Key considerations for blower specification

September 28, 2017

Aeration blowers used in wastewater treatment may use a significant amount of energy, but they are integral to the treatment process. It’s therefore important to get your choice of blower right first time to keep operations running smoothly and costs down. Adam Bond, Sales Manager Biological Treatment at Xylem, outlines the key considerations for businesses when it comes to blower specification, and explains the importance of a close relationship between customers and equipment providers.

Aeration is a key process within wastewater treatment plants, forming the secondary treatment stage after solids have been removed. The blower plays a big part in this process because it provides the air supply that metabolises organic compounds in the wastewater and removes carbon dioxide.

Many options currently exist on the market, some of which have been around for over 100 years. These are Positive Displacement Lobe Blowers and Screw Blowers, Integral Gear Type Centrifugal Blowers, Direct Drive Turbo Blowers, and Non-Contact Magnetic Bearing or Air Bearing Blowers. Each has its advantages and disadvantages and will be more suited to certain applications over others.

The main elements to consider when choosing the most suitable blower for a wastewater treatment plant are its total cost of ownership, efficiency, turn-down capability and maintenance requirements, in addition to the pressure, flow and size of your plant.

1.Total cost of ownership

In a typical wastewater treatment plant, blowers account for only 10 per cent of capital expenditure. Due to operating expenses, however, they can represent 40-80 per cent of the energy used at a treatment plant. It’s therefore important that you consider the lifetime cost of the blower, as well as its initial price. Non-contact blowers, particularly the air bearing blower, often have the lowest total cost of ownership due to their high efficiency and lack of frictional damage. The air bearing blower has the added bonus of being entirely automated, thus requiring little operator intervention, and can achieve high bearing lifetimes if triple nano silver coatings are used.

Integral gear type centrifugal blowers arguably have a higher cost of ownership because the control system needed to operate its parts is both complex and expensive. It also requires more plant space and good ventilation, as it is known for emitting a high amount of heat, noise and vibration. 

2. Efficiency 

As aeration blowers use a considerable amount of energy in a wastewater treatment setting, correctly matching the type of blower to the size of plant and varying conditions is crucial in order to achieve a return on investment. An initial way of understanding the potential payback period is to read the manufacturer’s guidance on how well the blower transfers the energy power into air power. This is not always a clear indicator of the blowers performance in practice, however, and a number of other points need to be considered, such as turn down capability to meet the needs of different operating conditions.

Direct Drive Turbo Blowers offer excellent efficiency because the turbo impeller is connected directly to the shaft, which makes it possible to eliminate all transmission losses, resulting in excellent efficiency. Direct drive turbo blowers are engineered using motor and variable frequency drive technology, which transforms 50 or 60 Hz of supplied power into high rotating frequencies.

Other blower types, such as the positive displacement screw and lobe blowers, often see efficiency losses through their transmission, motors, gears and variable frequency drives, with the lobe blower in particular often performing better on paper than it does in actual operation. Both also lose efficiency through increased pressure, although screw blowers are often more efficient than lobe blowers, which have no internal compression during the 360-degree rotation cycle.

3. Turn-down capability

Turndown is essentially a blower’s ability to reduce its air flow rate to meet demand, taking into account the maximum and minimum blower or system flow rates. It is important to gauge how a single blower or group of blowers are able to satisfy a change in future air demand, as this can alter considerably during the treatment process. Furthermore, turn down capability has a significant impact on the amount of energy a blower uses, thus affecting its efficiency.

Positive Displacement lobe blowers and screw blowers are renowned for their reliable turn-down capabilities when sized correctly, although they are often noisy and inefficient. The turn-down capabilities of these blowers are due, in part, to their simplicity. The lobe blower in particular has been used widely in aerating wastewater for more than 100 years, and is applicable to medium and small flows.

4. Repair and maintenance

Opting for a blower with minimal repair and maintenance requirements is important in reducing downtime, the costs of which can add up quickly. Depending on the model, blowers can be complicated to install, operate, and maintain, so it’s important to consider how frequently repair and maintenance is required, how complex it is, and ultimately if it is more economical to repair or replace.

Non-Contact Air Bearing Blowers are renowned for requiring minimal repairs and maintenance. They do not require any additional backup systems, are not negatively impacted by power failures, and are entirely self-controlled with no need for any safety devices to protect the bearing.

On the other hand, Integral Gear Type Centrifugal Blowers, which are used for higher flows, require a considerable amount of upkeep due to their flow inlet guide vane (IGV) and variable diffuser (VD). These components contain several mechanical parts requiring maintenance and the control system required for their operation is often complex and expensive.

Similarly, Non-Contact Magnetic Bearing Blowers can demand an intense maintenance regime because they use active magnets to position the motor rotor in the right position during both operation and at standstill. Despite offering good levels of efficiency and significant turn-down capability, constant monitoring of the rotor position means that this technology is sensitive to power failures. Overall the technology is affected by higher temperatures and requires several backup systems and control functions, making failure highly probable.

Positive displacement lobe blowers and screw blowers both have moderate repair and maintenance requirements but, due to their high levels of vibration, screw coatings can wear away and, over time, will need to be replaced.

With so much choice available it is important to talk to the experts before making a final decision. Choosing a blower is by no means a simple task, with many factors needing to be considered to meet the varying demands of different applications. Ultimately, the application and its potential conditions must be considered from the outset. Pressure and flow demands are of as much importance as cost, efficiency, turn-down ratio, and repair and maintenance.

Best practice always involves combining a customer’s knowledge of the application with the equipment provider’s knowledge of the blower technology. This will lead to the most suitable blower choice, with wastewater treatment plants reaping the long-term benefits of improved efficiency and reduced operational costs.