Mark Berube is a drives, motors and motion control product engineer at AutomationDirect.com in Cumming, Georgia. For more information, visit AutomationDirect.com.
To ensure the best system setup, end users must first consider operating conditions, then conduct thorough cost-benefit calculations.
A review of the system indicates that the pump normally operates with the throttling valve positioned to limit pump discharge to 100 GPM. The reduced flow rate represents 50 percent of the pump capacity, occurring 90 percent of the time.
Based on the affinity law, pump capacity is directly proportional to pump speed, so a reduction in speed to 50 percent will achieve an identical reduction in capacity/flow rate (see Equation 4).
Table 1 shows that, according to the pump affinity laws, reducing the flow by 50 percent cuts the pump head pressure to 25 percent of rating. A readily available VFD energy savings calculator can help determine the potential cost savings achieved by using a VFD.
Table 1. Pump affinity law effects
In this example, based on 4,160 hours of annual run time and a cost of $0.12 per kWh, annual energy consumption drops from 21 to 8 megawatt-hours when the pump is controlled with a VFD as opposed to the original control method using a throttling valve.
This represents an annual savings of $1,589 or 62.4 percent. With an estimated installed cost of $4,000 \uc0\u8232 for a 15-HP VFD, the payback period is 2.5 years.
Lower Costs & Simplicity
As the cost of electricity continues to rise, the need to reduce energy consumption becomes even more important. As demonstrated by the pump affinity laws, operating a pump at lower speeds can significantly reduce energy consumption.
Compared with operating a pump at full speed with a throttling valve, using a VFD to run a pump at the desired lower speed usually is a more efficient option. The VFD reduces energy consumption, eliminates the need for a throttling valve, simplifies piping design and installation, and cuts maintenance costs.