Document onmnBkqKgn8GaZeDqOvoV0L8

36 CHAPTER 3 1946 Guide Thermodynamics . 37 given temperature (below the critical temperature), though this requires that it have a definite pressure pi and that the coexisting condensed phase have the same temperature and pressure. The values of saturation pressure listed in Table 1 have been computed from the formulae of Goff and Gratch 2. Thermodynamic Properties of Water at Saturation Table 2 offers revisions to existing steam table data 3 with extension downward to --160 F. These revisions and extension were a necessary preliminary to the construction of Table 1. A detailed explanation of the' methods employed in constructing Table-2 is given in a paper 2 by John A. Goff and S. Gratch. As in Table 1 the temperature scale used as argument in Table 2 is the Fahrenheit scale defined in terms of absolute temperature T by Equation 1 whereas the Fahrenheit scale used as argument in exist ing steam tables is that derived from the International Centigrade scale by means of Equation 2. The symbols used as column headings in Table 2 are the same as those used in the steam tables and have the same mean ings; therefore, a Retailed explanation seems unnecessary. DEGREE OF SATURATION At given values of temperature and pressure the humidity ratio W of moist air can have any value between zero (dry air) and Ws (moist air at saturation). For convenience a parameter p called alternatively degree of saturation or percent saturation is introduced through the definition, W = (iiys (3) Obviously the degree of saturation p can "have any value from zero (dry air) to unity (moist air at saturation). To a degree of approximation within the estimated uncertainty of the data in Table 1 at temperatures below about 150 F, the volume v of moist air per pound of dry air at any degree of saturation p may be computed from the simple relation, = tfe + Ufas (4) To obtain comparable accuracy at temperatures above about 150 F it is necessary to add a correction term v as follows. p(l - p)A . 1 + aWe n (4a) where a denotes the ratio of the apparent molecular weight of dry air (28.966) to the molecular weight of water (18.016), namely, 1.6078. In- Table 3 are given, for each of several higher temperatures, the correspond ing value of the coefficient A, the value of p at which the correction term v attains its maximum value, and the maximum value of the correction term there attained. At temperatures below about 150 F. the enthalpy h of moist air . perpound of dry air at any degree of saturation p may. be computed from the simple relation, -/ h ~ fia + ' (5) by John A. Goff and S. Cratch.