Driving down energy costs

Two manufacturing firms have benefited from replacing fixed speed drives with variable speed alternatives, one company saving £130k a year in energy costs, the other can now offer its customers a more efficient compressor product. Dean Palmer reports By installing variable speed drives on its fume extraction plant fans, a brass foundry has saved itself a staggering £130,000 a year in energy costs. The company, Boliden MKM, makes copper and brass products and operates a brass casting plant near Walsall, West Midlands. As Ian Davey, engineering manager at the plant explained: "The orifice dampers we employed had to be closed to start the motors and then gradually opened up to the right position for operation. This was a complex method of control that led to a lot of energy loss in the system. As we have a variable rather than a fixed load, we decided to employ variable speed drives to regulate the speed of the fans." An earlier inquiry by ABB's partner Central Electrical revealed that Boliden was looking to save energy and reduce the effect of the Climate Change Levy on its business. Central Electrical carried out an energy audit on the filtration plants, measuring the energy usage over a typical week's production. Using variable speed drives from ABB, the results pointed to a potential energy savings of at least £25,000 a year on a 250kW fan and £15,000 a year for each of three smaller fans - an annual saving of £70,000 and a payback period of nine months. The audit covered the fume extraction filter plant for Boliden's brass casting process which consists of two Luhrfilter filtration plants, one with a 250kW fan and three 132kW fans. The airflow from the fans was controlled by restricting it using dampers. The turnkey contract, which involved removing the old damper system, supply and fitting of the drives, took two months to complete. Work was planned to keep breaks in production to a minimum. Fitting the drives also had other benefits besides reducing the energy bill. Boliden's swarf drying plant is served by three, 132kW extraction fans. A pressure transducer mounted in the ducting feeds back to the variable speed drive giving good control over the operating parameters as well as emissions. As Davey concluded: "As well as the large energy saving, the variable speed drives also help us with associated environmental issues." Since installing the drives, Boliden reported actual energy savings to be around £130,000 a year, a figure verified by the company's own energy management system. Other manufacturers are also reaping the rewards by replacing fixed speed drives with variable speed drives. Avelair, based in Bury St Edmunds, manufactures rotary screw compressors has recently switched to using Control Techniques' Unidrive SP variable speed drives. Brian Wood, managing director of Avelair commented: "The choice of Unidrive SP, with its integral PLC, has enabled us to offer a more compact, efficient and price-competitive product to our customers and the user is now assured of optimum efficiency at all times, with the compressor constantly matching output to demand." Avelair's VSD variable speed range of compressors has been on the market for two years and uses an inverter drive coupled to a PLC. "But the need for a separate drive and PLC, plus the associated costs of the complex wiring," explained Avelair's technical director Tim Shepherd, "were making the build costs expensive because we had to add PLC modules and almost every unit required a new layout for the control cubicle, all of which added costs." Shepherd said that with Unidrive SP, the company has now been able to standardise on layout and, by using the plug-in applications/PLC module, eliminate the need for a separate PLC, "while retaining all the flexibility and functionality we had before." The PLC is responsible for the complex procedures involved in starting up and closing down the VSD compressor to meet safety requirements and to monitor line pressure, control faults and the cooling fan, and to integrate the fault conditions from the drive itself to produce a visual warning of system or control faults. When starting, the compressor motor is accelerated to minimum speed, the cooling fan is started, the inlet solenoid valve remains closed while the control system checks for fault conditions. After 30 seconds, if the compressor's internal pressure is more than 1 bar, the system goes into running mode. In running mode, the compressor motor speed is controlled by the Unidrive SP in closed-loop speed control using its internal PID loop to maintain the set pressure. If the compressor runs at minimum speed (ie. no air required) for more than five minutes, the system automatically goes into idle mode to save energy. The air system only provides as much air as is required. Typically, a variable spped system will save more than 30%Unlike fixed speed drive systems that are usually set above the required pressure. Wood concluded: "The plug-in PLC has saved space, eliminated considerable wiring, removed a potential source of failure and cut costs all in one go. The small size of the drive has enabled us to reduce the size of the control cubicles on most of the units and control panel layouts have been standardised."