Industrial Mixing Industry Glossary
Comprehensive Terminology for Mixers, Agitators, Impellers, Fluid Dynamics & Process Engineering
A
- Aeration
- The process of introducing air or gas into a liquid during mixing, commonly for oxygen transfer, biological treatment, or gas-liquid reactions.
- Agglomeration
- The recombining of small, finely dispersed particles into larger clusters or particles, usually caused by surface forces. The opposite of dispersion.
- Agitation
- The mechanical motion applied to promote mixing, maintain uniformity, suspend solids, transfer heat, or facilitate mass transfer in a vessel. Also refers to the equipment performing this function.
- Agitator
- A complete mechanical device consisting of a drive, shaft, and impeller used to mix, suspend, dissolve, disperse, or homogenize fluids, slurries, or pastes inside a tank or vessel. Often used interchangeably with “mixer.”
- Anchor Impeller / Anchor Mixer
- A slow-speed impeller with a large diameter and anchor-like shape that closely conforms to the vessel bottom and walls. Used for high-viscosity fluids and heat transfer; often equipped with scrapers.
- Apparent Viscosity
- The effective viscosity of a non-Newtonian fluid measured at a specific shear rate and temperature. It varies with the applied shear conditions.
- Axial Flow
- Fluid movement parallel to the axis of the mixer shaft, creating top-to-bottom circulation in the tank. Down-pumping moves fluid away from the drive; up-pumping moves it toward the drive.
- Axial Thrust
- The vertical force generated by the impeller on the shaft, bearings, and gearbox during operation.
B
- Baffle
- Vertical plates or fixed internal elements mounted on or near tank walls to prevent vortex formation (solid-body rotation), increase turbulence, and improve mixing efficiency.
- Batch Mixing
- Mixing performed on discrete quantities of material. Ingredients are added, mixed for a set time, and the completed batch is discharged before the next cycle begins.
- Blade
- One of the individual vanes or wings on an impeller (not the entire impeller assembly).
- Blend Time / Mixing Time / Blending Time
- The time required to achieve a specified degree of homogeneity (uniform concentration) throughout the vessel contents after addition of components. Highly dependent on viscosity, impeller type, and power input.
- Blending
- The combining of two or more miscible liquids (or materials that mix readily) into a uniform mass without chemical reaction. Typically lower-shear than dispersion or emulsification.
- Bulk Fluid Velocity (BFV)
- A measure of the average large-scale circulation velocity in the tank, often calculated as pumping rate divided by cross-sectional area. Useful for assessing the level of agitation.
- Bypassing
- In continuous or semi-batch systems, the phenomenon where a significant portion of fluid does not spend adequate residence time in the high-intensity mixing zone.
C
- Cavern Formation
- A localized well-mixed zone surrounding the impeller in highly viscous or yield-stress fluids, while outer regions of the tank remain poorly mixed or stagnant.
- CFD (Computational Fluid Dynamics)
- Numerical simulation methods used to analyze flow patterns, velocity profiles, turbulence, shear zones, and dead zones inside mixing vessels.
- CIP (Cleaning-in-Place)
- Automated cleaning of vessels, pipes, and mixers using chemicals, heat, and water without disassembling the equipment. Critical for sanitary applications.
- Coagulation
- The process in which fine particles aggregate into larger clusters, often aided by chemical coagulants, to facilitate sedimentation or filtration (common in water treatment).
- Continuous Mixing
- A process in which materials are continuously fed into and discharged from the mixer, achieving steady-state operation (as opposed to batch mixing).
- Coverage
- The distance between the impeller and the free surface of the liquid. Proper coverage helps reduce vortexing.
- Critical Speed
- The rotational speed at which the natural frequency of the shaft/impeller system is excited, leading to excessive vibration and potential mechanical failure. Mixers should be designed to operate well away from critical speed and its harmonics.
D
- Dead Zone
- A poorly mixed or stagnant region inside a vessel where fluid motion is weak or absent, often in corners, near walls, or outside the impeller’s influence zone.
- Dilatant Fluid
- A non-Newtonian fluid whose viscosity increases as shear rate increases (shear-thickening). Opposite of pseudoplastic.
- Dispersion
- The process of breaking one phase (solid particles, liquid droplets, or gas bubbles) into discrete particles that are surrounded by a continuous phase (usually liquid). Requires sufficient shear to overcome cohesive forces.
- Dispersion Impeller / Sawtooth Impeller
- A high-speed disc with teeth around its periphery used for high-shear applications such as breaking down powders, wetting solids, or reducing particle/droplet size.
- Dissolution
- The process of dissolving solid particles into a liquid solvent until they become part of a homogeneous solution.
- Draft Tube
- A stationary hollow tube mounted concentrically above or around the impeller to enhance axial (vertical) flow and improve circulation efficiency.
- D/T Ratio
- The ratio of impeller diameter (D) to tank diameter (T). Optimal ratios vary with viscosity and application; higher ratios are typically needed for more viscous fluids.
E
- Emulsification
- The process of dispersing one immiscible liquid as fine droplets into another to form a stable emulsion (e.g., oil-in-water or water-in-oil). Often requires high shear and emulsifying agents.
- Emulsion
- A stable or temporarily stable mixture of two immiscible liquids in which one is dispersed as microscopic droplets within the other.
F
- Flash Mixer
- A high-intensity, short-residence-time mixer used to rapidly disperse small amounts of additive (such as coagulants) into a continuous stream, common in water and wastewater treatment.
- Flocculation
- A gentle mixing process that causes fine, destabilized particles to collide and form larger aggregates (flocs) that can settle or be filtered more easily.
- Flooding
- In gas-liquid mixing, the condition in which gas flow rate exceeds the impeller’s ability to disperse it, causing gas to accumulate around the impeller and severely reduce liquid circulation and mixing effectiveness.
- Flow
- The bulk movement of fluid generated by the impeller. High-flow applications emphasize rapid turnover and circulation rather than intense shear.
- Flow Number (NQ or Nq)
- A dimensionless number characterizing an impeller’s pumping capacity: \( N_Q = \frac{Q}{N D^3} \), where Q is volumetric flow rate, N is rotational speed, and D is impeller diameter.
- Freeboard
- The vertical distance from the liquid surface to the top of the vessel or tank.
- Froude Number (Fr)
- A dimensionless number comparing inertial forces to gravitational forces, relevant to vortex formation and free-surface behavior: \( Fr = \frac{N^2 D}{g} \).
G
- Gate Impeller
- An anchor-style impeller with additional horizontal and vertical blades, providing enhanced low-shear mixing and wall scraping for viscous materials.
- Gearbox Service Factor
- A numerical safety margin applied to gearbox selection based on operating conditions, duty cycle, and expected life. Values greater than 1.0 indicate the unit is underutilized relative to its rating; typical mixer gearboxes target ≥1.5.
H
- Helical Ribbon Impeller / Helical Blade
- A narrow ribbon that spirals around the shaft, used for very high-viscosity fluids. Produces strong axial flow and is effective for heat transfer and laminar mixing.
- High-Shear Mixer
- Equipment (often rotor-stator or sawtooth designs) designed to generate intense localized shear for particle size reduction, emulsification, or dispersion.
- Homogenization
- The process of reducing particle or droplet size and distributing components uniformly throughout a mixture to achieve consistent composition, texture, and stability.
- Homogenizer (Rotor-Stator)
- A high-speed device with a rotating rotor inside a stationary stator. Material is forced through narrow gaps at high velocity, producing intense mechanical and hydraulic shear.
- Hydrofoil Impeller
- A high-efficiency axial-flow impeller with airfoil-shaped blades. Provides high pumping capacity with relatively low shear and low power draw.
I
- Impeller
- The rotating component attached to the shaft that transfers energy from the drive to the fluid, generating flow and/or shear. Common types include propellers, turbines, paddles, anchors, hydrofoils, and dispersion discs.
- Impeller Diameter (D)
- The maximum outer diameter swept by the impeller during rotation.
- Impeller Tip Speed
- The linear velocity at the outer edge of the impeller: \( V_{tip} = \pi D N \).
- Inline Mixer
- A mixer designed to blend materials continuously within a pipeline or closed loop. Includes both dynamic (rotating) and static (no moving parts) designs.
J
- Just Suspension Speed (Njs)
- The minimum impeller rotational speed required to keep all solid particles suspended off the vessel bottom (no permanent sedimentation).
K
- Kneading
- Intensive mixing of high-viscosity pastes or dough-like materials using high shear forces, typically performed in sigma-blade, double-arm, or similar heavy-duty mixers.
L
- Laminar Flow
- Smooth, layered fluid motion occurring at low Reynolds numbers (typically Re < 10–100 in mixing contexts), where viscous forces dominate and there is little turbulence or intermixing of layers.
M
- Mass Transfer
- The movement of material between phases (e.g., gas into liquid during aeration or dissolution of solids), often enhanced by mixing.
- Mechanical Seal
- A sealing device at the point where a rotating shaft enters a pressurized or closed vessel, consisting of stationary and rotating faces held in contact to prevent leakage. Double seals are used for higher pressures or hazardous materials.
- Miscible
- Describes liquids that mix completely and remain homogeneous at any concentration without separation.
- Mixer
- General term for machinery that provides agitation or mixing. See also Agitator.
- Mixing
- The process of increasing the homogeneity of a system by imparting mechanical energy, reducing concentration gradients, temperature gradients, or phase segregation.
- Multiphase Mixing
- Mixing involving two or more phases (solid-liquid, liquid-liquid, gas-liquid, or combinations thereof).
N
- Newtonian Fluid
- A fluid whose viscosity is constant and independent of shear rate. The relationship between shear stress and shear rate is linear (e.g., water, mineral oils).
- Non-Newtonian Fluid
- A fluid whose viscosity changes with shear rate, time, or both. Includes pseudoplastic, dilatant, thixotropic, and rheopectic fluids.
O
- Off-Bottom Clearance
- The distance between the lowest point of the impeller and the bottom of the tank.
P
- Paddle Impeller
- A simple impeller consisting of flat blades (often two) with a large diameter relative to the tank. Produces primarily radial or tangential flow and is used at moderate speeds for blending and heat transfer.
- Pitched-Blade Turbine (PBT)
- An impeller with blades pitched at an angle (commonly 45°) that produces mixed axial and radial flow. Versatile for blending, solids suspension, and moderate shear applications.
- Power Number (NP or Np)
- A dimensionless number specific to impeller geometry that relates power consumption to density, speed, and diameter: \( N_P = \frac{P}{\rho N^3 D^5} \). Used for power calculations and scale-up.
- Propeller Impeller
- A helical, axial-flow impeller (typically 3 or 4 blades) used for low-viscosity fluids. Provides high pumping capacity with relatively low shear.
- Pseudoplastic Fluid
- A non-Newtonian fluid whose viscosity decreases as shear rate increases (shear-thinning). Common in many industrial slurries and polymer solutions.
- Pumping Rate (Q)
- The volumetric flow rate of fluid discharged by the impeller, typically calculated as \( Q = N_Q N D^3 \).
R
- Radial Flow
- Fluid movement directed outward perpendicular to the shaft axis, from the impeller toward the tank walls, where it is then redirected up or down. Characteristic of turbines such as the Rushton impeller.
- Residence Time
- The average time a fluid element spends in a continuous mixing system or reactor.
- Reynolds Number (Re)
- A dimensionless number representing the ratio of inertial to viscous forces: \( Re = \frac{\rho N D^2}{\mu} \). Characterizes flow regime (laminar, transitional, or turbulent) in mixing applications.
- Rotor-Stator
- A high-shear mixing head consisting of a high-speed rotating rotor inside a stationary stator with precisely controlled gaps. Used for homogenization, emulsification, and fine dispersion.
- Rushton Turbine / Rushton Impeller
- A classic radial-flow impeller consisting of a disc with six flat vertical blades. Widely used for gas dispersion and applications requiring high shear.
S
- Scrapers
- Hinged or flexible blades attached to the outer edge of an anchor or helical impeller that continuously wipe the vessel walls, preventing buildup and improving heat transfer.
- Service Factor
- See Gearbox Service Factor.
- Settling Rate / Settling Velocity
- The terminal velocity at which solid particles fall through a fluid under gravity. Critical for solids suspension design.
- Shear
- The stress or velocity gradient produced in a fluid when adjacent layers move at different speeds. High-shear regions near the impeller blades are responsible for most dispersive and emulsifying action.
- Static Mixer
- An inline mixer with no moving parts. Fixed internal elements (baffles, ribbons, or helical structures) repeatedly divide, rotate, and recombine the flowing fluid to achieve mixing.
- Suspension
- A mixture in which insoluble solid particles are distributed throughout a liquid. Maintaining suspension requires sufficient fluid velocity to counteract settling.
T
- Thixotropic Fluid
- A non-Newtonian fluid whose viscosity decreases over time under constant shear and recovers when shear is removed (time-dependent shear-thinning).
- Turbulent Flow
- Chaotic, irregular fluid motion with high Reynolds numbers (typically Re > 10,000–20,000 in mixing tanks), characterized by eddies and rapid mixing.
V
- Viscosity
- A measure of a fluid’s resistance to flow (internal friction). Absolute viscosity is expressed in poise or centipoise (cP); kinematic viscosity in stokes or centistokes.
- Vortex
- A swirling depression on the liquid surface caused by rotational motion. Controlled or prevented by baffles; excessive vortexing can draw air into the liquid or reduce mixing efficiency.
W
- Wetted Parts
- All components of a mixer that come into contact with the process fluid (impeller, shaft, seals in contact with product, etc.). Material selection is critical for corrosion resistance and sanitary requirements.
