Natural Draught
- This is produced with the help of chimney, hence, it is also called as chimney draught.
- The difference in static pressure by the chimney is produced due to the difference in weight of column of hot gases inside the chimney and the weight of equal column of cold air outside the chimney.
- Since the density of cold air outside is more than the density of hot gases in the chimney, it follows that pressure just outside the level of chimney base would be more than the pressure inside the chimney at its base.
- This pressure difference is responsible for the flow of air from the surroundings to the furnace grate.
- A chimney is shown in Figure
- Amount of draught produced by the chimney mainly depends upon the chimney height, temperature of surrounding cold air and that of the hot gases inside the chimney.
Referring to Figure
Let T0 = Temperature corresponding to 0° C = 273 K
T1 = Temperature of cold air, K
T2 = Mean temperature of hot gases inside the chimney, K
H = Height of chimney, m
h = Draught required in mm of water column
m = Mass of air supplied per kg of fuel
(m + 1) = Mass of flue gases, kg / kg of fuel
The equation for draught ‘h’ in mm of water is given by the equation,
h = 353 H [1 / T1 (m + 1 / m) . 1 / T2]
Draught losses : Equation (1) only represents the theoretical value of draught which depends upon chimney height and weather conditions. The draught decreases with the increase in
surrounding air temperature or with decrease in hot gas temperature. However, the actual draught available would be less than the theoretical draught due to :
- Friction losses offered by the flue gases in the passages.
- Energy required to impart velocity to flue gases.
- Losses in bends and curves in the flue gas passages.