PROPERTIES OF SUPERHEATED 



STEAM 



WHILE, as has been seen, the use of superheated steam is old, but 

 little was known until recently of its properties. 

 In the preparation and computation of a table giving the proper- 

 ties of superheated steam the important factor is its specific heat. Without this 

 value it is impossible to determine the total heat of superheated steam, that is, 

 the amount of heat that must be applied to bring saturated steam to any given 

 superheated condition. 



Regnault, in 1862, determined the specific heat of superheated steam, basing 

 his value, which he gives as 0.48, upon four series of experiments, all of which 

 covered approximately the same temperature range and all at atmospheric pressure. 

 While Regnault's experiments in no way proved that the value of specific heat 

 as given by him was independent of either pressure or temperature, this value 

 was accepted for some forty years and applied to higher pressures and temperatures 

 as well as to those within the range of his experiments. . 



With the revival of the use of superheated steam in the nineties, the assump- 

 tion that the specific heat was constant regardless of pressure and temperature 

 was found to be incorrect and a number of investigators turned their attention 

 to the subject. Grindley and Greissmann's experiments, in 1900, appeared to 

 indicate that the specific heat increased with the temperature and was independ- 

 ent of the pressure. Other investigators, however, proved this untrue. Among 

 those giving their services in this investigation were Lorenz, Linde, Holborn and 

 Henning, Callender, Carpenter, Thomas and Knoblauch and Jacob. The experi- 

 ments of the last two investigators were probably the most laborious and com- 

 prehensive and they made special efforts to eliminate the presence of moisture 

 in the steam in observations near the saturation point, an error which unavoidably 

 crept into other investigations. 



Table i* gives, in condensed form, the properties of superheated steam as 

 calculated by Lionel S. Marks and Harvey N. Davis. The values as given by 

 these authorities are considered reliable and are generally accepted in engineer- 

 ing practice. 



In determining the mean specific heat of superheated steam for various tem- 

 peratures and pressures and from these values the total heat, Messrs. Marks and 

 Davis use the values as determined by Knoblauch and Jacob, modifying their C,, 

 (or specific heat at constant pressure) curves in two respects ; the first consists 

 in modifications of the C p curves at low pressures near the point of saturation 

 because of thermodynamic evidence and because of Regnault's experiments at 

 atmospheric pressure ; the second modification is in the C p curves for high 

 degrees of superheat to follow Holborn and Henning's curve, which they con- 

 sider authentic. 

 * See pages 25 to 27. 



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