Volume of one unit mass of steam is thousand times that of water, When water is converted to steam in a closed vessel the pressure will increase. Boiler uses this principle to produce high pressure steam.
Conversion of Water to Steam evolves in three stages.
- Heating the water from cold condition to boiling point or saturation temperature – sensible heat addition.
- Water boils at saturation temperature to produce steam – Latent heat.addition.
- Heating steam from saturation temperature to higher temperature called Superheating to increase the power plant output and efficiency.
Sensible Heat Addition
Feed Water Pump.
The first step is to get a constant supply of water at high pressure into the boiler. Since the boiler is always at a high pressure. ‘Boiler feed water pump’ pumps the water at high pressure into the boiler from the ‘feed water tank’. The pump is akin to the heart in the human body.
‘Feed water heaters’, using extracted steam from the turbine, adds a part of the sensible heat even before the water enters the boiler.
Most of the sensible heat is absorbed in the Economiser. These are a set of coils made from steel tubes located in the tail end of a boiler. The hot gases leaving the boiler furnace heat the water in the coils. The water temperature is slightly less than the saturation temperature. From the economiser the water is fed to the ‘drum’.
Pre-Heating & Economiser
Latent Heat Addition
The drum itself a large cylindrical vessel that functions as the storage and feeding point for water and the collection point for water and steam mixture. This is the largest and most important pressure part in the boiler and weighs in the range 250 Tons for 600 MW power plant.
Boiling takes place in the ‘Water Walls’ which are water filled tubes that form the walls of the furnace. Water Walls get the water from the ‘downcomers’ which are large pipes connected to the drum. The downcomers and the water wall tubes form the two legs of a water column.
As the water heats up in the furnace a part of the water in the water-wall tubes becomes steam. This water steam mixture has a lower density than the water in the downcomers. This density difference creates a circulation of water from the drum, through the downcomers, water walls and back to the drum. Steam collects at the upper half of the drum. The steam is then sent to the next sections.
The temperature in the drum, downcomers and water wall is at the saturation temperature.
SuperHeat / ReHeat
Steam from the drum passes to the SuperHeater coils placed in the Flue gas path.. The steam temperature increases from the saturation temperature till the maximum required for operation. The superheated steam then finally goes to the turbine.Final Superheater temperatures are in the Range of 540 to 570 °C for large power plants and SuperHeated steam pressures are around 175 bar.
Steam from the exhaust of the first stage turbine goes back to the boiler for reheating and is returned to the second stage. Reheater coils in the flue gas path does the reheating of the returned steam. The reheat steam is at a much lower pressure than the super heated steam but the final reheater temperature is the same as the superheated steam temperature. Reheating to high temperatures improves the output and efficiency of the Power Plant. Final Reheater temperatures are normally in the range of 560 to 600 °C. Reheat steam pressures are normally around 45 bar.
SuperHeater / ReHeater
The above are the major water and steam circuit items in a boiler and are collectively called the ‘pressure parts’.