MUA-SVX01A-EN • Direct-Fired Make-up Air 59
Maintenance
Burner Operating Principles
Fuel gas is fed directly to the make-up air burner. The air stream’s kinetic energy
furnishes combustion air. It functions properly at the velocity and pressures
associated with usual ventilating systems.
The burner operates through a combination of velocity impact and suction
generated by the combustion-baffles. The shape of the baffles induce air through
the ports into the combustion zone. The air supply is constant, though only mixes
with the gas flowing from the burner ports. When a very small quantity of gas is
admitted to the burner, sufficient air to gas mixing takes place in the low-fire slot
within the burner casting. Combustion takes place in this zone. Since the burner
casting contains the low-fire zone, the casting shields the flame from
uncontrolled air entry.
Increasing gas supply moves the flame out to the intermediate fire zone for
additional air. At full capacity, mixing occurs at the larger air ports of the high-fire
zone, enhanced by air spilling over the end of the baffles. Reducing the gas
supply reverses the sequence. Flame recedes to the air-supply location until the
low fire zone is reached, Figure 19.
Figure 19. Direct-fired gas heater
Gas
Supply
IRFLOW
Combustion
Baffle
Indicated
Flame Length
High Fire
Zone
Low Fire
Zone
Intermediate
Zone
12” to 18” on
High Fire
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