What type of boiler used in thermal power plant

Industrial boilers are widely used in power generation and combined heat and power (CHP), chemical & process industries.They’re used mainly for generating steam at high pressures & temperatures for a variety of purposes.Its development began in the 18th century Industrial Revolution.Being fairly mature technology, today’s boilers have become very efficient at converting the thermal energy in coal, oil or natural gas to heat water & form steam at high pressure & temperature.

How does a thermal power plant work?

In the thermal power plant ,coal is the main fuel.coal is burned inside the combustion chamber of boiler.The products of combustion are nothing but gases.These gases which are at high temperature vaporize the water inside the boiler to steam.Some times this steam is further heated in a superheateras higher the steam pressure and temperature the greater efficiency the engine will have in converting the heat in steam in to mechanical work. This steam at high pressure and tempeture is used directly as a heating medium, or as the working fluid in a prime mover to convert thermal energy to mechanical work, which in turn may be converted to electrical energy.

Two types of boiler used in thermal power plant are as following:

Bolilers are classified as

Fire tube boilers and water tube boilers

In fire tube boilers hot gases are passed through the tubes and water surrounds these tubes. These are simple,compact and rugged in construction.Depending on whether the tubes are vertical or horizontal these are further classified as vertical and horizontal tube boilers.In this since the water volume is more,circulation will be poor.So they can't meet quickly the changes in steam demand.High pressures of steam are not possible,maximum pressure that can be attained is about 17.5kg/sq cm.Due to large quantity of water in the drain it requires more time for steam raising.The steam attained is generally wet,economical for low pressures.The outut of the boiler is also limited.

Water tube boilers : In these boilers water is inside the tubes and hot gases are outside the tubes.They consists of drums and

tubes.They may contain any number of drums (you can see 2 drums in fig).Feed water enters the boiler to one drum (here it is drum below the boiler).This water circulates through the tubes connected external to drums.Hot gases which surrounds these tubes wil convert the water in tubes in to steam.This steam is passed through tubes and collected at the top of the drum since it is of light weight.So the drums store steam and water (upper drum).The entire steam is collected in one drum and it is taken out from there (see in laout fig).As the movement of water in the water tubes is high, so rate of heat transfer also becomes high resulting in greater efficiency.They produce high pressure , easily accessible and can respond quickly to changes in steam demand.These are also classified as vertical,horizontal and inclined tube depending on the arrangement of the tubes.These are of less weight and less liable to explosion.Large heating surfaces can be obtained by use of large number of tubes.We can attain pressure as high as 125 kg/sq cm and temperatures from 315 to 575 centigrade.