Deaerator

Introduction

Deaerators serve three functions:

• Removal of oxygen in the water
• Heating the water
• Storage of deaerated and heated water

Corrosion

In water, the presence of dissolved gases, particularly oxygen and carbon dioxide, causes accelerated corrosion. The corrosion method is especially rapid at elevated temperatures such as are encountered in boilers and heat exchange equipment.  The primary function of the deaerator is to prevent this corrosion by removing the dissolved gases from all sources of water entering the boiler.  Figure 1 illustrates the expected oxygen content of water at various temperatures.

Iron goes into solution in pure water to a slight extent according to the formula⁽³⁾:

Fe + 2 H₂O → Fe(OH)₂ + H₂

but the ferrous hydrate (Fe(OH)₂) formed is alkaline and raises the pH value.  At a definite pH value further dissolving of iron is stopped.  However, if oxygen is present it immediately oxidizes the ferrous hydrate forming ferric hydrate (Fe(OH)₃) which is insoluble and precipitates, permitting more iron to go into solution and thus the reaction continues until all oxygen is dissipated.  It is evident, that the removal of oxygen and carbon dioxide from solution is important.

Mechanical Deaeration

Deaeration is the mechanical removal of dissolved gases from a fluid.   The process of deaeration is most frequently applied in boiler feed water heaters to protect piping, boilers and condensate equipment from corrosion.  The three principles of mechanical deaeration are:

• Heating
• Mechanical agitation
• Gas Removal

Water must be heated to full saturation temperature (ie. boiling point) corresponding to the steam pressure in the unit.  Theoretically, the solubility of any gas is zero at the boiling point of the liquid, complete gas removal is not possible unless the liquid is kept at the boiling temperature.

The heated water must be mechanically agitated by spraying, cascading over trays or atomization to expose maximum surface area to the scrubbing atmosphere.  This permits complete release of the gases since the distance that the gas bubble must travel for release is decreased.  Thorough agitation also overcomes tendencies of surface tension and viscosity to retain the gas bubbles and increases the rate of gas diffusion from the liquid to the surrounding atmosphere.

Adequate steam must be passed through the water to scrub out and carry away the gases after they are released from the liquid.  Extremely low partial gas pressures must be maintained to minimize the concentration of gases dissolved in the liquid.