Closed-impellers are also referred to as 'enclosed impellers'. This type of impeller has both a front and back shroud; the impeller vanes are sandwiched between the two shrouds. Closed-impellers are installed on radial flow centrifugal pumps and can be either single inlet, or double inlet.
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Single and Double Inlet Impellers
Closed-impellers are ideally suited for pumping liquids with a low amount of suspended bodies and a low viscosity (low resistance to flow). They can be used to pump liquids with a high amount of suspended solids, but the wear rate (rate the impeller erodes) will be high.
Closed-impellers are the most efficient type of radial flow impeller, as all flow is directed through the channels between the impeller vanes.
Radial Flow Centrifugal Pump
Compared to semi-open and open type impellers, the closed-impeller is the strongest, as the two shrouds increase the structural strength of the impeller.
Axial and radial thrusts created by closed impellers are not considerably high, which gives more flexibility when selecting bearings.
Axial and Radial Thrust
Typical applications for closed-impellers include fresh water and salt water systems.
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The below video is an extract from our Introduction to Centrifugal Pumps Online Video Course.
Bernoulli's principle states that if a steady flow is exposed to a change in area, then the pressure and velocity will change correspondingly.
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Example 1
If a pipe diameter increases, the pressure will increase, but the velocity will decrease.
Example 2
If a pipe diameter decreases, the pressure will decrease, but the velocity will increase.
Impellers are designed based upon Bernoulli's principle and the relationships between area, pressure and velocity.
Flow through the impeller vanes is radial. The impeller creates a negative pressure at the impeller eye (centre of the impeller) and this negative pressure draws liquid into the impeller. The liquid is thrown outwards radially due to the centrifugal force imparted onto it from the impeller. As the liquid flows through the vanes, the flow path area increases and there is a velocity decrease and a pressure increase.
Semi-Open Impeller
Notice on the above image that the distance between the channels (area between vanes) gradually increases as the vanes stretch towards the outer periphery. This gradual increase in area gives a gradual decrease in velocity and increase in pressure. The purpose of a volute casing and diffuser is to continue this velocity to pressure change in order to maximize the pressure and reduce the velocity as much as possible.
Impeller Surrounded by Volute Casing
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An open impeller is a device that consists of a hub with attached vanes and is mounted on a shaft. Open impellers are slightly less efficient than closed or semi-closed impellers.
However, the vane design has a design advantage as blade stresses are lower since the side plate is not fixed to the inlet. In a pump, the fluid enters from the inlet and is directed to the impeller's eye, where vanes add energy and direct it to the nozzle discharge. A relatively close clearance between vanes and a pump is necessary in order to prevent most of the fluids from flowing back.
You can compensate with wear on the bowl and the edges of the vanes by adjusting this clearance to maintain efficiency over time. More parts are visible, which makes open impellers easier to maintain & inspect for damage. The open impeller can be modified more easily to suit varying needs. They usually operate at a specific range of speeds and are typically faster & easier to maintain. For small pumps and applications that deal with solids, you would usually just use an open impeller. This is because sand locking occurs less frequently when using an open impeller.
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