Process Engineering: Pneumatic Conveying Moves Ahead
'Generally speaking, mechanical systems make sense with short, straight runs within a plant; they require less horsepower and often can be an inexpensive capital cost,' says Paul Solt, who owns Pneumatic Conveying Consulting, Allentown, Pa. 'But they are workbenches often higher-maintenance choice, endure dust generation or contamination of process material, and aren't as flexible when controlling plant-floor configurations. Given sufficient capital, there is nothing that cannot be pneumatically conveyed,' he says.
Greg Steele, executive vice president for Dynamic Air, E. Paul, Minn., says, 'Over the past 15-20 years, we've received a tremendous boost from EPA and OSHA. One by one, industries or applications where mechanical conveying already been the traditional choice have gone to pneumatic conveying. Several years or so ago, it was dry laundry detergent, the idea was rubber processing and more recently pharmaceuticals. With pneumatic conveying, you reduce emissions and lower plant housekeeping measures.'
Moreover, many new materials are more demanding to handle because of, for instance, reactivity or fire and health hazards; this also tips the scales toward pneumatic communities.
Because pneumatic systems run cleaner than mechanical ones therefore less susceptible to product contamination, recycling materials is more comfortable. A pneumatic system's generally smaller product holdup also is an advantage for utilize on a regular. Some conveying systems are designed to take care of different ingredients of their batch at different times, or must move the complete content of each batch together (which makes recycling of residues problematic). Mike Salvador, operations manager at Nu-Con Equipment, Chanhassen, Minn., says modern pneumatic systems need to be engineered with precise tolerances and should be capable of being disassembled, inspected and cleaned easily.
Entering another phase
No strict physical distinction marks the two main design options for pneumatic conveying: dilute-phase and dense-phase operation (vacuum conveying, which will be discussed later, can work in either regime). Dilute-phase conveying is simply lifting and blowing powders or particles down a line, usually at higher gas velocities. Dense-phase conveying involves calibrating the line pressure according to the physical characteristics of process material, hunting to achieve full fluidization of cloth in the line, but moving it at a lesser velocity.
'Dense-phase conveying was originally designed meet up with the problems associated the new handling of abrasive materials,' says Mike Weyandt, sales team leader for Nol-Tec Systems Inc., Lino Lakes, Minn. With typically velocities within dilute-phase conveying, particles can essentially sandblast holes into the elbows or connections of pneumatic systems, or can be degraded by impact with the walls of your conveying coats. 'Many applications are now being handled with dense-phase systems because of process requirements that limit the associated with product degradation or separation allowable,' according to him.
With dense-phase conveying, the designer strives to load the line with as much process material as possible and to try it at most energy-efficient condition, which 's just below the price that would cause pushing. Over long distances or higher loadings, pressure variations produce plugging and, to counter this, equipment vendors allow us various types of boosters that inject additional air when needed.
Dynamic Air, which been recently offering booster technology for several years, has become introducing the DC-5 Air Saver unit . 'Older boosters were simply integrated every 5 ft. or 10 your feet. along a line and supplied a steady pressure,' Steele says. Some designers also experimented with adding pressure sensors and process controls to the boosters. 'The DC-5 has a pressure-balancing valve that can regulate where pressure is applied automatically. In effect, the conveying line is constantly plugging and unplugging, which is the most efficient mode of operating.'
Meanwhile, Nol-Tec recently introduced its Air-Mizer injector, and that is designed to move automatically to regulate conveying velocity in the queue.
'The effectiveness of the dense-phase strategy is not guaranteed,' cautions Don McGlinchey, director of the Centre for Industrial Bulk Solids Handling at Glasgow Caledonian University in Scotland. 'Much varies according to the detail of the stressing mechanisms occurring during transport along with the material's responses. A dilute-phase system can be fairly forgiving to adjustments to material specification, but these tend to be more critical in dense-phase systems, in order to many operating problems.'
Pulling vacuum pressure
Technically, the only difference between a pressurized system and vacuum pressure system might former pushes material any line whereas the latter pulls the application. However, the two systems pose distinct requirements for inlet feeding and outlet dispensing, and the physical properties of the material being conveyed or the process layout tend to make a difference.
'A vacuum system would make the most sense whenever it's dispensing any single destination,' says Pneumatic Conveying's Solt. If there are several delivery points, the designer can split the flow of a pressurized system with diverter valves, which isn't as feasible with vacuum pressure system, he adds.
'Vacuum systems are limited in capacity by would like to know between atmospheric and vacuum pressures,' says Herman Purutyan, a vp at Jenicke & Johanson, a materials-handling consulting company in Westford, Mass. 'With a pressurized system, there is the design flexibility to increase the pressure often you have need of.'
There's certainly no vacuum most advantageous suppliers. For instance, IEDCO, Sewell, N.J., is now offering modular technology from Volkmann Vakuum Technik, Soest, Germany, to north of manchester American marketplace. IEDCO representatives say the technology is particularly suited to high-purity applications, such as pharmaceuticals or batch mixtures where it comes with concern about material left from a previous batch. An individual can obtain dilute or dense-phase conveying by selecting from among 60 different vacuum generators; tangential or radial suction ports accommodate material properties such as dryness or stickiness, respectively, to control flow personality. Discharge valves have their pneumatic components away from the conveyor body, allowing for quick cleaning in place, very popular IEDCO.
Cyclonaire Corp., York, Neb., addresses the stress question both ways. Its HC Series system, thanks to venturi tied to a blower, can use vacuum 1 part and pressurized flow in a new. A typical application would be vacuum unloading of, say, a railcar into a transfer vessel, according to your company. When the vessel fills, the system automatically cycles to a discharge mode, using this is equally blower to mention material along to its destination. Optional load cells allow these devices to weigh and batch the process flow.
Because the pressurized side of the software uses relatively low air pressures, machine is compatible to handling abrasive material. At cement-maker R-Con's Plant Low. 6 in Wichita, Kan., the HC system unloads railcars carrying cement into the plant, where more than 200 concrete blends are produced in different colors or compositions to handle architectural specifications. The system addressed one pressing problem the buildup of dust within unloading pit that were found to be caused by spillage from the screw-pump conveyor while raising capacity and productivity in the plant. 'Our railcars hold 80 to 84 tons each, as well as unload them in november 17 hours,' says Rick Heise, production owner. 'There's no need to get down in the pit, no dust, not even attempt to clean raise. When you time everything out, we're getting the job done much faster.'
Coming for the pipe
Most experts in pneumatic conveying agree that technology changes in the field been recently evolutionary rather than revolutionary. Nol-Tec's Weyandt says the company has introduced new broken-bag detectors and rotary feeders, which help streamline operation at the feed end of a conveying circle. Pelletron, Lancaster, Pa., now offers several varieties of engineered elbows to reduce abrasion resistance of a conveying string. Nu-Con has launched a compact rotary-valve tube selector permits a conveying system to feed multiple lines or to converge multiple lines into one; the unit has inflatable seals on all ports to ensure airtight conveying.
System design might be poised for potentially significant changes, as a result of emerging commercial use of a new software combination: the tying together of computational fluid dynamics (CFD) with so-called discrete element modeling (DEM).
CFD, which was used in commercial applications in caffeine industry above a decade, models continuous fluid phases such as air flowing around a shape. 'Our multiphase designs include been formerly solve particulate flow problems where the main focus has been the effect of fluid mechanics, rather then particle dynamics,' says Ahmad Khadari, a working manager at Fluent Inc., Lebanon, N.H.
'Classical applying this technology include cyclone separators, fluidized beds or classifiers.'
However, to handle a system from the perspective with the particles a 'discontinuous' phase another approach is needed, hence Dem. It originated, says John Favier, president of DEM Solutions, Edinburgh, Scotland, in geomechanics, where exercise routines, meal used to check out a problem like extra weight of a building atop a gravel bed. 'We've been constrained by accessibility of computational power,' according to. 'In the past, i was able to research thousands of particles in one system at a time; currently we can process regarding thousands, and very quickly we often be doing amount.'
At that level of detail, modeling the interparticle involving something exactly like a dense-phase flow or a complex geometry inside a conveying-system component becomes practical. This can eliminate previously necessary simplifications and potentially provide more accurate results.
DEM Solutions is advancing with several efforts to have built DEM address materials-handling problems in the commercial arena. Favier says the corporate has a task with John Deere, Moline, Ill., to examine the flow of grain particles in its harvesting equipment. It also formed an alliance this fall with Fluent to combine its program, called EDEM , with Fluent's CFD software.
Industry experts are choosing wait-and-see station. 'DEM, or CFD for that matter, aren't really 'there' yet,' says Jenicke & Johanson's Purutyan. 'A pneumatic conveying system can be very complex to model and, so far, all of the simplifying assumptions you must carry out limit the software's of a routine.' Instead, he recommends trying the particular process material in a test system and the shopping results for design.
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