Using Fog for Dust Suppression – One of many uses

Fog; microscopic sized water droplets suspended in the air – often an alien tool for many – is finding favour in a broad range of applications. It may not be a panacea for everything but it is proving highly useful in numerous areas.

​The Renby MicronFog™️ system objective was to use it for dust suppression. As experience with the technology has increased, a whole host of other uses have materialised in previously undreamt of applications.

• Haze: Visibility over 2km
• Mist: Visibility between 1km and 2km
• Fog: Visibility less than 1km

What is Fog?

At this time of year in The UK, you are often surrounded by it. That is; microscopic droplets of water suspended in the air. The Renby MicronFog™️ system recreates this phenomenon demand from pure drinking water, there are no additives. To be a fog, mist or haze the water droplet size is the same, it is just the quantity or density of water droplets that varies. The Met Office defines fog or mist by visibility i.e. quantifying the distance that you can see through it:

With MicronFog™️ we can produce anything from a fine haze to a thick fog with visibility falling to a few metres!

The key to fogging or misting is to keep the water droplet size small and therefore light enough to be suspended by the air molecules. To achieve this, the droplet size needs to be in the range 5 to 20 microns. With droplets in this size range, the fog hangs in the air and dwells for some time after the system has been stopped.

This effect cannot be achieved with a sprinkler system where the droplet size tends to be in the region 200 micron and droplets are heavy enough to fall straight to the ground. The small fog droplet size is important for several other reasons as well, more of which will come later.

How is Fog Generated?

Nature has developed a very effective way to produce fogs and mists naturally, by localised changes in temperature, resulting in air becoming saturated and the surplus moisture appearing as a mist. For an industrial process, fog is required upon demand, so a controlled approach is required that can be switched on or off at will.

There are in fact several ways to produce fog artificially. In an increasingly energy sensitive environment, it is important to opt for the lowest energy approach. The solution to this is to pressurise the water to significant pressures – 70 Bar / 1000 psi or more.

The water is then fed through lengths of tubing to a network of nozzles and forced through a tiny orifice in the nozzle to produce a plume of fog. The quality of the orifice is essential to producing an even, fine spray of fog.

There are many cheap nozzles available with rough edges around the opening. These tend to produce over-size droplets and are often identified by an irregularly shaped cone of fog emerging from the nozzle.

Renby MicronFog™️ nozzles use laser drilled ruby orifices to ensure a smooth, round opening every time. With openings as small as 85 microns, it is crucial that the water in the system is clean.

A two stage water filtration system is used to remove particles and sediment from the water. Without good filtration, the nozzles will block regularly and this extra stage pays dividends in longevity of the nozzles.

Likewise, hard water can cause problems, so every system is supplied with an electronic descaler that ensures lime does not precipitate out before the water leaves the nozzle. The electronic descaler also means that there are no softening chemicals to be topped up.

Fog Safety

Legionnaire’s Disease is another important consideration. If a system does not pay full attention to this then avoid it at all costs. It may be cheap in the short term, but who wants to be the one to explain to a family that they have lost a loved one as a result of saving a little bit of money?

As the water droplet size is so small, it and whatever it contains can easily enter the body, therefore, it is essential that the water used in the fog is extra safe. To this end, we insist that mains potable water is used as a supply and then take the supplemental step of further disinfection using ultra violet light. This is interlocked with the system control which prevents the system from operating if the UV lamp is not functioning or has reached the end of its life.

Energy Use

By using high pressure water, the energy requirement is approximately 1/8th of alternative techniques and uses relatively small amounts of power. All systems come with motor inverters to further reduce power consumption. As an example, a major onion packer uses a system that consumes less than 1.25kW power to suppress dust in a building with an area of 1100m2 and uses 28 nozzles.

At the other end of the scale, a biomass store with a floor area of 6000m2wet uses 5.5kW low flow nozzles. In both cases, the systems must not wet the product in the building.

This has been achieved through the use of careful nozzle placement and electronic controls on the system. Those familiar with dust collection systems will find these power figures remarkably low, making fog a cost effective solution for dust suppression in large warehouse type areas.

Dust Suppression with MicronFog™️

This a major use for fog. Many industrial processes generate dust. Once airborne, this will hang in the air for hours and can be breathed in by operatives, pollute the process and clog equipment.

Fog works as a dust suppressant due to the small droplet size. This is a similar size to the dust particle enabling the two to stick together when they collide. Typically, several dust particles adhere to the water droplet. As a result, the overall weight of the agglomerate becomes large enough to fall out of the air, removing the respirable airborne dust with it.

MicronFog™️ nozzles can be placed around the source and in certain cases can drop the dust back into the process. In other applications, for instance warehousing, a more general arrangement can be configured at roof height. Any airborne dust is then captured and dropped to the floor.

There is often concern about wetting the product but MicronFog™️ has been used successfully on dry powder bagging lines, potato and onion packing. In each of these applications, it is essential that product remains dry. If not, the product becomes unsaleable.

At present, there is a Government public education campaign about COPD (Chronic Obstructive Pulmonary Disease). Whilst smoking is a cause of this, breathing in workplace dust is another common cause. There are already regulations in place limiting exposure to silica dust to 0.1 mg/m3.

There is discussion about reducing this further to 0.05 mg/m3. Fog has been used successfully to bring the levels below this tighter requirement and will be an essential tool to meet the stricter requirement.

Humidification and Cooling

Another effect that occurs with fog is flash evaporation. Once again, due to the small droplet size, the fog droplet can rapidly evaporate into the air. As the flash evaporation occurs, there is a corresponding drop in temperature.

In a building, this can occur after a few minutes, but it has also been observed within seconds on a ducted system. For instance, when a fan is used to force air into a process, the compression can cause the air to increase in temperature.

By introducing fog downstream of the fan, cooling will occur due to flash evaporation. We have observed temperature drops from 40oC to 25oC within seconds on process air.

For food cultivation, the control of humidity is an essential ingredient for promoting crop growth.

MicronFog™️ replaced a low pressure spray system that was humidifying a group of polytunnels used for mushroom growing. Not only was the result a more precise control of humidity levels but it also reduced water usage as the older technique was wasteful.

When required, the system can be expanded to measure and control the humidity in each tunnel. This versatile function enables the system to provide precise humidity levels in each tunnel, responding to the crop requirements at each stage of the growth cycle.

Landscape and Architectural

Whilst not strictly an industrial process, the versatility of the system is such that it can produce great effects for art installations and sculptures. The ability to switch fog on and off at will facilitates the design of a whole new area of kinetic sculptures providing an immersive experience to visitors.

The system has been used to provide effects ranging from banks of fog for visitors at a tropical rainforest attraction to atmospheric effects for film sets. The image shows a kinetic sculpture designed by a leading UK sculptress, renowned for some of Europe’s largest kinetic sculptures.

Conclusion

The objective of this article has been to give an insight into the breadth of applications that can use fog. Fog offers a comprehensive range of options that can be tailored to address all manner of applications. When using fog in any application it is essential to ensure that due consideration is given to water safety. MicronFog Teconomical and low energy solution for all manner of uses.