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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 below 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.
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:
In this equation, A.in is taken to be the area of 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, which is shown in Figure 4 below.
Conclusion
Future Work
Bibliography
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