Recuperators recover heat from the furnace waste gases and convert it into usable heat that is put into the combustion air of a gas burner. They serve to pre-heat the unit, which in turn reduces the amount of fuel needed to heat the product. Compared to alternative methods, such as regenerative furnaces, the initial costs are lower. Recuperators can recover 70-80% of the waste heat, can pre-heat air up to 1200°F, increase flame temperature, and improve furnace efficiency.A failed or failing recuperator in a metallurgical furnace can cause significant loss of productivity and potential downtime. Understanding what causes a recuperator to fail is the first step in prevention, or at least a longer life cycle. A watchful eye for signs of potential issues is key.
In the steel industry, the first rows of tubes generally bear the brunt of the impact of high temperatures and the most corrosion from particles in the stream that can cause erosion and corrosion. Often, changing the first row to a material with better thermal properties can add significant life to the recuperator.
Planning solutions to recuperator failure
The first step towards a solution is a visual inspection. If the tube appears ‘green’ in color, sulfur is the culprit. We call it ‘green rot’. While it’s not the only cause of deterioration, it is a common one. Next, material analysis can be done to determine the changes in chemical composition of the tube. Nickel alloys in the tubes can become deteriorated by sulfur (or other corrosive elements) in the gas stream. This can destroy the nickel alloy quickly. Often there is a scale that has trapped some of the corrosive elements and that accelerates the deterioration. Higher Chromium content in the replacement tubes will enhance the corrosion resistance. Adding Aluminum to the metallurgy of the tube can help form a tenacious surface that resists oxidation.
We also look at the grain size of the material. Changes from the standard ASTM 4-6 to aught or 1 means the material is being exposed to temperatures beyond what they it can handle and is overheating. Again, a change in material composition might be suggested.
Quick replacement can protect productivity
Downtime due to a failed recuperator can have a devastating effect on productivity and profitability. So the faster a recuperator bundle can be replaced, the better. Alloy Engineering pioneered a removable module that can replace the first few rows of tubes, which is where the corrosion attack has the greatest impact. This reduces downtime, as well as the cost of replacing the entire recuperator.
Alloy Engineering is always available to do an on-site inspection to evaluate the conditions of recuperators and determine the cause of failure or make recommendations on material changes.