Deaeration in boiler feedwater is universally practised to treat water used in industrial boilers . Water fed to boilers is normally composed of raw make-up water and condensate returns , most commonly a mixture of the two. Both contain oxygen and carbon dioxide in solution in varying amounts . These gases, particularly at the elevated temperatures encountered in modern boilers, greatly accelerate the rate of corrosion and pitting of the steel surfaces within the boiler , as well as tube failures on piping and other plant apparatus in contact with the condensing steam.
Generally speaking, plants generating over 5,000 kg/h of steam can justify the installation of a Deaerator. If the percentage of make up to the system is above 50% and/or the CO2 content or alkalinity of the feedwater is high ( CO2 + HCO3 exceeding 100ppm ) the use of a Deaerator will result in worthwhile savings even below 5000 kg/h boiler capacity.
Deaeration is the mechanical removal of dissolved gasses such as oxygen and carbon dioxide from a fluid. Feedwater to the boiler should contain less than 0.005 cc of O2 per liter ( equals to 7 ppb) at all loads up to and including the rated capacity . This is the usual guarantee of Deaerator suppliers . In some cases actual Deaerator performance provides 0.002 to 0.003 cc per liter .
Unless a Deaerator is employed, chemical treatment may be necessary to remove last traces of oxygen. Powerhouse and sea going vessels, which use almost distilled water, still employ the use of oxygen scavengers for a final “ polishing” to remove oxygen to 0.00000 cc/ liter.
The modern Deaerator is the result of many years of development . The process involves the reduction in the solubility of dissolved gases in the water by raising its temperature. There are three main principles that must be met in the design of any Deaerator :
- The incoming water must be heated to the temperature corresponding to the saturated steam pressure inside the Deaerator. That is to say , that the water must be at the boiling temperature for the corresponding operating internal pressure , since theoretically, the solubility of O2 is zero at the boiling point.
- The heated water must be mechanically agitated by cascading over trays , packing, or by atomization through spray valves . By breaking down the water into thin films or droplets , the distance a bubble of gas has to travel to be released , is greatly reduced .
- Adequate steam must pass through the water to sweep out the gases after they are released , and to maintain an extremely low partial pressure of non-condensables . By maintaining an adequate steam flow through the Deaerator, the partial pressure of oxygen ( or other incondensable gases) in the steam is held at limits so low that the amount of these gases that can re –dissolve is immeasurable.
There are various types of Deaerators : counter-current flow type, atomizer type, parallel down-flow . Each design has advantages and disadvantages and the choice of type will be dependent on a number of factors.
The counter-current flow type is the most commonly used in power plants . The steam and water move in opposite directions and configuration may be horizontal or vertical , depending by required capacity to be treated ( see figures 1 & 2). These Deaerators can employ spray valves ( “spray” type) , or a combination of spray valves with trays ( “spray & tray” type) . A “spray” type is less costly ( only one vessel ) than a “spray & tray” ( usually two vessels , one for process named Tower and one for storage named Tank) because of the greater internal complexity of tray banks . Anyhow “spray & tray” is more reliable in terms of performance with high ability to deliver 7 ppb or less in boiler feed water . In addition, when a Deaerator is operated under varying load conditions or inlet water temperature, the “spray & tray “ type Deaerator gives the most satisfactory results over the entire operating range . This type is typically found in all power plants with a large scale of sizes , from small units ( around 4- 5 Tons/h ) to big and special sizes up to around 3,000 Tons/h or more as BFW treated capacity, each unit.
A variant of counter-current flow is the “packed type “ based on increasing the surface area to volume ratio of the feedwater for more efficient heat exchange between water and steam . Water is sprayed into a counter-flow steam atmosphere and it cascades through a packing composed by rings . The major removal of O2 takes place in the upper section with polishing in the lower section. Deaeration efficiency is high but this type can be used only in small plants ( under 50 Tons/h as BFW) .
The Atomizer type is made using one vessel in which incoming water is sprayed into the steam flow using a shower head . The water is then heated and deaerated by being directed past high velocity steam jets using a fixed or variable atomizer. They are cheaper to build but they are poorer performing when load changes rapidly.
Parallel down-flow type is characterized by steam flow parallel to the feed water flow. Also, potential liquid hold up in the tray bank is eliminated. More than 90% of deaeration is performed in the initial spraying of water into a steam atmosphere. The remaining dissolved gases are removed by the intimate contact of the incoming volume of pure steam flowing downward with the water , through the tray bank, resulting in effluent feed water with oxygen content of 7 ppb or less. This design is evidenced in old literature only , in my long experience I never saw a modern application in a power plant probably because its more complicated configuration of internals results in a higher costs , not convenient if compared with “spray & tray” design .
In conclusion , Deaeration is a critical operation in the boiler circuit. By removing dissolved gases , especially oxygen , the boiler materials are protected against corrosion and failure due to oxygen pitting . Many different designs have been developed with different advantages and disadvantages but the most important aspect to be considered by end users or contractors when they need to procure these kind of equipments is to select among suppliers which have a referenced experience in this special field .