Of all pumping technologies centrifugal pumps are the widest known thanks to their efficient handling of many fluids. However, the specification of the right centrifugal pump for an application involves various considerations; one being the choice of impeller/s. But what is an impeller?
You can find more information on our web, so please take a look.
The impeller is the rotating component within the centrifugal pump design, which transfers the energy from the pump&#;s motor to the fluid. It is made up of vanes that come off an open inlet at the centre, known as the eye, which create a centrifugal force as they spin to move the liquid from the casing to the discharge point.
There are several types of impeller, each of which offer different performance characteristics that make it more or less suitable than the others for a particular application. Given the vital part it plays in a centrifugal pump&#;s operation, you can see why the type and size of the impeller is an important factor in pump specification. But what exactly are the differences and when do you choose which?
This article will cover just that, but if you&#;d rather a quick summary, click to download our handy infographic!
The Types of Pump Impeller
1. Open impeller
As its name suggests, an open impeller has vanes that are open on both sides without any protective shroud. As they don&#;t have any support either side they tend to be weaker, and therefore are typically used in smaller, inexpensive pumps that aren&#;t operated under significant strain. Whilst they are able to handle a degree of solid content unlike closed impellers, they require a higher NPSH to operate without cavitation, damage and loss of efficiency.
2. Semi-open impeller
Semi-open impellers have a back-wall shroud that adds mechanical strength to the vanes, whilst remaining open on the other side. They are somewhat of a middle ground between open and closed impellers in terms of efficiency and NPSHr, making them suited to medium sized pumps with a small amount of soft solids. It&#;s important to note with semi-open impellers that the clearance between the vanes and the pump casing needs to be small, as if too large slippage and recirculation will occur.
3. Closed impeller
Now you&#;ve read about the open and semi-open impellers, you can probably guess that a closed impeller has enclosed at the back and front, providing maximum strength. They have a low NPSH required and provide a more efficient flow. However they are a more complicated, expensive design due to their reliance on close-clearance wear rings to reduce axial loads and help maintain efficiency. They are the most popular impeller for large pumps transferring clean liquid as they are prone to clogging when in contact with solids.
4. Vortex impeller
Unlike the three previous impellers discussed, vortex impellers are not channel impellers. In appearance they are similar to a semi open but have more space in the volute and work differently.
Its design is ideal for dirty fluids containing debris and stringy solids as it creates a whirlpool/vacuum which keeps any solids away from the impeller as the liquid is pulled through, therefore preventing damage to the internals. It&#;s minimal risk of clogging and solid handling capabilities are excellent, however efficiency is lower. For that reason, vortex impellers should only be selected when they have to be.
5. Cutter impeller
Like a vortex impeller, cutter impellers are designed to handle solids. However, they differ as rather than enabling the passing of solids like vortex impellers, they have sharp edged, scissor like vanes designed to grind and obliterate any solids before they enter the pump. Whilst efficiency is low, they are the ideal choice of impeller for the pumping of sewage and other waste where a channel impeller will clog.
The impact of impeller diameter
As well as the type of impeller, it is important to consider the impeller diameter when specifying a centrifugal pump as this can affect its performance. As you can see, the graph below shows multiple pump curves that represent different impeller sizes and the impact that these have on the flow and head of the pump model in question.
The larger the impeller, the higher the circumferential speed at the impeller output and therefore the greater the head and flow produced by the pump, and vice versa. With this being the case, impellers can be trimmed to meet the specific duty point required by the application at hand. Most centrifugal pumps&#; performance curves display the range of impeller trim sizes at which the pump can operate sufficiently. This is then used to determine the impeller diameter needed to achieve the performance requirements.
Impeller trimming is a lot cheaper than using a variable frequency drive to achieve the required duty point. However, the more an impeller is trimmed, the larger the clearance between it and the casing which results in efficiency losses. This is why there is a limit to which a pump's impeller should be trimmed.
Castle Pumps work with our customers to ensure our pump selection is based upon your requirements and therefore will select and trim impellers to suit. For centrifugal pump enquiries please contact our technical sales engineers on +44(0) 533 283.
Are you looking for a centrifugal pump?
View our range or contact our technical sales engineers who are available to discuss your process with you in detail, ensuring you get the right pump for your requirements.
View our range
No matter what you're mixing, impeller selection is critical. It plays a role in mixer specifications and your end product. Disperser blades, propellers, hydrofoils, and other common impeller designs make for a confusing choice.
Can this disperser blade give me the off-bottom suspension I need? How is a hydrofoil different from a propeller? What is a hydrofoil? Before you buy an impeller, you need to consider a few aspects of your product and process:
Process and Product Considerations
Impeller selection is often based on finding a balance between flow pattern, shear, and pumping capacity that suits your specific application. How do you know what balance you need to strike between the two? It depends on the desired outcome, ingredient properties, and vessel geometry among other factors.
Ingredient Properties
Are the liquids used in your application miscible or immiscible? How viscous are the liquids? Do the solids you are mixing in have a tendency to agglomerate? Processes combining miscible liquids don't often require high-shear impellers because the mixing is driven primarily by the flow of the liquid. In this case, it is better to go with a radial or axial flow impeller, depending on your process requirements. Impellers are specifically designed to be more efficient at certain viscosities.
Desired Outcome
If the end goal is simply to mix a soluble solid and a liquid, a propeller or hydrofoil will get the job done. If the end-goal is to suspend an insoluble material into a liquid, however, you will need a high-shear impeller. If you want to disperse a gas throughout the mixture, you will need a dispersion blade or other high-shear impeller.
Vessel Geometry
Most mixing tanks are cylindrical in design with the agitator coming down from the top of the tank with the impeller located near the bottom of the tank. Impeller diameters are typically configured for the specific application, but theoretically speaking, the impeller diameter should be about the same as the distance between the bottom of the impeller and the bottom of the tank.
Baffles also play a big role in tank geometry. Will the tank have baffles? Will the tank have 3 or 4 baffles? Baffles can drastically increase the efficiency and effectiveness of your mixer because they help create an axial flow pattern in the tank. Avoiding solid body rotation is an important part of impeller selection and tank geometry, and baffles can help with this.
Different Types of Impellers
Impellers come in all different shapes and sizes. They have different optimal speeds of operation, pumping capacity, and shear just to name a few attributes among many. It is important to get the right impeller for your application, so that you achieve the best results and the greatest efficiency possible.
This brief introduction to different impeller classes will help you get started down the right path to selecting the best impeller that will propel your project to success.
Axial Flow Impellers
Impellers in this class create an axial flow pattern. This is called an axial flow pattern because the liquids flows parallel to the axis around which the impeller rotates. Axial flow impellers typically pump the liquid in the tank downward using angled blades. This downward push on the liquids allows content in the top and the bottom of the tank to mix.
These impellers are most commonly used for heat transfer, solids incorporation, solids suspension, and blending among other applications. Within the axial flow impeller class, propellers and pitched blade turbines are the most common for low to medium viscosity applications.
Link to zen
Radial Flow Impellers
Radial flow impellers pump the liquid out toward the side of the tank along the radius of the impeller. As opposed to axial impellers, radial impellers don't have angled blades as this would force the liquid downward.
Instead of creating the two mixing loops of an axial flow pattern, radial impellers create four loops that divide the tank into quadrants.
Radial flow patterns are most useful for gas-liquid and liquid-liquid dispersion, but they can be used for other processes. Radial impellers can leave solid particles on the bottom of the tank if their is not enough power to create adequate off-bottom suspension.
But, what about the impellers themselves? How do I know if an impeller creates a radial flow? Generally speaking, there are two characteristics of radial impellers: open/disk and curved blade/flat blade.
- Disk -
Impellers with disks are used to create a more uniform radial flow pattern and when you need to prevent gases from rising along the shaft of the mixer.
- Open -
The right impeller below has a completely open space between each blade that allows for easy CIP, but provides less power than disk impellers.
- Curved -
Curved (a.k.a. backswept) blades like those on the right impeller allow materials caught on the blade to come off the blade as it rotates.
- Flat -
The flat blades on the left impeller provide more power and pumping capacity than the curved blades because of the angle at which the liquid is pushed off the blade.
Hydrofoil Impellers
Many of the recent advances in impeller technology have come in this category. Hydrofoils are designed to pair an axial flow pattern with low shear.
Hydrofoils are also distinguished by the twisting blade that has a similar look to a pitched-blade turbine. The twisting blade creates more consistency in how quickly fluids are moving when they are pushed by the blades.
Because the blades push fluids at a constant velocity across the blade, hydrofoils maximize pumping capacity while reducing the impeller's power number. Thus, hydrofoil impellers are a great option if you are restricted by the amount of power you can get to your mixer.
High Shear Impellers
Maybe you didn't know it at the time, but chances are that you've used a high-shear blade before. Think of a tablesaw blade or a circular saw blade when you think of a high shear impeller; that really is what they look like albeit with a few modifications to make them more efficient for solid-liquid dispersion and other other applications.
This impeller is one common design for disperser blades. It is designed to maximize shear, which is useful for breaking up solids that have agglomerated in your mixture.
Another high-shear impeller design is the bar turbine.
Bar turbine impeller design has lower shear
than other impellers designed for that purpose, but it still provides a useful amount of shear.
It is a disk impeller so that more 'bars' can be included with the impeller.
High shear impellers have very little, if any, pumping capacity and need to operate at high speeds to be effective. It is also fairly common to combine high shear and axial flow impellers to get the best of both worlds: high flow rates and high shear.
The company is the world’s best oem pump impeller supplier. We are your one-stop shop for all needs. Our staff are highly-specialized and will help you find the product you need.