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

Sp. Ht. =
Hsup - Hsat
––––––––––––––––––––
Ssup - Ssat
 (8)
WhereHsup=total heat of one pound of superheated steam at anypressure and temperature,
Hsat=total heat of one pound of saturated steam at same pressure, [Pg 138]
Ssup=temperature of superheated steam taken,
Ssat=temperature of saturated steam corresponding to the pressuretaken.

[TABLE 25]
MEAN SPECIFIC HEAT OF SUPERHEATED STEAM
CALCULATED FROM MARKS AND DAVIS TABLES
Gauge
Pressure		Degree of Superheat
50 60 70 80 90100110120130140150160170180190200225250
50.518.517.514.513.511.510.508.507.505.504.503.502.501.500.500.499.497.496
60.528.525.523.521.519.517.515.513.512.511.509.508.507.506.504.504.502.500
70.536.534.531.529.527.524.522.520.518.516.515.513.512.511.510.509.506.504
80.544.542.539.535.532.530.528.526.524.522.520.518.516.515.514.513.511.508
90.553.550.546.543.539.536.534.532.529.527.525.523.521.519.518.517.514.510
100.562.557.553.549.544.542.539.536.533.531.529.527.525.523.522.521.517.513
110.570.565.560.556.552.548.545.542.539.536.534.532.529.528.526.525.520.517
120.578.573.567.561.557.554.550.546.543.540.537.535.533.531.529.528.523.519
130.586.580.574.569.564.560.555.552.548.545.542.539.537.535.533.531.527.523
140.594.588.581.575.570.565.561.557.553.550.547.544.541.539.536.534.530.526
150.604.595.587.581.576.570.566.561.557.554.550.547.544.542.539.537.533.529
160.612.603.596.589.582.576.571.566.562.558.554.551.548.545.543.541.536.531
170.620.612.603.595.588.582.576.571.566.562.558.555.552.549.546.544.538.533
180.628.618.610.601.593.587.581.575.570.566.561.558.555.552.549.546.540.536
190.638.627.617.608.599.592.585.579.574.569.565.562.558.555.552.549.543.538
200.648.635.624.614.605.597.590.584.578.574.569.566.562.558.555.552.546.541
210.656.643.631.620.611.602.595.588.583.578.573.569.565.561.558.555.549.543
220.664.650.637.626.616.607.600.592.586.581.577.572.568.564.561.558.551.545
230.672.658.644.633.622.613.605.597.591.585.580.575.572.567.564.561.554.548
240.684.668.653.640.629.619.610.602.595.589.584.579.575.571.567.564.556.550
250.692.675.659.645.633.623.614.606.599.593.587.582.577.574.570.567.559.553

Factor of Evaporation with Superheated Steam—When superheat is present in the steam during a boiler trial, where superheated steam tables are available, the formula for determining the factor of evaporation is that already given, ([2]),[22] namely,

Factor of evaporation =
H - h
––––––––––
L

Here H = total heat in one pound of superheated steam from the table, h and L having the same values as in ([2]).

Where no such tables are available but the specific heat of superheat is known, the formula becomes:

Factor of evaporation =
H - h + Sp. Ht.(T - t)
––––––––––––––––––––––––––––––––––
L