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Fog, what is it?

What is a Fog?
A fog is a suspension of small liquid droplets in gas (generally air) characterized by its droplet size distribution (which can vary greatly). However, people tend to describe fogs qualitatively, using such descriptive terms as dry fog (10-15 micron volume mean diameter), wet fog (20-30 micron VMD), mist (30-60 micron), fine spray (above 60 micron), etc.

The stability of a fog can vary widely depending on the liquid (composition, vapor pressure, surface tension and density), particle size distribution, droplet density, air currents, sunlight, air temperature and condensation surfaces. Fortunately, you don't have to predict fog behavior. Just thinking about your application will usually indicate what kind of fog would be best (and sometimes even the best equipment to use).

For example, a "dry" fog of small particles is superior when droplets must diffuse widely, when wetting would be harmful and in gas contact applications (because smaller droplets have a greater surface to volume ratio). On the other hand, a wet fog would be better for dampening, particulate settling (dust control) and humidification. And high pressure spray nozzles (see next section) would likely suffer increased plugging and maintenance costs in a dusty or dirty environment.

How Are Fogs Made?
There are three common fog generation techniques (a fourth combines two of these):
Thermal foggers evaporate an oil-based liquid such as light kerosene on a hot surface. Upon exiting the nozzle, oil vapors condense into droplets to create a fine fog. Thermal foggers cannot dispense water-based products and are generally limited to outdoor use.

High pressure spray nozzles can deliver a wide range of particle sizes, depending on liquid pressure and nozzle opening. For fog output, typical liquid pressures are 500 - 1,500 psi, and orifices 0.005 inch or smaller. Spray nozzles can handle both oil- and water-based liquids but require high grade filtration to protect against nozzle plugging. Over time, they are also susceptible to nozzle wear and increased droplet sizes.

Mechanical foggers produce small droplets by turbulent air shear in the fogging nozzle. The typical air source is a high velocity fan, usually integrated with the fogging nozzle as a stand-alone component. Liquid pressures are low, and since nozzles have no small orifices, plugging is usually not a problem. However, exit noise (about the same as a vacuum cleaner) may make a mechanical fogger inappropriate when a noiseless discharge is required.

The air atomizing nozzle is a hybrid of the spray nozzle and the mechanical fogger. Liquid is first broken into coarse droplets with a low pressure nozzle, then further atomized with compressed air. The air atomizing nozzle therefore needs both a liquid pump and an air compressor. As an offset, its larger liquid orifices are less susceptible to plugging.


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