Document JJk13eRqV3JOepQrrNJo7Zy6e

52 CHAPTER 3 1952 Guide ment of the families of curves of constant dry-bulb temperature, wet-bulb temperature, volume, relative humidity and degree of saturation, are shown, in Fig. 4. Mass and energy balances deal only with net changes between definite states; the detailed history of a change is not involved. A chart used to facilitate such calculations must primarily aid in clearly establishing states. Lines drawn on the chart to connect different states need have Thermodynamics a a .a 8.a"a T-Tt ww 53 Ca) ce> DRY-BULB TEMPERATURE (0) Fio. 4. Arrangement op Families op Curves on A.S.H.V.E. PSYCHROMETRIC CHART no other, significance than being loci lines, that is, lines which contain .the two terminal points of a change according to the particular overall-conT ditions imposed. Loci lines are commonly called condition Unes, for the processes concerned.. On the A.S.H.V.E. chart a condition line is char acterized by the ratio {h% -- hi)/(Wi --LFj).......... An abridgment of the A.S.H.V.E. Psychrometric Chart appears in Fig. 5. A large size (24 x 32) chart will be found on the inside of the back cover. Thie chart is drawn for standard atmospheric pressure. Steady flow changed commonly involve pressure drops with the flow, but so long as these pressure drops, remain a small fraction of the barometric pressure, no appreciable, errors need arise from using a constant pressure chart. For many design problems, the use of. the standard pressure chart will not incur any undue error up to about 2000 ft above sea level. The region above the saturation curve is a two-phase region giving equilibrium states for water in both the liquid and vapor phases. The ordinate of a point in this region is the weight of water in both phases per pound of dry air in the vapor phase, neglecting any dissolved air Flo. 5. Abridgment op A.S.H.V.E. Psychrometric Chart in the condensed phase. The ordinate at the saturation curve, or the point at which the break in the isotherm through the point in question occurs, is the weight of water per pound of dry air in the vapor phase: Consequently, the difference between the two ordinates is the weight of condensed moisture per pound of dry air in the vapor phase. The shaded solid-liquid-vapor region at 32 F is an isothermal threephase zone, and separates the liquid-vapor zone from the solid-vapor zone. The temperature is 32 F throughout the shaded area.