Document 2JVqroaQExMXaLwZ3go0DzOo5

42 CHAPTER 3 1951 Guide noticed that the specific entropy of dry air has been assigned the value zero at 0 F and standard atmospheric pressure. -- 8, the difference between the entropy of moist air at saturation, per pound of dry air, and the specific entropy of the dry air itself, Btu per (pound of ary air) (Fahrenheit degree). 8a = entropy of moist air at saturation per pound of dry air, Btu per (pound of dry air) (Fahrenheit degree). Aw = specific enthalpy of condensed water (liquid or solid) at standard atmospheric pressure, Btu per pound of water. The specific enthalpy of liquid water has been assigned the value zero at 32 F, saturation pressure (0.088586 psia). sw * specific entropy of condensed water (liquid or solid) at standard atmospheric pressure, Btu per (pound of water) (Fahrenheit degree). The specific entropy of liquid water has been assigned the value zero at 32 F, saturation pressure (0.088586 psia). p. *= saturation pressure of pure water vapor, pounds per square inch or inches of Hg (absolute pressure). At a given pressure, moist air can be saturated at any temperature, though this requires that it have a definite humidity ratio W,, and that the coexisting condensed phase contain a definite, but very small, quantity of dis solved air. On the other hand, pure water vapor (steam) below the critical tempera ture, can be saturated at only one temperature for a given pressure. The values of saturation pressure listed in Table 2 have been computed from the formulas of Goff and Gratch. THERMODYNAMIC PROPERTIES OF WATER AT SATURATION Since water vapor at low pressures acts almost as a perfect gas, the enthalpy of water vapor should also be a function only of the temperature within these limits. Therefore, the enthalpy of the water vapor may be expressed as being approximately equal to the enthalpy of saturated vapor at the dry-bulb temperature of the mixture. Substituting these values in Equation 22, the enthalpy of the mixture becomes h - 0.24 i + Wh, . (25) where hg is the value of the enthalpy of saturated vapor at the temperature t, and is obtained from Table 3. Table 3 offers revisions to existing steam table data with extensions . downward to --160 F. These revisions and extensions were a necessary preliminary to the construction of Table 2. A detailed explanation of the methods employed in the construction of Table 3 is given in a paper - by John A. Goff and S. Gratch.6 As in Table 2, the temperature scale used as argument in Table 3 is the Fahrenheit scale defined in terms of absolute temperature T by Equation 24. The symbols used as column headings in Table 3 are the same as those used in steam tables, and have the same meanings. Properties of water above 212 F from Keenan and Keyes6 are given in Table 4. DEGREE OF SATURATION Degree of saturation has previously been defined as the ratio of-the actual humidity ratio to the humidity ratio of saturated air at the. same drybulb'temperature and barometric pressure. This may be stated mathe matically as W '-w. (26) Obviously the degree of saturation n can have any value from zero, (dry air) to unity (moist air at saturation). The degree of saturation is con- Thermodynamics , A3 8S8S SSSS gsg| 85SS'ssss ssss 2222 ISIS : 77 m i ii i i m i ii 11 1111 1111 1111 mm. E ntropy. Btu per (L b) (F) 4.0466 4.0326 4.0100 4.0067 3.9939 3.0812 3.9686 3.9660 3.0435 3.9311 3.018S 3.6065 1. i IIS! II III III II 11111! l* l i imiiii itiififi fislll 8ifISiSf 6m111i imKii 1m181i mIllsi 000.38 000.82 901.26 001.70 992.14 992.68 003 03 003.47 993.91 904.35 994.80 095.24 1 E nthalpy, Btu per lb Sat. Vapor *8 ISIS 1214.40 1214.60 1214.70 1214.82 ISIS 1215.12i l l 5 i fr III 1212.43 1212.65 1212.67 1212.79 1212.00 1213.02 1213.15 1213.26 1213.38 1213.49 1213.62 1213.73 ! J, 8K?S i . H|(aMas KS28 SSSS SS8S: SS28 3285 ZSZZ CSS" MM1 SMlsMs' 1 sMSS1 S sMssMs sMssMs s1 sMss1 SM55MS b 3's ^ M 1 %m sps ill Ii! Ill IIII ill t 3 ! i! llss Sp 3SS1 ill lil lil Hi! liS 1 0.01722 0.01722 0.01723 0.01723 0.01723 0.01723 0 01723 0.01723 0.01723 0.01723ss Sat. Solid t l> In. Hg Lb/Sq In. L 111 ill ii 3111 ISIS 1=1! nil m uM 5* 11 111! Ills ill ISIS ill sill mi II? -1S22 222 22S tzZZ ZZZZ S22S IsSS :'g25 IIII i i i l IIII iiii IIII IIII II l.l. 1 l 1 1 ii 0.01725