Exit Channel Design Program

The purpose of this program is to design the exit channel and weir that will keep a constant water level throughout the plant, which in turn will optimize the operation of the plant, as well as facilitating the monitoring of the plant.

Exit Channel Design Program Algorithm

Exit Channel Program Inputs
Exit Channel Program Outputs
Exit Channel AutoCAD Drawing Program

Algorithm

The exit channel prgram outlines the dimensions of the exit channel which carries water from the sedimentation tank to the distribution tank where. Chlorine in the is added to the water at the end of the exit channel before it enters the storage tank.
The exit channel program calculates three pieces of the design. It determings the dimensions of the exit channel it self, the size of the outlet weir, which controls flow to the storage tank and the plant water level, and the size and length of the sedimentation launders.

Exit Channel Dimensions

The length of the exit channel is found first. The channel is defined to span perpendicular to the sedimentation tanks, and thus its length must always be the total width occupied by all sedimentation tanks. The equation used is listed below.

Equation 1 - L.ExitChannel

The width of the exit channel is defined to be the same as the width of the inlet channel.

The depth of the exit channel is found at the end of the program after the head loss through the sedimentation tank launders can be determined.

Exit Weir Specifications

The exit weir, which controls the flow of water leaving the plant also regulates the water heights in the plant. The weir is a large diameter pvc pipe that functions freefall overflow weir. The equation used to determine the pipe size necessary is a deriviative of the fluid dynamics sharp crested weir equation. The equation was such that the length of the weir, over which water flows is the circumference of the pipe and the equation calculated the diameter of the pipe necessary for that length. The equation used can be found below.

This component must maintain some dimensional consistency with the other components of the plant. It consists of having inlet pipes that connect to each sedimentation tank, and a circular weir in the center that will transport the water to the storage tank. It is in the weir where water will be chlorinated.
The height of the leveling tank's wall will be the same as the height, measured from the platform of the plant, as the wall for the channel and the sedimentation tank, which is 70 cm.
The water level in the PLT is the difference between the water level in the channel and the total head loss through the launders:

An estimate of the weir diameter is obtained from the weir equation:

The height of the weir is obtained after the nominal pipe size is determined from the pipe database, and the actual height difference between the weir and the water level is subtracted from the water level measured from the bottom of the leveling tank.
The width of the plant leveling tank will have as an aesthetic reference the diameter of the chlorine tank and is calculated as the difference between the diameter of the chlorine tank and the weir diameter plus the launder diameter. The length of the plant must fit the launders and should provide space for concrete, therefore it is calculated as the sum of the diameter of the three launders, the weir diameter, and the assumed concrete space of 0.5 m:

Plant Elevation Design

The Plant Leveling Tank function determines all of the water level elevations throughout the plant. Any variables with HW indicate the depth of water. Any value Z indicates a height.
The program first assigns values from the inputs that are arrays to individual values so that it is easier to determine what they are based on the variable name.

The head loss through the launder is found in the launder function and the height of the water in the channel is set as a design assumption. From these the actual height of the water in the PLT is calculated from the height of the water in the channel minus three times the head loss through the launder since there are three launders. This algorithm is incorrect.

The number of orifices in the grit chamber are calculated by multiplying the sum of number of orifices at each height per riser by the number of risers in the grit chamber.

L.sedexits is the length of the exit pipes from the sedimentation tanks to the plant leveling tank.

The head loss through the weir, will determine the height of water in the level tank: The head loss through the weir in the plant leveling tank is calculated with the head loss equation for a weir in the fluids functions file. Head loss is also calculated through the total number of launder orifices and the head loss through the launder manifolds.

The height of the sed tank is found by the height of the water in the sed tank plus the height of the freeboard, which is a design assumption.

The height of the top of the plate settlers (lamella) in the sed tank is found by adding the height of the bottom of the plate settlers to the vertical projected height of the lamella at angle alpha.

The height of the launders centered in the water above the plate settlers is found by subtracting the height of the top of the plate settlers from water in the sed tank, dividing by two, and adding the height of the top of the plate settlers back again. This may be different from how the depth of the water above the launder is calculated in the sedimentation tank function. This should be checked.

The head loss through the pipes that take the water to the level tank needs to take into account that the flow won't divide quite equally between the pipes because of different total loss coefficients. We use conservation of mass and the fact that the head loss through each of the paths is the same to determine the actual head loss. First we calculate the minor loss coefficients for the exit pipes. This is based on the number of ninety degree turns in the pipe, which is a design assumption since it depends on the plant layout, times the Kel90 added to the Kexit for each pipe. Then the function HLsedexits is called on to calculate the head losses.

ZTsed is the height of the wall in the tank, calculated from the height of the water in the sed tank plus the freeboard.