Air Solubility in Water

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.

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 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.

Solubility Ratio

The solubility of air in water can be expressed as a solubility ratio:

S_a = m_a / m_w         (1)

where

S_a = solubility ratio

m_a = mass of air (lb_m, kg)

m_w = mass of water (lb_m, kg)

Henry's Law

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 = k_H p_g         (2)

where

c = solubility of dissolved gas

k_H = proportionality constant depending on the nature of the gas and the solvent

p_g = 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.

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 25^oC (77^oF)

• 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:

c_o = (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:

c_n = (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:

c_a = (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 25^oC (77^oF) can be summarized to:

Deariation

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.

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