The latest technology in radiant-tube inserts offers a cost-effective, proven alternative to other higher-cost energy-recovery solutions.
Emission reduction, energy efficiency and productivity are topics that appear regularly in articles published by Industrial Heating. This is expected because these topics impact every reader on a daily basis.
Emission Reduction, Energy Efficiency and Productivity
Metal-processing companies utilizing gas-fired radiant-tube furnaces continue to face increased environmental regulation of furnace emissions, increased quality requirements from customers and the ever-present need to continuously improve productivity. With every new issue of IH, operators look to see what new technology is being highlighted. What latest-and-greatest development is available to help heat-treating companies produce more with less?
Energy is typically one of the greatest costs in heat-treating businesses, so progressive companies are always looking for ways to get more output from the energy they buy. Over the years, great products have been featured, many of which look to reduce sources of waste.
One of the largest sources of wasted energy in heat treating is energy lost out of the exhaust of a combustion process. New products designed to reduce exhaust loss have received a lot of attention. Articles describing developments in burners and recuperators are published regularly.
In this article, a simple, high-value solution for radiant-tube productivity is presented. Ceramic radiant-tube inserts effectively reduce exhaust losses, increase the productivity of radiant-tube processes and reduce emissions per ton. The latest advancements in design make ceramic inserts a viable productivity option for any metal-processing company.
The concept of exhaust energy loss is understood. Combustion air is 80% nitrogen, so a portion of combustion energy is used to heat the inert nitrogen, which then just goes along for the ride and exits the tube as exhaust. This problem is depicted in a basic computational fluid dynamics (CFD) model of an exhaust tube (Fig. 1). In this model, combustion exhaust is entering the tube from the left. The exhaust gas entering the tube is very hot; illustrated as red in this model. Without inserts, this hot core of gas has minimal opportunity to transfer energy as it moves through the tube and exits at the right. It is cooler but still very hot.
Some convective heat transfer can be seen along the edges of the tube. Convective heat transfer is limited because exhaust flow is not very turbulent. Additionally, radiation is the primary method of heat transfer at operating temperatures, with the flame acting as the radiating media in the combustion leg. However, there is no radiating media in the exhaust stream. This hot core of gas leaving the process is the problem and the opportunity!
Inserts of different configurations and various materials have been present in the industry for a long time. Early designs recognized the hot core of gas moving through the tube and worked to break up that flow. Patents from as early as 1917 (U.S. Patent 1,246,583) describe “helical elements for use in heat interchanging apparatus,” meant to “break up or prevent the formation of continuous stream lines of the fluid thermal agent flow” and “thereby add materially to the rate at which heat is absorbed” (Fig. 4).
These early designs had simple spiral or helical shapes and were made of metallic ribbon. The spiral shape was meant to break up the core of gas and increase convective energy transfer in the tube. Unfortunately, the metallic material did not last in the combustion atmosphere. Other simple solutions were created in the shape of a large cross produced from refractory material. This product also worked to break the core of gases and increase convective heat transfer, but it was produced from a very low-emissivity material in a shape that created back-pressure in the tube.
Insert development continued with advances in material. The introduction of siliconized silicon-carbide inserts solved the material-degradation and the emissivity problems presented by previous iterations. These newer-version inserts – produced in a dense spiral shape similar to that originally described in 1917 –
increase convective heat transfer and impact radiant heat transfer. However, the spiral configuration significantly impacts pressure drop in the combustion system.
Most recently, a new design has been released to the market. This latest generation of insert technology combines the benefits of silicon-carbide material with a design that focuses on increasing radiant energy transfer as well as convective energy transfer, and it does so directionally. The nonsymmetrical wing-and-shell design drives more energy to the shell, which is always positioned toward the load.
The open cross section of the insert minimizes pressure drop within the tube and also allows operators to see the flame (Fig. 5 - lead image). The latest design, which is focused on increasing radiant heat transfer, is an improvement over previous designs because radiation is the primary method of heat transfer at temperatures above 1100°F (593°C). Optimization of the design and improvements in materials have maximized the effectiveness and minimized the back-pressure on the combustion system, making inserts a reliable, low-cost, proven technology for increasing performance of radiant-tube processes.
Radiant-tube inserts can be described as any object inserted into a radiant tube for the purpose of enhancing exhaust energy transfer to the load and reducing exhaust-gas waste. PSNERGY, LLC has developed the latest technology in ceramic radiant-tube inserts (RTIs). The effect of RTIs on the exhaust stream is shown in the CFD model (Fig. 3). Exhaust is entering the tube on the left; hot is illustrated as red. As this exhaust passes over the RTIs, a significant amount of energy is pulled from the stream and returned to the load. The exhaust exits significantly cooler; depicted as blue in this model.
Recognizing that furnace usage and system configurations vary widely, PSNERGY has published data proving significant improvements in energy available to the load with their design. One example of performance data (Fig. 2), from a 6-inch-diameter 309-SS U-tube with a 70-inch effective length firing an Eclipse TFB030 burner, shows improvements in energy transferred to the load in the range of 14-19% with the use of RTIs. A tube firing at 150,000 BTU/hour with inserts will deliver the same energy to the load as one firing at 200,000 BTU/hour without inserts. A tube firing at 250,000 BTU/hour with inserts will deliver more energy to the load than the same tube firing at 300,000 BTU/hour without inserts. Using more of the available exhaust energy allows furnace operators to increase production output per BTU of gas purchased while reducing emissions per ton of product produced.
RTIs can significantly increase the output of radiant-tube processes (Fig. 2). Radiant tubes with RTIs installed have been shown to deliver nearly 20% more energy to the load than tubes without. Although this level of productivity is common for other technologies, such as single-ended radiant tubes and recuperators, the cost of installation and maintenance must also be considered. Installing the latest burner or recuperator technology will certainly provide operators with productivity improvements. These are great products, but the cost to install can often be significant and require long process outages to change the combustion system, replace tubes, re-pipe combustion air or change gas lines.
RTIs are easily installed into furnaces, often during regularly scheduled downtime. The insert assembly is simply slid into the tube after removing the exhaust elbow or recuperator, if installed. Combustion-system tuning is recommended after installation of RTIs. Once installed, there is no need for maintenance; the inserts simply work. The ease of installation, relatively low cost per tube, no ongoing maintenance cost and large energy-to-load improvements make the latest RTI technology a proven value.
Productivity, Emission Reduction, Tube Life, Uniform Temperature
Increased energy to the load, delivering significant productivity increases and reducing emissions are certainly big drivers for installing RTIs in a process, but there are additional factors that must be considered. Tube life and process-temperature uniformity are also positively impacted by RTIs. The first question many will ask is, “How will inserts affect tube life?” RTIs positively impact radiant-tube performance and expected life.
The latest-technology inserts have been designed to maximize radiant and convective heat transfer while minimizing hot spots in the tube. The patent-pending, nonsymmetric inserts have been optimized to not exceed the heat-flux density limit of even the most basic radiant-tube material. Additionally, installing inserts in the exhaust portion of the tube increases temperature uniformity between combustion and exhaust legs.
Radiant-tube temperature profiles decrease from the burner to the exhaust of the tube. As the flame reaches the end of the combustion section, temperature starts to drop off. In a typical U-tube configuration without RTIs installed, the exhaust leg can be 150°F cooler than the combustion leg. The energy to maintain equivalent temperature with the combustion section is in the exhaust gas. However, there is no energy-transfer media to extract that energy. By installing RTIs, the exhaust-leg temperature increases, and the exhaust-to-combustion temperature difference drops, creating a very uniform temperature distribution across the entire tube.
Temperature uniformity is of great concern to heat-treating operations as customers require tighter temperature specifications. RTIs are an effective way to achieve greater uniformity. In addition to improving process quality, temperature uniformity also increases tube life by balancing thermal expansion of the combustion and exhaust legs.
In the U-tube configuration described (without inserts), thermal expansion will cause the hotter combustion leg to grow 0.100 inch more than the cooler exhaust leg, which causes considerable stress at the exhaust leg to the elbow joint, a typical failure point for U-tubes. By installing inserts, the exhaust-leg and combustion-leg temperatures are balanced, making the thermal expansion similar and significantly reducing the stress on the joint and extending tube life.
The latest technology in RTIs is proven to be a cost-effective way to improve heat-treating processes. This provides more energy to the load, reduces emissions per ton produced, improves temperature uniformity, increases tube life and reduces maintenance downtime. RTIs can be installed as stand-alone, high-value improvements to existing processes or used in conjunction with recuperators. If your operation can benefit from increased productivity, reduced gas consumption, improved temperature uniformity and reduced emissions, take a hard look at the value radiant-tube inserts can provide.