d) Nozzle Mixing. The nozzle mixing type generally provides a
higher turn-down ratio and requires lower pressure combustion air.
Oil Burners. Oil burners must atomize and vaporize oil to get the
oil into combustible form. Large burners usually prepare the oil for
combustion by first atomizing the fuel and injecting it into the combustion
Atomization is generally obtained by one of the following methods.
a) Horizontal Rotary Cup. In this method, a thin film of oil is
spun from the rim of a cup being rotated at high speed. The oil enters a cone
of high velocity primary combustion air where atomization is obtained. This
type burner is capable of being modulated over a wide range, but is seldom
used because of high maintenance costs and inefficiencies.
b) Mechanical Pressure Atomizing. This method expands the oil by
pressure through a small orifice thereby breaking it into a spray of very fine
droplets. This burner is not adaptable to appreciable modulation as a drop in
pressure results in rapid deterioration in atomizization efficiency.
Mechanical atomization requires the highest oil pressure and yields the
c) Steam Atomizing. In this method, oil is delivered to the
nozzle at pressures ranging from 40 to 150 psi (276 to 1035 kPa) with steam
pressure not less than 20 to 50 psi (138 to 345 kPa). In most cases, steam
atomizing can produce a turn-down of up to 10:1 at high efficiency. Steam for
atomization must be dry as moisture causes erosion which impairs burner
performance and causes pulsations which leads to loss of ignition. If steam
is wet, oil cannot be properly atomized regardless of the amount of steam
utilized. In general some superheat, possibly up to 50 degrees F (10 degrees
C), is desirable. The steam supply line should be of proper size, well
insulated, and adequately drained. The first cost of a steam atomizing system
is less, but it is about 1 percent less energy efficient than air atomization.
The steam atomizing burner has a higher turndown rate than the air-atomizing
types and is used whenever air atomization cannot obtain the same turndown
d) High-Pressure Air Atomizing. This method is similar to the
steam-atomizing type except air under pressure is used instead of steam.
High-pressure air-atomizing burners utilize air pressures of 30 psi (207 kPa)
and above and can operate within the same range of oil pressures and preheat
temperatures as the steam atomizing type.
e) Low-Pressure Air-Atomizing Type. This system generally
operates within 1/2 to 5 psig (3.5 to 34.5 kPa) air pressure and 5 to 20 psig
(34.5 to 138 kPa) oil pressure.
f) Low-Excess Air Burners. These burners are designed to operate
with excess air down around 5 percent. The burners are high in first cost,
and require excessive supervision and maintenance. Low-excess air burners are
used mainly to reduce nitrogen oxide (NOx) emissions.