Alan G. Dexter
Extension Specialist, Weed Control
University of Minnesota and
North Dakota State University, Fargo
Controlled droplet applicators (CDA's), also called rotary spray nozzles, use centrifugal force rather than hydraulic pressure to form spray droplets. Centrifugal force is supplied by a spinning cup or disc powered by a small electric motor. Several types of CDA's are available but the type used for ground broadcast application has the trade name Micromax and the following discussion will refer to this type.
The spray solution is injected at the bottom of the CDA spinning cup and is forced up grooves inside the cup. The liquid is formed into droplets on teeth at the upper edge of the cup and flung from the teeth in a circular pattern. The rubber drive belt can be placed on one of two pulleys to select the revolutions per minute of the cup. The CDA will operate at 2000 rpm to produce droplets of about 250 microns diameter or at 5000 rpm to produce droplets of about 75 microns diameter.
Research at the Canada Agriculture Research Station in Regina established that 82 inches spacing between CDA units gave the most uniform distribution when the units were operated at 2000 rpm, a height of 13 inches above the target, and a flow rate of 1 liter/min or 0.26 gal/mint This optimum spacing was determined with a stationary sprayer and no wind. Other observations indicate that a head wind, tail wind, or even the forward speed of the sprayer can cause spray droplets to deflect as they pass through the air. This deflection will prevent proper overlap and cause non-uniform coverage. Bouncing spray booms and applying spray solutions with different viscosities also have been reported to cause non-uniform coverage. Placement of CDA units 41 inches apart rather than 82 inches apart provides additional overlap which probably would help compensate for various factors that reduce uniformity of coverage.
Experiments at North Dakota State University compared weed control from equal rates of herbicides applied at 4 mph with a CDA at 2000 rpm, a CDA at 5000 rpm, or an 8001 nozzle at 40 psi. Weed control from the CDA at 5000 rpm(75 micron droplets) was generally less than from the CDA at 2000 rpm (250 micron droplets) or from the 8001 nozzle. Weed control from the CDA at 2000 rpm was generally similar to weed control from the 8001 nozzle. However, comparisons between flat fan and CDA nozzles should be made on an individual herbicide basis. For example, glyphosate (Roundup) has generally performed better through the CDA while a few reports with other herbicides have indicated reduced weed control from CDA applications as compared to flat fan nozzles. At this time, herbicide rates should not be reduced below labeled rates when using a CDA but future research may establish that rates of a few herbicides can be reduced. Spray drift was less with the CDA at 2000 rpm than with the 8001 nozzles but spray drift from the CDA at 5000 rpm was greater than from the 8001. Greater drift from the CDA at 5000 rpm may explain the reduced weed control as compared to the 8001 nozzles.
A CDA at 2000 rpm, 4 mph and a 0.26 gal/min flow rate will apply 4.8 gal/A with an 82 inch spacing or 9.6 gal/A with a 41 inch spacing. The 8001 nozzles with a 20 inch spacing and 40 psi will apply 7.4 gal/A at 4 mph. Thus, the CDA spaced at 41 inches for optimum coverage and operated at 2000 rpm for optimum weed control and minimum drift will use more water per acre than 8001 nozzles.
The advantages of controlled droplet applicators over flat fan nozzles for herbicide application appear to be reduced spray drift at 2000 rpm and improved weed control with a few herbicides. The main disadvantages are higher cost, greater complexity of the equipment, potential non-uniform spray coverage with the wide spacing between units, and potential increased spray drift with the 5000 rpm setting.
1981 Sugarbeet Research and Extension Reports. Volume 12, pages 70-71.