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NORA Oilheat Technicians Manual

Chapter 5 —Nozzles and Combustion Chambers 5-5 Figure 5-4: Nozzle spray patterns Figure 5-3: Nozzle spray droplets Hollow Cone Solid Semi-Solid Chapter 5 Nozzles and Chambers Oil Nozzle Type A Ordering Table 300 PSI reduces the droplet diameter by about 28%. Lower pressure means larger droplets that are much harder to vaporize and burn. While pressures greater than 100 PSI are sometimes desirable, never operate at less than 100 PSI. See Table 5-1 on following page. Spray pattern There are many different spray patterns offered by manufacturers. Although all spray patterns are hollow to some degree, nozzles are grouped into three general classifications—solid, hollow, and semisolid. See Figure 5-4. Hollow cone: As the name implies, the greatest concentration of droplets is at the outer edge of the spray, with little or no distribution in the center. Generally, hollow cone nozzles are used on low firing rate burners, particularly those firing less than 1 GPH. This is an important advantage in fractional gallonage nozzles, such as those used in mobile home furnaces, where cold, high viscosity oil may cause a reduction in spray angle and increases in droplet size. Hollow flames also tend to be quieter. Solid cone: Here the distribution of droplets is more uniform throughout the pattern. These nozzles work best when the air pattern of the burner is heavy in the center or where long fires are required. They provide smoother ignition for burners In Figure 5-3 we can see how the spray from a nozzle changes as the pressure increases. At low pressure, the cone shaped film is long and the droplets are large and irregular. As the pressure increases, the spray angle becomes better defined. Once a stable pattern is formed, any increase in pressure does not affect the spray angle directly in front of the orifice. However, at higher pressure, the angle of spray further away from the orifice does start to narrow by one to two degrees. This is because the droplets are starting to slow down due to air resistance and the air the spray draws in moves the droplets inward. This is the same effect that causes a shower curtain to be drawn into the shower spray. As you might expect, pressure increases cause a corresponding increase in the amount of oil flowing through the nozzle. A nozzle rated at one gallonper hour at 100 PSI will deliver about 1.23 gallons-perhour at 150 PSI. Increasing pressure also reduces the size of the droplets in the spray. For example, an increase from 100 to


NORA Oilheat Technicians Manual
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