Fig.10. Higher spray pressure increases the number of smaller droplets.
Spray pressure exerted on hydraulic nozzles provides the energy that breaks a stream of water into droplets, and thus is a key factor in regulating spray droplet size. Increased pressure generally causes a decrease in droplet size. One study with a flat-fan nozzle demonstrated that increasing the pressure from 20 psi to 40 psi more than doubled the volume of droplets in the 100 micron and smaller range (Fig. 10). Reducing pressure will reduce drift because larger droplets are formed, but operating nozzles below the recommended pressure will also reduce effective coverage and may result in poor distribution and improper spray overlap. The recommended pressure range for conventional flat-fan nozzles is about 30 psi to 50 psi. However, nozzle manufacturers have recently introduced nozzles that can be used at pressures as low as 15 psi with little reduction in coverage quality.