Static or Motionless Mixers

As the term implies, unlike dynamic mixers described in previous chapters which turn with the screw, static mixers have no moving parts. These mixers are not specific to plastics extrusion but are versatile chemical engineering devices used for pipeline fluid mixing over a wide viscosity range. They can also be used for chemical reactions at very high flow rates. 

In plastics extrusion they have advantages of ease of installation with no requirement to be attached to, or incorporated with, the extruder screw. As they need to be positioned between the extruder output flange and the die, the requirement for extra space may place them with the same retrofit disadvantages as add-on dynamic mixers. However, with no screw connection required they can be installed within any available space between extruder outlet and die inlet, even at right angles to the axial line of the extruder barrel, e.g., vertically for a blown film extrusion line. 

The two main applications are improved cross-sectional distributive mixing of additives such as pigments and temperature homogenisation. This includes melt cooling, for example, in foam extrusion. 

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Add-on s — Barmag add-on torpedo, Staro-mix , Maddox, Cavity Transfer Mixer (CTM), Twente Mixing Ring (TMR), the Extensional Flow Mixer (EFM), Dynamic Melt Mixer (DMX), static or motionless mixers (SM) from Koch, Ross, or Kenics, etc. 

Other mixers, viz. the static (or motionless) mixers were discussed in Part 9.1.5 Distributive mixing while the Extensional Flow Mixer (EFM) and the dynamic extentional flow Mixer (DEEM) were described in Part 9.1.6.4 Extensional Flow Mixer". 

When the flow is laminar, either single or multiphase, there is only one design class option static or motionless mixers. Other pipeline mixing devices described for turbulent flow are not usable for even the simplest mixing applications in the laminar regime. All rely on turbulence and cannot function at low Reynolds numbers. The only alternative technology is in-line dynamic mixers, which include extruders, rotor-stator mixers, and a variety of rotating screw devices. None of these has the benefits of simplicity and the little or no maintenance characteristic of static mixers. In-line mechanical mixers are discussed briefly later in the chapter. 

If the main stream has sufficient pressure, a static or motionless mixer should be used. 

Since the static or motionless mixer promotes radial flow of both momentum and heat, it will significantly enhance the heat transfer rate. Figure 7-30 shows static mixer heat transfer packing installed in the tubes of a multitube heat exchanger. 

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