DEAERATION: 

Removal of oxygen, carbon dioxide and other non-condensable gases from boiler feedwater is vital to boiler equipment longevity as well as safety of operation.  Carbonic acid corrodes metal reducing the life of equipment and piping.  It also dissolves iron (Fe) which when returned to the boiler precipitates and causes scaling on the boiler and tubes.  This scale not only contributes to reducing the life of the equipment but also increases the amount of energy needed to achieve heat transfer.  This is discussed in more detail in Chapter 5

The term given to the mechanical removal of dissolved gases is deaeration.  Mechanical deaeration for the removal of these dissolved gases is typically utilized prior to the addition of chemical oxygen scavengers.  Mechanical deaeration is based on Charles' and Henry's laws of physics.  Simplified, these laws state that removal of oxygen and carbon dioxide can be accomplished by heating the boiler feedwater which reduces the concentration of oxygen and carbon dioxide in the atmosphere surrounding the feedwater.

The easiest way to deaerate is to force steam into the feedwater, this action is called scrubbing.  Scrubbing raises the water temperature causing the release of O₂ and CO₂ gases that are then vented from the system.  In boiler systems, steam is used to "scrub" the feedwater as (1) steam is essentially devoid of O₂ and CO₂, (2) steam is readily available and (3) steam adds the heat required to complete the reaction.  For efficient operation, deaerating equipment must satisfy the following requirements: (1) Heating of the feedwater:  The operating temperature in the unit should be the boiling point of water at the measured pressure.  The pressure/temperature relationship is important since boiling must take place rapidly for quick and efficient removal of gases. If this temperature and pressure cannot be economically achieved then it is important to get as close to it as possible.  (2) Agitation decreases the time and heat energy necessary to remove dissolved gases from the water.  (3) Maximization of surface area by finely dispersing the water to expose maximum surface area to the steam.  This enables the water to be heated to saturation temperature quicker and reduces the distance the gases have to travel to be liberated.  (4) The liberated gases must be vented to allow their escape from the system as they are released.

 While the most efficient mechanical deaerators reduce oxygen to very low levels (.005cc/l or 5 ppb), even trace amounts of oxygen may cause corrosion damage to a system.  Consequently, good operating practice requires removal of that trace oxygen with a chemical oxygen scavenger such as sodium sulfite or hydrazine.  Free carbon dioxide can be removed by deaeration, but this process releases only small amounts of combined carbon dioxide.  The majority of the combined carbon dioxide is removed with the steam of the boiler, subsequently dissolving in the condensate, frequently causing corrosion problems.  These problems can be controlled through the use of volatile neutralizing amines or filming amines

TYPES OF MECHANICAL DEAERATORS:

1.  Tray Type Deaerators 

are composed of a deaerating section and a feedwater storage section.  Incoming water is sprayed through a perforated distribution pipe into a steam atmosphere where it is atomized.  There it is heated to within a few degrees of the saturation temperature of the steam.  Most of the non-condensable gases are released to the steam as the water enters the unit.  The water then cascades through the tray section, breaking into fine droplets, which immediately contact incoming steam.  The steam heats the water to the saturation temperature of the steam and removes all but a trace of oxygen.  Deaerated water falls to the feedwater storage section below and is protected from recontamination by a blanket of steam.  As the non-condensable gases are liberated, they as well as a small amount of steam are vented to atmosphere.  It is essential that sufficient venting is provided at all times or deaeration will be incomplete.

 2.  Spray Type Deaerators work on the same general principle as the tray types.  The spray-type deaerators do not use trays for dispersion of the water.  In this case, spring loaded nozzles located in the top of the unit spray water into a steam atmosphere which is heated to within a few degrees of the saturation temperature of the steam.  Most of the non-condensable gases are released to the steam, and the heated water falls to a water seal and drains to the lowest section of the steam scrubber.

The water is scrubbed by large quantities of steam and heated to the saturation temperature prevailing at this point.  The intimate steam to water contact achieved in the scrubber efficiently strips the water of dissolved gases.  As the steam-water mixture rises in the scrubber, a slight pressure loss causes the deaerated water temperature to remain a few degrees below the inlet steam saturation temperature.  The deaerated water overflows from the steam scrubber to the storage section below.

The steam, after flowing through the scrubber, passes up into the spray heater section to heat the incoming water.  Most of the steam condenses in the spray section to become part of the deaerated water.  A small portion of the steam, vented to atmosphere, removes non-condensable gases from the system.

3.  Spray/Tray Type Deaerators are a combination of the above with a steam spray nozzle sending the water over the trays.

4.   Feedwater Tanks are another form of mechanical deaerators normally found in small firetube and watertube boiler systems due to cost considerations.  These less expensive systems are limited by design as they are operated at atmospheric pressure with feedwater temperatures ranging from 180⁰F - 212⁰F; while deaerators operate under pressure allowing for higher temperatures and more efficient oxygen removal.

Like deaerators, feedwater tanks operate by forcing steam into the feedwater which scrubs oxygen and carbon dioxide gases that are then vented to atmosphere.

Steam enters the bottom of the tank agitating the feedwater as it rises to the top of the tank, and finally is vented along with the liberated gases.  The temperature is normally controlled as high as possible without causing pump problems which occurs when the Net Positive Suction Head (NPSH) is too low.  Steam bubbles form and fill the pump cavity causing vibration, a condition know as cavitation.  This condition may cause serious damage to the feedwater pump and jeopardize steam production.  The most practical potential solution for cavitation is the installation of a slipstream, which allows a portion of the high pressure feedwater to recirculate to the suction side of the pump where it lowers the temperature and eliminates the boiling and cavitation.  The slipstream will not always work leaving the choices of increasing the NPSH by increasing the distance between the tank and the pump, or sizing a new pump properly. Practically speaking, most feedwater tanks are controlled between 180⁰F - 200⁰F and rely more on the assistance of a chemical oxygen scavenger for complete oxygen removal. 

ECONOMIZER:

An economizer removes additional Btu’s from the stack gasses by circulating the deaerated boiler feedwater through a series of bent tubes in the stack.  This translates into a "free" source of energy from the boiler operation.  Finned tube economizers are less costly and more efficient as the "fins" are a source of heat transfer as well as the tubes.  Economizers in watertube boilers typically increase the efficiency of the boiler 4-10% which is usually less than a one year payback.  Due to the higher efficiencies of firetube boilers the payback is usually longer and therefore economizers are not used as frequently on them.  An economizer can also be a useful means of increasing the steam capacity of a boiler.

The use of high sulfur oils, particularly #6 oil, is very corrosive on the economizer tubes.  This can be improved by increasing the temperature of the feedwater to the economizer and the use of soot blowers but the life of an economizer in that environment is limited to about 2-3 years.  A bare tube economizer is easier to keep free of the corrosive sulfur but requires more tubes to achieve the same efficiency as a finned tube economizer

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