Rapid Mix Tube Design Modifications

Agalteca Rapid Mix Tube Design Modifications

Please note: This page only describes and documents the changes and modifications that were made to the Rapid Mix Tube design for the customization of the Rapid Mix tube to the Agalteca plant. To view the original design parameters and theory behind the tube design, please see the original Rapid Mix Tube Design page. The theory behind the design is detailed here, including the design of the macro and micro scale mixing orifices, and parameters such as flow rate, energy dissipation, and target head losses are also included.

The Rapid Mix Tube designed during the Fall 2009 semester was installed in the new Agalteca plant in March 2010. The original Rapid Mix Tube design was very general, and certain modifications had to be made to customize the tube to the Agalteca plant. A second, modified MathCAD file was created for the Agalteca plant to serve a few different purposes:

1. We need to be able to adjust the total head loss through the entire plant to adjust for any extra head loss created in the plant. The easiest way to do this is to change the number of orifices in the Rapid Mix Plate: adding more orifices will reduce the total amount of head loss, and creating a plate with fewer orifices will increase the total head loss through the plant. The following two plots, created in MathCAD, show the variation of head loss with the number of orifices in the micro scale mixing plate for the given maximum Agalteca flow rate of 6.3 L/s and the target energy dissipation rate of 1 W/kg:

Figure 1. Plot of the Head Loss through the micro scale mixing plate against the number of orifices of 7/8" diameter drilled in the plate. As can be seen in the above plot, as the number of orifices in the plate increases, the head loss decreases, showing that increasing the number of orifices in the plate is an effective way of decreasing the head loss through the plate.

Figure 2. Plot of the Head Loss through the micro scale mixing plate against the number of orifices of 7/8" diameter drilled in the plate for a reduced range or number of orifices from 12 to 17, the range of numbers that would be feasible for the micro mixing plate in Agalteca. This plot shows the values of the head loss created through the plate for each number of orifices in the plate, as well as the range of head losses that would be observed in the Agalteca plant for each number of orifices.

Calculation of Target Head Loss and Flocculator Head Loss

The MathCAD file created allows the user to easily input the existing plant parameters such as plant flow and measured head loss, as well as the existing plate characteristics, including the number and diameter of the existing orifices. Using this data, the MathCAD file calculates the total head loss created through the current plate, as well as the target head loss through the plant, and produces the number of extra orifices that must be drilled or taken away from the existing plate to achieve the target head loss through the plant.

1. We also need to measure the head loss through the flocculator in the Agalteca plant, and the MathCAD file produced will also estimate that given the total measured head loss through the plant. Because the head loss through all other plant components can be measured or easily modeled, the head loss through the flocculator simply becomes the difference between the total plant head loss and the sum of the head losses of all other plant components.
2. The target head loss through the plant will change with varying flow rates, so to adjust correctly the number of orifices in the Rapid Mix Plate, we must know the target head loss through the plant at the time of measurement. This calculation was also added to the MathCAD file for greater accuracy.

Energy Dissipation Rate Concerns

The energy dissipation rate created through the micro scale mixing plate is dependent upon the total number and area of the micro scale orifices in the micro scale mixing plate, as well as the flow rate through the plant, as shown in the following equation:

Because the flow rates through plant may vary based on the available source flow, we need to know how the energy dissipation rate will change with these changing flow rates. The following plot was generated in MathCAD to display how energy dissipation rates will change with varying flow rates through the orifice plate in place in Agalteca:

Figure 3. Plot of the Energy Dissipation rate through the micro scale mixing orifice as it varies with flow rates through the plant. This plot was generated using the orifice diameter of 7/8" and 16 orifices in the plate, which is used in the Agalteca plant.

Another detail that was addressed in the Agalteca Rapid Mix Tube installation was the delivery of the dosed alum into the Rapid Mix Tube itself. To accomplish this delivery, the end of the flexible tubing running off of the doser's level arm was fitted with a small plastic elbow, then attached to the edge of the large scale mixing orifice using a small plastic clip. This allows the alum to be delivered directly into the flow of water entering the tube, improving mixing and reducing the chance that alum will escape into the entrance tank.

This delivery tube will always be submerged in the water in the entrance tank, thereby eliminating the mixture of air, water, and alum that has occurred in the previous dosing designs. This delivery modification was made in an effort to reduce the amount of air introduced to the dosing system, which will theoretically reduce any problems with floating flocs created in the flocculator, reducing plant performance. The following photo was taken at the Agalteca plant showing the new dosing delivery system. Note that the tank was drained at this point for installation of the micro mixing orifice plate, and this is why the delivery elbow is not submerged; under normal operating conditions, this elbow would be submerged and would eliminate the addition of air to dosing of alum with the raw water.

Figure 4. Photo of the attachment of the aluminum sulfate dosing tube to the top of the Rapid Mix Tube in the Agalteca plant. The wire in the left of this photo is attached to the Rapid Mix Plate; this allows for easier removal of the plate as the operator can simply pull it out of the tube instead of reaching into the tank to retrieve the plate.

Deliverables

• Modified MathCAD file to achieve target head loss using rapid mix plate adjustments
  • This MathCAD file is used to adjust the rapid mix plate orifice number and sizing to create the target head loss through the plant. It can also estimate the head loss through the flocculator, a variable of interest, as well as adjust the allowable orifice sizes for the rapid mix plate based on the actual maximum flow rates for the plant, which likely differ from those estimated for the design.