In relative terms pumps are efficient machines, most operating near 75% efficiency.
Because of this relatively high efficiency it is easy to dismiss the potential energy savings that exists by optimizing pumping systems.
A pump is very sensitive to how it is operated and it is the pumping system that has the greatest influence on pump energy use.
Energy can account for as much as 75% of a pump's total cost of ownership. A pump converts mechanical energy into pressure energy, which is imparted into a fluid, which in creates flow.
A pump can be very efficiently designed yet it can be operated in an inefficient manner.
Typically, pumps run at a fixed speed with a throttling valve on the pump discharge to regulate pump output.
Think of this as operating your car with the gas peddle mashed to the floor and regulating your speed by applying the brake... not an efficient method of control.
A variable speed drive system adjusts the pump output to meet the system requirements.
As the system requires less water, the system delivers less water. Like smoothly operating the gas peddle on your car... always just giving it enough gas to control speed... an efficient method of control.
Typical Variable Speed Drive System Configuration:
The variable speed drive system includes a speed controller (AC frequency drive) that adjusts the speed of the pump motor. Flow varies directly with the speed of the motor. Pressure varies with the square
of the speed of the motor (one half speed = 1/2 flow and 1/4 pressure).
A pressure transducer senses the supply line pressure and sends a signal to the drive control. The drive control interprets the transducer signal and sends a speed signal to the pump motor.
The motor speed is varied to maintain a constant line pressure. As the line pressure increases due to mold temperature controller cooling valves, heat exchanger valves and chiller condenser valves closing,
the water flow demand decreases. When this occurs, the drive control slows the motor speed to reduce flow and pressure. When line pressure decreases below the set point (caused by valves opening) the drive control
increases the motor speed to increase the flow rate and pressure.
Why Use A Variable Speed Drive System?
Because it saves energy and money. Here's how: electricity is expensive. The figure below shows the approximate cost of one brake horsepower (bhp) at various electrical costs per kilowatt-hour (kwh).