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20/03/13 Air Solubility in Water
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Air Solubility in Water
Amount of air that can be dissolved in water - decrease with temperature - increase withpressure
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The amount of air that can be dissolved in water increase with the system pressure and decrease with thetemperature.
Deaeration
When fresh water is heated up, air bubbles start to form. The water can obviously not hold the dissolved air with
increasing temperature. At 100 oC (212 oF) water starts to boil - the bubbles are formed by evaporated water orsteam. 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
The 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)
Henry's Law
Solution of air in water follows Henry's Law - "the amount of air dissolved in a fluid is proportional with thepressure of 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 the gas
The solubility of oxygen in water is higher than the solubility of nitrogen. Air dissolved in water containsapproximately 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:
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 0.0589 0.0750 0.0990 0.1201
100 0.0136 0.0334 0.0536 0.0730 0.0928 0.1126
110 0.0126 0.0314 0.0501 0.0699 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
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20/03/13 Air Solubility in Water
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210 0.0004 0.0155 0.0306 0.0457 0.0607 0.0758
Example - Calculating Air Dissolved in Water
Air dissolved in 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 is the sum of Nitrogen and Oxygen:
ca = (0.0089 g/litre) + (0.0138 g/litre)
= 0.0227 g/litre
~ 0.023 g/kg
Calculating 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
Deariation
For maximum deaeration the water should be heated up to 212 oF (100 oC) at atmospheric pressure. This iscommon in steam systems where fresh water is supplied to the system through an heated deaeration tower on thetop of the condensate receiver tank.
It is also common to install deaeration devices on the hot sides of heat exchangers in heating distribution systemsto force the dissolved air out of the system.
Note! Since the maximum deaeration is limited by the minimum static pressure and maximum temperature in thesystem - 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.
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