power plant engineering

Depending are two types of boiler depending on pressure:- Positive pressure :- When the pressure in combustion area of the boiler is more than atmospheric pressure, normally forced draft boilers, HRSG are positive pressure. Negative pressure :-When the pressure in combustion area of the boiler is less than atmospheric pressure. This is achieved in balanced draft boilers by using Forced draft fans to force air into boiler (pressuring it)and Induced draft fans to remove air from the boilers to create negative pressure. In general negative pressure boilers are preferred as any flames or exhaust from boiler don't have a tendency to spread outside as the pressure inside is lower. The air ingresses inside incase of small leaks between boiler tube fins( large boiler are not perfectly sealed and have small gaps between fins) and contains the flames in the boiler. It also reduces the risk of explosions in boilers w.r.t. positive pressure boilers.

Deaerator at height not only provides Available suction head at boiler feed pump at higher side than required NPSH but also makes recirculation valve discharge to remain in liquid state and not the gaseous state. because feed water temperature to the BFP (and also the temperature after the throttling in ARC valve) is lower than the saturation temperature of the return line of re-circulation to deaerator.

The overall coal plant efficiency ranges from 35 % to 48 %. The value varies with the technology, material, parameters (Temp., Pressure) used. At condenser comes the big loophole in plant efficiency. The steam is condensed for reuse. During this process the latent heat of condensation is lost to the cooling water. This is the major loss and is almost 40 % of the energy input.