I.C.E. BMA-136 HBR - Manual (Page 9)

8
OPERATING PRINCIPLES OF THE RAW GAS BURNER
The raw gas burner is designed to operate in a duct of flowing fresh air. Fuel gas is fed directly to the burners; kinetic energy
of the air stream furnishes combustion air. The burner must be installed to fire with, and parallel to, the air flow. By virtue of
velocity impact and suction generated by the diverging shape of the combustion baffles, air is induced into the air ports in the
combustion zone. The air supply is constant though only that which mixes with the gas flowing from the burner ports, takes
part in combustion.
When a very small quantity of gas is admitted to the burner, sufficient mixing takes place in the low fire slot within the burner,
casting and combustion takes place in this zone. Since the low fire zone is contained within the burner casting it is effectively
shielded from fire disrupting uncontrolled air entry.
As the gas is increased the flame progresses into the intermediate fire zone where an additional supply of air is available. High
or full capacity, mixing occurs at the larger air ports of the high fire zone augmented by air spilling over the end of the baffles.
On a reduction of gas supply the reverse sequence takes place. The flame receding to a location of lesser air supply until the
low fire zone is reached. The system above is suitable for a turn down range of approximately 30 to 1.
With the suction by the blower there is a pressure in the gas manifold of less that zero at low fire. Therefore, when checking
the manifold pressure you will find that the pressure will range from approximately 4” W.C. to less than zero, when the unit is
modulating from high to low fire.
EXAMPLE FOR CALCULATING C.F.M.
Example for Calculating The Amount of Air and Gas in a Direct Fired Make-Up Air Unit Pull Through Type
YOU WILL NEED: A pressure differential gauge (Manometer) (Magnehelic) Thermometer -30 °F - 200 °F.
All units are factory set with a profile opening around to burner sized for 2950 F.P.M. velocity. Due to more or less external
static pressure the velocity may not be within this range on start up of unit. The pressure drop should be checked to insure the
unit is operating around this velocity.
A pulley adjustment or change should be done to bring the velocity within operating range. If an air balance has been done and
the C.F.M. verified to be correct as stamped on the rating plate and the velocity across the burner is not correct the profile area
should be adjusted.
If velocity is higher than 2950 FPM then the profile area should be increased. This can be achieved by readjusting the top and
bottom profile plates.
The profile area is stamped on the rating plate, but to get to the free area you will have to deduct the space taken up by the
burner.
If Midco burner is used deduct .65 sq. feet for each 1-foot section, or .33 sq. feet for each 6” section.
If Maxon burner is used deduct .45 sq. feet for each 1-foot section, or .23 sq. feet for each 6” section.
Using a magnehelic differential gauge across the burner profile will give you the pressure drop and using the burner capacity
chart will tell you what velocity you have through the profile. If the unit is operating between .45 W.C. and .65 W.C. it is
considered to be within operating range, as the low air switch is factory set to make at .25 W.C. and the high profile switch is
set to open at .95 W.C.
This should allow for a wide leeway before the unit will lock out due to low air / high air velocity across the profile plate.
Under normal servicing the tubes from the air switch should be checked to insure they are free of any moisture or dirt as this
could cause the burner to lockout on the flame safeguard relay as both switches are in the flame rod circuit.
If moisture is entering the tubes they may have to be repositioned to a lower area on the cabinet. Care must be taken to insure
that they will operate to shut down the unit if the velocity is out of the operating set points.