Dust is a common by-product of many industrial processes. How serious a problem dust is depends upon the process and the nature of the dust. At one end of the scale, production of dust can be merely an unpleasant inconvenience - at the other end of the scale it can pose a severe risk to humans and equipment. At the more severe end of the scale the reduction of dust contamination is often mandated by regulatory legislation, so effective dust control is a business-critical application.
There are five key engineering considerations when finding the right spray solution for your dust control application.
- Do you need preventative or symptomatic control?
- What is the dust particle size?
- What are air flows in the dusty environment like?
- Do you need to avoid wetting?
- What coverage do you need?
Preventative control stops dust becoming airborne before it becomes a problem. A spray is applied to the surface that is liable to generate dust, wetting it. The moisture prevents dust from forming. Symptomatic control attempts to remove dust once it is already airborne by spraying a finely atomised mist so that the droplets of the spray will hit dust particles and remove them.
The fine particles that make up a dust will tend to flow past suspended water droplets if the droplets are considerably larger. If the spray is too fine because droplets are smaller than the dust particles then dust suppression will not be optimum.
The motion of air in the dusty environment can have a significant impact on the efficacy of a spray. Fine sprays are more easily carried and disrupted by air flows so, in a windy environment for example, solutions such as a nozzle which delivers a broad spectrum spray or which utilises compressed air to give the spray more momentum are needed.
In many dust suppression applications it is undesirable to cause excessive wetting to the source of the dust. For example, processing dried food stuffs may cause dust and, whilst this needs to be prevented, the source material needs to remain dry. In order to achieve this, an equilibrium between the delivery of the dust-suppressing fluid being applied and the removal of the fluid by evaporation needs to be established.
Often dust is produced over a large area and so any dust suppressing spray needs to be distributed at a wide angle. An alternative strategy to achieve the necessary spray coverage is to project the spray using fans or air atomising nozzles.
Nozzle selection for dust suppression applications
Air Atomising Nozzles
These nozzles use a secondary fluid or gas to break up the primary spraying fluid into an exceptionally fine spray (atomisation). The secondary fluid is usually air, but can be any gas.
Fine sprays at very low flow rates
An advantage of these over direct pressure nozzles is that they can form a finely atomised spray pattern at very low flow rates. With impingement, spiral and whirl chamber designs the energy required to break up the fluid flow into a hollow cone pattern comes directly from the fluid being sprayed. This means that there is a natural restriction to minimum flow rates. With air atomisation a secondary source of energy is introduced in the form of pressurised gas. This is used to break up the fluid into the spray pattern. As such, atomisation and spray patterns can be maintained with much lower fluid flows from this type of nozzle.
Greater Control
With air atomising nozzles a greater degree of control can be exerted over the size of the droplets by varying the air pressure. This variation is achieved independently of flow rate so the droplet size mix can be adjusted to match the dust particle size.
Improved Reach
Air atomisers also can help overcome problems of spray drifting. The fine sprays required for effective dust capture are very prone to being swept away from the target area by crosswinds. If the spray is produced by a standard hydraulic nozzle then the vast majority of the fluid's internal energy will have been used to break it up into droplets. This means the spray has very little momentum and hence is at the mercy of any air currents. Air atomising nozzles use compressed air to break up the fluid but also use the air pressure to project the spray forwards, meaning they can deliver very fine sprays many metres, even in windy conditions.
Spiral Nozzles
Spiral nozzles work by impacting the fluid onto a protruding spiral shape causing it to shear and break apart into droplets.
A relatively small average droplet size can be achieved with spiral nozzles. But within the mix of droplet sizes there exist some larger droplets. These tend to concentrate in concentric rings of higher spray density. This means that spiral full cone nozzles do not produce a very even spray but the presence of rings of heavier droplets serves to carry the smaller droplets along, meaning that spirals can still deliver fairly fine sprays to areas unreachable by other nozzles. In dust suppression applications this means they are suitable for more windy environments. If wetting is a problem then the low flow rate spiral variants may be required.
Axial Whirl Nozzles
Axial whirl nozzles break up the fluid into a spray pattern by setting the fluid into a whirlpool motion inside the nozzle by forcing it through a shaped internal vein. The droplet sizes are relatively coarse compared to the other dust suppression nozzles discussed in this section and so this would mean they are only suitable for dust prevention applications, i.e. they are useful for wetting surfaces.
If a moving conveyor of product requires wetting, a series of narrow angle nozzles might be appropriate. For large areas of wetting, wider spray angle axial whirls will be required, although it is likely that low flow rate variants should be selected to avoid over-wetting.
Misting Nozzles
Small orifice misting nozzles
Small orifice misting nozzles are suitable for symptomatic dust control as they produce sprays with a large number of droplets of a similar size to many common dust particles. The sprays from these nozzles have almost no impact and momentum. As such, they would be unsuitable for dust suppression in windy environments. The limiting factor for this method of producing a mist is blocking of the orifice. As the production of the mist is entirely due to the small orifice it has a very small free passage and any small contaminants in the fluid may cause a blockage. Effective filtration is required if there is any chance of contamination by particles.
Impingement misting nozzles
With this design of nozzle the fluid is ejected from the nozzle orifice and impacts upon a pin in the direct path of the water flow. This breaks up and disrupts the flow into a fine spray cone. As the fluid is being broken up by impact this allows for a larger orifice, thus allowing for more viscous or 'dirty' fluids to be sprayed into a fog pattern. One potential downside of this design is the propensity for dripping to occur under the impingement pin.
This design of nozzle is ideal for producing very fine droplets and produces the finest droplets of any direct pressure nozzle. For finer droplets still, however, pressurised air (or other gas) needs to be used to atomise the fluid so air atomising solutions will be more suitable.
For more information or advice on the best dust suppression solution for your application, please contact SNP’s technical engineers on 01273 400092 or email info@spray-nozzle.co.uk.
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