Phantom Air Blast Sprayer Conversion to Spinning Disk Introduction The Phantom airblast sprayer is typical of airblast sprayers used in Desert Locust control. However, this sprayer (and similar types) were originally designed for use as orchard sprayers at high volumes (5 l/ha or more), and are not very well suited to locust control. The high velocity airblast was originally intended to aid penetration of droplets into fruit tree canopies. There are two inherent problems with this type of machine when adapted for ULV spraying. Firstly, as they are designed for high volumes, achieving the consistent low flowrates that are required for Desert Locust control can be difficult. Secondly, drop production is by air shear, in which the liquid is injected into a high velocity airstream when shatters the liquid into small drops. However, the nature of this atomisation process is inefficient, resulting in a wide droplet spectra both very large and very small droplets are produced. These large droplets constitute a significant volume of the total spray, and due to their size, will fall close to the emission point. The very small droplets on the other hand, have a tendency to drift for many hundreds of metres. In both cases, loss of the product through inappropriate droplet size can be a serious constraint to achieving optimal control of the target insect at the minimum dosage. Spinning disk atomisers are widely used in ULV spraying, as they are able to produce droplets within a narrow spectra, and it is possible to select the required droplet size by altering the speed of rotation of the disc. This Technote describes a series of trials in which a Phantom airblast sprayer has been converted so that droplet production is by spinning disc atomiser. System Components A Phantom airblast sprayer consists of a large tank (6 l), and to produce the airstream and pump energy, a diesel engine. Assorted pipework and valves control the flow of liquid to the outlet of the airstream. At the exit of the high velocity air, 5 airblast nozzles are arranged in a pattern of 4 outer and one central nozzles. In ULV spraying, only the centre nozzle is used. The Micron X-1 spinning disc atomiser was specifically designed for use in high velocity airstreams, either aircraft mounted or in airblast orchard sprayers in order to apply low volumes of liquid. It consists of a impeller driving a disc (Plate 1). Speed of rotation at a given windspeed can be controlled by altering the angle of the blades of the impeller. Normally the X-1 mounts directly onto a hydraulic nozzle body. In the case of the Phantom, there was no convenient system of mounting an X-1, so additional pipework was made up to allow the X-1 to be mounted centrally at the exit of the airstream. For safety reason, a plastic housing was mounted around the X-1 (Plate 2). Field Trials A series of drop deposition field trials were conducted with converted and unconverted machines in order to examine whether any improvement in deposition was apparent. Standard physical model deposition trials methodologies were used. Oil sensitive cards were placed at
5 m intervals at 9 o to the wind direction. The spray material used was diesel, with Uvitex OB included to allow easier visualisation of the droplets on the cards. In one comparative trial, pipecleaners were used to collect spray, which was then washed off and used to measure the volume of spray deposited. Rotational speed of the X-1 was measured using an electronic tachometer. Application was made crosswind, at 12 km/hr. Flowrate in all cases was 1 lmin -1, equivalent to a volume application rate of 1 l ha -1 at a velocity of 12 km hr -1 and a track spacing of 1 m (typical parameters used in Desert Locust control). Emission height was 2m The area chosen for the trials was a flat area, without any vegetation. Results Trial 1 Drops Deposited X-1 Disc Speed 16 RPM Windspeed X-1 6.92 ms -1 Windspeed Unconverted 7.88 ms -1 6 Trial 1 5 4 3 2 1
Trial 1 - Volume Deposited (Relative Values) 1 Volume Deposited - Phantom Sprayer Trial 1 9 Volume Deposited - Relative Values 8 7 6 5 4 3 2 1
Trial 2 Drops Deposited X-1 Disc Speed 7 RPM Windspeed X-1 2.5 ms -1 Windspeed Unconverted 2.59 ms -1 5 Trial 2 4 3 2 1
Trial 3 Drops Deposited X-1 Disc Speed 7 RPM Windspeed X-1 8.32 ms -1 Windspeed Unconverted 8.62 ms -1 16 Trial 3 14 12 1 8 6 4 2
Trial 4 Drops Deposited X-1 Disc Speed 7 RPM Windspeed X-1 8.49 ms -1 Windspeed Unconverted 7.84 ms -1 16 Trial 4 14 12 1 8 6 4 2
Discussion This series of work has highlighted some interesting points. It is clear that in Trials 1 and 2, the deposit obtained, in terms of number of drops cm -1 was better with the X-1 than he unconverted machine; however, in both cases the total number of drops collected from both versions was comparatively low. In the case of Trial 1, a relatively high windspeed, coupled with very small droplets produced as a result of the high rotational speed of the X-1 could have meant that most droplets remained airborne and were not deposited in the target zone. However, the number of drops from the unconverted machine was also low, and if compared to Trials 3 and 4 at similar windspeeds, seems surprising. There is no obvious explanation for this. For Trials 3 and 4, under comparatively high windspeeds (about 8 ms -1 ), there was little to choose between the versions, although in Trial 3 the deposit from the unconverted machine did not tail off as rapidly as with the X-1; a similar pattern, although not as pronounced, was seen in Trial 4. Only one volume study has been undertake to date. This was done during Trial 1. The result indicates that there is a difference in the deposit patterns between the X-1 and the unconverted machines when volume of spray is considered. The deposit from the unconverted machine is characterised by two major peaks (at about 4 and 7 m downwind), and smaller peaks at 2 and 13 m downwind. The X-1 version has a more even pattern with a sole major peak at about 6 m downwind and minor peaks at 15 and 35 m. This would suggest that there is a more homogenous droplet distribution from the X-1, as the peaks in deposit from the unconverted machine may arise from only a single large drop being caught on the collectors. Two problems were noted with the converted machine related to engineering. A clip on the initial X-1 fitted to the Phantom failed at moderate airspeeds, causing the X-1 to become detached at high rotational speeds. Fortunately, the protective housing prevented any further damage (although the X-1 was destroyed). However, the protective housing itself causes problems with droplets from the X-1 hitting the protective mesh and dripping off. Obviously the is not acceptable, and solutions are being actively sought. Further Work The work to date has indicated that there is potential for improving the deposit from the Phantom sprayer by converting it to use the X-1 as the atomiser, in place of air shear nozzles. However, to date the data obtained is not sufficiently clear to allow definitive conclusions to be drawn. Further trials are required to examine whether any improvement in deposit justifies the costs of conversion.