This is the seventh of eight articles from our nozzle knowledge series. In this article, we look at what adding air into a nozzle does. Prefer to watch a video version, please click here. Or, if you would like to download a pdf version, click here.
The basic working principle of an air atomising nozzle is that there is no longer a single feed into the nozzle. Air is introduced alongside the liquid due to be sprayed and this creates a fast moving air flow that entrains, atomises and shapes the spray. The basic spray properties are dramatically changed.
Air atomisers will tend to produce smaller droplets than other spray nozzles. With very high pressures from hydraulic nozzles, it is possible to get somewhere close to them. But air atomisers are an efficient way to achieve very small drop sizes at relatively low fluid pressures. The drop size is no longer dictated by the fluid pressure. The primary factor is the amount of air and air pressure that’s put in. Air pressure is the primary factor in atomisation and droplet size.
SPRAY PATTERN
With air atomising nozzles, variable patterns are created depending on the air fluid mix. Most hydraulic nozzles need the spray pattern to be stable. It varies a little bit with pressure in some nozzles. A great variation of spray angles and patterns can be generated from a single air atomising nozzle, depending on the air fluid mix. Some designs even have a separate air feed for the shaping of the spray and dictation of the spray angle. The air feed is independent from the atomising air feed which is used to control the droplet size. Overall, there is a great degree of control due to the second variable that plays with the air.
The velocity of the drops is dramatically increased because they are entrained in the fast moving air flow and will maintain a velocity quite a lot further from the nozzle. But because they're so finely atomised, there isn’t much of an effect on impact. Air atomisers wouldn’t usually be used in any kind of application that required impact. Air doesn't have a positive effect on impact per se, because any of the gains made are lost by the atomisation effect. However, it does have a noticeable effect on reach.
When comparing a misting nozzle with an air atomising nozzle, the misting nozzle rapidly loses its coherence. It only forms a coherent spray pattern very close to the nozzle. The air atomising nozzle holds its spray pattern a lot longer before it descends into a fog. This is ideal for applications such as dust control that need the spray projected across large areas.
For coating applications that have air currents moving around the factory and a finely atomised coating spray being sprayed, it is possible that the hydraulic nozzle, even very close to the target, could get wafted off target. Air atomisers have that entrainment within the air to help keep things on target.
LIQUID DISTRIBUTION
With hydraulic nozzles, centrifugal effects and other similar factors will often naturally separate out some of the big droplets and smaller droplets. This results in spray pattern variation. Depending on the nozzle design, this variation mean quite a big variation. But air atomisers create a more even distribution because the weight of the drops and centrifugal type forces do not have such an effect. The pattern is also more even.
For more technical nozzle and spray information, head to the resources page of our website here.
About SNP
SNP aims to take the hassle out of procuring small engineering components - spray nozzles and spray systems - with expert, friendly, fast advice and product specification.
SNP supplies spray nozzle solutions to industries worldwide, including the largest engineering, food processing, chemical and petrochemical companies, as well as a vast array of small and medium sized enterprises (SMEs). The company’s product range includes tank washing nozzles, hydraulic nozzles, air atomising nozzles, custom piping assemblies and spray accessories as well as disinfecting and washdown systems.
SNP is as skilled in advising design engineers at the beginning of projects as it is recommending and supplying replacement products that may be needed in a hurry. The company can also provide innovative custom solutions, including using non-standard materials. SNP holds ISO 9001 certification.
SNP is an exclusive distributor for US spray nozzle manufacturer BETE, for pre-treatment specialist Uni-Spray, specialist paper industry nozzle manufacturer ML Gatewood and tank cleaning systems manufacturer Dasic. In addition, SNP supplies specialist spraying equipment from Guarany. Orders are handled quickly and efficiently from SNP’s own dedicated fulfilment facilities in the Midlands.
01273 400092
info@spray-nozzle.co.uk
www.spray-nozzle.co.uk
Five ways to prepare for your first day
If I may add my own personal Tip No. 6 it goes something like this: From time to time a more senior member of staff will start explaining something...