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NORA Oilheat Technicians Manual

Figure 5-11: Comparison of warm vs. cold oil on nozzle flow rates spray becomes narrower. The flame is longer, thinner, bigger, and less stable. This creates incomplete combustion that means increased smoke and soot. It is also harder to light cold oil, so ignition is delayed, and if the viscosity is very high, flame out and no heat result. See Figure 5-11. Outside, above-ground storage tanks suffer most dramatically from the problems of cold oil. Let’s say you do a tune up on a sunny hot summer day. The temperature of the oil in the tank is 80°F. You adjust the burner to run clean at that viscosity. As the temperature drops, the oil becomes much thicker and the amount of oil flowing out of the nozzle increases, causing the burner to over-fire. Not enough air is delivered and smoke increases. The angle of the spray decreases and the fire gets longer and REDO Figure 5-12: Nozzle droplet size in relation to temperature Colder Oil Causes More Oil to Flow from the Tip of the Nozzle Droplet Spray Liquid Oil Cone Nozzle Tip 5-12 Nozzles and Combustion Chambers Average Droplet Size Increases as Oil Temperature Drops Warm Oil Oil spins rapidly Large air core Lower oil flow Cold Oil Oil spins slowly Small air core Higher oil flow Fueloil temperature, degrees Chapter 5 Nozzles and Chambers Swirl Chamber % Change in Mean Droplet Diameter Fueloil temperature, degrees What makes the viscosity of oil increase? Temperature is the main factor in changing oil viscosity. As the temperature of the oil goes down, the viscosity goes up. The viscosity of No.2 oil is 35 SSU (seconds saybolt universal) at 100°F. When the temperature drops to 20°F, the viscosity goes to 65 SSU. See Figure 5- 10 on previous page. The effects of increased viscosity can be confusing. As the viscosity of the fuel flowing through a nozzle increases, so does the flow rate. Here is how it happens. As higher viscosity oil passes into the nozzle through the tangential slots and into the swirl chamber, the rotational velocity slows down. As a result, the walls of the tube of oil leaving the nozzle orifice are thicker—more oil enters the chamber and the oil droplets are bigger. The result is that the flame front moves out into the chamber and the angle of the


NORA Oilheat Technicians Manual
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