Power Plant Boiler

The boiler generates high pressure steam by transferring the heat of Combustion in various heat transfer sections. This part of the article series briefly describes the flow and arrangement of the heat transfer sections in a boiler. In line diagrams help make the concept clear.

 Basics

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.

Pre-Heating

'Feed water heaters’, using extracted steam from the turbine, adds a part of the sensible heat even before the water enters the boiler.

economizer.

 Most of the sensible heat is absorbed in the Economizer. 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 economizer the water is fed to the 'drum'.

Latent Heat Addition

Drum.

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.

Water Walls

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 ‘down comers’ which are large pipes connected to the drum. The down comers 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.

WaterWalls