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Full text of "The Flow Of Gases In Furnaces"

HEAT CAPACITY AND  CALORIFIC INTENSITY CURVES   351
(d)  The thermal receptivity and mass of the substance to
which heat is imparted ;
(e)   The temperature differential established;
(/)  The velocity of flow of the gases;
(g) The pressure in the combustion chamber.
The accompanying curves show the. possibilities, as regards
heat transfer, of the respective combustibles when burned with
various amounts of excess air and various amounts of preheat
for the air and the fuel, when this latter does not contain hydro-
carbons dissociated by preheating. The chemical composition
of these combustibles is given in Table 6. In the case of the
liquid and pulverized fuels the amount of air or steam required
for atomizing purposes is given in the caption of the curve. One
of the interesting facts revealed is that the use of steam instead of
air for atomizing oil results in reducing the temperature obtainable
by 170 C. (306 F.). This is due to the fact that the same
amount of air is required in both cases, and the steam merely
increases the volume and weight of the products of combustion.
At the same time it may be that it is much more economical to use
the steam directly in the atomizing instead of indirectly in com-
pressing the air for atomizing. A steam jet is notoriously inefficient.
In a marine installation where space and weight have to be con-
sidered, a Great Lakes boat would use a steam jet floating in her
boiler feed supply. On salt water, where boiler make-up is
obtained by evaporation, an air compressor would exhaust to the
condenser and evaporator capacity would be saved or the air blast
fans would be run by motors. Steam jets would increase evap-
orator capacity and heat consumption. Mechanical atomizing
will avoid fans or compressors but will necessitate the use of oil
heaters and the pumps necessary to create the oil pressure required
for the spray nozzles. Assuming a 20 per cent excess air supply, the
comparison on p. 352 can be made from the curves. While the
difference in favor of air atomizing is only 1 per cent of the heat
released it is available in producing steam at boiler efficiency, from
1 to 9 per cent of the steam generated will be used in atomizing.
The oil will evaporate from 11 to 14 times its weight of water.
With air atomizing the temperature in the firebox will approxi-
mate 1885 C. (3280 F.) and with steam 1710 C. (3110 F.).
The actual temperature realized will be about 100 C. or 200