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Glossary of Mixing Terms
| Term | Definition |
|---|---|
| Agglomeration | The recombination of small dispersed particles into larger particles, usually caused by a change in surface forces following a change in conditions. It is the opposite of dispersion. |
| Agitator | A machine made up of a drive, a shaft and an impeller, used to agitate the contents of a vessel. Also called a mixer. |
| Anchor Impeller | A large-diameter impeller shaped like an anchor, whose outline closely follows the bottom and walls of the vessel. See Types of Mixer Impellers. |
| Apparent Viscosity | The resistance to flow of a non-Newtonian fluid at a given point on its shear stress / shear rate curve. |
| Axial Flow | Fluid movement parallel to the shaft, usually from the top to the bottom of the vessel. |
| Axial-Flow Impeller | An impeller, such as a pitched-blade turbine with blades at 45°, that produces flow in the axial direction. See Types of Mixer Impellers. |
| Baffle | A fixed plate mounted on or near the tank wall to prevent swirling and promote vertical flow. See Tank Baffles. |
| Blade | One of the blades of an impeller, not to be confused with the complete impeller. |
| Blend Time | The time needed to mix the contents of a vessel to the required uniformity. It depends largely on the viscosity of the products. |
| Blending | Combining two or more miscible liquids into a uniform mixture. |
| Beavertail Baffles | Wide baffles used when the tank itself cannot be modified (for example, when baffles cannot be welded to the wall). Typically two beavertail baffles are suspended from the mixer bridge into the tank. |
| Bulk Density | The density of a granular material including the voids between the particles. It is always lower than the true density. |
| Centipoise / Centistoke | Common units of dynamic and kinematic viscosity. See Viscosity. |
| Consistency | The resistance of a fluid to deformation under shear stress. |
| Critical Speed | The rotational speed that matches the natural frequency of the shaft and impeller assembly. It can also be affected by the mounting structure. |
| Crystallisation | The formation of solid crystals from a liquid solution. It is the opposite of dissolution. |
| Density | The mass per unit volume of a substance. See Specific Gravity. |
| Dilatant Fluid | A fluid whose viscosity increases as the shear rate increases. It is the opposite of a pseudoplastic fluid. |
| Dispersion | A two-phase system in which one phase is broken up into discrete particles or droplets surrounded by the second phase. In mixing, the second phase is usually a liquid. |
| Draft Tube | A fixed open tube mounted concentrically around the impeller to increase vertical flow in the vessel. |
| Drive | The part of the agitator that converts the power supplied into mechanical energy for mixing. The drive can be electric or pneumatic, direct-drive or geared. |
| Dry Well | A collection chamber that catches any lubricant leaking from the drive oil seals, so that it cannot fall into the product. It can be fitted with an oil-leak sensor. |
| Emulsion | A colloidal dispersion of two or more immiscible liquids. |
| Flash Mixer | A mixer that blends small quantities of additive into a continuous flow with a very short residence time, typically for dosing flocculation chemicals in water and wastewater treatment. |
| Flocculation | A gentle mixing process in which fine particles collide and agglomerate into larger flocs, which can then be separated from the liquid more easily. |
| Gate Impeller | An anchor impeller with additional horizontal and vertical blades. See Types of Mixer Impellers. |
| Helical Gears | Gears with helical (angled) teeth, used to connect parallel or non-intersecting shafts. |
| Homogenisation | Mixing the components until the composition is the same throughout the batch. |
| IBC | Intermediate bulk container: a reusable industrial container, usually 1,000 L, used for the transport and storage of liquids such as chemicals and solvents. |
| IBC Mixer | An industrial mixer designed to be mounted on an IBC. |
| Impeller | The part of the mixer that transmits force to the product being mixed. See Types of Mixer Impellers. |
| Krebs Unit (KU) | A common unit for expressing the consistency of paints and other coatings. |
| Laminar Flow | Flow made up of long, smooth, parallel layers moving in the same direction, with little mixing between them. See Turbulent Flow and Viscosity. |
| Liquid | A substance that is neither a solid nor a gas, which flows and can be poured, for example water. |
| Liquid Solution | A liquid in which a substance (solid, gas or another liquid) has been completely dissolved. |
| Mechanical Seal | A pressure seal where the shaft enters the vessel. It consists of a stationary ring and a ring that rotates with the shaft, held together by springs or by the process pressure. For higher pressures, a double mechanical seal with two seals in a pressurised housing can be used. |
| Mixer | A machine designed to provide agitation. See Agitator. |
| Mixing | The process of increasing the uniformity of a product, usually by applying mechanical energy. |
| Newtonian Fluid | A fluid whose rate of flow is proportional to the applied shear stress: its viscosity is constant and does not depend on the shear rate. See Viscosity. |
| Non-Newtonian Fluid | A fluid whose rate of flow is not proportional to the applied shear stress: its viscosity varies with shear rate, time, or both. See Dilatant Fluid, Pseudoplastic Fluid, Thixotropic and Viscosity. |
| Paddle | A two-bladed impeller, usually with a diameter greater than 60% of the tank diameter. |
| Particle Size | The average diameter of the particles in a solids suspension application, usually given in microns (µm). |
| Pitch | The angle of an impeller blade relative to the horizontal plane. See Types of Mixer Impellers. |
| Poise | The CGS unit of dynamic viscosity. One poise equals 100 centipoise. See Viscosity. |
| Power Number | A dimensionless number used to calculate the power drawn by an impeller. |
| Propeller | An impeller with three or four helical blades that produces axial flow. See Types of Mixer Impellers. |
| Proximity Factor | A correction factor used in impeller power calculations to allow for geometric variations, such as the clearance between the impeller and the tank bottom, or between the impeller and the liquid surface. |
| Pseudoplastic Fluid | A liquid or slurry whose viscosity decreases as the shear rate increases (shear-thinning). |
| Pumping Rate | The volumetric flow rate discharged by the impeller at a given speed. |
| Radial Flow | Fluid movement perpendicular to the shaft, usually from the centre of the tank towards the wall. Radial flow is needed in heat transfer and gas dispersion applications. |
| Radial-Flow Impeller | An impeller, such as a flat-blade turbine with vertical blades, that discharges fluid radially. See Types of Mixer Impellers. |
| Rapid Mixer | See Flash Mixer. |
| Residence Time | In a continuous process, the average time a component spends in the mixing zone. |
| Reynolds Number | A dimensionless number used to characterise fluid flow: the ratio of inertial forces to viscous forces. |
| Scrapers | Hinged or flexible blades fitted to the outer edge of an anchor impeller to scrape the vessel walls and prevent product build-up. |
| Service Factor | A rating applied to gearboxes, based on operating hours, type of drive and duty required. |
| Settling Velocity | The velocity of a particle falling freely through a fluid under gravity. |
| Shear | In liquid mixing, the action that produces mixing at molecular and microscopic level. Shear may be desirable in some applications and undesirable in others. |
| Slinger | A ring fitted to the shaft above the liquid level to prevent liquid from splashing up the shaft towards the seal or the drive. |
| Slurry | A mixture of a liquid and insoluble solids. See Solids Suspension. |
| Solids Suspension | Keeping insoluble solids suspended in a liquid, typically a slurry. There are three degrees of suspension: 1. Uniform – even the heaviest particles reach the surface and are evenly distributed. 2. Off-bottom – all particles are lifted off the tank bottom, but not distributed up to the surface. 3. On-bottom motion – particles are kept moving, but some may settle on the tank bottom. |
| Solids Wetting | Dispersing solid particles so that each particle is coated with a film of liquid. |
| Specific Gravity | The density of a material relative to the density of water under standard conditions. See Density. |
| Spiral Bevel Gears | Gears with curved teeth, used to connect intersecting shafts. |
| Helical Ribbon Impeller | An impeller made of narrow ribbons wound in a spiral around a central shaft, attached to it by several arms. Its diameter is usually close to that of the vessel. |
| Stabilising Fins | Additional fins fitted to an impeller to direct the flow it creates and counteract shaft deflection. They allow longer shafts without a bottom steady bearing. |
| Steady Bearing | A radial shaft-support bearing mounted on the vessel bottom to reduce deflection of long shafts. |
| Stuffing Box | A pressure seal where the mixer shaft enters the vessel, made of a housing around the shaft filled with several rings of braided packing. It often includes a means of lubricating the packing. |
| Swirl | Rotation of the liquid around the mixer shaft, with little actual mixing within the liquid. |
| Thixotropic | Describes a material whose viscosity decreases gradually over time at a constant shear rate, unlike a material whose viscosity changes immediately with shear rate. |
| Turbine | A multi-bladed impeller with relatively short blades. The impeller-to-tank diameter ratio is typically 0.2 to 0.5. See Types of Mixer Impellers. |
| Turbulent Flow | Flow in which the fluid undergoes irregular fluctuations, with velocity constantly changing in magnitude and direction. It is the opposite of laminar flow. See Viscosity. |
| Viscosity | A measure of the resistance of a fluid to flow when a force is applied. – Dynamic (absolute) viscosity is measured in centipoise (cP). Water at room temperature has a viscosity of about 1 cP. – Kinematic viscosity is measured in different ways depending on the instrument and can be converted to centistokes (cSt). Centipoise = centistokes × density of the fluid. |
| Viscosity Cup | A standard instrument for measuring kinematic viscosity. Common types include DIN, ISO and Shell cups. |
| Viscosity Factor | A correction factor applied to the standard power draw of an impeller to allow for a high-viscosity liquid. |
| Vortex | When an agitator swirls the liquid, a low-pressure zone forms at the surface, like a whirlpool. This is called a vortex. |
| Wet-Out | The dispersion of solid particles so that each particle is coated with liquid or dissolved in the solution. |
| Worm Gears | Gears giving a high speed reduction between non-intersecting shafts set at 90° to each other. |