Spray tips and application quality with knapsack sprayers

According to data from the Rural Registry and Dieese, Brazil has more than 7 million rural properties, of which around a third are smaller than 10 ha. Even on these properties there is a need for mechanization, aiming to increase production capacity and efficiency.

In this scenario, small and large farmers use practices to increase the effectiveness of the use of fertilizers and phytosanitary products, investing in equipment to apply these inputs to crops to preserve productivity.

On properties with a smaller area, smaller machines are primarily used in crops, such as portable equipment, which includes a manual knapsack sprayer. However, this type of equipment is common in properties of all sizes, being considered for different activities, in fundamental work or for small services, with a rural property that does not have at least one manual knapsack sprayer being rare. Therefore, in Brazil this equipment is sold daily by the thousands and used by the millions.

There is little information to help the farmer in choosing the best sprayer model suited to the reality of the crop, and even less in relation to certain parts that make up this equipment, such as spray tips. However, these parts make all the difference to the quality of the application and optimization of the use of this equipment.

Manual knapsack sprayers generally come factory equipped with a spray tip known as a “flat iron”, which consists of a steel disc with a central hole, associated with a core of skewed holes, responsible for the formation of the conical jet.

Models with flat jet nozzles or “fan”, as they are better known, normally have a more uniform and symmetrical spray pattern, compared to the conical jet, and with lower risks of drift, given the size of the droplets.

The working pressure of the sprayers also affects the size of the droplets, and the higher the pressure, the smaller the droplet size. Pressure also affects the spray angle and coverage. Therefore, it is appropriate to work within the recommended pressure range.

 

Spray equipment and improvements in application techniques

Spraying is the main way of applying phytosanitary products to crops, to control weeds, diseases and pests that affect crop productivity.

There are several models of sprayers, from the simplest, such as the manual backpack, to modern self-propelled ones.

Manual knapsack sprayers are often used on small properties for services on a smaller application scale, since their reservoir is limited to a volume that, in general, varies from 5 to 20 liters.

The preference for this equipment among farmers is justified by its availability, low cost and versatility of use. On the other hand, one of the disadvantages is the repetitive physical effort used in work operations, to activate the spraying given by a lever associated with a pumping piston.

This equipment has been improved over the years. The insertion of a pressure valve is considered a significant change, as it provides standardization in working pressure, with the effect of less variation in the jet angle and droplet size.

Despite the evolution in relation to the technology associated with equipment for applying phytosanitary products, the adaptation of the use of this equipment in the field occurs slowly. Its use does not always occur with good agronomic practices, requiring development in operational procedures, the adequacy of accessories and ergonomic aspects.

Faced with the challenge of environmental safety and economic viability of the production system, it is essential to use technology and application resources aimed at optimizing the products applied, since with correct use it is necessary to avoid crop losses due to diseases and pests, as well as minimizing damage from drift.

Drift is one of the main causes of loss of phytosanitary products, with consequences for human poisoning and environmental contamination. Drift can be defined by the movement of a product in the air during or after application to a location other than the planned target. Therefore, it is any particle or product resulting from the application that does not reach the target or that does not remain there.

With spray drift, in addition to the potential environmental risk, unexpected effects may occur on cultivated plants, such as poisoning and changes in growth and leaf anatomy, with damage to the development and growth of seedlings, with a negative effect on crop productivity.

Selecting the best configuration of spray tip models, working pressure and even adjuvants is essential to avoid environmental contamination and loss of the products used.

According to Prof. Tomomassa Matuo, described in his classic book “Application Techniques for Agricultural Pesticides” (1990), the spray tip is the main component of application equipment, as it is the item responsible for the production and dispersion of drops with the product in a spray. , deserving all the care in its selection.

 

Spray tips and working pressure

Spray tips are associated with precision and safety in the application of phytosanitary products. The use of the appropriate and correctly functioning spray tip should result in maintaining the efficiency of the treatment, reducing the need for reapplications.

With distinct volumetric distribution characteristics, the spray tips have specific characteristics for certain application conditions. Therefore, choosing the most suitable spray tip will depend on different work situations.

For the most correct selection of the tip, one must consider the product to be applied and its mode of action (systemic or contact), whether the application will be pre- or post-emergence, in a total area or in bands. Meteorological conditions (wind, temperature and relative humidity) and the sensitivity of the treated crop or its surroundings must also be considered during applications.

The spray tip models feature different jet patterns. The most common are: flat jet (or “fan”), empty cone and full cone.

Cone jet spray nozzle models come in two basic variations: the empty cone and the full cone. The empty cone spray tip generally breaks up the liquid into smaller droplets and operates at higher pressures. These characteristics make empty cone models recommended for applications on more leafy plants, such as crops in full vegetative development. This application deserves special care because smaller droplets are more susceptible to drift. Empty conical spray nozzle models are generally chosen by producers because they result in greater target coverage. However, as smaller droplets are more subject to evaporation and drift, these commonly result in less uniform distribution of the spray over the plants, compared to flat jet models.

Regarding spraying with flat jet models, there are several options, such as those with an extended plane and deflector for full-area application and a continuous plane for directed application. Deflector plane jet models feature a wide angle flat jet pattern, with pressure changes having a significant effect on the width of the spray pattern.

There are also air induction spray tip models that produce droplets in the extremely coarse to ultra coarse class and can reduce spray drift by an additional 70%.

Once the model has been defined, it is necessary to pay attention to the pressure in the sprayer's hydraulic circuit, which changes the size of the drops, the flow rate and the angle of the jet.

Within certain limits, increasing pressure means decreasing droplet size, increasing flow rate and spray jet angle. The variation in the syrup flow respects a quadratic relationship. In other words, to double the flow it is necessary to quadruple the pressure. However, an increase in working pressure in these proportions implies much smaller droplets, modifying the entire transport and deposition characteristics of the drops, with a greater risk of drift. Furthermore, higher pressures can result in greater wear on the spray tips. Therefore, it is recommended to respect the limits indicated in each manufacturer's catalogs, with the performance data normally provided in spray tip catalogs.

Manual knapsack sprayers typically work with pressures ranging between 15 and 60 psi. Due to the proximity of the operator to the spray jet, this variation can mean very different exposures during application.

In this context, to reduce spray drift, nozzles that produce larger drops, operating at lower pressures, can guarantee greater safety for operators.

In relation to prices, when considering the qualitative and operational aspects, the differences justify the use of models with better performance. However, adjustments to the equipment can be evaluated, such as modifying the pumping system using wider pistons to increase the liquid displacement capacity, or even pumping with electric motors. In addition, pressure control valves and syrup agitation systems are available that can help maintain the desired quality for the application.

All of this will help with the results of the application, aiming to maintain the efficiency of the products and the safety of the applicators and the surrounding environment. Good agronomic practices will certainly contribute to the sustainability of the activity.

Alcides Marangoni Junior, Prof. Occupational Health and Safety – IFMA, São Luis, MA; Marcelo da Costa Ferreira, Prof. Application Technology – UNESP, Jaboticabal, SP