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Rapid Mix Tube

Abstract

During the fall 2009 semester, the main focus of the Rapid Mix Tube subteam was the development and design of a rapid mix tube system to provide adequate large and small-scale mixing of the aluminum sulfate dose with the raw water source entering the plant. The system developed in the fall 2009 semester is designed for the Agalteca plant, but the ultimate design can be modified to fit into future AguaClara plants. The design of the system evolved multiple times throughout the semester, and the current system was developed to improve upon the main problem of those initial designs: access to the small scale rapid mix orifice to clean it in case of clogging. The design of the tube was thus tailored to fix this problem, and a MathCAD file calculating orifice sizes and head loss through the system was developed.

Rapid Mix Tube Design Process

The initial rapid mix system proposed for the Agalteca plant was much different than the system designed this semester. As can be seen in Figure 2 below, water from the entrance tank flows into a pipe that carries it into the flocculation tank. Rapid mix is achieved in this system when the water flows through an orifice at the end of the pipe leading into the flocculation tank, allowing small-scale mixing of the aluminum sulfate with the raw water to occur before reaching the flocculation tank. One of the main problems with this system is the location of the rapid mix orifice; it is submerged in the bottom of the flocculation tank, making it very difficult to reach or remove. Flow to the plant would have to be stopped and the flocculation tank drained at least partially to remove and clean this orifice if it ever clogged or needed to be replaced or exchanged. Another problem with this design is that the exit tube taking water from the entrance tank to the flocculation tank is flush with the side wall of the entrance tank and is located quite deep in the tank. Thus, for flow to the plant to be stopped, the entrance tank would have to be emptied completely.

The current design for the rapid mix tube was developed to address the problems created by the initial design. A schematic of the new design is shown in Figure 3. This new rapid mix tube system consists of two separate 'stages,' a large-scale mixing process in the first portion of the system, and small-scale mixing process in the second portion. The tube protrudes up into the entrance tank to help regulate flow through the plant-flow through the plant will cease once the water level in the entrance tank reaches the top of the rapid mix tube, allowing the water already in the tank to be stored if there is low source flow or the plant needs to be cleaned.

Figure 3. Schematic of the current Rapid Mix Tube system.

Water enters the top of the tube through the large-scale mixing orifice, where it is dosed with the aluminum sulfate and begins the rapid mix process. This orifice is in place to create large scale mixing in the first section of the tube. The design of this orifice size is based on the exit loss coefficient through the orifice, K. The target K value for this orifice is 2, which provides the best mixing in the first section of the tube for large-scale rapid mix. To calculate the necessary area and diameter of the large scale mixing orifice, the following equation was used:

Solving for A.in, which will be the area of the stream of water entering the tube through the orifice, we obtain:

In this equation, A.in is taken to be the area of contracted flow through the orifice, which is the area of the large-scale mixing orifice multiplied by the vena contracta coefficient, which accounts for the contraction of flow through an orifice. The equation the describes this is as follows:

Figure 4 illustrates the effect of the water contraction flowing through an orifice.

Figure 4. Diagram showing the area used for A.in in the Exit loss coefficient equation.

A.out in the above equations is taken to be the area of the pipes used in the system since the water is allowed to outlet freely into these pipes.

Conclusion

Future Work

Bibliography

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Deliverables

What do you plan on "handing in" by the end of the semester? Designs, programs, separate reports for publication, etc.

  • MathCAD file detailing the Rapid Mix Tube design specifications including orifice sizes and head losses through the system
  • Schematic drawing of the Rapid Mix Tube system as well as how it fits into the greater plant design
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