SUPERHEATED STEAM

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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