Since the realisation that dust was an environmental issue, arresting it with water has been the inevitable solution, as it is easy to obtain, store and deliver at the point of dust creation.
Dust has many forms and sizes, depending upon the material from whence it comes, how it is formed and the environment that allows its formation.
Nozzle selection in dust suppression is not an exact science due to the very nature of dust itself; it is in a continual state of flux, its size, volume, velocity and intensity determined by its environment and production outputs.
Dust can be formed from various sources, although it is commonly associated with dry soils, or as a result of hard rock materials. Those wishing to look at spray nozzles for dust suppression should ensure that adding water to their process or plant will not create further problems at that point or further on, or contaminate critical media.
Dust size and type identification and the type of operating environment are two factors that should determine if spray nozzle or water droplet suppression can be used. Generally speaking, nozzles are used to pre-wet material or to suppress the dust when it leaves the confines of barrier type suppression systems (eg enclosed transfer points on conveyors).
In this discussion, we refer to temporary, small and medium spray nozzles, not larger spraying devices, such as stockpile sprays or water cannons. Therefore this discussion relates to applications smaller than large scale stockpile spraying, stacker reclaimers or large scale loading facilities that require large area dust management.
Before describing how we can select the best nozzle for the best possible outcome, a discussion about the basic types, their construction and limitations is required.
There are two groups of nozzles. The first are pressure atomising nozzles. This group has two major categories: flat spray or fan pattern and a conical or cone pattern.
The former group can be broken into full cone patterns and hollow cone patterns, meaning the cone shape they produce is either full of water droplets or simply the outer shape such as with the hollow cone, the reason for which is discussed later.
The second group are air atomising or two fluid nozzles, as they mix the medium (water) with compressed air to form small droplets. Air atomising nozzles, such as the XA series, have advantages in that the droplet size can be controlled by air flow and pressure. Unlike pressure atomising nozzles, they need consistent quality air flows and filtration but in some applications requiring small droplets, these nozzles may be the only option.
To a large extent, application-based nozzle selection will dictate the spray nozzle type. However, there are some basic principles common to all use of nozzles in dust suppression applications, as follows.
WATER QUALITY
In mine site applications, filtering water to ensure its quality creates more maintenance issues than it overcomes. However, some circumstances dictate that the water be as clean as possible; this is certainly the case for nozzles with small orifices or ports, or where the position of the spray nozzles are such that regular maintenance is difficult and therefore must operate until an annual shut-down is scheduled.
OPERATING PRESSURE AND NOZZLE FLOW
Nozzles are metering devices; they produce a given flow at a given pressure. To produce certain droplet sizes, a guaranteed pressure and flow must be provided to the nozzle. Consideration must be given to pipe sizing, pump selection and the dedication of both to the dust suppression system. A simple pressure gauge at the point of delivery can be a helpful tool.
MATERIAL OF CONSTRUCTION
Depending on the operating pressure, the water quality and operating environment, the selection of the most appropriate construction material must be considered. Generally speaking, if water quality is poor, high in salinity or has suppression additives, the harder corrosion resistant materials are used, such as stainless steel, cobalt alloys or ceramics. Plastics can be employed successfully at a reduced cost but in harsh environments and water quality, orifices can wear quickly, destroying spray patterns and flow rates and, importantly, droplet sizes.
NOZZLE PLACEMENT
Generally, one of the things that assist in nozzle selection are the performance tables for the nozzles themselves. They provide the flow for each nozzle at given water or air/water pressure. Also provided will be the spray angle for that nozzle at a given pressure. Once determined, a spray angle coverage table can be consulted, which gives the coverage for that nozzle, at various distances away from the nozzle.
{{image2-a:r}}After this selection process, the number of nozzles required can be determined, and the total number and capacities used to determine pump and/or compressor size and piping requirements.
Nozzle selection and placement will depend on the application at hand and its location. Generally, application types can be broken into
three areas, and will use associated nozzle spray pattern types to suit. If the application is static, then a conical nozzle may cover a larger cross-sectional area. If the product moves under the nozzle, such as on a conveyor, then a fan spray, employed at a right angle across the conveyor, can be considered.
Mostly, the narrower the spray angle, the more penetration into the product sprayed, thus giving secondary dust arrest. It should be noted that, in some cases, such as a coal conveyor or similar, the dust suppression system may need to be sized to account for fire protection functions. Therefore, other system requirements will need examining, eg:
- At the mine site, eg dump points, roads.
- At the processing stage, eg crushing, screening, conveying.
- Loading /handling, eg conveying, transferring, loading.
DROPLET SIZE
As spray droplets do most of the airborne dust particle arresting, correct nozzle selection is critical. Smaller droplets are used for finer dust particles or fine rising dust. These can be achieved with hollow cone spray nozzles as opposed to full cone pattern nozzles, at the same given pressure. However, pressure atomising nozzles that employ high pressures can also produce small droplets. These are best suited in protected areas, where wind drift will not remove droplets from the target, or where dust is extremely fine.
Most of the time, nozzles that form a fog-like droplet size will need the best filtration possible, as their outlet orifice size will be small. However, there is a family of ?impingement? nozzles that employ an impingement wire at the outlet orifice. As they have no internal devices to block and are run from higher pressure pumps, this family of nozzle is gaining more acceptance by dust suppression experts the world over. Although fog systems use less water, the pure volume of material that is producing the dust must be considered. Although companies are mindful of water conservation, some applications need bigger volumes of water to do the job!
Full cone pattern nozzles can be employed in primary crushing, washing and even double as a fixed deluge fire system, but their design should be such as to avoid premature blockage. The best nozzle for this application is the spiral design, or ?pig?s tail?, nozzle. As water deflects off the helical spirals, it produces sets of annular rings, forming a conical pattern. Best known for their low clog attributes, spiral nozzles are available in full and hollow cone patterns. Previously unavailable are ?miniature spiral nozzles?, which give the added advantage of small droplet sizes and low clog resistance, as compared to similar flow designs.
It is commonly accepted that flat fan nozzles are useful in creating ?water curtains? around transfer or loading points and can be beneficial as an adjunct to hard curtains and extraction systems. Due to their design, fan nozzles resist clogging compared to their conical partners. They can also assist in the secondary application of dust suppression of chemicals, belt washing and increased penetration into the material, keeping it wetter longer as it transfers to other stations or conveyors. This series of nozzle can also be supplied in kit form, with special quick attach plastic pipe clamps for quick installations or temporary dust suppression application on machines that are mobile.
Stuart Morgan is a technical sales advisor at Spray Nozzle Engineering.