A pneumatic conveying system transfers dry bulk solid materials or powders through an enclosed pipeline using pressure differential and gas flow (usually air), generated by an air movement device such as a fan, roots blower or compressor.

Pneumatic conveying provides a cost-effective way to handle and transfer powdered and bulk granular materials easily with very little loss. It is suitable for a range of process industries such as; Food and Beverage, Pet Food, Chemicals and Detergents, Renewables and Specialist Materials.

Pneumatic conveying can be separated into two categories, dilute phase and dense phase.

Dilute phase: where conveying gas velocities are typically in excess of 17-18 m/s and conveying pressure or vacuum is low; usually below 0.1 barg. The material particles remain wholly suspended in the conveying gas stream and the concentration or solids loading ratio (mass of solids/mass of gas) is relatively very low. Dilute phase conveying is ideal for materials with low bulk density and that is non-abrasive. Typical air movement devices for this type of conveying would be a Fan or Side Channel Blower/Exhauster.

Common Dilute Phase applications:

Materials with light bulk densities

Non-abrasives such as flour

Products that are not easily breakable

Dense Phase: Dense phase, where conveying gas velocities are typically in the range 6-18 m/s and conveying pressures are above 1 barg. At higher conveying velocities, the material particles can still be substantially in suspension, however, at the lower velocities, the material will be predominantly in contact with the conveying pipe and will move in waves or plugs. This type of conveying is more commonly used to convey materials over longer distances and at higher throughput, where 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 in order to minimise either material damage or system damage. Typical air movement devices for this type of conveying would be a compressor of liquid ring pump.

Common Dense Phase applications:

Products with heavy bulk densities

Abrasive products, such as sugar

Friable products, such as pet food post-extruder and carbon black prills

Blended products, such as detergents

Products that do not require continuous delivery to their destination, such as plastic pellets

• Flexible layout, a pneumatic conveying system can be routed around existing equipment, giving more flexibility than a mechanical conveying system.
• Can run vertically or horizontally over a long distance.
• As the system is enclosed, there is protection against dust emissions to the atmosphere and also protection of the conveyed material from external contaminants.
• Pneumatic conveying systems can be easier and more cost effective to maintain than a mechanical conveying system – fewer moving parts.
• The ability to maintain a controlled atmosphere around the material.
• Minimise system wear from abrasive materials and damage to fragile materials.
• Take up less floor space so are easier to locate.
• Can have multiple pick up and discharge points.

The things that you need to consider when looking at your application.

• Particle size distribution and shape
• Air Humidity
• Moisture content
• Angles of flow, slip, and repose
• Density
• Fluidisation
• Hardness / Friability
• Temperature
• Concentration
• System throughput
• System distance

Unfortunately, there is no industry standard for measuring these operating phases. So just because a pneumatic conveying system has a rotary airlock valve, it is not necessarily operating in dilute phase, and just because a system has a transporter, it is not necessarily operating in dense phase. However, you can use these rules of thumb for determining a pneumatic conveying system’s operating phase :

Most dilute-phase pressure systems operate below 15 psi (typically between 4 and 8 psi), while most dense-phase pressure systems run above 15 psi.

Most dilute-phase vacuum systems operate below 12 inches mercury (typically between 8 and 12 inches mercury), while most dense-phase vacuum systems run above 12 inches mercury (typically between 12 and 14 inches mercury).

Depending on the conveyed material, most pressure and vacuum dilute-phase systems have an air velocity between 3,500 and 9,000 fpm and most pressure and vacuum dense-phase systems have a 3,000-fpm or lower air velocity.

In a dilute-phase system, the material velocity is nearly the same as the air velocity. In a dense-phase system, especially one with slug flow, the average material velocity is much slower than the air velocity. In either system, the material can’t move faster than the air.

One caution: When you are talking to a dense-phase system supplier about selecting a new system, make sure that the material velocity numbers the supplier is using are clearly defined. Some suppliers use air velocity and material velocity numbers interchangeably. Make sure you know what numbers the supplier is talking about before you accept the supplier’s material velocity claims.

  Once you have decided on a dilute or dense phase pneumatic conveying system and determined whether it will operate under pressure or vacuum, have the system supplier run pilot-plant tests of the proposed system in the supplier’s testing facility. The supplier should conduct the tests with the same material the installed system will handle in your plant and simulate your field conditions as closely as possible. This includes configuring the pilot- plant system, if possible, with the same conveying line routing, length, and number of bends that the installed system will have and running the tests under your plant’s ambient air temperature and humidity conditions.

  Before the supplier runs the tests, you should define several test criteria. Basic criteria include whether the proposed system conveys your material, at what rate it conveys the material, and how much air the system consumes. You also need to define criteria specific to your application. If, for instance, you are conveying a friable material and are concerned about material attrition, you need to define how much attrition is acceptable and then accurately verify how much attrition occurs during the tests. The supplier will use the pilot-plant test results to size the conveying system that is installed in your plant. Make sure that you understand exactly how the supplier will use the test results to size your installed system and, in particular, how the supplier will use the data to calculate the installed system’s material conveying rate and air consumption.

  A pneumatic conveying process is ideal for fine, dry powders. We have designed and installed systems for a variety of petrochemical, food and industrial products, including:

• Soy flour
• Wheat flour
• Starch
• Sugar
• Cement powder
• Carbon black
• Coal fines
• Sands
• Metal powders
• Granular materials
• Pelletized materials

Pneumatic systems also work very well with batch ingredients when you need to weigh out a certain amount of material or when you need a product to cool a bit during conveyance.

There are two main modes of transport in pneumatic conveying – dilute and dense phase.

These modes have advantages and disadvantages for transport depending upon which factors or effects you are considering, such as energy consumption, pipeline abrasive wear, particle attrition, or reliability. This webinar will review the basic understanding of each technology, along with effective selection tools to help you best determine an appropriate method for reliable pneumatic transport.

Pressure systems introduce compressed air at the system inlet in order to push the material through the piping; vacuum systems apply a vacuum at the delivery end in order to pull the material through the piping. Pressure and vacuum systems can be used for dense (high pressure/low velocity) or dilute (low pressure/high velocity) phase operation.

Pressure Systems

The basic components of a pressure system are a rotary air lock feeder valve, a high pressure air compressor system or a low-pressure positive displacement blower or fan to serve as the power source. A pressure vessel, the conveying line, and the receiver make-up the balance of the system. Systems using high-pressure compressed air, operate with pressures above 15 psig, usually with a beginning pressure of about 45 psig and an ending pressure near atmospheric pressure. Low-pressure displacement blowers or fans supply a beginning pressure below 15 psig and an ending pressure near atmospheric pressure.

First, the materials is charged into the pressure vessel through the rotary air lock. Once the pressure vessel is filled, the inlet and vent valves close and seal, and high-pressure air is gradually introduced into the pressure vessel. The high-pressure air conveys the material to the receiver, where the air and the material are separated by a filter or other system. Valves and sensors control the air pressures and velocities. When the predetermined low pressure setting is reached at the end of the conveying cycle, the air supply is turned off and the residual air volume purges the pressure vessel and the conveying line.

Pressure conveying systems are generally preferable when transporting heavier materials longer distances. Pressure conveying systems can be fairly costly, however, since they require special equipment, like a rotary valve to introduce material into the air stream at the inlet and extra components to remove the air at the discharge end through a vent system.

Vacuum Systems

The basic components of a vacuum system are the pick-up nozzle, the conveying line, the receiver, and the vacuum generator, which is the power source. The vacuum generator creates the required negative pressure to pull the material through the conveying line and into the receiver. A number of devices, including a regenerative blower, a compressed air driven eductor (Venturi) unit, a plant central vacuum using liquid ring vacuum pumps or low-pressure blowers, or a positive displacement vacuum pump, can serve as the vacuum generator. The maximum negative pressure generated and the overall capability of the system, as well as the system efficiency and general operating characteristics, are determined by the type of vacuum generator used.

The air flow created by the vacuum generator moves the material through the conveying piping and into the receiver. There, gravity causes the material to drop into the receiver hopper. Internal filters separate the material from the air to remove any dust and protect the vacuum generator. Delivery of the material from the receiver to its final destination (e.g., process vessels or a packaging line) may be accomplished using a number of methods dependent on application suitability, including a dumpgate simple slide valves, pneumatically operated dump gates, or air lock rotary valves.

Vacuum systems are usually preferred for transporting materials that may tend to pack or plug in a pressure system. They are also a good choice when space is at a premium; for example, attaching a pressure system rotary valve in the limited space below a hopper rail car may be impractical. However, vacuum conveyors are not a good option if you need to transport materials long distances. Because they operate with pressures at or below atmospheric pressure (14.7 psig), vacuum conveyors are limited to a maximum horizontal distance of 50 feet and a maximum horizontal distance of 200 feet. The effective horizontal distance is also reduced by vertical distances and piping bends.