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:
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$$
\Delta h = 2\sigma } \over {\rho gr - {{2r} \over 3}
$$