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.       
              

Aquastat

     An aquastat is a device used in hydronic heating systems for controlling water temperature. To prevent the boiler from firing too often , aquastats have a high limit temperature and a low limit. If the thermostat is calling for heat , the boiler will fire until the high limit is reached , then shut off (even if the thermostat is still calling for heat). The boiler will re-fire if the boiler water temperature drops below a range around the high limit. The high limit exists for the sake of efficiency and safety. The boiler will also fire (regardless of thermostat state) when the boiler water temperature goes below a range around the low limit , ensuring that the boiler water temperature remains above a certain point. The low limit is intended for tankless domestic hot water , it ensures that boiler water is always warm enough to heat the domestic hot water. Many aquastats also have a "diff" control which determines the size of the range around the "low" and/or "high" controls.
            Aquastst is a registered Trademark of Honeywell International Inc.

Variations - Package boiler

      The term "package" boiler evolved in the early - to mid - 20th century from the practice of delivering boiler units to site already fitted with insulation , electrical panels , valves and gauges. This was in contrast to earlier practice where little more than the pressure vessel was delivered and the ancillary components were fitted on-site.

Variations - Reverse flame

      In homage to the Lancashire design , modern shell boilers can come with a twin furnace design. A more recent development has been the reverse flame design where the burner fires into a blind furnace and the combustion gasses double back on themselves. This results in a more compact design and less pipework.

Variations - Water tubes

      Fire-tube boilers sometimes have water-tubes as well , to increase the heating surface. A Cornish boiler may have several water-tubes across the diameter of the flue (this is common in steam launches). A locomotive boiler  with a wide firebox may have arch tubes or thermic syphons. These increase the heating surface and give additional support to the brick arch.
      Not all shell boilers raise steam ; some are designed specifically for heating pressurized water.

Horizontal Return Tubular boiler

      Horizontal Return Tubular (HRT) has a horizontal cylindrical shell , containing several horizontal flue tubes , with the fire located directly below the boiler's shell , usually within a brickwork setting.

Horizontal Return Tubular boilers from the Staatsbad Bad Steben GmbH

Vertical Fire-tube boiler

      A vertical fire-tube boiler (VFT) , colloquially known as the "vertical boiler" , has a vertical cylindrical shell , containing several vertical flue tubes.

Taper boilers

      Certain railway locomotive boilers are tapered from a larger diameter at the firebox and to a smaller diameter at the smokebox end. This reduces weight and improves water circulation. Many later Great Western Railway and London , Midland and Scottish Railway locomotives were designed or modified to take taper boilers.

Locomotive boiler

      A locomotive boiler has three main components : a double walled firebox ; a horizontal , cylindrical "boiler barrel" containing a large number of small flue-tubes ; and a smokebox with chimney , for the exhaust gases. The boiler barrel contains larger flue-tubes to carry the superheater elements , where present. Forced draught is provided in  the locomotive boiler by injecting exhausted steam back into the exhaust via a blast pipe in the smokebox.
      Locomotive-type boilers are also used in traction engines ,  steam rollers , portable engines and some other steam road vehicles. The inherent strength of the boiler means it is used as the basis for the vehicle : all the other components , including the wheels are mounted on brackets attached to the boiler. It is rare to find superheaters designed into this type of boiler and they are generally much smaller (and simpler) than railway locomotive types.
      The locomotive-type boiler is also a characteristic of the overtype steam wagon , the steam-powered fore-runner of the truck. In this case , however , heavy girder frames make up the load-bearing chassis of the vehicle and the boiler is attached to this.

Scotch marine boiler

      The Scotch marine boiler differs dramatically from its predecessors in using a large number of small-diameter tubes. This gives a far greater heating surface area for the volume and weight. The furnace remains a single large-diameter tube with the many small tubes arranged above it. They are connected together through a combustion chamber - an enclosed volume contained entirely within the boiler shell - so that the flow of flue gas through the fire-tubes is from back to front. An enclosed smokebox covering the front of  these tubes leads upwards to the chimney or funnel. Typical Scotch boilers had a pair of  furnaces , larger ones had three. Above this size , such as for large steam ships , it was more usual to install multiple boilers.

Side-section of a Scotch marine boiler: the arrows show direction of flue gas flow; the combustion chamber is on the right, the smokebox on the left

Lancashire boilers (Fire-tube boilers)

      The Lancashire boiler is similar to the cornish , but has two large flues containing the fires. It was the invention of William Fairbairn in 1844 , from a theoretical consideration of the thermodynamics of more efficient boilers that led him to increase the furnace grate area  relative to the volume of water.
      Later developments added Galloway tubes (after their inventor , patented in 1848) , crosswise water tubes across the flue , thus increasing the heated surface area. As these are short tubes of large diameter and the boiler continues to use a relatively low pressure , this is still not considered to be a water-tube boiler. The tubes are tapered , simply to make their installation through the flue easier.

Lancashire boiler in Germany

Cornish boilers (Fire-tube boilers)

      The earliest form of fire-tube boiler was Richard Trevithick's "high-pressure" Cornish boiler. This is a long horizontal cylinder with a single large flue containing the fire. The fire itself was on an iron grating placed across this flue , with a shallow ashpan beneath to collect the non-combustible residue. Although considered as low-pressure (perhaps 25psi) today , the use of a cylindrical boiler shell permitted a higher pressure than the earlier "haystack" boilers of Newcomen's day. As the furnace relied on natural draught (air flow) , a tall chimney was required at the far end of the flue to encourage a good supply of air (oxygen) to the fire.
      For efficiency , the boiler was commonly encased beneath by a brick-built chamber.Flue gases were routed through this , outside the iron boiler shell , after passing through the fire-tube and so to a chimney that was now placed at the front face of the boiler.

Operation

Schematic diagram of a "locomotive" type fire-tube boiler

      In the locomotive-type boiler , fuel is burnt in a firebox to produce hot combustion gases. The firebox is surrounded by a cooling jacket of water connected to the long , cylindrical boiler shell. The hot gases are directed along a series of fire tubes or flues , that penetrate the boiler and heat the water thereby generating saturated ("wet") steam. The steam rises to the highest point of the boiler , the steam dome , where it is collected. The dome is the site of the regulator that controls the exit of steam from the boiler.
      In the locomotive boiler , the saturated steam is very often passed into a superheater , back through the larger flues at the top of the boiler , to dry the steam and heat it to superheated steam. The superheated steam is directed to the steam engine's cylinders or very rarely to a turbine to produce mechanical work. Exhaust gases are fed out through a chimney , and may be used to pre-heat the feed water to increase the efficiency of the boiler.
      Draught for firetube boilers , particularly in marine applications , is usually provided by a tall smokestack. In all steam locomotives  , since Stephenson's Rocket , additional draught is supplied by directing exhaust steam from the cylinders into the smokestack through a blastpipe , to provide a partial vacuum. Modern industrial boilers use fans to provide forced or induced draughting of the boiler.
      Another major advance in the Rocket was large numbers of  small-diameter fire tubes (a multi-tubular boiler) instead of a single large flue. This greatly increased the surface area for heat transfer , allowing steam to be produced at a much higher rate. Without this , steam locomotives could never have developed effectively as powerful prime movers.

Fire-tube boiler

      A fire-tube boiler is a type of boiler in which hot gasses from a fire pass through one or more tubes running through a sealed container of water. The heat of gasses is transferred through the walls of the tubes by thermal conduction , heating the water and ultimately creating steam.
      The fire-tube boiler developed as the third of the four major historical types of boilers : low-pressure tank or "haystack" boilers , flued boilers with one or two large flues , fire-tube boilers with many small tubes and high-pressure water-tube boilers. Their advantage over flued boilers with a single large flue is that the many small tubes offer far greater heating surface area for the same overall boiler volume. The general construction is as a tank of water perforated by tubes that carry the hot flue gasses from the fire. The tank is usually cylindrical for the most part - being the strongest practical shape for a pressurized container and this cylindrical tank may be either horizontal or vertical.

Sectioned fire-tube boiler from a DRB Class 50 locomotive. Hot flue gases created in the firebox (on the left) pass through the tubes in the centre cylindrical section, which is filled with water, to the smokebox and out of the chimney (far right).

       This type of boiler was used on virtually all steam locomotives in the horizontal "locomotive" form. This has a cylindrical barrel containing the fire tubes , but also has an extension at one end to house the "firebox". This firebox has an open base to provide a large great area and often extends beyond the cylindrical barrel to form a rectangular or tapered enclosure. The horizontal fore-tube boiler is also typical of marine applications , using the scotch boiler. Vertical boilers have also been built of the multiple fire-tube type , although these are comparatively rare : most vertical boilers were either flued or with cross water-tubes.

History of Supercritical Steam Generation

      Contemporary Supercritical Steam generators are sometimes referred as Benson boilers. In 1922 , Mark Benson was granted a patent for a boiler designed to convert water into steam at high pressure.
      Safety was the main concern behind Benson's concept. Earlier steam generators were designed for relatively low pressures of up to about 100 bar (10,000kpa ; 1,450psi) ,  corresponding to the state of the art in steam turbine development at the time. One of their distinguishing technical characteristics was the riveted water/steam separator drum. These drums were where the water filled  tubes were terminated after having passed through the boiler furnace.
      These header drums were intended to be partially filled with water and above the water there was a baffle filled space where the boiler's steam and water vapour collected. The entrained water droplets were collected by the baffles and returned to the water pan. The mostly dry steam was  piped out of the drum as the separated steam output of the boiler. these drums were often the source of boiler explosions , usually with catastrophic consequences.
      However , this drum could be completely eliminated if the evaporation separation process was avoided altogether. This would happen if water entered the boiler at a pressure above the critical pressure (3,206) ; was heated to a temperature above the critical temperature (706 degrees F) and then expanded (through a simple nozzle) to dry steam at some lower subcritical  pressure. This could be obtained at a throttle valve located downstream of the evaporator section of the boiler.
      As development of Benson technology continued , boiler design soon moved away from the original concept introduced  by Mark Benson. In 1929 , a test boiler that had been built in 1927 began operating in the thermal power plant at Gartenfeld in Berlin for the first time in subcritical mode with a fully open throttle valve. The second Benson boiler began operation in 1930 without a pressurizing valve at pressures between 40and 180 bar (4,000 and 18,000 kpa ; 580 and 2,611 psi) at the Berlin cable factory. This application represented the birth of the modern variable-pressure Benson boiler. After that development , the original patent was no longer used. The Benson boiler name , however , was retained.
     Two current innovations have a good chance of winning acceptance in the competitive market for once-through steam generators :

          * A new type of heat-recovery steam generator based on the Benson boiler , which has operated
              successfully at the Cottam combined-cycle power plant in the central part of England ,

          * The vertical tubing in the combustion chamber walls of coal-fired steam generators which combines
              the operating advantages of the Benson system with the design advantages of the drum-type boiler.
              Construction of a first reference plant , the Yaomeng power plant in china , commenced in 2001.

Supercritical steam generators

      Supercritical steam generators (also known as Benson boilers) are frequently used for the production of  electric power. They operate at "supercritical pressure". In contrast to a "subcritical boiler" , a supercritical steam generator operates at such a high pressure (over 3,200psi/22.06MPa or 220.6bar) that actual boiling ceases to occur , and the boiler has no water - steam separation. There is no generation of steam bubbles within the water , because the pressure is above the "critical pressure" at which steam bubbles can form. It passes below the critical point as it does work in the high pressure turbine and enters the generator's condenser. This is more efficient , resulting in slightly less fuel use. The term "boiler" should not be used for a supercritical pressure steam generator , as no "boiling" actually occurs in this device.