There was only one venturi-type spray nozzle on the market when I first wrote about them in 1996 – the GreenLeaf TurboDrop. It looked like an interesting way to reduce spray drift, but at $24 per tip, most sprayer owners shuddered.

Since then, other nozzle manufacturers have introduced their own versions of the venturi nozzle. And GreenLeaf has added a TurboDrop XL model, designed for lower pressure operation, to its lineup. The tips included in the chart on the next page are the main ones sold in the U.S., but there are other versions being sold in Canada and Europe that are just starting to be sold in the U.S.

The price of these tips has dropped from $24 per tip to about $7-$9 per tip. The original TurboDrop is now down to about $14 per nozzle.

That still may seem pricey compared to conventional spray tips. But it's cheap compared to other drift-reduction technology, according to Tom Wolf of Agriculture & Agri-Food Canada. He points out that venturi nozzles can cost from $2.15 to $11 per foot of boom. An air-assist setup could cost $70-$140 per foot, while a shroud over the boom would cost $14-$35 per foot of boom.

“This nozzle technology is available to everybody for relatively little money,” says Wolf. He says it is a cheap way to reduce drift even if it looks expensive. You may get $1,000 tied up in nozzles on a big sprayer, he says, but the nozzles alone determine how well the sprayer will work.

“I believe these nozzles are revolutionary in their ability to reduce spray drift,” says Wolf. The venturi nozzles produce a spray that is far coarser than what most sprayer operators are used to. They typically produce droplets with a Volume Median Diameter (VMD) in the range of 400 to 600 microns, even at spray pressures of 60-80 psi. And they practically eliminate driftable fines – the droplets smaller than 100-200 microns.

How they work
The venturi nozzles differ in design, but all work on the same principle. A preorifice meters the flow of the liquid into a mixing chamber. An internal venturi creates negative pressure inside the nozzle body, and air is drawn in through aspiration holes and mixed with the spray liquid. The liquid mixture that comes out the spray tip contains large air bubbles. The venturi body determines the flow rate while the discharge orifice creates the spray pattern. The discharge tip is typically twice as large as the metering orifice to accommodate the air-filled droplets.

Manufacturers claim that these nozzles are able to provide good spray coverage with large drop sizes because these air-filled droplets shatter or explode on impact to provide coverage comparable to conventional tips.

The Greenleaf TurboDrops are two-piece units that allow you to use exit tips in a variety of spray patterns. The original TurboDrops were designed primarily for custom applicators and large farmers, says Will Smart of Greenleaf Technologies. It has a ceramic metering orifice that should wear significantly longer (up to 50,000 acres) than stainless steel or polyacetal at higher spray pressures.

The TurboDrop XL, on the other hand, has a plastic metering orifice that can be expected to last for 5,000-10,000 acres of spraying. It was designed to accommodate a lower pressure range.

The TurboDrop tips come apart by hand for cleaning and allow users to add their own tips for different spray patterns. “A lot of our customers keep TeeJet tips on the shelf,” says Smart.

The Spraying Systems AI (Air Induction) TeeJet is a slender nozzle about 1 inch long with a built-in stainless steel flat fan tip. The AI nozzles have two air induction openings in the barrel and a removable polymer pre-orifice. Wolf points out that you will probably need tools to remove the metering orifice for cleaning from these nozzles.

The AI TeeJet nozzles now come in sizes up to 10. Spraying Systems recommends AI tips when spraying at pressures of 30 psi or more. At lower spray pressures, the company recommends Turbo TeeJet or XR TeeJet tips.

While most of the venturi nozzles are similar in design, the AI TeeJets produce a bit tighter spectrum of droplet sizes, according to Marty Heyen of Spraying Systems.

The Delavan Raindrop Ultra has a slender molded plastic body with an orifice of 303 stainless steel. It has a removable, plastic metering orifice. It fits any standard nozzle cap and produces 70-80% less driftable fines than conventional nozzles, according to James Blowers of Delavan.

“It is a very simple design,” Blowers says. It has two air intakes, one on each side. The preorifice easily pops out of the nozzle body to allow for easy cleaning.

The Lurmark Ultra-Lo-Drift tip, marketed by Precision Fitting and Valve, has two flow-metering holes. It's a compact nozzle that can be used at pressures as low as 15 psi and still hold a pattern. It fits into a conventional flat fan cap.

Complex designs
Venturi nozzles have a complex design that makes it difficult to predict any trends in droplet size, says Al Womac, ag engineer at the University of Tennessee. Unlike conventional nozzles, it's hard to make general statements about the droplet size you can expect with various combinations of flow rate and pressure. Bigger nozzles don't always make bigger droplets, he says.

Womac has done a series of droplet size studies with the various venturi tips and expects the results to be published soon. He has also done a series of droplet size measurements using actual chemical mixtures instead of water in the tank. He says switching chemicals was a more significant factor in determining the droplet size and percentage of driftable fines than changing types of venturi nozzles.

Measuring droplets containing chemicals and surfactants can produce different results than you would get with water only, Womac says. His earlier research (Successful Farming, October 1997, p. 52) indicates that the surfactant effect on droplet size is not the same for all nozzle designs.

Womac and colleagues' recent study indicates that the pesticide formulation (including the amount of surfactant) can influence droplet size more than the choice of nozzles. He found, for example, that the venturi tips produced smaller droplets and more driftable fines with Liberty than with Roundup or Gramoxone.

Are they effective on pests?
The easy answer is that they appear to be quite effective. Wolf enlisted a number of chemical companies to set up trials comparing one venturi nozzle (randomly assigned for the different trials) to a standard flat fan TeeJet and a Turbo TeeJet tip. He says his trials show that the venturi nozzles have very good efficacy with herbicides.

There are performance differences among the types of herbicides, however. Wolf's Canadian group tested the tips with 19 different herbicides. The tests showed the venturi nozzles work well with sulfonylureas, imidazolinones, growth regulators, Roundup and Gramoxone. He says you need to be cautious when using them to apply the “fops and dims” – the postemergence grass herbicides – and the contact-type broadleaf herbicides such as Basagran, Blazer and Buctril.

Wolf's trials showed that weeds that are difficult to wet, such as wild oats, foxtail and lambsquarters, present a challenge when spraying these contact herbicides with these venturi tips. He suggests keeping the pressure, volume and herbicide rate up for best control of these weeds.

While normal adjuvants and surfactants work well with venturi nozzles, Wolf says using a drift-control adjuvant will disrupt the spray pattern.

Need a pressure range
There doesn't seem to be much argument that these nozzles produce larger droplets and fewer fines. There are differences in how they respond to variations in pressure. The TurboDrop XL and Lurmark nozzles can both be used at 15 psi and still hold the pattern, while the original TurboDrop, Spraying Systems AI and Raindrop Ultra work best with at least 40 psi.

Wolf prefers to see these nozzles used at higher pressures. He says if your sprayer can't produce at least 60 psi, you probably shouldn't be considering the venturi nozzles. Higher pressure allows for pressure drops through a spray controller if you need to slow down.

If you have a nozzle calling for a minimum of 40 psi and you run at that pressure, pressure drops that occur when you slow down may seriously disrupt the spray pattern. You need to run the nozzles at a high enough pressure to allow for changes in speed.

When these tips were first introduced, there had been concern about the air intakes plugging in dusty conditions, but that has not been a major problem, the manufacturers say.