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Full text of "The Flow Of Gases In Furnaces"

DESIGN OF OPEN-HEARTH FURNACES               233
possible, say, 5 m (16.4 ft) as a minimum. The flues below the
checker should be proportioned to permit free flow to the passes
at a low velocity. If desired, the height of these flues may be
stepped down, proportionally to the distance from the inlet.
This, however, introduces complications in the brickwork with
comparatively little gain.
The vertical velocity of the gases leaving the checkerwork
may be considerable and it is necessary to provide sufficient space
above the checker to get rid of the eddies which will be formed.
Unless sufficient space is provided these eddies will cause con-
siderable interference with the flow of the gas from the checker
chamber into the slag pocket, converting what should be a smooth
regular flow into a series of bursts or blow-throughs. These bursts
may have entirely different periods in the air and gas chambers
and result in considerable irregularity of combustion and waste
of fuel.
In any regenerative fired furnace it is impossible to reduce the
temperature of the waste gases leaving the regenerator to the
temperatures of the incoming air and gas. This results in a
considerable loss of heat up the chimney. Frequently one-third
of the heat passes uselessly up the stack, and the amount of heat
lost is more often in excess of this value than it is less.
The theoretical temperature of the waste gases at the base of
the stack is about 300. The actual temperature at the base
of the stack ranges from 600 to 1000, and rarely runs below 700.
This would seem an ideal opportunity to install a steam boiler
and recover a portion of the heat, but until a comparatively few
years ago such an installation was not considered, although the
installation of boilers on puddling furnaces had been common
practice for many years. To-day few open-hearth furnaces are
built without waste heat boiler equipment and it is extremely
probable that such boilers could be profitably installed in con-
nection with many of the other furnaces.
Considerable notice has been taken of these waste heat installa-
tions in the various technical papers, but it has been in the main
simply tabulations of the installations as made with very little
real information as to the underlying reasons which led to the
selection of the particular equipment installed. A noticeable
departure from this practice was presented by the paper of
Thomas B. Mackenzie before the Iron and Steel Institute, in 1918.