Radiant tube style furnaces are common in the heat-treating industry because they offer the opportunity to elevate the temperature of a product in a neutral or specially designed atmosphere. Essential to providing consistent product quality, reliability, affordability and improved environmental performance, these furnaces should be maintained to assure normal and consistent operations and whenever possible, be optimized using new technology. As many industry experts would agree, the seduction to install new technologies can be overwhelming, but, it is important to remember that the latest devices must be implemented as part of a disciplined furnace operation optimization program.
Thermocouples are used for precise and accurate furnace temperature measurement and are essential for proper observation and control. Their relative accuracy has a direct impact on fuel use, combustion efficiency and product quality. Thermocouple life is strictly dependent on time at temperature. To prevent failure, it is necessary to develop and maintain timelines and schedule procedures for inspection, testing and periodic replacement of the thermocouple and lead wires. Timelines can greatly vary, from years in low-temperature applications to as little as once a month in critical high-temperature, corrosive atmosphere environments.
Thermocouple placement is equally important and ideally would be located near the physical center and mass center of the load. In many cases, this is difficult or impossible to achieve. Common practice is to locate the thermocouple near the center of the load and the heat source. Uniformity surveys can help identify the ideal location for the control thermocouple.
Process controls also are fundamental to ensuring that the furnace is optimized for quality and energy-efficient production. Good process controls primarily should consist of verifying that the thermocouples are accurate together with the installation and management of proper tuning parameters. Thermocouple-based control systems should be calibrated to traceable national standards as frequently as once per month. Tuning of the control parameters is not always well understood, but remains an important aspect of system control and efficiency. For example, a common criterion is to adjust the controller so that the system has a decay ratio of one-quarter, resulting in a good compromise between fast system response and little delay before achieving a steady state condition.
Combustion adjustment and enhancement of radiant tube furnaces is a particularly tough challenge due to tube damage that can occur from excessive temperatures. Significant temperature differences along the tube length can lead to thermal stress-related failures. This particularly is true for recuperated burner systems where the highly turbulent burner leg gases inside the tube can peak at temperatures as high as 4000F (2205C), while the exhaust leg gases become laminar with very little accompanying heat transfer. In high-temperature applications (1400F or 760C and above], radiation is the primary method of heat transfer from fuel to the tube and then to the load. However, because fuels such as natural gas contain little carbon, they are not very effective at radiation. Under such conditions, it normally would be desirable to adjust the combustion somewhat fuel rich so that the carbon slowly burns, thus contributing more radiant heat along the length of the tube. Unfortunately, this same excess carbon is responsible for material degradation of the tube as well as fouling of the heat transfer surfaces.
The radiant heat problem is not new, and partial premix burners have been used to provide a compromise and allow most combustion systems to operate as high as 3% excess oxygen and still have radiant features. One new technology that recently has been developed is the installation of silicon graphite (SiG) inserts into the exhaust leg of the radiant tube. The inserts stabilize and circulate the gases as they travel down the tube while serving as a radiant body within the tube. Use of inserts improves the convection and radiant heat transfer from the gases to the tube by allowing the exhaust leg of a radiant tube to match the heat transfer rate of the burner leg. The result is better temperature uniformity, less distortion of the alloys and less carbon buildup while realizing significant production improvements and fuel savings.
It seems relatively obvious, and common sense suggests that a successful furnace operation and optimization program consists of a balance between ensuring the efficiency and environmental performance of the existing design prior to, during and after the addition of a new technology.