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Mixing Terminology

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Common Mixing Terminology

Axial Flow

Flow of fluid that runs parallel to the axis of the mixer shaft. If the flow moves away from the drive unit, it is considered "down pumping". Flow moving toward the drive unit is considered "up pumping".


Solid surfaces placed in the path of the fluid flow generated by the impeller. Often placed against the walls in round tanks, these surfaces help to prevent vortexing and enhance vertical turnover in the tank.

Bulk Fluid Velocity (BFV)

(Ratio) The cross sectional area of the vessel divided by the pumping rate in a tank with a 1:1 aspect ratio. Useful calculation of the level of agitation in the tank.


The distance between the impeller and the surface of the fluid. Coverage is necessary to prevent vortexing. In most applications, the minimum desired level of coverage is two times the impeller diameter (D), Min Desired Coverage = 2D.

Critical Speed

The natural frequency of vibration of the mixing system is called Critical Speed. When operated near this speed, the vibrations in the system can cause extreme shaft deflection and/or failure. Mixing systems should not be designed to operate at speeds near the Critical Speed or its harmonics.

D (variable)

(Ratio) The relationship between the Impeller Diameter (D) and the Tank Diameter (T). Testing has shown that as viscosity increases, the D/T ratio must also increase for optimal mixing.


(Ratio) The relationship between the Impeller Diameter (D) and the Tank Diameter (T). Testing has shown that as viscosity increases, the D/T ratio must also increase for optimal mixing.


A measurement of the movement generated in the fluid being mixed. The pumping rate of the rotating impeller is usually regarded as a measurement of flow. Applications with high flow exhibit rapid turnover of the material, but low levels of Shear. Generally, shear and flow are inversely related.

Flow Number (Nq)

The Flow Number is a measurement of the pumping capacity of a specific impeller design.

FMB (acronym)

Acronym: From Mounting Base. Used to specify the location of the impeller relative to the base of the mount. Determines required mixer shaft length.


The distance from the surface of the fluid to the top of the tank or vessel. Adding the Freeboard to the Coverage is useful in determining the proper shaft length of a mixer.


The device which actually contacts the fluid and accomplishes mixing. A wide array of standard designs exist with specific flow and shear characteristics desirable for different applications.

Impeller Diameter (D)

Measurement of the outer diameter of the mixing impeller in motion.

Impeller Speed (N)

Rate of rotation of the impeller, in revolutions per minute (rpm).

Np (variable)

See Power Number.

Nq (variable)

See Flow Number.

Power Number (Np)

The Power Number (Np) is a constant, specific to each impeller design, that represents the power requirements for the use of that impeller.

Off Bottom

The distance between the mixing impeller and the bottom of the tank. In most applications, the desired Off Bottom distance is one to two times the impeller diameter (D), so in some applications, Optimal Off Bottom = D.

Pumping Rate (Q)

Measurement of the actual flow rate generated by a specific impeller rotating at a known speed. Usually measured in gallons/minute or liters/minute.

Q (variable)

See Pumping Rate.

Radial Flow

Flow of fluid that runs perpendicular to the axis of the mixer shaft. Radial flow impeller generally move fluid from above and/or below the impeller and pump them outward to the walls of the tank where the material is diverted up and/or down along the wall.

Reynolds Number

A dimensionless number that represents the magnitude of fluid motion or agitation generated in a mixing application. This number is used in calculating the flow and power draw in applications with viscosity levels higher than water.

Service Factor (SF)

A measurement that suggests a lifespan adjustment to the expectations of a mechanical system. For example, if a gearbox which has a 1.0 SF rating under constant use with a 5HP motor is actually used in a system with a 2.5HP motor under constant use, the gearbox has a service factor of 2.0 suggesting that the expected lifespan of the gearbox has been substantially increased because it is being underutilized in the current configuration. In general terms, a system with a Service Factor less than 1 is overstressed and will fail prematurely, while a system with a Service Factor greater than 1 is underutilized and will last longer than expected. Typically, mixer gearboxes should be designed with a service factor of 1.5 or greater.

Settling Rate

The terminal velocity of the particles falling through the fluid, usually measured in feet/minute or meters/minute.


A measurement of the stress produced on the fluid being mixed. Applications with high shear have areas where neighboring particles have significantly different velocities causing the intermixing of fluids, dispersing of gases, or breakdown of the cohesiveness of liquid droplets. Generally, Shear and Flow are inversely related.

Specific Gravity

The ratio between the density of fluid and the density of water.

T (variable)

See Tank Diameter.

Tank Diameter (T)

Measurement of the outer diameter of a cylindrical mixing tank.


A measurement of the "thickness" of a fluid. Usually measured in CentiPoise (Cp), this "thickness" is actually a measurement of the internal fluid friction (resistance to motion) of the fluid.


Typically undesirable, this rapid swirling of the fluid pulls the fluid surface down around the shaft until the surface contacts the impeller, which allows ambient air to become part of the mix. Vortexing substantially reduces the life of the mixer and does not promote good mixing.

Z (variable)

The overall height of the fluid in the mixing vessel. (Note: Sometimes used to denote the overall height of the mixing vessel itself.)