130 STEAM BOILERS, ENGINES, AND TURBINES
gas, into the other body, the second leg being open to the atmosphere
where the gauge is fixed.
Evidently the difference between the pressures of the two vessels,
or the two air passages, or an air passage and the atmosphere, will be
measured by the height of the water in the gauge. It is of course
not necessary that the water shall be 1 square inch in section. It
may be of any sectional area, provided it is graduated accordingly.
For ordinary boiler work, pressures up to 2 inches water-gauge are
employed, and of the total pressure required, the fuel takes from
0-2 up to 18 inch of water-gauge, according to the substance. The
following table, given by Mr. Hutton, shows the draught required for
the different kinds of fuel. As will be seen, straw, wood, and the
free burning coals require the smallest pressure to drive the air
through them, large coal also requires a small pressure compared
with small coal, and the anthracites require the largest pressure of all.
Straw.
Wood.
Sawdust
Kind of fuel.
Peat (light)
>
(heavy)
TABLE XIV.
Sawdust mixed with small coal
Steam coal (round)
Slack (ordinary).
"
(very small)
Coal dust
Semi-anthracite coal
Mixture of breeze and slack.
Authracite (round)
Mixture of breeze and coal dust
Anthracite slack.
Pressure required in inches,
water-gauge.
"
0-2
0:3
0.35
•
0.4
'
0.5
0.6
0.4 to 0.7
0.6 0.9
33
0.7 1.1
"
0-8 1-1
"2
0.9 1.2
21
10 1.3
33
0.2 1.4
"
1-2 1.5
J
13 1.8
""
It was mentioned in the first chapter, that a certain quantity of
oxygen is required for the complete combustion of every kind of
fuel, and it may be mentioned that the minimum quantity of air
required to furnish the oxygen for the complete combustion of 1 lb.
of carbon, oxidizing to carbonic acid, is approximately 12 lbs. In
practice, however, it is never possible to work to exactly these
conditions, and it is usual to reckon upon a supply of 24 lbs. of air
to each pound of the fuel to be consumed.
The volume of the gases produced from the combustion of carbon,
providing that it is completely oxidized to carbonic acid, is the same
as that of the air, before the carbon combined with the oxygen, at
the same temperature. The volume of the nitrogen, at any given.
temperature, remains unchanged of course; and the volume of the
carbonic acid formed by the combination of the carbon with the
