Economiser &Air pre-heater

What is economiser?“A Device , which increases the temprature of feed water using waste heat of flue gases leaving the boiler through chimney.

It is used for stationaryBoilers.

Economiser

• construction

• working

•advantages•disadvantages

Construction of economisersConsists of a large numbers of vertical cast irons (steel water pipes)

One end of vertical pipe connected with horizontal pipe A & another end with B.

Working of economisersThe feed water enters into economiser through the bottom pipe A.

Then, it comes into top pipe B through the vertical pipes.

After that ,it comes into boiler for further working.

Working of economisersThe hot flue gases coming from boiler pass over the vertical tubes of economiser.

OBSERVE : The heat transfer takes place from flue gases to cold water rising in vertical tubes of economiser.

Working of economiser The heated feed water is supplided to the boiler from STOP VALVE.

Effects of working The soot of flue gases which gets

deposited on the vertical pipes REDUCES EFFICIENCY of the economiser.

TO PREVENT SOOT DEPOSIT ,the scrapers S move up & down by chain and gears to keep the external surface of the pipe clean(for better HEAT TRANSFER)

Effects of working A BLOW OFF COCK VALVE is provided at back end of vertical pipes to remove sediments deposited in the bottom boxes .

A SAFETY VALVE provided at the end of the pipe A for the safety of the pipes.

Advantages Higher feed water temperature tend to

reduce boiler thermal stersses, so life of boiler increased.

Economisers utilized waste heat from flue

gases, so fuel consumption of boiler reduces.

Increased evaporate capacity of boiler.

Disadvantages Economisers placed at passage of the flow of flue gases , so pressure drop take place of flue gases (loss of draught)

Air preheaterAir-preheater: An Essential

Element of Fuel Fired Steam Power Plants

What is air preheater?A device, which increases the tempurature of air

before it supply to the furnance using heat from flue gases passing through chimney.

Types of air preheater

Recuperative type air preheater

Regenerative type air preheater

Recuperative turbulent type air preheater

• Construction

• Working

• Advantages• Disadvantages

Construction An air preheater is installed between economisers and the chimney .

It consists of large numbers tubes which arranged in the path flue gases shell .

Inlet air is at lower end .Baffles are provided inside air

preheater .The soot hoppper is provided at

the bottom .

Working Hot flue gases enters into tubes

from the top of the shell and leaves from to the chimney.

The inlet air at R.Tem. Is admitted into the shell with the help of the fan.

The air passes upward around the tubes in the opposite direction to the flow of the hot gases.

Working

The heated air leaves at the upper end of the tube bank absorbing heat from the hot flue gases.

The soot hopper provided at the bottom is used to collect soot during cleaning operation of the tubes.

Baffles increases the total path length of air and thus increase heat transfer .

Advantages of air preheatingPreheated air increases combustion rate and then increases steam generations rate of boiler.

Due to higher temperature of air, furnance temperature increases, so low grade coal can be burnt efficiently .

Advantages of air preheatingAir heated by heat of exhaust gases. It reduces fuel consumptions . Therfore thermal efficiency of boiler increased.

Disadvantages of air preheating There are formations of clinkers , on the grate due to higher combustion temperature.

Natural draught is not possible with air pre heater because of temperature of flue gases is reduced , also pressure drop take place in the flow of the flue gases. therfore forced draught is required.

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k you

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