Fig. 1. "U," "W" and straight radiant tubes


A practical starting point to understand radiant tubes is to review the tube temperature and heat transfer of a single-pass radiant tube having a burner located on one end and an exhaust on the other. Such tubes may have many shapes, the most common being “U,” “W” and “straight” (Fig. 1).

Fig. 2. Typical radiant-tube temperature distributions

Radiant-Tube Fundamentals

With these conventional tube-firing configurations, the tube temperature is not uniform. The temperature profile is characterized by an increase in temperature from the region near the burner tip to a maximum temperature that is typically located in the firing leg or at the first return bend in the case of “U”- and “W”-style radiant tubes. If the tube is long enough, the temperature then decays along the length, reaching a minimum at the exhaust end of the tube. Typical tube temperature-distribution profiles for the three common radiant tubes are shown in Figure 2.

To further understand the issues presented by non-uniform tube temperature distributions, we need to examine the service limitations. The most common failure is a penetration of the tube wall – often preceded by bulging or sagging – in the region of highest temperature. Typically, the reason for this failure is that the mechanical loading experienced by the tube exceeds the high-temperature creep strength of the material. For a well-designed radiant-tube installation, the applied mechanical load is the result of the weight of the tube plus thermally induced stresses. In cases where allowances for differential thermal expansion of the tube and/or enclosing furnace structure are not accounted for, additional thermal stresses may be imposed.

Fig. 3. Creep strength of RA330 radiant-tube alloy

The strength of the tube needs to be sufficient to resist the stresses and have a reasonable safety margin. When reviewing the material characteristics for radiant tubes, the most limiting is the creep strength. This temperature-dependent ability to resist stress over a period of time falls off rapidly near typical operating temperatures. Figure 3 illustrates the creep strength of a typical 330 alloy commonly used for many high-temperature radiant tubes. While there are many metallic material choices with higher creep strength, they all exhibit similar behavior. Ceramic radiant tubes are sometimes considered to allow operation at higher temperatures, but their brittle nature calls for increased attention to stress-eliminating mounting-structure design and protection from impact from furnace loads or charging mechanisms.

The failure of radiant tubes typically occurs where the temperature is at its highest and the material at its weakest. Where a tube is used within “safe limits” and we consider the lack of uniformity of the typical radiant tube, only a small portion of the “safe” tube is used at its full potential. A common and often cost-effective approach is to design to exceed the maximum creep temperature and accept a shortened life. Generally, the industry has accepted this approach – with typical radiant-tube service lives ranging from one to five years – and the cost consequences of downtime and replacement.

Radiant-tube temperature non-uniformity underutilizes the heating capability and thus also limits the production throughput of the heat-treating furnace. The dominant mode of heat transferred from the surface of the tube to the furnace is by radiation. Many heat-treating furnaces are equipped with recirculating fans to help distribute heat to the load, but these play a small role in the overall heat transfer when compared to radiation given its 4th-power dependence on absolute temperature.

Fig. 4. Typical radiant-tube heat-flux distributions

Figure 4 shows typical heat-flux profiles for the common radiant-tube shapes. From this we can see a very high contribution of heat from the highest-temperature region and very little contribution from the balance of the tube. In essence, only a small portion of the tube is effectively utilized for heat transfer.

The Ideal Radiant Tube

Having reviewed the fundamentals and limitations of radiant tubes, we can now begin to characterize an ideal radiant-tube system. First, respecting the material strengths, the entire tube should be selected so that the developed stresses are below the acceptable creep strength to achieve the desired life/cost balance. Additionally, the heat flux should be consistent along the length of the tube for maximum utilization and minimized surface area (and thus cost) for the desired heat input. Clearly, the idealized radiant tube would have a perfectly uniform temperature along the length. This ideal tube would have minimal stresses yielding a long service life and transfer the maximum amount of heat to the furnace.

Practical experience has shown that with improvements in temperature uniformity the service life may be two to three times that of a conventional tube of the same material. Depending on the type of process, the increase in heat is typically from 10-30%.

Fig. 5. Increased heat ramp

The Impact on Furnace Production

There can be many limitations on the rate of production of the typical heat-treating furnace. Complex part geometries and metallurgical considerations may dominate the rate of application of heat. For many products, however, the production rate is governed by the rate at which heat can be delivered to the furnace – limited by the radiant tube.

For continuous furnaces, the radiant tubes in the heating section of the furnace can be improved to add heat and increase the “ramp rate” of the product temperature from inlet to soaking temperature. An increased ramp rate results in getting the product to temperature in a shorter distance along the furnace, allowing a longer length (and thus residence time) for soaking the product. If the actual soak time for the product stays the same, the throughput of the furnace can be increased as the soaking zone has essentially been lengthened. The concept of an increased ramp rate is illustrated in Figure 5. Depending on the soak time required, production-rate increases of 10-30% are common.

In batch furnaces, the typical cycle involves heating the furnace and product from an initial temperature to a final soaking temperature. For batch processes, the time required to reach the soaking temperature is often a key limitation. Adding extra heat during the heating period will shorten the time to reach soaking temperature, thereby reducing the total cycle time for the furnace. Again, depending on the required soaking time, the expected increases are on the order of 5-20%.

Experienced combustion-equipment manufacturers and/or furnace suppliers can help to estimate the expected production increases by a thermal analysis of the furnace and its operation. Current performance data will be extracted from the furnace to anchor a baseline scenario against which the improvements will be benchmarked.

Radiant-tube life also plays an important role in the production rate of the furnace. As tubes begin to fail, the decision has to be made to either “blank” it out of service or shut the furnace down to make the necessary repairs. Out-of-service radiant tubes reduce the available energy, resulting in a reduction in the throughput for furnace-limited products. Downtime obviously reduces the available production time. In either case, extending the service life of radiant tubes can have a very positive impact on the production rate of the heat-treating furnace.

Fig. 6. Installation of regenerative radiant-tube burners

The Commercial "Idealized Radiant Tube"

Innovations in radiant-tube burners and types of tubes during the last two decades have led to several options that very closely approach the idealized radiant tube.

One combustion style involves complete replacement of the single-pass tubes analyzed in this article by recirculating radiant tubes. These use high-velocity burners to recirculate high volumes of spent combustion products through the tube, diluting the combustion temperature and increasing the temperature uniformity. The downside of these systems is that the existing radiant tube requires replacement, resulting in additional costs and downtime.

Another style of burner exists for application on conventional radiant tubes. Compact regenerative burners are installed in pairs directly onto the existing single pass “U” and “W” tubes (Fig. 6). In this system, one burner will be firing into the radiant tube while the companion burner at the other end is in exhaust mode and collecting energy in a regenerative heat-storage bed. After a short time period, the firing and exhausting modes of the individual burners are reversed, thereby recovering energy previously stored in the exhaust cycle and delivering it as preheated combustion air to the firing burner of the radiant tube. While available heat – the common measure for energy efficiency in combustion – of the regenerative systems is typically in the 85% range, the greatest benefit is commonly the resulting tube temperature uniformity. Utilizing regenerative burners such as North American’s TBRT III (see the description in the sidebar) can typically increase heat-transfer rates by 10–50% depending on location within the furnace.

Another advanced technology for existing straight tubes is a new variant of the classical single-ended recuperative radiant tube. With this system, a burner and center tube are installed into one end of an existing (or new) straight-through radiant tube, and the other end is capped. The high-efficiency recuperative burner initiates combustion in the annulus between the inner tube and the radiant tube, transferring energy directly to the outer tube, which radiates into the furnace. This diffuse annular combustion method, combined with integral flue-gas recirculation, provides enhanced temperature uniformity and allows greater average heat input per unit area of the outer tube. Figure 7 illustrates this RASERT (reverse annulus single-ended radiant tube) system that was developed jointly by the Gas Technology Institute and The North American Mfg. Co. See the sidebar for an update on this technology.

Fig. 7. RASERT radiant-tube burner

Conclusion

Advancements in combustion equipment provide powerful new alternatives to conventional once-through radiant tubes that allow one to rethink classical design rules and typical, expected production output from existing heat-treating furnaces. Results are realized by increased thermal transfer, extended radiant-tube life or both. Increases in production typically dominate the economics when considering a new radiant-tube combustion system.

Although this article focuses on the positive effect on production, the advanced combustion systems mentioned all operate at very high combustion efficiencies and will lead to energy savings over older, conventional systems. Regenerative burners represent the highest practical combustion efficiencies, and thus fuel savings, but advanced recuperative systems such as the RASERT also yield significant energy savings.

In summary, advancements in radiant-tube technologies afford the heat-treat industry new methods for higher production, fuel efficiency and reduced downtime for tube replacement.IH

For more information:Dennis E. Quinn is senior product manager–regenerative and indirect combustion systems for The North American Manufacturing Company, 4455 East 71st Street, Cleveland, Ohio 44105; tel: 216-271-6000 X417; fax: 216.641.7852; email: dennisquinn@namfg.com; web: www.namfg.com

Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: thermal transfer, radiant tubes, creep temperature, continuous furnace, regenerative burner, annulus

SIDEBAR 1: TBRT III Compact Regenerative-Burner System for Radiant-Tube Processes

The North American Manufacturing Company’s TBRT III compact TwinBed regenerative radiant tube offers the increased production, significantly reduced fuel consumption and unequaled radiant-tube-life benefits realized by the users of over 3,000 earlier-generation TBRT burners.

The TBRT III’s robust ceramic heat regenerator is mounted in the section of the radiant tube within the furnace wall, keeping the heat inside where it belongs and reducing ambient temperatures around the furnace. The honeycomb bed design, housed in a stout silicon-carbide holder, provides the optimum combination of high thermal efficiency and long-term durability. This high-efficiency bed typically yields energy savings of up to 60%.

Alternate firing of each end of the radiant tube with the TBRT III assures uniform temperature, even with large “W”-style radiant tubes. This uniform tube temperature yields increases in heat liberated without raising maximum temperatures. The result: greater production with up to a 15-year radiant-tube life.

Key system components are integrated into the burner structure, facilitating quick and efficient installation and making the TBRT III an ideal candidate for both new and retrofit applications. On-board components include cycling valves, limiting valves and flow measurement. The integrated pilot assures reliable ignition over the operating range of the TBRT III burner.

The benefits of TBRT III are being utilized to boost production on two major steel-strip annealing furnaces. Several other heat-treating customers in Europe and the U.S. are currently evaluating the increased production potential.

The benefits can be realized on your existing radiant tube, minimizing both production downtime and installation costs. The TBRT III is available in three sizes with capacities up to 750,000 Btu/hr input.

SIDEBAR 2: RASERT Single-Ended-Burner System for Radiant-Tube-Fired Processes

North American’s new RASERT delivers enhanced production and improved energy efficiency to heat-treating users. The RASERT (reverse annulus single-ended radiant tube) burner system reverses conventional thinking about firing straight radiant tubes. Unlike traditional single-ended radiant tubes (SERTs), the RASERT initiates combustion in the annulus formed between the radiant tube and the central return tube. Patented by the Gas Technology Institute (GTI) and jointly developed and licensed by The North American Manufacturing Company, the RASERT offers increases in production, energy savings of up to 50% and extremely low emissions when compared to conventional radiant-tube systems.

Sometimes complex issues have simple solutions, and that is the case for the improved heat transfer from the RASERT. Burning in the annulus releases the heat immediately adjacent to the radiant tube, eliminating the dual-step heat transfer from the inner to the outer tube of conventional SERTs. Combined with diffuse combustion and internally recirculated flue gases, the RASERT delivers the uniform tube temperatures required to boost production for the heat-treating process.

The RASERT is an excellent example of collaboration encompassing a research organization, an industrial-equipment supplier and end users who all participated throughout the development, initial field testing and commercialization. Following successful testing on a zone at California Steel Industries, Inc.’s (CSI) No. 1 continuous galvanizing line, the RASERT was commercially launched and found its way into additional continuous heat-treating applications. Combined with other modifications, a 60-tube conversion on a thin-strip processing line resulted in a production increase exceeding 30%. Continuing collaborative projects include a second zone conversion at CSI and an evaluation program on a Surface Combustion Allcase® furnace at Akron Heat Treating Co.

Co-development of this technology was sponsored by several natural-gas distribution companies through Utilization Technology Development Corp., NFP (UTD), an Illinois not-for-profit company that funds technology development and demonstration activities related to end-use applications. The California Energy Commission sponsored the second zone at CSI to investigate the operating flexibility of the technology. The activities at Akron Heat Treating Co. are being sponsored by the Ohio Department of Development (ODOD) through an award from the State Technologies Advancement Collaborative (STAC) program funded by the U.S. Department of Energy (DOE), the Energy Solutions Center (ESC) and GTI’s Sustaining Membership Program (SMP).