Surface Tension
The affect of surface tension has the potential to seriously hinder the effective and accurate dosing of alum in AguaClara plants. Surface tension is caused by the cohesive forces of liquid molecules at the water surface. The concept of surface tension can be visualized by the figure below.
Figure 1: The affect of surface tension
As can be seen in the figure above, a certain head, h, is required to overcome surface tension and form a water droplet. The graph above indicates this relationship between the height required to form the water droplet and the diameter of an orifice subject to the water head.
The relationship shown in the graph above is represented by the formula:
Unknown macro: {latex}
$$
\Delta h = {{2\sigma } \over {\rho g
Unknown macro: {d over 2}
}} - 2{d \over 2 \over 3}
$$
where:
Unknown macro: {latex}
$$\Delta h$$
= the height of water above the orifice
Unknown macro: {latex}
$$\sigma $$
= Surface tension, force/length
Unknown macro: {latex}
$$\rho $$
= density of alum, mass/volume
Unknown macro: {latex}
$$d$$
= Diameter of the orifice, length
The affect of surface tension can manifest itself in errors in the calculated head values in the dosing system. A certain headloss on the lever arm corresponds to a certain flow rate of alum. If surface tension increases the necessary head required to achieve this flow rate, underdosing will result. This concept can be shown in the figure below:
