RATIONAL CONSTRUCTION OF FURNACES
ingots will be plunged into a pocket of cold and stagnant gas and
therefore will not be heated uniformly, the top of the ingots being
hotter than the bottom.
A number of designs for pit furnaces have been seen by the
author, but none of them were perfectly satisfactory, and in many
of them there were gross violations of the physical laws in regard
to the carrying off of the waste gases. Fig. 112 shows a pit
furnace of the " new Siemens system " of a very costly design.
These pits are only supplied with regenerators for the air supply,
regenerators for the gas being omitted. Therefore, the products
of combustion cannot pass out of the heating chamber without
passing through the air regenerators, which have a port sill or
bridge wall extending to a height of 600 mm above the hearth
of the pit (refer to the dotted line A}. It is very clear that the
hearth of these pits forms a pocket which will be filled with colder
gases. Nothing can be done to save this furnace and make it heat
properly. The walls were lowered between the pits until the
height h was equal to the thickness of the stream under the in-
verted weir, as calculated by Yesmann's formula. After the
furnace was reconstructed in this manner the pits worked, but
the velocity of the gas in the heating chamber was equal to the
velocity under a normal inverted weir, that is to say, one under
which the height h equals 1 m, the velocity v being 6 m per second
(refer to page 52). A velocity as high as this is unsatisfactory.
The hot gases did not remain in the heating chamber a sufficient
length of time to give up their heat to the ingots. For this reason
the fuel consumption of these pits was very large. The best means
of reconstructing this furnace consists in removing the bridge wall
in front of the regenerator for air, so that the bottom of this port
will be as nearly as possible at the level of the hearth of the pits.
Fig. 113 shows Siemens type pit furnace at the Salda (Oural)