How to optimise the energy efficiency of pumps using VFDs

6th December 2021  |  Drive systems

More than 90% of the total cost of ownership of a pump comes from energy consumption, outweighting servicing, maintenance and replacement parts.

There are various approaches used to control the flow rate of a pump, but they all come with their own inefficiencies. However, there are tried and tested solutions for efficiently managing the performance of pumps in the form of Variable Frequency Drives (VFDs) – also known as Variable Speed Drives (VSDs).

VFDs, which have been available since the 1980s, are used to regulate motor speed in applications including conveyor systems, fans and pumps. This article explains why a VFD is the most energy-efficient method of controlling pump flow, as well as other ways they help reduce the TCO of a pump.

Methods for optimising energy efficiency in simple systems

Traditional approaches to designing pump control solutions can be highly energy efficient in applications where little operational variation is required. For example, if the pump is running at maximum flow, then it is highly efficient. But introducing some variation in the flow causes the energy efficiency to drop away. Here are some commonly used approaches to controlling flow and saving energy:

  • On/off method: as its name suggests, fluid flow through the system is controlled by running the pump at maximum flow rate and then stopping it. Industrial pumps are generally designed to run at optimal efficiency when at the maximum flow rate, so when the pump is running it is energy efficient. However, there is no flow rate control using this method and in practice pumps rarely run at full rate. In addition, the water hammer generated when the pump suddenly stops can cause seal and joint damage further reducing energy efficiency, and repeated stop-starts damage the motor windings.
  • Multiple pumps: using two or more pumps in series or parallel enables engineers to vary flow capacity, alongside providing greater reliability and redundancy when the application demands it. Pumps in series must be carefully specified to optimise performance. If one pump fails, the overall efficiency falls, because the second pump’s operating conditions are no longer optimal, so its efficiency falls.
  • Two pumps in parallel do increase the flow rate but do not necessarily generate double the flow. Two pumps mean increased energy loss due to higher fluid friction, lower flow rate per pump and compromised pump efficiency. Pumps that operate too far outside of normal operating parameters are more likely to fail.
  • Mechanical valves: valves added to the inlet (suction) side, or the outlet (delivery) side can be closed, throttling and reducing flow. To increase flow, the valve is opened. This method is often used when the head has been calculated too high in error, so the pump and motor are oversized. The pressure drop across the valve as it closes increases friction and the steeper system curve results in higher power loss. Overall, pump efficiency falls, particularly with over-specified equipment.

These pump flow control methods do provide practical solutions in some applications with specific requirements, where either control, energy efficiency or both are not primary factors. But when sustainability and TCO are priorities, there are more effective solutions.

How to optimise the energy efficiency of pumps using VFDs

Simply put, pumps with VFD motor starters are more energy efficient in most pumping applications because they provide variable flow rate control that delivers energy efficiencies during operation. Furthermore, VFDs are also intrinsically more efficient because of the affinity laws.

When coupled with appropriate sensors and transducers, the VFD-controlled pump motor will provide pumping power according to the demand requirements of the system, rather than at either peak operating output or at a suboptimal and lower efficiency output. Running motors constantly at full speed lowers operational lifespans.

If the motor is not subject to the electrical overload and mechanical shocks of on/off and reversing flow control, instead soft starting and stopping, less strain is placed on the motor and the wider system, protecting seals, joints and pipelines. When damaged, these system elements can interrupt flow and reduce efficiency. The undamaged mechanical elements of the motor will run smoothly without additional friction and wear. An added benefit of reduced pump motor shocks for reducing TCO is lower maintenance requirements.

The affinity laws, or pump laws, are a range of formulae with variables including head, flowrates, impellor diameter and shaft speeds. Even allowing for the underlying assumptions, the laws clearly demonstrate that running a pump motor at lower rates reduces energy consumption. A 25% decrease in pump motor speed can reduce energy consumption by 60%.

VFDs are the most energy efficient method of controlling flow. When combined with the maintenance reductions and reduced failure risk that VFD pump motors enjoy, the initial investment in VFDs leads to a much lower TCO, as well as optimising pump energy efficiency.

Dalroad’s specialists will help you find the right inverter drive for your industrial pump

Dalroad helps end users and electrical contractors in a wide range of industries to select the best VFD for their pump. Contact our team of experts today for no-obligation advice and recommendations.

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