Oxygen scavengers

      Oxygen scavenging chemicals are very often added to the deaerated boiler feedwater to remove any last traces of oxygen that were not removed by the deaerator. The most commonly used oxygen scavenger is Sodium Sulfite (Na2SO3). It is very effective and rapidly reacts with traces of oxygen to form Sodium Sufite (Na2SO4) which is non-scaling. Another widely used oxygen scavenger is hydrazine (N2H4).

Deaeration steam

      The deaerators in the steam generating systems of most thermal power plants use low pressure steam obtained from an extraction point in their steam turbine system. However , the steam generators in many large industrial facilities such as petroleum refineries may  use whatever low-pressure steam is available.

Spray-type deaerator

      As shown in figure ,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 he bottom of the vessel.

A schematic diagram of a typical spray-type deaerator

      The boiler feedwater is sprayed into section (E) where it is preheated by the rising steam from the sparger. The purpose of the feedwater spray nozzle (A) and the preheat section is to heat the boiler feedwater to its saturation temperature to facilitate stripping out the dissolved gases in the following deaeration section.
      The preheated feedwater then flows into the dearation 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 may 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.

Tray-type deaerator

A schematic diagram of a typical tray-type deaerator

    The typical horizontal tray-type deaerator has a vertical domed deaeration section mounted above a horizontal boiler feedwater storage vessel. Boiler feedwater enters the vertical dearation section above the perforated trays and flows downward through the perforations. Low-pressure dearation steam enters below the perforated trays and flows upward through the perforations.  Some designs use various type 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 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 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 feedwater warm. External insulation of the vessel is typically provided to minimize heat loss.

Type of deaerators

    There are many different horizontal and vertical deaerators available from a number of manufacturers and the actual construction details will vary from one manufacturer to another.

Deaerator

      A deaerator is a device that is widely used for the removal of air and other dissolved gases from the feed water to the steam-generating boilers. In particular , dissolved Oxygen in boiler feed waters will cause serious corrosion damage in steam systems by attaching to the walls of metal piping and other metallic equipment and forming oxides (rust). Water also combines with any dissolved Carbon dioxide to form carbonic acid that causes further corrosion. Most deaerators are designed to remove oxygen down to levels of 7ppb by weight (0.005cm³/L) or less.
            There are two basic type of deaerators . the tray-type and the spray-type.

    * The tray-type (also called the cascade-type) includes avertical 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.  

British Thermal Unit (Btu)

       The British thermal unit (symbol BTU or Btu) is a traditional unit of energy equal to about 1,055 joules. It is approximately the amount of energy needed to heat 1 pound (0.454kg) of water (exactly one tenth of a UK gallon or around 0.1198 US gallons) from 39 to 40°F (3.8 to 4.4°C). The unit is most often used in the power , steam generation , heating and air conditioning industries. In scientific context the BTU has largely been replaced by the SI unit of energy , the joule , though it may be used as a measure of agricultural energy production (BTU/kg). It is still used unofficially in metric English-speaking countries (such as Canada) and remains the standard unit of classification for air conditioning units manufactured and sold in many non-English-speaking metric countries.
      In north America , the term "BTU" is used to describe the heat value (energy content) of fuels and also to describe the power of heating and cooling systems , such as furnaces , stoves , barbecue grills and air conditioners. When used as a unit of power , BTU per hour (BTU/h) is the correct unit , though this is often abbreviated to just "BTU".
      The unit MBTU was defined as one thousand BTU , presumably from the Roman numeral system where "M" stands for one thousand (1,000). This is easily confused with the SI mega (M) prefix , which multiplies by a factor of one million (1,000,000). To avoid confusion many companies and engineers use MMBTU to represent one million BTU. Alternatively a therm is used representing 100,000 or 105 BTU and a quad as 1015 BTU.