Energy-Saving Methods for Pumps and Fans

1. Overview

Historical International Copper Association surveys and studies reported that electric motors accounted for more than half of energy consumption in the sectors examined. The reference period and sector boundaries were not identified, so this should not be treated as a current universal statistic. Improving motor-system efficiency can reduce electricity costs and improve economic performance.

Energy efficiency can lower production costs and improve product competitiveness. Reducing energy consumption also reduces pressure on natural resources and associated emissions, contributing to environmental protection and climate action.

ITVC has applied ABB ACS310, ACS550 and, particularly, ACH550 drives in energy-saving projects in Vietnam. These are historical product examples; current equipment should be selected according to the application's requirements and the manufacturer's current range and support.

The applications discussed are mainly pump and fan drives, which are widely used and can offer substantial opportunities for energy savings. Examples include ventilation and flue-gas fans in cement plants and thermal power stations, and HVAC systems in buildings.

2. Characteristics of pump and fan drives

For centrifugal pumps and fans under suitable similarity conditions, with friction-dominated systems and approximately constant efficiency, load torque varies approximately with the square of rotational speed.

Flow rate varies approximately in proportion to rotational speed under these conditions.

M ∝ n²

M ∝ F²

Mechanical shaft power is torque multiplied by angular velocity. If n is expressed in revolutions per minute, use the conversion below:

P = M × ω = (2π/60) × M × n

Thus, under the stated assumptions, power varies approximately with the cube of speed and flow rate:

P ∝ n³ ∝ F³

For example, reducing flow from 100 m³/h to 80 m³/h gives an ideal power ratio of 0.8³ = 0.512, or approximately 51.2% of the original power. Actual electrical savings depend on static head, system curves, efficiency and operating conditions.

3. Actual power consumption

Actual input power exceeds useful hydraulic or air power because of motor and pump/fan inefficiencies and system losses. Pipe or duct friction, bends, branches and installed equipment contribute to these losses. These include relatively fixed losses and controllable losses associated with throttling valves or pressure-control devices.

Conventional pump and fan systems often use valves, dampers or inlet controls to adjust output to process requirements. Depending on the equipment and installation, control may be applied at the inlet or outlet.

4. Saving energy with variable-speed drives

When demand is below design flow, throttling control can leave substantial losses even though total power consumption decreases. Different control arrangements produce different losses. Reducing avoidable throttling losses can therefore provide significant savings.

A suitable variable-speed drive adjusts pump or fan output by changing motor speed. Where the system design permits, existing regulating valves or dampers can be opened further, reducing avoidable throttling losses. Fully open valves still have some pressure loss, and required safety, isolation and process controls must be retained.

The operating strategy must be assessed for the actual application, including start-up, minimum flow, pressure and equipment limits. Under the ideal cube-law assumptions, a 20% flow reduction corresponds to about a 49% power reduction. Historical examples cited savings exceeding 40% compared with outlet-valve control; that comparison is illustrative and must be verified by measurement and system analysis rather than promised for every installation.

Appropriate variable-speed control can also reduce unnecessary mechanical loading and wear on valves and other components, potentially lowering maintenance costs. The result depends on correct selection, installation and operating conditions.

Reference: U.S. Department of Energy — Pump Systems