Inlet Channel Design Program

The purpose of the inlet channel program is to determine the dimensions of the inlet channel, the dimensions of circular ports in the inlet chimneys that deliver water from the inlet channel into the sedimentation tank, and the dimensions of the weir in the inlet channel that will regulate the height of the water in the inlet channel and flocculator. The inlet channel will run along the inlet end of the sedimentation tanks, such that its length will be equal to the combined widths of all the sedimentation tanks plus the thickness of the walls between the tanks and the thickness of the walls that separate the bays within the sedimentation tank channels.

The Inlet Channel Weir

The purpose if the inlet weir is to regulate the height of water in the inlet channel and flocculator. This is accomplished by a relation between the width of the weir and the head loss over the weir, which is governed by the equation:

Where W is the width of the weir and H is the head loss over the weir. The weir can either be placed parallel or perpendicular to the length of the channel.

Parallel Weir

The parallel weir will be placed in the inlet channel such that it will satisfy two constraints. First that on the side of the weir that delivers water to the inlet chimneys, the width of the channel will be wide enough so that the energy dissipation rate associated with the channel geometry will be greater than the energy dissipation rate at the end of the flocculator and will also be wide enough to fit the ports leading to the inlet chimneys plus an additional 3 cm of space on each side of the port. The second constraint for placing the weir will be that on the side of the weir that delivers water to waste, the channel will be wide enough to handle the flow of water in the event that all sedimentation tanks are shut off and all of the water coming from the flocculator must go to waste; it must be able to handle the entire flow of the plant.

Perpendicular Weir

The perpendicular weir design is placed inside the inlet channel such that the energy dissipation rate associated with the inlet chimneys will be much greater than the energy dissipation rate in the open inlet channel. A perpendicular weir also must be placed so that it can handle the entire flow of the plant if all the sedimentation tanks are turned off and all water is going to waste.

Inlet Channel Design Algorithm

Inlet Channel AutoCAD Drawing

Algorithm

The length of the channel is a function of the number of sedimentation tanks, and the thickness of the walls between the sedimentation tanks.

The cross sectional area of the inlet channel on the side of the weir that delivers water to the inlet chimneys is determined using the A.Port function in Fluids Functions

Therefore the height of the water in the inlet channel is equal to the square root of the area of the channel.

The first constraint for the width of the inlet channel on the side of the weir that delivers water to the chimneys is to consider the energy dissipation rate. The area of the inlet channel and the height of the water in the channel have already been calculated, and so the width may be determined:

Next the size of the ports that take the flocculated water down into the inlet chimneys and to the sedimentation ports is calculated through the A.Port function in Fluids Functions.

The diameter of the hole that delivers the water from the inlet channel into the inlet chimneys is calculated using an array that loops through the available pipe size diameters and rounds the needed diameter up to the next available diameter through the function found in the Pipe Database.

The circular hole through the inlet chimneys must have at least 3 cm of space around it, and so the second constraint on the width of the inlet channel on the side of the weir that delivers the water to the chimneys is that it must be at least as wide as the diameter of this port, plus the 3 cm of space needed on each side of the port.

Therefore the width of the inlet channel on the side of the weir delivering water to the chimneys is set to be the maximum of the width determined by the allowable energy dissipation in the inlet channel, and the width needed by the circular port in the inlet chimney.

The height of the inlet channel is equal to the water height in the inlet channel plus the plant freeboard height, so that the water is not at the very top of the channel.

As mentioned earlier, the inlet channel weir will serve to regulate the height of the water in the inlet channel. On one side of the weir, the water is being delivered to the inlet chimneys. On the other side of the weir, water will go to waste when the sedimentation tanks are shut off. The width of this side of the weir is determined to be able to handle the flow in the case that all of the sedimentation tanks are shut off and all of the water must go to waste; it must be able to handle the entire flow rate of the plant.

To do this, the inlet channel must have a width that will yield a desired head loss. Taking advantage of the fact that head loss is proportional to the width of the channel, the width of the inlet channel on the side of the weir that delivers water to waste is determined through an iterative solution that compares the channels width and calculated head loss to a target head loss and then adjusts the width until the target head loss is reached. This algorithm can be found in Fluids Functions.

The width of the entire inlet channel is now the sum of the width of the channel delivering water to the inlet chimneys, the thickness of the weir, and the width of the channel delivering water to waste:

Stopping the Flow of Water into the Sedimentation Tank

The inlet chimneys carry water from the inlet channel into the bottom of the sedimentation tank. However, sometimes it is necessary to shut off the flow of water into a sedimentation tank in order to clean it, make repairs, or to close it off if the plant is producing an excess of clean water. For this purpose, control pieces are needed to stop the flow of water into the chimneys. Individual control pieces for each chimney are advantageous because a particular sedimentation tank can be shut off while still allowing the rest of the plant to function normally.

The control piece consists of three parts:

1. A PVC pipe to fit into the coupling of the inlet chimney

2. A cap to cover the top of the PVC pipe

3. A thin, long tube that comes out of the cap and extends above the height of the water in the inlet channel to allow air to escape
 
The constraint for designing this control piece is the diameter of the coupling in the inlet chimney, which is derived from the variable D.SedChimneyPipe, the diameter of the chimney itself. With this variable, and using the information in the pipe database that specifies PVC coupling and cap sizes for a particular PVC pipe size, the dimensions of the rest of parts (1) and (2) of the control piece are determined. The specifications for the long tube are that it is less than 1 inch in diameter and that its height extends above the height of the water in the inlet channel (HW.InletChannel), so that air can flow out of it, and it is easy to pull out.

These chimney plugs are one half of the control pieces being designed to allow for the shutting off and re-filling of a sedimentation tank.