User Guide

12
Effects of Pressure
On Nozzle Performance
Normally, 100 psi is considered satisfactory for the fixed pressure
supplied to the nozzle, and all manufacturers calibrate their
nozzles at that pressure.
It is interesting to observe the sprays of a nozzle at various
pressures. See Figures 11-13. At the low pressure, the cone-shaped
film is long and the droplets breaking off from it are large and
irregular. Then, as the pressure increases, the spray angle becomes
better defined. Once a stable pattern is formed, any increase in
pressure does not affect the basic spray angle, measured directly in
front of the orifice.
At higher pressure, however, you will note that beyond the area of
the basic spray angle, the movement of droplets does make a
slight change in direction—inward. That’s because at this point
the air pressure outside the spray cone is higher than that on the
inside, which deforms the spray inward.
Pressure has another predictable effect on nozzle performance. As
you would expect, an increase in pressure causes a corresponding
increase in the flow rate of a nozzle, assuming all other factors
remain equal. This relationship between pressure and flow rate is
best shown in the table on page 13.
Increasing pressure also reduces droplet size in the spray. For
example, an increase from 100 to 300 psi reduces the average
droplet diameter about 28%. One last word on the subject: if
pressure is too low, you may be under-firing the burner.
Efficiencies may also drop sharply because droplet size is larger
and the spray pattern changed. If pressure isn’t carefully checked,*
the marking on the nozzle becomes meaningless. Pressures of
more than 100 psi are sometime desirable, but rarely do burners
operate at anything less.
Figure 11 Spray at 10 psi pressure
* Pressure can be reduced between the pump and the chamber
by clogged filters in the line or the nozzle. Check pressure
whenever reduced, not just at the pump.