Finned tubes of tubular heating furnace have fallen off
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Dear experts in the group, what causes the fin tubes to fall off in tubular heating furnaces? I’m seeking an explanation; thank you all! The basic situation is as follows: The chimney of the natural gas tubular heater often emits black smoke. The air valves have been opened to full capacity, and the preheating temperature has been reduced from the designed value of 635°C to 615°C to keep the system operating, which affects the acetylene yield. The flue gas temperature increased from 150°C to 270°C, resulting in flue gas emissions that did not meet the standards, with persistent excess levels of CO. During the shutdown for maintenance in August 2016, it was found that the finned tubes were severely corroded and falling off, blocking the convective air ducts, with a tendency for the condition to worsen. The relevant technical parameters are as follows: 1. Total sulfur content in the fuel gas: Due to frequent changes in the gas quality, the total sulfur content in the fuel gas varies between 1–30 mg/m3. (The concentration in the new area is usually below 10 mg/m3; it rose above 20 mg/m3 after June 2016.) 2. Designed flue gas temperature: 186℃ ； Design thermal efficiency: 89.2%. Under normal operation, the actual smoke exhaust temperature is 130–170°C; in worse conditions, it can reach 270°C. The actual thermal efficiency is 90%. 3. At 100% load: Designed fuel gas flow rate: 337 Nm3/h ； Excess factor: 1.2 ； Flue gas flow rate: 4176 Nm3/h ； The medium being heated is natural gas, with a flow rate of 7107 Nm3/h and an inlet temperature at room temperature.
To provide further clarification on my question: both the base tube and the fins of the finned tube are made of 321 material. The base tube is not damaged, with no signs of corrosion; however, the fins have carbonized and fallen off, to the point where they can be broken with just a finger.
Is it possible that it’s a problem with the device itself? ?
Based on the description, it should be a finned tube with thin steel strips for the fins; these fins are very thin and not resistant to high temperatures. It is recommended to replace the heat transfer elements with those of this specification.
At present, the main suspicion is that the fin material is substandard; we are waiting for material analysis tests
The temperature inside the flue is not high, usually around 200 or 300 degrees
It’s impossible to determine what the cause is. Which possibility do you think is more likely?
The device itself? Which one specifically are you referring to?
Based on your description, I believe that 1) the sulfur content in the fuel gas should be compared with the design specifications; it is not advisable to deliberately lower the exhaust temperature of the flue gases, as doing so may lead to dew point corrosion. 2. The reason why the waste heat temperature of natural gas cannot increase is not necessarily due to a lack of fuel; it’s also very likely that the heat exchange capacity has declined. By checking the equipment when it is stopped, if it is found that the fins have fallen off, this proves that the inability to raise the temperature is caused by a decrease in the heat exchange coefficient. 3. Generally, there is an excess of oxygen in such heating furnaces, so CO should not be present in the flue gases. 4. It seems that this heating furnace is a non-forced draft furnace; therefore, it is essential to adjust the burner flame properly during operation to prevent under-oxygen combustion.
The flue gas temperature is not deliberately reduced either; it remains at that level during actual operation. Since the fuel gas contains very low levels of sulfur (essentially no more than 10 mg/m3), there is no issue of dew point corrosion – evidence of this is the fact that, after years of operation, the finned tubes remain undamaged, with no signs of corrosion or pits； The reason the temperature of the material being heated doesn’t rise is that the fins have fallen off, blocking the flue and resulting in poor heat exchange ； An excessive level of CO in the flue gas also occurs after the fins fall off ； The furnace is a naturally ventilated furnace (without a blower), and there are no measures to regulate the air flow during operation (as there are no control valves in the air ducts, and the dampers remain fully open). The oxygen content in the flue gas is designed to be around 3.6% (with an excess coefficient of 1.2), so there should be no issue with under-oxygen combustion. Are there any other reasons for the shedding of your fins? Thank you