Rapid Mix Chamber Design Program

Introduction

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The AguaClara team is making changes in the arrangement of the Rapid Mix system.  This page will document and describe the current changes in the Rapid Mix system and the algorithm for preparing an AutoCADD drawing of the Rapid Mix system. 

The suspended particles in the water are removed through a process known as sedimentation.  The rate of sedimentation is increased by increasing the diameter of the particles through sticking them together.  This is done in a process known as flocculation. Alum is the coagulant of choice used to cause the particles to undergo flocculation.  This has to be done rapidly as to ensure the aluminum sulfate does not precipitate before it can be used by the particles.  We also need even distribution of alum to the molecular level. 

The rapid mix system is designed to accomplish this.  The rapid mix design consists of piping leading from the entrance tank to the flocculator entrance.  The piping contains two orifices.  One orifice is for macro-mixing and the other orifice is for micro-mixing.  The orifices decrease the cross sectional area of the flow allowing for a higher velocity and thus turbulence.  Turbulence, measured in energy dissipation rate, is associated with the formation of eddies which mix the alum to the length scale at which viscosity overrides the formation of eddies with a larger energy dissipation rate being associated with a smaller eddies.  The length scale at which the eddies can mix the alum to is known as the Kolmogorov length scale. 

There are two main forms of mixing.  One is macro-mixing and the other is micro-mixing.  Macro-mixing mixes the alum to the length scale at which micro-mixing can start.  Micro-mixing distributes the eddies to the length scale that molecular diffusion can finish the task. 

Design Algorithm

Design Assumptions Design Program<!-- /* Font Definitions */ @font-face

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-->In the current design, we have two orifices on the same pipe segment.  The micro-mixing orifice is two pipe diameters below the macro-mixing orifice.  This ensures adequate mixing time for macro-mixing to take effect.  The orifices for both equations are sized using the equations of minor loss coefficient for a submerged orifice. 

Research has estimated that for a macro-mixing orifice, we should have   a minor loss coefficient of 1.3.  This is what we are using for our design.  In the case of the micro-mixing orifice, we should strive for an energy dissipation rate of 1 W/kg.  In the future we plan to use the micro-mixing orifice as a flow measurement device and allow the orifice to have a head loss of 20-50 cm.  From here, we derive the orifice size. 

In the case of a macro-mixing orifice, each pipe diameter allows for one orifice size.  An increase in flow-rates will give the macro-mixing orifice a significant head loss.  In the algorithm, if there is significant head loss (2 cm or above), then the piping is upgraded to the next size.