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The purpose of this program is to design the exit channel and the exit 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 plantand carry treated water into the distribution tank.

Exit Channel Design Program Algorithm

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Exit Channel Program Outputs
Exit Channel AutoCAD Drawing Program

Algorithm

The exit channel prgram program 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 as it enters the storage distribution tank.

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.

The program also determines the outlet weir dimensions. The outlet weir controls flow to the storage tank and determines the water level throughout the entire plant. The last piece of the plant calculated by this program is 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.

\large
$$
L_{ExitChannel} = (N_{SedTanks} )(W_{Sed} ) + (N_{SedTanks} + 1)(T_{PlantWall} )
$$

The width of the exit channel is designed based weir orientation and the constraint that the head loss in the open channel must be very small compared with the head loss in the launder orifices to ensure that the flow is uniformly distributed between sedimentation tank bays. To ensure this the head loss in the channel was chosen to be twenty times smaller than the head loss in the launder. Using the head loss equation this ratio was converted to use as an input the ratio of flow between the sedimentation tank with shortest route to the exit channel and the tank with the longest route to the channel.

\large
$$
h_L  = C_{p_1 } {{Q_1 ^2 } \over {2gA^2 }}
$$
$$
C_{p_1 } Q_1 ^2  = C_{p_2 } Q_2 ^2
$$

The variable Π _loss was assigned a value of 1/20, representing the ratio of head loss between the channel and the orifice. The flow ratio was then solved as a function of the loss ratio.

\large
$$
h_{L_{shortpath} }  = {1 \over {\Pi _{loss} }}
$$
$$
h_{L_{longpath} }  = {1 \over {\Pi _{loss} }} + 1
$$
$$
\Pi _{QSedTanks}  = \sqrt {{{{1 \over {\Pi _{loss} }}} \over {{1 \over {\Pi _{loss} }} + 1}}}
$$
$$
\Pi _{loss}  = {{1 - \Pi _{QSedTanks} ^2 } \over {\Pi _{QSedTanks} ^2 }}
$$

For simplicity the channel is assumed to be square (w=b). The code starts with the width equal to the inlet channel width, then iterates to find the smallest width that fulfills the head loss requirement. The larger of either a W _{ExitChannelMin} or the returned value from the iteration is returned. W _{ExitChannelMin} is needed to ensure that the tank is large enough for a person to put their hand in the channel to cap the launders. This value is approximately equal to 1.5*(ND.launder), where ND.launder is the diameter of the launder pipe.

The depth of the water in the exit channel is lower than the depth of water in the sedimentation tank by the head loss in the effluent launder. That value is set to approximately 5 cm. The channel walls are as high as the walls in the sed tank and thus the freeboard for the channel is the freeboard in the rest of the plant (10 cm) plus the head loss in the launder (5 cm).

Exit Weir Specifications

The exit weir controls the flow of water leaving the plant and regulates the water heights throughout the plant. The width of the exit weir is designed based on the following equation:

\large
$$
W = {3 \over 2}{Q \over {K_{VC} \sqrt {2g} H^{{3 \over 2}} }}
$$

Where W is the width of the weir and H is the head loss over the weir. There are two possible orientations for the weir: perpendicular to the length of the channel, or parallel.

Parallel Weir

The parallel weir design has the weir placed parallel to the length of the channel and attached to the inner half of the channel. Water flows in from the launders into the inner box created by the weir, and then overflows into the outer channel and into the distribution tank. The channel with the parallel weir is designed so that the outer channel that flows to the distribution tank satisfies the condition that the head loss in the open channel be far less than the head loss in the launder orifices.

Image Added

Perpendicular Weir

The weir will be placed in the exit channel such that the head loss in the open channel is very small compared with the head loss in the launder orifices so that the flow between sedimentation tank bays is uniformly distributed. The weir must also be designed so that a constant water level is obtained throughout the entire plant.

Image Added

Sedimentation Tank Exit Launders

Water that leaves the sedimentation tank exits at the top of the tank through holes drilled in pvc pipes that run the length of the tank. The holes drilled in the sides of the pipe ensure uniform flow of water up through the tank. The water exits into the exit channel. The launder parameters to be determined are the launder length, pipe size and orifice size.
The first step to designing the sedimentation launders is to determine the length of the launders. The launders are defined to run the entire length of the sedimentation tank, minus the widths of both the inlet and exit channels.

The size of the launder pipe is dependent on the length of the pipe, the flow through the pipe, an estimated head loss available, and the given acceptable ratio of the flow in the last orifice to the flow through the first orifice. This ratio is assumed to be 0.8 for this program. As a reference, a ratio of 1 indicates that the flow is exactly the same in the first and last holes. Less than 1 indicates some discrepancy. This function (ND.Manifold) is defined in the fluids functions program and returns the nominal diameter for the sedimentation launder (ND.SedLaunder).

The actual head loss through the manifold is found based on the actual diameter of the launder. The function determining head loss is also referenced from the fluids functions program.

The diameter of the launder orifices is calculated from the orifice equation, based on creating uniform flow and designed to have a head loss that is equal to the total head loss designed for the launder minus the frictional head loss through the launder pipe. This equation is also referenced from the fluids function programs. The holes size is important because we need uniform flow to ensure good sedimentation, and the smaller the holes the better the flow uniformity. But smaller holes lead to more head loss which in turn makes the sedimentation tanks deeper and more expensive. This equation reflects the compromise between these parameters of head loss and uniform flow.

The launder should be placed at a height that is as close to the top of the sedimentation tank as possible, but still always under water. The height of launder is placed below the water level in the sedimentation tank by the total head loss through the launder.

The height of the water in the exit channel can now be determined and is defined to be the height of water in the sedimentation tank less the head loss through the sedimentation tank launder. The depth of the exit channel is depth of the water in the exit channel plus the plant free board of 10cm.

The height of the plant weir is the height of the water in the exit channel less the height of the lip of water over the weir.

{float:left|border=2px solid black}
[!ExitChannelFrontView.png|width=600px!|ExitChannelFrontView.png]
{float}

{float:left|border=2px solid black}
[!ExitChannelSideView.png|width=600px!|ExitChannelSideView.png]
{float}

{float:left|border=2px solid black}
[!ExitChannelRendered.png|width=600px!|ExitChannelRendered.png]
{float}

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