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A design of a typical steam boiler for electric power generation?

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February 23, 2012 1:41PM

In a Power generating plant, steam is produced in a boiler. The boiler is effectively a huge kettle. Generally a boiler has the following subassembly layout: high pressure water flow (>200 bar g, >250 DEG C) enters the economizer inlet headers and then thru the economizer heat exchange pipes to arive in the economizer outlet headers. Main role of the economizer is heat up the water. After the economizer the water runs thru the headers and heat exchange pipes of the evaporator. Here the water vaporizes in to steam. Steam then runs through the headers and heat exchange pipes of generally 2 - 3 superheaters. Between superheaters and after the last superheater is a water injection point to control the temperature of the superheated steam.

This layout would be in case of drum boiler, used in small power plants (electric power output not more then 100MW). Now the drum is large barell on top of the boiler where water and steam are separated. In case of large MW rated power plants the boiled does not have a drum. I must also say that when a boiler has a drum it is called a multiple pass boiler. That means that water is circulated through the vaporizer in 2,3 or more cycles until is fully transformed in to steam. In case of large MW rated power plants water passes through the vaporizer only one time and is fully transformed in to steam. This type of boilers are called " once through" boilers or "Benson" type after the name of the inventor.

Now to have a view about the actual layout of the heat exchange surfaces here are some facts :

- furnace, wih burners, is place where the flame or fire is located. Here temperature is very high ( >900DEG C). Heat surfaces are superheaters. Main heat exchange is done through radiation. Professionals calls this heat exchanger as radiation superheater.

- convective superheaters. Called like this because heat exchange is particularly by convective means. The flue gases are cooler here and this is an array of pipes that the flue gases washes them. As I mentioned earlier it can be 2-3 convective superheaters.

Steam that comes out from superheaters is run through pipes to the turbine inlet valves. In large MW rated power plants, steam that comes out from the high pressure turbine body is reheated in the boiler. Reheaters are also heat exchange surfaces that are generally intercalated with the superheaters. After reheating the steam is run to the intermediate pressure turbine body.

To continue with the heat exchange surfaces layout in boiler, flue gases after washing the superheaters and/or reheaters will wash the evaporator and then the economizer before actually exiting the boiler body. To recover as much heat from the flue gases, after exiting the boiler, flue gases are run through a rotarry or tube heat exchanger. Here, air that is fed to the burners is heted. This way remanent heat of the flue gases is put back in to boiler making it more efficient.

So to summarize a boiler is made of ( in order of flue gases pass) :

- furnace (radiation superheater, pipes are run by high pressure steam)

- convective superheaters ( pipes are run by high pressure steam)

- evaporator (pipes are run by a mixture of water and steam)

- economizer ( pipes are run by water )

Unlike a huge kettle, modern boilers use tubes filled with water and steam. Contained within a furnace.