HUMIDIFICATION ABSORPTION DISTANCES
ANCE LEARNING
an external source such as electricity or natural gas to convert water to steam, which is injected into the air stream; an adiabatic system works by dispersing a mist of fi ne cold water droplets or aerosol into the air stream; it uses heat from the surrounding air to convert this mist into vapour. For this reason, when specifying an adiabatic unit, designers should take extra care to ensure suffi cient heat is available in the airstream to vaporise the water spray being added. Droplet separators or eliminator plates should be fi tted beyond the absorption distance to remove any stray droplets. Regardless of the type of humidifi er,
there are many elements that need to be considered in the humidifi cation system’s design. The amount of moisture air can hold
is related to its dry-bulb temperature. An increase in dry-bulb temperature will lead to a corresponding increase in the air’s ability to hold water vapour. The air is saturated when no more water vapour can be added to the air. Generally, the more vapour that needs to be added to the air the higher its temperature will
need to be and the longer absorption distance required. Absorption distance is defi ned as
the dimension from the point where moisture is discharged from the humidifi er lance or nozzle to the point at which wetting will not occur. Beyond this distance, obstructions will remain dry unless they are cooler than the airstream such as in a cooling coil, for example. The absorption distance required for
a particular installation is dependent on many variables. The temperature of the airstream will affect the absorption distance, because cool air absorbs less moisture than warm air and therefore requires a longer distance for absorption. Smaller sized water droplets are absorbed more quickly; steam droplets are the smallest, mist droplets from adiabatic humidifi ers are larger. The temperature difference between the water droplets and the air stream will also impact absorption distance. The speed of the air stream also has an
impact on absorption distance. The faster the airstream the greater the distance covered before full evaporation can
Designers should ensure airfl ow is even across the humidifi er, otherwise this can affect the air’s ability to mix the droplets or vapour
www.cibsejournal.com February 2013 CIBSE Journal 45
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