Closed type impellers are also referred to as enclosed type impellers. This type of centrifugal pump impeller has both a front and back shroud; the impeller vanes are sandwiched between the two shrouds. Closed type impellers are installed on radial pumps and can be either single inlet, or double inlet.
Single and Double Inlet
Closed type impellers are ideally suited for pumping liquids with a low amount of suspended bodies and a low viscosity (low resistance to flow). This type of impeller can be used to pump liquids with a high amount of suspended solids, but the wear rate (rate the impeller erodes at) will be high.
Closed type impellers are the most efficient type of impeller as all flow is directed through the channels between the impeller vanes.
Compared to the semi-open and open type impellers, the closed type is mechanically the strongest as the two shrouds increase the mechanical strength of the impeller.
Axial and radial thrusts created by the closed impeller are not considerably high.
Typical applications for this type of impeller would include fresh water and salt water systems.
Bernoulli’s principle states that if a steady flow is exposed to a change in area, then the velocity and pressure will change correspondingly.
If a pipe diameter increases, the pressure will increase but the velocity will decrease.
If a pipe diameter decreases, the pressure will decrease but the velocity will increase.
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 thus there is a velocity decrease and a pressure increase.
Notice on the below 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 maximise the pressure and reduce the velocity as much as possible.
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