ECONOMISERS,AIR PREHEATER

ECONOMISERS

An economiser is a heat exchanger  which raises the temperature of the feed water leaving the highest pressure feed water heater to about the saturation temperature corresponding to the boiler pressure.This is done by means of hot flue gases leaving the superheater at a temperature varying from 370°C to 620 °C.It consists of a series of steel tubes through which feed water flows.Hot flue gases pass over the tubes and transfer some of their heat to the feed water.The advantages of an economiser are as follows:
1.It reduced the consumption of coal.
2.It improves thermal efficiency.
 Although an economizer requires  additional floor area ,extra investment and more maintenance cost,it has become an integral part of all modern thermal power plants.

AIR PREHEATER

 The heat carried with the flue gases coming out of economiser is further utilised for preheating air before supplying to the combustion chamber.It has been found that an incease of 20°C in the air temperature increases tha boiler efficiency by one percent.In modern power plants a preheater is a necessary  equipment for supplying hot air to dry the coal in pulverised fuel systems to facilitate grinding and stisfactory combustion of fuel in furnace.The main advantages of air preheaters are as follows:
(i) improve combustion
(ii)effective use of low grade(high ash content) fuel
(iii)saving in fuel combustion
(iv) increased steam generation capacity of the boiler
Air preheaters are usually installed with steam generators that burn solid fuels.They are rarely used with gas or oil fired units.Air preheaters are not essential for the operation of steam generator.Their installation depends upon the economic advantage.on the other hand economisers are used for most boiler burning coal, liquid or gas whether or not air preheater is provided.

SUPERHEATER

The steam produced in the boiler is nearly saturated.The superheater is a heat exchanger to remove the last traces of moisture(1 to 2 %) from the saturated steam coming out of the boiler and to increase its temperature sufficiently above saturation temperature. superheating increases the overall efficienty. In addition,it reduces the moisture content in the last stages of the turbine.Therefore ,there is lesser erosion of turbine blades.the heat of combustion gases is utilized for superheating.The steam is superheated by transferring the heat of combination gases either by convention,or by radition,or by combined convention and radiation.