Amount of air that can be dissolved in water – decrease with temperature – increase with pressure
The amount of air that can be dissolved in water increase with the system pressure and decrease with the temperature.
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 or steam. If the water is cooled down at then again reheated, bubbles will not appear until the water starts to boil. The water is deaerated.
The solubility of air in water can be expressed as a solubility ratio:
Sa = ma / mw (1)
Sa = solubility ratio
ma = mass of air (lbm, kg)
mw = mass of water (lbm, kg)
Solution of air in water follows Henry’s Law – “the amount of air dissolved in a fluid is proportional with the pressure of the system” – and can be expressed as:
c = pg / kH (2)
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 contains approximately 35.6% oxygen compared to 21% in air.
Solubility of Air in Water
|Temperature (oF)||Gauge Pressure (psig)|
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)
- 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
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 an 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.