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 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 it enters the storage tank.
The exit channel program calculates three pieces of the design. It determines 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.

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 free fall overflow weir. The equation used to determine the pipe size necessary is a derivative 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 calculated diameter is rounded up to next available pipe size.

In order to calculated the height of the weir needed the lip height of the water over the edge of the weir has be found.

The height of the weir is then found by subtracting the this water lip height from the water level in the exit channel. The height of the water in the exit channel depends on the about of head loss in the sedimentation launders, and thus is calculated at the end of the program.

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 create uniform flow of water up through the tank. The water exits into the exit channel. 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 tanks, 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 hole to the flow through the first hole. This function (ND.Manifold) is defined in the fluids functions program and returns the nominal diameter for the manifold (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 referenced from the fluids functions program.

The diameter of the launder holes 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 reference from the fluids function programs.

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 is defined to be the height of water in the sedimentation tank less the head loss through the sedimentation tank launder.

The height of the plant weir is defined to be the height of the water in the channel less the height of the lip of water over the weir.