The pressure drop is largely independent of the tower diameter , but it depends significantly on the nature of the packing. Different packing have different free cross sectional areas. The free cross sectional area , which is related to the void volume increases with packing size for a single type of packing , but differs between packing of the same nominal size , but differrent forms
The introduction of liquid into the tower increases the frictional resistance and viscous drag affecting gas flow and so the pressure drop is greater in the wet tower than in the dry tower. The pressure drop is related directly to liquid flow rate by an equation of the form
However , as the liquid rate and/or gas rates increase the restriction on gas flow suddenly become more significant. At this point , the loading point the influence of gas rate or velocity on pressure drop becomes more significant. The slope of the grape of P vs G increases and the pressure drop rapidly increases , ever for small increments of gas flow. This continues until gas can no longer pass freely up the column , nor liquid pass freely down it. The frictional drag becomes so great that the downward flow of liquid is substantially impeded and the column is said to flood. This can be observed visually . The visual flooding point is sometimes slightly higher than that estimated from pressure drop measurements.