Developing strategies to reduce spray drift in pneumatic spraying in vineyards: Assessment of the parameters affecting droplet size in pneumatic spraying

Abstract

Pneumatic sprayers are widely used in vineyards due to their very fine droplet size, which makes the drift risk to become an important problem to be considered. The aim of this study was to assess the effect of the spout diameter at the release point on the spray droplet size and uniformity achieved for different liquid flow rates (LFR) and air flow rates (AFR). A test bench was developed to simulate a real pneumatic sprayer under laboratory conditions, and it was empirically adjusted to match the air pressure conditions as closely as possible to real working conditions. Two positions of insertion of the liquid hose, the conventional position (CP) and an alternative position (AP), were tested for three LFRs, 1.00, 1.64, and 2.67Lmin, and four AFRs, 0.280, 0.312, 0.345, and 0.376ms. The air speed decrease between the two insertion points of the liquid hose was measured. A Malvern SprayTec® instrument was used to measure the droplet size, and the D50, D10, and D90 parameter values were obtained. The relative SPAN factor (RSF) was also calculated. A model to predict variations in D50 was fitted using the aforementioned parameters. The results show that variations in the diameter of the spout significantly change the droplet size, producing a mean increase of 59.45% in D50 and similar increases in D10 and D90. The model developed to predict variations in D50 has a very high degree of accuracy (R=0.945). The relative decrease in the air speed along the spout did not present significant differences for the different airflow rates tested. The results of the study show that the droplet size produced in pneumatic spraying can be modified easily by varying the air spout dimensions. This should be taken into account by manufacturers from a design point of view.

Assessing the influence of air speed and liquid flow rate on the droplet size and homogeneity in pneumatic spraying

“…The trial was performed at laboratory facilities of the Department of Agricultural, Forest, and Food Sciences (DiSAFA) of the University of Turin (Grugliasco, Turin, Italy). The methodology used for the droplet size characterization and the trial deployment is carefully described and detailed in previous works . Very briefly, the laboratory arrangement (Fig.…”

“…(c)) and it was aligned with the longitudinal axle of the nozzle, and set at 500 mm distance from the spout (Fig. (c)); based on preliminary tests, 500 mm is the minimum distance at which the droplets size values stay constant and stable.…”

“…Based on information previously acquired, the rotary speed of both test bench and real sprayer Cima 50 Plus 400L need to be different to match the same air pressure conditions. Thus, the relationship between test bench's fan rotary speed and sprayer's fan rotary speed was used to translate usual field‐working conditions to the bench rotation.…”

“…Therefore, the same correlation between both rotation speeds, obtained in a previously deep investigation of cannon nozzle type, was used to set the test bench close to the operative field conditions (Fig. ) …”

“…However, the liquid flow rate is not easy to change, as it depends on the forward speed, set by the operator. Other options that may be considered are (i) the change of the of the air spout size, as the larger the air spout's outlet section, the slower the outcoming air, or (ii) as recently suggested by Miranda‐Fuentes et al ., to change the position of the elements that release the liquid into the air stream. In any case, it is easy to appreciate that, for a given pneumatic sprayer configuration, the modification of the droplet size is very difficult to achieve.…”