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


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