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Positive displacement Blowers (Lobe, Roots, screw) for pneumatic conveying : an Engineering Guide

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Section summary
1. What is a blower used for ?
2. Design of rotary lobes blowers
3. Performances of rotary blowers in pneumatic conveying applications
4. Important design considerations
5. Other designs of blowers
6. Manufacturers of air blowers

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1. What is a blower used for ?

Positive displacement blowers applications

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

2. Design of rotary lobes blowers

How does a Roots Blower work ?

Rotary lobe blowers are equipped with 2 rotors, by rotating and imbricating in each other, the rotors are bringing a pocket of air from the suction pressure to the discharge pressure. Such blower is often called "Roots blower". Roots are positive displacement blowers. Standard designs have only 2 lobes (as on the figures) while more recent designs have 3 lobes which allows to have a better efficiency.

Considering the small clearances in between the housing and the rotor, the air handled must be particularly clean, it will not accept any dust as it 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

Which pressure can reach a Roots blower ?

Rotary lobe 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 -0.3 to -0.4 bar g in vacuum. One must also consider that the compression will heat up the air which can be of importance for pressure dilute phase conveying and may require the use of an aftercooler. The performance of a blower is given on blower performance curves.

4. Important design considerations

How to chose an air blower for pneumatic conveying ?

Rotary lobe 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

How many types of blower are there ?

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.

What are the different kinds of blowers used in pneumatic conveying and their characteristics

Blower Type	Description	Applications and Advantages
Rotary Lobe Blowers	- Use rotating lobes to trap and move a fixed volume of air.	- Ideal for low-pressure applications in pneumatic conveying systems.
	- Known for durability and reliability.	- Kaeser’s OMEGA series blowers with three-lobe rotors are popular.
Centrifugal Fans	- Also known as centrifugal blowers.	- Provide high airflow at relatively low pressures.
	- Commonly used in dilute-phase conveying systems.	- Efficient for supplying conveying air to the system.
Roots Blowers	- Trap air in a fixed volume and discharge it against system pressure.	- Suitable for pneumatic conveying due to various advantages.
	- Used in both positive and negative conveying systems.
Screw Blowers	- Deliver efficient and reliable low-pressure air.	- Used for conveying, aeration, and process applications.
	- Easy to maintain and deliver oil-free air.	- Often included in blower packages.

Feature	Rotary Lobe Blowers (Roots)	Screw Blowers	Side channel Blowers
Picture
Operating Principle	Positive displacement	Positive displacement	Dynamic displacement
Design	Two-lobed or three-lobed	Screw rotor	Impeller with side channels
Efficiency	High	High	Moderate to High
Pressure Range	Up to 1 bar g	Up to 1.2 bar g	Up to 1.5 bar g
Vacuum Capability	Up to -0.5 bar g	Up to -0.6 bar g	Up to -0.7 bar g
Maintenance	Regular lubrication, filter replacement	Regular lubrication,filter replacement	Infrequent maintenance
Energy Efficiency	Moderate to High	High	Moderate to High
Noise Level	Moderate	Low	Moderate
Application Flexibility	Suitable for lean phase and dense phase conveying	Suitable for both lean and dense phase	Suitable for low to moderate pressure applications
Environmental Consideration	Oil-free options available	Oil-free options	Oil-free options

6. Manufacturers of air blowers

Positive displacement blower manufacturers

In order to get quotes, prices of air blowers for pneumatic conveying, the reader can refer to the following companies :

• Aerzen compressors
  
• Robuschi compressors
  
• Mapro blowers (also for side channel blowers)
  

(Note that PowderProcess.net has no link with those companies)