When should the exhaust fan be turned on during the heating of the furnace?

The exhaust fan should be turned on before drying the furnace, to create a certain negative pressure throughout the system.

For oven drying, natural ventilation is usually sufficient; there is no need to use exhaust fans.

Personal understanding: It’s best to turn on the exhaust fan during furnace drying in order to dry the refractory lining in the flue. The temperature of the flue gas leaving the preheater should not be too low; the principle is that the temperature of the flue gas at the exit of the preheater should be higher than the designed exhaust temperature, in order to prevent corrosion of the preheater.

Small heating furnaces generally do not have blowers or exhaust fans; they rely on flue dampers and the opening degree of air valves to regulate the amount of natural ventilation and the draft in the chimney, thereby controlling the negative pressure inside the furnace chamber； Large heating furnaces are generally equipped with blowers and exhaust fans. Whether the furnace is preheated or not, as long as the heating furnace is lit, it is necessary to control the negative pressure in the furnace chamber before lighting it. If the negative pressure in the furnace is too high or even turns positive, flashback will occur ； If the negative pressure in the furnace is too low, the burner cannot be ignited. Since the original poster asked when to turn on the exhaust fan during the heating of the furnace, it is likely that the furnace is quite large and is equipped with both an exhaust fan and a blower. In such cases, the exhaust fan should be turned on first before starting the furnace, in order to remove the combustible gases from the furnace chamber and keep their concentration within acceptable levels. After that, the blower is turned on, and the airflow rates of both fans are adjusted so that a certain negative pressure is maintained in the furnace chamber before the furnace is started.

I have been involved in the heating-up process of hydrocracking units. First, the blower is started to purge the furnace, and flue dampers are adjusted to create a negative pressure. Then, the pilot light is ignited, followed by the main burners. Once the temperature at the inlet of the air preheater reaches around 300 degrees, the exhaust fan is turned on. This is because the temperature at the outlet of the air preheater must not drop below 120 degrees, in order to prevent corrosion due to dew points in the preheater.

I have been involved in the heating-up process of hydrocracking units. First, the blower is started to purge the furnace, and flue dampers are adjusted to create a negative pressure. Then, the pilot light is ignited, followed by the main burners. Once the temperature at the inlet of the air preheater reaches around 300 degrees, the exhaust fan is turned on. This is because the temperature at the outlet of the air preheater must not drop below 120 degrees, in order to prevent corrosion due to dew points in the preheater.

If it is a small boiler that uses wood for heating, there is no need to turn on the exhaust fan at first; natural ventilation is sufficient. Later, as a small amount of coal is added, the exhaust fan needs to be turned on, as the metals in fluidized bed or chain grate boilers require protection against localized overheating. You should make clear which boiler.