User Guide

Figure 12 Spray at 100-psi pressure
Figure 13 Spray at 300-psi pressure
Effects of Viscosity
On Nozzle Performance (Also see page 26)
One of the most important factors affecting nozzle performance is
viscosity, technically defined as a measure of resistance to flow
within a liquid. More commonly, viscosity is thought of in terms
of “thickness.” For example, a gallon of gasoline can be poured
through the spout of a can much faster than a gallon of tar. That’s
because the tar has a much higher viscosity than gasoline, or
greater resistance to flow.
Strangely enough, the opposite is true to nozzle applications. As
we will see in a minute, with an increase in viscosity, nozzle flow
rate also increases.
Temperature is the main factor in changing oil viscosities. It works
something like a scale (Fig. 14). As the temperature goes down, the
viscosity goes up. Take No. 2 fuel oil for example: at a temperature of
100°F, it has a viscosity of 35 SSU (Seconds Saybolt Universal). But
when the temperature drops to 20°F, the viscosity increases to 65 SSU.
An outside storage tank may contain cold oil, and cold oil can
cause problems. Here’s what happens: the thick oil passes into the
nozzle, through the slots, and into the swirl chamber. Since it is
more viscous, the rotational velocity is slowed down. This causes
a thickening of the walls in the cone of oil as it emerges from the
orifice, so the nozzle actually delivers more fuel and larger
droplets (see Figures 15 and 16). And as a result, the flame front
moves away from the burner head. In severe cases, atomization
may be so poor that the fuel cannot be ignited. Or if it is ignited, it
often produces a long, narrow and noisy fire that burns off the
back wall of the combustion burner.
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