Balancing turndown consideration with TCO?

Do you know that the biggest factor that determines the best blower for a WWT plant is its ability to cope with changing air demands throughout its lifespan??

Thus, it is essential to consider how different technologies will perform in your WWT plant, based on changing air demands, turndown range and the solutions design.

With the average lifecycle of a WWT plant being 20 to 25 years, specifying blowers for this length of service can prove to be a challenge. Decision makers not only need to plan for a plant’s existing capacity, but for future growth too. And if we consider the solutions that were available 20 years ago and how dramatically technology has evolved since then, it’s clear that selecting a solution for this length of time is not straight-forward.?

A common problem we see is that many WWT plants end up over-sizing solutions to try to accommodate future demands, as well as lowering any risks of having a downsized plant.?

By considering blowers’ turndown in the selection process, you can optimize to meet the demands of your plant now, or in the next 5 to 15 years.

Get the full picture of how to choose the most efficient and cost-effective blower system for your site?

Turndown is an essential factor that can help accommodate changes in demand over a period of time. Nevertheless, turndown is consistently overlooked when specifying blower systems, and yet can have a big impact on a system’s whole life costs.

What is turndown?

‘Turndown’ refers to the blower’s ability to reduce its air flow rate quickly and efficiently to meet the changing air demands of a site. Turndown considers the maximum and minimum blower or system flow rates, and could be written as the following equation…?

Turndown % = (Max capacity – Min capacity x 100) / Max capacity?

Shortly, greater turndown means more flexibility for a wastewater treatment to handle unexpected demands.?

Greater turndown aims that the system is better equipped to meet a site’s lowest air requirements, without wasting energy, while delivering the flexibility to achieve larger air demands at peak times or to meet long-term needs.?

Ultimately, no matter how good the performance of a system is, having a blower or a system generate unnecessary air is still a significant waste of energy.?

Another consideration is that greater turndown also provides more operational flexibility, enabling a system to effectively satisfy air demands with fewer blower units.

Forecasting for the future?

When selecting blower technology, it’s critical to have an understanding of the equipment’s efficiency capabilities over time – over a 25-year period, to be precise.?

The architecture and design of each type of technology will further impact, limit and improve turndown over time, so this needs to be factored in when choosing a system too.?

A blow-by-blow comparison of different technologies

Rotary lobe blowers?

Available for a lower capital cost, a rotary lobe blower is an oil-free technology are well-known for their reliable turndown range when sized properly to a site’s demands and are often ideally suited to applications requiring medium to small air flows.?

These machines are generally less efficient than screw and turbo solutions.?

Rotary lobe blowers do tend use more energy during their service life, leading to greater operating costs for plants. Their simple design, however, means they are relatively low-cost to maintain.?

Screw blowers

This oil-free technology (3x5 or 4x6 profile rotor) boasts increased efficiency due to its internal compression, which can be further enhanced with a frequency converter.?

Nevertheless, screw technology can often see efficiency losses through its transmission, motors, gears and frequency converters, which means they can appear to be not as efficient as turbo technology.???

However, what screw blowers do offer is a wide turndown range due to the volumetric’ machine, rather than a ‘dynamic’ one like turbo or centrifugal technology.

A WWT plant’s varying working pressure is easy for screw technology to handle, delivering a real benefit to sites that demand a wide turndown range.

Turbo blowers

Using a high-speed motor and drive, this oil-free technology is popular because of its high performance, taking advantage of either air foil or magnetic bearings to deliver excellent energy efficiency. With no mechanical efficiency losses and limited maintenance operations due to contactless transmission, it appears on paper that the turbo generally offers the best blower solution available.?

When operating at its ‘sweet spot’ – the ideal pressure and flow specified by an engineer – the turbo’s performance is impressive. But, when turndown is factored in too, it becomes apparent that the turbo will not always be operating in these optimal conditions. Once air demand fluctuates, the turbo blower will no longer be as efficient as perhaps first anticipated.?

Turndown scenarios??

The best means of analysing the correct blower technology is to compare different case Turndown scenarios.?

To illustrate the energy consumption and financial impact of both approaches, Robuschi White paper presents a fictional but realistic case study example, with two potential turndown scenarios.??

? Scenario 1: Fixed flow: No fluctuation in total flow capacity considered during the blower selection process, with the decision made based same parameters. In this case, only the most challenging flow requirement is considered in order to calculate the TCO. This reflects the expected cost of ownership if the blowers operate from day one and for the next 20 years at their design capacity.?

? Scenario 2: Variable Flow: Fluctuation in flow capacity considered for the plant’s operating years until 20 years. Here, the calculation accounts for the flow parameter variability, where flow demand increases gradually to match the increasing demands of the aeration process.??

The precise figures presented in the Robuschi white paper allow us to conclude that a WWT plant would be better just purchasing the number of blowers it needs for a set period. The plant’s consumption could be reviewed over time, with additional units purchased in the future to help increase the plant’s capacity. This investment would be based on more accurate data, too.??

It might even be that a mix of different blower technologies would help provide the best solution for your site.?

As shown in the scenario comparison in the white paper, the demand fluctuation has a significant impact on energy consumption. This offers a more realistic approach, compared to the worst-case scenario of running at maximum capacity demand from the beginning of the plant’s operation.?

No ‘one-size-fits-all’?

Throughout the whitepaper, you will see there is no ‘one-size-fits-all’ approach to choosing the right blower technology for your WWT plant. It will depend on a variety of differing factors, from the design of the blower system and site conditions, to air demands throughout the lifetime of the WWT plant and the turndown range of equipment during this time.?

In order to specify the best blower solution for your site, you need to balance both turndown considerations with a TCO mindset, taking into account a plant’s day-to-day demands, operating conditions and the available budget to determine which technology is the right one for you. Only by doing this will you be able to identify the most efficient and cost-effective long-term solution.??

Would you like to know more??Download now the dedicated white paper “Balancing turndown considerations with TCO”?

Hurry up, contact our sales representatives to have the help and support of a trusted blower manufacturer and specialist, who can work closely with you to undertake a long-term cost analysis model for your WWT plant.?