Page 327 - Boiler_Operators_Handbook,_Second_Edition

312 Boiler Operator’s Handbook

is restricted the fuel-air mix there will be richer in fuel
than the mixture passing around the diffuser so the igni-
tion point is usually aligned with the diffuser and it can
be altered by changing the position of the diffuser. On a
typical gas or oil fired burner the diffuser normally has
two positions, one for firing gas and one for firing oil.
The reason for the different positions has nothing to do
with the diffuser itself and everything to do with where
the fuel enters the burner. In the typical burner oil is ad-
mitted in the center at an oil nozzle and gas is admitted
through a gas ring or spuds on the outside of the burner.
The diffuser positions must be switched to control the
primary air ratio for each fuel. When that’s the case,
a semi-permanent marking should be applied to the
adjustment for each fuel position so an operator knows
the diffuser is properly located. Paint a ring around the
diffuser guide pipe with arrows pointing to the point
where it enters the burner and label them for each fuel.
An inexperienced operator positioned a diffuser
improperly on a boiler in south Baltimore in 1999. The
Figure 10-44. Burner register
pipe wasn’t marked but he knew it was pushed in for
firing oil so he pushed the diffuser guide pipe all the
way in, as far as it would go. The burner failed to light the flame. On some boilers the registers are modulated
several times until enough oil had accumulated in the along with the air and fuel controls to alter the shape of
furnace to feed the explosion! the flame for different loads.
An induced draft oil or gas fired boiler doesn’t You probably won’t see a burner register throttled
have a forced draft fan and doesn’t need any provisions down for better mixing today because we’ve learned that
to supply the air to the burner but will still need means rapid mixing makes for quick burning, hotter fires and
of controlling the distribution of air. Single burner boil- more NO production. The register burner is being re-
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ers are typically fitted with a screen or perforated plate placed by the axial flow burner which is designed to min-
to provide uniform flow of air to the burner. Burners on imize turbulence but ensure even distribution of air to the
multiple burner units are typically fitted with a register, flame front. The original concept of the axial flow burner
a set of bent damper blades that form a circle around the was developed in England in concert with the Royal Air
burner inlet (Figure 10-44). Some are independently set Force to improve performance of multiple burner boilers
with a locking bolt or screw on each blade while others at the English Air Force Bases and included creation of a
are fitted with linkage attached so the blades turn uni- venturi throat for each burner that not only improved air
formly and the flow of air to each burner can be adjusted flow distribution but also provided a means of measur-
while maintaining an even flow of air around the burner. ing the air flow at each burner to allow final tuning of air
Burner registers will not only serve as a means to distribution through them (Figure 10-45).
throttle air flow they also deflect the air to create a swirl One advantage of the venturi is it creates a large
in the air. That produces additional turbulence for better static pressure to velocity pressure conversion at the
mixing. Sometimes two registers are employed, one to burner inlet, most of which is recovered in the diverging
supply air around the outside of the fire and one for air section. The velocity conversion tends to balance the air
supplied to the center, around the diffuser. When they flow through each burner to improve air distribution on
are used, dual registers typically produce swirls in the multiple burner boilers. Control of air flow, including
opposite direction for better mixing. Another function shutting off burners on axial flow units is achieved by
of the burner registers and diffuser is flame shaping. a damper that forms a sliding sleeve at the inlet of the
Modern package boilers have very small furnaces and venturi. Most low NO burners applied to single burner
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older sterling boilers have short furnaces. To prevent boilers can’t benefit from the venturi design so other
flame impingement on the furnace walls the burner means are used to improve air distribution.
register and diffuser position combinations help shape Large single burner and multiple burner boilers