The basic function of pressure nozzles is to convert the pressure energy from the high pressure   pump into kinetic energy into droplets or a thin film, the stability of which is determined by the properties of the liquid such as viscosity, surface tension, density and quantity per unit of time, and by the medium into which the liquid is sprayed. The spray pattern is a function of the operating pressure. Capacity of spraying water is usually assumed to be directly proportional to the square root of the pressure as delineated below;

Flow rate (kg/h) = K × √P  

As rule-of-thumb it can be established that higher viscosity, liquid density and surface tension and lower pressure will result in bigger particles.

Many proposed correlations have been reported, but delineated below is by far the most accurate:

  

Where:

d_s = volume particle mean diameter of the spray (microns)
Ơ = surface tension of liquid (dynes/cm)
P = nozzle pressure (p.s.i.)
µ = viscosity of liquid (poises)
PL = liquid density (gm/cc)
Q = volumetric feed rate / unit of time
K_n = nozzle constant (depending on spray angle)
d_o = orifice diameter (inches)