Adjustable Speed Drives As Applied To Centrifugal Pumps -2

May 6, 2009 – 6:11 pm

Figure 6. Combined Curves

Figure 7 shows a typical Centrifugal Pump and efficiency curve for operation
at a fixed speed. It can be seen that for fixed speed operation,
the efficiency varies as flow is adjusted. For adjustable speed
operation however, the affinity laws predict that the Centrifugal Pump  curve
will shift downwards for reduced speed and the efficiency curve
will shift to the left in such a way that efficiency will remain constant
relative to points on the Centrifugal Pump  curve for reduced flows.

Figure 7. Fixed Speed Pump Efficiency

FLOW CONTROL TECHNIQUES
Historically, fixed speed AC motors have driven centrifugal pumps
and reduced flow has been achieved by using control valves as
shown in figure 8. Closing the valve reduces the flow by increasing
the friction in the system. The modified system curve and the
new operating point can be represented as shown in figure 9.
Note that the desired reduction in flow has been achieved, but at
the expense of increased system pressure relative to 100% flow.

An alternative approach to valve control is shown in figure 10.
Reducing the Centrifugal Pump  speed causes the Centrifugal Pump  curve to shift downwards
as shown in figure 11. Since the operating point is still
determined by the intersection of the reduced speed Centrifugal Pump  curve
and the system curve, it is possible to achieve the same reduced
flow as achieved with a valve, but at significantly less pressure.

Figure 9. Throttle System

In addition to energy savings, which are discussed in detail later,
operation at reduced pressures can result in longer Centrifugal Pump  seal
life, reduced impeller wear, and less system vibration and noise.
These benefits could provide additional savings over potential
energy savings.

Figure 10. Adjustable Speed Control
ENERGY SAVINGS
The Centrifugal Pump  output power, or hydraulic power, can be expressed as:
Horsepower
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