Natural Draught with advantages and disadvantages applications in Natural Draught:
Natural draught system employs a tall chimney as shown in figure. The chimney is a vertical tubular masonry structure or reinforced concrete. It is constructed for enclosing a column
of exhaust gases to produce the draught. It discharges the gases high enough to prevent air pollution. The draught is produced by this tall chimney due to temperature difference of hot gases in the chimney and cold external air outside the chimney.
Where H- Height of the Chimney (m)
pa –Atmospheric pressure (N/m2)
p1 –Pressure acting on the grate from chimney side (N/m2)
p2 –Pressure acting on the grate from atmospheric (N/m2)
Due to this pressure difference (p), the atmospheric air flows through the furnace grate and the flue gases flow through the chimney. The pressure difference can be increased by increasing the height of the chimney or reducing the density of hot gases.
Merits of Natural Draught
1. No external power is required for creating the draught.
2. Air pollution is prevented since the flue gases are discharged at a higher level
3. Maintenance cost is practically nil since there are no mechanical parts.
4. It has longer life.
5. Capital cost is less than that of an artificial draught
Demerits of natural draught
1. Maximum pressure available for producing draught by the chimney is less.
2. Flue gases have to be discharged at high temperature since draught increases with the increase in temperature of flue gases.
3. Heat cannot be extracted from the flue gases for economizer, superheater, air pre-heater, etc. since the effective draught will be reduced if the temperature of the flue gases is decreased.
4. Overall efficiency of the plant is decreased since the fluid gases are discharged at higher temperatures.
5. Poor combustion and specific fuel consumption is increased since the low velocity of air affects thorough mixing of air and fuel.
6. Not flexible under peak loads since the draught available for a particular height of a chimney is constant.
7. A considerable amount of heat released by the fuel (about 20%) is lost due to flue gases.
Natural draught system is used only in small capacity boilers and it is not used in high capacity thermal plants.