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Roots Blowers

Positive displacement blowers especially adapted for pneumatic conveying

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Section summary
1. Introduction
2. Design
3. Performances
4. Important design considerations
5. Other designs of blowers

1. Introduction

Blowers constitute the air movers in many pneumatic conveying systems running lean phase. They can be placed at the beginning of the line, delivering an overpressure, or at the end of the line to produce a vacuum. The most common type of blower used for this purpose is the Roots blower, a positive displacement blower that can be operated either in pressure or vacuum without specific changes.

2. Design for rotary lobes blowers

Roots blowers are equipped with 2 rotors, by rotating and imbricating each other, the rotors are bringing a pocket of air from the suction pressure to the discharge pressure. Older designs of rotors have 2 lobes while newer one are using 3 lobes per rotor, which allows to reach a better efficiency for dilute phase pneumatic conveying. Roots blower are positive displacement (volumetric) blowers.

Considering the small clearances in between the housing and the rotor, the air handled must be particularly clean, it will not accept any dust would could lead to a damage of the blower. In order to avoid dust entering the blower, such blowers are equipped with a filter at the suction.

For hygienic applications, oil free systems must be specified.

3. Performances of rotary blowers in pneumatic conveying applications

Roots blowers can reach max 1 bar g in pressure and max -0.5 bar g in vacuum. Maximum operating pressures of pneumatic conveying blowers are therefore often set at 0.7-0.8 bar g in pressure and -0.5 bar g in vacuum.

Table 1 : Operating and maximum pressure of Roots Blower in pneumatic conveying line service

Roots blower service type Typical operating pressure Typical maximal pressure
Pressure 500 to 600 mbar 700 to 800 mbar
Vacuum -350 to -400 mbar -500 mbar

4. Important design considerations

Roots blowers are volumetric, they will always take a volume of air and bring it to new conditions. Therefore when the system is used in pressure, it must be considered in the design that, if the discharge pressure is increasing, the volume delivered will decrease, leading to a lower air speed in the pipeline. Sufficient margin must be considered so that the air speed in pressure is still high enough to convey in lean phase (speed > saltation or choke speed for the product conveyed).

5. Other design considerations

The energy efficiency of the blower technology chosen is another criteria of choice. Hybrid or screw blowers can have a payback of around 2 years compared to classics roots blowers if they are used intensively.