3. Required excess air must be maintained at a
minimum to reduce stack thermal loss.
4 . Flame propagation temperature must be
Vaporization within the burner is generally
confined to small domestic services, such as water
heating, space heating, and cooking, and to some
industrial processes. Burners for this purpose are
usually of the pot type with natural or forced draft,
gravity float-type feed control, and hand or electric
ignition. Kerosene, diesel oils, and commercial oils of
grades Nos. 1 and 2 are suitable fuels because they
vaporize at relatively low temperatures.
If oil is to be vaporized in the combustion space in
the instant of time available, it must be broken up into
many small particles to expose as much surface as
possible to the heat. This atomization is done in three
1. By using steam or air under pressure to break the
oil into droplets
2. By forcing oil under pressure through a suitable
3. By tearing an oil film into tiny drops by
Primary combustion air is usually admitted to the
furnace through a casing surrounding the oil burner.
The casing is spiral-vaned to impart a swirling motion
to the air, opposite to the motion of the oil. Three types
of burners used for atomization are the steam- or
air-atomizing burner, the mechanical-atomizing
burner, and the rotary-cup burner.
Burners should be piped with a circulating fuel
line, including cutout, bypass, pressure-relief valves,
and strainer ahead of the burner. Burners should be
accessible and removable for cleaning, and the orifice
nozzle plates should be exchan geable to compensate
Steam-atomizing burners are simpler and less
expensive than the air-atomizing type and are usually
used for locomotive and small power plants. They
handle commercial grade fuel oils Nos. 4, 5, and 6 and
require a steam pressure varying from 75 to 150 psi.
The oil pressure needs to be enough to carry oil to the
burner tip, usually from 10 to 15 psi. Burners using air
as the atomizing medium are designed for three air
pressure ranges: low pressure to 2 psi, medium
pressure to 25 psi, and high pressure to 100 psi.
Figure 4-54 shows a steam-atomizing burner of the
external mixing type. In view (A), the oil reaches the
tip through a central passage and whirls against a
sprayer plate to break up at right angles, view (B), to
the stream of steam. The atomizing stream surrounds
the oil chamber and receives a whirling motion from
vanes in its path. When air is used as the atomizing
medium in this burner, it should be at 10 psi for light
oils and 20 psi for heavy oils. In view (C), combustion
air enters through a register; vanes or shutters are
adjustable to give control of excess air.
The burner is universally used except in domestic
or low-pressure service. Good atomization results
when oil under high pressure (to 300 psi) passes
through a small orifice and emerges as a conical mist.
The orifice atomizing the fuel is often aided by a
slotted disk that whirls the oil before it enters the
for a wide range in load demand.
Steam-Atomizing and Air-Atomizing Burners
The burners consist of a properly formed
jet-mixing nozzle to which oil and steam or air is piped.
The conveying medium mixes with fine particles of
fuel passing through the nozzle, and the mixture is
projected into the furnace. Nozzles may be of the
external or internal mixing type, designed to project a
flame that is flat or circular and long or short. A burner
should be selected to give the form of flame that is most
suitable for furnace conformation. Nozzles should be
Figure 4-55 shows a mechanical-atomizing
burner. View (A) is a cross section of the burner; view
(B) shows the central movable control rod that varies,
through a regulating pin, the area of tangential slots in
the sprayer plate and the volume of oil passing through
the orifice; view (C) shows a design with a
wide-capacity range, obtained by supplying oil to the
burner tip at a constant rate in excess of demand. The
amount of oil burned varies with the load; the excess is
Horizontal Rotary-Cup Burner
The burner (fig. 4-56) atomizes fuel oil by tearing
it into tiny drops. A conical or cylindrical cup rotates at
high speed (about 3,450 rpm), if motor driven. Oil
moving along this cup reaches the periphery where
centrifugal force flings it into an airstream. It is
suitable for small low-pressure boilers.
positioned so there is no flame impingement on the
furnace walls and so combustion is completed before
the i-lame contacts the boiler surfaces.