Superheated steam, as already stated, is steam the temperature of which
exceeds that of saturated steam at the same pressure. It is produced by
the addition of heat to saturated steam which has been removed from
contact with the water from which it was generated. The properties of
superheated steam approximate those of a perfect gas rather than of a
vapor. Saturated steam cannot be superheated when it is in contact with
water which is also heated, neither can superheated steam condense
without first being reduced to the temperature of saturated steam. Just
so long as its temperature is above that of saturated steam at a
corresponding pressure it is superheated, and before condensation can
take place that superheat must first be lost through radiation or some
other means. Table 24[20] gives such properties of superheated steam for
varying pressures as are necessary for use in ordinary engineering
practice.
Specific Heat of Superheated Steam--The specific heat of superheated
steam at atmospheric pressure and near saturation point was determined
by Regnault, in 1862, who gives it the value of 0.48. Regnault's value
was based on four series of experiments, all at atmospheric pressure and
with about the same temperature range, the maximum of which was 231.1
degrees centigrade. For fifty years after Regnault's determination, this
value was accepted and applied to higher pressures and temperatures as
well as to the range of his experiments. More recent investigations have
shown that the specific heat is not a constant and varies with both
pressure and the temperature. A number of experiments have been made by
various investigators and, up to the present, the most reliable appear
to be those of Knoblauch and Jacob. Messrs. Marks and Davis have used
the values as determined by Knoblauch and Jacob with slight
modifications. The first consists in a varying of the curves at low
pressures close to saturation because of thermodynamic evidence and in
view of Regnault's determination at atmospheric pressure. The second
modification is at high degrees of superheat to follow Holborn's and
Henning's curve, which is accepted as authentic.
For the sake of convenience, the mean specific heat of superheated steam
at various pressures and temperatures is given in tabulated form in
Table 25. These values have been calculated from Marks and Davis Steam
Tables by deducting from the total heat of one pound of steam at any
pressure for any degree of superheat the total heat of one pound of
saturated steam at the same pressure and dividing the difference by the
number of degrees of superheat and, therefore, represent the average
specific heat starting from that at saturation to the value at the
particular pressure and temperature.[21] Expressed as a formula this
calculation is represented by
H_{sup} - H_{sat}
Sp. Ht. = ----------------- (8)
S_{sup} - S_{sat}
Where H_{sup} = total heat of one pound of superheated steam at any
pressure and temperature,
H_{sat} = total heat of one pound of saturated steam at same
pressure,
S_{sup} = temperature of superheated steam taken,
S_{sat} = temperature of saturated steam corresponding to the
pressure taken.