Variable torque applications often involve pumps and fans. However, a distinction has to be made in the case of pumps. Although the most popular types of centrifugal pumps have a quadratic torque (variable torque) characteristic, eccentric, vacuum or positive displacement pumps have a constant torque characteristic.
High energy saving potential
The energy-saving potential of pumps and fans is very high as these machines follow the affinity laws:
Q1/Q2 ~ (n1/n2)
Flow is proportional to speed
H1/H2 ~ (n1/n2)²
Pressure or head is proportional to square of speed
Power is proportional to cube of speed
The flow Q increases linearly with increasing speed (rpm), while the pressure/head H increases quadratically and the power consumption P increases cubically. In theory a reduction in speed of 20% results in an energy reduction of 50%.
In many fan and pumps systems, swirl flaps, dampers or throttles are used for controlling the flow of the system. If a centrifugal pump is controlled using a throttle valve, throttling moves the machine’s working point along the pump characteristic. The reduction in energy requirement achieved is minimal compared with the pump’s nominal operating point.
If a fan/pump is speed-controlled, the operating point moves along the system characteristic. This moves the unit out of its best efficiency point and efficiency will typically decrease slightly but the energy saving due to the reduced speed is still much higher compared to throttle or other mechanical controls. In real applications, the achieved energy savings will differ from the theoretical because losses in piping and duct -work result in a basic load and thus additional losses.
In pump applications often a minimum speed (application and pump type/make related) is required to avoid sedimentation of solids and ensure sufficient lubrication of the pump. If the range between minimum speed and speed for the maximum required power is too big the system can be cascaded. When pumps are cascaded, one speed-controlled pump covers the base load. If consumption increases, the frequency converter will switch in more pumps sequentially. The pumps accordingly operate at maximum efficiency whenever possible. Pump control ensures that the system is always as energy-efficient as possible. In some applications, more than one pump is speed controlled. Cascades can be used in a similar way for other applications like fans or compressors.