PWM systems are becoming more common on boom sprayers recently and there are increasing number of growers who are first-time users of these systems and need to make nozzle selection decisions for their systems. While nozzle selection for a PWM system is somewhat similar to the nozzle selection process for a sprayer without PWM, there are few additional things that the growers need to consider because of how a PWM system works and regulates flow rate as compared to the sprayer using a standard rate controller.
PWM sprayer: Travel speed fluctuations are common during pesticide applications. On a boom sprayer equipped with a rate controller, target application rate – with changes in travel speed – is maintained by regulating flow rate – which is primarily accomplished by changing spray pressure. This becomes a problem when spray pressure reaches outside the (narrow) operating range and deteriorates the spray pattern and uniformity. To overcome this issue, Pulse Width Modulation (PWM) systems control flow rate by varying the duty cycle (proportion of the time that nozzle is on; see pictures below) whereas the spray pressure stays constant throughout the boom. In short, a PWM sprayer provides a better control over spray pressure while still maintaining target application rate with ground speed changes, thus enabling consistent and accurate spray application.
Sizing a nozzle for PWM system: The first step in sizing a spray nozzle for PWM system is to understand duty cycle of a pulsing solenoid – a key parameter used to regulate flow rate. The duty cycle is the proportion of the time that the solenoid is open/on, means the nozzle is spraying. Typical duty cycle ranges are between 20 and 100%. A 100% duty cycle means a spray nozzle delivering maximum flow whereas a nozzle operating at a 20% duty cycle will deliver about one-fifth of the max. flow (100% duty cycle). During application, the PWM system automatically adjusts duty cycle to maintain target application rate with changes in ground speed. While lower duty cycles (<20%) are possible, they are not recommended due to inconsistent spray quality and pattern.
Since duty cycle is directly related to ground speed, it is important to understand during nozzle selection that you want to aim for an average travel speed that keeps the duty cycle of the system between 60% and 80%. This means that the nozzle is open roughly about 2/3 of the time and it also provides plenty of flexibility for the system to adjust duty cycle if the travel speed increases or decreases considerably below the average speed.
Besides application rate and droplet size, one of the important parameters for nozzle selection for PWM systems is average ground speed as it determines the minimum and maximum speed that you can spray without exceeding the selected nozzle’s pressure rating. Once you have selected an average speed, you can calculate the minimum (20% duty cycle) and maximum speeds (100% duty cycle), and then use the nozzle selection/tabulation charts in the same way as we normally do for selecting nozzles for sprayers without PWM.
Let’s work through an example to illustrate this process. A grower wants to select a nozzle for his PWM sprayer to apply 10 GPA (20” nozzle spacing) at 10 mph and the droplet size requirement for this application is Medium droplets. Assuming 10 mph as an average speed around 75% duty cycle, the minimum speed will be 2.7 mph and the maximum speed will be 13.3 mph. Now using the GreenLeaf Technologies nozzle selection chart below, we will first select the application rate (10 GPA), then move down the 75% column to find the speeds that are closest to the selected average speed of 10 mph. Now moving left on the chart, the selected speeds fall within the 40 to 60 PSI range for the 04 nozzle, and within the 30 to 40 PSI range for the 05 nozzle. Since we need medium droplets for this application, we can see there are only two nozzles (BPDF, 04 at both 40 and 50 PSI, and BP, 04 at 60 PSI) that will provide the desired droplet size. Given the options here, the best solution for 10 GPA at 10 mph is BPDF, 04 nozzle at 50 PSI. Remember there can be two to three different nozzles that fits the desired application so try to select the one that is more closer to your target application parameters and/or provides more application flexibility.
TeeJet has a similar chart for sizing PWM nozzles but instead of listing travel speeds at all 25, 50, 75 and 100% duty cycles, it displays minimum and maximum speeds under each application rate. In this case, you will again use your average speed at a selected duty cycle (remember to keep this between 60 and 80%) to calculate the minimum and maximum speeds, and select the nozzle that best fits the speed range and droplet size requirement. Using the same example above, we can see that there are two nozzles (TT and TTJ60, 04 at 50 PSI) that can provide 10 GPA and the desired droplet size in the selected speed range.
Wilger also has a tool “Tip Wizard” (available as both online and a smartphone app) for selecting the right nozzle for both conventional and PWM sprayers. The application asks for the target rate, sprayer speed, PWM system, nozzle spacing and droplet size, and provide nozzle choices best suited for that application. This application can be used for sizing nozzles for any brand as it asks the user about the type of PWM system (most PWM system options offered today are listed) available on the sprayer as they have different pressure drops across the solenoid.
Lastly, remember that though PWM systems have some flexibility in terms of maintaining the target application rate within a wider speed range, make sure to avoid using nozzles and/or making applications that require the system to operate at a duty cycle lower than 50% or more than 90% for prolonged periods. In that case, we would reconsider a different nozzle using the new average ground speed so the system can operate within the preferred duty cycle and to utilize the full capabilities of the PWM system.