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450 NOTES ON STEAM.
NOTES ON STEAM.
When water is heated and converted into steam of atmos-
pheric pressure, one cubic foot of water will make 1646 cubic
feet of steam. (The common expression that " a cubic inch of
water makes a cubic foot of steam " is not strictly correct, as a
cubic foot contains 1728 cubic inches.)
The specific gravity of steam at atmospheric pressure, when
compared with water is, therefore, "TgTg = 0.000608.
The weight of one cubic foot of steam at atmospheric
pressure will, therefore, be 0.000608 X 62.5 = 0.038 pounds. At
any other pressure the weight per cubic foot of steam is given
in Table No. 82.
Saturated steam is steam at the temperature of the boiling
point which corresponds to its pressure. Saturated steam does
not need to be wet steam, as the word saturated does not mean
that the steam is saturated with water, but it means that it is
saturated with heat ; that is to say : the temperature under the
given pressure cannot possibly be any higher as long as the
steam is in contact with water, because if more heat is added
more water will be evaporated, and if the volume is kept con-
stant, as in a steam boiler, both the pressure and temperature
will increase simultaneously.
High pressure steam is steam the pressure of which greatly
exceeds the pressure of the atmosphere.
Low pressure steam is steam the pressure of which is less
than the atmosphere, and also steam having a pressure equal
to, or not greatly above, the atmospheric pressure.
Wet steam is steam which contains water held in suspen-
sion mechanically.
Dry steam is steam which does not contain water held in
suspension mechanically.
Super-heated steam is steam which is heated to a tempera-
ture higher than the boiling point corresponding to its pressure.
It cannot exist in contact with water, nor contain water, and
resembles a perfect gas. Vertical boilers with tubes through
the steam space (such as the Manning boiler) give slightly super-
heated steam ; but if steam is to be super-heated to any consid-
erable extent it must be passed through a super-heater, which
usually is in the form of a coil of pipes subjected to the hot
gases in the uptake from the boiler.
The sensible heat of steam is the temperature which can
be measured by a thermometer.
The latent heat of steam is that heat which is absorbed
when water of any given temperature is changed into steam of
the same temperature.
When water is evaporated under pressure the sensible heat
will increase and the latent heat will decrease. For instance,
at atmospheric pressure the sensible heat is 212 degrees, and the
latent heat of evaporation is 966 B. T. U., but at 100 pounds
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