Positive displacement blowers for pneumatic transport
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# Types of pneumatic transport
# Conveying phases
# Dilute Phase transport
# Dense Phase transport
# Air mover
# Roots Blower
# Roots Blower performance curve
# After Cooler
# Airlock Rotary Valve
# Product inlet / Injector
# Choking velocity
# Conveying speed / velocity
# Air volumetric and mass flowrate
# Pipe Equivalent Length
# Solids particle velocity in pipe
# Solids Breakage
#Pipe Diameter or Bore
# Selecting dilute or dense phase
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. 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
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.
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