While many boiler rooms and power plants have been swapping out old coal and oil-fired burners/boilers for natural gas and bio-fuels, there still will always be the need for oil burners/boilers in the world. With this need, come the multiple options of how to burn the oil. While there are many different variations, the two main methods of oil atomization for the burner are pressure and air atomization.
Pressure atomization depends on the oil pressure inside the nozzle tip to spray a fine mist of oil, very similar to a Windex spray bottle. The micronized oil droplets are flung into burner head, where they are thoroughly mixed with the combustion air and ignited. As mentioned above, the pressure at the oil nozzle is the key factor in the atomization process, therefore your oil pump and pressure regulator are the key components in this system. The pump needs to be able to meet the gallons per hour (gph) requirement for the burner/boiler to meet their load capacity. The pressure regulator is set in accordance to the firing rate which is normally between 100-300 (psi). The turndown ratio for a pressure atomizing burner is normally only 3:1 or 4:1.
Air atomization adds another variable to the equation. Like pressure atomization, the oil is pumped through the system and into the oil nozzle. There, the oil is sheared by a intersecting stream of air. These two elements are mixed rapidly and forced out the nozzle tip into the burner head where they mix with the combustion air and ignited. As mentioned above, not only is the oil pressure of importance, as seen in the pressure atomization, but the injection air is as well. The oil and atomizing air are both varied based on the firing rate of the burner. While the oil pressure will remain around 100 (psi), the atomizing air pressures can range from 5-75 (psi) based on the design of the burner and firing rate. The turndown ratio for an air atomized burner is normally 6:1 or 8:1.
Both main forms of oil atomization are acceptable and widely used throughout the industry. Depending on the application and resources available one may be better suited than the other. Pressure atomization requires a more powerful pump and motor assembly to create the amount of pressure at the oil tip, while air atomization does not. Air atomization requires an air compressor to be able to create the atomizing air needed to shear the oil in the tip while pressure atomization does not. The whole piping rack for the oil is not as complicated and requires fewer parts for the pressure atomization while air atomization requires piping for air, solenoid valves, air pressure regulators, etc. Air atomization allows for a better turndown ration, which will allow a burner to run at a lower rate while keeping up with the load, while the pressure atomized burner will have to run at a higher or lower rate and either vent the excess steam or cycle on and off to keep the base load. As mentioned before, the application and resources available will determine whether pressure or air atomization will be the better choice for an oil burner application.
Robert Bohn, Mechanical Engineer