At the heart of a centrifugal pump is a spinning impeller housed in a volute casing (Figure 24.1). The rotation of the impeller imparts velocity to fluid as it is cast out from the impeller vanes against the walls of the volute, eventually finding its way to the casing outlet. Fluid exiting the casing creates suction at the central inlet of the impeller, drawing fluid into the casing from the inlet. This suction induces liquid flow into the pump but requires either gravity or slight pressure to feed the liquid. Although there are exceptions, centrifugal pumps require priming to operate successfully.
Variable Frequency Drive
A variable frequency drive (VFD) is a system for controlling the rotational speed of an alternating current electric motor. It controls the frequency of the electrical power supplied to the motor. A variable frequency drive is a specific type of adjustable-speed drive. Variable frequency drives are also known as adjustable frequency drives (AFD), variable speed drives (VSD), AC drives, or inverter drives. Its job is to control the motor input frequency and voltage and, therefore, control the pump. Some of the benefits of VSDs include:
In general, it is best to “over feed” the pump inlet, i.e., use 2.5 to 3-inch piping or hose to supply a 2-inch inlet. For good pump performance, do not “starve” the pump inlet with too small a supply line or too low hydrostatic head (gravity). Starved pumps will cavitate, which is quite loud and physically destroys the pump components. To avoid cavitation, the available net positive suction head (NPSH) in the system should always exceed the required NPSH of the pump. This suction head can be supplied by gravity, i.e., stationing the pump under the tank it is drawing from or by pressure applied to the supply tank.
Choosing the correct centrifugal pump for brewery application is critical for maximizing long-term performance. The wrong pump will not only operate inefficiently, but can fail prematurely because it is not ideally suited to the application’s conditions. Identifying the best pump for the application starts with examining the pump curve, which indicates how a pump will perform against specific rates of pressure head and flow.
How to Read a Pump Performance Curve
Curves typically include performance metrics based on pressure, flow, horsepower, impeller trim, and Net Positive Suction Head Required (NPSHr). Centrifugal pump curves are useful because they show pump performance metrics based on head (pressure) produced by the pump and water-flow through the pump. Flow rates depend on pump speed, impeller diameter, and head. Head is the height to which a pump can raise water straight up. Since water creates pressure or resistance, at predictable rates, the head can be calculated as the differential pressure that a pump has to overcome in order to raise the water.
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