Pneumatic Hopper Discharge Flow Aid Devices
Vessels of many shapes and sizes have since been built through a process of trial and error, to store a huge range of bulk materials in virtually all spheres of industry. Particle technology has evolved considerably over the last 50 years, to produce a coherent set of theories and design guidelines.
However, there are many challenging materials whose behaviour cannot be easily accommodated hopper design i.e., fibrous, spring-like, wet / moist, stick & tacky, visco-elastic, highly compressible, caking prone and very fine bulk materials. For these types of materials, applications where conventional design does not provide an acceptable solution and retrofit situations where operating difficulties are encountered, discharge aids or discharge systems are often used to secure discharge and empty the contents of silos.
These devices are almost invariably selected based on past industrial experience or by a process of trial & error. Selection must take into account constraints that are imposed by the process and the available space. There is usually a significant economic penalty for incorrect or sub-optimal choice of a discharge system in terms of extended commissioning costs, production delays and interruptions, loss of output, product wastage, loss of quality, increased maintenance and manual attention cost.
Due to the many industries these flow problems are found, and their specific requirements, a Fresco Systems engineer will visit site to discuss your particular requirements in detail. If the facility into which a system is being considered exists the engineer will carry out a detailed survey, if a new facility we would work from project drawings. We have 20 plus years of combined experience in the design of discharge aids or discharge systems, with installations across a whole spectrum of industries from pharmaceutical and production facilities to mineral processing and power generation.
The objective of this paper is to summarise information on pneumatic air flow aid discharger device that are currently prevalent in industry. An effort is also made to provide guidelines for their selection and specification as part of an integral design, or for retrofit to overcome operating problems.
Discharge aids may be defined as devices that stimulate or improve bulk solids flow out of bulk storage container. Items may be installed downstream of discharge aids to provide a means to shut off or regulate the flow of bulk solids. Slide gate valves and feeders are examples of discharge controllers. A discharge system can either be integrated with the silo or installed as an add-on, depending on the design and reason for its inclusion.
It is important to distinguish between the basic objectives of discharger aids and those of feeders to avoid misapplications. The primary purpose of a discharger aid is to promote flow, not necessarily to regulate it, and without regard to the order of zone discharge. A feeder, on the other hand, depends on the material flowing reliably to its inlet. Feeders influence the flow regime developed in the storage container and will not function if flow in the bin is unreliable. A feeder and its supply hopper are therefore an integral system.
Types of Flow Problems
The more common forms of flow difficulty are concerned with the restriction of flow, either complete or erratic stoppages, or a delivery rate less than that required. Circumstances also arise where the discharge rate is in excess of requirements, uncontrollable, in an unsuitable condition for handling, process or use or is incomplete. These difficulties arise for a number of different reasons, such as:
- Arching: where the product forms a blockage over the outlet and flow ceases. Two basic types of arch can create a stable obstruction over a hopper outlet. One is that created by the bulk strength of a cohesive material being able to span the dimension of the opening. The other is when lumps come together to make a continuous structural across the orifice by virtue of the contact points offering a static relationship that makes a continuous load path as in a bridge.
- Rat Holing (Piping): where material empties from a central core above the outlet up to the surface level of the stored material but no further product collapses into the empty flow channel.
- Irregular flow: where the discharge rate is erratic or subject to cyclic variations, that is not compatible with the specific process requirements of the operation.
- Flushing: a form of uncontrollable flow, generally due to the presence of excess air or gas in the voids that dilates the bulk material to a weak condition with virtually zero shear strength.
- Static zones: where subsequently problems occur due to deterioration of flow property or product quality because of extended residence time or Residue material unable to discharge by gravity.
- Segregation: that leads to flow or processing difficulties or loss of quality.
All discharge aids work using one or more of the following principles:
- Dilate the material to enhance flow. The flow function of dilated material exhibits significantly lower unconfined yield strength (see Figure 3) thereby making it flow better. (Air injection may be used to dilate the bulk or inhibit time consolidation due to settlement).
- Induce stresses that exceed the strength of the bulk material. (Vibration and mechanical agitators may be used to deform the bulk).
- Reduce the friction between particles and the wall of flow channel. (Change the surface finish to a contact friction of lower value)
- Modify the flow regime to one more favourable to flow.
- Alter the bulk material flow properties by additives or surface modifiers. (Inhibit particle to particle adhesion or ‘caking’).
Pneumatic Discharge Aids
A wide range of pneumatic discharge aids are available in the market, namely
- Aeration or fluidising pads, fluidising hoppers
- Directed air-jet type (continuous and pulsed)
- Pneumatically inflated dischargers or air pillows
- Air cannons
Aeration or fluidising pads and fluidising hoppers
These discharge aid rely on dilation of bulk material (increase in inter-particle separation) by injecting air in the interstitial space between the particles. Powders tend to behave like fluids when fully aerated, but total fluidisation is not essential to promote the flow of fine particulate material, in fact doing so can result in the powder being difficult to control or not be in a suitable state for packing. Bulk materials comprised of particles of size less than 75 microns (-200 mesh), or with at least a 25% fraction less than 75 microns, (-200 mesh), are suitable candidates for aeration. However, powders with particles mostly less than 10 microns are very slow to settle, but difficult to re-fluidise, since they then exhibit channelling behaviour. Good air dispersion may be re-achieved by pulsing large airflow rates that creates shock waves to cause massive agitation.
There are two main techniques of employing product aeration:
- Air injection during discharge – This works by reducing the materials bulk strength and particle wall friction, particularly near the outlet region.
- Continuous air slide injection during storage – This works by inhibiting de-aeration and the gain of bulk strength of the whole mass due to time settlement.
It should be determined whether the bulk material has a tendency to flush/flood or flow uncontrollably in fluidised state. In such cases, option #2 is more suitable. The amount of air required to avoid high strength gain of fine powders due to time settlement is very small, but the technique is not appropriate for products that rapidly de-aerate (particle size greater than 200 microns).
Excessive fluidisation can result in bubbling and the elutriation of fines. It can also aggravate the segregation of coarse and fine fractions within the hopper.
Aeration or fluidisation pads are easily mounted on existing hoppers as retrofits, multi-layer metal mesh or woven media is typically used as air distributor. Uniform air distribution is achieved by maintaining a large pressure drop across the media. The air consumption is typically 8.5 m3/min per square metre of pad area. These inject air only when discharge is required. They generate a pressure differential between the injection points and the hopper outlet, providing both a driving force and a supply of air to satisfy the void demand of bulk expansion for flow.
Bulk control can be achieved by use of an aeration pad that covers the whole container base. Dilatation of the bulk improves the materials ‘flowability’ by reducing both wall friction and inter-particle cohesion. Activation of the entire hopper section allows a shallow hopper design to be employed. This may be supplied with a low, controlled-volume injection during storage, to stabilise the flow condition whilst the material is static, and increase the degree of aeration by injecting a higher rate of air for discharge. It is critical to supply oil free, clean and dry air for aeration to avoid product contamination. Appropriate arrangements must also be made to exhaust excess air and contain entrained dust at the top of the bin/silo.
Directed jets can be effective in using the kinetic energy of air-jets to dislodge material from surrounding hopper wall and provide better gas dispersion through turbulence generation (Figure 1). The effective radius of these jets is limited to 1- 2 feet. Therefore, the jets must be placed in effective locations or multiple units need to be installed on the hopper wall to avoid dead zones. These jets can be timed and pulsed to minimise gas consumption. It is critical to supply oil free, clean and dry air to avoid contamination and prevent plugging of fine nozzles. The crucial flow region for discharge is that near the outlet, because the smaller span at this location is the most likely place for stoppages to form. Clearing this region, or part of the periphery of the orifice, is equivalent to having a larger opening that can be sufficient for the remaining contents to discharge.
Pneumatically Inflated Dischargers or Air Pillows
These are flexible bladders mounted on the cone or inclined walls of the bin/silo. Upon pressurisation (typically 1 to 3 bars), the flexible bladders expand and force the material towards the centre. They are helpful in breaking ratholes or “brittle arching”. These devices should not be used when the hopper outlet is closed or where the material is unable to flow, as local compaction will aggravate the flow difficulties, or with sharp or abrasive products that can puncture or wear through the flexible diaphragm.
Air cannons (or blasters) are designed to inject blasts of high pressure gas, (up to 10 bars), in a short duration (typically fractions of a second). The shockwave traveling through the bulk solid provides a substantial force to break an arch or a rathole. Air cannons must be located where the stored material can be moved into an empty flow channel. Typical application includes use with sticky, wet, adhesive, fine, caking and fibrous materials. These devices are also used to knock sticky or adhesive materials and residual pockets of material from the walls of a bin/silo. The force created by discharging air cannons is directly proportional to the reservoir pressure. The duration of the pressure pulse depends on both the size of the reservoir and the initial air pressure. The blast from air cannons or blasters can be directed either tangentially, (along the wall), or into the bulk material at various angles. Various shapes of nozzles are available to create different dispersion patterns. When operating multiple air cannons, those at the bottom should be fired first, and the other moving progressively upwards at regular intervals. These devices should not be used for continuous operation. They are most useful for restarting flow after long downtime, after a process upset or for terminally clearing the bin after gravity flow has cleared what will discharge of its own accord. Every blast causes a reactionary force on the silo wall, so reinforcement of walls near the blasters fittings must be considered, especially for retrofit situations. Large chunks of caked or consolidated material may be dislodged from a wall, arch or rathole, to generate significant impact stresses within the silo. The silo and any associated equipment must accommodate such conditions.
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