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Mixing Glossary

Axial Mixing: The blending of fluid along its direction of flow.  Static mixers by themselves do very little axial mixing.  This means that a static mixer can only blend the components in a particular plane passing through the housing.  Bartlett Engineering can design supplemental devices that can be used with static mixers to improve axial mixing.

BMV: The BMV style mixer element consists of corrugated plates stacked in alternating orientations which caused the fluid flow paths to "cross" so that many zones of micro turbulence are created.  This style of mixer is most common because it has a high mixing efficiency to pressure drop ratio, and in turbulent flow applications it achieves mixing in a short distance which is particularly useful in applications which require high pressure or high alloy housings.  This style mixer traces its roots to the well established Koch Engineering "SMV".  

BMX: The BMX style mixing element consists of crossed flat blades which cause many instances of flow stream cross sliding and splitting.  The BMX element is ideal for high viscosity applications such as plastic blending.  The origin of the BMX element is found in the Koch Engineering "SMX" style element.

Radial Mixing: The blending of fluid streams perpendicular to the direction of flow.  Bartlett Engineering static mixers are very effective at this type of mixing.

Mixer Element:  The "element" is internal component that causes fluid stream blending.  Our most common elements are the BMV (Bartlett, mixer, vane) and the BMX (Bartlett, mixer, “X” style) elements.  We almost always recommend removable elements because the additional cost for us is minimal, and a non-removable element should be considered to be not cleanable.

Mixer Housing:  The unit which holds the mixing elements.  Bartlett Engineering offers housings in any material in which pipe can be obtained (high nickel alloys, TFE lined, PVC, fiberglass) or some materials in which pipe cannot be purchased, but which can be fabricated from rolled and welded material.

Mixing Efficiency:  The index of how well a mixer has mixed the entering flow streams.  It is the quotient of the standard deviation of the variation in composition divided by the portion of the flow stream that makes up the added material.  It is commonly written as “sigma over 'x' bar.”  Although this unit can be used as a quick gauge of the thoroughness with which a mixer will mix two streams, the weakness of the mathematics must be kept in mind when two streams of nearly equal proportions are mixed.  As an example, when two equal streams are mixed the value of “x bar” is 0.5.  When a large standard deviation (sigma) is divided by a large “x bar” then a small (and ostensibly good) mixing efficiency number seems to be the result.  The value of the mixing efficiency can be misleading in instances of nearly equal quantities of the mixed streams.  A perfect example of this is an epoxy putty mixing application where nearly equal flow rates are mixed.  As a result, an unacceptably large variation in homogeneity would be hidden when it was divided by a large value of “x bar.”  In epoxy mixing applications the variation of the outlet stream needs to be more homogeneous than the typical mixing efficiency of 0.05.

Reynolds Number:  The ratio of the dynamic fluid forces to the viscous fluid forces.  Though the units of the numbers used do not literally compare the dynamic to the viscous forces of a fluid flowing through, over, or around an object; research data compiled using this formula allows the prediction of the velocity at which laminar flow ends and turbulent flow begins.

Turbulent Flow: Fluid flow characterized by microscopic tumbling of a fluid.  This tumbling is why turbulent flow is most effective for mass or heat transfer.  Some mixing is achieved in turbulent flow, but to achieve effective mixing without a static mixer, pipe lengths of up to 100 diameters or more may be required.

Laminar Flow:  Flow characterized by the sliding of fluid layers.  An example would be epoxy in a pipe at low velocity.  If a colored stream were added to this flowing epoxy, the colored stream would remain straight and in its location as the epoxy traveled down the pipe for many pipe diameters.  During laminar flow, heat transfer is primarily by conduction, and mass transfer is primarily by diffusion.  Both of these are very slow mechanisms.

Vane Type Element:  The Bartlett Engineering BMV element.  This element creates crossing flow channels that cause microscopic tumbling of the fluid between the two flow streams.  The mechanism which produces mixing is this tumbling which creates random trading of fluid from stream to stream.  In low viscosity applications, the BMV element produces the best mixing with the least pressure drop of any element design.

“X” Type Element:  The Bartlett Engineering BMX element.  This element consists of crossed bars at an angle to the flow stream (i.e., an “X”) which causes the flow stream to be cut and recombined.  At each cross section of the element, the flow stream is separated into 10 to 100 separate streams.  When compared to mixer elements which only separate the flow stream into two streams, it is easy to understand why the degree of mixing per element for the BMX is the best of any mixer available for laminar flow.  In laminar flow, pressure drop increases with every square inch of area over which the flow must travel.

Static mixers which require additional elements to achieve the same mixing effectiveness will have an increased pressure drop proportional to the additional metal area inserted into the flow stream.

We continue to add to this Static Mixer Glossary but if you have questions which are not answered here, please call one of our Static Mixer Specialists for personal assistance.

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