# Cut Centrifugal Load Energy Costs

A simple calculation determines how upgrading a centrifugal load to a variable-frequency drive can cut costs

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Many organizations want to save energy because of increasing costs for electric power. Costs for electricity vary widely across the U.S., with most utility rates rising steadily for a variety of reasons.

For example, in the San Francisco Bay area, one wastewater treatment plant was charged \$0.24 per kWh at non-peak times. On Long Island, N.Y., some companies are paying that rate or higher. The U.S. national average is around \$0.10 per kWh non-peak, with industrial users often paying penalties for excess demand.

VFDs for savings

To save on electrical costs, many companies are using variable-frequency drives (VFDs) to control centrifugal loads. In addition to energy savings, VFDs also provide better control of the connected load, often improving performance and extending equipment life.

A centrifugal load is any device that rotates around a center point, such as an impeller driven pump or a fan, and the mechanical characteristics of these devices make them ideal candidates for energy savings.

These devices follow what’s known as the Laws of Centrifugal Load, or the Affinity Laws, which can be expressed as follows. As the speed of the device increases, the torque or ft-lb requirements increase to the square of the speed increase. Understanding this mechanical phenomenon is important, as it shows just how vital speed control via a VFD or other means can be in reducing energy consumption.

Reduced speed

What happens if we reduce the speed? The torque requirements of the device go down to the square of the speed change. In other words, if a centrifugal pump or fan turns at half speed — or 30 Hz instead of 60 Hz — the torque requirement is approximately 25 percent at half speed as compared to full speed.

Electrical demand goes down, too, typically by a significant amount. The VFD maintains a V/Hz ratio at the motor of approximately 7.5/1 for 460 V, but if the speed is reduced, the power requirement is reduced.

Here are two examples using electric motors to drive a centrifugal pump. Example 1 is value engineered to reduce initial capital expenditures and is the less expensive solution in terms of up-front costs.

Example 2 is designed for maximum energy savings and uses a VFD to minimize total life cycle costs.

The following data apply to both examples: \$0.10 per kWh electricity cost, no static head pressure to overcome, cycle time of once per year, NEMA B premium-efficiency 4-pole motor, and an impeller pump needing 300 ft-lb of torque at 1,750 rpm.

Example 1: Soft-start motor and throttling valve

The soft-start motor drive, the motor, the pump and the impeller are all sized to produce the minimum adequate gpm flow rate. The motor runs at constant speed, and the flow rate is controlled by a throttling valve.

100 hp motor x 0.746 kW/hp = 75 kW
75 kW x 8,760 annual hours of use x \$0.10 per kWh = \$65,700 energy cost per year

Example 2: Variable-frequency drive

Same application, but the motor is increased to 125 hp to ensure adequate gpm is maintained at a 40 percent lower speed. The flow rate is controlled by changing the speed of the variable-frequency drive, so there’s no need for a throttling valve.

125 hp motor x 0.746 kW/hp = 93 kW
40% x 93 = 37 kW (power reduction at 40% slower speed)
93 kW - 37 kW = 56 kW (average power consumption)
56 kW x 8,760 annual hours of use x \$0.10 = \$49,056 energy cost per year

Annual savings: \$65,700 - \$49,056 = \$16,644

At higher electric power rates of \$0.24 per kWh, the annual savings would be \$157,680 minus \$117,734, or \$39,946.

The savings are offset somewhat by the increased cost of the larger motor and the VFD. A soft starter for this application would cost about \$1,300, a 100 hp NEMA B motor would be about \$4,500, and a throttling valve costs about \$4,000. The total cost of these three components is therefore \$9,800.

A 125 hp motor and a VFD would cost about \$13,500. Therefore, the cost difference is about \$3,700, or about 22 percent of the first year’s savings. This means that the payback period for the VFD option in this case is only about three months.

You may experience more or less energy savings, depending on your particular system and your local power cost, but this formula is an excellent place to start.