Properties of Air

 

The following is a list of the properties of air;

 

Air has weight.

Air is under pressure.

Air has temperature.

Air has a volume.

Air usually contains some water vapour.

Air usually has some velocity (speed).

General Properties of Air.

Weight:

Because the weight of air varies with pressure and temperature it has to be defined accurately. The following figures may be used.

The weight of dry air (no moisture content) at 0 deg C and under a normal atmospheric pressure of 1013 mbar is 1.293 kg/m3.

The weight of dry air (no moisture content) at 0 deg C and at a pressure of 1000 mbar (1 Bar) is 1.275 kg/m3.

 

Density of Air at 50% and 100% saturation

 

 

Temperature (oC)

Specific

Volume

(m3/kg)

@ 100% sat.

Density

(kg/m3)

@ 100% sat.

Specific

Volume

(m3/kg)

@ 50% sat.

Density (kg/m3)

@ 50% sat.

-2.0

0.7716

1.2960

0.7696

1.2994

-1.0

 

 

0.7726

1.2943

0.0

0.7780

1.2854

0.7757

1.2892

1.0

 

 

0.7787

1.2842

2.0

 

 

0.7817

1.2793

3.0

0.7877

1.2695

0.7848

1.2742

4.0

 

 

0.7879

1.2692

5.0

0.7944

1.2588

0.7910

1.2642

6.0

 

 

0.7941

1.2593

7.0

0.8011

1.2483

0.7972

1.2544

8.0

 

 

0.8003

1.2495

9.0

0.8080

1.2376

0.8035

1.2446

10.0

 

 

0.8067

1.2396

11.0

0.8151

1.2268

0.8099

1.2347

12.0

 

 

0.8131

1.2299

13.0

0.8224

1.2160

0.8163

1.2250

14.0

 

 

0.8196

1.2201

15.0

0.8299

1.2050

0.8229

1.2152

16.0

 

 

0.8262

1.2104

17.0

0.8376

1.1939

0.8296

1.2054

18.0

 

 

0.8330

1.2005

19.0

0.8456

1.1826

0.8365

1.1955

20.0

 

 

0.8399

1.1906

21.0

 

 

0.8434

1.1857

22.0

 

 

0.8470

1.1806

23.0

 

 

0.8506

1.1756

24.0

 

 

0.8542

1.1707

25.0

 

 

0.8579

1.1656

26.0

 

 

0.8617

1.1605

27.0

 

 

0.8655

1.1554

 

Pressure:

Air is under pressure; this is caused by gravity.

Air pressure at sea level is approximately 1013 mbar, which is about the same as 14.7 psi.

The reason for this pressure is because there is so much air stacked up on top of it. If you were higher up, say in and aeroplane, the air pressure outside the 'plane would be much lower.

We know that the air pressure at 18,000 ft (about 5500 metres) is approximately half that at sea level.

At 32,000 ft (about 10,000 metres) the air pressure is only a quarter of that at sea level.

The reason for the reduction in pressure is because there is less air stacked up on top at these high altitudes.

Temperature:

Air has temperature, an obvious statement really. Like most things around us, air expands when it gets hot and contracts when it gets cold. Temperature has an effect on Volume, and that Volume has an effect on Pressure. This is the basis of the Characteristic Equation, as shown below;

 

P1V1 / T1    =   P2V2 / T2

Where;

P          =          pressure (N/m2)

V         =          volume (m3)

T          =          temperature ( o Kelvin) i.e. oC  +  273

 

An important thing to remember is that whenever we use Temperatures and Pressures in a calculation, they are always absolute values i.e. degrees Kelvin and pressure in N/m2.


Volume:

Air occupies a specific volume. This volume is inter-related with pressure and temperature. If you squeeze air into a smaller space the air gets hotter. This is easily demonstrated when you pump up a bicycle tyre. The harder you pump, the hotter the air gets and the hotter the hand pump gets.

Because the amount of air contained within a box will vary with temperature and pressure, it is necessary to qualify the temperature and pressure.

Air usually contains some Water Vapour:

Air behaves a bit like a sponge, if there's any water around it will try to absorb it.

Like a sponge it can only hold just so much water before it becomes saturated. Again like a sponge, if you squeeze it (compress it) the water will drip out.

A dry sponge doesn't have any water in it; therefore it has a relative humidity of 0%.

A soaking wet sponge can't take in any more water because it's already saturated. Therefore this sponge has a relative humidity of 100%.

 

A sponge half full of water can take on board just as much again. This sponge has a relative humidity of 50%.

 

If you pick up the saturated sponge and squeeze it, water drips out. If you dip it in a bucket of water, keeping it squeezed, it doesn't absorb any water out of the bucket. We've demonstrated that by compressing the sponge we have reduced its ability to hold water. Although a squeezed sponge holds less water than an un-squeezed sponge, the squeezed sponge is nevertheless 100% saturated.

Air acts in the same way. If air is ‘squeezed’ or compressed in a compressor it can't hold as much water vapour as atmospheric air of the same temperature.

 

Hot air also has the ability to hold more water than cold air, however if you cool down hot air which has a 100% RH, the water vapour condenses out into liquid.

The amount of water vapour contained in air can be found from a psychrometric chart or tables.

Air usually has some Velocity:

You can see this every day, leaves getting whipped up by the breeze and being blown down a road. Outside air velocity is a function of wind strength.

The velocity of air in a room may be low at 0.25 m/s or much higher in a compressed air pipe.

In engineering terms air is a fluid and has fluid properties.

General Properties:

Air is a mixture of gasses, mainly nitrogen and oxygen. The typical composition of natural air is as follows.

 

Component

Mass % (dry air)

Volume % (dry air)

Oxygen

23.14

20.9476

Nitrogen

75.52

78.084

Argon

1.288

0.934

Carbon dioxide

0.048

0.0314

Hydrogen

0.000003

0.00005

Neon

0.00127

0.001818

Helium

0.000073

0.000524

Krypton

0.00033

0.000114

Xenon

0.000039

0.0000087

 

Air also contains water vapour and hard matter such as dust, microbes and pollen. These variables depend upon climatic conditions, which vary worldwide. The table therefore reflects the European average dry gas content of air, which may vary slightly in your area.

 

The mean molecular weight of dry air is approximately 28.97.

The molecular weight of water is only 18.

This means that volume by volume, moist air is lighter than dry air.

The amount of energy required to heat up dry air is less than the amount to heat up the same volume of wet air. This is because of the difference in Specific Heat between dry air and water vapour.

Everything has a Specific Heat. This is the amount of energy that is required to heat up a given mass of stuff compared to the amount of energy required to heat up the same mass of pure water. If we keep the pressure the same, the Specific Heat is termed Cp.

Cp for 100% water saturated air at atmospheric pressure is about 2000 Joules/kg deg K (or 2.00 kJ/kg deg K)

Cp for dry air at atmospheric pressure is 1020 Joules/kg deg K (or 1.02 kJ/kg deg K)

 

In real terms this means that it takes almost twice the amount of energy to heat up the water vapour in wet air, than it would take to heat up an equal number of molecules of dry air.

 

Temperature (oK)

Temperature

(oC)

Specific Heat Capacity (Cp)

of Dry Air

at constant pressure

(kJ/kg K)

250

-23

1.0031

275

+3

1.0038

300

27

1.0049

325

52

1.0063

350

77

1.0082

375

102

1.0106

400

127

1.0135

450

177

1.0206

500

227

1.0295