Draught System in Power Plant: Classification, Merits and Demerits
Table of Contents
In a thermal power plant, draught is a crucial component. The draught system serves the following purposes:
- To provide the furnace with the necessary amount of air for fuel combustion.
- To draw the combustion products through the system.
- To remove burnt products from the system.
So Draught is the negligible pressure differential between the fuel bed (furnace) and the outside air needed to keep the air flowing continuously and discharge the gases through the chimney to the atmosphere. Using a chimney fan, steam jet, air jet, or a combination of these can create draughts.
Classification of Draught System
1. Natural draught: In this, only chimney is used for producing the draughts.
2. Artificial draught: The draught is produced by steam jet or by fan.
- Steam jet draughts: Steam jet is used for creating draught in the system.
- Induced draughts: The flue gas is sucked through the system by a fan or steam jet.
- Forced draughts: The air is forced into the system by a blower or steam jet.
- Mechanical draughts: Fan or blower is used for creating draught in the system.
Natural draught
A tall chimney is built to take advantage of natural draft. The chimney is a vertical reinforced concrete or tubular masonry structure. It is designed to contain a column of exhaust gases in order to create draught. The flue gases are released at a sufficient height to prevent air pollution. The temperature difference between the hot gases inside the tall chimney and the cold atmospheric air outside the chimney is what causes the natural draught.
Let
- H – height of the chimney above the grate level in m
- Pa – atm. pressure in N/m2
- wa and wg – weight densities of atm air and hot gases
The pressure at grate from chimney side in N/m3
P1 = Pa + wgH
The pressure at grate from atmospheric side.
P2 = Pa +waH
Since the density varies with chimney height, the average density is taken into account when computing.
The net pressure acting on the grate is given by
The pressure difference in chimney should be between 10 to 12 mm of water head.
Merits
- Less air pollution results from the high level of gas discharge.
- No external power is required.
- It has longer life.
- Maintenance cost is practically low.
- Capital cost is less than the artificial draught.
Demerits
- Flue gases have to be discharged at high temperature for better draught resulting in wasting of heat.
- Maximum pressure available for producing draught is less.
- Overall efficiency of the plant is reduced because the gases are discharged at high temperature.
- Heat cannot be extracted for economizer, superheater (or) air-pre heater since the effective draught would be reduced.
- Poor combustion and increased specific fuel consumption.
- Not flexible under peak load.
Artificial draught
Modern power plants should have flexible draught systems that are independent of atmospheric conditions and can adapt to changing loads. So Currently, 20,000 tons of steam are produced per hour by modern power plants. Draft fans must be used in order to accomplish this, and by doing so, the height of the chimney will be decreased. When the required draught is greater than 40 mm of water, the artificial draught is more cost-effective. The three types of artificial drafts are forced, induced, and balanced drafts.
Forced draught
The blower in this draught system is situated close to the grate at the boiler’s base. Electricity or steam power the blower.
The forced fan forces air into the furnace, and the air preheater and economizer force flue gases into the chimney. Because the pressure of the hot gases and air in this draught system is higher than atmospheric pressure, this system is known as a positive draught system. In order to reduce air pollution, the chimney releases the hot gases at a higher level.
Merits
- The fan requires less space and power because it can handle cold air.
- No need of water cooled bearings because the air being handled is cold air.
- Because the system’s pressure is higher than atmospheric, less air leaks into the furnace.
Demerit
- It is not possible to open the furnace for firing or inspection because doing so could cause the high pressure air to try to blow out suddenly and cause the furnace to stop.
Induced draught
A blower is positioned close to (or) at the base of the chimney in an induced draught. Electricity or steam power the fan. The fan draws the furnace’s flue gas out, partially vacuuming the furnace. In order to facilitate fuel combustion, atmospheric air is thus forced to flow through the furnace. Through the chimney, the flue gases drawn by the fan are released into the atmosphere.
Merits
- The chimney height can be reduced since it discharges flue gases only.
- So After recovering the majority of their heat in an economizer and an air pre heater, the gases could be released at a lower temperature.
Demerits
- Water cooled bearings are needed for the fan to with stand high temperature of the flue gases.
- The furnace cannot be opened for firing or inspection because doing so would allow cold air to enter, which would stifle combustion and potentially lose heat.
- Air leakage into the furnace is possible since the pressure inside the furnace is below atmospheric
Balanced draught system
When the furnace is opened for firing in this induced draught system, the cold air enters the furnace and thins the combustion. When the forced-draughts furnace is opened for use, the high-pressure air will attempt to blow out suddenly, which could cause the furnace to stop. As a result, if the two systems are used independently, the furnace cannot be opened for firing or inspection.
To get around the aforementioned issues, balanced draught—a combination of forced and induced draughts is used. So Two blowers are used in this, one at the base of the boiler and the other at the base of the chimney. In the balanced draught, the pressure distribution is uniform. Above atmospheric pressure, the air beneath the grate is under pressure. This promotes even combustion. Air pressure is less than atmospheric above the grate. This facilitates the quick removal of hot flue gases from the combustion zone. The furnace’s internal pressure is almost atmospheric. Therefore, if the doors are opened for firing and inspection, there will be no blow out of flames or entry of outside air into the furnace.