ASHRAE FUNDAMENTALS SI CH 1-2013 Psychrometrics.pdf

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 . 1.8Thermodynamic Wet-Bulb and Dew-Point Tem

2、perature . 1.9Numerical Calculation of Moist Air Properties 1.9Psychrometric Charts 1.10Typical Air-Conditioning Processes 1.12Transport Properties of Moist Air . 1.15Symbols . 1.15SYCHROMETRICS uses thermodynamic properties to ana-Plyze conditions and processes involving moist air. This chapterdisc

3、usses 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 for t

4、hermodynamicproperties 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 standard

5、atmospheric pressure for a temperature range of50 to 50C. 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 not n

6、ormally 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 (1965) l

7、ists 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 Hyl

8、and 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 offset b

9、ydecreases 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 Mohr

10、and Taylor (2005) value for the universal gasconstant isRda= 8314.472/28.966 = 287.042 J/(kgdaK) (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 state

11、 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 droplets). T

12、he relative molecular mass ofwater is 18.015 268 on the carbon-12 scale. The gas constant forwater vapor isRw= 8314.472/18.015 268 = 461.524 J/(kgwK) (2)U.S. STANDARD ATMOSPHEREThe temperature and barometric pressure of atmospheric air varyconsiderably with altitude as well as with local geographic

13、andweather conditions. The standard atmosphere gives a standard of ref-erence for estimating properties at various altitudes. At sea level,standard temperature is 15C; standard barometric pressure is101.325 kPa. Temperature is assumed to decrease linearly withincreasing altitude throughout the tropo

14、sphere (lower atmosphere),and to be constant in the lower reaches of the stratosphere. The loweratmosphere is assumed to consist of dry air that behaves as a perfectgas. Gravity is also assumed constant at the standard value, 9.806 65m/s2. Table 1 summarizes property data for altitudes to 10 000 m.P

15、ressure values in Table 1 may be calculated fromp = 101.325(1 2.25577 105Z)5.2559(3)The equation for temperature as a function of altitude ist =15 0.0065Z (4)whereZ = altitude, mp = barometric pressure, kPat =temperature, CThe preparation of this chapter is assigned to TC 1.1, Thermodynamics andPsyc

16、hrometrics.Table 1 Standard Atmospheric Data for Altitudes to 10 000 mAltitude, m Temperature, C Pressure, kPa500 18.2 107.4780 15.0 101.325500 11.8 95.4611000 8.5 89.8751500 5.2 84.5562000 2.0 79.4952500 1.2 74.6823000 4.5 70.1084000 11.0 61.6405000 17.5 54.0206000 24.0 47.1817000 30.5 41.0618000 3

17、7.0 35.6009000 43.5 30.74210 000 50 26.436Source: Adapted from NASA (1976).1.2 2013 ASHRAE HandbookFundamentals (SI)Equations (3) and (4) are accurate from 5000 m to 11 000 m.For higher altitudes, comprehensive tables of barometric pressureand other physical properties of the standard atmosphere, in

18、 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 theInternational Temperature Scale of 1990 (ITS-90). This ideal scalediffers slightly from

19、 practical temperature scales used for physicalmeasurements. For example, the standard boiling point for water (at101.325 kPa) occurs at 99.97C on this scale rather than at the tra-ditional 100C. Most measurements are currently based on ITS-90(Preston-Thomas 1990).The following properties are shown

20、in Table 2:t = Celsius temperature, based on the ITS-90 and expressed relativeto absolute temperature T in kelvins (K) by the followingrelation:T = t + 273.15Ws= humidity ratio at saturation; gaseous phase (moist air) exists inequilibrium with condensed phase (liquid or solid) at giventemperature an

21、d 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, m3/kgda.vas= vs vda, difference between specific volume of moist air at saturation and that of dry air, m3/kgda, at same pressur

22、e and temperature.vs= specific volume of moist air at saturation, m3/kgda.hda= specific enthalpy of dry air, kJ/kgda. In Table 2, hdais assigned avalue of 0 at 0C and standard atmospheric pressure.has= hs hda, difference between specific enthalpy of moist air atsaturation and that of dry air, kJ/kgd

23、a, at same pressure andtemperature.hs= specific enthalpy of moist air at saturation, kJ/kgda.sda= specific entropy of dry air, kJ/(kgdaK). In Table 2, sdaisassigned a value of 0 at 0C and standard atmospheric pressure.ss= specific entropy of moist air at saturation kJ/(kgdaK).THERMODYNAMIC PROPERTIE

24、S OF WATER AT SATURATIONTable 3 shows thermodynamic properties of water at saturationfor temperatures from 60 to 160C, calculated by the formulationsdescribed by IAPWS (2007). Symbols in the table follow standardsteam table nomenclature. These properties are based on ITS-90.The internal energy and e

25、ntropy of saturated liquid water are bothassigned the value zero at the triple point, 0.01C. Between thetriple-point and critical-point temperatures of water, two states (sat-urated liquid and saturated vapor) may coexist in equilibrium.The water vapor saturation pressure is required to determinea n

26、umber 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 are within 300 ppm of the latest IAPWS formulations. Forhigher accuracy, developers of software and ot

27、hers are referred toIAPWS (2007, 2008).The saturation pressure over ice for the temperature range of100 to 0C is given byln pws= C1/T + C2+ C3T + C4T2+ C5T3+ C6T4+ C7ln T (5)whereC1= 5.674 535 9 E+03C2= 6.392 524 7 E+00C3= 9.677 843 0 E03C4= 6.221 570 1 E07C5= 2.074 782 5 E09C6= 9.484 024 0 E13C7= 4

28、.163 501 9 E00The saturation pressure over liquid water for the temperature rangeof 0 to 200C is given bylnpws= C8/T + C9+ C10T + C11T2+ C12T3+ C13ln T (6)whereC8= 5.800 220 6 E+03C9= 1.391 499 3 E+00C10= 4.864 023 9 E02C11= 4.176 476 8 E05C12= 1.445 209 3 E8C13= 6.545 967 3 E+00In both Equations (5

29、) and (6),pws= saturation pressure, PaT = absolute temperature, K = C + 273.15 The coefficients of Equations (5) and (6) were derived from theHyland-Wexler equations. Because of rounding errors in the deriva-tions and in some computers calculating precision, results fromEquations (5) and (6) may not

30、 agree precisely 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 thesame temperature. Consequently, pscan be used in equations inplace of pwswith very little error:ps= xwspwhere xwsis the mole fractio

31、n of water vapor in saturated moist airat temperature t and pressure p, and p is the total barometric pressureof moist air.HUMIDITY PARAMETERSBasic ParametersHumidity ratio W (alternatively, the moisture content or mixingratio) of a given moist air sample is defined as the ratio of the massof water

32、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 (18.015 268/28.966 = 0.621 945):W = 0.621 945xw/xda(8)Specific humidity is the ratio of the mass of water vapor tototal mass of the moist air sample: = Mw/(Mw+ Md

33、a) (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 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

34、)where v is the moist air specific volume, m3/kgda, as defined byEquation (26).Psychrometrics 1.3Table 2 Thermodynamic Properties of Moist Air at Standard Atmospheric Pressure, 101.325 kPaTemp., CtHumidity RatioWs, kgw/kgdaSpecific Volume, m3/kgdaSpecific Enthalpy, kJ/kgdaSpecific Entropy, kJ/(kgdaK

35、)Temp., Ctvdavasvshdahashssdass60 0.0000067 0.6027 0.0000 0.6027 60.341 0.016 60.325 0.2494 0.2494 6059 0.0000076 0.6055 0.0000 0.6055 59.335 0.018 59.317 0.2447 0.2446 5958 0.0000087 0.6084 0.0000 0.6084 58.329 0.021 58.308 0.2400 0.2399 5857 0.0000100 0.6112 0.0000 0.6112 57.323 0.024 57.299 0.235

36、4 0.2353 5756 0.0000114 0.6141 0.0000 0.6141 56.317 0.027 56.289 0.2307 0.2306 5655 0.0000129 0.6169 0.0000 0.6169 55.311 0.031 55.280 0.2261 0.2260 5554 0.0000147 0.6198 0.0000 0.6198 54.305 0.035 54.269 0.2215 0.2213 5453 0.0000167 0.6226 0.0000 0.6226 53.299 0.040 53.258 0.2169 0.2167 5352 0.0000

37、190 0.6255 0.0000 0.6255 52.293 0.046 52.247 0.2124 0.2121 5251 0.0000215 0.6283 0.0000 0.6283 51.287 0.052 51.235 0.2078 0.2076 5150 0.0000243 0.6312 0.0000 0.6312 50.281 0.059 50.222 0.2033 0.2030 5049 0.0000275 0.6340 0.0000 0.6340 49.275 0.066 49.209 0.1988 0.1985 4948 0.0000311 0.6369 0.0000 0.

38、6369 48.269 0.075 48.194 0.1943 0.1940 4847 0.0000350 0.6397 0.0000 0.6397 47.263 0.085 47.179 0.1899 0.1895 4746 0.0000395 0.6425 0.0000 0.6426 46.257 0.095 46.162 0.1854 0.1850 4645 0.0000445 0.6454 0.0000 0.6454 45.252 0.107 45.144 0.1810 0.1805 4544 0.0000500 0.6482 0.0001 0.6483 44.246 0.121 44

39、.125 0.1766 0.1761 4443 0.0000562 0.6511 0.0001 0.6511 43.240 0.136 43.104 0.1722 0.1716 4342 0.0000631 0.6539 0.0001 0.6540 42.234 0.153 42.081 0.1679 0.1672 4241 0.0000708 0.6568 0.0001 0.6568 41.229 0.172 41.057 0.1635 0.1628 4140 0.0000793 0.6596 0.0001 0.6597 40.223 0.192 40.031 0.1592 0.1583 4

40、039 0.0000887 0.6625 0.0001 0.6626 39.217 0.215 39.002 0.1549 0.1539 3938 0.0000992 0.6653 0.0001 0.6654 38.212 0.241 37.970 0.1506 0.1495 3837 0.0001108 0.6682 0.0001 0.6683 37.206 0.269 36.936 0.1464 0.1451 3736 0.0001237 0.6710 0.0001 0.6711 36.200 0.301 35.899 0.1421 0.1408 3635 0.0001379 0.6738

41、 0.0001 0.6740 35.195 0.336 34.859 0.1379 0.1364 3534 0.0001536 0.6767 0.0002 0.6769 34.189 0.374 33.815 0.1337 0.1320 3433 0.0001710 0.6795 0.0002 0.6797 33.183 0.417 32.766 0.1295 0.1276 3332 0.0001902 0.6824 0.0002 0.6826 32.178 0.464 31.714 0.1253 0.1232 3231 0.0002113 0.6852 0.0002 0.6855 31.17

42、2 0.516 30.656 0.1211 0.1189 3130 0.0002345 0.6881 0.0003 0.6883 30.167 0.573 29.593 0.1170 0.1145 3029 0.0002602 0.6909 0.0003 0.6912 29.161 0.636 28.525 0.1129 0.1101 2928 0.0002883 0.6938 0.0003 0.6941 28.156 0.706 27.450 0.1088 0.1057 2827 0.0003193 0.6966 0.0004 0.6970 27.150 0.782 26.368 0.104

43、7 0.1013 2726 0.0003532 0.6994 0.0004 0.6998 26.144 0.866 25.278 0.1006 0.0969 2625 0.0003905 0.7023 0.0004 0.7027 25.139 0.958 24.181 0.0965 0.0924 2524 0.0004314 0.7051 0.0005 0.7056 24.133 1.059 23.074 0.0925 0.0880 2423 0.0004761 0.7080 0.0005 0.7085 23.128 1.170 21.958 0.0884 0.0835 2322 0.0005

44、251 0.7108 0.0006 0.7114 22.122 1.291 20.831 0.0844 0.0790 2221 0.0005787 0.7137 0.0007 0.7143 21.117 1.424 19.693 0.0804 0.0745 2120 0.0006373 0.7165 0.0007 0.7172 20.111 1.570 18.542 0.0765 0.0699 2019 0.0007013 0.7193 0.0008 0.7201 19.106 1.728 17.377 0.0725 0.0653 1918 0.0007711 0.7222 0.0009 0.

45、7231 18.100 1.902 16.198 0.0685 0.0607 1817 0.0008473 0.7250 0.0010 0.7260 17.095 2.091 15.003 0.0646 0.0560 1716 0.0009303 0.7279 0.0011 0.7290 16.089 2.298 13.791 0.0607 0.0513 1615 0.0010207 0.7307 0.0012 0.7319 15.084 2.523 12.560 0.0568 0.0465 1514 0.0011191 0.7336 0.0013 0.7349 14.078 2.769 11

46、.310 0.0529 0.0416 1413 0.0012261 0.7364 0.0014 0.7378 13.073 3.036 10.037 0.0490 0.0367 1312 0.0013425 0.7392 0.0016 0.7408 12.067 3.326 8.741 0.0452 0.0317 1211 0.0014689 0.7421 0.0017 0.7438 11.062 3.642 7.419 0.0413 0.0267 1110 0.0016062 0.7449 0.0019 0.7468 10.056 3.986 6.070 0.0375 0.0215 109

47、0.0017551 0.7478 0.0021 0.7499 9.050 4.358 4.692 0.0337 0.0163 98 0.0019166 0.7506 0.0023 0.7529 8.045 4.763 3.282 0.0299 0.0110 87 0.0020916 0.7534 0.0025 0.7560 7.039 5.202 1.838 0.0261 0.0055 76 0.0022812 0.7563 0.0028 0.7591 6.034 5.677 0.356 0.0223 0.0000 65 0.0024863 0.7591 0.0030 0.7622 5.028

48、 6.193 1.164 0.0186 0.0057 54 0.0027083 0.7620 0.0033 0.7653 4.023 6.750 2.728 0.0148 0.0115 43 0.0029482 0.7648 0.0036 0.7684 3.017 7.354 4.337 0.0111 0.0175 32 0.0032076 0.7677 0.0039 0.7716 2.011 8.007 5.995 0.0074 0.0236 21 0.0034877 0.7705 0.0043 0.7748 1.006 8.712 7.707 0.0037 0.0299 10 0.0037900 0.7733 0.0047 0.7780 0.000 9.475 9.475 0.0000 0.0364 01 0.004076 0.7762 0.0051 0.7813 1.006 10.198 11.203 0.0037 0.0427 12 0.004382 0.7790 0.0055 0.7845 2.011 10.970 12.981