Pump systems use a lot of energy, and much of it's wasted. Many of these systems are oversized, while the motors operate at a fixed speed. Pumping applications with variable loads offer a promising area for dramatic cost savings. Additional benefits include increased reliability and productivity, as well as longer equipment life.
Pump sizing and variable loads
Pump systems are often oversized. If a single pump is installed for a variety of applications, it's typically sized to meet the largest demand, making it too large for most operations; this wastes energy.
In addition, many pumping applications require a variation in flow or pressure, while the motors operating the pumps run at a fixed speed. You can save energy and money by using control methods to reduce the power driving the pump during periods of low demand.
Controlling flow with variable frequency drives
Traditional flow-control methods include throttling valves and bypass lines. Although these are effective in some cases, they often waste energy and increase operating costs. When properly applied, variable frequency drives (VFDs) can significantly lower energy costs, improve system reliability and minimize pump system life-cycle costs.
Motors are more than 90% efficient when running at their rated load. However, efficiency can drop to as low as 60% when running at part load. VFDs vary motor speed by varying the input power. Slowing a pump in this manner saves energy more effectively than allowing the motor to run at a constant speed and then restricting or bypassing the flow.
VFDs also allow motors to start slowly and speed up (soft starting). This reduces mechanical stress on the motor, as well as voltage sags that can occur when it starts too quickly. Voltage sags can dim lights and cause other equipment to shut down or restart.
Advantages and disadvantages of VFDs
For many facilities, variable speed pumps provide a number of potential benefits — energy savings, improved process performance by correcting for small variations in flow and pressure, as well as reduced overall pump wear, particularly in bearings and seals.
Variable speed pumps typically cost more than fixed speed units, but the cost savings provide a return on your investment. However, structural resonance in variable speed systems can cause excessive vibration, which can damage equipment. Harmonics generated by VFDs may cause problems with other equipment on the supply circuit. VFDs work best with inverter-rated motors which are hardened to high-frequency noise and heating from reduced airflow at low speeds.
Pump system upgrades helps glass plant shine
Technology improvements at Visteon Corporation's automotive glass plant reduced the demand for process cooling water. This resulted in the pump system providing more water than needed. The larger, older pumps were replaced with small units fitted with VFDs. The result was an annual savings of $280,000 in reduced energy and water costs. At a total cost of $350,000, the project paid for itself in less than 15 months.