Boiler

Water Circulation System

Contents of Presentation • • • • •

Theory of circulation Types of circulation Economizer Water walls Drum and its internals

Circulation in Boiler •The steam generator has to produce steam at highest purity, and at high pressure and temperature required for the turbine. •.Water must flow through the heat absorption surface of the boiler in order that it may be evaporated into steam . •Natural circulation is the ability of water to circulate continuously, with gravity and changes in temperature being the only driving force known as "thermal head“.

• The ratio of the weight of water to the weight of steam in the mixture is called Circulation Ratio. •Because the density of the cold water is greater, it descends in the 'downcomer' towards the lower bottom ring header, displacing the warmer water up into the front tubes.

Criticality of Drum Level control • Boiler drum level control is critical for both plant protection and equipment safety and applies equally to high and low levels of water within the boiler drum. • The purpose of the drum level controller is to bring the drum up to level at boiler start-up and maintain the level at constant steam load. • A dramatic decrease in this level may uncover boiler tubes, allowing them to become overheated and damaged. • An increase in this level may interfere with the process of separating moisture from steam within the drum,

Forced Circulation •However, when the pressure in the water-tube boiler is increased, the difference between the densities of the water and saturated steam falls, consequently less circulation occurs. • To keep the same level of steam output at higher design pressures, the distance between the Bottom ring header and the steam drum must be increased, or some means of forced circulation must be introduced. Therefore natural circulation is limited to boiler with drum operating pressure around 175 Kg/cm2.

Natural Circulation & Forced Circulation Natural Circulation  The downcomer contain relatively cold water, whereas the riser tube contain steam water mixture ,whose density is comparitively less .this density difference is the driving force ,for the mixture. (thermo-siphon principle)  Circulation takes place at such a high rate that the driving force and frictional resisitance in water wall are balanced.

Natural Circulation

Forced Circulation

Forced Circulation  Beyond 180 Kg/cm2 of pressure, circulation is to be assisted with mechanical pumps, to overcome frictional losses. To regulate the flow through various tubes, orifice plates are used. This system is applicable in the high sub-critical regions (say 200 Kg/cm2).

Economizer Boiler Economizer are feed-water heaters in which the heat from waste gases is recovered to raise the temperature of feed-water supplied to the boiler.

Economizer • • • •

The economizer preheats the feed water by utilizing the residual heat of the flue gas. It reduces the exhaust gas temperature and saves the fuel. Modern power plants use steel-tube-type economizers. Design Configuration: divided into several sections : 0.6 – 0.8 m gap

Finned Economizers

Advantages of Economizer 6oC raise in feed water temperature, by economizers corresponds to a 1% saving in fuel consumption

220 C reduction in flue gas temperature increases boiler efficiency by 1%

Boiler Drum • It is an enclosed Pressure Vessel • Heat generated by Combustion of Fuel is transferred to water to become steam • Process: Evaporation Steam volume increases to 1,600 times from water and produces tremendous force

DRUM Lifting in progress

DRUM SERVES TWO MAIN FUNCTIONS  SEPERATING STEAM FROM THE MIXTURE OF WATER AND STEAM 

IT HOUSES ALL EQUIPMENTS USED FOR PURIFICATION OF THE STEAM AFTER BEING SEPERATED FROM THE WATER

Downcomers • There are six down comers which carry water from boiler drum to the ring header. • They are installed from outside the furnace to keep density difference for natural circulation of water & steam.

WATER WALLS • Heating and evaporating the feed water supplied to the boiler from the economisers. • These are vertical tubes connected at the top and bottom to the headers. • These tubes receive water from the boiler drum by means of downcomers connected between drum and water walls lower header.

• Approximately 50% of the heat released by the combustion of the fuel in the furnace is absorbed by the water walls.

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