What is an impeller?
An impeller is used in centrifugal pumps to transfer mechanical energy from the pump motor to the pumped fluid. Centrifugal pump manufacturers connect the impeller to the motor using a metallic shaft. The impeller has several vanes that start from the impeller center (impeller eye) as shown below.
Figure: Centrifugal pump impellers.
When the electric motor is turned on, it spins at high rpm. Since the impeller is connected to the motor via a shaft, the impeller also rotates at the same high speed. Rotation of the impeller creates a vacuum inside the pump. As such, the fluid in the storage tank is forced into the pump due to a low-pressure (vacuum). The fluid enters the pump via the suction pipe and meets the spinning vanes. The vanes add kinetic energy to the liquid while moving it radially outwards towards the casing. The fluid proceeds to the casing, where its kinetic energy is reduced while the pressure energy is increased. The pressure energy obtained by the fluid is used to move the liquid to its destination.
Figure: How centrifugal pump impellers work to transfer liquid.
Types of impellers in a centrifugal pump
There are various types of impellers meant for different applications, flow rates, and pressure levels. The types are:
Closed impeller
The closed impeller is designed with protective shrouds on both sides. These shrouds help to enhance high strength. As such, these impellers can pump significant flow rates and high pressure. They also have high efficiency, unlike open impellers. The challenges associated with this impeller are that it is complex and thus expensive to design and manufacture since they use close-clearance rings to reduce hydraulic loads. Centrifugal pump manufacturers create this impeller type to pump clean fluids in large pumps. Using this impeller is dirty, or liquids with suspended solids will cause clogging.
Figure: Closed impeller.
Open impeller
The open impeller has its vanes open on both the left and right sides without a shroud. Without the protective covering, this impeller is weak relative to the closed type. Thus, centrifugal pump manufacturers recommend this impeller for use in inexpensive, small pumps operating under minimal strain. However, unlike closed-type impellers, these impellers have the advantage of handling fluids with suspended solids.
Figure: Open impeller.
Semi-open impeller
This impeller is designed with a back-wall shroud but remains open on the second side. The single shroud helps to enhance its strength, making it stronger than the open impeller. The efficiency of this impeller is relatively higher than that of the open type but slightly lower than that of the closed type. Centrifugal pump manufacturers design this impeller for use in medium-sized pumps. Such pumps are capable of handling fluids with small and soft solids.
Figure: Semi-open impeller.
Cutter impeller
A cutter impeller is a special type meant to shred solids in the pumped fluid. They are thus designed with sharp edges on their vanes, enabling them to grind any solids contained in the liquid. They have low efficiency compared to other impellers. However, they perform very well when used to pump sewage and other related wastes where other impellers would clog.
Figure: Cutter impeller.
Vortex impeller
This is the impeller, which resembles a semi-open impeller. It is designed for handling dirty fluids with stingy solids and debris. It creates a whirlpool while keeping the solids away as the fluid is pumped. This design helps to prevent the impeller internals from damage. It has low efficiency but minimal clogging chances and excellent solids handling capabilities.
Figure: Vortex impeller.
Factors to consider when selecting centrifugal pump impeller
Flow rate. For significant flow rates, a large pump is recommended. Large pumps have large and robust impellers enough to withstand hydraulic stresses due to large fluid flows. Such impellers are mostly closed or semi-open type.
Type of fluid. Some impellers are best for handling clean liquids, while others can take stingy and dirty fluids. Clean fluids need closed impellers, while dirty and waste materials need cutter/open and vortex impellers.
Head. The head is the height at which the fluid needs to be lifted. For large heads, large-sized impellers and pumps are used.