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Figure 3. Schematic of the current Rapid Mix Tube system.
Large-Scale Mixing Orifice Design
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.
Water flows down the pipe after entering through the large-scale mixing orifice, then flows down the length of the pipe, which should be at least as long as several diameters of the pipe to provide adequate mixing. The Agalteca plant will be using 0.152 m (6-in.) diameter PVC pipes, and thus the length of the large-scale rapid mix tube will be between 0.457 m and 0.762 m (18-in. to 30-in.) to provide enough diameters of length for large scale mixing.
Small-Scale Mixing Orifice Design
After water flows into the rapid mix tube and through the top section of the pipe to achieve large-scale mixing, it reaches the small-scale mixing orifice. The area of the small-scale mixing orifice is designed to achieve a target head loss, providing a mechanism to measure the level of water in the plant to assist in the correct dosing of the plant's raw water source with aluminum sulfate. The equation used to calculate the area of the small-scale mixing orifice is as follows:
Conclusion
Future Work
Bibliography
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