These rules are just for preliminary checks & estimates and should not be followed as design criteria, because design criteria is always different from thumb rules depending on specific conditions & detailed design calculations.
- Packed towers almost always have lower pressure drop than comparable tray towers.
- Packing is often retrofitted into existing tray towers, to increase capacity or separation. Thus same size of packed towers can handle more than tray towers.
- For gas flow rates of 500 ft3/min (14 m3/min) use 1 in (2.5 cm) packing, for gas flows of 2000 ft3/min (57 m3/min) or more, use 2 in (5 cm) packing.
- Ratio of tower diameter to packing diameter should usually be less than 15.
- Due to the possibility of deformation, plastic packing should be limited to an unsupported depth of 10-15 ft (3-4 m) while metallic packing can withstand 20-25 ft (6-7.5 m).
- Liquid distributor should be placed every 5-10 tower diameters along the length for pall rings and every 20 ft (6.5 m) for other types of random packing.
- Packed columns should operate near 70% flooding.
- Height Equivalent to theoretical stages (HETS) for vapor liquid contacting is 1.3-1.8 ft for 1in pall rings and 2.5-3 ft for 2.0 in pall rings.
- Design pressure drop should be as follows