84 APPLICATION OF THE LAWS OF HYDRAULICS
from which
/*5 = 0 m 716 CD and fc JL™^?§6 = o mm 155.
The total of these pressures and losses will be
1. In the ash pit................... <5i =0 mm 016
2. In the grate openings............ 62 = 0 046
3. In the fuel bed.................. <53 = 0 090
4. In the upper part of the firebox.... 54 = 0 027
5. Over the bridge wall............. 85 = 0 155
8 =0 mm 334
Therefore, the hydrostatic pressure required at the level of
the grate will be, as a minimum, + 0 mm 334 of water column.
At the temperature £=1200°, the weight of 1 cu m of gas in
the furnace will be
Therefore a column of this gas 1 m in height will give a hydro-
static pressure of
1.29-0.246 = 1 mm 044 of water.
Then, using x to express the height in millimeters of the column
of gases corresponding to the 'pressure of a water column of
0 mm 334 in height, the following proportion can be written :
x ^0.334
. 1000~~1.0447
from which : x = 3 19 mm, which is the minimum vertical distance
of the grate bars below the hearth of the furnace.
(e) Waste Gas or Smoke Flue.— As the hot gases from the
furnace are to be passed under a boiler, there is no necessity for
increasing their velocity of flow. Increasing their velocity would
necessitate a decrease in the gas pressure below the boiler by
means of the draft supplied by the chimney. But such a decrease
in the pressure is always accompanied by the sucking in of the
cold outside air, which reduces the temperature of the gases and
decreases the production of steam; it is therefore desirable to
(1) Refer to Appendix III.