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Air Movement in a Humidified Space

Air movement – quite similar to the movements which we know from the weather around us – occur also in any closed space which is not totally isolated from theoutside – e.g. in a glasshouse, a winter garden, in a cold storage space, in a conference room, but also inside the cabinet of a electronic device. If the climatic conditions of the closed space is changed with regards to the “outside” – e.g. by means of heating, cooling, humidification or desiccation – a local “micro-climate” will be created. This microclimate is not only influenced by the artificial processes, which are installed, but also by outside influences like sunlight, insulation or intake of fresh air. According to the intended use of the space the responsible person will try to create an optimal microclimate, which is beneficial for the “inhabitants” (plants, animals, mushrooms, stored goods). In case it is necessary to humidify the air – by means of a fogging system – a natural air circulation will occur. Everybody knows that hot air has a lower specific weight than cold air.

Hot air will rise up (thermic lift), cold air will fall down. Furthermore we know that hot air can take up more water vapor as well as that hot, unsaturated air will cool down by evaporating fog droplets. With most fogging systems the feeder-pipes together with the fogging nozzles are mounted in the upper parts of the treated space.. Following the application of fog the temperature of the surrounding air will cool down by adiabatic cooling and will sink downwards. It will replace the warmer air layers which have been located so far near the floor which will rise towards the ceiling. Usually a rotating air current will be formed after a short while – parallel to the feeder pipes carrying the nozzles. The rotation will be faster, if the treated air layers are hot and dry and slower if the air is already cool and humid  Therefore larger or smaller water droplets entering the air do NOT induce the air movement, but alone by the cooled air layers which will sink down! In most cases a thorough mixing of air layers in a treated space will occur automatically by means of the adiabatic cooling and resulting convection currents; if not needed for other purposes a fan for moving the air will not be necessary.

Dew-Point

The air tries to take up water in the form of water vapor until it is “saturated”. The saturation amount is dependent of the air-temperature.
If the saturation amount has been taken up – i.e. the saturation amount is contained in the air – it is said that the “Saturation Point” is reached.
In the discussion so far we have taken for granted that we have knowledge about the air temperature. Now let us take an opposite point of view:

  • Suppose we have a volume of air with a known amount of water vapor, yet do not know its temperature.
  • Looking up the “Steam Tables” we can deduct the temperature, at which the air would be saturated.
  • This temperature is called the “Saturation-Temperature” or “Dew-Point”.
  • The dew point is therefore the temperature at which a volume of air with a certain amount of water vapor has reached its maximal water contents – 100%.
  • This temperature may be increased or decreased by heating or cooling.
  • If the temperature increases (with constant water content) it will no longer be saturated and we arrive at relative moisture with values below 100%.
  • If the temperature is decreases, e.g. by cooling, the humidity should increase; it cannot, however exceed 100%, as the air was already saturated with water vapor.
  • The volume of air therefore cannot carry the total amount of water anymore and the superfluous amount of water will fall out – it “condenses”.

An example from Meteorology:

A low-pressure area carries rain-clouds (saturated with water vapor) northwards – from the Mediterranean Sea to the Alps.
When the clouds arrive at the southern slopes of the mountains they will be pushed upwards – into layers of air, which get cooler with increasing elevation. Due to the lowered temperature the dew point will be under run, the superfluous amount of water will fall out as rain. Rain will persist until the air in the cloud cover again reaches saturation.

 

“Evaporation Cooling”

Air can take up water in the form of water vapor. If water is present the laws of physics always try to reach equilibrium and evaporate water to be carried by the surrounding air up to the point when the air is “saturated” with water vapor.
The term “Evaporation Cooling” describes an effect, which occurs during the evaporation of liquids in general. Whenever a liquid evaporates the process needs some additional energy to mobilize the molecules of the liquid and thereby the surrounding matter is deprived of this energy, which results in a cooling effect. The scientific term for this effect is “adiabatic Cooling”.
The process of evaporation and therefore the cooling effect will be enhanced greatly if a stream of dry air transports off the emerging water vapor, so that at any moment in time enough unsaturated air is available to enhance evaporation.

Examples:
Remember how cold you can feel if you have been swimming on a hot and dry day and get out of the water into a windy environment and the water clinging to your skin dries off. Or just dip a finger in glass of water, blow air about your finger and feel how cool this feels.  In case no air movement is present, the surrounding air is quickly saturated, no more evaporation and no more cooling will occur.  If you want to utilize a fogging system to cool by means of evaporation cooling the best effect is not reached by cooling of the air in the cooled area but on the surface of the goods which are to be cooled (plants in a conservatory, vegetable storage etc.). It is important to notice, that the influx of dry air is moderate, otherwise the cooling effects may be too small.

 

Relative Humidity

What does “Relative Humidity” really mean?
One of the major properties of the air around us is the ability to contain water in the form of vapor. The air surrounding us is never really dry – it contains more or less water vapor. This amount of water, measured usually in grams per cubic meter or in grams per weight (g/kg) depends strongly on the temperature of the air. Hot air can take up more water vapors, cold air less.
The maximal amount of water vapors which air of a certain temperature can contain is called the “Saturation Amount”. This means that air at a certain temperature carries 100% of the amount of water is can possibly contain – it is “saturated”. The saturation amount depends on the temperature. Values for the saturation amount have been determined experimentally and can be looked up in so-called “Water-Tables”.

1 m3 air at a temperature of +30°C has a saturation amount of 30,39 g water / m3, whereas 1m3 water at a temperature of +3° can carry only 5,9 g water.
Usually the air around us is not really saturated; sometimes it is crisp and dry, sometimes hot and humid. The well being of all organic life – plants, animals and people – depends on the degree of saturation of the surrounding air.
In all cases the amount of moisture in the air can be given only as a relative figure – depending on the saturation amount of the current temperature.

Weather forecast 30. August, 12:00 hrs. : Air temperature +30°C, relative moisture 65%.
Air at +30°C has a saturation amount of 30,39 g/m3, 65% of this value are currently there, which means 30,39 x 0,65 = 19,75 g/m3

Some recommended values derived from practical experiences:
Application: Relative Humidity:
Propagation of cuttings in Forestry 96% – 60%
Conservatories of Botanical Gardens
(warm- and cold houses)
85%
Cold-storage of Fruit and Vegetable 90 – 94%
Winter gardens, private Gardening 75 – 85%
For people indoors – for instance Hospitals 52 – 55%
Print Shops, Paper Storage 55 – 65%