BOILER
Definition
As per Indian Boiler Act 1923, Boiler is defined as any closed vessel exceeding 22.75 liters in capacity which is used exclusively for generating steam under pressure and includes any mounting or accessories attached to such vessel, which is wholly or partially under pressure, when steam is shut off.
A good Boiler should have some essential qualities.
1. Capable to meet large load fluctuations.
2. Fuel efficient i.e. to generate maximum steam with minimum fuel consumption.
3. Ability to start-up quickly.
4. Easy in maintenance and inspection.
5. Occupy less floor space.
6. Lower friction loss in water and flue gas circuit
7. Little attention for operation and maintenance.
Systems in a Boiler
A Boiler mainly contains following systems :
1. Feed water system.
2. Steam system.
3. Air system.
4. Flue gas system.
5. Fuel handling system.
6. Ash handling system.
Boiler Mountings
Fittings on a Boiler which are required for its safe and efficient operation are called mountings. These are as follows :
1. Safety valve
2. Water level sight glass (gauge glass)
3. Pressure gauge
4. Blow down valve
5. Main steam stop valve
6. Feed water check valve (NRV)
7. Air Vent
8. Start-up vent
9. Manhole
Boiler Accessories
The devices which are used in a Boiler as an integral part and help to run the Boiler efficiently are called Boiler Accessories. These are :
1. Super heater
2. De-super heater
3. Economizer
4. Air Pre-heater
5. Soot Blower
6. Feed Pump
7. ID and FD fans
8. Ash Removal system
9. Fuel supply system
10. Dosing system
11. Deaerator
Steam Generation In A Boiler..contd
In a Boiler fuel is burnt to get heat energy which is converted from chemical energy stored in a fuel. This heat energy is utilized to produce steam from feed water.
Fuel is fired in the furnace of the Boiler. Different fuel is used in different Boilers. Accordingly furnace is designed. Water tubes are arranged around the furnace and flue gas path. Water tube arrangement made around the furnace is called as water wall. Feed water is circulated in these tubes. Water comes to water wall from Boiler drum, and circulated back to drum after absorbing heat. Due to difference in density which is created due to difference in temperature, water circulates in these tubes naturally. Therefore, it is called Natural Circulation.
During circulation of water in tubes, steam is generated and collected at the upper part of the Drum. This is called Saturated Steam corresponding to Boiler drum pressure. This steam is further heated in Superheaters and becomes superheated steam.
Boiler Drum is filled with fresh feed water. The feed water, before entering into drum is heated at Economizer. Economizer is placed at the flue gas path. Most of the heat of the flue gas is utilized inside the Boiler. Still considerable amount of heat energy is available in it. This heat is utilized in Economizer to heat up the feed water.
For burning of fuel, required Oxygen is obtained from atmospheric air. Air is required in Boiler furnace for combustion. This is supplied by Forced Draught (FD) fan. This air is heated at air pre-heater (APH) before being sent into furnace. If cold air is used then there will be loss in energy. Air pre-heater is placed at the flue gas path after Economizer. Air pre-heater is a heat exchanger which exchanges the heat of flue gas to the cold air, which is to be used in furnace. By heating the air, burning of fuel is easier and loss of energy is minimized. If hot flue gas would not be used to heat up feed water at Economizer and air at Air Pre-heater then it would escape into atmosphere.
Finally the flue gas passes through Electrostatic Precipitator (ESP) and exhausted to atmosphere through chimney. At ESP the dust particle in the flue gas is trapped and clean gas escapes to atmosphere.
Ash which is produced in the Boiler due to combustion of solid fuel is collected at Boiler bottom and also in Economizer, Air Pre-heater and ESP. This ash is disposed off with the help of suitable ash handling system.
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