passage through the tower the packing can be a monolithic structure manufactured for a specific tower but it is much more frequently manufactured as discreet pieces from less than 1 inch to about 3 inches in size the size of the packing is selected depending on the diameter of the tower the amount needed to fill a section to the desired depth is dumped into the column to create the packed tower since water is fed to the top of the packed section and allowed to flow downward over the packing while air is caused to flow upward past the falling water it is called counter current the selection of the packing is critical to the performance of an air stripper the selection includes decisions on the type of packing manufacturer and specific design the material of construction and the nominal size a good packing promotes a high interfacial area between the water and the air as the water follows a tortuous path downward through the packed section of the tower it is important that the individual pieces not become entangled with each other bridge because that can lead to a high pressure drop and or preferential pathways for the water to follow rather than breaking up the flow and creating newly exposed surface area for mass transfer an.