Agitators

Chapter 1: What are Agitators?

Agitators are equipment used to homogenize media inside a tank. They operate by rotating immersed impellers at a controlled speed, known as revolutions per minute (RPM). The impeller's action induces flow and shear within the tank, facilitating the homogenization of single or multi-component media. This ensures that the media flows uniformly and maintains a consistent pattern.

Agitators serve various functions in industrial plants, including:

• Homogenizing solutions and suspensions to achieve a uniform consistency
• Maintaining solutions in a mixed state and preventing concentration gradient
• Dispersing a gas into a liquid solvent
• Promoting chemical reaction inside a reactor
• Maintaining a consistent temperature of the solution inside a vessel
• Promoting heat transfer to a jacket

Agitators are capable of handling liquid, gaseous, and solid media such as granules and powders. They are also effective with slurries, suspensions, and highly viscous liquids. However, choosing the right type, size, and design of agitator is critical, depending on the specific characteristics of the media. Factors such as viscosity and sensitivity to shear stress must be carefully evaluated when selecting an agitator. Agitators find extensive applications across various industries, including food and beverage, pharmaceuticals, agriculture, biotechnology, paints, and water treatment.

Agitators vs. Mixers

While the terms “agitators” and “mixers” are often used interchangeably, they have distinct meanings. Mixers are designed to rapidly blend two or more components together, regardless of whether they are of the same or different phases (e.g., solid-liquid, liquid-liquid, gas-liquid). Components typically enter a mixer in a “pure” state and exit combined with other components. On the other hand, agitators are used to maintain homogeneity and equilibrium within an existing mixture. They prevent the formation of concentration and temperature gradients, ensuring uniform consistency throughout the mixture.

Chapter 2: What are the parts of an agitator?

Agitators typically consist of three main components:

Motor Component

The motor drives the agitator assembly, producing the necessary torque to induce controlled flow and shear within the media. The power requirements of an agitator depend on various factors such as:

• Viscosity, specific gravity, and solid content of the media
• Speed or rpm of the impeller
• Impeller diameter
• Power number of the impeller
• Number of impellers

Shaft Component

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.

Impeller Component

The impeller is the most critical component of agitators, as it determines the flow pattern, efficiency of the homogenizing process, and mixing parameters. Responsible for directly exerting energy onto the materials being mixed, the impeller induces fluid flow and shear patterns through its rotation. It is the component primarily responsible for 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 are classified into two main types: open-blade and disc-type. An open-blade impeller has blades directly connected to the shaft with open spaces between each blade, making it easier for cleaning-in-place processes. A disc-type impeller consists of a disc with attached blades, creating a more uniform radial flow, commonly used in gas dispersions.

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Chapter 3: What are the typical flow patterns associated with agitator impellers?

Impellers are categorized based on the flow patterns they generate, often exhibiting one dominant flow pattern alongside others.

Axial Flow Impellers

Axial flow impellers cause the media to flow parallel to the impeller’s axis of rotation. These impellers have blades angled at less than 90 degrees to the plane of rotation, facilitating a "top-to-bottom cyclic" flow pattern within the tank. Fluid near the top is pushed downward by the impeller until it reaches the bottom, where it spreads across the tank floor and then flows upward along the wall before being drawn back into the impeller. This movement effectively mixes the fluid throughout the tank, preventing solids and solutes from settling at the bottom.

Axial flow impellers are suitable for solid suspensions and low to medium viscosity fluids. They exhibit low shear characteristics, making them ideal for shear-sensitive media like non-Newtonian fluids or those affected by stress-induced viscosity changes. They are also utilized in heat transfer applications and are typically installed in tanks with high liquid levels to induce substantial vertical currents.

Radial Flow Impellers

Radial flow impellers cause the media to flow perpendicular to the impeller’s axis of rotation, generating a "side-to-side flow pattern" within the tank. Fluid ejected from the impeller moves towards the tank walls, then either upward or downward before returning to the impeller's center. This continuous motion thoroughly mixes the tank contents. Unlike axial flow impellers, radial flow impellers lack angled blades directing fluid downward, necessitating baffles to minimize vortex formation and swirling.

Radial flow impellers induce high shear and less overall flow due to their sideways fluid movement. They are effective for blending viscous liquids and for gas-liquid and liquid-liquid dispersions. These impellers are commonly used in elongated tanks for low-level mixing applications.

Tangential Flow Impellers

Tangential flow impellers direct the media to flow in a circular path around the shaft, causing the fluid to rotate around the vessel along with the impeller blades. This rotational motion results in minimal vertical flow as the fluid impacts the tank wall. Tangential flow impellers are characterized by low shear forces.

These impellers are typically employed for blending highly viscous media and promoting stratification within the tank.

Close Clearance Impellers

A close clearance impeller belongs to a category of impeller types characterized by a high diameter ratio, typically around 80% of the tank diameter. These impellers maintain a small clearance between their outer edges and the tank wall. This design allows them to gently scrape sticky products that may accumulate on the tank walls, thereby improving product homogeneity and preventing fouling, which can reduce heat transfer efficiency.

Close clearance impellers are specifically used for low-speed laminar blending of highly viscous liquids, typically those with viscosities greater than 50,000 cP. They are commonly employed in industries handling paints, inks, adhesives, grease, polymer solutions, and other viscous products.

Examples of close clearance impellers include anchor, paddle, gate, and helical ribbon impellers. These impellers typically have diameters that are about 80% of the tank diameter. In contrast, other types of impellers like turbine impellers and propellers generally have diameters approximately one-third of the tank diameter.

Chapter 4: What are the different types of agitators used in?

Paddle Agitators

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 relative to the plane of rotation, creating an axial flow pattern. This design is commonly used for homogenizing suspensions and is a variation of the paddle agitator.

Anchor Agitators

Anchor agitators feature impellers shaped like anchors, typically with a U-shape that matches the tank's contour. They predominantly generate a tangential flow pattern but can incorporate angled blades on their horizontal supports to produce an axial flow.

These agitators are used for blending and heat transfer of highly viscous liquids. Their impellers create a laminar, low-shear flow, making them ideal for mixing shear-sensitive media. Anchor agitators are among the most economical options for laminar flow agitation and are suitable for tanks with rounded or conical bottoms. The impeller design allows for low clearance with the tank wall.

Helical Ribbon Agitators

Helical ribbon agitators feature a helical impeller blade mounted on a shaft by rods. These impellers serve as an alternative to anchor impellers and are designed to generate an axial flow pattern. They provide a higher fluid contact area, making them effective for mixing fluids with higher viscosities.

Double Helical Ribbon Agitators

Double helical ribbon agitators feature two helical blade flights running through the shaft in opposite directions. This design improves the mixing of more viscous fluids. They are also used in heat transfer applications and are regarded as the best impeller for high-viscosity laminar flow. The impeller can be designed with low clearance to the tank wall.

Agitators With Screw Impellers

Screw impellers are close-clearance devices with a helical flight attached directly to the impeller shaft. They provide excellent top-to-bottom turnover and are used for blending high-viscosity and shear-sensitive media.

Propeller Agitators

Propeller agitators primarily produce an axial flow pattern, though they can also create a tangential flow. The fluid is displaced and accelerated along the length of the tank as the impeller blades draw it in. The inclination of the impeller blades affects how the fluid is deflected. These blades are tapered towards the shaft to minimize centrifugal force and maximize axial flow.

Operating at medium to high speeds, propeller agitators often use marine propellers.

They are commonly used for homogenizing, dispersing, and suspending low-viscosity products. In solid-liquid suspension systems and chemical reactors, propeller agitators help prevent solids from settling at the tank bottom. They can be installed in unbaffled tanks, either vertically inclined from the centerline or positioned off-center.

Turbine Agitators

Turbine agitators serve as an intermediary between propeller and paddle agitators. They typically have larger diameters compared to propeller agitators and combine centrifugal and rotational motion. Turbine agitators are used in emulsification and dispersion processes where high-speed flow of the media is required. They provide a good balance between flow and shear, and are usually operated at high speeds. These agitators can handle a wide range of material viscosities while maintaining high mixing efficiency.

The types of turbine agitator impellers are as follows:

• Straight blade turbine impellers consist of two to eight flat vertical blades. These turbine impellers generate a radial flow pattern and high shear. They are used in solid suspension systems, heat transfer applications, and moving solutes from the bottom of the tank.

  

  
  

• Pitched blade turbine impellers have flat angled blades. The most common type is a four-blade turbine that makes a 450 angle with the vertical. They provide a combination of axial and radial flow; the axial flow is more dominant than the latter. They generate high shear and have good mixing efficiency. They are used in gas dispersions and solid suspensions.

  

  
  

• Rushton turbine impellers consist of flat vertical impeller blades arranged symmetrically around the circumference of a horizontal disc. These impellers generate a radial flow pattern. Rushton turbine impellers are effective for gas dispersion applications. The gas is sparged and captured below the rotating disc and then diverted in high turbulent regions near the blades. They can also be used in gas-liquid contacting and mixing.

  

  
  

• Smith turbine impeller is a variation of a Rushton turbine impeller, in which semi-circular or curved blades are attached instead of flat blades. This impeller is designed for gas-gas and gas-liquid dispersions and emulsions. It has a lower power requirement and larger gassing duty before flooding than Rushton turbine impellers.

  

  
  
• Curved blade turbine impellers, also known as sweptback turbine impellers, consist of curved blades mounted vertically to the central hub. These impellers also produce a radial flow pattern. They have a lower power requirement and produce less shear compared to flat blade turbine impellers. They are used in low level mixing, solid suspension systems, and heat transfer.

Agitators with Retreat Curve Impellers

Retreat curve impellers are designed with three curved blades that have rounded edges and corners, making them suitable for coating with glass. This coating helps to prevent corrosion and contamination, which is crucial in industries such as food, beverage, and pharmaceuticals. These impellers mainly produce radial flow, while the axial flow is affected by the ratio of the impeller diameter to the clearance from the tank bottom.

They are particularly effective for achieving uniform dispersion in solid-liquid and slurry mixtures. The rounded corners help reduce turbulence and ensure low shear, which is ideal for shear-sensitive materials. Typically, retreat curve impellers are used at lower speeds.

Agitators with Hydrofoil Impellers

Hydrofoil impellers feature two to four narrow, tapered, and cambered blades, with the three-blade configuration being the most common in industrial applications. The blade angle increases from the tip to the hub, creating an axial flow pattern. These impellers are designed to maximize fluid flow while generating low shear and consuming minimal energy, making them more efficient than pitched blade impellers. Hydrofoil impellers are also more economical than propellers for large diameter tanks.

Standard hydrofoil impellers are ideal for mixing, suspension, and flocculation in low viscosity fluids. They are also effective for shear-sensitive media, such as high-biomass slurries.

Wide blade hydrofoil impellers have a higher solidity ratio compared to standard hydrofoils, which is the ratio of the total blade area to the area of the circle encompassing the impeller. Due to their larger contacting area, these impellers are well-suited for gas-liquid dispersions. Although they require more power than standard hydrofoil impellers, their power requirement is still lower than that of pitched blade impellers.

Agitators with Dispersion Blade Impellers

Dispersion blade impellers feature a disc with sharp outer blades or teeth designed to break down agglomerations of solids and viscous liquids into fine particles. The sawtooth design is commonly used in various industries. Over time, the outer blades become sharper due to abrasion with the media. These impellers operate at high speeds to generate high shear and turbulent flow. Typically, dispersion blade impellers are made from durable materials like 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.

Agitators with Coil Impellers

Coil impellers utilize springs as their impeller blades, generating a primarily radial flow pattern. The springs are designed with high mechanical rigidity to overcome the resistance from solids at the bottom of a suspension during mixing. Additionally, coil impellers help prevent solids from settling at the bottom of the tank.

Chapter 5: What are the different configurations of agitators?

The types of agitators, based on their configuration when installed in a mixing tank, are as follows:

Top Entry Agitators

Top entry agitators are installed above the liquid level, typically at the centerline of a baffled vessel. They are well-suited for wide tanks with small aspect ratios (i.e., the ratio of liquid level to tank diameter) of about 1:1. These agitators can be positioned at an angle from the vertical to effectively suspend solids within the solution. They are suitable for sealed tanks and can be mounted using either a plate mount or flange mount. Their design and location facilitate easy dismantling and cleaning.

The top entry configuration is the most common type used in industrial mixing applications.

Side Entry Agitators

Side entry agitators are mounted on the sidewall of a tank and are ideal for situations where the tank's width is significantly greater than the liquid level. They are also used in tanks with low ceiling clearance, which prevents the installation of top-entry agitators. However, side entry agitators often result in less consistent mixing because the fluid is directed along the tank walls. During maintenance, the fluid must be removed. Additionally, side entry agitators generally have a higher power requirement compared to top-entry agitators.

Bottom Entry Agitators

Bottom entry agitators are positioned at the bottom of the tank with a short shaft directly linked to the motor's driveshaft. They are highly effective for mixing materials that tend to settle at the tank's bottom. These agitators are commonly used in large tanks and are well-suited for homogenizing mixtures like oils, milk, juices, and similar substances.

Conclusion

• Agitators are equipment that induces flow and shear to a fluid or material, which causes the fluid to homogenize.
• Mixers are used to blend two or more components rapidly. Agitators ensure homogeneity and equilibrium in an existing mixture.
• The main components of an agitator are the motor, shaft, and impeller. The impeller is considered as the most critical component that primarily determines the flow pattern, efficiency of the homogenizing process, and others.
• Agitator impellers can be classified based on the flow pattern they produce. These flow patterns are axial, radial, and tangential flows.
• The types of agitators are paddle agitators, anchor agitators, helical ribbon agitators, propeller agitators, turbine agitators, agitators with screw impellers, retreat curve impellers, hydrofoil impellers, dispersion blade impellers, and coil impellers.
• The configurations of an agitator when it is installed in a mixing tank are top entry, side entry, and bottom entry.

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