Midco G69B - Manual (Page 4)

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FIGURE 5 Conventional BoilerÑ Small Firebox FIGURE 6 Conventional BoilerÑLarge Firebox
foot or less. Use the entire floor as a combustion chamber
a and cover the water leg base to 6" from the bottom. Floor
construction should conform to Figures 1 to 3.
Firing door installations will usually entail removal of the
grates. Figure 8 shows a typical installation. A steel plate
floor can be supported from the grate lugs, if suitable. In
any case, the floor must be rigid enough to prevent
sagging, being supported by brick piers as necessary.
If firing over the grate, level it off with rubble of common
brick or firebrick if necessary; then cover over with
insulating block before laying the refractory floor.
The hearth of worm feed stokers can be used as a floor
base. Level off the hearth to a suitable height, remove the
worm feed mechanism and fill the tuyere with rubble.
Cover at least the tuyere area with block insulation; if using
castable refractory, pour a new floor over a leveling
surface of sand. The external stoker parts should be
removed to eliminate clutter under the burner.
The burner mounting in the door must be rigid and
refractory lined as shown in Figure 8. The burner nozzle
must not protrude directly into the combustion chamber.
The refractory liner must extend beyond or be flush with
the burner face as shown in Figure 4. A sheet metal rim
should encase the refractory up to the inside of the boiler
wall. It should be intermittently bent inward to retain the
refractory, or other means of anchoring should be
provided. The refractory lined combustion chamber can be omitted in
"Scotch Marine" and "Steam Generator" boilers or warm
air furnaces that include no ashpits. The burner is fired
directly into the heat exchanger, requiring no refractory
unless the combustion chamber is so short that flame
would impinge excessively on the rear heat exchanger wall
(this particularly important in a warm air furnace).
Refractory protection is recommended if the length of the
primary chamber is less than 20% larger than the length
given in Table 1. In any case, the burner entry wall must
be refractory lined if it is not a heat exchanger surface.
Firing Door Installations
It is advantageous, on occasion, to fire through the boiler
firing door. For example, pitting can be avoided on low
base boilers, damage to the burner can be avoided if
basement flooding is prevalent, or the combustion
chamber volume can be reduced in boilers with unusually
large ashpits.
Great care in planning will be required for firing door
applications since flame impingement on boiler surfaces is
more probable. Do not fire a boiler which contains a drop
section directly in the path of the flame, or do not fire over
a water grate in a smokeless type boiler. The firebox
length must be great enough to exceed the combustion
chamber lengths given in Table 1 by at least 20%.
When raising the floor, maintain sufficient firebox volume
to limit the heat release to 50,000 BTU per hour per cubic
FIGURE 7 Tubular Combustion Chambers