Air atomising nozzles for coating applications

Air atomising nozzles operate by mixing a pressurised air stream with the fluid.  The air causes the fluid to atomise into a fine spray.  As air is the primary atomising agent fluid pressure can be kept low allowing for very low flow rates of fine sprays.  In addition the air stream helps to direct the resulting spray and carry it towards the target i.e. it is less likely to drift.  These properties make air atomising nozzles very well suited to a variety of coating applications.

Droplet size

The small droplet sizes achievable with air atomising nozzles means that more even coatings can be delivered.  The smaller a spray's average droplet size the more evenly it will spread out when it hits its target surface.  Without air atomisation the only way to achieve very fine droplets is to increase the fluid pressure but this will also increase the flow from the nozzle and so may result in overdosing or unacceptable pressures. 

Viscous fluids

The spraying of fluids with viscosities much higher than water can present problems.  They tend to need to be sprayed at higher pressures as more energy is required to eject them through the nozzle orifice. When mixing with air, as they are with air atomising nozzles, the resulting turbulent flow within the nozzle can cause viscous fluid to bubble or expand causing problems with spray pattern formation.  A solution this is to mix the air with the fluid after ejection from the nozzle.  These external mix air atomising nozzles allow for viscous fluids to be formed into a good, well-atomised spray pattern.

Air actuation

All our air atomisers can be fitted with air-actuated shut-off systems.  The nozzle has a spring-loaded pin that blocks the orifice preventing spraying.  The spring is only overcome by the presence of an air flow of 1.5 bar pressure or above.  Once the spring pressure is overcome spraying starts.  This means that the fluid feed can be kept pressurised at all times right up to the nozzle tip allowing for a very responsive on/off spraying cycle.

The air actuation system can be connected to an independent air line or can be from the same air feed as the atomising air.  Note: with a single air feed the lower limit of atomising air pressure is 1.5 bar i.e. the air pressure required to overcome the spring action.

Air atomisers – internal mix

The final flow pattern shape will be affected by the mix of air and fluid passed through the nozzle. Changing either of these two variables will affec flow rate atomisation level and pattern shape.  A subgroup of internal mix atomisers would be siphon or gravity-fed nozzles.  With these nozzles the fluid feed is not pressurised.  Fluid is drawn into the nozzle by a siphoning action cause by a reduction in air pressure in the siphon tube which is due to the pressurised air flow in the connected nozzle.  The flow rate of siphon-fed nozzles is controlled by the air flow as well as the height of the nozzle above or below the fluid reservoir.  These nozzles can achieve very low flow rates and do not have any fluid under pressure which can be a safety advantage when dealing with hazardous fluids such as acids.

Air atomising – external mix

The flow rate from these nozzles is determined by the fluid pressure alone.  Atomisation level and pattern shape will be determined by the varying levels of air and fluid pressure.

Air atomising – independent atomising feed

These nozzles have a fluid feed and two independent external mix air feeds.  One air feed controls the level of atomisation the other dictates the shape of the spray pattern.

The flow rate of these nozzles is determined by fluid pressure alone.  Atomisation level is determined by the mix of atomising air and fluid pressure but the shape of the spray remains unaffected.  The shape of the spray will be determined by the pressure of the second air feed. 

Coating engineering considerations

Coating nozzles


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