Conventionally BOILER is a pressurised vessel which converts water in to steam. According to modern definition of boiler “it is a heat exchanger whose work is to produce high pressure steam from water”. In this heat exchanger, the hot side is the mixture resulting from the combustion of the fuel & air and the fluid on cold side of this heat exchanger is water, entering as liquid and leaving as steam, an imperfect gas. (*we are discussing about the Sub-critical Boiler)
Before we further discuss about the boiler, let us discuss about the types of Boiling and those are :-
Nucleate Boiling
Film Boiling
The initial stages of heating involve ‘sub cooled ‘ water heating. I.e. below the saturation temp. The water in contact with the tube begins to evaporate, and forming bubble of steam. But as the temp. Of surrounding water has not reached saturation temp. , the bubble collapse & give latent heat to surrounding water. This process continues and temperature of water increases. When temp of water reaches saturation temp then bubble will not collapse and water will start boiling. This is called nucleate boiling. The entire boiler should work with Nucleate boiling condition.
In case as discusses above, if the temp gradient or the heat flux is higher than prescribed value then the bubble will be formed very rapidly and they will not move towards the lesser temperature part of the fluid and will collapse and will form a thin steam film inside tube surface. As the steam works as a poor conductor of heat, further heat transfer to water will not take place properly and the tube metal will get heated up. This type of boiling is called Film boiling. So we should avoid this condition in order to protect the boiler tube from overheating.
Let’s see some construction details of modern boiler. Generally most of the higher capacity boilers in thermal power plants are corner fired. As discussed earlier that boiler is nothing but a heat ex-changer. So Some of the basic parts of boiler is listed here, (Boiler drum, Down comer, Bottom ring header, Water wall tubes, Top header, Super heater, Reheater, Economiser, Air preheater,BCP)
Let’s see what the functions of each component are.
- Boiler drum: generally people think it is use to store the water, but actually it is use to separate the water from the steam by the help of Gravity and Centrifugal force. Water and steam mixture from top header comes in through the riser tubes into the drum where steam is separated and sent to superheater for further heating, and water is kept in drum for further use in cycle and sent through the down comers.
- Down comer:These are nothing but big diameter pipes which are connected from drum to bottom ring header. But remember one thing, these pipes are not in the heating zone, this are outside of furnace area.
- Bottom ring header: it is a circular pipe where all the water walls and down comers are connected.
- Water walls: water walls are the tubes which absorbs radiant heat from fire ball. These walls are surrounded to the fire ball for maximum use of generated heat.
- Top header: water walls are connected to top header which collect heated water and steam mixture and send it to the drum through riser tubes
- Superheater: it is nothing but number of coils which absorbs convective heat of flue gases generated. It will increase the temp of dry saturated steam coming from drum.
- Reheater: it is also a heat exchanger where steam coming from hp turbine will further heated to increase the efficiency. This provision is made in boiler capacity more than 60MW.
- Economiser:It is an heat exchanger from where water is heated but in which steam formation not takes place.
- Air preheater: This is last heat recovery device in which air is heated to increase combustion efficiency.
- BCP: BCP stands for boiler circulating pump. it is mainly connected in between the down comers and these are provided in both Sub-critical as well as Super-critical boilers.
This article is just regarding the basic function and components of the Boiler and later we will discuss about the various components of the Boiler in brief.
(*Here we have explained mainly regarding the Sub-critical Boiler)
