Traditionally impellers have been classified as either axial or radial flow. The distinction is shown in Figure 4.
Axial flow creates an overall top-to-bottom flow pattern along the shaft. Radial flow is
characterized by a radial discharge from the impeller, and dual recirculation loops above and
below the impeller. We need to keep in mind that these are highly idealized flow patterns
typically only true for water-like fluids. The flow patterns will change significantly with fluid properties.
Although this type of classification has been around for a number of years, you cannot
determine much about the success of a mixer design from these idealized flow patterns.
Perhaps a somewhat equivalent but more useful way to classify impellers is on their design
strength between pumping and producing turbulence in the impeller region.
Impellers which convert the power draw to bulk flow, have larger blade surface area, profiled
blades to reduce turbulence, and generally pump in the axial direction.
Impellers which convert the power draw into intense turbulence in the impeller region have
small blade area or vertical orientation of the blades. Some impellers can be found in the
middle of the spectrum, providing some pumping as well as turbulence in the impeller region.
This classification is important as some processes, such as blending, would favor
an impeller with pumping as the design strength, whereas other processes, such as
emulsification, would favor turbulence generation.
One of the most common types of impellers seen in bench scale equipment is the propeller.
These are similar to marine propellers. An example is shown in Figure 5.
The original design intent is to produce axial flow in water to provide propulsion for watercraft.
They are not common in larger process tanks since they are made from a casting
manufacturing method. This makes larger versions extremely heavy and expensive.
Since it is unlikely that a propeller will be used in the full-scale process, it is best not to use it in the bench or pilot scale process either.
The Rushton turbine is shown in Figure 6.
It is named after its inventor, but is sometimes more commonly called a disk turbine.
The Rushton turbine was originally developed for gas dispersion. The disk surface redirects the
gas flow out radially through the blades where it is dispersed. In recent years more efficient
curved bladed disk turbines have replaced the Rushton turbine for gas dispersion processes.
We will discuss this more, later in the gas/liquid chapter.
A turbine has flat blades attached to a hub on the mixer shaft. The blades can be vertical or pitched. Examples are shown in Figure 7.
The vertical blade turbine is sometimes called the flat blade turbine. For pitched turbines,
the most common angle is 45°, although 30° and 60° are also available.
The pitched turbine generates less turbulence than the vertical turbine blade, but provides more pumping.
The turbine can also provide pumping instead of turbulence by making the blades
wider. The blade width to diameter ratio is usually around 0.2.
Turbines can have six or eight blades, but little benefit has been found to exceed more than from four blades, the most common design.
The turbine is easy and cheap to manufacture. Thet can be made in any machine shop.
They a wide range of uses. Because of this, they have become the workhorse of the industry.
A low viscosity hydrofoil is shown in Figure 8.
We can seen to picture that the blades are curved and twisted as we move from the shaft out to the tip.
The intention is to streamline the blade shape and reduce the formation of turbulence in the
wake of the blade. These same principles are used to make cars more fuel efficient through
streamlined body shapes. The principle is shown in Figure 9.
The reduction in turbulence makes these impellers better at pumping the fluid.
It also allows the power input to occur at higher RPM, resulting lower torque designs. This may result in lower equipment cost.
As we mentioned earlier, reduction in turbulence is not always desirable and the potential to misuse these impellers is large.
Generally these impellers should only be used with low viscosity water-like fluids.
These impellers should not be used for complex or viscous fluids because of the small blade area.
An example of the wide blade hydrofoil is shown in Figure 10.
The blade area is large and the impeller has a high solidity ratio. What does this mean?
If we hold the impeller up to a light, we see almost no light passing through. The blade surface occupies most of the cross sectional
area. This impeller, because of its large blade area, is an excellent pumper and produces
little turbulence. It is a good choice for viscous, shear thinning and yield stress fluids.
Research has also shown that it is effective in gas dispersion processes in an upside down configuration because of the high solidity ratio.
Close clearance agitators are characterized by a large D/T ratio. The impeller diameter is almost as large as the tank diameter.
The most common type is the U-shaped anchor shown in Figure 11.
Often scrapers are attached to the anchor to create a scraped wall agitator.
The anchor literally scrapes the tank wall clean as the impeller passes. This is needed for highly
viscous fluids, especially if a tank jacket is used to cool or heat the fluid.
Another type of close clearance impeller is the helix If the helix has a solid interior, it resembles a screw.
resembles a screw. A ribbon helix, on the other hand, has an open interior. They come in either
a single or double design. The helix works like a positive displacement device. The rotation of the
helix pushes the fluid down in the axial direction. Often a draft tube is used with smaller diameter
helices. The helix or screw is placed inside the tube as shown in Figure 12.
Fluid enters the tube and is pushed down by the helix. Fluid exits the bottom of the draft tube and
travels back up the tank along the outside of the tube. The presence of the tube allows the use of
a smaller diameter helix, while maintaining a strong downward pumping design.
Anchors, screws, and helices are used for various viscous, sheer thinning and yield stress fluids.