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The equation above is important since it defines the relationship between plant flow and head loss through the plant, which is important as well for our dosing apparatus. The CDC uses a lever arm with a float and counterweight to relate the dosing to the changes in the entrance tank water level, which is a function of the influent flow rate. An increase in head loss links the chemical flow rate to the plant flow rate and the chemical dose (mg/L) will be constant as plant flow varies.As can be seen above, the equation which describes the flow of water through plant and the orifice equation which relates flow of alum in the dosing system both have the same relationship between flow rate and head loss. Since the flow rate for both the plant and alum flow are each related by the square root of the head loss, the two flows can be linked through the float in the entrance tank. Any rise
For a detailed step by step description of the steps involved with measuring the plant flow rate please see flow measurement section of the Chemical Dose Controller Manual.
The dosing tube must be designed to minimize major losses so that minor losses dominate head loss. The amount of major losses due to the friction in the dosing tube will vary based on the flow of alum going through the dosing tube, with higher head loss occurring at higher alum flow rates. As a result, it is not desirable to set the size of the orifice based on the varying major losses through the dosing tube, so the head loss caused by the orifice needs far outweigh major losses so the friction in the tube can be ignored. As a result, the dosing tube needs to have a large enough diameter so that the effects of major losses are not significant when compared to the minor losses causes by the orifice. The graph below illustrates the relative contributions of head loss caused by a large 2 mm orifice cap as opposed to the major losses through the dosing tube.

Figure 1: Major loss contribution to the total head loss

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