1、1.1CHAPTER 1PSYCHROMETRICSComposition of Dry and Moist Air . 1.1U.S. Standard Atmosphere 1.1Thermodynamic Properties of Moist Air 1.2Thermodynamic Properties of Water at Saturation 1.2Humidity Parameters 1.2Perfect Gas Relationships for Dry and Moist Air . 1.12Thermodynamic Wet-Bulb and Dew-Point Te
2、mperature . 1.13Numerical Calculation of Moist Air Properties 1.13Psychrometric Charts 1.14Typical Air-Conditioning Processes 1.16Transport Properties of Moist Air . 1.19Symbols . 1.19SYCHROMETRICS uses thermodynamic properties to ana-Plyze conditions and processes involving moist air. This chapterd
3、iscusses perfect gas relations and their use in common heating,cooling, and humidity control problems. Formulas developed byHerrmann et al. (2009) may be used where greater precision isrequired.Herrmann et al. (2009), Hyland and Wexler (1983a, 1983b), andNelson and Sauer (2002) developed formulas fo
4、r thermodynamicproperties of moist air and water modeled as real gases. However,perfect gas relations can be substituted in most air-conditioningproblems. Kuehn et al. (1998) showed that errors are less than 0.7%in calculating humidity ratio, enthalpy, and specific volume of satu-rated air at standa
5、rd atmospheric pressure for a temperature range of60 to 120F. Furthermore, these errors decrease with decreasingpressure.COMPOSITION OF DRY AND MOIST AIRAtmospheric air contains many gaseous components as well aswater vapor and miscellaneous contaminants (e.g., smoke, pollen,and gaseous pollutants n
6、ot normally present in free air far from pol-lution sources).Dry air is atmospheric air with all water vapor and contaminantsremoved. Its composition is relatively constant, but small variationsin the amounts of individual components occur with time, geo-graphic location, and altitude. Harrison (196
7、5) lists the approximatepercentage composition of dry air by volume as: nitrogen, 78.084;oxygen, 20.9476; argon, 0.934; neon, 0.001818; helium, 0.000524;methane, 0.00015; sulfur dioxide, 0 to 0.0001; hydrogen, 0.00005;and minor components such as krypton, xenon, and ozone, 0.0002.Harrison (1965) and
8、 Hyland and Wexler (1983a) used a value 0.0314(circa 1955) for carbon dioxide. Carbon dioxide reached 0.0379 in2005, is currently increasing by 0.00019 percent per year and is pro-jected to reach 0.0438 in 2036 (Gatley et al. 2008; Keeling andWhorf 2005a, 2005b). Increases in carbon dioxide are offs
9、et bydecreases in oxygen; consequently, the oxygen percentage in 2036 isprojected to be 20.9352. Using the projected changes, the relativemolecular mass for dry air for at least the first half of the 21st centuryis 28.966, based on the carbon-12 scale. The gas constant for dry airusing the current M
10、ohr and Taylor (2005) value for the universal gasconstant isRda= 1545.349/28.966 = 53.350 ftlbf/lbdaR (1)Moist air is a binary (two-component) mixture of dry air andwater vapor. The amount of water vapor varies from zero (dry air) toa maximum that depends on temperature and pressure. Saturation isa
11、state of neutral equilibrium between moist air and the condensedwater phase (liquid or solid); unless otherwise stated, it assumes aflat interface surface between moist air and the condensed phase.Saturation conditions change when the interface radius is very small(e.g., with ultrafine water droplet
12、s). The relative molecular mass ofwater is 18.015268 on the carbon-12 scale. The gas constant forwater vapor isRw= 1545.349/18.015268 = 85.780 ftlbf/lbwR (2)U.S. STANDARD ATMOSPHEREThe temperature and barometric pressure of atmospheric air varyconsiderably with altitude as well as with local geograp
13、hic andweather conditions. The standard atmosphere gives a standard of ref-erence for estimating properties at various altitudes. At sea level,standard temperature is 59F; standard barometric pressure is14.696 psia or 29.921 in. Hg. Temperature is assumed to decreaselinearly with increasing altitude
14、 throughout the troposphere (loweratmosphere), and to be constant in the lower reaches of the strato-sphere. The lower atmosphere is assumed to consist of dry air thatbehaves as a perfect gas. Gravity is also assumed constant at the stan-dard value, 32.1740 ft/s2. Table 1 summarizes property data fo
15、r alti-tudes to 30,000 ft.Pressure values in Table 1 may be calculated fromp = 14.696(1 6.8754 106Z)5.2559(3)The equation for temperature as a function of altitude ist = 59 0.00356620Z (4)whereZ = altitude, ftp = barometric pressure, psiat =temperature, FThe preparation of this chapter is assigned t
16、o TC 1.1, Thermodynamics andPsychrometrics.Table 1 Standard Atmospheric Data for Altitudes to 30,000 ftAltitude, ft Temperature, F Pressure, psia1000 62.6 15.236500 60.8 14.9660 59.0 14.696500 57.2 14.4301,000 55.4 14.1752,000 51.9 13.6643,000 48.3 13.1734,000 44.7 12.6825,000 41.2 12.2306,000 37.6
17、11.7787,000 34.0 11.3418,000 30.5 10.9149,000 26.9 10.50610,000 23.4 10.10815,000 5.5 8.29620,000 12.3 6.75830,000 47.8 4.371Source: Adapted from NASA (1976).1.2 2013 ASHRAE HandbookFundamentalsEquations (3) and (4) are accurate from 16,500 ft to 36,000 ft.For higher altitudes, comprehensive tables
18、of barometric pressureand other physical properties of the standard atmosphere, in both SIand I-P units, can be found in NASA (1976).THERMODYNAMIC PROPERTIES OF MOIST AIRTable 2, developed from formulas by Herrmann et al. (2009),shows values of thermodynamic properties of moist air based on theInter
19、national Temperature Scale of 1990 (ITS-90). This ideal scalediffers slightly from practical temperature scales used for physicalmeasurements. For example, the standard boiling point for water (at14.696 psia) occurs at 211.95F on this scale rather than at the tra-ditional 212F. Most measurements are
20、 currently based on ITS-90(Preston-Thomas 1990).The following properties are shown in Table 2:t = Fahrenheit temperature, based on the ITS-90 and expressedrelative to absolute temperature T in degrees Rankine (R) by thefollowing relation:T = t + 459.67Ws= humidity ratio at saturation; gaseous phase
21、(moist air) exists inequilibrium with condensed phase (liquid or solid) at giventemperature and pressure (standard atmospheric pressure). Atgiven values of temperature and pressure, humidity ratio W canhave any value from zero to Ws.vda= specific volume of dry air, ft3/lbda.vas= vs vda, difference b
22、etween specific volume of moist air at saturation and that of dry air, ft3/lbda, at same pressure and temperature.vs= specific volume of moist air at saturation, ft3/lbda.hda= specific enthalpy of dry air, Btu/lbda. In Table 2, hdais assigned avalue of 0 at 0F and standard atmospheric pressure.has=
23、hs hda, difference between specific enthalpy of moist air atsaturation and that of dry air, Btu/lbda, at same pressure andtemperature.hs= specific enthalpy of moist air at saturation, Btu/lbda.sda= specific entropy of dry air, Btu/lbdaR. In Table 2, sdaisassigned a value of 0 at 0F and standard atmo
24、spheric pressure.ss= specific entropy of moist air at saturation Btu/lbdaR.THERMODYNAMIC PROPERTIES OF WATER AT SATURATIONTable 3 shows thermodynamic properties of water at saturationfor temperatures from 80 to 300F, calculated by the formulationsdescribed by IAPWS (2007). Symbols in the table follo
25、w standardsteam table nomenclature. These properties are based on ITS-90.The internal energy and entropy of saturated liquid water are bothassigned the value zero at the triple point, 32.018F. Between thetriple-point and critical-point temperatures of water, two states (sat-urated liquid and saturat
26、ed vapor) may coexist in equilibrium.The water vapor saturation pressure is required to determinea number of moist air properties, principally the saturation humid-ity ratio. Values may be obtained from Table 3 or calculated fromthe following formulas (Hyland and Wexler 1983b). The 1983 for-mulas ar
27、e within 300 ppm of the latest IAPWS formulations. Forhigher accuracy, developers of software and others are referred toIAPWS (2007, 2008).The saturation pressure over ice for the temperature range of148 to 32F is given byln pws= C1/T + C2+ C3T + C4T2+ C5T3+ C6T4+ C7ln T (5)whereC1= 1.021 416 5 E+04
28、C2= 4.893 242 8 E+00C3= 5.376 579 4 E03C4= 1.920 237 7 E07C5= 3.557 583 2 E10C6= 9.034 468 8 E14C7= 4.163 501 9 E00The saturation pressure over liquid water for the temperature rangeof 32 to 392F is given bylnpws= C8/T + C9+ C10T + C11T2+ C12T3+ C13ln T (6)whereC8= 1.044 039 7 E+04C9= 1.129 465 0 E+
29、01C10= 2.702 235 5 E02C11= 1.289 036 0 E05C12= 2.478 068 1 E09C13= 6.545 967 3 E+00In both Equations (5) and (6),pws= saturation pressure, psiaT = absolute temperature, R = F + 459.67The coefficients of Equations (5) and (6) were derived from theHyland-Wexler equations, which are given in SI units.
30、Because ofrounding errors in the derivations and in some computers calculat-ing precision, results from Equations (5) and (6) may not agree pre-cisely with Table 3 values.The vapor pressure psof water in saturated moist air differs neg-ligibly from the saturation vapor pressure pwsof pure water at t
31、hesame temperature. Consequently, pscan be used in equations inplace of pwswith very little error:ps= xwspwhere xwsis the mole fraction of water vapor in saturated moist airat temperature t and pressure p, and p is the total barometric pressureof moist air.HUMIDITY PARAMETERSBasic ParametersHumidity
32、 ratio W (alternatively, the moisture content or mixingratio) of a given moist air sample is defined as the ratio of the massof water vapor to the mass of dry air in the sample:W = Mw/Mda(7)W equals the mole fraction ratio xw/xdamultiplied by the ratio ofmolecular masses (18.015268/28.966 = 0.621945
33、):W = 0.621945xw/xda(8)Specific humidity is the ratio of the mass of water vapor tototal mass of the moist air sample: = Mw/(Mw+ Mda) (9a)In terms of the humidity ratio, = W/(1 + W) (9b)Absolute humidity (alternatively, water vapor density) dvis theratio of the mass of water vapor to total volume of
34、 the sample:dv= Mw/V (10)Density of a moist air mixture is the ratio of total mass to totalvolume: = (Mda+ Mw)/V = (1/v)(1 + W) (11)where v is the moist air specific volume, ft3/lbda, as defined byEquation (26).Psychrometrics 1.3Table 2 Thermodynamic Properties of Moist Air at Standard Atmospheric P
35、ressure, 14.696 psiaTemp., FtHumidity RatioWs, lbw/lbdaSpecific Volume, ft3/lbdaSpecific Enthalpy, Btu/lbdaSpecific Entropy, Btu/lbdaFTemp., Ftvdavasvshdahashssdass80 0.0000049 9.553 0.000 9.553 19.218 0.005 19.213 0.04593 0.04592 8079 0.0000053 9.578 0.000 9.578 18.977 0.005 18.972 0.04530 0.04528
36、7978 0.0000057 9.603 0.000 9.604 18.737 0.006 18.731 0.04467 0.04465 7877 0.0000062 9.629 0.000 9.629 18.497 0.006 18.490 0.04404 0.04402 7776 0.0000067 9.654 0.000 9.654 18.256 0.007 18.250 0.04341 0.04339 7675 0.0000072 9.680 0.000 9.680 18.016 0.007 18.009 0.04279 0.04277 7574 0.0000078 9.705 0.0
37、00 9.705 17.776 0.008 17.768 0.04216 0.04214 7473 0.0000084 9.730 0.000 9.730 17.535 0.009 17.527 0.04154 0.04152 7372 0.0000090 9.756 0.000 9.756 17.295 0.009 17.286 0.04092 0.04090 7271 0.0000097 9.781 0.000 9.781 17.055 0.010 17.045 0.04030 0.04027 7170 0.0000104 9.806 0.000 9.806 16.815 0.011 16
38、.804 0.03968 0.03966 7069 0.0000112 9.832 0.000 9.832 16.574 0.012 16.563 0.03907 0.03904 6968 0.0000120 9.857 0.000 9.857 16.334 0.012 16.321 0.03845 0.03842 6867 0.0000129 9.882 0.000 9.882 16.094 0.013 16.080 0.03784 0.03781 6766 0.0000139 9.908 0.000 9.908 15.853 0.014 15.839 0.03723 0.03719 666
39、5 0.0000149 9.933 0.000 9.933 15.613 0.015 15.598 0.03662 0.03658 6564 0.0000160 9.958 0.000 9.959 15.373 0.017 15.356 0.03601 0.03597 6463 0.0000172 9.984 0.000 9.984 15.132 0.018 15.115 0.03541 0.03536 6362 0.0000184 10.009 0.000 10.009 14.892 0.019 14.873 0.03480 0.03475 6261 0.0000198 10.034 0.0
40、00 10.035 14.652 0.020 14.632 0.03420 0.03414 6160 0.0000212 10.060 0.000 10.060 14.412 0.022 14.390 0.03360 0.03354 6059 0.0000227 10.085 0.000 10.085 14.171 0.023 14.148 0.03300 0.03293 5958 0.0000243 10.110 0.000 10.111 13.931 0.025 13.906 0.03240 0.03233 5857 0.0000260 10.136 0.000 10.136 13.691
41、 0.027 13.664 0.03180 0.03173 5756 0.0000279 10.161 0.000 10.161 13.451 0.029 13.422 0.03120 0.03113 5655 0.0000298 10.186 0.000 10.187 13.210 0.031 13.180 0.03061 0.03053 5554 0.0000319 10.212 0.001 10.212 12.970 0.033 12.937 0.03002 0.02993 5453 0.0000341 10.237 0.001 10.237 12.730 0.035 12.695 0.
42、02942 0.02933 5352 0.0000365 10.262 0.001 10.263 12.490 0.038 12.452 0.02883 0.02874 5251 0.0000390 10.288 0.001 10.288 12.249 0.040 12.209 0.02825 0.02814 5150 0.0000416 10.313 0.001 10.314 12.009 0.043 11.966 0.02766 0.02755 5049 0.0000445 10.338 0.001 10.339 11.769 0.046 11.723 0.02707 0.02695 49
43、48 0.0000475 10.364 0.001 10.364 11.529 0.049 11.479 0.02649 0.02636 4847 0.0000507 10.389 0.001 10.390 11.289 0.053 11.236 0.02591 0.02577 4746 0.0000541 10.414 0.001 10.415 11.048 0.056 10.992 0.02532 0.02518 4645 0.0000577 10.439 0.001 10.440 10.808 0.060 10.748 0.02474 0.02459 4544 0.0000615 10.
44、465 0.001 10.466 10.568 0.064 10.504 0.02417 0.02400 4443 0.0000656 10.490 0.001 10.491 10.328 0.068 10.259 0.02359 0.02341 4342 0.0000699 10.515 0.001 10.517 10.087 0.073 10.015 0.02301 0.02283 4241 0.0000744 10.541 0.001 10.542 9.847 0.078 9.770 0.02244 0.02224 4140 0.0000793 10.566 0.001 10.567 9
45、.607 0.083 9.524 0.02187 0.02166 4039 0.0000844 10.591 0.001 10.593 9.367 0.088 9.279 0.02129 0.02107 3938 0.0000898 10.617 0.002 10.618 9.127 0.094 9.033 0.02072 0.02049 3837 0.0000956 10.642 0.002 10.644 8.886 0.100 8.787 0.02015 0.01990 3736 0.0001017 10.667 0.002 10.669 8.646 0.106 8.540 0.01959
46、 0.01932 3635 0.0001081 10.693 0.002 10.695 8.406 0.113 8.293 0.01902 0.01874 3534 0.0001150 10.718 0.002 10.720 8.166 0.120 8.046 0.01846 0.01816 3433 0.0001222 10.743 0.002 10.745 7.926 0.128 7.798 0.01789 0.01757 3332 0.0001298 10.769 0.002 10.771 7.685 0.136 7.550 0.01733 0.01699 3231 0.0001379
47、10.794 0.002 10.796 7.445 0.144 7.301 0.01677 0.01641 3130 0.0001465 10.819 0.003 10.822 7.205 0.153 7.052 0.01621 0.01583 3029 0.0001555 10.845 0.003 10.847 6.965 0.163 6.802 0.01565 0.01525 2928 0.0001650 10.870 0.003 10.873 6.725 0.173 6.552 0.01509 0.01467 2827 0.0001751 10.895 0.003 10.898 6.48
48、5 0.184 6.301 0.01454 0.01409 2726 0.0001857 10.920 0.003 10.924 6.244 0.195 6.050 0.01398 0.01351 2625 0.0001970 10.946 0.003 10.949 6.004 0.207 5.797 0.01343 0.01293 2524 0.0002088 10.971 0.004 10.975 5.764 0.219 5.545 0.01288 0.01234 2423 0.0002213 10.996 0.004 11.000 5.524 0.233 5.291 0.01233 0.01176 2322 0.0002345 11.022 0.004 11.026 5.284 0.246 5.037 0.01178 0.01118 2221 0.0002485 11.047 0.004 11.051 5.044 0.261 4.782 0.01123 0.01060 2120 0.0002632 11
