Expert ForumsMixing

Topic: Re: What is your advice on a helical ribbon mixer/transport device?

posed by 
After seeing the attached sketch (click here for the sketch), I have a better appreciation for the application and problem.  The idea of a helical ribbon mixer/transport device is a good one.  However, depending on the flow rate into the bin and the rotational speed of the helical ribbon, the problems could be moved from the center of the bin to the ends.  The outer flight will have the greater influence on the transport of powder.  The transport capabilities will be approximately in proportion to the relative diameters of the respective ribbons.  The pitch of the helical ribbons and the rotational speed must be matched to the feed and transport rates of the powders.  The long pitch length means that each rotation will push the powder a long distance down the bin.  Too much speed and the powder may be transported and packed against the ends of the bin, rather than accumulating in the center. The other possible issue has to do with bin size and stickiness of the powder.  Starting a helical ribbon in a settled bed of solids is always a problem, since the initial torque requirements can be high.  In a small bin, the initial torque may be overcome without much trouble.  In a large bin, with a ribbon diameter greater than 12 inches (1/3 meter), starting torque and structural strength of the helical ribbon may be more of a problem, especially with the open design.  Powder loads on helical ribbon impellers are difficult to predict.  In many cases, the only way to operate a helical ribbon mixer is to add the powder with the mixer running.  Restarting a mixer in settled solids, especially sticky ones, may be impossible.  If size and flowability are a problem, a paddle mixer design may be an effective alternative to a helical ribbon.
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