What is a Agitator ? and What are the Types of Agitators
- A propeller or a turbine in a tank is the most commonly used equipment for operations involving liquid-liquid and to some extent liquid-solid mixing.
- In liquid-liquid mixing, a system may contain liquids with or without solids that are not viscous (e.g., light oils) liquids with or without solids that are viscous but pourable (e.g. paints, heavy oils) and liquids with solids that form stiff pastes (oil-bound distempers).
- The usual form of equipment is a vertical vessel fitted with an agitator (i.e., an agitated vessel). The height of the vessel ranges from 1.5 to 2 times the diameter. The impeller diameter is usually one-third of the tank diameter.
- An agitated vessel is a vertical, cylindrical vessel fitted with an agitator. The agitator is driven by an electrical motor directly or through a speed reducing gear box. It is provided with inlet and outlet connections, coil, jacket, etc.
Types of Agitators : Agitator Meaning : Agitator in Vessel
- In the agitated vessel, the impeller creates a flow pattern, causing the liquid to circulate through the vessel and return ultimately to the impeller.
- An agitator is a combination of the impeller and shaft, i.e., impeller attached to the shaft. There are various types of agitators and so the types of agitator. When we say turbine impeller, it is also termed as turbine agitator. The terms Impeller and Agitator are used interchangeably.
What is a Agitator ? : Agitator Animation
Types of Agitator
There are two types of agitators :
- Axial flow Agitators and
- Radial flow Agitators.
- Axial flow Agitators make an angle of less than 90° with the shaft. They generate flow currents parallel to the axis of shaft.
- Radial flow agitators have blades parallel to the axis of the shaft. They generate flow currents in tangential (tangential to the circular path) or radial directions (perpendicular to the shaft).
- Agitators are further classified into three sub-types as :
- Propeller Agitators
- Paddle Agitators
- Turbine Agitators
- Helical Agitators
- Anchor Agitators
- Propellers and pitched blade turbines are axial flow agitators, whereas paddles, flat blade, curved blade, disc flat blade turbines are radial flow Agitator.
- Axial Agitators or impellers are used at high speeds to promote rapid dispersion and used at low speeds to keep solids in suspension. Radial flow agitators are used for large scale mixing of solid/ liquid suspension.
Propeller Agitators
- A propeller agitator is an axial-flow, high speed impeller commonly used for low viscosity liquids. It may be mounted centrally, off-centre or at an angle to the vessel. It is simple and portable. The diameter of propeller usually lies between 15 to 30% of the diameter of the vessel.
- A typical propeller agitator is shown in Fig. The most common propeller is a standard three bladed marine propeller. A propeller is shaped with a tapering blade to minimise the effect of centrifugal force and produce maximum axial flow.
Types of Agitator : Agitator Meaning : Propeller Agitator
- Small propellers rotate at full motor speeds, whereas large ones rotate at a speed of 400 to 800 r.p.m.
- Propellers may also be mounted near the bottom of the cylindrical wall of a vessel as shown in Fig. for blending low viscosity fluids or suspending slow settling sediments in very large tanks.
- Propeller drives the liquid straight down to the bottom of the vessel, at the bottom the stream spreads radially in all directions towards the wall, then the liquid flows upward along the wall, and finally returns to the suction of impeller from the shaft. Such a flow pattern is shown in Fig. These agitators are used in situations where strong vertical currents are desired, e.g., for suspending heavy particles.
Types of Agitator : Side Entering Propeller
Types of Agitator : Flow Pattern in Vessel
Turbine Agitators
- Various types of turbine agitators are shown in Fig. Pitched blade turbine is an axial flow impeller while curved blade and flat blade turbines are radial flow agitators.
Turbine Agitator
- They are capable of creating a vigorous mixing action due to centrifugal and rotational motions generated by them. A stator ring surrounding this impeller gives an efficient mixing action.
- The blades of the impeller may be attached to a central hub or to a central disc. The diameter of the impeller is kept between one-third and one-sixth of the vessel diameter. The blade length is one-fourth of the impeller diameter. With a central disc, it is 1/8th of the impeller diameter. The blade angle of curved blade turbine may be between 30 to 60o. The impeller speed usually ranges from 50 to 250 r.p.m.
- Turbines are very effective over a wide range of viscosities (upto 104 cP).
- Turbine agitators drive the liquid radially against the wall, where the stream divides into two portions. One of the portions flows downward to the bottom and then returns to the centre of impeller from below, while other flows upward towards the surface and finally returns to the impeller from the above (See Fig. Types of Agitators).
- Turbines are especially effective in developing radial currents, but with a baffled vessel they also induce vertical flow currents. To avoid vortexing and swirling with turbines, baffles or a diffuser ring can be used
- It is a common practice to locate the agitator at a height not less than one agitator / impeller diameter length from the bottom of a vessel and it should be submerged in the liquid at a depth equal to twice the diameter of agitator / impeller at low speeds and four times at high speeds. When the depth of the liquid is more than twice the agitator diameter, it may be advisable to use two agitators on the same shaft.
Paddle Agitators
- Paddle agitators with two or four flat blades are very common. The blades of these agitators are usually vertical and extend close to the vessel wall. They are simply pushers and cause the mass to rotate in laminar swirling motion with practically no radial flow along the paddle blades or any axial flow (vertical motion). The circulation is poor and the mixing action is insufficient. These rotate with a speed ranging from 20 to 150 r.p.m. The total length of this impeller lies between 50 to 80% of the inside diameter of the vessel (commonly 80% of the diameter). The width of the blade is 1/4 to 1/10th of the paddle diameter.
Paddle Agitators
- In some designs, the shape of blades is similar to the shape of the bottom of a vessel so that they scrap the surface or pass over it with a close clearance. Such a type of paddle is known as an anchor agitator. Anchors are very useful for preventing deposits on a heat transfer surface as in reaction vessels and are commonly employed for obtaining improved heat transfer in high viscosity fluids but are poor mixers.
- Gate and anchor are used to sweep the entire peripheral area of the vessel, both walls and bottom.
Helical Agitators
They are known as helical agitators because the blades are placed in a helix shape. Helical agitators, which resemble threaded screws, are axial flow agitators that generate strong motion within the vessel or tank. This type of agitator is commonly used in the polymer industry.
Anchor Agitators
Anchor agitators are commonly used for mixing very viscous and non-Newtonian fluids. Their name refers to the form of the impeller, which resembles an anchor. They are usually installed in tanks or containers with conical or rounded bottoms.
Selection Criteria of Industrial Agitators
- The phase that will be mixed (single-phase or multiphase).
- The precise function is required (blending, dissolving, dispersion, heat exchange, chemical reaction, crystallization, emulsification, suspension, etc).
- The mixing process (batch size, the time required for agitation, material addition sequence, etc).
- The characteristics of the materials to be mixed.
- The viscosity of the initial ingredient and the viscosity of the finished product.
- Solids solubility and concentration used.
- Process outcomes that are desired.
- Properties that are corrosive or flammable.
Data required for the design of an Agitator
The following data are required for the industrial design of an agitator:
- Agitation's purpose/function.
- Cycle of mixing.
- Foaming proclivity.
- Individual physical properties and quantities of materials to be mixed.
- Dimensions of the tank/vessel/reactor, Preferably with a sketch.
- Working hours.
- Electrical responsibilities.
- Construction Material.
Applications of Industrial Agitators
Industrial agitators are used in a variety of applications, including
- Viscous products and non-Newtonian liquids are mixed in a tank.
- Liquids that are clean or lightly contaminated.
- Slurries that are fibrous and non-fibrous.
- Liquids with a high solid or gas content.
FAQs
How many types of agitators are there?
There are two types of agitators one is Axial flow agitators and another is Radial flow agitators.
What is hydrofoil agitator?
The hydrofoil is a 3 bladed high efficiency impeller.
What is the difference between agitator and stirrer?
Stirrer is a device used to stir while agitator is one who agitates.