Document eowg52Y3xV94Je9mM6jJ1004

28 CHAPTER 3 1960 Guide Table 6 .... Coefficient* A, B ,C Appearing in Equations 34, 35, 36, Maximum Values of Corrections Defined by Equations 34, 35, 36. Degree of Saturation at Which These Three Maxima Occur, (u.. Maximum Value of Correction Defined by Equation 37, and Degree of Saturation at Which This Maximum Occurs, {Standard Atmoiphorie Frassora) (F) A (ftyib*) B C iau (Btu/lb.) (Btu/F/lb.) (ftVlb.) Umax lu, (Btu/lb.) (Blu/F/lb*) (Btu/F/lh.) Pm 96 0.0018 0.0268 0.00004 0.0004 0.0059 0.00001 0.0042 0.0096 0.0650 0.00009 0.00020 0.0010 . 0.0022 0.0155 0.0332 0.00002 0.00005 0.4878 0.3149 0:00042 0.0047 0.0693 0.00009 0.4691 0.00091 0.0099 0.1418 0.00019 0.4511 0.00207 - 0.0207 0.2903 0.00037 0.4213 0.00567 0.0151 0.6180 0.00076 0.3662 0.0333 0.3129 DETERMINATION OF THERMODYNAMIC PROPERTIES OF MOIST AIR FROM TABLE 2 DATA The Table 2 data are given for either perfectly dry or com* pletely saturated air. Convenient interpolation will yield magnitudes of the volume, enthalpy, or entropy of mixtures between these limits. Hie basis of this interpolation is the degree of saturation, denoted by the symbol u- (Degree of satu ration plays a role closely similar to steam quality in the steam tables.) Interpolation formulas am the following; ">++ 8 ft * ft. + ph,, + ft (31) (32) = * + + 4 -+ a xO - u)A " 1 + aW# (33) (34) a ratio of apparent molecular weight of dry air (28.966) to the molecular weight of water (18.016) 1.6078. tures are selected to be of practical value in the graphical solution of engineering problems. Two kinds of psychrometrie charts are most used today. The first is plotted on rectangular coordinates of dry-bulb temperature and humidity ratio; the second is plotted on oblique-angle coordinates of enthalpy and humidity ratio. While the same problems can be solved with either chart, de tailed computational procedures and language differ slightly. The ASHRAE Chart, a copy of which is included with The Goins, is of the second type. This chart iB based on the ther modynamic data of Goff and Gratch, as given in Table 2. fig. 4 is a skeleton sketch showing the oblique coordinate system of enthalpy and humidity ratio, which provides the framework upon which other curve families are plotted to give important properties. The k, W coordinates were chosen because steady-state energy and mass balances, as introduced in the beginning of this chapter, are expressible and solvable in terms of these two properties only. Restriction of the chart to a constant pressure of 1 standard atmosphere is conven-, tioaal; actually the chart is usable for pressure variations up to roughly 1 in. mercury from the standard. K g(l -- plB 1 + aWtP (35) <*(l -- it)C I 1 + oW.m (36) a * 0.1579 1(1 + ftoWi) logi(l 4~ paW, -- f*aW, logiaw) (37) - x(l + aWt) log(l + alT,)! Table 6 gives magnitude of the factors A, B, and C, and also an indication of the maximum magnitudes attained by the various correction terms in Equations 34, 35, and 36. If moist air were a mixture of perfect gases, the terms 8, ft, and 8 would be sero. The term 2 is a mixing entropy change, which is the consequence of partial-pressure variations with degree of saturation at fixed system pressure and exists even for per fect gases. Note particularly that the corrections to direct interpola tion based upon degree of saturation are small below about 150 F and usually they may be neglected. This gives quanti tative substance to the common approximation that perfectgas relationships are reasonably valid below roughly 150 F and at pressures near 1 atmosphere. THE ASHRAE PSYCHROMETRIC CHART A psychrometrie chart is a graphical representation of the thermodynamic properties of moist air. Its distinctive fea A. linot of <ontftiaf h or* ttrtughf, partdld and jnefiond at an oblkfua angle. The angle h cSatan for ccnwuaoc*- A. Top Cm it fhn noxtaKm humidify fetio W to be coitadaced. C. TWi comar point it tha aathafpy of dry air al the minimum dry-buib torn* peruhiia to ba conddnmd. D. Thit comar point it ardtatpy of dry oir of maximum dry-bwfb temperature to ba tontkJarod. L foat.of constant W or* draight, parotid, and horizontal. fig. 4.... Bosk Coordinates of ASHRAE Psychrometrie Chart Thermodynamics The ft, IV oblique coordinates were first used in 1923 by Moilier,18-u who also showed one way to modify the chart for various pressures. The scale units, coordinate angle, and range of the ASHRAE Chart have been chosen for convenience, and reasonable ac curacy. The temperature range 0 to 125 F covers the mostused range of air-conditioning problems. Similar charts can be drawn for other temperature ranges. The five additional curve families plotted on the chart are shown schematically in Fig. 5, and these families provide the necessary additional data to solve the usual types of problems. An abridgment of the ASHRAE Pstchsometkic Coast is shown as Fig. 6. A large chart is inside of the back cover. The feature to be considered next is the enthalpy-humidity difference ratio,-(A* -- Aij/.(JP -- Wi), where the subscripts indicate a change from state ! to state 2 on the chart. It is essential to recognise that mas and energy balances are formulated in terms of net changes between defined states; tile detailed history or process path of a change is not involved. Graphical analysis of these mass and energy balances is in dispensably aided by the use of loci lines, which are drawn to contain ail possible locations of the two states being oon- 29 sidered, state 1 and state 2, according to the particular overall conditions imposed upon the problem. These loci lines are commonly called condition lines, and in the ASHRAE Chart the slope of a condition line is given by the ratio (ft* -- ftj) (W* -- IFt). The protractor scale on the chart, used with drafting equipment for drawing parallel lines, permits either the setting of a desired condition-line slope or the numerical evaluation of this slope for different given problem data. The region above and to the left of the saturation tine on the ASHRAE Chart is a two-phase region giving equilibrium states when water is present either in both the liquid and the vapor phase or both the solid and the vapor phase. Humidity ratio here represents total water present (both phases) in pounds per pound of dry air. The three curve families in this region are enthalpy, humidity ratio, and temperature; the temperature lines are continuations of the wet-bulb lines, and the wet-bulb and dry-buib temperatures are identical. Moist air in the two-phase region always is saturated, and its vapor-phase humidity ratio is that for saturation at the pre vailing temperature and pressure. The liquid or solid water present, expressed as pounds of liquid or solid per pound of dry, air, is found as the difference between the humidity ratio Wat-bdb tamparatwa Gnat ora (insight but they ore not parotid, to &o region wbara liquid water is prestorf, tb* wet-butb and dry-Mb fampara- htrat ere tha tamo. tioa 31). Rg. 5 .... Arrangement of Families of Curves on ASHRAE Psychrometrie Chart