Plant & Works Engineering September 2022 Issue

Focus on: Compressed Air Process, Controls & Plant bearing. These systems eliminate mechanical wear, extending the life of the blower and their ability to support the shaft from zero speed keeps the HST efficient and wear-free through frequent on- off cycles. Turndown requirements The air demand in a wastewater treatment process can vary significantly depending on the volume of water entering the system. Optimum performance requires the ability to adjust blower output as conditions change. Volumetric blowers can typically “turndown” to 30% of their full output; reducing the speed of the motor reduces the output of the blower and the energy consumed by the system. Multi-stage centrifugal compressors have a narrower operating range, allowing turndown to 60% or 70% of full output before efficiency begins to drop off significantly. Single- stage centrifugal blowers using adjustable vanes at the inlet and/or outlet and high-speed turbo compressors using variable speed operation can reach between 30-50% turndown from maximum flow, depending on pressure. For applications where demand varies beyond the turndown capabilities of a single compressor, operators usually install multiple units, switching them on or off in sequence, according to demand. This approach optimises energy efficiency and improves the overall availability of the system, since spare capacity is available outside peak times should a blower be unavailable. Reducing maintenance costs The different types of blowers will call for different types of maintenance. The gear box of the single-stage centrifugal blower will need regular attention according to a strict service and maintenance program. The screw blower design will need to be checked regularly to avoid costly breakdowns and often an exchange of the core elements can be expected at some point. The Roots type of blower is often not cared for properly and hence is prone to seizing up. If that happens, it is usually repairable but may lose significant volumetric tightness and as a result efficiency will suffer. High-speed designs with magnetic bearings do not have wearing parts and Edward Paro, Portfolio Manager Compressors & Aeration for Sulzer, says the HST turbocompressor is unique in not containing any liquid. This limits physical maintenance tasks to checking and exchanging air filters. Introducing more efficient and more reliable blowers can have a significant benefit to the operational and maintenance budgets. For example, Paro says in one case where HST blowers were introduced, maintenance costs were reduced by 95% while in another, energy costs were reduced by 30%. One installation saved EUR 12’000 per blower in annual maintenance costs. In fact, Paro highlights that the installation of four HST units at one site in Europe saw the combined energy and maintenance costs drop by EUR 175’000 per year. In municipal wastewater treatment applications, Sulzer often installs 10 to 15 HST turbo blowers at a single site. Some recent large-scale projects have involved up to 40 units. In each case, their efficiency as well as their reliability have surpassed expectations and treatment plants have made considerable savings in terms of operational and maintenance costs. September2022 www.pwemag.co.uk Plant & Works Engineering | 25