This article takes an in-depth look at agitators.
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Agitators are equipment used to homogenize media inside a tank. They work by rotating immersed impellers at a controlled speed, also called revolutions per minute (RPM). The work exerted by the impeller induces the flow and shear of the media inside the tank, causing a single or multi-component media to homogenize. The flow of the media is kept at a uniform rate and pattern.
Agitators can perform diverse functions in any industrial plant, which include:
Agitators can handle liquid, gaseous, and solid (e.g., granules, powders) media. They can also work with slurries, suspensions, and highly viscous liquids. However, selecting the appropriate agitator type, sizing, and design for the specific nature of media is crucial. Viscosity and sensitivity to shear stress are essential considerations when selecting an agitator. Agitators are widely used in many industries, including food and beverage, pharmaceutical, agricultural, biotechnology, paint, and water treatment industries.
The terms “agitators” and “mixers” are often used interchangeably, but they technically do not mean the same thing. Mixers are equipment that rapidly blends two or more components together. These components may be of the same or different phases (e.g., solid-liquid, liquid-liquid, gas-liquid). When a component enters a mixer, it is often in a “pure” state and leaves combined with other components. On the other hand, agitators maintain homogeneity and equilibrium in an existing mixture. They prevent the formation of concentration and temperature gradients and ensure uniform consistency in a mixture.
Agitators consist of three main components:
The motor drives the agitator assembly. It produces the required torque to induce flow and shear to the media at a controlled degree. The power requirement of an agitator depends on many factors such as:
The shaft is connected to the motor’s driveshaft and transmits the torque to the impeller. Couplings, end caps, and other devices are used to build the shaft assembly. Sealings are also used to prevent material build-up.
The impeller is considered the most critical component of agitators that determine the flow pattern, efficiency of the homogenizing process, and mixing parameters. It is responsible for exerting energy directly to the materials being mixed. It induces fluid flow and shear patterns as it rotates. This component does the actual mixing.
Impellers mainly consist of a hub and blades. The hub is directly connected to the shaft through a shaft key and a grub screw. The agitator blades are attached to the hub by welding or screwing. A welding connection is more hygienic because it prevents material-build on the fasteners and fittings. There could be more than one impeller installed in an agitator shaft. The number of impellers depends on the type of impeller, the maximum height of the media inside the vessel, vessel diameter, and specific media gravity. This information is also necessary when determining the spacing between the impellers.
Impellers may be classified as an open- or disc-type. An open blade impeller has blades directly connected to the shaft. There are open spaces between each blade which makes cleaning-in-place easier. A disc-type impeller consists of a disc with attached blades. It creates a more uniform radial flow and is used in gas dispersions.
Impellers are classified based on the flow pattern they create. An impeller can produce more than one flow pattern, with one flow pattern more dominant than the other.
Axial flow impellers cause the media to flow parallel to the impeller’s axis of rotation. These impellers have angled blades that make less than 900 to the plane of rotation. The rotation of the impeller causes a “top-to-bottom cyclic” flow pattern. The fluid at the upper portion of the tank is forced to flow down until it is deflected at the tank bottom. It then spreads over the tank bottom and subsequently flows up along the wall before being drawn by the impeller. This motion mixes the fluid at the top and bottom of the tank and prevents the solids and solutes from settling at the tank bottom.
Axial flow impellers are suitable for solid suspensions and low to medium viscosity fluids. Their low shear characteristics make them ideal for shear-sensitive media (i.e., non-Newtonian fluids or fluids changing viscosity due to applied stress). They are also used for heat transfer applications. Axial flow impellers are typically installed in tanks with high liquid levels where large vertical currents are desired.
Radial flow impellers cause the media to flow perpendicularly to the impeller’s axis of rotation. The rotation of the impeller causes a “side-to-side flow pattern.” As the fluid ejects from the impeller, it flows towards the tank’s walls. It then moves either upward or downward until it is drawn again to the impeller's center. This motion is repeated to incorporate the contents of the tank thoroughly. As opposed to axial flow impellers, radial flow impellers do not have angled blades that force the fluid downwards. Baffles are essential when using these impellers to minimize vortex formation and swirling motion.
Radial flow impellers produce a high degree of shear and less flow since the fluid flows sideways. Hence, they are suitable in blending viscous liquids and for gas-liquid and liquid-liquid dispersions. Radial flow impellers are used for low-level mixing inside elongated tanks.
Tangential flow impellers cause the media to flow in a circular path around the shaft. The fluid rotates around the vessel together with the impeller blades. A very low vertical flow is produced as the fluid hits the tank wall. These impellers induce low shear.
Tangential flow impellers are used in blending highly viscous media and stratification.
Close clearance impeller is a category of impeller types with a high diameter ratio (ratio of the impeller diameter to the tank diameter). The diameters of these impellers are about 80% of the tank diameter, making a small clearance between the tank wall and the outer edges of the impeller. These propellers can slightly scrape the sticky products building up on the tank walls, thereby improving product homogeneity and preventing fouling, reducing heat transfer rate.
Close clearance impellers are used in the low-speed laminar blending of highly viscous liquids (greater than 50,000 cP). The fluids these impellers handle include paints, inks, adhesives, grease, polymer solutions, and other viscous products.
Examples of close clearance impellers are anchor, paddle, gate, and helical ribbon impellers. The diameter of other impeller types, such as turbine impellers and propellers, generally have one-third of the tank diameter.
Paddle agitators consist of two flat paddle-shaped impeller blades extending to reach the tank walls. They are used if no extensive axial and radial flow is required. These impellers can produce a laminar low shear flow and are used for low viscosity liquid mixing, crystallization, dissolution, and heat transfer. They are typically operated at low speeds and predominantly give a tangential flow pattern. Secondary blades can be installed on the paddle blades to enhance the mixing of more viscous materials.
The impeller blades are inclined from the plane of rotation to create an axial flow pattern. This variation of paddle agitator is typically used for homogenizing suspensions.
Anchor agitators have impellers that resemble the shape of an anchor. They typically have a U-shape that matches the contour of the tank. They generate a predominantly tangential flow pattern, but angled blades can be incorporated on their horizontal supports to create an axial flow.
Anchor agitators are used for blending and heat transfer of highly viscous liquids. Their impellers generate a laminar low-shear flow; hence, they are used to mix shear-sensitive media. Anchor agitators are considered the most economical laminar flow agitator. They are suitable for tanks with rounded or conical bottoms. The impeller can be designed to have a low clearance with the tank wall.
Helical ribbon agitators have a helical impeller blade fixed in the shaft by rods. These impellers are an alternative to anchor impellers which can generate laminar flow. These agitators create an axial flow pattern. They generally have a higher fluid contact area to mix fluids with higher viscosities.
Double helical ribbon agitators are designed to have two helical blade flights running through the shaft in opposite directions. The additional flight enhances the mixing of more viscous fluids. These impellers are also used in heat transfer applications and are considered the best high viscosity laminar flow impeller. The impeller can also be designed to have a low clearance with the tank wall.
Screw impellers are close clearance impellers with a helical flight directly attached to the impeller shaft. They provide an excellent top-to-bottom turnover. They are used in blending high viscosity and shear-sensitive media.
Propeller agitators mainly produce an axial flow pattern, though tangential flow can also be produced. The fluid is displaced and accelerated longitudinally after the impeller blades draw it. The deflection of the fluid depends on the inclination of the impeller blades. The impeller blades are tapered towards the shaft to minimize centrifugal force and maximize axial flow.
Propeller agitators are operated at medium to high speeds. Marine propellers are typically used for propeller agitators.
Propeller agitators are used in homogenization, dispersion, and suspension of low viscosity products. They are used in solid-liquid suspension systems and chemical reactors to prevent solids settling at the tank bottom. They may be installed in unbaffled tanks when the propeller is inclined vertically from the centerline or in an off-center position.
Turbine agitators are an intermediate between propeller and paddle agitators. They usually have larger diameters than propeller agitators. These agitators combine centrifugal and rotational motion. They are used in emulsification and dispersion processes in which the media is required to flow at high speeds. They offer a good balance between flow and shear. They are typically operated at high speeds. They can handle a wide range of material viscosities and still provide a high mixing efficiency.
The types of turbine agitator impellers are the following:
Retreat curve impellers consist of three curved blades with rounded edges and corners that can be easily coated with glass material. The glass coating can prevent corrosion and contamination, critical in the food, beverage, and pharmaceutical industries. Retreat curve impellers primarily create a radial flow pattern; the axial flow depends on the diameter ratio and the clearance of the impeller from the tank bottom.
Retreat curve impellers are used in achieving uniform dispersion in solid-liquid and slurry media. The rounded corners can prevent turbulence and generate low shear, making them suitable for shear-sensitive media. These impellers typically operate at low speeds.
Hydrofoil impellers are composed of two to four narrow tapered and cambered blades. The three-blade configuration is the most common in industries. Their blade angle increases from the tip to the hub. These impellers generate an axial flow pattern. Hydrofoil impellers maximize the fluid flow while producing a low shear rate and consuming the least energy, making them more efficient than pitched blade impellers. Hydrofoil impellers are more economical than propellers when used in tanks with large diameters.
Standard hydrofoil impellers are used in mixing, suspension, and flocculation systems involving low viscosity fluids. They are also used in shear-sensitive media such as high-biomass slurry.
Wide blade hydrofoil impellers have a higher solidity ratio compared to standard hydrofoils. The solidarity ratio refers to the ratio of the total blade area to the area of the circle circumscribing the impeller. These hydrofoil impellers are suitable for gas-liquid dispersions due to their large contacting area. The power requirement for these impellers is higher than standard hydrofoil impellers. However, the power requirement of a pitched blade impeller is still greater.
Dispersion blade impellers consist of a disc with sharp outer blades or teeth at its edges which break down agglomerations of solids and viscous liquids into fine particles. The sawtooth design is common in industries. The outer blades sharpen through use because of their abrasion with the media. These impellers are operated at high speeds to achieve high shear and turbulent flow. Dispersion blade impellers are typically made of hard metals such as carbide and stainless steel.
Dispersion blade impellers are commonly used in solid-liquid or liquid-liquid dispersion. They are used in dispersing pigments in a viscous paint mixture. They are also used in emulsification and grinding applications.
Coil impellers have springs that act as the impeller blades. They primarily create a radial flow pattern. The spring has high mechanical rigidity to overcome the resistance given by the solids at the bottom of a suspension during mixing. These impellers are also used to prevent solids from settling at the bottom of the tank.
The types of agitators based on their configuration when it is installed in a mixing tank are the following:
Top entry agitators are installed at the overhead of the liquid level, commonly at the centerline of a baffled vessel. They are ideal for wide tanks with small aspect ratios (i.e., the ratio of liquid level to tank diameter) of about 1:1. They may be installed with an angle offset with the vertical to suspend solids in the solution effectively. They are suitable for sealed tanks and can be attached by plate mount or flange mount. Their structure and position allow easy dismantling and cleaning.
The top entry is the most common configuration found in industrial mixing.
Side entry agitators are installed on the sidewall of a tank. They are used when the width of the tank is much greater than the liquid level. They are also used in tanks with low ceiling clearance, which does not permit the installation of a top-entry agitator. However, less consistent mixing is achieved because the fluid is pushed along the walls of the tank. The fluid content must be drawn during maintenance. The power requirement of side entry agitators is also higher than top entry agitators.
Bottom entry agitators are installed at the bottom of the tank. They have a short shaft that is directly connected to the motor's driveshaft. Bottom entry agitators are used for mixing the materials which tend to settle at the tank bottom. They are commonly used in large volume tanks. They are used in homogenizing mixtures such as oils, milk, juices, and others.
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