Reprinted from WORLD CEMENT Bulk Materials Handling Review 2005

IntroductionDynamic Air Inc. is a specialist in the pneumatic con-veying of dry, bulk solids. Applications range fromfood to poison, from light fumed silica to heavy pow-dered metals. All these applications, however, haveone thing in common, the necessity to control theconveying velocity in order to control particle degra-dation and conveying pipe wear, minimise air con-sumption, and eliminate pipe line plugging.

What is pneumatic conveying?Pneumatic conveying is nothing more than creating apressure differential along a pipeline and moving abulk material along with the air as the air movestowards the area of lower pressure. This can be donewith a vacuum inducer, or with compressed air beinginjected into one end of, or along the pipeline.

Dilute phase vs. dense phaseThe two most distinct categories of pneumatic con-veying can be described aseither low pressure (dilutephase) or high pressure (densephase) systems.

Dilute phase pneumaticconveying systems utilise pres-sure differentials less than 1atmosphere. These systems useeither positive or negativepressure to push or pull mate-rial through the conveying lineat relatively high velocities(Figure 1). They are describedas low pressure/high velocitysystems which have a high airto material ratio.

Dense phase pneumaticconveying systems, utilise pres-sure differentials above 1atmosphere. These systems usepositive pressure to push mate-rials through the conveying line

at relatively low velocities (Figure 2). They aredescribed as high pressure/low velocity systems whichhave a low air to material ratio.

Why dense phase?By definition, dense phase pneumatic conveying sim-ply means using a small amount of air to move a largeamount of bulk material in closely associated slugsthrough a conveying line, much like extruding. Unlikedilute phase conveying systems that typically use lar-ger amounts of air to move relatively small amountsof material at high velocities in suspension, densephase offers the enormous advantage of efficientlypushing a much denser concentration of bulk solids atrelatively low velocities through a conveying line.

Pneumatic conveying systems forcementCement plants need to transfer large quantities ofmaterial, often over long distances. As a result, the

Gregory J. Steele, P.E., Dynamic Air Inc., USA,describes the new generation of dense phasepneumatic conveying systems.

Figure 1. Dilute phaseconveying system.

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power consumption of the conveying system is ofprime importance. In pneumatic conveying, thepower consumption is a function of the systemdesign, transfer rate, and conveying length. A con-veying system, therefore, that transfers 100 tph ofcement over a distance of 200 m will use about thesame power as a system of the same design and con-veying the same material at a rate of 200 tph over adistance of 100 m. So, from this it is evident that thelength of a pneumatic conveying system has the sameimpact on power consumption as the rate.

Traditionally, cement has been handled and con-veyed by medium phase pneumatic conveying sys-tems such as impeller screw pumps. These systems,

however, are plagued with problems such as highmaintenance and high air consumption. The impellerscrew pump requires two motors to deliver cement:the screw drive motor, and the air compressor motor.In the case of the 100 tph and 200 m convey lengthexample, the screw drive motor requires about 93 kWand the air compressor motor requires about 186 kW.So, the screw motor consumes one third of the totalpower of the system, or 50% of the power of the aircompressor. If a pneumatic conveying system could beutilised that eliminates the screw motor, dramaticsavings in power consumption would be observed.

Power consumption savings do not stop with thecost of electricity. There are also many other benefits

associated with lower power con-sumption. First, if a conveying sys-tem uses less power, there is lessmaintenance. Compare the Form-ula 1 race car to the economy car.Which car needs more mainte-nance? Which car gets more lifefrom the tyres? From the engine?Pneumatic conveying systems areno different. If two pneumaticconveying systems must conveythe same material over the samedistance, and one system uses lesspower than the other, then thelower power consumption systemwill have less wear, will need asmaller dust filter, and will createless pollution.

A new generationThrough research and develop-ment, a new dense phase pneu-matic conveying concept has beenborn, the Dense Phase Full LineConcept with DC-5 Air Saver tech-nology (Figure 3). The DC-5 AirSaver technology is a means bywhich compressed air is automati-cally injected along the conveyingpipe in small amounts in responseto the conditions inside the con-veying pipe.

This new technology reducesthe air consumption to theabsolute minimum by allowingthe system to convey with theconveying pipe line at maximumdensity. This means that the con-veying pipe is as full as possiblewith cement, and not flowingdilute with cement as a vacuumcleaner would. This maximumdensity conveying technique hasthree main advantages. Firstly,because the conveying pipe line isso dense with cement, the air can-not slip past the cement, which is a common inefficiency in

Reprinted from WORLD CEMENT Bulk Materials Handling Review 2005

Figure 2. Dense phase conveying system.

Figure 3. Full LineConcept™ convey-ing system withDC-5 Air Savers.

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pneumatic conveying systems. If the slip is eliminated,efficiency is improved. Secondly, when the conveyingpipe is at maximum density, only a small percentageof the particles are in contact with the conveying pipeat any given time. The majority of the particles are inthe interior of the pipe, therefore not abrading thepipe. So, this significantly decreases pipe wear.Thirdly, by increasing the pipe density, the conveyingvelocity can be decreased for a given transfer rateand pipe diameter. The transfer rate, Q, in kg/s can beexpressed as:

If the conveying density is increased, therefore,while holding all of the other variables constant, theconveying velocity will decrease.

Benefits of velocity controlVelocity control is another byproduct of this newtechnology. Since compressed air is injected alongthe conveying pipe, the DC-5 Air Saver technologycan provide very accurate control of the conveyingvelocity. This is important for three reasons. Firstly,similar to a car, energy efficiency in a pneumaticconveying system is velocity dependent. In general,the slower the conveying velocity, the better the airefficiency (Figure 4). Secondly, pipe wear increaseswith velocity, and is proportional to the velocitycubed. So, if the velocity is doubled, the wearincreases eight times. Thus, even small reductions invelocity can provide dramatic improvements in pipewear. Thirdly, and maybe the least intuitive, is that aslower conveying velocity reduces the chance of con-vey pipe plugging. Most would think the opposite.After all, if one does not want a car to get stuck inthe mud, one had better go very fast through it. Theopposite is true for pneumatic conveying systems.The reason for this is that a pneumatic conveyingsystem must change directions through the use ofpipe bends. When the conveyed material reachesthe pipe bend, it decelerates due to the friction ofthe bend. If the material is moving too fast whenentering the bend, it will decelerate rapidly, causingthe conveyed material to compact like a brick, andform a plug. If the conveying velocity is very slow,however, the deceleration will be negligible. A sim-ple illustration of this can be provided. At the speedof a Formula 1 race car, one must brake hard atevery turn. If you rode a bicycle on the same track,though, one would hardly notice the turn since thebicycle speed is so slow. This is what the pneumaticconveying system is trying to achieve.

Improved reliabilityFinally, the DC-5 Air Saver technology has providedanother very significant benefit, reliability. As thecompressed air is injected along the conveying pipeline, should the system tend to block or plug, the DC-5 Air Saver technology automatically adjusts thecompressed air distribution to break the plug. Thesystem automatically reacts to the conveying condi-tions and can provide tremendous leverage to dis-lodge a blockage at any point along the conveyingpipe. Although most tend to focus on the air efficien-cy aspect of the conveying system, reliability is maybeof even more importance. A robust conveying systemwill allow the conveying system to function evenwhen the material to be conveyed is not ideal.

How does it work?A series of air inlet controls, called DC-5 Air Savers,are mounted on the conveying line. Each one allowspart of the conveying air to enter the conveying line.They have the dual purpose of being able to controlthe conveying velocity, as well as prevent systemplugging. In effect, the Air Saver will reduce the totalresistance of the system. The effect of each providesan incremental force. The total number of Air Saversacting in unison produces a significant total force.

The Full Line Concept™ (with the DC-5 Air Savers)operates as follows: Material fills the transporterfrom a storage silo through an inflatable seated but-terfly valve. Displaced air is vented up through a ventvalve to allow easier filling. Once the transporter isfilled, as signalled by a high level control or weighingdevice, the inlet and vent valves close and seal. Thenthe outlet valve opens and compressed air is intro-duced into the transporter to displace the conveyedmaterial. All other compressed air used for conveyingis added through the DC-5 Air Savers spaced alongthe conveying line.

When the transporter is completely empty, as sig-nalled by a low level control in the transporter, the com-pressed air is turned off and material stops in the con-veying line. Since the conveying line does not purge

Reprinted from WORLD CEMENT Bulk Materials Handling Review 2005

Figure 4. Airefficiency vs.convey ingvelocity.

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itself clean, the high velocity normally seen in the con-ventional type system during the purge cycle is totallyeliminated, thus making the Full Line Concept systemideal for abrasive and/or fragile materials.

Also, since the line always remains full, there is notime lost in emptying and filling the conveying line.Air consumption is drastically reduced making the sys-tem ideal for long conveying line distances and wherea single material is being conveyed. The system can,however, operate as a conventional system when theconveying line needs to be purged for maintenanceor if product changeover is necessary.

Other componentsThe valves that seal the transporter must be able toopen and close on cement, and be able to provide abubble-tight seal. Even the smallest of leaks will pro-gressively damage the valve as the air (and abrasivecement) exit the small leak at sonic velocity. The use ofinflatable seat valves such as the Posi-flate butterflyvalve (Figure 5) have dramatically improved the relia-bility of pneumatic conveying systems by allowing 1 to3 million cycles between service intervals.

The method of joining the pipe seems like aninsignificant point, but it is critical from an air effi-ciency point of view. A typical flanged joint, forexample, allows joint misalignment. On a long con-veying system, this adds up to significantly more pres-sure drop (and horsepower). So, the use of self-align-ing couplings such as the Tuf-Lok pipe coupling(Figure 6) ensures that the piping system is perfectlyaligned and smooth on the interior joints.

The PLC control system is maybe one of the mostimportant components of a pneumatic conveying sys-tem. It is the brains of the system. Proper monitoringand control of the system, therefore, is vital to thesystem functioning as designed. A modern pneumaticconveying system should have PLC controlled pres-sure regulation. This means that the PLC can changethe system pressure automatically in response to theoperation. For example, if a pneumatic conveying

system needs to convey to two destinations, one thatis 30 m in length and another that is 300 m in length,the pneumatic conveying system should operate at alower pressure for the 30 m length destination.

Selection of dust filtration equipmentAt the end of every pneumatic conveying system mustbe a dust filter. The cement dust must be separatedfrom the air. Dust filtration requirements for densephase pneumatic conveying systems are differentthan for central dust collection systems. For example,a dense phase pneumatic conveying system does notprovide a steady state air flow to the dust filter. In adense phase pneumatic conveying system, the con-veyed material flows in slugs separated by pockets ofair. These pockets of air are pressurised. When theseair pockets reach the destination silo they rapidlydepressurise, resulting in high instantaneous air flowto the dust filter. Thus, it is important that the dustfilter is designed to accommodate this situation. Ingeneral, dust filters with venturi cleaning mecha-nisms should be avoided because of the high pressuredrop associated with them. Modern conveying sys-tems use cartridge filters (Figure 7) to reduce pollu-tion, reduce space, and provide easier maintenance.Dust filters, such as the one shown in Figure 7, canprovide filtration efficiencies of 99.999+%, and typi-cally provide emissions below 5 mg/m3.

ConclusionAs cement producers become more concerned withthe energy efficiency, as well as the reliability of theirconveying systems, they will demand more advancedconveying systems to reduce their operating costs. Asupplier of pneumatic conveying systems today,therefore, has to provide a high technology solutionto move material reliably and with the minimumamount of compressed air. Although slightly morecostly, the savings in operating costs often pay for theadvanced technology pneumatic conveying system injust a few years._________________________________�

Reprinted from WORLD CEMENT Bulk Materials Handling Review 2005

Figure 7. Modu-Kleen Series669 bin vent filter.

Figure 6. Tuf-Lok self-aligning pipecouplings.

Figure 5. Posi-flate inflatableseated butterfly valve.

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