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Friday, November 29, 2019

Venturi eductors are an effective way to move powders, pellets and granular bulk solids in a process known as lean phase (dilute) pneumatic conveying. They are widely used in the cement, food processing, mining, power generation and other major industries. Venturi eductors are also known as ejectors, injectors, jet pumps and venturi pumps.

What are venturi eductors and what do they do?

A venturi eductor is the section of a pneumatic conveying system that transports a given product (powders, pellets and granular bulk solids) over a fixed distance and height. These devices use the venturi principle to entrain low pressure air with a high pressure air stream. The mixture converts to an intermediate pressure as it discharges through the length of the eductor. Venturi eductors can pneumatically convey up to 15 tonnes of product per hour over distances up to 100 meters. As they have no moving parts, they generally operate maintenance-free for many years.

How do they work

A venturi eductor converts the pressure energy of a high pressure air stream (the motive) into velocity. The motive expands through the eductor nozzle to create a vacuum. This entrains the low pressure air that contains the product and creates suction. The two air masses and product are mixed and become re-pressurised to an intermediate pressure within the eductor. As the re-pressurisation occurs and the velocity is converted back into pressure, the product is transported through the eductor to finally discharge from the outlet nozzle.

Venturi eductors in pneumatic conveying systems

Venturi eductors in pneumatic conveying systems

How are venturi eductors used in pneumatic conveying systems?

Venturi eductors require only a positive pressure from a suitable blower to drive the conveying process. They are an effective alternative to using rotary valves or electric pumps as the mechanism to drive the product transport.

Venturi eductors are superior to rotary valve-based conveying systems when the product is abrasive, fine, fragile or irregularly shaped. These devices can safely operate in hazardous or high temperature environments where electrical pumps cannot.

Venturi eductor usage determines the blower type and output level

The output level of a blower connected to the venturi eductor is the dominant factor used when designing a pneumatic conveying system. The blower output can be easily adjusted to meet the transport requirements of a given product over a given length. One blower with a sufficiently high level of output can be used to drive a pneumatic system that has many eductors. Further, pneumatic systems can be configured so that a venturi eductor outputs via multiple outlets (typical in large dust collectors).

Roots blowers

Roots blowers (positive displacement) are the most common type used in venturi eductor-based systems. They work by trapping and discharging air between impeller lobes within a housing. For more information on roots blowers you can read how to choose the right roots blowers for your pneumatic conveying system.

Regenerative blowers

Venturi eductors can also be combined with regenerative blowers mounted on a mobile cart to make a portable pneumatic conveying system. Regenerative blowers have an impeller that spins within its housing, which has both an inboard and outboard channel. For more information on regenerative blowers, you can read centrifugal blowers vs regenerative blowers.

Multiple venturi eductors driven by single blower

Multiple venturi eductors driven by single blower

Replacing rotary valve-based systems

The replacement of rotary valve-based systems with venturi eductors can be feasible in certain situations. Venturi eductors make for simpler, more reliable conveying systems that can better transport sensitive or irregular products. Also, they provide significant improvement, such as:

  • Aversion of dust emissions and leakage
  • Elimination of rotary valve wear
  • Provide clean-outs when avoiding cross-contamination is critical
  • Permit reclamation of discarded product
  • Reduction of product damage and degradation

Determining if venturi eductors are a superior alternative in new plant

It is ideal to engage experienced design experts when implementing installation of a new pneumatic conveying system. They can determine from the start whether the system can work with venturi eductors. This will avoid having to replace at a later date a system that was first designed with rotary valves.

Checking feasibility when considering retrofitting in existing plant

It is recommended that experienced design experts be engaged when considering a retrofit, and replacement of an existing rotary valve-based system with a venturi eductor-based one. Retrofitting existing pneumatic conveying systems with eductors can have some technical and commercial challenges. Detailed analysis of the existing blower, pipe geometry, dust collection and conveyed rate material is required to check feasibility.

Elimination of blowback issues

All rotary valves have some level of blowback (fugitive dust or leakage). The blowback from abrasive or fine product can cause extreme wear to rotary valves. They suffer from bridging and housekeeping problems and can pose explosion risks. Slide gates can be used to reduce blowback in rotary valves but this is not always possible.

Elimination of product damage

As venturi eductors have no moving parts they do not cause the product damage that is typical when using rotary valves. Movement of abrasive, fine, fragile or irregularly shaped products through a rotary valve can result in mechanical shearing, smearing or degradation.

Venturi eductors can eliminate rotary valves in pneumatic conveying systems

Venturi eductors can eliminate rotary valves in pneumatic conveying systems

What products are venturi eductors used to convey?

Below is a list of some of the bulk powder, pellets and granular products conveyed with venturi eductor based systems.

  • Acid powders
  • Activated carbon
  • Aluminium compounds
  • Ammonium compounds
  • Anticaking agents
  • Anti-perspirants
  • Apple bits
  • Almonds
  • Asbestos
  • Charcoal
  • Cement
  • Coal
  • Dry adhesives
  • Flakes
  • Fly ash
  • Limestone
  • Pellets
  • Rice hulls
  • Sweeteners

Need help with eductors for your pneumatic conveying system?

For more than 30 years Pneuvay has been providing help to the Australian industry on venturi eductor-based pneumatic conveying systems. We have experience working with a broad range of Australian companies, designing, installing and maintaining venturi eductor systems that:

  • Minimise damage to sensitive products
  • Maximising conveying rates for sensitive products
  • Keep maintenance costs to a minimum
  • Reduce dust-related OH&S risks

If you have any questions in relation to venturi eductors, feel free to call us on 1300721458 or send us a message via our Pneuvay website contact page or Pneuvay Facebook page.

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