Main Factors Affecting Spray

All the data relating to our nozzles contained in our catalog are based on spraying water at standard temperatures. When spraying liquids other than water, performance is likely to be different. The following presentation describes how various liquid properties and operating conditions affect performance.

CAPACITY

A nozzle’s flow rate capacity is primarily dependent of the operating pressure. For hydraulic nozzles, this relationship is straightforward. Capacity C increases in direct proportionality to the square root of the operating pressure P; that is C ~ P1/2. For air assisted nozzles, in which both air pressure and liquid pressure play a role, this simple relationship does not generally hold although the trend of increasing capacity with pressure does.

Capacity is affected by other factors, including the specific gravity and viscosity of the liquid. In general the capacity is related to specific gravity by the equation:

Capacity of Liquid = Capacity of water x Specific gravity^1/2

Viscosity also plays a role in capacity. Increasing viscosity reduces the turbulence of the rotational flow inside full-cone and hollow cone nozzles at a given pressure since the internal flow velocity decreases. The net effect is an increase in capacity, although usually, at the expense spray pattern integrity. Other types of nozzles, such as flat-jet nozzles, which do not rely on rotational flow, generally show a decrease in capacity with increasing viscosity, simply because of a lower exit velocity.

The effects of viscosity on capacity were discussed in the previous paragraph. Viscosity is a property of liquids that measures the intermolecular attraction between its molecules. The greater this attraction (higher viscosity), the greater will be the resistance for the molecules to move over one another, which is what occurs during liquid flow. High viscosity liquids have a profound effect on the spray pattern. In general the pattern deteriorates and the spray angle narrows considerably compared to the equivalent value for water.

Viscosity is highly temperature dependent. Small increases in a liquid’s temperature can dramatically reduce its viscosity.

VISCOSITY

TEMPERATURE

The liquid temperature is an important factor in nozzle performance since it has a direct bearing on the other elements that affect performance; namely specific gravity, viscosity, and surface tension. The chart below summarizes these effects.

SURFACE TENSION

This property of liquids refers to the behavior of those molecules that lie at or near its surface and are in contact with a different medium (either a solid surface or air.) The surface tension is essentially a force at this interface that minimizes the potential energy of all the molecules involved. Water has a very high surface tension. Most all other liquids exhibit lower values. The “beading up” of water on a glass surface is indicative of a high surface tension. Adding soap or some other surfactant to water dramatically lowers the surface tension as evidenced by the water now “spreading out” over the glass surface.