Frequency Converter for Pumps

The affinity laws apply to pumps as they do to fans. To understand frequency converter applications applied to pumps, it is a good idea to review the basics of how pumps are applied in general.

Figure 1 shows a pump curve relative to a hydronic system curve. The pump curve describes the “head” (or pressure) versus flow characteristics of a particular pump. The curve shows that the pump will produce limited flow , at point “B”, if it is applied to a system with a large differential pressure required to lift the water/glycol and overcome resistance to flow. Larger flow rates are achieved with this pump if pressure differential is reduced- as at point A. To determine where along the pump curve the pump will operate requires information provided by the system curve. The system curve show the characteristics of the piping system. It shows the “friction head’ as it increases with flow. Friction head is the measure of resistance to flow provided by the pipe, valves, elbows and other system components.

STATIC HEAD

The head required at zero flow is a called static head or lift. Figure 2 shows the combined friction and static head curve for the system. The static head is the amount of feet of elevation the pump must lift the water regardless of flow. Another way to look at is as the work needed to overcome gravity. The intersection of the pump and system curves shows the natural or design operating point for the system. At this point pump pressure matches system losses. The intersection would generally be chosen to ensure that the pump is operating at or near its best efficiency.

PUMP ENERGY SAVINGS WITH FREQUENCY CONVERTERS

Applying a frequency converter to a pump to reduce speed and therefore flow, causes the pump curve to shift down as shown in figure 4. Since the operating point is still the new reduced pump curve and the system curve, the same reduced flow (note 700 GPM from 1200 GPM in figures) is achieved as with a valve. This flow is achieved but at a reduced pressure. Operating at reduced pressures can result in longer pump seal life, reduced impeller wear and less system vibration and noise.

Due to the affinity laws, power is greatly reduced at reduced flows thus offering significant savings by frequency converter.