Anti-bridging devices are also known as anti-bridge devices, discharge aids or agitators. They help to control and promote the flow of bulk solids that are subject to behaviours that impede flow, such as caking with very fine powders. Bridging (or arching) occurs when an empty space forms at the outlet of the storage vessel and the remaining bulk material forms a span or barrier across this space, causing the material to stop flowing. An agitator can help to introduce the product from storage into feeders or conveyor systems. Restricted flow or blockages are the most common kinds of material flow problems.

Common anti-bridging techniques include causing the storage bin to vibrate, agitating the product, using forced extraction or blowing air into the storage inlet (aeration).

Vibratory devices have been found to have only limited and inconsistent effectiveness, and may cause noise, pollution or settling. Agitating devices such as rotating vertical pipes fitted with hanging chains are sometimes effective. Hoppers that are off-centre or with angled bottoms help to improve flow, and when bridging occurs they can be bumped with rubber mallets to correct the problem.

Anti-bridging devices require considerable maintenance and may drop some of their parts into the product flow, possibly causing damage to downstream equipment and bridging.

Types of Anti-bridging Devices

Pneumatic anti-bridging devices can be divided into four main types: air cannons, inflated dischargers or air pillows, directed air-jet devices and aeration / fluidising devices.

Directed air jets use high-velocity air streams to free material from the wall of the hopper and also create turbulence, which improves dispersion. They are only effective for short distances and must therefore be carefully placed to prevent “dead zones”, usually near the outlet. Air use can be minimised by pulsing and timing the jets. Clean, dry air is needed to avoid nozzle clogging and material contamination.

Aereation / fluidising devices help to dilute the bulk material by injecting air into it, thus improving flow. Partial fluidisation makes the material behave more like a free-flowing fluid. Complete fluidisation is not desirable, because it can make control of the material more difficult, and excessive aeration can also cause elutriation (separation of fine particles by density) or bubbling. This technique is suitable for materials consisting of small particles (at least 25% of particles under 75 microns). Dispersion for extremely small particles of under 10 microns can be difficult to fluidise after settling, but pulsing airflow to create shock waves can help.

Air may be injected either during discharge or continuously during storage. The former reduces particle bulk strength, while the latter prevents it from building up. Continuous aeration is more useful if flow of the fluidised material is difficult to control, but is not suitable for large particles, which can de-aerate very quickly. To ensure an even air distribution, a large pressure drop is usually maintained across the air distribution medium, which can be a mesh or woven fabric. Sometimes, a low, continuous injection takes place during storage, while higher air volumes are injected during discharge.

Aeration pads can easily be retrofitted on existing hoppers. Care should be taken to ensure proper exhaustion of excess air and to introduce only clean, dry air into the material to be aerated. Dust containment at the top of the storage container is also necessary.

Inflated air pillows or bladders are placed on the slanting section of the bin walls and inflated to break certain kinds of arching and ratholes. When the material cannot flow at all (e.g. due to a closed outlet), the use of these devices can cause them to wear or puncture, or worsen flow.

Air cannons or air blasters inject high-pressure jets, usually in durations of less than a second. This produces a shockwave that breaks up material which is blocking flow or adhering to container walls. They can be used for sticky or moist materials. However, they can only be installed where the material can be shifted into empty flow channels. Different configurations regarding the direction of airflow and the nozzle shape are possible. They are best used to restart flow after process downtimes or for clearing material left over after most of the material has been discharged by gravity, not for continuous use. Air cannons can create significant stresses on silo walls and other equipment, a factor that must be taken into account when installing them.

Choosing an Anti-bridging Device

The constraints imposed by the process type and the amount of space available will determine the type of anti-bridging device selected. While theoretical guidelines can help, experience remains the best guide for preventing bridging. In many difficult cases, trial and error must be used. Even successful trials do not always lead to successful implementations. A poor choice of anti-bridging device can incur significant costs: production downtime or delays, reduced outputs and product waste, poor product quality, high maintenance costs and labour use, and commissioning costs.

To select an anti-bridging device, the following factors need to be taken into account:

• Bulk material properties and variability regarding cohesion, particle size distributions, friability, moisture content, combustibility etc.
• Process requirements (space constraints, installation, feed demand, maintenance etc.)
• Retrofitting constraints, where applicable (maintenance access, operation interruptions, safety, cost, additional equipment requirements etc.)
• Trial, scale test or field test results

In general, pneumatic anti-bridging devices are suitable for cohesive and/or fluidisable powders. Caked materials, large particles, explosive dusts, and particles with excessive moisture contents are not well-suited for the application of this kind of anti-bridging device.