Other Considerations for Designing a Good Oil Burner
Oil burners are typically designed to operate at oil pressures in the 40-100 psig range. Below about 35 psig, it is very difficult to make an oil burner operate properly because of the reduced potential for mixing air into the combustion zone. Operating oil burners at low fuel pressures produces large, soft, unstable flames.
A practical minimum for stable combustion in an oil burner is about one-million Btu/hr. This limit is based on the fluid mechanics of atomization, practical orifice sizes, and pressures. Oil simply won’t turn down nearly to zero like gas will. Again, oversizing burners should be avoided because it will limit turndown capability.
In combination oil/gas burners, the most common mistake when it comes to oil burning is simply to add “a little” oil only to selected burners already firing near capacity on gas. Even at minimum oil pressure, you will probably add a substantial firing increment that will almost surely air starve those selected burners, giving substantially longer flames. More than one tube has been ruptured doing that.
Even if the burners are not firing near capacity, don’t burn only a little oil and a lot of gas in any individual burner. Do just the opposite. If the oil pressure is low, the oil mixes poorly with air and may have to wait to burn until it gets out of the gas combustion zone. Poor mixing and waiting give longer flames. Also, below about two-million Btu/hr oil heat release, the oil burner is expected to produce a poor flame (soft, large, nearly unstable), even if the oil is not competing with gas for the available air.