Refractory insulation is a key component in industrial furnaces and ovens. However, the high operating temperatures of such process heating equipment create extreme stresses from expansion and contraction, eventually causing the insulation to wear out. Areas prone to refractory failure include welded joint sections, corners and seams and burner areas.
If damaged insulation isn't replaced, heat loss will increase. Under normal conditions, up to 10% of the total heat input is lost through insulated surfaces, according to the U.S. Department of Energy (DOE).
Other potential problems from poor insulation include:
- Overheating of the heat exchanger or other components, damaging the system
- Soot production and interference with combustion product flow occur, reducing efficiency
- Furnace shell can burn through, releasing dangerous gases, flames and extreme heat
- Structural weakening can occur, leading to catastrophic failure
Covering your heat losses
You can significantly reduce these heat losses by:
- Reviewing process requirements to select appropriate insulation
- Properly designing, installing and preconditioning insulation systems during start-up
- Operating equipment to specifications and performing periodic maintenance
Infrared thermography helps find damaged insulation and should be a part of your preventive maintenance program. Inspect insulation during the start-up cycle of a furnace or when there is a change in process. Document any hot spots and potential areas susceptible to failure for future reference and monitoring. Examine every accessible part of a furnace. Also, record the absolute temperature to compare to the maximum allowable shell temperature of the furnace.
The material strength of refractory insulation is also reduced when the insulation reacts with chemicals found in the furnace atmosphere, such as sulfur, sodium, potassium and hydrocarbons. When hydrocarbons react with iron oxides, spalling (cracking of concrete and other structural materials) can occur from thermal shock. Cold crush or room temperature strength testing verifies the strength of the installed material.
Corundum, a type of aluminum oxide, is another problem. When liquid aluminum is oxidized by either exposure to air or another source of oxygen, corundum crystals form and are deposited onto furnace walls. The deposits are often removed by scraping, which can damage the refractory lining inside the furnace. Corundum can also penetrate the lining wall, causing stress fractures and making the refractory more susceptible to thermal cycling or thermal shock.
Maintaining proper furnace pressure (usually negative) is also critical to avoiding degradation of refractory insulation. If not maintained, air enters the furnace, damaging the insulation. Even a small gap around the furnace door can bring in cold air through the bottom.
Calculating insulation's impact on heat loss
Various free tools can help you estimate the cost and savings of replacing insulation. With 3E Plus software, you can select and calculate insulation thickness for a variety of conditions.
Also, process heating assessments through the DOE's MEASUR tool allow you to survey process heating equipment, identify the most energy-intensive users and find energy-saving opportunities.
Consider the use of artificial intelligence in your operations
When properly configured, artificial intelligence (AI) can assist in the early detection of insulation failure, potentially saving maintenance costs and extending equipment life.