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Pneumatic Conveying Systems for Food Processing in Canada

Granular and powdered solids are transported through gas streams in many sectors of the chemical industry. This article examines the advantages and challenges of pneumatic conveying systems and offers guidelines for design engineers.

When considering pneumatic conveying, two key concepts must be kept in mind. First, the system pressure decreases along the length of the pipeline, causing the specific volume of the gas to increase, along with its velocity. These changes can limit the effective conveying distance. Second, a minimum velocity is required to carry the solids. This velocity depends on factors such as the gas-to-solid ratio, the densities of the gas and solids, particle size and shape, and the pipe’s diameter.

Key Components of Pneumatic Conveying Systems

A pneumatic conveying system typically consists of four main components:

  1. Prime Mover: This includes compressors, fans, blowers, or vacuum pumps that generate the gas stream. Engineers must determine the required gas flow rate and pressure (whether positive or negative) to move the material.
  2. Feed Mechanism: Solids are introduced into the gas stream using vacuum systems or specially designed valves, pumps, or blow vessels. The solids must be rapidly accelerated to the conveying velocity, which creates a significant pressure drop.
  3. Conveying Piping: The design and sizing of the pipes (including slopes and bends) must align with the properties of the solids and the desired conveying rate(s).
  4. Disengagement: After transportation, the solids are separated from the gas stream using cyclone separators or fabric filters. This stage also involves collecting the solids (e.g., into a bin) and managing the gas stream (either as waste or for recycling).

Types of Pneumatic Conveying Systems

Pneumatic conveyors are generally classified into two types: dilute-phase and dense-phase.

  • Dilute-Phase Systems: In this system, solid particles are suspended in the carrier gas and conveyed at high velocity to prevent settling or blockages, especially in vertical sections. Dilute-phase systems are the most common type, characterized by:
    • High velocities (700–1,000 m/min or 2,400–3,200 ft/min for fine powders or granules).
    • High attrition due to particle collisions.
    • Pressure typically <100 kPa (15 psig).
    • Low system cost.
    • Larger pipe sizes.
    • Low solid-to-gas mass ratios (up to 15).
  • Dense-Phase Systems: Here, solids are conveyed at a lower velocity, flowing as slugs or plugs. These systems are ideal for materials like plastic pellets or cement that have high gas permeability or retention. Dense-phase systems are characterized by:
    • Low velocity (typically 200 m/min or 600 ft/min).
    • Low attrition.
    • High pressure (typically 100–600 kPa).
    • Higher system cost.
    • Smaller pipe sizes.
    • High solid-to-gas mass ratios (100 or more).

Benefits of Pneumatic Conveying Systems

Pneumatic conveying offers several key advantages:

  • Minimal exposure to the environment: The system can operate in a closed loop, reducing product contamination.
  • Long-distance transport: It is effective for conveying material over distances up to 1,000 meters.
  • Versatility: Suitable for handling multiple products from multiple sources to multiple destinations.
  • Protection of sensitive materials: Ideal for transporting products sensitive to air or moisture.
  • Space-efficient routing: Pneumatic systems can navigate complex elevations and obstacles more easily than mechanical conveyors.
  • Seamless integration: Pneumatic conveyors interface well with other transportation modes like trucks, railcars, and ships.
  • Reliability: Fewer moving parts mean lower maintenance and higher reliability.
  • Multiple product handling: A single pipeline can be used for transporting different products.
  • Security: Pipeline systems are ideal for high-value products as they provide a secure means of transportation.
  • Automation: Pneumatic conveyors can be easily automated and controlled for better operational efficiency.

Challenges of Pneumatic Conveying

Despite their benefits, pneumatic conveying systems also face certain challenges:

  • Product degradation: Poor system design can result in material damage during transport.
  • Wear and tear: Components, such as pipes, may experience wear due to abrasion, increasing maintenance needs.
  • Limited long-distance capabilities: The expansion of gas over long distances can reduce efficiency, making pneumatic conveying unsuitable for distances greater than 1,000 meters.
  • High power consumption: The energy requirements for pneumatic systems can be substantial, especially in high-volume or long-distance applications.
  • Expertise required: Proper system design, operation, and maintenance require skilled personnel to ensure optimal performance and minimize risks.

In conclusion, while pneumatic conveying systems provide many advantages in terms of efficiency, versatility, and automation, they also come with design and operational challenges that need to be addressed to ensure long-term success and cost-effectiveness.

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