Series of Orifices Design
Summary
Another proposed design was to use a series of orifices within a tube that supplies the alum stock to the untreated water. Given the head loss that these orifices would provide, we needed to determine if the resulting alum flow would be slow enough to dose the water appropriately. To do this, we allowed a 20-percent error (since the dose was not intended for flocculation) and looked at the range of flow given this allowable error and the constant decrease in alum solution level. Unfortunately, we found that the resulting flow of alum from this design was too large for our purposes.
Calculations
Download the Series of Orifices Design MathCAD file here.
The head loss in the tube was determined using the equations for major and minor losses. The equations used in these calculations were found in online AguaClara notes and in Frank M. White's Fluid Mechanics (6th Edition).
The flow through the alum doser exit tubing was determined given user-defined parameters for the dimensions of the alum stock tank, such as height of tank (and thus, height of alum level) and diameter of exit orifice (and thus, diameter for the attached plastic tubing).
Next, using the previously determined flow through the exit tubing, the flow through each of the orifice plates can be easily determined.
Then, head loss through both the tubing and orifice plates are calculated given user input for width and number of orifice plates along with the length of the desired tubing. This total head loss through the system is the summation of the head loss through the orifices plates and the head loss through the wider, encompassing tubing.
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Using this information, the necessary flow of alum from the doser based on dimensions given above is determined. Using a stock dose concentration of 1.5 mg/L, the necessary stock concentration is found as well.
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The following section calculates the flow of alum given varying alum stock heights (of 5 inches to 25 inches). The final graph depicts results showing that if alum stock height varies greatly, the flow remains generally within the acceptable flow error. This error stems from the fact that we are not using alum for flocculation.
