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The effects of pipe bends in pneumatic conveying
PBE-News reader Shivkumar K. Dalal asked: What effect will an increase in pipe bends have on dense- and dilute-phase pneumatic conveyors?
Airdusco, says: This question reflects an important issue in successful pneumatic conveyor design, whether for dilute- or dense-phase systems (or those in between). Since the answer to this issue is complex and relates to a variety of system design issues, I'm going to summarize with five "Rules-Of-Thumb" (ROTs), which are applicable to the majority of applications. However—and this is a very important "however"—there are always exceptions that must be considered.
ROT#1: Increasing the number of elbows increases the system's differential pressure requirements. This is both logical and common sense. But you also must consider multiple additional factors, including any new elbow's radius angle and centerline radius, the material flow velocity, and the material's characteristics. For some materials, such as plastic pellets, proper design can allow for a significant number of elbows, while for others, such as zinc oxide, even a few turns can cause unwanted problems. It's normally beneficial to minimize the number of turns in a system.
ROT#2: The more abrupt the turn (or the shorter its centerline radius), the higher the pressure loss. This also seems logical and common sense. However, as pneumatic conveying consultant and PBE pneumatic conveying columnist Paul Solt has proven, sometimes even an abrupt, 90-degree T elbow can be best (such as for highly abrasive materials), and in some cases, very long radius elbows can actually add to the pressure drop compared to elbows with a more reasonable turn radius.
ROT#3: The routing with the fewest turns is best. Again, this appears logical and is generally true. However, there are multiple examples where this isn't true:
- Conveying up an inclined plane: Although this may be the shortest path between two points, it can create unwanted pressure losses in the system. When conveying up an inclined plane, gravity tends to pull the material down to the bottom of the piping, and the material tends to stay in a straight line, eventually impacting the piping due to the angle of rise. This results in a "pitch-and-catch" conveying, where the material falls in and out of the conveying airstream, requiring significant energy to re-entrain the material. Short distances may have a minimal negative effect, while a long inclined plane will result in significant, unnecessary pressure loss. In many cases it's best to ignore the "hypotenuse" routing and use the horizontal/vertical route with 90-degree elbows. Also, using excessively long radius elbows can actual result in this same "inclined-plane" energy loss.
- Installation: Remember, someone has to install this piping and access it for maintenance. The shortest routing may reduce energy losses, but it can also create a situation where installation is costly and access is almost impossible.
ROT#4: The higher the conveying velocity, the higher the pressure loss for the turn. This ROT is basically valid. The carrying airflow pressure losses are based on the airflow velocity squared. In addition, each material has its own minimum carrying velocity that affects the total pressure loss at the turn.
ROT#5: Never make more than two turns in close physical proximity. Experience has proven this to be a valid approach when designing piping layouts. Remember, neither the conveying air or the conveyed material has "brakes." For either to change direction they literally impact the elbow walls and rebound back and forth until the conveying air re-entrains the material into the flow. This takes time and distance. Since this happens at each turn, placing elbows in close proximity can result in insufficient energy to re-entrain the material into the flow, resulting in plugging. In fact, pneumatic conveying system plugging often occurs just after a turn.
Although dense-phase conveying is less affected by turns (for a multitude of reasons) than dilute-phase conveying, a turn should only be used when truly needed. Perhaps the best advice is to imagine the material flow in your mind as it flows through the system routing. If you can imagine a problem at a location, it should be investigated and improved. Keep it simple and avoid the complex where feasible.
Jack Osborn is the engineering manager for Airdusco. He will be presenting two conference sessions at PBE's 2012 Southeast Conference & Exhibition, to be held March 13-15 at the Cobb Galleria Centre in Atlanta, Georgia.
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