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234                                  APPENDIX VII

One of the operating difficulties with the open-hearth furnace
lies in the fact that explosions of gas are likely to occur whenever
the furnace is reversed. This difficulty is also met in all regenera-
tive furnaces. These explosions vary in their intensity from slight
puffs to heavy explosions and are due to the gas trapped in the
gas regenerator meeting the air that is drawn into the stack flue.
When the furnace is connected directly with the stack the puff of
the explosion passes up the chimney and is rarely noticed. Similar
explosions are not infrequent in blast furnace practice, and
experience in that line has demonstrated the absolute necessity
of providing explosion doors to relieve explosion pressures, as well
as the necessity of making all the flues and settings gas tight and
building them with buck stays of sufficient strength to stand the
explosion stresses.
When boilers or economizers are connected with the furnace
these explosions become of serious import and unless relief valves
are provided of sufficient area to prevent excessive rises in pressure
the settings will be damaged and numerous cracks will admit cold
air, greatly reducing the efficiency of the waste heat installation.
Another cause of unsatisfactory results with waste heat boilers
arises from the loss of sensible heat by the gases in passing through
the flues. These flues are generally underground and close to the
surface, and the ground above the flue is frequently so hot that
it remains dry, except when very heavy rains occur. Exactly
what the heat loss from this source will be depends upon the
construction of the flue, the depth below the surface and the
length of the flue. With waste heat utilization it is desirable
that the flues should be well insulated and as short and direct as
The waste-gas flues are frequently far from tight, and when
waste-heat boilers are installed the air leakage into the flues is
much more serious than when they connect directly to the stack.
Low temperatures at the bottom of the chimney in most metal-
lurgical high-temperature furnaces should be viewed with suspicion,
until checked by an analysis of the waste gases.