Perhaps you are aware of the Blueray furnace system. A yellow flame at bes gets no more than 75 percent efficiency, while a blue flame is supposed to get 90 percent combustion as the hot gases are pulled back around a baffle in the ignition area, preheating the oil so that it burns hotter and better. Thus, all the hydrocarbon is burned and there is no soot. A less hot yellow flame, by contrast, has unburned hydrocarbon and the resulting soot coats the boiler and prevents if from fully heating the water. However, I'm wondering if each time the burner starts up there might be a short lapse of time before the hot gases are available to preheat the oil, thereby resulting in a yellow flame that eventually builds up soot. Henry B. Dunseith Mattapoisett, Mass.
I've talked with Don Lynch, project manager for research and development at Blueray Systems, Inc., Mineola, New York.
First of all, he says, the efficiencies in your letter are not exactly correct. Yellow- flame, steady-state efficiency depends not just on the burner performance, but also on the boiler or furnace performance as well -- whether it's hot air, hot water, steam, etc.
"Our boiler can get a steady-state efficiency of up to 87 percent," he reports. "The CO [carbon dioxide] level, which is a measure of how effective the burner is, runs between 13.5 and 14.5 percent by volume in the field. Under laboratory conditions, however, it runs up to 15.3." The maximum that can theoretically be obtained, he adds, is 15.4 percent using No. 2 fuel oil.
Typical yellow flames can achieve about 14 percent in the lab but are never recommended to be set higher than 14.25 percent, according to Mr. Lynch.
The flame of a blue-flame unit is yellow for no longer than three seconds when the unit first comes on. The oil, as you say, is preheated in a blue-flame unit so that it vaporizes before it ignites. In other words, it's just like burning gas instead oil.
Further, as you say in your letter, when soot builds up in a heater it insulates the unit and the stack temperature goes up, thus reducing the efficiency of the heating system.
A blue-flame unit also is said to have cleaner smoke than a yellow-flame unit. Using a test procedure for measuring the smoke, a blue-flame unit is rated at zero -- or no discoloration at all. A 10 is totally black smoke. A yellow flame is considered to be set up properly if it has a level of 1 or higher.