Droplet spectrum at different vapour pressure deficits1¹

Espectro de gotas em diferentes déficits de pressão de saturação de vapor d’água

ABSTRACT

An efficient pesticides spraying depends a lot in psychrometric conditions, mainly if it is using fine droplets, once climatic conditions may cause droplets evaporation and consequent financial loss to the farmer. Thus, the aim of this work was to determine the droplet spectrum depending on the vapour pressure deficits. The work was carried out inside of a climate chamber to obtain the vapour pressure deficits (VPD_air). The laser particle analyzer, model Spraytech, was used to determine the droplet spectrum, and the experiment was conducted in factorial scheme 5 x 20, consisted of five working pressures (100; 200; 300; 400 and 500 kPa) and twenty VPD_air (2.3; 3.2; 4.2; 5.6; 7.0; 7.4; 9.5; 11.7; 12.7; 15.8; 16.4; 16.9; 21.2; 22.1; 22.2; 28.1; 29.7; 36.9 39.4 e 51.6 hPa), in completely randomized design with five replications. There is influence of VPD_air on droplet spectrum behavior. Increasing the VPD_air reduces the percentage of sprayed volume comprised by droplets with diameter between 100 and 200 µm, between 200 and 300 µm, between 300 and 400 µm, between 400 and 500 µm and between 500 and 600 µm. Increasing VPD_air increases the VMD, Dv₉₀, SPAN and the percentage of sprayed volume comprised by droplets larger than 600 µm.

Key words:
Droplets diameter; Droplets evaporation; Hydraulic spraying; Spraying technology

RESUMO

Uma pulverização de agrotóxicos eficiente depende muito das condições psicrométricas do ar, principalmente quando se utiliza de gotas finas, uma vez que condições climáticas adversas podem gerar evaporação de gotas e consequente prejuízo ao agricultor. Assim, objetivou-se com este trabalho determinar o espectro de gotas em diferentes déficits de pressão de saturação de vapor d’água no ar. O experimento foi realizado dentro de uma câmara climática para permitir a obtenção dos déficits de pressão de saturação de vapor d’água no ar (DPV_ar). Para a determinação do espectro de gotas, empregou-se o analisador de partículas a laser modelo Spraytech, e o ensaio foi conduzido em esquema fatorial de 5 x 20, constituído por cinco pressões de trabalho (100; 200; 300; 400 e 500 kPa) e vinte DPV_ar (2,3; 3,2; 4,2; 5,6; 7,0; 7,4; 9,5; 11,7; 12,7; 15,8; 16,4; 16,9; 21,2; 22,1; 22,2; 28,1; 29,7; 36,9 39,4 e 51,6 hPa) no delineamento inteiramente casualizado e com cinco repetições. Houve influência do DPV_ar no comportamento do espectro de gotas. O aumento do DPV_ar reduziu a porcentagem do volume pulverizado composta por gotas com diâmetro entre 100 e 200 µm, entre 200 e 300 µm, entre 300 e 400 µm, entre 400 e 500 µm e entre 500 e 600 µm. O aumento do DPV_ar aumentou o DMV, o Dv₉₀, o SPAN e a porcentagem do volume pulverizado composto por gotas maiores que 600 µm.

Palavras-chave:
Diâmetro de gotas; Evaporação de gotas; Pulverização hidráulica; Tecnologia de aplicação

INTRODUCTION

One of the most important barriers to obtaining quality spraying is evaporation of the droplets before they reach, or even before they are absorbed by the target, whether this be the crop or a weed. This can result in the inefficient or ineffective application of pesticides, leading to losses for the farmer, impacts on the environment and the poisoning of people.

The evaporation of droplets, together with drift, can represent a 45% loss in the volume being applied (CHAIM et al., 1999), and although droplets can be lost by evaporation under any weather conditions, losses are less at lower temperatures and under conditions of higher relative humidity (COUTINHO; CORDEIRO, 2004).

Nascimento et al. (2012) observed that spraying with fine to medium droplets resulted in significantly lower deposition at times when the temperature is 34.2 °C and the relative humidity is 29%, compared to when the temperature is 18 °C and the relative humidity is 78%, but saw no significant differences for sprays of large droplets. Balan et al. (2008) and Souza; Castro and Palladini (2007) obtained similar results, noting that when spraying with fine droplets, deposition of the mixture was reduced for an increase in temperature and a decrease in relative humidity.

Such evaporation take place because the conditions of low relative humidity and high air temperature cause the rapid evaporation of droplets, and consequently increase losses due to drift (TOBI et al., 2011). Drift occurs mainly with droplets smaller than 100 so that these droplets serve as a more efficient indicator of drift than the VMD (ARVIDSSON; BERGSTROM; KREUGER, 2011).

Therefore, in order to prevent evaporation, the ideal time for spraying should be when psychrometric conditions show air temperatures of between 15 and 30 °C and a minimum range of 3 to 7 km h^-1, in order to avoid convective air currents when the speed is low, and drift at higher speeds (RAETANO, 2011).

It is not only the psychrometric conditions of the air and the diameter of the droplets that influence evaporation, but also the surface to be deposited, such as leaves with hydrophobic, hydrophilic, waxy or hairy surfaces, and the droplet composition, with these containing surfactants or drift reducers (GIMENES et al., 2013; XU et al., 2010a; XU et al., 2010b; XU et al, 2011; YU et al, 2009a; YU et al., 2009b).

The vapour pressure deficit in the air (VPD_air) is often used in studies related to droplet evaporation to characterise the psychrometric conditions of the air, this being the difference between the pressure of saturated water vapour in the air and the partial pressure of water vapour, which are both directly related to the relative humidity and air temperature (RODRIGUES et al., 2011; VIANELLO; ALVES, 2012). Arvidsson, Bergstrom and Kreuger (2011) state that the rate at which water droplets evaporate depends almost entirely on the droplet diameter, and on the VPD between the droplet surface and the surrounding air.

Due to the great influence of the psychrometric conditions on spraying, the aim of this work was to determine the droplet spectrum as a function of the vapour pressure deficit, in order to guarantee effectiveness and maximise efficiency in the application of pesticides.

MATERIAL AND METHODS

The experiment was carried out at the Department of Agricultural Engineering on the campus of the Federal University of Viçosa in the state of Minas Gerais, Brazil. The climate in the region, according to the Koppen classification, is type Cwa, i.e. mesothermal (warm temperate), with rainy summers and dry winters, and an average temperature for the warmest month of over 22 °C (VIANELLO; ALVES, 2012).

The experiments were conducted inside a climate chamber in order to obtain the psychrometric conditions used in the study. The chamber has a volume of 9.7 m³, and can control relative humidity in the range of 30 to 90%, and the air temperature from 0 to 40 °C.

The vapour pressure deficit in the air (VPD_air) was calculated with the Tetens equation (RODRIGUES et al., 2011; VIANELLO; ALVES, 2012), and obtained from the difference between the saturation pressure of water vapour in the air (e_s) and the partial pressure of water vapour (e). To calculate e_s, the value for air temperature was considered, as per Equation 1.

where: e_s = saturation pressure of water vapour in the air (hPa); and t = air temperature (°C).

The value for e was then determined from the relative humidity and the value for e_s.

where: e = partial pressure of water vapour in the air (hPa); and RH = relative humidity (%).

Finally, the VPD_air was obtained as shown below, by the difference between e_s and e.

where: DPV_air = vapour pressure deficit (hPa).

The conditions of temperature and relative humidity to obtain the DPV_air were set up intentionally to represent situations both favourable and unfavourable to spraying, in order to evaluate the effect of the temperature and relative humidity on the spray. Temperature values of 20, 25, 30, 35 and 40°C were therefore combined with values for relative humidity of 30, 50, 70 and 90%, making it possible to evaluate the evaporation of the sprayed liquid under 20 distinct conditions of DPVair (2.3, 3.2, 4.2, 5.6, 7.0, 7.4, 9.5, 11.7, 12.7, 15.8, 16.4, 16.9, 21.2, 22.1, 22.2, 28.1, 29.7, 36.9, 39.4 and 51.6 hPa).

To measure the psychrometric conditions of the air inside the climate chamber, a humidity and temperature probe was used (model HMP60, Vaisala, Woburn, MA, USA), which has a measuring range of 0 to 100% and -40 to + 60°C. The analogue signals of the electrical voltage from the two sensors were converted into digital signals by means of a microcontroller (model ATmega328, Duemilanove, Arduino, Ivrea, Turin, Italy) and then sent to a computer by serial data transmission.

The working pressures used in determining the droplet spectrum were 100, 200, 300, 400 and 500 kPa. These results were obtained with a stationary sprayer (model S-12, Yamaho, Diadema, SP, Brazil), having a rotation of 800 rpm, nominal flow rate of 12 L min^-1, power of 0.75 to 1.12 kW, and maximum pressure of 3516 kPa. The sprayer was driven by an electric motor (model F56H, Weg, Jaragua do Sul, Santa Catarina, Brazil), with a rotation of 3,570 rpm and power of 1.5 kW. The droplets were analysed by a laser particle analyser (model Spraytech, Malvern Instruments Ltd., Malvern, Worcestershire, UK). This instrument has a focal length of 750 mm, and uses the technique of laser diffraction for measuring the size of the sprayed droplets. This technique measures the intensity of light that is scattered as a laser beam passes through a sprayed jet. The data are then analysed to calculate the size of the droplets that created the scattering profile. The analyser was calibrated to count droplets in the size class of 0.10 to 2,500 µm.

Study of the droplet spectrum included the following indicators: VMD, D_V10, Dv₉₀, span, and the percentage of spray volume containing droplets smaller than 100 µm (%V < 100), droplets with a diameter between 100 and 200 µm (100 < %V < 200), a diameter between 200 and 300 µm (200 < % V < 300), a diameter between 300 and 400 µm (300 < % V < 400), a diameter between 400 and 500 µm (400 < % V < 500 ), a diameter between 500 and 600 µm (500 < V% < 600), and finally, droplets greater than 600 µm (% V > 600).

The liquid was sprayed over the particle analyser in such a way that the entire jet from the nozzle passed transversely across the light beam emitted by the laser. To do this, a spray boom was mounted on the analyser, and an electric motor installed at the end of the boom to allow the boom to revolve, ensuring the complete analysis of the jet from the nozzle. The spray boom consisted of a nozzle and pressure gauge. The electric motor installed at the end of the boom was a type CEP, of 12 V and 7.5 A, with a torque of 25 to 48 mN, and a maximum speed of 10.47 rad s^-1 (100 rpm).

For the experiment, five similar hydraulic nozzles were used (model Lurmark LD11002, Hypro EU Ltd, Longstanton, Cambridgeshire, UK), which were chosen at random from a set of 20 nozzles, as per ISO 5682-1 (1996). These nozzles were positioned on the spray boom at a distance of 0.50 m from the laser beam emitted by the particle analyser, representing the height of the spray boom in relation to the crop. The water used throughout the experiment was obtained directly from a tap, as is recommended by ASAE standard S572.1 (2009).

The experiment was carried out in a 5 x 20 factorial scheme, comprising the 5 working pressures and the 20 values for DPVair, in a completely randomised design, with five replications. The data were evaluated using response surface methodology. The models were chosen based on the significance of the regression coefficients by t-test, adopting a significance level of 10% for the coefficient of determination and for the behaviour of the variance under study.

RESULTS AND DISCUSSION

The VMD, which is the primary parameter studied in the droplet spectrum, increased with the increase in VPD_air and reduction in the operating pressure, with the lowest value being 152.77 µm at a pressure of 500 kPa and VPD_air of 2.3 hPa, and the highest being 256.92 µm at a pressure of 100 kPa and VPD_air of 51.6 hPa (Figure 1a). In a similar way to the VMD, the Dv₉₀ and the span increased with the increase in VPD_air and reduction in working pressure (Figures 1c and 1d). Yet the Dv₁₀ displayed a negative trend with the increase in working pressure and VPD_air. At high pressures however, an increase in VPD_air generated an increase in Dv₁₀, this increase being produced by the total evaporation of the small droplets (Figure 1b).

Figure 1
Estimates of the volumetric median diameter, the droplet diameter below which is found 10% of the total volume of sprayed liquid, the droplet diameter below which is found 90% of the total volume of sprayed liquid, and the span, with estimates of the percentage of spray volume containing droplets smaller than 100 µm, droplets with a diameter from 100 to 200 µm, a diameter from 200 to 300 µm, a diameter from 300 to 400 µm, a diameter from 400 to 500 µm, a diameter from 500 to 600 µm, and droplets greater than 600 µm, for the VPDair and working pressure. (a) VMD; (b) Dv10; (c) Dv90; (d) span; (e) %V < 100; (f) 100 < %V <200; (g) 200 <%V < 300; (h) 300 <%V < 400; (i) 400 < %V < 500; (j) 500 < %V <600; (k) % <600

With the increase in VPD_air a reduction is expected in droplet diameter, since such an increase favours evaporation; but results show that the VMD increased due to a complete evaporation of the very fine droplets. Alvarenga et al. (2013), working at a pressure of 1,055 kPa and with a JA-2 nozzle, reported that the Dv₁₀ and Dv₉₀ were reduced with the increase in VPD_air. However, the droplet spectrum was determined using water-sensitive labels, which are not reliable when evaluating a droplet spectrum, since in that situation the droplet diameter is measured from spots that form on the labels, so if two or more droplets should be deposited at the same place, the droplets will coalesce, and the actual diameter will be overestimated.

The span depends on the values of Dv₁₀ and Dv₉₀. When there is a great difference between these values, the span is increased. Alvarenga et al. (2014) however, observed a reduction in the span from the JA-2 nozzle when increasing the VPD_air from 7.6 to 26.3 hPa; but these authors used water-sensitive labels to evaluate the droplet spectrum. The application of pesticides should be carried out with hydraulic nozzles that have a low span, with a value close to or less than one, since this value ensures that more droplets of a similar diameter are produced.

Keeping the VPD_air constant, the percentage of spray volume comprised of droplets smaller than 100 µm increases with the increase in working pressure. However, maintaining the pressure constant at low pressures, the increase in VPD_airproduces an increase in the percentage of spray volume composed of droplets smaller than 100 µm. At high pressures, the increase in VPD_airleads to a reduction of this percentage (Figure 1e). This reduction is derived from the complete evaporation of the smaller droplets, and consequently increases the value of Dv₁₀.

The percentage of spray volume composed of droplets with a diameter between 100 and 200 µm was reduced with the increase in VPD_air and reduction of the working pressure (Figure 1f). This shows that nozzles that produce fine droplets actually have poor deposition on the target, as found by Balan et al. (2008), Nascimento et al. (2012) and Souza; Castro and Palladini (2007).

The increase in both VPD_air and working pressure reduced the percentage of spray volume composed of droplets with a diameter between 200 and 300 µm, between 300 and 400 µm, between 400 and 500 µm, and between 500 and 600 µm (Figure 1g, 1h, 1i e 1j). Evaporation is helped by raising the VPD_air; therefore, a decrease in the percentage of spray volume composed of droplets of these diameters, for an increase in VPD_air, means that the sprayed droplets are evaporating. Probably, such evaporation is only not greater because part of the volume lost through the evaporation of droplets in the diameter range of 200 to 300 µm is compensated by the evaporation of droplets in the range of 300 to 400 µm, and so on.

The percentage of spray volume composed of droplets greater than 600 µm increased with the increase in VPD_air and reduction in the working pressure. However, the VPD_air had a much greater influence on this variable than did the working pressure (Figure 1k).

An increase in the spray volume comprising droplets greater than 600 µm with the increase in VPD_air occurs due to the high evaporation of smaller droplets. Thus by eliminating the small droplets, there is a reduction in the total spray volume, and consequently, an increase in the percentage of spray volume composed of extremely coarse droplets, since these are less affected by evaporation compared to the finer droplets.

This increase in the percentage of spray volume consisting of droplets larger than 600 µm explains the increase seen in the values of VMD and Dv₉₀ for an increase in VPD_air, these consequently altering the span.

Moreover, the reduction in droplet diameter with an increase in working pressure takes place because as the pressures increases, the amount of liquid that crosses the nozzle orifice is greater, resulting in greater fractionation of the liquid, and the increased production of droplets with reduced diameters.

CONCLUSIONS

1. The increase in VPD_airreduces the percentage of spray volume composed of droplets with a diameter from 100 to 200 µm, from 200 to 300 µm, 300 to 400 µm, 400 to 500 µm, and from 500 to 600 µm, and increases the VMD, Dv₉₀, span and percentage of spray volume made up of droplets greater than 600 µm;

2. Because the droplet spectrum changed with the VPD_air, these data show that under critical psychrometric conditions, the climate has a direct influence on the behaviour of the sprayed droplets; before each application of pesticide therefore, it is necessary to be aware of the psychrometric air conditions to ensure that spraying the pesticide is effective and efficient.

• Extraído da Dissertação de Mestrado do primeiro autor e apresentada no Programa de Pós-Graduação em Engenharia Agrícola da Universidade Federal de Viçosa

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Publication Dates

• Publication in this collection
  Mar 2016

History

• Received
  09 Apr 2014
• Accepted
  24 Sept 2015