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A '''deaerator''' is a device that is widely used for the removal of air and other dissolved [[gas]]es from the feedwater to steam generating [[boiler]]s. In particular, dissolved [[oxygen]] in boiler feedwaters will cause serious corrosion damage in steam systems by attaching to the walls of metal piping and other metallic equipment and forming [[oxide]]s (rust). It also combines with any dissolved [[carbon dioxide]] to form carbonic acid that cause further corrosion. For those reasons, oxygen should be removed down to levels of 7 ppb by weight (0.0005 cm³/L} or less.<ref name=Spirax>[http://www.spiraxsarco.com/resources/steam-engineering-tutorials/the-boiler-house/pressurised-deaerators.asp#head5 Pressurized deaerators]</ref><ref>[http://www.termochimica.com/deaerator.html Deaerator Presentation]</ref> | A '''deaerator''' is a device that is widely used for the removal of air and other dissolved [[gas]]es from the feedwater to steam generating [[boiler]]s. In particular, dissolved [[oxygen]] in boiler feedwaters will cause serious corrosion damage in steam systems by attaching to the walls of metal piping and other metallic equipment and forming [[oxide]]s (rust). It also combines with any dissolved [[carbon dioxide]] to form carbonic acid that cause further corrosion. For those reasons, oxygen should be removed down to levels of 7 ppb by weight (0.0005 cm³/L} or less.<ref name=Spirax>[http://www.spiraxsarco.com/resources/steam-engineering-tutorials/the-boiler-house/pressurised-deaerators.asp#head5 Pressurized deaerators]</ref><ref>[http://www.termochimica.com/deaerator.html Deaerator Presentation]</ref> | ||
There are two basic types of deaerators, the tray-type and the spray-type:<ref name=Spirax/><ref>{{cite book|author=Robert Thurston Kent (Editor in Chief)|title=Kents’ Mechanical Engineers’ Handbook|edition=Eleventh edition (Two volumes)|publisher=John Wiley & Sons (Wiley Engineering Handbook Series)|year=1936|id=}}</ref><ref>{{cite book|author=Babcock & Wilcox Co.|title=Steam: Its Generation and Use|edition=41st edition|year=2005|id=ISBN 0-9634570-0-4}}</ref><ref>{{cite book|author=Thomas C. Elliott, Kao Chen, Robert Swanekamp (coauthors)|title=Standard Handbook of Powerplant Engineering|edition=2nd edition|publisher=McGraw-Hill Professional|year=1997|id=ISBN 0-07-019435-1}}</ref> | There are two basic types of deaerators, the tray-type and the spray-type:<ref name=Spirax/><ref>[http://www.stork-thermeq.nl/spraytype-deaerator-operating.aspx Spray-type Deaerator Operating Principal]</ref><ref>{{cite book|author=Robert Thurston Kent (Editor in Chief)|title=Kents’ Mechanical Engineers’ Handbook|edition=Eleventh edition (Two volumes)|publisher=John Wiley & Sons (Wiley Engineering Handbook Series)|year=1936|id=}}</ref><ref>{{cite book|author=Babcock & Wilcox Co.|title=Steam: Its Generation and Use|edition=41st edition|year=2005|id=ISBN 0-9634570-0-4}}</ref><ref>{{cite book|author=Thomas C. Elliott, Kao Chen, Robert Swanekamp (coauthors)|title=Standard Handbook of Powerplant Engineering|edition=2nd edition|publisher=McGraw-Hill Professional|year=1997|id=ISBN 0-07-019435-1}}</ref> | ||
* The ''tray-type'' (also called the ''cascade-type'') includes a vertical domed deaeration section mounted on top of a horizontal cylindrical vessel which serves as the deaerated boiler feedwater storage tank. | * The ''tray-type'' (also called the ''cascade-type'') includes a vertical domed deaeration section mounted on top of a horizontal cylindrical vessel which serves as the deaerated boiler feedwater storage tank. | ||
Revision as of 15:59, 15 July 2008
A deaerator is a device that is widely used for the removal of air and other dissolved gases from the feedwater to steam generating boilers. In particular, dissolved oxygen in boiler feedwaters will cause serious corrosion damage in steam systems by attaching to the walls of metal piping and other metallic equipment and forming oxides (rust). It also combines with any dissolved carbon dioxide to form carbonic acid that cause further corrosion. For those reasons, oxygen should be removed down to levels of 7 ppb by weight (0.0005 cm³/L} or less.[1][2]
There are two basic types of deaerators, the tray-type and the spray-type:[1][3][4][5][6]
- The tray-type (also called the cascade-type) includes a vertical domed deaeration section mounted on top of a horizontal cylindrical vessel which serves as the deaerated boiler feedwater storage tank.
- The spray-type consists only of a horizontal (or vertical) cylindrical vessel which serves as both the deaeration section and the boiler feedwater storage tank.
Types of deaerators
There are many different horizontal and vertical designs available from a number of manufacturers, and the actual construction details will vary from one manufacturer to another. Figures 1 and 2 are representative schematic diagrams that depict each of the two major types of deaerators.
Tray-type deaerator
The typical horizontal tray-type deaerator in Figure 1 has a vertical domed deaeration section mounted above a horizontal boiler feedwater storage vessel. Boiler feedwater enters the vertical dearation section above the perforated distillation trays and flows downward through the perforations. Low-pressure dearation steam enters the below the perforated trays and flows upward through the perforations. Some designs use various types distillation packing, rather than perforated trays, to provide good contact and mixing between the steam and the boiler feed water.
The steam strips the dissolved gas from the boiler feedwater and exits via the vent at the top of the domed section. Some designs may include a vent condenser to trap and recover any water entrained in the vented gas. The vent line usually includes a valve and just enough steam is allowed to escape with the vented gases to provide a small and visible telltale plume of steam.
The deaerated waster flows down into the horizontal storage vessel from where it is pumped to the steam generating boiler system. Low-pressure heating steam, which enters the horizontal vessel through a sparger pipe in the bottom of the vessel, is provided to keep the stored boiler feedwater warm. External insulation of the vessel is typically provided to minimize heat loss.
Spray-type deaerator
The operation of a spray-type deaerator, in contrast to the tray-type principle, is based on pre-deaeration. Water is first sprayed into the steamed space of the tank, followed by final deaeration in the water volume of the tank, where steam is brought in close contact with the water to be deaerated.
The pre-deaeration is achieved by a condensate sprayer. Under all operating conditions, the sprayer assures heating up of the condensate to saturation temperature and ensures a very large area for mass transfer. As the solubility of oxygen in water at saturation conditions is practically zero, oxygen transfers from the water droplets to the surrounding steam. As the steam condensates on the water, the concentration of oxygen in the direct vicinity of the sprayer is increased making it possible to vent a small amount of steam with a relatively high oxygen concentration.
Final deaeration in the water tank is achieved by injection of steam deep under the water level. A proper designed steam rake, taking into account the hydro-dynamics in the water tank, will result in optimal contact between water and steam, allowing the oxygen to move from the water to the steam in accordance with Henry's law.
Deaeration steam source
The steam is generally tapped from an extraction point of the turbine to improve the cycle efficiency. The deaerator therefore is also termed as one of the feedwater heaters in the turbine cycle. It is also possible to use flash steam. Since the deaerator is always hot, sufficient insulation is provided to minimize the heat loss.
References
- ↑ 1.0 1.1 Pressurized deaerators
- ↑ Deaerator Presentation
- ↑ Spray-type Deaerator Operating Principal
- ↑ Robert Thurston Kent (Editor in Chief) (1936). Kents’ Mechanical Engineers’ Handbook, Eleventh edition (Two volumes). John Wiley & Sons (Wiley Engineering Handbook Series).
- ↑ Babcock & Wilcox Co. (2005). Steam: Its Generation and Use, 41st edition. ISBN 0-9634570-0-4.
- ↑ Thomas C. Elliott, Kao Chen, Robert Swanekamp (coauthors) (1997). Standard Handbook of Powerplant Engineering, 2nd edition. McGraw-Hill Professional. ISBN 0-07-019435-1.