Page 99

NORA Oilheat Technicians Manual

Chapter 5 Nozzles and Chambers Figure 5-15: Air trapped in nozzle line (1) Air bubbles trapped in oil line are compressed while oil is under pressure during firing period. (2) Same air bubbles expanding after burner has shut down and oil pressure has been released. Expanding air forces oil through nozzle orifice causing after-drip and after-fire. This illustration reproduced with permission by McGraw- Hill Companies from “Domestic and Commercial Oil Burners,” Charles Burkhardt, Copyright, 1969, Third Edition, published by McGraw-Hill, Inc. Chapter 5—Nozzles and Combustion Chambers 5-15 area is still hot, this oil burns with a smoky fire. If the combustion area is not hot enough, the oil drips out and collects in the bottom of the chamber. When the burner comes back on, all this extra oil lights and results in smoke, soot and rumbles. There are three basic causes of afterdrip— 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. The first is easy to check. Install a reliable pressure gauge in the nozzle discharge port of the fuel unit. Start the burner and let it run for the duration of the safety timing cycle. When it locks out, the pressure should drop about 20% and hold indefinitely. If it fails to stabilize and slowly descends to zero, you know the pressure-regulating valve in the pump is no good and the pump should be replaced. If air is trapped in the nozzle line or adapter, it will cause an after-drip. See Figure 5-15. A bubble of trapped air will be compressed to 1/7th its original volume by the 100 PSI pressure of the oil. When the burner shuts off, the pressure eases back to normal and the air bubble expands back to its original volume. This rapid expansion pushes oil out of the nozzle, causing an after-drip for several seconds. This can lead to delayed ignition, sooted heat exchangers and the smell of fumes. This condition is diagnosed by looking into the combustion chamber at burner shut down. If there is no view port, you can perform the same check by tilting the transformer back and looking through the combustion head. If air is present, check for air leaks using the procedure described in the Chapter on Fuel Units. Expansion of oil in lines can also cause after-drip. For every degree F of temperature rise, there is a .04% expansion of volume. After-drip occurs when the burner shuts down and the temperature of the oil in the nozzle line and adapter rise because of the heat from the appliance. Hard refracto


NORA Oilheat Technicians Manual
To see the actual publication please follow the link above