الفصل الرابع

4-1.Introduction:
The flow through a Centrifugal turbomachines is primarily in the radial direction, outward for a pump and usually inward for a turbine. Centrifugal turbomachines have thin raised curved radial surfaces attached to the face of a rotating disk. The unit, called an impeller, rotates inside a casing or shroud. At the outer edge of the impeller the shroud opens up into a scroll or volute. Figure (4-1) shows the principle of a centrifugal turbomachine.
In a centrifugal pump the fluid enters axially at the center, or eye, of the impeller. The fluid then flows radially out through the passages on the face of the rotating impeller. The impeller increases the head of the flow by increasing both the static pressure and kinetic energy. The flow leaves the impeller and is diffused by passing through the annular space between the casing and the impeller. Sometimes a set of stationary diffusing vanes is located just outside the impeller. After being diffused, the high static pressure low kinetic energy flow is collected in the spiral volute or scroll.
The kinetic energy of a liquid coming out of an impeller is obstructed by creating a resistance in the flow. The first resistance is created by the pump casing which catches the liquid and slows it down. When the liquid slows down the kinetic energy is converted to pressure energy.
It is the resistance to the pump s flow that is read on a pressure gauge attached to the discharge line. A pump does not create pressure, it only creates flow.