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Once the materials and fittings are decided upon, this information needs to be incorporated into the Automated Design Tool. The goal is for the design tool to eventually include the CDC and to have a material list and AutoCAD drawing of the installation included as part of a standard design package.
Design Upgrade of the NDC
An NDC was installed in the water treatment plant in Algateca. This was the first time an NDC was used in a real world application. This device was installed on top of the entrance tank and was only designed to administer alum to the plant. Aglateca is currently using an LDC for the administration of chlorine.
The ultimate goal is to design an NDC that is a stand-alone device which is capable of controlling all liquid chemical feeds for the plant. Currently, this includes alum and chlorine but will eventually include saturated lime. The mechanics of this design have yet to be worked out and are the challenge of this sub-team. Ideas and concepts to help with the initial stages of design:
- "Stand-alone" means the NDC would no longer be installed on top of the entrance tank but would be more centrally located in the plant. The float for the NDC could be in an 8" diameter pipe which is connected to the bottom of the entrance tank via a 1/2" PVC pipe. The use of the 8" pipe would require that the float move in a pure vertical motion but would significantly save on space.
- The current float attachment point (one end of the lever) does not provide pure vertical motion . As the lever responds to min and max flow rates, the vertical location of the chain shifts approximately 15 cm. Connecting the float to a pulley instead of a lever arm would provide true vertical motion, allowing the float to move vertically withing the confines of an 8" pipe. This could be accomplished using stainless steel bike chain and sprockets (both available through McMaster Carr) or any other type of pulley-cable arrangement.
- A dosing lever is still needed, even in the new design. The dosing lever would eventually need to accommodate three process chemicals. Triple-scale, triple-orifice for three chemicals all mounted on one dosing lever seems quite cumbersome. One solution is to build a structure to support a shaft that is supported between two bearings. One end of the shaft could be connected to the center point of the pulley and the dosing arms would be connected to the shaft in a fashion similar to our current installation.
Additionally, Monroe recently provided a document with his thoughts on some of the mechanics involved. The complete document can be found in CDC Summer 2010 Google Docs but to summarize a few key points:
- The float could be attached by cable to a pulley mounted on the ceiling. The cable would be connected to the dose controller side of the lever arm since the cable (after returning from the ceiling) and will move in the opposite direction as the water level changes. The range of lever motion would be adjusted by moving the attachment position. The slight angle of the cable from the pulley on the ceiling to the lever would be inconsequential.
- This float could be in the entrance tank rather than in a separate connected tank! Two pulleys mounted on the plant ceiling could transfer this motion to the CDC allowing the CDC to be placed wherever is most convenient. Each pulley adds a potential friction point as corrosion and dirt accumulate.
- Have a float attached to a cable that wraps around a very large pulley that is connected to a lever arm. This introduces error into the lever displacement. The pulley changes the equation of motion. Instead of applying a change in height to the point of connection with the lever, the float applies a change in angular rotation. These would be similar for small angle displacements, but there will be significant error for large displacements.
- The float could be a large diameter, shallow float with a small submerged weight that gives it stability. The float must have a large submerged AREA so that the depth of submergence is not changed by more than perhaps 1 cm when the dosing slides are moved. The buoyant weight of the slides (including the vertical drop tubes) will be larger if they are not submerged in water. Thus the float area will need to be larger if the CDC isn't mounted on top of the entrance tank (where the drop tubes were partially submerged).