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Why Is A De-Aerator Used On Ships?

written by: Raunekk • edited by: Lamar Stonecypher • updated: 8/19/2009

Learn how remove dissolved oxygen is removed from the feed water going to the boiler

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    De-aerator

    If a system consists of a condenser, then it will definitely have a deaerator attached to it. The condensation process that begins in the condenser is not complete without using a de-aerator.

    De-aerator is used to remove any dissolved oxygen in the feed water. The oxygen if not removed, may lead to corrosion of the internal parts of the system. Thus a de-aerator completely removes the air and vapour from the system, which helps in reducing the maintenance of the system. Moreover, de-aerator also acts as a feed heater.

    The heating of feed water solves two functions :

    • It removes the dissolved oxygen from water
    • Raises the temperature of the feed, in order to increase the boiler efficiency

    The deaerator that also works as a feed water heater is of a direct contact type. In this type, the feed water is heated almost to the boiling point, which releases the dissolved gases from the boiler.

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    Construction and Working

    The structure of a de-aerator is that of a large hollow cylinder with other essential parts.

    The main parts of a de-aerator are :

    • Spray valve or nozzles
    • eductor or ejector
    • storage tank

    The spray valve is attached at the top most area.The feed water enters the spray valve and passes through a number of nozzles to spread inside the whole of de-aerator. This is done to facilitate the falling of most of the incoming feed water unto the de-aerating cone assembly, which is heated by the incoming high temperature steam. The cone assembly provides a larger surface area of contact with the heating system.

    The heated feed water slides down the conical surface to pass through a narrow opening or the central passage. The narrow passage works as a ejector, which removes small amount of steam generated in the system. This is possible as the steam has higher volume. The heated feed water and the condensed steam gets collected into a storage tank located at the bottom of the de-aerator. The design is so made that the incoming high temperature steam circulates through out the de-aerator to cool the feed water, gets condensed in return and mixes with the feed water at in the storage tank.

    Heating of feed water also gives rise to the release of other dissolved gases, which leaves the system through a vent connection. The vent connection is connected to a vent condenser or a de-vapouriser outside of the system. If there is any passage of water vapour though the vent, it gets condensed through the vent condenser and returns to the system.

    de-aerator  

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    "Flash-Off" Problem

    The main problem any de-aerator faces is that of "flash off". The feed water in the de-aerator, when heated is very close to the temperature and pressure of the steam. If any kind of pressure drop occurs in the feed water, the water will immediately flash off into steam, which may result in gassing. Gassing leads to formation of vapors in the feed pump suction. This problem can be averted by placing the de-aerator high up in the machinery spaces, which helps in providing a positive suction head to the feed pumps.

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    Image Credits

    Fundamentals of Thermodynamics - Borgnakke & Sonntag

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