Rotary atomisers

Rotary atomisation produces the most uniform atomisation of any of the aforementioned techniques, and produces the smallest maximum particle sise. It is almost always used with electrostatics and at lower rotational speeds the electrostatics assist the atomisation. At higher rotational speeds the atomisation is principally mechanical in nature and does not depend on the electrical properties of the coating material. If the viscosity of a coating material is sufficiendy low that it can be deUvered to a rotary atomiser, the material can generally be atomised. The prime mover is usually an ak-driven turbine and, provided that the turbine has the requked power to accelerate the material to the angular velocity, Hquid-dow rates of up to 1000 cm /min can be atomised using an 8-cm diameter beU. 

Rotary atomisation produces an excellent surface finish. The spray has low velocity, which allows the electrostatic forces attracting the paint particles to the ground workpiece to dominate, and results in transfer efficiencies of 85—99%. The pattern is very large and partially controlled and dkected by shaping ak jets. The spray when using a metallic cup has relatively poor penetration into recessed areas. Excessive material deposited on the edges of the workpiece can also be a problem. 

Pressure used in pressure atomisers Centrifugal energy used in rotary atomisers Gaseous energy used in twin-fluid or blast atomisers... 

Figure 16.21. Characteristic rotary atomisers (a) Sharp-edge flat disc (b) Bowl (c) Vaned disc (d) Air-blast...

Although wide drop-size distributions can sometimes be an advantage (Hislop, 1983) the large numbers of small drops produced in hydraulic nozzle sprays can result in spray drift and inadequate targeting (Miller, 1993). Rotary atomisers (Bals, 1975) can reduce the breadth of drop-size distributions and provide a more targeted size distribution. They have not been widely adopted in broad-acre ground crops, because their spray volumes and drop trajectories often cause control difficulties, but they have found some acceptance in orchard sprayers. 

Hand-held rotary atomiser application systems are able, in some circumstances, to operate at volume application rates that are low enough to enable operation with formulations that do not require dilution prior to use. In this case, the container of formulated product can be connected directly to the application system using an arrangement that minimises any loss or dripping when full containers are loaded or empty ones removed. 

Figure 5.11 CDA rotary atomiser herbicide sprayer. Photo Micron Sprayers...

Figure 8.5 CDA atomisers (courtesy of Micron Ltd) the Ulva+ on the left represents the culmination of many years of small rotary atomiser development, and is robust and versatile. The Microdyne atomiser (section right) underwent much testing, but was never commercially marketed...

Figure 8.7 Droplet size spectra water +0.1% Agral, atomised by the Herbi rotary atomiser, compared with three hydraulic nozzles. The hydraulic nozzles were scanned diagonally through the centres of the spray fan (as indicated in the centre right diagram)...

Figure 8.7 shows the droplet size spectra produced by nozzles in the study by Thornhill et al. (1995). They achieved lowest contamination by controlling pressure at 100 kPa, which, like the newer low-drift nozzles such the Turbo Teejet , produce larger spectra than standard flat fan atomisers. However, these settings simply shift the droplet size spectra out of the size range known to be most efficient for pesticides (e.g. Matthews, 1992 Knoche, 1994). The only way to reduce drift and maintain efficient dose transfer is to narrow the droplet spectrum with the optimum range illustrated using nozzles such as the Herbi rotary atomiser. 

In cooperation with the Karlsruhe Institute of Technology (Prof. Schuchmann) and the Technical University of Dortmund (Prof. Walzel), the impact of atomisation on the oil droplet size distribution was investigated using a pneumatic nozzle or a rotary atomiser. Variation of the protein content at a fixed dry matter... 

Fig. 2.22 Median of the spray droplet size of single and bilayer emulsions atomised at different energy inputs by (left) pneumatic nozzle or right) rotary atomiser. Reproduced and adapted with permission from [51]...

Atomisation was performed with three different atomisers namely a Caldyn pneumatic nozzle (1.9 bar, Caldyn CSL A, 2.5 mm) for 20 pm droplets, a Niro rotary atomiser (4.1 bar, 24,000 rpm. Type 010084-0001, Niro Atomiser, Copenhagen, Denmark) to generate droplets of 50 pm and a self-constructed laminar operated rotary atomiser (LamRot [29]) for the production of 130 pm droplets. A closed feed stock vessel was used to avoid an increase of the feed concentratiOTi by evaporation of water. Inlet temperatures were set to 100 °C and 140 °C, which lead to a corresponding outlet temperature of 70 C and 100 °C, respectively [31],... 

The impact of droplet size was studied with a pilot scale spray dryer in order to investigate the influence of droplet size on the morphology of spray dried mannitol carrier particles irrespective of the size and drying capacity of the spray tower. Three different atomisers as mentioned in Sect. 2.2.1 were used to generate droplets of approximately 20, 50 and 130 pm. Table 14.3 gives the droplet size distributions measured in an offline laser diffraction system. The Caldyn nozzle shows the smallest mean droplet sizes with X5o,3 = 19.1 pm, followed by the Niro rotary atomiser with X50.3 =48.9 pm and the LamRot with X50.3 = 130.2 pm. Width of size distribution decreases in the same order [31, 49]. 

Table 14.4 shows the appropriate particle size distributions for 70 and 100 °C outlet temperature. As expected from droplet size results, the LamRot atomiser generated the largest mannitol particles followed by the Niro rotary atomiser and the Caldyn nozzle. 

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