Gracias Entrance Tank Design

Abstract

As the flow rate through an AguaClara plant is increased, the plants design will need to be altered. More specifically, to achieve the required amount of energy dissipation for adequate alum mixing, we would need to redesign the entire entrance tank. Since the flow is so much greater than that of previous plants, the orifice exiting the entrance tank will need to be much larger. This presents a problem as the alum orifice will not be as large, and thus we would require a greater amount of mixing. This problem will be achieved by using two hydraulic drops.

Gracias Entrance Tank Design

Design Schematic

Rapid Mix Chamber

The AutoCAD file for this preliminary design is attached to this page. 

The rapid mix chamber is made of concrete and is attached to one side of the entrance tank. The water within the entrance tank initially enters the horizontal channel through a rectangular orifice. This rectangular orifice will help provide the necessary energy dissipation to create the large eddies that are necessary for global mixing. The water that enters this rectangular orifice will experience free fall into the horizontal channel. Then, another waterfall occurs when the water flows from the horizontal channel into the vertical channel. The water that flows into the vertical channel will be in free fall. The water will experience will experience a contraction at the rectangular orifice that leads into the flocculation tank. This will allow for the necessary energy dissipation to achieve the molecular diffusion of the alum.

Design Assumptions and Specifications

The majority of the assumptions made for this design were the same as in past designs.

When designing the entrance tank, we will assume that the upflow velocity is 700 m/day. This design constraint is based on the work of other subteams, as it is approximately the velocity used to backwash a filter. Using this assumption, along with the flow rate through the plant, it was determined that the tank should have a cross sectional area of 6.171 m ². If we then assume that the tank has square dimensions, we find that the length of one of the sides of the tank is 2.484 m.

For the design of the entrance tank orifice, we will assume that the maximum height of the water from the center of the entrance tank orifice's width is 20cm. Since we are dealing with an orifice, we will also assume that the cross sectional area of the water, as it passes through the orifice, is reduced by a factor of 0.62. Finally, we will assume that the smallest width that is practical to build given that the tank will be built out of cement, is 4cm.

Assuming major headloss is negligible at the upper level of the entrance tank, we used the minor headloss equation along with the orifice flow equation to calculate the orifice area. We substituted the velocity variable in the minor headloss equation with the **** relationship to be able to solve for the vena contracta area of the orifice. This area, though, is the cross sectional area of the contracted water as it is passes through the orifice. To calculate the actual orifice area we divide the vena contracta area by the vena contracta coefficient. Given the width constraint we can calculate the length of the orifice.

The design of the orifice that leads into the flocculator must provide the necessary energy dissipation to achieve the required molecular diffusion of the alum in the water. Energy dissipation rate from minor head loss has to be greater than diffusion requirements: ε = 0.5-1.0W/kg. The following equation is used to determine the minimum energy dissipation required to overcome the diffusion requirements.

The minimum energy dissipation is calculated to be approximately to be 0.733 W/kg. We assume a minimum energy dissipation of 0.8 W/kg is required at the flocculation entrance orifice to be safe during our design process.