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Full text of "The Flow Of Gases In Furnaces"

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336                                  APPENDIX IX
forced only to the extent of turning a sufficient amount of water
into steam to occupy the full area of the hottest tubes at their
highest point. Any further forcing with this type of boiler will
cause it to destroy itself.
Exactly what the last word in boiler design will be is hard to
say. Present designs leave much to be desired, not only in the
circulation of the water, but in the manner in which the hot gases
pass through the setting and come in contact with the heating
surface. The ruling temperature of boilers is very low. Con-
vection currents in water are set up with very low-temperature
differences, and as the temperature of the mass increases the
convection head caused by one degree difference in temperature
increases. These physical principles do not cause design dif-
ficulties and it is probably for this reason that boiler engineering
has settled into a rut and the mechanical details of the connections
entirely overshadow the importance of increasing the heat-
absorbing capacity.
HEATING or the application of heat to industrial and domestic
processes is an engineering problem of importance. A study of
existing installations reveals many interesting discrepancies in
practice. In writing about heating furnace a great deal has
been said regarding the importance of permitting the heat to
soak in. It is well known that a time factor is necessary. The
velocity of gas travel is very low in many furnaces. But when
boiler settings are considered, a great deal is said regarding the
importance of high gas velocities as contributing to efficient
The ideal condition is found between these two extremes.
In boiler practice the problem is to pass the gases by the recipient
surface at a velocity which will permit them to give up or transfer
all of the heat they carry between two temperature limits. Their
initial or high temperature limit is fixed by the firebox or com-
bustion temperature. Their final or low temperature limit is
fixed by the ruling temperature, or temperature of regime, of the
boiler plus the temperature required to supply a temperature head
necessary for the transfer of heat. This final temperature of the