Exit Channel Design Program

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 and carry treated water into the distribution tank.

Exit Channel Design Program Algorithm

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. Chlorine is added to the water at the end of the exit channel as it enters the distribution tank. 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.

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.

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.

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).

The height of water is dependent on the head loss through the sedimentation tank launders, which has not yet been determined. The depth of the exit channel is thus found at the end of this algorithm.

Exit Weir Specifications

The exit weir controls the flow of water leaving the plant and regulates the water heights throughout the plant. There are two possible designs for the weir.

One design has the weir perpendicular to the length of the channel. With the perpendicular weir design, the width of the exit channel is designed based on the head loss constraint stated above and the following equation:

The other 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 to the distribution tank satisfies the head loss constraint.

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.