined parameters, one can easily compare the performance of different mixer designs. It is, of course, natural to favor the mixer that produces the required thrust at the lowest power consumption. The ration of thrust to input power is expressed as (R). Thrust is measured in Newtons (N) and power is measured in kilowatts (kW). Thus, the thrust per power ratio, R, attains the unit N/kW.
N/kW, though not dimensionless, is a true wire-to-water characteristic. The thrust-to-power ratio for today’s Flygt submersible mixer portfolio ranges between 150 N/kW for the small direct driven compact mixers, up to 1500 N/kW for the large, slow geared mixers.
Comparing mixer designs in terms of thrust-to-power ratios, large and slow mixer propellers always have much higher R than small, fast propellers. In a theoretical model if we scale up a small and speedy mixer (with R = 150 N/kW) to the same size as a large, slow-geared mixer (with R = 1500 N/kW), then run it at the same speed as the large mixer, the R for the upscaled mixer would be about 600 N/kW.
This clearly illustrates that more efficient mixing systems can be reached by going large and slow. It also illustrates why, when comparing different mixer designs in terms of thrust to power ratio, it’s important that propeller diameters be the same.