Archimedes’ Screws (normally called screws) are widely used by the water industry, usually used for pumping at the inlet of Water Recycling Centres to lift the flow and allowing a subsequent gravity fall through the works. These screws are known for their efficiency, simplicity, ability to handle high suspended solids and capacity to cope with a wide range of flows.
In the past, tests with inverter control have shown the screws demonstrate a constant kWh/m³ over a wide speed range and flows. This was thought to mean that varying the speed of a screw would have little benefit to power consumed. Compare this to a (low static head) centrifugal pump application where a 20% reduction in speed (and flow) leads to a 50% saving in power consumed. It is not surprising, given this data, that inverter control of centrifugal pumps has been a remarkable success.
In early 2017, Anglian Water’s Northern Energy Engineer, Wayne Barley, in conjunction with ABB’s inverter partner, Inverter Drive Systems Ltd (IDS), decided to install a temporary inverter on an inlet screw at the Water Recycling Centre at Newton Marsh, Tetney, where there are three screws in parallel each fitted with a motor rated at 90kW. To reduce starting current and stresses, these motors were started using variable voltage soft starters.
It was observed that when a screw stops liquid runs down the screw. Wayne Barley said, “I often wondered how much energy is wasted by the loss of liquid when a screw stopped and then it had to be re-pumped”.
Wayne Barley and IDS investigated methods of controlling the screw to avoid this wasteful part of the pumping cycle. After many control method experiments, we settled on a method of control. The power consumed by all three screws was then measured over a period of months and compared to previous data. The results were impressive – a saving 13.7% power overall. This was a payback, purely in energy terms, of less than three years!
There are other significant benefits of inverter controlled screws. Even under soft start control the current drawn was 600A; with inverter control it was no more than 180A! This also results in less mechanical stresses, increasing the life of the V belts, gearboxes and the screw’s bearings. This naturally gives longer life to the equipment, less downtime and less risk of non-compliance.
Blaise Ford of IDS worked with Wayne Barley on this project and said, “It was pleasing to find a successful control method of Archimedes’ Screws that saves energy.” ABB’s new water industry specific ACQ580 inverters are ideal to carry out this control with their new IPC software. Wayne went on to say, “When you add up all the inverter benefits of screw control, it becomes a no-brainer”.
Results from this new inverter control method of the Inlet Screws at Hibaldstow Water Recycling Centre showed a 32% power saving and Flag Fen’s RAS Screws saved 19.4% power.
To find out more about inverter control of screws contact IDS engineers on 0115 944 1036 or by email [email protected] IDS are ABB’s leading inverter AVP centrally located in Ilkeston, Derbyshire.