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HOW TO CONTROL SPRAY CHARACTERISTICS

There are several different types of sprayers used by professional pest managers: trigger sprayers, which are pressurised by pulling the spray trigger; compression sprayers, which are pressurised by being pumped up; and backpack sprayers, which are either pressurised manually or powered by a petrol or battery-driven motor. These are the types most commonly encountered.

There are several different types of sprayers used by professional pest managers: trigger sprayers, which are pressurised by pulling the spray trigger; compression sprayers, which are pressurised by being pumped up; and backpack sprayers, which are either pressurised manually or powered by a petrol or battery-driven motor. These are the types most commonly encountered.

To make a sprayer work, it needs at least a tank, a pump of some nature, a trigger/release valve and a spray nozzle (see Figure 1).

Fundamental of components of a sprayer
Figure 1: Fundamental components of a sprayer

To create a spray, you need pressure and, most importantly, the right nozzle. With a quality nozzle, you can significantly influence the resulting spray on three levels: spray pattern, spray volume and droplet size.

1. Spray pattern

The spray pattern is determined by the physical characteristics of the nozzle selected. There are various nozzles on the market, which can create different spray patterns. Depending on the surface being treated, pest managers can select one of four primary spray patterns (see Figure 2).

Four primary spray patterns
Figure 2: Four primary spray patterns

Typically, if you need to cover a flat surface such as walls or ceilings, a fan jet nozzle provides the best coverage and delivers an ideal and regular distribution of the spray. For treating inside cracks and crevices a jet nozzle is preferred. A hollow cone nozzle is ideal when treating three-dimensional objects such as plants and bushes, as the hole in the centre prevents overspray occurring at the centre of the treated objects, and helps ensure more even and regular coverage. (A full cone nozzle has the same application as the hollow cone, it is just second best as the coverage is less even, with overspray occurring at the centre.)

Adjustable nozzles allow you to vary between a full cone pattern through to a jet. This gives maximum flexibility, but compromises on the quality of the individual spray patterns. For example, an adjustable nozzle cannot create a hollow cone shape, so the full cone is one of the compromises made when using an adjustable nozzle.

Another important factor is the material the nozzle is made from. The sharper the edges of the outlet hole, the better the spray pattern. Therefore, high-end nozzles will feature a stainless steel hole plate, or will be entirely made of brass. Plastic nozzles tend to provide more rounded edges to the hole, which influences the spray pattern shape negatively.

2. Spray volume

The spray volume is determined by the pressure available and the diameter of the nozzle outlet. Mathematically, the flow rate is directly proportional to the nozzle diameter. Professional nozzles have a defined characteristic and should come with a table or diagram that shows the flow rate related to the pressure. Table 1 shows the flow rate of a high-end hollow cone nozzle, which is available in five versions, each showing a different relationship to the various available pressures.

Pressure Nozzle Diameter and Flow Rate
0.8mm 1.0mm 1.3mm 1.5mm 1.65mm
200kPa 0,34 L/min 0,45 L/min 0,65 L/min 0,77 L/min 0.84 L/min
400kPa 0,47 L/min 0.62 L/min 0.90 L/min 1.07 L/min 1.17 L/min
600kPa 0.56 L/min 0.75 L/min 1.08 L/min 1.30 L/min 1.42 L/min
800kPa 0.64 L/min 0.85 L/min 1.24 L/min 1.49 L/min 1.63 L/min
1000kPa 0.70 L/min 0.95 L/min 1.38 L/min 1.66 L/min 1.82 L/min

 

Table 1: Spray volumes that can achieved using variable nozzles and pressure

 3. Droplet size

Droplet size is a consequence of the available pressure, the type of nozzle selected and the flow rate the nozzle provides.

The available pressure is the level of pressure offered by the sprayer you are using, which typically varies between 100kPa and 1000kPa (with 100kPa being equal to 1 bar). Petrol-driven sprayers may reach pressures up to 2500kPa, but in practice this is usually well beyond the pressure required. A pressure range between 100-600kPa is optimal for most of the work performed by a professional pest manager.

A choice needs to be made between the different spray patterns for a nozzle and the nozzle hole size. Based on the nozzle’s characteristics, as shown in Table 1, the flow rate for the specific nozzle can be determined. The larger the nozzle diameter, the higher the flow rate, e.g. at 600kPa, a 0.8mm nozzle provides a flow rate of 0.56L/min, whilst a 1.65mm nozzle provides 1.42L/min.

Increasing the pressure also increases the flow rate of the same nozzle, e.g. increasing the pressure of a 0.8mm hollow cone nozzle from 200kPa to 1000kPa increases the flow rate from 0.34L/min to 0.70L/min.

The size of the nozzle and the pressure not only determine the flow rate, they also determine the droplet size. The higher the pressure, the smaller the droplet size!

With this set up it becomes clear that a larger nozzle with a high flow rate should be selected for creating a mosquito protection zone, where large droplets are required. The same is true if spraying a herbicide where we wish to avoid spray drift. In both cases, we would ideally use a 1.65mm hole size nozzle with the sprayer adjusted, if possible, to a low pressure of 150-200kPa. If we require a very fine spray, so the droplets hang longer in the air to directly make contact with the insects, a small nozzle with a low flow rate is optimal, e.g. a 0.8mm hole size nozzle with the pressure adjusted to 600kPa or even up to 1000kPa.

On-the-job considerations

Of course, there are some additional parameters to consider when selecting nozzles for a job. If you need to spray a large volume of liquid, such as when performing a termite treatment, you need to choose a large nozzle so you can work faster, but keep the pressure lower to create a larger droplet size to minimise spray drift. On the other hand, if you need to cover large surfaces e.g. with herbicides, a double nozzle holder or even a spray boom with the sprayer at a low pressure is recommended.

The latest range of Li-ion-battery-driven backpack and wheelbarrow sprayers on the market provide electronically controlled pressure regulators, which makes it easy to adjust the pressure steeples between 100-1000kPa and provide a constant flow rate.

For manual compression sprayers, constant flow valves allow pest managers to use a pre-set pressure, e.g. at 145kPa. These valves are fitted between the nozzle and the lance and allow the flow rate to be kept constant over the complete working pressure range. Typically, with a quality compression sprayer the pressure varies from around 400kPa in the beginning to 150kPa at the end, before re-pumping. With cheap sprayers, this range is even greater, and the lower pressures more quickly reached (see Figure 3).

Figure 3: The effect of using a constant flow valve on a compression sprayer vs backpack sprayer

Using a constant flow rate valve also helps to improve safety when applying chemicals, since you avoid the high flow rate peaks when the compression sprayer is first pumped up to the maximum, compared to at the end when the pressure is low.

Understanding your equipment and these relatively straightforward calculations will help you achieve the right spray output for the job at hand, which can be the difference between a job well done and a potential callback.

Steve Broadbent, Ensystex Australasia with Jörg Lembachner, Birchmeier Sprühtechnik AG