MechChem Africa March 2017

⎪ Products and industry news ⎪

Aerator drives and the single source advantage

The supply of ten SEW-Eurodrive DRN IEC motors with MC-series gearboxes for the aerators at a local wastewater treatment plant represents the first project success for the company’s new IE3-rated motors since their release into South Africa last year. MechChem Africa talks to head of projects, Rudi Swanepoel.

S EW-Eurodrive has recently completed a delivery of ten cus- tomised aerator drive systems destined for a wastewater treat- ment plant being upgraded in Meyerton. “Springing fromour core agitator gearbox business, wewere approachedby anOEM for wastewater treatment, management systems and services that was handling the upgrade on behalf of thewater board,” says Swanepoel. Having designed the wet-end of the aeration system, the OEM contracted SEW-Eurodrive for the supply of the drive ends of these systems, each consisting of a verticallyhungMC-seriesgearboxcoupled to a DRN IEC motor. Aeration is the critical stage of the activated sludge treatment process, a process that oxidisesbiologicalmatter and removes nutrients such as nitrogen and phosphorus. Aeration supplies the oxygen required to accelerate sludge activation. “Simply put, an impeller is used to churn up the surface and bubble air into the wastewater,” Swanepoel explains.

Surface aerator impeller vanes are used to pull wastewater up from the tank anddischarge it horizontallyoutsideof the impeller rim. A liquidplume is created that maximises the contact area between the air and the water, causing air and oxygen to be continuously mixed, dissolved and circulated through the wastewater being treated. “We supplied gearboxes from our mixer and agitator (MC) range, coupled to the motors. Ten drive systems in total were supplied, six with 45 kW DRN IEC motors and four with 55 kW motors,” Swanepoel says. TheMCgearbox range is purpose built for mixing, agitating and aerating applica- tions. “Our portfolio of high-torque and robust industrial gear units is designed to meet themost rigorous torque and shock- loading requirements,” says Swanepoel. Large axial and radial forces occur along the impeller shaft during these pro- cesses and SEW-Eurodrive has overcome this problem with its extended bearing distance (EBD) design. This extends the

life of the bearingswithin the gearbox and prevents theneed for oversizedgearboxes or additional support bearings. “Wedesignour gearboxes for abearing life of 100000hours, which is just short of 10 years of operation. Thermal ratings are also excellent. We don’t get overheating, particularly on the MC range, even under high ambient temperatures,” Swanepoel adds. For surface aeration applications the MC gearbox is installed vertically above the tank, but for mixing or agitation use, these units can also be installed horizon- tally (on the side of the tank) or under the tank in special cases, and both left and right output shaft positions are possible. SEW-Eurodrive uses a calculation program designed specifically to select a gearbox suitable for the application. “We use this program to determine whether a gearbox selection is adequate, based on the calculated torques, loads and bending moments. This is particularly important when it comes to aerators and mixers. So these are not off-the-shelf products, but have been selected specifically for the system in question,” he says. Swanepoeltells MechChemAfrica about gear ratios used for theMeyertonaeration project: “The 45 kWunits are fittedwith a gearbox with a 32.22 reduction to rotate the impeller at around 46 rpm from the four pole, 1 470 rpmasynchronousmotor speed,” Swanepoel responds, adding that the more powerful 55 kW units turn the impeller a little slower at 41 rpm from a higher ratio gearbox (35.78). “The impeller speeds and gearbox torques are all very carefully selected so that the impellers can continuously sup- ply the optimum quantity of oxygen to the process. Too little oxygen slows the

SEW-Eurodrive recently supplied ten of its DRN IEC motors directly coupled to MC-series gearboxes for the aerators at a local wastewater treatment plant.

36 ¦ MechChem Africa • March 2017