Applications on pneumatic:Pneumatic conveying

Pneumatic conveying

Pneumatic conveyance in pipelines

Pneumatic conveying is the transport of bulk materials through a pipeline by air pressure or vacuum. Materials that can be handled mnge from asbestos with a bulk density of 100 kg/m 3 to crushed stone with a density of 1500 kg/m. The advantages of pneumatic conveying over mechanical conveying include safer working conditions (clean atmos­ phere and reduced fire hazards), greater flexibility, freedom from contamination and the ease in which a change of direction can be achieved.

It has to be admitted that the cost of air compression is likely to be higher than that of pure mechanical transport, particularly when the transported material requires a high degree of purity in the air. Fragile materials may not be suitable for pneumatic conveying, but most other materials and some manufactured components lend themselves to this form of transport.

The following types of pneumatic conveyors are available:

• Vacuum system, similar in principle to a domestic vacuum cleaner.

• Low pressure system, up to I bar.

• Medium pressure system, from I bar to 3 bar.

• High pressure system 3 bar to 8 bar.

• Pulse phase system.

• Combination vacuum/pressure systems.

• Air activated gravity conveyor.

The choice of a suitable system will depend primarily on the material to be transported-its density, particle size, moisture content and abrasiveness. There is a great deal of skill needed in choosing a suitable conveying method, so anyone contemplating installing a system would be well advised to approach a company with a wide experience in the various techniques. One can do no more here than indicate some of the factors that should be considered.

Vacuum systems

This uses a high velocity (up to 40 m/s) airstream to suspend the material in the pipe, using a vacuum up to 400 mbar.

The materials suitable for vacuum transport are dry, pulverised and crushed granular with a small particle size and a low density. Under ideal conditions, it is claimed that the conveying distance can be as much as 500 m, but in practice the maximum distance is likely to be rather less. Optimum design would require the internal pipe diameter to increase in steps along its length so as to stabilize the velocity. As in all systems, the limitation on conveying length is the pressure loss that occurs through pipeline friction. With a vacuum system the pressure loss can be no more than the vacuum depression, but with a pressure system, the loss through friction can always be compensated by increasing the positive pressure. The energy consumption will be between 1.5 and 5 kW hr per tonne of material, depending on the density and conveying distance. This makes it the most expensive method in power consumption, but the simplicity of installation and low capital cost makes it appropriate for many situations.

Vacuum systems are ideal where several pick-up points are required in one line. Another advantage is the ease with which material can be introduced into the pipeline. The simplest way is from an open container such as a ship's hold where the material is admitted with the air. In this case the material may have to be lifted through a considerable height and the system is then known as a pneumatic elevator.

When the material is in a hopper, there are several forms of feed device. One such is a rotary feeder which has a star wheel rotating in a close-fitting housing. For dry materials which flow easily, a simple on/off valve can be used and the material falls into the pipe by gravity. A positive low pressure may also be used to fluidize the hopper.

The suction is usually generated by a turbo blower (centrifugal or axial). The same equipment can be used for both suction and low pressure conveying systems.

Low pressure systems

The distinction between low, medium and high pressure systems is related more to the means adopted for producing the pressure than to their application to different materials. Some materials can be conveyed at all pressure regimes, but the main application of low pressure conveying is for dry, low density materials. Turbo blowers or Roots-type blowers are commonly used for pressures up to 1 bar. For pressures up to 0.3 bar a simple fan may be used. The velocity in the pipe is limited to 20m/s, so this method is more appropriate for fragile materials than the vacuum method. The power used is between 0.5 and 3.0 kW hr per tonne, depending on density and distance.

Because of the positive pressure in the line, special methods have to be used to introduce the material into the pipeline against the positive internal pressure. Close fitting feed mechanisms are required for two reasons: that air should not be wasted through leakage; and when dusty or unpleasant materials are being handled, they are not blown out of the hopper into the atmosphere. The system is suitable where there is only a single pick-up point with the option of multiple discharge points.

When using a pressure below about 0.2 bar, it is possible to use a venturi-type pick up, provided that the material is suitable and is carefully metered; a venturi feed cannot handle plugs of material or deal with an excess of capacity that would inhibit the venturi effect. A rotary feeder is the customary form of injecting the material into the pipe, but care must be taken that the feed mechanism does not damage the material, Figure 1.

Medium pressure systems

In this system, the material has to be forced into the line through a feed pump. It is most successful when dealing with materials which can be fluidised and then behave like viscous liquids (known as fluid solids); dry and fine powders are most suitable. The most successful pump for this purpose is a rotating screw, of which several makes are available.

The Mono pump is one type in which a specially shaped rotor gyrates in a casing and so causes pockets of fluidized powder to be drawn in at the intake and pumped into the pipeline. Another type is the Fuller-Kinyon pump, which is a rotating screw conveyor incorporating a non-return valve at the delivery, Figure 2. Any pump used for this purpose has to be chosen to resist the abrasive or corrosive action of the powder. The pump chosen has to operate against the pressure in the line and be well sealed to prevent leakage of the air back into the intake.

High pressure systems

When the air pressure is high, the material is transported in a dense phase compared with medium and low pressure systems which use a dilute phase. Dense phase means that the fluidized material moves as a compact slug along the pipe. It is suitable both for powders that may be fluidized and for coarse and wet materials. It may also be used for a range of materials such as manufactured components and slaughterhouse residue. The material/air mass ratio is in excess of 50:1, so it is reasonably economical in the use of air. One method of introducing powdered material into the air is through a blow-tank, Figure 3. The tank has to be designed as a pressure vessel. The method is essentially a batch (non-continuous) method, although it is possible to use twin tanks and switch between the two, approximat­ ing to a continuous feed.

Pulse phase systems

This is a type of medium pressure (between I and 2 bar) system where the material is transported in discrete plugs. Material/air ratios in excess of300: 1 have been recorded; the air consumption is low and so this can be a very economical method. In Figure 4, air is injected into the vessel to fluidize the material; beyond the discharge valve at the base of the hopper is an air knife which injects pulses of air into the conveying line, and as a result the material is divided into plugs. When the full batch of material has been transported, the vessel is returned to atmospheric conditions, the inlet valve opens and the cycle repeats automatically.

Combination vacuum/pressure systems

This is a useful system when conveying from several pick-up points to several discharge

points. The pick-up region is under vacuum and the delivery region is under pressure. The same blower can be used for both regions, but this places restrictions on the maximum positive pressure that can be generated, so it is more common to use both an exhauster and a blower and keep the two pressure regions separate.

This method (Figure 5) can be used when the material is to be transported over a distance with a vertical component. Air is used to fluidize the material which then move along the incline under gravity. In normal conditions, a powder runs down an incline only when the angle of the incline is larger than the angle of repose. If the powder is fluidized, the natural angle of repose is reduced. As an example, a powder which has an angle of repose of 40° will require a chute set at an angle of 45°, but when fluidized the powder will flow down a chuteof2.5°. Even material which cannot be fluidized in the conventional sense may still benefit from a supply of air to the underside of a porous trough, which reduces the coefficient of friction.

 

The material has to be separated from the air at the delivery point. Usually, a cyclone is needed for primary separation, with the outlet filtered. It is desirable to retain all the

material in the line and avoid unpleasant dust, so an efficient filter system has to be devised. For materials that do not cause problems if a small proportion escapes with the exhausting air, a fabric bag is satisfactory. The cyclone is connected to a hopper from which the material is taken by a rotary valve, this is similar to the method shown for introducing material into the pipe as illustrated in Figure I. Another useful device which is suitable for vacuum lines is the vacuum valve, Figure 6.

Calculations on the power absorbed in pneumatic conveying

As indicated above, the design of conveying systems depends on practical experience. The conveying speed for various materials and the most suitable pressure regime cannot always be predicted without trials. However it may be helpful to indicate some of the theoretical concepts that can be used to analyse the power required. The treatment below is applicable to dilute phase systems (pressure or vacuum) only.

The pressure difference from one end of a pipeline to another is due to:

• Acceleration of the powder from rest

• Pipeline friction

• Changes of direction

• Gravitational forces.

The pressure difference required to accelerate the powder from rest is given by

where M> is the pressure difference, F1 is the pick-up factor, V is the air velocity and p is the density of the powder/air mixture.

Usually, because the weight of the material being conveyed is so much greater than that of the air, the mixture density is accurately given by:

For some materials that are commonly conveyed, the value of the saturation is quoted; this is the reciprocal of the density. The pick-up factor varies with the feed design method, but it usually lies between 2 and 3; a value of 2.5 is customarily taken. The actual air velocity can be used, calculated from volumetric flow and pipe diameter. The ideal velocity for the common conveyed materials can often be obtained from published data. Once the material has been picked up, it is conveyed along the pipe at constant velocity.

The pressure drop caused by pipeline friction is calculated in a similar way to that of turbulent flow of a homogenous tluid .

F2 is the conveying factor, L the pipeline length, D the internal diameter.

F2 depends on a large number of factors- the size, shape and density of the particles, but primarily on the velocity of the flow. Figure 7 may be used if test results are unknown, with a generous factor of about 50% to account for uncertainty.

Changes in direction result in a pressure loss (for a single right angle bend) of

Note that the above equations are correct with consistent units. In particular with L, D and H in metres, V in rn/s, P in N/m 2, W is in watts. If P is required in bar, the correct conversion factor must be used.

There will be additional power required for feeding material into and extracting it from the pipeline, to overcome leakage and pressure drop through the separation cyclones at the end of the conveyance.

Air lift pumps

This is a method of pumping water from the bottom of a well by injecting air at the base of the pumping tube. The air mixes with the water and produces a mixture which has a lower mean density than the water alone, so the static head of the water can be used to force the mixture to a considerable height. This system is less efficient than other forms of deep-well pumps; usually the compressor power required will be about 2 to 3 times that of a shaft driven pump. It is however useful for temporary installations and where it has to deal with a mixture of sand, grit or coarse mud. In this latter role it is commonly used for silt removal around submerged wrecks or for dredging. The size of solid objects that can be handled can exceed half the diameter of the pipe.

The size of the rising pipe must be chosen correctly. The velocity should not be so great that there is too large a friction loss and not so small that the air rises through the water rather than lifting it. At the point of injection the velocity should be between 2 and 4 m/s and at discharge between 4 m/s and 14 m/s. The velocity is calculated from the combined volume of the air and water and the cross-sectional area of the pipe, the volume changing with pressure. For large lifts, in order to keep the velocity between the above limits, it may be necessary to increase the pipe diameters in steps towards the surface. The air pressure must be selected so that it is larger than the local static water pressure at the point of injection.

Figure 8 shows the most common arrangement of air-lift pumping with the air supply pipe external to the discharge pipe. This is the most efficient system, but it takes up more room in the well than the alternative arrangement where the air pipe is enclosed within the discharge pipe. The latter can accommodate a larger diameter of pipe up to the bore of the well and so can handle more water for a given well size; if there are variations in water level, adjustments for efficient operation can be easily made by altering the length of the air line.

Compressed air for blending and mixing

Powders which can be fluidized, can usually be mixed in the fluidized state more efficiently than by mechanical means; one example is the blending of sand ground to different size gradings. Blending by air produces a more uniform mixture than by mechanical blending.

There are practical limitations when attempting to blend powders with different densities and particle sizes. One manufacturer claims that mixing can be achieved with a density ratio of 5:1 and a size range of 10:1. Powders can be mixed:

• By feeding them in the required proportions into the same conveying pipeline.

• In a silo with aeration pads in the base.

• In a silo with air-jet mixers.

Mixing of powders and liquids can be achieved by spraying the liquid in a fine mist over a fluidized bed.

The volume of air required depends on density, particle size and rate of mixing required. It is best determined by experiment.

Compressed air can also be used for mixing liquids and slurries. This can be simply done by admitting air through a perforated pipe in the base of the mixing tank. For this application, recommended air flows are available, depending on the submergence of the pipe below the surface and the cross-sectional area of the tank. Table 4 can be used for an initial estimate of the air flow required.

The values of air flow are for moderate agitation. For complete agitation multiply the table values by 2 and for violent agitation multiply by 4. The holes admitting the air should be evenly distributed over the base of the tank. The total orifice area can be calculated from the nozzle equations given in the chapter on Pipe Flow. The back pressure on the nozzle will be that of the head of water.