Why is the design flow velocity for chimneys set at 15 m/s? !

Heat tower effect – pressure difference, economic flow velocity, preventing water carryover; there are other considerations as well, but I’m too lazy to look them up.

Typically, the outlet inner diameter should be determined based on the optimal designed outlet flow velocity. The flue gas exit velocity is related to the engineering design concept of \"economic flow velocity\" and the rationality of chimney height. From the perspective of atmospheric pollutant emissions and dispersion, increasing the exhaust velocity appropriately, while ensuring that the design requirements for the exhaust stack are met, helps to boost the momentum of the flue gas and pollutants and reduce their concentration at the ground level. However, if the exhaust smoke velocity is too high, it can lead to excessive pressure in the air supply and exhaust systems, which is not economical; moreover, a rapid entrainment effect occurs at the chimney outlet ； Furthermore, an excessively low exhaust gas velocity can cause the flue gas to flow downward at the chimney outlet, resulting in poor exhaust flow. This hinders the proper emission and rapid dispersion of the flue gas, affecting not only the normal operation of the related exhaust equipment and the optimization of its economic and technical design but also leading to localized severe pollution due to the spread of smoke. It is only reasonable when the two are in balance.

The general provisions regarding the smoke velocity at the outlet of exhaust stacks can be found in Section 5.3.5 of the \"Technical Guidelines for Air Pollution Control Projects\" HJ2000-2010, under the topic of emission of polluted gases. It states that the outlet diameter of the exhaust stack should be determined based on the outlet flow velocity, which is preferably around 15 m/s. When a steel pipe chimney is used and its height is large or the amount of flue gas is high, the exit flow velocity can be appropriately increased to around 20 m/s–25 m/s. ” The determination of the flue gas outlet flow rate shall also comply with the requirements of relevant codes and standards for engineering design, fire protection design, environmental protection design, etc. For example, the \"Technical Specifications for Dust Removal in the Cement Industry\" (HJ434-2008) stipulate that: \"The outlet diameter of the exhaust stack should be determined based on the gas exit flow velocity, which can range from 10 to 16 m/s.\" The general provisions regarding the smoke velocity at the outlet of exhaust stacks can be found in Section 5.3.5 of the \"Technical Guidelines for Air Pollution Control Projects\" HJ2000-2010, under the topic of emission of polluted gases. It states that the outlet diameter of the exhaust stack should be determined based on the outlet flow velocity, which is preferably around 15 m/s. When a steel pipe chimney is used and its height is large or the amount of flue gas is high, the exit flow velocity can be appropriately increased to around 20 m/s–25 m/s. ” **The determination of the flue gas outlet flow rate shall also comply with the requirements of relevant codes and standards for engineering design, fire protection design, environmental protection design, etc. For example, the \"Technical Specifications for Dust Removal in the Cement Industry\" (HJ434-2008) stipulate that: \"The outlet diameter of the exhaust stack should be determined based on the gas exit flow velocity, which can range from 10 to 16 m/s.\"

For power station boilers and industrial boilers that generate large amounts of flue gas emissions, the flue gas exit velocity at their exhaust stacks should be around 10 m/s to 30 m/s, with 15 m/s being the most appropriate value；

There are regulatory requirements; it’s sufficient to follow them. Why ask why? Do you want to break those regulations and create problems for yourself in the future?