Entrance Tank Design Program
The purpose of this program is to design the dimensions of the entrance tank based on the flow rate of the plant and the head loss required for rapid mixing. The Entrance Tank is constructed as a separate channel before the flocculator and in current schematics is located offset with the chemical storage tanks a distance from the flocculator enough to provide a walkway.
A view of just the tank, without the rapid mix system. The thickness of tank wall is T.PlantWall.
The entire entrance tank, with rapid mix system is shown below. The macro-mix orifice is the cap on top, while the orifices, pipes, and elbows account for a head loss that will provide a total head loss of 40 cm (HL.PlantTotal, an Expert Input) throughout the plant.
Entrance Tank Design Program Algorithm
Entrance Tank Program Inputs
Entrance Tank Program Outputs
Entrance Tank AutoCAD Drawing Program
Algorithm
The entrance tank is designed to be a separate channel before the flocculator to house the rapid mix system and provide the head necessary to drive water through the plant.
The area of the entrance tank is calculated based on the flow rate through the plant and the required up-velocity through the entrance tank (found in Design Assumptions).
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The water level in the entrance tank is designed to meet requirements for rapid mix, as well as head loss due to flow from the entrance tank to the flocculator. These variables can be found in the Rapid Mix, Flocculation, Sedimentation Inlet Slopes, and User Inputs design programs.
The height of the entrance channel is based on the elevation of the tank and providing a distance of two pipe diameters beneath the tank, along with room for a pipe radius and thickness of concrete. The tank's base does not need to extend to the base of the flocculator. Because it can be elevated, the flocculator can be dug into the ground and less concrete needs to be poured.
The length of the entrance tank (L.Et) is the square root of the entrance tank area, which is determined from plant flow rate and required up-flow velocity through the entrance tank. The width of the entrance tank is chosen so that it meets the geometry requirements, as well as the width required for rapid mix. In the Rapid Mix, the width of the entrance channel needs to be at least the the spacing between the floc baffles in the first channel (S.FlocBaffle0).
Rapid Mixer
The rapid mix system is designed to mix the alum and raw water on both the macro and micro-mixing scale.
The rapid mix system is centered in the middle of the entrance tank (RM.origin) and consists of a pipes with interfaces that produce head losses through changes in diameter. The desired total head loss throughout the entire plant (HL.PlantTotal) is 40 cm and the head loss for the rapid mix system (h.totalRM) is determined by subtracting the head losses throughout the plant from this value. The head loss for the macro-mixing orifice is constrained at 5 cm (MacroMHConstraint), so the micromixing orifice size and number accounts for the head loss discrepancies that need to be met. The head loss at the macro-mixing orifice should suffice to mix the water and alum on a macro scale, while the micro-mixing orifices mix on a molecular (micro) level.
The elevation of the rapid mix system is determined from the height of the weir at the end of the sedimentation tank, so when there is no head there is no flow through the plant. Only a water level above the rapid mix system can drive the plant. The bottom of the rapid mix system is at a height the thickness of the plant concrete, and the size of an elbow above the bottom of the plant origin. The length of the pipe is determined in the Rapid Mixer design program. The nominal diameter of the pipes and elbows that are part of the rapid mix system is determined from the required head loss desired throughout the plant and the length of pipe and elbow needed to provide the rapid mix system and connect to the flocculator.
Since the entrance tank is situated next to the chemical storage tanks, the system must be connected to the flocculator with additional pipes than just the pipe that runs vertically through the entrance tank. The pipe must span the walkway between the flocculator and the entrance tank and if there is an even number of flocculation tanks, the pipe must run the entire length of the tank (L.Sed). This total length - that is, the length of the pipe in the tank, and the length of the pipe joins this pipe (via elbows) to the flocculation tank is the length that is used in the head loss considerations that calculate the nominal diameter of the pipe system as well as the diameter of the macromixing orifice and the size and diameter of the micromixing orifice.
The macromixing orifice is a cap that sits atop the pipe in the entrance tank and has a hole in it of the size of the macromix orifice diameter. A length of two pipe diameters is necessary to ensure adequate mixing. A coupling will allow the pipe to run through the bottom of the tank. To ensure enough room for mixing, the micromixing interface will be located at the junction of the pipe and the first elbow.
Macro-Mix Orifice:
