Chapter 28

Pesticide Application and Equipment in Greenhouses

Sprayer Components

Sprayers come in a large range of types and sizes, from small, hand-held sprayers to large, self-propelled machines. While there is such a large variety, there are some basic components that are found on nearly all types of sprayers. The basic components used in liquid application systems include tank, pumps, filtration devices, fans, and nozzles.

Tank

The spray tank should be of an appropriate size for the type of sprayer used and the volume of pesticide mixture required for the area to be sprayed. The shape of the tank should allow for easy access for filling and ease of drainage and cleaning.

Pump (liquid flow)

The pump must deliver the necessary flow to all nozzles at the desired pressure to ensure uniform distribution. When selecting a pump, consider the pressure ranges the pump can handle, the gallons per minute it can supply, its resistance to corrosive damage from pesticides, ease of priming, and power source availability. It’s a good idea to choose a slightly oversized pump.

Agitation System

Many chemical formulations consist of fine powders or particles that need to be held in suspension in the chemical mix. If the mix is left to stand, these particles may settle on the bottom of the tank. A system to agitate or mix the chemical is therefore required. This is usually achieved by recirculating some of the spray mix back to the spray tank.

Filtration System

Filters are required to prevent nozzle blockage. Blockage results in wasted time, increased risk of chemical exposure if nozzles or filters require cleaning in the field and poor coverage in the field if individual nozzle blockages are not detected. Factors such as the water source, pesticide formulation and pump agitation capability influence the type of filtration system required for the sprayer.

Pressure Regulators and Control Valves

Liquid flow rate and pressure to nozzles must be controlled to ensure that sprayer output is consistent. This is generally achieved by use of pressure regulators and/or control valves. These may be operated manually or electronically, particularly for the larger sprayers.

Fans

Fans are used to move spray into plants to enhance the uniformity of pesticide deposition on the plants. Air movement also displaces leaves and branches, which aids spray penetration and increases the exposure of surfaces to spray. The air stream helps to atomize the spray and assures spray droplet velocity, which increases impingement (sticking) of very small spray droplets to the plant canopy. There are three main types of fans: axial flow, centrifugal, and cross-flow.

Nozzles

The term “nozzle” is used in a wider sense of any device through which spray liquid is emitted, broken up into droplets and dispersed over the target. The nozzle regulates the flow rate, atomizes (breaks up) the mixture into droplets, and disperses the droplets in a specific pattern. Nozzle selection is one of the most important decisions to be made related to pesticide applications. The type of nozzle affects not only the amount of spray applied to an area, but also the uniformity of the applied spray, the coverage obtained on the sprayed surfaces, and the amount of drift. Each nozzle type has specific characteristics and capabilities and is designed for use under certain application conditions. Nozzles usually have several components, including a body, cap, strainer, disc and core (orifice and whirl plate). Nozzles vary according to capacity (gpm or gph), spray pattern angle, and shape of spray pattern. Unfortunately, no one nozzle can cover every type of application.

Nozzle Materials

Nozzles are made from several materials. The most common are brass, nylon, stainless steel, hardened-stainless steel, tungsten carbide, thermoplastic, and ceramic. There are advantages and disadvantages with each type of material. Brass nozzles are relatively inexpensive, but they wear rapidly with abrasive materials, such as wettable powders and liquid fertilizers. Stainless steel and hardened stainless steel are the most resistant to wear, but their expense discourages some users. Frequent replacement of brass nozzles usually makes their use more costly in relation to the area sprayed.

Nozzle Nomenclature

There are many types of nozzles available, with each providing different flow rates, spray angles, droplet sizes and patterns. Some of these spray tip characteristics are indicated with a four- or five-digit number designation on the tip. Remember, when replacing tips, be sure to purchase the same tip number, thereby ensuring your sprayer remains properly calibrated. For example, the TeeJet “11004 nozzle” has a 110-degree spray angle and applies 0.4 GPM at the rated pressure of 40 psi (See Figure 28.1).

Nozzle Types

Nozzles can be categorized as follows: (1) hydraulic nozzles, (2) air-shear nozzles, and (3) rotary atomizers (controlled droplet applicators). Hydraulic nozzles create droplets by the expenditure of hydraulic pressure; air-shear nozzles require the moving air to create a venturi effect, which pulls the liquid into the air stream and forms droplets; and rotary atomizers use centrifugal force generated by a rotating cage or disc to create droplets.

Hydraulic Nozzles. Hydraulic nozzles operate on the principle of driving a liquid under pressure through an orifice considerably smaller than the diameter of the feed line. The change from large to small diameter results in a large increase in the liquid’s velocity, which in turn causes the stream of liquid exiting the nozzle to become unstable and to break up into small drops. Hydraulic nozzles consist of a body, cap, filter, and tip. There are many types of hydraulic nozzles available for spraying pesticides. Major nozzle manufactures market several ranges, each available with an extensive set of differenct flow rates, distribution patterns and spray angles so that users can select the correct nozzle for a specific application.  On all hydraulic nozzles, the pressure of liquid at the orifice will have an effect on the flow rate, spay angle, and the droplet size. Hydraulic spray nozzles fall into three basic categories: (1) cone, (2) flat-fan, and (3) air induction.

Air-Shear (Air-Assisted) Nozzles.Some air-blast sprayers use the high-speed air discharge to break up the spray liquid into droplets rather than orifice nozzles and pressure (See Figure 28.5). Discharging the spray directly into the airstream against the air flow produces the smallest droplets but if discharged at 90 degrees to the air-stream the droplets are larger, and even larger if discharged with the air flow. Faster airblasts produce smaller droplets. An increase in the liquid flow rate or a reduction in airblast causes the formation of larger droplets, while decreasing the liquid flow or increasing the air flow produces smaller droplets.

Crontrolled Droplet Applicators. Controlled droplet application is a term used to describe a new method of applying pesticides (See Figure 28.6). Controlled droplet application (CDA) technology uses centrifugal force instead of hydraulic pressure to produce with a narrow range of droplets of the same size. Conventional spray nozzles produce droplets that vary widely from small droplets that may drift or evaporate before reaching the target, to large droplets that concentrate too much of the pesticide in one spot.

Droplet Size Classification

The American Society of Agricultural and Biological Engineers (ASABE) developed a second color scheme used in nozzle literature and on pesticide labels that describes spray droplet sizes. This scheme defines droplet size ranges or categories using the Volume Mean Diameter (VMD). Droplet sizes within any spray are never completely uniform, so VMD is used as an indicator of the average droplet diameters within a spray. Pesticide manufacturers use the droplet size categories on pesticide labels to specify the optimum droplet size for a product. Nozzle design features, including pre-orifices, mixing chambers, and Venturi inlets, can have dramatic effects on the VMD and the range of droplet sizes produced by a nozzle. Generally speaking, combinations of wide fan angles, smaller nozzle orifice sizes (lower flow rates), and high pressures result in smaller droplets. Refer to Table 28.1 for the designation of droplet size ranges in microns.

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