When it comes to selecting a mixer for your blending application, one of the key pieces to the puzzle is impeller selection. After all, it is the component that physically does the mixing. Impellers are selected for a particular application by their unique shear and fluid flow patterns, resulting in just the right mix. Impeller types can be categorized into two basic categories: axial flow, and radial flow.
Axial flow impellers have an up and down flow pattern, ideal for applications where solids suspension or stratification is a challenge. The flow pattern produced by typical axial flow impeller produces an excellent top to bottom motion when the agitator is center mounted, and the vessel is fully baffled see Fig 1B.
If the baffles are removed, the fluid in the vessel will swirl and vortex Fig 1Aresulting in a rather poor mix. Propeller: The propeller is typically used in mixers that are small and portable. This type of impeller tends to be heavy and quite expensive in larger sizes. Pitched Blade: A pitched blade impeller is used when a balance of flow and shear is required. It is especially useful in applications where 2 or more liquids are blended, and is well suited for situations with low bottom clearance or low liquid submergence.
Mixing 101: Optimal Tank Design
Hydrofoil: The hydrofoil impeller offers the best high flow design. It is known for its low turbulence, and is excellent for shear sensitive applications. Radial flow impellers generate a side to side flow pattern. Like the axial flow impellers, adding baffles reduces the swirling and vortexing motion in the vessel Fig 2Btherefore increasing the level of agitation inside.
Radial Impeller: These types of impellers are typically available in 4 or 6 blade designs. They are known to provide more shear and less flow per unit of applied horsepower than axial flow designs, and in comparison, radial flow impellers do not have a high tank turnover flow. They are sensitive to viscosity, which makes them an excellent impeller in dispersion applications like pigment pastes or caulking compounds. Learn about the 9 most common mixing scenarios, mixing design calculations, and tips for more efficient mixing.
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Log In Sign Up. Agitation and mixing. Suryakant Randeri. Agitator is the main part, like an impeller in a pump to give mechanical energy to liquid. Suitable for mixing of low viscose liquids, particle suspension and heat transfer enhance. Propeller small diameter, high speed, large flow rate and low head. Helical ribbon large diameter and mixing range, low speed, low head.
Special design for high viscosity liquid. Suitable for high viscosity liquids and capable of preventing the deposit on tank wall. The liquid level on tank center will fall to form a forced vortex. The high the speedthe deep the vortex. Sometimes gas is absorbed from lower liquid level to disturb operation. Solution 1 install baffles on tank wall. Especially for particle suspension. For agitation operation, the useful flows are axial and radial, not the tangential.
Convective flow breaks the liquid into large drops macro mixing ; the drops are then broken into smaller ones by vortex deformation inter-drop mixing those vortex breakage and deformation will increase or renew the contacting area between drops with different concentration and promote the molecular diffusion. A fully homogeneous mixing depends on molecular diffusion.
Agitation Scale up
Typically, axial impellers promote bulk motion while radial ones promote instead shear stresses. Processes promoted by mixing may be classified on the basis of their sensitivity to bulk motion or shear stress promotion: Bulk motion controlled processes — those which do not need to create new interface blending, heat transfer promotion or which must allow the availability of the actual interface for exchange processes solid suspension.
Shear rate controlled processes — those which efficiency rely on the generation of inter-phase exchange surface gas- liquid and liquid-liquid dispersions. Mixing mechanism of homogeneous systems low viscosity liquids Large vortex is broken into small ones by shearing effect.In our article on 4 Impeller Types and Their Applicationswe provided an overview on the most common types of impellers used in industrial mixing.
Our focus on impellers is due to the fact that they are the part of the mixer that does the actual mixing: as they rotate they create fluid flow. These flow patterns are the primary considerations when designing a mixer because creating the right flow pattern is critical to achieving the desired result.
The most common flow patterns in mixing are axial down and up and radial side to side flow. These flow patterns also describe the generic classes of impellers: axial and radial. Axial down and up pumping is an important flow pattern because it addresses two of the most common challenges in mixing; solid suspension and stratification. In this process both the superficial and annular velocities can be calculated to determine and control the level of mixing. If your axial impeller is causing swirling or vortexing, check out our previous Mixing posting Baffled by Baffles?
The pitch blade impeller is the most versatile impeller and was the standard until the development of the airfoil. A more technical benefit is the laminar flow created by the camber of this impeller.
This camber reduces turbulence shear substantially. That is why it is selected for shear-sensitive applications as well. As you can see, the first step to meeting your mixing objective is to identify your desired flow pattern, which is dictated by the impeller. Next, you must consider the 4 main factors for configuring your mixer. From there, you must address the mechanical requirements of your mixing process: pumping or flow.
The mechanical requirements to produce flow or torque per unit volume, the most important relationship in mixing, are explained next. Submit a quote request online, or call us directly at to find out how we can optimize the quality of your application results. Dynamix Agitators incorporates over 45 years of industrial mixing experience into its industrial mixer models and the processes they are part of.
Our mixers and agitators are configured to suit your process and application. Posted on September 20, Axial Flow Impellers Axial down and up pumping is an important flow pattern because it addresses two of the most common challenges in mixing; solid suspension and stratification.Sonic boom in missouri today
Here are some of the most common axial impellers: A. Pitch Viscosity : 0 — 50, cps The pitch blade impeller is the most versatile impeller and was the standard until the development of the airfoil. Radial Flow Impellers Unlike axial impellers, radial impellers are commonly selected for low level mixing known as a tickler blade or elongated tanks.
They typically give high shear rates because of their angle of attack. They also have a relatively low pumping number, making them the most sensitive to viscosity. Radial impellers do not have a high tank turnover flow like axial flow impellers. Impeller Selection As you can see, the first step to meeting your mixing objective is to identify your desired flow pattern, which is dictated by the impeller. Mixing Terms Glossary Shear Stress Parallel-acting force where 2 layers inside the fluid slide against each other.
This is in contrast to compression perpendicular-acting forcetension stretching forceand torsion twisting force. Power Number NP Constant used to calculate power draw, unique to each type of mixing impeller. Pumping Number NQ Constant used to calculate flow or pumping rate, derived empirically for each unique shape of impeller.
Superficial Velocity Velocity of the fluid being pumped downward by the impeller.
Annular Velocity Rate at which the liquid is traveling upwards inside a tank past the impeller. Stratification Separation of 2 immiscible liquids due to density variation.In industrial process engineeringmixing is a unit operation that involves manipulation of a heterogeneous physical system with the intent to make it more homogeneous.
Familiar examples include pumping of the water in a swimming pool to homogenize the water temperature, and the stirring of pancake batter to eliminate lumps deagglomeration. Modern industrial processing almost always involves some form of mixing. With the right equipment, it is possible to mix a solid, liquid or gas into another solid, liquid or gas. A biofuel fermenter may require the mixing of microbes, gases and liquid medium for optimal yield; organic nitration requires concentrated liquid nitric and sulfuric acids to be mixed with a hydrophobic organic phase; production of pharmaceutical tablets requires blending of solid powders.1971 dodge dart demon
The opposite of mixing is segregation. A classical example of segregation is the brazil nut effect. The type of operation and equipment used during mixing depends on the state of materials being mixed liquid, semi-solid, or solid and the miscibility of the materials being processed.
In this context, the act of mixing may be synonymous with stirring- or kneading-processes.Agitation : Types of impellers
Mixing of liquids occurs frequently in process engineering. The nature of liquids to blend determines the equipment used. Single-phase blending tends to involve low-shear, high-flow mixers to cause liquid engulfment, while multi-phase mixing generally requires the use of high-shear, low-flow mixers to create droplets of one liquid in laminarturbulent or transitional flow regimes, depending on the Reynolds number of the flow.
Turbulent or transitional mixing is frequently conducted with turbines or impellers ; laminar mixing is conducted with helical ribbon or anchor mixers. Mixing of liquids that are miscible or at least soluble in each other occurs frequently in process engineering and in everyday life. An everyday example would be the addition of milk or cream to tea or coffee. Since both liquids are water-based, they dissolve easily in one another.
The momentum of the liquid being added is sometimes enough to cause enough turbulence to mix the two, since the viscosity of both liquids is relatively low. If necessary, a spoon or paddle could be used to complete the mixing process.
Blending in a more viscous liquid, such as honeyrequires more mixing power per unit volume to achieve the same homogeneity in the same amount of time. Blending powders is one of the oldest unit-operations in the solids handling industries.
Mixing and Agitation
For many decades powder blending has been used just to homogenize bulk materials.Search This Blog. Mixing and Homogenization. Theory of mixing, solid-solid, solid-liquid and liquid-liquid mixing equipments, homogenizers. Mixing may be defined as a unit operation in which two or more components, in an unmixed or partially mixed state, are treated so that each unit particle, molecule etc.
In the production of tablets, capsules, sachets and dry powders two or more powders or granules are mixed.Clockwise and counterclockwise rotations
Mixing may aim at producing a change that is physical, for example the solution of a soluble substance. In case of dissolving a solid in a solvent mixing will take place by diffusion but the process will be slow. In this case agitation makes the process rapid.
In case of emulsions and creams two immiscible liquids are mixed where one liquid is dispersed into other. In suspension and pastes solid particles are dispersed in a liquid by mixing. Mixing will usually encourage and control at the same time a chemical reaction, so ensuring uniform products. Mixtures may be divided into three types that differ fundamentally in their behavior:. Positive mixtures are formed from materials such as gases or miscible liquids, which mix spontaneously and irreversibly by diffusion and tends to approach a perfect mix.
There is no input of energy required. If enough time is available the mixing is complete. In general, such materials do not present any problems in mixing. In negative mixtures, after mixing, the components will tend to separate out. If this occurs quickly, then energy must be continuously input to keep the components in dispersed state.
Negative mixtures are more difficult to form and a higher degree of mixing efficiency is required. Neutral mixtures are static in their behavior, the components having no tendency to mix spontaneously, nor do they segregate when mixed.
It has been generally accepted that solids mixing proceeds by a combination of one or more of the following mechanisms:. A relatively large mass of material is moved from one part of the powder bed to another - this is called convection. As a result of forces within the particulate mass, slip planes are set up. Depending on the flow characteristics these can occur singly or in such a way that it give rise to laminar flow. When shear occurs between regions of different composition and parallel to their interface, it reduces the scale of segregation by thinning the dissimilar layers.
Shear occur in a direction normal to the interface of such layers is also effective since it too reduces the scale of segregation. Such as exchange of positions by single particles result in reduction of the intensity of segregation. Diffusive mixing occurs at the interfaces of dissimilar regions that are undergoing shear and therefore results from shear mixing. In tumbling mixers, rotation of the vessel imparts movement to the materials by tilting the powder until the angle of the surface exceeds the angle of repose when the surface layers of the particles go into a slide.
A common type of mixer consists of a container of one of several geometrical forms, which is mounted so that it can be rotated about an axis. The resulting tumbling motion is accentuated by means of baffles or simply by virtue of the shape of the container. The drum type, cubical-shaped, double-cone and twin shell blenders are all examples of this class of mixers. Drum-type blenders with their axis of rotation horizontal to the centre of the drum are used quite commonly.
Cubical and polyhedron shaped blenders with the rotating axis set at various angles also are available.Apart from the actual mixer, the design of the mixing tank is the single most important factor in producing a successful result in any process. Incorrect positioning negatively affects the performance of a mixer, the quality of a product, and may even be detrimental to the performance life of the mixer drive.
When looking at tank designs, vertical cylindrical, and square or rectangular tanks are most common. In the process of selecting the optimal tank design for an application there are certain rules of thumb to observe. For most mixing applications the ideal liquid level to tank diameter ratio is 0. A ratio that is too small does not allow proper axial mixing in the tank. Anything less than a 0. Any time that the liquid level to tank diameter ratio exceeds 2.
As the mixer shaft length extends, so does the price of the mixer. For example, if we take a typical 5,gallon tank based on the ideal scenario and the slim tank scenario, the following will occur:.
Vertical cylindrical tanks are the most common type of tank in use. A key consideration for cylindrical tanks is to ensure that they are either baffled or offset-mounted to prevent swirling from occurring. However, for larger tanks, it is much more cost effective to install baffles than to invest in a more expensive, more heavy-duty mixer that is offset-mounted. Rectangular tanks have an equivalent diameter that can be calculated by taking the square root of Length x Width and multiplying it by 1.
However, rectangular tanks are not recommended for solid suspension because packets of solids will form in the corners. Therefore, a greater level of mixing is required than is needed in a cylindrical tank of equal size to achieve a similar result.
Some tanks will have a round dish or cone bottom. Below are some standard guidelines about approaching mixing for these tanks.
Rectangular tanks are self-baffling, so the use of baffles is a requirement when using cylindrical tanks only. If an agitator is center-mounted in an un-baffled cylindrical tank, it produces a swirling motion, which is very inefficient. As an example, imagine two particles swirling in a circular motion, they will always be chasing each other and will not mix. The optimal installment is comprised of four baffles within a tank, but the use of three baffles is sufficient for most applications.
Baffles should fully extend the length of the tank, leaving some space at the bottom to avoid the build-up of solids. For more detailed information on the use of baffles and optimal arrangements, see our article Mixing Baffled by Baffles. Submit a quote request online, or call us directly at to find out how we can optimize the quality of your application results.
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Generally, agitation refers to forcing a fluid by mechanical means to flow in a circulatory or other pattern inside a vessel. Mixing usually implies the taking of two or more separate phases, such as a fluid and a powdered solid or two fluids, and causing them to be randomly distributed through one another.
There are a number of purposes for agitating fluids, some of which are briefly summarized: 1. Blending of two miscible liquids, such as ethyl alcohol and water.
Dissolving solids in liquids, such as salt in water. Dispersing a gas in a liquid as fine bubbles, such as oxygen from air in a suspension of microorganisms for fermentation or for the activated sludge process in waste treatment.
Suspending of fine solid particles in a liquid, as in the catalytic hydrogenation of a liquid, where solid catalyst particles and hydrogen bubbles are dispersed in the liquid. Agitation of the fluid to increase heat transfer between the fluid and a coil or jacket in the vessel wall.
The height of liquid is approximately equal to the tank diameter. An impeller mounted on a shaft is driven by an electric motor. A typical agitator assembly is shown in Fig.
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