What is the appropriate range for selecting the flue gas flow rate and air flow rate in a tubular air preheater?

In vertical air preheaters, the gas flow velocity is 10~14 meters per second； The air flow velocity is 45% to 55% of the flue gas flow velocity.

The flue gas flow velocity is generally set at 10–14 m/s. For the air flow velocity, considering that the lateral scouring of the tube banks by air results in a higher heat transfer coefficient, the air flow velocity can be taken as half of the flue gas flow velocity, i.e., W_air/W_flue = 0.5, which is approximately 5–7 m/s

In the series book \"Boiler Structure and Design\", the recommended optimal flue gas velocity for vertical tubular air preheaters is 9~13 m/s； To maintain a reasonable ratio between the air velocity and the flue gas velocity, so that the heat transfer coefficients on the flue gas side and the air side are similar and a higher heat transfer coefficient can be achieved, the air velocity is generally controlled to be 1/2 of the flue gas velocity. Another configuration of the tubular air preheater is the horizontal air preheater, which is characterized by the tubes being arranged horizontally; flue gas flows horizontally outside the tubes, while air flows vertically inside them. Due to its less compact structure and larger size, it is difficult to maintain an appropriate flue gas velocity, and the expansion sealing is also complicated; therefore it is generally not used, and no recommended range for flue gas and air velocities exists.

To increase the heat release coefficient on the flue gas side, raise the wall temperature, and prevent ash accumulation, the flow velocity of the flue gas in the air preheater should be greater than that of the air. However, too high a flue gas velocity can cause structural difficulties and lead to an increase in flue resistance, with larger pipe walls and lower exit temperature and pressure. When a higher preheated air temperature is required, the temperature and pressure at the air outlet will be lower, which results in a very low average temperature and pressure in the air preheater. This necessitates a large heating surface area, which is not economical. Moreover, when the preheated air temperature reaches a certain level, the temperature and pressure at the air outlet may drop to zero; in such cases, it is impossible to achieve the desired hot air temperature using only a single-stage air preheater. To ensure economical use of the heated surfaces, the temperature and pressure at the air outlet side (i.e., the flue gas inlet side) should generally not be lower than 25–30ºC. For this reason, when burning low-quality fuels, bituminous coal, and in liquid slag furnaces where a hot air temperature of over 300ºC is required, and a tubular air preheater is used, it is necessary to adopt a two-stage arrangement combining a economizer and an air preheater.

The flue gas flow rate is generally set at 9–11 m/s, while the air flow rate is half of the flue gas flow rate.