Comparison of chimney draft calculation formulas in various reference materials
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Recently, I’ve noticed that the formulas for calculating chimney draft in various sources don’t quite match. I’d like to discuss this matter here; I would also appreciate any guidance from those with more experience: According to the \"Practical Design Manual for Boiler Rooms\", S=H*(rou
Recently, I’ve noticed that the formulas for calculating chimney draft in various sources do not agree with each other. I’d like to discuss this matter and seek advice from those who are more knowledgeable: In the \"Practical Design Manual for Boiler Rooms\", the formula is S=H*(ρ_air*273/（273+t_air）-ρ_smoke*273/（273+t_smoke）). This formula is clearly incorrect, as it does not include the acceleration due to gravity nor takes into account atmospheric pressure corrections. \"Principles and Calculations of Boilers (Second Edition)\": S = H * (1.2 – ρ_smoke * 273 / (273 + t_smoke)) * P_atmosphere / 760 * g. This formula takes an ambient air temperature of 20 degrees Celsius into account, and it includes a correction for atmospheric pressure, making it quite comprehensive ； HG 20680-1990: S = H/0.029 * (1/(273 + t_air) – 1/(273 + t_fume)) * P_atmosphere. This formula takes into account atmospheric pressure corrections; when calculating the density difference, it assumes that both the density of air under standard conditions and the density of fumes are 1.293 kg/m3. This is a simplified assumption, and as a result the calculated value will be lower than the actual value, resulting in a conservative design. HG 20680-2011: S=8*H(ρ_air – ρ_smoke)*g/10. This is the new standard, but I don’t understand this formula. According to the standard’s explanations, both ρ_air and ρ_smoke refer to densities under standard conditions; generally, we take ρ_air as 1.293 and ρ_smoke as 1.34. In that case, the value calculated using this formula turns out to be negative. This formula seems very different from the previous ones. I’m not sure whether it’s a mistake in the standard or due to certain assumptions being made. I would appreciate guidance from experts or those with knowledge in this area. Thank you.
This post was last edited by chalisen on 2019-11-16 at 20:10. Hello, you have in-depth knowledge regarding the suction force for boiler dust removal. Have you compared the four formulas you mentioned? Which one yields the highest results, which one the lowest, and which one is the most accurate? On which page is S=8*H(ρ_air-ρ_smoke)*g/10 in HG 20680-2011? I haven’t found it yet!
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The so-called suction force is the pressure difference resulting from density differences; it is determined by the data from actual operating conditions, and calculations can be done using those actual values.