There is a saying that time is money. This is certainly true for manufacturing facilities that are faced with the high costs of not producing during furnace maintenance programs. That's why reducing downtime is a strong driving force within outage schedules. When maintenance work includes monolithic refractory, there is a need to go through a controlled and sometimes lengthy heat-up schedule. Without a controlled heat-up, there is the risk of the refractory lining not achieving desired properties, or worse, being damaged during the heat-up process. In severe cases, there is risk the refractory lining could spall and even explode, potentially damaging other components.
Improved refractory technologies allow for much faster furnace firing schedules, providing a great opportunity to reduce downtime. However, with the ability to fire in faster, there are more aspects of the process that need to be addressed for an optimized and safe firing process.
Castable refractories contain water in some form or another. As the refractory heats up, this water turns to steam and is driven out of the lining. Essentially, the speed of heatup is controlled by the steam pressure that builds within a lining and the lining material's ability to withstand or dissipate the buildup of pressure. This sounds simple enough, but when attempting to push firing schedules to their limit, this can get quite complicated. The quantity and type of liquid being used and the material's porosity, heat capacity and conductivity will all directly affect how fast the lining can be fired. Additionally, the total quantity of material being used, the lining thickness and configuration, location within the unit and type of unit all factor into how fast a lining can be fired.
Let's look at the mechanics of what is happening to a green lining during heatup. As the temperature inside a furnace increases, a temperature gradient is created throughout the refractory lining. The hot face of the refractory lining will be exposed to a higher temperature that decreases through the lining to the shell of the furnace. During heat-up, this temperature gradient forces free moisture back toward the cold face and the shell. With gravity, the water tends to accumulate at low points within the furnace. For the water to be removed from the system, it either must be allowed to escape through weep or vent holes or, if that is not an option (as with pressurized vessels), then the liquid must convert to steam and work its way back out through the pores of the refractory lining.
Conventional heat-up schedules tend to be slower and let this moisture escape over an extended time. If the heat-up schedule is to be accelerated, then the process must account for the faster release of moisture. Air flow over the surface of the refractory is critically important to allow for rapid removal of steam from the lining. Sufficient air flow over the refractory surface ensures that the air above the refractory does not become moisture-saturated. Saturated air would limit further steam from escaping the lining. The flow of dry air over the refractory also creates a Venturi effect, enabling steam to escape more readily. This is critical for minimizing heat-up times. Convection within the unit is also critical. This will happen with an air exchange rate of at least 30 times the furnace volume per hour while maintaining positive pressure.
There is not a single heat-up schedule for any specific material. Work with the material supplier to develop the best heat-up schedule for your specific lining configuration. Steady state as well as transient heat-flow analysis with understanding of the critical water release temperature points of the material will help determine the optimal heat-up schedule. Ensure burner equipment can handle the temperature control needed and supply the necessary air exchange. Lastly, install thermocouples and pressure probes strategically within the unit to monitor and control for uniform temperature and pressure throughout the process.
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