Superheated steam boilers

A superheated boiler on a steam locomotive

     
          Most boilers produce steam to be used at saturation temperature ; that is , saturated steam. Superheated steam boilers vaporize the water and then further heat the steam in a superheater. This provides steam at much higher temperature , bur can decrease the overall thermal efficiency of the steam generating plant because the higher steam temperature requires a higher flue gas exhaust temperature. There are several ways to circumvent this problem , typically by providing an economizer that heats the feed water , a combustion air heater in the hot flue gas exhaust path or both. There are advantages to superheated steam that may and often will , increase overall efficiency of both steam generation and its utilisation: gains in input temperature to a turbine should outweigh any cost in additional boiler complication and expense. There may also be practical limitations in using wet steam , as entrained condensation droplets will damage turbine blades.
      Superheated steam presents unique safety concerns because , if any system component fails and allows steam to escape , the high pressure and temperature can cause serious , instantaneous harm to anyone in its path. Since the escaping steam will initially be completely superheated vapor , detection can be difficult , although the intense heat and sound from such a leak clearly indicates its presense.
      Superheater operation is similar to that of the coils on an air conditioning unit , although for a different purpose. The steam piping is directed through the flue gas path in the boiler furnace. The temperature in this area is typically between 1,300-1,600 degree Celsius (2,372-2,912°F). Some superheaters are radiant type ; that is , they absorb heat by radiation. Others are convection type , absorbing heat from a fluid such as a gas. Some are a combination of the two types. Through either method , the extreme heat in the flue gas path will also heat the superheater steam piping and the steam within. While the temperature of the steam in the superheater rises , the pressure of the steam does not : the turbine or moving pistons offer a continuously expanding space and the pressure remains the same as that of the boiler. Almost all steam superheater system designs remove droplets entrained in the steam to prevent damage to the turbine blading and associated piping.

Hydronic Boilers

Hydronic Boilers

      Hydronic boilers are used in generating heat for residential and industrial purposes. They are the typical power plant for central heating systems fitted to houses in northern Europe (where they are commonly combined with domestic water heating) , as apposed to the forced-air furnaces or wood burning stoves more common in North America. The hydronic boiler operates by way of heating water/fluid to a preset temperature (or sometimes in the case of single pipe systems , until it boils and turns to steam) and circulating that fluid throughout the home typically by way of radiators , baseboard heaters or through the floors. The fluid can be heated by any means...gas , wood , fuel oil etc.. , but in built-up areas where piped gas is available , natural gas is currently the most economical and therefore the usual choice. The fluid is in an enclosed system and circulated throughout by means of a motorized pump. The name "boiler" can be a misnomer in that , except for systems using steam radiators , the water in a properly functioning hydronic boiler never actually boils. Most new systems are fitted with condensing boilers for greater efficiency. These boilers are referred to as  condensing boilers because they condense the water vapor in the flue gases to capture the latent heat of vaporization of the water produced during combustion.
   
    Hydronic systems are being used more and more in new construction in North America for several reasons. Among the reasons are :

* They are more efficient and more economical than forced-air systems (although initial installation can be
   more expensive , because of the cost of the copper and aluminium).
* The baseboard copper pipes and aluminium fins take up less room and use less metal than the bulky steel
  ductwork required for forced-air systems.
* They provide more even , less fluctuating temperatures than forced-air systems. The copper baseboard pipe
   hold and release heat over a longer period of time than air does , so the furnace does not have to switch off
   and on as much.(Copper heats mostly through conduction and radiation , whereas forced-air heats mostly
   through forced convection. Air has much lower thermal conductivity and volumetric heat capacity than
   copper , so the conditioned space warms up and cools down more quickly than with hydronic)
* They tend to not dry out the interior air as much as forced air systems , but this is not always true. When
   forced air duct systems are air-sealed properly and have return-air paths back to the furnace (thus reducing
   pressure differentials and therefore air movement between inside and outside the house) this is not an issue.
* They do not introduce any dust , allergens , mold or (in the case of a faulty heat exchanger) combustion by
   products into the living space.

Forced-air heating does have some advantages , however.