Blowers are an often underestimated, but critical piece of equipment in sewage and wastewater treatment. Typically used to provide aeration in activated sludge plants and to promote aerobic digestion, they also keep solids suspended in channels and in aerated grit chambers, which is critical to a proper treatment ecosystem.
With several blower technologies on the market, it can be difficult for operators and engineers to determine which one is right for their facility and budget. CAPS National Sales Manager for low pressure air products, Nick Drull, leads us through the core technologies in the market and gives some advice on what to consider when making your selection.
What are the main considerations when selecting a blower?
In a nutshell, the main considerations are blower effectiveness and efficiency. In a typical wastewater treatment plant, blowers account for only ten per cent of capital expenditure, but due to operating expenses they can represent 40–80 per cent of the plant energy bill. Therefore, it is essential to invest in the appropriate blower to maximise effectiveness and minimise energy expenditures.
What are my options?
There are four main types of blower that are used in the wastewater treatment industry; positive displacement rotary lobe (also known as tri-lobe or roots), positive displacement rotary screw, gear-driven and multistage centrifugal, and high speed turbo. Let’s take a closer look at the different technologies, highlighting their individual advantages and limitations.
Positive displacement rotary lobe
Positive displacement rotary lobe type (PD) blowers have been the industry’s workhorse for many years. They are very simple to operate; the flow changes by varying the speed with the variable frequency drive (VFD). PD blowers have a very large turndown range—that is the ability of a blower to go from design point to its minimum point—typically around 25 per cent. So if the blower has a 1000m3/hr rating, it can typically turn down to around 250m3/hr.
Turndown is a very significant factor because a plant’s aeration needs will vary. When selecting a blower, you need to know whether it will be able to satisfy possible changes in air demand. A PD blower can easily adapt to changes in pressure and temperature. Other advantages are the initial capital investment, installation and footprint requirements are comparatively low when compared to other technologies.
Indeed, the technology does come with its limitations. The blower is much less efficient as compared to other technologies and particularly as the blower speed is reduced. Depending on the application, this can increase your total cost of ownership over the equipment lifecycle.
Maintenance requirements are considered to be moderate for this type of blower compared to other technologies.
Positive displacement rotary screw
Rotary screw blowers have been around for many years but traditionally they have been used for pressures higher than typically required in a wastewater plant. Rotary screw blowers have a similar turn down rate as rotary lobe blowers but with the advantage of being more efficient.
Screw blowers actually compress the air inside the casing of the machine itself, which is a more efficient process than the lobe blower which just conveys the air and relies on backpressure. The limitation of this technology for water treatment applications is that these efficiency gains are reduced when operating at lower pressures.
As with lobe blowers, maintenance requirements are considered to be moderate for rotary screw blowers when compared to other technologies.
Gear-driven and multistage centrifugal
Centrifugal blowers use an entirely different method to operate. They work through dynamic compression which converts kinetic energy into potential energy. The advantage of this method is that you can achieve a great deal of flow at very high efficiency, particularly at the design point.
There are two main types of centrifugal blowers; single stage gear-driven and multistage. Single stage gear-driven centrifugal blowers, whilst requiring a large footprint, and higher capital investment and maintenance requirements, provide excellent longevity and reliable operation with minimal downtime potential.
In multistage centrifugal blowers, the above process gets repeated a number of times, allowing it to operate at relatively low speed—essentially the same speed as the motor which is typically 3000rpm.
High speed turbo
Centrifugal technology evolved to the high speed turbo blower. This technology often incorporates air foil bearings, permanent magnet motor and an onboard motor control system. The whole system is self-contained and comes in a wide variety of sizes, ranging from 50HP up to 700HP.
High speed turbo blowers provide very high levels of efficiency with some models reaching up to 75 per cent total efficiency. One contributor to the high efficiency of this technology is the turbo impeller being connected directly to the shaft, which eliminates transmission losses.
For a high speed turbo to operate effectively and efficiently, it needs to run within the sweet spot of the performance map, avoiding surge and choke while still satisfying system air requirements. They are most efficient when operating at, or close to, design point, performing cost-effectively in applications with narrow swings in turndown. You are better off designing your system with a large quantity of smaller blowers that can operate close to their design point as opposed to one large blower operating over a wide turndown range.
Although they don’t take up a lot of space, they are sensitive to contamination and sudden pressure swings. For high ambient environments, they are available with a separated electrical enclosure to provide excellent reliability and longevity as the electrical enclosure is then located in a clean air conditioned room and not exposed to the high temperatures. They use less energy than many other blower technologies, despite higher initial costs.
This type of blower has extremely low routine maintenance costs as all that is required is air filter cleaning and replacement, which plant maintenance staff can perform with ease.
So, which blower should I select?
All blower technologies have their advantages and, in the right operating conditions and application, can deliver both performance and efficiency. Best practice combines a customer’s knowledge of the application with the equipment provider’s knowledge of the blower technology.
When selecting a blower for your specific application, you should first start with the system. Determine your system flow requirements; your min, your max, your average. Then look at pressure — whether it is consistent or variable and just how much that variation is.
Then you have to look at site conditions. For instance, your elevation which impacts the density of air. Consider the ambient range, space allocations and also what your control requirements are (i.e. whether the system is on/off or off a VFD, etc). You also need to consider the environmental conditions (i.e. corrosive, dusty, etc), your power supply (is it consistent, clean and reliable or prone to voltage fluctuations/harmonics, etc).
Once you have narrowed down your options based on the system, then you can conduct a cost-benefit analysis that includes both the capital expenditure as well as the operating variables. When making your decision, consider daily and seasonal swings in oxygen demand, fouling and aging of diffusers, air flow control and turndown capabilities, total blower efficiency and energy consumption over time, mode of operation, blower accessories and plant set up.
The fact is, there is no simple rule of thumb when it comes to selecting a blower. Making the right selection requires looking at the bigger picture and taking all variables into careful consideration.
How can CAPS help?
The best outcome for the customer is when we draw on their knowledge of the application and use that information to select a blower that delivers the greatest long-term benefits of improved efficiency and reduced operational costs. CAPS carries all four technologies from industry-proven brands including Pedro Gil, Ingersoll Rand, Next Turbo Technologies, Neuros and more.
Having access to all technologies means that we can truly offer the best fit that caters to your requirements.
If you already own or operate a blower, CAPS can also assist by providing preventative maintenance or even the option of a complete overhaul if the blower is reaching the end of its operational life.
We have nine branches across Australia ready to assist you during the selection process. Contact us on 1800 800 878 or visit caps.com.au to find your nearest branch location.