Usually, it is hard to find a supplier for humidification columns such as that shown in Fig. 29.3, so they are typically custom-made from a design supplied by the purchaser. Within the interior of humidification columns the water flows downwards through a bed of packing that ensures a high superficial area of contact between the air and water, in order to guarantee saturation at the air outlet.
The column diameter is chosen obeying the criteria of minimum pressure drop and avoidance of flooding. Flooding, namely the accumulation of water at the top of the column, happens in packed humidification columns when the air velocity through the column becomes large enough to stop the liquid from flowing down the bed. The height of the column necessary to ensure saturation can be calculated from mass and energy balances. Once the dimensions are determined then the pressure drop across the column can also be calculated.
Sources of advice on how to design humidification columns are given in the Further Reading section at the end of the chapter.
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