A dearearator is a device widely used for the removal of air and other dissolved gases from the feedwater to steam generating boilers. In particular, dissolved oxygen in boiler feed waters will cause a 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 causes further corrosion. Most deaerator are designed to remove oxygen down to levels of 7 ppb by weight (0.0005 cm³/L) or less.
There are two basic types of deaerators, the tray type and the spray.
- 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 feed water storage tank.
- The spray type consists only of a horizontal (or vertical) cylindrical vessel which serves as both the deaeration section and the boiler feed water storage tank.
Types of Deaerator
There are two types of Deaerators.
- Tray type deaerator
- Spray type deaerator
The typical horizontal tray type deaerator in Fig. has a vertical domed deaeration section mounted above a horizontal boiler feed water storage vessel. Boiler feed water enters the vertical deaeration section above the perforated trays and flows downward through the perforations. Low pressure deaeration steam enters below the perforated trays and flows upward through the perforations. Some designs use various types of packing material, 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 feed water 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 water 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 feed water warm. External insulation of the vessel is typically provided to minimize heat loss.
Spray Type Dearearator
As shown in Fig. the typical spray type deaerator is a horizontal vessel which has a preheating section (E) and a deaeration section (F). The two sections are separated by a baffle (C). Low pressure steam enters the vessel through a sparger in the bottom of the vessel.
The boiler feed water is sprayed into section (E) where it is preheated by the rising steam from the sparger. The purpose of the feed water spray nozzle (A) and the preheat section is to heat the boiler feed water to its saturation temperature to facilitate stripping out the dissolved gases in the following deaeration section.
The preheated feed water then flows into the deaeration section (F), where it is deaerated by the steam rising from the sparger system. The gases stripped out of the water exit via the vent at the top of the vessel. Again, some designs include a vent condenser to trap and recover any water entrained in the vented gas. Also again, 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 boiler feedwater is pumped from the bottom of the vessel to the steam generating boiler system.
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