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Specifying the correct absorption distance for a humidifier is essential if wet and dripping ductwork is to be avoided as Andie Chessun explains
Of the many elements that need to be considered in the design and installation of a room's or building's humidification system, one of the most important is absorption distance.

With the correct absorption distance, mist or steam introduced to the airstream by the humidifier can be evaporated or mixed sufficiently to prevent the water droplets precipitating or the water vapour condensing in areas of turbulence caused by obstructions such as dampers, turning vanes or an attenuator.

Get the absorption distance wrong, however, and incomplete absorption will result in poor humidification control. Worse, it could result ductwork becoming wet, which could lead to rusting, or even to water dripping into offices or industrial spaces. Wet surfaces are also susceptible to microbial growth, which can give rise to health issues. However, with a basic understanding of the factors that affect absorption it is possible to enjoy the benefits of humidification without the need to worry about wet ducts.

Humidity is fundamental to ensuring a comfortable, healthy environment in which people can live and work. Humidification is about increasing the amount of water vapour in the air. Correct levels of humidity will improve occupancy comfort, reduce static electricity and provide the optimum environment for material handling and industrial processes.

There are two main types of humidifier: steam (isothermal) and cold water humidifiers (adiabatic), which spray unheated aerosols or water droplets. Regardless of the type of humidifier, there are many elements that need to be considered in design and installation of a humidification system in a room or a building's HVAC system.

As a general rule, a humidification load is based on the amount of outside, or make-up air, entering a space. A kilogram of water needs about 2,500kJ of energy to change to vapour. This change of phase can occur either isothermally or adiabatically. An isothermal humidification system uses heat from an external source such as electricity or natural gas to convert water to steam, which is injected into the air stream. An adiabatic humidification system disperses a mist of fine water droplets or aerosol into the air; it uses heat from the surrounding air to convert this mist into vapour.

The amount of moisture air can hold is related to its dry-bulb temperature. As dry-bulb temperature is increased, so is the air's ability to hold water vapour. When air is saturated, no more water vapour can be added to the air without some condensing out as liquid. The more vapour that needs to be added, the warmer air or the longer absorption distance needed.

Absorption distance is the dimension from the leaving the humidifier nozzle to the point at which wetting will not occur. Beyond this distance obstructions in the airstream will remain dry, unless they are cooler than the airstream.

Absorption distance depends on many variables. The temperature of the airstream impacts on absorption distance because cool air absorbs less moisture than warm air and will require longer absorption distances. The smaller the size of the water droplets, the more quickly they will be absorbed; steam droplets are smallest, mist droplets in adiabatic humidifiers are generally larger. Absorption distance is also affected by the temperature difference between the droplets and the air stream.

Impact on absorption

Air speed also has an impact on absorption, higher speeds provide less time for evaporation. Distances can be shortened by increasing the number of humidifier injection nozzles to maximise dispersion into the airflow. However, uneven airflow across the humidifier can affect the air's ability to mix the droplets or vapour, which can negate this advantage.

While all these variables impact on absorption distance the basic rule is the longer the unobstructed length of ductwork after the humidifier the more time the droplets will have to evaporate. For steam humidifiers, the absorption distance is the point at which the steam has been absorbed into the air steam. For spray and atomising humidifiers, eliminator plates should be fitted beyond the absorption distance.

Recommended absorption distances are available from manufacturers for each type of humidifier. They can be calculated accurately by using either an absorption distance nomogram or an absorption distance.

Once the absorption distance is known the location of the humidifier can be determined. For adiabatic units extra care needs to be taken to ensure sufficient heat is available to vaporise the water spray being added, which in most systems will be downstream of the heating coil and upstream of the cooling coil.

Where space in an air handling unit is limited, a simple device such as Hygromatik's Vortex Steam module can be placed in front of a steam manifold to spin the air; this will help it mix with the steam to shorten absorption distances by up to 50 per cent.

Finally, with the absorption distance correctly specified the last challenge for any designer is to ensure that the humidifier is installed correctly on site. Absorption distances are the same regardless of whether you are a specifier or installer! Too often humidifiers are installed in the most convenient location, which is not always in the most appropriate location. However, with the system designed correctly and the humidifier properly installed a comfortable, healthy, humidified environment awaits.

The author is national sales manager at HygroMatik and group chair of FETA's HEVAC humidity group
3 January 2013

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