Pneumatic Conveying

Types of pneumatic conveying systems

There are several methods of transporting materials using pneumatic conveying.

In general, they seem to fall into these main categories: dilute phase, dense phase, and semi-dense phase.

Dilute phase pneumatic conveyors

Dilute phase pneumatic conveyors apply low pressure to move bulk material to send it to its destination at high speed through the conveying line. Dilute phase pneumatic conveyors can be either vacuum or pressure focused depending on the material to be transferred.

The dilute-phase system can be designed in three ways:

1. Positive pressure system - Positive pressure systems operate above atmospheric pressure and convey bulk materials from a single source or multiple sources to one or multiple destinations, over medium distances, and with greater capacity than possible using vacuum systems.
2. Negative pressure or vacuum system - Negative pressure or vacuum systems operate with air pressures below atmospheric pressure. Negative pressure (vacuum) is generally used to convey material from multiple sources such as storage vessels, process equipment, trucks, and rail cars, to an individual or various destinations.
3. Combination of positive-negative system - This pull-push dilute phase system incorporates the advantages and benefits of both negative and positive pressure arrangements in a single design. These systems are used with multiple material entry points and delivery points.

Dilute phase pneumatic conveying systems are ideal with:

• Non-abrasive and non-fragile materials with a light density
• Certain batching systems
• Examples: flour, cornstarch

Dense phase pneumatic conveyors

Dense phase pneumatic conveyors offer the enormous advantage of effectively "pushing" a much thicker concentration of bulk solids at relatively low velocities through a conveying line.

The dense phase pneumatic conveyor is more commonly used to convey materials over longer distances and at higher throughputs. The limitations of air movement devices such as fans and roots blowers become prohibitive.

This method is also commonly used to convey fragile or abrasive materials at low velocities to minimize material damage or system damage. The low velocity of the dense phase means minor wear and tear on both the system and the materials being conveyed.

Dense phase pneumatic conveyors: contrary to dilute phase pneumatic conveyors, dense phase pneumatic conveyors apply high pressure achieving low speed. This is mainly because the material transferred is otherwise susceptible to coming apart.

Dense phase pneumatic conveying systems are ideal with:

• Very fragile materials and mixtures, as the low speed and lower air volumes prevent materials from breaking down.
• Best for low- or high-rate transfer of abrasive or brittle materials, as the slow convey speed reduces abrasion of conveying lines.
• Slightly hygroscopic materials, without requiring the introduction of air drying equipment, because air quantities needed for the dense phase are significantly lower than semi-dense and dilute.
• Examples: Sand, sugar, carbon black prill

Semi-dense phase pneumatic conveying

Semi-dense phase pneumatic conveyors: semi-dense phase pneumatic conveyors are a middle-ground solution developed to convey material at medium velocity.

As an intermediary step between dense and dilute phases, the semi-dense phase conveys materials using a medium velocity. Only a portion of materials is suspended in the air stream.

Semi-dense phase pneumatic conveying systems are ideal with:

• Products in an aeratable state
• Abrasive or friable materials
• Examples: cement, fly ash

Choosing a pneumatic conveying system design

The decision of which pneumatic conveying system design is best for your operation should be based on the following questions:

• What kind of material is being transported?
• Is material or piping degradation a concern?
• What is the purpose of the material transfer system?
• Do you want to move the material?
• Do you want to transfer more material than your current system can handle
• Are you more concerned with reliability and efficiency or the gentleness of transport?

To answer these questions, you need to consider several material handling characteristics, including conveyability, optimum air-to-material ratios, buildup tendencies, flowability, and degradation.

Knowing how a material behaves during pneumatic conveying allows for the accurate specification of conveying mode (dilute or dense phase), gas flow and solids loading, and type of gas mover (for example, blower, fan, or compressor).

This Technology Zone offers an insight into the latest innovations in pneumatic conveying system design.