Draught System in Thermal Power Plant

Draught System

The function of draught system in thermal power plant is to extract the non combustible products after completely combustion happened in the boiler and then sent into atmosphere at safe elevation through chimney.

Following are the classifications of Draught System's 

1. Natural Draught System
2. Mechanical Draught System

Natural Draught System :-

In this system the draught is created by height of the chimney due density difference between the hot flue gases inside the chimney and cold air out side the chimney. The density of hot flue gas is less then the cold air outside the chimney. The natural draught system is lower efficient then mechanical draught system.

Mechanical Draught System:- 

It is the draught artificially produced by rotary fans. And they are classified as follows

1. Induced Draught
2. Forced Draught
3. Balanced Draught

Induced Draught System:-

Induced draught fan is generally located at the out let of dust collector and base of the chimney. The fan takes hot flue gases from the boiler through Economizer, Air-preheater and dust collector and delivers it to the chimney into the open atmosphere. The fan used for this process is known as ID Fan.

The ID Fan creates negative pressure inside the furnace or suction discharge the gases out after complete combustion  from the furnace and it also helps to proper combustion. Since ID Fan can handle the hot flue gases, they have more corrosion and erosion problems even when used with dust collectors. The most common type of fans used as ID Fans are the backward inclined blade centrifugal fans. The controlling of flue gas flow is maintained by the help of inlet pneumatic damper adjustment, fan speed controlled by VFD, Fan speed controlled by scoop coupling.

Forced Draught System:-

The fan used for this process is known as FD Fan.  The FD Fan is draws air from the atmosphere and forces it into the furnace through a air preheater. These fans are located near the boiler and clean area to draw the clean air and push high pressure air  to the combustion chamber, where it mixes with fuel to produce positive pressure. 

The most common centrifugal fans used as FD fans are backward curved and airfoil  centrifugal fans. To control the air flow into the furnace through inlet pneumatic damper control,fan speed controlled by VFD, Fan speed controlled by scoop coupling. 

The FD fan handling the cold air so, it operation and maintenance cost is less comparatively ID Fan. Corrosion and Erosion problems are less.

Balance Draught System:-

In this system the both ID and FD fans are operating simultaneously and draught can be maintained inside furnace such that the +/- 5mmwc. The operating efficiency of boiler with this system is high. The FD fan push the fresh cold atmospheric into the furnace and makes furnace pressure positive due to which the flame coming out of the furnace through manholes, peep holes, expansion  joints. The ID Fan maintains the negative pressure ( below the atmospheric pressure ) inside the furnace such that flame not coming out of the furnace, it makes boiler operation continuously.

Advantageous of Mechanical Draught:- 

The mechanical Draught having a following advantageous,

1. Easy to control the combustion and evaporation.
2. Increase in evaporate power of boiler. 
3. Combustion efficiency of boiler is increases.
4. It reduces  the chimney height.
5. Prevention of smoke.