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Amount of air that can be dissolved in water - decreases with temperature and increases with pressure
The amount of air that can be dissolved in water increases with pressure and decreases with temperature.
Deaeration
Word Air 1 7 30
When fresh water is heated up air bubbles start to form. The water can obviously not hold the dissolved air with increased temperature. At 100 oC (212 oF) water starts to boil - the bubbles are formed by evaporated water or steam. If the water is cooled down and then again reheated, bubbles will not appear until the water starts to boil. The water is deaerated.
Solubility Ratio
Solubility of air in water can be expressed as a solubility ratio:
Sa = ma / mw (1)
where
Sa = solubility ratio
ma = mass of air (lbm, kg)
mw = mass of water (lbm, kg)
Sa = solubility ratio
ma = mass of air (lbm, kg)
mw = mass of water (lbm, kg)
Henry's Law
Solubility of air in water follows Henry's Law - 'the amount of air dissolved in a fluid is proportional to the pressure in the system' - and can be expressed as:
c = pg / kH (2)
where
c= solubility of dissolved gas
kH= proportionality constant depending on the nature of the gas and the solvent
pg= partial pressure of gas (Pa, psi)
c= solubility of dissolved gas
kH= proportionality constant depending on the nature of the gas and the solvent
pg= partial pressure of gas (Pa, psi)
The solubility of oxygen in water is higher than the solubility of nitrogen. Air dissolved in water contains approximately 35.6% oxygen compared to 21% in air.
Solubility of Air in Water
Solubility of air in water - expressed as ratio of absorbed air volume to water volume:
Solubility (vair/vwater) | ||||||
---|---|---|---|---|---|---|
Temperature (oF) | Gauge Pressure (psig) | |||||
0 | 20 | 40 | 60 | 80 | 100 | |
40 | 0.0258 | 0.0613 | 0.0967 | 0.1321 | 0.1676 | 0.2030 |
50 | 0.0223 | 0.0529 | 0.0836 | 0.1143 | 0.1449 | 0.1756 |
60 | 0.0197 | 0.0469 | 0.0742 | 0.1014 | 0.1296 | 0.1559 |
70 | 0.0177 | 0.0423 | 0.0669 | 0.0916 | 0.1162 | 0.1408 |
80 | 0.0161 | 0.0387 | 0.0614 | 0.0840 | 0.1067 | 0.1293 |
90 | 0.0147 | 0.0358 | 0.0569 | 0.0780 | 0.0990 | 0.1201 |
100 | 0.0136 | 0.0334 | 0.0532 | 0.0730 | 0.0928 | 0.1126 |
110 | 0.0126 | 0.0314 | 0.0501 | 0.0689 | 0.0877 | 0.1065 |
120 | 0.0117 | 0.0296 | 0.0475 | 0.0654 | 0.0833 | 0.1012 |
130 | 0.0107 | 0.0280 | 0.0452 | 0.0624 | 0.0796 | 0.0968 |
140 | 0.0098 | 0.0265 | 0.0432 | 0.0598 | 0.0765 | 0.0931 |
150 | 0.0089 | 0.0251 | 0.0413 | 0.0574 | 0.0736 | 0.0898 |
160 | 0.0079 | 0.0237 | 0.0395 | 0.0553 | 0.0711 | 0.0869 |
170 | 0.0068 | 0.0223 | 0.0378 | 0.0534 | 0.0689 | 0.0844 |
180 | 0.0055 | 0.0208 | 0.0361 | 0.0514 | 0.0667 | 0.0820 |
190 | 0.0041 | 0.0192 | 0.0344 | 0.0496 | 0.0647 | 0.0799 |
200 | 0.0024 | 0.0175 | 0.0326 | 0.0477 | 0.0628 | 0.0779 |
210 | 0.0004 | 0.0155 | 0.0306 | 0.0457 | 0.0607 | 0.0758 |
Example - Air Dissolved in Water
The amount of air dissolved in water - mass of air to volume/mass of water - can be calculated with Henry's law.
Henry Law's Constants at a system temperature of 25oC (77oF)
- Oxygen - O2 : 756.7 atm/(mol/litre)
- Nitrogen - N2 : 1600 atm/(mol/litre)
Molar Weights
- Oxygen - O2 : 31.9988 g/mol
- Nitrogen - N2 : 28.0134 g/mol
Partial fraction in Air
- Oxygen - O2 : ~ 0.21
- Nitrogen - N2 : ~ 0.79
Oxygen dissolved in the Water at atmospheric pressure can be calculated as:
co = (1 atm) 0.21 / (756.7 atm/(mol/litre)) (31.9988 g/mol)
=0.0089 g/litre
~ 0.0089 g/kg
Nitrogen dissolved in the Water at atmospheric pressure can be calculated as:
cn = (1 atm) 0.79 / (1600 atm/(mol/litre)) (28.0134 g/mol)
=0.0138 g/litre
~ 0.0138 g/kg
Since air mainly consists of Nitrogen and Oxygen - the air dissolved in the water can be calculated as:
ca =(0.0089 g/litre) + (0.0138 g/litre)
= 0.0227 g/litre
~ 0.023 g/kg
Calculating the air dissolved in water for some other pressures at temperature 25oC (77oF) can be summarized to:
Pressure, abs (atm) | 1 | 2 | 3 | 4 | 5 | 6 |
Dissolved Air in Water (25oC) (g/kg) | 0.023 | 0.045 | 0.068 | 0.091 | 0.114 | 0.136 |
Dissolved Oxygen in Fresh Water
Deaeration
For maximum deaeration the water should be heated up to 212 oF (100 oC) at atmospheric pressure. This is common in steam systems where fresh water is supplied to the system through a heated deaeration tower on the top of the condensate receiver tank.
It is also common to install deaeration devices on the hot sides of heat exchangers in heating distribution systems to force the dissolved air out of the system.
Note! Since the maximum deaeration is limited by the minimum static pressure and maximum temperature in the system - the best deaeration result is achieved in positions at the hottest and highest levels of the systems - and/or at the suction side of pumps.
Related Topics
- Material Properties - Material properties for gases, fluids and solids - densities, specific heats, viscosities and more
Related Documents
Word Air 1 7 35
- Air - Composition and Molecular Weight - Dry air is a mechanical mixture of nitrogen, oxygen, argon and several other gases in minor amounts
- Air - Density at varying pressure and constant temperatures - Figures and tables showing changes in air density at pressure varying from 1 to 10 000 bara (14.5 - 145000 psi) and constant, selected temperatures. Figures are given in different scales.
- Air - Density, Specific Weight and Thermal Expansion Coefficient at Varying Temperature and Constant Pressures - Online calculator, figures and tables showing density, specific weight and thermal expansion coefficient of air at temperatures ranging -100 to 1600 °C (-140 to 2900 °F) at atmospheric and higher pressure - Imperial and SI Units
- Air - Diffusion Coefficients of Gases in Excess of Air - Diffusion coefficients, D12, for gases in large excess of air at temperature varying from 0 - 400 °C
- Air - Dynamic and Kinematic Viscosity - Online calculator, figures and tables showing dynamic (absolute) and kinematic viscosity of air at temperatures ranging from -100 to 1600°C (-150 to 2900°F) at pressure ranging from 1 to 10 000 bara (14.5 - 145000 psia) - SI and Imperial Units
- Air - Prandtl Number - Figures and table showing changes in Prandtl number for air with changes in temperature and pressure
- Air - Properties at Gas-Liquid Equilibrium Conditions - Figures and tables showing how the properties of air changes along the boiling and condensation curves (temperature and pressure between triple point and critical point conditions). An air phase diagram are also given.
- Air - Specific Heat at Constant Temperature and Varying Pressure - Figures and table showing isobaric (Cp) and isochoric (Cv) specific heat of air at constant temperature and varying pressure ranging 0.01 to 10000 bara
- Air - Thermal Conductivity - Online calculator, figures and tables showing air thermal conductivity at varying temperature and pressure, SI and imperial units.
- Air - Thermal Diffusivity - Figures and tables showing dry air thermal diffusivity at varying temperarure and pressure, SI and Imperial units
- Feed Water Treatment to Avoid Corrosion - Make-up water to steam boilers should be treated with oxygen scavengers to avoid serious corrosion problems
- Oxygen - Density and Specific Weight - Online calculator, figures and tables showing density and specific weight of oxygen, O2, at varying temperature and pressure - Imperial and SI Units
- Oxygen - Dynamic and Kinematic Viscosity - Online calculator, figures and tables showing dynamic and kinematic viscosity of oxygen, O2, at varying temperature and pressure - Imperial and SI Units
- Oxygen - Solubility in Fresh Water and Seawater - Solubility of oxygen in equilibration with air in fresh water and seawater (salt water) - pressures ranging 1 - 4 bar abs
- Oxygen - Thermophysical properties - Chemical, Physical and Thermal Properties of Oxygen - O2
- Oxygen Gas - Specific Heat - Specific heat of Oxygen Gas - O2 - at temperatures ranging 175 - 6000 K
- Total Dissolved Solids - TDS - In a boiler generating steam impurities from the feedwater will concentrate in the boiling water
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