Chimney Height:
where h is draught in mm of water, H is chimney height in metre, Ta is temperature of outside air in Kelvin, Tf is flue gas temperature in kelvin, m is mass of air supplied per kg of fuel in kg.
If Hf is height of hot gas column in metre
Mass Flow Rate of Hot Flue Gas:
Condition for Maximum Discharge:
Example 4.12: Calculate minimum chimney height required to produce a draught of 25 mm of water column, if 20 kg of air is required to burn 1 kg of fuel on the grate. The minimum temperature of gas inside the chimney is 360°C and that of atmospheric air is 28°C.
Solution:
Draught in mm of water is given by
where h is draught in mm of water, H is chimney height in metre, Ta is temperature of outside air in Kelvin, Tf is flue gas temperature in Kelvin, m is mass of air supplied per kg of fuel in kg.
Example 4.13: Calculate the height of a chimney to produce a static draught of 18 mm of water. The temperature of flue gas inside the chimney is 300°C and temperature of air outside chimney is 15°C. The value of R for air = 0.287 kJ/kg K and for flue gas = 0.26 kJ/kg K.
Solution:
Similarly, density of flue gas at 300°C is
Example 4.14: Calculate mass of air used per kg of fuel burnt in a boiler for chimney height of 35 m. Drought produced in terms of water column is 20 mm. The temperature of flue gas inside the chimney is 365°C and that of air outside the chimney is 32°C. Also, calculate the draught produced in terms of height of column of hot flue gas in metres.
Solution:
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