AN ANALYSIS OF SPECIFIC HEAT DATA FOR WATER AND WATER VAPOR IN THE CRITICAL REGION. Technical Report No. 8

Abstract

A comprehensive review of the available data for the specific heat at constant pressure of water and water vapor is given for pressures from 14.22 to 4300 psia and temperatures from 680 to 800 deg F. ln addition a description is given of the various experimental techniques employed by different investigators to carry out measurements of this property. Deviations of up to 20% are shown to be in existence between the measured values of specific heat of different investigators. The estimated apparent over-all error in the specific heat measurements was found always to be within 5%. In view of the actual deviation it would appear that the various estimates that were made on the apparent over- all error are somewhat optimistic. An attempt was made to show that excessive random and systematic errors may arise in the critical region when values obtained from mean specific heat measurements are considered as being identical to the instantaneous values oi specific heat. There are no specific heat formulations C/sub p/ = C/sub p/ (P, T), in existence for the part of the critical region where the specific heat first increases more » with tennperature, attains a maxi mum finite value, and then subsequentiy decreases with temperature. Consequently, the smoothed values of the specific heat at constant pressure of various steam tables were derived by graphical interpolation of different experimental data. Deviations between the various smoothed data for the specific heat at constant pressure approach and even exceed 30%. This is especially true in the regions of maxima of various isobars. These large deviations are due, in part, to the error introduced by graphical interpolation and also to the previously mentioned deviations among the experimental data. The specific heat was derived in the critical region from P-V-T data. The deviations between the derived and measured values were generally between 10 to 20%. Near the critical point, at supercritical pressures, and in the region of maximum specific heats along various isobars, the deviations were larger. However, except for one point, even these deviations were entirely within 40%. Finally it is shown that the pressure and temperature relation corresponding to the maximum values of specific heat along isobars is different to that corresponding to specific heat along isotherms. This is significant insofar as it was previously thought by other investigators that the locus of points on a pressure temperatare diagram defining ( partial delta C/sub P// partial delta P)sub T/ = 0 was identical to the locus of points defining ( partial delta C/sub P// partial delta T)/sub P/ = 0. The physical significance of the region enclosed by the locus of points on a pressure temperature diagram defining ( partial delta C/sub P// partial delta T)sub P/ = 0 and enclosed by the above two curves is the boundary between the liquid and vapor phases in the supercritical region. (auth) « less