Speed control for pumps can give fast payback
Installations with large pumps running at constant speeds stand to benefit from being controlled with variable-speed drives. Steve Ruddell explores the potential.
When it comes to achieving a quick solution to high energy consumption, low-voltage AC drives are hard to beat. Not only are they relatively quick to install in existing applications, but they can also save money immediately and with relatively low payback times of only a few months in some cases can very rapidly make a major impact on their operator’s bottom line.
Installing low-voltage AC drives as part of improving the energy efficiency of a building can be done in three ways — replacement, upgrade or retrofit.
Replace
Frequently, AC drives and AC induction motors are installed to replace slip-ring motors, direct-on-line (DOL) starting or soft-starters. There is not much difference in price between a small drive and soft-start anymore. Replacing with an AC drive means that the benefits of energy savings and improved process control come as part of the package.
Old soft-starters can suffer from reliability problems, particularly if they are damp. If an unreliable soft-starter is replaced with an AC drive, the improvement in energy efficiency can quickly pay for the replacement. It is also possible to change out old PLCs (programmable logic controllers) and let the logic in the AC drives do the number-crunching.
Upgrade
The next choice to be considered is whether the AC drive is actually giving you what you want. Drive technology moves on, and today’s AC drives offer a whole raft of new functions that were only dreamed about in the early years. In many cases this does not mean you have to throw away your existing drive. Depending on its age, you could decide to upgrade the technology to add functionality, either by downloading the latest software or applying a hardware update.
There are many reasons why this may be a good course of action. It could be that faster production rates are needed or more process intelligence is required, including exact savings expressed in kWh, CO2 emissions or in monetary terms. In fact an upgrade can have a major impact on the actual process by bringing new levels of efficiency and end-product quality.
So an existing AC drive can often be upgraded to the latest software or hardware to improve the performance of the application. Drive upgrades are designed for improving the performance and extending the functionality and the lifetime of the drive to provide end-users with the best possible return on their investments.
Many ABB drives, for example, need only a simple upgrade of firmware before they can benefit from new software such as our pump-control program, which makes it possible to have two pumps running at low speed rather than one running flat out.
Retrofit
In a retrofit project, existing cabling, motors and cabinets often remain in use, while the drive or drive system is replaced either completely or partly. ABB calls this the classic phase, and it is here when the product support and access to spares is being tapered off.
This was the case in a project for the UK’s largest indoor water park, Sandcastle Water Park in Blackpool, which is now saving £13 000 per annum, an improvement of 40%, on the running costs of its water-circulation pumps following the installation of four ABB variable-speed drives (VSDs).
Four 15 kW pumps are used to circulate 418 m3/h of water through the pool. The circulation pumps used to run constantly with no form of speed control. The pumps were originally used in a 2-duty/2-standby configuration, with peak periods using a 3-duty/1-standby configuration. Running costs were nearly £33 000 per annum.
Following connection of four ABB variable-speed drives, the circulation system was run in both a 3-pump and 4-pump configuration at part speed. An original power consumption of 34.8 kW fell to 21 kW for three pumps and 17.5 kW for four pumps, giving an estimated saving of £13 000 and £16 000, respectively.
Radway Control Systems, a distributor of ABB Drives then installed four 15 kW VSDs to run the pumps as a permanent measure. It was found that running three pumps at a lower speed gives better efficiency than running two at full speed. This is due to the cube law, where the power needed to run the motors is the cube of the speed. Reducing speed thus dramatically lowers the power drawn.
With a £15 000 investment, the project had a 15-month payback on 3-pump operation.
Wherever and however you use or choose to use VSDs, a service contract incorporating a planned maintenance programme will ensure that drives continue to deliver peak energy efficiencies throughout their lives.
Steve Ruddell is division manager for discrete automation and motion with ABB.
