DISTA
The correct absorption distance is essential for a humidifi er to function effectively and to avoid dripping ductwork, explains Andie Chessun, group chair of FETA’s HEVAC humidity group
f the many factors that need to be considered in the design and installation of a building or factory
humidifi cation system, one of the most important is humidifi er absorption distance. With the correct absorption distance, mist or steam introduced to the airstream by a humidifi er can be evaporated, or mixed suffi ciently, to prevent water droplets precipitating or water vapour condensing at obstructions, where the airfl ow is more turbulent, such as dampers, turning vanes, attenuators or fi lters. The wrong absorption distance, however, will lead to incomplete absorption. Not only will this lead to poor humidifi cation control but incomplete absorption can result in ductwork becoming wet, which could lead to it rusting or even to water dripping from ducts into occupied rooms or industrial spaces. Worse, wet surfaces are susceptible to mould and microbial growth, which could affect occupants’ health or result in contamination of
industrial processes. With a basic understanding of the factors that affect absorption, however, it is possible to enjoy all of the benefi ts of humidifi cation without the need to worry about wet ductwork. Humidity is fundamental to ensuring
a comfortable, healthy environment in which people can live and work. Correct levels of humidity improve occupancy comfort, reduce static electricity and provide the optimum environment for material handling and industrial processes. Humidifi cation is about increasing the amount of water vapour in the air – see box: four reasons why humidity control is important. As a general rule, the humidifi cation load is the amount of moisture required to treat outside – or make-up – air supplied to a space. There are two main types of humidifi er: steam (isothermal) and cold water humidifi ers (adiabatic). A kilogram of water needs about 2500kJ of energy to change to vapour; this change of phase can occur either isothermally or adiabatically. An isothermal humidifi cation system uses heat from
44 CIBSE Journal February 2013
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