Some things are inherently confusing: the definition of "pie" comes to mind. Others shouldn't be confusing, but still are somehow. Take pneumatic conveying. In theory, it's simple, since there are really only two categories: If the conveyed...
January 13, 2011 by Canadian Plastics
Some things are inherently confusing: the definition of “pie” comes to mind. Others shouldn’t be confusing, but still are somehow. Take pneumatic conveying. In theory, it’s simple, since there are really only two categories: If the conveyed material is suspended in air throughout the pipeline, the mechanism meets the definition of dilute phase conveying. If it’s conveyed at low velocity in a non-suspension mode, on the other hand, the system falls under the umbrella of dense phase conveying.
What could be easier?
The reality isn’t quite so simple, and there’s a lot of room for misunderstandings and flat-out bad choices. So how does a plastics processor make the wise pick? Step one is to understand some of the pluses and minuses of each conveying style. Step two is taking a look at the particular process specifications of your own shop that will tip your design choice toward one mode or the other.
This method uses high gas velocities at low pressures. The conveying gas volume and velocity are sufficient to keep the material that’s being transported in suspension. The material is being conveyed in a continuous manner, and is not accumulating on the bottom of the conveying line at any point.
For dilute phase conveying, a relatively high conveying air velocity must be maintained. The typical velocity of dilute phase systems is in the range of 5,000 ft./min. to 8,000 ft./min. It’s not a constant velocity, though; there’s a pick-up velocity at the beginning of the conveying and a terminal velocity at the end, as well as acceleration throughout the length of the line.
Let’s start with your wallet. “Because dilute phase systems are relatively simple, the upfront investment costs are very economical compared to dense phase,” said Shawn Werner, chief engineer at Vortex Valves.
It gets better. “Almost any material can be conveyed in dilute phase suspension flow through a pipeline, regardless of the particle size, shape or density,” said Paul Solt of Pneumatic Conveying Consultants. “A dilute phase system is easy to maintain, as well. I have much less call to repair them than for dense phase systems.”
Turns out, there’s a price to be paid for all of that high velocity. “There’s a lot of degradation of the conveyed pellets that can occur during dense phase conveying because the velocity is so high, which results in the generation of dust and streamers,” said Heinz Schneider, president and CEO of Pelletron Corporation. “Also, the use of dilute phase for abrasive products results in wear and tear of the conveying line and the pipe elbows.”
This high velocity can also show up on power bill. “Because of the higher power requirements of high velocity conveying, dilute phase is less energy efficient than dense phase,” said Shawn Werner.
Dense phase conveying takes several different forms, which makes it difficult to define neatly. For plastics processing, think of it as a system that moves material – ideally plastic pellets – with low velocity and high pressure through the pipes, with the pellets settling and accumulating on the bottom of the horizontal conveying line. The pellets are dragged along, and may flow in intermittent surges.
The typical velocity of dense phase systems is in the range of 400 ft./min. to 2,000 ft./min. As with dilute phase, there’s a pick-up velocity and terminal velocity at the start and finish of the line, and acceleration throughout.
Lower velocity conveying translates into less wear and tear of material and the system. “There is almost no degradation of the material during dense phase – this is one of the qualities that it’s best known for,” said Shawn Werner. “For this reason, material that’s friable or that can easily smear is well suited for this system. And abrasive products also tend to fare better in dense phase because the velocity is lower.”
Energy efficiency is on everyone’s minds nowadays, and dense phase delivers – maybe. “Dense phase is advertised as more energy efficient than dilute phase, and generally this is true, if the system has been properly designed,” said Paul Solt. “The upper part of dense, just below saltation velocity, uses the lowest energy, but usually needs a compressor; and if the compressor isn’t properly matched to the usage a processor might not save any energy.”
Dense phase has a few built-in restrictions: It’s only possible if the air-supply pressure is relatively high or the conveying distance is relatively short. “Even if a material exhibits the physical properties necessary for dense-phase conveying, for low pressure or long distance conveying it will only be possible to convey a material in dilute phase,” Paul Solt said.
Also, in large-scale dense phase systems with large pipe diameters and long distances, installation of special pipe supports and additional steel structures might be necessary to compensate for the pipe forces. “These factors can increase the installation and maintenance costs,” said Heinz Schneider.
MAKING A CHOICE
So, now that the two modes are less mysterious, how do you select the better system for your needs? First, ignore the hype. “Since the introduction of dense phase conveying in the 1970s, there’s been an advertising war between the two modes,” said Paul Solt. “Too many vendors sell one type of system or the other, and recommend that type for any material handling situation.”
Also, processors sometimes bring their own biases to the table. “I’ve been approached by many processors asking for one particular conveying mode before they know, for example, the properties of the material to be conveyed,” Solt continued.
A good second step is to realize it’s not always an easy decision. “There’s no simple answer as to which conveying system is best for a specific application,” said Heinz Schneider. “The choice depends on a combination of factors, such as conveying distance, pressure availability, the characteristics of the material to be conveyed, and the processor’s own economic considerations.”
Let’s start with conveying distance, which is usually linked with pressure. Since conveying-line pressure drop is almost directly proportional to distance, long-distance conveying – around one mile – equates to high pressure, particularly if a high material flowrate is required. Turns out, we’ve just eliminated one system. “For dense phase applications, the use of high pressures over long distances isn’t very convenient,” said Shawn Werner. “As a consequence, most long-distance pneumatic conveying typically is handled in the dilute phase.”
A second and very big – determinant is the properties of the materials to be conveyed. “If a customer tells me they want a conveying system, my first questions will be about the particle size and the density of their material,” said Paul Solt. “From that alone, I can tell what system to install. It simply doesn’t make sense to select either dilute or dense phase when the material is unknown.” It’s worth repeating: Almost any material can be conveyed in dilute phase, either powder or resin; abrasive materials, however, tend to do better in dense phase because the velocity is lower.
Last, but definitely not least, is the question of money. “Many materials are in-between: moderately abrasive and capable of being handled by either dilute or dense phase,” said Shawn Werner. “In this instance, selecting one mode over the other can come down to life-cycle costing. On one hand, the equipment will wear out faster in a dilute phase system, requiring replacement parts; on the other, dense phase systems have higher initial costs, but won’t require replacement parts at the same rate.”