Selecting the optimal blower for the water industry [white paper extract]

The use of blowers in water industry applications plays a decisive role when it comes to establishing an efficient and sustainable water supply. Blowers are used for a multitude of different applications in the water management sector, whether it’s supplying oxygen for biological purification processes at wastewater treatment plants, filtration and cleaning of water treatment systems, or maintaining the pressure in the pipework at wastewater pumping stations. 

The product range is every bit as diverse as the applications for which blowers can be used, which therefore begs the following question before making an investment: rotary lobe, rotary screw or turbo blower? Here, the operator must focus on the individual requirements in terms of pressure, flow rate and operating hours, in order to achieve the highest possible energy savings by meticulous selection of the right blower type.

Low-pressure applications in the water industry generally range from a differential pressure of 0.4 bar to 0.8 bar. This requirement can be achieved by a number of different compressor technologies, but beware: energy, maintenance and investment costs can often vary considerably. Therefore, a precise analysis of the prevailing operating conditions and resulting life-cycle costs is absolutely essential. Only when the operator’s exact requirement is known can the right decision be taken in view of energy consumption, dependability and operating costs. 

For the low-pressure range, rotary lobe, rotary screw or turbo blowers may be used. Each of these three technologies has its own distinct advantages, but to be frank, one or two disadvantages as well. 

The data for the different blower types are often not far apart, and the decision as to which is the best in terms of energy can only be made following a calculation and comparison of the life-cycle costs. This calculation must be based on comparable performance data.

Rotary lobe blowers operate by compressing air inside a rotating housing, usually via a three-lobe rotor. The primary and secondary rotors have the same cross section. Pressure build-up does not occur in the blower block, but in the downstream process line by means of the constant pushing of the air mass against the prevailing resistance in the subsequent process. 

Rotary lobe blowers are durable, less cost-intensive systems recommended for differential pressures up to 0.5 bar. Depending on the model size, blower speed is from 2,000 to 6,000 rpm. A maximum control range of 1:3 is achievable, and on systems equipped with a frequency converter, isentropic efficiency is between 45 and 60%. 

This type of blower is characterised by its robustness and low initial investment costs; however, due to energy reasons, in the water industry it is best suited to applications with low to medium pressure requirement and short run times.

Rotary screw blowers operate with a helical primary and a secondary rotor that rotate in opposite directions to one another, thereby compressing the air within the screw threads. This means that the air is pre-compressed inside the blower airend before being expelled into the pressure piping. If this internal pre-compression has been well matched to the required pressure in the process piping, less compression work is needed – this makes the rotary screw blower more efficient. 

These high-efficiency systems are suitable for differential pressures between 0.5 and 1.1 bar, and achieve a flow rate control range of 1:4. Rotary screw blowers equipped with integrated frequency converter attain an isentropic efficiency significantly higher than that of rotary lobe blowers at 60 to 78%, which remains very stable even with a varying flow rate. Currently, models are available with a flow rate range of 5 to 165 m3/min. Depending on the model size, blower speeds from 3,000 to 12,000 rpm are usual. 

These blowers are known for their high performance. In the water industry, they are suited to applications with medium to high pressure requirement. They are recommended for aeration processes that combine the requirements of a high number of operating hours, a broad control range and a constant efficiency curve over the flow rate.

Unlike rotary lobe and rotary screw blowers, turbo blowers are dynamic compressors. Pressure build-up occurs due to an increase in the flow velocity through the turbo impeller and its subsequent conversion to pressure in the diffuser. 

Single-stage turbo blowers are suitable for differential pressures between 0.4 and 1.3 bar. Impeller speed ranges between 20,000 and 30,000 rpm. Magnetic bearing turbo blowers achieve an isentropic efficiency between 60 and 78%. In contrast with positive displacement blowers, peak efficiency is achieved in a narrower range and is more heavily dependent on the respective pressure and flow rate. The flow rate control range on turbo blowers also varies more heavily with the pressure; at design pressure, a maximum control range of 1:3 is achievable. This must be taken into account in advance during the planning stage, in order to select a blower with the most efficient and complete overlap possible within the station network. 

Turbo blowers are especially efficient and powerful, and are best suited for applications with very high air demand – such as aeration processes in the water industry, where the flow rates that can be achieved using rotary screw blowers may not be sufficient.

Unfortunately, it’s not just a question of selecting the right blower type. This is because when comparing different quotations, numerous stumbling blocks remain hidden. The reason?

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