Water & Wastewater Treatment

32 | january 2015 | WWT | www.wwtonline.co.uk In the know Getting to grips with blowers Performing under pressure Whether in water or wastewater applications, choosing the right low-pressure blower for the right job is crucial F or industries that deal with water and wastewater treatment, the breadth of blower technology available today can be confusing. In the past, the means of providing a supply of low pressure air for purposes such as water treatment aeration, pneumatic conveying and other industry applica- tions required relatively simple blower equipment. But with reliability, perfor- mance and energy efficiency higher on the agenda than ever before, specifiers should address a series of key considera- tions before committing to a purchase. There are a number of important fac- tors to take into account when making the right choice of low pressure blower equipment. Which is best, positive displacement or dynamic / centrifugal? There are two main methods of air com- pression: positive displacement, which creates pressure by reducing air volume, and dynamic or centrifugal compression, which uses speed to create pressure by turning kinetic energy into static energy. Positive displacement blowers usually handle lower flows than centrifugal blowers, but provide a bigger turndown, which maintains a steady level of ef- ficiency. Centrifugal machines operate with bigger flows and can be optimised at specific points. The choice depends on the application. For processes that require wide variations of flow, positive displacement blowers are ideal; whereas those that require a big flow with little variation might find a centrifugal blower to be better suited. However, it's not unusual to combine the two types. For Rob boughton LoW Pressure saLes Manager, aTLas CoPCo Zs series blowers installed at a northumbrian Water site example, a centrifugal machine might look aƒer the base load, while the posi- tive displacement machine manages variations of flow. Is operating range important? In positive displacement, the market has been traditionally led by lobe blowers, which are commonly used for aeration in wastewater treatment plants or for pneumatic conveying within a factory. Low pressure applications invariably feature fluctuations in air demand for the process so you need to know that turndown on conventional lobe blowers can incur increased energy penalties. However, a blower incorporating more modern screw technology can oper- ate from 100% capacity down to 25% with very little change in specific power requirement. how much air can actually be delivered? Make sure that you know the actual volume of air the blower package deliv- ers, not just the air intake figure stated by the manufacturer, which is a common practice in the lobe blower market. The air output is a critical factor in sustain- ing essential oxygen levels within any aerobic process and, what's more, it is a requirement of industry standard ISO1217 that performance data is stated as FAD (Free Air Delivery) – the actual volume of air at the blower outlet. Do you need to eliminate pres- sure drops in the system? It is a proven fact that lobe blowers have to work harder, and consequently less efficiently, than screw technology blow- ers to deliver the same output. That is because the delivered air is compressed externally and depends on back pressure from the connected pipe work – or, in water treatment applications, water resistance in the aeration tank – in order to maintain the required air flow and pressure. Bear in mind that resistance to the internal air flow can lead to substan- tial pressure drops and increased energy consumption. In contrast, a rotary screw type blower demands up to 30% less energy because its proven internal compression principle is far more energy efficient. how can power transmission losses be minimised? Belt and pulley transmission from the motor shaƒ to the compression element has been the industry standard system for some time now but the downside is the need for constant maintenance and frequent replacement. A blower with close-coupled integrated gearbox trans- mission offers a more efficient alternative and can gain more than 3% in energy savings. What about discharge temperature? The discharge air temperature of a screw blower is, on average, some 5% cooler than that from a lobe blower. A lower operating temperature can benefit the aerobic process in wastewater aeration applications and allow for more options on the pipework layout. Is low noise operation essential? Environmental impact is a vital consider- ation for many blower application sites.

• Cover