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NORA Oilheat Technicians Manual

Chapter 5 Nozzles and Chambers 120°. Yet another way to help with this problem is to blend kerosene or additives with heating oil. Thermal stability If you find a fuel failure but the filter and strainer are clean and the nozzle is plugged with coke (a dull black substance), the problem is probably thermal stability. Oil can become unstable in the prolonged presence of heat, particularly when in contact with copper and other “yellow” metals. As the oil sits in the nozzle and drawer assembly and its temperature rises, it can form coke. This is more of an installation issue than a fuel issue. There are three basic causes of after-drip—a defective pump shut off valve, air entrapped in the nozzle line, and oil expansion in the nozzle line caused by excessive radiated heat at shut down. Nozzles should not get hot. If the nozzle is hot enough to overheat the oil, you probably have either a bad draft situation, an old hard brick chamber reflecting excessive heat back on the nozzle after shut down, an after-drip problem, or a draw assembly and end cone sticking into the chamber. There are good mechanical fixes for these problems—i.e. post purge, draft inducers, interrupted ignition, ceramic chamber liners, and end cone amulets, to name a few. If you encounter a thermal stability problem, find out what is causing the nozzle to get hot. The problem is most likely to occur after burner shutdown. Check the over-fire draft after shut-down. Check to see if the draft regulator closes after shutdown. If it stays open, it will reduce draft over-the-fire needed to cool the nozzle. Check electrode settings and the type of chamber. Check to be sure that 5-14 Nozzles and Combustion Chambers the end-cone is flush or slightly recessed from the chamber face. Check for afterdrip. Any of these problems could be the cause of your thermal instability. Another cause of overheating is a hard brick chamber. When replacing an old nonflame retention burner with a new flame retention burner, it is tempting to leave the old chamber in place. The problem is, new burners have much higher flame temperatures than the old burners. It did not matter with the old burner’s cool flame that the hard brick chamber held its heat for hours. The white-hot flame from the new burners, however, heats the chamber to very high temperatures. When the burner shuts off, the old chamber reflects all this heat back and overheats the nozzle. The solution is to replace the old chamber or line it with a ceramic liner. In some cases, with big old dry base boilers, you can fill in the old chamber and install the new burner in the clean-out door, firing against a target wall—essentially creating a wet base boiler. Many units are very tight or operate without a chimney and offer little or no over-fire draft. The best way to avoid nozzle overheating in these situations is motor-offdelay (commonly called post-purge.) Using a solenoid valve, the primary control shuts off the flow of oil but keeps the burner running for a few minutes, blowing air from the burner air intake through the air tube and past the nozzle—keeping it cool. Nozzle after-drip The quickest way to soot up a heat exchanger is nozzle after- drip. This happens when oil drips from the nozzle orifice after the burner shuts down. If the combustion


NORA Oilheat Technicians Manual
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