We’re continuing our series on selecting blower technology for industrial wasterwater treatment applications with an overview of the technology behind positive displacement blowers. You can read the previous articles from the series by choosing “wastewater” under industries or by following the link.
Once air demand has been calculated, evaluation of blower technologies can begin. Several technologies, including positive displacement and centrifugal blowers, are available for wastewater aeration and mixing.
Positive Displacement (PD) Technology
The positive displacement (PD) blower is an American invention pioneered by the Roots brothers in the late 1850s. PD machines operate with flow and pressure as independent variables. At constant speed and if pressure increases, the flow rate is largely uneffected. The efficiency curve, or specific energy ratio (SER), of PD machines is typically close to linear. This means that the efficiency of the machine is similar across the operating range which makes PD blowers the ideal choice for applications that require large variations in flow or pressure. There are two main types of positive displacement blowers:
- Roots, Lobe-Style Blower –The “Roots” or lobe-style blower that was invented in the 1800s remains very much unchanged today; the primary advancement in lobe technology over the past 150 years has not been improved efficiency, but reduced noise. Bi-lobe blowers produce very high amplitude waves at a low frequency; to reduce the amplitude and increase the frequency of the waves, a third lobe was added. While this innovation decreased efficiency slightly, it reduced noise levels to below 85 dBA as mandated by OSHA.
- Rotary Screw Blower – In an effort to increase efficiency of positive displacement blower technology, the rotary screw blower was introduced to the market in 2009. The rotary screw blower uses a helical screw profile instead of lobes to create internal compression of the inlet air. The screw rotor profile serves a dual purpose. First, the internal compression of air reduces energy consumption by 30 percent compared to traditional lobe blowers. Second, by eliminating the pulses caused by lobe technology, screw blower noise levels are typically below 75 dBA which is three to five times quieter than conventional tri-lobe blowers.
Rotary screw blowers can also reach higher pressures than lobe blowers, and often have lower operating temperatures due to the reduced slip of the rotor profile. As a result of lower operating temperature, variable speed screw blowers usually have a much wider operating range (more turndown) than lobe blowers. On the other hand, lobe blowers come in a wide variety of sizes, so higher flow rates can usually be achieved with a lobe blower than a rotary screw blower.
Stay Connected
We’ll continue our series on selecting blower technology next week with a discussion on centrifugal technologies. In the meantime, explore Atlas Copco’s blower technologies by visiting Atlas Copco’s efficiency blowers microsite. Subscribe to The Fifth Utility and receive an email newsletter every Friday morning with links to the week’s articles.
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