Spray Nozzle Selection and Use

Part 2, Section 1: Pest Management

Pest Management

Pesticides and Their Application

Spray nozzle selection and use

Manufacturers of spray equipment have developed new "second generation" nozzles that provide additional protection against spray drift by reducing the amount of very fine drops created by the spray pattern. The design objective for a spray nozzle is to create a pattern of spray drops of a specific size distribution and spray distribution pattern. With a new nozzle, the application pattern from a series of nozzles in a row is evenly distributed across the spray path if the nozzles are set at the recommended height above the crop canopy. The nozzle orifice is a precisely engineered device that creates the drops of a certain average size at a specific pressure and with a designed spray distribution pattern, such as a flat fan spray. If the nozzle is damaged or worn, the average size of the droplet is changed with many more fine droplets created, and the spray distribution pattern is modified so that the uniformity of the spray along the boom is reduced.

With the increasing number of crop protection chemicals available, the applicator must be able to select a nozzle that is appropriate not only for the chemical (contact or systemic) but also for the application rate. In addition, environmental conditions such as wind and relative humidity affect the potential for spray drift, and this must be considered in selecting the appropriate nozzle.

A variety of spray nozzles are available to provide the applicator with a means of getting the right size spray drops to reduce spray drift, and to deliver the appropriate liquid flow rate so that the field application speed can be maintained. Using damaged or worn nozzles is potentially damaging to the crop and also not economical. The cost of new nozzles often can be recovered with chemical cost savings from one or two applications. Some of the new nozzles available are:

1. Modified flat fan for better drop size control over wider variation of pressures.
2. "Turbo" nozzles that create larger drops but maintain a consistent drop size throughout a wider range of pressures.
3. Air injection nozzles that create extra large drops for reduced spray drift. The drop contains a small bubble of air that bursts on contact with the leaf, spreading out the drop contents after contact, but keeping a large drop size during travel from the nozzle to the leaf.

One strategy for refitting sprayers is to use a rotating nozzle body that holds from three to five nozzles of different types. A specific nozzle appropriate for the type of chemical and drop size required can be selected by rotating the nozzle body without changing the calibration or pressure settings of the sprayer.

Spray nozzle selection procedure

1. From the chemical application label, determine the application rate in gallons per acre with recommended tank mix of water and active ingredient. The chemical label will often recommend spray drop size or type of nozzle.
2. Here is where the control for spray drift starts. If you have more than one nozzle to select from, select a nozzle size and type considering the wind and relative humidity conditions you expect to encounter in the field. Very low relative humidity causes the spray drops to evaporate more quickly and allows the active ingredient (as a vapor) to drift across or off the field. Drop sizes less than 200 micrometers (50 micrometers equals the diameter of a human hair) evaporate quickly and are subject to spray drift in even light to moderate wind. Spraying in winds of more than 4–5 miles per hour is not recommended. Manufacturers' catalogs provide data on the drop sizes produced by each nozzle at different spraying pressures.
3. From the manufacturer's catalog tables for the type of nozzle mounted on your sprayer, select the operating pressure that will create spray drops that are not subject to drift. Higher spray pressures increase the gallons per minute (GPM) output from the nozzle, but create smaller spray drop sizes. The goal is to balance the GPM from the nozzle without making driftable spray drops. Select the GPM output from the nozzle, based on drop size and pressure. You may want to purchase new nozzles if you decide to upgrade to second generation nozzles of a different capacity and type. Always make it a practice to use the lowest spray pressure possible without disrupting the spray pattern.
4. Select a desired spraying speed or spraying pressure. You will be balancing these two factors to get the right drop size and sprayer speed to reduce spray drift, while still getting the recommended application rate.
5. You now have a preliminary spray speed in MPH or GPM per nozzle, the sprayer nozzle spacing in inches, and the gallons per acre (GPA) recommended application rate. Use either of the following formulas, depending on whether you want to set the pressure and calculate the spraying speed, or set the speed and calculate the GPM per nozzle, which sets the sprayer pressure:
   
   If the calculated spraying speed is too slow for you, you must increase pressure (which may increase spray drift), or select a different size or different type of nozzle.
   
   For example, you could increase pressure by changing from a flat fan (medium-sized drops) to a Turbo TeeJet, which would provide a coarse drop size at increased pressure. You could then use an increased GPM for the nozzle and drive the sprayer faster.

If the required nozzle GPM is larger than the catalog value GPM that gives the right drop size, you can slow down the sprayer speed or choose a different nozzle type (providing larger drop sizes). This allows the application rate per acre to be met with a slower sprayer speed and reduced nozzle pressure, or the required GPM per nozzle to be met with increased pressure.

For example, if your sprayer had a flat fan TeeJet XR8004 installed and you wanted the nozzle to provide coarse-sized drops, the maximum pressure that would do this is 20 psi, and the nozzle would deliver 0.28 GPM at that pressure. But the required GPM at a spray speed of 6 MPH is 0.4. This can be delivered at 40 psi, but the drop size at that pressure is medium-sized. (There is a potential for spray drift with this drop size, because of expected winds and low relative humidity.) To solve this problem, you could choose an air injection nozzle, which would provide 0.4 GPM but also would provide a very coarse drop size at 40 psi.

The selection of nozzle type, nozzle capacity, and durability of material is becoming more important as the selection and variety of crop protection chemicals increases. The spray equipment supplier and nozzle manufacturer's catalogs can provide important advice on the proper selection of nozzles. Consult your local Cooperative Extension agent for fact sheets that provide additional guidance in nozzle selection.