The liquid head pressure drop includes the effects of surface tension and of the frothing on the tray. It is typically represented as the product We know that flow = velocity  area, so we can calculate the flow area from the known vapour flow rate and the desired velocity (a fraction of flood). This area needs to be increased to account Correlations are available for the aeration factor (); a value of 0.6 is good for a wide variety of situations. The height of liquid on the tray is the sum of the weir height and the height of liquid over the weir. The total height can be p L  p V p V ⎝ l weir⎠ calculated directly from the volume of liquid on the tray and its active area. Another approach is to back the height out of a version of the Francis weir equation (which relates flow-off a tray to liquid height and weir length). One version, for a straight weir, in units of inches and gal/min is: 2/3 minimum reflux ratios because this is approximately the region of minimum operating costs (more reflux means higher reboiler duty).