Atmospheric air
Atmospheric air is a mixture of gases, water vapour and impurities (both solid and gaseous). The proportions of the important constituents for dry air at sea level are given in Table 2.1. This table may be considered representative of air at all the altitudes usually experienced in fan engineering.
|
Constituent |
Chemical symbol |
% by Volume |
% by Weight |
|
Nitrogen |
N2 |
78.09 |
75.52 |
|
Oxygen |
°2 |
20.95 |
23.15 |
|
Argon |
Ar |
0.93 |
1.28 |
|
Carbon dioxide |
CO, |
0.03 |
0.04 |
|
Also traces of helium, hydrogen, krypton, neon, ozone etc. |
|
Table 2.1 Constituents of atmospheric air |
Table 2.1 shows that air is primarily a mixture of nitrogen and oxygen,(both of which are diatomic gases) with molecular weight calculated from the average constituents.
For purposes of uniformity, standard air has been defined as air with a density of 1.2 kg/m3 and an absolute viscosity of 18.19 x 10-6 Pa. s. This is substantially equivalent to air at a temperature of 20 °C, 50% relative humidity and a barometric pressure of
101.325 kPa. The ratio of specific heats, (y), is taken to be 1.4, which is the expected value for a perfect diatomic gas.
The temperature and barometric pressure of atmospheric air vary widely with weather conditions and geographical location, most noticeably altitude. In order to simplify design, a standard atmosphere has been defined. This gives the atmospheric pressure, temperature and therefore, density with altitude. (See Table 2.2.)
|
Altitude M |
Temperature °C |
Atmospheric Pressure KPa |
Gas density kg/m3 |
|
0 |
15.00 |
101.32 |
1.230 |
|
100 |
14.35 |
100.13 |
1.215 |
|
200 |
13.70 |
98.94 |
1.201 |
|
300 |
13.05 |
97.77 |
1.189 |
|
400 |
12.40 |
96.61 |
1.177 |
|
500 |
11.76 |
95.46 |
1.166 |
|
600 |
11.11 |
94.32 |
1.155 |
|
700 |
10.46 |
93.20 |
1.145 |
|
800 |
9.81 |
92.08 |
1.134 |
|
900 |
9.16 |
90.98 |
1.123 |
|
1000 |
8.51 |
89.88 |
1.112 |
|
1100 |
7.86 |
88.80 |
1.102 |
|
1200 |
7.21 |
87.72 |
1.091 |
|
1300 |
6.56 |
86.66 |
1.080 |
|
1400 |
5.90 |
85.61 |
1.069 |
|
1500 |
5.25 |
84.56 |
1.058 |
|
1600 |
4.60 |
85.53 |
1.047 |
|
1700 |
3.95 |
82.50 |
1.037 |
|
1800 |
3.30 |
81.49 |
1.026 |
|
1900 |
2.65 |
80.49 |
1.016 |
|
2000 |
2.00 |
79.49 |
1.006 |
|
2100 |
1.35 |
78.51 |
0.996 |
|
2200 |
0.70 |
77.54 |
0.986 |
|
2300 |
0.53 |
76.57 |
0.976 |
|
Altitude M |
Temperature °C |
Atmospheric Pressure KPa |
Gas density kg/m3 |
|
2400 |
-0.60 |
75.62 |
0.967 |
|
2500 |
-1.25 |
74.68 |
0.957 |
|
2600 |
-1.90 |
73.74 |
0.948 |
|
2700 |
-2.55 |
72.82 |
0.938 |
|
2800 |
-3.20 |
71.91 |
0.929 |
|
2900 |
-3.85 |
71.00 |
0.919 |
|
3000 |
-4.50 |
70.11 |
0.909 |
|
3100 |
-5.15 |
69.23 |
0.900 |
|
3200 |
-5.80 |
68.35 |
0.890 |
|
3300 |
-6.46 |
67.48 |
0.880 |
|
3400 |
-7.11 |
66.62 |
0.871 |
|
3500 |
-7.76 |
65.77 |
0.862 |
|
Table 2.2 Standard atmospheric data versus altitude |
The density of atmospheric air is also a function of the humidity. Although the change in density with humidity is not large, it is often significant and air system designers should be cognizant of these changes. Remember that increasing humidity lowers the density since water vapour is lighter than dry air. The density of saturated air for various barometric and hygrometric conditions is shown in Table 2.3.
|
Dry- Bulb Temp °C |
Density of saturated air for various barometric pressures and dry bulb temperatures — kg/m3 |
|||||
|
Barometric pressure kPa |
||||||
|
97 |
98.5 |
100 |
101.5 |
103 |
104.5 |
|
|
-2.0 |
1.244981 |
1.263273 |
1.282390 |
1.302927 |
1.324194 |
1.340401 |
|
-1.0 |
1.239396 |
1.258667 |
1.277753 |
1.297353 |
1.319731 |
1.335505 |
|
0.0 |
1.234260 |
1.254012 |
1.272975 |
1.292141 |
1.315018 |
1.330532 |
|
1.0 |
1.229423 |
1.249325 |
1.268119 |
1.287163 |
1.310140 |
1.325506 |
|
2.0 |
1.224768 |
1.244618 |
1.263236 |
1.282324 |
1.305166 |
1.320447 |
|
3.0 |
1.220207 |
1.239902 |
1.258360 |
1.277553 |
1.300147 |
1.315376 |
|
4.0 |
1.215680 |
1.235188 |
1.253510 |
1.272800 |
1.295123 |
1.310307 |
|
5.0 |
1.211147 |
1.230483 |
1.248697 |
1.268037 |
1.290121 |
1.305254 |
|
6.0 |
1.206587 |
1.225792 |
1.243921 |
1.263247 |
1.285157 |
1.300224 |
|
7.0 |
1.201994 |
1.221119 |
1.239179 |
1.258431 |
1.280239 |
1.295225 |
|
8.0 |
1.197375 |
1.216468 |
1.234459 |
1.253595 |
1.275367 |
1.290260 |
|
9.0 |
1.192743 |
1.211838 |
1.229752 |
1.248752 |
1.270533 |
1.285328 |
|
10.0 |
1.188116 |
1.207227 |
1.225045 |
1.243920 |
1.265728 |
1.280428 |
|
11.0 |
1.183512 |
1.202631 |
1.220330 |
1.239113 |
1.260938 |
1.275553 |
|
12.0 |
1.178948 |
1.198047 |
1.215603 |
1.234343 |
1.256148 |
1.270693 |
|
13.0 |
1.174432 |
1.193466 |
1.210866 |
1.229616 |
1.251342 |
1.265837 |
|
14.0 |
1.169963 |
1.188879 |
1.206131 |
1.224925 |
1.246506 |
1.260970 |
|
15.0 |
1.165527 |
1.184277 |
1.201420 |
1.220251 |
1.241632 |
1.256073 |
|
16.0 |
1.161092 |
1.179644 |
1.196770 |
1.215560 |
1.236712 |
1.251125 |
|
17.0 |
1.156606 |
1.174968 |
1.192231 |
1.210795 |
1.231747 |
1.246101 |
|
18.0 |
1.151991 |
1.170232 |
1.187875 |
1.205877 |
1.226746 |
1.240975 |
|
19.0 |
1.146325 |
1.164887 |
1.182780 |
1.200987 |
1.222584 |
1.237641 |
|
20.0 |
1.141813 |
1.160033 |
1.78197 |
1.196304 |
1.217665 |
1.232675 |
|
21.0 |
1.137279 |
1.155335 |
1.173591 |
1.191607 |
1.212804 |
1.227740 |
|
22.0 |
1.132735 |
1.150742 |
1.168962 |
1.186898 |
1.207980 |
1.222830 |
|
23.0 |
1.128188 |
1.146207 |
1.164311 |
1.182174 |
1.203177 |
1.217939 |
|
24.0 |
1.123646 |
1.141691 |
1.159639 |
1.177435 |
1.198380 |
1.213061 |
|
25.0 |
1.119111 |
1.137164 |
1.154946 |
1.172681 |
1.193576 |
1.208190 |
|
26.0 |
1.114582 |
1.132601 |
1.150234 |
1.167912 |
1.188756 |
1.203320 |
|
27.0 |
1.110055 |
1.127983 |
1.145503 |
1.163126 |
1.183912 |
1.198445 |
|
28.0 |
1.105523 |
1.123300 |
1.140754 |
1.158323 |
1.179039 |
1.193559 |
|
Dry- Bulb Temp °C |
Density of saturated air for various barometric pressures and dry bulb temperatures — kg/m3 |
|||||
|
Barometric pressure kPa |
||||||
|
97 |
98.5 |
100 |
101.5 |
103 |
104.5 |
|
|
29.0 |
1.100978 |
1.118548 |
1.135988 |
1.153503 |
1.174134 |
1.188656 |
|
30.0 |
1.096404 |
1.113730 |
1.131206 |
1.148664 |
1.169195 |
1.183730 |
|
31.0 |
1.091787 |
1.108856 |
1.126408 |
1.143808 |
1.164226 |
1.178775 |
|
32.0 |
1.087106 |
1.103942 |
1.121596 |
1.138932 |
1.159230 |
1.173786 |
|
33.0 |
1.082339 |
1.099014 |
1.116769 |
1.134037 |
1.154213 |
1.168756 |
|
34.0 |
1.077460 |
1.094100 |
1.111930 |
1.129122 |
1.149185 |
1.163679 |
|
35.0 |
1.072440 |
1.089240 |
1.107079 |
1.124186 |
1.144155 |
1.158549 |
|
36.0 |
1.067247 |
1.084478 |
1.102216 |
1.119229 |
1.139139 |
1.153361 |
|
37.0 |
1.061846 |
1.079865 |
1.097342 |
1.114250 |
1.134151 |
1.148108 |
|
38.0 |
1.056198 |
1.075460 |
1.092459 |
1.109249 |
1.129210 |
1.142784 |
|
Table 2.3 Density of saturated air at various temperatures and barometric pressures |
Posted in Fans Ventilation A Practical Guide