Document 5LZDXQdY4JnmDkVdpZo6pbw2e

56 CHAPTER 3 1951 Guide veniently used to interpolate values for the enthalpy, specific volume, and entropy of moist air from the data of Table 2. Within the estimated precision of the data of Table 2, at temperatures below 150 F, the volume v, enthalpy h, and entropy s, of moist air per `pound of dry air at any degree of saturation p may be computed from the simple relations: v = v, + pv,, (27) h = h, + ph,, (28) s = s. + ps,, (29) Thus, the degree of saturation is used in conjunction with Table 2 in the same manner as the quality is used with tables of the thermodynamic properties of steam. Correction of Table 2 for Temperatures Above 150 F The simple relations expressed in Equations 27, 28, and 29 give the properties of unsaturated air with satisfactory precision for most engineer ing design problems. Above 150 F, when greater precision than obtained by Table 2 is required, these simple relations can be adjusted by the addi tion of supplementary terms. To correct the volume, it is necessary to add a correction term v which is defined as n(l -- m)A 1 + aWpt (30) where a denotes the ratio of the apparent molecular weight of dry air (28.966) to the molecular weight of water (18.016) and is equal to 1.6078. Table 5 gives the values of the coefficient A for several higher temperatures, the value of p at. which the correction term 6 attains its maximum value, and the maximum value of v term there attained. The correction term for the enthalpy is - _ g(l -- p)B _ 1 + aWpt (31) Table 5. Coefficients A, B, C Appearing in Equations 30, 31, 32] Maximum . Values of Corrections Defined by Equations 30, 31, 32. Degree of Saturation at Which These Maxima Occur, Maximum Value of Correction Defined by Equation 33. Degree of Saturation at Which This Maximum Occurs, (Standard Atmospheric Pressure) l <F) A (ftVJbft) B (Btu/lba) ' C (Btu/F/ lb.) Pmax mviM (BtS/iba) (B^p/ (B*mtua/xF/ Jba) . 96 112 128 144 160 176 192 0.0018 0.0042 0.0096 0.0215 0.0487 0.1169 0.3363 0.0286 0.0650 0.1439 0.3149 0.6969 1.636 4.608 0.00004 6.00009 0.00020 0.00042 0.00091 0.00207 0.00567 0.0004 0.0010 0.0022 0.0047 0.0099 0.0207 0.0451 0.0069 0.0155 0.0332 0.0693 0.1418 0.2903 0.6180 0.00001 0.00002 0.00005 0.00009 0.00019 0.00037 0.00076 0.4925 0.4878 0.4805 0.4691 0.4511 0.4213 0.3662 0.0015 0.0025 0.0040 0.0065 0.0106 0.0179 0.0333 0.3650 0.3632 0.3602 0.3557 0.3485 0.3363 0.3129 \ Thermodynamics 57 Table 5 gives the values of the coefficient B and maximum values of h the mf""'"" values occurring at the same degree of saturation as v. "'Corrections for the entropy consist of two terms: S which is defined as p(1 -- p)C 1 + aWpt (32) and S, the so-called mixing entropy, which contributes the larger part of the error.' The mixing entropy is defined as 1 = 0.1579 [(1 + paW.) logn(l + paW, -- paW,\oga (jt) - /i(l + aW.) logu(l + aW,] (33) Table 5 lists the values of the coefficient C, the maximum values of s and s and the values of p at which they occur. The maximum s occurs at the same degree of saturation as the maximum values of 6 and 7t. Fig. 3. Basic Coordinates of A.S.H.V.E. Psychrometric Chart THE ASHVE PSYCHROMETRIC CHART A psychrometric chart is a graphical representation of the thermo dynamic properties of moist- air. To be of real value in the solution of engineering problems, it must have distinctive features which aid in problem analysis. An examination of psychrometric mass and energy balances shows that no properties other than enthalpy and humidity ratio are required for the solution of problems. It is logical, then, to use these properties as the coordinates of a pisychrometric chart. This was initially done by Mollier in 1923,7 8 and is the arrangement followed in the chart included with the Guide. The A.S.H.V.E. Psychrometric Chart is based on the best thermodynamic data available today, namely, those of Goff and Gratch, as given in Table 2. The chart is plotted on oblique coordinate's of enthalpy and humidity ratio; the enthalpy axis making an angle of approximately '40 deg with the humidity ratio axis. This is shown in Fig. 3. For practical use, the inclusion of dry-bulb and wet-bulb temperatures, . volumes, and indices of the condition of the air in relation to saturation, is necessary for locating and describing states on the chart. The arrange-