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The Physics of Humidification

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basic Physics

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Humidity

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High Pressure Fogging

What is fog really and which physical effects can be utilized with fog ?

Without discussing the various types of fog which are known to meteorology we can say that fog consists of finest water droplets which are so small that they are able to hover in air saturated with water vapour ( see also Relative Humidity ).

Natural fog occurs with droplets of some 0,12 mm (120 micron), for utilization in various areas of interest the diameter of the droplets should not exceed a diameter of 30 micron (30 thousands of a millimetre).

The most frequent application of fog is simply humidification. The central criterion for successful humidification is the "surface of evaporation". This means that the smaller the droplets are the larger their combined surface gets and therefore the fast the water is able to evaporate into the surrounding air.

Fast evaporation makes controlling the process much easier ( refer to remark 1 )

Fogging techniques are also used often to wet surfaces - for instance to water plants by this means, for dust binding, etc). This can be done with water alone or using the water as a carrier medium for additives - like disinfectants, pesticides, fungicide, ... In other cases simply a fluid medium is atomized to aid some physical or chemical process - for instance an acid.

Generally known and experienced is the physical process of evaporative cooling. By means of evaporation of water in hot and dry air heat energy is used up whereby this effect can be utilized for cooling purposes (refer to Evaporative Cooling )

The physical effect that water gives off heat when it freezes is not as good known as evaporative cooling. The phenomenon can be used to protect fruit orchards very efficiently from hoar-frost
( refer to Frost Protection )

In Architecture and Artwork the last years have shown that fog is often used to create interesting visual effects; however depending on the environment and the desired effect the used medium is not always water-fog, but also frozen carbon dioxide (dry ice) and alcohol-fog (disco-fog).

Artificial creation of Hovering Fog :

To create artificially a free-floating fog it is necessary to provide water droplets with a maximal diameter of 35 micron (thousands of a mm). To achieve this objective a substantial amount of energy is needed (refer to remark 2).
The needed energy may be provided by various means - e.g.

o In Hospitals hot steam is utilized - mainly for reasons of sterility;
this by far the most expensive method.

o Home-appliances with small throughput are utilizing the so called "piezoelectric effect" (oscillating crystals) and generate "cold steam" by means of ultrasonic oscillations.

o Other systems utilize a strong air current from a fan moving across an unheated water surface.
With "pad cooling" and "air washers" an air current is pushed thru a layer of water droplet
This method unfortunately creates the risk of germs and bacteria growing in the reservoir-water, which may be expelled with the cooled and moistened air (e.g. legionary disease, ...). To avoid this risk the water has to be laced with disinfectants.

o Dual-Media Nozzles utilize for atomization of the main medium ( for instance water, otherwise acid or something else) a carrier medium - usually air or technical gasses and work by means of the "Venturi Principe" by creating suction with a partial vacuum

o Single-Medium Nozzles work with rather high pressures (up to 120 bar) and atomize the medium directly by means of special nozzles which are manufactured from Plastic, metal or even ceramics. Most nozzles are either vortex- or impact-nozzles.

Which Processes are used by Plantfog ?

The processes utilized by Plantfog to create hovering fog are in most cases based on high pressure pumps driven by motors. According to the extent and application profile we provide pumps in pressure stages of

o 20 bar - the small systems "tropic", "exotic" and "c-flo"
[ ... please read more at Low Pressure Systems ]

o aggregates up to 100 bar (High Pressure Systems)

The fog itself is created in special vortex nozzles developed by Plantfog in 1986 which are manufactured from high-grade stainless-steel alloys.

For certain applications mainly in the areas of agriculture and fruit plantations we recommend a special venturi nozzle developed during 2003 (refer to Agriculture).

Another interesting area is the application for medicinal and pharmaceutical purposes where the needed feeding pressure is provided by technical gasses from high pressure flasks (without pumps or electrical connections).

remark 1:
A water spherical droplet with a diameter of 1 mm has a surface of 3,1415mm².
If this droplet is atomized into particles of some 20 thousands mm diameter (20 micron) some 125.000 micro-droplets will appear whose total volume is the same as the droplet of 1 mm diameter - their combined surface area however will be 157,07 mm² , about 50 times larger !

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remark 2:
For single-medium nozzles the following rule applies: the higher the operating pressure of a good nozzle is the finer the exhausted fog will be created.

We want to stress this rule because some products still maintain the opinion they can provide decent fog at tap-pressure (some 4 to 6 bar) or with very low pressures of some 10 bar.

To a certain degree this is not really wrong.

Each decent nozzle will provide at any applied pressure a certain percentage of fine particles (aerosols). The main quality-parameter for fog is however the spectrum of droplet diameters, where microscopic particles should dominate.
This spectrum can be attained only at high pressures.
Therefore we want to emphasize that these low-pressure systems should not be called fogging systems - instead they are spraying- or sprinkling-systems because the majority of droplets are not small enough to create a hovering fog.
We do not want to downrate such systems - they have a lot of defined areas of application.
We want however to emphasise the difference because often customers do not have enough knowledge of basic physics to be able to assess the performance of a system.